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Onion Peeling for CDMA-- Symmetric Rate Under RMS-Bandwidth Constraints
T. Guess and M. K. Varanasi, Proc. of the 1996 Conf. on Information Sciences and Systems, pp. 584-589, Princeton University, Princeton, NJ (March 1996).
Abstract Multiuser coding is a relatively new field of research, whereas single-user coding is well established. For this reason there has been an interest in methods which allow the users to choose single-user codes independently of each other. This paper considers the synchronous, code-division multiple-access (CDMA) channel, where the users are root-mean-square (RMS) bandlimited. If the signature waveforms are all identical, the discrete- time model reduces to the conventional, multiple-access Gaussian channel. The other extreme is when the signature waveforms are all orthogonal (e.g., time-division multiple-access (TDMA)) in which case the discrete-time model is a bank of single-user channels. The latter clearly allows single-user coding at the expense of bandwidth, while the former has been discussed in  and  where information-theoretic arguments have shown that sequential decoding (onion peeling) can be used in such a way as to achieve maximum rate-sum capacities. The mediate ground lends itself to sequential decoding based on soft outputs from a multiuser detector. The result is a sequential decoder for CDMA with a degree of freedom that determines to what extent the signature waveforms are correlated. In other words, there is a give-and-take relationship between coding and spreading, and this can be exploited using the results on signal design in  to achieve significant gains over the extremes of linearly- dependent and orthogonal signaling.
Last modified: 2001-09-09T23:02