The speech processing in GSM is divided into two sections: a) Transmitter section, b) Receiver Section

The Transmitter section is composed of the following stages of processing:

• Speech Coding: The GSM speech coder is based on the Residually Excited Linear Predictive Coder(RELP),which is enhanced by including a Long Term predictor(LTP).This coder provides 260 bits for each 20ms block of speech, which yields a bit rate of 13 Kbps. The GSM speech coder takes advantage of the fact that in normal conversation, each person speaks on average for less than 40% time.
• Channel Coding: Channel coding improves transmission quality when interference, multipath fading, and Doppler shift are encountered. As a result, the bit error rate and frame error rate are reduced, but throughout is also reduced. Four kinds of coding are used in GSM: Convolutional codes, Fire codes, Parity check codes and concatenation codes.
• Bit Interleaving: Interleaving scrambles and/or spreads a sequence of bits before transmitting them. The sequence of bits is put back in order at receiving end.
• Encryption: The Bit interleaved data is then encrypted to provide higher level of security and protection from theft.
• Burst Assembly. The data then obtained is formed in the form of burst and not continuous form.
• Modulation: The modulation scheme used by GSM is 0.3 GMSK(Gaussian Minimum Shift Keying), where 0.3 describes the 3 db bandwidth of the Gaussian pulse shaping filter with relation to the bit rate.

i.e BT=0.3

The Receiver section is composed of the following stages of processing:

• Demodulation
• Delay Equalisation
• Burst Assembly
• Decryption
• Bit De interleaving
• Channel Decoding
• Speech Decoding

Exactly the opposite process is done to extract the speech out of the received bits which are demodulated, equalised, decrypted, de interleaved, channel decoded and speech decoded.