I haven't tried implementing Differential Quadrature Phase Shift Keying on PSoC.
Commenting on the transmitter, I feel that 2 approach can be followed:
1) Two look-up tables should be written for the two waveforms (Inphase and Quadrature). Two VDACs should be fed with the two Look up table values using DMA depending upon the symbol to be transmitte (00,01,10 or 11). The output of the two VDACs can be added using the Opamp with external resistors. This resultant waveform will be the required DQPSK signal.
2) The look-up table should have provision for all 4 symbols (00,01,10 and 11). When a given pattern is to be transmitted, a single VDAC should be fed with the Look-up table values using DMA.
The transmission rate will be limited by the rate of update of the VDAC.
These are the ways I can think of. There may be other techniques to do this too. Let us know of your requirement.
I am not aware of the demodulation techniques used for DQPSK signal.