2005 IEEE 16th International Symposium on Personal, Indoor and Mobile Radio Communications

Investigation of Diversity and BeamformingTechniques for UMTS High Speed Downlink

Packet AccessYejian Chen, Joachim Speidel, Cornelis Hoek, and Stefan Russ

Abstract-This paper deals with the High Speed DownlinkPacket Access (HSDPA) in UMTS/FDD system. The transmitdiversity and beamforming techniques are focused on, namelySpace Time Transmit Diversity (STTD), Closed Loop mode 1transmit diversity, conventional beamforming and closed loopbeamforming with 2 transmit antennas. The link level systemperformance of the various methods is investigated andcompared for the fading environment. At low user speeds, closedloop transmit diversity turns out to be superior. STTD has thelowest system complexity and achieves an acceptableperformance at high SNR. Both beamforming schemes are thegood solutions for high speed mobile terminal at low SNR.

I. INTRODUCTIONIN the recent UMTS specification Release 5, the HSDPA

concept is systematically designed, which is one of the mostimportant features for the 3G WCDMA mobilecommunications system. The target of an enhanced 3G system,is a variable data rate and an increased packet data throughput,whose data rate shared goes up to 10 Mbits/s [1], [2], [3], [4].

It was recognized from the very beginning that thedownlink technology will be a critical point for the 3Gsystems, if many users have to be accommodated at highcommunication quality and high data rate simultaneously [6].Although all the requirements above cannot be satisfiedideally in one communications system, the increase of thesystem efficiency is still quite an important research field.

In the HSDPA concept, the multiple antenna array offersone possibility to increase the downlink capacity. Thistechnology exploits multiple antennas in transmit and receiveside with associated coding, modulation and signal processingto enhance capacity, coverage and throughput of the wirelesssystem.

In this paper, the performance gain of transmit diversity andbeamforming techniques is investigated for a HSDPA system.The organization of this paper is as follows. In section II themodel of the investigated system is described. Subsequently,the performance of the system is demonstrated in section III.

Y. Chen and J. Speidel are with the Institute of Telecommunication.Department of Electrical Engineering and Computer Science, StuttgartUniversity, Germany. (Email: chenat.inuc.uni-StLttetart.de, speidcavijnue.Lni-stuttgart.de)

C. Hoek and S. Russ are with Alcatel Research & Innovation, Stuttgart,Germany. (Email: cnrnc1s.lcutci.de, stQfan.russ0.-va1curtel.ide)

Finally, the conclusion is done in section IV.

II. LINK LEVEL SYSTEM MODEL

A. HSDPA ConceptIn the HSDPA concept, the crucial techniques include

adaptive modulation and channel coding, Hybrid AutomaticRepeat reQuest (HARQ) and fast scheduling with the goal toincrease throughput, reduce delay and achieve high peak datarates. The physical layer is structured in three types ofchannels to achieve the HSDPA features, namely, High SpeedDownlink Shared Channel (HS-DSCH), High Speed SharedControl Channel (HS-SCCH) and Uplink High SpeedDedicated Physical Control Channel (HS-DPCCH) [4].The strategy to achieve high data rate contains

* Fixed, relatively small Spreading Factor (SF), equal to16, for user data

* High order modulation method, 16 QAM in addition toQPSK

* Short interleaving interval, namely, Transmission TimeInterval (TTI) 2 ms

* Up to 15 multi-codes utilization in parallel for a singleuser or multiple users

The strategy to guarantee the communication qualityencompasses* Fast physical layer (L 1) retransmission combining* Strong channel coding mechanism, i.e. turbo coding

B. Physical Channel ArrangementIn the HSDPA link level system model, all the physical

channels, which are defined in Release'99, remain. In the Fig.1, the HSDPA link level simulator is depicted. Fig. 2 showsthe generic downlink transmitter in more detail. Besides theHS-DSCH and the HS-SCCH the physical channels can besummarized as follows,* DPCH: it acts as the relative low speed dedicated data,

e.g. speech. Its spreading factor is variable. Thecorresponding bit rate is variable up to 2 Mbits/s.

