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The most important loss in deep space communication is due to path loss and the main restriction of the link in transmit power. We cannot use a modulation scheme with a huge alphabet because it path loss and the additional noise during the path will cause too much damages on the signal, therefore the probability of errors would be to big. We have to deal with small alphabet modulation scheme that’s why we have chosen Differential Offset Quaternary Phase Shift Keying (DOQPSK). We use Differential coding in order to prevent error at the receiver due to phase shifts and Offset coding in order to reduce the maximum phase-shift from 180° to 90°.
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