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Recently I have come across different variation of the sane modulation techniques. I am quite confused about the advantages of different constellations. In particular, what are the advantages of using

Differential BPSK over BPSK?

Any thoughts?

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  • $\begingroup$ "Any thoughts?" Yes. When is your homework due? $\endgroup$ – Dilip Sarwate May 26 '15 at 13:23
  • $\begingroup$ This is not my homework, if you notice my questions are at higher level than homeworks.@DilipSarwate $\endgroup$ – Tyrone May 26 '15 at 13:32
  • $\begingroup$ I have not noticed that your questions (here as well as on math.SE) are at a higher level than homework questions. I have a lifetime's experience in creating homework questions and asking them of students and am perfectly capable of judging the level of questions. The flood of recent questions from you (more than 25 since mid-April) supports my hypothesis that you are taking a course in digital communications (and possibly a separate one in coding theory) and are getting all your homework answers here (and on math.SE). $\endgroup$ – Dilip Sarwate May 26 '15 at 14:06
  • $\begingroup$ As I said this is not my homework with all respect to your experience. I am trying to learn. If you believe this annoying for you, please do not answer and avoid my questions. @DilipSarwate $\endgroup$ – Tyrone May 26 '15 at 14:48
  • $\begingroup$ Regrettably, many of the questions that you have inundated this forum (and math.SE and stats.SE) with are in my areas of interest. I have lived all my life with the firm belief that there is no such thing as a stupid question, but I am thinking that maybe this belief needs some updating. $\endgroup$ – Dilip Sarwate Jun 2 '15 at 20:37
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Differential BPSK

As the information is encoded in the phase difference of two consecutive symbols, phase estimation can be omitted in systems where the carrier phase can be assumed to be constant during two symbols. Specifically, it can be used in a self-coherent receiver which mixes one symbol with its predecessor in order to reconstruct the phase difference between the two symbols. In such a scheme, no local oscillator is required which simplifies the system (e.g. frequency synchronization is not needed).

Non-differential (coherent) BPSK

In differential BSPK, two symbols are "added" and as a consequence the noise of the two symbols adds up. This results in a 3 dB sensitivity gain of coherent BPSK. In other words, coherent BPSK is more robust against noise.

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  • $\begingroup$ Thank you Deve. What do you mean by two symbols are added? If I want to send a 0 I send s0 and if I want to send bit 1 I send s1. Not sure what you meant by added. Do you mean at the receiver? $\endgroup$ – Tyrone May 26 '15 at 13:43
  • $\begingroup$ Now this is another question, namely "How does differential BPSK work?" which cannot be answered by comments. Can you read up on differential modulation and come back with specific questions? $\endgroup$ – Deve May 26 '15 at 13:46
  • $\begingroup$ Sure, but the term "added" you used in the non-differential BPSK and not in the differential case? $\endgroup$ – Tyrone May 26 '15 at 13:47
  • $\begingroup$ I used it referring to differential BPSK. It's a sloppy formulation, therefore I put it inside quotes. $\endgroup$ – Deve May 26 '15 at 13:49
  • $\begingroup$ Sorry I meant your answer should have been: In non-differential (coherent) BSPK, two symbols are "added" and as a consequence the noise of the two symbols adds up. This results in a 3 dB sensitivity gain of coherent BPSK. In other words, coherent BPSK is more robust against noise $\endgroup$ – Tyrone May 26 '15 at 13:52

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