Achievable information rates, and the coding-spreading tradeoff in finite-sized synchronous CDMA systems

We compute achievable rates for synchronous code-division multiple-access (CDMA) systems and study the associated coding-spreading tradeoff problem using these results. We assume random spreading sequences and the computed achievable rates are averaged over ensembles of spreading sequences. Unlike most prior work, which analyzed the spectral efficiency of large CDMA systems under Gaussianity assumptions (inputs and multiple access interference), we make no such assumptions. In order to display the coding-spreading tradeoff, we plot the required minimum SNR for reliable transmission as a function of information rate. It is shown that the coding-spreading tradeoff favors all coding (i.e., no spreading) when the optimal multiuser detector (MUD) is employed, whereas for systems with suboptimal MUDs and single-user decoding, there generally exist optimal code rates. We also provide simulation results on the performance of LDPC-coded synchronous CDMA systems which approach the information-theoretic limits we have computed.