GALDINO, J. F.; http://lattes.cnpq.br/3588063339399737; GALDINO, Juraci Ferreira.
Abstract:
This work deals with the design of techniques to combat the effects of additive noise
and intersymbol interference (ISI) on receivers, for frequency-selective fading channels.
Those techniques are suitable for blind reception and training sequence aided reception.
Concerning the blind reception, this thesis focuses on receivers that use the maximum
likelihood criterion to detect symbol sequences employing per-survivor processing
(MLSE-PSP), for mobile communications channels subjected to time-variant frequency
selective fading (TVFSF). Special attention is given to the choice of the adaptive filtering
algorithm used by the receivers. The analysis of the performance of those receivers
shows that they are severely affected by burst errors in the detection processing.
In order to overcome this problem, a procedure is implemented to detect the occurrence
of errors in digital receivers. This procedure is based on a new concept introduced
in this thesis: filtering diversity. This procedure is computationally simple, works in
time-variant channels, presents good performance and can be used for any reception
scheme.
Other important theme treated in this thesis is the analytical evaluation of the
steady-state behavior of the adaptive filtering algorithm LMS {Least Mean Square) on
the identification of channels subjected to the effect of TVFSF. Closed-form expressions
are obtained for the mean squared error (MSE) and for the optimum step of the LMS.
Relating to the processing to combat the effect of the additive noise, the use of
denoising techniques is proposed as an alternative method to the conventional schemes,
which use linear time-invariant filters. The denoising techniques are implemented in
the Wavelet transform domain and they take advantage of the parsimonious property
of this transform.
The performance of the proposed front-end scheme is analyzed considering two aspects:
the analytical one, for AWGN channels, and the use of simulation, for frequencyselective
channels. In the first case, approximations are obtained for the system bit
error probability in terms of the parameters of the Wavelet transform and the denoising
technique. For frequency-selective channels, many configurations of decision-feedback
equalizers are analyzed. It is observed that the proposed scheme outperforms the
conventional ones, mainly for the case when the channel frequency response presents
notches in the frequency band of the transmitted signal.