adjacent sub-carriers on each OFDM CHAPTER 10 The IEEE 802. 16m Physical Layer (Part II) symbol are paired, encoded, and transmitted from two BS antenna ports where the encoding is defined as follows: (10-37) where (xk, -xx+1) and (xk+1,xx) are transmitted from antenna port 1 and 2, respectively. Note that the spatial rate of the SFBC codes in this case is one. 10.4.3 MIMO Receivers The orthogonal space-time block coding transforms a vector detection problem into a number of scalar detection problems. Similar extensions can be made to frequency-selective fading MIMO channels. Therefore, receiver design techniques such as Zero-Forcing (ZF), Minimum-Mean Square Error (MMSE) estimation, and (optimal) Maximum Likelihood (ML) sequence detection can be utilized [15,41]. The use of transmit diversity techniques, such as delay diversity or frequency-offset diversity, transforms a MISO channel into a number of SISO channels and allows the application of SISO receiver architectures. For a general space-time trellis code, a vector Viterbi decoder can be employed. The space-time trellis coding in general provides improved performance over orthogonal space-time block coding at the expense of receiver complexity. The problem encountered by a receiver for spatial multiplexing is the presence of multi-stream interference since the signals sent from different transmit antennas interferes with each other. Note that in spatial multiplexing the different data streams are transmitted in the same channel and occupy the same resources in time and frequency. For the sake of simplicity we restrict our attention to the case where N > Nt. The optimal detection for spatially-multiplexed signals is to use maximum likelihood criterion. The ML receiver performs vector decoding and is optimal in the sense of minimizing the error probability. Assuming equally likely, temporally uncoded vector symbols, the ML receiver generates an estimate of the transmitted signal vector as follows: = argmin S y - (10-38) where the minimization is performed over