LOPES, W. T. A.; LOPES, Waslon Terllizzie Araújo.
Resumo:
This thesis deals with techniques to improve the performance of communication
systems that use modulation diversity (MD), which is based on the combination of
a suitable choice of the reference angle of an MPSK constellation with independent
interleaving of the symbol components. The MD technique is applied to the transmission
of vector-quantized images over a fading channel. The thesis also covers robust
vector quantization (VQ), in which a VQ codebook is made robust against the channel
errors by means of an index assignment (IA) procedure. In this way, the quality of the
reconstructed images is enhanced.
Concerning the MD technique, many important aspects are evaluated, such as:
optimum rotation angle for QPSK, 8PSK and 1GPSK constellations; study of the influence
of the channel estimation errors on the system performance (a relevant problem,
which in general is not covered in the literature on MD); the impact of the Doppler
effect on system performance is studied and a trade-off between the interleaving depth
and the error probability is stablished; explanation of the performance gain of the MD
technique by using a novel geometric interpretation: The Ferris Wheel Effect.
Regarding robust vector quantization, a new figure of merit for the index assigment
problem is proposed in this thesis. This figure of merit is used j o i n t ly with the simulated
annealing algorithm to assign binary indexes to the VQ codevectors.
In this work, the M D and IA techniques are evaluated, as well as, the combination
of both, in terms of the quality of the reconstructed imagens after transmission over
a fading channel. Simulations results show that the combination of the MD and IA
leads to the best system performance.
This thesis also presents a new method for calculating the bit error probability
(BEP) of modulation schemes subject to Rayleigh fading. In this method the communication
channel is seen as an additive noise channel where the noise is modeled as
the ratio between a Gaussian random variable (r.v.) and a Rayleigh r.v. The method
consists of using the cumulative density function of that additive noise to obtain exact
expressions for the bit error probability.