Channel Coding

 Channel Coding:

The speech coder output bits are grouped for error protection, according to their significance in the speech quality. The quality of speech produced by encoding the 260 bits in a frame are divided into three classes. They are:

(i) Class Ia: 50 bits (Most sensitive to bit errors) 

(ii) Class Ib: 132 bits (Moderately sensitive to bit errors)

(iii) Class II: 78 bits (Least sensitive to bit errors).

In class Ia bits, there are 3 parity check (CRC) bits added to them for detecting the non-correctable errors at the receiver.

The class Ib 132 bits along with the first 53 (50 Ia bits + 3 parity bits) are reordered and appended by four trailing zero bits making a data block of 189 bits.

The block is encoded for error protection using 1/2 rate convolutional encoder with constraint length K = 5. It provides a sequence of 378 bits.

The last 78 bits do not have any error protection. They are concatenated to the existing sequence to form a block of 456 bits in a 20 ms frame.

The error protection coding method increases the data rate of GSM speech signal with channel coding to 22.8 Kbps. The figure shows the error protection for speech signals.