>....but you expect to see same PeBit for QPSK when the SNR is 3.01 dB higher
>than for BPSK because, at the same symbol rate, they occupy the same
>bandwidth and the signal energy needs to be twice as big for QPSK to have
>the same energy per bit as BPSK.


While the above is a perfectly true statement, it glosses
over the fact that the BPSK system conveys one bit per
symbol interval, whereas the QPSK system conveys two
bits per symbol interval. If the *symbol* rate is the same
(i.e. the bandwidth is the same) and the signal power is
the same for both systems, then the QPSK system has
smaller Eb/No ratio and therefore poorer error-rate
performance (as noted above), but double the data rate
(in bits per second). If the *bit* rate is the same and the
signal power is the same, then both systems have the
same BER but the BPSK system uses twice the bandwidth.

In comparing apples and oranges, it is best to set the apples
to equal one another and compare the oranges, or set the
oranges equal to one another and compare the apples....