GSM: The base-station network

( Part of Henrik Kaare Poulsen's GSM pages )

The secret behind GSM

It is not difficult to create a radio system which can communicate over long distances. For example, public (one way) radio systems broadcasts with high power to cover large areas. National or regional stations cover larger areas than local stations, because they use higher power. Two-way radio systems also use high power to communicate over long distances. This is for example the case for the popular walkie-talkies.

However, the laws of physics dictates, that there is only a limited number of radio channels (frequencies) available. So if one radio station is using a specific frequency with high power, no other stations in the vicinity can use the same frequency. So, only a limited number of public radio stations or walkie-talkies can operate at the same time.

GSM: A cellular system

In the figure, a cellular network - eg. GSM - with 4 frequencies is illustrated. Each color represents a given frequency, and the colored area represents the geographical area covered by that frequency. This way, the same frequency can be used over and over again. In order to limit the number of base stations required, each station is assigned more than one frequency. In the example, if we have 16 channels available, they could be distributed between the cells as follows:

SITE	FREQUENCIES
Site 1	1	2	3	4
Site 2	5	6	7	8
Site 3	9	10	11	12
Site 4	13	14	15	16
Site 5	1	2	3	4
...	...	...	...	...

As explained earlier, there are 124 channels (frequencies) available in GSM 900. These are normally distributed between up to 3 GSM operators, each having 40 channels. To achieve a reasonable quality in GSM, 4 frequencies (as in the figure) are not enough: between 9 and 12 frequencies are necessary. This means that we can assign each GSM base station 4 different frequencies, and reuse these at something like every 10th bases station. Assuming a dense GSM network with, say, 2km between base stations, 10 base stations would cover around 40 square kilometers. In these 40 square kilometers there can then be 280 simultaneous calls or between 5 and 10 simultaneous calls per square kilometer.

If less capacity is needed, GSM sites can be placed further apart. In GSM the limit is 35 kilometers between base station and handset. If more capacity is needed, GSM sites can be placed closer - eg. down to maybe 200 meters apart.

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