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
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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
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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)
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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
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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
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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
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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
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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
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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
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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
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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
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(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
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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
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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
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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
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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/
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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.
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Questions?Questions?
Thank You!