QS-CDMA: A potential air interface candidate for 4G wireless communications

The paper addresses the problem of constructing signature sequences for quasi-synchronous CDMA systems. The major feature of QS-CDMA is that the duration of any signature waveform is significantly longer than the maximal signal delay with respect to a common clock. In contrast to completely asynchronous CDMA systems for which all aperiodic correlations must be taken into consideration, in case of quasi-synchronous CDMA, it is sufficient to focus on a small window around the zero shift, the so-called interference window. This simplifies sequence design and allows us to reduce both the inter-symbol interference and multiple access interference drastically, thereby enhancing the system performance tremendously. This is all the more important since construction of sequences for completely asynchronous CDMA systems is a notoriously difficult problem and no such sequence sets are known. Consequently, random sequences must be used that are known to induce a significant capacity loss. In this paper, we present two possible methods for obtaining sequences with small aperiodic correlation sidelobes in the vicinity of the zero shift. We discuss advantages and disadvantages of QS-CDMA and W-CDMA, the major air interface for third generation mobile communications systems. Some simulation results are presented.