  This paper presents an analysis of how to calculate bit error ratio (BER)
with physical explanation for optically pre-amplified DPSK receivers using
optical Mach-Zehnder interferometer (MZI) demodulation and balanced detection.
It is shown that BER calculation method for this kind of receivers is different
from the conventional calculation method used widely for IM/DD receivers. An
analytical relationship in receiver sensitivity between DPSK receivers using
MZI demodulation with balanced detection and IM/DD receivers (or DPSK receivers
using MZI demodulation and single-port detection) is given based on the
Gaussian noise approximation. Our calculation method correctly predicts the
3-dB improvement of receiver sensitivity by using balanced detection over
single-port detection or IM/DD receivers. Furthermore, quantum-limited DPSK
receivers with MZI demodulation are also analyzed.
