Signature sequence optimization in asynchronous CDMA systems

We characterize the user capacity, i.e., the maximum number of supportable users at a common SIR target level for a fixed processing gain, of a single-cell symbol asynchronaus CDMA system. We show that the user capacity of an asynchronous system is the same as the user capacity of a synchronous system; that is there is no loss in user capacity due to asynchrony. Optimum signature sequences are those that minimize the total squared asynchronous correlation (TSAC) among the users, and depend on users' delay profile. Optimum received powers of the users are equal, and the optimum linear receiver filters in any observation window of size M > 1 symbols (i.e., M-shot MMSE filters) are one-shot matched filters. We present iterative and distributed signature adaptation algorithms where, at each iteration, only one user updates its signature sequence to decrease the TSAC of the set.