[IEEE 2011 28th National Radio Science Conference (NRSC) - Cairo, Egypt (2011.04.26-2011.04.28)] 2011 28th National Radio Science Conference (NRSC) - Downlink interference mitigation

28th NATIONAL RADIO SCIENCE CONFERENCE (NRSC 2011)

April 26-28, 2011, National Telecommunication Institute, Egypt

Downlink Interference Mitigation for Two-Tier LTE Femtocell Networks

Hany Ismail, Essam Sourour Faculty of Engineering, Alexandria University, Alexandria, Egypt, [email protected]

Faculty of Engineering, Alexandria University, Alexandria, Egypt, [email protected]

ABSTRACT

Femtocells are low-power, very small-service-area (e.g. home or office environment) cellular base stations that will significantly impact the cellular landscape in the next several years. One of the most important open technical issues related to femtocells concerns the impact of femtocells on the performance of the conventional macro-cell system and the whole two-tier system [1], in this paper, a study of LTE wireless mobile system is proposed; First System Performance is modelled and simulated for the effect of insertion of closed subscriber group femto Access Points (F APs) into the network and how it affects the overall performance of the systems.

Then a study of the effect on the macro-cell neighboring users for the cases of the L TE system with random resources assignment, and the case of a new resources assignment procedure proposed for the closed subscriber group F APs based on the Fractional Frequency Reuse (FFR) of the macro base stations.

The performance of the model is evaluated throughout system level simulations and it is shown that insertion of F APs has some benefits and some drawbacks on the network, and it is also shown that the using of the proposed assignment procedure overcomes some of these drawbacks.

Keywords: LTE, Femtocell, Fractional frequency reuse, Closed subscriber group, Interference mitigation.

I. INTRODUCTION

It is a known fact that nowadays nearly 60% of phone calls take place indoors [2]. Hence the provision of high indoor coverage and capacity at low costs is of high interest for network operators.

Femtocells have emerged as a solution to increase both the capacity and coverage while reducing both capital expenditures and operating expenses [3], One of the most interesting open technical questions concerns how best to optimize the performance of networks including both femtocells and macro-cells [1].

The outline of the paper is as follows. System infrastructure is introduced in Section II. Access methods, propagation model, fractional frequency reuse, resources assignment, and adaptive modulation and coding techniques are introduced and explained in sections III,IV,V,VI,VII respectively. Performance evaluation is presented and discussed in section VIII, and fmally conclusions are presented in section IX.

II. SYSTEM MODEL INFRASTRUCTURE:

A number of randomly distributed indoor and outdoor environments with macro-cells, femtocells, and mobile stations are defined.7 macro-cells environment, designated by X, each with Omni directional antenna are considered.

An indoor environment is a two floor building of 3 meters height per floor and a circular region of 20 m in diameter surrounded by walls that attenuate all signals traversing them by 10 dB. Inside each indoor environment the number of indoor users is defmed and is classified into macro-cell users and femtocell users.

Indoor environments without femtocells are designated by circles, while indoor environments including femtocells are designated by squares, as shown in figure 1. There are also 30 edge macro-cell base stations designated by X.

Virtual users are assigned to these edge base stations depending on the average occupancy of the area under study.

All other locations on the map are outdoor environments. And there are a number of mobile stations of outdoor users designated by dots.

Each femtocell works in a Closed Subscriber Group (CSG). The macro-cell downlink transmit power is set such that the received signal to noise ratio at the cell edge is 20 dB.

The system operates at a centre frequency of 3.5 GHz, both the bandwidth of 10 MHz and the bandwidth of 15 MHz are considered in the simulation, both the macro base stations and the femto access points share the same bandwidth.

A neighbour is a macro-cell mobile user located within the range of the building containing closed access FAP.



x x x x

x x

x x x Area of Interest for pe�rrnanc. analysis

o Indoor Environment with FAP �x:' Macro Base station o Indoor Environment without FAP • .' Mobile User

Fig. 1: Snap shot for the simulated network.

