The frequency spectrum is one of the most fundamental resources of a mobile network operator (MNO). It is the means of allowing mobile operators to transmit and receive signals in any geographical location so that their customers can get access to mobile services. The spectrum defines the frequency bands allocated to mobile operators within a country, which they can then use to create frequency channels. These frequency channels allow our mobile phones to send and receive communication to and from the nearest base stations at specified frequencies. The deployments of GSM networks around the world have used various frequency bands. Let’s have a look below to review the original and the extended versions of the GSM frequency bands.
The original frequency band for GSM networks ranged from 890 MHz to 915 MHz for the uplink and 935 MHz to 960 MHz for the downlink. This frequency band is known as Primary GSM band or P-GSM. The primary GSM band was later extended in order to add 10 MHz to both the uplink as well as the downlink. The extended band is known as Extended GSM or E-GSM and it ranges from 880 MHz to 915 MHz for the uplink and 925 MHz to 960 MHz for the downlink.
GSM frequency channels
In GSM networks, a term ARFCN is used for frequency channels which stands for Absolute Radio Frequency Channel Number. The frequency band in GSM is divided into pairs of frequency channels. Each pair represents one frequency for the uplink (mobile phone to base station) and one frequency for the downlink (base station to mobile phone). There are various “flavours” of GSM but in this article, we will only touch upon GSM900. In the original GSM900 networks, the primary frequency band (P-GSM) was used to create ARFCNs from 1 to 124.
The extended band added a further 10 MHz both to the uplink and the downlink. In this extended version, the ARFCNs were created in two steps. First, the P-GSM band was used to create ARFCNs from 0 to 124, and then ARFCNs from 975 to 1023 were created using the extra 10 MHz in the uplink and downlink i.e. 880-890 MHz and 925-935 MHz.
In both P-GSM and E-GSM, each ARFCN represents a pair of frequencies one for the uplink and one for the downlink. Each frequency (uplink frequency and downlink frequency) has a bandwidth of 200 kHz.
Frequency Division Duplex
GSM networks use FDD or Frequency Division Duplex which means that separate frequency bands are used for the uplink and the downlink. The opposite of FDD is TDD (Time Division Duplex) where the uplink and downlink are separated by time slots in the same frequency band. For the multiple access (allow multiple people to use mobile services), GSM networks use a combination of two radio access techniques, Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA). This way, the frequency band is used much more efficiently by first breaking the frequency band down into frequency channels (ARFCNs) and then breaking the channels further into time slots. GSM networks have a higher bandwidth as compared to their earlier counterparts i.e. 1G technologies like AMPS, D-AMPS, NMT, TACS, etc.). This higher bandwidth makes it easier for GSM networks to allow more services as compared to the earlier networks.