1) This is a follow up question to the most up-voted answer to this question.
2) The accepted answer is that "QPSK transmits 1 bit per Hz at passband".
3) The theory found in books seems to agree with this answer.
4) But this defies my logic and my experience.
5) My logic and experience tells me that QPSK transmits 2 bits per Hz at passband.
6) From experience, I know that in order to acquire 1 MHz of complex bandwidth (at baseband), I need to set the IQ rate of my receiver to 1 Mega samples per second. (i.e. IQ rate = Complex Bandwidth, and keep in mind that the IQ rate usually needs to be slightly higher to account for the roll-off of an anti-aliasing filter. But let's assume that there is no filter for simplicity.)
7) At baseband, this is the equivalent of 500 KHz of bandwidth if you don't consider the negative frequencies.
8) And at passband, the total bandwidth will be 1 MHz because the negative frequencies "appear" as the frequencies on the "lower" side of the carrier frequency and they are now taken into consideration on the amount of bandwidth, effectively "doubling" the bandwidth.
9) With an IQ rate of 1 Mega samples per second, I can send 2 bits on each sample (because it is QPSK).
10) So the effective data rate is 2 Mbps for a bandwidth of 1 MHz at passband (in my experience).
11) Refer to image below, describing relationship between data rate and bandwidth in my experience.
12) Other people seem to have obtained similar results to mine:
A) Refer to the last video on these student experiments: They show a transmission of 2 Mbps with a 1 MHz QPSK signal.
B) In this video a 4 KHz data rate signal results in a 2 KHz QPSK 3dB spectrum.