Matlab system level simulation is made to study the pre-mentioned system. Indoor environments are randomly distributed on the map, so that indoor environments with femto access points are located within the area under study. Indoor mobile users are randomly distributed within the range of their indoor environment (buildings). Outdoor mobile users are randomly distributed in the outdoor environment.

Distances from each mobile user (femto and macro mobile users) to all the base stations (macro and femto base stations) is calculated, then the propagation loss between mobile users and base stations is calculated according to the distance, and the type of the link (outdoor to indoor, ... etc.) where shadowing is taken into account, as shown in the propagation model section IV.

According to the propagation loss, each macro-cell mobile user tries to connect to the macro base station with the lowest propagation loss, where each femtocell mobile user tries to connect to its closed subscriber group femto access point.

Signal to interference and noise ratio (SINR) is calculated for each mobile user, and so modulation technique, coding rate and throughput are calculated according to tables 2 and 3 in section VII.

A user is considered to be an outage in one of two cases, the case of lack of resources when the system doesn't have enough resources to assign to the mobile user, and the case of low SINR when resources have already been assigned to the mobile user, but it cannot connect to the network due to its low SINR resulting from poor signal (and/or) high interference.

III. ACCESS METHODS:

Selection of an access method has great effects on the performance of the overall network, the two main Types of access methods are the closed access method where only a subset of the users, can connect to femtocells. This model is referred to as CSG (Closed Subscriber Group) and Open Access method where all customers of the operator have the right to make use of any femtocell.

As the closed access method blocks the use of the femtocell Resources to a subset of the users, a new set of interfering signals is defmed, hence; the deployment of CSG femtocells makes the problem of interference mitigation even more complex. The deployment of open F APs would solve this issue, but bringing security and sharing concerns to the customer, furthermore when users move across areas with large numbers of open F APs, the number of hand overs and thus the signalling in the network increases.

Finally, a combination of these two types is called Hybrid Access method where a limited amount of the femtocell resources are available to all users, while the rest are operated in a CSG manner, hybrid access techniques can be seen as a trade-off between open and closed approaches .However, the number of shared resources must be carefully tuned to avoid a large impact in the quality of service of the femtocell customers [4].

IV. PROPAGATION MODEL:

The radio links between the Base stations, FAPs and Users (MOU, MIU, and FIU)l are divided into outdoor, indoor, and outdoor-to-indoor links according to the radio environment. The propagation loss L caused by slow fading at the Downlink is modelled based on an ITU path loss model [3]:

1 MOU stands for Macro-cell Outdoor User, MIU stands for Macro-cell Indoor User, F AP stands for Femto Access Point, FlU stands for

Femtocell Indoor User.



• Indoor Link (F AP -+ FlU):

L = 103 d3.71OS/l0 • Outdoor link (Macro-cell BS -- MOU):

L = 1049(�)4 /31OS/I0 1000 • Outdoor-to-indoor link (Macro-cell BS -+ MIU, FAP -+ MOU):

L = 104.9(�)4 /31OS/l01OPLII0 1000

(1)

(2)

(3)

Where PL, d, and f are the building wall penetration loss (dB), the amount of transmitter-receiver separation (m), and the frequency (MHz) respectively. S represents log-normal shadowing with a standard deviation of 8 dB [3].

V. FRACTIONAL FREQUENCY REUSE:

Since OFDMA systems are very sensitive to co-channel interference (CCI) from neighboring cells caused by the use of the same frequency channel [5], to combat the effect of the (CCI) in the cell edge we use Fractional Frequency Reuse.

Capacity is optimized if the neighboring cells do not transmit at all; alternatively, they may transmit only at low power (probably to users in the inner parts of the neighboring cells) to avoid creating strong interference to the scheduled user in the fIrst cell [6].

A Hybrid Mix of Reuse One and Reuse Three Schemes has been proposed , The idea is to use a frequency reuse of one at the cell centres where interference is low, and a frequency reuse of three at the cell edges where users are more subject to interference, as illustrated in fIgure 2.

