Analysis of Interference and Performance in Heterogeneously

Analysis of Interference and Performancein Heterogeneously Deployed LTE systems

MATTIAS BERGSTROM

Master of Science ThesisStockholm, Sweden 2010

Analysis of Interference and Performancein Heterogeneously Deployed LTE systems

MATTIAS BERGSTROM

Master of Science Thesis performed at

Wireless Access Networks, Ericsson Research

September 2010

Supervisor: Konstantinos DimouExaminer: Ben Slimane

KTH School of Information and Communications Technology (ICT)Department of Communication Systems (CoS)

CoS/RCS 2006-TRITA-ICT-EX-2011:6

c© Mattias Bergstrom, September 2010

Tryck: Universitetsservice AB

Abstract

Heterogeneous network deployment has been advocated as a mean to enhancethe performance of cellular networks, but at the same time heterogeneous de-ployments give rise to new interference scenarios which are not seen in homoge-neous deployments. This report includes five studies pertaining heterogeneousnetwork deployments which is based on simulations of LTE in high detail onthe lower layer protocol stack. In the first study it is investigated if results fromsimulated systems with ideal deployments can be generalized to realistic lowpower node deployments, which is seen to be the case.

Three heterogeneous network configurations, specified by 3GPP, were com-pared to a macro-only system. It is observed that the gain from low powernodes is strongly connected to the distribution of UEs. If the UE distributionis uniform the UE throughput gain is below 100 % while if the UEs are highlyclustered a UE throughput gain of 400 % is achieved.

The configuration with uniform UE distribution was further analyzed and itwas seen that in a low load system the average UE throughput gain from lowpower nodes is below 20 %. In a low loaded system with uniform UE distributionadding low power nodes is not a good way of enhancing the system performance.

A study investigating the gain of low power node range extension showedthat SINR problems arise if the range of the low power nodes is extended,however the system as a whole gets increased throughput. The same appliesfor UE throughput. The main reasons are macro layer offloading & reducedinterference created by the macro layer.

It is showed that if more low power nodes are added the UE throughput gainper low power node increases. It is also showed that a system with two rangeextended low power nodes outperforms a system with four low power nodeswithout range extension. Inter-low power node interference is seen not to be aproblem in the simulated system configurations.

iii

Acknowledgements

I would like to express my gratitude to my supervisor, Konstantinos Dimou,for his valuable input, guidance and commitment through this project. Kon-stantinos has always been supportive and found time for discussions around theproject.

I am thankful to Johan Lundsjo, manager at RAN Architecture & Protocols,for giving me the opportunity to do this project here in Ericsson.

I would also like to thank my examiner, Ben Slimane, my colleagues; PeterMoberg, Gunnar Mildh, Michael Eriksson and Robert Baldemair for their inputand discussions around the topic of this project and Jessica Ostergaard forreminding me to go home after too long days in the office.

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Contents

1 Introduction 1

1.1 The wireless system . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.1.1 First generation . . . . . . . . . . . . . . . . . . . . . . . . 1

1.1.2 Second generation . . . . . . . . . . . . . . . . . . . . . . 1

1.1.3 Third generation . . . . . . . . . . . . . . . . . . . . . . . 2

1.1.4 Fourth generation . . . . . . . . . . . . . . . . . . . . . . 2

1.2 Problem statement . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.3 Thesis outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2 What is interference? 5

2.1 Frequency hopping . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.2 Spatial multiplexing . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.3 Beam forming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.4 Interference cancellation . . . . . . . . . . . . . . . . . . . . . . . 8

3 Fourth Generation cellular networks 11

3.1 Long Term Evolution . . . . . . . . . . . . . . . . . . . . . . . . . 12

3.1.1 OFDM . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

3.1.2 Spectrum flexibility . . . . . . . . . . . . . . . . . . . . . 13

3.1.3 Multiple antenna technology . . . . . . . . . . . . . . . . 13

3.1.4 Hybrid ARQ with soft combining . . . . . . . . . . . . . . 13

3.2 LTE-Advanced . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

3.2.1 Carrier aggregation . . . . . . . . . . . . . . . . . . . . . . 14

3.2.2 Higher order MIMO . . . . . . . . . . . . . . . . . . . . . 14

3.2.3 Coordinated Multi-Point transmission and reception . . . 14

3.2.4 Heterogeneous network deployment . . . . . . . . . . . . . 15

4 New interference scenarios in Heterogeneous Networks 17

4.1 Downlink . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

4.1.1 Low power eNB interference to macro UE . . . . . . . . . 18

4.1.2 Macro eNB interference to low power node UE . . . . . . 18

4.2 Uplink . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

4.2.1 Macro UE interference to low power eNB . . . . . . . . . 18

4.2.2 Low power node UE interference to macro eNB . . . . . . 18

4.3 Crucial factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

4.3.1 Cell association . . . . . . . . . . . . . . . . . . . . . . . . 19

4.3.2 P0 offset . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

vii

viii Contents

5 Impact of misplacement of low power nodes 23

5.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

5.2 Simulation details . . . . . . . . . . . . . . . . . . . . . . . . . . 23

5.2.1 Performance Measurements . . . . . . . . . . . . . . . . . 23

5.2.2 Configurations . . . . . . . . . . . . . . . . . . . . . . . . 25

5.2.3 System parameters . . . . . . . . . . . . . . . . . . . . . . 25

5.2.4 Traffic model . . . . . . . . . . . . . . . . . . . . . . . . . 26

5.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

5.3.1 Performance overview . . . . . . . . . . . . . . . . . . . . 27

5.3.2 User distribution . . . . . . . . . . . . . . . . . . . . . . . 27

5.3.3 Interference . . . . . . . . . . . . . . . . . . . . . . . . . . 28

5.3.4 SINR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

5.3.5 Cell Throughput . . . . . . . . . . . . . . . . . . . . . . . 32

5.3.6 UE Throughput . . . . . . . . . . . . . . . . . . . . . . . 35

5.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

6 Analysis of 3GPP system configurations 41


6.1.1 Configurations . . . . . . . . . . . . . . . . . . . . . . . . 41


6.1.3 Traffic model . . . . . . . . . . . . . . . . . . . . . . . . . 42


6.2 Uplink results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43


6.2.2 Cell throughput . . . . . . . . . . . . . . . . . . . . . . . 44

6.2.3 Interference . . . . . . . . . . . . . . . . . . . . . . . . . . 48

6.2.4 SINR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

6.2.5 UE Throughput . . . . . . . . . . . . . . . . . . . . . . . 52

6.3 Downlink results . . . . . . . . . . . . . . . . . . . . . . . . . . . 53


6.3.2 Cell throughput . . . . . . . . . . . . . . . . . . . . . . . 54

6.3.3 SINR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

6.3.4 UE Throughput . . . . . . . . . . . . . . . . . . . . . . . 59

6.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

6.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

7 Analysis of 3GPP system configurations - Low load 63


7.1.1 Configurations . . . . . . . . . . . . . . . . . . . . . . . . 63


7.1.3 Traffic model . . . . . . . . . . . . . . . . . . . . . . . . . 63


7.2 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

7.2.1 SINR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

7.2.2 Cell throughput . . . . . . . . . . . . . . . . . . . . . . . 65

7.2.3 UE Throughput . . . . . . . . . . . . . . . . . . . . . . . 67

7.3 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

Contents ix

8 Analysis of 3GPP system configurations - Range extension 718.1 Simulation details . . . . . . . . . . . . . . . . . . . . . . . . . . 71

8.1.1 Configurations . . . . . . . . . . . . . . . . . . . . . . . . 718.1.2 System parameters . . . . . . . . . . . . . . . . . . . . . . 718.1.3 Traffic model . . . . . . . . . . . . . . . . . . . . . . . . . 718.1.4 User distribution . . . . . . . . . . . . . . . . . . . . . . . 72

8.2 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 728.2.1 Cell Throughput . . . . . . . . . . . . . . . . . . . . . . . 728.2.2 Interference . . . . . . . . . . . . . . . . . . . . . . . . . . 728.2.3 SINR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 768.2.4 UE Throughput . . . . . . . . . . . . . . . . . . . . . . . 818.2.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . 89

8.3 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89

9 Analysis of 3GPP system configurations - Multiple low powernodes 919.1 Simulation details . . . . . . . . . . . . . . . . . . . . . . . . . . 91

9.1.1 Configurations . . . . . . . . . . . . . . . . . . . . . . . . 919.1.2 System parameters . . . . . . . . . . . . . . . . . . . . . . 929.1.3 Traffic model . . . . . . . . . . . . . . . . . . . . . . . . . 929.1.4 User distribution . . . . . . . . . . . . . . . . . . . . . . . 92

9.2 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 929.2.1 Cell Throughput . . . . . . . . . . . . . . . . . . . . . . . 929.2.2 Interference . . . . . . . . . . . . . . . . . . . . . . . . . . 939.2.3 SINR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 959.2.4 UE Throughput . . . . . . . . . . . . . . . . . . . . . . . 97

9.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 999.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99

10 Conclusions, proposal and future work 10310.1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10310.2 Proposal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105

10.2.1 Existing ICIC schemes . . . . . . . . . . . . . . . . . . . . 10510.2.2 Fractional Frequency Reuse . . . . . . . . . . . . . . . . . 10510.2.3 Proposed scheme . . . . . . . . . . . . . . . . . . . . . . . 108

10.3 Proposed further studies . . . . . . . . . . . . . . . . . . . . . . . 11010.4 Alternative technology . . . . . . . . . . . . . . . . . . . . . . . . 111

Bibliography 113

List of Tables

3.1 Cell spectral efficiency requirements in IMT-Advanced. . . . . . . 113.2 Cell edge user spectral efficiency requirements in IMT-Advanced. 12

5.1 System parameters. . . . . . . . . . . . . . . . . . . . . . . . . . . 265.2 Uplink throughput. The numbers in the parentheses are the gains

compared to the reference case. . . . . . . . . . . . . . . . . . . . 275.3 Percentage of UEs connected to the low power nodes. . . . . . . 295.4 Macro PRB utilization. . . . . . . . . . . . . . . . . . . . . . . . 29

6.1 3GPP heterogeneous network deployment configurations. . . . . 416.2 User distribution and macro PRB utilization. . . . . . . . . . . . 436.3 FTP upload time. . . . . . . . . . . . . . . . . . . . . . . . . . . 446.4 Uplink throughput. The numbers in the parentheses are the gains

compared to the reference case. . . . . . . . . . . . . . . . . . . . 456.5 FTP download time. . . . . . . . . . . . . . . . . . . . . . . . . . 546.6 Downlink throughput. The numbers in the parentheses are the

gains compared to the reference case. . . . . . . . . . . . . . . . . 556.7 Gains from adding low power nodes in the different configurations

compared to the reference case. . . . . . . . . . . . . . . . . . . . 61

7.1 User distribution between macro eNB and low power nodes andmacro PRB utilization. . . . . . . . . . . . . . . . . . . . . . . . . 64

7.2 Average uplink SINR per UE. . . . . . . . . . . . . . . . . . . . . 657.3 Average downlink SINR per UE. . . . . . . . . . . . . . . . . . . 657.4 Average uplink cell throughput per cell. . . . . . . . . . . . . . . 677.5 Average downlink cell throughput per cell. . . . . . . . . . . . . . 687.6 Average uplink UE throughput per UE. . . . . . . . . . . . . . . 687.7 Average downlink UE throughput per UE. . . . . . . . . . . . . . 69

8.1 User distributions and macro PRB utilization. . . . . . . . . . . . 748.2 Gains from 8 dB range extension for the different configurations. 89

9.1 User distributions and macro PRB utilization. . . . . . . . . . . . 929.2 Spectral efficiency vs. number of low power nodes per macro cell

area. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 939.3 UE throughput gain and UE throughput gain per low power node.

Measured on the fiftieth percentile. . . . . . . . . . . . . . . . . . 1009.4 Gains from different number of low power nodes without and with

8 dB range extension compared to the reference case. . . . . . . . 100

xi

List of Figures

2.1 Interference between two terminals TA and TB . . . . . . . . . . . 52.2 Example of beam forming. . . . . . . . . . . . . . . . . . . . . . . 8

3.1 Representation of bandwidth resources in LTE. . . . . . . . . . . 123.2 Examples of carrier aggregation. . . . . . . . . . . . . . . . . . . 143.3 Example of beam forming. . . . . . . . . . . . . . . . . . . . . . . 153.4 Joint processing of signals. . . . . . . . . . . . . . . . . . . . . . . 16

4.1 Heterogeneous deployment example. . . . . . . . . . . . . . . . . 174.2 Interference from low power eNB to macro UE. . . . . . . . . . . 184.3 Interference from macro eNB to low power node UE. . . . . . . . 194.4 Interference from macro UE to low power eNB. . . . . . . . . . . 194.5 Interference from low power node UE to macro eNB. . . . . . . . 204.6 Illustration of RSRP and path loss based cell association. . . . . 21

5.1 Distribution of UEs between macro and low power nodes. . . . . 285.2 Interference received by base stations. . . . . . . . . . . . . . . . 305.3 CDF - average low power node uplink UE SINR. . . . . . . . . . 315.4 CDF - average macro uplink UE SINR. . . . . . . . . . . . . . . 325.5 CDF - average uplink UE SINR including all UEs. . . . . . . . . 335.6 Average uplink cell throughput per cell. . . . . . . . . . . . . . . 335.7 Different low power node cell sizes depending on distance to

macro node. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 345.8 CDF - average uplink low power node cell throughput. . . . . . . 355.9 CDF - average uplink macro cell throughput. . . . . . . . . . . . 355.10 CDF - average uplink macro cell area throughput. . . . . . . . . 365.11 CDF - average uplink low power node UE throughput. . . . . . . 375.12 CDF - average uplink low power node UE throughput. . . . . . . 375.13 CDF - average uplink UE throughput including all UEs. . . . . . 38

6.1 User distribution between macro eNB and low power nodes inconfiguration 1, 4a and 4b. . . . . . . . . . . . . . . . . . . . . . 42

6.2 Average uplink cell throughput per cell. . . . . . . . . . . . . . . 456.3 Path loss from one macro eNB and two low power nodes. The

cell borders are marked with vertical lines. . . . . . . . . . . . . . 466.4 CDF - average uplink low power node cell throughput. . . . . . . 476.5 CDF - average uplink macro cell throughput. . . . . . . . . . . . 476.6 CDF - average uplink macro cell area throughput. . . . . . . . . 486.7 Time average uplink interference per PRB per cell. . . . . . . . . 48

xiii

xiv List of Figures

6.8 CDF - average low power node uplink UE SINR. . . . . . . . . . 506.9 CDF - average macro uplink UE SINR. . . . . . . . . . . . . . . 506.10 CDF - distance from macro eNBs to their macro UEs. . . . . . . 516.11 CDF - average uplink UE SINR including all UEs. . . . . . . . . 516.12 CDF - average uplink low power node UE throughput. . . . . . . 526.13 CDF - average uplink macro UE throughput. . . . . . . . . . . . 536.14 CDF - average uplink UE throughput including all UEs. . . . . . 536.15 Average cell throughput. . . . . . . . . . . . . . . . . . . . . . . . 556.16 CDF - average downlink low power node cell throughput. . . . . 566.17 CDF - average downlink macro cell throughput. . . . . . . . . . . 566.18 CDF - average downlink macro cell area throughput. . . . . . . . 576.19 CDF - average low power node downlink UE SINR. . . . . . . . . 576.20 CDF - average macro downlink UE SINR. . . . . . . . . . . . . . 586.21 CDF - average downlink UE SINR including all UEs. . . . . . . . 596.22 CDF - average downlink low power node UE throughput. . . . . 606.23 CDF - average downlink macro UE throughput. . . . . . . . . . . 606.24 CDF - average downlink UE throughput including all UEs. . . . 61

7.1 Average uplink SINR per UE. . . . . . . . . . . . . . . . . . . . . 657.2 Average downlink SINR per UE. . . . . . . . . . . . . . . . . . . 667.3 Average uplink cell throughput per cell. . . . . . . . . . . . . . . 667.4 Average downlink cell throughput per cell. . . . . . . . . . . . . . 677.5 Average uplink UE throughput per UE. . . . . . . . . . . . . . . 687.6 Average downlink UE throughput per UE. . . . . . . . . . . . . . 69

8.1 User distribution between macro eNB and low power nodes inconfiguration 1, 4a and 4b with and without 8 dB range extension. 72

8.2 Average cell throughput per cell. . . . . . . . . . . . . . . . . . . 738.3 Time average uplink interference per PRB per cell. . . . . . . . . 758.4 Average SINR per UE. . . . . . . . . . . . . . . . . . . . . . . . . 778.5 CDF - average low power node UE SINR. . . . . . . . . . . . . . 788.6 Path loss from macro eNB and macro UE. . . . . . . . . . . . . . 798.7 Uplink interference from macro layer to low power node layer. . . 798.8 Downlink interference from macro layer to low power node layer. 798.9 CDF - average macro UE SINR. . . . . . . . . . . . . . . . . . . 808.10 CDF - average UE SINR including all UEs. . . . . . . . . . . . . 828.11 5 percentile SINR. . . . . . . . . . . . . . . . . . . . . . . . . . . 838.12 CDF - average low power node UE throughput. . . . . . . . . . . 848.13 CDF - average macro UE throughput. . . . . . . . . . . . . . . . 858.14 CDF - average UE throughput including all UEs. . . . . . . . . . 868.15 Example of a system map for configuration 4a. . . . . . . . . . . 878.16 Legend to figure 8.15. . . . . . . . . . . . . . . . . . . . . . . . . 888.17 5 percentile UE throughput. . . . . . . . . . . . . . . . . . . . . . 88

