GSM (Global System for Mobile Communications) is the European standard for mobile telephones and CDMA (Code Division Multiple Access) is the North American standard. GSM is digital, and in addition to full duplex voice communications, GSM offers SMS (Short Message Service), voicemail, fax and other features like caller ID and call forwarding. In the RF (Radio Frequency) spectrum, specific bands of frequencies have been assigned a specific uses such as television and cellular telephone.
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GSM Frequency Bands
Several frequency bands, a specific range of frequencies, are allocated to GSM mobile phone use. These bands are 450MHz, 850MHz, 900MHz, 1800MHz and 1900MHz. Both the 450MHz and the 900MHz bands have an extended GSM (EGSM) band added. This added band provides increased spectrum. The complete GSM frequency band cable is available at the referenced "RF Café" website.
Uplinks and Downlinks
To enable GSM full duplex operation, each GSM channel contains a pair of frequencies. One uplink frequency (mobile phone to tower) and one downlink frequency (tower to mobile phone). This allows the user to talk (transmit) and listen (receive) simultaneously, just like a landline telephone or normal conversation. The uplink and the downlink frequencies each have a 20MHz bandwidth. The uplink and the downlink frequencies are separated by a guard band called the offset. This offset varies for each GSM band. Offset for the most popular GSM bands are GSM 450 - 10MHz offset, EGSM 450 - 10MHz offset, GSM 850 - 45MHz offset, GSM 900 - 45MHz offset EGSM 900 - 45MHz offset, GSM 1800 - 95MHz offset and finally, GSM 1900 has a 80MHz offset.
Frequency Division Multiple Access (FDMA)
The GSM cellular network uses a technique called FDMA. This frequency division multiple access technique divides each band of the GSM spectrum into individual carrier frequencies. Each carrier is separated by 20MHz. FDMA does just what it says; it divides the frequencies to allow multiple access. Employing FDMA is the first step in allowing each user to have a specific channel that will not conflict with another user on the same network.
Absolute Radio Frequency Channel Number (ARFCN)
The next step is to take this FDMA frequency division and assign it to individual channels. Each ARFCN channel has a dedicated pair of frequencies. One frequency for the uplink and one frequency for the downlink.
Calculating Uplink/Downlink Frequencies
To calculate the exact uplink and downlink frequency, you must know the frequency band, the ARFCN and the offset. Here's an example using GSM 900MHz band:
GSM 900: Up = 890.0 + (ARFCN * .2) and Down = Up + 45.0
Given the ARFCN 72, and we know the offset is 45MHz for the GSM900 band: Up = 890.0 + (72 * .2) Up = 890.0 + (14.4) Up = 904.40MHz Down = Up + Offset Down = 904.40 + 45.0 Down = 949.40MHz
The uplink/downlink pair for GSM900 ARFCN72 is 904.40/949.40 (MHz)
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