--- title: "GSM frame structure; GSM vs CDMA" subject: "Wireless Communication" chapter: 8 key: "semester_7/wireless_communication:gsm-frame-vs-cdma" source: https://www.bctengineer.com/q/semester_7/wireless_communication/gsm-frame-vs-cdma site: BCT Engineering --- # GSM frame structure; GSM vs CDMA Wireless Communication · Chapter 8: Wireless Systems and Standards - Asked in: 2076 Bhadra, 5+3 marks - Video: [Wireless Technology | Tutorial #23 | GSM Frame Structure](https://www.youtube.com/watch?v=kvtoswzO8wc) — RANJI RAJ - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/wireless_communication/gsm-frame-vs-cdma - Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-8 ## Question Explain the frame structure of GSM. Differentiate between GSM and CDMA standards. ## 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. ## Answer #### 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). ```text 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):** ```text |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 | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How do GSM and IS-95 CDMA differ in handoff? A. GSM uses hard handoff; CDMA uses soft and softer handoff B. Both use hard handoff C. GSM uses soft; CDMA uses hard D. Neither supports handoff Answer: A. CDMA with reuse of 1 can connect to several BTSs at once. (Key point: Handoff) 2. What frequency reuse factor does CDMA use compared with GSM? A. N = 7 versus N = 1 B. N = 1 versus N = 3-7 C. N = 1 for both D. N = 4 for both Answer: B. All CDMA cells share the 1.25 MHz carrier; GSM needs cluster reuse. (Key point: Reuse) 3. Which is correct for power control rates? A. GSM 800 Hz, CDMA 2 Hz B. Both 800 Hz C. GSM about 2 Hz, CDMA about 800 Hz D. Both 2 Hz Answer: C. CDMA needs fast power control to fight the near-far problem. (Key point: Power control) 4. How do GSM and CDMA handle multipath? A. GSM uses an adaptive equalizer; CDMA uses a RAKE receiver B. GSM uses RAKE; CDMA uses equalizer C. Both use only guard bands D. Neither handles multipath Answer: A. Equalizer with training sequence for GSM; RAKE combining resolvable paths for CDMA. (Key point: Multipath handling) ## Related in Wireless Communication - [GSM and CDMA standards; GSM architecture](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Agsm-cdma-standard-architecture) — Wireless Communication, chapter 8 - [CDMA merits/demerits; GSM frame efficiency](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Amultiple-access-cdma-frame-efficiency) — Wireless Communication, chapter 7 - [GSM frame structure, traffic and control channels](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Agsm-frame-traffic-control-channels) — Wireless Communication, chapter 8, asked 4× - [GSM channel structure and TDMA frame efficiency](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Agsm-channel-structure-efficiency) — Wireless Communication, chapter 8 - [GSM frame structure and channels during a call](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Agsm-frame-call-channels) — Wireless Communication, chapter 8 - [GSM NSS components and frame hierarchy](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Agsm-nss-frame-hierarchy) — Wireless Communication, chapter 8 ## Related in other subjects - [Work breakdown for frame structure quantities](https://www.bctengineer.com/question.md?key=semester_7%2Festimating_and_costing%3Awork-breakdown-quantity-frame) — Estimating and Costing, chapter 2 - [Flowchart vs structure chart](https://www.bctengineer.com/question.md?key=semester_6%2Fobject_oriented_analysis_and_design%3Aflowchart-vs-structure-chart) — Object Oriented Analysis and Design, chapter 1 - [Monolithic vs micro-kernel structure](https://www.bctengineer.com/question.md?key=semester_6%2Foperating_system%3Amonolithic-vs-microkernel) — Operating System, chapter 1 - [Primitive vs non-primitive data structure](https://www.bctengineer.com/question.md?key=semester_4%2Fdata_structure_and_algorithm%3Aprimitive-vs-nonprimitive) — Data Structure and Algorithm, chapter 1