9.1 Average cell throughput per cell. . . . . . . . . . . . . . . . . . . 939.2 Spectral efficiency vs. number of low power nodes per macro cell

area. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 949.3 Time average uplink interference per PRB per cell. . . . . . . . . 949.4 Average SINR per UE. . . . . . . . . . . . . . . . . . . . . . . . . 969.5 CDF - average low power node UE SINR. . . . . . . . . . . . . . 96

List of Figures xv

9.6 CDF - average macro UE SINR. . . . . . . . . . . . . . . . . . . 979.7 CDF - average low power node UE throughput. . . . . . . . . . . 989.8 CDF - average macro UE throughput. . . . . . . . . . . . . . . . 989.9 CDF - average UE throughput including all UEs. . . . . . . . . . 99

10.1 Performance evaluation of ICIC schemes. . . . . . . . . . . . . . 10610.2 Static reuse ICIC scheme. . . . . . . . . . . . . . . . . . . . . . . 10710.3 Fractional Frequency Reuse ICIC scheme. . . . . . . . . . . . . . 10710.4 Allocation order based ICIC scheme. . . . . . . . . . . . . . . . . 10810.5 FFR scheme protecting UEs in range extended region of OA low

power node cells. . . . . . . . . . . . . . . . . . . . . . . . . . . . 10910.6 FFR scheme protecting UEs in range extended region of CSG low

power node cells. . . . . . . . . . . . . . . . . . . . . . . . . . . . 11010.7 Reception of transmission grant and downlink data transmission

simultaneously. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111

List of Abbreviations

AMPS Advanced Mobile Phone System

CA Carrier Aggregation

CB Coordinated Beam Forming

CoMP Coordinated Multipoint transmission and reception

CS Coordinated Scheduling

CSG Closed Subscriber Group

CSG Closed Subscriber Group

eNB E-UTRAN Node B

FDD Frequency-Division Duplexing

FDMA Frequency-Division Multiple Access

FFR Fractional Frequency Reuse

HARQ Hybrid Automatic Repeat Request

HeNB Home E-UTRAN Node B

HII High Interference Indication

ICIC Inter-cell Interference Coordination

ITU International Telecommunication Union

JP Joint Processing

JT Joint Transmission

LTE Long Term Evolution

LTE-Advanced Long Term Evolution-Advanced

MIMO Multiple-Input-Multiple-Output

NAT Network Address Translation

NMT Nordic Mobile Telephone

OA Open Access

xvii

xviii List of Figures

OFDM Orthogonal Frequency-Division Multiplexing

OI Overload Indication

PRB Physical Resource Block

QAM Quadrature Amplitude Modulation

QPSK Quadrature Phase-Shift Keying

RE Range extension

RNTP Relative Narrowband Downlink TX Power

RSRP Reference Signal Received Power

SIC Successive Interference Cancellation

SINR Signal-to-Interference-plus-Noise Ratio

TDD Time-Division Duplexing

TDMA Time-Division Multiple Access

TTI Transmission Time Interval

UE User Equipment

UMTS Universal Mobile Telecommunications System

WCDMA Wideband Code-Division Multiple Access

Chapter 1

Introduction

1.1 The wireless system

The usage of cellular systems has been growing since the systems got deployedin the 1980s. The International Telecommunication Union (ITU) estimated 4.6billion mobile subscriptions globally in 2009. In recent years the cellular systemshave also started to be used for data traffic and in 2008 the number of mobilebroadband subscriptions overtook the number of fixed broadband subscriptions.

What we want to achieve with a cellular system is to offer connections to theusers anywhere at any time. The user demands of the cellular systems have alsoincreased as the years have passed and new network architecture and technolo-gies are needed. After the first generation of cellular system was introduced inthe 1980s a new generation has come about around once a decade. The fourthgeneration cellular systems is planned to be deployed in 2011.

1.1.1 First generation

The first generation of cellular systems, 1G, was introduced in the 1980s andwas targeting voice communication. 1G systems are analogue where the usersare separated in the frequency domain, so called Frequency Division MultipleAccess (FDMA). NMT and AMPS are examples of 1G systems.

1.1.2 Second generation

The second generation of cellular systems, 2G, was digital. The digitalization ofthe system made it possible to send data traffic, enabling low rate data servicessuch as SMS. The 2G systems also had higher capacities than the precedinganalogue system because of the digitalization. The traffic could be compressedand multiplexed also in time, so called Time division Multiple Access (TDMA).This gave more degrees of freedom which increased the capacity because ofhigher utilization of the bandwidth. Compared to 1G systems, where a channelwas assigned a terminal even during times when it did not transmit, the secondgeneration technologies could let several users transmit in parallel through time.GSM is the most widespread 2G system.

1

2 Chapter 1. Introduction

1.1.3 Third generation

In the third generation cellular systems, 3G, the throughput was further in-creased which made services such as video calls possible. One of the mostused 3G technologies is Universal Mobile Telecommunications System (UMTS)which uses Wideband Code-Division Multiple Access (WCDMA) to separatethe users. WCDMA uses near-orthogonal codes to spread the terminals signalsover a wider bandwidth making their signals look like Gaussian noise to eachother. Since the terminals all use the same bandwidth, in which their signalsappears as noise to each other, adding a terminal effectively adds noise. A newterminal can be added to the system as long as the noise is not exceeding acritical level. WCDMA is therefore said to have a soft terminal limit comparedto a hard terminal limit as in the case with TDMA or FDMA where there is afixed number of channels.

1.1.4 Fourth generation

For the fourth generation of cellular systems, 4G, the requirements are fur-ther increased and will have peak data rates of 100 Mbps for downlink and 50Mbps in uplink. One promising technology to meet the 4G-standard is LongTerm Evolution-Advanced (LTE-Advanced). LTE-Advanced is an evolution ofa technology named LTE which has not fully met the requirements to be calleda fourth generation technology. The requirements are found in [1].

Key technologies in LTE-Advanced that are making it possible to meet therequirements are Carrier Aggregation, multiple antennas, heterogeneous deploy-ment and coordinated transmissions between different base stations. LTE andLTE-Advanced are described in more detail in section 3.1 and 3.2 respectively.

1.2 Problem statement

The bandwidth used in radio communication is a scarce commodity and as thedemands on the networks increase there is a need to make more efficient use ofthe bandwidth. To enhance the performance of cellular networks the followingdeployment approaches have been suggested; denser macro base station de-ployment, more advanced macro base stations and heterogeneous deployments.Macro base stations are expensive and might take long time to deploy. Hetero-geneous deployments is an alternative in which lower power base stations aredeployed where there are clusters of users with high traffic demands or in areaswhere the macro base stations has bad coverage. The low power base stationsare cheaper and can be deployed without making a big impact on the rest ofthe network. On the downside, new interference scenarios follows heterogeneousdeployments. This report will discuss interference scenarios and performanceproblems associated with heterogeneous deployment. Possible countermeasureswill be presented and assessed.

1.3 Thesis outline

This report has the following structure.

1.3. Thesis outline 3

In chapter 2 a background to what interference is and how it arises will bepresented. Common ways to mitigate interference in cellular systems will alsobe explained.

Chapter 3 gives an introduction to LTE and LTE-Advanced and their maintechnologies.

Chapter 4 explains the new interference scenarios associated with heteroge-neous network deployment.

Five studies have been performed for this report. First a study investigatingthe impact of misplacement of low power nodes is found in chapter 5. In simu-lations, unlike the reality, the placement of low power nodes is often ideal. Thepurpose of the study is to see how ideal versus non-ideal deployment affects thesystem performance.

3GPP has presented a set of system configurations which should be consid-ered when simulating heterogeneous networks. In chapter 6 these configurationshave been simulated and the performance has been analyzed to find possibleproblems related to heterogeneous deployments.

In one of the configurations it was seen that adding low power nodes willnot give much gain. In chapter 7 this configuration has been further analyzed,this time with lower load to see the benefits from adding low power nodes inthat configuration.

To increase the gain from the low power nodes their cell sizes can be in-creased, so called range extension. A study pertaining range extension is foundin chapter 8.

In chapter 9 a study is presented where the number of low power nodes isvaried to see how the spectral efficiency and other performance measurementsare affected. Another question this study answers is how serious interferencebetween low power nodes is for the performance.

Conclusions, proposal and future work is found in chapter 10.1. The ma-jor interference problem seen arised when the range of low power nodes wasextended. An ICIC-scheme is proposed to mitigate this interference. JointScheduling between Home eNBs and macro eNBs is proposed as future work.

Chapter 2

What is interference?

The capacity C of a communication channel with bandwidth B, such as thechannel between a mobile phone and a base station, follows equation 2.1 ac-cording to Shannon’s Theorem.[2] SINR is the Signal-to-Interference-plus-NoiseRatio and is discussed below.

C = B × log2 (1 + SINR) (2.1)

If a transmitter TA transmits a signal to its desired receiver RA, at the sametime as a transmitter TB transmits a signal, not only will RA receive TAs signalbut also the signal from TB . See figure 2.1. At the receiver the signals willsuperposition and from RAs point of view TBs signal will be interference. Sig-nal quality is in general quantified with Signal-to-Interference-plus-Noise Ratio(SINR). High interference leads to low SINR meaning low quality of the wantedsignal.

TA

TB

RA RB

Figure 2.1: Interference between two terminals TA and TB .

In digital communication the receiver is trying to detect the transmitteddata. The lower the SINR is the harder it is for the receiver to correctly detectthe transmitted signal. When the SINR is below a threshold correct detectionis not possible. This means that if the number of simultaneously transmittingusers within a bandwidth is too high no detections will be correct.

In cellular networks the served area is divided in to smaller zones called cells.A cell will have a base station and the terminals in the cell will be connected to

5

6 Chapter 2. What is interference?

its base station. To make efficient use of the bandwidth different cells can usethe same bandwidth. This reuse of bandwidth introduces some interference asthere is a possibility of terminals in different cells using the same bandwidth atthe same time. To counter interference different methods can be used, some ofwhich are discussed in this chapter.

2.1 Frequency hopping

When a terminal gets assigned a channel it can either be assigned a free channelor a channel used by other terminals in other cells. If assigned a free channelthe terminal will not experience any interference. If assigned a channel usedby another terminal they will interfere each other until one stops transmitting.To counter this problem the terminals can at regular time intervals changechannel. There will be a possibility of another collision but since the terminalswill only stay in their channel for a limited time they will only be affected bythe interference until the next frequency hop.1 The effect of frequency hoppingcan be seen as spreading the interference through time.

What is needed? The transmitter and the receiver need to agree on thehopping pattern.

• Pros

– Interference gets averaged though time which gives a more reliabletransmission.

• Cons

– Transmitters and receivers need some complexity to make them ableto change frequency during transmission.

– The transmitter and receiver needs to communicate in advance toagree on the hopping pattern.

2.2 Spatial multiplexing

The principle of spatial multiplexing is to increase the number of available trans-mission channels between transmitter and receiver. This can be achieved by hav-ing multiple transmitting and receiving antennas, a so called MIMO antennasetup. According to Shannon’s theorem the capacity is given by:

C = B × log2 (1 + SINR)

In a MIMO system with Nt antennas at the transmitter and Nr antennas atthe receiver, theoretically, NL = min(Nt, Nr) different, uncorrelated paths canexist between them. The capacity of each channel is:

C = B × log2

(1 +

Nr

NLSINR

)1As long as the terminals are not unlucky and jumps to the same channel again.

2.3. Beam forming 7

This gives a total capacity of:

C = B ×NL × log2

(1 +

Nr

NLSINR

)In theory, the capacity is increasing linearly with the number of channels,

which can be created by adding antennas.[3]In order for the receiver to demultiplex the data from the links it needs

to know the properties of the created channels. This is achieved by havingthe transmitter transmit a known reference signal. The receiver estimates thechannel properties from the received version of the reference signal and thentells the transmitter how it should code the data onto the antennas in order toget the best transmission.[4]

What is needed? Multiple antennas at the transmitter and receiver areneeded. The receiver also needs to do channel estimation and feed it backto the transmitter.

• Pros

– Theoretical linear increase of the capacity within a given bandwidth.

• Cons

– Complex antenna structures.

– Channel estimation is required.

– Communication between the transmitter and received is needed.

2.3 Beam forming

Beam forming is to change the antenna beam pattern by use of array antennas.The phase and amplitude of the signal is adjusted at each antenna element toform the beam pattern. The antenna beam pattern can be changed so thatthe main lobe is pointed towards a desired transmitter/receiver to achieve highantenna gain or to point the nulls in direction of undesired transmitters/receiversto avoid interference, see figure 2.2.[3]

To form the antenna beam the antenna array needs several elements spacedsufficiently far apart. Due to size limitations of mobile terminals beam formingis not suitable for terminals.

What is needed? Array antennas and feedback of measurements to the trans-mitter which are used to adjust the beam pattern is needed.

• Pros

– Transmitted power can be reduced due to higher antenna gains inmain lobe.

– Interference can be reduced.

• Cons

8 Chapter 2. What is interference?

Figure 2.2: Example of beam forming.

– Advanced antenna structure with multiple antenna elements is needed.

– Pre-coding of the signal needs to be done before transmission.

– Not suitable for mobile terminals.

– Needs to sense the direction of the mobile terminals.

– Signaling between the terminal and the base station is needed.

2.4 Interference cancellation

In cellular networks several users can use the same bandwidth at the same timeand therefore interfere each other. If a receiver can estimate the interferingsignals they can cancel the interference by subtracting it. There are severalways of doing this, one of which is called Successive Interference Cancellation(SIC).

In SIC the transmitters are given different code words with which they en-code the signals before transmission. The receiver will try to demodulate anddecode one of the signals from the received compound signal to extract its mes-sage. If successfully extracted the message is re-encoded, re-modulated andsubtracted from the original signal. The procedure is repeated until all signalshave been extracted.

As signals get subtracted the SINR is getting higher in each recursion. Themost effective way of extracting the signals is therefore by starting with thehighest SINR signal.[5] If a decoding error is made the wrong signal will besubtracted which will destroy the compound signal and the error will in thatsense propagate to the next step.

What is needed? The receiver needs to know how each signal is modulatedand encoded in order to decode and demodulate them. The structure differs de-pending on which cancellation method is used and can be more or less complex.

2.4. Interference cancellation 9

• Pros

– Ability to extract multiple signals which are interfering each other.

• Cons

– Complex receiver structure.

– Delay due to signal processing.

– Not always possible to decode.

Chapter 3

Fourth Generation cellularnetworks

The International Telecommunication Union (ITU) has set the requirements forthe fourth generation telecommunication systems, also called IMT-Advanced.[1]The requirements are as follows:

• Peak spectral efficiency of 15 bit/s/Hz and 6.75 bit/s/Hz in downlinkand uplink respectively1 .

• Data latencies of maximum 10 ms in both uplink and downlink.

• Latencies of maximum 50 and 150 ms for intra- and inter-frequency han-dovers respectively.

• Scalable bandwidth up to 40 MHz.

• Increased cell spectral efficiency according to table 3.1. The test environ-ments are described in [6].

Test environment Downlink (bit/s/Hz/cell) Uplink (bit/s/Hz/cell)

Indoor 3 2.55Microcellular 2.6 1.8Base coverage urban 2.2 1.4High speed 1.1 0.7

Table 3.1: Cell spectral efficiency requirements in IMT-Advanced.

• Increased cell edge user spectral efficiency according to table 3.2. The testenvironments are described in [6].

• Interworking with other radio access systems.

• Unicast and multicast broadcast services.

1Assuming an antenna configuration of downlink 4 × 4, uplink 2 × 4

11

12 Chapter 3. Fourth Generation cellular networks

Test environment Downlink (bit/s/Hz/cell) Uplink (bit/s/Hz/cell)Indoor 0.1 0.07Microcellular 0.075 0.05Base coverage urban 0.06 0.03High speed 0.04 0.015

Table 3.2: Cell edge user spectral efficiency requirements in IMT-Advanced.

As discussed in section 1.1.4 LTE-Advanced is one of the most promisingtechnologies to reach the requirements for a fourth generation wireless com-munication system. The focus in this report is on heterogeneous deploymentsin LTE-Advanced and we will, in section 3.2, look in to more details aboutLTE-Advanced.

LTE-Advanced is an evolution of LTE which will be described first.

3.1 Long Term Evolution

Long Term Evolution (LTE) is an air interface for cellular networks which isdefined by 3GPP. The main components of LTE are introduced in this section.

3.1.1 OFDM

In LTE Orthogonal Frequency-Division Multiplexing-based (OFDM) transmis-sion schemes are used for both uplink and downlink transmission. OFDM canbe seen as a combination of TDMA and FDMA where the time is divided into timeslots and frequency is divided into a large set of orthogonal narrow-band channels called sub carriers. Twelve sub carriers are grouped togetherinto a Physical Resource Block (PRB), see figure 3.1. This separation of UserEquipments (UEs) means that is no interference between UEs within a cell butintercell interference exists.

f

t

Physical Resource Block

Figure 3.1: Representation of bandwidth resources in LTE.

Before transmission the transmitter parallelizes the signal to several lowerrate signals which gets modulated using QPSK, 16 QAM or 64 QAM. Each

3.2. LTE-Advanced 13

low rate signal will be transmitted on a separate sub carrier. The receiverthen demodulates the signals and recreates the original signal before performingdetection.

3.1.2 Spectrum flexibility

LTE supports both Frequency-Division Duplexing (FDD) and Time-DivisionDuplexing (TDD) to separate uplink and downlink communication.

Which band and bandwidth used by LTE is not specified in the standard.This implies that operators can deploy LTE in a variety of frequency bands. Anoperator which previously deployed GSM in the 900 MHz spectrum can deployLTE there instead and because the bandwidth is not specified the transitionfrom GSM to LTE can be done gradually.[4]

3.1.3 Multiple antenna technology

As discussed in chapter 2, it is beneficial to have several antennas for beamforming and spatial multiplexing. In LTE the terminals (UE in 3GPP terms)and base stations (eNB in 3GPP terms) supports up to two and four antennasrespectively.[7][8]

3.1.4 Hybrid ARQ with soft combining

To cope with errors created in non ideal channels Hybrid ARQ (HARQ) isutilized in LTE. The transmitted data is coupled with two sets of redundantbits. One set of which is used by the receiver to first try to correct errors andanother set which later is used to detect uncorrected errors.

