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channel encoding in GSM


Channel Encoding

Channel coding is performed to detect and, if possible, correct errors that occurred during
the transmission. It adds redundancy bits to the original information in order to detect
and correct errors. GSM uses both a block code (parity encoding) and a convolutional
code. The coding differs for the data, speech and control channels. Since we are only
modeling the speech channel of the GSM system, speech channel encoding

The 260 bits of a GSM speech frame are divided into three different classes according to
their function and importance. The most important class is the class Ia, containing 50 bits.
Next in importance is the class Ib, which contains 132 bits. The least important is the
class II, which contains the remaining 78 bits. The different classes are coded differently.
First of all, the class Ia bits are block-coded (parity encoding). Three parity bits, used for
error detection, are added to the 50 class Ia bits. The resultant 53 bits are added to the
class Ib bits. Four zero bits are added to this block of 185 bits (50+3+132). A
convolutional code, with r = 1/2 and K = 5, is then applied, obtaining an output block of
378 bits. The class II bits are then added, without any protection. An output block of 456
bits is finally obtained.





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