Comparing equalizers and multiuser detectors for CDMA downlink systems

In CDMA systems, the multiuser detector has been recognized as a solution to the multiple access interference (MAI) problem. The multiuser detector uses information about multiple users jointly to detect each individual user better. On the other hand, a channel equalizer which simply equalizes the channel without any information from the other users can also effectively suppress MAI, especially in a CDMA downlink. The equalizer displays an excellent performance even in a heavily loaded CDMA system. The paper presents a performance comparison between equalizers and linear multiuser detectors in a CDMA downlink system. It is found that the equalizers are almost as good as the linear multiuser detectors. It is shown that the performance of the equalizer (ZF or MMSE) is approximately equal to the performance of the corresponding linear multiuser detector for any multipath channel in a fully loaded CDMA downlink system when orthogonal spreading codes are used.