Praise for Fundamentals of WiMAX

5.9 Advanced Techniques for MIMO 193Table 5.2 Summary of MIMO TechniquesTechnique (N r , N t ) Feedback? Rate r a CommentsReliability-Enchancement Techniques (r ≤1)SelectioncombiningMaximal ratiocombiningSpace/time blockcodesTransmit selectiondiversityDOAbeamformingLinear diversityprecodingEigenbeamformingGeneral linearprecodingOpen-loop spatialmultiplexingBLASTGeneral linearprecodingN r ≥ 1N t = 1N r ≥ 1N t = 1N r ≥ 1N t > 1N r ≥ 1N t > 1N r ≥ 1N t ≥ 1N r + N t> 2N r ≥ 1N t > 1N r ≥ 1N t > 1N r > 1N t > 1N r > 1N t > 1N r > 1N t > 1N r > 1N t > 1Open loop r = 1Open loop r = 1Increases average SNR by (1 + 1/2 +1/2 + ... 1/N r)Increases SNR to γ Σ = γ 1 + γ 2 + ... +γ NrOpen loop r ≤1 Increases SNR to γ Σ = γ ||H|| F /N tClosed loop: Feedbackdesired antennaindexOpen loop if N t = 1Closed loop if N t > 1or used for interferencesuppressionr = 1usually (r < N t )r = 1Precoding TechniquesSame SNR as selection combiningCan suppress up to (N r – 1) + (N t – 1)interference signals and increase gainin desired direction. Ineffective inmultipath channelsSpecial case of linear beamforming;Closed loop: Feedbackchannel matrix r = 1 only one data stream is sent. IncreasesSNR to γ Σ = γ ||H|| FClosed loop: Feedbackchannel matrix1 ≤r ≤min(N r ,–LN t –L)Can be used to both increase desiredsignal gain and suppress L interferingusersSimilar to eigenbeamforming, but interferingsignals generally not suppressed;Closed loop: Feedbackchannel matrix 1 ≤r ≤min(N r , N t ) goal is to send multiple data streamsSpatial MultiplexingOpen loop r = min(N r , N t )Open loop r = min(N r , N t )Closed loop: Feedbackchannel matrix 1 ≤r ≤min(N r, N t )Can receive in a variety of ways: linearreceiver (MMSE), ML receiver,sphere decoder. If N r > N t , select bestN r antennas to send streamsSuccessively decode transmittedstreamsSame as preceding; both a precodingtechnique and a spatial-multiplexingtechniquea. r is similar to the number of streams M but slightly more general, since r < 1 is possible for some of thetransmit-diversity techniques.