Algebraic-phase scrambling sequences for code-spread code-division multiple-access

In this paper, we first present a modified code-spread code-division multiple-access (CS-CDMA) scheme, named phase-scrambling CDMA (PS-CDMA), for a Gaussian multiple access channel (MAC). In PS-CDMA, we realize the code-spreading using a low-rate serially concatenated code consisting of a convolutional code and a repetition code. Then, we use complex user-specific algebraic-phase scrambling sequences to distinguish users. The receiver is based on the iterative multiuser decoding suggested in [5,8]. Next, we design complex algebraic phase scrambling sequences to mitigate multiple access interferences for overloaded PS-CDMA, i.e., the number of users is greater than the spreading factor. By analyzing extrinsic information transfer (EXIT) curves and the trajectories, we demonstrate that PS-CDMA using the proposed scrambling sequences has faster iterative decoding convergency and better system performance, when compared to the PS-CDMA using random-phase scrambling sequences and some previously known multiple access schemes.