The nearest neighbour receiver for the downlink of a DS-CDMA system

We consider the downlink of a DS-CDMA communication system which employs BPSK modulation and is subject to the effects of a stationary multipath channel. A multi-user detector, consisting of a matched filter bank is considered and we demonstrate an example in which the resulting space requires a non-linear decision boundary. Such a boundary may be realised via an RBF network, but the complexity of this approach increases rapidly with the number of active users. The behaviour of the RBF decision boundary as the background noise is reduced motivates our proposal for a receiver which makes its estimate simply by taking the weight associated with the centre which is nearest to the soft output vector from the matched filter bank. Simulations of the performance of this nearest neighbour receiver show very close agreement with the conventional Gaussian RBF network. Interpreting the nearest neighbour boundary as a subset of the Voronoi diagram for the centres of the network allows an axiomatic method for discarding the number of centres which are considered in the comparison, and the performance of a receiver based on this reduced set of centres is shown to provide a compromise between the excellent error performance of the RBF network, and the computational simplicity of a linear filter.