* OCNS: it denotes the Orthogonal Channel NoiseSimulator, which acts as the low speed dedicated data ofother users in the same cell. It simulates the intracellularinterference in the link level simulator. In the HSDPAsystem model, 6 OCNS channels are maximally allowedto be active simultaneously.

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* CPICH: it denotes the Common Ppurpose of channel estimation,complete transmission power is allc

After over-sampling, these physicalspread by corresponding spreading fact(physical channels generates the data strlthe antenna. For example, in a multiplediversity technique, the original d4transmitted normally by the first anteniprocessed according to corresponding cthen fed into the second antenna.

Delica,t Du High Spd Shad1DPCH) DataS-DSCH)

.2on /T1 2nB/7 11

Ii-.

'ilot Channel. For theabout 10% of the

)cated to CPICH.1~--ZC~ - DPCH (t) CDPCH (t) +IHS-SCCH XHS-SCCH (t).CHS-SCCH (t) + FIH XCPICH (t). CCPICH (t) +

cnannel signals are 6 1 1ors. The sum of these E OCNSXOCNS, COCNS( i= 1,2 (1)cam, which is fed into j=O J2antennas system with where xHSDSCH (t) , XHSSCCH (t) , XDPCH (t) , XOcNS (t) andata stream will be XCPICH(t) denote the signals after over sampling and signalna, and in parallel be

prcsigathNoeBseFg.2liversity scheme. It is processing at the Node B, see Fig. 2, CHSDSCH (t), CHS-SCCH (t)CDPCH(t) COcNs(t) and CCPICH(t) denote the corresponding

Sp"SuhledHDhta Dd sDa spreading and scrambling codes. The energies of the different(HS-DSCH) (DPCH)

2.ns/TTI 2T0" l physical channels are 6HS DSCH'9HS SCCH I 8DPCH 6£CNS and_4 1i -CPICH -In (1), 15 HS-DSCH codes and 6 OCNS channels areiia tPiggUE: Sio ePegUaE:I PhysiChll Si.lSep-o activated. The weight coefficients w and wON areIDei-k-mvig * d. F-n_ V . . --s*^sOrn

_ Pby-jci Cbannc SenSW.".-o , I' Dcird-g

/11 * HARQ Fn-s . R-c Ma..>t)< * ~T.wb.D.odi.g :Ch*DCid_T C o eBcTai BitsD,ion

Desectio * CRC D.-.toI . CRC Dac6-i

Node Base station (Node B) User Equipment (UE)Number of Antenna L = I or 2 Number of Antenna M=

Figure 1: Structure ofUMTS/FDD Link Level System

determined by diversity schemes or beamforming schemesrespectively.

In STTD scheme, the Alamouti space time encoding isapplied for the HS-DSCH codes, HS-SCCH and DPCH [5]. Inclosed loop transmit diversity mode 1, the weights w, areadjusted according to the uplink feedback information. Thephase adjustment is achieved by the second weight w2, and

jw21= I holds. The weight of the first antenna branch isconstant, namely, w,=1. The same weights are used byDPCH. On HS-SCCH only STTD is used, no complexweighting is applied. A more detailed description can befound in [1] and [2].

In Fig. 3 and Fig. 4, the beam pattern for conventionalbeamforming and the principle of closed loop beamformingare illustrated. In Fig. 3, Node B places several beam patternssimultaneously for 7 users. With the closed loop beamformingscheme in Fig. 4, one out of the four pattems is activated. ForSTTD and closed loop diversity an antenna configuration withuncorrelated antennas is used, e.g. a cross-polarized antenna.For the beamforming schemes closely spaced antennas,typically with spacing of half a wavelength are used. Forconventional beamformning, the antennas must be calibrated.For closed loop beamforming no calibration is needed.

