--- title: "MSK, GMSK and OFDM modulator/demodulator" subject: "Wireless Communication" chapter: 4 key: "semester_7/wireless_communication:msk-gmsk-ofdm-modem" source: https://www.bctengineer.com/q/semester_7/wireless_communication/msk-gmsk-ofdm-modem site: BCT Engineering --- # MSK, GMSK and OFDM modulator/demodulator Wireless Communication · Chapter 4: Modulation and Demodulation in Mobile Communication - Asked in: 2070 Magh, 8 marks - Video: [WC U3 2 MSK and GMSK](https://www.youtube.com/watch?v=2bIR3cO5wSw) — ECE Web Content - RCET - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/wireless_communication/msk-gmsk-ofdm-modem - Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-4 ## Question What are MSK and GMSK modulation techniques? Draw the block diagram of OFDM modulator and demodulator and explain them. ## Short answer MSK is continuous-phase FSK with h=0.5 and smooth plus or minus 90 degree phase changes; GMSK adds a Gaussian filter for a compact spectrum. The OFDM modulator maps bits, takes N-point IFFT, adds cyclic prefix and up-converts; the demodulator removes CP, takes FFT, equalizes each subcarrier and demaps. ## Answer #### MSK and GMSK - **MSK** is continuous phase FSK with modulation index h = 0.5 (tone spacing R_b/2). Phase changes smoothly by ±π/2 per bit; constant envelope; can be seen as OQPSK with half-sine pulse shaping. - **GMSK** is MSK whose data first passes a Gaussian low-pass filter (BT = 0.3 in GSM). This makes the spectrum very compact while keeping the constant envelope. #### OFDM modulator ```text bits->[Map QAM]->[S/P]->[IFFT]->[P/S]->[Add CP] N-pt | v RF <-[Up-conv]<-[DAC] ``` - Bits are mapped to QAM/PSK symbols and split into N parallel streams. - The **IFFT** places each symbol on a separate orthogonal subcarrier (spacing 1/T_s) and produces one time-domain OFDM symbol. - After P/S conversion a **cyclic prefix** longer than the channel delay spread is added, then DAC and RF up-conversion. #### OFDM demodulator ```text RF->[Down-conv]->[ADC]->[Remove CP]->[S/P]->[FFT] | v bits<-[Demap]<-[P/S]<-[One-tap equalizer (pilots)] ``` - After down-conversion and sampling, the CP is removed (it carries the ISI). - The **FFT** separates the subcarriers: Y_k = H_k X_k + N_k. - Each subcarrier is equalized by dividing by its channel estimate H_k, then demapped to bits. OFDM turns a frequency-selective channel into many flat sub-channels, so it handles high data rates in multipath (Wi-Fi, LTE, 5G). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What describes MSK? A. FSK with h = 1 and phase jumps B. QAM with Gaussian filter C. Multicarrier modulation with an IFFT D. Continuous phase FSK with h = 0.5 and constant envelope; OQPSK with half-sine shaping Answer: D. MSK phase changes smoothly by +/-pi/2 per bit. (Key point: MSK) 2. What value of BT does GSM use for GMSK? A. 0.3 B. 3 C. 0.03 D. Infinity Answer: A. BT = 0.3 gives a compact spectrum; BT = infinity is MSK. (Key point: GMSK in GSM) 3. What does the IFFT do in the OFDM modulator? A. Removes ISI after reception B. Places each symbol on a separate orthogonal subcarrier, forming a time-domain OFDM symbol C. Spreads symbols with a PN code D. Filters data with a Gaussian response Answer: B. The IFFT modulates all N subcarriers at once; then a cyclic prefix is added. (Key point: OFDM modulator) 4. After the FFT in the demodulator, how is each subcarrier equalized? A. By a long adaptive time-domain filter B. By multiplying with a PN code C. By dividing by its channel estimate H_k (one tap) D. By the Viterbi algorithm Answer: C. Y_k = H_k X_k + N_k, so one division per subcarrier works. (Key point: OFDM demodulator) ## Related in Wireless Communication - [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 - [Digital vs analog modulation; GMSK](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Adigital-modulation-gmsk) — Wireless Communication, chapter 4 - [MSK modulation and DSSS](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Amsk-dsss) — Wireless Communication, chapter 4 - [Short note: GMSK modulation technique](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Agmsk-short-note) — Wireless Communication, chapter 4 - [Space diversity techniques with block diagrams](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aspace-diversity-block-diagrams) — Wireless Communication, chapter 5 ## Related in other subjects - [QPSK modulator, demodulator and signal space](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Aqpsk-mod-demod-signal-space) — Communication Systems, chapter 7, asked 2× - [FSK modulator, demodulator and signal space](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Afsk-mod-demod-signal-space) — Communication Systems, chapter 7, asked 2× - [Why DPSK over PSK; DPSK modulator, demodulator, signal space](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Adpsk-vs-psk-mod-demod) — Communication Systems, chapter 7 - [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×