Blind adaptive multiuser detection for asynchronous dual-rate DS/CDMA systems

In this paper, the authors consider an asynchronous direct-sequence code di vision multiple access (DS/CDMA) system wherein users are allowed to transm it their symbols at one out of two available data rates. Three possible acc ess schemes are considered, namely, the variable spreading length (VSL), th e variable chip rate (VCR), and the variable chip rate with frequency shift (VCRFS) formats. Their performance is compared for the case that a linear one-shot multiuser receiver is employed. It is also shown that detection of the users transmitting at the higher rate requires a periodically time-var ying processing of the observables. Moreover, the problem of blind adaptive receiver implementation is studied, and a cyclic blind recursive-least-squ ares (RLS) algorithm is provided which is capable of converging to the peri odically time-varying high-rate users detection structure. Numerical results show that the proposed receivers are near-far resistant, and that the VCRFS access technique achieves the best performance. Finally, as to the adaptive blind receiver implementation, computer simulations hav e revealed that the cyclic RLS algorithm for blind adaptive high-rate users demodulation outperforms the conventional RLS algorithm in most cases of p rimary importance.