- 2076 Bhadra · 5+3 marks
GSM frame structure; GSM vs CDMA
Short answer
GSM uses FDMA/TDMA with 200 kHz carriers, 8 slots, GMSK, hard handoff and SIM; CDMA uses DS-CDMA with 1.25 MHz carriers, QPSK/OQPSK, N=1, soft handoff and fast power control. GSM relies on adaptive equalization; CDMA uses RAKE receivers.
Explain the frame structure of GSM. Differentiate between GSM and CDMA standards.
Frame structure of GSM
GSM uses FDMA + TDMA: each 200 kHz carrier is divided into 8 time slots, gross bit rate 270.833 kbps (bit period 3.69 µs).
Hyperframe = 2048 superframes = 3 h 28 min 53.76 s
|
Superframe = 26 x 51-MF = 51 x 26-MF = 6.12 s
|
+------------------+--------------------+
26-multiframe 51-multiframe
(traffic) 120 ms (control) 235.4 ms
|
TDMA frame = 8 slots = 4.615 ms
| TS0 | TS1 | TS2 | TS3 | TS4 | TS5 | TS6 | TS7 |
|
Time slot = 156.25 bits = 576.9 µs
Normal burst (one time slot):
|T | Data |S| Training |S| Data |T |Guard|
|3 | 57 |1| 26 |1| 57 |3 |8.25 |
- Tail bits (3+3): all zeros; let the equalizer/Viterbi decoder start and end in a known state, and give time for power ramp.
- Data (2 × 57): coded speech/data bits.
- Stealing flags (1+1): show whether the data half-burst carries traffic or has been "stolen" for FACCH signalling.
- Training sequence (26): known pattern in the middle of the burst, used by the adaptive equalizer to estimate the channel.
- Guard period (8.25 bits ≈ 30 µs): prevents overlap of bursts from mobiles at different distances.
Other bursts: frequency correction burst (FCCH, all zeros), synchronisation burst (SCH, long 64-bit training), access burst (RACH, short, long guard of 68.25 bits), dummy burst.
- 26-multiframe (120 ms): frames 0–11 and 13–24 TCH, frame 12 SACCH, frame 25 idle.
- 51-multiframe (235.4 ms): carries FCCH, SCH, BCCH, CCCH and SDCCH control channels on TS0 of the beacon carrier.
GSM vs CDMA standards
| Point | GSM | CDMA (IS-95) |
|---|---|---|
| Multiple access | FDMA/TDMA | DS-CDMA |
| Carrier bandwidth | 200 kHz | 1.25 MHz |
| Users per carrier | 8 (full rate) | ~20–60 (interference limited) |
| Modulation | GMSK | QPSK (forward), OQPSK (reverse) |
| Frequency reuse | N = 3–7 (e.g. 4/12) | N = 1 |
| Handoff | Hard | Soft and softer |
| Capacity | Hard limit | Soft limit |
| Power control | Slow (2 Hz) | Fast (800 Hz) |
| Speech codec | RPE-LTP 13 kbps | QCELP 8/13 kbps, variable |
| Multipath | Adaptive equalizer | RAKE receiver |
| User identity | SIM card | Originally handset (ESN); later R-UIM |
| Synchronisation | Asynchronous BTS | BTS synchronised by GPS |
✓ Answer checked by Claude Opus 5.5 (high effort)
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