"Advancing the future of wireless communications"

CODE OPTIMIZATION IN CDMA SYSTEMS
Sennur Ulukus and Roy D. Yates

For single cell synchronous code division multiple access (CDMA) systems, both the information theoretic capacity and the network capacity have been identified. In both cases, it was shown that if the number of users, is no more than the processing gain, then orthogonal signatures are optimal while if then signatures which satisfy the Welch bound on the total squared correlation with equality (called WBE sequences) are optimal. In both cases, the optimal signature sequences are the ones that minimize the total squared correlation (or equivalently the total interference in the system). The basic idea in this work is to update one signature sequence at a time that decreases (to be more exact, that does not increase) the total squared correlation. There are many ways to do that. This talk presents one such algorithm, which, at each step, replaces one signature from the set with the normalized linear MMSE receiver filter corresponding to that signature. Since the MMSE filter can be obtained by a distributed algorithm for each user, the proposed algorithm is amenable to distributed implementation. In this case, each user updates its own signature sequence by making some local measurements and the whole signature sequence set converges to an optimum one where both the information theoretic and network capacity are maximized.

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