7

2t0o

LCPICH panem I PICRH pattem 2COCNS,

Figure 2: The generic downlink transmitter structure to support HSDPA with2 transmit antennas

The transmitted data streams on both antennas are given by,15

Xi (t) = W;~- EV£HS-D~SCH, XHS-DSCH, (t) CHS-DSCH (t)+ W,*=l J~~~~~~~~~~~~~

0o 0 Interferers

/~~~~~~~....... - s-.

- 20sG

044 ., ;-

/0 *,,,

, < ' i:, .2l

-

/ 03

~UE

Figure 3: Beam pattern of conventional beamforming

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120.- ...[ m$f\

0.D.4 o -. wf \2wj2,01 -: - 00 /iW W= [I r2e ]

{ ~~~~ ~~~~~~~~~ I ' F2ezj

w= I ,Fe- 4|2?0

Figure 4: Principle of closed loop beamforming

C. Channel ModelIn the channel model for the link level simulation, the UE

and the other 6 OCNS interferers locate in one 1200 sector ofa cell, namely from -600 to 600. In the vicinity of the UE,the objects reflect the original signal from the Node B. Thereceived signal at the UE is the superposition signal fromdifferent paths. The original signal experiences a differentdistortion and attenuation by these multipaths. In the link levelsimulator of this paper, the Rayleigh fading channel ismodeled, namely, 4-tap Rayleigh channel and 6-tap Rayleighchannel. In principle, the extra delay components are causedby increased speed of the UE in an environment richscattering and reflections. For the reason of the UE's mobility,6-tap Rayleigh channel is defined as Vehicular A fadingchannel and 4-tap Rayleigh channel is defined as Pedestrian Afading channel in WCDMA standard. Additionally in thespatial domain, an angular spread is assumed for individualtaps at the Node B, with 0fA5= 100. The baseband equivalentchannel impulse response between transmit antennas m andthe receiver can then be modeled as,

h. (t, t) = Zam (rn, t)ejor J)S( -,, )n=l

N

=I im(inn,t)(i(r - an)n=l

(2a)

m=1,2

with 2im(rn t) = a,n, (a, t)ej9-( 4) . The function am(rn t) and

Om (n,, t) denote the attenuation factor and phase of mthantenna in the nth path respectively, rnn denotes the nth tapdelay.In Fig. 5 the envelope of the baseband equivalent channel

impulse response is depicted for the UE with 3km/h underPedestrian A fading channel. In principle Ihm(r,t)l goes to

infinity because of the Dirac impulse in (2a). However, to givea graphical representation, the magnitudes of the impulses inFig. 5 are drawn proportional to ain (inn ,t)j .

D. UE ReceiverAt the UE receiver, the channel coefficients im(i,z,t)

n=l.. , N, are assumed to be constant during one UMTS

slot interval PT1;0t . t< (P +I)TIt, with ThOrt =0.667ms,VP E Z, namely an(Tn",t) =in,,p. The further simplificationcan be applied for (2a) as,

N

h.t (Z, n)= I iinn.A ( -rn ) -n=l

(2b)

Fnc0o: Seonence*.Mewn AnvOnIcapon

c

14,(r,t)l 1 5 .7.

I

1.t.5

2,01

lo.' 2,0204

r(sec)

t .2

zsM ; _

US.Z s

2.5

3.o2t (sec)

Figure 5: Channel model for Pedestrian A fading channel (3km/h)

The received signal at the UE is,L

y(t) = x.(t) *h.(V,t) +n(t)m=i

(3)

with L transmit antennas. The signal processing at the UEside takes place each UMTS slot Ts,.. During time interval

PT,I>t < t < (P + I)T,,ot, the equation (3) can be formulated as,L N

y(t) = a0,n,- X. (t - r., ) + n(t) .

mA n=(4)

Under the assumption of pairwise perfect orthogonality ofthe despreading, the corresponding physical channels can bereconstructed. The estimated kth HS-DSCH signal is obtained,after despreading and down sampling, with 1 < k < 15, as

