GSM RF Planning
There are various GSM frequency bands as mentioned below. For our discussion of GSM RF(Radio frequency) planning we will consider GSM 900's P-GSM frequency band.
GSM Frequency bands versus ARFCN
| GSM Frequency Band | GSM Frequency range | ARFCN(Channel Number) |
|---|---|---|
| GSM450 Band | 450 to 458MHz (Uplink), 460 MHz to 468 MHz (Downlink) | 259 to 293 |
| GSM 480 band | 478 to 486 MHz(Uplink), 488 MHz to 496 MHz(Downlink) | 306 to 340 |
| GSM850 | 824 to 849MHz(Uplink) 869 MHz to 894 MHz (Downlink) | 128 to 251 |
| GSM900(P-GSM) | 890 to 915MHz(Uplink) and 935 to 960 MHz(Downlink) | 1 to 124 |
| GSM900(E-GSM) | 880 to 915 MHz(Uplink) and 925MHz to 960 MHz(Downlink) | 975 to 1023, 0 to 124 |
| GSM900(R-GSM ) | 876 to 880 MHz(Uplink) and 921 to 925MHz(Downlink) | 940 to 974, 0 to 124 |
| GSM1800 (called DCS1800) | 1710 to 1785 MHz(Uplink) 1805 MHz to 1880 MHz(Downlink) | 512 to 885 |
| GSM1900 (called PCS1900) | 1850 to 1910MHz(Uplink) and 1930 MHz to 1990 MHz(Downlink) | 512 to 810 |
As mentioned in the table above there are about 124 channels in P-GSM. 174 ARFCNs in E-GSM
and 374 ARFCNs in DCS1800.
Let us understand how we can obtain using following formula.
F(Uplink)=890+0.2*n, where n varies as mentioned from 1 to 124 and
is referred as ARFCN(Absolute Radio Frequency Channel Number).
F(Downlink)=F(Uplink)+45MHz
45MHz is the referred as duplex spacing, which is the difference between
uplink and downlink frequency channels.
GSM Radio Frequency planning
As frequency spectrum is the scarce resource and need to be optimally used in GSM network or any other cellular network.
Hence proper radio frequency planning is required to meet the need.
First of all the areas are divided into cells,where in cells can employ omnidirectional or directional antennas to broadcast the signals to the mobile users.
For our discussion we will assume cell divided into three sectors.
Each cell employ one Base station(e.g. BTS).
If the number of BTSs allocated are N,and each cell is allocated M channels,
Hence total ARFCNs(T) allocated in this GSM system can be obtained as follows.
T= 3*M*N
Frequency reuse can be applied before radio frequency planning to efficiently use the channels(ARFCNs) in deploying the GSM network.
ARFCNs (RF Frequencies) are reused in non adjacent cells.
GSM Frequency Re-use patterns
The frequencies used in GSM radio frequency planning are divided among different frequency groups.
GSM uses re-use patterns of 4/12 and 3/9 in most of the GSM installations.
4/12 refers to 12 frequency groups and 4 base stations, which means available GSM network frequencies are divided into 12 frequency groups across 4 BS sites. It has been assumed that there are 3 cells interfaced with each BS.
Here frequency groups are designated as A1, B1, C1, D1, A2, B2, C2, D2, A3, B3, C3, D3.
The 4/12 GSM frequency layout is shown in the figure 1.
Frequency allocation to GSM channels
In GSM, there are two main categories of channels, signalling and traffic. Signalling channels help network maintain resources,allocate resources(frequency and time slot), call establishment,call release and convey system related critical informations (in the form of SIs) to the mobiles in the cell. Traffic channels carry user voice/data(email,sms,browsing). Signalling channel category is referred as BCCH frequency and traffic channel category is referred as TCH frequency(non BCCH frequency). To serve the purpose each cell is assigned one BCCH frequency(ARFCN) in the downlink and one BCCH frequency in the uplink. Each cell is assigned with one or more TCH frequencies in the downlink as well as uplink, based on the capacity of users required in the cell.
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