Published by Australian Academy of Science
KEY TEXT Mobile phones – communications on the go This topic is sponsored by the Australian Government's National Innovation Awareness Strategy.

You will get more from this topic if you have mastered the basics of electromagnetic radiation – these links will take you to an annotated list of sites with helpful background information.

The main difference between a standard telephone and a mobile phone is that a standard telephone converts the sound waves of your voice into electrical signals which are sent along telephone lines and a mobile phone converts the sound waves of your voice into radio waves which are transmitted through the air.

Both types of phones require a transmission network. Standard phones use a network of wires; mobile phones use a network of antennas.

Base stations connect mobile phones to the communication system

Base stations consist of a number of antennas that both transmit and receive signals from mobile phones. These antennas are mounted on towers, or on the sides or roofs of tall buildings. After receiving the signal from a mobile phone, the base station then transmits the signals to a 'switching centre' - a telephone exchange for mobile phones. Here the call is connected - 'switched' – either to another mobile phone or to the telephone network. (Although this sounds like a lengthy process, it is virtually instantaneous.)

Base stations are in 'cells'

A mobile phone system requires a number of base stations. Each base station sits in the middle of a geographical area known as a 'cell'. (Mobile phones are also known as 'cellular' phones.) The number of calls (or 'traffic') that a base station can handle at any one time is limited by engineering design constraints. In order to operate the mobile phone network as efficiently as possible, base stations are located so as to maximise the number of calls that can be connected during peak use periods. Therefore, the geographic size of a cell depends on the traffic during these periods. Cells in populated areas, with many mobile phone users, will be smaller than cells in less populated areas.

As you cross the boundary of one cell, the cell next to it will automatically take over. This is called a 'hand-over' or 'hand-on', and it is controlled (usually without you noticing) by a computer in the switching centre. The switching centre knows which cell your mobile is in, and switches it to the next cell if you move across a boundary.

Your call will continue to get 'handed on' to each cell in turn until you reach the edge of the last boundary. Then you will be out of range of any cells and your call won't be transmitted.

Mobile phones 'check in' regularly to base stations

An important thing to remember is that mobile phones cannot talk directly to each other without going through a base station first.

• When you switch on a mobile phone it will send out a coded message saying something like: 'I am here, is anybody listening?'

• If the phone is inside a cell boundary there will be a base station receiver in range, and it will send back a message saying: 'Yes I can hear you, tell me what your number is and I will let you know if any messages have come through for you.'

• When the base station at the centre of the cell has got the number of the mobile handset, it will then send out more messages. This time the base station will communicate (by radio waves or telecommunication cables) with a central computer in the switching centre and say: 'I have a mobile phone number in my cell right now, are there any messages for it?'

• If there is a message, the central computer will direct it through to the cell's base station transmitter, which will then broadcast it across its cell, where it can be picked up by the mobile phone.

• If there are no messages, the computer will keep a record of which cell the phone is in, and if a message arrives later it will know where to send it. When it is switched on, a mobile phone talks to cell base stations at regular intervals. This means the central computer should always know where to send messages.

Mobile phones in Australia used to operate on two systems, analogue and digital. The analogue Advanced Mobile Phone System (AMPS), was first established in 1987 and it formed the basis for the early expansion of mobile phone use across the nation. It used frequencies in the range 825-890 megahertz. Within this range of frequencies, there were 666 channels for receiving and 666 for transmitting.

The digital Global System for Mobile Telecommunications (GSM), which has been in use in Australia since 1993, uses the frequencies 890-960 megahertz. Within this range of frequencies, there are 124 channels for receiving and 124 for transmitting. (The digital system has the advantage that each channel can handle up to eight calls simultaneously.)

Dialling the future

In the early 1990s, the Australian government passed legislation ruling that GSM would take over from AMPS in the year 2000. However, because GSM has a much smaller range than AMPS, it is difficult for this system to provide adequate mobile phone coverage in country areas. Telstra upgraded its AMPS network to a new digital technology, Code Division Multiple Access (CDMA). CDMA uses the same frequencies as AMPS and can provide the same coverage to country areas but it offers the benefits of digital technology. There is some debate whether CDMA is an adequate replacement for the analogue system in regional Australia.

Another change on the horizon that could be important for those in remote areas is the launch of more telecommunications satellites. We may see some base stations replaced by 'low Earth orbit' satellites (LEOs), which will circle the Earth low enough to pick up your call. The LEO will then re-transmit your message just like a base station. This technology is not fully developed yet, but expect to see some more major advances in mobile phones before too long.

Mobile phones and health

Mobile phones are very useful and are growing in popularity. But some people are concerned that there is a possible health risk associated with mobile phone use. The mobile phone network uses radio waves (also called radiofrequency radiation), which is a part of the electromagnetic radiation spectrum. Some mobile phone users are concerned that having the phone's antenna close to their head exposes them to levels of radio waves that will increase the risk of brain cancer.

With the research that has been done so far, scientists generally agree that radio waves cannot cause cancer, but there is some debate about whether they can make an already existing cancer grow faster. However, as far as digital or analogue mobile phones are concerned, there is no clear evidence in the existing scientific literature that the use of these devices poses a long-term health hazard.

Research into these health issues continues around the world. The Australian government has provided about $4 million so that research on possible links between mobile phones and cancers may continue at a faster pace. At the international level, the World Health Organization has initiated an Electromagnetic Fields Project. The project, designed to standardise international research efforts and to increase our knowledge in this particular area, should be completed by 2001.

Precautionary options

Options to reduce exposure are available for mobile phone users who are concerned about the possible health risks. Minimising call times is obvious, but using an earphone/microphone cable attached to the mobile phone allows its use away from the user's head. Mobile phone covers that claim to shield the user's head are also available. Research to support this claim has not been published and phone manufacturers have stated that such covers inhibit the correct function of the phone.

CREDITS

Related Nova topic:

Wireless but not clueless

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