After that the receiver has performed the correction of possible errors anddetected whether the transmission was successful or not it will send a reportto the transmitter of the outcome. In case of an erroneous transmission thetransmitter resends the data.

In HARQ the erroneous packets are discarded. A packet with errors canhowever contain some valuable information which would be lost if the packet isdiscarded. To avoid this waste a modification of the Hybrid ARQ scheme hasbeen done. Hybrid ARQ with soft combining will save erroneous packets to becombined with retransmitted packets. The combination of two or more packetswill be more reliable and will have higher chance of a successful detection.[4]

3.2 LTE-Advanced

The LTE standard does not fully reach the ITU requirements for a 4G systemand is sometimes called 3.9G. LTE-Advanced is, however, planned to reach thoserequirements. 3GPP’s aim is to have peak data rates of 1 Gbps in downlink and500 Mbps in uplink in a bandwidth of 100 MHz. The spectrum efficiency willthen be 30 bit/s/Hz and 15 bit/s/Hz in downlink and uplink respectively. Thekey components that will make this possible are, among others, Carrier Ag-gregation, higher order MIMO, Heterogeneous network deployment and CoMPwhich are described below.[9]


LTE-A LTE-A LTE-A LTE-A LTE-Af

(a) Multiple component carriers

LTE-A LTE-A LTE-A

Other services

f(b) Separated component carriers

Figure 3.2: Examples of carrier aggregation.

3.2.1 Carrier aggregation

In LTE the bandwidth can, as discussed in section 3.1.2, change in size. Thebandwidth can be as narrow as around 1 MHz up to 20 MHz. Something whichis new for LTE-Advanced is that it can be deployed using several frequencybands, adjacent or not, see figure 3.2a and 3.2b. The concept is called CarrierAggregation (CA) in 3GPP terms where the bands used are called componentcarriers.

Carrier Aggregation will be backward compatible with LTE UEs. LTE UEswill, however, only be able to use one component carrier at one time whileLTE-Advanced UEs can use several to reach higher data rates. Carrier Aggre-gation is an important component in reaching higher data rates in the sensethat operators can deploy LTE-Advanced in frequency bands they already ownand gradually migrate to LTE-Advanced as described in section 3.1.2 instead ofbuying new bandwidth for LTE-Advanced.

3.2.2 Higher order MIMO

Multiple-Input-Multiple-Output (MIMO) antenna configurations refer to theexistence of multiple antennas at the transmitter and receiver. With multipleantennas multiple channels can be created between the transmitter and receiverfor so called spatial multiplexing described in section 2.2. MIMO was includedin the LTE standard with support for four antennas at the base station and twoantennas in the UE. In LTE-Advanced it will be possible to have eight antennasat the base station and four in the UE, or even more.

3.2.3 Coordinated Multi-Point transmission and reception

Coordinated Multi-Point transmission and reception (CoMP) is a technologyaimed to improve coverage of high data rates, cell edge performance as well asoverall system performance. The principle of CoMP methods is to have sev-eral eNBs coordinating their transmissions. There are two categories of CoMP;

3.2. LTE-Advanced 15

Coordinated Scheduling/Coordinated Beam forming (CS/CB) and Joint Pro-cessing/Joint Transmission (JP/JT).

The first type, Coordinated Scheduling/Coordinated Beam forming, meansthat the involved eNBs are coordinating the access to the resource blocks in away so that interference will be avoided. If, for example, one eNB is communi-cating with an edge UE the neighboring eNB should then avoid schedule one ofits edge UEs at the same time. Beam forming can also be used in a way so thatthe eNBs coordinate their beams not to interfere with each other. See figure3.3.

Figure 3.3: Example of beam forming.

In Joint Processing/Joint Transmission several cooperating eNBs are trans-mitting to one single UE. The data which is going to be transmitted to the UEtherefore needs to be available at all involved eNBs. Interference can be avoidedby having the cooperating eNBs process the signals in a way so that interferingsignals will destruct at the UE. To achieve this, a lot of signaling is needed tobe sent over the back haul at the same time as the eNBs have access to thechannel conditions.[9] See figure 3.4.

One difficulty with CoMP is that if we want the eNBs to cooperate they needto be able to exchange messages within a few milliseconds to not be obsoletewhen arriving. This put latency and throughput restrictions on the connectionsbetween the nodes.[10]

3.2.4 Heterogeneous network deployment

Heterogeneous network deployment refers to a network where eNBs of differenttransmit powers, i.e. different cell sizes, is distributed in a nonuniform mannerthroughout the served area. To increase the performance and offer higher datarates it is possible to add eNBs with low output power at heavy loaded areaswhere the signal from the macro eNB is weak. Below is a description of thebase stations in LTE-Advanced is specified.


Figure 3.4: Joint processing of signals.

• Macro eNB is the top level node. The UEs should be able to reach amacro base station from anywhere within the service area. The transmitpower is typically around 43dBm. The macro eNBs are connected to eachother with a dedicated back haul connection.

• Relay eNB is a low power (23 − 30 dBm) eNB with a over-the-air backhaul connection to the serving macro eNB.

• Pico eNB is a low power eNB which has a dedicated back haul connec-tion. The transmit power is usually around 23 − 30 dBm. The nodes aredeployed by the operator.

• Femto eNB, or Home eNB (HeNB), as they also are called, are low powernodes that the users can buy and deploy where they need. Femto eNBs areconnected to the rest of the network through the Internet. Since the users,instead of the operators, deploy femto eNBs planning is not possible forthe femto eNBs. The femto eNBs can operate in two modes; open accessor Closed Subscriber Group (CSG). If operating in open access any UEcan connect to the node while in the CSG mode only authorized UEs canconnect. The owner of a femto eNB can for example give access to itsfamily and friends.[11]

Chapter 4

New interference scenariosin Heterogeneous Networks

Heterogeneous network deployment both has benefits and drawbacks. It is bene-ficial to add low power nodes where the macro eNBs signal has problem reaching,inside buildings for examples. It is also beneficial to add low power nodes inhigh user density areas to support the high traffic. On the other hand, new in-terference scenarios are created which are not seen in traditional homogeneousdeployments. Section 4.1.1 to 4.2.2 describes four interference scenarios relatedto heterogeneous deployments. Section 4.3 discusses how cell association andthe UE target output power can be adjusted to mitigate interference.

Low power node 1

Low power node 2

Low power node 3

Figure 4.1: Heterogeneous deployment example.

17

18Chapter 4. New interference scenarios in Heterogeneous

Networks

4.1 Downlink

4.1.1 Low power eNB interference to macro UE

Low power nodes and macro eNBs normally use the same spectrum. Becauseof this, a macro UE close to a low power node might receive a stronger signalfrom the low power node than from the macro eNB which results in low SINR.This effect gets worse in cases when the distance to the macro eNB is big andwhen the macro UE is close to the low power node. See figure 4.2.

Macro UE

Low power node UE

Figure 4.2: Interference from low power eNB to macro UE.

4.1.2 Macro eNB interference to low power node UE

In case a low power node is close to the macro eNB the UEs connected to thelow power node can get interference from the macro eNB. Since the macro eNBhas higher output power than low power nodes there can be cases when the lowpower node UEs gets a stronger signal from the macro than from the low powernode. The closer the low power eNB is to the macro eNB the stronger this effectgets. See figure 4.3.

4.2 Uplink

4.2.1 Macro UE interference to low power eNB

Low power nodes will receive interference from macro UEs. The further a UEgets from the serving eNB the higher power it transmits in order to reach theeNB. This effect gets stronger when the low power node is on the macro celledge. See figure 4.4.

4.2.2 Low power node UE interference to macro eNB

When a low power eNB is close to the macro eNB the signals from the UEs inthe low power cell can reach the macro eNB and therefore create interference.

4.3. Crucial factors 19

Low power node UE

Macro UE

Figure 4.3: Interference from macro eNB to low power node UE.

Macro UE

Low power node UE

Figure 4.4: Interference from macro UE to low power eNB.

This is shown in figure 4.5.

4.3 Crucial factors

Aside from the factors given in section 4.1.1 to 4.2.2 other factors can affectthe interference in the system, such as cell association and P0 offsets discussedbelow.

4.3.1 Cell association

As discussed, high interference can arise when UEs are close to an eNB that theyare not connected to. Therefore cell selection in heterogeneous networks is animportant factor to the system performance. The task of assigning UEs to basestations is non-trivial and there is no universally optimal way of solving the task.

20Chapter 4. New interference scenarios in Heterogeneous

Networks

Macro UE

Low power node UE

Figure 4.5: Interference from low power node UE to macro eNB.

If, for example, the cell association is optimized for downlink transmissions theupload transmissions will suffer and vice versa.

To optimize the downlink performance the UE should be assigned to thebase stations from which the strongest signal is received. In this way the higherpower a base station is transmitting the bigger the cell gets. This approach incell association is called Reference Signal Received Power (RSRP).

To optimize uplink transmissions the UEs should be assigned to the basestations to which the path loss is lowest. This way of path loss based cellassociation will make the UEs connect to the base station which will have thebest potential to receive it.

Figure 4.6 shows these two ways of association UEs with the low powernodes. If RSRP cell association is used the low power node cell will be smallerhaving the blue cell border. If path loss based cell association is used the cellborder will be larger and have the red cell border. In either case the UEs in theyellow region will create or receive interference.

If RSRP cell association is used the UEs in the yellow region will be con-nected to the macro eNB for optimal downlink performance. As seen in thefigure the UEs in the yellow region will be closer to the low power node butconnected to the macro eNB. This means that the low power node will receive astronger version of their signal than the macro eNB and uplink performance isnot optimal. They will also create interference to the low power node describedin section 4.2.1.

If path loss based cell association is used the UEs in the yellow region willconnect to the low power nodes. In this case they will be connected to the basestation which will get the strongest version of their transmitted signal whichwill optimize uplink performance. In downlink there will be problems. The UEsin the yellow region gets a stronger signal from the macro eNB compared to thelow power node and the signal received from the macro eNB is interference tothem.

A compromise between RSRP and path loss based cell association is to useRSRP with offsets. When comparing the received power from two base stations,

4.3. Crucial factors 21

Figure 4.6: Illustration of RSRP and path loss based cell association.

say a macro eNB and a low power eNB, an offset is added to the measuredreceived power from the low power node resulting in that the UEs will withhigher probability connect to the low power node. This can be thought of asenlarging the low power cells without changing their output power and is calledrange extension (RE).

4.3.2 P0 offset

As we saw earlier in this chapter the difference in output power between lowpower nodes and macro nodes creates interference problems. A macro UE justoutside the cell border of a low power node can create strong uplink interferenceto the low power node. See section 4.2.1.

To overcome this problem the low power node can tell its UEs to increasetheir output power to fend the high interference.[12]

Chapter 5

Impact of misplacement oflow power nodes

The following study will show how misplacement of low power nodes within ahot zone will affect the performance of the system.

5.1 Background

In cellular networks users tend to gather in certain areas, such as in a shoppingmall or a busy square, forming so called hot zones. To support the high trafficin a hot zone a low power base station can be deployed in it.

Hot zones are often modeled in an ideal manner as a perfect circle in whicha low power node is placed in the center. In reality a hot zone is defined bythe location of the UEs. The shape and location of hot zones therefore changeas the UEs move and the low power nodes are in general not located in thecenter of the hot zones. The aim of this study is to see how the performanceis affected by having non perfect deployment compared to perfect deploymentof low power nodes within hot zones. To investigate this there is a need to seehow the distribution of UEs between the low power nodes and the macro nodestogether with the SINR distributions are changing in the different deploymentscenarios. The conclusions obtained for uplink are applicable to downlink aswell.

5.2 Simulation details

To perform the simulations in this report a simulation tool which simulatesLTE in high detail on the lower layer protocol stack has been used. Systemparameters such as traffic model, propagation model and deployment are inputin the simulator and the output has then been processed in MatLab.

5.2.1 Performance Measurements

In this section details about the performance measurements are described. Theperformance measurements are calculated in the same manner in all studies in

23

24 Chapter 5. Impact of misplacement of low power nodes

this report.

PRB utilization

In each Transmission Time Interval (TTI) the PRB utilization is calculatedby dividing the number of PRBs used for transmission by the total number ofPRBs, according to equation 5.1. The PRB utilization is averaged over thewhole simulation time.

PRB utilization =number of PRBs used for transmission

total number of PRBs(5.1)

Interference

The base stations will sum the total received power under a time t seconds.After t seconds the interference is calculated by subtracting the power of usefulsignal from the total power. The interference is calculated according to equation5.2 and is averaged over time, PRB and cell and presented in dBm.

Interference = 10 × log10 (Total received power − Useful signal power) + 30(5.2)

The time t is 0.2 seconds in these simulations.

SINR

The SINR is the useful signal in a transmission divided by the interference plusnoise, see equation 5.3. The SINR is presented in dB.

SINR =Useful signal power

Interference(5.3)

The SINR is averaged over a time t = 0.2 s

Cell throughput

The cell throughput is calculated by starting a timer and having a countercount the number of received bits. After a time t the simulator calculates thethroughput according to equation 5.4 after which the number of received bits isset to zero before the counter is restarted.

Cell throughput =Number of received bits

t(5.4)

Where t = 0.2 s.

UE throughput

The UE throughput is calculated in a similar way as the cell throughput, seeequation 5.5.

UE throughput =Number of received bits

t(5.5)

Where t = 0.2 s.

5.2. Simulation details 25

5.2.2 Configurations

Two cases have been simulated. First the low power nodes have been placed, asthey often are in simulations, in the center of the hot zone, from here on refereedto as bingo deployment. Thereafter the low power nodes have been placedrandomly within the hot zones, referred to as random deployment. Within thehot zones 50 % of the users are placed, while the rest of the users are distributedrandomly within the system.

• No low power nodes. (Reference case)

• Bingo deployment. One hot zone per macro cell area where 50 % of theUEs are placed. A low power node is deployed in the center of each hotzone.

– 0 dB Range extension



• Random deployment. One hot zone where 50 % of the UEs are placed. Alow power node is deployed at a random location within the hot zone.




5.2.3 System parameters

Range extension has been achieved by changing the cell association algorithm.The UEs measure the received signal power from the all base stations fromwhich they receive a signal. For all low power nodes an offset is added to thereceived power. The UEs then connect to the base station which has the highestvalue.

The system parameters are found in table 5.1. The reason for not havingshadow is to make the simulations run faster.

The propagation model is defined by the following two equations. The gainfrom a macro eNB to a UE is follows equation 5.6 and the gain from a low powernode to a UE follows equation 5.7.

Gain = −35.3 − 3.76 × 10 log10 (distance) + 14 − min

(12

(angle

70360 × 2π

), 20

)(5.6)

Gain = −50.6 − 3.67 × 10 log10 (distance) (5.7)

where distance is the distance from a UE to its base station and angle is theangle between the UE and middle of the base station antenna beam.


Parameter ValueDeployment

Number of macro base stations 7Number of cells per macro base station 3Hot zone radius 40 mCell radius 167 mMacro to macro distance 500 mMinimum LPN to LPN distance 75 mMinimum LPN to macro distance 75 m

ResourcesBandwidth 10 MHzNumber of PRBs 50

PropagationMacro propagation factor - 3.76Macro attenuation constant - 35.3Low power node propagation factor - 3.67Low power node attenuation constant - 50.6Shadow fading -

Base station specificsNoise figure 5 dBMacro base station output power 40 WMacro base station antenna elements (per cell) 2Low power base station output power 1 WLow power base station antenna elements 2Transmit antenna ports 1Receive antenna ports 2

UE specificsSpeed 0 m/sOutput power 0.2 WNoise figure 9 dBUE antenna elements 2Transmit antenna ports 1Receive antenna ports 2

MiscellaneousUE scheduling algorithm Round robin

Table 5.1: System parameters.

5.2.4 Traffic model

The traffic model is chosen to comply with the Poisson based traffic model 1specified in [9]. Users arrive in the system following a Poisson distribution withan arrival intensity of λ users per second. They upload or download one FTPpacket of fixed size and then disappear from the system.

• λ: 150 UE/s system wide. (7.14 UE/s/cell)

• FTP packet size: 100 kByte

5.3. Results 27

This traffic model was chosen in order generate fixed offered traffic regardlessof how the system performs in different situations. The traffic model generatesthe following offered traffic.

• 120 Mbps system wide.

• 5.712 Mbps per macro cell area.

Simulation time is 100 seconds during which 14947 UEs was created, i.e.149.47 UEs / second.

5.3 Results

The following results were obtained by computer simulations. Only uplink per-formance has been analyzed in this study.

5.3.1 Performance overview

In table 5.2 the throughput performance has been summarized.

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Macro cell area5.5

5.8 5.8 5.8 5.8 5.8 5.8throughput (Mbps) (5%) (5%) (5%) (5%) (5%) (5%)

Macro cell5.5 4.4 4.6 3.2 3.5 2.4 2.4

throughput (Mbps)

Low power node- 1.4 1.2 2.7 2.4 3.4 3.4

throughput (Mbps)

Spectral efficiency0.55

0.58 0.58 0.58 0.58 0.58 0.58(bps/Hz/Macro cell area) (5%) (5%) (5%) (5%) (5%) (5%)

5 % UE0.014

0.78 0.73 1.0 0.98 1.1 1.1throughput (Mbps) (5500%) (5100%) (7000%) (6900%) (7800%) (7800%)

50 % UE1.0

1.5 1.4 1.6 1.6 1.7 1.7throughput (Mbps) (50%) (40%) (60%) (60%) (70%) (70%)

95 % UE1.6

1.8 1.8 1.9 1.8 1.9 1.9throughput (Mbps) (15.5%) (15.5%) (18.8%) (15.5%) (18.8%) (18.8%)

Table 5.2: Uplink throughput. The numbers in the parentheses are the gainscompared to the reference case.

