A MAC Protocol for UWB Very Low Power Mobile Ad-hoc Networks based on Dynamic Channel Coding with Interference Mitigation
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; in contrast, interference from inside the exclusion region should be combatted. We use pulse position modulation, as is commonly done with UWB. The very low power assumption implies that pulses are infrequent. This property, together with a demodulation scheme that cancels much of the interfering energy, allows us to mitigate interference from nodes in the exclusion region. This scheme is entirely local to a destination and involves no protocol action. In addition, sources constantly adjust their channel codes (thus their bit rates) and send incremental redundancy as required. Contention between sources sending to the same destination is solved by a private MAC protocol, i.e., a set of interactions that involve only the nodes that want to talk to the same destination. Carrier sensing is not used, as it is practically impossible with UWB. We show by simulation that we achieve a significant increase in network throughput, compared to traditional MAC protocols that are separated from the physical layer.