RFGSM Traffic Channel Processing in the Physical Layer
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This article covers traffic channel processing through the physical layer for various GSM channels as per the 3GPP TS 45.003 standard. Specifically, we’ll look at:
- Speech channel at Full rate (TCH/FS)
- Speech channel at Half rate (TCH/HS)
- Speech channel at Enhanced Full Rate (TCH/EFR)
- Adaptive Multi rate speech channel at full rate (TCH/AFS)
- Adaptive Multi rate speech channel at half rate (TCH/AHS)
- Wideband Adaptive Multi Rate Speech Channel at full rate (TCH/WFS)
- Data Channel at full rate 12.0 Kbits/s radio interface rate (TCH/F9.6)
The goal of this article is to provide a block schematic view of traffic channel processing (speech/data).
Speech channel at Full rate (TCH/FS)
Speech channel at Half rate (TCH/HS)
Speech channel at Half rate (TCH/EFR)
Adaptive Multi rate speech channel at full rate (TCH/AFS)
This section describes the coding for the different frame formats used for TCH/AFS.
- SID_UPDATE: Used to convey comfort noise parameters during DTX (Discontinuous Transmission).
- SID_FIRST: Marker to define the end of speech and the start of DTX.
- ONSET: Used to signal the Codec mode for the first speech frame after DTX.
- SPEECH: Speech frames
- RATSCCH: Frames used to convey RATSCCH (Radio Access Technology Selection Cell Change Handover) messages
| Codec Mode | Rate | No. of input bits to Conv.Encoder | No. of output bits from Conv.Encoder | No. of punctured bits |
|---|---|---|---|---|
| TCH/AFS12.2 | 1/2 | 250 | 508 | 60 |
| TCH/AFS10.2 | 1/3 | 210 | 642 | 194 |
| TCH/AFS7.95 | 1/3 | 165 | 513 | 65 |
| TCH/AFS7.4 | 1/3 | 154 | 474 | 26 |
| TCH/AFS6.7 | 1/4 | 140 | 576 | 128 |
| TCH/AFS5.9 | 1/4 | 124 | 520 | 72 |
| TCH/AFS5.15 | 1/5 | 109 | 565 | 117 |
| TCH/AFS4.75 | 1/5 | 101 | 535 | 87 |
Adaptive Multi rate speech channel at half rate (TCH/AHS)
This section describes the coding for the different frame formats used for TCH/AHS.
- SID_UPDATE: Used to convey comfort noise parameters during DTX
- SID_UPDATE_INH: Used to inhibit the second part of a SID_UPDATE frame if there is a speech onset
- SID_FIRST_P1: First part of the marker to define end of speech, start of DTX
- SID_FIRST_P2: Second part of the marker to define end of speech, start of DTX
- SID_FIRST_INH: Used to inhibit the second part of a SID_FIRST_P1 frame if there is a speech onset
- ONSET: Used to signal the Codec mode for the first speech frame after DTX
- SPEECH: Speech frames
- RATSCCH_MARKER: Marker to identify RATSCCH frames
- RATSCCH_DATA: Frame that conveys the actual RATSCCH message
| Codec Mode | No. of input bits to Conv.Encoder | Rate | No. of output bits from Conv.Encoder | No. of punctured bits |
|---|---|---|---|---|
| TCH/AHS7.4 | 129 | 1/2 | 266 | 78 |
| TCH/AHS7.4 | 126 | 1/2 | 260 | 64 |
| TCH/AHS6.7 | 116 | 1/2 | 240 | 40 |
| TCH/AHS5.9 | 108 | 1/2 | 224 | 16 |
| TCH/AHS5.15 | 97 | 1/3 | 303 | 91 |
| TCH/AHS4.75 | 89 | 1/3 | 285 | 73 |
Wideband Adaptive Multi Rate Speech Channel at full rate (TCH/WFS)
This section describes the coding for the different frame formats used for TCH/WFS.
- SID_UPDATE: Used to convey comfort noise parameters during DTX
- SID_FIRST: Marker to define end of speech, start of DTX
- ONSET: Used to signal the Codec mode for the first speech frame after DTX
- SPEECH: Speech frames
- RATSCCH: Frames used to convey RATSCCH messages
| Codec Mode | Rate | No. of input bits to Conv.Encoder | No. of output bits from Conv.Encoder | No. of punctured bits |
|---|---|---|---|---|
| TCH/WFS12.65 | 1/2 | 259 | 526 | 78 |
| TCH/WFS8.85 | 1/3 | 183 | 561 | 113 |
| TCH/WFS6.60 | 1/4 | 140 | 576 | 128 |
Data Channel at full rate 12.0 Kbits/s radio interface rate (TCH/F9.6)
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
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