Equalization and RAKE detection in wideband ISI channels: towards rapproachement

The maximum-likelihood (ML) detector for direct-sequence spread-spectrum signaling over ISI channels is developed for both symbol-length and long spreading sequences. The approach is based on a delay-domain orthogonal decomposition of a joint specular/diffuse dispersive multipath channel. The detector, which extends the Ungerboeck symbol-spaced structure, demonstrates the path diversity inherent in spread spectrum ML detection. The RAKE detector is shown to be the ML detector under a set of ideal conditions, one of which cannot be obtained in practice. As an intermediate result, the DS/SS decision-feedback equalizing detector is shown to be a suboptimum approximation of the ML detector.