Sum-Rate Optimal Multicode CDMA Systems: An Equivalence Result

In this paper, the sum rate of a multicode code-division multiple-access (CDMA) system with asymmetric-power users is maximized, given a processing gain and the power profile of users. Unlike the sum-rate maximization for a single-code CDMA system, the optimization requires the joint optimal distribution of each user's power to its multiple data streams as well as the optimal design of signature sequences. The crucial step is to establish an equivalence of the multicode CDMA system to restricted frequency-division multiple-access (FDMA) and time-division multiple-access (TDMA) systems. The CDMA system has upper limits on the numbers of multicodes of users, whereas the FDMA system has upper limits on the bandwidths, and the TDMA system has upper limits on the duty cycles of users. The equivalence facilitates the complete characterization of the maximum sum rate of the multicode CDMA system and also provides new insights into single- and multicode CDMA systems in terms of the parameters of the equivalent FDMA and TDMA systems.