Quadrature Amplitude Modulation means changing both :
2017
Quadrature Amplitude Modulation means changing both :
- A.
Frequency and phase of the carrier.
- B.
Frequency and Amplitude of the carrier.
- C.
Amplitude and phase of the carrier.
- D.
Amplitude and Wavelength of the carrier.
Attempted by 101 students.
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Correct answer: C
Correct answer: Amplitude and phase of the carrier. Quadrature Amplitude Modulation (QAM) encodes information by changing both the amplitude and the phase of a carrier signal.
QAM uses two orthogonal carriers (cosine and sine) whose amplitudes are varied; these amplitudes are called the in‑phase (I) and quadrature (Q) components.
Each unique pair of I and Q amplitudes corresponds to a symbol with a distinct amplitude and phase.
Example: 16‑QAM uses 16 distinct amplitude–phase combinations to represent 4 bits per symbol.
Why the other choices are incorrect:
Frequency and phase of the carrier: Changing frequency is not part of QAM. Phase-only changes describe phase shift keying (PSK); frequency changes describe frequency-shift keying (FSK) or FM.
Frequency and amplitude of the carrier: This mixes frequency and amplitude modulation, which is not how QAM encodes symbols. QAM specifically pairs amplitude changes with phase changes.
Amplitude and wavelength of the carrier: Wavelength is just the inverse of frequency; QAM is defined by amplitude and phase variations, not by wavelength.