We study ultra wideband (UWB) communications over dense multipath channels with M-ary frequency shift keying (FSK) data modulation and low complexity receivers. We model the signal transmission-reception taking into account the effects of both the frequency response of the antenna system and the frequency selectivity of the multipath channel. We derive an expression for the bit error rate (BER) when detection is achieved using a pair of passband filters. While other low complexity binary receivers reported in the literature present a large signal-to-noise ratio (SNR) degradation with respect to the binary coherent detection, using our signal design and filter-based detection the loss with respect to the binary coherent detection is only 3.5 dB using a binary receiver with no channel estimation.