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

MC-CDMA 87first iteration. The optimum joint detection techniques MLSE and MLSSE perform almostidentically and outperform the other detection techniques. The SNR-degradation with theoptimum detection techniques compared to the matched filter bound (lower bound) iscaused by the superposition of orthogonal Walsh–Hadamard codes, resulting in sequencesof length L that can contain up to L − 1 zeros. Sequences with many zeros performworse in a fading channel due to the reduced diversity gain. These diversity losses canbe reduced by applying rotated constellations, as described in Section 2.1.4.4. An upperbound of the BER for MC-CDMA systems applying joint detection with MLSE andMLSSE for the uncorrelated Rayleigh channel is derived in Reference [19] and for theuncorrelated Rice fading channel in Reference [25]. Analytical approaches to determinethe performance of MC-CDMA systems with interference cancellation are shown in References[25] and [30].The FEC coded BER versus the SNR per bit for single-user detection with MRC, EGC,ZF, and MMSE equalization in MC-CDMA systems is presented in Figure 2-16. It can beobserved that rate 1/2 coded OFDM (OFDMA, MC-TDMA) systems slightly outperformrate 1/2 coded MC-CDMA systems with MMSE equalization when considering caseswith full system load in a single cell. Furthermore, the performance of coded MC-CDMAsystems with simple EGC requires only an SNR of about 1 dB higher to reach the BERof 10 −3 compared to more complex MC-CDMA systems with MMSE equalization. Witha fully loaded system, the single-user detection technique MRC is not of interest inpractice.10 010 −110 −2BER10 −310 −4MRCZFEGCMMSEMC-CDMA single-user boundOFDM (OFDMA, MC-TDMA)10 −50 1 2 3 4 5 6 7 8 9 10E b /N 0 in dBFigure 2-16 FEC coded BER versus SNR for MC-CDMA with different single-user detectiontechniques: fully loaded system; channel code rate R = 1/2; QPSK; Rayleigh fading