In this work we explore flexible modulations to allow demodulation using receivers with different complexity and cost. More specifically, we study pulse-based ultra wideband (UWB) communications over channels with dense multipath effects using frequency shift keying (FSK) data modulation. Our aim is to explore the performance tradeoffs between high and low complexity receivers. For this purpose we determine the performance of coherent, non-coherent and mismatched (e.g., non-coherent detection using templates consisting of a single path) demodulators. We take into account the influence of the frequency response of the antenna system and the effects of the frequency selectivity of the multipath channel. Given a specific channel condition, we derive expressions for the bit error rate for coherent, noncoherent and mismatched reception of correlated FSK signals with unequal energies, and calculate the signal-to-noise ratio (SNR) degradation for different receiver's complexities with a given FSK frequency deviation We show that UWB-FSK has a SNR degradation similar to other low-complexity receivers studied in the literature¹.