Simulator for the LTE Link Level Performance Evaluation

5/20/2018 Simulator for the LTE Link Level Performance Evaluation

Simulator for the LTE Link Level PerformanceEvaluation

Stojan Kitanov1

1University for Information Science and Technologies St. Paul, The ApostleBuilding at ARM

Ul. Partizanska B.B.Ohrid 6000

Republic of Macedonia

Abstract.The concepts of 4G have already been much discussed by a number

of global research initiatives and at the moment are in the process ofstandardization. Since the 4G concepts have already moved to the

standardization phase, it is time to start working on the building blocks of theNext Generation of Wireless and Mobile Networks, 5G. LTE provides high

data rates that are needed for the availability of new services and mobile

applications. Therefore LTE is seen as a potential technology candidate for 5G.

This paper describes an LTE link level simulation, which tests the performanceof an LTE link level channel. The simulator offers to carry out single-downlink,

single-cell multi-user, and multi-cell multi-user simulations. The simulation

results will contribute for the network optimization of LTE (and 5G in the

future). Additionally, by adding new functionalities to the simulator, theresearchers can create and test different network scenarios.

Keywords: 5G, 4G, WiFi, WiMAX (802.16), LTE, NGN (Next Generation

Networks), MATLAB, simulator.

1 Introduction

The road towards 4G (4th

Generation of Wireless Networks) has been the driving

force behind a number of global research initiatives in the last few years. The

concepts of 4G have already been much discussed and at the moment are in the

process of standardization. Since the 4G concepts have already moved to the

standardization phase, it is time to start working on the building blocks of the Next

Generation Wireless and Mobile Networks(referred to as 5G). 5G should make an

important difference and add more services and benefit to the world over 4G. 5G

should be more intelligent technology that interconnects the entire world without

limits. LTE provides high data rates that are needed for the availability of new

services and mobile applications and meets the demands for 5G. Additionally LTEsupports inter-technology mobility, i.e. the ability to support movement of a device

between differing radio access network types. Therefore LTE is seen as a potential

technology candidate for 5G Networks.

ICT Innovations 2011 Web Proceedings ISSN 1857-7288 107

L. Kocarev (Editor): ICT Innovations 2011, Web Proceedings, ISSN 1857-7288 ICT ACT http://ictinnovations.org, Skopje, 2012


This paper describes an LTE link level simulation, which tests the performance of

an LTE link level channel. The simulator offers to carry out single-downlink, single-cell multi-user, and multi-cell multi-user simulations. Additionally, the simulator can

be utilized to perform real-world measurements with a testbed. It also provides a

possibility for parallel processing of the simulations, and thus reduces the processing

time. The simulation results will contribute for better network optimization of an LTE

network (as well as 5G networks in the future). Additionally, by adding new

functionalities to the simulator, the researchers can create and test different network

scenarios.

The paper is structured as follows. A brief overview of related LTE simulation

platforms and technologies is given in Section 2. A description of the link level

simulation model is provided in Section 3. Furthermore, all relevant simulation

assumptions are introduced those which are applicable for both technologies, and

those being different. The link level simulation results covering SISO, MIMO

diversity and MIMO spatial multiplexing scenarios are shown in Section 4. The

simulation results are discussed in detail, and explanations for the different behaviorof LTE under certain conditions are derived. Finally, the main conclusions are drawn

in Section 5.

2 Related LTE Simulation Platforms and Technologies

A channel simulation is very important in every communication system in order to

evaluate its performance as well as to improve, or optimize it by changing the

systems parameters. So, in order to make the simulator useful it is necessary to make

the channel simulation as realistic as possible, evaluating various methods and

procedures to implement the most appropriate one for each case. In the case of LTE

networks, the simulation has to take into account all the phenomena introduced by the

wireless communication.Realistic performance evaluations of LTE in terms of coverage, capacity and

quality in a multi cell system require standard compliant simulators. For that reason,

several commercially available LTE simulators have been developed, for example [1-

3]. Additionally Large mobile communication equipment vendors have implemented

their own proprietary simulators. However, simulation has been extensively used

within the research and development of telecommunications, as an engineering tool

for design, implementation and optimization of radio networks for a very long time.

