Summary and Conclusions Multiuser MIMO Systems

5.9 Advanced Techniques for MIMO 193

Table 5.2 Summary of MIMO Techniques Technique
N
r
, N
t
Feedback? Rate r
a
Comments Reliability-Enchancement Techniques r
≤1
Selection combining
N
r
≥ 1 N
t
= 1 Open loop
r = 1 Increases average SNR by 1 + 12 +
12 + ... 1N
r
Maximal ratio combining
N
r
≥ 1 N
t
= 1 Open loop
r = 1 Increases SNR to
γ
Σ
= γ
1
+ γ
2 + ... +
γ
Nr
Spacetime block codes
N
r
≥ 1 N
t
1 Open loop
r ≤ 1
Increases SNR to γ
Σ =
γ ||H||
F
N
t
Transmit selection diversity
N
r
≥ 1 N
t
1 Closed loop: Feed-
back desired antenna index
r = 1 usually r N
t
Same SNR as selection combining
DOA beamforming
N
r
≥ 1 N
t
≥ 1 N
r
+ N
t
2 Open loop if N
t
= 1 Closed loop if N
t
1 or used for interfer-
ence suppression r = 1
Can suppress up to N
r
– 1 + N
t
– 1 interference signals and increase gain
in desired direction. Ineffective in multipath channels
Precoding Techniques
Linear diversity precoding
N
r
≥ 1 N
t
1 Closed loop: Feed-
back channel matrix r = 1
Special case of linear beamforming; only one data stream is sent. Increases
SNR to γ
Σ =
γ ||H||
F
Eigenbeam- forming
N
r
≥ 1 N
t
1 Closed loop: Feed-
back channel matrix 1
≤ r ≤ minN
r
,–L N
t
–L Can be used to both increase desired
signal gain and suppress L interfering users
General linear precoding
N
r
1 N
t
1 Closed loop: Feed-
back channel matrix 1
≤ r ≤ minN
r
, N
t
Similar to eigenbeamforming, but inter- fering signals generally not suppressed;
goal is to send multiple data streams
Spatial Multiplexing
Open-loop spatial multiplexing
N
r
1 N
t
1 Open loop
r = minN
r
, N
t
Can receive in a variety of ways: lin- ear receiver MMSE, ML receiver,
sphere decoder. If N
r
N
t
, select best N
r
antennas to send streams BLAST
N
r
1 N
t
1 Open loop
r = minN
r
, N
t
Successively decode transmitted streams
General linear precoding
N
r
1 N
t
1 Closed loop: Feed-
back channel matrix 1
≤ r ≤ minN
r
, N
t
Same as preceding; both a precoding technique and a spatial-multiplexing
technique a. r is similar to the number of streams M but slightly more general, since r 1 is possible for some of the
transmit-diversity techniques.
194 Chapter 5 • Multiple-Antenna Techniques

5.11 Bibliography

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