FDMA

Frequency-Division-Multiple-Access(FDMA) is a method of multiple accessing where a given RF bandwidth is divided into smaller frequency bands called subdivisions. Each subdivision has its own IF carrier frequency. A control mechanism is used to ensure that two or more earth stations do not transmit in the same in the same subdivision at the same time  essentially, the control mechanism designates a receiver station for each of the subdivisions. In demand-assignment systems, the control mechanism is also used to establish or terminates the voice-band links between the source and destinations earth stations. Consequently, any of the subdivision may be used by any of the participating earth stations at any given time.

 

If each subdivision carriers only one 4khz voice band channel, this is known as a single channel per carrier system . When  several voice band channels are frequency division multiplexed together to from a composite baseband signal comparied of groups, supergroups, or even mastergroups, a wider subdivision is assigned . This is referred to as multiple-channel per carrier. Carrier frequencies ans bandwidths for FDM/FM satellite systems using multiple-channel-per-carrier formats are generally assigned and remain fixed for a long period of time.

 

This is referred to as fixed assignment multiple access(FDM/FM/FAMA). An alternate channel allocation scheme is  demand-assignment, multiple access(DAM). Demand assignment allows all users continuous and equal access of the entire transponder bandwidth by assigning carrier frequencies on a temporary basis using a statistical assignment process. The first FDMA demand-assignment system for satellite was developed by com-sat for use on the Intelsat series IVA and V satellites.

 

Spared DAMA  satellite system. SPADE is an acronym for single-channel-per-carrier PCM multiple-access demand -assignment equipment. With SPADE 800 PCM encoded voice -band channels separately QPSK modulate an IF carrier signal. Each 4khz voice band channel is sampled at an 8khz rate and converted to an eight-bit PCM code. The produces a 64kbps PCM code for each voice band channel. The PCM code from each voice band channel QPSK modulates a different IF carrier frequency . With QPSK the minimum required bandwidth is equal to one half the input bit rate. The output of each QPSK modulator requires a minimum bandwidth is a 13khz guard band between pairs of frequency -division-multiplexed channels.
 

The IF carrier frequency being at 52.0225mhz and increase in 45 kHz steps to 87.9775 MHz .

The entire 36mhz band is divided in half, producing two 400 channel bands . For full-duplex operation, 40045-khz channels are used for one direction of transmission, and 400 are used for the opposite direction. Also channel 1,2, and 400 from each band are left permanently vacant. This reduces the number of usable full-duplex voice band channels to 397. The 6-ghz c-band extends from 5.725 Ghz to 6.425 ghz . This allows for apporoximately 1936-mhz RF channels per system. Each RF channel has a capacity of 397 full duplex voice band channels.