EFFECT OF TRANSMITTER AND RECEIVER IMPAIRMENTS ON THE PERFORMANCEOF MIMO IN HSDPA

Bo GoranssonJ, Stephen Grant2

, Erik LarssonJ, and Zhu Feng3

Ericsson Research

IStockholm, Sweden; 2Research Triangle Park, NC, USA; 3Beijing, China{bo.goransson, stephen.grant, erik.larsson, feng.zhu} @ericsson.com

ABSTRACT

In this contribution we study the impact of transmitter andreceiver impairments on the throughput for MIMO and64QAM transmissions in an evolved HSDPA system. Asimple additive white Gaussian noise model is used tomodel impairments in the transmitter and receiver chain.The investigation is performed through computersimulations both on link and system level. It is found thatthe impact from transmitter and receiver distortions is rathersmall for MIMO with 16QAM while the impact when64QAM is used is more pronounced.

Index Terms- EVM, HSDPA, MIMO, WCDMA,UMTS, 64QAM

1. INTRODUCTION

Recently 3GPP has finalized Rel-7 of the WCDMAspecification. The focus (on layer I) of this release has beento improve the data rate and throughput for HSDPA (HighSpeed Downlink Packet Access) by introducing a number ofnew features such as higher order modulation and MIMO.By introducing the possibility to use 64QAM, the downlinkpeak-rate has been increased by 50% from approximately14Mbps to over 21Mbps in a 5MHz channel. Thestandardized 2x2 MIMO scheme can provide bitrates up to28Mbps. In the coming release, Rel-8, this is even furtherextended to over 42Mbps by allowing the combination ofMIMO and 64QAM.These high bitrates require a high available SNR. This callsfor well planned networks and low load in the networks. Italso puts rather strict requirements on the quality of thetransmitted signal and on the receiver front end.In this contribution we investigate the effects of transmitter(Tx) and receiver (Rx) impairments on the throughput of anevolved HSDPA network.The main source for Tx impairments are the Tx filters andnon-linearities and clipping in the power amplifier. Acommon way to model the Tx impairments is to use Error

Vector Magnitude (EVM) as a measure. EVM describeshow much the transmitted signal deviates from the idealsignal. This is usually expressed as a percentage relative tothe total transmitted power. In the WCDMA standard,minimum requirements are set by the performancespecification 25.104 [1]. It should be noted that otherrequirements such as peak code domain error should befulfilled as well. This is, however, not addressed in thiscontribution. In the current version of the specification it isstated that the EVM should not be worse than 12.5% whenthe base station is transmitting a signal containing 16QAM.The main sources of receiver impairments are the receivefilters, phase noise and I/Q imbalance. The impairmentscreated by the transmitter and receiver will limit theperformance of features such as 64QAM and MIMO whichrequire a high SNR to provide large gains.The impairments are described through and extra additivenoise term at both the Tx and Rx modeling the EVM. Theimpact on throughput and capacity is studied through linkand system simulations when applying MIMO and 64QAMto HSDPA.

2. ERROR VECTOR MAGNITUDE (EVM)

The impairments induced through imperfections in thetransmitter and receiver chains can be modeled throughextra additive noise terms at both the Tx and Rx. Aschematic description of the transmitter and the impairmentmodel used herein is shown in Figure I. Here a white noisemodel is used whereby complex Gaussian noise is addedafter the transmit filter(s). First the pilots (PI and P2) areadded to the modulated, spread and root-raised cosinefiltered signals (SI and SI). Here we assume that the powerof the signals on each antenna is the same. The signals E Iand E2 are independent complex Gaussian white noiseprocesses that model the transmitter distortion.The variance of the white noise process on the mth Tx

antenna is given by O'~ =cTxPm where Pm is the average

transmitted power on the mth antenna (signal and pilots)

978-1-4244-2046-9/08/$25.00 ©2008 IEEE 496 SPAWC 2008

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-2 dB (63%)

3 equipower voice users-10dB (10%)

15

GCNS modelPilot powerHS-PDSCH power

#spreading codes

and finally plotted vs. SNR. Only the Pedestrian A powerdelay profile is considered since the combination of MIMOand 64-QAM is likely to be deployed only in lightlydispersive scenarios.The receiver used for these studies is similar to theGeneralized-RAKE receiver for MIMO systems describedin [5], except it is extended to incorporate the pre-codingused in the Rel-7/8 MIMO scheme.The following additional parameters have been usedthroughout the link simulations.

