MMSE multiuser detection for asynchronous dual-rate direct sequence CDMA communications

The authors consider an asynchronous DS/CDMA system wherein users are allowed to transmit their symbols at one out of two available data-rates. Besides the variable spreading length (VSL) access technique, described in the literature, two other types of access methods are considered, namely the variable chip-rate (VCR) and the variable chip-rate frequency shifted (VCRFS), wherein users with different data-rates are accommodated by means of signature waveforms with different chip-rates. The authors derive, through an unified approach valid for each of the above access techniques, an MMSE-based multiuser detector, and show that detection of the users transmitting at the higher data-rate requires a periodically time varying processing. As to the performance analysis, results show that the VCRFS access technique presents the best performance, as well as that enlarging the processing window length and sampling at rates larger than the chip-rate yield beneficial effects on the system performance