WiMAX
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Overview
This paper is part of the Wireless Standards White Paper Series.
WiMAX (Worldwide Interoperability for Microwave Access) is based on the IEEE standard 802.16. The standard, also called WirelessMAN, was created by the WiMAX Forum. It aims to provide “last mile” broadband wireless access and has the potential to replace many of the current telecommunications infrastructures. This document discusses the technical specifications of the WiMAX standard, including strengths and weaknesses.
Table of Contents
WiMAX Variations
There are several different flavors of WiMAX, but most commercial interest is in the 802.16d and 802.16e standards. The 802.16d revision, also called Fixed WiMAX, allows non-line-of-site (NLOS) service from a base station to a subscriber and uses the orthogonal frequency division multiplexing (OFDM) modulation scheme. It does not cater to hand off between base stations.
Support for mobility in WiMAX is added with the 802.16e revision, also known as Mobile WiMAX. It uses the orthogonal frequency division multiplexing access (OFDMA) modulation scheme, which is a multiuser version of OFDM. Mobility is achieved with a network optimized hard hand off.
Technical Specifications
The WiMAX standard offers a variety of fundamentally different design options, depending on the revision. The revisions cater to different deployment scenarios and applications. The figure depicts a subset of the OSI model for WiMAX.
WiMAX OSI Model
The WiMAX MAC layer uses an intelligent scheduling algorithm that allows subscriber stations to compete only once for initial entry into the network. The subscriber is then assigned an access slot by the base station. This slot can enlarge and contract to allow Quality of Service (QoS) control, but it remains assigned to the subscriber station. This is different from the Wi-Fi standard, where distant stations can have service interruptions from closer stations. Additionally, it allows more stability under overload conditions, which ultimately results in better bandwidth efficiency.
The physical/data link layer is based on OFDM, a multicarrier modulation scheme that uses a large number of closely spaced orthogonal (perpendicular) subcarriers to carry data. The subcarriers overlap in frequency but are designed not to interfere with each other. Carriers are separated using a fast Fourier transform (FFT) algorithm. Each subcarrier is then modulated with a conventional modulation scheme such as QAM or QPSK but at a low symbol rate. Each subcarrier can uphold data rates comparable to single-carrier modulation schemes in the same bandwidth.
OFDMA is a multiuser version of OFDM that was added in the 802.16e revision of WiMAX. It assigns subsets of subcarriers to individual users, which allows several users to simultaneously transmit at low data rates. Additionally, it allows control over the data rate and QoS for each user. The 802.16e revision furthermore added support for multiple input/multiple output (MIMO) smart antenna technology. This technology breaks individual signals into subchannels so they can support many users at once. Multiple antennas are used to increase throughput and range. This collectively allows WiMAX to carry more data traffic.
The original WiMAX standard (IEEE 802.16) specified WiMAX for the 10 to 66 GHz range, but later specifications were added for the 2 to 11 GHz range.
WiMAX Strengths
- Long-range connectivity (two to five miles)
- Data rates up to 134 Mb/s
- High Quality of Service (QoS)
- Robust security
- Support for mobility
- Advanced antenna technology
WiMAX Weaknesses
- Very long-range connectivity (greater than five miles) – this is realistically achievable only with a high-quality external antenna and line-of-site service to the base station
- Lack of available high-quality spectrum
Applications
WiMAX has the potential to be used in a variety of fixed and mobile applications.
Fixed WiMAX Applications
Broadband Connection to Rural Homes/Metropolitan Areas – There is potential for a new broadband Internet market serving rural homes where wired solutions are too expensive. WiMAX could provide this connection to these “last mile” homes. Additionally, WiMAX could serve as a metropolitan area network (MAN) to provide Internet connection to broad areas or even whole towns.
T1 Emulation – Worldwide, commercial buildings can have limited access to T1 fiber-optic connections. Because of the lower costs and high quality of service, WiMAX is a capable solution for this business market.
Backhaul for Wi-Fi Hotspots – Currently Wi-Fi hotspots use wired broadband connections to an Internet backbone. WiMAX could offer cheaper and faster solutions to connect to these backbones.
Mobile WiMAX Applications
Nomadic Broadband – Nomadic access would allow users to seamlessly take their connections from one location to another.
4G Cellular Networks – WiMAX is a contender to compete with other standards to be a fourth-generation network for cellular providers.
WiMAX Testing Solutions
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