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

90 MC-CDMA and MC-DS-CDMA3.0Spectral efficiency in bit/s/Hz2.52.01.5QPSK8-PSK16-QAMQPSK, OFDM8-PSK, OFDM16-QAM, OFDM1.00.54 5 6 7 8 9 10 11 12 13 14E b /N 0 in dBFigure 2-19 Spectral efficiency of MC-CDMA and OFDM (OFDMA, MC-TDMA): fully loadedsystem; Rayleigh fading; BER = 10 −4Section 2.1.5.1). The performance of an MC-CDMA system with different loads and MRCin the synchronous uplink is shown in Figure 2-20. It can be observed that due to the lossof orthogonality between the user signals in the uplink only moderate numbers of activeusers can be handled with single-user detection.The performance of MC-CDMA in the synchronous uplink can be significantly improvedby applying multi-user detection techniques. Various concepts have been investigated inthe literature. In the uplink, the performance of MLSE and MLSSE closely approximatesthe single-user bound (one user curve in Figure 2-20) since here the Walsh–Hadamardcodes do not superpose orthogonally and the maximum diversity can be exploited [47].The performance degradation of a fully loaded MC-CDMA system with MLSE/MLSSEcompared to the single-user bound is about 1 dB in SNR.Moreover, suboptimum multi-user detection techniques have also been investigated forMC-CDMA in the uplink, which benefit from reduced complexity in the receiver. Interferencecancellation schemes are analyzed in References [1] and [32] and joint detectionschemes in References [5] and [49].To take advantage of MC-CDMA with nearly orthogonal user separation at the receiverantenna, the pre-equalization techniques presented in Section 2.1.6 can be applied. Theparameters of the MC-CDMA system under investigation are presented in Table 2-3.