5.3.2 User distribution

The number of UEs in the hot zones is 50 % in all configurations. To cover thewhole hot zone means that we should see 50 % of the UEs connection go the


low power nodes.1 In figure 5.1 and table 5.3 the percentage of UEs connectionto the low power nodes is displayed.

No low power nodeBingo Random Bingo 8dB Random 8dBBingo 16dBRandom 16dB0

500

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Num

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User distributions

Macro usersLow power node users

Figure 5.1: Distribution of UEs between macro and low power nodes.

From table 5.3 we can see that when RSRP is used without any offset 25 %and 20 % of the UEs are connection to the low power nodes in the bingo andrandom case respectively. This means that 50 % and 40 % of the hot zone iscovered by the low power cell. When the offset is increased a larger portion ofthe hot zones are covered by the low power nodes and in the case of 16 dB rangeextension we can see that the whole hot zone is covered. Comparing the valuesin the bingo and random deployment cases it is seen that the bigger the cell is,i.e. the larger offset is used, the smaller the impact of misplacement is on thenumber of UEs connecting to the low power nodes.

The PRB utilization in the macro layer is compiled in table 5.4.

5.3.3 Interference

Figure 5.2 is showing the average interference received by the base stations.The difference in interference between bingo and random deployment is due

to different number of UEs connecting to the low power nodes. The relationbetween number of UEs connecting to the low power node and the interferenceis discussed below.

Low power eNB

A decrease in interference to the low power nodes is observed as the offsets getslarger. This is explained by that a low power node gets the strongest interferencefrom macro UEs surrounding the cell. The number of UEs in the hot zones is

1The UE which are not placed in the hot zones intentionally are randomly distributedthroughout the system area. There is a chance that a UE not chosen to be placed in the hotzone are placed there anyway. This means that to cover the whole hot zone a low power nodeshould actually have more than 50 % of the UEs connected to it.

5.3. Results 29

Bin

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Bin

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Ran

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0 dB 25 % 20 % 25 %8 dB 46 % 39 % 18 %16 dB 59 % 56 % 5 %

Table 5.3: Percentage of UEs connected to the low power nodes.

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80 % 65 % 70 % 48 % 54 % 39 % 41 %PRB utilization

Table 5.4: Macro PRB utilization.

the same regardless of the offset abut what differs is the number of UEs whichare absorbed by the low power nodes. In the case without offset, there will bea large number of surrounding UEs which are connected to the macro eNB andthe interference is -93.6 dBm and -94.1 dBm in the Bingo and Random caserespectively. If an offset is added those surrounding UEs are absorbed by thelow power node and therefore will not interfere to it and in case of a 16 dBrange extension the whole hot zone is covered and the interference is reducedto -111 dBm and -110 dBm. This effect was earlier explained in section 4.2.1.

Macro eNB

There are two factors affecting the interference to macro eNBs as the offsetchanges. The dominant interferers to a macro eNB are the edge UEs in neigh-boring cells and the UEs connected to low power nodes within its own cell.

• By increasing the offsets of a low power node, hence assigning more UEsto it, there will be more possible interferers to the macro eNB. UEs whichearlier were intra cell UEs have become inter cell UEs when absorbed bythe low power nodes and therefore will interfere with the macro cells.

• On the other hand, in neighboring cells edge UEs are absorbed by the low


Low power node Macro Macro cell area−130

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Ref0 dB Bingo0 dB Random8 dB Bingo8 dB Random16 dB Bingo16 dB Random

Figure 5.2: Interference received by base stations.

power node as well. Those UEs will be closer to their serving eNB andwill transmit with less output power and therefore interfere less.

From the interference reduction we can conclude that the interference addedby the low power node UEs is smaller than the reduction of interference fromthe neighboring cells.

It can be seen that the interference from neighboring cells decreases andcompensates for the interference from the low power nodes.

Overall

Summing the interference received by the low power nodes and the macro nodesshows that it is possible to get lower interference than in the homogeneousdeployment case if the range is extended.

It can be concluded that the interference depends on how many UEs arehanded over to the low power node. If a low power node is misplaced it willhave fewer UEs connecting to it and therefore the interference will be stronger.

The difference in interference between bingo and random deployment isaround 1 dB in all configurations.

5.3.4 SINR

Which modulation scheme (QPSK, 16 QAM or 64 QAM) can be used for atransmission depends on the SINR level. With high SINR higher modulationschemes can be used, hence utilizing the bandwidth more efficiently. In thissection the SINR for the UEs is analyzed. The SINR is calculated from equation5.8.

SINR =S

I +N(5.8)

5.3. Results 31

Low power node users

In figure 5.3 a CDF over the SINR for UEs connected to low power nodes isshown. When the range is extended the following two things will happen.

1. The average distance from the low power nodes to their UEs will increasegiving an average higher path loss and lower SINR.

2. The interference decreases which will give higher SINR, mainly to the edgeUEs. An explanation to why edge UEs are mostly affected by the uplinkinterference reduction is found in section 8.2.3.

For the high percentiles the SINR seems to decrease when using range exten-sion. The reason for this is described in point 1. Worth noting is that the UEswho were connected to the low power node in the case without range extensionwill get higher SINR when range extension is applied due to lower interference.

In the lower percentiles the edge UEs are found. In the case without rangeextension the edge UEs are closer to the low power node compared to the caseswith range extension. When the range is extended the edge UEs will have higherpath loss which is reducing the SINR but at the same time range extensionreduced the interference and since the interference reduction is larger than thehigher path loss a higher SINR is achieved. The path loss from the low powernode to its edge will be 8 or 16 dB when the range is extended. At the sametime the interference will in those cases be 10.4 and 17.4 dB lower respectivelyresulting in a gain.

−20 −10 0 10 20 30 400

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Uplink UE SINR − Low power node UEs

0 dB Bingo0 dB Random8 dB Bingo8 dB Random16 dB Bingo16 dB Random

Figure 5.3: CDF - average low power node uplink UE SINR.

Macro users

Figure 5.4 shows a CDF over the macro UE SINR. When deploying a low powernode the number of UEs connecting to it will depend on its distance to the macro


eNB. A low power node on the edge of the macro cell will absorb more UEs thana low power node deployed close to the macro eNB. This means that the lowpower nodes will, on average, absorb more edge UEs compared to center UEs.The observed gain in SINR in the low percentiles is not a direct gain but rathera gain coming from removing edge UEs from the macro cells which thereforewill not be present in the macro SINR CDF.

The higher the offset is the more UEs will be absorbed by the low powernodes and the bigger gain is seen.

The UEs in the high percentiles are those close to the macro eNB. ThoseUEs are not as likely to be absorbed by the low power nodes and will only gainfrom lower interference.

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Figure 5.4: CDF - average macro uplink UE SINR.

All users

A CDF for all UEs average SINR is shown in figure 5.5. The SINR is higherwhen the low power nodes are deployed in the center of the hot zones. Wealso see that the importance of bingo deployment is also reduced as the offsetincreases.

5.3.5 Cell Throughput

In this section the cell throughput is discussed.

Averages

Figure 5.6 shows the average cell throughput. We see the effect of the low powernode offloading the macro cells. In the reference case the served traffic was 5.54Mbps but when the low power nodes are deployed the served traffic increasedto 5.81 Mbps.

5.3. Results 33

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FUplink UE SINR − All UEs


Figure 5.5: CDF - average uplink UE SINR including all UEs.

Low power node Macro Macro cell area0

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Uplink Cell Throughput − Averages


Figure 5.6: Average uplink cell throughput per cell.

Low power cell

The location of the hot zones have been chosen randomly in these simulationswhich lets us see the effect of having low power nodes both close to and far awayfrom the macro base station. In figure 5.8 we see a CDF of the throughput forthe low power nodes. The lower part of the CDF represents low power nodecells located close to the macro eNB and are therefore small, see figure 5.7a.These low power nodes absorb few UEs and therefore will have low throughput.In the upper part of the CDF are low power nodes located on the macro cell


(a) Small low power node cell due to shortdistance to macro eNB.

(b) Large low power node cell due to longdistance to macro eNB.

Figure 5.7: Different low power node cell sizes depending on distance to macronode.

edge which therefore are large, see figure 5.7b. Those cells will absorb manyUEs and therefore have high throughput.

Looking at the solid lines, the case without RSRP offset, it is observed thatthe difference between random and bingo deployed low power nodes is smallfor the low power nodes close to the macro eNB. For low power nodes on themacro cell edge, on the other hand, there is a bigger difference between the twocases. A low power node close to the macro eNB is small and few UEs are tobe connected to it. Moving a low power node away from the center of the hotzone then has small effect. The further the low power node gets from the centerof the macro eNB the more important the deployment is.

There is a turning point where the low power node cell gets big enough tocover the whole hot zone. If the low power cell covers more than the hot zonea misplacement is not as critical as it would be with a smaller cell. This is seenin the upper part of the CDF for 8 dB offset.

Looking at the 16 dB offset case the low power nodes are big enough to coverthe whole hot zones even if they are moved and therefore will not suffer fromlow power node misplacement.

Macro cell

As the low power node is increasing in size more UEs are getting assignedto it and offloaded from the macro eNB. This effect is seen in the macro cellthroughput CDF in figure 5.9. The CDF is an inverted version of the low powercell throughput CDF.

Overall

The macro cell area throughput is the throughput of the macro cell and its lowpower node cell. A CDF for the macro cell area throughput is seen in figure5.10. What is observed is that no distinct difference is seen between the caseswhen the low power node is randomly deployed or bingo deployed within thehot zone. All cases which have low power nodes deployed are having higher

5.3. Results 35

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Figure 5.8: CDF - average uplink low power node cell throughput.

1 2 3 4 5 6

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Figure 5.9: CDF - average uplink macro cell throughput.

macro cell area throughput than the reference case due to the offloading of themacro eNB, as we saw from the average cell throughput in figure 5.6.

5.3.6 UE Throughput

What is interesting for the users is which throughput they get. The throughputdepends on two parameters; the SINR and the number of PRBs available for


3.5 4 4.5 5 5.5 6 6.5 7 7.5

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Figure 5.10: CDF - average uplink macro cell area throughput.

the UEs. In this section the UE throughput is shown.

Low power node users

In figure 5.11 a CDF with the average UE throughput for the low power nodeUEs is found. The UEs in the low percentiles are UEs on the edge of the lowpower nodes. Those UEs are limited by the SINR and the throughput thenfollows the SINR curves. The UEs in the center of the low power node cellsare having high SINR and will instead be limited by the available bandwidthresources. For the UEs limited by bandwidth no significant gain is obtained byincreased SINR.

Macro users

In figure 5.12 a CDF over the average throughput for macro UEs is found. TheUEs connected to the macro eNBs are both gaining from higher SINR and morebandwidth resources. It was seen that the SINR increases as the offsets increaseat the same time as the macro eNBs gets offloaded which gives more PRB perUE to the UEs which remains connected to the macro eNB.

All users

A CDF including all UEs is shown in figure 5.13. Adding low power nodesincrease the throughput and the gain is higher if the low power node is deployedin the center of the hot zone. The difference between the bingo and randomdeployment is smaller when offsets are added. With 0 dB offset the 50 percentileUE throughput is 3.5 % higher in the bingo case compared to the random case,while with 16 dB offset this number is 1.2 %. The edge UEs are suffering morefrom misplacement and the corresponding numbers are 7.5 % and 1.8 %.

5.4. Conclusions 37

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FUE throughput − Low power node UEs


Figure 5.11: CDF - average uplink low power node UE throughput.

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5.4 Conclusions

This study aimed to show if the results obtained in simulations with ideal lowpower node placement can be generalized and applies to real networks where thelow power node placement is not ideal. The system that has been simulated isquite extreme when considering the UE distribution where 50 % of the UEs arelocated in one hot zone per cell. The reason for this setup is to get distinct re-sults. A more commonly used system setup was earlier simulated without seeing


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Figure 5.13: CDF - average uplink UE throughput including all UEs.

any significant different between the bingo and random deployments meaninglow impact of the placement of low power nodes. In the simulations done in therest of this report more commonly seen system setups have been used. We canthen conclude that the results obtained for the simulations can be generalizedand applies to real networks which have a random error in the low power nodeplacement.

• Overall UE SINR gain with perfect deployment compared to random de-ployment.

– More UEs are absorbed by the low power nodes and therefore getshigher SINR.

– The UEs who are not absorbed by the low power nodes are benefitingfrom less interference.

• Small difference in UE throughput between the bingo and random deploy-ment cases. The more the range of the low power nodes is extended theless it matters if it is perfectly deployed or not.

• The more UEs connecting to the low power nodes the more the macroeNB gets offloaded resulting in more PRBs per UE in the macro cell.

• Deploying a low power node gives higher effect if it is deployed on edge ofa cell.

– If it is not easy to find the hotspot center the best thing is to deployin a direction away from the serving macro eNB.

• A more uniform user experience is achieved with offsets.

5.4. Conclusions 39

Note: Earlier simulations with a less extreme case with lower photzone showedeven less difference between the bingo and random deployments. In a realisticsystem where photzone is lower the difference between bingo and random deploy-ment is expected to be even lower.

Chapter 6

Analysis of 3GPP systemconfigurations

It has been discussed in chapter 4 different interference scenarios that can arisewhen adding lower power nodes to cellular systems. To see the effects of lowpower node deployment a study has been carried out examining three differentsystem configurations specified by 3GPP.

This study will show where problems arise and ways of improvement will begiven.


The details of the simulated systems are described in the coming sections.


3GPP has in [9] specified a set of configurations which should be consideredwhen analyzing heterogeneous deployments in LTE, see table 6.1.

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Number of low power nodes 0 2 2 2photzone - 0 4/15 2/3

Table 6.1: 3GPP heterogeneous network deployment configurations.

In the reference case a traditional homogeneous network with only macrobase stations is used for comparison. The configuration 1, 4a and 4b two hotzones are located at random positions within each macro cell area with a col-located low power node. Within those hot zones a percentage of the users,

41

42 Chapter 6. Analysis of 3GPP system configurations

specified by photzone, are placed while the rest of the users are distributed ran-domly within the system.


The system parameters are the same as in chapter 5.

6.1.3 Traffic model

The traffic model is the same as in chapter 5 with the following parameters.

• λ: 13 UE/s system wide. (0.62 UE/s/Cell)

• FTP packet size: 2 MByte

The traffic model will generate the following offered traffic.



Simulation time is 100 seconds during which 1254 UEs was created, i.e. 12.54UEs / second.


In the different configurations photzone, i.e. the clustering factor is changing. Asphotzone increases more UEs are placed in the vicinity of the low power nodes andmore UEs are therefore connecting to them. The distribution of UEs betweenmacro eNB and low power nodes is shown in figure 6.1.

Ref Conf 1 Conf 4a Conf 4b0

200

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UE distributions

Macro UEsLow power node UEs

Figure 6.1: User distribution between macro eNB and low power nodes in con-figuration 1, 4a and 4b.

6.2. Uplink results 43

A compilation of the UE distributions and PRB utilization is shown in table6.2. First it can be seen that less than half of the area of the hot zone is coveredby the low power node. This means that there are many UEs residing justoutside the low power node cell.1

Note: The UEs not intentionally placed in a hot zone are evenly distributedthrough the system area meaning that there is a 7 % chance they will be placedin the hot zones anyway.2

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UEs in hot zones - - 37 % 72 %UEs connected to LPN - 6 % 18 % 34 %LPN coverage of hot zone - - 49 % 47 %

Macro uplink PRB utilization 95 % 94 % 90 % 77 %Macro downlink PRB utilization 93 % 91 % 80 % 57 %

Table 6.2: User distribution and macro PRB utilization.

The macro PRB utilization is calculated with equation 5.1. The macro PRButilization depends on the number of macro UEs. We see that when the macrocell is offloaded by the low power nodes the macro PRB utilization goes down.

In the reference case and the low clustered cases the macro PRB utilizationis very high meaning that the PRB per UE ratio will be low. Low throughputis expected for the macro UEs due to the heavy load. It can be concluded thatit is a need to free resources in the macro eNB.

The macro PRB utilization is lower in downlink compared to uplink. Theoutput power of the macro eNB is 40 W with means that the output power perPRB is 0.8 W. Comparing this to 0.2 W which is the output power of the UEstells us that the signal in downlink is stronger than in uplink resulting in higherSINR and higher throughput per PRB in downlink. Since the offered load is thesame for uplink and downlink simulations a higher throughput per PRB resultsin lower PRB utilization.

To make a fair comparison between the configurations the PRB utilizationis measured in macro eNBs only.

6.2 Uplink results

In this section the performance for uplink transmissions is discussed. For uplinktransmissions each UE uploads one FTP packet according to the traffic modelin section 6.1.3.

1This is a potential source of high interference to the low power nodes as described inchapter 4.

2The hot zone area is 7 % of the macro cell area.



The lifetimes, or FTP delays as they also will be referred to, for each configu-ration are found in table 6.3. The FTP delay is the time taken from that theUE sends the first data of a packet until it has transmitted the whole packetand disappear from the system. In the same table the percentage of UEs whofinishes their transmission before the simulation ends are shown.

The numbers in table 6.3 are approximates because the simulator logs thedelay only for UEs which finishes their upload before the simulation time ends.Therefore the actual delays are expected to be longer than stated in the table.

