| The paper focuses on the research and implementation of data transmission system for HF communications in extremely low SNR. This transmission system provides an emergent communication means under severe electromagnetic conditions of strong interference and noises where conventional communication means are completely disabled. And data transmission with very low rate can be achieved in this system.Firstly, the paper introduces the date transmission scheme based on DS_BPSK which was existed. The data transmission works under the condition of SNR = ?10 dB~?20dB. Secondly, the paper introduces some techniques to improve the performance of the data transimission which was existed. In spread spectrum techniques, the technique of chaotic spread-spectrum is used. The balances, autocorrelations and correlations of three chaos-maps are studied and simulated. The Improved Logistic-map and m sequence is acted as spreading pseudorandom sequence. The simulations of two spreading sequences show that the new spreading sequence improves the synchronization and credibility of the data transimission system. At the receiver,technique of the suppression of narrow-band interference in time domain is adopted to eliminate the effect of narrow-band interference effectively and enhance the system capability to reject the narrow-band interference. In the techniques of error control, the CRC is used to reinforce the capability of system error detection. RS coding instead of BCH coding combined with helix interleaving techniques have better performance on correcting random and burst errors. Consequently, the Packet Error Rate of system is reduced.Finally, the schemes of this data transmission system’s transmitter and the filter for suppression of narrow-band interference have been implemented on SEED-DEC6713 hardware platform. The simulated results show that the system can work correctly. |
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