Table 1: Link simulation parameters

E 2 TransmitterDistorsion

P2Pilots

DataStream 2

and crx =(% Tx EVM/I00)2 is the fraction of this

appearing as distortion power (same fraction on both Txantennas).

DataStream 1 ,-, ,-,

The throughput of MIMO in HSDPA as a function of SNRhas been simulated for different values of Tx and Rx EVM.As already pointed out, the focus of this contribution is toshow the impact of EVM for 2x2 MIMO (Rel-7/Rel-8).However, as a comparison we also include results for 2-wayreceiver diversity (RxDiv). Results for 16QAM and 64QAMcan be found in Figure 2 and Figure 3, respectively.We see that the impact of radio impairments on the peakthroughput is quite moderate for MIMO+ I6QAM (Rel-7),whereas it may become more severe for MIMO+64QAM(Rel-8). It is further noticed that MIMO is more sensitive toEVM than RxDiv. This is expected since MIMO can beconsidered as a form of higher-order modulation (2 streams64-QAM is like I-stream 4096-QAM, but with aconstellation that has an irregular shape depending on thechannel realization). With the increased effectivemodulation order of MIMO, the constellation points arecloser together than with RxDiv, thus more sensitive toEVM.It can also be observed that MIMO is more sensitive to RxEVM than Tx EVM. The main reason for this is that thenoise added at the transmitter (modeling Tx EVM)experiences the same frequency- and time-selective fadingas the desired signa1. In contrast, the noise added at thereceiver (modeling Rx EVM) does not experiencefrequency-selective fading (only time-selective fading).Hence if the channel frequency response contains notches,the desired component of the received signal can becomeburied in the Rx EVM noise - but not the Tx EVM noise- thus explaining the increased sensitivity to Rx EVMcompared to Tx EVM.

Figure 1: Transmitter model used in this contribution.

The main source of impairment in the receiver is the radiofront end. The impairments stem from I1Q-imbalance,receiver filters, and phase noise. The receiver impairmentmodel is defined in a similar way as the Tx impairmentmodel above. We add additional noise modelling the RxEVM on top of the AWGN modelling thermal noise. Thevariance of the noise process on the nth receive antenna is

given by (j~ =cRx Pn where ~ is the instantaneous power

(conditioned on the channel realization) received from bothTx antennas, and cRx is the fraction of this appearing as

distortion power (defined in an analogous way as cTx ).

3. LINK SIMULATIONS

To investigate the impact of EVM on the throughput ofMIMO in HSDPA we have conducted a set of linksimulations. The impairments are modeled through EVM asdescribed above.

3.1. Link simulation setup

In the link simulations, average throughput is obtained bygenerating many realizations of the MIMO fading channel,calculating the corresponding receiver combining weightsfor each realization over a range of SNR values, and thencomputing the corresponding post-combining SINRs foreach channel realization/SNR combination taking intoaccount the Tx and Rx distortion. The SINRs are thenmapped to instantaneous data rates using a modulation andcoding scheme (MCS) table spanning the range of ratesavailable in Rel-7/8 MIMO. A given MCS is assumed to besupported as long as the post-combining SINR is greaterthan or equal to the corresponding 10% block-error rate(BLER) switching point obtained from off-line AWGNsimulations. The instantaneous data rates (summed acrossdata streams) are then averaged over all channel realizations

Channel estimationReceiver algorithms

3.2. Link simulation results

IdealIdeal

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Average Throughput

46dBm (40 W)

File download

Round Robin

0%,7% modelled as AWGN

Dual antenna GRAKE2 [5]

1.4W

3kmph

0%, 7%, 9% modelled as AWGN

2W

2GHzSCM, Suburban macro [3], [4]

0.5 between cells, 1.0 between sectors

50m

Hexagonal grid, 19 cell sites, 3 sectorsper site

500m

20dBL=128.1 + 37.610g10(.R), (R in km)

8dB

Traffic model

BSTxEVM

DE RxEVM

DE Receiver

Scheduler

CCHpower

Total BS TX power

Pilot power

Channel modelCarrier frequency

Inter-site distance

Correlation distanceof Shadowin

DE speed

Shadowing std

Path loss

Shadow correlation

Penetration Loss

Cellular layout

No code multiplexing is used in these simulations; henceonly one user is scheduled in each TTL It is further assumedthat the CQI is perfectly estimated, but a delay of one TTl isapplied to the CQI report. It is supposed that all usersdownload a file of same size, and that a user leaves thesystem when it has downloaded a complete file. So, the userthroughput is defined as a weighted mean throughput of allusers. The weighted throughput is calculated as

TP= lUiIIIbu '

ueU

where U is the set of all users, bu is the bit rate of user U, andlUI is the number of users.

