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Ultra-wideband Narrowband Interference Cancellation and Channel Modeling for Communications

Ultra-wideband Narrowband Interference Cancellation and Channel
Modeling for Communications
Brian Michael Donlan
(ABSTRACT)
Interest in Ultra-wideband (UWB) has surged since the FCC’s approval of a First Report
and Order in February 2002 which provides spectrum for the use of UWB in various
application areas. Because of the extremely large bandwidth UWB is currently being
touted as a solution for high data rate, short-range wireless networks. An integral part of
designing systems for this application or any application is an understanding of the
statistical nature of the wireless UWB channel. This thesis presents statistical
characterizations for the large and small scale indoor channel. Specifically, for large
scale modeling channel frequency dependence is investigated in order to justify the
application of traditional narrowband path loss models to UWB signals. Average delay
statistics and their distributions are also presented for small scale channel modeling.
The thesis also investigates narrowband interference cancellation. To protect legacy
narrowband systems the FCC requires any UWB transmission to maintain a very low
power spectral density. However, a UWB system may therefore be hampered by the
presence of a higher power narrowband signal. Narrowband interferers have a much
greater power spectral density than UWB signals and can negatively affect signal
acquisition, demodulation, and ultimately lead to poor bit error performance. It is
therefore desirable to mitigate any in-band narrowband interference. If the interferer’s
frequency is known then it may simply be removed using a notched filter. It is however
of more interest to develop an adaptive solution capable of canceling interference at any
frequency across the band. Solutions which are applied in the analog front end are
preferable to digital backend solutions since the latter require extremely high rate
sampling. The thesis therefore discusses two analog front-end interference cancellation
techniques. The first technique digitally estimates the narrowband interference (this is
possible because the UWB signal is not being sampled) and produces an RF estimate to
perform the narrowband cancellation in the analog domain. Two estimation techniques,
an LMS algorithm and a transversal filter, are compared according to their error
performances. The second solution performs real-time Fourier analysis using transform
domain processing. The signal is converted to the frequency domain using chirp Fourier
transforms and filtered according to the UWB spectrum. This technique is also
characterized in terms of bit error rate performance. Further discussion is provided on
chirp filter bandwidths, center frequencies, and the applicability of the technology to
UWB.


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