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The theoretical performance for
coherent and differential PSK is shown here for an additive
white Gaussian noise limited channel. The BER probability for coherent PSK is exactly
the same as that derived for bipolar baseband transmission in Chapter 3, and the PSK
modulation/demodulation process can be viewed as a convenient mechanism for obtaining a
bandpass representation of the equivalent baseband source. It should not be forgotten,
however, that the baseband-to-bandpass transformation decreases the maximum bandwidth
efficiency of the data link from 2 bits/second/Hz to 1 bit/second/Hz for binary signalling in
both cases. (We will see in Chapter 6 that Quadrature Phase
Shift Keying (QPSK) a four-symbol PSK scheme can allow us to get back to an
efficiency of 2 bits/second/Hz for the bandpass case without degrading the performance
compared with binary PSK.) It can be seen that there is little performance penalty between fully coherent PSK (assuming a perfect 'jitter-free' reference) and the simpler DPSK implementation. This margin reduces further if differential encoding and decoding is used with coherent PSK to overcome the phase ambiguity within the carrier recovery process which introduces some double bit errors. |
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