Hybrid-Selection Optimum Combining

Wireless Systems Research Department

AT&T Labs - Research

Hybrid-Selection Optimum Combining

Jack H. Winters

Moe Win


Middletown, NJ

USA

May 7, 2001


OUTLINE

• Introduction

• Single Link Results

• Cellular Performance

• Conclusions


Diversity TechniquesSelection:

• Select antenna with the highest received signal power

Output

W1

WM

Output

Maximal ratio combining:

• Weight and combine signals to maximize signal-to-noise ratio (M-fold diversity)


Diversity Techniques (cont.)

W1

WM

Output

Optimum combining:

• Weight and combine signals to maximize signal-to- interference-plus-noise ratio (SINR)

• Suppresses K<M-1 interferers with M-K diversity gain


Hybrid Diversity Techniques

• RF chains are the most expensive component• Therefore, select L out of M antennas

• H-S/MRC: Select L antennas with the highest S/N and combine with maximal ratio combining

• Proposed Technique: Hybrid-Selection/Optimum Combining (H-S/OC)

• Select L out of M antennas that maximize SINR with optimum combining


H-S/OC Issues

• Unlike H-S/MRC, no known technique to find best set of L => exhaustive search of all M!/(L!(M-L)!) combinations required

• Need to know interference level => training sequences


H-S/OC for L=1

• Selection based on SINR versus S

• Rayleigh fading of desired and K equal-power interferers

• 5 dB gain for M=10


H-S/OC for L>1

• K equal-power interferers

• K=L-1: interference suppression with increasing diversity gain as M increases

• K=L: for L=M, only a few dB better than MRC, but increasing improvement of H-S/OC over H-S/MRC as M increases

• Improvement even for K=L+1


Cellular System Results

• ANSI-136 model

• 98 cells for N=7, 36 cells for N=1,3,4 (3 sectors/cell)

• Computer simulation for outage probability (BER > 2 X 10e-2) with 60% loading


Uplink Results

• 10% outage requires:

• L=1:

• M=2 for N=3,4

• M=15 for N=1

• L=2:

• M=4 for N=1


Downlink Results

• Results similar to, but slightly better than, uplink results

• L=3 performance at:

• L=2, M=4

• L=1, M=16

• As L increases, outage decreases faster with M


Conclusions

• Proposed Hybrid-Selection/Optimum Combining

• Suppresses K>L-1 interferers if L<M

• Improvement increases with M even for M>>L

• For ANSI-136, dramatically reduces outage

– 7-fold capacity increase with L=1, M=12 or L=2, M=4

• Suppresses multiple strong interferers and obtains diversity gain with a small number of receiver chains

Hybrid-Selection Optimum Combining

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