Difficulty: Medium
Correct Answer: PSK
Explanation:
Introduction / Context:Detection techniques are classified as coherent (requires carrier phase reference) or non-coherent (does not require carrier phase reference). Different modulation schemes have different requirements.
Given Data / Assumptions:
Concept / Approach:PSK encodes data in the phase of a carrier wave. Phase can only be correctly interpreted if the receiver has a reference to the carrier's phase, requiring coherent detection.
Step-by-Step Solution:
ASK can use envelope detectors (non-coherent).FSK can use frequency discriminators (non-coherent).PSK cannot be decoded without phase synchronization.Verification / Alternative check:Digital communication textbooks confirm that BPSK and QPSK require coherent detection, while non-coherent detection works for ASK/FSK.
Why Other Options Are Wrong:
ASK: envelope detection works.FSK: slope/energy detectors allow non-coherent decoding.Both (a) and (c): wrong because only (a) strictly requires coherent detection.Common Pitfalls:
Assuming FSK also strictly requires coherent detection.Final Answer:
PSK
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