--- title: "Need for shift keying; ASK vs FSK vs PSK" subject: "Communication Systems" chapter: 7 key: "semester_6/communication_systems:shift-keying-ask-fsk-psk" source: https://www.bctengineer.com/q/semester_6/communication_systems/shift-keying-ask-fsk-psk site: BCT Engineering --- # Need for shift keying; ASK vs FSK vs PSK Communication Systems · Chapter 7: Digital Modulation Techniques - Asked in: 2064 Shrawan (CS I), 3+5 marks - Video: [Digital modulation: ASK, FSK, and PSK](https://www.youtube.com/watch?v=qGwUOvErR8Q) — Sunny Classroom - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/communication_systems/shift-keying-ask-fsk-psk - Whole chapter: https://www.bctengineer.com/md/semester_6/communication_systems/chapter-7 ## Question Why do we use shift keying technique in communication system? Distinguish between ASK, FSK and PSK. ## Short answer Shift keying transmits digital data over bandpass channels, enabling small antennas, multiplexing, and noise control. ASK varies amplitude (poor noise immunity), FSK varies frequency (constant envelope, better than ASK), PSK varies phase (constant envelope, best noise performance but requires coherent detection). ## Answer #### Why shift keying is used **Shift keying** (digital carrier modulation) means switching the amplitude, frequency or phase of a high-frequency sinusoidal carrier according to the binary data. It is used because: - **Bandpass channels:** radio, satellite, microwave and telephone channels pass only a band of frequencies around some centre frequency; baseband pulses (with energy near DC) cannot go through them directly. - **Practical antenna size:** antennas must be about $\lambda/4$ long; a high carrier frequency makes them small. - **Multiplexing:** different users can be given different carrier frequencies (FDM). - **Noise and interference control:** FSK and PSK keep a constant envelope and so resist amplitude noise, and the scheme can be chosen to trade bandwidth for power. - **Long-distance transmission** over wireless and existing analog lines (modems). #### ASK vs FSK vs PSK | Point | ASK | FSK | PSK | |---|---|---|---| | Parameter changed | Amplitude | Frequency | Phase | | Signal for 1 / 0 | $A\cos\omega_ct$ / 0 | $A\cos\omega_1t$ / $A\cos\omega_2t$ | $A\cos\omega_ct$ / $-A\cos\omega_ct$ | | Envelope | Varies (on-off) | Constant | Constant | | Bandwidth (approx.) | $2R_b$ | $\lvert f_1-f_2\rvert+2R_b$ (largest) | $2R_b$ | | Noise immunity | Poor | Better than ASK | Best | | $P_e$ (coherent) | $\tfrac12\text{erfc}\sqrt{E_b/2N_0}$ | $\tfrac12\text{erfc}\sqrt{E_b/2N_0}$ | $\tfrac12\text{erfc}\sqrt{E_b/N_0}$ | | Detection | Envelope or coherent | Non-coherent or coherent | Coherent only (or DPSK) | | Complexity | Simplest | Moderate | Most complex (carrier recovery) | | Signal space | 1-D, points 0 and $\sqrt{2E_b}$ | 2-D, orthogonal points | 1-D, antipodal $\pm\sqrt{E_b}$ | | Typical use | Optical fibre, low-rate links | Low-speed modems, paging | Satellite, Wi-Fi, high-speed modems | ```text data : 1 0 1 ASK : /\/\/\ ______ /\/\/\ FSK : /\/\/\ /\ /\ /\/\/\ (fewer cycles for 0) PSK : /\/\/\ \/\/\/ /\/\/\ (phase flips) ``` ($E_b$ is the average energy per bit; for ASK the "1" bit has energy $2E_b$. On this basis coherent ASK has the same $P_e$ as coherent FSK, but its decision threshold depends on the received amplitude, so it suffers more from amplitude noise and fading. If $E_b$ is instead taken as the energy of the "1" bit, the ASK result is written $\tfrac12\text{erfc}\sqrt{E_b/4N_0}$.) ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why can baseband pulses not be sent directly on radio or satellite channels? A. Baseband pulses are too weak in amplitude B. These channels pass only a band around a centre frequency, not frequencies near DC C. Digital pulses cannot be amplified D. Antennas require square pulses Answer: B. A carrier shifts the signal spectrum into the passband of the channel. (Key point: Reason for shift keying) 2. Which scheme has the largest bandwidth? A. ASK, about 2Rb B. PSK, about 2Rb C. FSK, about |f1 - f2| + 2Rb D. All are exactly the same Answer: C. FSK spreads over two tones as well as the keying bandwidth. (Key point: Bandwidth comparison) 3. Which statement about the envelope is correct? A. ASK has constant envelope B. PSK has a varying envelope C. FSK has an on-off envelope D. FSK and PSK have constant envelopes, so resist amplitude noise better than ASK Answer: D. ASK is on-off so its envelope varies; FSK and PSK keep constant amplitude. (Key point: Envelope and noise) 4. Which gives the best coherent error probability for same Eb/N0? A. PSK: (1/2)erfc sqrt(Eb/N0) B. ASK: (1/2)erfc sqrt(Eb/N0) C. FSK: (1/2)erfc sqrt(Eb/N0) D. They are all equal Answer: A. PSK has distance 2sqrt(Eb); FSK has Eb/2N0 and ASK Eb/4N0 inside the erfc. (Key point: Error performance) ## Related in Communication Systems - [ASK and FSK modulators/demodulators; PSK with envelope detector](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Aask-fsk-mod-demod-psk-envelope) — Communication Systems, chapter 7 - [Coherent ASK and PSK error; ASK needs double power](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Aask-psk-error-double-power) — Communication Systems, chapter 9 - [Communication system block diagram; need for modulation](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Acomm-block-diagram-needs-modulation) — Communication Systems, chapter 1 - [PSK generation and detection; GMSK](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Apsk-generation-detection-gmsk) — Communication Systems, chapter 7 - [Threshold effect in FM; error probability of coherent PSK](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Athreshold-effect-psk-error) — Communication Systems, chapter 9 - [Error probability of coherent ASK](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Aask-coherent-error-probability) — Communication Systems, chapter 9, asked 2× ## Related in other subjects - [ASK, FSK and PSK modulation](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Aask-fsk-psk) — Data Communication, chapter 5, asked 3× - [M-ary QAM; 16-QAM vs 16-PSK](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Amary-qam-vs-psk) — Wireless Communication, chapter 4 - [Need for equalization and basic technique](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aequalization-need-basic) — Wireless Communication, chapter 5 - [Why do we need DFT](https://www.bctengineer.com/question.md?key=semester_7%2Fdigital_signal_analysis_and_processing%3Awhy-need-dft) — Digital Signal Analysis and Processing, chapter 7, asked 6×