Synchronization and detection of spread spectrum signals in multipath channels using antenna arrays

This paper deals with the synchronization of spread spectrum signals in a CDMA communication system, while carrier lock is assumed to have been achieved. For satisfactory performance, the error in code timing must be less than a chip-length because of the sharp autocorrelation function that most codes possess. This task is further complicated because of the typical multipath channels that users in wireless communication systems transmit over. We detail an algorithm that exploits spatial diversity at the receiver using antenna arrays to estimate the code timing as well as direction of arrival (DOA) of a particular user even in the presence of strong multipath. Using antenna arrays, we extend a subspace based technique proposed by Bensley and Aazhang (see GLOBCOM, December 1994) to synchronize to (and therefore detect the packets of) more users than is currently feasible. The timing and DOA estimates are then used in the detection of the bits transmitted by a given user. Maximum likelihood single-user antenna array based techniques for synchronization of spread spectrum signals have been addressed before by Dulgos and Scholtz (1989), but these techniques aren't suitable for conditions of high traffic (because the users' timings are acquired sequentially and a training sequence is required). We propose the design of an antenna-array based algorithm for multiuser acquisition and detection without the need for a priori training. A number of detection strategies employing antenna arrays in mobile cellular systems have been discussed, but these schemes do not address the issue of synchronization to the different users.