Multiuser detection for CDMA systems with nonlinear modulation

Most of the extensive literature on multiuser detection for direct-sequence code division multiple access (CDMA) focuses on systems with (direct or differential) linear modulation. There are a number of existing CDMA systems, however, that do not fall within this setting. In this paper, we consider the general problem of multiuser detection for CDMA with nonlinear modulation. We have two basic goals: (i) To characterize the best possible performance obtainable using multiuser detection. To this end, we consider the maximum likelihood (ML) receiver, which assumes knowledge of the timing and complex amplitudes for all users, and the generalized likelihood ratio test (GLRT), which requires knowledge only of the timing for all users. Both receivers have complexity exponential in the number of users. (ii) To obtain low-complexity multiuser detectors that outperform the conventional single user detector (which ignores the presence of the multiple-access interference). We propose two new decision projection algorithms with complexity linear in the number of users.