Hybrid acquisition of direct sequence CDMA signals

Abstract

In direct sequence code division multiple access (DS-CDMA) systems, signal acquisition is necessary before communication can commence. Recent work has shown that the problem of acquisition may be even more restrictive than the problem of error control in limiting the capacity and performance of these systems. 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 studyhybrid 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 that none of the phases are correct, in which case another group of phases is tested. Two new hybrid acquisition schemes are presented; the first is based on a fixed-sample-size weighted MAP test, and the second is based on anM-ary sequential hypothesis test called MSPRT. It is shown that the weighted MAP-based scheme hypothesis test called MSPRT. It is shown that the weighted MAP-based scheme outperforms a standard MAP-based scheme. It is also shown that considerable performance gain can be obtained using sequential testing. In particular, it is shown through numerical examples that MSPRT-based schemes are several times faster than corresponding fixed-sample-size schemes.