Channel Coding

GPRS employs different channel coding to achieve different error correcting capabilities and achieving different data rates. The different data rates used and their performance is shown in table 2.1 below.

Table 2.1: Channel coding schemes for logical traffic channels in GPRS (Source: Bettstetter et. all)

Coding	Pre-Code	Info bits	Parity	Tail	Output	Punctured	Code	Data
Scheme	USF	without	Bits	Bits	conv	bits	Rate	Rate
		USF	BC		encoder
CS-1	3	181	40	4	456	0	1/2	9.05
CS-2	6	268	16	4	588	132	2/3	13.4
CS-3	6	312	16	4	676	220	3/4	15.6
CS-3	12	428	16	-	456	-	1	15.6

On the PDTCH one of the above coding schemes can be chosen, depending on quality of channel. Under very bad channel conditions, we may use CS-1 and get a data rate of 9.05kbit/s per GSM slot with very reliable coding. Under very good channel conditions we transmit without convolutional coding and achieve a data rate of 21.4kbit/s. With eight time slots we achieve a maximum data rate of 172.2kbit/s.

After encoding, codewords are interleaved into a block interleaver of depth 4. On the receiver side, the codewords are de-interleaved. The decoding is performed by Viterbi Algorithm.