Technical data

www.westermo.com Theoretical and general applications 87

Cell structures

Base stations are positioned to give maximal coverage. The area covered by a base

station is called a cell.

The entire GSM network is organised with cells of varying sizes. A cell can

cover areas with a radius of 200 metre (656 ft) up to areas with a radius of

~30 km (18.64 mi). This depends on where the base station is located and

the surrounding environment.

Other factors that affect the installation are, among others, the output

power and whether the base station is located in an environment that is

problematic for radio traffic. The cell structure results in the reuse of fre-

quencies in the base stations. In the figure opposite the frequency A1 can be

reused in the third ring without the risk of crosstalk between cells with the

same frequency.

If you travel through an area it is necessary to switch between the cells through

which you pass. This is known as handover.

Radio transmissions between MS and BSS

In the eighties when the GSM specification was drawn up, the ITU (International

Telecommunication Union) reserved two frequency bands of 25 MHz for GSM radio

transmissions:

… 880–915 MHz “uplink” transfer from MS to BSS.

… 925–960 MHz “downlink” transfer from BSS to MS.

The development within mobile communication has resulted in the need to use

multiple frequencies to satisfy demand. Today there are five standardised frequencies

400, 850, 900, 1800 as well as 1900 MHz. The latter frequency is generally used in the

USA and in some parts of Asia, while 900 and 1800 are more globally used.

E21

C10

C11

C12

C13

C14

C15

C16

C17

C18

C19

D2 D2

B A C K