Equalization and Multi-User Detection for OFDM-Based CDMA Schemes

Mobile radio communication systems are affected by the frequency and time selectivity of radio channel due to multipath propagation and Doppler shifts. In an Orthogonal Frequency Division Multiplexing (OFDM) based Code Division Multiple Access (CDMA) system, the spreading schemes will suffer loss of orthogonality between codewords of different users. In this thesis, basic system model of several OFDM-based CDMA system is described and their performance is simulated and compared. In the basic system model, simple single-user detection without equalization is used. It is found that TimeFrequency Localized CDMA (TFL-CDMA) outperforms Multi-Carrier CDMA (MCCDMA) and Multi-Carrier Direct-Sequence CDMA (MC-DS-CDMA), by localizing chips to a rectangular time-frequency grid. Improved detection techniques used to restore the orthogonality between different users’ codewords, such as equalization and low-complexity Multi-User Detection (MUD), are investigated. Their performance is compared with the basic system model and the single-user system (Multiple-Access-Interference (MAI) free system). It is observed that both equalization and MUD can improve system performance significantly. It is also found that 7-user MUD outperforms 2-user MUD and equalization, at the cost of the highest complexity of the receiver. Using well localized TFL-CDMA schemes and 7-user MUD together, system performance is found to quite close to the BER of corresponding single-user system. In addition, equalization is shown to have better performance than 2-user MUD, although its complexity is less than the latter.