Question

$\text{The message bit sequence to a DPSK modulator is "1, 0. 1, 1, 0". This data is differentially encoded with unknown logic gate (EX-OR/EX-NOR) and unknown starting reference bit. Carrier phase of}"\pi "\text{is assigned to logic - 0 and carrier phase of "0" is assigned to logic - 1 after encoding the data. If the carrier phase corresponding to the first two message bits (from left side) are},\pi 0\text{respectively, then the carrier phases corresponding to the remaining three message bits will be respectively}$

• A. $0,\pi ,\pi$
• B. $0,0,\pi$
• C. $\pi ,\pi ,0$
• D. $\pi ,\pi ,0$

Answer 1

The correct option is B $0,0,\pi$

$\text{It is given that the carrier phase for first two encoded bits are}\pi ,0.\text{So first two encoded bits are 0,1.}$
The logic diagram of DPSK encoder can be given as,

Finding the logic gate:

If we observe clearly,

$d_0(\text{logic)}{b}_1=d_1$
$0(\text{logic)}0=1$

So the possible logic is, EX-NOR.
So, the complete encoding can be given as,

$\text{So, the carrier phases corresponding to the remaining three bits are}0,0,\pi$