Coherent Interference Cancellation

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 [1] and [2] 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 [5] to achieve significant gains over the extremes of linearly- dependent and orthogonal signaling.

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1. Cover, T. M., Thomas, J. A., Elements of Information Theory. New York: Wiley, 1991.
2. Rimoldi, B., Urbanke, R., "Onion Peeling and the Gaussian Multiple Access Channel," Submitted to IEEE Trans. Inform. Theory.

5. D. Parsavand and M. K. Varanasi, "RMS Bandwidth Constrained Signature Waveforms that Maximize the Total Capacity of PAM-Synchronous CDMA Channels," IEEE Trans Commun., COM-44:1, (January 1996), pp. 65-75.