Modem AFSK coerente e completamente digital para módulo TT&C padrão CubeSat

• Ano de defesa: 2021
• Autor(a) principal: Pacelli, Rubem Vasconcelos
• Tipo de documento: Dissertação
• Tipo de acesso: Acesso aberto
• Idioma: por
• Palavras-chave em Português: AFSK; CubeSat; Sincronismo; Demodulação coerente; Algoritmo de Viterbi
• Link de acesso: http://www.repositorio.ufc.br/handle/riufc/60259

The telecommunication and service subsystem aims to perform the telecommand, telemetry, and tracking communications, functionalities indispensable for any satellite mission. Due to legacy and compatibility issues, the AFSK (Audio Frequency-Shift Keying) modulation with non-coherent detection has been widely used in TT&C (Telemetry, Tracking, and Command) modules for the telecommand signals uplink. Despite the simplicity, this approach has low efficiency in terms of power. In this dissertation, a new all-digital AFSK modem with coherent detection aimed to CubeSat satellites is presented. The modulator and the demodulator are described mathematically and their architectures are analyzed through blocks diagram. The system operates in AWGN (Additive White Gaussian Noise) channel, in addition to phase and symbol time impairments. In the demodulator, the Viterbi algorithm is used to perform the maximum likelihood sequence detection of the transmitted bits. For synchronism, the proposed phase and symbol time estimators operate in decision-directed mode, that is, the bits detected by the Viterbi algorithm are feedback to the system in order to estimate the phase and time offsets. It is also derived an upper bound for the bit error rate which serves as a benchmark for system performance, obtained via computer simulation. One observes that the limit derived in this dissertation provides a good approximation of the bit error obtained in computer simulation. The model performance is also compared to the theoretical probability error of the non-coherent AFSK system, giving a gain of 5 dB, approximately. When the model is exposed to different scenarios of time and/or phase offset, it is possible to notice that the estimators are able to mitigate such impairments. The estimator’s quality is assessed in terms of its variance and is compared to the modified Cramér-Rao bound, obtaining reasonable results.