- When GSM was launched in the early 1990's, only the half
rate coder was available.
- This operated at a basic rate of 13 kbps
which became 22 kbps gross when the coding for forward error correction
was added.
- The full rate codec operates at 11.4 kbps gross,
almost doubling the capacity and spectral efficiency of' a network vital to satisfy
anticipated customer demand and to cope with the huge number of new customers.
- GSMs coding principle is known as RPE-LPE (Regular
Pulse Excitation - Long Term Prediction), and sampled the analogue signal with a 13
bit uniform PCM, at 8000 samples per second.
- A sophisticated Discontinuous Transmission
(DTX) scheme is used; to minimize radiated power; for battery efficiency and spectral
efficiency.
- This features voice activity detection, with insertion of
comfort noise during periods of 'silence', and speech frame substitution, to minimize
distortion due to interference.
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