Multi-Carrier and Spread Spectrum Systems: From OFDM and MC ...

304 Additional Techniques for Capacity and Flexibility EnhancementStream 0Modulation−DetectionStream 0Stream 1Modulation...−Detection...Interference estimationStream 1Stream M − 1Modulation−DetectionStream M − 1Transmitter Receiver (L = M)Figure 6-2V-BLAST transceiverTwo basic variants of BLAST are proposed [12, 28]: D-BLAST (diagonal BLAST) andV-BLAST (vertical BLAST). The only difference is that in V-BLAST transmit antennam corresponds all the time to the transmitted data stream m, while in D-BLAST theassignment of the antenna to the transmitted data stream is hopped periodically. If thechannel does not vary during transmission, in V-BLAST the different data streams maysuffer from asymmetrical performance. Furthermore, in general the BLAST performanceis limited due to the error propagation issued by the multi-stage decoding process.As illustrated in Figure 6-2, for detection of data stream 0, the signals transmitted fromall other antennas are estimated and suppressed from the received signal of the data stream0. In References [3] and [4] an iterative decoding process for the BLAST architecture isproposed, which outperforms the classical approach.However, the main disadvantage of the BLAST architecture for mobile communicationsis the need for high numbers of receive antennas, which is not practical in asmall mobile terminal. Furthermore, high system complexity may prohibit the large-scaleimplementation of such a scheme.6.2.2 Space–Time CodingAn alternative approach is to obtain transmit diversity with M transmit antennas, where thenumber of received antennas is not necessarily equal to the number of transmit antennas.Even with one receive antenna the system operates. This approach is more suitable formobile communications.The basic philosophy with STC is different from the BLAST architecture. Insteadof transmitting independent data streams, the same data stream is transmitted in anappropriate manner over all antennas. This could be, for instance, a downlink mobilecommunication, where in the base station M transmit antennas are used while in theterminal station only one or few antennas might be applied.