Coding-spreading tradeoff in LDPC-coded CDMA with turbo multiuser detection

We consider the problem of coding-spreading tradeoff in code-division multiple-access (CDMA) systems employing multiuser joint decoding. In particular, we assume the users employing the capacity-achieving low-density parity-check (LDPC) codes and at the receiver, turbo multiuser detection is used to implement joint decoding. We solve the coding-spreading optimization based on the extrinsic information signal-to-noise ratio (SNR) evolution curves for the soft-input soft-output (SISO) multiuser detectors and the SISO LDPC decoders. Two types of SISO multiuser detectors are treated, namely, the soft interference cancellation minimum mean-square-error (SIC-MMSE) detector and the soft interference cancellation matched filter (SIC-MF) detector. Moreover, both single-cell and multicell scenarios are considered. For each of these cases, we are able to characterize the extrinsic information SNR analytically, for both finite-size systems and for the so-called large systems where asymptotic performance results must be evoked. Our analysis indicates that the SIC-MMSE-based system offers a significant gain in spectral efficiency compared with the SIC-MF counterpart, in both single-cell and multicell scenarios. This is in contrast to the single-user decoding case, where it has been shown that the MMSE detector offers little advantage over the conventional matched-filter in terms of capacity in multicell scenario. Moreover, the results on coding-spreading tradeoff for finite-size systems and large systems match very well.