RICHARD A. QUINNELL, CONTRIBUTING EDITOR

WHAT IS WiMAX?
The term WiMAX—Worldwide Interoperability for Microwave Access—refers to a series of wireless networking standards under the umbrella designation of IEEE 802.16. The specifications have undergone a significant evolution since their inception in 2001, growing in scope and augmenting the capability of systems built to the standards. The two most significant standards are IEEE 802.16d or fixed WiMAX, also called IEEE 802.16-2004, and IEEE 802.16e or mobile WiMAX. These are the standards to which existing and emerging products are designed and to which the WiMAX Forum (www.wimaxforum.org) is certifying components and system.

A WiMAX system has two major elements, as shown in Figure 1. The infrastructure equipment, residing in the base station, provides the backhaul link to the Internet as well as Internet Protocol (IP) links to the end-user equipment. The end-user equipment can perform any of several functions. The end-user unit may be a server for a local network that uses WiMAX to link to the Internet. It could also be an Internet phone, a music or gaming system, a video player, or some combination of all these functions.

The WiMAX communications scheme offers several configurations in order to support the widest possible range of applications. Using orthogonal frequency division multiplexing (OFDM) radios and a scheduling algorithm to control access, WiMAX provides a high-bandwidth connection to multiple simultaneous users at several quality-of-service (QoS) levels, shown in the Table. The combination of bandwidth and QoS options allows a system to readily handle data, voice, and video data streams.

The WiMAX radio channels offer data rates as high as 72 Mbits/second to users or connections as far as 50 km. There is a tradeoff between bandwidth and distance, however, and the achievable capacity of links depends on whether the installation is line-of-sight (LOS) or non-line-of-site (NLS). The LOS installations offer the maximum bandwidth and range and generally are fixed installations that use provider-installed directional antennas. NLS installations are typically restricted to 5-15 km but allow customer installation and use "smart" antennas that provide adaptive beam-forming.

The standards for fixed and mobile WiMAX differ somewhat in the radio links as well as some of the other protocol layers. They aim at different markets, however, so their incompatibility may not be an issue for equipment developers. Fixed WiMAX is the technology currently being deployed. Many industry observers believe, however, that mobile WiMAX will become the dominant installation because it can serve both fixed and mobile users.

What's Needed in Base Stations
The function of a WiMAX base station is to establish and manage links with a large number of users, providing them with connection to an Internet backbone. The station consists of an antenna, radio, and server. Each offers several design alternatives.

Antenna design will depend on the type of connections the base station is to provide. Line-of-sight connections to users, such as apartment and office buildings, require directional antennas. Non-line-of-sight connections, such as to mobile users and residences with customer-installed equipment, require omni-directional antennas with adaptive beam-forming capability.

The radio will vary with the country of installation. No single frequency band has been allocated worldwide for use in wireless broadband, so a single radio design will not serve all markets. Radio bands at 2.3 GHz, 2.5 GHz, 3.5 GHz, and 4.9 GHz have been licensed in various countries for WiMAX use. Some unlicensed radio bands are also available, including 900 MHz, 2.4 GHz, and 5.8 GHz. The standards support radio frequencies up to 11 GHz for fixed WiMAX and up to 6 GHz for mobile WiMAX.

The same server section will work with all antenna and radio combinations, but there are differences between fixed and mobile WiMAX systems. Mobile WiMAX protocols include provisions for roaming handoff and for authorization, authentication, and accounting functions. There are also provisions for digital rights management targeting mobile entertainment applications. As a result, developers should ensure that the equipment and components they are using conform to the correct specification: 802.16d supports fixed systems while 802.16e targets mobile systems.

Regardless of the standards supported, base station servers have common design requirements. A prime requirement is low power per channel. Base station equipment is typically installed in small enclosed spaces where heat buildup can become significant. System designers will want the equipment to support as many users as possible without creating excessive power demands. Typically, systems implemented solely in software have high power demands due to the processor speeds required. Thus, minimizing power per channel typically requires some form of dedicated acceleration hardware to help offload from the processors functions such as beam forming, encryption, and user scheduling.

At the same time, developers will want to retain a measure of design flexibility. The market is relatively new, and additional applications with new system requirements are continually appearing. A system designed to be programmable will offer service providers a quick upgrade path for tracking market changes. This requirement is at odds, however, with the need to keep power per channel at a minimum. One solution to this conflict is to use programmable logic (FPGAs) to implement the acceleration hardware. This allows developers to retain design flexibility while keeping power demands down.

Another key requirement for base station equipment design is rapid time to market. Implementing a WiMAX network will require establishment of a base station grid in the target market area. Once those grids are established, however, the market for base station equipment reduces to replacements and upgrades. Thus, designers creating base station equipment who are late to market will find their opportunity has passed.

One way to help speed development time is to work with component vendors that offer substantial software support. Many of the functions in a WiMAX base station are generic, so do not represent added-value opportunities for designers. Obtaining pre-certified software from the processor vendor can save significant development effort as well as free designers to concentrate on unique applications. Development tool support from component vendors can also help ease the design task.

As with other portable equipment, power is a key design issue for WiMAX. As with the base stations, keeping power demands low will require a combination of hardware and software implementation for functions. Unlike base stations, however, the end user equipment has a relatively short market life and significant price sensitivity. Thus ASICs and other dedicated hardware are more cost effective than in base stations.

Antenna design can be a complex issue in end-user equipment. Units intended for fixed location or stationary use employ high-performance external antennas. Portable equipment, however, needs internal antennas. Because many portable WiMAX systems will also incorporate other wireless technologies, such as Bluetooth, wireless USB, or WiFi, there may be multiple antennas and radio frequencies in use. Developers will want to consider combining antennas (see Figure 2) or radio units to keep unit costs down.