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Average UE lifetime (s) 20.7 17.1 11.1 5.64Average macro UE lifetime (s) 20.7 18.4 13.5 7.42Average LPN UE lifetime (s) - 1.95 2.05 2.35

Finished upload 60.4 % 63.0 % 78.42 % 93.0 %Finished upload macro 60.4 % 61.0 % 74.5 % 93.0 %Finished upload LPN - 97.3 % 97.7 % 97.4%

Table 6.3: FTP upload time.

It is desired to have a uniform user experience in the network. It is foundfrom table 6.4 that the low percentiles of UEs have very low throughput inthe reference case and in low clustered cases. There is a big difference in UElifetimes for the macro and low power node case in the no or lightly clusteredcases. Deploying low power nodes in a system with uniform UE distributionwill benefit only a few users, this might not be desired by the operators.

6.2.2 Cell throughput

In figure 6.2 the average throughput per cell is seen. The difference between theconfigurations is the number of UEs gathered around the low power nodes. Themore UEs cluster around the low power nodes the more UEs connect to theminstead of the macro eNBs; hence their traffic will go through the low powernode instead. The low power node is said to offload the macro eNB. We see thiseffect by comparing the low power node and the macro throughput where thelow power nodes throughput is increased at the same time as the macro eNBsthroughput decreases when the UEs gather around the low power nodes.

The macro cell area throughput, which is the throughput for the macro celltogether with its two low power nodes throughput, is seen to increase when theUEs cluster around the low power nodes. This indicates that the system wascongested and by offloading the macro eNB the system gets less congested andmore data can get through.


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Macro cell area7.4

8.0 8.6 9.3throughput (Mbps) (8%) (16%) (26%)

Macro cell7.4 7.5 7.1 6.1

throughput (Mbps)Low power node

- 0.28 0.79 1.6throughput (Mbps)


0.8 0.86 0.93(bps/Hz/Macro cell area) (8%) (16%) (26%)

5 % UE0.12

0.14 0.28 1.0throughput (Mbps) (17%) (130%) (730%)

50 % UE0.53

0.72 1.4 3.3throughput (Mbps) (36%) (160%) (520%)

95 % UE2.7

7.4 8.7 8.8throughput (Mbps) (170%) (220%) (230%)

Table 6.4: Uplink throughput. The numbers in the parentheses are the gainscompared to the reference case.


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RefConf 1Conf 4aConf 4b


Low power cells

If RSRP cell association is used the UEs will connect to the base station fromwhich they get the strongest signal from. A macro eNB is in general placed at a


high altitude, on a mast or on the top of a building. A low power node, on theother hand, is located on street level where the signal suffers larger attenuationcompared to the signal from a macro base station. The path loss for low powernodes is therefore modeled with a larger attenuation factor than a macro eNB.This affects the size of the low power node cell. The size of the low power nodecell also depends on the distance from the macro base station as seen in figure6.3. The low power node to the left in the figure is larger than the one to theright.

−300 −200 −100 0 100 200 300−160

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Macro eNBLow power eNb

Figure 6.3: Path loss from one macro eNB and two low power nodes. The cellborders are marked with vertical lines.

A low power node cell close to a macro eNB is small and will absorb few UEswhile a low power node on the edge of a macro cell will be larger and absorbmore UEs. Figure 6.4 shows a CDF of the cell throughput for the low powercells. We can see the effect of having low power node cells of different sizes.In the high percentiles are low power nodes that have absorbed many UEs, i.e.located on the edge of the macro cell, while those low power nodes in the lowpercentiles are those close to the macro eNB. When many UEs are located inthe hot zone, such as in configuration 4b, more UEs are connected to the lowpower nodes and the cell throughput increases.

Macro cells

The macro cell throughput, seen in figure 6.5, shows the effect of the low powernodes offloading different amounts of traffic. Macro cells which are offloadedmuch traffic by their low power cells are seen in the low percentiles. Macro cellswith low power nodes close to the macro base stations are not offloaded a lotand will be having higher throughput and seen in the high percentiles.

Macro cell area throughput

In figure 6.6 the macro cell area throughput is seen. The macro cell area through-put is the sum of the macro cell throughput and the two low power nodesthroughput. It can be observed that in case the UEs are clustering around thelow power nodes the macro cell area throughput is increased, meaning more of


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Figure 6.4: CDF - average uplink low power node cell throughput.

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Figure 6.5: CDF - average uplink macro cell throughput.

the offered traffic gets through the system, this from the offloading of the heavyloaded macro cell.

When the simulator clusters the UEs it randomly selects UEs which areplaced in the hot zones. Which hot zone the UEs are placed in is choosenrandomly and UEs might end up in a different cell due to the clustering. Thiseffect is seen in the lower percentiles in figure 6.6 where some cells gets lowerthroughput due to that UEs has been moved to other cells.


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Figure 6.6: CDF - average uplink macro cell area throughput.

6.2.3 Interference

Figure 6.7 is showing the average interference received by the base stations. Theinterference is calculated with equation 5.2.


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Figure 6.7: Time average uplink interference per PRB per cell.


Low power nodes

The major interferer to low power nodes are surrounding macro UEs. For thelow power nodes there are two effects competing.

• As the UEs gets handed over to the low power nodes there are going tobe fewer macro UEs and therefore less interference.

• The low power nodes are not covering the whole hot zone. This meansthat as the UEs cluster in the hot zone more UEs are going to be locatedjust outside the low power node cells and cause heavy interference.

The fairly stable interference levels between the different cases indicate thatthe two effects are more or less cancelling each other.

Macro eNB

The interference received at the macro eNB decreases as the clustering factor,photzone, increases. Interference to macro eNBs are coming from UEs in neigh-boring cells and UEs in the own low power node cells. By clustering there will bemore low power node UEs interfering the macro eNB. At the same time there isalso a reduction of interference from the neighboring cells where previous macroUEs are handed over to the low power node and therefore will interfere less dueto lower transmit power. The created interference to a macro eNB by handingover UEs to its own low power node is compensated for by the reduction ofinterference from neighboring cells.

Macro cell area

In the right part of figure 6.7 the interference averaged over all base stationsis shown. It can be concluded that the interference increases when deployinglow power nodes. The average is higher because of the high interference to lowpower nodes.

6.2.4 SINR

Assuming constant noise over time the SINR depends on signal strength andinterference. The signal strength depends on the path loss.

Low power node UEs

The SINR for the UEs connected to the low power nodes is shown in figure 6.8.It is found that the SINR for the UEs is not deviating a lot as the UEs clusterto different degrees and for most UEs kept above 5 dB.

Macro UEs

The SINR for the UEs connected to the macro eNBs is shown in figure 6.9. Again is seen as the UEs cluster. Partially this is due to the reduced interferenceand partially due to that the UEs absorbed by the low power nodes is no longerincluded in this CDF. Since the low power cells are bigger when located on theedge of the macro cell there will be more macro edge UEs absorbed by the low


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Figure 6.8: CDF - average low power node uplink UE SINR.

power nodes. The absorbed UEs are removed from this CDF and we thereforesee a gain of the SINR in the lower end of the figure.

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Figure 6.9: CDF - average macro uplink UE SINR.

Aside from the UEs who are put in the hot zones intentionally, due to theclustering factor, the UEs will have the same location in all simulations. Tofurther see that the center UEs are not absorbed a CDF with the distance fromthe macro eNBs to their UEs is shown in figure 6.10. We see that the UEs closeto the macro eNB is not handed over while those further away are seen to be


absorbed by the low power nodes.

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Figure 6.10: CDF - distance from macro eNBs to their macro UEs.

All UEs

A CDF including all UEs is found in figure 6.11. An overall SINR increase isseen as the UEs cluster around the low power nodes. The gain in 50 percentileis 0.69 dB, 2.43 dB and 4.67 dB in configuration 1, 4a and 4b respectively.

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Figure 6.11: CDF - average uplink UE SINR including all UEs.


6.2.5 UE Throughput

To get high UE throughput a high SINR is needed at the same time as there hasto be sufficient PRBs available. We have seen that this system is heavy loadedand we can expect the PRB to UE ratio to become a problem.

Low power node UEs

Figure 6.12 shows a CDF for the throughput of UEs connected to the low powernodes. In figure 6.8 it was seen that the SINR was almost the same for allconfigurations. In the throughput we see a drop of throughput as the low powernodes cluster, this due to fewer PRBs per UE in the low power node as moreUEs get absorbed.

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Macro UEs

The macro PRB utilization in the macro eNB, table 6.2, is very high. Whenthe system is this congested there are not much bandwidth resources availableand the UE throughput is limited by the available PRBs.

When UEs gets handed over to the low power nodes the PRB per UE ratioincreases in the macro cell and the UE gets higher throughput, see figure 6.13.

All UEs

Figure 6.14 shows the CDF for the UE throughput including all UEs in thesystem. Comparing the reference case with configuration 1 we see that the gainis small when adding low power nodes to a system with uniform UE distribution.

6.3. Downlink results 53

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Figure 6.13: CDF - average uplink macro UE throughput.

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Figure 6.14: CDF - average uplink UE throughput including all UEs.

6.3 Downlink results

In this section the performance for downlink performance is discussed. Fordownlink transmissions each UE downloads one FTP packet according to thetraffic model in section 6.1.3.



In table 6.5 are the FTP download times in downlink shown. The figures intable 6.5 are approximates because the simulator logs the delay only for UEswhich finish their upload. Therefore the actual delays are expected to be longerthan stated in the table.

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Average UE lifetime (s) 17.2 13.6 7.39 3.32Average macro UE lifetime (s) 17.2 14.6 8.55 3.88Average LPN UE lifetime (s) - 2.42 2.41 2.22

Finished upload 64.4 % 71.8 % 85.5 % 95.4 %Finished upload macro 64.4 % 71.8 % 85.5 % 95.4 %Finished upload LPN - 97.3 % 96.5 % 96.3%

Table 6.5: FTP download time.

Below, in table 6.6, the downlink throughput has been compiled. In thereference case it can be seen that the throughput is very low for many UEs.When low power nodes are introduced the throughput gets somewhat higher,but only when the UE cluster around the low power nodes a meaningful gain isseen.


In figure 6.15 the downlink cell through is shown. The same effects are applyingfor downlink as for uplink. The downlink cell throughput is overall higher thanthe uplink cell throughput.

Low power nodes

The low power cell throughput in downlink is similar to that of the uplink. Seefigure 6.16.

Macro eNBs

Also the macro cell throughput for downlink is similar to the uplink. See figure6.17.

Macro cell area throughput

It naturally follows that the macro cell area throughput is also similar to thatof uplink. See figure 6.18.


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Macro cell area7.9

8.6 9.1 9.5throughput (Mbps) (9%) (15%) (20%)

Macro cell7.9 8.0 7.5 6.3

throughput (Mbps)Low power node

- 0.27 0.79 1.6throughput (Mbps)


0.86 0.91 0.95(bps/Hz/Macro cell area) (9%) (15%) (20%)

5 % UE0.14

0.19 0.48 1.7throughput (Mbps) (36%) (240%) (1100%)

50 % UE0.69

1.1 2.7 6.1throughput (Mbps) (60%) (290%) (780%)

95 % UE5.8

8.5 11 13throughput (Mbps) (47%) (90%) (120%)

Table 6.6: Downlink throughput. The numbers in the parentheses are the gainscompared to the reference case.


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Figure 6.15: Average cell throughput.

6.3.3 SINR

SINR in downlink is the signal power the UEs receive from the base station theyare connected to divided by the signal power from other base stations presented


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Figure 6.16: CDF - average downlink low power node cell throughput.

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Figure 6.17: CDF - average downlink macro cell throughput.

in dB. In this section the SINR for the low power node UEs and macro UEs arefirst shown in separate CDFs and then a CDF containing all UEs.

Low power node UEs

The simulator used can not provide measurements of the downlink interference.Since the average distance from the base stations to the UEs is the same for thedifferent configurations so is the average path loss. The SINR then follows from


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Figure 6.18: CDF - average downlink macro cell area throughput.

the interference.

The SINR for the low power node UEs is seen in figure 6.19. The macrobase station will use strong transmit power in order to reach the edge UEs. Thefewer macro UEs there are the less interference is expected to the low powernode UEs.

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Figure 6.19: CDF - average low power node downlink UE SINR.


Macro UEs

Also for the macro UEs the average distance, and therefore the path loss, tothe macro eNB is unchanged as the UEs cluster. The SINR for the macro UEsthen depend on their received interference. See figure 6.20.

The interference to macro UEs comes from neighboring cells with their lowpower nodes and also the low power nodes in the own cell. As more UEs getshanded over to the low power nodes the interference to the macro UEs from thelow power nodes increase. At the same time, the interference from neighboringcells decreases when the low power nodes absorb edge UEs.

From a macro UEs point of view; the strongest interference comes from theneighboring macro eNB when it communicates with its edge UEs. If the edgeUEs can get absorbed by the low power nodes the heavy interference is reduced.Explained in figure 6.3 the low power node cells at the edge of the macro cellare larger than those close to the macro eNB. This means that that there are alarger proportion of edge UEs that will be absorbed compared to UEs close tothe macro eNB.

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Figure 6.20: CDF - average macro downlink UE SINR.

All UEs

A CDF for all UEs in the system is seen in figure 6.21. In all cases with lowpower nodes has higher SINR compared to the reference case. The gain is smallwhen the UEs are not clustered but as the UEs cluster more the gain increases.The gain in 50 percentile is 0.32 dB, 1.45 dB and 4.08 dB in configuration 1, 4aand 4b respectively.


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Figure 6.21: CDF - average downlink UE SINR including all UEs.

6.3.4 UE Throughput

Also in downlink the throughput is expected to be limited by the PRB to UEratio as more UEs had no SINR problems.

Low power node UEs

A CDF for the low power node UEs throughput is found in figure 6.22. Thethroughput is, as discussed, depending on the SINR and the available bandwidthresources. We know that the number of UEs connecting to the low power nodesis small and the delays are low, see table 6.2 and 6.5. This implies that thereare few occasions when there are more than one UE connected to a low powernode at one time. The throughput for the low power node UEs therefore followsfrom the SINR shown in figure 6.19.

Macro UEs

A CDF for the throughput for the macro UEs is shown in figure 6.23. Wehave seen, in figure 6.20, that the downlink SINR is not low in any of theconfigurations. For the macro UE throughput there is an apparent problem.Aside from when the UEs are strongly clustered the UE throughput is low formany UEs. The low throughput is due to congestions in the macro cell.

All UEs

In figure 6.24 is a CDF including all UEs throughput.


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Figure 6.22: CDF - average downlink low power node UE throughput.

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Figure 6.23: CDF - average downlink macro UE throughput.

6.4 Summary

The performance of the system depended on the distribution of the UEs. Intable 6.7 the gains compared to the reference case is compiled.

6.5. Conclusions 61

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Figure 6.24: CDF - average downlink UE throughput including all UEs.

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Uplink Cell throughput +8 % +16 % +26 %Downlink Cell throughput +9 % +15 % +20 %Uplink interference +3.5 dB +6 dB +4,8 dBUplink SINR +0.44 dB +1.38 dB +2.77 dBDownlink SINR +0.34 dB +1.46 dB +4.18 dBUplink UE throughput +63 % +200 % +400 %Downlink UE throughput +57 % +180 % +370 %

Table 6.7: Gains from adding low power nodes in the different configurationscompared to the reference case.

6.5 Conclusions

Simulations of the configurations defined by 3GPP in [9] shows that introducinglow power nodes achieves gains in all configurations.

The gain mainly comes from the offloading the heavy loaded macro base sta-tions. The more the UEs are clustering around the low power nodes the highergain is seen. In the case when the UEs are uniformly distributed (configuration1) the low power nodes are not absorbing many UEs and the gain is low (8 %and 9 % in uplink and downlink respectively). When the UEs are clusteringaround the low power nodes the gain increases (26 % and 20 % in uplink and


downlink respectively in configuration 4b).Low power nodes receive strong interference from surrounding macro UEs.

This increase in interference is compensated for by lower path loss and the SINRis still higher for the low power node UEs compared to macro UEs.

The macro base stations get less interference when more UEs are absorbed bythe low power nodes; -97.6 dBm in configuration 1 and -102 dBm in configuration4b. This because the interference from UEs on the macro cell edge in neighboringcells are reduced when the low power nodes absorb them.

We see overall better system performance when low power nodes are added.As seen in the summary, the gain is relatively small when the UE distributionis uniform but when more UEs are clustering around the low power nodes thegain is higher.

Chapter 7

Analysis of 3GPP systemconfigurations - Low load

In chapter 6 it has been investigated different configurations of heterogeneousnetworks. It was investigated how deployment of low power nodes impactsthe performance when the UEs were clustering around the low power nodes todifferent degrees under heavy user load. In case of uniform UE distribution thegain was small.

To further see the value of deploying low power nodes in a network wherethe UEs are uniformly distributed another study has been carried out with thesame set up as in chapter 6 but with lower load.




In this study only configuration 1 has been compared to the reference case.Details are found in section 6.1.1.



7.1.3 Traffic model

The traffic model is the same as in chapter 5 with the following parameters:

• λ: 7.77 UEs/s system wide. (0.37 UE/s/Cell)

• FTP packet size: 2 MByte

The traffic model will generate the following offered traffic.


63

64 Chapter 7. Analysis of 3GPP system configurations - Low load




The user distribution and the macro PRB utilization is found in table 7.1.

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UEs connected to LPN - 6 % - 6 %

Macro uplink PRB utilization 70 % 65 % 95 % 94 %Macro downlink PRB utilization 52 % 46 % 93 % 91 %

Table 7.1: User distribution between macro eNB and low power nodes andmacro PRB utilization.

7.2 Results

In the following section configuration 1 will be compared with the referencecase. The results from the study with the high load traffic model are displayedin parallel as comparison.

