---
title: "Short note: GMSK modulation technique"
subject: "Wireless Communication"
chapter: 4
key: "semester_7/wireless_communication:gmsk-short-note"
source: https://www.bctengineer.com/q/semester_7/wireless_communication/gmsk-short-note
site: BCT Engineering
---

# Short note: GMSK modulation technique

Wireless Communication · Chapter 4: Modulation and Demodulation in Mobile Communication

- Asked in: 2070 Bhadra, 3 marks
- Video: [Gaussian Minimum shift Keying-GMSK](https://www.youtube.com/watch?v=AU00f3jEXHo) — deva Sena
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/wireless_communication/gmsk-short-note
- Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-4

## Question

Write a short note on GMSK modulation technique.

## Short answer

GMSK is constant-envelope MSK whose data passes through a Gaussian low-pass filter, smoothing phase and reducing side lobes. The BT product trades spectrum compactness against ISI; GSM uses BT=0.3, allowing efficient nonlinear amplifiers and coherent or non-coherent detection.

## Answer

**Gaussian Minimum Shift Keying (GMSK)** is a constant-envelope digital modulation obtained by passing NRZ data through a **Gaussian low-pass filter** before MSK (FM with modulation index h = 0.5).
```text
data->[Gaussian LPF (BT)]->[FM/VCO, h = 0.5]-> GMSK
```
- The filter smooths the phase path, so side lobes and adjacent-channel interference are much lower than MSK.
- **BT product** (3 dB filter bandwidth × bit period) sets the trade-off: smaller BT gives a narrower spectrum but more ISI. GSM uses BT = 0.3.
- Constant envelope allows efficient non-linear (class C) amplifiers.
- Can be detected coherently (like MSK) or non-coherently with an FM discriminator; GSM uses an MLSE equalizer to handle the ISI.
- Used in GSM (270.833 kbps in 200 kHz), DECT and, in GFSK form, Bluetooth.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. In GMSK, what is the purpose of the Gaussian low-pass filter placed before the frequency modulator?
   A. It smooths the phase path so side lobes and out-of-band power fall, while the envelope stays constant
   B. It removes the carrier so the signal can be detected non-coherently
   C. It makes the envelope vary so the signal can use linear amplifiers
   D. It increases the modulation index above 0.5 to widen the spectrum
   Answer: A. The Gaussian filter smooths the NRZ data, so the phase changes gradually. This gives a compact spectrum with lower side lobes and a still-constant envelope. (Key point: Gaussian filter role)

2. What happens when the BT product of a GMSK filter is made smaller (for example from infinity to 0.3)?
   A. The spectrum becomes wider but ISI decreases
   B. The spectrum becomes narrower but ISI increases
   C. Both the spectrum width and ISI decrease
   D. The envelope stops being constant
   Answer: B. Smaller BT means a narrower Gaussian filter, so the spectrum is more compact but each bit spreads into neighbours (more ISI). BT = infinity is plain MSK. (Key point: BT product)

3. What modulation index does the FM stage of GMSK use?
   A. h = 1
   B. h = 0.3
   C. h = 0.5
   D. h = 2
   Answer: C. GMSK is MSK with a Gaussian pre-filter, and MSK uses h = 0.5. BT = 0.3 is the filter product, not the index. (Key point: Modulation index)

## Related in Wireless Communication

- [Digital vs analog modulation; GMSK](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Adigital-modulation-gmsk) — Wireless Communication, chapter 4
- [GMSK transmission and detection](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Agmsk-transmission-detection) — Wireless Communication, chapter 4, asked 5×
- [MSK, GMSK and advantages of spread spectrum](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Amsk-gmsk-spread-spectrum-advantages) — Wireless Communication, chapter 4
- [MSK as CPFSK/OQPSK; compare OQPSK, MSK, GMSK](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Amsk-oqpsk-gmsk-comparison) — Wireless Communication, chapter 4
- [MSK, GMSK and OFDM modulator/demodulator](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Amsk-gmsk-ofdm-modem) — Wireless Communication, chapter 4

## Related in other subjects

- [Line coding in PCM and the right technique](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aline-coding-pcm-solution) — Telecommunications, chapter 3
- [BPSK modulation and signal space diagram](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Abpsk-signal-space) — Communication Systems, chapter 7
- [Design goals of digital modulation; coherent BPSK](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Adesign-goals-bpsk) — Communication Systems, chapter 7