Research On Diversity Receiving Technology Of HF Frequency Selective Fading FSK Signal

Since the beginning of the 20 th century,short-wave communications have been an important part of military communications.Digital frequency shift keying(FSK)frequency-modulated signals are widely used in short-wave and other low-speed data communications.Because short-wave signal transmission mainly depends on ionospheric reflection,channel quality is subject to ionization Layer changes and noise,FSK signal has obvious frequency selective fading,and the performance of single-channel demodulation will have a bottleneck.Therefore,this article focuses on the use of multi-channel diversity technology to improve the demodulation performance of short-wave FSK signals.When multiple channels have independent channel fading,the combined reception of multiple signals can effectively compensate each other's fading,thereby improving the demodulation performance of short-wave FSK signals.First,the existing signal presence detection methods for single-channel signals are studied,and the time domain class detection algorithm based on time domain energy and the frequency domain detection algorithm based on Fast Fourier Transform(FFT)are introduced.Domain method statistics have low computational complexity and are easy to implement but have poor effect on the detection of fading channel.Frequency domain method has good detection effect on the presence of fading channel signals but loses time domain resolution.Subsequently,the joint detection method of multi-channel signals was studied,and a multi-channel signal detection scheme based on short-time Fourier transform and a multi-channel signal detection scheme based on energy entropy ratio were proposed.Then,this article has a research on different merging strategies,mainly including merging timing and weighting schemes.The choice of combining timing includes waveform combining at the front end and combining based on demodulation metric values at the signal demodulation stage.A waveform combination scheme based on phase error correction is proposed,and a variety of weighting coefficients are studied for weighted combination based on metric values.A method for estimating the signal-to-noise ratio of multiple FSK signals under short-wave channels is introduced,and the signal-to-noise ratio is used as a weight The coefficients combine the metric values of multiple signals.At the same time,an entropy-based weighted combination scheme is also proposed.The simulation shows that the weighted combination based on entropy in the short-wave channel has obvious performance advantages over equal gain combination.Finally,according to the second-generation automatic link establishment(2G-ALE)technology specified in Appendix A of the US shortwave adaptive communication standard MIL-STD-188-141 B,a diversity reception simulation link was built.Golay codes are used to study decoding schemes,and a maximum likelihood decoding method based on soft metrics is proposed.According to the ALE signal transmission format,a set of soft demodulation-based joint demodulation and decoding schemes with optimal demodulation performance is reversed,and the multi-channel signal soft metrics are combined to achieve ALE signal diversity reception processing,which has obvious gain than single-channel signals.