Reasearch On The Key Techniques Of HF OFDM Systems

Orthogonal Frequency Division Multiplexing (OFDM) technology has various advantages such as robust characteristic under multipath environment and high speed transmission rate.It has been broadly applied in wireless LANs ,digital Audio/Video broadcasting and digital TV. Modern HF communication is paid more attentions, owing to its advantages such as low cost, convenience and long distance communications .However ,the HF channel is multipath and Rayleigh fading and the band is narrow.That has impact on the speed and reliability of transmission. Therefor,it is very significant to apply OFDM in high speed HF communication.In this paper, taking account of multipath propagation, fading and Doppler shift on the shortwave channel,the key technology in OFDM are studied.A a serial of simulation results and some analysis are given ,and then concrete design scheme is proposed. The system is realized mainly based on the DSP and FPGA. The primary endeavor are as follows:1.In the condition of packet transmition mode,the synchronization solution is discussed .Then the acquisition method is put forward on low SNR terrible HF channels.2.The channel estimation of OFDM in HF communication is studied.The methods of LS and LMMSE are introduced.Based on the Two methods ,the performance comparison under 2-path channel between the two following methods:the lowpass filter in time-domain and the Delay-Subspace.At last the two methods are applied in the system.3.In order to come to higher transmion speed,the density of pilots is reduced.In the high-speed scheme ,the method of iterative joint signal detection algorithm of channel estimation and decoding using decision feedback is discussed.The direct decision feedback and the soft information feedback are compared.And the result of simulation shows that the method of feedback is better then the traditional method.4.The realization method of key technology of the OFDM communication system is designed,and the system scheme of hardware and software is set up.The scheme is applied in th plateform of DSP and FPGA successfully.The above methods of the OFDM communication system provides a significant scheme in OFDM application fields, and has significant impact on OFDM communication system.