INTRODUCTIONBroadband wireless access (BWA) has gained a particular attention during the past few years. The widely successful IEEE 802.11 wireless LAN (WLAN) technologies were attempted in BWA applications along with a host of proprietary solutions. When the WLAN technologies were examined closely, it was evident that the overall design and feature set were not well suited for outdoor BWA applications. In response to this need, the IEEE 802 committee set up a working group in 1999 to develop a new standard for BWA applications, IEEE 802.16. Later, another industrial association, the Worldwide Interoperability for Microwave Access (WiMAX) Forum [1], was formed to promote the 802.16 standards by defining the interoperability specifications between 802.16 products from different vendors. Thus, IEEE 802.16 networks are also often referred to as WiMAX networks.The first IEEE 802.16 standard [2] was completed in October 2001, and addressed radio frequency bands from 10 to 66 GHz; thus, line of sight (LOS) is required between a base station (BS) and subscriber stations (SSs). In order to support more customers less expensively, an amendment called IEEE 802.16a was ratified in January 2003 by operating the physical (PHY) layer at lower frequency bands from 2 to 11 GHz, and thus allowing the possibility of non-LOS (NLOS) operation. In October 2004 the new standard 802.16-2004 [3] was published, which is actually an amalgamation of 802.16 and 802.16a specifying interoperable air interfaces from 2 to 66 GHz with a common medium access control (MAC) layer. Recently, the 802.16e standard [4,5] was also ratified in December 2005, allowing upgrade from fixed BWA systems to mobile service provisioning up to vehicular speeds.The 802.16 WiMAX air interface supports two operational modes: a mandatory point-tomultipoint (PMP) mode and an optional mesh mode. In PMP mode, a centralized BS controls all communications among the SSs and the BS, whereas in the mesh mode, SSs can also serve as routers by cooperative access control in a distributed manner. In this article we focus on centralized PMP mode, which is thought to be able to provide better quality of service (QoS) performance than distributed mesh mode; hence, PMP mode is the first choice of WiMAX operators. In a downlink subframe of PMP mode, the BS transmits a burst of MAC protocol data units (PDUs) using time-division multiplexing (TDM);
ABSTRACTThe WiMAX standard specifies a metropolitan area broadband wireless access air interface. In order to support QoS for multimedia applications, various bandwidth request and scheduling mechanisms are suggested in WiMAX, in which a subscriber station can send request messages to a base station, and the base station can grant or reject the request according to the available radio resources. This article first compares two fundamental bandwidth request mechanisms specified in the standard, random access vs. polling under the point-to-multipoint mode, a mandatory transmission mode. Our results demonstrate that random access outperforms p...