WiMAX’s strong industry backing, standards-based approach, and mobility support bring a new horizon to wireless broadband. Zakhia Abichar, Yanlin Peng, and J. Morris Chang WiMAX: The Emergence of Wireless Broadband he much-anticipated technology of that focus on technical,regulatory,and marketing WiMAX, the Worldwide Interoperability aspects.The certification working group has devel- for Microwave Access, aims to provide oped a WiMAX product certification program, business and consumer wireless broad- which aims to ensure interoperability between Tband services on the scale of the Metropolitan WiMAX equipment from vendors worldwide. Area Network (MAN).WiMAX will bring a stan- The certification process also considers interop- dards-based technology to a sector that otherwise erability with the High Performance Radio depended on proprietary solutions.The technol- Metropolitan Area Network (HiperMAN), the ogy has a target range of up to 31 miles and a tar- European Telecommunications Standards Insti- get transmission rate exceeding 100 Mbps and is tute’s MAN standard. Such interoperability is expected to challenge DSL and T1 lines (both possible because 802.16 and HiperMAN each expensive technologies to deploy and maintain) were modified to include features from the other; especially in emerging markets. now, they share the same physical layer (PHY) and medium access control (MAC) layer specifica- EVOLUTION tions.The WiMAX Forum, through its Regulatory WiMAX is the commercialization of the IEEE Working Group, is also in discussions with gov- 802.16 standard, an evolving standard initiated at ernments worldwide about spectrum regulations. the National Institute of Standards and Technolo- gies in 1998 before being transferred to the IEEE APPLICATIONS to form Working Group 802.16. In June 2004, the WiMAX’s attributes open the technology to a working group won approval for the latest 802.16 wide variety of applications (see Figure 1). With standard for fixed wireless access, known as IEEE its large range and high transmission rate,WiMAX 802.16-2004.In December 2005,an extension that can serve as a backbone for 802.11 hotspots for addresses mobility also won approval as IEEE connecting to the Internet.Alternatively,users can 802.16e-2005. connect mobile devices such as laptops and hand- Throughout WiMAX’s development,the WiMAX sets directly to WiMAX base stations without Forum, which comprises a group using 802.11. Developers project this configura- of industry leaders (Intel,AT&T, tion for the WiMAX mobile version, which will Inside Samsung, Motorola, Cisco, and provide users broadband connectivity over large others), has closely supported coverage areas compared with 802.11 hotspots’ Resources and promoted the technology. moderate coverage. Mobile devices connected The group’s workforce is divided directly to WiMAX base stations likely will along multiple working groups achieve a range of 5 to 6 miles, because mobility 44 IT Pro July ❘ August 2006 Published by the IEEE Computer Society 1520-9202/06/$20.00 © 2006 IEEE Authorized licensed use limited to: University of North Carolina at Charlotte. Downloaded on January 28, 2009 at 15:29 from IEEE Xplore. Restrictions apply. makes links vulnerable. The technology can also provide fast and Figure 1. WiMAX applications. cheap broadband access to markets that lack A WiMAX base station can serve terminals infrastructure (fiber optics or copper wire), in a business district and residential area such as rural areas and unwired countries. Currently,several companies offer proprietary and on moving vehicles. solutions for wireless broadband access,many of which are expensive because they use Business district chipsets from adjacent technologies, such as 802.11. Manufacturers of these solutions use the physical layer and bypass the medium access control layer by designing a new one. Unlike these proprietary solutions,WiMAX’s standardized approach offers economies of scale to vendors of wireless broadband prod- ucts, significantly reducing costs and making the technology more accessible. Many com- panies that were offering proprietary solu- Radio tower tions, however, have participated in the Residential area WiMAX Forum and now offer WiMAX- Highway based solutions. WiMAX can be used in disaster recovery scenes where the wired networks have bro- ken down. In recent hurricane disasters, WiMAX networks were installed to help in recovery missions. Similarly,WiMAX can be used as backup links for broken wired links. FIXED ACCESS WITH IEEE 802.16-2004 interference (WiMAX users interfere with each other). The initial version of the 802.16 standard specified oper- The transmit power control scheme adjusts the transmis- ation in frequency ranges between 10 GHz and 66 GHz. sion power to reduce the interference with neighboring These high frequencies have more available bandwidth transmitters. and less risk of interference. Most technologies, such as The 802.16 standard was designed mainly for point-to-mul- 802.11, prefer lower frequencies because of better multi- tipoint topologies, in which a base station distributes traffic