Performance comparison of NLMS-based adaptive single-user receivers for direct-sequence spread spectrum CDMA systems
The capacity of a direct-sequence code division multiple access (DSSS-CDMA) system is limited by multiple access interference (MAI) and the near-far problem. Multiuser receivers and single-user receivers provide two approaches to mitigate these problems. The former do cancel the interference and enhance system capacity, but have large computational requirements and require the knowledge of MAI parameters. Adaptive single-user receivers, however, do not require knowledge of MAI parameters and need fewer computations. This thesis discusses a wide range of adaptive single-user receivers found in the literature and presents their bit-error-rate (BER) performance results under a unified framework to provide a basis of comparison. Due to the cyclostationary nature of the DSSS signal, the signals can exhibit a correlation with frequency-shifted versions of themselves. In order to exploit the spectral correlation, a receiver needs to be an implementation of the periodically time-dependent adaptive filter (TDAF), which is also known as the frequency-shift (FRESH) filter. The difference between the different receivers is in how much of the spectral correlation they exploit. It is shown that fractionally-spaced adaptive receivers, which exploit fully or in partial spectral correlation, perform better than the adaptive receivers that cannot exploit this correlation. Multipath results presented for two-ray channels indicate that fractionally-spaced adaptive receivers act as RAKE receivers.