7.2.1 SINR

Uplink

The average uplink UE SINR is seen in figure 7.1 and in table 7.2. Adding lowpower nodes gives great gains in SINR for those UEs who connect to them, UEswhich are absorbed by the low power nodes gets a gain of 5.04 dB and 5.45 dBin the low and high load case respectively. The UEs which are not absorbed bythe low power nodes also get a higher SINR but the gain is lower; 0.31 dB and0.12 dB in the low and high load case respectively. Averaging the gain over allUEs gives a gain of 0.59 dB and 0.44 dB.

Downlink

In downlink the SINR gain for the UEs is also small. The average gain in UESINR in downlink is 1 dB and 0.34 dB in the low and high load case respectively.See figure 7.2 and table 7.3.

7.2. Results 65

Low power node UEs Macro UEs All UEs0

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Figure 7.1: Average uplink SINR per UE.

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Low load 8.26 dB 8.85 dB 0.59 dBHigh load 7.06 dB 7.50 dB 0.44 dB

Table 7.2: Average uplink SINR per UE.

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Low load 11.6 dB 12.6 dB 1 dBHigh load 7.82 dB 8.16 dB 0.34 dB

Table 7.3: Average downlink SINR per UE.


One reason for adding low power nodes is to create gains by offloading themacro eNBs. When the UEs are uniformly distributed high offloading is hardto achieve.



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Figure 7.2: Average downlink SINR per UE.

Uplink

Only six percent of the UEs connected to the low power nodes in configuration1. This low offloading suggests only a small gain in served traffic. In figure 7.3and table 7.4 we see that in the simulations there was no gain in the low loadcase and 8 percent gain in the high load case. Worth noting is that in the lowload case almost 100 % of the offered traffic is served.


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7.2. Results 67

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Table 7.4: Average uplink cell throughput per cell.

Downlink

Similar to uplink, the downlink shows a no gain in served traffic in the low loadcase while a gain of 9 % when the system is has a high load. See figure 7.4 andtable 7.5. Also in downlink almost all offered traffic is served in the low loadcase.


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Figure 7.4: Average downlink cell throughput per cell.

7.2.3 UE Throughput

Uplink

For heavy loaded systems the average UE throughput is low due to congestion.The low power nodes are only offloading six percent of the UEs but whichstill gives a high percentage gain; see figure 7.5 and table 7.6. Even if a highpercentage gain of 63 % is seen in the highly loaded case the macro cells arestill congested and the throughput is low.

We see a gain in UE throughput of 18 % even though there was no gainin cell throughput in the low load case. This means that all offered traffic is


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Table 7.5: Average downlink cell throughput per cell.

served in the low load case and deploying low power nodes will only increasethe throughput for the UEs, not the cells.


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Table 7.6: Average uplink UE throughput per UE.

7.3. Conclusions 69

Downlink

The downlink UE throughput, as the uplink UE throughput, gets higher whenadding low power nodes. See figure 7.6 and table 7.7. In case of a heavy loadedsystem the percentage gain is high but the offloading is not high enough to givehigh throughput for the macro UEs. When the system has a lower load and themacro cells are less congested the presence of low power nodes are not highlynoticeable.


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0

7700

000

6200

000

6700

000

1400

000

1800

000

6200

000

6900

000

1400

000

2200

000T

hrou

ghpu

t (bp

s)

Downlink UE Throughput − Averages


Figure 7.6: Average downlink UE throughput per UE.

Ref

eren

ceca

se

Con

figu

rati

on1

Gai

n


Table 7.7: Average downlink UE throughput per UE.

7.3 Conclusions

In this chapter configuration 1 with two low power nodes has been comparedto the reference case. In the previous chapter it was seen that the gain fromintroducing low power nodes in a heavy loaded system with uniform UE distri-bution gave low gain in the system throughput. The gain was low because the


low power node was not able to offload many UEs from the macro cell. The lowpower nodes absorbed 6 % of the UEs.

This study has shown that when the system load is lower there is no increasein served traffic from introducing low power nodes. This is natural because whenthe macro cell is not heavy loaded offloading is not necessary.

The UE throughput was also compared and we see that the when the systemload was high the UE throughput was very low in the reference case. By intro-ducing two low power nodes there was a high percentage gain in UE throughputof 63 % and 57 % in uplink and downlink respectively. This shows that in aheavy loaded system a small offloading can give a high percentage gain in UEthroughput.

When the system load was lower the UE throughput gain from adding lowpower nodes was also lower; 18 % and 11% in uplink and downlink respectively.

Chapter 8

Analysis of 3GPP systemconfigurations - Rangeextension

There is a way to compromise between uplink and downlink performance. If wewant to optimize downlink performance the UEs should be assigned to the basestations from which the highest power is received (RSRP based cell association).If uplink performance should be optimized the UEs should be assigned to thebase station to which the path loss is lowest (path loss based cell association).

To compromise between the two extremes the range can be extended byusing RSRP cell association with an offset for the low power nodes. If theoffset is zero the UEs are connecting to the base station which gives highestreceived power, i.e. best downlink performance. As the offset is increased a cellassociation closer to path loss based cell association is approached.




The configurations simulated is the same as in chapter 6.



8.1.3 Traffic model

The traffic model is the same as in chapter 6.

71

72Chapter 8. Analysis of 3GPP system configurations - Range

extension


In the different configurations photzone, i.e. the clustering factor is changed.As photzone increases more UEs are placed in the hot zones and more UEs aretherefore connecting to the low power nodes. The cell association offset is alsoincreasing the number of UEs connecting to the low power nodes by extendingthe cell range. The distribution of UEs between macro eNBs and low powernodes is shown in figure 8.1 and table 8.1.

As expected the coverage of the hot zones is increasing with range extension,from below 50 % to below 100 %.

0

500

1000

1500

Num

ber

of U

Es

UE distributions

RefConf 1

Conf 1 REConf 4a

Conf 4a REConf 4b

Conf 4b RE

Macro UEsLow power node UEs

Figure 8.1: User distribution between macro eNB and low power nodes in con-figuration 1, 4a and 4b with and without 8 dB range extension.

8.2 Results


When increasing the RSRP offsets both the uplink and downlink macro cell areathroughput increases. This because the heavy loaded macro eNBs gets offloadedand more data can get through the system. See figure 8.2.

The highest gain in macro cell area throughput is seen in configuration 1, 0.8Mbps and 0.6 Mbps in cell throughput gain in uplink and downlink respectively.The reason for the higher gain in configuration 1 compared to configuration 4bis due to that a high percentage of the offered traffic is served in configuration4b and an increase is not as easy to achieve as in configuration 1.

8.2.2 Interference

The uplink interference is seen in figure 8.3. As described in section 4.2.1, macroUEs interfere the low power nodes. The closer the macro UEs are to the lowpower nodes the stronger the interference gets. In case the low power node isclose to the macro cell edge the surrounding macro UEs are using high transmitpower to reach the macro eNB and the low power node gets strong interference.

8.2. Results 73


1

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9

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ough

put (

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000 92

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094

0000

096

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ough

put (

bps)



(d) Downlink 8 dB offset

Figure 8.2: Average cell throughput per cell.


extension

Ref

eren

ce

Con

figu

rati

on

1

Con

figu

rati

on

1R

E

Con

figu

rati

on

4a

Con

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rati

on

4aR

E

Con

figu

rati

on

4b

Con

figu

rati

on

4b

RE

UEs in- - - 37 % 37 % 72 % 72 %

hot zonesUEs connected

- 6 % 16 % 18 % 35 % 34 % 61 %to LPNLPN coverage

- - - 49 % 95 % 47 % 85 %of hot zones

Macro uplink95 % 94 % 92 % 90 % 78 % 77 % 41 %

PRB utilizationMacro downlink

93 % 91 % 82 % 80 % 55 % 57 % 24 %PRB utilization

Table 8.1: User distributions and macro PRB utilization.

When the UEs are clustering inside the hot zone more of them are going to beabsorbed by the low power node. As a result there are fewer macro UEs whichcan cause interference.

At the same time, when no offset is used the low power cells are not coveringthe whole hot zone meaning that there will be many macro UEs surroundingthe low power nodes. In configuration 1 there will be few macro UEs residingjust outside the low power node cell but there will be many macro UEs in total.In configuration 4a there are fewer macro UEs in total but the number of macroUEs surrounding the low power nodes is higher. The sum of these two effectsis that the interference is increased. In configuration 4b the number of macroUEs is even lower and even though there is more surrounding macro UEs thetotal interference is lower than in configuration 4a.

The macro UEs surrounding the low power nodes can be absorbed by ex-tending the range of the low power cells so that they cover the whole hot zone.When the whole hot zone is covered the interference to the low power nodesgets lower the more UEs are placed in the hot zones. In configuration 1 thelow power nodes receive -96.3 dBm while in configuration 4b the correspondingnumber is -102 dBm.

The interference to the macro base stations comes partially from low powernode UEs partially from neighboring cells. The interference from the neighbor-ing cells comes primarily from the macro UEs. As macro UEs gets absorbedby the low power nodes the interference from the neighboring cell gets lower.At the same time UEs in the own cell will be handed over to low power nodesand interference is created. The sum of these two effects is seen to result in lessinterference. Naturally, range extension will then reduce the interference to themacro eNBs.

The downlink interference has not been obtained in these simulations.

8.2. Results 75


−120

−110

−100

−90

−92.

4 −89.

8

−91

−97.

4

−97.

6

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7

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−91.

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−92.

6

Inte

rfer

ence

(dB

m)





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Inte

rfer

ence

(dB

m)



(b) Uplink 8 dB offset



extension

8.2.3 SINR

The SINR depends on the interference and the received signal power. As fadinghas been removed in these simulations the received signal power depends ondistance from the UEs to the serving eNB.

Averages

The average SINR is seen in figure 8.4.

Uplink For the low power nodes, expanding the range leads to a greater aver-age distance to their served UEs. In configuration 1 the SINR is seen to decreasewith 2 dB due to a greater path loss which the decrease in interference does notcompensate for. For the more clustered cases the interference reduction is largerand the SINR is kept stable.

The average distance from macro eNB to macro UE is not affected by anextended range which makes the average macro SINR increase as the interferencedecrease.

The average SINR for all UEs is seen to increase with 0.41 dB, 1.19 dBand 1.77 dB in configuration 1, 4a and 4b respectively when range extension isapplied.

Downlink The low power node UEs receives interference when the macroeNB is transmitting to its UEs. The interference received by the low powernodes is related to the PRB utilization in the macro cells in table 8.1. HighPRB utilization in the macro cell gives high interference to the low power nodesand low PRB utilization in the macro cell gives low interference to the lowpower nodes. Without range extension, figure 8.4b, the SINR is higher in allconfigurations compared to the reference case. Range extension will make thelow power nodes absorb UEs further away from the base station. The range isextended with 8 dB and therefore the edge UEs will have 8 dB higher path lossin the range extended case and the low output power of the low power nodeswill not be able to support the edge UEs with high SINR in configuration 1 and4a, while in configuration 4b the interference is reduced enough to increase theSINR for the low power node UEs. See figure 8.4d.

For the macro UEs, the SINR is seen to increase for all configurations whenthe range is extended. Once again, the average distance from the macro eNBto the macro UEs is unchanged when the range is extended meaning that theincrease in SINR for the macro UEs is due to interference reduction.

The average SINR is reduced in configuration 1 but increased in configura-tion 4a and 4b.

Low power node UEs

Uplink In figure 8.5a and figure 8.5c the uplink SINR is shown without rangeextension and with 8 dB range extension. In both graphs the lower end ofthe CDF are the edge UEs while in the higher end of the graph has the UEsclose to the low power node. Adding an offset, figure 8.5c, will make the lowpower node absorb UE which are further away from the base station. The UEsin the range extended region are having a large distance to the base station

8.2. Results 77


5

10

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20

0

12.5

12.5

12.8

7.05

7.17 7.59 8.

32

7.06 7.5 8.

44

9.83

SIN

R (

dB)





5

10

15

200

9.07 10

12.5

7.82 8.11 9.

14

11.7

7.82 8.16 9.

28

12

SIN

R (

dB)





5

10

15

20

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12.6

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7.05 7.4 8.

03

9.55

7.06 7.

91

9.63

11.6

SIN

R (

dB)





5

10

15

20

0

1.87

7.35

14.7

7.82 8.

93

11.9

18.5

7.82

7.77

10.3

16.1

SIN

R (

dB)




Figure 8.4: Average SINR per UE.


extension

−5 0 5 10 15 20 250

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SINR (dB)

CD

FUplink UE SINR − Low power node UEs



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F




Figure 8.5: CDF - average low power node UE SINR.

meaning lower SINR. In configuration 1 the interference reduction will not beable to compensate for the further distance while in configuration 4a and 4b theinterference reduction is higher and the SINR increases.

For the UEs close to the low power node, in the upper part of the CDF, theinterference is small compared to the signal power the base station receives andthe interference has low impact on the SINR.

Downlink In downlink we see that in configuration 1 and 4a many UEs aregetting very low SINR when range extension is applied. In configuration 1 thePRB utilization in downlink is high which gives high interference. In configura-tion 4b the PRB utilization is lower and the interference is therefore lower.

The high interference, in combination with large UE-to-base station distancegives low SINR for many UEs in configuration 1. In configuration 4b the in-terference is reduced enough to compensate for the longer UE to base stationdistance.

Looking at the low power node UE SINR curves for uplink and downlink,figure 8.5a and 8.5b, we see that their shapes differs. In uplink the edge UEs

8.2. Results 79

suffer more from the interference than the center UEs. In downlink all UEssuffer more or less the same. The explanation for this will be seen in figure8.6. The signal from the macro base station will be attenuated according to thegreen curve. The signal from the macro UE will be attenuated according to theblue curve.

−300 −200 −100 0 100 200 300−140

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−100

−80

−60

−40

−20

Low power node cell border

Low power node


Distance (m)

Pat

h ga

in (

dB)

Macro eNBMacro UE

Figure 8.6: Path loss from macro eNB and macro UE.

In uplink the interfering signals to the low power nodes comes from sur-rounding macro UEs. As we can see if figure 8.7 the signal from the macro UEwill affect the low power node UE close to the cell border more than the lowpower node UE close to the low power node.

−50 0 50 100 150 200−140

−120

−100

−80

−60

−40

−20


Low power node


Distance (m)

Pat

h ga

in (

dB)

Macro UELPN UE

Figure 8.7: Uplink interference from macro layer to low power node layer.

In downlink the interfering signal comes from the macro eNB. As seen infigure 8.8 the attenuation of the signal is more uniform through the low powernode cell and the UEs are affected more uniformly.

−250 −200 −150 −100 −50 0 50 100 150 200−140

−120

−100

−80

−60

−40

−20


Low power node


Distance (m)

Pat

h ga

in (

dB)

Macro eNBLPN UE

Figure 8.8: Downlink interference from macro layer to low power node layer.


extension

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FUplink UE SINR − Macro UEs



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Figure 8.9: CDF - average macro UE SINR.

Macro UEs

Uplink Comparing the SINR curves for the different configurations in figure8.9a a gain is seen when the UEs cluster. The gain comes both from lower macroPRB utilization and that the absorbed UEs are removed from this CDF. Whendeploying a low power node on the edge of the macro cell it will be larger thefurther away from the macro eNB it is placed, see figure 6.3. This means thatthere is more cell edge macro UEs getting absorbed by the low power nodesthan cell center macro UEs which results in the apparent gain in SINR for theedge macro UEs.

With range extension, in figure 8.9c, the effects described above are simplygetting stronger.

Downlink The same reasoning applies to downlink as for uplink for the macroSINR. See figure 8.9b and 8.9d

8.2. Results 81

All UEs

In this section the trade off between uplink and downlink performance is seenwhen the range is extended.

Uplink It is beneficial for uplink transmission to let the UEs connect to theclosest base station. With range extension this path loss based way of associateUEs with the base stations is approached and we see a SINR gain in uplinkfrom figure 8.10a to 8.10c. A reduction of interference which follows from thelowered PRB utilization in the macro eNB will also contribute to higher SINR.

Downlink For downlink transmission, RSRP cell association is optimal mak-ing the UEs connect to the base station from which they get the strongest signal.By adding offsets the system is deviating from the RSRP cell association and wewould expect reduced SINR in downlink. In figure 8.10d we see that 10 % and7 % of the UEs have an SINR below zero in configuration 1 and 4a respectively.Those UEs are the ones in the range extended region of the low power nodeswhich has worse downlink condition to the low power nodes compared to themacro resulting in low SINR.

Fifth percentile UE SINR

This section shows the SINR for the fifth percentile UEs. See figure 8.11.

Uplink When the range is extended the uplink interference decreases for allconfigurations. The more the UEs cluster the larger the reduction is. Comparingthe bars to the right in figure 8.11a and 8.11c a significant SINR reduction of 4.06dB is seen in configuration 1. In configuration 4a and 4b the SINR increases by0.44 dB and 2.72 dB respectively as a result of lower interference in the systemwhen the macro PRB utilization is reduced.

Downlink In downlink the SINR is reduced by 3.5 dB and 1.75 dB when therange is extended in configuration 1 and 4a respectively. This reduction is aresult of very low SINR for the edge low power node UEs which suffered fromhigh path loss. The interference reduction was not able to compensate for thisreduction. In configuration 4b the interference reduction is much larger andcompensates for the higher path loss.

8.2.4 UE Throughput

Low power node UEs

A CDF for the low power node UE throughput is found in figure 8.12.

Uplink The UE throughput depends on SINR and the number of availablePRBs per UE in a cell. We saw that the low power node uplink SINR was moreor less unchanged for the different configurations but since the low load nodeload is higher in configuration 4b the UE throughput is lower compared to theother configurations. When the range is extended the nodes absorb more UEswhich means heavier load and the UE throughputs decrease. In configuration


extension

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Figure 8.10: CDF - average UE SINR including all UEs.

