Questions: Phase Shift Keying Modulation

Question 1

An engineer switches a communication link from BPSK to QPSK while keeping the same symbol rate and carrier power. What changes?

• A. Spectral efficiency doubles — 2 bits per symbol instead of 1 — with no change in required bandwidth
• B. Bit error rate improves, because more phase states provide better noise immunity
• C. Required bandwidth doubles to accommodate the additional phase states
• D. Carrier power per symbol must increase to maintain adequate phase separation

Answer: (A)

QPSK encodes 2 bits per symbol versus BPSK's 1 bit, but uses the same bandwidth (bandwidth depends on symbol rate, not modulation order). Spectral efficiency therefore doubles. Option 1 (BER improves) is the opposite of the truth: QPSK's four constellation points are closer together than BPSK's two, so it is more susceptible to noise for the same carrier power per symbol. Option 2 (bandwidth doubles) confuses bandwidth with constellation complexity — bandwidth is set by the symbol rate, which hasn't changed.

Question 2

A BPSK receiver loses lock on carrier phase. What is the consequence for demodulation?

• A. Demodulation fails — coherent PSK detection requires a phase reference to determine which constellation point the received signal is closest to
• B. The receiver automatically switches to envelope (noncoherent) detection, recovering bits with higher error rate
• C. Error rate doubles, but roughly 50% of bits are still correctly recovered by chance
• D. Demodulation is unaffected because BPSK is inherently phase-insensitive

Answer: (A)

PSK encodes information entirely in phase. Coherent demodulation works by measuring the phase of the received signal relative to a reference and assigning it to the nearest constellation point. Without carrier phase recovery, the 'nearest point' comparison has no reference — the decision boundary is undefined. Option 3 (50% correct) is plausible but wrong: with no phase reference, symbol decisions are essentially random. This is precisely why carrier phase recovery (via a PLL or other scheme) is described as the central practical challenge of PSK systems.

Question 3

In QPSK, Gray coding is used so that adjacent constellation points differ by exactly two bits, limiting burst errors when noise causes misdetection.

• T. True
• F. False

Answer: False

Gray coding ensures adjacent constellation points differ by exactly ONE bit — not two. The purpose is exactly as stated (limiting error severity), but the mechanism is one-bit differences. When noise causes a detection error (misidentifying a QPSK point as a neighbor), Gray coding ensures only a single bit error results rather than two. With natural binary mapping, diagonal neighbors could differ by two bits, doubling the bit error cost of certain detection errors.

Question 4

As M increases in M-ary PSK, each doubling of M adds one more bit per symbol transmitted but requires higher SNR to maintain the same bit error rate.

• T. True
• F. False

Answer: True

With M phases equally spaced on a circle, adding more phases (doubling M) increases bits per symbol by 1 but shrinks the angular distance between adjacent points. Smaller angular separation means a smaller noise margin — the receiver must more precisely measure phase to distinguish neighbors. Each additional bit per symbol costs approximately 6 dB of required SNR in M-ary PSK, which is why practical systems switch to QAM rather than continuing to increase M.

Question 5

Why do engineers switch from M-ary PSK to QAM for high spectral efficiency rather than simply increasing M indefinitely in PSK?

Model answer: In M-ary PSK, all constellation points lie on a circle of fixed radius — they can only be spread angularly, so as M increases, points crowd together and angular separation shrinks rapidly. QAM uses both phase AND amplitude variation, placing points across a 2D grid. For the same average power and number of constellation points, QAM achieves larger minimum distances between points than PSK, yielding better noise immunity at the same spectral efficiency. Beyond 8-PSK, QAM constellations (16-QAM, 64-QAM) outperform M-PSK at the same spectral efficiency, making higher M-ary PSK impractical.

The geometric insight is key: PSK constrains points to a 1D manifold (a circle), while QAM uses the full 2D plane. This allows QAM to pack more points at larger minimum distances for the same average power. Modern high-throughput systems (Wi-Fi, 5G) use 256-QAM or 1024-QAM rather than M-ary PSK precisely for this reason.