Performance of multicarrier CDMA in frequency-selective fading via statistical physics

This correspondence extends previous work on direct-sequence code-division multiple access (DS-CDMA) to give a single-user characterization of multicarrier (MC) CDMA systems. Results on the error performance and information rate of a family of multiuser detectors for MC-CDMA are obtained using the replica method, which was originally developed in statistical physics. The central result is the "decoupling principle", namely, an MC-CDMA channel with frequency-selective fading followed by a generic multiuser detection front end can be decoupled into a bank of single-user fading channels in the large-system limit. Thus, conditioned on one\´s fading coefficients, each user essentially experiences an equivalent single-user Gaussian channel with a degradation in the signal-to-noise ratio (SNR) in lieu of multiaccess interference. A set of joint equations is identified, which determines the degradation for each user, known as the multiuser efficiency. The spectral efficiencies under both optimal joint decoding and separate single-user decoding following multiuser detection are obtained analytically. The result applies to arbitrary input distribution and SNRs, and to optimal multiuser detection as well as various suboptimal schemes