Hence, a diverse range of simulation software and frameworks exist, both free and

commercial, which are able to model complex wireless network systems.

One of the better renowned network simulators is OPNET [4], which is a software

suite containing simulation technologies for network and wireless network simulation

modeling. OPNET also offers data visualization, GUI supported modeling, result

prediction, monitoring and application optimization. OPNET arrives with a

commercial license and requires some detailed implementations to be implemented inC/C++ programming languages [4].

Another publicly available network simulator isNs 2 [5], which is a discrete event

simulator maintained as an open source project, originating from UC Berkeley. Ns 2

108 ICT Innovations 2011 Web Proceedings ISSN 1857-7288



provides support for the simulation of TCP, routing and multicast protocols over

wired and wireless networks and is primarily targeted for UNIX systems, even thoughit may be built and run on Microsoft Windows with Cygwin support [5].

WinProp Software Suite [6] is a commercial software suite, containing tools for

radio network planning and mobile radio wave propagation simulations, supporting

models of indoor and outdoor environments with different infrastructures. It supports

several network standards such as 2G, 3G, wireless LANs and WiMAX [6].

WarnSim [7], is a simulator for circuit switched wide area radio networks such as

Land Mobile Radio System (LMR), Personal Communication System (PCS) and

Public Safety Wireless Network(PSWN). The simulator is developed in C# .NET and

hence only currently runs on Microsoft Windows platforms with the .NET framework

installed [7].

The computation and visualization software suite MATLAB [8] is another

application extensively used for simulator implementations in radio network

simulation. Several publicly available LTE technology related simulators developed

for MATLAB also exists. One such LTE simulator is developed at the ViennaUniversity of Technology [9, 10]. It is available for free under an academic, non-

commercial use license and allows researchers to compare algorithms in a

standardized system. Since the source code of all functions is also provided, highest

flexibility for changes and additions as well as support for different platforms is

guaranteed. Therefore this simulator can be considered as a potential simulator for 5G

Networks. Next section reviews the structure of this LTE simulator, as well as the

potential research applications.

3 LTE Simulator Structure

The LTE link level simulator from the Vienna University of Technology [10]

consists of the following functional parts: one transmitting eNodeB, N receiver UserEquipments (UEs), a downlink channel model over which only the Downlink Shared

Channel (DL-SCH) is transmitted, signaling information, and an error-free up-link

feedback channel with adjustable delay. Possible modulations for the DL-SCH are 4-

QAM, 16-QAM, and 64-QAM. The elements of the simulator are shown in Figure 1

and more details are provided in [11].

Fig. 1. Overall simulator structure [11]




At the moment this LTE simulator has implemented the following basic features:

multi-user and HARQ. CQI (Channel Quality Indicator) feedback, and AMC(Adaptive Modulation Coding), i.e. dynamic assignment of MCSs (Modulation and

Coding Schemes) are under development. The following MIMO modes are supported:

Transmit Diversity, OLSM, and Single-Input Single-Output (SISO), Transmission

Diversity (TD) and Open Loop Spatial Multiplexing (OLSM) modes. Currently

Closed Loop Spatial Multiplexing (CLSM) mode is under development. AWGN, Flat

Rayleigh, Block Fading, Fast Fading and uncorrelated Pedestrian B channel are some

of the channel models the following are currently being implemented in the simulator.

The simulator, however, is already prepared for channel-adaptive simulations, that is,

the feedback channel, as well as all MIMO schemes are fully implemented; only

methods for calculating the feedback are missing [11].

