Hello Sahil Singh,
for viewing the link you need to have a research gate account. I am attaching herewith the words what I was suggested.
According to him,
1. in conventional DQPSK scheme, a delay of 1 symbol is required for direct detection1..
2. sample the waveform at 2.56 THz, with 256 samples in each of 128 symbols for a sequence length of 12.8 ns. The symbol rate is 10 GBaud,
with 2 symbols/bit for an aggregate bit rate of 20 Gbit/s.
3. the symbol rate 5 GBaud with 64 symbols each of 200 ps period? In this case your demodulator MZ delay of 2/Bit rate is correct.
4. A 2.5 GHz pulse train would explain the zeros in the eye.
So these are the main points arose by Alan.
Regards,
Dhiman