Link-layer Cooperation Based on Distributed TDMA MAC for Vehicular Networks
Vehicular ad hoc networks (VANETs) are a special kind of communication networks, where vehicles communicate with each other or with stationary road side units. VANETs are expected to support a large spectrum of mobile distributed applications that range from vehicle-collision warning and vehicle-traffic alert dissemination (safety applications), to file-sharing and location-aware advertisements (infotainment). As communication nodes are organized in an ad hoc manner to form a communication network, VANETs possess special characteristics, such as the highly dynamic network topology with high node mobility and frequent link breakage and stringent service requirements for high priority for delay sensitive safety messages, as compared with the general mobile ad hoc networks (MANETs). Hence, directly applying the existing communication protocols designed for MANETs may not be reliable and efficient in VANETs. Thus, communication protocols in VANETs are required to support the wide range of applications and their strict service requirements. This seminar presents link-layer cooperative frameworks to improve transmission reliability and network throughput in the existing distributed TDMA MAC protocols for VANETs.
We present a link-layer node cooperation scheme for VANETs, referred to as Cooperative ADHOC MAC (CAH-MAC). In CAH-MAC, neighboring nodes cooperate to increase the performance by utilizing unused time slots to retransmit failed packets. Throughput improvement is achieved due to the usage of idle time slots that are wasted in the absence of node cooperation. In addition, as a packet is retransmitted earlier by a relay node, transmission delay and packet dropping rate are reduced. Numerical results demonstrate that CAH-MAC performs better when the number of nodes in a two-hop neighborhood is moderate as compared with the total number of time slots available in a frame. Furthermore, performance gain is significant for a moderate channel condition.
We study the effects of a dynamic networking environment, in the presence of high relative mobility and channel fading, on the performance of CAH-MAC. It is observed that, system performance degrades due to cooperation collisions. To tackle this challenge, we present an enhanced CAH-MAC (eCAH-MAC) scheme. In eCAH-MAC, using different types of packet, as per the operation performed, and by delaying or suspending some relay transmissions, cooperation collisions can be avoided and cooperation opportunities can be efficiently utilize without disrupting the normal operations of the distributed TDMA MAC. Through mathematical analysis and simulations, we observe that the proposed node cooperation framework is capable of avoiding cooperation collisions by suspending a cooperative relay transmission phase, which allows more nodes, seeking time slots, to efficiently reserve unused time slots.
The proposed node cooperation frameworks enhance the performance of distributed TDMA MAC and make it more robust to tackle the dynamic networking conditions in VANETs.