When using this fractional reuse scheme, upon the arrival of a user, it is allocated a sub-channel within the frequency band that corresponds to its position in the cell. As the location of the mobile cannot be precisely known, in a real system the choice is based on the path loss: a threshold on the path loss is fIxed and terminal equipments with a path loss larger than this threshold are assigned a sub-channel within the frequency reuse three bandwidth [7].

Pot(ier

"'bdL-= •• '" u

flf2f3 Fig. 2: Fractional Frequency Reuse (3).

VI. RESOURCES ASSIGNMENT:

There are two radio frame structures for LTE: frame structure type 1 for full duplex and half duplex FDD, and frame structure type 2 for TDD, frame structure type 1 - which is our interest - consists of ten 1 ms sub-frames, each composed of two 0.5 ms slots, for a total duration of 10 ms. The Frame Structure type 1 is the same in the uplink and downlink in terms of frame, sub-frame, and slot duration although the allocation of the physical signals and channels is quite different. Uplink and downlink transmissions are separated in the frequency domain.

The smallest time-frequency unit used for downlink transmission is called a resource element, defIned as one symbol on one sub-carrier. A group of 12 sub-carriers contiguous in frequency and one slot in time form a Resource Block [8].

The relationship between the channel bandwidth of an L TE carrier (BW Channel) and the maximum number of Resource Blocks that can be allocated within a L TE RF channel - known as Transmission bandwidth confIguration (NRB) - is shown in Table 1 [9].



Table 1: Transmission bandwidth configuration in L TE.

BW Channel [MHz] NRB

In our scenarios femto Base stations reuse the same frequency as the macro Base stations, and only one resource block is assumed to be assigned to each user. The assignment sequence for the macro Base Stations is thought to be random, while the Assignment sequence for the femto Access Points is made according to the FFR of the closest (most effective) macro base station, as follows:

If the F AP is located within the range of the inner frequencies (close to the macro base station), it uses the Outer frequencies first in the assignment procedure. And if the F AP is located within the range of the outer frequencies (far from the macro base station), it uses the Inner frequencies first in the assignment procedure .This way the system decreases the probability of interference between the femto Users and the neighboring macro-cell Users as they use different frequencies, as illustrated in fig3.

c:::J lImf'l" Frf'qlltuciu

S �lobiJf' sl:.lioll

o FAP

Fig. 3: FAP Resources assignment based on FFR.

VII. ADAPTIVE MODULATION AND CODING (AMC):

Adaptive modulation and coding (AMC) has been proposed in most of the current wireless standards to archive higher system capacity, adaptive modulation is used for maximizing the data throughput of the subcarriers that are allocated to the users. This is based on the SINR measurements of each sub-carrier followed by the application of the modulation scheme that maximizes the spectral efficiency while at the same time maintaining an acceptable BER.

The User Equipment uses the CQI (Channel Quality Information) to tell the network of the highest data rate it can accept. Based on this information, the scheduler chooses a coding and modulation format for the next TTl (Transmission Time Interval) that would work better in poor conditions [10], The Downlink SINR requirements are shown in Table 2 [11], and the downlink bit rates (Mbps) for LTE according to the AMC used and the channel bandwidth are shown in Table 3 [12].

Table 2: Downlink SINR requirements.

Modulation Coding SINR+IM (dB) 2

QPSK 112 4.5

I6QAM 112 10.9

I6QAM 3/4 14.3

64QAM 3/4 21.5

Table 3: Downlink peak bit rates (Mbps).

AMC 1.6 MHz 3.0 MHz 5.0 MHz 10 MHz 15 MHz 20 MHz QPSK 112 0.8 2.2 3.7 7.4 11.2 14.9

16 QAM 112 1.5 4.4 7.4 14.9 22.4 29.9 16 QAM 3/4 2.3 6.6 11.1 22.3 33.6 44.8 64 QAM 3/4 3.5 9.9 16.6 33.5 50.4 67.2

2 Where Implementation Margin (1M) is included to account for the difference in SINR requirement between theory and practicable

implementation, This can include degradation of the signal due to any (digital) processing of the signal before the demodulator (such as filtering and re-sampling) and the use of a non-ideal demodulator, as well as the diversity gain being less than 3 dB .



