GSM Control Channel(Signalling channel) Processing through physical layer
This article on GSM control channel(signalling channel) processing through physical layer covers processing of SACCH,FACCH/F,FACCH/H,BCCH,PCH,AGCH,CBCH,NCH,SDCCH,RACH and SCH channels as per 3GPP TS 45.003 standard.
SACCH control channel processing
As shown in the figure following steps SACCH information flow through physical layer.
• SACCH information consists of total 184 bits, to which 40 bits are added using shortened binary cyclic code(Fire code). This gives out about 224 bits.
• 4 bits are added as tail bits, which gives 228 bits
• This 228 bits are passed through Convolutional Encoder(C.E.) of rate 1/2, which gives 456 bits. Go and G1 polynomials are defined in the 3GPP specifications.
• This 456 bits are passed through block interleaver and mapped to 4 GSM bursts.
• This data is passed for GMSK modulation and passed to RF for upconversion and amplification before transmitted to the air. This Channel gets mapped to its position in a bigger GSM 51 frame multiframe structure.
• Two bits as mentioned in the figure,hl(B) and hu(B) are flags used for indication of control channel signalling.Both are set to 1 for a SACCH control channel.
Fire code generator polynomial, g(D) = (D23 + 1) (D17 + D3 + 1)
FACCH full rate control channel processing
• 184 bits are processed through block code in similar way as
processed for SACCH channel.
• Block coded data will go through Convolutional encoding similar way as SACCH is processed.
• Interleaving is performed as performed for TCH/FS
• 456 interleaved data bits are mappped to 8 consecutive bits (same as TCH/FS).
hu(B) equal to 1 for first 4 bursts when even numbered bits stolen.
hl(B) = 1 for the last 4 bursts when odd numbered bits stolen.
FACCH half rate control channel processing
• 184 bits are passed through block coder and convolutional encoder same as
processed for SACCH channel.
• The encoded data is processed through interleaver same as TCH/FS.
• The interleaved data is mapped on 6 consecutive bursts.
hu(B) equal to 1 for the first 2 bursts when even numbered bits stolen.
hu(B) equal to 1 and hl(B) = 1 for the middle 2 bursts when all bits stolen.
hl(B) equal to 1 for the last 2 bursts when odd numbered bits stolen.
BCCH/PCH/AGCH/CBCH/NCH/SDCCH Control Channel Processing
For BCCH,PCH,AGCH,CBCH,NCH and SDCCH, Processing is done in similar way as done for SACCH control channel.
RACH control channel processing
RACH will have either 8 or 11 information bits to carry.
Encoding process for 8 bits is mentioned in the figure.
• 6 bits of BSIC data is added with 6 parity bits to give out 12 bits which gets added with 2 initial bits of information.
• 4 tail bits are added to 14 bits of this data to give out 18 bits.
• 18 bits of data is passed through Convolutional encoder of rate 1/2 same as TCH/FS to give out 36 encoded data.
SCH control channel processing
• 25 information bits of SCH data are added 10 parity bits to give 35 bits.
• 4 tail bits are added to this 35 bits to give out 39 bits.
• 39 bits of data is passed through Convolutional encoder to give out 78 bits same as processed for TCH/FS.
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GSM Standard References
3GPP TS 45.002 describes burst building and burst multiplexing
3GPP TS 45.003 describes coding and interleaving
3GPP TS 45.004 describes differential encoding and modulation
3GPP TS 45.005 describes transmitter,receiver and antenna part
3GPP TS 43.020 & 23.221 describes encryption/ciphering part