7rots51 all you have to do is recognize that the S/N
ratio, in the equation given by Borber, depends
on the modulation(in your case, QPSK) and the
level of error you will want it to be. In others words,
for example for a Pe=10^-6 ( one bit error in a
million) the S/N ratio required could be 15 dB, by the
the table or by the equation of Pe=f(S/N).