Research On And Realization Of Key Techniques For HF Middle And Low Rate Digital Transmission

HF channel is a typical time-varying channel with severe frequency-selective fading and doppler spreading phenomenon.So,low rate data transmission based on spread spectrum technique is an important way to guarantee link available under bad channel situation.The key receiving techniques of serial mode low rate 75bps data transmission systems based on America military standard 110A are studied in this paper.Two different depreading schemes are theoretically analysed and verified through simulation results.The problems appeared in frequency shift estimation are well solved using window technique.A simple maximum ratio combining method easy to realize for project is presented and its comparison to selective combining is done.An improved multipath capture scheme is given to change the situation that the capture probability of mulitipath is a bit low.A synchronization scheme and simple soft information calculate means are introduced based on the characteristic of data transmission standard.In the last part,burst waveform BW0 and BW1 transmitting HF link control command are introduced,also,decoding algorithm and key points in hardware realization of tail-biting convolutional code are researched.Finally,bit error rate of 75bps data transmission system and packet error rate of BW0 are tested by HF channel simulator MDM-3001,and the results indicate that the performance meets the requirements.