Channel coding in GSM uses the 260 bits from speech coding as input to channel coding and outputs 456 encoded bits.
The 260 bits are split according to their relative importance:
- Block 1: 50 very important bits
- Block 2: 132 important bits and
- Block 3: 78 not so important bits
The first block of 50 bits is sent through a block coder, which adds three parity bits that will result in 53 bits. These three bits are used to detect errors in a received message.
The 53 bits from first block, the 132 bits from the second block and 4 tail bits (total = 189) are sent to a 1:2 convolutional coder which outputs 378 bits. Bits added by the convolutional coder enable the correction of errors when the message is received.
The bits of block 3 are not protected.
--
Satya Sravan
No comments:
Post a Comment