Waslon Terllizzie A. Lopes

Performance of a rotated QPSK based system in a fading channel subject to estimation errors

This paper presents a performance analysis of a modified QPSK scheme for transmission over fading channels. Interleaving and rotation of the signal constellation are applied with the objective of improving its performance. It is shown that the proposed system outperforms the original scheme even in the presence of channel estimation errors.

Bit error probability of M-QAM and I × J-QAM modulation schemes in Nakagami fading

In this paper closed-form expressions for the bit error probability of M-QAM and I times J-QAM modulation schemes in Nakagami fading are derived. The classical optimum detector for signals under additive white Gaussian noise and the bit mapping consistency of QAM signals submitted to Gray mapping are considered.

Evaluation of optimal and sub-optimal speech noise reduction wiener filters

This paper presents a comparative evaluation of Wiener optimal and sub-optimal finite impulse response filters, which allows a balance between noise reduction and distortion insertion, by setting a parameter α. This is done observing an Automatic Speech Recognition (ASR) system error rate. The ASR system in these paper is tested for Brazilian Portuguese. The percentage of correctly recognized words is obtained for speech signals subject to Additive White Gaussian Noise (AWGN), for an SNR ranging from zero to 20 dB, using filtered speech signals. To evaluate the distortion effect caused by filtering, the filtered version of clean speech signals is processed by the recognizer, and it is observed that the error rate decreases with the reduction of the parameter α. The application of a suboptimal filter, with α = 0.7, produces the highest recognition rate. The observed improvement is 10% for the lowest SNR and 14% for the highest SNR. It is observed that the output SNR increases with parameter α.

Effect of channel estimation errors on the performance of two MPSK detection schemes subject to Rayleigh fading

This paper presents a performance analysis of two MPSK detection schemes for flat Rayleigh fading channels with imperfect channel estimation. Closed-form expressions for the bit error probability (BEP) of both schemes are presented considering QPSK modulation and for higher level MPSK modulation, such as 8PSK, 16PSK and 32PSK, the BEP is obtained by simulation. It is shown that they present the same error probability even in the absence of ideal channel estimation and a complexity analysis is carried out to determine the best scheme in terms of the number of operations (multiplications, divisions, subtractions and comparisons) executed by each detector.

On the performance of M-QAM for Nakagami channels subject to gated noise

This paper presents a mathematical analysis of the joint effects of additive white Gaussian noise and gated noise in an M-ary Quadrature Amplitude Modulation communication system subject to Nakagami fading. This channel model is suitable to describe scenarios in which the Nakagami channel is affected by noisy signals originated from switching electronic circuits present in industrial environments, for example. Novel and exact analytic expressions for the bit error probability of the system are derived and computer simulations corroborate the analytic results.

Genetic algorithm optimization applied to the project of MIMO systems

This paper presents a technique to increase the data throughput in Multiple Input Multiple Output (MIMO) systems. The technique uses a meta-heuristic to optimize the data throughput and to choose the best solution. The optimization is based on Genetic Algorithms (GA), with the objective of finding out the best antenna configuration to achieve the highest data throughput from the variation of the distance between the antenna array elements and the number of antennas. Simulation results show an increase of more than 15% in the data throughput considering 4×4 MIMO channels.

Cognitive spectrum sensing via statistical tests in Rice fading channels

Spectrum sensing is an important tool for the expansion of cognitive radio. This paper evaluates the use of statistical tests to optimize the spectral detection of available frequency bands for opportunistic occupation. Statistical characteristics of the transmitted signals over a channel subject to Rice fading are selected to achieve better cognitive sensing performance. More precisely, Jarque-Bera, Kurtosis and Skewness statistical tests are compared via simulation in terms of their detection probabilities in non-fading and in Rice fading channels. Simulation results indicate that these statistical tests are useful, and can be effectively applied to cognitive wireless channels subjected to fading.

A Quasi Random Symbol Interleaving Technique Applied to Image Transmission By Noisy Channels

Image vector quantization (VQ) is sensitive to channel errors because they can lead to spurious blocks and consequent quality degradation of transmitted images. One way to increase the robustness of VQ to channel errors is to perform an index assignment to the codevectors. For wireless channels with Rayleigh fading, another challenge is the occurrence of burst errors. This work proposes the application of low discrepancy sequences to the interleaving of symbols, to improve the quality of reconstructed images considering those errors. Results obtained by preference tests, based on visual inspection of the reconstructed images, attest the success of the technique.

Spectrum sensing via statistic tests in Rayleigh fading channels

Cognitive radio applications are limited by the scarcity of available frequency bands for telecommunication services. The improvement of the spectrum sensing to enable a better sharing of the frequency spectrum among different users is a major challenge in wireless communications based on cognitive radio. This paper evaluates the use of statistic tests to optimize the non-cooperative cognitive spectrum sensing. Statistical characteristics of the transmitted signals are selected to achieve better cognitive sensing performance. More precisely, Jarque-Bera, Kurtosis and Skewness statistic tests are compared in Rayleigh fading channel and non-fading scenarios. Simulation results indicate that these statistic tests can be effectively applied to cognitive wireless channels that are subjected to fading.

Cognitive spectrum sensing based on statistical tests in fading channels

The cognitive radio concept is fostered by the scarcity of frequency bands for wireless communication. Several new techniques and algorithms are being proposed to deal with the spectrum occupancy in cognitive channels. The goal is to enable a better sharing of the frequency spectrum among different users. This paper evaluates the performance of statistical tests to optimize the spectrum sensing for fading channels. Statistical characteristics of the transmitted signals can be used to achieve better cognitive sensing performance. Jarque-Bera, Kurtosis and Skewness statistical tests are evaluated when subject to fading effects. Rayleigh, Nakagami-m, Rice and Lognormal fading channels are simulated. Results indicate that these statistical tests can be applied to cognitive wireless channels even when subject to fading.