Example 5.7
A binary PSK modem uses a preamble data word to
achieve symbol synchronization in the receiver. This symbol timing reference, however,
drifts between resynchronization bursts so that the timing is in error by 10% when the
next synchronisation update is received. If the PSK detector uses an integrate and dump
filter, and the symbols are not shaped, what will be the worst-case degradation on the
effective received symbol energy due to the timing error?
Solution
With reference to the diagram below, the 10% timing
error in the symbol sampling time will result in the voltage at the output from the
integrator when sampled being reduced by 90% of the optimum value. This in turn will
result in a fall of 81% (0.9 x 0.9) in the effective symbol energy, equivalent to a
0.91 dB degradation in Eb/N0 performance for the modem.
