MCI cancellation for multicode wideband CDMA systems

Multicode code division multiple access (CDMA) is a new transmission scheme for flexible and high-speed data communications. The basic idea of multicode CDMA, is to assign multiple channelization codes to any given user. If these channel codes are orthogonal, the self-interference among them can be eliminated in an additive white Gaussian noise (AWGN) channel. However, in a multipath environment, these intrauser signals from different delay paths no longer maintain orthogonality and, thus, cause interference, i.e., multicode interference (MCI), to each other. In high-speed data networks, where the number of users is much less than in the voice networks, the MCI may represent a large portion of the total interference and has a great impact on the system performance. A complex spreading multicode wideband CDMA receiver with RAKE structure and multistage MCI cancellation is studied. By pilot aided channel estimation, the MCI associated with the reference user is regenerated and subtracted from the received signal by a cancellation factor of λ. A complete and consolidated theoretical analysis is presented to show that the system performance is significantly improved by the MCI cancellation. The optimal cancellation factor in the kth stage is approximated by λ_opt^(k)⩽1-2P_e^(k-1) , where P_e^(k-1) the error probability of the (k-1)th stage. The optimal value of each stage can be chosen from 0.5 to 0.85 for a wide range of signal to noise ratios