Table 2: System simulation parameters

The simulation assumptions used here are similar to thoseused for the evaluation of EUTRA (LTE) in 3GPP [2]. Themain parameters are given in Table 2 below.

40

40

35

35

30

30

10

10

....... (0,0)40 -+- (7,0)

- (0,7)35 - (7,7)

-...- (9,7)en 30 -.- (7,9)Q..a~25"'··············'·····'5Q.

"6, 20 I- , , .. ".II'.~""''-':::J

2~ 15

Average Throughput

....... (0,0)40 -e- (7,0)

-(0,7)

35 - (7,7)......- (9,7)

en 30 -.- (7,9)Q..a~25'5Q.

~20:::J2~ 15

15 20 25SNR, lo/Ioc (dB)

Figure 3: Throughput as a function of SNR when using64QAM; The marks in the legends represent (Tx, Rx)EVM. Solid lines show 2x2 MIMO, whereas dashed lines(lower group of curves) show RxDiv.

15 20 25SNR, lo/Ioc (dB)

Figure 2: Throughput as a function of SNR when using16QAM; The marks in the legends represent (Tx, Rx)EVM. Solid lines (upper group of curves) show 2x2MIMO, whereas dashed lines (lower group of curves)show RxDiv.

4. SYSTEM SIMULATIONS4.2. System simulation results

To further investigate the impact of EVM, the model shownin Figure 1 has been implemented in a dynamic systemsimulator. The system simulator contains detailed models ofvarious aspects of a WCDMA network. For example,functions such as power control, link adaptation and variousradio network algorithms are included. In addition to thetraffic channel (HS-DSCH) the simulator also explicitlymodels control channels such as HS-SCCH and A-DCH.

4.1. System simulation setup

We study two different scenarios here: a heavily loadedsystem and a lightly loaded system. The antenna diagramused in these simulations is taken from a commerciallyavailable antenna [6] where the antenna pattern in thevertical domain has been incorporated. The 3D antennashave 10° tilt (5° mechanical and 5° electrical tilt).

In the first scenario, 1 user/cell (on average) is served inevery transmission time interval (TTl), which is 2ms inHSDPA. The Rx EVM is fixed at 7% and 0% while the TxEVM is varied.

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Table 3: MIMO+64QAM usage

Table 4: User throughput loss (Rx EVM 7%)

I I

90 - - - +- - - - - - ~ - - - ~ - - -1 I 1 1

80 - - - 1 - - - -1- L --.l1 1 1 1

1 1 1

70 - - - + - - -I - - - - +- - -I 1 I

o 5 10 15 20 25 30 35 40

User throughput (Mbit/s)

Figure 5: User throughput cumulative distributionfunction (CDF) for MIMO+64QAM in a low load

scenario: legends show (Tx, Rx) EVM.

When comparing the throughput including effects fromEVM to a system without any Tx or Rx impairments there isa significant loss in throughput. However, if the throughputfor different Tx impairments are compared relative to asystem with, e.g., 7% Rx impairments (as shown in Table 4)only a rather marginal throughput loss is expected even inthe scenario where 64QAM is used more often.

5. CONCLUSIONSIn this contribution we present some results on the impact ofEVM when MIMO and/or 64QAM is applied to a HSDPAnetwork. Link and system simulation results indicate thatthe performance will be affected by EVM. This is mostapparent when 64QAM is used together with MIMO. To

60

~u.. 50oo 40

30 --t--. (0,0) 64QAM MIMO-._--. (7,0) 64QAM MIMO

20 -- (0,7) 64QAM MIMO---- (7,7) 64QAM MIMO

10 ----, (9,7) 64QAM MIMO- ..- (7,9) 64QAM MIMO

User Throughput CDF100 r---~------.-------.----------r~-~dIIIl"~"._

To increase the fraction of 64QAM usage, the load of thesystem is decreased to 0.2 served users/cell. This meansthat, on average, one user is served every 5th TTl in one cell.Figure 5 shows the corresponding throughput results for thislow-load scenario. Comparing with the previous case, thereis now a substantial use of 64QAM (see Table 3). We alsonote that the impact from Tx EVM is more severe as shownin Table 4. Obviously, the best user throughput (90th

percentile) is influenced more than mean user throughput(50th percentile) because the best users always have a higherSINR, and hence are transmitting with 64QAM more oftenthan the mean users.- ....-. (0,0) 64QAM MIMO

