Final Report for Department for Business, Innovation and Skills and Department for Culture, Media and Sport
Radio spectrum: what is it, how is it used and what are the constraints?
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- Navigate this page:
- What is radio spectrum
- The ‘sweet spot’ for commercial uses of spectrum
- Analogue and digital transmission
- One-way and two-way transmission
- Managing spectrum use in the UK
- Dealing with the problem of interference
Radio spectrum: what is it, how is it used and what are the constraints?Mobile handsets, TVs, radios, Wi-Fi devices and other wireless communications equipment all rely on radio spectrum. This section explains: what radio spectrum is how information is transmitted – including the benefits of moving from analogue to digital the difference between one-way transmission (e.g. TV) and two-way transmission (e.g. mobile communications) the constraints around the use of spectrum, including the problems of interference. What is radio spectrum?Radio waves are a form of electromagnetic radiation with a wavelength that is longer than visible light. They travel at the speed of light, which is approximately 300 000km per second (186 000 miles per second) in free space. Radio waves can be grouped according to the length of the wave, and so may be referred to as ‘medium wave’ or ‘long wave’ radio. The higher the frequency, the shorter the wavelength (since frequency multiplied by wavelength equals the speed of light). The frequency of the wave cycle is measured in hertz (Hz), where one hertz equals one cycle per second. The usable part of the radio spectrum extends from around 3kHz (3000 hertz, equivalent to a wavelength of around 100km) to 300GHz (300 billion hertz, equivalent to a wavelength of around 1mm) The radio spectrum is conventionally divided into eight frequency bands, starting with Very Low Frequency (VLF), which extends from 3kHz to 30kHz, and ending with Extra High Frequency (EHF) at 30GHz to 300GHz (see Figure 2 .1). Each of these eight successive bands contains ten times as much spectrum as the one immediately below it. This is the fundamental reason why the low-frequency bands can only be used to support relatively low-bandwidth applications. Figure 2.1: Spectrum usage [Source: Analysys Mason, 2012, based on an earlier version by Ofcom and PwC] 
The ‘sweet spot’ for commercial uses of spectrumSpectrum can only be put to commercial use if certain prerequisites are met. The economics of a mobile telecoms network, for example, are heavily dependent on the distance that the signal can travel. Lower-frequency signals will fare better than higher-frequency signals over longer distances, and in addition will give superior indoor reception. This gives lower-frequency spectrum some attractive characteristics, but also implies a number of other disadvantages (such as a need for larger antennas). These characteristics, coupled with the shortage of bandwidth in certain bands, have led to the emergence of a ‘sweet spot’ for many commercial applications, which broadly coincides with the Ultra High Frequency (UHF) band, i.e. 300MHz to 3GHz. Radio signals in this frequency range have a typical range of a few kilometres to a few tens of kilometres, work well indoors (particularly at the low end of the band) and require an antenna size that can be readily incorporated into handheld equipment. Within the UHF band, sub-1GHz spectrum is generally more highly prized than spectrum in the 1–3GHz range because it has an appreciably longer range (meaning that fewer base stations are required to cover a given area) and penetrates buildings better (making it suitable for indoor coverage).