VIII. PERFORMANCE EVALUATION:

The spacing between base stations is 300 meters. Power control is assumed for the F APs that grantees power satisfying a modulation of 64QAM with a coding rate of 3/4 for each femto user with a power ceiling of 11100 of the macro base station power. Fractional frequency reuse inner area is a circle of radius 4110 of the spacing between base stations, and the outer frequencies power is half the inner frequencies power.

Part 1: First, we study how far the insertion of femto Access Points into the network affects the overall performance of the network. A scenario of a network of 50 buildings is proposed, each building contains 6 macrocell users, and we study the performance when some of these buildings use F AP and only 3 mobile users are assumed to be able to connect to each F AP, 150 macro outdoor users are considered.

450 ----------------------------.... --.... - ...... --.. --------.. ----------------------------.----....... - ----.. -------_ ... ---------------------------.. --

'*' 40 ��.!.L •. '-;:'::: ____ ' __ ' ___ ' _____ " _____ " ___ ' __ " ' __ ' __ ----.---.--.-.-.. -..... QJ 3S ------ �.!- .•. � .• ::--------------------------------. tID !9 30

� ..... � 25 --------- -- -.-.---.---.-.-.-.. -.-.-----:

,., ..... -;;-;; -;- ::

-: : -:�- -- ---

N 20

� 15 -.---------------.-----... -.------.-.-... ---=�-�--� � 10 ---------------------------------.--

Q.

i400 '":" 350 � :::> '00 � '"

a. 250 ... ::I a. 200

-f. ::::J 150 �

.£; 100 .....

� 50 � � 0 '-------------------

w � � � � � � � �

Where,

Number of Femto access points ••• Overall 10 MHz -Overall 15 MHz

Number of Femto access points

••• Overal110 MHz -Overal115 MHz

Fig. 4: (a) Percentage total Outage for the system, (b) Average Throughput for the system.

Outages Caused by lack of resources + Outages Caused by SINR PercentageTotal Outage = --------'------''--------------''----Number of users L User Throughput

I no. of users PercentageTota Outage = --"--------Number of Users

(4)

(5)

From the above results it is clear that by inserting more F APs in the network the overall performance of the network increases, as some of the macro-cell users connect to the F APs and get higher throughput, and they also leave their resource blocks empty for other macro-cell users, and this decreases the outage cased by lack of resource blocks.

Part 2: Second, two Scenarios are proposed to show the effect of using F APs on the neighboring macro-cell users in an L TE network, Each scenario is made for two cases, the case of conventional F AP random resource assignment and the case of the suggested F AP resource assignment. Case 1:

Scenario 1: in this scenario we study the performance of both the femto users and the macro-cell users when increasing the number of macro-cell users near FAPs (neighbours), while maintaining the number of femto indoor users constant.20 FAPs are considered each with 10 FlUs. 10 homes without FAPs are considered each with 5 macro indoor users.40 macro outdoor users. Results are shown below:

4' � �------------------

� 35 ....

... . .. .... ..... .. . .. .... . .. ... . .. . ..

•••.•••

•• • . • �'-"-"-'-'-'-.x ••••••

,x ..•••. � � 30

••• _

•••• �

.... ... .. "'" 6 � ... . . . . .. ,x ..•• .. 20 � r.��-----::::;;= __ -��--N �

¥".... -

!: �� 10 �-�- ��-----------� , �� ---------------

=

o -* __ "* __ �_._*" __ *" __ � ___ .� ___ +<_ ., <--_--=-_--"-_--=-_--"'-----"�O _-=.:"'-----""-4 _-::16

Number of Neighbors per FAP

••• Macro Users 10 MHz- -Femto Users 10 MHz-overall10MHz

.)(. Macro Users 15 MHz�Femto Users 15 MHz�verall15MHz

500 -;; ]- 450 )6r. ______ )4c _____ M. ______ ...:_ -. __ .... ___ �. ___ . -.)410 -. ____ )(_ '" - 400 � 350 Q:j 300

0.. ; 250 0..

