New rule promotes automated frequency-shifting methods to prevent spectrum clashes with commercial, government users.
A recently adopted international standard protects military radar and scientific satellite transmissions against potential interference from wireless local area networks. It provides a toolkit and guidelines for manufacturers to modify their products to switch automatically to alternate channels when these signals are detected.
Preventing conflicts between civilian and government communications and navigation systems is an increasingly important and complex issue as commercial wireless devices become more capable of roaming across national borders. Regulatory bodies are weaving a growing web of rules and protocols to stay abreast of this constantly changing situation.
One such effort is being promulgated by the Institute of Electrical and Electronics Engineers (IEEE) Standards Association, an international standards-setting group for industry based in Piscataway, New Jersey. In September, it approved an amendment to its 802.11a wireless local area network (WLAN) standard that prevents these networks from obstructing radar, Earth exploration satellite service (EESS) and space research service (SRS) transmissions.
The amendment, 802.11h, calls for WLANs and other devices to detect the presence of radars and EESS and SRS systems and to protect them from interference by switching to an unused channel or reducing transmission power. IEEE 802.11h creates a common toolkit for avoiding spectrum conflicts that permits manufacturers to produce compliant equipment that can interoperate with similar products from other suppliers.
According to Carl R. Stevenson, chair of the 802.18 Radio Regulatory Technical Advisory Group, Emmaus, Pennsylvania, the proposal’s genesis came from European spectrum management initiatives. Speaking as a private expert, Stevenson explained that the European Radio Commission (ERC) issued a decision called ERC (99)23. In it, member nations agreed to allocate two areas of spectrum for high-performance WLANs in the 5150- to 5350-megahertz and 5470- to 5725-megahertz ranges.
The ERC recognized that a globally harmonized spectrum allocation in the 5-gigahertz band for wireless systems would benefit both the industry and the user communities, Stevenson explains. A major factor in this decision was the perceived benefit derived from the economies of scale in building globally acceptable products that can roam freely across domains. “If I go to Europe, I’d like my wireless LAN to work with the infrastructure in my company’s offices there. But in order to do so, it has to be acceptable in the regulatory sense. In other words, it has to be legal for whatever is in my notebook computer to operate there and to interoperate with the existing infrastructure,” he says.
Because globally harmonized allocations and regulations facilitate free international wireless roaming, the capability was a major item on the agenda at this summer’s World Radiocommunication Conference (WRC) in Geneva, Switzerland. There it was adopted and immediately became a part of International Telecommunications Union (ITU) regulations. “In some sense, it’s the globalization of what the Europeans had already decided to do on a regional basis. But it did make a lot of sense on a worldwide basis,” Stevenson says.
But these mobile systems cannot impede other devices or networks. To prevent interference problems, 802.11h highlights two mitigation technologies, dynamic frequency selection (DFS) and transmit power control (TPC). The standard and the feature allowing users to apply it in an interoperable fashion were important issues at the WRC, he says. The ITU recommended a capability for WLANs to monitor their local electromagnetic environment and to react accordingly to protect incumbent services.
These suggestions became the 802.11h toolkit that permits manufacturers to modify their products. If a compliant WLAN network cell detects a radar signal, it will immediately stop operating on that frequency and switch to another channel. He explains that the goal is to avoid the potential for the combined energy from a large number of WLAN devices to interfere with radar systems by raising the electromagnetic noise floor.
Stevenson notes that the DFS and TPC systems were originally conceived within the 802 community as a means of preventing interference between adjacent WLAN cells. It became apparent that a radar detection function would have to be created to preclude any potential interference and gain sharing in the frequency bands, he adds.
Although he knows of no recorded instances of interference, Stevenson explains that ITU compatibility studies recognized the potential for large numbers of WLAN devices to limit the range of some radars in ways their operators found unacceptable. The studies identified DFS as the most mature mitigation technique available for WLAN systems. He offers that, while it is the first such method available, it is not the only mitigation technique that may be used to protect radar and other systems.
A great deal of activity also took place in the United States prior to this summer’s WRC. The U.S. Defense Department and business groups reconciled the radar interference issues in the 5-gigahertz band after commercial firms indicated they wanted to use more of the band for WLANs. The government was concerned about the effect large numbers of wireless networks would have on its military radar systems. But the issue was not a contentious one, Stevenson explains. Industry and the U.S. National Telecommunications and Information Administration conducted joint simulation studies to develop the parameters now specified in the ITU regulations.
The WRC resolution designated 455 megahertz of spectrum in the 5-gigahertz band for WLAN use with restrictions to protect radars and other services. Another issue was restrictions on the outdoor use of the 5250- to 5350-megahertz band. European delegates wanted to shield sensitive systems such as EESS. A compromise was reached by promoting WLAN use primarily for indoor purposes in Europe in the 5150-to 5350-megahertz band, while the U.S. secured and upgraded a secondary radar allocation in the 5350- to 5650-megahertz range to primary status and gained new allocations for Earth exploration and space sciences systems (SIGNAL, September, page 71).
Although the international community has agreed to globally harmonized allocations at 5 gigahertz for WLANs, this does not immediately change existing U.S. Federal Communications Commission (FCC) rules in any way, Stevenson cautions. The FCC currently has a notice of proposed rule making on the matter that is out for comment. He adds that the notice was issued before the WRC met in July to jumpstart the process in the United States. The United States already uses access to the 5150- to 5350-megahertz band but not the 5470- to 5725-megahertz band. “The FCC’s notice of proposed rule making seeks to add the 5470- to 5725-megahertz band into its unlicensed national information infrastructure rules,” he says.
Stevenson notes that the 802.11h standard is mature—to the point where vendors such as WLAN chip and equipment manufacturers can modify their devices to comply with the new standard before it passes through the FCC’s comment resolution process.
The subject of cognitive radio also is on the horizon, foreshadowed by techniques such as DFS. Stevenson notes that the U.S. Defense Advanced Research Projects Agency is working on cognitive radio technologies through its Next Generation Communication program (SIGNAL, page 35). “I think it’s reasonable to state that there is interest within industry in cognitive radio, but there currently is not an active task group in IEEE 802 developing a standard for cognitive radio,” he says.
The technology is still not mature enough for serious regulatory action. Stevenson observes that the FCC currently has issued only notices of inquiry based on its Spectrum Policy Task Force report. However, he is confident that cognitive radio is within the reach of today’s technology.
Additional information on IEEE standards is available on the World Wide Web at http://www.standards.ieee.org.