Iterative Signature Adaptation for Capacity Maximization of CDMA Systems

For single cell synchronous code division multiple access (CDMA) systems, both the information theoretic capacity and the network capacity have been identi ed. In both cases, it was shown that if the number of users, N , is no more than the processing gain, L, then orthogonal signatures are optimal while if N > L, then signatures which satisfy the Welch bound on the total squared correlation with equality (called WBE sequences) are optimal. This paper presents an algorithm which iteratively updates the signatures in a distributed fashion, starting from an initial set of signatures. Under mild conditions on the initial set of signatures, we prove that the algorithm converges to a set of orthogonal signatures if N L and to a WBE set if N > L. At each step, the algorithm replaces one signature from the set with the normalized linear MMSE receiver lter corresponding to that signature. Since the MMSE lter can be obtained by a distributed algorithm for each user, the algorithm is amenable to distributed implementation.