1. 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

2. 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)

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

4. 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

5. 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.

6. 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

7. 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”

8. 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

9. Slow-Fading : before & after of beam-forming

10. 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.

11. 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.

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

13. Simulation Results

14. 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

15. Simulation Results

16. Simulation Results-Cont.

17. Simulation Results-Cont.

18. 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.

19. 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.

20. 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

21. 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
S. M. Alamouti, “A simple transmitter diversity scheme for wireless communications,” IEEE J. Select. Areas Commun., vol. 16, pp.1451–1458, Oct