WiMAX technology encompasses broadband wireless equipment which is designed in compliance with the IEEE 802.16 standard and certified by the WiMAX Forum. The IEEE 802.16e standard leverages several differences and enhancements over the 802.16-2004 standard to support mobile subscribers.
Scalable orthogonal frequency division multiple access (SOFDMA): Introduced in the 802.16e amendment over fixed WiMAX’s OFDM, SOFDMA supports scalable channel bandwidths from 1.25 to 20 MHz, using quadrature amplitude modulation (QAM; 16QAM or 64QAM) or quaternary phase shift keying (QPSK) modulation. SOFDMA enables additional resource allocation flexibility and adaptively optimized multiuser performance.
Advanced antenna technologies: MIMO PHY layer techniques have the potential to significantly increase bandwidth efficiency based on the premise that operation occurs in a rich scattering multipath environment. 802.16e defines optional support for such advanced antenna technologies. Major advantages of MIMO include diversity gains, multiplexing gains, interference suppression, and array gains. The inclusion of MIMO techniques alongside flexible subchannelization and adaptive modulation and coding (AMC) enables mobile WiMAX technology to improve system coverage and capacity.
In addition, 802.16e presents many advanced features for performance enhancements, such as handover support, quality-of-service (QoS) support, and energy savings mechanisms for handheld support, etc.
Multihop relay: Another milestone in the development of WiMAX was the introduction of multihop relay running as the 802.16j multihop relay project, which targets on OFDMA PHY layer and Medium Access Control (MAC) layer enhancements for licensed bands to enable the operation of RSs. The objectives of 802.16j are to enhance coverage, throughput, and system capacity by specifying 802.16 multihop relay capabilities and functionalities of interoperable RSs and BSs. Several technical topics were focused on, mainly including relay concepts, frame structure, network entry, bandwidth request, security, mobility management, routing, path management, interference control and radio resource management, etc.
The concept of multihop relaying is already well developed in the fixed telecommunications world. Microwave radio relays have been widely used to transmit digital and analog signals over long distances, with examples including telephony and broadcast television. With the evolution of mobile networks, wireless relays have been further developed for cellular transmission. Analog repeaters are sometimes used in cellular systems to extend coverage into regions that are uncovered by the standard network. Digital relaying for cellular applications was initially investigated to enhance coverage for delay-insensitive traffic. More recently, Streaming21 of the United States announced the availability of a 3G relay server in mid-2006, a carrier-grade mobile streaming solution that allows mobile operators and content providers to deliver multimedia contents to mobile phone subscribers over GPRS and 3G networks. Now, the relay concept is being further developed in 802.16j to supporting both digital repeater and decode-forward (DF) relaying with various techniques, such as cooperative relaying, intelligent radio resource management (RRM) for radio resource reuse, smart antenna on RSs for direction-controlled transmission, relay grouping, etc.
While the IEEE and the WiMAX Forum strive to address the technological challenges of high mobile, NLOS WiMAX services, large throughput and coverage, etc., commercial service providers face additional operational challenges including spectrum limitations, security vulnerabilities, and QoS implementations. Many researchers have been working toward developing mechanisms that provide highly efficient mobile WiMAX, which is the core topic in this chapter. Further development is expected in a newly approved project from the IEEE, namely 802.16m—Advanced Air Interface to meet IMT-advance requirements. It is targeting data rates of 100 Mbps for mobile applications and 1 Gbps for fixed applications, cellular, macro- and microcell coverage, with currently no restrictions on the RF bandwidth.
