Research On Multicarrier Transmission Technique For Mobile Satellite Communication Systems

The mobile satellite communication systems can provide stable communication to users in areas without the mobile terrestrial communication systems, such as forest, desert and ocean, and areas where the mobile terrestrial communication systems are disrupted by natural disasters, such as earthquake, tsunami and fire. The mobile satellite communication systems have the advantages of both the large coverage area and the high deployment flexibility. With the development of communication techniques, mobile satellite communication systems have become an important part of the future global communication networks. In order to meet the growing demand for data rates, mobile satellite communication systems will evolve from single carrier systems to multi?carrier systems.Multi-carrier transmission technique determines the transmission method of data information and affects the implementation of the time-frequency synchronization module and the other modules. However, the nonlinear amplifier characteristics of the satellite, and the carrier frequency offset (CFO) as well as the carrier phase offset (CPO), which are caused by the Doppler effect and the non-ideal oscillator, will cause the signal distortion. The unwanted signal distortion will result in the bit error rate(BER) performance deterioration. Therefore, it is of great significance to study the multi-carrier waveform design technique and the corresponding synchronization technique. In this thesis, the multi-carrier transmission techniques in mobile satellite communication systems are studied. The main research results are as follows:(1) Orthogonal frequency division multiplexing (OFDM) technique has the capability of providing high spectral efficiency. However, the high peak-to-average ratio (PAPR) of OFDM technique will decrease the energy efficiency of the satellite systems. Constant envelope OFDM (CE-OFDM)technique can reduce the PAPR to OdB. However, the CE-OFDM technique can only reach nearly 50% spectral efficiency of the OFDM technique. If the bandwidth resource is limited, CE-OFDM technique is difficult to guarantee the system capacity. In order to improve the spectral efficiency and maintain a low PAPR, the quasi-constant envelope OFDM (QCE-OFDM) technique is proposed in this thesis. The transceiver structure based on the phase shifting and the Taylor series expansion is proposed.The receiver with iterative detection based on amplitude-phase demodulator is proposed to improve the BER performance of the QCE-OFDM technique with high modulation index and high order modulation constellation. Simulation results show that the QCE-OFDM technique can improve the spectral efficiency and keep the PAPR of the waveform being lower that 3dB.(2) The Doppler effect and the non-ideal oscillator will cause the CFO and the CPO. The CFO and CPO will further result in the BER performance deterioration. In order to suppress these disadvantages, the joint CFO and CPO estimation method based on the phase demodulator is proposed. According to the linear relationship among CFO and CPO as well as data symbols, the minimum variance unbiased (MVU) estimators and the corresponding Cramer-Rao bounds (CRBs) are presented. In addition, the optimal pilot symbol structure is designed according to the minimum CRB criterion. Simulation results show that the estimation accuracy of the proposed method is close to the CRB.(3) The modulation index is an important parameter of the QCE-OFDM technique. The modulation index affects both the spectral efficiency and the energy efficiency. It should be noted that the spectral efficiency will increase while the energy efficiency will decrease with the increasing of the modulation index. Therefore, how to choose the appropriate modulation index to improve the system performance is worth researching. In this thesis, a system performance model is proposed and it considers the energy efficiency and the spectral efficiency as two factors.The modulation index selection strategy is to choose the appropriate modulation index to maximize the system performance. The relationship between these two factors and the modulation index is analyzed and the system performance curves are given by simulation.