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LTE System Level Simulation with Matlab

Ying Li1, Fei Yu

1,

1. Zhengzhou Information Science and Technology Institute

Zhengzhou, China

hawk.hawk@163.com

Shu-lei Zheng2, Chun-lei Yang

1

Zhengzhou Surveying and Mapping Institute

Zhengzhou, China

AbstractWith the rapid development of 3GPP and its related

techniques, evaluation of system level performance is in great

need. However, LTE system level simulator is secured as

commercial secrets in most 3GPP members. In this paper, we

introduce our Matlab-based LTE system level simulator

according to 3GPP specifications and related proposals. We

mainly focus on channel model and physical abstract of

transmission. Brief introduction of every part is given and

physical concept and analysis are given.

Keywords-LTE; System Level; Simulation; Matlab

I. INTRODUCTIONThe 3GPP LTE and LTE-Advanced are the most popular

standards [1] in future wireless networks with most sponsorsand members. In order to study key structure and newmethods in 3GPP LTE standards, the LTE system levelsimulation is in great need.

The LTE system level simulation is a statistical-basedsimulation tool used to evaluate total system performanceand new algorithms. 3GPP has established a set ofsimulation parameters and evaluation methods so that itsmembers can build the platform themselves.

Studies on LTE system level simulation shows that thecomplete platform is secured as commercial secret for most3GPP members. For academic study and evaluation of 3GPPLTE standards and related algorithms, we build our ownMatlab-based platform.

This paper is organized as follows. Part gives a brief

introduction of 3GPP LTE system level simulator. In part,

channel model is specified. In part, transmission function

is introduced and conclusion is given in part .

II. OVERVIEW OF THE PLATFORMThe LTE system level simulator is designed and

implemented by different 3GPP members themselves, so theprocess and system details may have some difference. Infigure 1, we give the block diagram of the platform.

In figure 1, we consider the green part physical set part,the blue part link transmission part. The physical set part isthe fundamental of the platform. In this part, parameterinitialize and simulation control are presented. Similar in linktransmission part, this part will help finish transmission

process of each user in given cycle. Of course, in this part,adaptive modulation and coding (AMC), HARQ, Scheduling

and system to link level mapping (SL to LL) is performed forevery scheduled user to finish the process.

The pseudo-code of our matlab-based simulator is shown

below, the bold indicates the function of the process.

(channel, users, BSs)

each frame

Move UEs

each timeslot (TTI)

each BS do

scheduling, HARQ

each user in BS do

1. Calc SINR2. AMC3. SL to LL mapping4. Send feedbackIII. CHANNEL MODEL

The channel model for mobile wireless communication isregarded as a time, frequency and spatial correlated channel

This work is supported by New-generation Mo ile Wireless

Communication Network of Nationa National Sci-Tech Major SpecialItem. (No. 2009ZX03007-003)

BS Init

MS Init

Channel Init

TTI Cycle BS Cycle

AMC

HARQ

Scheduling

SL to LL

Save &

Out ut

Figure 1 Block Diagram of 3GPP LTE System Level Simulation

978-1-4244-7255-0/11/$26.00 2011 IEEE

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model. The commonly used channel model by 3GPP and itsmembers is called spatial channel model (SCM) [2] andSCM-Enhanced (SCME) [3].

Channel model is very important for system levelsimulation. Different from link level simulation, the systemlevel simulation concentrates the complete and total system

performance and evaluation. The basic element of systemlevel simulation is called a drop in which a snapshot will

be taken for every element in the simulation environment. Inthis paper, we take SCM as an example. The SCM model

presents an accurate way to express time, frequency andspatial influenced channel gain factor that can reflect realfading transmission environment.

The link between the u th receive and s th transmitantenna in given time sample t can be described as follows,where n denotes the n th path of overall 6 multi-path, M is

the total number of subpath, nPis the power of the nth path,

SF is the lognormal shadow fading , , ,n m AoD is the AoD

for the m th subpath of the nth path, , ,n m AoA is the AoA for

the m th subpath of the nth path, )(,, AoDmnBS

G is the BS

antenna gain of each array element, )( ,, AoAmnMSG is the MS

antenna gain of each array element.

