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 signalto-noise ratios (SNRs). In this paper, we consider the application of superposition coding to increase the system capacity 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 by 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 techniques under both averageand fixedpower 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.