One reason for the relatively slow uptake of 5GHz 802.11a Wi-Fi, compared to its 2.4GHz cousin 802.11g, has been the complexity of licensing regulations affecting the 5GHz band - also an issue for WiMAX in the unlicensed spectrum. Two extensions to the 802.11 standard are easing the situation and should boost the adoption of the 'a' variant. One is the 802.11j specification, ratified last week, which supports the Japanese opening of the 4.9GHz band for Wi-Fi; the other is 802.11h, already finalized but mandatory in many areas from the start of 2005, which defines mechanisms to prevent WLans interfering with radar.
The 802.11j extension has gained new importance since the US regulator relaxed the rules on devices allowed in the 4.9GHz band, which in the US is reserved for public safety. The new FCC rules mean that standard 802.11a products can be easily adapted for this lucrative market and 802.11j, which allows equipment to adapt between the 4.9GHz and 5GHz frequencies, will be equally applicable to this sector as to Japan.
The new 802.11j will pave the way to the development of products that shift intelligently between 4.9GHz and 5GHz frequencies, allowing travellers to and from Japan to use their equipment seamlessly. To this end, important components of 802.11j include the ability to change channel widths and dynamically modify radio capabilities.
According to the IEEE, the new spec "allows 802.11 networks to communicate and move to any new frequency, change the spectrum footprint to improve performance or user capacity, and communicate new rules and operating parameters to support both indoor and outdoor modes. Depending on the manufacturer, IEEE 802.11 products may be upgraded to use IEEE 802.11j features to take advantage of these new capabilities."
The ratification of 802.11j is expected to increase Japanese enterprise interest in Wi-Fi and give companies more confidence to adopt 802.11a. Some emergency services departments in the US are already trialling products using 802.11j, which will also be relevant to the US public safety band.
Atheros, which was one of the developers of 802.11j, was the first to announce supporting products, though Intel already offers pre-standard support in its chipsets.
Japan has always adopted a different approach to 5GHz from other countries. As well as its 4.9GHz decision, it is the only country to open bands for WLan at the lowest end of the 5GHz band (5.03-5.09GHz) but in 2003 had opened nothing above 5.25GHz.
The Japanese agency MPHPT is, however, keen to boost usage of wireless communications in unlicensed space. Last year it laid down policy to stimulate uptake of WLans and, in particular, fixed wireless. These include a new spectrum refarming policy, allowing for streamlined reallocation of frequencies to broadband wireless and WLan, with compensation for former holders. It is also planning a simplified registration scheme for WLan operators. In 2003, Japan agreed to open up the 5.25-5.35GHz frequencies but is still considering whether to come into line with international guidelines above 5.47GHz. It expects to open up the 5.47-5.725GHz bands progressively over the 2005-2008 period but has no plans for the so-called Upper Band above 5.725GHz.
Japan allocates spectrum specifically for broadband fixed wireless at 5GHz, unlike the US, and also aims to finalize rules on coexistence of WLan and fixed wireless in the middle bands by the end of 2005. For instance, WLans may be restricted, in the 5.25-5.35GHz band, to indoor use to leave the outdoor applications to fixed wireless.
802.11h products appear
Another important Wi-Fi extension, 802.11h, is already in place to comply with European regulations regarding 5GHz and, in particular, to prevent Wi-Fi devices interfering with the incumbent users of the band, military and other radar. This will become mandatory in Europe and the US at the start of next year and a rush of new products is likely to appear soon.
Early into the fray is Austria-based intellectual property company NewLogic, which has announced an implementation that it claims enhances on the base 802.11h specification and should appeal to US authorities, which have yet to set guidelines on radar avoidance (though they will mandate 802.11h as a base from 2005). NewLogic, a supplier of reference designs and cores for WLan chips, says its latest platform combines all three Wi-Fi variants, modems, radio and an enhanced radar detection algorithm in one Cmos chipset. The algorithm is compliant with 802.11h but improves on its basic functionality, claims the company, which also supports the 802.11i and 802.11e extensions in its design.
It says that it is difficult to detect radar pulses in the way necessary to support 802.11h. "The biggest issue is not to detect radar but to avoid false detections," the company explains. This is important since, once a detection is made, the channel must be blocked to WLan transmissions for at last half an hour. If the false alarm rate is high, performance will be severely degraded. NewLogic claims an exceptionally low false alarm rate, while detecting radar pulses with 99% probability, by using additional processing concepts in addition to the standard method of measuring energy levels of received pulses to determine the proximity of radar.
Marketing manager Mana Coste said that low false alarm rates are "a performance parameter that system vendors must not underestimate in the selection of a WLan solution, at the risk of finding themselves with a compliant solution, but with unhappy customers and a high return rate of their equipment."
Support for 802.11h will be required in all 802.11a-compliant systems by the beginning of 2005 in Europe and will also be applicable to the US in order to avoid interference with military radar, even in the absence of formal regulation.
