Why phase modulation is popular for data links Pick the primary reason phase modulation (and related PSK schemes) is widely used for digital data transmission over radio channels.

Introduction / Context:
Digital radio systems must contend with noise, fading, and interference. The choice of modulation directly affects bit-error rate (BER) performance and spectral efficiency. Phase modulation and its digital forms (BPSK, QPSK, etc.) are staples of satellite and terrestrial microwave links due to their robustness.

Given Data / Assumptions:

• Data transmission in noise-limited and fading-limited channels.
• Amplitude fluctuations can be significant (e.g., scintillation, multipath fades).
• Coherent or differential detection available for PM/PSK.

Concept / Approach:

PSK and PM encode information in the signal phase rather than amplitude. Many noise processes primarily affect amplitude; by using limiters and coherent/differential detection, the receiver can maintain low BER at a given Eb/N0 compared with simple AM schemes. This resilience translates into practical performance advantages under fading and interference.

Step-by-Step Solution:

Verification / Alternative check:

Link simulations and BER curves show BPSK/QPSK outperform simple ASK/AM at the same Eb/N0. This is standard in satellite modems and microwave radio design.

Why Other Options Are Wrong:

(a) The phase range alone is not the reason; (c) demodulation requires coherent loops or differential methods and is not trivial; (d) highest data rate depends on bandwidth, SNR, and coding; (e) ISI requires filtering and equalization, not guaranteed by phase modulation itself.

Common Pitfalls:

Assuming PM always beats all schemes—performance depends on constraints like spectral masks and receiver complexity.

Final Answer:

Because it offers strong resistance to amplitude-noise and fading compared with AM