Hybrid acquisition schemes for direct sequence CDMA systems

The acquisition of chip timing in direct sequence code division multiple-access (DS/CDMA) systems must occur before the demodulation of incoming messages can begin. Recent work has shown that the problem of acquisition may be the dominant factor limiting the capacity of DS/CDMA systems and networks. Passive matched filters and parallel search schemes have been shown to be able to acquire signals rapidly, but they do so at the cost of high or prohibitive complexity. In contrast, straight serial search schemes have lower complexity but acquire the signal much more slowly. In this paper, we present several hybrid active correlation schemes that provide flexibility in the trade-off between acquisition speed and complexity. These techniques test several phases concurrently, and either decide that a particular phase is correct, in which case the decision is verified by a binary hypothesis test, or it is decided that none of the phases are correct, in which case another group of phases is tested. Several fixed sample size (FSS) tests are considered, as well as a test based on a new M-ary sequential hypothesis test called the MSPRT. It is found that the MSPRT provides the best performance.