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GSM Power Control & Power Class

 GSM Power Control & Power Class

The transmitted power levels and power control of 2G GSM mobiles is of great importance as it affects a number of areas of performance.

The 2G GSM standard defined groups referred to as the GSM power class designations. These have been allocated to indicate the power capability of various mobiles.

The base station controls the power levels of the mobiles linked to it. This reduces battery life and reduces the levels of interference . Also the performance of the base station is not compromised by high power local mobiles which may tend to mask out weaker mobiles further away from the base station.

GSM power levels

The base station controls the power output of the mobile, keeping the GSM power level sufficient to maintain a good signal to noise ratio, while not too high to reduce interference, overloading, and also to preserve the battery life.

A table of GSM power levels is defined, and the base station controls the power of the mobile by sending a GSM "power level" number. The mobile then adjusts its power accordingly. In virtually all cases the increment between the different power level numbers is 2dB.

The accuracies required for GSM power control are relatively stringent. At the maximum power levels they are typically required to be controlled to within +/- 2 dB, whereas this relaxes to +/- 5 dB at the lower levels.

The power level numbers vary according to the GSM band in use. Figures for the three main bands in use are given below:

GSM power level table for GSM 900
Power level numberPower output level dBm
239
337
435
533
631
729
827
925
1023
1121
1219
1317
1415
1513
1611
179
187
195
GSM power level table for GSM 1800
Power level numberPower output level dBm
2936
3034
3132
030
128
226
324
422
520
618
716
814
912
1010
118
126
134
142
150
GSM power level table for GSM 1900
Power level numberPower output level dBm
3033
3132
030
128
226
324
422
520
618
716
814
912
1010
118
126
134
142
150

GSM Power class

Not all mobiles have the same maximum power output level. In order that the base station knows the maximum power level number that it can send to the mobile, it is necessary for the base station to know the maximum power it can transmit. This is achieved by allocating a GSM power class number to a mobile. This GSM power class number indicates to the base station the maximum power it can transmit and hence the maximum power level number the base station can instruct it to use.

Again the GSM power classes vary according to the band in use.

GSM Power Class NumberGSM 900GSM 1800GSM 1900
Power level number Maximum power output Power level number Maximum power output Power level number Maximum power output
1PL030 dBm / 1WPL030 dBm / 1W
2PL239dBm / 8WPL324 dBm/ 250 mWPL324 dBm / 250 mW
3PL337dBm / 5WPL2936 dBm / 4WPL3033 dBm / 2W
4PL433dBm / 2W
5PL529 dBm / 800 mW

GSM power amplifier design considerations

One of the main considerations for the RF power amplifier design in any mobile phone is its efficiency. The RF power amplifier is one of the major current consumption areas. Accordingly, to ensure long battery life it should be as efficient as possible.

It is also worth remembering that as mobiles may only transmit for one eighth of the time, i.e. for their allocated slot which is one of eight, the average power is an eighth of the maximum.

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