Code-timing synchronization in DS-CDMA systems using space-time diversity

The synchronization of a desired user transmitting a known training sequenc e in a direct-sequence (DS) asynchronous code-division multiple-access (CDM A) system is addressed. It is assumed that the receiver consists of an arbi trary antenna array and works in a near-far, frequency-nonselective, slowly fading channel. The estimator that we propose is derived by applying the m aximum likelihood (ML) principle to a signal model in which the contributio n of all the interfering components (e.g., multiple-access interference, ex ternal interference and noise) is modeled as a Gaussian term with an unknow n and arbitrary space-time correlation matrix. The main contribution of thi s paper is the fact that the estimator makes efficient use of the structure of the signals in both the space and time domains. Its performance is comp ared with the Cramer-Rao Bound, and with the performance of other methods p roposed recently that also employ an antenna array but only exploit the str ucture of the signals in one of the two domains, while using the other simp ly as a means of path diversity. It is shown that the use of the temporal a nd spatial structures is necessary to achieve synchronization in heavily lo aded systems or in the presence of directional external interference. (C) 2 001 Elsevier Science B.V. All rights reserved.