The 802.11h standard defines mechanisms that allow 802.11a devices to comply with the ITU Recommendation M.165 on interference avoidance. Both specifications are based on two agile radio techniques for reducing interference, Dynamic Frequency Selection (DFS) and Transmit Power Control (TPC). The main significance of 802.11h is that it enables vendors to create a 'world mode' 802.11a product that addresses interference rules in all major markets.
Similar mechanisms will be devised for 802.16. While the main objective is to avoid conflict with incumbent users such as the military, the 802.11h standard and its 802.16 equivalent will also help reduce interference between all unlicensed and/or secondary use equipment.
Some countries, such as the UK and Hong Kong, and probably Japan, allow interim arrangements for avoiding interference until full DFS/802.11h is implemented, in order not to stall the market, but products incorporating 802.11h will be fundamental to global unification of 802.11a.
Some products supporting 802.11h are already available and they will be mainstream in early 2005. DFS and TCP support are mandatory in Europe and North America and most other countries are bringing in similar rules following WRC 2003.
DFS detects other devices using the same radio channel and switches WLan operation to another channel if necessary. It handles interference avoidance with other devices, such as radar or other WLans, and uniform usage of channels. A Wi-Fi access point (AP) specifies that it uses DFS in the frames used by WLan devices to find APs. When the device associates with that AP, a list of channels that it can support is provided, and the AP uses this data to determine the best current channel. The list is drawn from the APs measuring channel activity to see whether there is other radio traffic in its channels.
TPC reduces interference by reducing the radio transmit power that WLan devices use. TPC can also be used to manage power consumption of wireless devices and the range of access points. Maximum power limits are set within the access point and are imposed on any device that associates with it. The AP controls the maximum power limit for its whole segment by tracking the transmit power capability of each connected station. Radio power in the segment can be adjusted to reduce interference while maintaining adequate WLan link margins.
As a by-product, DFS and TCP should stimulate interest in 802.11a because both mechanisms can make the management and operation of WLans more efficient.
International 5GHz spectrum policy
There are still anomalies and inconsistencies in 5GHz policy round the world that are deterrents to 802.11a vendors and adopters. For vendors, there is a need for more flexible radios - on the lines of 802.11j - to enable them to develop a single product that can adapt to variations of licensing law in different countries. Otherwise they are denied the economies of scale that come from creating one product for all markets - economies that are important in the price sensitive world of Wi-Fi.
International spectrum policy for 5GHz was not harmonized until the World Radio Congress of 2003 (WRC-03), which established international guidelines, but not all countries have yet adopted these, and others, including Japan, have retained their own variations. WRC-03 allocated the 5.47-5.725GHz band for WLans, following European precedents for HiperLan, in addition to the 5.15-5.35GHz frequencies, a total of 455MHz of new global spectrum for WLans.
This established a common set of rules that mean most countries, which have all opened different sections of this swathe of spectrum, will unify policy and open the whole range by 2007. Some countries will also have further bands, notably Japan in 4.9-5.0GHz, and the US, which has reserved 75MHz of spectrum in 5.85-5.925GHz for its Dedicated Short Range Communications Service, aimed at roadside and vehicular communications and to be based on 802.11a. However, the Wi-Fi Alliance may not certify 5GHz devices below 5.15GHz or above 5.85GHz, at least until after 2005.
The chief remaining area of confusion for Wi-Fi and WiMAX is in the so-called Upper Band, at 5.725-5.85GHz. This is allocated by the International Telecommunications Union to ISM (industrial, scientific and medical) but is also open in many countries for WLans, though not officially part of the ITU allocation for WLan.
This Upper Band is not uniformly available across Europe or in Japan and European nations will roll it out at very different and often unspecified paces, while Japan has not made a final decision on opening it up at all. This upper band is mainly relevant to outdoor 802.11a systems delivering hotzone or broadband wireless access services, and to 802.16.
The Wi-Fi vendors are also likely to stay clear of the Upper Band because of its overlap with 802.16, unless they are specifically creating long distance, outdoor base stations. This Upper Band has a higher 4W EIRP, which has not been verified in all countries (for instance, India, parts of Europe, Korea).
This band is allocated to fixed wireless as the primary service in India, Korea, Australia, Sri Lanka, Malaysia, Japan, Thailand and Vietnam and so is likely to be dominated by WiMAX rather than Wi-Fi. Higher powered, point-to-multipoint devices, notably those conforming to the 802.16 standard (WiMAX), will mainly be allowed to operate in unlicensed 5GHz space in the ISM band (5.725-5.850GHz). Here it can coexist with 802.11a. In some countries where the lower parts of the 5GHz band are open too, 802.11a may be kept out of the ISM band.
Copyright © 2004, Wireless Watch
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