1. "Spatial Multiuser Access OFDM With Antenna Diversity and Power Control” Mobiles, M Ti Antennas for ith user Base Station, M R antennas

2. Contents System Diagram Why OFDM The Matrix Algebra Multi-User Detection Results Power Control Comparison to other methods

3. System Diagram (single user) H(f) (= U  V H ) V(f 0 ) N-point IFFT N-point FFT V(f N-1 ) U H (f N-1 ) U H (f 0 ) x(f N-1 )y(f N-1 ) x(f0) y(f0) Channel ReceiverTransmitter

4. Why OFDM? Using OFDM:

5. (f) = H(f) (f) H(f) = U(f)  (f)V H (f) y(f) = U H (f)H(f) (f) = U H (f)U(f)  (f)V H (f)V(f)x(f) =  (f)x(f) The Matrix Algebra y(f) = U H (f) (f) (f) = V(f)x(f)

6. Multi-User Detection I (rec’d signal) In “real” channel, λ 1 >> λ 2 (subscript refers to user number) With spatially matched prefiltering, spatially matched output filtering: K users x, y now scalars, u now vector

7. Multi-User Detection II (CCI, noise) Let Then: y(f) = R(f)  (f)x(f) + z(f) Where

8. Multi-User Detection III (methods) Linear Methods: Detection statistic (f) = W(f)y(f) W(f) = I (conventional) = R -1 (f) (decorrelator) = (R(f) +  2  -2 (f)) -1 (MMSE) Non-Linear Methods ZF-DFE, MMSE-DFE, Multi-stage MUD In simulations, multistage detector with interference cancellation used. Claim is that Pe of <.01 before coding will be fine after coding

9. Results Rayleigh Fading 5 Transmit, 5 Receive Antennas 2 Multipath model 2 Users MMSE detection, Multistage with interference cancellation

10. Power Control Waterfill optimal if single-user Multi-user case: Subject to: With:  j = power of jth user, P e = Probability of error,

11. Power Control Algorithm Waterfill for all K users if: –Perfect interference cancellation, or –BER constraint is satisfied When interference kicks in: –Do not assign further energy, instead, use it on other channels.

12. Power Control Results Pe < 0.01 on all active subchannels

13. Comparison to Other Methods: Has path diversity vs Beamforming Space Time Equalizer: W(f) = [H*(f)H(f)] -1 H*(f) –Noise enhancement when signal fades –Since channel gain (  ) not present in SVD, channel model updates less frequently, and is less prone to channel estimation errors –SVD less prone to near/far because of spatial isolation.

14. Summary: OFDM for Flat Fading Typically one spatial mode per user per frequency Receiver spatially separates multiple users on a frequency Apply traditional detection methods Power Control has similar look and feel to other systems