Downlink-Assisted Uplink Zero-Forcing for Uplink Zero-Forcing for TDD Multiuser MIMO Systems

Petri Komulainen

GIGA Seminar 4.12.2008

Contents

Introduction to MU-MIMOIntroduction to MU MIMOTDD systemGreedy beam selectionTx-Rx UL-DL zero-forcingIterative ZF solutionPil d iPilot designNumerical resultsConclusionConclusion

4 Dec 2008 CWC | Centre For Wireless Communications 2

Introduction to Multiuser MIMO

Multiple antennas in base antennas in base station and mobilesM lti l ti l Multiple spatial beams (modes) per user allowedRich scattering environment, non-line-of-sightnon line of sightGeographic direction not b iobvious

4 Dec 2008 CWC | Centre For Wireless Communications 3

TDD system: Assumptions

Uplink and downlink channel reciprocityUL d DL f f b dUL and DL frames on same frequency bandIn OFDMA, same subcarriers used for UL and DLTX uses the knowledge of CSI obtained during RX

Space division multiple access (SDMA)A b f i t i th ti /f fA number of users co-exist in the same time/frequency frameBeam/user selection (scheduling)

Opportunistic rate allocationConstant (sum) TX power with per frame variable rates

Relatively small cells and low mobilitySimultaneous users are symbol synchronousSimultaneous users are symbol-synchronousNo inter-symbol-interference (cyclic prefix)

4 Dec 2008 CWC | Centre For Wireless Communications 4

TDD system: Signal modely gDL and UL RX signals:

TX-precoder

Ter-l 1

Channel

TX-amplitude (diagonal)

Dataminal 1

Base node

Ter-minal 2Nb = 3b

Nu = 2

4 Dec 2008 CWC | Centre For Wireless Communications 5

Greedy beam selectiony

Greedy [4,5] user/beam selection by BSselection by BSThe same set of beams are active in both directions

Beams spatially compatible in DL Beams spatially compatible in DL are compatible also in UL

Selection is based on user-Selection is based on user-specific eigenmodes

Select consecutively, one at a timetimeAt each step, select eigenbeam with largest component orthogonal to previously selected

Beamforming gain when SNR lowMultiplexing gain

gbeamsStop selecting if calculated capacity reduces or when all D F’ d

Multiplexing gain when SNR high

DoF’s are used

4 Dec 2008 CWC | Centre For Wireless Communications 6

Tx-Rx UL-DL zero-forcing ZF criterion:g

No restrictions for the

ZF criterion:

number of antennas in active terminals [1]

UL is based on reversal of DL signal processing chain

D li kDL receiver orthonormalized into UL precoder

Downlink

p

Tx power + rate ll ti d allocation, and

scheduling/beam selection decoupled from Tx

Uplink

precoder design

4 Dec 2008 CWC | Centre For Wireless Communications 7

SU-MIMO channel

Iterative ZF solution

Closed form Closed form solution here [2]

Iteration starts from user-specific eigenmodes [3]Ideally DL receiver is matched filter; active interference suppression not mandatory => single user processing by suppression not mandatory => single-user processing by terminalsIn practice multi-user interference exists in DL due to imperfect CSI > interference suppression is beneficialimperfect CSI => interference suppression is beneficial

4 Dec 2008 CWC | Centre For Wireless Communications 8

Pilot signalsg

The same precoded pilots used for the purposes of Rx a p od d p o u d o pu po oand TxIn DL, precoded pilots associated with the active data beams are sufficientbeams are sufficientIn UL, append the pilot precoder matrix to be unitary => UL pilot overhead grows with the aggregate number of user antennas (as with all MU-MIMO schemes)

pilot precoder matrixdata precoders

additional precoders

4 Dec 2008 CWC | Centre For Wireless Communications 9

Pilot signals: Rx responsesg p

Pilot responses are the only reference for spatial Pilot responses are the only reference for spatial multiuser processing

Modified equivalent ZF criterion becomes

unitary

4 Dec 2008 CWC | Centre For Wireless Communications 10

Summary of UL-DL procedurey p

User Terminal (UL TX):U1

User Terminal (UL TX):Estimate DL pilot RX responses (Lk pilot streams per user)SVD for the personal N xL channel or

U2BASE NODE

SVD for the personal NuxLk channel or receiverWaterfill (WF) over each user’s Lk eigenmodes for data

U3

for dataUL pilots TX via Lk data precoders and additional Nu-Lk pilot precoders with equal powers => unitary pilot precoder matrix

K=3Nb=4Nu=2

L1 = 1L2 = 0L3 = 2

Base Node (DL TX):Estimate all UL pilot RX responses (Nu pilot streams per user)Estimate all UL pilot RX responses (Nu pilot streams per user)Perform beam selection and iterative multiuser ZF processingWaterfill (WF) over the chosen beams => Transmit Lk data streams per user

