Research On Wideband Ground-Air Communications With Low Elevation

With the development of the aviation terrace, the demands for wideband ground-air communications are rapidly increasing. It is an urgent and important task to build several data transmission links to satisfy the requirement of various missions executed by many kinds of aviation terraces. As a node of a wideband communication network, aircraft payload needs various functions, such as relay,repeat/forward, resource scheduling, data switching and data distribution, etc., where the key technique is the wideband ground-air link.When the elevation of the antenna in the ground station is very low, the multipath effect, resulted from the reflection of ground surface features, appears to be serious, multipath time delay is rather large (hundreds of symbol periods), and the Doppler effect is remarkable. This is because that its transmission distance is very large and the moving speed of the aircraft is much greater than that of the vehicle. Therefore, its multi-path fading is characterized as fast varying frequency selective, so that anti-fading transmission in this case is a challenging issue.For the sake of exploring the most suitable method for anti-multipath transmitting in a wideband ground-air communication with low elevation, this dissertation proposes a model of ground-air channels to reflect their characteristics. After analyzing the limitations of the existing anti-multipath techniques, which are commonly used in the conventional wideband mobile communications, a novel transmission scheme is proposed, where the Filter Banks for Frequency Domain Equalization (FB-FDE) is employed as the kernel technique for anti-multipath fading. Theoretical analysis and simulation experiments show that the FB-FDE scheme behaves well and less complexity for engineering implementation. Besides, the dissertation researches on several issues involved in the FB-FDE applied to such cases, such as channel estimation, multiple access and narrowband interference (NBI) suppressing, etc. The author's main contributions are as follows.1. The dissertation discusses the methodology about modeling the channel of wideband ground-air communications with low elevation, and proposes a Rice fading channel model by synthetically considering several factors, such as Rice factor, the time-variation of the channel parameters, the power spectrum distribution of azimuth and time delay, the Doppler spreading spectrum property as well as the correlation between multiple antennas. Moreover, the channel model simulating method and some simulation results are given for its evaluation.2. After analyzing the transmission mechanism and principle of OFDM and SC-FDE systems, the dissertation indicates the limitations of the two techniques applied to anti-multipath fading in the wideband ground-air communication. A scheme of transmission systems based on FB-FDE is proposed, where the prototype filter design as well as the signal reproducing is novel. In researching on FB-FDE and its implementation, a synchronization method and a diversity reception algorithm are proposed. Through theoretical analysis and simulation experiments, the performance of the proposed FB-FDE system and the conventional SC-FDE system are compared, which can validate the feasibility of the FB-FDE to be applied to the wideband ground-air communication as a kernel technique.3. After theoretical analyzing, the dissertation indicates that the FB-BDE system over-performs the conventional SC-FDE or OFDM systems in the ability adaptive to rapid variation of the channel. However, the FB-FDE system can not exert its superiority if it is combined with Least Mean Squares (LMS) adaptive algorithm because of LMS's slow convergence. If Recursive Least Squares (RLS) algorithm instead of LMS is employed to the FB-FDE system, its computational complexity is too high( O ( M 2)grade) although it has faster convergence. In the dissertation, a modified RLS algorithm is proposed and applied to FB-FDE system, where the RLS not only has much faster convergence performance than LMS, but also has very low implementation complexity to be similar as LMS. Analysis and simulations show that the modified RLS algorithm can guarantee the performance of the system; moreover, it can also be applied to SC-FDE system.4. A multiple access and networking scheme based on star-shape topology is proposed, which can satisfy two kinds of requirements for ground-air communications in networking, where the one is composed of multiple ground stations with one aircraft-loaded hub station, the other is composed of multiple aircraft-loaded terminals with one hub ground station. In the proposed system, the forward link is an synchronous transmission link, using FB-FDE based M-ary multi-codes spreading multi-access mode, and it has good performance in channel efficiency and dynamic adaptive ability for channel variation; while the reverse link is a asynchronous transmission link based on Orthogonal Code Division Multiplexing(OCDM) combined with FB-FDE, and a novel block coding method for improving transmission performance of the OCDM is also proposed.5. Through analyzing the performance of suppressing narrowband interference of two systems, i.e. FB-FDE and SC-FDE systems when they are combined with a similar NBI suppressing method, the dissertation indicates that the former has stronger NBI suppressing ability than the latter.Finally, the dissertation summarizes the research results and indicates some issues which worth further endeavor in engineering realization.