Newest Developments: Wireless Access Systems at 5 GHz
By Charles T. Glass

The challenge by Commerce Secretary Don Evans "to promote our country's economic growth while protecting national security and public safety," was recently met by NTIA, in cooperation with the Federal Communication Commission, the Department of Defense, and a number of industry representatives, when they completed a robust agreement to promote a new international allocation at 5 GHz.

"I am very pleased that the participants from the federal government and the private sector have reached a consensus that satisfies both of their interests," said Assistant Secretary of Commerce and NTIA Administrator Nancy J. Victory. "In achieving this, the Bush Administration has continued its goal of stimulating the economy and ensuring the national defense and preserving the leadership of the U.S. high-tech sector. Based on these changes, the United States is now able to formalize its position with respect to earth exploration satellite systems, mobile and radiolocation services at 5 GHz and will now fully support these allocations," Victory said.

This is one example of the major developments in 5 GHz Wireless Access Systems (WAS) in the United States and Europe during the last year. Wireless access systems are a class of devices that provide broadband communications, including Radio Local Access Networks (RLANS), that can lead to various high speed Internet and other networking applications between fixed and mobile terminals, either indoor or outdoor. Figure 1 presents the WAS concept. While there are similar devices already operating in many bands, such as 2.4 GHz, this article is limited to the proposed new operations in the 5 GHz band. For more information about WAS, see ITU-R Recommendation M.1450, "Characteristics of Broadband Radio Local Area Networks."

The ITU, under WRC-2003 Agenda Item 1.5, will consider a breakthrough spectrum-sharing technology, allowing a mobile allocation for WAS in the 5 GHz band where WAS will share with services that use radars. Currently, the U.S. rules for National Information Infrastructure devices allow WAS use of the 5 GHz band on a non-protected basis at 5150- 5350 and 5725-5825 MHz. In Europe, WAS has been authorized as a short range device called High Performance Radio LAN (Hiperlans) at 5150-5350 and 5470- 5725 MHz (see ERC DEC (99)23). WRC-2003, will allow 5 GHz to be used by WAS devices worldwide.

Most of the 5 GHz band is used on a primary basis by radiodetermination and the earth exploration satellite and space research (active) services; and on a secondary basis, by radiolocation as well as the amateur and amateur-satellite services. The radar operations at 5 GHz are vital. The radars perform a variety of important functions that require the unique spectrum propagation qualities at 5 GHz, including tracking of objects such as the space shuttle and weather rockets, national defense, navigation and ground mapping.

Recent technological developments made successful sharing possible between the existing allocated services and WAS in a large portion of the 5250-5825 MHz band. Spectrum sharing between the radars and WAS at 5 GHz can be accomplished by employing a new breakthrough technology known as dynamic frequency selection (DFS). DFS uses the same principle as listen-beforetransmit communications systems, but operates automatically and has a much faster response time. The NTIA was instrumental in developing and validating the DFS technique as a method to allow sharing. This was accomplished through analysis employing rigorous simulation over a two-month period. Without proper specific characteristics for DFS, however, sharing will still not be feasible.

The following values are required for DFS to allow successful spectrum sharing at 5 GHz:

  1. a DFS detection threshold of -64 dBm for WAS devices operating at a total effective isotropic radiated power (e.i.r.p.) of 200 milliwatts to 1 watt, and -62 dBm for devices operating at an e.i.r.p. of less than 200 milliwatts, measured over a period not to exceed 1 microsecond, as normalized to a 0 dBi gain antenna. These measurements must be accomplished during quiet periods between or within each WAS frame or packet;
  2. a channel non-occupancy period of 30 minutes to ensure that fixed radars will be protected for any channel in which the DFS detection threshold has been exceeded;
  3. a channel availability check time of 60 seconds upon initial startup or monitoring of WAS on any particular channel to ensure all radars present around a WAS are detected prior to it utilizing a channel; and
  4. a DFS Channel Move Time of no more than 10 seconds. DFS Channel Move Time is the period that WAS systems will have to move off of the channel once the DFS detection threshold has been exceeded. It takes an average of 200 milliseconds for all normal traffic to suspend and then intermittent control signals can continue for up to 10 seconds.

To ensure non-interference, administrations should authorize only those WAS devices meeting the aforementioned parameters to be operated in the 5 GHz band (5250-5350 MHz and 5470-5725 MHz). The ITU-R Draft New Recommendation on this subject is in the approval process (ITU-R Circular Letter 8/LCCE/120, see ).

These critically important spectrum sharing parameters should be given sufficient regulatory status by placing them into a WRC Resolution that is referenced by an allocation table footnote. The issues around this mobile allocation will be decided at the World Radiocommunication Conference in Geneva, Switzerland which meets June 9 to July 4, 2003.

Figure 1: Wireless Access System Concept
Figure 1: Wireless Access System Concept

Access Points (AP) - access points provide coverage of an area (cell). The operations to maintain proper sharing conditions is done at the cell level. The e.i.r.p. of the AP is dynamically variable depending on where the mobile terminal is located.

Backbone Network - the network that ties the Wireless Access System components together and connection to the internet.

Mobile Terminal - user devices, normally a wireless network card or imbedded wireless network chip, most operating at less than 50 milliwatts.

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