Figure 2 shows the principle of SCM channel model.This figure is cited from [2]. It is clear that physicaltransmission environment is represented using this propermodel.

BS

AoDn,

, ,n m AoD

AoDmn ,,

BS

N

NCluster n

AoAmn ,,

, ,n m AoA

,n AoA

MS

MS

v

BS array broadside

MS array broadside

BS array

MS direction

of travel

MS array

Subpath m

v

Figure 2 Principle of SCM

( ) ( )[ ]( )

( ) ( )( )( )( )

=

+

=

M

m

vAoAmn

AoAmnuAoAmnMS

mnAoDmnsAoDmnBS

SFnnsu

tjk

jkdG

kdjG

M

Pth

1

,,

,,,,

,,,,,

,,

cosexp

sinexp

sinexp

)(

v

1

( )

A. Antenna PatternAs is shown in equation (1), the gain of both basestation

and mobile station is calculated using interpolation ofantenna pattern. The antenna pattern describes the antennagain in different directions shown in figure 2.

3-Sector Scenario

Figure 3 Antenna Pattern of 3-Sector Cell

Figure 3 shows a 3-sector antenna pattern of thesimulator. The left picture shows the broadside of the givensector and the right picture shows the physical antenna gain

pattern. The 3-sector scenario with three 120-degreedirectional antennas is a commonly used scenario in 3GPPLTE system level simulation.

At user side, an omnidirectional antenna is used toreceive signal. In equation (1), both BS gain and MS gain aregetting from interpolation using angle of arrival (AOA) [4].

B. Pathloss and ShadowingBoth pathloss and shadowing are the large scale

influence of a given channel model. Different from the smallscale influence such as Doppler Effect, large scale

parameters represents the influence of a relatively long time,e.g. 1 second.

In the area of wireless communications, pathloss is afunction of distance. Same in 3GPP, there are 3scenariosusing different pathloss model shown in figure 4[5]. Detaileddescription is given in reference [6].

0 200 400 600 800 100080

90

100

110

120

130

140

150

meter

Pathloss/dB

Pathloss In Different Scenario

Urban Macro

Urban Micro

Suburban Micro

Figure 4 Pathloss in Different Scenario

The shadow fading is the parameter that reflectsshadowing in transmission environment. It is a lognormaldistribution and in 3GPP specifications, we will not changeany of them.

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IV. TRANSMISSION FUNCTIONThe transmission function is a physical abstract of real

transmission process. There are four parts of this function:adaptive modulation and coding (AMC), HARQ, Schedulingand SL to LL mapping.

A. AMCIn this sub function, different combination of modulationand coding scheme is performed in every given resource

block using SL to LL mapping method [7] [8].

The RBI to BLER [9] is independent with modulation. Ifthe symbol number of the chunk is fixed, the RBI (received

bit information) of the chunk is still related to themodulation. Therefore, we generate the throughput map andcode rate threshold for each permitted modulation, whichonly considered the case that BLER, is no larger than TargetBLER. These mapping information are given as system

parameters. In the AMC module, we first get the RBI foreach permitted modulation. Then, we get the modulation andthroughput by interpolation in RBI and maximization in

modulation. The code rate is obtained by comparison withthe RBIR.

Figure 5 Block Diagram of AMC

The function of AMC is to decide better modulation andcoding scheme to perform the transmission, so functionaltests in AWGN channel is necessary. Figure 6 shows thesimulation result of AMC in AWGN channel. We can easilyinfer that with the increase of SNR, AMC module is decidingthe proper modulation and coding scheme to performtransmission under the instructions we set. The red lineindicates different modulation scheme and the green line isthe code rate. The blue one correspond the throughput in

AWGN channel, we can see the step of the working modulein which will prove the accuracy of this module.

-10 -5 0 5 10 15 20 25 300

1

2

3

4

5

6

snr(dB)

AMC throughput in AWGN channel

throughput/10000

modulation

code rate

Figure 6 Verification of AMC

B. HARQ and SchedulingHARQ and scheduling are function of resource allocation.

In each given timeslot, basestation has some resource blocksto assign.

