Understanding Concurrent Transmissions over Ultra-Wideband Complex Channels

Ultra-wideband (UWB) devices operate only on a few frequency channels and commonly lack clear channel assessment capabilities: this makes it difficult to support several devices operating concurrently within a single network or to avoid coexistence issues with other UWB-based systems operating in close proximity. To address this issue, the IEEE 802.15.4 standard proposes the use of complex channels (i.e., diverse combinations of frequency channels and preamble codes) to enable multiple orthogonal transmissions. However, existing studies have shown that concurrent UWB transmissions on different complex channels are unreliable and incur high packet loss. In this paper, we investigate and shed light on the reason for this packet loss. We then present concrete methods to boost the reliability of concurrent UWB communications over different complex channels and demonstrate their effectiveness experimentally. In detail, we show that the synchronization and clock frequency offset among concurrent transmitters as well as the employed physical layer settings can be used to increase communication performance over different complex channels. Our study shows the feasibility of more than eight concurrent UWB transmissions on the same frequencies, sustaining a packet reception rate above 99% while retaining the ability to carry out centimetre-accurate ranging.