-.-._. (7,0) 64QAM MIMO-- (0,7) 64QAM MIMO----, (7,7) 64QAM MIMO---_. (9,7) 64QAM MIMO--0--. (7,9) 64QAM MIMO

User Throughput CDF100 r---:I--:-----:----T~:::d ....~,••••~

1 1 I

90 - - - I - - - ~~"''''''JII_ - - - r - - - I - - -

I 1 I I

80 - - - + - - - - - - - ~ - - - -:- - - - ~ - - -~I - - -

I 1 1 I 1

70 - - - ~ - - - - - - - L - - - -.J - - -I 1 1

I 1 1

60 - - - -t - - - - - t - - - -j

I 1 I

I 1 I---T ---1---1

1 1 I

I L J L __ ~ _I 1 I 1

I I 1

30 - - - - - - -I - - - I- - - - --t - -1 1 I

1 1 I I20 - - I - - - -1- - - - I - - - I - -

1 1 1 1

10 - -+----:--- ~---~--1 I 1 1

u.. 50oo 40

Table 4 gives throughput losses from Tx EVM and Rx EVM(7%). "Mean" and "Best" indicate mean user throughput(50th percentile) and best user throughput (90th percentile) inFigure 4, respectively.

o 5 10 15 20 25 30 35 40

User throughput (Mbit/s)

Figure 4: User throughput cumulative distributionfunction (CDF) for MIMO+64QAM in a high load

scenario: legends show (Tx, Rx) EVM.

Table 3 shows the percentage of time when 64QAM is usedfor the different scenarios studied here. From this it is clearthat for the scenario with high load (1 user/cell), 64QAM isonly used in a rather small fraction of the total transmissiontime. Since the EVM hits harder on 64QAM than on16QAM or QPSK, very little throughput loss is seen. On theother hand, the gain by applying 64QAM in this scenario ismodest.

Figure 4 depicts the user throughput distribution for thishigh-load scenario. It can be noted that the impact ofTx andRx distortion is fairly small in this case as shown in Table 4.The main reason for this almost negligible impact is that64QAM is used quite infrequently. Comparing the (7,0)curve with the (0,7) curve in Figure 4, the conclusion is inline with the link simulation results that Rx EVM has alarger impact than Tx EVM.

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achieve the very highest bitrates, a very clean transmittedsignal is needed. When the modulation is limited to 16QAM(as in Rel-7) much less impact of the EVM is seen. Withrealistic values on EVM (Tx = 9%, Rx= 7%) only amarginal impact is noted in this case.It can further be noted that the Tx distortion model usedhere assumes a fixed EVM, while in practice the EVMproblem is more severe for higher output powers. This ismainly due to the clipping distortion which is morepronounced for higher output powers. Hence, usersrequiring less than maximum output power will experiencesignificantly less Tx EVM.

6. REFERENCES

[1] 3GPP TS 25.104 V7.9.0, Base Station (BS) radio transmissionand reception (FDD) (Release 7).

[2] 3GPP TR 25.814 V7.1.0, Physical layer aspects forevolved Universal Terrestrial Radio Access (UTRA).

[3] 3GPP TR 25.996 V6.1.0, Spatial channel model forMultiple Input Multiple Output (MIMO) simulations.

[4] G. Calcev, D. Chizhik, B. Goransson, S. Howard, H. Huang,A. Kogiantis, A.F. Molisch, A.L. Moustakas, D. Reed, H. Xu, "AWideband Spatial Channel Model for System-Wide Simulations",IEEE Trans. On Veh. Tech., pp 389-403, March 2007.

[5] SJ. Grant, KJ. Molnar, and G.E. Bottomley, "GeneralizedRAKE Receivers for MIMO Systems", in Proc. IEEE VTC'03­Fall, Orlando, FL, October 2003.

[6] Kathrein 742215, multi-band F-pane1171Q-2170 MHz, dualcross polarization 45° slanted, half-power beam width 65°,Adjustable electrical downtilt 0-10°.

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