The ultra-wideband (UWB) communication systems have attracted both academic research and commercial interests due to their potential high throughput and precise ranging capability. Convoluitonal codes are widely used in different proposals for the high-speed UWB communication systems. In this paper, a novel Viterbi decoder architecture is studied for the multiband orthogonal frequency division multiplexing (MB-OFDM) UWB system . In the Viterbi decoder, sliding block, 2-step lookahead and 2 parallel techniques are combined to achieve the highest desired data rate. For lower data rates, it is possible to disable some parts of the decoder for power saving by proper analysis of the effects of puncturing on word length and trace back length. At the same time, in add-compare-select (ACS) unit, pipelined most-significant bit (MSB) first ACS unit is also utilized to shorten the length of the critical path.