GSM stand for Global System for Mobile communications.It is a digital network that allows mobile phones users communicate over the air interface through frequency spectrum. Unlike the traditional PSTN (Public Switch Telephone Network) which requires copper wire connection from the telephone switching office to a telephone handset at your home or office, GSM technology doesnt needs copper wires.
GSM Frequency System: There is P-GSM 900 where P stands for Primary. It was the first system developed with a bandwidth of 25MHz. There is also E-GSM 900 where E stands for Extended, due to the limited bandwidth capacity of the P-GSM 900. The P-GSM was extended by 10MHz, therefore the E-GSM 900 has a bandwidth capacity of 35MHz.
Then there is GSM 1800 which has a bigger bandwidth of 75MHz and the last is GSM 1900 with a bandwidth of 60MHz.
Allocation of GSM Frequencies to Mobile Operators. Assignment and allocation of frequencies to mobile operators is the sole responsibility of the National Telecommunications Regulator. GSM frequencies are assign to each Telecom operator according to availability of the bandwidth and operator's bandwidth requirements. Where the number of operators are more, the regulators may decide to re-assign or allocate the frequencies to enable all operators operate the service.
Figure1: GSM Frequency Spectrum, bandwidth, Carrier, No of radio channels per bandwidth.
GSM CHANNEL CALCULATION: Let see how to calculate the total number of channels in the various GSM spectrum as indicated in Figure1.
No. of channels in a given frequency = bandwidth / Carrier Separation. The channel separation is nothing than the distance between two frequency channels being transmitted on the same direction.
Channel separation is needed to avoid information from one channel overlapping into other channel(adjacent channel). It is very important to understand. Imaging 2 PVC pipes; one carrying clean water and another waste water and there is a leakage from the two pipes. Therefore carrier separation is require between 2 frequencies to avoid interference.The GSM Carrier separation is fixed at 200KHz.
The P-GSM bandwidth = 25MHz
Carrier Separation = 200KHz
No of channels in P-GSM 900 = 25MHz / 200KHz
= 25,000 KHz / 200 KHz
= 125 Channels.
These spectrum licences are sold or auction to telecom operators at price depending on the bandwidth capacity for a duration, for example 10 years. After 10 years expiration, an operator is require to renew the licence again also at a fees. This is how government make money from the sale of the spectrum be it GSM, UMTS, LTE, NR or 5G in the Mobile telecommunication industry.
Figure2. Show the variations of E-GSM 900 spectrum bandwidth which a National Telecommunication Regulator or Commision allocate to GSM Operator.
