Interplay between TVWS and DSRC: Optimal strategy for QoS of safety message dissemination in VANET

Vehicular safety system has been drawing considerable attention as a key application of Vehicular Ad-hoc NETworks (VANETs). In the safety system, there are two types of safety messages: Emergency Safety Messages (ESM) and Periodic Beacon Messages (PBM). The ESM has to be disseminated to vehicles within specified area and meet latency and delivery ratio requirements, while the PBM is transmitted periodically with vehicle information such as position, speed and direction. For vehicular communications, Dedicated Short-Range Communication (DSRC), the de facto standard, has normally been used. However, the one-hop transmission range of the DSRC is so short that a multi-hop dissemination is required in order to cover a large dissemination area of ESM. The multi-hop dissemination with the limited number of DSRC channels induces channel collision and network congestion. Moreover, the coexistence with PBMs aggravates collision and congestion of DSRC channels, which makes it hard to satisfy the requirements of the ESM dissemination. To overcome the limitation of the DSRC, we utilize an extra TV White Space (TVWS) band that has a large communication range for ESM disseminations, and exploit a DSRC band as a control channel for TV channel rendezvous. In this paper, we propose and analyze a distributed channel usage scheme between DSRC and TVWS bands for quality of service (QoS) of ESM disseminations under the existence of PBMs. Our scheme employs TVWS Channel Rendezvous Algorithm (TCRA) that is based on available TVWS channel map, ensuring that vehicles within a dissemination area select the same channel with the ESM sender. To compensate ESM reception failures in a TVWS band, our scheme adopts On-Demand Recovery Algorithm (ODRA) that uses a DSRC band for an ESM retransmission to reception failure vehicles. Through an in-depth simulation study, we show that the proposed scheme satisfies ESM requirements of latency and delivery ratio, and outperforms legacy DSRC systems with tw- interfaces under various vehicle speeds in highway area.