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GMSK modulation
Case of frequency correction burst
Solving modulation equations
One bit change
Normal duration burst



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Illustration for SACCH,SDCCH,BCCH,AGCH,PCH

Illustration for SACCH/SDCCH/BCCH/PCH/AGCH [Under GSM]

Input is 184 information bits:



Parity (40) and tail (4) bits (Ref 45.003:4.1.2):



Convolutional coding is applied to 228(=184+40+4) bits, 1/2 rate coding with following formulas (Ref 45.003:4.1.3):

G0 = 1 + D3 + D4
G1 = 1 + D + D3 + D4

Dy is cyclically shifted (by y bits) version of original bit sequence.

G0 = u(k) + u(k - 3) + u(k - 4)


G1 = u(k) + u(k - 1) + u(k - 3) + u(k - 4)


G0 and G1 (228 bits each) are put together (G0 at even bits and G1 at odd bits) as below, totaling 456 bits.



The 456 bits are re-ordered and partitioned in 4 bursts, 114 bits in a burst (Ref 45.003:4.1.4).


Above bits are encrypted (using CK) and then actual bursts are formed. Normal burst have following format (Ref 45.002:5.2.3):

Tail bits(3) Info bits(57) Steal bit Training sequence(26) Steal bit Info bits(57) Tail bits(3)

Below are actual normal burst data (Ref 45.002:5.2.3,45.003:4.1.4/5):


References: GSM book by Mouly and Pautet.

© Copyright Samir Amberkar 2013