If a carrier modulated by a digital bit stream has one of the possible phases 0°, 90°, 180° and 270°, then…

Question

If a carrier modulated by a digital bit stream has one of the possible phases 0°, 90°, 180° and 270°, then the modulation is called

Accepted Answer

Correct answer: B — Concept: In phase-shift keying (PSK) the information rides entirely in the phase of a constant-amplitude carrier. A PSK scheme that uses M distinct, equally spaced phase states is called M-ary PSK: each transmitted symbol selects one of those M phases, the states sit 360°/M apart, and a symbol therefore carries log2M bits. The count of allowed phase states is what names the scheme. Application: The stem enumerates exactly four allowed phase values — 0°, 90°, 180° and 270° — and describes the carrier by nothing else, so the scheme is identified by those phase states alone. They are spaced 360°/4 = 90° apart, so M = 4 and each symbol carries log24 = 2 bits: the four bit pairs 00, 01, 10 and 11 map one-to-one onto the four phases. Four-phase PSK is the quadrature case, named QPSK. Contrast with the other options: Scheme What carries the data Why the four listed phases do not name it BPSK Phase; two states 180° apart, 1 bit per symbol Only two phase values exist in the scheme, so four distinct phase values cannot be represented at all. QAM Amplitude and phase jointly; named by total constellation point count (16-QAM, 64-QAM) The stem lists phase values with no amplitude levels, so the phase-only naming applies. The four-point 4-QAM constellation does coincide with QPSK, but a four-point scheme described purely by its phases takes its PSK name. MSK Frequency; two tones separated by half the bit rate, with continuously gliding phase The phase glides by ±90° across each bit interval instead of resting at fixed states, so there is no set of four instantaneous phases to enumerate. Four equally spaced phase states, 2 bits per symbol: QPSK.

Answer

A. BPSK

Answer

B. QPSK

Answer

C. QAM

Answer

D. MSK

Scheme	What carries the data	Why the four listed phases do not name it
BPSK	Phase; two states 180° apart, 1 bit per symbol	Only two phase values exist in the scheme, so four distinct phase values cannot be represented at all.
QAM	Amplitude and phase jointly; named by total constellation point count (16-QAM, 64-QAM)	The stem lists phase values with no amplitude levels, so the phase-only naming applies. The four-point 4-QAM constellation does coincide with QPSK, but a four-point scheme described purely by its phases takes its PSK name.
MSK	Frequency; two tones separated by half the bit rate, with continuously gliding phase	The phase glides by ±90° across each bit interval instead of resting at fixed states, so there is no set of four instantaneous phases to enumerate.

If a carrier modulated by a digital bit stream has one of the possible phases…