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

MC-CDMA 85Table 2-2MC-CDMA system parametersParameterValue/characteristicsSpreading codesWalsh–Hadamard codesSpreading code length L 8System loadFully loadedSymbol mappingQPSK, 8-PSK, 16-QAMFEC codes Convolutional codes with memory 6FEC code rate R and FEC decoder 4/5, 2/3, 1/2, 1/3 with Viterbi decoderChannel estimation and synchronization PerfectMobile radio channelUncorrelated Rayleigh fading channelThe performance of the MC-CDMA reference system presented in this section is applicableto any MC-CDMA system with an arbitrary transmission bandwidth B, an arbitrarynumber of sub-systems Q, and an arbitrary number of data symbols M transmitted peruser in an OFDM symbol, resulting in an arbitrary number of sub-carriers. The numberof sub-carriers within a sub-system has to be 8, while the amplitudes of the channel fadinghave to be Rayleigh-distributed and have to be uncorrelated on the sub-carriers of asub-system due to appropriate frequency interleaving. The loss in SNR due to the guardinterval is not taken into account in the results. The intention is that the loss in SNRdue to the guard interval can be calculated individually for each specified guard interval.Therefore, the results presented can be adapted to any guard interval.2.1.10.2 Synchronous DownlinkThe BER versus the SNR per bit for the single-user detection techniques MRC, EGC,ZF, and MMSE equalization in an MC-CDMA system without FEC coding is depictedin Figure 2-14. The results show that with a fully loaded system the MMSE equalizationoutperforms the other single-user detection techniques. ZF equalization restores theorthogonality between the user signals and avoids multiple access interference. However,it introduces noise amplification. EGC avoids noise amplification but does not counteractthe multiple access interference caused by the loss of the orthogonality between the usersignals, resulting in a high error floor. The worst performance is obtained with MRC,which additionally enhances the multiple access interference. As reference, the matchedfilter bound (lower bound) for the MC-CDMA system is given. Analytical approaches toevaluate the performance of MC-CDMA systems with MRC and EGC are given in Reference[55], with ZF equalization in Reference [51] and with MMSE equalization in [25].Figure 2-15 shows the BER versus the SNR per bit for the multi-user detection techniqueswith parallel interference cancellation, MLSE, and MLSSE applied in an MC-CDMA system without FEC coding. The performance of parallel interference cancellationwith adapted MMSE equalization is presented for two detection stages. The significantperformance improvements with parallel interference cancellation are obtained after the