XdJHS-DSCH, (t) = SHS-DSCH 8HS DSCH0 1 nPWLm=1 n=l

XHS-DSCH, (t - -,,) + nHS-DSCH, (t) (5)The spreading factor of HS-DSCH is denoted by SHS-DSCH 'In(5) a noise term HS-DKOH, (t) still remains, which consists ofthe thermal noise and the intracellular interference. After thedespreading ofCPICH we obtain,

XdPC (t) S=1~s IL N -Xd,CPICH(t)I=CHVFECPICH EL1: E PXCPICH,, (t t

Lm=l n=l+fCPICH (t). (6)

Channel estimation is done with the CPICH. To show theprinciple, we consider the received signal of the 1-tap fadingchannel at the UE, if 2 transmit antennas are used at the NodeB, i.e. L = 2 and N = 1 in (6). Furthermore perfectsynchronization at the UE is assumed to estimate the tap delayz- . For the convenience of the derivation, the noise termkCPtCH(t) will be neglected here. With these simplifications,

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the despread CPICH signal becomes,2

Xd,CPICH (t) SCPICH VCPCH2 amIPXCPICH. (t)2 m=l

(7)

SCPICH Ia1pp(t)+ SCPtCH a-'p2PP )2 ~~~~~2where p1 (t) and p2 (t) denote two CPICH signals, namely,

pI (t) = XCPICH, (t) and P2 (t) = XCPICH, (t), which are orthogonal.The fading coefficients of the channel are estimated as,

,P S XdCPCH (t)p (t)dt (8a)CPICH ECPICH S lot

2P 1 1P XdCPICH(t)P2 (t)dt (8b)CPICH CPICH Slot O

Maximum Ratio Combining (MRC) helps

signal for the given example, namely L = 2despread kth HS-DSCH signal then is,

Xd,HS-DSCH, (t) SHS-DSCH CHS DSCH. am P Mm=l

For the closed loop transmit diversity

scheme, the same HS-DSCH signals willtransmitted by two antenna branches. Therefsignal is,

XHS-DSCH, (t)I

SHS-DSCH 6-HS-DSCHIia"1ipW;Mn=l

SHS-CPICH CPICH

Table 1: Link Level Simulation Parameters

to reconstruct the In Figs. 6 to 7 the throughput of the HSDPA system is2 and N =1. The shown as a function of the signal power to interference noise

ratio Ior/Ioc . The complete transmission power is denoted byIor. Throughout the simulation, its value is 10% CPICH

Vm XHS DSCH, (t) (9) power, 2.5% DPCH power, 1% OCNS power. The residualpower is reserved for HS-DSCH signals. The power of the

and beighted nd interference is denoted by Im . With QPSK or 16 QAM for 10

Fore, the estimated HS-DSCH codes, the theoretical maximum throughput is 3.6Mbps or 7.2 Mbps respectively, by implementing turbo codewith code rate 3/4. The HARQ process is applied, namely the

Xd,HS-DSCH,(t) HS-DSCH can be retransmitted maximally for 4 times. Forclosed loop mode 1, 4% Feed-Back Error Ratio (FBER) isassumed in uplink transmission. The ideal antenna verification

TS10t is implemented at the UE [2].SHS-DSCH X HS-DSCH, E W fXdCP[CH (l)Pm (t)dt

m=l O

Xd,HS-DSCH, (t) * (1 0)

For the STTD, the space time decoding will be done forXd,HS DSCH, (t) after the despreading according to [5]. Then theMRC can be implemented similarly to (10).

III. SIMULATION RESULTSIn this section simulation results are presented.

simulation parameters are listed in Table 1.

