Advantages of QPSK?

Question

The advantage of the Quadrature Phase Shift Keying (QPSK) modulation versus the Binary Phase Shift Keying (BPSK) one is well known. It is the possibility to transmit in the same frequency band twice more information, while the number of errors and the Eb/No relation are the same. The probability of errors can be estimated as

where erfc is the tabulated error function [1], Eb is the energy per 1 bit, No is the noise spectral density in (W/Hz).
We will show that the above advantage of the QPSK method is not valid under certain conditions which are important for satellite communication system.

Answer 1

The disadvantage of QPSK relative to BPSK is that it is more sensitive to phase variations. See http://www.sasltd.com/T/faq/faq2.html.

Check out the paper at http://www.aeroastro.com/publications/SSC02-II-4.pdf comparing satellite communication modulation choices. Note that this is for inter-satellite communication, so consider that satellite to ground communication would have to account for much more atmospheric effects.

Answer 2

Advantages Of Qpsk

Answer 3

(I'm not really sure what the question is...but I'll try.)

You already state the common advantage of QPSK modulation over that of BPSK, so you have already answered your own question. However, you go to state that there may be some disadvantage of QPSK in satellite channels under some conditions.

This may be true due to the non-linear amplifier characteristic of the satellite. This property may force the QPSK channel spacing within the satellite channel (or transponder) to actually be farther apart than what would be normally required for BPSK. Then, it could be argued that the overall transponder capacity for QPSK could then be lower than that of BPSK.

It is not clear to me however that this would be true, certainly not always. Though maybe for very small bandwidth QPSK channels (relative to the transponder bandwidth) this might be the case.

Honestly, this is hard to conjecture without test or simulation data.