..t:. 200 '" B 150 � 100 � � 50 � �

�'I-X... _ II Sf ••••

::::��:::::::�:: ::::� •••••• ·X ••••••• " •••••• 'X •••• �x . . . ... ........ � ........ .............--

10 12 14 Number of Neighbors per FAP

••• Macro Users 10 MHz- -Femto Users 10 MHz-Overalll0MHz

·M Macro Users IS MHz�Femto Users IS MHz .... OveralllSMHz J

Fig.5 : Senariol Casel results . (a) Percentage Outage caused by interference, (b) Average Throughput.



Scenario 2: in this scenario we study the performance of both the femto users and the macro-cell users when increasing the number of femto indoor users per F AP, while maintaining the number of neighbours constant, .20 F APs are considered each with 12 macro-cell neighbors. I 0 homes without F APs are considered each with 5 macro indoor users.40 macro outdoor users. Results are shown below:

so --------------------------------------------------------------------------------------------------------------------------------_.

*- 45

OJ .., boO

� 3

5

6 30

700 0. � 600

5::: 500 :::> II

OJ 25 boO '" '0 1: OJ 15 U Qj 10 Co.

cE 400 S Q. 300

� co i! 200 �

II

�::�:::::::�::::::�:::::::�:::::::� �---�----�---�---�---�---�----x

10 Il 14 ,.

� 100 !! " � 10 12

Number of Femto Users per FAP Number of Femto Users per FAP

• • • Macro Users 10 MHz- -Femto Users 10 MHz-OveralllOMHz

oK Macro Users 15 MHz�Femto Users 15 MHz�Overall15MHz

••• Macro Users 10 MHz - -Femto Users 10 MHz -OveralllOMHz

.)+ Macro Users 15 MHz�Femto Users 15 MHz�Overalt lSMHz

Fig.6 : Senario2 Casel results . (a) Percentage Outage caused by interference, (b) Average Throughput. Where,

Number of Outages Caused b SINR PercentageTotal Outage = !Y Number of users

L User Throughput A Th h no. of users verage roug !]Jut= ...:..:...c""'--'.c.:..:..:...'--____ _

Number of Users

14

(6)

(7)

,.

From the above results of Case I scenarios 1 and 2 it is clear that any increment in the number of either the neighboring macro-cell users or the number of femto indoor users affects both the average Throughput and the Outage caused by interference of the macro-cell users badly. And this is because the probability of interference increases as the probability of sharing the same resource blocks increases.

It is also clear that the average Throughput and the Outage of the femto indoor users are slightly affected by the change in either the neighboring macro-cell users or the number of femto indoor users. And this is because of the short distances between the F APs and their femto indoor users and because of the wall penetration loss that weakens the macro-cells' signal.

It is also clear that the behaviour of the system looks very similar when using either 10 MHz or 15 MHz bandwidth, But the bandwidth of 15 MHz always show better performance. And this is because it offers more resources and this reduces the probability of sharing the same resources (Le. the probability of interference).

Case 2: This time we use the FFR based assignment procedure stated above in section VII for the same two scenarios

mentioned earlier, and with the same parameters and configurations.

Scenario I results are shown below: "

� 30 OJ :f 25 ':i a 20 OJ � 15 1: OJ 10 � � ,

. . .. .... .. . . . ..... .. .. .. . ... •••••••••

.� •••••• <M •••••• �

.0· . .. ••••••••

.->< •••••• �

••

;::.;.' ..

.. : .::::: �

.....

10 12

Number of Neighbors per FAP

14

••• Macro Users 10 MHz--Femto Users 10 MHz-OveralllOMHz

.)(0 Macro Users 15 MHz�Femto Users 15 MHz�OveralllSMHz

,.

700 - ..... ..... _- ------... ............ . ----. --------- ... --.... .

