Subject : Mobile Communication

Topic : GSM

Difficulty : Medium

Speech coding:

• GSM speech coder is RELP (Residually Excited Predictive Coder), which is enhanced by including a Long Term Predictor (LTP).
• The coder provides 260 bits for 20ms blocks of speech, which yields a bit rate of 13kbps.
• GSM system operates in Discontinuous Transmission mode (DTX) by incorporating a Voice Activity Detector (VAD) in speech coder. This mode provides a longer battery life and reduces instantaneous radio interference since GSM transmitter is not active during silent periods.
• A Comfort Noise Subsystem (CNS) is used at receiver which adds background acoustic noise to compensate for the annoying switched muting which occurs due to DTX.