There's no such thing as Differential BPSK. If you consider BPSK you have two states (0 and 1). If the data is at 0 you have a signal that is in phase, and if the data is at 1 the signal is then shifted 180 Degrees. That's all there is to it. The idea of using Differential data is to look at the difference between two signals, but you really can't have a differential when you only have two states. You may be referring to Differential QPSK or pi/4 DQPSK where the difference between two symbols is how the phasor gets shifted accordingly. For BPSK, this doesn't exist.