GSM Channel Coding

GSM uses a combination of block and convolutional coding methods. First some of the
information bits are block coded, building a block of information each with an associated
block check sequence (BCS). All the block coded bits (including the BCS) are passed through
a convolutional coder to form the final coded bits
53 bits

GSM (TCH/F) Channel Coding

The reason for this ‘double coding’ is that ideally forward convolutional coding will detect
and correct all errors. However, if the data is damaged beyond repair, block coding is used to
ignore the data and request a retransmission of the corrupted data block.
The GSM coding scheme is described as ‘concatenated’. as divides the data into three
prioritised sections, depending on the importance of the data to the speech characteristics. It
then applies different levels of coding to each, as shown.. The resultant code is then put
together (concatenated) for transmission.
The convolutional coder described previously is that used by GSM. The purpose of the
addition of 4 tail bits in the previous diagram is to ensure that the registers in the coder are
flushed after each 260-bit block has been coded.