Effect of spreading and training on sum-capacity in overloaded synchronous CDMA

This work addresses the issue of sum-capacity in an overloaded, uplink, synchronous code-division multiple access (CDMA) system in a quasi-static Rayleigh-fading environment when the receiver does not have a priori knowledge of the fading gains. Part of each fading block is reserved for channel training and the remainder of the block is used for data transmission. Two modes of operation are considered: one in which the users are allowed to update their spreading sequences based on the channel estimates, and one in which their sequences cannot be modified. For both schemes we derive the optimal training scheme and then evaluate a lower bound on sum-capacity. When all the users have the same average received power, we consider the effects on this bound caused by the training-spreading-coding tradeoff.