Cyclically permutable codes for rapid acquisition in DS-CDMA systems with asynchronous base stations

There has been significant interest in cellular systems employing direct sequence (DS) code division multiple access (CDMA), with no synchronization requirement between base stations (BS), especially in the context of the so-called third-generation (3G) wireless systems. In this case, since neighboring BSs use different spreading sequences, the mobile must first determine the spreading sequence being used by the BS(s) from which the mobile receives sufficiently strong signals. To facilitate acquisition, the total set of spreading sequences is divided into groups, and the group identification is transmitted over a synchronization channel. This paper describes a novel mechanism for transmitting the group identification (ID) using cyclically permutable (CP) codes; this scheme results in significant reduction in the search space, allowing for rapid acquisition. The technique described is currently a component of the radio access technology in the 3G partnership project (3GPP) proposal to the International Telecommunications Union (ITU) for 3G wireless systems (IMT-2000). An analysis of the three-stage search process required for acquisition and cell search is also presented, and simulation results are shown to demonstrate the efficacy of this approach.