Simulation Research Of HF Adaptive Transmission System Based On OFDM

HF communication is a telecommunication way which depends on the reflection of the atmospheric ionosphere for information transmission.And it has been developed maturely in both civilian and military uses.With the development of communication technology and new business,and the higher and higher need for HF communication system,introducing new technology to promote the performance and quality of the system is quite positive.Firstly,the current situation of HF OFDM system development and the adaptive modulation technology are introduced,and the basic principles of OFDM system and adaptive modulation technology are briefly given in this thesis.The four adaptive modulation algorithms in single-user OFDM system including Hughes hartogs algorithm,Chow algorithm,Fischer algorithm and SBLA algorithm are discussed.The bit and power allocation of these algorithms are simulated under multipath Rayleigh fading channel,and the performance and the complexity of different algorithms are analyzed and compared as well.Secondly,the systematic structure diagram and parameters of HF adaptive transmission system based on OFDM are given.Then the BER of the whole system with different adaptive modulation algorithms are simulated.The simulation results are as follows: first,HughesHartogs algorithm is of the best performance among the sub-carrier-based adaptive modulation algorithms;next,the smaller sub-band width brings better performance in SBLA algorithm;last,the performance of the system applying adaptive modulation technology is greater than the system without the same application.Finally,to improve the frequency spectrum efficiency,a processing strategy of OFDM symbol is given when the CP length is less than the maximum multipath delay,and this strategy is applied to the HF adaptive system based on OFDM.For multipath channel,ISI and ICI will occur in effective OFDM symbol when CP length is not enough.Decision feedback processing and CP compensation technology are used to solve ISI and ICI respectively in the thesis.The simulation results show that the performance of the system with new strategy is not as good as that of the system with traditional CP length,but the spectral efficiency is improved obviously.So a compromise scheme about the performance and the spectral efficiency can be selected for the actual system.