• The transmitter (modulator) of a DEPSK system is identical to the DPSK transmitter.

• The differential phase shift keying (DPSK) is a modulation of BPSK.

• Fig shows the block diagram of DPSK generator and the relevant waveforms.

Operation:

• d (t) represents the data stream which is to be transmitted .It is applied to one input of an EX-OR logic gate.

• The EX-OR gate output “b (t)” is delayed by one bit period Tb and applied to the other input of the EX-OR gate. The delayed output is represented by “$b(t- T_b )$”

• Depending on the values of “”d (t)” and “$b(t- T_b )$” ,the EX-OR gate produces the output sequence “b (t)” .The waveforms for DPSK generator have been drawn by arbitrarily that in the first interval b (0)=0.

• Output of EX-OR gate is then applied to a bipolar NRZ level shifter which converts “b(t)” to a bipolar level signal b’(t) as shown in the fig.

• This bipolar NRZ signal b’(t) is then multiplied with the carrier signal to produce the DPSK signal. The DPSK output signal is mathematically expressed as:

  $V_{DPSK} (t) = b’(t) * \sqrt{2 P_s} cosω_C t$

• So when b(t) =1 , b’(t) =1 hence

  $V_{DPSK} (t) = $b'(t)$ * \sqrt{2 P_s} cosω_C t$

• That means no phase shift has been introduced.

  Bt when b(t) =0, b’(t)= -1 hence

• Thus 180 degree phase shift is introduced to represent b(t) =0

• The block diagram of DEPSK receiver is shown below. It shows that the signal b (t) is recovered from the received signal, using the synchronous demodulation technique. This is same as the BPSK detection. Once the signal b (t) is recovered, it is applied to one input of an EX-OR gate.

• The signal b(t) is also applied to a time delay circuit and the delayed signal $b(t- T_b )$ is applied to the other input of the EX-OR gate as shown below

• If b(t)= $b(t- T_b )$ then output of the EX-OR gate will be 0

  d (t) =0 ………if b$ (t) = b(t- T_b )$

And if b(t) = $b(t- T_b )$ then output of the EX-OR gate will be 1

d (t) =1 ……… b(t) = b$(t- T_b )$