8.2. Results 83


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SIN

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Low power node UEs Macro UEs All UEs−8

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Downlink UE SINR − 5th Percentile



Figure 8.11: 5 percentile SINR.


extension

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FUE downlink throughput − Low power node UEs



Figure 8.12: CDF - average low power node UE throughput.

1 the SINR also decreased when the range was extended which will also reducethe UE throughput.

Downlink Aside from fewer PRBs per UE in the low power nodes when therange is extended the SINR was also greatly reduced when the range is extended.This effect is clear when comparing figure 8.12b and 8.12d. In configuration 1the loss in 50 percentile is 67 % and in configuration 4a 35 %, in configuration4b the SINR gain is overcoming the higher load and there is no difference in 50percentile UE throughput.

Macro UEs

A CDF for the macro UE throughput is found in figure 8.13.

Uplink The low UE throughput in the reference case which is seen in figure8.13a is due to congestions in the macro layer. When the macro eNB is offloadedby the low power nodes the PRB per UE ratio is increasing giving higher UEthroughput in the macro eNB. See figure 8.13c. When the UEs are uniformly

8.2. Results 85

0 2 4 6 8 10 12 14

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F




Figure 8.13: CDF - average macro UE throughput.

distributed, configuration 1, the gain of range extension is 64 %. In configuration4a and 4b the macro UE throughput gain is 91 % and 100 % respectively.

Downlink The same effect as in uplink also applies to downlink when thesize of the low power node cells is increasing. The downlink UE throughputgain in 50 percentile from extending the range is 98 %, 152 % and 100 % inconfiguration 1, 4a and 4b respectively.

All UEs

A CDF including all UEs in the system is seen in figure 8.14. The gain in the50 percentile uplink UE throughput is 80 %, 150 % and 91 % in configuration1, 4a and 4b respectively. The corresponding gain in downlink is 82 %, 93 %and 48 %.

Fifth percentile UE throughput

This section shows the fifth percentile of UE throughput. Worth noting is thatthe correlation between having the lowest SINR and the lowest throughput is


extension

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Figure 8.14: CDF - average UE throughput including all UEs.

8.2. Results 87

not very strong. Figure 8.15 shows a system map over a system in configuration4a. A legend to the figure is found in figure 8.16. It is seen that the UEs whohave low SINR not necessary have low throughput and vice versa.

−1000 −800 −600 −400 −200 0 200 400 600 800−800

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System map: Configuration 4aBlue circle: SINR < 10%Red triangle: TP < 10%

Figure 8.15: Example of a system map for configuration 4a.

The fifth percentile UE throughput is found in figure 8.17.

Uplink From the CDF for the low power node UE throughput, figure 8.12, wesee that there is some statistical uncertainty in the low percentiles but what isclear is that the UE throughput goes down in the low percentiles when the rangeis extended. For configuration 1 the increase in load is not as big as for the otherconfigurations but there is also a drop in SINR which reduces the throughput.This is also seen when comparing figure 8.17a and 8.17c. The 5 percentile lowpower node UE throughput is reduced by 17 % and 19 % in configuration 1 and4a respectively.

The macro edge UEs will get increased throughput when the range is ex-tended due to offloading of the macro layer.

Downlink Range extension clearly reduced the 5 percentile SINR in downlinkin configuration 1 and 4a. This is also reflected in their throughput comparingfigure 8.17b and 8.17d. The 5 percentile downlink low power node UE through-put is reduced by 84 % and 54 % in configuration 1 and 4a respectively.

As for uplink the macro edge UEs will get higher throughput in downlinkwhen the macro layer is offloaded.


extension

(a) Macro eNB (b) LPN

(c) Macro UE (d) LPN UE

(e) Low SINR UE(f) Low through-put UE

Figure 8.16: Legend to figure 8.15.


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Figure 8.17: 5 percentile UE throughput.

8.3. Conclusions 89

8.2.5 Summary

We have seen different gains in different configurations. A summary of the gainswhich range extension brings to the system is found in table 8.2.

Con

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Avg. uplink cell throughput +10 % +8.1 % +3.2 %Avg. downlink cell throughput +7.0 % +3.3 % +1.1 %Uplink interference -2.9 dB -8.0 dB -10 dBAvg. uplink SINR +0.41 dB +1.19 dB +1.77 dBAvg. downlink SINR -0.39 dB +1.02 dB +4.1 dBAvg. uplink UE throughput +71 % +69 % +44 %5% uplink UE throughput +64 % +230 % +150 %Avg. downlink UE throughput +41 % +51 % +32 %5% downlink UE throughput +95 % +210 % +82 %

Table 8.2: Gains from 8 dB range extension for the different configurations.

8.3 Conclusions

One reason for deploying low power nodes is to offload the macro layer. Tomake the low power nodes absorb more UEs, hence make more efficient use ofthe low power nodes, an offset can be added in to them in the cell associationalgorithm. In this study we have seen that adding offsets gives different gainsdepending on how the UEs are distributed. The highest percentage gain in cellthroughput is seen in configuration 1 where the UEs are uniformly distributed.Also, the average UE throughput increased with 44 % to 71 % in uplink and 32% to 51 % in downlink when the range was extended due to more efficient useof the bandwidth resources.

When the range is extended UEs in the range extended region is forced toconnect to the low power nodes even though they have a better link to the macrobase station. This made the SINR for these UEs to go down and 10 % and 7 %of the UEs had an SINR below zero in configuration 1 and 4a respectively, eventhough the interference in the system is reduced when the range is extended. Ifthe UEs in the range extended region could be protected from interference byan intercell interference mechanism their SINR is expected to increase.

Even though there are some UEs suffering from low SINR in some situationsthere is an overall gain in UE throughput by extending the range of the lowpower nodes in all configurations.

Chapter 9

Analysis of 3GPP systemconfigurations - Multiplelow power nodes

In chapter 7 the gain from adding two low power nodes to a system with uni-form UE distribution was studied. Later in chapter 8 we saw the interferenceproblems the low power node UEs in the range extended region had. In thischapter the studies in chapter 7 and 8 will be taken one step further. We willin this chapter see how adding more low power nodes to the system will affectthe performance. How the deployment of low power nodes affects the spectralefficiency will be shown and if interference between low power nodes createsproblems.




The following configurations have been compared which complies with configu-ration 1 defined by 3GPP.

• No low power nodes. (Reference case)

• 2 low power nodes.

• 2 low power nodes with 8 dB range extension.





91

92Chapter 9. Analysis of 3GPP system configurations - Multiple

low power nodes



9.1.3 Traffic model

The traffic model is the same as in chapter 6.



In table 9.1 we see the distribution of UEs between the macro and low powernode layer. The number of UEs which connects to the low power nodes increaseslinearly with increased number of low power nodes both with and without rangeextension. The macro PRB utilization follows from the number of UEs absorbedby the low power nodes. As the transmit power of the base stations is higherthe PRB utilization is lower in downlink compared to uplink.

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UEs connected- 6 % 16 % 12 % 32 % 18 % 45 %

to LPN

Macro uplink96 % 95 % 92 % 94 % 81 % 91 % 63 %

PRB utilizationMacro downlink

93 % 91 % 82 % 87 % 60 % 80 % 40 %PRB utilization

Table 9.1: User distributions and macro PRB utilization.

9.2 Results


The cell throughput is found in figure 9.1. Naturally the macro cell throughputdecreases as more low power nodes are deployed which will offload the macroeNBs. The throughput per low power node does not depend on the numberof low power node, rather the size of the low power node cells. Without rangeextension the low power nodes have a throughput of around 0.3 Mbps, withrange extension the low power node cell throughput is around 0.8 Mbps.

When more low power nodes are added the macro cell area throughputnaturally increases. In the case with 6 low power nodes and range extension alloffered traffic is served.

9.2. Results 93


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Figure 9.1: Average cell throughput per cell.

The spectral efficiency can be calculated from the macro cell area throughputand the bandwidth using equation 9.1. Figure 9.2a and 9.2b shows the spectralefficiency as a function of the number of low power nodes and the results isquantified in table 9.2.

Spectral efficiency =macro cell area throughput

available bandwidth(bit/s/Hz/cell) (9.1)

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Uplink spectral efficiency 0.74 0.8 0.88 0.86 0.97 0.91 0.99Downlink spectral efficiency 0.79 0.86 0.92 0.92 0.97 0.95 1

Table 9.2: Spectral efficiency vs. number of low power nodes per macro cellarea.

9.2.2 Interference

The uplink interference is seen in figure 9.3. Downlink interference was notpossible to obtain.

Low power nodes

In earlier studies it was seen that the strongest interference to the low powernodes comes from the surrounding macro UEs. We can here see that the inter-ference decreases as more low power nodes are deployed. We can conclude that


low power nodes

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Figure 9.2: Spectral efficiency vs. number of low power nodes per macro cellarea.

Low power nodes Macro nodes All nodes−130

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the inter-low power node interference will not be a problem in heterogeneousnetworks.

In chapter 8 we saw that adding an offset to the low power nodes in the cellassociation algorithm will drastically reduce uplink interference. This is alsoseen in these simulations.

Macro eNBs

The interference to macro eNBs comes, as earlier discussed, from neighboringcells, especially from their edge UEs. Having the low power nodes absorb UEswill reduce the interference to the macro base stations. Therefore the inter-ference to the macro base stations is reduced when deploying more low powernodes and when offsets are added.

9.2. Results 95

9.2.3 SINR

The SINR depends on the interference and the received signal power. As thefading has been removed in these simulations the received signal power dependson the distance from the UEs to the serving eNB.

Averages

The average SINR is seen in figure 9.4.

Uplink Without range extension the low power node SINR is stable around13 dB as more low power nodes are deployed. It was earlier seen that the uplinkinterference to the low power nodes was reduced when adding offsets. Eventhough the interference is reduced the low power node uplink SINR is reducedwith 3 dB to 1 dB when the range is extended due to higher path loss.

For the macro UEs the average path loss is fixed regardless of the number oflow power nodes or range extension. The average SINR therefore follows fromthe interference and is increased 0.1 dB to 0.6 dB without range extension and0.3 dB to 1.8 dB with 8 dB range extension.

Overall, a gain in uplink SINR is achieved both by using range extensionand adding low power nodes.

Downlink Also in downlink the low power node SINR is stable as more lowpower nodes are deployed without range extension.

As earlier seen the downlink SINR for the UEs in the range extended regionof the low power nodes is low due to a weak link to the low power node andinterference from the macro eNB. The average low power node downlink SINRis reduced by 7.5 dB, 4.7 dB and 1.9 dB with 2, 4 and 6 low power nodes permacro cell area.

As in uplink, the downlink macro SINR follows from the interference. Themacro UEs which gets the strongest interference are those on the edge of themacro cell which gets interference from the neighboring macro base station.When the macro PRB utilization is reduced so is the interference and the SINRis increased at the same time. The macro downlink SINR gain is 0.3 dB, 0.7 dBand 1.5 dB without range extension and 1.2 dB, 3.2 dB and 7.2 dB with 8 dBrange extension.

Low power node UEs

Uplink In figure 9.5a we see the low power node UE SINR in uplink. Eventhough the interference is reduced when the range is extended the SINR isdecreasing in all configurations due to higher average path loss. Adding lowpower nodes reduced the interference as well but the average path loss fromthe low power nodes to their UEs remains unchanged which gives higher SINRwhen more low power nodes are deployed.

Downlink The low power node SINR in downlink, figure 9.5b, is not changingwhen adding low power nodes. It will, however, decrease when extending therange of the low power nodes because the UEs in the range extended region is


low power nodes


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Figure 9.4: Average SINR per UE.

forced to connect to the low power node even though they have better down-link conditions from the macro eNB. This effect was described in chapter 8.Interesting to see is that the SINR problem for the UEs in the range extendedregion is reduced when more low power nodes are deployed, this due to lowerinterference when the macro PRB utilization is reduced.

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Figure 9.5: CDF - average low power node UE SINR.

Macro UEs

Uplink Figure 9.6a shows the CDF for the macro SINR in uplink. There aretwo effects which affects the uplink macro SINR. First, the macro UEs getshigher SINR due to a reduction of interference as the macro PRB utilization isreduced when low power nodes absorb UEs. The other effect is that the UEswhich are absorbed by the low power nodes will disappear from this CDF andsince the low power nodes mostly absorbs low SINR UEs there is an apparentgain in SINR in the lower percentiles.

9.2. Results 97

Downlink Figure 9.6b shows the CDF for the macro UE SINR in downlink.The path loss is unchanged for the different configurations and the CDFs followfrom the interference. We see that the interference is reduced when low powernodes offloads the macro layer due to lower macro PRB utilization.

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Figure 9.6: CDF - average macro UE SINR.

9.2.4 UE Throughput

Low power node UEs

A CDF for the low power node UE throughput is seen in figure 9.7.

Uplink In figure 9.7a we see the uplink low power node UE throughput. Thethroughput depends on the SINR and the available bandwidth resources. With-out range extension the number of available PRB per UE in the low power nodesis high as well as the SINR. When the range of the low power nodes is extendedthere will be fewer available PRBs per UE as well as lower uplink SINR, es-pecially with 2 and 4 low power nodes per macro cell area. The lower SINRtogether with fewer available PRBs per low power node UE is the reason for thelower low power node UE throughput when the range is extended. The averageUE throughput is 8.1, 7.9 and 8.0 Mbps and the reduction is 12 %, 6 % and 4% due to the 8 dB range extension with 2, 4 and 6 low power nodes per macrocell area respectively.

Downlink In downlink we also see the effects of the SINR reduction whenadding the offset, see figure 9.7b. The downlink SINR will decrease drasticallywhen the range of the low power nodes is extended. The reduction of low powernode uplink UE throughput due to range extension is 68 %, 44 % and 23 %with 2, 4 and 6 low power nodes per macro cell area.

Macro UEs

A CDF for the Macro UE throughput is seen in figure 9.8.


low power nodes

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Figure 9.7: CDF - average low power node UE throughput.

Uplink In the reference case the macro cell is heavy loaded which is alsoreflected in the uplink macro UE throughput. When low power nodes are addedthe macro cell will be offloaded and macro UE throughput will increase. Theincrease in macro UE throughput depends on how many UEs the low powernodes absorbs. When the range of the low power nodes is increased the numberof absorbed UEs increase and so will the number of available PRBs per macroUE resulting in higher macro uplink UE throughput.

Downlink Also in downlink the macro UE throughput, figure 9.8b, dependson the offloading of the macro cells and the same reasoning applies in downlinkas for uplink.

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Figure 9.8: CDF - average macro UE throughput.

9.3. Summary 99

All UEs

A CDF for the UE throughput including all UEs in the system is seen in figure9.9. When more low power nodes are added the system gets more resourcesand the UE throughput increases. To make best use of the added resourcesthe load should be uniformly distributed between the nodes which we have seenrange extension can be a tool in achieving. Even though range extension createdSINR problems for UEs in the range extended region of the low power nodesthe system as a whole benefits both from the 8 dB offset and the added lowpower nodes. In table 9.3 the gain in the fiftieth percentile is quantified.

Interesting to see is that the gain in UE throughput per low power nodeincreases as more low power nodes are deployed both without and withoutrange extension. Also worth noting is that, when comparing the dashed greencurve with the solid red curve, is that most UEs are better of in a system withtwo range extended low power nodes than a system with four low power nodeswithout range extension.

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Figure 9.9: CDF - average UE throughput including all UEs.

9.3 Summary

In table 9.4 the gains in different configurations compared to the reference caseis summarized.

9.4 Conclusions

The system was benefiting from adding low power nodes. Interference betweenlow power nodes has been shown not to be a problem. The major interferencearises when the macro UEs are communicating with the macro eNB. By of-floading the macro cells the interference decreases both to low power nodes andmacro nodes.

Range extension will make the low power nodes absorb more UEs whichwill offload the macro cell but at the same time range extension creates SINR


low power nodes

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Uplink UE34 % 145 % 89 % 504 % 183 % 957 %

throughput gainUplink UE throughput

17 % 73 % 22 % 126 % 31 % 159 %gain per LPN

Downlink UE45 % 204 % 161 % 552 % 320 % 900 %

throughput gainDownlink UE throughput

22 % 102 % 40 % 138 % 53 % 150 %gain per LPN

Table 9.3: UE throughput gain and UE throughput gain per low power node.Measured on the fiftieth percentile.

2L

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Avg. uplink+8.1 % +19 % +16 % +32 % +23 % +34 %

cellthroughput

Avg. downlink+8.9 % +16 % +16 % +23 % +20 % +27 %

cell throughput

Uplink+4.2 dB -0.1 dB +4.3 dB -2.1 dB +3.4 dB -4.8 dB

interference

Avg. uplink+0.4 dB +0.7 dB +0.9 dB +2.1 dB +1.6 dB +2.9 dB

SINR

Avg. downlink-0.4 dB +0 dB +0.8 dB +1.5 dB +1.6 dB +4.2 dB

SINR

Avg. uplink+65 % +180 % +135 % +380 % +220 % +560 %

UE throughput

5 % uplink+17 % +92 % +50 % +530 % +160 % +1230 %

UE throughput

Avg. downlink+57 % +120 % +115 % +280 % +190 % +420 %

UE throughput

5 % downlink+35 % +165 % +86 % +690 % +240 % +1470 %

throughput

Table 9.4: Gains from different number of low power nodes without and with 8dB range extension compared to the reference case.

problems for UEs in the range extended region of the low power nodes, especiallyin downlink. Those UEs received a weak signal from the low power nodes andthe interference was strong. By reducing the number of macro UEs the macroPRB utilization was reduced with lower interference as a result and the SINR

9.4. Conclusions 101

problem became smaller.Adding low power nodes will not only remove interference it will naturally

also increase the cell throughput and spectral efficiency in the macro cell areadue to more bandwidth resources.

The UE throughput was seen to increase as more low power nodes weredeployed and the UE throughput gain per low power node also increased asmore low power nodes were added. In case of two low power nodes they gave aUE throughput gain of 17 % and 22 % each in uplink and downlink respectively.The corresponding numbers for a system with six low power nodes per macrocell was 31 % and 53 %.

