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

"Spatial Multiuser Access OFDM With Antenna Diversity and Power Control”. Base Station, M R antennas. Mobiles, M Ti Antennas for ith user. Contents. System Diagram Why OFDM The Matrix Algebra Multi-User Detection Results Power Control Comparison to other methods.

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

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

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

  3. 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)

  4. Why OFDM? Using OFDM:

  5. 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)

  6. 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

  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, Pe = 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)]-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.

  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

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