The Research Of Modulation And Demodulation Techniques In Deep Space Communications

Due to the complexity of deep space communication environment and the shortage of spectrum resources currently, one of the most effective and easily implemented technologies is the efficient modulation technology. As a quasi-constant envelope modulation with excellent spectral and strong anti-interference performance, Feher Quadrature Phase Shift Keying(FQPSK) becomes the recommended standard of deep space communications. Moreover, the combination of modulation techniques, channel coding techniques and cooperative communications is an effective method to meet the requirement of bit error rate(BER), spectral efficiency and data transmission speed in deep space communications. The power efficiency and/or bandwidth efficiency can be improved by exploiting spatial diversity and coding gains at the same time.The research about FQPSK baseband waveform suitable for deep space channel and corresponding demodulation schemes aims at overcoming the slope discontinuity of the quasi-constant envelope modulation FQPSK baseband waveform and the limitations of conventional demodulation schemes. Based on cooperative translation and coded modulation technologies, a low-complexity suboptimal detection scheme is proposed. The main works and innovations of this paper are summarized as following:(1) Considering the slope discontinuity of the original FQPSK baseband waveform, a set of baseband waveform with continuous slope at any time has been designed in this paper. The relationship between baseband waveform and spectral performance has been researched, and the simulation result shows that the improved baseband waveform can improve the spectral performance of modulating signal.(2) The specific channel model can be used to research the suitable demodulation scheme of FQPSK. By introducing Maximum A Posteriori(MAP), the soft information decision formula which derived under two different channel models can be applied separately the baseband waveform before and after improving, and then the performance of the two baseband waveform in three demodulation schemes are simulated. The simulation results show that the MAP demodulation algorithm can utilize the particularity of FQPSK more efficiently than the regular demodulation schemes, and has the better BER performance, furthermore, it proved that the MAP demodulation performance can be improved through improving baseband waveform.(3) We derive a low-complexity iterative Viterbi algorithm(IVA) that operates in a decision-directed manner for the suboptimal maximum likelihood(ML) noncoherent detection. We investigate the noncoherent block detection techniques for block-coded MPSK modulation in cooperative DF relay systems. Following the introduction to the system model, we summarize the optimal ML noncoherent block detection at the destination, and present a suboptimal ML noncoherent block detection by simplifying the detection metric, then we derive a low-complexity IVA. By simulations for three-node cooperative QPSK systems, we demonstrate that the IVA can approach well the BER performances of the exhaustive detection. Moreover, the IVA for a(7,4)-coded QPSK can approach the BER performances of the coherent detection.