The frequency-shift (FRESH) filter is a structure which exploits the spectral correlation of cyclostationaryudsignals for removing interference and noise from a wanted signal. As most digitaludcommunication signals are cyclostationary, FRESH filtering offers certain advantages for interferenceudrejection in a communications receiver.udThis thesis explores the operation and application of FRESH filters in practical interference scenarios.udThe theoretical background to cyclostationarity is clarified with graphical interpretationsudof what cyclostationarity is, and how a FRESH filter exploits it to remove interference. The effectsudof implementation in a sampled system are investigated, in filters which use baud rate related cyclostationarity,udleading to efficiency improvements. The effects of varying the wanted signal pulseudshape to enhance the cyclostationarity available to the FRESH filter are also investigated.udA consistent approach to the interpretation of the FRESH filter's operation is used throughout,udwhile evaluating the performance in a wide range of realistic channel conditions.udVLF radio communication is proposed as one area where interference conditions are particularlyudsuitable for the use of FRESH filtering. In cases of severe adjacent channel interference it is foundudthat a FRESH filter can almost completely remove the interferer. The effects of its use with anudimpulse rejection technique are also investigated.udFinally, blind adaptation of FRESH filters through exploitation of carrier related cyclostationarityudis investigated. It is found that one existing method loses the advantage of FRESH filtering overudtime invariant linear filtering. An improvement is proposed to the latter which restores its performanceudto that of a trained FRESH filter, and also reveals that carrier related cyclostationarity canudbe exploited, in some cases, by a simpler method.udJ.
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