We propose a MAC protocol for very low radiated power (1 microwatt) ultra-wide band (UWB) mobile ad-hoc networks. Unlike traditional approaches, our protocol fully utilizes the specific nature of the physical layer of UWB. This makes it possible to reach a reasonably high bit rate per source in spite of the low power, while being fully distributed. We use dynamic channel coding as an efficient way to adapt to varying channel conditions due to mobility and interference. Our design is based on existing theoretical findings that state that it is optimal to allow interfering sources to transmit simultaneously, as long as they are outside a well-defined exclusion region around a destination, and to adapt the channel code to this interference