TAILIEUCHUNG - Wireless Communications over MIMO Channels phần 4

biện pháp về hiệu suất, SINR người nhận điện thoại di động có thể được sử dụng. Nó đã được thể hiện trong (Tạ Đình Phong và Viswanath năm 2005) là tính hai mặt tồn tại giữa uplink và downlink. Vì vậy, các bộ lọc MMSE, được biết đến để tối đa hóa SINR nhận, | INFORMATION THEORY 89 measure of performance the SINR at the mobile receivers can be used. It has been shown in Tse and Viswanath 2005 that a duality exists between uplink and downlink. Therefore the MMSE filter which is known to maximize the SINR at the receiver would be the optimum linear prefilter at the transmitter too. There also exists an equivalent transmitter structure for successive interference cancellation at the receiver. Here nonlinear precoding techniques such as the Tomlinson-Harashima precoding have to be applied Fischer 2002 . Downlink with Single Transmit and Multiple Receive Antennas In environments with a single transmit antenna at the base station and multiple receive antennas at each mobile superposition coding and receive beamforming with interference cancellation is the optimal strategy and maximizes the SINRs at the receive filter outputs. Considering the two-user case both transmit signals x k are assumed to have identical powers Es Ts. The receive filters are matched to the spatial channel vectors hu and deliver the outputs ru k h k -ZLn k h x k n k . l h H l h n At each mobile superposition decoding has to be applied. Without loss of generality we can assume that h1 H2 h2n2 holds so that the rates each user can support are R1 C1 log 1 h1 2 N0 J R2 CMUI log 1 l h 2Es-2 . 2 2 l h2H2Es 1 No J Hence user two decodes only its own signal disturbed by thermal noise and the signal of user one. On the contrary user one first detects the signal of user two subtracts it from the received signal and decodes its own signal afterwards. Downlink with Multiple Transmit and Receive Antennas Finally we briefly consider the multiuser MIMO downlink where transmitters and receivers are both equipped with multiple antennas. Here the same strategies as in the multiuser MIMO uplink have to be applied. For each user the base station transmits parallel data streams over its antennas. With full CSI at the transmitter linear prefiltering in the zeroforcing or

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