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 20 ms 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 throughput 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.
The Receiver section is composed of the following stages of processing:
- Delay Equalisation
- Burst Assembly
- 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.
Bit inter leaver in GSM is used to scramble and/or spreads a sequence of bits before transmitting them. The Sequence of bits is put back in order at the receiving end.
Interleaving schemes in GSM:
A code word of 456 bits could be spread into following format:
- For full bursts: divide 456 bits into four parts, each one filling up a whole bursts.
- Eight Half Bursts: divide 456 bits into 8 parts, each one filling up half a burst. Four parts share with the previous and four parts with new partial code word.
Advantages of Interleaving is as follows:
- It converts burst errors into random errors and is very effective for data transmission
- It helps minimise the effects of sudden fades on the received data.