An in-depth survey which covers the research activities and open issues from physical layer to applications, and from mobility management to power management, from designs with single channel single radio device to those with multiple channel multiple radio devices. However, most of the existing researches about wireless mesh networks are based on IEEE 802.11 standard. The routing and MAC layer protocols designed for 802.11 standard-based wireless mesh network cannot be used directly or efficiently for WiMAX mesh networks. And not much work has been done on WiMAX mesh networks. In this section, we will introduce the routing and scheduling related work on WiMAX mesh networks.
COORDINATED DISTRIBUTED SCHEDULING
The main idea of the coordinated distributed scheduling is to coordinate the transmission of MSH-DSCH messages over transmission opportunities in a collision-free manner. Through the exchanges of collision-free MSH-DSCH over control subframes, collision-free data slot reservations in the data subframes can be achieved.
To achieve the goal of collision-free MSH-DSCH transmission, nodes will exchange 2-hop or 3-hop neighborhood scheduling information with each other. Because nodes shall run the scheduling algorithm independently, a common algorithm has been specified in the standard for each node in the neighborhood to calculate the same schedule. The algorithm is random and predictable by dynamically constructing the seeds of a random number generator for each node according to a common rule. In particular, the seed for a given node is constructed based on its unique node ID and the index of the candidate transmission opportunity. The most important parameters that have significant impacts on the performance of coordinated distributed scheduling are Xmt Holdoff Exponet (3 bits) and Next Xmt Xm (5 bits). They can be used to control the contention on the transmission opportunities and improve bandwidth utilization.
Due to the importance of the coordinated distributed scheduling on the network performances, an analytical framework is needed to assess the performance of the scheduling scheme. Cao et al. analytically investigated how the channel contention is correlated with the total node number, exponent value, and network topology. With the assumption that the transmit time sequences of all the nodes in the control subframe form statistically independent renewal processes, they developed methods for estimating the distributions of the node transmission interval and connection setup delay. The analytical method will be helpful for evaluating upper layer performance like throughput and delay. They implemented the coordinated distributed scheduling module in NS-2 and showed that their analytical model is quite accurate under various scenarios, including both single hop and multihop networks.
Based on Cao’s analytical model, Bayer et al. presented an enhancement of the model. In particularly, they evaluated the scalability of the coordinated distributed scheduling. A scalability problem was observed that leads to poor performance in dense networks and aggravates QoS provisioning. The problem may result from the election-based transmission timing mechanism for scheduling the transmission of MSH-DSCH messages. They propose a dynamic adaptation mechanism to counteract the scalability problem, in which the parameter Xmt Holdoff Exponet is dynamically and locally adjusted according to the network contention and the status of a node. The Next Xmt Xm is used as a contention indicator. If Next Xmt Xm used by the node or its neighbors exceeds a specified threshold, the Xmt Holdoff Exponet is increased. The status of a node is defined according to its transmission activity, if it is BS or if it is a sponsor node. Significant UDP throughput increase is observed with the application of the adaptation mechanism for both single and multiple hop network scenarios.
In the 802.16 standard and the above analytical work, it is assumed that the control messages can be transmitted without collision in the extended neighborhood (2-hop or 3-hop). However, such kind of interference model may not hold in practice. Zhu and Lu investigate the performance of coordinated distributed scheduling under a realistic interference model [12]. Extensive simulations were conducted to evaluate the reception collision performance of the scheduling mechanism. It was reported that the collision ratio of control messages can be as high as 20 percent for 2-hop extended neighborhood. They studied how to deal with the collision problem by appropriate configuration of parameters such as Xmt Holdoff Exponent.
