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by W.A. SteerááPhD
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GSM phone signal analysis

After coming by some new information, my curiosity was aroused with regard to what exactly a digital GSM mobile phone transmits. This document summarises what I found out, and actually measured.

Background

Contemporary 'digital' mobile phones in Europe operate using the GSM (Global Systems Mobile) system. There are two frequency bands allocated to GSM mobile phones, one at 900MHz, and one at 1800MHz. GSM uses a combination of frequency division multiple access (FDMA) and time division multiple access (TDMA). What this means in reality is that within each band there are a hundred or so available carrier frequencies on 200kHz spacing (the FDMA bit), and each carrier is broken up into time-slots so as to support 8 separate conversations (the TDMA bit). Correspondingly, the handset transmission is pulsed with a duty cycle of 1:8; and the average power is one eighth of the peak power. Once a call is in progress, the phones are designed to reduce the radiofrequency (RF) output power to the minimum required for reliable communication - under optimum conditions, the power can be set as low as 20mW (one hundredth of full power). Battery consumption and radiation output of the handset is further reduced by using 'discontinuous transmission' (DTX); the phone transmits very much less data during pauses in the conversation.

Base station transmitHandset transmitPeak handset power
used in the UK primarily by
GSM900
935-960MHz
890-915MHz
2 watts
BT Cellnet and Vodafone
GSM1800 (PCN)
1805-1880MHz
1710-1785MHz
1 watt
Orange and One2one

The basic handset transmission consists of carrier bursts of 0.577ms duration, repeating every 4.615ms, giving a repetition rate of 216.7Hz. The data is encoded using a phase modulation scheme known as 'Gaussian Minimum Shift Keying' (GMSK) and a symbol duration of about 4Ás, which doesn't affect the carrier amplitude. Owing to the coding and control protocols, every 26th pulse is omitted during a conversation, leading to a component in the output modulation at 8.33Hertz.

Experimental

I rigged up a simple circuit (crystal-set type non-tuned diode demodulator) and recorded the signal amplitude using a standard PC soundcard line-in socket.

The figure below represents the amplitude of the a GSM mobile phone (Vodafone) transmission on reception of an incoming call.

The interpretation of the call progress is my own, based on limited knowledge. Note how the transmitter power is reduced once the call has been set up and the conversation begins. Note also the 'gaps' during the silence in the conversation, where the transmitted signal returns to a 'holding' signal -- which looks superficially similar to the signal during ringing.

The slow and smooth variation in amplitude is caused by moving the phone relative to my receiver during the measurement, and is not significant!


This represents a section of the voice transmission, the timescale of the whole plot being a little over 1 second.


Expanding the timescale further, the detail of the pulse structure is revealed. The pulses come at 4.62ámillisecond intervals (approx. 217Hz frequency), each lasting 0.57ámilliseconds. This gives a mark:space ratio of 1:7, allowing up to eight calls to be time-multiplexed (TDMA) onto the same carrier frequency. Every 26th pulse is omitted, causing an 8.3Hz periodicity in the signal.

The plot below shows some detail of the effect of the discontinuous transmission.

Possible health issues

I'm trying to continue to 'sit on the fence', and not give an opinion on the possible health-implications of mobile phones. The general consensus of the scientific community and the relevant radiation- protection bodies is that there is no significant evidence of a health risk from mobile phones. Nevertheless, some people still claim to suffer headaches and other symptoms which they blame on their phone. Long term effects, of course, can only be observed after a long time. The official line is basically that they are safe, but some caution wouldn't go amiss. The emitted RF energy will be much reduced if you use the phone in a good signal area (eg line of sight to the base-station), whereas use in a poor signal area like inside a lift (elevator) or a tunnel will result in the phone using a much higher power.

You might want to consider the following points:

Further reading


Created: February 2001
Last modified: 23 March 2002

Source: http://www.techmind.org/gsm/

©2001 William Andrew Steer
andrew@techmind.org