Q 1.17 In which way are digital methods superior to analogue on satellite links?
Digital methods offer the following advantages over analogue satellite links.
Digital coding improves robustness of radio frequency to noisy satellite signals, typical of a satellite receiver, to deliver high quality reproduction – advanced coding schemes such as Turbo coding can approach theoretical limits in channel utilization (Shannon limit).
Digital processing can regenerate digital signals in regenerative satellite transponders and retransmit them afresh thereby decoupling up and down links – thus providing high power efficiency and noise rejection that leads to smaller earth stations and improved interference rejection compared to non-regenerative and transponders carrying analogue traffic.
Digital techniques allow encryption to offer highly secure communication.
Voice coding has reached such sophistication that communication is possible at bit rates down to 2.4 kb/s.
Digital systems do not differentiate between signal types – thus voice, computer data, imagery and digital signals from other sources can be multiplexed into a single transmission stream thereby improving transmission efficiency.
Advanced digital techniques can stuff useful data between speech or data pauses to fully utilise otherwise wasted bandwidth – typically a voice channel is occupied for about 40% of time.
Large scale device integration has allowed massive all round reduction in cost, size and efficiency enabling introduction of hand-held user terminals; on-board digital processing allowing features such as traffic dependent beam reconfiguration in real time, software reconfigurable transmitter and receivers, etc.
Digital systems can be reprogrammed locally or remotely thus improvements can be introduced without need of changing hardware.
Digital multiple access techniques such as Time Division Multiple Access (TDMA), Time Division Multiplexed (TDM) streams make highly efficient use of shared satellite transponders.
All modern satellite systems are digital due to overriding advantages of digital systems.