A Complete list of simulation parameters for the LTE simulator is given in the

documentation of the LTE simulator [12]. Below are given some of the important

parameters:

number of UEs to simulate, number of eNodeBs (cells) that will be simulated. Currently support for

multiple eNodeBs is not yet implemented,

Channel Quality Indicators (CQIs) - set of MCSs that are used for thesimulation. According to [13], 15 different CQIs are specified,

transmission modes: single antenna, TD, OLSM, CLSM, and MultiuserMIMO (not yet implemented); these modes are defined in TS 36.213-820

Section 7.1, page 12 [13],

carrier center frequency in Hz,

Bandwidth: 1.4 MHz, 3 MHz, 5 MHz, 10 MHz, 15 MHz, and 20 MHz.These bandwidths are equivalent to 6, 15, 25, 50, 75, and 100 Resource

Blocks (RBs) respectively,

number of parallel Hybrid-ARQ (HARQ) processes. The maximum value,according to [14] is 8,

maximum number of HARQ retransmissions, not including the originaltransmission. Valid values are 0, 1, 2 or 3,

Subcarrier Spacing: 15 kHz, or 7.5 kHz (just for MBSFNbasedmulticast/broadcast transmissions). Tests were so far performed using the 15

kHz spacing,

cyclic prefix length [15]. Either normal or extended for MBSFN-basedmulticast/broadcast transmissions, and

type of channel used. Some of the the available ones are: AWGN (AdditiveWhite Gaussian Noise channel), flat Rayleight channel, as well as tap-delay

based channel models: PedA, PedB, PedBcorr, VehA, VehB, TU, RA, and

HT [16, 17]

The possible research applications of the LTE simulator are given in Figure 2.

Depending on the application we distinguish between three different classes of

simulations that differ greatly in computational complexity. Furthermore, thesimulation code can be utilized to perform real-world measurements with a testbed

[18]. Additionally the simulator can perform parallel processing of the simulations,

and thus reduces the processing time.




Fig. 2. Overview of different possible simulation scenarios in the LTE simulator [11]

The single-downlink simulation only covers the link between one base-station and

one user-equipment [11]. Such a set-up allows for the investigation of

channel estimators, channel tracking,

channel prediction,

synchronization algorithms, for example [19, 20],

MIMO gains,

AMC feedback including feedback mapping optimization, for example [21],

receiver structures [22], neglecting interference and impact of the scheduling,

modeling of channel encoding and decoding [23], and

physical layer modeling [24, 25] crucial for system level simulations.The single-cell multi-user simulation covers the links between one base-station and

multiple users [11]. This set-up now additionally allows for the investigation of

receiver structures, taking the influence of the scheduling into account,

multi-user precoding [26],

resource allocation and scheduling, and

multi-user gains.The multi-cell multi-user simulation is by far the most computationally demanding

scenario and covers the links between multiple base-stations and multiple users [11].

This set-up allows for the realistic investigation of

interference-aware receiver techniques [27, 28],

interference management, including cooperative transmissions [29], and

network based algorithms like joint resource allocation and scheduling.The LTE simulator can also be used to provide the signal processing part for a

testbed [30 - 33]. Therefore, in the LTE simulator the transmitter and receiver sub-

functions are clearly separated required for real-world measurements utilizing

physically separated testbed transmitters and receivers. Testbed measurements also

inherently offer another desirable degree of realism. The impact of RF impairments

on the LTE link performance can be investigated easily for different components. In

simulations, such an analysis would require a very detailed RF modeling, like in [34].




4 LTE Link Level Simulator Results

This section provides some simulation results that were obtained using the

standard compliant LTE link level simulator described in Section 3. The common

simulation settings for the results presented in the next two sections are summarized

in Table 1. The simulation results test the performance of an LTE transmission on an

uncorrelated PedB channel and flat Rayleigh channel for several transmission modes.

Since currently the feedback calculation for the Closed Loop Spatial Multiplexing

(CLSM) mode is not implemented, only the Single-Input Single-Output (SISO),

Transmission Diversity (TD) and Open Loop Spatial Multiplexing (OLSM) modes

are simulated.

In a MIMO-Orthogonal Frequency Division Multiplexing (OFDM) transmission

system the SNR is the measurement for channel quality information and is a key

factor of link error prediction. The SNR in the simulator is defined as the subcarrier

SNR (that is the sum of the data subcarrier signal powers divided by sum of the noisepowers received on all data subcarriers) [11].

