A-VeMAC：一种自适应车载 MAC 协议外文翻译资料

 2022-08-12 16:31:07

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Abstract

This paper proposes an adaptive vehicular MAC protocol, called A-VeMAC, for VANETs. A-VeMAC is a multichannel MAC protocol based on VeMAC. Like VeMAC, it employs a TDMA mechanism, and supports one control channel and several service channels. For the control channel, each frame is partitioned into two disjoint sets of timeslots, which are associated with vehicles moving in opposite directions, respectively. Unlike VeMAC, which equally partitions each frame, the frame partitioning with A-VeMAC is not equal. Instead, it can adaptively vary with the vehicle traffic conditions in opposite directions. The purpose is to better support unbalanced vehicle traffic conditions in opposite conditions. With A-VeMAC, a vehicle will first make an adjustment of frame partitioning based on the current traffic conditions in opposite directions before it attempts to reserve a timeslot. After the frame partitioning adjustment is completed, it will determine a set of timeslots that are available for it to reserve and then randomly selects one for reservation from the available timeslots. For the service channels, A-VeMAC employs the same access mechanism used in VeMAC. Simulation results show that A-VeMAC can well support both balanced and unbalanced vehicle traffic conditions, and can achieve a better performance than VeMAC in terms of the channel utilization and the access collision rate, in particular, under unbalanced traffic conditions.

Key Words—A-VeMAC；adaptive protocol；MAC；vehicular ad hoc network

I. INTRODUCTION

Medium access control (MAC) is a critical issue in the design of a vehicular ad hoc network (VANET) [1]. In a VANET, multiple nodes may need to transmit data via a common radio channel simultaneously, which would cause data collision and thus affect the access performance of the network. In order to avoid potential data collision, each node must employ a MAC protocol to coordinate the channel access of multiple nodes and handle collision problems. Due to the unique characteristics of VANETs, however, such as high node mobility, frequent topology change, and predictable mobility model [2], existing MAC protocols for mobile ad hoc networks cannot directly be employed in a VANET. To achieve good access performance, it is necessary to design efficient MAC protocols for VANETs.

The IEEE 802.11 p [3] is a typical MAC protocol for VANET, which employs CSMA/CA as the basic channel access mechanism and can deal with high node mobility and frequent topology change. However, the CSMA/CA mechanism of IEEE 802.11 p has several limitations. First, for a broadcast service, the access mechanism neither uses an RTS/CTS exchange during the access procedure nor uses an acknowledgement message after a node receives a message. This makes it difficult to handle the hidden terminal problem, i.e., a collision occurs at a node when two other nodes which are not in each others one-hop neighborhood communicate with the node simultaneously, and thus would reduce the message delivery ratio of a network. Second, it employs a carrier sense multiple access with collision avoidance (CSMA/CA) mechanism, in which every node owns the same opportunity to access the channel. Even if a node has a critical safety message to send, it cannot access the channel immediately if the channel is busy at that time. Therefore, IEEE 802.11 p cannot ensure timely delivery of critical safety messages.

In contrast, a TDMA-based MAC protocol can address this problem to a certain degree. The ADHOC MAC protocol proposed in [4] is a typical one which is based on a completely distributed access mechanism, the Reliable R-ALOHA (RR-ALOHA) [5], and operates in a time-slotted structure. With ADHOC MAC, each node can acquire an exclusive timeslot to transmit its messages, which can improve timely delivery of safety messages. Moreover, each node must transmit frame information (FI) in its timeslot to indicate the status of each timeslot and thus can handle the hidden terminal problem. Due to the node mobility, however, merging collisions may occur frequently [6]. In that case, each node involving a merging collision will lose its timeslot and then attempts to reserve a new timeslot, which would lead to more access collisions. Such collisions would increase the time for a vehicle to reserve a timeslot, and thus reduce the efficiency of channel access and degrade the delay and throughput performance in message delivery. Another major limitation of ADHOC MAC is that it is a single channel protocol, not suitable for the seven DSRC channels.

To address the limitations of the ADHOC MAC protocol, a VeMAC protocol was proposed in [7], which is able to support multi-channels. VeMAC supports one control channel and several service channels. The control channel is used for transmitting high-priority short information and control information, while the service channels are used for transmitting safety or non-safety related applications messages. According to [8], merging collisions are more likely to occur among vehicles moving in opposite directions, due to the high relati

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