Frequency Division Multiple Access (FDMA) Techniques

FDMA is a type of channelization protocol. This bandwidth is divided into various frequency bands. Each station is allocated a band to send data and that band is reserved for the particular station for all the time which is as follows:

Frequency Division Multiple Access (FDMA) consists of two main techniques.

1. Multi-channel-per-carrier (MCPC) transmission
2. Single-channel per carrier (SCPC) transmission

Multi-Channel-Per-Carrier Transmission

• Analog multiplexing is used at earth station in the early stages of communication, this helps in combining large numbers of telephone channels into a single baseband signal and thus modulating into a single RF carrier.
• Using frequency division multiplexing (FDM) telephone signals can be combined in a group of channels, this is done by  shifting the baseband frequency to a higher frequency.
• Upto 1800 telephone channels in a satellite are multiplexed using the FDM,  thus making wide baseband that occupies bandwidth of 8 MHz.
• The wide baseband signals are then modulated onto the RF carrier using FM i.e. Frequency Modulation.
• Different RF carrier for each and every earth station is used for frequency modulations.
• A common transponder for transmission is been shared from various earth stations for the FDM-FM-RF. This technique is called as FDM-FM-FDMA.
• So has various different telephone channels are been multiplexed for transmission over a single RF carrier, it is known as Multi-channel-per carrier (MCPC) transmission.
• The below attached images show fixed assigned FDM-FM-FDMA scheme for the two C-band transponders.

Transponder 1

Transponder 2

Single-Channel-Per Carrier Transmission

• When a single signal on a carrier is been send via the earth station, this FDMA access technique is called as “Single-channel-per carrier (SCPC) transmission”.
• Hence if a system in which large number of small earth station are used, for example mobile telephones, which access via a single transponder using FDMA is called a Single-channel-per carrier Frequency division multiple access scheme (SCPC-FDMA).
• The SCPC systems can be reconfigurable, this depends on the traffic condition which is been accumulated to that particular system, thus making it compatible with the demand assignment systems.
• So when the link is been activated then only the carrier for SCPS channel is transmitted.
• This helps in reducing the transponder power consumption.

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