Vehicular ad hoc networks (VANETs) are playing an important role in Intelligent Transportation Systems (ITS), which undertake a large number of key vehicular safety and traffic efficiency applications. Wireless access design for VANETs is always a challenging work since VANETs have some special properties such as rapidly changing topology, limited spectrum resource and predictable moving path. In this dissertation, the wireless access algorithms of VANETs are carefully studied from four aspects: multi-channel access with changeable channel interval, protocol sequence allocation, vehicle differentiation access and intra-cluster service channel resource allocation.
The main contents and innovations of this dissertation are shown as follows.
1. To overcome the weak points that collision probability is high and channel resource allocation is unreasonable in wireless access in vehicular environments (WAVE) protocol, a multi-round elimination contention based multi-channel access algorithm is proposed. In this algorithm, the control channel (CCH) interval is divided into three phases: roadside unit (RSU) broadcast phase, safety message broadcast phase and service channel (SCH) reservation phase. Specially, in the first phase, the RSU broadcasts coordination packets which include slot allocation information to vehicular nodes around; In the second phase, vehicular nodes carry out collision-free broadcast; In the third phase, vehicles contend with each other for SCH resource on the basis of the multi-round elimination contention process which could significantly decrease transmission collision probability and provide more successful reservations. Besides, a round-bye based vehicle traffic differentiation scheme is used to deal with multi-priority access problem in the third phase. Vehicles who have high priority can stay out of the contention in the first round of the elimination process, hence, they have high probability winning in the end. Considering the dynamic property of VANETs, the proposed algorithm can adaptively adjust the length of the CCH interval and the value of the round number for the improvement of the system throughput. Theoretical analysis and simulation results show that the proposed algorithm can take full use of the CCH and SCH resource, effectively decrease transmission collision probability in the SCH reservation phase, and exhibit its superiority in saturated throughput compared to the variable CCH interval (VCI) MAC and the WAVE MAC.
2. To overcome the drawbacks that protocol sequence allocation in VANETs heavily relies on the help of roadside units and that lots of sequence resource is wasted owing to the one-to-one scheme, two adaptive sequence allocation schemes are proposed. Specifically, the rectangle-cell (R-C) scheme is proposed for neighboring nodes to occupy sequences without overlapping on straight roads. Furthermore, the hexagon-cell (H-C) scheme is proposed to handle the sequence allocation problem on city roads. The sum of generalized prime sequences which builds the foundations for the proposed schemes is fully studied. Besides, the algorithms for vehicles to generate cell marks and occupy sequences without any inter-vehicle interference are given in detail. Simulation results show that both the R-C scheme and the H-C scheme can take full advantage of the sequence resource and exhibit their superiority in sequence utilization and throughput compared to the one-to-one scheme.
3. To overcome the slot wastage problem in traditional time division multiple access (TDMA) based VANETs, a two-phase based user differentiation algorithm (UDA) for slot allocation is proposed. Vehicular nodes in this algorithm are divided into two types: the backbone nodes and the ordinary nodes. In the first phase, every vehicular node tries to occupy one time slot with the same behavior, and at the end of the first phase based on the received broadcast packets during one frame part of the ordinary vehicular nodes can be upgraded to backbone nodes. During the second phase the backbone nodes carry out a greedy behavior to contend for more time slots while ordinary nodes give up slot contention. The slot occupation period and the dynamically backbone node selection strategy are given in detail. Moreover, the performance of average channel utilization and broadcast delay have been analyzed. Analysis and simulation results show that the proposed UDA has a better performance in the two indexes listed above compared to the one-to-one scheme without user differentiation.
4. To overcome the wick points that lots of SCH resource is wasted in cluster based VANETs, an adaptive SCH slot allocation algorithm is proposed. This algorithm is composed of two parts: slot pre-allocation period and slot reallocation period. In the first part, cluster members occupy SCH slots according to the position of the slot which has been reserved on the CCH. In the second part, cluster members contend with each other to occupy the unreserved slots via polling. Besides, an improvement algorithm which enables cluster members to occupy slots on the basis of the cluster stabilization time is proposed to deal with the problem that the resource of some cluster members will always be harmed due to the constant occupation order in the second part. Simulation results show that the proposed adaptive slot allocation algorithm can take full use of the SCH resource and exhibit a high level of long-term fairness among cluster members.