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Transceiver architectures for fast fading (V-BLAST family). Transceiver architecture for slow fading (D-BLAST). Multiple antennas in networks: SDMA | 8. MIMO II: Capacity and Multiplexing Architectures Outline Transceiver architectures for fast fading (V-BLAST family) Transceiver architecture for slow fading (D-BLAST) Multiple antennas in networks: SDMA Transmitter and Receiver CSI Can decompose the MIMO channel into a bunch of orthogonal sub-channels. Can allocate power and rate to each sub-channel according to waterfilling Analogy with OFDM Major difference: In MIMO, the U and V matrices depend on the channel H. In OFDM, the IDFT and DFT matrices do not. Receiver CSI Only The channel matrix H and its singular values i2's are random and unknown to the transmitter. Has to fix a Q and a power allocation independent of H. Q=I and uniform power allocation is optimal in many cases. It is not trivial to come up with capacity-achieving architectures. Capacity Can write: Slow fading: Fast fading: Fast Fading Capacity for I.I.D. Rayleigh Fading d.o.f. determines the high SNR slope. Fast Fading Capacity: . | 8. MIMO II: Capacity and Multiplexing Architectures Outline Transceiver architectures for fast fading (V-BLAST family) Transceiver architecture for slow fading (D-BLAST) Multiple antennas in networks: SDMA Transmitter and Receiver CSI Can decompose the MIMO channel into a bunch of orthogonal sub-channels. Can allocate power and rate to each sub-channel according to waterfilling Analogy with OFDM Major difference: In MIMO, the U and V matrices depend on the channel H. In OFDM, the IDFT and DFT matrices do not. Receiver CSI Only The channel matrix H and its singular values i2's are random and unknown to the transmitter. Has to fix a Q and a power allocation independent of H. Q=I and uniform power allocation is optimal in many cases. It is not trivial to come up with capacity-achieving architectures. Capacity Can write: Slow fading: Fast fading: Fast Fading Capacity for I.I.D. Rayleigh Fading d.o.f. determines the high SNR slope. Fast Fading Capacity: Low SNR nr – fold power gain at low SNR Nature of Performance Gain At high SNR (d.o.f. limited): min(nt,nr)-fold d.o.f. gain. MIMO is crucial. At low SNR (power limited): nr-fold power gain. Only need multiple receive antennas. At all SNR, min(nt,nr)-fold gain due to a combination of both effects. System Question Should one blindly overlay MIMO technology on CDMA universal reuse systems? These systems operate at low SINR. MIMO gain is mainly receive antenna power gain. Having multiple transmit antennas may not be necessary. Interesting implication on the uplink: expensive to have many antennas at the mobile. However mobile antennas are useful for the downlink. They can also be used to suppress out-of-cell interference and provide diversity. Transceiver Architecture: V-BLAST Can get the performance gain by sending independent coded streams at each of the Tx antennas and joint ML decoding. Is this surprising? Question: How to get the d.o.f. gain even when streams interfere .