Camrer Frequency 2 GHzChip Rate 3.84 Mchip/s

Channel Symbol Rate DPCH: 30 ksymbol/sInformation Bit Rate DPCH: 12.2 kbit/s

TTI DPCH: 20 msHS-DSCH: 2 ms

Power Control OFFRake Fingers for Channel Model 4

DPCH: Convolutional Code,Channel Coding Code Rate 1/3

HS-DSCH: Turbo Code, CodeRate 3/4

Oversampling Factor 8Spreading Factor for DPCH 128

Slot Format of DPCH 11. [1]Spreading Factor for CPICH 256

Spreading Factor for HS-DSCH 16

The

rV. CONCLUSIONIn this paper, multiple antennas are introduced to the

enhanced 3G system. Two transmit antenna arrangements are

implemented to achieve transmit diversity or beamforming,namely, STTD, closed loop transmit diversity mode 1,conventional beamforming or closed loop beamforming. BERof the various schemes are investigated and compared by linklevel computer simulation. For closed loop transmit diversityand closed loop beamforming, the weight generation isdetermined by means of the uplink feedback. Therefore, inPedestrian scenario both schemes outperform STTD andconventional beamforming. With increasing speed of the UE,STTD and conventional beamforming tum out to be the bettersolutions. At high velocity, e.g. 120 km/h, the improvementsby beamforming or diversity are both limited, since precisereal time channel estimation can hardly be achieved.

Besides the general characteristics of diversity andbeamforming, the performance, which relates to the HSDPAconcept, will be also briefly concluded. In the HSDPA system,the short TTI interleaving interval and the small spreadingfactor limit the time diversity gain and the CDMA processinggain. If the higher order modulation scheme is applied, a

relatively large signal-to-noise ratio is needed to achieve highthroughput.

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Uplink feedback rate 1500 HzUplink feedback delay 1 slot

Modulation DPCH: QPSKHS-DSCH: QPSK or 16 QAM

Number ofOCNSs 6Angular Spread (TAS = 100

Total CPICH power 10% of /orNumber of parallelHARQ processes on 6

HS-DSCHMaximum number of

transmissions of a Transport 4Block

Number of HS-DSCH codes 10

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REFERENCES

[1] 3GPP TS 25.211 v 5.5.0, 3d Generation Partnership Project; TechnicalSpecification Grouip Radio Access Network; Physical Channels andMapping of Transport Channels onto Phvsical Channels (FDD),September 2003.

[2] 3GPP TS 25.214 v 5.7.0, 3d Generation Partnership Project. TechnicalSpecification Group Radio Access Network; Physical Laver Procedures(FDD), December 2003.

[3] 3GPP TS 25.858 v 5.0.0, 3d Generation Partnership Project; TechnicalSpecijication Grotup Radio Access Network; High Speed DownlinkPacket Access, Physical Layer Aspects (FDD), March 2002.

[4] H. Holma, A. Toskala, WCDMA for UMTS, Radio Access For 7ThirdGeneration Mobile Communications, John Wiley & Sons, Ltd, 2002

[5] S. M. Alamouti, A Simple Transmit Diversity Techniqte for WirelessCommunications, IEEE Journal on Select Areas in Communications, vol.16, No. 8, October 1998, pp. 1451-1458.

[6] L. C. Godara, Application ofAntenna Array to Mobile Communications,Part I; Performance Improvement, Feasibility, and SvstemConsiderations, Proceedings of the IEEE, vol. 85 No. 7, July 1997, pp.1029-1060.

[7] L. C. Godara, Application ofAntenna Array to Mobile Communications,Part HI. Beam-Forming and Direction-of-Arrival Considerations,Proceedings of the IEEE, vol. 85 No. 8, August 1997, pp. 1I 95-1245.

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Figure 6: Throughput for HSDPA system, under Pedestnran A Rayleighfading channel, QPSK, -16dB DPCH and -20dB OCNS, HARQ on, 40%

FBER with ideal antenna verification

WM-10.

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-- --- +----4

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2TX CL SesoanonogV*-0 2 Tx C4tZvenonS Beanftomw-Thesard*nSe MaOnoxThrouw

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Figure 7: Throughput for HSDPA system, under Vehicular A Rayleighfading channel, QPSK, -16dB DPCH and -20dB OCNS, HARQ on, 4%

FBER with ideal antenna verification

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