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

64 MC-CDMA and MC-DS-CDMA2.1.5 Detection TechniquesData detection techniques can be classified as either single-user detection or multi-userdetection. The approach using single-user detection detects the user signal of interestby not taking into account any information about multiple access interference. In MC-CDMA mobile radio systems, single-user detection is realized by one tap equalization tocompensate for the distortion due to flat fading on each sub-channel, followed by userspecificde-spreading. As in OFDM, the one tap equalizer is simply one complex-valuedmultiplication per sub-carrier. If the spreading code structure of the interfering signals isknown, single-user detection is sub-optimum since the multiple access interference shouldnot be considered in advance as noise-like. The sub-optimality of single-user detection canbe overcome with multi-user detection where the apriori knowledge about the spreadingcodes of the interfering users is exploited in the detection process.The performance improvements with multi-user detection compared to single-userdetection are achieved at the expense of higher receiver complexity. The methods ofmulti-user detection can be divided into interference cancellation (IC) and joint detection.The principle of IC is to detect the information of the interfering users with single-userdetection and to reconstruct the interfering contribution in the received signal beforesubtracting the interfering contribution from the received signal and detecting the informationof the user of interest. The optimal detector applies joint detection with maximumlikelihood detection. Since the complexity of maximum likelihood detection grows exponentiallywith the number of users, its use is limited in practice to applications with asmall number of users. Simpler joint detection techniques can be realized by using blocklinear equalizers.An MC-CDMA receiver in the terminal station of user k is depicted in Figure 2-5.2.1.5.1 Single-User DetectionThe principle of single-user detection is to detect the user signal of interest by not takinginto account any information about the multiple access interference. A receiver withsingle-user detection of the data symbols of user k is shown in Figure 2-6.R 0yinverse OFDM. . .R L−1parallel-to-serialconverterrsingle-userormulti-userdetectord^(k)d^Figure 2-5MC-CDMA receiver in the terminal stationrequalizerGudespreaderc (k)*n (k)quantizerd^(k)Figure 2-6MC-CDMA single-user detection