Signature sequence and training design for overloaded CDMA systems

The focus of this paper is on training and signature sequence design in an overloaded synchronous CDMA system. The channel fading gains are assumed to be unknown and are estimated using training sequences. We derive a maximum-likelihood (ML) estimator and design sequences to minimize the mean-squared error (MSE) of the estimate. Two design scenarios are considered. One case assumes that the spreading sequences are fixed due to existing system constraints and optimal training sequences are designed using an iterative algorithm in order to minimize the MSE of the estimate. Performance of the iterative algorithm is examined using Welch-bound equality (WBE) sequences as the pre-designed spreading sequences. In the other scenario, spreading sequences and training sequences are designed jointly to minimize the MSE of the estimate. The jointly designed training and spreading sequences achieve optimum performance in the sense of minimizing the MSE. It is observed that when WBE sequences are used as spreading sequences the performance of the iterative algorithm is close to optimum, or even optimum, in certain situations