GPRS Coding Schemes

In the present data speeds (3G) discussing about GPRS sounds low, but i wish to write about the four coding schemes that are used in GPRS

The channel encoding process in GPRS consists of two steps: first, a cyclic code is used to add parity bits, followed by coding with a possibly punctured convolution code. Here, the Coding Schemes CS-1 to CS-4 specifies the number of parity bits generated by the cyclic code and the puncturing rate of the convolution code.

In Coding Schemes CS-1 through CS-3, the convolution code is of rate 1/2, i.e. each input bit is converted into two coded bits. In Coding Schemes CS-2 and CS-3, the output of the convolution code is punctured to achieve the desired code rate i.e., 1/2. In Coding Scheme CS-4, no convolution coding is applied. Modulation technique used in these coding schemes is GMSK. 

  

Overview of all the four coding schemes

Channel Coding Scheme	CS-1	CS-2	CS-3	CS-4
Pre-cod. USF	3	6	6	12
Info bits without USF	181	268	312	428
Parity bits BC	40	16	16	16
Tail bits	4	4	4	-
Output convolution encoder	456	588	676	456
Punctured bits	0	132	220	-
Code rate	1/2	~2/3	~3/4	1
Data rate kbit/s	9.05	13.4	15.6	21.4
Maximum data speed with 8 time-slots	72.4 kb/s	107.2 kb/s	124.8 kb/s	171.2 kb/s

Utilization

• CS - 4 which is fastest is used near a BTS while CS - 1 is used when the MS is far away from BTS. 
• CS - 4 can achieve a user speed of 20.0 Kbit/s per time slot, but has 25% of normal cell coverage whereas CS - 1 gives 98% of normal cell coverage with 8 kbps speed.

Which coding scheme to choose ?

• The choice of coding scheme depends on the condition of the channel provided by the mobile network.
• If the channel is very noisy, the network may use CS-1 with only 8 kbps speed to ensure higher reliability while if the channel is providing a good condition, the network could use CS-3 or CS-4 to obtain optimum speed.

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Satya Sravan