Research On OFDM Transmission Technique For Broadband LEO Satellite Communication System

Air-space-ground of integrated information communication network becomes focus field of research in the world communications technology. The broadband LEO satellite mobile communication system will play an important role in providing services to mobile terminal user anytime and anywhere, which becomes a significant component of the air-space-ground integrated information network. The broadband LEO satellite mobile confronts with the frequency selectivity channel fading characteristic, and bandwidth resources are inefficient. Therefore the special transmission technique which could not only counteract anti-channel frequency selective fading, but also has high bandwidth utilization efficiency is needed. OFDM (Orthogonal Frequency Division Multiplexing) is a multicarrier transmission technology, which could counteract the anti-channel frequency selective fading, and has high bandwidth utilization efficiency. Simultaneously subcarrier may be flexibly allocated to realize asymmetrical transmission for uplink, downlink of the broadband LEO satellite system. Therefore utilization of OFDM for broadband LEO satellite mobile communication to improve system performance is an important research topic.In this dissertation, OFDM transmission technique for broadband LEO communication system is researched. The dissertation mainly focuses on the following aspects.Firstly, the applicability research of OFDM transmission technique based on broadband LEO satellite communication system. The channel statistical property of typical simulation scene for broadband LEO satellite has been described. DFT-S OFDM (Discrete Fourier Transformation Spread Orthogonal Frequency Division Multiple) transmission system for uplink broadband LEO satellite and OFDM transmission system for downlink broadband LEO satellite have been established. According to LEO satellite mobile channel characteristic, Analysis shows comparison of anti-muti channel fading, anti-Doppler shift, PAPR (Peak to Average Power Ratio), anti-AWGN (Additive White Gaussian Noise) channel based on DFT-S OFDM and OFDM transmission system separately. It proves that OFDM transmission technique is suitable for the broadband LEO satellite communication system, which is the key point for the following researches.Secondly, signal detecting algorithms have been researched for uplink broadband LEO satellite system. Because traditional detecting algorithm in frequency domain is not suitable for tracking channel, the improved RLS (Recursive Least Squares) algorithm for single user signal detecting has been proposed, which is quite suitable for tracking fast LEO satellite changing channel. In view of the multi-user interference caused by different frequency offsets for uplink LEO satellite, the precision of tradition PIC (Parallel Interference Cancellation) algorithm or the SIC (Successive Interference Cancellation) algorithm is not high, and the iterative numbers are excessively large, which is not suitable for the broadband LEO satellite. OWPIC algorithm has been proposed for cancelling multi-user interference, which simplifies the iterative step, improves the precision.Thirdly, signal detecting algorithms have been researched for downlink broadband LEO satellite system. The larger frequency offset destroys the orthogonality of sub-carrier and introduces interference. For the complexity of tradition cancellation algorithms is high, and frequency offset scope that can be captured is limited, TF-LMS (Time and Frequency Domain Least Mean Square) algorithm has been proposed to cancel the interference introduced by large frequency offset with low complexity. In view of serious multi-channel fading such as urban service scene for broadband LEO satellite, FSAGE (Frequency Space Alternating Generalized Expectation Maximization) algorithm is proposed to cancel the interference introduced by frequency offset and channel fading, which firstly estimates and pre-cancel the frequency offset, then tracks the disturbance introduced by both frequency offset and channel fading. In order to reduce redundant information and enhance the transmission efficiency for broadband LEO satellite communication system, MCMA-RLS (Modified Constant Modulus and Recursive Least Squares Algorithm) blind algorithm has been proposed, which does not need to use training sequence, and has fast convergence.Fourthly, resources allocation method for LEO satellite OFDM system. In order to enhance power utilization for uplink mobile user terminal, user power control algorithm with satellite power detecting has been proposed, which reduces the power loss effectively for users. In view of ratio proportion for different user terminals and optimizing transmission rate in downlink broadband LEO satellite transmission system, a low complexity average resource allocation algorithm united with power, sub-carrier, bit has been proposed, which firstly guarantees ratio proportion of user terminals with low complexity in average way, and fulfills the maximized rate in a certain extent.Fifthly, analysis of throughput of OFDM transmission technique for broadband LEO communication system has been given. Broadband LEO link transmission system has been firstly established combined with AMC (Adaptive Modulation and Coding), and performance of throughput for mobile user terminal has been shown. Compared with MF-TDMA transmission technique for broadband LEO communication system, OFDM is superior in broadband LEO satellite.