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Mobile Phone Base Stations - How Mobile Phone Networks Work

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Introduction

This document is designed to give a very brief explanation of how today's mobile phone networks work. It explains what is meant by radio communication, and describes how mobile phone networks, using the cellular radio concept, operate. It also gives details of the technologies currently used by mobile phone networks in the UK.

The Radiocommunications Agency (RA) also produces two documents, 'Mobile Phones: Jargon Explained' and Frequently Asked Questions (FAQs), that will be helpful if you require more information on this subject.

Radiocommunication

Mobile phones may be a relatively new technology, but radio has been used as a means of communication for over a hundred years. Marconi made the very first radio transmission in 1895. Within thirty years radio was being used on a daily basis for broadcasting and for two-way radio communication by the military and the police. Today, a little over a hundred years since Marconi's first transmission, 60% of the UK population - around 40 million people - enjoy the benefits of mobile phone use.

What is a radio wave?

Mobile phones and their base stations transmit and receive signals using electromagnetic waves (also referred to as electromagnetic fields, or radio waves). Electromagnetic waves are emitted by many natural and man-made sources and play a very important part in our lives. We are warmed by the electromagnetic emissions of the sun and we see using the part of the electromagnetic spectrum that our eyes detect as visible light. All electromagnetic radiation consists of oscillating electric and magnetic fields and the frequency, which is the number of times per second at which the wave oscillates, determines their properties and the use that can be made of them. Frequencies are measured in hertz or Hz, where 1 Hz is one oscillation per second, 1 kHz a thousand, 1 MHz is a million, and 1 GHz, is a thousand million. Frequencies between 30 kHz and 300 GHz are widely used for telecommunication, including broadcast radio and television, and comprise the radio frequency band.

In the UK, AM radio uses frequencies between about 180 kHz and 1.6 MHz, FM radio ranges from 88 to 108 MHz, and TV ranges from 470 to 854 MHz. Cellular mobile services operate within the frequency ranges 872-960 MHz, 1710-1875 MHz and 1920 - 2170 MHz. Waves at higher frequencies but within the RF region, up to 60 GHz, are referred to as microwaves and have a wide variety of uses. These include radar, telecommunication links, satellite communications, weather observation and medical diathermy.

RA produces a Radio Frequency Allocation Information Sheet, which gives details of the types of services operating in any particular band. This can be obtained from our website.

How radio communication works

A radio frequency wave used for radio communication is referred to as a carrier wave. The radio frequency carrier wave of any system is produced by the transmitter as a sine wave. A sine wave conveys very little information since it simply repeats over and over. However, it can be switched on and off and this was the technique used in the earliest radio transmissions which used Morse code.

If the radio wave is to convey more information, such as speech or computer data etc., this information has to be added to the carrier wave in some way, a process known as modulation. The modulation process involves some feature of the carrier wave being varied in accordance with the information transmitted. For example, for AM (amplitude modulation) transmission, the electrical signal from a microphone produced by speech or music is used to vary the amplitude of the carrier wave, so that at any instant the size or amplitude of the Radio Frequency carrier wave is made proportional to the size of the electrical modulating signal. Figure 1 below demonstrates this concept.

Figure 1: Amplitude Modulation

Figure 1: Amplitude Modulation

There are many different types of modulation technique, each with different characteristics, and each suitable for different applications. You might be familiar with the frequency modulation (FM) used for radio broadcasting, or the digital techniques used by mobile phones. All work by varying some property of the carrier wave in a way by which the information to be communicated can be conveyed or carried by the radio frequency carrier wave.

Mobile Phone Networks

Base Stations and handsets

A mobile phone sends and receives information (voice messages, fax, computer data, etc) by radio communication. Radio frequency signals are transmitted from the phone to the nearest base station and incoming signals (carrying the speech from the person to whom the phone user is listening) are sent from the base station to the phone at a slightly different frequency. Base stations link mobile phones to the rest of the mobile and fixed phone network.

Once the signal reaches a base station it can be transmitted to the main telephone network, either by telephone cables or by higher frequency radio links between an antenna (e.g. dish) at the base station and another at a terminal connected to the main telephone network.

'Cellular' Radio

Each base station provides radio coverage to a geographical area known as a cell. Base stations are connected to one another by central switching centres, which track calls and transfer them as the caller moves from one cell to the next. Diagram 2 below shows the cell structure of a mobile phone network1. An ideal network may be envisaged as consisting of a mesh of hexagonal cells, each with a base station at its centre. The cells overlap at the edges to ensure the mobile phone users always remain within range of a base station. Without sufficient base stations in the right locations, mobile phones will not work.

The size of each cell depends on three factors. First, the local terrain; radio signals are blocked by trees, hills and buildings. Second, the frequency band in which the network operates (in general, the higher the radio frequency, the smaller the cell). Third, the capacity (i.e. number of calls) needed in any given area. Base stations are typically spaced about 0.2-0.5 km in towns and 2-5 km apart in the countryside.

If a person with a mobile phone starts to moves out of one cell and into another, the controlling network hands over communications to the adjacent base station.

Figure 2: 'Cellular' Radio

Figure 2: 'Cellular' Radio

Why are so many base stations required?

Transmitted signal strength falls off rapidly with distance from base stations, and mobile phones require a certain minimum signal strength to ensure adequate reception. The current generation of GSM base stations cannot communicate over distances greater than 35 km because the delay in receiving radio signals becomes too great. However, the decline of signal strength with distance places a practical limit on coverage of around 10 km. For these reasons an extensive network of base stations is needed to ensure coverage throughout the UK.

Why can't one base station serve my town?

Radio spectrum is a precious natural resource with many different demands upon it (for example, radio and TV broadcasting, emergency communication, navigation aids etc). Consequently the amount made available to each mobile phone operator is limited and this means base stations can only carry a limited number of calls at any one time.

To accommodate the steadily increasing volume of users, network operators have to use the limited number of radio frequencies licensed to them to support the maximum number of mobile phone users. This is achieved by re-using any given radio frequency many times in a network and carefully controlling base station power so that signals arising in different parts of the network do not interfere with each other. This concept of frequency re-use is illustrated in figure 3. The cells are grouped into clusters, with the frequencies allocated to a particular cell within a cluster not being re-used until the corresponding cell in adjacent clusters. This gives a repeating pattern of cells and clusters which can be expanded to provide national coverage.

To increase the capacity of their networks, operators have to build additional base stations and thus reduce cell size. It is for this reason that one large base station cannot serve a whole town.

Figure 3: Frequency Re-use

Figure 3: Frequency Re-use

Types of Mobile PhoneTechnology

Cellular radio networks operate in one of three bands in the UK; 900 MHz, 1800 MHz and 2.2 GHz ,using two different technologies, GSM and UMTS. A little detail on these two technologies is given below:

GSM

Global system for Mobile Communications or Groupe Speciale Mobile.

The international, pan-European operating standard for the current generation of digital cellular mobile communications. It enables mobile phones to be used across national boundaries. In the UK this technology operates in the 900 MHz and 1800 MHz frequency bands.

RA publishes details of the technical requirements for GSM technology operating in the UK in the form of UK Interface Requirement [IR2014]

UMTS

Universal Mobile Telecommunication System

The next generation of mobile phone technology, expected to result in widespread use of video phones and access to multimedia information. In the UK this technology operates in 2 GHz region.

RA publishes details of the technical requirements for UMTS technology operating in the UK in the form of UK Interface Requirement [IR2019]

1While cells are generally thought of as regular hexagons, making up a 'honeycomb' structure, in practice they are irregular due to site availability and topography.

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Public Telecommunication Networks Unit - April 2001

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