The fundamental difference between Amplitude-Shift Keying (ASK) and Frequency-Shift Keying (FSK) lies in which property of the carrier wave is modified to encode data. In ASK, the amplitude of a constant-frequency carrier wave is varied to represent binary data (e.g., high amplitude for '1', low/zero amplitude for '0'). In contrast, FSK maintains a constant amplitude carrier wave but shifts its frequency between two or more discrete values to represent the data (e.g., frequency f1 for '1', frequency f2 for '0'). This distinction is the defining characteristic of each modulation scheme.

B. FSK is more sensitive to noise than ASK

This is incorrect. ASK is generally more susceptible to noise and interference because these phenomena often manifest as amplitude variations, which can be misinterpreted as data by an ASK receiver.

C. ASK does not work at high frequencies

This is false. Both ASK and FSK are used across a wide spectrum, including high-frequency (HF), very-high-frequency (VHF), and ultra-high-frequency (UHF) bands, depending on the application.

D. ASK can carry more data than FSK

This statement is not universally true. The data rate depends on factors like available bandwidth, signal-to-noise ratio, and the specific variant of the modulation. Neither scheme is inherently superior in all scenarios.

1. Madhow, U. (2014). Introduction to Communication Systems. Cambridge University Press.

Page 168, Section 5.1: "In amplitude shift keying (ASK), the transmitted signal corresponding to bit b is sb(t) = Ab cos(2πfc t), where A0 and A1 are two different amplitudes."

Page 173, Section 5.2: "In frequency shift keying (FSK), the transmitted signal corresponding to bit b is sb(t) = A cos(2πfb t), where f0 and f1 are two different frequencies near the carrier frequency fc."

2. Gallager, R. G. (2008). Course 6.450, Principles of Digital Communications I, Lecture Notes. MIT OpenCourseWare.

Lecture 10, Page 10-1: "In ASK, the information is carried in the amplitude of a sinusoid... In FSK, the information is carried in the frequency of a sinusoid."

3. Proakis, J. G., & Salehi, M. (2008). Digital Communications (5th ed.). McGraw-Hill.

Page 166, Section 4.2.2: Discusses the higher susceptibility of ASK to noise compared to FSK, stating, "amplitude-shift keying is a linear modulation method and, consequently, it is suboptimal in terms of power-efficiency." FSK is a non-linear modulation which provides better performance in the presence of noise.