In current LTE systems, error-correction mechanism ofphysical layer is implemented using CC or IR. Figure 7 givesthe brief introduction of HARQ. Once the simulation cyclereaches the scheduler, the scheduler will make a schedulingof all RBs pending to be arranged. HARQ packet is a high-

priority process. That means if a packet is a re-transmissionpacket, it is firstly scheduled. If the re-transmission block isempty, data packet is to be transmitted.

Figure 7 Block Diagram of HARQ

The scheduling module is the very close to HARQ. ForHARQ packet is send before all data packet. The commonlyused scheduler is proportional fairness scheduler.

In every given TTI, consider M is the schedulingcoefficient. The definition of M is:

( )( )

( )

currenti

average

T tM t

T t= (2)

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1 1( ) ( ) (1 ) ( 1)

average current AverageT t T t T t

N N= +

(3)

Where N is the timing window of simulation whichreflect the tolerance of latency of users, and alpha is thefairness factor that denotes the fairness of algorithms.

C. SL to LL MappingIn LL simulation, people focus on a point-to-point link

and its detailed algorithms. Different in SL simulation,people focus on the whole links in the simulation system andevaluate system features such as capacity, throughput, etc.,which will cause large amount of calculations if full linkactivity is performed.

In order to solve the problem, LTE standardizationproposed an effective mapping method between SL and LL.In given wireless multipath fading channels, the key point ofthe mapping method is to figure out the effective signal to

noise-and-interference ratio (SINR) valueeffSINR from

subcarrier SINRs, and then search the basic LL SINR-BLER(Block Error Rate) table to find the estimate SINR value.The link level SINR-BLER table is got from the link levelsimulation result under Additive White Gaussian Noise(AWGN) channel and this mapping is unique [10].

The mapping method can be described as follows:

( ) ( )k AWGN eff

BLER SINR BLER SINR= (4)

The left side of the equation1 means the real BLER in

channel model (e.g. SCME), and the right side is thesimulated BLER value in AWGN channel. The effectiveSINR value is defined in equation (3).

1

1

1( ( ))

N

eff n

n

SINR I I SINRN

=

= (5)

Where ( )I x is the subcarrier SINR combining function,

and n denotes the n th subcarrier. Figure 1shows themapping process between SL to LL.

The SL to LL part then finish the mapping and return theblock error rate to system to accomplish the simulation ofgiven timeslot.

Figure 8 SL to LL Mapping Scheme

V. CONCLUSIONIn this paper, we give a brief introduction of 3GPP LTE

system level simulation and present the details of ourMatlab-based simulator. It is a significant platform forevaluation and study of key technique of LTE and LTE-Awhich will break the monopoly of 3GPP members.

REFERENCES

[1] 3GPP TSG-RAN-1 Meeting #37,OFDM-HSDPA System levelsimulator calibration (R1-040500)

[2] 3GPP TR 25.996 v8.0.0 (2008-12), Spatial Channel Model for MIMOsimulations

[3] 3GPP, TS36.211 (V8.5.0), Evolved Universal Terrestrial RadioAccess (E-UTRA); Physical Channels and Modulation

[4] Josep Colom Ikuno, Martin Wrulich, Markus Rupp: System levelsimulation of LTE networks, IEEE VTC 2010

[5] D. S. Baum, J. Salo, G. Del Galdo, M. Milojevic, P. Kysti, and J.Hansen, An interim channel model for beyond-3G systems, in Proc.IEEE VTC05,Stockholm, Sweden, May 2005.

[6] R1-030999, RAN WG1 #34, Considerations on system performanceevaluation of HSDP using OFDM modulations [S].

[7] Mumtaz S, Gamerio A, Rodriguez J, et al. EESM for IEEE 802.16e:WiMaX[C]. Portland: 7th IEEE/ACIS International Conference onComputer and Information Science, 2008: 361-366.

[8] IEEE c802.16e 141r3, CINR measurement using EESM method [C],IEEE 802.16e contribution, 2005

[9] YuanGao,HongYi Yu, An Enhanced System Level to Link LevelMapping Method for 3GPP LTE System Level Simulation,CSIE2011

[10] 3GPP2-C30-20030429-010,Effective-SNR Mapping for ModelingFrame Error Rates in Multiple-state channels