Research On Application Of Time-frequency Modulation In Short Wave Communication

Time-Frequency modulation is a two-dimensional combination modulation technology developed on the basis of Frequency Shift Keying(FSK)and Time Shift Keying(TSK),that is,the symbol is divided into several continuous slots,in which one or more coherent carrier frequencies are arranged to convey information.Due to the characteristics of time diversity and frequency diversity,it has good resistance to channel fading and has been widely used in short-wave communications.However,the symbol design of the traditional time-frequency modulation technology is simple,and the number of bits carried by each symbol is limited.It is difficult to meet the requirements of a short-wave communication system with a higher transmission rate and supporting rate adaptation and waveform adaptation.Simply reducing the symbol symbol length will cause a sharp decline in reception performance,and expanding the spectrum bandwidth is also constrained by the increasing scarcity of spectrum resources.Demodulation research is mainly based on hard decision,which is difficult to combine with channel coding technologies such as LDPC codes and Turbo codes using soft information to decode,which limits the further improvement of the performance of the time-frequency modulation system.Based on the above problems,we propose Time Frequency Shift Keying with Index Modulation(TFSK-IM),and theoretically deduce its system performance under AWGN channel and Rayleigh fading channel based on the non-coherent reception.By expanding the index dimension of the signal space,the information rate and spectral efficiency can be improved without expanding the frequency spectrum.In order to achieve system rate adaptation,an optimal TFSK-IM signal mode design method is proposed in this paper,which can effectively realize the optimal performance TFSK-IM signal mode design at a specified information rate.For orthogonal and non-orthogonal TFSK-IM symbols,a soft information calculation method is proposed.Combining the bit-likelihood information(LLR)calculated based on this method with LDPC code to soft demodulate can bring significant coding gain and improve the performance of the time-frequency modulation system.