0. � 600

� 500 :::> � 400 � � 0. '00 � o 200 � � 100

�

� .......... �==�==�===�===� u u�uu

,l

uuuuumm

ll

uuuuuuuuuuu

�

':':::::: :�::::::�:::::: �:::::: �:::::: �::::::�::::::�

10 12 14

Number of Neighbors per FAP •••

Macro Users 10 MHz- -femto Users 10 MHz-OveralilOMHz

0)(0 Macro Users IS MHz-t<-Femto Users IS MHz .... OveraillSMHz

Fig.7 : Senariol Case2 results . (a) Percentage Outage caused by interference, (b) Average Throughput.

,.



Scenario 2 results are shown below:

"

�.==�===�===�===�===�===�====x

700 � ... f---",,*,..,=,,-===,*=,-,,,-,===-,,*,-,,-,===,,,::tt:,-,,,-===,,,*,-,,,-===,..:i<.,=,,--,,,--,,=,..:t5,,,===-,,=,-,t.<,,---� soo f----------------------

.-ft. 400 ��==� j 300 h � g,o � ••• M •••••• M •••••• )G. •••••• x •.••... x ...••• ok � 200 ... � 100 f----------------------

10 12 14 16 ,. j 1. 12 14 Number of Femto Users per FAP

• • • Macro Users 10 MHz--Femto Users 10 MHz-overalllOMHz

.� Macro Users 15 MHz�Femto Users 15 MHz,*,Overall15MHz

Number of Femto Users per FAP

••• Macro Users 10 MHz - -Femto Users 10 MHz-overalilOMHz

0)(' Macro Users IS MHz1(-Femto Users 15 MHz�verall15MHz

Fig.8 : Senario2 Case2 results . (a) Percentage Outage caused by interference, (b) Average Throughput.

From the results of Case 2 scenarios 1 and 2 it is has been noticed that the results of Case 2 are similar to the results of Case 1, But the results of Case2 show better performance as it increases the macro-cell users average throughput and decreases macro-cell users percentage outage, and so the overall performance of the system is improved.

To show the advantages of case 2 over Case 1, the percentage gain of the system for scenarios I and 2 is calculated as follows:

. (CaselOutage - Case20utage) GamOutage = (8) CaselOutage

• _ (Case2Throughpuf- CaselThroughpu) GamThroughpuf- --------='-'--------=--'--

CaselThroughpuf And this is shown in the following four figures: Scenario I results:

30 --- - --- - - ----. -_. _. .-

"

H 25 " o .� 20 C

., 4. " f;" e 30 ..c ... " ,5

(9)

.. w:.:�:.::.::.::.::.::.::.::.::.::.::.::.;:.;:.::.:w ........ .... _--

� 15 g>., .!9 10 C 1l

C 2. 'iij \!l 15 g>., .!9 10 C

�I -� ,

Q.

,. 12 Number of Neighbors per FAP

-Macro Users 10 MHz'" OveralilOMHz

�Macro Users 15 MHz- -OveralllSMHz

14 ,. � 5 "

Q.

,. 12 Number of Neighbors per FAP

-Macro Users 10 MHz··· Overalll0MHz

�Macro Users 15 MHz- -Overall 15MHz

Fig.9 : Senariol Percentage gain. (a) Outage caused by interference, (b) Average Throughput. Scenario 2 results: ,. CII � Z5 ':; o 2. .!: .: 15 IV

� CII ,. .. IV 1: CII

.. " ::I ... � 20 e � 15

l-.!: c ,.

.; � CII ' .. IV ..

14 ,.

:= CII � . f--- -./=--------------------"" ., 10 12


-Macro Users 10 MHz- - - Overalll0MHz


14 ,. i ·5 r. 10 12


-Macro Users 10 MHz··· Overalll0MHz


Fig. to : Senario2 Percentage gain. (a) Outage caused by interference, (b) Average Throughput.

14 ,.



