Radio Waves in Mobile Telephony
A mobile phone network operates with two communicating elements – the handset and the base station. The mobile phone handset transmits radio waves to the base station, and these carry the voice of the phone user. Similarly, the base station transmits radio waves to the mobile phone, and these carry the voice of the person the phone user is listening to. The base station passes the signals to and from the phone network.
Mobile Phone Networks in the UK
Four mobile phone networks have been operating in the UK since the early to mid-1990s using the digital Global System for Mobile (GSM) operating system. These are known as second-generation (2G) systems, as they followed on from the first-generation analog networks, which have now been shut down.
A third generation of mobile phone systems (3G) is becoming available in the UK. The four existing 2G operators are rolling out new 3G networks across the country and there is also a new fifth operator deploying a network. The UK network operators are:
- O2 (formerly BT Cellnet),
- Vodafone,
- T-mobile (formerly One2One),
- Orange, and
- Hutchison 3G.
Cellular Radio
Each base station handles all of the mobile phone calls taking place in a region around it known as a cell. Cells are sometimes thought of as regular hexagons making up a ‘honeycomb’ structure; however, their shapes are irregular in practice because:
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- Features of the local terrain, such as trees, hills, and buildings, can block or weaken the radio signals
- Network operators cannot always put masts where they want to because suitable sites are not available
- Smaller cells are needed where there is a greater density of users, e.g., in city centers
The technology places limits on the maximum coverage area for base stations because the radio signals can only travel so far before they become too weak to be received. Using higher transmit powers lets the radio signals travel further, but beyond around 35 km, the time taken for the signals to travel between the phones and GSM base stations becomes too great.
Base stations also have limited capacity in terms of how many mobile phone calls they can handle at the same time. The number of transmitters installed on a base station determines the capacity, and the number of phone users in its cell determines the level of demand. Adding extra transmitters gives more capacity to a base station, but there is a limit to how many can be used.
Given the above constraints, cells for GSM base stations are generally 1-10 km in diameter in rural areas and a few hundred meters in diameter in urban areas. When a person using a mobile phone travels across the country, radio communications are passed from one base station to another, allowing continued communication. The radio signals never have to travel more than a few kilometers.
Some areas of the country have two layers of network coverage, with macrocellular base stations having antennas mounted high up on masts above the buildings and microcellular base stations having antennas mounted much closer together and closer to the street level to give extra capacity to the network where the demand for phone use is high.
Microcellular base stations are also used to enhance capacity in locations such as airports, railway stations, and shopping centers. Sometimes picocellular base stations are used to provide coverage inside particular buildings.
Frequencies used for Mobile Telephony
There are two quantities used to describe simple radio signals and these are the frequency and the field strength. Radio receivers tune in to signals according to their frequency, and signals must have sufficient field strength for them to be received. The field strength can be specified either as the electric field strength or the magnetic field strength in the wave.
The field strength of radio waves oscillates in time, as shown in the following graph, and the frequency is the number of cycles in a field strength that occur every second.
When considering people’s exposure to radio waves from base stations, it is more usual to consider the power density than the field strength because this is more closely related to the quantities in which restrictions on exposure are specified. Power density is proportional to the average value of the field strength squared.
The use of different frequencies for different radio signals ensures they do not interfere with each other. The mobile phone networks in the UK use frequencies close to 900 MHz, 1800 MHz, and 2000 MHz, where the unit MHz (megahertz) is a million cycles per second. These frequencies are above those used for broadcast television and radio and below those used for microwave communications links. Typical frequencies used for radio communications are shown in the diagram below.
What Health Concerns?
The increasing use of mobile phones has been accompanied by concerns about possible harmful effects on health arising not only from exposure to the radio waves that are produced by the phones but also from the base stations that serve the phones. It was in order to respond to such concerns that the Government asked the Chairman of NRPB to set up the Independent Expert Group on Mobile Phones (IEGMP) in 1999. The Group was chaired by Sir William Stewart FRS FRSE and published its report in May 2000.
IEGMP carried out a wide-ranging investigation not only focussing on the relevant areas of science but also considering the planning process and regulatory matters in respect of people’s exposure. Accordingly, it made recommendations aimed toward Government, Industry, and the NRPB, as well as on research requirements.
In addition to IEGMP, other expert groups have reviewed the science relating to mobile telephony and health. Individuals have also expressed their opinions on science, and these sometimes differ from the consensus viewpoint. As part of IEGMP’s work program, many individual scientists were invited to give oral evidence, and all citizens were free to submit written evidence.
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An update on progress on mobile telephony and health subsequent to the publication of the IEGMP report was published by Stather (2003) in the Radiological Protection Bulletin.
