Impact of 5.9 GHz spectrum sharing on DSRC performance

To increase the amount of contiguous spectrum available for unlicensed use, there is interest in both the United States and Europe to allow secondary users on the 5.9 GHz band allocated for Intelligent Transportation Services. Under standard spectrum sharing rules, secondary users such as Wi-Fi are required to avoid harmful interference to primary users such as DSRC devices. Compared to conventional spectrum sharing scenarios, such as unlicensed devices sharing TV whitespaces, the safety-critical nature of DSRC transmissions places stricter requirements on the effectiveness of spectrum sharing mechanisms. In this paper, we analyze this spectrum sharing problem to identify its fundamental challenges and derive interesting network sharing scenarios. We also evaluate two recently proposed spectrum sharing mechanisms, Detect & Vacate and Detect & Mitigate, to understand their performance in these challenging scenarios. We identify that both mechanisms suffer from a delayed detection problem, which can be effectively improved by extending interframe idle periods. We further find that due to the unilateral hidden terminal problem, Detect & Mitigate can introduce up to 30% extra packet loss to DSRC transmissions after detecting the presence of DSRC devices. However, Detect & Vacate leaves the band after detecting DSRC, minimizing the impacts on DSRC transmissions.