Wireless Systems Design

Wi-Fi Popularity Extends To 5 GHz

Recent events may show the industry how to dramatically alter the landscape for 802.11a products in the 5-GHz band.

In recent years, the wireless-local-area-network (WLAN) products based on the IEEE 802.11 standards have become wildly popular. In fact, products based on this technology—also known as Wi-Fi—have been listed among the most encouraging areas of high-tech growth. To date, most Wi-Fi products worldwide have been limited to spectrum in the 2.4-GHz band. 802.11a products, which utilize the 5-GHz band, have not achieved the same success. Mainly, this trend is due to the large variation in regulatory requirements and spectrum availability worldwide.

Recent events may alter this landscape and significantly increase the number of 802.11a products. The first is the harmonization of 5-GHz spectrum, which came about as a result of the World Radiocommunications Conference. Ratification of IEEE 802.11h—a standard to reduce interference in the 5-GHz band—stands as the other compelling event.

Together, these two achievements will allow products that utilize the 5-GHz band to achieve the same economies of scale that aided the success of earlier products in the 2.4-GHz band. Previously, manufacturers had to create an unwieldy array of products to match a patchwork environment. Now, they will be able to concentrate more efficiently on one standards-based solution.

Initial Wi-Fi adoption has been limited to products that use the 802.11b standard. Most recently, though, the products that utilize the 802.11g standard also have tasted success. Both of these technologies operate in the 2.400-to-2.4835-GHz band. This spectrum is allocated on a worldwide basis for mobile services including WLANs. Most countries allow WLAN products to operate on an unlicensed or license-exempt basis.

Allowing unlicensed devices to operate in the 2.4-GHz band has reduced the barriers to entry and encouraged innovation. This 83.5 MHz of unlicensed spectrum unleashed Wi-Fi and mobile computing. As Wi-Fi popularity has increased, however, the need for additional spectrum has become apparent.

The challenge of addressing the spectrum shortage fell to the International Telecommunication Union. From June 9 to July 4, 2003, member states sent official government delegations to the World Radiocommunication Conference (WRC) in Geneva. This effort was significant. After all, conferences to harmonize spectrum usage worldwide are only held every three to four years. Plus, the drafting and promoting of official positions is a lengthy and often arduous process.

During the conference, WLAN use in the 5-GHz band was one of the most contentious issues addressed. The delegates needed to balance WLAN equipment's popularity with the need to protect equipment already operating under previous primary allocation.

Debate over how best to achieve this balance lasted for months during pre-conference activities. Finally, the conference allocated the spectrum from 5.150 to 5.350 GHz and 5.470 to 5.725 GHz on a primary basis to the mobile service. It is to be used for the implementation of wireless access systems (WASs), including WLANs.

The conditions under which WLANs will be allowed to share the spectrum are defined in ITU Recommendation M.165. This document defines the agile radio techniques of dynamic frequency selection and transmit power control. Originally, these mechanisms were developed in response to European regulatory requirements, which governed WLAN use in the 5-GHz spectrum as embodied in ETSI EN 301 893 V1.2.2 (2003-2006).

For the basic conditions for spectrum sharing, look to TABLE 1. The resolution provides a win-win solution for incumbent spectrum users and WLAN users alike. Incumbent spectrum holders benefit from protection against interference with equipment like radar. At the same time, consumers worldwide will enjoy harmonized spectrum for Wi-Fi usage.

How does this resolution help the user? As countries around the world implement the WRC resolution, there will be a large increase in the amount of 5-GHz spectrum that is available for WLAN use by 802.11a products. This increase should encourage product development and innovation. In addition, the regulations governing WLAN use in 5 GHz should become very similar in many countries. Having common frequencies and spectrum-usage rules will free business travelers and others from having to purchase multiple product versions for use in different locations.

Take the United States, for example. The Federal Communications Commission (FCC) has already completed the process of rulemaking that would allow 802.11a products with 802.11h-based mitigation measures to operate in the 5.470-to-5.725-GHz band. Having an additional 255 MHz of spectrum means that 24 non-interfering channels are now available for use by 802.11a products in the U.S. By comparison, the 2.4-GHz band only has a total of three non-overlapping channels.

Yet spectrum harmonization alone is not enough. In order for manufacturers to achieve economies of scale, other regulatory requirements, such as mitigation measures, must be similar from region to region. FCC officials and their counterparts worldwide are starting to define the rules for WLAN use in the 5-GHz band. They must ensure that the other devices that are legally operating in that band do not suffer from harmful interference from WLAN products. To that end, it's desirable to harmonize mitigation measures and other requirements.

