Opportunistic Beam-forming with Limited Feedback دانشگاه صنعتي اصفهان دانشكده برق و كامپيوتر Opportunistic Beam-forming with Limited Feedback نوروز معتمدي ارائه مقاله تحقيقي در درس “ راديو نرم افزاری ” مدرس: دکتر محمد جواد اميدی نيمسال بهار 1387-1386

Table of contents Introduction Multi-user Diversity System Model Multi-user Diversity Proportional Fair Scheduling Opportunistic Beam-forming Simulation Results Limited Feedback Comparison with space-time code (Alamouti Code)

System Model MIMO Communication Downlink Flat fading N: number of receive antenna M: number of transmit antenna n: number of user Downlink Flat fading

Multi-user Diversity Diversity in wireless comm. arises form independent path Multi-user diversity arises from independent fading channels across different users Fundamental difference : “Multi-user diversity takes advantage of rather than Compensate fading” But it has problem : Fairness and Delay Amount of diversity gain depends mainly on Range of SIR fluctuation Rate of SIR fluctuation in window Number of active users

What is the sum capacity with channel state feedback? At any time transmit full power only to the best user. Channel tracked by receiver and SNIR fed back to BS.

Proportional Fair Scheduling At time slot t, given user’s average throughputs T1(t),T2(t),… in past window current request rate R1(t), R2(t), R3(t)… transmit to the user k* with the best Tk(t) updated by an exponential filter symmetric channel statistics greedy policy of transmitting to the mobile with the highest requested rate

Opportunistic Scheduling Question : Is it possible to dictate nature to have more fluctuation ? Idea : Artificially induce channel SIR fluctuations that have “Larger range and Faster time schedule fluctuations”

Random vs. True Beam-forming If the gains h1k and h2k are known at the transmitter true (Coherent) beam-forming Same signal transmitted over all antennas Different power & phase in time

Slow-Fading : before & after of beam-forming

Random Beam-forming: Consideration The rate of variation power & phase in time It should be fast to provide full channel fluctuations within the latency time scale of interest The variation should be slow enough to allows the channel to be reliably estimated by the users and the SNR fed back. to ensure that the channel seen by the users does not change abruptly and thus maintains stability of the channel tracking loop.

Opportunistic Beam-forming: slow fading Dumb antennas can approach the performance of true Beam-forming When there are many users in the systems with less feedback.

Opportunistic Beam-forming : fast fading Improve performance in fast fading Ricean channel

Simulation Results

Limited Feedback The BS sets a threshold β for all users. Each user will send a “1” to the BS if their channel gain exceeds the threshold, otherwise a “0” is sent. The BS selects randomly from among eligible users. If all the feedback bits received by the BS are zero, then no signal is transmitted in that interval or randomly pick a user for transmission to avoid waste. Throughput of this scheme scales as M log log nN. increasing the number of receive antenna has no significant impact on the throughput

Simulation Results

Simulation Results-Cont.

Simulation Results-Cont.

Comparison with space-time code : Performance Slow Fading: Alamouti: diversity gain dumb antennas: diversity gain plus 3 dB power gain Fast Fading: Alamouti: reduces channel fluctuations and thereby reduces the multi-user diversity gain. dumb antennas: keeps the fluctuations the same in Rayleigh fading and increases the fluctuations in Rician fading.

Comparison with space-time code : Complexity Alamouti: requires two separate pilots to estimate the multi-antenna channel. special encoder/decoder. Dumb Antennas: only requires a single pilot to estimate the overall channel SNR. no special encoder/decoder. In fact the mobiles are completely oblivious to the existence of multiple transmit antennas.

Acknowledgment Some slides have been taken from the following presentations Opportunistic Communication: A New System Design Tse, Opportunistic Communication: Smart Scheduling and Dumb Antennas, 2002 Viswanath, Opportunistic bema-forming: Dumb Antennas and smart scheduling, 2001

References P. Viswanath, D. N. C. Tse, and R. L. Laroia, “Opportunistic beamforming using dumb antennas,” IEEE Trans. Inf. Theory, Vol.48, No.6, pp.1277–1294, June 2002. D.Tse, P.Viswanath, Fundamentals of Wireless Communication, Cambridge University Press, 2005. S.Sanayei, A.Nosratinia, “Opportunistic Beamforming with Limited Feedback”, IEEE Transaction on Wireless Communication”, Vol.6, No.8, August 2007. S. Sanayei and A. Nosratinia, “Exploiting multiuser diversity with only 1-bit feedback,” in Proc. IEEE Wireless Communication and Networking Conference (WCNC), New Orleans, LA, March 2005. M. Sharif and B. Hassibi, “On the capacity of MIMO broadcast channel with partial side information,” IEEE Trans. Inf. Theory, vol. 51, no. 2, pp. 506–522, Feb. 2005. S. M. Alamouti, “A simple transmitter diversity scheme for wireless communications,” IEEE J. Select. Areas Commun., vol. 16, pp.1451–1458, Oct. 1998.