Table 1. Basic Settings used for the simulation of LTE

Parameter Value

Number of User Equiments (UEs) 1

Bandwidth 1.4 MHzRetransmissions 0 and 3

Channel Type Flat Rayleigh, PedB uncorrelated

Filtering

Receiver TypeSimulation Length

Transmit modes

Block Fading

Soft Sphere Decoder5000 frames

SISO, TD (21, and 42) and OSLM (42)

To obtain the Block Error Ratio (BLER) and throughput for the Modulation andCoding Scheme (MCS) corresponding to each CQI value, AWGN simulations were

performed. The MCS determines both the modulation alphabet and the Effective

Code Rate (ECR) of the channel encoder. Figure 3 shows the BLER results of CQIs

1-15 without using HARQ. Each curve is spaced approximately 2 dB from each other.

-20 -10 0 10 20 3010

-3

10-2

10-1

100

BLER, 1.4MHz, SISO AWGN, 5000 subframes

BLER

SNR [dB]

CQI 1

CQI 2

CQI 3

CQI 4

CQI 5

CQI 6

CQI 7

CQI 8

CQI 9

CQI 10

CQI 11

CQI 12

CQI 13

CQI 14

CQI 15

Fig. 3. BLER curves obtained from SISO AWGN simulations for all 15 CQI values.

From CQI = 1 (leftmost) to CQI = 15 (rightmost)




The BLER curves for a flat Rayleigh channel, for the number of retransmissions

equal to 0 (HARQ = 0) and 3 (HARQ = 3) are shown in Figure 4 and 5, respectively.It can be noticed that SNR shifts to the left due to the presence of HARQ. Similar

simulation results can be obtained for PedB (uncorrelated Pedestrian B) channel.

-10 -5 0 5 10 15 2010

-3

10-2

10-1

100

BLER, CQI 7, PedB, 5000 subframes, flat rayleigh, 0 re-tx

BLER

SNR [dB]

SISO

TxD 2x1

TxD 4x2

OLSM 4x2

Fig. 4. BLER, flat Rayleigh channel, no HARQ

-10 -5 0 5 10 15 2010

-3

10-2

10-1

100

BLER, CQI 7, PedB, 5000 subframes, flat rayleigh, 3 re-tx

BLER

SNR [dB]

SISO

TxD 2x1

TxD 4x2

OLSM 4x2

Fig. 5. BLER, flat Rayleigh channel, 3 retransmissions

In Figure 6, the throughput curves are plotted for every CQI value. Here, HARQ is

switched off and no retransmissions are performed. The SNR gap from the achievable

capacity is around 2 dB for most of the CQI values. Similar throughput performance

is achieved if HARQ is enabled. The reason for the similar performance is that in an

AWGN channel the switching between the modulation and coding schemes can be

done perfectly and hardly any retransmissions are required (although allowed if

necessary).

In Figures 7 and 8, the data throughput of SISO, 21 transmit diversity (TxD), 42

transmit diversity, and 42 Open Loop Spatial Multiplexing (OLSM) is compared

when transmitting over a flat Rayleigh channel. In this simulation we set the CQI to a

fixed value of seven and the maximum number of HARQ retransmissions set to zero

(Figure 7) and the maximum number of HARQ set to three (Figure 8).

The maximum throughput values achieved by the different MIMO schemes in

Figure 7 and 8 depends on the number of transmit antennas and on the number of datastreams (layers). If more transmit antennas are utilized for the transmission, more

pilot symbols are inserted in the OFDM frame and thus lower maximum throughput

can be achieved. In the case of OLSM, two spatially separated data streams are




transmitted thus leading to twice the maximum throughput of the 42 TD system.

Note that the results in Figure 10 were obtained without channel adaptive pre-coding.An additional gain of the TD schemes can therefore be expected when the PCI is

utilized. Similar simulation results can be obtained for an uncorrelated ITU Pedestrian

B channel.