On September 19, 2003, the IEEE approved an amendment to the IEEE 802.11 WLAN standard to implement ITU-R Recommendation 1652. This amendment, IEEE 802.11h, has two main elements. They include dynamic frequency selection (DFS) and transmit power control (TPC). Dynamic frequency selection detects the presence of a primary service and switches the WLAN to a clear frequency. Transmit power control reduces the total power on a Wi-Fi network. The Wi-Fi Alliance plans to add interoperability testing for 802.11h mitigation measures to its certification program in 2004.

The WRC demands that a channel be vacated when a WLAN device receives a signal in a channel exceeding -62 or -64 dBm. One way to fulfill this requirement is with dynamic frequency selection. This technique is similar to the way that conversations are handled via walkie talkies. Public-safety personnel frequently detect other users on a particular channel. When the traffic reaches a certain level, they ask their partners to switch to another channel so they can continue their conversation uninterrupted.

DFS allows Wi-Fi products to avoid co-channel interference. Among the other potential uses of DFS is the uniform utilization of available channels. This aspect will become increasingly important as 802.11a products become more common. The elements that are associated with DFS are shown in TABLE 2.

Dynamic frequency selection works in the following manner: A station provides a list of supported channels to the access point. The access point then approves or rejects the request based on channel information. Whenever an access point schedules a quiet interval in order to test for radar, stations may not transmit on that channel during the quiet interval (FIG. 1).

An access point also can request measurements from a station via a measurement request. The station responds by sending a measurement report to the access point (FIG. 2). Lastly, an access point may utilize a channel-switch announcement to inform stations when they should switch to a specified channel. The station subsequently responds by switching to the new channel (FIG. 3).

The second element of 802.11h, transmit power control (TPC), is a method of lowering the transmit power used by a WLAN system. TPC lowers the "volume" of the signal on a network. WLAN access points and stations that are located near each other do not need to use the same transmit power as access points and stations that are close to the limit of their range.

Transmit power control was designed to reduce WLAN interference with satellite services in the 5-GHz band. Other potential uses of TPC include range control and the reduction of power consumption. The elements that are associated with TPC are shown in TABLE 3.

TPC is utilized in the following manner: A station sends the minimum and maximum transmit-power information (the station's power capability) to the access point (FIG. 3 ). This step occurs during the association or reassociation attempt. The access point then determines if the station meets the local regulatory requirements (power constraint). It either approves or denies the request.

Next, the access point sends a request for transmit-power and link-margin information (transmit-power-control request) to the station. The station responds by sending the information (transmit-power-control report) to the access point. The station adapts its transmit power based upon input from the access point and/or other sources.

The WRC resolution and the ratification of IEEE 802.11h form a powerful union. This combination bodes well for the success of 802.11a Wi-Fi products. In fact, the two events are likely to facilitate the spread of Wi-Fi from the 2.4-GHz band to the 5-GHz band in the upcoming year. Consequently, 802.11a and tri-mode (802.11a/b/g) products should become more prevalent in the future.

To date, regulations regarding the use of the 5-GHz spectrum have not been globally harmonized. This fact has hampered the development of a "world-mode" product. With different portions of the spectrum approved for WLAN use in different countries, manufacturers have been faced with two options: Either provide products that meet each individual country's regulatory requirements or do not provide 802.11a products at all.

With the 802.11h amendment to the 802.11a PHY and MAC layer, interference mitigation becomes a reality in the 5-GHz band. Manufacturers are now poised to implement one standards-based solution to address interference concerns around the world. This solution is vastly superior to implementing multiple technologies.

The combination of harmonized-spectrum allocations and mitigation measures will allow for greater economies of scale in 802.11a and dual-band products. These cost savings will be passed on to consumers, making Wi-Fi even more affordable. Many companies will likely produce a "world-mode" product that will cover the 5.15-to-5.25-GHz; 5.25-to-5.35-GHz; 5.47-to-5.725-GHz; and 5.725-to-5.825-GHz channels with appropriate 802.11h interference-mitigation techniques. Such a product would fulfill numerous countries' regulatory requirements. Business travelers, in particular, will soon benefit from having a product that is capable of working in multiple regulatory domains worldwide.

To date, Wi-Fi has been tremendously successful. Thanks to harmonized spectrum and standards-based interference-mitigation measures, that popularity is about to extend to 5 GHz.

Intel Corp.
200 Mission College Blvd., P.O. Box 58119, Santa Clara, CA 95052-8119; (408) 765-8080, www.intel.com.

TAGS: Mobile
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