Questions: Modulation: Amplitude, Frequency, and Phase Shift Keying

Question 1

A wireless sensor in an industrial environment is subject to amplitude fading — signal strength fluctuates unpredictably as it reflects off metal surfaces. An engineer must choose between ASK, FSK, and BPSK. Which scheme is LEAST suitable for this environment?

• A. ASK, because amplitude fading directly corrupts the amplitude-encoded information
• B. FSK, because frequency changes are distorted by amplitude fading
• C. BPSK, because phase detection requires a very stable amplitude reference
• D. All three are equally affected, since amplitude fading degrades signal energy regardless of modulation type

Answer: (A)

ASK (including on-off keying) encodes information in the amplitude of the carrier. When the channel introduces amplitude fading, the received signal amplitude no longer reliably reflects the transmitted symbol — a '1' may arrive looking like a '0'. FSK encodes information in frequency: fading changes signal strength but not which frequency is present, so the receiver can still detect the correct symbol. PSK encodes in phase, also robust to amplitude variations. Option D reflects the common misconception that fading affects all modulations equally — what matters is whether fading corrupts the information-bearing parameter, and only ASK has that vulnerability.

Question 2

What are the two primary reasons for modulating a baseband signal onto a high-frequency carrier before wireless transmission?

• A. To compress the signal and reduce file size; and to allow the receiver to filter noise more easily
• B. To shift the signal spectrum to frequencies where antennas are physically practical; and to enable multiple signals to share the same medium via different carrier frequencies
• C. To encrypt the data so the signal cannot be intercepted; and to reduce the bandwidth required
• D. To increase the bit rate beyond what the baseband signal supports; and to eliminate the need for carrier synchronization

Answer: (B)

The two fundamental reasons are: (1) frequency shifting for practical antenna design — a 1 kHz audio signal has a 300 km wavelength requiring an impossible antenna, while 100 MHz has a ~1.5 m wavelength; and (2) frequency division multiplexing — different carrier frequencies allow many signals to coexist on the same medium without interfering. Options A, C, and D describe effects that are either wrong (modulation does not compress or encrypt) or not primary motivations for modulation.

Question 3

In Binary Phase Shift Keying (BPSK), transmitting a '0' bit uses a carrier signal with zero amplitude.

• T. True
• F. False

Answer: False

BPSK is a phase modulation scheme, not amplitude. Both '0' and '1' are transmitted at the same amplitude — what changes is the phase. A '1' is transmitted as cos(2πf_c t) (0° phase) and a '0' as cos(2πf_c t + π) = −cos(2πf_c t) (180° phase shift). The common confusion is with ASK on-off keying, where a '0' IS transmitted as zero amplitude. The distinction matters: BPSK's constant amplitude makes it far more robust to fading than ASK, which is why PSK is preferred in noise-prone environments.

Question 4

Higher-order modulation schemes like 16-QAM transmit more bits per symbol than BPSK, making them more spectrally efficient but also more susceptible to noise and channel impairments.

• T. True
• F. False

Answer: True

16-QAM encodes 4 bits per symbol using 16 amplitude-phase combinations; BPSK encodes 1 bit per symbol using 2 phase states. More bits per symbol means higher data rate in the same bandwidth — higher spectral efficiency. However, the 16 signal points must be packed more closely together in signal space, making them harder to distinguish when noise or channel distortion shifts a symbol. Higher-order modulation requires a higher signal-to-noise ratio to achieve the same bit error rate. This fundamental tradeoff drives the adaptive modulation schemes in modern 4G/5G systems.

Question 5

Why does shifting a baseband signal to a higher carrier frequency enable more efficient wireless transmission, even though the information content is unchanged?

Model answer: Antenna efficiency depends on wavelength — an antenna radiates most efficiently when its length is a significant fraction of the signal's wavelength. A 1 kHz audio signal has a 300 km wavelength, requiring a physically impossible antenna. Shifting that signal to 100 MHz (wavelength ~3 m) makes a practical ~1.5 m antenna feasible. Additionally, higher frequencies enable frequency division multiplexing: different transmitters use different carrier frequencies, allowing many signals to coexist on the same physical medium without interference. Baseband transmission cannot achieve this since all baseband signals occupy the same low-frequency region.

This is why modulation exists as a concept at all — baseband transmission of information is physically impractical for wireless channels. The information travels just as well at high frequency as low frequency, but the physical constraints of antennas and spectrum sharing are radically different. Every wireless communication system, from AM radio to 5G, exploits this principle.