"Spatial Multiuser Access OFDM With Antenna Diversity and Power Control”

"Spatial Multiuser Access OFDM With Antenna Diversity and Power Control” Base Station, MR antennas Mobiles, MTi Antennas for ith user

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

System Diagram (single user) Channel Transmitter Receiver x(f0) V(f0) N-point IFFT N-point FFT UH(f0) y(f0) H(f) (= UVH) x(fN-1) V(fN-1) y(fN-1) UH(fN-1)

Why OFDM? Using OFDM:

The Matrix Algebra (f) = H(f) (f) H(f) = U(f)(f)VH(f) y(f) = UH(f)H(f) (f) = UH(f)U(f) (f)VH(f)V(f)x(f) = (f)x(f) y(f) = UH(f) (f) (f) = V(f)x(f)

Multi-User Detection I (rec’d signal) • In “real” channel, λ1 >> λ2 (subscript refers to user number) With spatially matched prefiltering, spatially matched output filtering: • K users • x, y now scalars, u now vector

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

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

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

Power Control • Waterfill optimal if single-user • Multi-user case: Subject to: With: j = power of jth user, Pe = Probability of error,

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.

Power Control Results • Pe < 0.01 on all active subchannels

Comparison to Other Methods: • Has path diversity vs Beamforming • Space Time Equalizer: W(f) = [H*(f)H(f)]-1H*(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.

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

"Spatial Multiuser Access OFDM With Antenna Diversity and Power Control”