IX. CONCLUSIONS

In this paper, the performance of a network containing macro base stations and femto base stations was investigated using Matlab system level simulation, and a new assignment sequence was proposed to overcome the drawbacks of the system. Simulation results showed that:

• Insertion of Femto Access Points in the network improves the overall performance of the network, as it increases the coverage of indoor users, and reduces the user demands on macro-cells.

• Although, Femto Access Points have the above mentioned benefits, they also have some drawbacks, as it strongly increases the interference at the macro-cell users neighboring to the F APs.

• The proposed assignment sequence technique improves the performance of the neighboring macro-cell users, as it decreases the probability of sharing the same resources.

• Generally, the improvement resulting from the proposed assignment sequence technique increases as the number of femto indoor users or the number of neighboring macro-cell users or both increases, till it reaches the point when the safe resources (those which don't suffer from interference) ends, and then this improvement decreases because the probability of sharing the same resources increases.

• The behaviour of the system looks very similar when using either 10 MHz or 15 MHz bandwidth, But the bandwidth of 15 MHz always show better performance, because it offers more resources and this reduces the probability of sharing the same resources.

• The proposed assignment sequence technique shows convergent performance for both bandwidths.

REFERENCES:

[ I] David Choi, Pooya Monajemi, Shinjae Kang, and John Villasenor "Dealing with Loud Neighbors: The Benefits and Tradeoffs of Adaptive Femtocell Access" 'IEEE Global Telecommunications Conference (Globecom)'Dec.2008, page(s): I - 5.

[2] Alvaro Valcarce, David Lopez-Perez, Guillaum De La Roche, and Jie Zhang "Limited access to OFDMA femtocells" Personal, Indoor and Mobile Radio Communications Symposium (PIMRC), Tokyo, Japan, September2009.

[3] Han-Shin Jo, Cheol Mun, June Moon, and Jong-Gwan Yook "Interference Mitigation Using Uplink Power Control for Two-Tier Femtocell Networks ", IEEE Transactions on wireless communications , Vol. 8, No. 10, October 2009, Page(s):4906 - 4910.

[4] Guillaume de la Roche, Alvaro Valcarce, David Lopez-Perez, and Jie Zhang "Access Control Mechanisms for Femtocells" IEEE Communications Magazine Volume: 48. January 2010, Page(s): 33 - 39.

[5] Abdelhalim Najjar, Noureddine Hamdi and Ammar Bouallegue "Fractional Frequency Reuse Scheme With Two and Three Regions for Multi-cell OFDMA Systems ", 17th Telecommunications forum TELFOR 2009, Serbia, Belgrade, page(s): 319 -322.

[6] Stefania Sesia, Issam Toufik, Matthew Baker "LTE - The UMTS Long Term Evolution from Theory to Practice ", John Wiley & Sons Ltd, 2009, Chapter 12, Page(s): 294 - 299.

[7] Yang Xiao "WiMAX / Mobile Fi Advanced Research and Technology ", Auerbach Publications, Taylor & Francis Group, LLC, 2008, Chapter 6, page(s):143 -144.

[8] "Agilent 3GPP Long Term Evolution: System Overview, Product Development, and Test Challenges" Application Note.

[9] "UMTS Long Term Evolution (LTE) Technology Introduction", Application Note IMAlll_2E, Rohde & Schwarz.

[10] Ajay R. Mishra " Cellular technologies for emerging markets 2G, 3G and beyond", John Wiley & Sons, Ltd, 2010, Chapter 5, page: 132.

[11] Stefania Sesia, Issam ToufIk, Matthew Baker "LTE - The UMTS Long Term Evolution from Theory to Practice ", John Wiley & Sons Ltd, 2009, Chapter 22, Page 521.

[12] Harri Holma and Antti Toskala "LTE for UMTS - OFDMA and SC-FDMA Based Radio Access ", 2009 John Wiley & Sons Ltd, Chapter 9, Page(s): 213 - 214.

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[IEEE 2011 28th National Radio Science Conference (NRSC) - Cairo, Egypt (2011.04.26-2011.04.28)] 2011 28th National Radio Science Conference (NRSC) - Downlink interference mitigation