Superposition Coding in the Downlink of CDMA Cellular Systems

Superposition coding techniques can be used to improve communications efficiency in a variety of scenarios. These techniques are particularly appropriate for applications in which there are spatial or temporal disparities in received signal-to-noise ratios (SNRs). In this paper, we consider the application of superposition coding to improve the throughput of the forward link in a CDMA cellular system using orthogonal spreading sequences. By taking advantage of power disparities that are created by power control on the forward link, the system can transmit to a larger number of users by transmitting to multiple users on a single spreading sequence and using superposition coding. We use an abstraction of a cellular communication system in which Shannon capacity or the optimal capacity equations for broadcasting on an additive white Gaussian noise channel are used to determine the required transmit power to a user or users. We evaluate the user capacity achieved with the proposed superposition coding technique under both average-and fixed-power constraints. We compare the performance of the proposed technique to that of a system that employs generalized Welch bound equality (GWBE) sequences. The results indicate that superposition coding can significantly increase the user capacity of a cellular system and that broadcasting is much more effective than using GWBE sequences.