The 5GHz radio spectrum and licensing

Radio transmissions are, in general, regulated by national organisations, Ofcom in the UK, the CRTC in Canada, the FCC in the United States, and so forth. The rules vary to an extent and are coordinated in Europe with CEPT and internationally with the ITU. In Europe, for the most part, CEPT recommendations on the use of different frequencies are transformed into directives by the European Commission and then implemented in national law by the member states. In general, and where it can be helped, CEPT won’t produce recommendations that are at odds with ITU policies and close attention is paid to existing usage when new rules are being developed, as they were in the late 1990s and early 2000s for the 5GHz band, which is of particular interest for community wireless networks.

The whole process is quite complicated, and if you look at the Ofcom rules, they are pretty simple and easy to read, but they defer to the European rules and a host of standards from the ETSI and elsewhere, and pretty soon it is easy to get lost in thousands of pages of documents. This is an attempt to explain the relevant parts of the rules in a simple way and lay out what you need to know to build wireless networks out of readily available commodity hardware, with pointers to the official documents where the actual rules are laid out in excruciating detail.

The basic rule is, respect your neighbours which means, in particular, do not cause interference. This principle underlies most of the EC Directive 1999/5/EC which is concerned with making sure equipment certified for use in the EU does not cause harmful interference. To accomplish this, Ofcom uses the powers from the Wireless Telegraphy Act 1949 to sell licenses for the use of various bands. Some bands, like 5GHz, are shared between different services. Where a band is shared it typically has a primary and some secondary services. The secondary services must not interfere with the primary ones.

The image at right shows what the 5GHz spectrum looks like. It’s actually pretty busy with all kinds of radar services and such. The 802.11a or 802.11n parts of the band are described as “Radio LAN”. Radar transmits in short bursts and wanders all over the band. Because of this, it is feasible to use the band when there is no nearby radar actively transmitting. To facilitate this, there are some extra requirements that have to be met in order for the general public to use the band.

The first requirement is another of the general rules of radio operations. It’s pretty simple: only use the minimum power needed to establish and maintain communications. The particular arrangement for these bands is formalised and elaborated in the ETSI EN 302 502 standard where a Transmit Power Control (TPC) mechanism is described. In short, the equipment must automatically lower its transmit power to the minimum that is needed. If an 802.11a radio does not support this feature it should not be used in the UK (unless it is already very low power). If your radio has an option to enable or disable TPC, it must be enabled.

The next requirement is also a codification of another rule of radio ops: listen before you transmit. It arrangement is called Dynamic Frequency Selection (DFS). This means the radios listen for other transmissions and if necessary switch to a different channel to avoid causing interference. As with TPC, DFS must be supported by the equipment and must be enabled.

The Radio LAN sub-band is further divided into three parts, A, B and C. Different conditions apply to each of these. The A band is for indoor use only to avoid interference with the mobile satellite uplink service. It is also limited to a low power, 200mW, but does not require a license. The A band should not be used for outdoor backbone links.

The B band is more useful for community networks. There is a higher power limit of 1W. Some early documents suggest that it is only to be used for mobile or nomadic, however the current rules covering its use, IR 2006 makes no mention of nomadic operations and only says it may be used indoors or outdoors (IR 2006 also covers the A band).

The C band requires a license. This license permits installations at specific places and is not very expensive or difficult to obtain. The output power is still greater at up to 4W and there are also some restrictions on how point-to-multipoint or sectorised distribution antennas must be installed – roughly they must not be pointed upwards at an angle of greater than 15 degrees with respect to the tangent plane to the earth, or don’t point the antennas at aeroplanes. The conditions on use of the C band are laid out in IR 2007.

There is also a little notch carved out of the C band that is not available for use, between 5795 and 5815 MHz. This is reserved for road transport telematics.

The power limits are treated in a specific way, in terms of equivalent isotropically radiated power or (EIRP). This mouthful means that the transmitter and the antenna are treated together as one unit and the power is calculated as follows. The limit (say 4W for the C band) is for a theoretical antenna that radiates uniformly in all directions. If you replace that antenna by one, say, that has a gain of 3dBi (double) in one direction, and radiates nothing in the opposite direction, you are allowed to transmit half the power. This means you can’t put an 18dBi dish (64x) on a 4W transmitter in order to achieve the equivalent of a 250W transmitter in the direction you are interested in. That would be cheating.

Converting dBm / dBW to watts

Manufacturers of radios and antenna almost always helpfully quote the power output in terms of dBm (or dBW for larger radios). The regulations are in terms of watts so what to do? It is annoying, but the wikipedia page for EIRP explains it fairly well. If you know the power output of the radio, the cable loss (just say 0) and the gain of the antenna, when expressed in dB you can just add them up, \(P_E = P_T - L_c + G_a\).

So suppose you have a Ubiquiti Rocket M5 and you’ve maxed the power output to 27dBm and pluged into the theoretical ideal antenna. What is its power output in terms of watts? The gain of the ideal antenna is by definition 0, since it is the reference. So in this case, we have \(27 = 10 log (P_w) \) where \(P_w\) is the power output of the radio in mW. Rearanging, \(P_w = 10^{27/10} = 501 mW = 0.5 W\).

Now lets say we keep the power maxed and and connected it to a 34dBi dish. This adds up to a 61dBm signal or, doing the same arithmetic 1.25kW EIRP – well in excess of the regulatory limit!

What are the regulatory limits in terms of dBw? Doing the calculation in reverse for 200mW, 1W and 4W we get 23dBm for band A, 30dBm for band B and 36dBm for band C. So whatever you do, make sure the settings on your radio plus the gain of your antenna add up to less than those numbers!

Apply for a 5.8GHz Ofcom License

Application for a license to use band C is done by filling in a form and submitting it to Ofcom. Once they have replied and given you login details, as this is for fixed sites only you will have to provide the location of each radio (terminal), which includes client installations. The fee is £1/terminal/year with a minimum fee of £50.

Summary

• Do not transmit outside of bands where you are entitled to – this means only using band B and using bands B and C if you have a license to do so.

• Do be considerate to other band users by listening before transmitting and switching to another channel if necessary (DFS).

• Do use the minimum possible transmit power and use TPC to have the equipment automatically adjust.