Many GSM cellular phones support three bands (900/1,800/1,900 MHz or 850/1,800/1,900 MHz) or four bands (850/900/1,800/1,900 MHz), and are usually referred to as tri-band and quad-band phones, or world phones.
In Europe, GSM was initially specified only for operation in the 900 MHz band between 890 and 915 MHz in the uplink direction and between 935 and 960 MHz in the downlink direction,
‘Uplink’ refers to the transmission from the mobile device to the network and ‘downlink’ to the transmission from the network to the mobile device. The bandwidth of 25 MHz is split into 125 channels with a bandwidth of 200 kHz each.
It soon became apparent that the number of available channels was not sufficient to cope with the growing demand in many European countries. Therefore, the regulating bodies assigned an additional frequency range for GSM, which uses the frequency band from 1710 to 1785 MHz for the uplink and from 1805 to 1880 for the downlink. Instead of a total bandwidth of 25 MHz as in the 900 MHz range, the 1800 MHz band offers 75 MHz of bandwidth, which corresponds to 375 additional channels. The functionality of GSM is identical on both frequency bands, with the channel numbers, also referred to as the Absolute Radio Frequency Channel Numbers (ARFCNs), being the only difference.
While GSM was originally intended only as a European standard, the system soon spread to countries in other parts of the globe. In North America, analog mobile networks continued to be used for some time before second‐generation networks, which included the use of the GSM technology, were introduced. As the 900 MHz and the 1800 MHz bands were already in use by other systems the North American regulating body chose to open frequency bands for the new systems in the 1900 MHz band and later on in the 850 MHz band.
The GSM standard is also used by railway communication networks in Europe and other parts of the world. For this purpose, GSM was enhanced to support a number of private mobile radio and railway‐specific functionalities and this version is known as GSM‐R. The additional functionalities include the following:
As GSM‐R networks are private networks, it has been decided to assign a private frequency band in Europe for this purpose, which is just below the public 900 MHz GSM band. To use GSM‐R, mobile phones need to be slightly modified to be able to send and receive in this frequency range. This requires only minor software and hardware modifications. To be also able to use the additional functionalities described above, further extensions of the mobile device software are necessary.
Example: ARFCN =12 compute uplink and downlink frequency?
In Europe, GSM was initially specified only for operation in the 900 MHz band between 890 and 915 MHz in the uplink direction and between 935 and 960 MHz in the downlink direction,
‘Uplink’ refers to the transmission from the mobile device to the network and ‘downlink’ to the transmission from the network to the mobile device. The bandwidth of 25 MHz is split into 125 channels with a bandwidth of 200 kHz each.
It soon became apparent that the number of available channels was not sufficient to cope with the growing demand in many European countries. Therefore, the regulating bodies assigned an additional frequency range for GSM, which uses the frequency band from 1710 to 1785 MHz for the uplink and from 1805 to 1880 for the downlink. Instead of a total bandwidth of 25 MHz as in the 900 MHz range, the 1800 MHz band offers 75 MHz of bandwidth, which corresponds to 375 additional channels. The functionality of GSM is identical on both frequency bands, with the channel numbers, also referred to as the Absolute Radio Frequency Channel Numbers (ARFCNs), being the only difference.
While GSM was originally intended only as a European standard, the system soon spread to countries in other parts of the globe. In North America, analog mobile networks continued to be used for some time before second‐generation networks, which included the use of the GSM technology, were introduced. As the 900 MHz and the 1800 MHz bands were already in use by other systems the North American regulating body chose to open frequency bands for the new systems in the 1900 MHz band and later on in the 850 MHz band.
The GSM standard is also used by railway communication networks in Europe and other parts of the world. For this purpose, GSM was enhanced to support a number of private mobile radio and railway‐specific functionalities and this version is known as GSM‐R. The additional functionalities include the following:
- The Voice Group Call Service (VGCS). This service offers a circuit‐switched walkietalkie functionality to allow subscribers who have registered to a VGCS group to communicate with all other subscribers in the area who have also subscribed to the group. To talk, the user has to press a ‘push to talk’ button. If no other subscriber holds the uplink, the network grants the request and blocks the uplink for all other subscribers while the push to talk button is pressed. The VGCS service is very efficient, especially if many subscribers participate in a group call, as all mobile devices that participate in the group call listen to the same timeslot in the downlink direction. Further information about this service can be found in 3GPP TS 43.068
- The Voice Broadcast Service (VBS). It is similar to VGCS, with the restriction that only the originator of the call is allowed to speak. Further information about this service can be found in 3GPP TS 43.069
- Enhanced Multi‐Level Precedence and Preemption (EMLPP). This functionality, which is specified in 3GPP TS 23.067 [18], is used to attach a priority to a point‐to‐point, VBS or VGCS call. This enables the network and the mobile devices to automatically preempt ongoing calls for higher priority calls to ensure that emergency calls (e.g. a person has fallen on the track) are not blocked by lower priority calls and a lack of resources (e.g. because no timeslots are available).
As GSM‐R networks are private networks, it has been decided to assign a private frequency band in Europe for this purpose, which is just below the public 900 MHz GSM band. To use GSM‐R, mobile phones need to be slightly modified to be able to send and receive in this frequency range. This requires only minor software and hardware modifications. To be also able to use the additional functionalities described above, further extensions of the mobile device software are necessary.
How to calculate Uplink/Downlink Frequency?
GSM900
- The base-station receives (Uplink frequency) f1(n)=890.2+(n-1)×0.2 MHz
- The base-station sends (Down link frequency) f2(n)＝f1(n)＋45 MHz
Example: ARFCN =12 compute uplink and downlink frequency?
- frequency uplink = 890.2+(12-1) x 0.2 = 892.4 MHz
- frequency downlink = 892.4 + 45 = 937.4 MHz
DCS1800
- The base-station receives: f1(n)＝1710.2 +(n-512) × 0.2 MHz
- The base-station sends：f2(n)＝f1(n)＋95 MHz
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