Research On Time-Frequency Modulation In Wideband Short Wave Communication

Shortwave(HF)wideband communication is one of the development directions of the next generation of shortwave communication.HF wideband channel modeling and broadband modulation technology are important research contents.Time-Frequency Shift Keying(TFSK)is a modulation technique widely used in HF narrowband systems and has good anti-fading performance due to its frequency diversity feature.This paper studies the application of TFSK technology in HF wideband communication,aiming to build an HF wideband channel model and research on the bit error rate(BER)performance of the TFSK system in the channel,including the following aspects:1.In the simulation of the ITS HF wideband channel model,the calculation of the delay power profile(DPP)parameters is complicated,which causes inconvenience in the simulation.This paper proposes a method for curve fitting using the Nakagami-m distribution probability density function,in which the calculation of the fitting parameters is based on the root mean square delay spread.The fitting method avoids a lot of complexity caused by iterative calculations when calculating DPP parameters,reduces the simulation overhead,and has a low fitting error.2.Perform research on TFSK theory,analyze the symbol design,bit error performance,and spectral efficiency of TFSK.Aiming at the problem that the increase of frequency diversities in TFSK reduces the spectral efficiency,this paper proposes to apply Frequency Shift Chirp Modulation(FSCM)with similar frequency diversity characteristics to shortwave communication and analyzes its theory and related performance.For the multipath interference of TFSK and FSCM in HF channel,the receiving structure of frequency domain equalization is studied,and the BER performances of TFSK and FSCM system under HF wideband channel as well as different interference are given,and the results show that the increase of frequency diversities improves the BER performance.3.Perform research on channel coding technology in shortwave communication systems.Aiming at the problem of needing bit likelihood information when applying channel codes such as LDPC codes,the soft-decision demodulation algorithms of TFSK and FSCM are studied.A soft-decision demodulation algorithm for FSCM based on non-coherent reception is proposed.The BER performances of TFSK and FSCM systems using different codes as well as different decoding algorithms under the HF wideband channel are given.Simulation results show that soft decision provides better BER performance than hard decision,and good codes such as LDPC codes provide more coding gains.