Quality-Of-Service Scheduling for WiMAX Networks

Nicola Scalabrino, D. Miorandi, Francesco De Pellegrini, R. Riggio, Imrich Chlamtac, and E. Gregori

The broadband wireless world is moving toward the adoption of WiMAX (the commercial name of the IEEE 802.16 standard) as the standard for broadband wireless Internet access. This will open up a very large market for industry and operators, with a major impact on the way Internet access is conceived today. On the other hand, the emergence of innovative multimedia broadband services is going to impose severe quality of service (QoS) constraints on underlying network technologies. In this work, after a brief review of the IEEE 802.16 standard, we intend to present an in-depth discussion of its QoS support features. We point out the scheduling algorithm as the critical point in QoS provisioning over such networks, and discuss architectural and algorithmic solutions for an efficient support of multimedia flows. Performance measurements obtained from an experimental test-bed are also presented. The chapter concludes with a description of the key research challenges in the area, and provides a roadmap for the research in the field.

Introduction

The IEEE 802.16 standard, promoted by the WiMAX (Worldwide Interoperability for Microwave Access) forum, will be the leading technology for the wireless provisioning of broadband services in wide area networks. Such technology is going to have a deep impact on the way Internet access is conceived, by providing an effective wireless solution for the last mile problem.

The market for conventional last mile solutions (e.g., cable, fiber, and so on) presents indeed high entrance barriers, and it is thus difficult for new operators to make their way into the field. This is due to the extremely high impact of laborintensive tasks (i.e., digging up the streets, stringing cables, and so on) that are required to put the necessary infrastructure into place. On the other hand, the market is experiencing an increasing demand for broadband multimedia services , pushing toward the adoption of broadband access technologies. In such a situation, broadband wireless access (BWA) represents an economically viable solution to provide Internet access to a large number of clients, thanks to its infrastructure-light architecture, which makes it easy to deploy services where and when it is needed. Furthermore, the adoption of ad hoc features, such as self-configuration capabilities in the Subscriber Stations (SSs) would make it possible to install customer premises equipment without the intervention of a specialized technician, so boosting the economical attractiveness of WiMAX-based solutions. In this context, WiMAX is expected to be the key technology for enabling the delivery of highspeed services to the end users.

Typical BWA deployments will rely on a point-to-multipoint (PMP) architecture, as depicted in Figure 1a, consisting of a single Base Station (BS) wirelessly interconnecting several SSs to an Internet gateway. The standard also supports, at least in principle, mesh-based architectures, like the one plotted in Figure 1b. While WiMAX-based mesh deployments could play a relevant role in the success of such technology, the current standard is far from off ering a real support to such architecture. Therefore, we intend to restrict the scope of our work to the PMP architecture only.

Figure 10.1: Typical WiMAX system configuration—(a) point-to-multipoint; (b) mesh.

Figure 10.1: Typical WiMAX system configuration—(a) point-to-multipoint; (b) mesh.

In terms of raw performance, WiMAX technology is able to achieve data rates up to 75 Mb/s with a 20 MHz channel in ideal propagation conditions. But regulators will often allow only smaller channels (10 MHz or less) reducing the maximum bandwidth. Although 50 km distance is achievable under optimal conditions and with a reduced data rate (a few Mb/sec), the typical coverage will be around 5 km in non-line-of-sight conditions and around 15 km with an external antenna in a line-of-sight situation. Moreover, such a wide coverage makes it possible, and economically viable to provide broadband connectivity in rural and remote areas, a market which is usually not covered by traditional service providers.

The fundamental requirements for WiMAX to define itself as a possible winning technology are data reliability and the ability to deliver multimedia contents. Indeed, the provision of QoS guarantees will be a pressing need in the next generation of Internet, to enable the introduction of novel broadband multimedia applications. Users are actually getting more and more interested in broadband applications (e.g., video streaming, video conferencing, online gaming, and so on) that require assurances in terms of throughput, packet delay and jitter, to perform well. This applies also to WiMAX networks, which have also to face all the problems related to the hostile wireless environment, where time-varying channels and power emission mask constraints make it difficult to provide hard QoS guarantees. This entails the definition of a Medium Access Control (MAC) protocol which is able to effectively support such multimedia applications, while on the other hand, it efficiently exploits the available radio resources. The IEEE 802.16 standard encompasses four classes of services, with different QoS requirements and provides the basic signaling between the BS and the SS to support service requests/grants. However, the scheduling algorithms to be employed in the BS and the SS are not specified and are left open for the manufacturers to compete.

In this paper, after a brief review of the standard fundamentals, we will provide an in-depth overview and discussion on the QoS support provided by WiMAX technology. Particular attention will be devoted to scheduling algorithms for WiMAX networks. We will survey the existing literature, and point out some common issues involved in well-known technologies (e.g., wireless ATM), from which a system designer can draw to design an efficient scheduler without starting from scratch. Performance measurements obtained from an experimental test-bed are also presented. This chapter concludes with an overview of the actual research challenges, pointing out and detailing the most promising directions to pursue for research in this field.