Chapter 10

Conclusions, proposal andfuture work

10.1 Conclusions

In this report heterogeneous deployment has been studied in LTE-Advanced.Low power nodes have been deployed to enhance the performance of the system.A set of studies has been carried out to see in which situations problems arises.The main conclusions from the studies are summarized here.

Misplacement study

First, in chapter 5, it was investigated how a misplacement of the low powernodes affected the performance. The purpose of the study was to see howmuch the results seen in simulations with ideal deployment deviated from morerealistic situations where the low power nodes have a random error in theirdeployment. It was found that the difference between ideal deployment andrandom deployment depended on the size of the low power node cells. If therange of the low power node cells is extended misplacement will not affect thesystem performance significantly. Without range extension a 3.5 % differencebetween ideal and random low power node deployment was seen which couldbe considered low. The results obtained from simulations with ideal low powernode deployment therefore also apply to more realistic deployments too.

Analysis of 3GPP system configurations

In chapter 6 configuration 1, 4a and 4b from [9] was compared to the case withonly macro base stations. In all configurations the low power nodes brings again to the system compared to the macro-only case. The SINR levels increasesand so does the UE throughput. The UEs which gets absorbed by the low powernodes naturally gets high throughput and the UEs who remains connected tothe macro eNBs will also get higher throughput since they get higher SINR dueto less interference and more available bandwidth resources.

A higher gain was observed in configuration 4b compared to the other con-figurations. This because the gain is related to the number of UEs the lowpower nodes absorbs and there are more UEs absorbed by the low power nodes

103

104 Chapter 10. Conclusions, proposal and future work

in configuration 4b as the UEs are clustered. In configuration 1 only 6 % of theUEs connected to the low power nodes and the gain was therefore small.

Even though the low power nodes received strong interference from the sur-rounding macro UEs the low power node SINR was still higher than the macroSINR since the low path loss was compensating for the increased interference.The macro eNB gets less interference when more UEs get absorbed by the lowpower nodes due to lower macro PRB utilization.

Analysis of 3GPP system configurations - Low load

In chapter 7 configuration 1 was further analyzed. It was seen in chapter 6that the gain from deploying low power nodes in a system with uniform UEdistribution was low since the low power nodes only absorbed 6 % of the UEs.The gain in cell throughput was 8 % and 9 % in uplink and downlink respectivelywhile the gain in average UE throughput was 63 % and 57 %. The gain in UEthroughput was high because the absorbed UEs had a very high throughputwhich increased the average UE throughput.

The same systems were simulated with a low load traffic model as well andthen no gain in cell throughput was seen, this because a system that has a lowload and serves all offered traffic can not gain in macro cell area throughput.Even though the cell throughput was not increasing there was a gain in averageUE throughput of 18 % and 11 % in uplink and downlink respectively. Againthis gain mostly comes from the few UEs which get absorbed by the low powernodes while the macro UEs which do not get absorbed do experience a largegain.

Analysis of 3GPP system configurations - Range extension

In the study in chapter 8 the range of the low power nodes was extended tofurther increase the gain brought by the low power nodes. An RSRP offset of8 dB was used which made the low power nodes absorb more UEs. It was seenthat the more the UEs cluster around the low power nodes the higher gain rangeextension gives.

When the range of the low power nodes is extended the interference wasreduced. The interference reduction depended on the number of UEs the lowpower nodes absorbed. At the same time range extension forces UEs in therange extended region to connect to a low power node even though they havebetter downlink conditions to the macro base station. These UEs got very lowdownlink SINR due to a weak downlink and high interference. In configuration1 and 4 10 % and 7 % of the UEs had an SINR below zero respectively, whilein configuration 4b the interference was reduced enough to overcome the weakdownlink and the UEs in the range extended region got higher SINR from therange extension.

Even though the UEs in the range extended region of the low power nodeshad very low SINR, looking the system as a whole it performed better withrange extension in all configurations.

Analysis of 3GPP system configurations - Multiple low power nodes

In the earlier studies it was seen that adding two low power nodes to the systemwas increasing the performance of the system. In chapter 9 it was seen how the

10.2. Proposal 105

system performance was affected when adding more low power nodes.As seen in chapter 8 that the UEs in the range extended region of the

low power nodes had downlink SINR problems, especially when the UEs wereuniformly distributed. In this study we saw that this problem got smaller whenthe more low power nodes was added due to lower macro PRB utilization.

Adding low power nodes increased the UE throughput, and interesting tosee was that the UE throughput gain per low power node was higher the morelow power nodes there were.

Interference between low power nodes was not seen to be a problem.

10.2 Proposal

As seen in chapter 8 there is a need to protect the UEs in the range extendedregion of the low power node cells. This can be done, for example, with Inter-Cell Interference Coordination (ICIC).

ICIC is a technique where the cells are scheduling their UEs in a way so thatinterference is mitigated. There are different ways of doing this. The principleis to put restriction in the scheduling so that neighboring cells do not scheduleUEs to collide in time and frequency and therefore will not interfere each other.

For an ICIC-scheme to bring gain to the system the avoidance of collidingUEs has to compensate for the lower trunking efficiency in the system.

If two cells are scheduling UEs in the same PRBs they will collide and getlower SINR which forces them to use a lower order modulation scheme andtherefore lower throughput. This should be compared to the case when restric-tions are put on the UEs so that they can only use a subset of the resources. Ifthe UEs get higher throughput by using fewer PRBs with less interference, com-pared to more PRBs with higher interference, the collision should be avoided.

Figure 10.1 shows an example of this. In figure 10.1a no restriction is madeon the scheduling and the transmissions in cell 1 and 2 will collide. If an ICICscheme is used the transmissions can be separated so that no collision will occur,see figure 10.1b. In the case of collision a lower throughput per PRB will beachieved but the cells will have more PRBs to schedule their UEs on.

10.2.1 Existing ICIC schemes

Below some existing ICIC-schemes are presented which separates the UEs infrequency. There are also ICIC schemes which separate UEs in time but willnot be discussed here.

Static reuse

In static reuse the frequency bandwidth is divided in to a number of non-overlapping bands. Each cell is assigned one of these bands. The assignmentof the bands is done in a way so that neighboring cells are not using the samebands. As in figure 10.2 the red cells can only use the red part of the bandwidth.

10.2.2 Fractional Frequency Reuse

As an alternative to static reuse, in Fractional Frequency Reuse (FFR) differentareas of the system uses different parts of the bandwidth. Scheduling collisions


Cell 1

Cell 2

PRB 1

PRB 2

PRB 1

PRB 2

(a) No ICIC is used and transmissions will col-lide in frequency.

Cell 1

Cell 2

PRB 1

PRB 2

PRB 1

PRB 2

(b) ICIC is used and collisions will be avoided.

Figure 10.1: Performance evaluation of ICIC schemes.

that would cause heavy interference can be avoided. One FFR setup is shownin figure 10.3. UEs in the center of a cell can use the whole spectrum whileedge UEs only can use part of the spectrum. The interference from and to edgeUEs is in general strongest. Having different cells schedule their edge UEs ondifferent frequencies reduces interference.

Allocation order based schemes

In this class of ICIC schemes the scheduler is scheduling the UEs from a startingPRB index. Different cells start scheduling from different PRB indexes. Thisscheme does not have any hard restriction on PRB usage as the static reusescheme has. If a base station needs to use all PRBs it will simply continue theallocation even if that means it will step over another cells offset. An illustrationof this scheme is found in figure 10.4.

Randomized scheme

This is similar to the allocation order based schemes. The difference is that theoffset used by each base station is chosen autonomously in a random mannerby each base station. The base stations will choose a new random offset with

10.2. Proposal 107

Figure 10.2: Static reuse ICIC scheme.

Figure 10.3: Fractional Frequency Reuse ICIC scheme.

a given time interval. The starting offsets will then change over time and theinterference will therefore be averaged over time. The randomized selection ofthe starting offsets will take away the need for the base stations to communicatewith each other.


#1 #2 #3

Figure 10.4: Allocation order based ICIC scheme.

Schemes based on X2-signalling

The connection between the eNBs can be used for signaling to counter interfer-ence. 3GPP have proposed three messages that can be sent. [13] [14]

• Uplink High Interference Indication (HII) is a message sent between eNBsover the X2 interface to help them to avoid uplink collisions. The HIImessage is a bitmap representing on which PRBs high interference usersare planned to be scheduled on. High interference users are generally celledge UEs. The action which the receiving eNB is going to take is notstandardized but it is beneficial if it is avoiding to schedule cell edge userson the PRBs which it knows a neighboring cell is going to schedule anedge user on.

• Each eNB is measuring the interference plus noise on each PRB and send-ing them to the neighboring eNBs in a message called Uplink OverloadIndication (OI). This info can be used for power control so as the inter-ference could be lowered in the uplink.

• Relative Narrowband Downlink TX Power (RNTP) is a message contain-ing an estimate of the transmission power a eNB is going to use in comingPRBs. This message is sent to the neighboring eNBs. The 3GPP standardhas not specified what actions the receiving eNBs should take. One possi-bility could be to adjust the reuse patterns within the system dynamicallywhere the eNBs are working together to minimize the number of collisionsin downlink.

10.2.3 Proposed scheme

We have seen that the UEs in the range extended region of the low power nodesget very low downlink SINR in some situations due to weak downlink conditionsto the low power node and strong interference from the macro node, which isa problem. To counter this problem the UEs in the range extended region canbe separated in frequency from the macro UEs by aid of an FFR-scheme whichwill be proposed in this section.

Through this report the low power nodes have operated in Open Access-mode (OA) where all UEs can connect to all base stations, as opposed to Close

10.2. Proposal 109

Subscriber Group-mode (CSG) where UEs need to be a member of a CSG inorder to connect to it. The details of the proposed scheme will depend on whichmode the low power nodes operate in.

Open Access mode

The proposed FFR scheme is seen in figure 10.5. The UEs in the range extendedregion (blue region) get scheduled on blue resources and edge macro UEs (greenregion) are scheduled on the green resources. UEs in the center of the macroand low power node cells can use the whole bandwidth. Alternatively the macroeNB can schedule UEs on resources in the blue region but with reduced transmitpower.

Center macro/LPN

Edge macro

Edge LPN

Figure 10.5: FFR scheme protecting UEs in range extended region of OA lowpower node cells.

Closed Subscriber Group mode

If the low power nodes operate in CSG-mode there might be situations whennon CSG members are located within the low power node cell. These UEs areexpected to give very strong interference to the low power node and should beseparated in frequency from the UEs in the CSG. To achieve this, the previousscheme can be altered as in Figure 11 6. The UEs in the low power nodes cellswhich are not members of the CSG can only be assigned resources in the yellowregion. They are separated from the CSG UEs at the same time as the LPNedge UEs are separated from the edge UEs of the macro cell.


Center macro

Edge macro

CSG Center LPN

Non-CSG LPN

CSG Edge LPN

Figure 10.6: FFR scheme protecting UEs in range extended region of CSG lowpower node cells.

Details of the scheme

There is a need to investigate appropriate parameters of this scheme such as thenumber of PRBs in each bandwidth region, the size of the center/edge regionsetc. This is a topic of a future study.

10.3 Proposed further studies

To further mitigate interference the following is an alternative CoordinatedScheduling (CS) scheme between macro and HeNBs that have potential to in-crease the uplink throughput. As discussed in chapter 4, a macro UE close to alow power node will create strong interference to the low power node UEs if theyare scheduled on the same PRBs. To avoid that the UEs gets scheduled on thesame resources ICIC could be used. ICIC algorithms can lower the interferencebut also reduces the trunking efficiency. If the macro and low power eNB cancommunicate and perform CS, the resources can be used more efficiently. CSrequires low latency and high throughput connections between the involved basestations. HeNBs do not have such connections but instead are connected to therest of the network over the internet. An internet connection is not reliable orfast enough to be used for CS.

An alternative approach is introduced below. In this schedule the HeNBwill not perform the scheduling of its UEs but will only receive their data. The

10.4. Alternative technology 111

macro eNB will perform the scheduling both in the macro cell and HeNB cellin the following manner:

1. The HeNB UE sends a resource request, not to the HeNB, but to themacro eNB. The HeNB will recognize that one of its UEs is transmittinga request and will expect a grant coming from the macro eNB.

2. The macro eNB will schedule the HeNB UE and send a transmission grantto the UE. The HeNB will overhear the grant and know with which trans-mission format and on which PRB the UE will transmit.

3. The UE will transmit the data to the HeNB while the macro eNB cancancel the UEs signal.

There is expected to be gains by having the macro eNBs aware of on whichPRBs the HeNB UEs will use and then avoid scheduling its own UEs in thesame PRBs. At the same time the macro eNB will be aware of how the lowpower node UEs signals will look like and there is a possibility for the macroeNB to cancel out the interference from the HeNB UEs.

A difficulty with this scheme is that the HeNBs needs to receive the grantin the downlink where it at the same time will transmit to its UEs. Figure 10.7shows this. The PRB with the grant is next to the PRBs which are used todownlink transmissions and to create hardware able to do this is challengingand expensive.

Downlink data transmissionGrant

Power

Frequency

Figure 10.7: Reception of transmission grant and downlink data transmissionsimultaneously.

10.4 Alternative technology

As a way of enhancing the performance of the cellular networks Home eNBs areproposed to be deployed. The Home eNBs are to be deployed by the users in,for example, their homes or in offices where there is a need for high throughputand good coverage.

Many of today’s UEs have, aside from the common cellular network connec-tion, also Wi-Fi access. The Wi-Fi access can be used to enhance the perfor-mance in the same way as a Home eNB. There are a few drawbacks however.


When a UE is connected to a Home eNB it will have its own IP address whilewhen connected to a Wi-Fi access point it will be behind a NAT which makesit hidden to the network outside the access point. The operator can then notauthenticate the UE preventing it to access services in the cellular network.

To counter this problem the software in the Wi-Fi access points can be mod-ified so that the operators have control over it and have a separate connectionto each of the connected UEs which would solve the authentication problem.

Another drawback is that a handover between the cellular systems to Wi-Ficannot be performed. This means that if a user is in their home and connectedto the Wi-Fi access point the connections will terminate if the user moves out ofreach of the access point. If connected to a Home eNB the user would seamlesslybe handed over to the macro eNB if it looses connection to the Home eNB.

There are several benefits with the Wi-Fi approach compared to Home eNBs.First of all is the economical perspective. A Wi-Fi access point is today verycheap and they are already widely spread. Instead of buying a Home eNBowners of a Wi-Fi access point can simply update the software. It is also easierfor a Wi-Fi access point to utilize services such as media servers and printers.Another benefit with the Wi-Fi access points is the spectrum aspect. Thespectrum used by Wi-Fi access points is unlicensed and the operators do notneed to pay for the spectrum at the same time the UEs connected to Wi-Fiwill not interfere with the UEs connected to the cellular network. A drawbackfrom using unlicensed spectrum is that the operators have no control over thespectrum and no guarantees can be given to the users. That no guarantees canbe given is also true for Home eNBs since the users are deploying them but sincethey will operate in spectrum that the operator owns the operator has a higherlevel of control.

Bibliography

[1] ITU-R. M.2134 requirements related to technical performance for imt-advanced radio interface(s). Technical report, ITU-R, 2008.

[2] C.E. Shannon. A mathematical theory of communication. The Bell SystemTechnical Journal, 27:379–423, 1948.

[3] David Tse and Pramod Viswanath. Fundamentals of Wireless Communi-cation. Cambridge University Press, 2004.

[4] Erik Dahlman, Stefan Parkvall, Johan Skold, and Per Beming. 3G Evo-lution: HSPA and LTE for mobile broadband. Elsevier Ltd., first edition,2007.

[5] T. Guess M.K. Varanasi. Optimum decision feedback multiuser equaliza-tion with successive decoding achieves the total capacity of the gaussianmultiple-access channel. Technical report, Asilomar Conference on Signals,Systems, and Computers, 1997.

[6] ITU-R. Report m.2135: Guidelines for evaluation of radio interface tech-nologies for imt-advanced. Technical report, ITU-R, 2008.

[7] 3GPP. Ts 36.101: 3rd generation partnership project; technical specifica-tion group radio access network; evolved universal terrestrial radio access(e-utra); user equipment (ue) radio transmission and reception. Technicalreport, 3GPP, 2010.

[8] 3GPP. Ts 36.104: 3rd generation partnership project; technical specifica-tion group radio access network; evolved universal terrestrial radio access(e-utra); base station (bs) radio transmission and reception. Technical re-port, 3GPP, 2010.

[9] 3GPP. Tr 36.814: 3rd generation partnership project, technical specifica-tion group radio access network, further advancements for e-utra physicallayer aspects. Technical report, 3GPP, 2009.

[10] Lingjia Liu, Jianzhong (Charlie) Zhang, Jae-Chon Yu, and Juho Lee. In-tercell interference coordination through limited feedback. InternationalJournal of Digital Multimedia Broadcasting, 2010:7, 2010.

[11] 3GPP. Tr 25.820: 3rd generation partnership project; technical specifi-cation group radio access networks; 3g home nodeb study item technicalreport. Technical report, 3GPP, 2008.

113

114 Bibliography

[12] Ericsson and ST-Ericsson. R1-102619 ul power control in hotzone deploy-ments. Technical report, 3GPP, 2010.

[13] 3GPP. Ts 36.423 : 3rd generation partnership project, technical specifica-tion group radio access network, evolved universal terrestrial radio accessnetwork (e-utran), x2 application protocol (x2ap). Technical report, 3GPP,2009.

[14] Harri Holma and Antti Toskala. LTE for UMTS, OFDMA and SC-FDMAbased Radio Access. John Wiley & Sons Ltd., first edition, 2009.

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