Channel Model Simulation And Spread Spectrum OFDM For HF Communication

HF (high frequency) radio propagation in HF communication systems involves time continuous signals in the 3-30MHz band. The emitted wave is reflected on the atmospheric layers for transmission over distances of the order of 1000 km. The HF communication systems has the advantages of simple device, long distance transmission, low cost, good flexibility and survivability, and is widely used for military, maritime, and aeronautical systems and mondial broadcasting. In recent years, with the development in researches of HF signal transmission, characteristics of atmospheric layers and HF channels, a lot of achievements are gained about HF Adaptive Frequency Selection, adaptive equalization. HF communication has improved a lot in resisting muti-path fading, high data-rate to bandwidth ratio and high data transmission rate.HF channel modeling, simulation and modulation and other new technology are the focus of researches in HF communication. Channel simulator is an effective and convenient mean for the design and performance evaluation of HF communication systems. Valid HF channel models are the basis of implementation of hardware HF channel simulator, system design and performance evaluation. The Orthogonal Frequency Division Multiplexing and spread spectrum communication technique can improve the data-rate to bandwidth ratio and data transmission rate, and also increase the abilityIn this thesis, HF channel modeling and simulation, OFDM modulation technique and spread spectrum OFDM in the HF communication are investigated, theoretical analysis and simulation experiments are also carried out.First of all, the main characteristics of HF communication are summarized and the research result about HF channel and key technologies are of HF communication illustrated in detail. The HF channel is a typical time-varying channel, suffering from multi-path delay, fading, Doppler frequency shift and spread, interferences and noises. The HF Radio Channel is very difficult to characterize as it has non-stationary fading, impulsive noise and man-made interference. The thesis presents the variety of HF channel models usually used. The most usually used Watterson model is illustrated in detail, including its model structure, assumption and expression. Then the software simulation of the Watterson model is presented. An improved Watterson model in which the time-delays and Doppler frequency shifts are both modeled as random variables with Gaussian distributed function is proposed. Simulation results prove the validity of the proposed HF channel simulator and provide some new insight into the time-varying properties of the HF channel. The proposed model can be used into the simulation of adaptive HF communication system and is much helpful to the design of hardware-based simulator.Motivated by the application of spread spectrum technology and digital signal processing techniques to high-frequency (HF) communication systems, advanced HF systems that operate over wide bandwidths (on the order of 1 MHz or more) are needed. The thesis summarized the main wideband HF channel models as well as the hardware simulators, and the most famous ITS wideband HF channel model is presented in detail. A real-time software simulator of wideband HF (high frequency) channel model is presented. Using the tapped delay line structure of narrowband Watterson model, the time delay profile of the wideband HF channel model is sampled into discrete points, dividing each propagation path into series of sub-paths. The deterministic phase function and stochastic modulation function is applied. The sum of all sub-paths forms the output signal. The analytical expression of the proposed HF channel model is presented, too. Simulation results show the validity of the proposed simulation model. The resulted wideband HF channel simulator is easy to deal with the simulation of HF channel with varying parameters and is convenient to be implemented in the hardware simulator of HF.Channel adaptive technologies, mainly referring to the real time frequency selection and the frequency adaptive technology, are wildly used in modern HF communications, making the HF communication system work on the best frequency to adapt to the vary of atmospheric layers and resist the interferences of neighbor radio station.HF communication electronic defense technology mainly refers to HF spread spectrum, including the HF frequency hopping, direct sequence spread spectrum, aiming to deal with the problem of less security and ability of anti-interference.Modulation and demodulation is one of the key aspects of HF communication. OFDM is the most frequency-efficient transmission system known so far. It has the advantages of fast transmission rate and the strong ability to anti multi-path fading. It combines the digital modulation, digital signal processing and multi-carrier transmission technique together, which makes it much more competitive in frequency usability, power usability and system complexity. It is one of the most potential high speed communication techniques of the next generation.The thesis applies the OFDM technique to HF communication. Based on the software simulation of wideband HF channel, a suit of comparatively intact HF OFDM system design is proposed. The system design is analyzed by analytical expression and validated by software simulation. The improvement of system performance is mainly discussed by simulation results. Spread spectrum technique has stronger ability of resisting jamming, narrow band, and multi-path interference, and it also has the characteristics of low probability of intercept. This thesis applies the spread spectrum technique into the OFDM HF communication system. The proposed algorithm combines OFDM modulation with direct spread spectrum, making the system can accommodate large variation of HF channels. The software simulation is implemented. Simulation results show that it can not only effectively enhance the transmission rate and capacity of the HF communication system but also effectively suppresses the multi-path fading and interferences.Finally, the main directions of future research in HF communication are presented in the thesis. The advanced digital signal processor can be used to implement the high performance real-time channel simulator, and test word should be performed to get more channel measurements. The DSS-OFDM used in HF communication can enhance the system performance a lot, but because of the inherent characteristics of HF channel, there are many problems to be solved, such as the peak-to-average power ratio, the source coding, channel coding, error coding used in HF communication.