Overview of the Mobile WiMAX Specification

IEEE 802.16 Standard

The IEEE 802.16 standard, which includes Medium Access Control (MAC) and physical (PHY) layer specifications, aims at supporting Internet services over wireless metropolitan area networks (WMAN). It is also an alternative to traditional wired networks, such as Digital Subscriber Line (DSL) and cable-modem. There are two modes defined in WiMAX networks: point-to-multiple-points (PMP) mode and mesh mode.

In the PHY layer, the IEEE 802.16 standard adopts the orthogonal frequency division multiplexing (OFDM), which is a multicarrier modulation scheme. The IEEE 802.16 standard has two OFDM-based modes: OFDM and orthogonal frequency division multiplexing access (OFDMA). Both of these technologies allow subcarriers to be adaptively modulated (e.g., QPSK, 16-QAM, and 64-QAM), depending on transmission distance and noise. Moreover, OFDMA has scalability to provide efficient use of bandwidth.

The MAC layer of IEEE 802.16 standard was originally designed for the PMP mode. On the later amendments of the IEEE 802.16a and the IEEE 802.16d, the mesh mode was included. The IEEE 802.16a adopts OFDM to provide greater spectral efficiency and to mitigate interference. IEEE 802.16b covers most of the quality of service (QoS) aspects. The IEEE 802.16e introduces scalable OFDMA into the standard, and supports mobile communications. With handover mechanisms, WiMAX is thus able to support mobile communications at vehicular speeds. We summarize the history of the evolution of the IEEE 802.16 standard in Figure 1.

Figure 1: Evolution of the IEEE 802.16 standard.

The IEEE 802.16 working groups on broadband access standards developed the IEEE 802.16 WirelessMAN standard for WMANs. On the other hand, the WiMAX forum was formed in June of 2001 to ensure interoperability among 802.16 products from different vendors. These groups and their activities may help popularizing WiMAX networks and systems by bringing vendors together and improving the specifications.

According to the IEEE 802.16 specification, the non-line-of-sight (NLOS) transmission range is 4 miles. With the combination of soft-switch technologies, a WiMAX network can work as a wireless "last mile" and make a viable alternative to the Public switched telephone network (PSTN) for VoIP services. In addition, a WiMAX network can work as a point-to-point backhaul trunk with a transmission capability of 72 Mbps at a transmission distance over 30 miles. With its technological advantages of throughput, power, transmission range, and versatility, WiMAX might be a strong competitor of other technologies, such as WiFi and 3G. Therefore, from both economical and technical points of view, WiMAX could be an appealing choice for broadband wireless services.

WiMAX Network Architecture

WiMAX has an IP-based wireless access architecture, which contains three parts: user terminal devices, access service network (ASN), and core service network (CSN). A user terminal device can be a fixed or portable/mobile terminal device, which supports the fixed/nomadic/mobile usage scenarios. Each device can establish a connection link to a WiMAX Base Station (BS), and perform authentication and registration through an access gateway in the CSN. The system architecture is illustrated in Figure 2.

Figure 2: Internet Protocol (IP)-based wireless access architecture of WiMAX.

A mobile WiMAX network has a similar architecture as a cellular network, where PMP links are between each BS and multiple Subscriber Stations (SSs). Each BS provides frequency and timing reference to SSs for synchronization purpose. The detailed MAC layer protocols and message sequences will be described later.