-20 -10 0 10 20 300

1

2

3

4

5

6throughput, 1.4MHz, SISO AWGN, 5000 subframes

throughput[Mbps]

SNR [dB]

CQI 1

CQI 2

CQI 3

CQI 4

CQI 5

CQI 6

CQI 7

CQI 8

CQI 9

CQI 10

CQI 11

CQI 12

CQI 13

CQI 14

CQI 15

Fig. 6. Throughput performance over an AWGN channel for individual CQIs without HARQ

-10 -5 0 5 10 15 200

0.5

1

1.5

2

2.5throughput, CQI 7, PedB, 5000 subframes, flat rayleigh, 0 re-tx

throughput[Mbps]

SNR [dB]

SISO

TxD 2x1

TxD 4x2

OLSM 4x2

Fig. 7 Throughput performance over a flat Rayleigh channel (CQI 7, no retransmissions)

-10 -5 0 5 10 15 200

0.5

1

1.5

2

2.5throughput, CQI 7, PedB, 5000 subframes, flat rayleigh, 3 re-tx

throughput[Mbps]

SNR [dB]

SISO

TxD 2x1

TxD 4x2

OLSM 4x2

Fig. 8. Throughput performance over a flat Rayleigh channel (CQI 7, 3 HARQ retransmissions)

The simulation results show that the simulator is functioning. The simulation

results will contribute for better network optimization of an LTE network. In future,

more focus should be dedicated on multi-cell multi-user simulation scenarios. This




includes interference-aware receiver techniques [27, 28], interference management,

including cooperative transmissions [29], network based algorithms like jointresource allocation and scheduling, as well as, inter-technology mobility, i.e. the

ability to support movement of a device between differing radio access network types.

Since LTE is seen as a potential technology candidate for 5G Networks (Next

Generation Mobile and Wireless Networks), the simulation results from this LTE

Link Level Simulator will contribute for better network optimization of 5G networks.

Additionally, by adding new functionalities to the simulator, the researchers can

create and test different network scenarios for LTE as well as future 5G networks.

5 Conclusion

This paper provides means of how to simulate the link level of an LTE network.

Firstly, related LTE simulation platforms and technologies were discussed in Section2. Then from all these LTE simulation platforms and technologies, the simulator from

Vienna University of Technology [10] was chosen as the most convenient simulator

for research purposes. This is because the simulator is available for free under an

academic, non-commercial use license and allows researchers to compare algorithms

in a standardized system. Since the source code of all functions is also provided,

highest flexibility for changes and additions as well as support for different platforms

is guaranteed. An overview of the structure of this simulator, as well as the possible

research applications were given in Section 3. The simulator tests the performance of

an LTE link level channel. The simulator offers to carry out single-downlink, single-

cell multi-user, and multi-cell multi-user simulations. Additionally, the simulator can

be utilized to perform real-world measurements with a testbed. It also provides a

possibility for parallel processing of the simulations, and thus reduces the processing

time. The LTE link level simulation results were discussed in Section 4. The

simulation results tested the performance of an LTE transmission on an uncorrelatedPedB channel and flat Rayleigh channel for several transmission modes: the Single-

Input Single-Output (SISO), Transmission Diversity (TD) and Open Loop Spatial

Multiplexing (OLSM) modes. The BLER curves and the Throughput performance

results were displayed. The simulation results show that the simulator is fully

functioning. The simulation results will contribute for better network optimization of

an LTE network. In future, more focus should be dedicated on multi-cell multi-user

simulation scenarios. This includes interference-aware receiver techniques [27, 28],

interference management, including cooperative transmissions [29], network based

algorithms like joint resource allocation and scheduling, as well as, inter-technology

mobility, i.e. the ability to support movement of a device between differing radio

access network types.

Since LTE is seen as a potential technology candidate for 5G Networks (Next

Generation Mobile and Wireless Networks), the simulation results from this LTE

Link Level Simulator will contribute for better network optimization of 5G networks.Additionally, by adding new functionalities to the simulator, the researchers can

create and test different network scenarios for LTE as well as future 5G networks.




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Simulator for the LTE Link Level Performance Evaluation