4 Dec 2008 CWC | Centre For Wireless Communications 11

DL pilots TX via data precoders but with equal powers

Numerical results: Simulation setupNumerical results: Simulation setup

Frequency flat fading, independent between antennasf fWaterfilling power allocation, assuming knowledge of N0

Common UL-DL power allocation rule:

Acronym Note Explanation

ZF Iterative proposed schemes

UL and DL co-exist, greedy beam selection

ZF Closed form

Best user SVDbench-marks

Single-user MIMO by SVD precoding for the strongest user, UL and DLmarks ,

Non-precoded UL only, no precoding or power allocation, stream per TX antenna for the selected

4 Dec 2008 CWC | Centre For Wireless Communications 12

(strongest) users => all DoFs used

Numerical results: Achievable rate

The achievable data rate per stream s for user k can be evaluated from the SINR seen after transmitter-channel-receiver chain:

Receiver TX precoder

TX power

Inter-stream interference exists due to non-ideal CSI in both TX and RX

4 Dec 2008 CWC | Centre For Wireless Communications 13

Performance: Ideal CSI

SNRP

kk∑

=0

SNRN

=

Nb = 4, Nu = 2, greedy beam selectionIdeally UL and DL are equal in performance given same power Ideally UL and DL are equal in performance, given same power constraints

4 Dec 2008 CWC | Centre For Wireless Communications 14

Performance: Estimated CSI and ZF RX

{K=4, Nb=4, Nu=2}, noisy CSI estimated from pilot onlyEstimated CSI in RX vs Estimated CSI in RX and TX PN kpilot∑Estimated CSI in RX vs. Estimated CSI in RX and TXOnly ZF processing used here

Zero-forcing RX in BS Least-norm RX in UE (is ideally ZF when precoding was perfect)

0pilotSNR

Nk

p ∑=

Least norm RX in UE (is ideally ZF when precoding was perfect)

UL RX suffers from UL pilot power distribution

4 Dec 2008 CWC | Centre For Wireless Communications 15

Performance: Estimated CSI and LMMSE RX

{K=4, Nb=4, Nu=2}, noisy CSI estimated from pilot onlyEstimated CSI in RX vs. Estimated CSI in RX and TXLMMSE RX used here 0

pilotSNRN

PNk

kpilot∑=

Only non-precoded UL seems to really benefit

4 Dec 2008 CWC | Centre For Wireless Communications 16

Conclusion

Low overheadCo existence of UL and DL taken into accountCo-existence of UL and DL taken into accountSingle set of pilots sufficient for both coherent RX and TX precoding

SimplicityDecoupled MIMO channels accommodate simple power/rate allocation, as well as coding and modulation, gIdeally, terminals perform single-user MIMOLinear TX and RX processing

PerformancePerformanceBeamforming gain from TX CSI also in uplinkOpportunistically uses all available degrees of freedomZero-forcing means suboptimal use of MU-MIMO channel capacity

Loss is small due to user/beam selection/

4 Dec 2008 CWC | Centre For Wireless Communications 17

References

[1] P. Komulainen, A. Tölli, M. Latva-aho, and M. Juntti, “Downlink Assisted Uplink Zero-Forcing for TDD Multiuser MIMO Systems,” accepted for publication in

l k fIEEE Wireless Communications & Networking Conferece, WCNC 2009, Budapest, Hungary, April 2009.

[2] Q. H. Spencer, A. L. Swindlehurst and M. Haardt, “Zero-Forcing methods for [2] Q. H. Spencer, A. L. Swindlehurst and M. Haardt, Zero Forcing methods for Downlink Spatial Multiplexing in Multiuser MIMO Channels,” IEEE Trans. on Signal Processing, vol. 52, no. 2, pp. 461–471, Feb. 2004.

[3] Q H Spencer and A L Swindlehurst “A hybrid approach to spatial [3] Q. H. Spencer and A. L. Swindlehurst, A hybrid approach to spatial multiplexing in multiuser MIMO downlinks,” EURASIP J. Wireless Comm. and Netw., vol. 2004, no. 2, pp. 236–247, Dec. 2004.

[4] A. Tölli and M. Juntti, “Scheduling for multiuser MIMO downlink with linear processing,” in Proc. IEEE Int. Symp. Pers., Indoor, Mobile Radio Commun., vol. 1, Berlin, Germany, Sep. 2005, pp. 156–160.

[5] G. Dimic and N. Sidiropoulos, “On downlink beamforming with greedy userselection: Performance analysis and a simple new algorithm”, IEEE Transactions on Signal Processing, vol. 53, no. 10, pp. 3857–3868, 2005.

4 Dec 2008 CWC | Centre For Wireless Communications 18

Questions?Questions?

Thank You!