Group gain design for overloaded CDMA

In overloaded CDMA decision-feedback receivers may have a bit error probability of at least one user that "floors", i.e. cannot be reduced to zero when the signal-to-noise ratio increases. This problem is a consequence of the overloaded nature of the CDMA system, characterized by a very high correlation among users\´ signatures and corresponding high multiuser interference that the receiver is not able to cancel. In this paper we show that "floors" can be avoided by properly controlling each users\´ gain and propose an algorithm that, grouping users, sets gains in such a way that all users have a BER that vanishes. Simulations show that group decision-feedback receivers with simple conventional receivers as constituent blocks, have performances very close to those of the optimal ML receiver.