--- title: "Digital vs analog modulation; GMSK" subject: "Wireless Communication" chapter: 4 key: "semester_7/wireless_communication:digital-modulation-gmsk" source: https://www.bctengineer.com/q/semester_7/wireless_communication/digital-modulation-gmsk site: BCT Engineering --- # Digital vs analog modulation; GMSK Wireless Communication · Chapter 4: Modulation and Demodulation in Mobile Communication - Asked in: 2079 Bhadra, 2+6 marks - Video: [GMSK-Gaussian minimum shift keying P05 ECE23](https://www.youtube.com/watch?v=tIQ4ytrpqsM) — Miss Cheska - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/wireless_communication/digital-modulation-gmsk - Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-4 ## Question Mention advantages of digital modulation over analog modulation. Explain the transmission and detection process of GMSK modulation scheme with block diagram and constellation diagram. ## Short answer Digital modulation offers better noise immunity, error coding, encryption, easy multiplexing, and higher spectral efficiency than analog. GMSK passes NRZ data through a Gaussian LPF (BT=0.3), integrates it to form phase, and modulates I/Q carriers. The receiver uses coherent I/Q demodulation with Viterbi equalization; the constellation has constant envelope with phase moving by plus or minus 90 degrees per bit. ## Answer #### Advantages of digital over analog modulation - Better noise and fading immunity; signals can be regenerated. - Supports error detection/correction coding, encryption and source coding. - Easy multiplexing of voice, data and video (TDMA, CDMA, OFDM). - Higher spectral and power efficiency, and cheaper VLSI/DSP implementation. #### GMSK modulation **Gaussian MSK** is MSK in which the bipolar NRZ data passes through a **Gaussian low-pass filter** before frequency modulation with index h = 0.5. The filter smooths the phase trajectory, which reduces side lobes and out-of-band power. The 3 dB bandwidth–bit period product **BT** controls the shape: GSM uses BT = 0.3, and BT = ∞ gives MSK. ```text h_G(t) = (√π/α) exp(−π² t² / α²), α = 0.5887 / B ``` **Transmitter** ```text NRZ -> [Gaussian LPF] -> [Integrator] -> φ(t) data (BT = 0.3) | +---------+----------+ v v [cos φ] [sin φ] | | cos ωct->(x) sin ωct->(x) +------(+)-----------+ | GMSK ``` - The Gaussian filter spreads each bit over about 3 bit periods (controlled ISI). - The filtered signal is integrated to give the phase φ(t); I = cos φ, Q = sin φ modulate quadrature carriers. (A simpler form feeds the filtered data directly to a VCO with h = 0.5.) **Receiver** ```text GMSK->[BPF]-+->(x)->[LPF]->[Sample]->I-+ | cos | | [MLSE / ]->data +->(x)->[LPF]->[Sample]->Q-+ [decision] sin ^ [Carrier & clock recovery] ``` - Coherent I/Q demodulation recovers the phase; I and Q are sampled at alternate bit instants as in MSK. - Because the Gaussian filter adds ISI, GSM receivers use a Viterbi (MLSE) equalizer. Non-coherent detection with a limiter–discriminator is also possible. **Constellation (phase states)** ```text Q • | •------+------• I | • Constant envelope; phase moves smoothly by ±π/2 per bit. ``` **Merits:** constant envelope (class-C amplifiers, long battery life), narrow spectrum (low adjacent channel interference), used in GSM, DECT and Bluetooth (GFSK variant). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which is an advantage of digital over analog modulation? A. It supports error-correcting coding, encryption and signal regeneration B. It always needs less bandwidth than analog C. It cannot be multiplexed with other signals D. It is immune to all noise without coding Answer: A. Digital signals can be regenerated, coded for error control, encrypted and easily multiplexed, giving better noise and fading immunity. (Key point: Digital advantages) 2. What does the Gaussian filter do to the GMSK phase trajectory? A. It makes it jump by pi at each bit B. It smooths it, reducing side lobes and out-of-band power C. It makes the phase constant D. It makes the envelope vary with data Answer: B. Smoother phase transitions give a more compact spectrum, at the cost of controlled ISI spread over about 3 bit periods. (Key point: Gaussian filter effect) 3. In the I/Q GMSK transmitter, what comes right after the Gaussian LPF? A. A PN code multiplier B. An IFFT block C. An integrator giving phi(t), then cos(phi) and sin(phi) on quadrature carriers D. A frequency synthesizer hopping the carrier Answer: C. The filtered data is integrated to the phase phi(t); I = cos phi and Q = sin phi modulate the quadrature carriers and are summed. (Key point: Transmitter chain) 4. Why is an MLSE/Viterbi equalizer used after I/Q demodulation of GMSK? A. To despread the PN code B. To remove the cyclic prefix C. To restore the lost carrier amplitude D. To handle the ISI added by the Gaussian filter Answer: D. Coherent I/Q demodulation recovers the phase, but the Gaussian filter's ISI is resolved by GSM's Viterbi equalizer. (Key point: Receiver) ## Related in Wireless Communication - [GMSK transmission and detection](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Agmsk-transmission-detection) — Wireless Communication, chapter 4, asked 5× - [QPSK transmission and detection](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aqpsk-transmission-detection) — Wireless Communication, chapter 4, asked 5× - [Short note: GMSK modulation technique](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Agmsk-short-note) — 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 - [Reasons for modulation; digital communication block diagram](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Amodulation-reasons-digital-adv-duplex) — Communication Systems, chapter 1 - [Digital vs analog communication; DCS block diagram](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Adigital-vs-analog-dcs-block) — Communication Systems, chapter 1 - [Analog communication block diagram; modulation and its reasons](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Aanalog-comm-modulation-reasons) — Communication Systems, chapter 1, asked 3× - [Modulation; analog communication system block diagram](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Amodulation-analog-block-diagram) — Communication Systems, chapter 1, asked 3×