The diversity gain degradations due to fading correlation and unequal average power are investigated for practical, two-branch postdetection selection diversity reception. The average bit error rate (BER) of pi/4-shift QDPSK is theoretically analyzed taking into account additive white Gaussian noise (AWGN), cochannel interference, and multipath channel delay spread; exact diversity gain degradations are calculated. This paper also presents very simple and useful approximate expressions for the gain degradations. When the fading correlation and the power ratio between the two antennas are given by h12(\h12\ less-than-or-equal-to 1) and q(less-than-or-equal-to 1), respectively, the total gain degradation is D = 5log{q(1 - \h12\2)} dB if the errors are caused by AWGN, while it is 5log{2q(1 - \h12\2)/(1 + q2)} dB if cochannel interference and delay spread are the cause of errors.
