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Mobile telephony and health background information

Radio waves in mobile telephony

Mobile phone and base station transmission

A mobile phone network requires two communicating elements - the handset and the base station. The handset transmits radio waves to the base station, and these carry the -communication of the phone user. Similarly, the base station transmits radio waves to the mobile phone and these carry the communication of the person or service the phone user is connected to. The base station passes the signals to and from the phone network.

Mobile Phone Networks in the UK

Four mobile phone network providers have been operating in the UK since the 1990s using the digital Global System for Mobile (GSM) standard. These are known as second generation (2G) networks, as they followed on from the first generation analogue networks, which have been shut down.

The four existing 2G operators introduced the third generation (3G) of mobile phones systems using the Universal Mobile Telecommunications System (UMTS) standard across the UK in 2001 and were joined by a fifth operator deploying solely an additional 3G network. Other mobile phone companies provide what are termed “virtual networks”, in that they purchase services from the five network operators. All of the network operators are planning to roll out fourth generation (4G) networks to provide higher data streaming services. The UK network operators are:

*As of 2011 T-mobile and Orange have merged their UK networks to better utilise their infrastructure, forming an operator called Everything Everywhere (EE)

Cellular Radio

Base stations, mobile phones and cells

Each base station handles all of the mobile phone communications 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:

  • 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 centres

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 kilometres (km), the time taken for the signals to travel between the phones and GSM base stations becomes too great. 3G signals have an even shorter range due to propagation delay at higher frequencies.

Base stations also have limited data capacity in terms of how many mobile users they can accommodate at the same time. The number of transmitters installed on a base station determines the capacity and the number of phone users (and their data requirements) 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 base stations are generally 1-10 km in diameter in rural areas and a few hundred metres in diameter in urban areas. When a person using a mobile phone travels across the country, the radio communications are passed from one base station to another allowing continued communication. The radio signals never have to travel more than a few km.

Some areas of the country have additional layers of network coverage with macro cellular (macrocell) base stations having antennas mounted high up on masts above the buildings. Microcellular (microcell) base stations with antennas mounted much closer together and closer to street level give extra capacity to the network where the demand for phone use is high, for instance in locations such as airports, railway stations and shopping centres. Sometimes even smaller pico cellular (picocell) base stations are used to provide coverage inside particular buildings.

In some areas where data demands are very high, more than one base station are used to provide data streaming to the one mobile handset.

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. 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 field strength that occur every second. The unit of frequency is the Hertz (Hz).

Field strength and frequency of radio waves 

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 unit of power density is the watt per square metre (Wm-2).

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, 1800 and 2100 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.

 

Radio communications - typical frequencies


Last reviewed: 24 July 2013