We study ultra wideband (UWB) communications over dense multipath channels using M-ary frequency shift keying (FSK) data modulation with both coherent and non-coherent detection. We present an M-ary FSK signal design to balance energy and correlation variations due to the frequency selectivity of the multipath channels and the frequency response of the antenna system. We evaluate the bit error rate (BER) taking into account the frequency domain effects of the antenna and the channel. We calculate the signal-to-noise ratio (SNR) improvement for different values of M and channel conditions. Using the binary coherent receiver as a baseline, we provide numerical results showing that a coding gain larger than 6 dB is possible for large M using a coherent receiver with just two correlators.