Frequency shift filtering for cyclostationary signals.
The frequency-shift (FRESH) filter is a structure which exploits the spectral correlation of cyclostationary
signals for removing interference and noise from a wanted signal. As most digital
communication signals are cyclostationary, FRESH filtering offers certain advantages for interference
rejection in a communications receiver.
This thesis explores the operation and application of FRESH filters in practical interference scenarios.
The theoretical background to cyclostationarity is clarified with graphical interpretations
of what cyclostationarity is, and how a FRESH filter exploits it to remove interference. The effects
of implementation in a sampled system are investigated, in filters which use baud rate related cyclostationarity,
leading to efficiency improvements. The effects of varying the wanted signal pulse
shape to enhance the cyclostationarity available to the FRESH filter are also investigated.
A consistent approach to the interpretation of the FRESH filter's operation is used throughout,
while evaluating the performance in a wide range of realistic channel conditions.
VLF radio communication is proposed as one area where interference conditions are particularly
suitable for the use of FRESH filtering. In cases of severe adjacent channel interference it is found
that a FRESH filter can almost completely remove the interferer. The effects of its use with an
impulse rejection technique are also investigated.
Finally, blind adaptation of FRESH filters through exploitation of carrier related cyclostationarity
is investigated. It is found that one existing method loses the advantage of FRESH filtering over
time invariant linear filtering. An improvement is proposed to the latter which restores its performance
to that of a trained FRESH filter, and also reveals that carrier related cyclostationarity can
be exploited, in some cases, by a simpler method.