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I am very confused about how to get the transmit sequence of the DPSK signal.

In my lecture notes, teacher give as the following example.

Assume the following rule is apply

"no change" of sign for information 1
"change" of sign for information 0

if the data sequence is "0000 1010 1110 0011"
then the transmitted sequence is "+ -+-+ +--+ +++- +---"

if the data sequence is "1101 0000 0111"
then the transmitted sequence is "- --++ -+-+ ----"

I cannot understand why the data sequence is 0000 and the transmit sequence is -+-+ instead of ++++? and When should I use + or - in the transmitted sequence?

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    $\begingroup$ Because the signal changes sign when the data is a 0. The key is that it doesn't matter what the signal starts as (+ or -), what matters is how it changes. $\endgroup$ – Jim Clay Dec 7 '12 at 19:03

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