#### Example 5.8 | #### Question 5.12

## BER performance for PSK

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. |