As the number of vehicle accidents increases, car manufacturers and academic researchers have developed a vehicular safety system. The key component of the safety system is vehicular communications, by which vehicles exchange their local status information with neighbor vehicles and disseminate a warning message within a specified area. The challenge lies in satisfying stringent communication requirements of the safety system, extremely reliable packet delivery and low communication latency. Unfortunately, the current de-facto standard for vehicular communications, Dedicated Short-Range Communication (DSRC) cannot meet the requirements especially in high vehicle density. Specifically, high operation frequency of the DSRC causes serious signal attenuation in Non Line-Of-Sight (NLOS) conditions and the frequent exchanges of vehicle status information make the network easily congested.

To overcome this challenge, we exploit extra TV White Space (TVWS), which Federal Communications Commission (FCC) allowed unlicensed users to access in the absence of license user's activities. The use of extra TVWS helps to improve the safety system because 1) adoption of extra frequency resource can mitigate the network congestion and 2) the excellent TVWS signal propagation improves the reachability of the safety message dissemination. Thus, in this doctoral dissertation, we design and evaluate TVWS-based Cognitive Radio Network (CRN) systems as a solution for realizing the vehicular safety system. First, we evaluate various vehicular MAC protocols using network simulator (NS-2) and investigate whether the MAC protocols are feasible to the vehicular safety system when only using a frequency band allocated to the DSRC (i.e., DSRC band). Next, we propose a TVWS-based CRN system that is specialized for the safety message dissemination in the safety system. More specifically, we utilize an extra TVWS band for emergency safety message dissemination, and exploit a DSRC band for 1) the exchange of control data and 2) the compensation of the reception errors. Finally, we propose a TVWS-based CRN system that supports reliable safety message dissemination in a NLOS corner. Specifically, for reliable dissemination in the NLOS corner, the proposed system leverages the excellent propagation characteristics of a TVWS signal and further improves the reliability by adopting a novel retransmission mechanism. Through in-depth simulation studies, we show that the proposed system outperforms previous system designs and supports the safety systems well.