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

314 Additional Techniques for Capacity and Flexibility EnhancementsOFDMH 0H 1IOFDMIOFDMH *0r 0H *1r 1ΣrFigure 6-14 OFDM with MRC receiver; L = 2With the received signalsr 0 = H 0 s + n 0 ,(6.13)r 1 = H 1 s + n 1the diversity gain achievable with MRC can be observed as follows:r = H ∗ 0 r 0 + H ∗ 1 r 1= (|H 0 | 2 +|H 1 | 2 )s + H ∗ 0 n 0 + H ∗ 1 n 1. (6.14)6.3.2.2 Delay and Phase DiversityThe transmit diversity techniques delay, phase, and time-variant phase diversity presentedin Section 6.3.1 can also be applied in the receiver, achieving the same diversity gainsplus an additional gain due to the collection of the signal power from multiple receiveantennas. A receiver with phase diversity is shown in Figure 6-15.6.3.3 Transmit/Receive Diversity Performance AnalysisThe gain in SNR due to different transmit diversity techniques to reach the BER of3 × 10 −4 with M transmit antennas compared to one transmit antenna over the numberof antennas M is shown in Figure 6-16. The results are presented for a rate 1/2coded OFDM system in an indoor environment. Except for sub-carrier diversity withoutinterleaving, promising performance improvements have already been obtained with twotransmit antennas. The optimum choice of the number of antennas M is a tradeoff betweencost and performance.e jΦ 0, nsOFDMIOFDM. . .e jΦ M − 1, nΣrIOFDMFigure 6-15Phase diversity at the receiver