Channel Coding

 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.

Error protection for speech signals

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