Resumo-O desempenho de sistemas e redes de comunicações em canais com desvanecimento variante no tempo é significativamente afetado não apenas pela taxa de erro de bits média, mas também pela ocorrência de erros em surtos, que podem degradar bastante a operação de protocolos de comunicações de níveis superiores. Diversos trabalhos recentes têm mostrado que a técnica de modulação adaptativa é uma ferramenta eficaz para maximizar a eficiência espectral neste tipo de canal, atendendo a requisitos de desempenho de taxa de erro. No entanto, até onde vai o conhecimento dos autores o impacto desta técnica na geração de surtos de erros não tem sido investigado. Neste trabalho é feita uma comparação de desempenho entre a técnica de modulação adaptativa e técnicas convencionais de modulação fixa em canais com desvanecimento plano variante no tempo, sob o enfoque da geração de erros em surtos. Várias estatísticas usualmente empregadas na análise deste tipo de erro são levantadas por simulação, sob diferentes condições de transmissão. Os resultados obtidos mostram que o emprego de modulação adaptativa reduz sobremaneira a geração de erros em surtos, podendo fornecer para protocolos de níveis superiores um processo de erros de bit com vários parâmetros estatísticos próximos dos de um canal sem memória. Palavras-Chave-Modulação Adaptativa, Erros em Surtos, Canais RayleighAbstract-The performance of communications systems and networks over time-varying fading channels is significantly affected not only by the average bit error rate (BER), but also by the occurrence of burst errors that may seriously degrade the operation of upper-layer communication protocols. Several recent works have shown that the adaptive modulation technique is an effective tool to maximize the spectral efficiency subjected to BER constraints in this kind of channel. However, the impact of this technique in the generation of burst errors has not been addressed so far, to the best of our knowledge. A performance comparison between the adaptive modulation technique and conventional fixed modulations in time-varying flat-fading channels is presented in this work, with focus on the burst-error generation. Several statistics usually applied in the analysis of burst-error discrete channels are obtained by simulation, under different transmission conditions. The obtained results show that the adaptive modulation technique significantly reduce the occurrence of burst errors and can provide to the upper-layer protocols an error process that has several statistical parameters close to that of a memoryless channel.
A new Hidden Markov Model (HMM) for burst errors is proposed. This model is based on a compact representation of the error sequence in terms of succeeding pairs of clusters and gaps lengths. Its Markov chain has two classes of states associated to the generation of gaps and clusters lengths, respectively. The proposed model may be characterized by few parameters. An algorithm for ML ("Maximum Likelihood") estimation of these parameters on the grounds of the EM ("Expectation-Maximization") approach is derived. Some preliminary results of performance evaluation show that the model and the estimation algorithm here presented provide a flexible and efficient tool for capturing and reproducing statistics of interest in the context of burst errors modelling.
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