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Wireless Communication Basics

Which digital modulation scheme represents data by changing the phase while keeping the carrier's amplitude and frequency constant?

  1. APSK
  2. BASK
  3. CFSK
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Answer: A. PSK

Digital modulation changes the amplitude, frequency or phase of a sinusoidal carrier according to the data being sent. ASK changes the amplitude, FSK the frequency and PSK the phase. QPSK, which uses four phases, can send 2 bits per change (per symbol). QAM changes both amplitude and phase.

  • A. Correct. PSK (phase-shift keying) represents data by differences in phase.
  • B. ASK (amplitude-shift keying) represents data by changing the carrier's amplitude.
  • C. FSK (frequency-shift keying) represents data by changing the carrier's frequency.
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  3. Which is a general property when the modulation is changed from 16QAM to 256QAM at the same symbol rate?
  4. For a channel with a bandwidth of 20 MHz and an S/N (signal-to-noise ratio) of 30 dB, which is closest to the channel capacity (the theoretical upper limit on the error-free transmission rate) given by Shannon's theorem?
  5. Which describes OFDM (orthogonal frequency-division multiplexing)?