GSM Power Control & Power Class

Power levels and power control is important within 2G GSM: power control & power classes have been introduced to help manage the power transmitted and base station expectations.


GSM primer includes:
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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 number Power output level dBm
2 39
3 37
4 35
5 33
6 31
7 29
8 27
9 25
10 23
11 21
12 19
13 17
14 15
15 13
16 11
17 9
18 7
19 5



GSM power level table for GSM 1800
 
Power level number Power output level dBm
29 36
30 34
31 32
0 30
1 28
2 26
3 24
4 22
5 20
6 18
7 16
8 14
9 12
10 10
11 8
12 6
13 4
14 2
15 0



GSM power level table for GSM 1900
 
Power level number Power output level dBm
30 33
31 32
0 30
1 28
2 26
3 24
4 22
5 20
6 18
7 16
8 14
9 12
10 10
11 8
12 6
13 4
14 2
15 0

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 Number GSM 900 GSM 1800 GSM 1900
  Power level number Maximum power output Power level number Maximum power output Power level number Maximum power output
1     PL0 30 dBm / 1W PL0 30 dBm / 1W
2 PL2 39dBm / 8W PL3 24 dBm/ 250 mW PL3 24 dBm / 250 mW
3 PL3 37dBm / 5W PL29 36 dBm / 4W PL30 33 dBm / 2W
4 PL4 33dBm / 2W        
5 PL5 29 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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