RFGSM Packet Switched Channel Processing Through Physical Layer
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Introduction
This article delves into the processing of packet-switched channels within the GSM (Global System for Mobile Communications) framework, focusing on the physical layer. We will explore GPRS (General Packet Radio Service) channel coding schemes (CS1, CS2, CS3, CS4) and EGPRS (Enhanced GPRS) channel coding schemes (MCS1, MCS2, MCS3, MCS4, MCS5, MCS6, MCS7, MCS8, MCS9).
Coding Parameters for GPRS Modulation and Coding Schemes (CS1, CS2, CS3, CS4)
GPRS employs various coding schemes to mitigate the effects of noise, interference, and fading in the air interface. These schemes offer a trade-off between data rates and robustness; higher data rates generally correspond to lower robustness and vice versa. The four primary coding schemes, CS1 through CS4, are detailed in the following table:
| Scheme | Code Rate | USF | Pre-coded USF | Radio Block (exclude USF/BCS) | BCS | Tail Coded Bits | Punctured Bits | Data Rate (kbps) |
|---|---|---|---|---|---|---|---|---|
| CS1 | 1/2 | 3 | 3 | 181 | 40 | 4 | 456 | 9.05 |
| CS2 | 2/3 | 3 | 6 | 268 | 16 | 4 | 588 | 13.4 |
| CS3 | 3/4 | 3 | 6 | 312 | 16 | 4 | 676 | 15.6 |
| CS4 | 1 | 3 | 12 | 428 | 16 | - | 456 | 21.4 |
USF: Uplink State Flag, BCS: Block Check Sequence
EDGE/EGPRS Modulation and Coding Schemes
In EGPRS, the radio interface remains the same as in GPRS. The concepts of the 52-frame multiframe, physical channels (PDCH), logical channels (PBCCH/PCCCH/PRACH/PDTCH/PTCCH and PACCH), and logical channel to physical channel mapping are also consistent with GPRS.
The primary difference lies in the modulation and coding schemes used, as outlined in the table below:
| Modulation-Coding Scheme | Modulation | Code Rate | Max. Throughput (kbps) | Raw Data within Radio Block | RLC Blocks/Radio Block | Family |
|---|---|---|---|---|---|---|
| MCS1 | GMSK | 0.37 | 8.8 | 178 | 1 | C |
| MCS2 | GMSK | 0.49 | 11.2 | 226 | 1 | B |
| MCS3 | GMSK | 0.53 | 14.8 | 298 | 1 | A |
| MCS4 | GMSK | 0.66 | 17.6 | 354 | 1 | C |
| MCS5 | 8-PSK | 0.76 | 22.4 | 450 | 1 | B |
| MCS6 | 8-PSK | 0.85 | 29.6 | 594 | 1 | A |
| MCS7 | 8-PSK | 0.92 | 44.8 | 2 x 450 | 2 | B |
| MCS8 | 8-PSK | 1.0 | 54.4 | 2 x 546 | 2 | A |
| MCS9 | 8-PSK | 1.0 | 59.2 | 2 x 594 | 2 | A |
GMSK: Gaussian Minimum Shift Keying, 8-PSK: 8-Phase Shift Keying, RLC: Radio Link Control
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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