 all possible transmit vector symbols S. Let Z denote the alphabet size of the scalar constellation transmitted from each antenna, a complete implementation requires an exhaustive search over a total of ZN1 vector symbols or hypotheses, making the decoding complexity of this receiver grow exponentially with the number of transmit antennas. The recent development of fast ML detection algorithms reduces the computational complexity of the ML decoder. The ML receiver realizes N1 diversity order for the HE scheme and N X N diversity order for the VE scheme [15,41]. An alternative non-linear approach to detection of spatially multiplexed signals is Successive Interference Cancellation (SIC) by assuming that the spatially-multiplexed signals are separately coded (i.e., horizontal encoding or multi-codeword transmission). As shown in Figure 10-28, in an SIC receiver, one of the spatially multiplexed signals is initially demodulated and decoded. On successful decoding, the data is re-encoded and subtracted from the received signals. The second spatially multiplexed signal can then be demodulated and decoded in the absence of interference from the first detected signal. The subtraction would ideally improve the signal-to-interference ratio. The procedure 10.4 Multi-antenna transmission schemes First Decoded Demodulation Decoding Signal Re-encoding Second Decoded Demodulation Decoding Signal Re-encoding Demodulation Decoding Nrth Decoded Signal FIGURE 10-28 The structure of a successive interference cancellation MIMO receiver is iterated until all spatially multiplexed signals are demodulated and decoded. It is noted that the first signals to be decoded using this approach are subject to higher interference levels compared to those that are decoded later in the process. Therefore, the ith signal to be decoded must be more robustly coded than the (i + 1)th signal. This can be achieved in multi-codeword transmission by applying different modulation and coding schemes to different spatially multiplexed signals. The latter sch