Performance of synchronous multirate CDMA via statistical physics

New results are reported on the performance of synchronous multirate code-division multiple access (CDMA) systems. It has been shown using a statistical physics approach that, under arbitrary inputs and signal-to-noise ratios (SNRs), a CDMA channel followed by a generic multiuser detection front end can be decoupled into a bank of Gaussian single-user channels in the large-system limit. This work extends the analysis to multirate CDMA where users may employ a combination of adaptive modulation, variable spreading factor, and multicode schemes to transmit at different data rates. It is found that the decoupling principle still holds, i.e., each user experiences an equivalent single-user Gaussian channel, where the degradation in the SNR, known as the multiuser efficiency, is obtained analytically by solving some fixed-point equations. This result applies to optimal multiuser detectors as well as many well-know suboptimal detectors. The simple large-system characterization of multirate CDMA encompasses many previous results as special cases