Analogue and digital transmissionRadio waves can be used to transmit information in analogue or digital form. Medium wave radio is typically broadcast as analogue transmission, in which the amplitude of the carrier signal is varied continuously so that it replicates the amplitude of the original sound wave. In digital communication the information to be transmitted is coded as a series of binary digits before being transmitted. Generally speaking, digital transmission makes much more efficient use of radio spectrum. For example, in the case of TV, an analogue system would require around eight times as much spectrum as a digital system to transmit an equivalent signal. The transition from analogue to digital broadcasting in terrestrial TV not only allows more channels to be transmitted than before (including some HD channels, each of which requires four to five times the bandwidth of a standard-definition channel), but also allows more spectrum to be made available for mobile communications (the so-called ‘digital dividend’). One-way and two-way transmissionSome applications of spectrum only require information to be sent in one direction (e.g. radio and TV broadcasting) while other applications require two-way communication (e.g. mobile telecoms). Generally speaking, a particular frequency can only be used to send information in one direction at a time, otherwise the signals interfere with each other. In a walkie-talkie this is accommodated by using a single channel alternately for transmission and reception, with a push-to-talk button on each user terminal. However, for mobile telecoms (and a variety of other applications) it is desirable to have simultaneous two-way (duplex) communication. The traditional way of enabling duplex communications, dating back to the time when most radio communications systems were analogue, is to use two different frequencies. This is called frequency division duplexing. To achieve this, spectrum is typically allocated as a paired block of spectrum comprising two separate bands (usually of equal size). One band is designated for transmission from the base station to the terminal, while the other is designated for transmission from the terminal to the base station. This arrangement is efficient when roughly equal amounts of information need to be transmitted in each direction (as in a voice call), but it does not make very efficient use of spectrum if the information flow is highly asymmetrical (as it typically is when someone is browsing the web or streaming audio or video content). However, with modern digital electronics it is also possible to divide a single radio channel into a large number of short time slots, used alternately for transmitting and receiving. This approach, known as time-division duplexing, avoids the need for paired spectrum. Moreover, many standards for time-division duplex communications allow the amount of capacity in each direction of transmission to be altered by varying the proportion of timeslots allocated to transmit and receive, which improves the efficiency of spectrum use in the case of asymmetric information flows. Wi-Fi is a widely used application of time-division duplex technology. Today, for legacy reasons, paired spectrum still tends to be more valuable than unpaired spectrum. For example, in recent 4G auctions in Europe, 2×5MHz of paired spectrum has typically sold for more at auction than 10MHz of unpaired spectrum, although this is by no means a universal trend, and may change in future based on changing demands for spectrum and increasing use of asymmetric data, for which the use of unpaired spectrum offers some benefits. Managing spectrum use in the UKThe civilian use of spectrum in the UK is managed by Ofcom, which is responsible for issuing licences (and making certain bands available on a licence-exempt basis), developing policies to ensure that spectrum is used efficiently, and consulting on new uses of spectrum. In the licensed bands, such as those used by broadcasters and mobile network operators, the licences specify how the spectrum can be used – for example, how much power can be used for transmissions. Licence-exempt bands – such as those used for Wi-Fi – can be used by anybody, provided that the equipment used meets a specific standard designed to ensure that it does not cause interference to other users of the same or neighbouring spectrum. There are also various other regulatory models for spectrum access, falling somewhere between licensed and licence-exempt, and typically involving multiple users being granted concurrent rights of use in a given frequency band, with obligations on systems to self-coordinate. Examples of these in the UK include a small block of 1800MHz spectrum (referred to as the ‘DECT guard band’) and spectrum in the 5.8GHz range. In future, we expect to see a greater use of shared methods of spectrum access in certain bands, including innovative models for sharing spectrum between licensed and licence-exempt users. This matter is discussed later in this report.
Dealing with the problem of interferenceRadio transmitters operating in identical or adjacent frequencies will cause and suffer from interference. In analogue communications systems this may result in the sound becoming distorted or another channel breaking through; while in digital data communications it can cause data packets to become corrupted, resulting in the need for re-transmission which reduces the effective throughput of the link. In radar systems interference may prevent the objects from being detected or located with precision. It is for this reason that the majority of the radio spectrum in the UK and other countries is licensed to specific organisations for specific purposes, and often in specific locations. This is also the reason why radio licences usually specify technical parameters such as the maximum power that can be transmitted. In the UK it is Ofcom that has responsibility for monitoring the airwaves to identify cases of interference and take action against illegal broadcasters and the use of unauthorised wireless devices. Licence-exempt spectrum is open for general use, and is constrained by a relatively low maximum power threshold which limits the range of transmission and thus reduces the potential for interference. In addition, the standards developed for licence-exempt equipment are designed to mitigate the effects of interference by, for example, making devices able to ‘hop’ between a number of different frequencies within the permitted band. For licensed applications, interference mitigation techniques are also becoming increasingly sophisticated. For example, first- and second-generation (2G) public mobile networks minimised interference by using different frequencies in adjacent cells, but third-generation (3G) networks use the same frequency in all cells, relying instead on advanced digital signal processing techniques to maintain optimal transmission power levels at all times and to distinguish between data and background noise. In 4G systems, there are options to use the same frequency in all cells or different frequencies in adjacent cells. The ability to use the same spectrum in adjacent cells is one factor that has contributed to an increase in the spectral efficiency of public mobile networks, i.e. the amount of information that they can carry in a channel of specified bandwidth.
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