Signal Analysis & Synthesis

Analogue Modulation

Analogue signal processing

Digital signal processing

Digital modulation

Communication System

TDMA

In time division multiple access system, satellite transponder is used only by one earth-station at a given instance of time. This system has the advantage of limiting of the producing inter-modulation products resulting due to multiple carrier frequencies as in FDMA.Here, the telephone signals are sampled and converted into PCM at a sampling frequency of 8,000 Hz. Each earth-station, therefore, produces a PCM digit stream, the speed of which is dependent upon the number of telephone channels handled by that station. The PCM signals from each station are interleaved in time. The combined signal at the satellite input is amplified by a signal wideband amplifier. Signals from each earth station are re-timed and re-radiated as a signal burst per frame period.

The below illustration shows signals at different points in the UP and DOWN direction.

TDMA system requires stable timing to be maintained between different earth stations. Since the transmission path involved is long, it is extremely difficult to achieve accurate timing.

TDMA frame structure

To overcome this difficulty an alternate method of providing a guard time between signals corresponding to different earth-stations is employed. However, this method has a drawback that it requires storage and re-timing of the bit stream before and after transmission. It is to be noticed that TDMA technique is not yet fully exploited commercially.