This paper investigates the Multi-band OFDM (MB-OFDM) based physical (PHY) layer proposal for IEEE 802.15.3a working group on short-range high data-rate Ultra-wide-Band (UWB) communications. An overview of the MB-OFDM PHY layer architecture with its various parameters is presented and the optimal choice of critical parameters is discussed. Next, we derive the theoretical un-coded Bit Error Rate (BER) of MB-OFDM over the Rayleigh fading channel. Performance results over realistic IEEE UWB channel models are analyzed and compared to a pulsed-OFDM based approach.  Although pulsed-OFDM was presented in the literature as an enhancement to the MB-OFDM approach, simulation results showed that with same redundancy-factor, both systems give almost similar BER performance.