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I wonder why there are still some modems using BPSK as their modulation. Is there any benefit over QPSK (power, bandwidth, synchronization techniques, etc.)?

I have been searching and found these:

  1. BPSK has a bit more PAPR than QPSK
  2. With the same power, BPSK has 3 dB more energy on its in-phase component (QPSK has 3 dB less energy on its in-phase and quadrature but uses quadrature to compensate)
  3. They both have the same probability of error vs Eb/N0 performance
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2 Answers 2

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From a hardware perspective, BPSK in general lends itself to less complex modulators, demodulators and carrier-phase-recovery circuits when compared to QPSK, due to the need for a quadrature not being strictly required.

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From Dr. Fuyun Ling:

QPSK needs twice of SNR, but it can transmit twice of bits in the same unit time. Thus, they have the same Eb/No, so in this respect they are equivalent. In the spectrum efficiency is not an issue, e.g., in the case of spread spectrum, BPSK could be preferable. Of course, if higher spectrum efficiency is needed, QPSK or higher modulation should be used.

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