path propagation (meaning that signals can better handle to many subscriber stations that are mounted on rooftops. obstacles). On high frequencies, signals can’t diffract Researchers are also working to enhance receivers so that around obstacles and often require line-of-sight deploy- operators can install subscriber stations indoors.The point- ment in which the transmitter and receiver are directly to-multipoint configuration uses a scheduling mechanism opposite each other with no obstacles. Line-of-sight that yields high efficiency because stations transmit in their deployment requires more base stations to circumvent scheduled slots and don’t contend with one another. Also, obstacles, and thus increases deployment costs. unlike 802.11, WiMAX doesn’t require stations to listen to The standard’s subsequent additions specify operation at one another, because they encompass a larger area. This lower frequencies, between 2 GHz and 11 GHz, in both scheduling design suits WiMAX networks because sub- licensed and license-exempt bands. Commercial interest scriber stations might aggregate traffic from several com- in WiMAX focuses mainly on these frequencies because of puters and have steady traffic, unlike terminals in 802.11 the ease and cost of deployment. The initial WiMAX hotspots, which usually have bursty traffic. Forum certification profiles are in the 3.5 (licensed) and In addition to the point-to-multipoint mode, 802.16 sup- 5.8 GHz (license-exempt) bands. Because of possible inter- ports a mesh mode, where subscriber stations can com- ference in those ranges, the standard specifies schemes to municate directly with one another. The mesh mode can adjust the chosen frequency and transmission power for help relax the line-of-sight requirement and ease the improved performance. The dynamic frequency selection deployment costs for high-frequency bands by allowing scheme chooses the frequency that allows high perform- subscriber stations to relay traffic to one another. In this ance, and this scheme differentiates between primary- case, a station that doesn’t have line-of-sight with the base user interference (WiMAX users interfere with other station can get its traffic from another station. Figure 2 technologies using the same spectrum) and cochannel illustrates a WiMAX network with a mesh topology. July ❘ August 2006 IT Pro 45 Authorized licensed use limited to: University of North Carolina at Charlotte. Downloaded on January 28, 2009 at 15:29 from IEEE Xplore. Restrictions apply. WIRELESS BROADBAND tions aren’t required to listen to one another, because this Figure 2. Mesh topology listening might be difficult to achieve in WiMAX envi- deployment. Mesh deployment ronments. The base station schedules subscriber station relaxes the line-of-sight transmissions in advance through a flexible frame struc- ture. Stations need to contend only when they access the requirement and helps reduce channel for the first time.The reduced contention increases deployment costs by efficiency and allows one WiMAX base station to serve a requiring fewer base stations. large number of stations. In contrast, 802.11 terminals usu- Stations in area B do not have ally have bursty,or intermittent, traffic and contend every line-of-sight with the base station. time before transmitting. This contention decreases effi- ciency as the number of stations increases. They receive their traffic from Duplexing,a station’s concurrent transmission and recep- stations in areas A or C. tion,is possible through time division duplex and frequency A division duplex. In TDD, a station transmits then receives (or vice versa) but not at the same time.This option helps reduce subscriber station costs, because the radio is less N o lin complex. In FDD, a station transmits and receives simul- e-o f-sig taneously on different channels. ht The 802.16 MAC layer supports quality of service (QoS) for stations through adaptive allocation of the uplink and downlink traffic. Radio tower B Finally, the MAC of 802.16 supports different transport technologies such as Internet Protocol version 4 (IPv4), n IPv6, Ethernet, and Asynchronous Transfer Mode (ATM). nectio n This lets service providers use WiMAX independently of esh co M the transport technology they support. C SPECTRUM ISSUES Uniform spectrum allocation makes it possible to opti- mize radio performance for the allocated spectrum.Thus, IEEE 802.16-2004 physical layer because the radio accounts for a major portion of equip- For the bands in the 10- to 66-GHz range, 802.16 defines ment costs, spectrum allocation greatly impacts those costs. one air interface with a single-carrier modulation called The WiMAX Forum expects that initial deployment will WirelessMAN-SC. The PHY design for the 2- to 11-GHz occupy frequency bands in the 5 GHz (license-exempt) range (both licensed and license-exempt bands) is more and 2.5 GHz (licensed) bands. complex because of interference. Hence, the standard sup- ports burst-by-burst adaptivity for the modulation and • License-exempt 5 GHz. The frequency ranges of interest coding schemes and specifies three interfaces.