Thursday, May 27, 2010

GSM 217Hz TDMA noise

Bring a GSM cellphone near some low grade audio equipment and you can hear some very irritating noises. GSM or Global System for Mobiles uses FDMA, TDMA and SDMA for providing network access to multiple users. This each achieves as follows: Base Transceiver Stations within Cells (a fundamental unit of geographical areacovered by the GSM service provider) surrounding a given cell will all use frequencies different from each other (Frequency Division Multiple Access). The maximum amount of power transmitted by BTS as well as mobile units within a given cell is limited and so cells separated by a considerable geographical area can both reuse the same frequency for transmission (Space Division Multiple Access). And finally within a given cell, mobile units will use Time Division Multiple Access so that man users can be served by the same Base Transceiver Station. Of course different frequencies are used for uplink and downlink. Depending on the country, GSM operates in 800MHz to 900MHz or 1800MHz to 1900MHz bands.

With regards to TDMA, a GSM multiframe lasts 120 milliseconds consists of 26 frames. each of these frames itself last 4.61538 milliseconds and have 8 timeslots each. During a simple conversation, each of these is allotted to a different user - so a cell phone (in an active call) sends a burst of data every 4.61538 milliseconds which results in a 217 Hertz pulse waveform which lies in the audible range and gets coupled into unshielded audio circuits operating in the vicinity of the cell phone.

Of course if you bring an oscilloscope probe near a cellphone and make a call, you can observe these bursts too:



The horizontal scale is 20 milliseconds, you can note that the time between two bursts is approx 4.6 milliseconds. The above waveforms have been captured on a Tektronix TDS3044B Digital Phosphor Oscilloscope with 350MHz bandwidth. While I was trying to get a proper waveform for the 217Hz noise, I ended up capturing some vivid patterns on the scope. These too were captured while the cellphone was on an active call. Note that these waveforms have been improperly captured and cannot be used to deduce much useful information about the signal:





I was also able to capture a clean waveform of the noise induced by a cell phone in a nearby landline telephone - note that the duration of 4.6 milliseconds between two successive bursts is clearly seen. The signal was captured by touching the probes directly to the two wires of the telephone lines.



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