Routing and Scheduling for WiMAX Mesh Networks

In addition to the mandatory implementation of point-to-multipoint (PMP) mode, WiMAX networks can be optionally configured to work in Mesh mode, to achieve increased reliability, coverage, and reduced network costs. Although IEEE 802.16 standards specify several quality of service (QoS) schemes and related message formats for WiMAX networks, the problems of scheduling algorithms for both PMP and Mesh mode are left unsolved. Routing algorithms for WiMAX networks are outside the scope of the standard work as well. In this chapter, we investigate the issues of routing and scheduling for WiMAX mesh networks. We overview the mesh mechanisms specified in the IEEE 802.16 standard and survey the existing research on scheduling and routing for WiMAX mesh networks. Then both distributed and centralized routing algorithms are studied, and their effectiveness on alleviating potential network congestion is compared. The scheduling problem is mathematically modeled by taking into account the interference constraints. Solutions are developed which can maximize the utilization of network capacity subject to fairness constraints on the allocation of scarce wireless bandwidth.

INTRODUCTION

In today’s telecommunications, networking and services are changing in a rapid way to support next generation Internet (NGI) user environment. Wireless networks will play an important role in NGI. Wireless broadband networks are being increasingly deployed and used in the last mile for extending or enhancing Internet connectivity for fixed or mobile clients located on the edge of the wired network.

With high data rate, large network coverage, strong QoS capabilities, and cheap network deployment and maintenance costs, WiMAX is regarded as a disruptive wireless technology and has many potential applications. It is expected to support business applications, for which QoS support will be a necessity. Depending on the applications and network investment, WiMAX network can be configured to work in different modes, point-to-multipoint (PMP) or Mesh mode. An illustration of PMP and mesh mode is presented in Figure 1. For example, the network can have a simple base station (BS) working in PMP mode and serving multiple subscriber stations (SSs) if the potential SSs can be covered by the BS. Mesh topology is an optional configuration for WiMAX networks. In the Mesh mode, traffic demands are aggregated at a set of SS nodes which are equipped with 802.16 interfaces. Subsequently, the traffic demands at SS nodes are delivered to a set of BSs nodes which functions in the PMP mode. These BS stations can be connected by a backhaul and connected to Internet access point (IAP) nodes. An amendment to the IEEE 802.16 specifications (where WiMAX is based) is IEEE 802.16j, Multihop Relay Specification. IEEE 802.16j is being developed by IEEE 802.16’s Relay Task Group. It is expected to extend reach/coverage through relaying. 

 

Figure 1: WiMAX (a) PMP network and (b) mesh network architectures.

Wireless mesh network offers increased reliability, coverage, and reduced network costs. There are extensive research, standardization, and commercial development activities on mesh networks. For example, several IEEE special task groups have been established to define the requirements for mesh networking in wireless personal area networks (WPANs), wireless local area networks (WLANs), and wireless metropolitan area networks (WMANs). A brief description of the standardization activities. Wireless mesh networks can be a prospective solution for broadband wireless Internet access in a flexible and cost-effective manner. However, wireless mesh networks also raise a number of research challenges, e.g., network routing, scheduling, QoS support, network management, etc. Those challenges are faced by WiMAX mesh networks without exception. WiMAX Mesh mode is defined with OFDM for frequency between 2 and 11 GHz and time division multiple access (TDMA) is used in the Medium Access Control (MAC) layer to support multiple users. Unlike the single hop wireless networks, routing algorithms are required to determine routes for the connections between a SS and a BS. As WiMAX networks operate synchronously in a time-slotted mode, it is also necessary to allocate time slots without collision over the network to achieve assigned bandwidth for each connection. More challenging is that the routing and scheduling for WiMAX networks are tightly coupled. The routing and scheduling problem for WiMAX networks is different from 802.11-based mesh networks. In the 802.11-based mesh networks, the MAC layer is contention-based; routing algorithms and MAC layer protocols can be designed and operated separately.

Although IEEE 802.16 standards specify several QoS schemes and related message formats, the problems of scheduling algorithms for both PMP and Mesh mode are left unsolved. Routing algorithms for WiMAX networks are outside the scope of the standard work as well. In this chapter, we will investigate the issues of routing and scheduling in WiMAX mesh networks. Both distributed and centralized routing algorithms will be studied, and their effectiveness on alleviating potential network congestion will be compared. The scheduling problem will also be mathematically modeled by taking into account the interference constraints. Solutions are developed to maximize the utilization of network capacity subject to fairness constraints on allocation of scarce wireless resource among the SSs.