Link Enhancement Technology In High Frequency Sky-wave Communication

The High-Frequency communication has been widely used in long range military communication systems. The HF radio waves propagate over long distances due to reflection from the ionosphere layers in the upper atmosphere. For contemporary HF systems, the channel often exhibits the characteristics of low SNR, time and frequent selective fading, which cause severe burst errors.This thesis is aims at the crucial techniques for improving the sky-wave performance in wideband HF communication. An improved multicarrier 2-D spread system using differential detection or coherent detection for low SNR links are proposed, which can effectively obtain time-frequent domain gain and considerably promote the BER performance. In addition, an MIMO-OFCDM system is proposed, which can effectively improve the system performance. At last, the corresponding distributed MIMO system timing and frequent synchronization method is designed.The layout of the thesis is presented as the following:Chapter One comprises the overview of the development of domestic and foreign HF communications. Some key technologies, including OFDM, spread spectrum and diversity technologies are given, and followed by the main objective of this thesis.Chapter Two describes the basics of HF sky wave channel, including transmission characteristics of the ionosphere, the channel fading, noise and channel interference. Then some models of HF channel are proposed, eventually a possible model channel is made up, which is presented as the channel platform to set up a simulation system.Chapter Three focuses on a two-dimensional time-frequency domain spread spectrum OFDM method, which can obtain time and frequent domain diversity gain at the same time. In addition, the coherent detection and differential detection are proposed in the system. By the system design and performance simulation, the method is verified under three sky-wave channels, which effectively improves the performance of the HF link.Chapter Four introduces the distributed multi-antenna systems in HF communication. This chapter proposes a new MIMO-OFCDM system, and the practical synchronization for multi-antenna is discussed. We design a new preamble based on CAZAC sequence and the corresponding timing and frequency synchronization scheme are proposed. Simulations verify that the method can effectively improve system performance and increase the transmit rate.Chapter Five reviews the previous chapters, and discusses some possible developments of the HF sky-wave communication system.