GSM: Radio Coverage

( Part of Henrik Kaare Poulsen's GSM pages )

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Signal Strength

If the signal strenght at the mobile phone from the GSM radio station is sufficient to place and maintain a call, the mobile phone is sad to be "in coverage". In a celluar system the communication between the GSM radio station and the mobile phone is two-way. However, the system is designed so that if there is enough signal strengh at the mobile phone from the GSM radio station, then there is also enough signal strenght at the GSM radio station from the mobile station.

The signal strength received by the mobile phone depends on the distance from the GSM radio station and the environment between the mobile phone and the GSM radio station.

The signal strength decreases exponentially with the distance from the GSM radio station as shown in the figure:

If the area between the GSM radio station and the mobile phone is flat, open landscape, the signal strenght decreases as shown in the figure above. If the area is hilly, the signal strenght behind the hills is attenuated even further. Trees or houses in the path between the GSM radio station and the mobile phone will also decrease the level of the signal.

If the mobile phone is used in urban environments, the so-called multipath fading is also contributing to attenuation of the rado signal. The signal can be reflected once or several times between the buildings. If the signal from the GSM radio station is reflected from two different buildings, the resulting signal may be amplified (if the two signals are in phase) or it may be attenuated (if the two signals are not in phase).

This phenomena can also be observed when receiving normal FM radio in a car, when being far from the radio station. In one place the signal may be good, but after moving a meeter or two (towards a red light!) the signal may be completely gone.

Designing a cellular system

When designing a cellular system, the signal strenght plays a major role. One tries to place GSM radio stations in such a way that there is enough signal to operate a mobile phone anywhere. However, as shown in the section on the Base Station Network, the stations are placed in a cellular pattern, where frequencies are reused. Hence - optimally - one GSM radio station should cover only one cell.

In areas with little mobile phone traffic - such as rural areas - one tries to put the GSM radio stations where they give the best coverage - usually on top of hills. In areas with much mobile phone traffic - such as urban areas - one tries to put the GSM radio station in such a way that their signal does not disturb the neighbouring cells - usually on low buildings where higher buildings "shields" the signal towards the other cells.

In the efforts to make sure that two GSM radio stations does not disturb eachother, one may incidentially risk not to have coverage in an area where someone wishes to use their mobile phone.

Rolling out a cellular system

Building a cellular GSM network with hundreds or thousands of GSM radio stations is a large undertaking. Obviously all GSM radio stations cannot be build at the same time. Hence even in a well-planned cellular system coverage my be lacking - simply because not all GSM radio stations have been build yet.

Want to read about GSM in a book?

GSM System Engineering	GSM System Engineering The secrets of GSM. Includes description of the radio environment and all protocols. Buy it from Amazon.com
Or buy other GSM related books here

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