Research On Ultra Reliable And Highly Secure Transmission Technology For OTFS

In the future,6G will penetrate into human life with unprecedented breadth and depth and become the infrastructure of information society.In this background,the security and reliability issues it faces will accordingly involve all aspects of society,especially in high-speed mobile wireless communication scenarios such as high-speed railroad(HSR),unmanned aerial vehicle(UAV),vehicle-to-everything(V2X),and low earth orbit(LEO).In the above scenarios,the contradiction between “dynamic,open wireless channel” and “high reliability and high security requirements” is becoming increasingly prominent,and the failure to ensure high security and reliability of wireless transmission may cause social and economic property loss,privacy leakage of citizens,or threaten the national infrastructure and information security.In high-mobility wireless scenarios,wireless communication is characterized by high mobility,high-frequency bands,complex access environments,and the coexistence of ”frequency selectivity” and ”fast time-variation” in wireless channels,which makes the Doppler effect one of the key factors constraining the reliability and security of transmission.Therefore,the study of wireless transmission technology with high universality,endogenous anti-Doppler and high security characteristics has become one of the hot research directions for 6G.Oriented to the high reliability and high secure transmission requirements of future highmobility wireless communications,this paper studies in depth the emerging modulation technology with endogenous anti-Doppler characteristics,orthogonal time frequency space(OTFS),and based on the analysis of its reliability,covertness,and security rate,the corresponding transmission strategy is designed to realize the improvement of transmission reliability and security.The main research contents of this paper are summarized as follows:1.After extensive research and comparison of existing mainstream anti-Doppler solutions,OTFS,an emerging delay-Doppler domain modulation technique,is introduced.Firstly,the fundamental reason for the endogenous anti-Doppler property of OTFS is explained in terms of the transmission mechanism of radio waves in wireless channels.Further,the modulation and demodulation process of OTFS is explained in detail,the end-to-end relationship between its transmitted and received signals is given.Finally,the performance of OTFS,including the received signal-to-noise ratio and outage probability,is analyzed by mathematical tools such as probability theory and matrix theory,which provides a theoretical basis and an analytical framework for realizing the high reliability and high secure transmission of OTFS.The corresponding analysis and simulation results verify the superiority of OTFS over Orthogonal Frequency Division Multiplexing(OFDM)modulation in high mobility wireless communication scenarios.2.The OTFS transmission system with frequency-domain power allocation is designed for high reliability transmission requirements.Then,the closed-form solution of the received signal-to-noise ratio under frequency-domain zero-forcing linear(FDZF)equalizer is derived,and the frequency-domain optimal power allocation(OPA)scheme is designed.For the OTFS transmission system with frequency domain minimum mean squared error(FD-MMSE)equalizer,the closed-form solution of signal to signal plus interference plus noise ratio(SSINR)is derived,and the corresponding OPA scheme is obtained by Lagrange multiplier method.The results show that the OTFS transmitter needs to allocate more power to the poorer subchannels,which is exactly the opposite of the water filling algorithm used in OFDM.Finally,the BER simulation results of the proposed OPA schemes are shown and analyzed,and compared with the conventional schemes such as equal power allocation and water filling to verify the effectiveness of the schemes.3.To prevent the leakage of transmission messages,the physical layer security performance of OTFS transmission is analyzed.For the typical unicast-multicast joint transmission scenario with high security requirements in 6G networks,the OTFS-based unicast-multicast joint physical layer security transmission system model is proposed.Then,a low-complexity OTFS-based unicast-multicast joint transmission scheme is proposed by using multi-channel OTFS.Further,the closed expressions for the security outage probability and security rate of unicast is derived by solving the approximate probability distribution function of the sum of inverse gamma distribution variables.Finally,the effects of mobility,resource allocation and other factors on unicast security performance are analyzed,and the following conclusions are drawn: 1)the threat of low-mobility eavesdropper to unicast security is greater;2)the more timefrequency domain resource blocks are allocated to unicast,the greater the achievable security rate on a single subchannel;3)when the mobility of eavesdropper is higher than that of unicast user,the eavesdropper is prone to channel estimation error due to the failure of guard interval,resulting in a floor effect in its signal-to-noise ratio and consequently enhancing unicast security.4.In order to secure the behavior of OTFS transmission and fundamentally eliminate potential attacks and eavesdropping,the covertness of OTFS transmission in highmobility scenarios is analyzed.First,the OTFS covert communication system under mobile eavesdropper detection is modeled by introducing the stochastic geometry theory,and two different detection strategies for eavesdropper are discussed: 1)singlepoint detection;2)area detection.Then,for the single-point detection strategy,the covert throughput of this system is analyzed by integrating the two conflicting constraints of transmission covertness and reliability.For the area detection strategy,the detection error probability of eavesdropper is derived by modeling the fused signals as shot noise,and the closed-form solution for the covert throughput is given on this basis.Finally,the differences between the two strategies,single-point detection and area detection,are discussed.The analysis reveals that area detection does not always outperform single-point detection.However,area detection is more effective in limiting covert throughput than single-point detection,which requires the transmitter to stay away from the entire detection area rather than a specific detection point.5.It is not sufficient to secure the transmission by passively adjusting the system parameters according to the environment and user requirements.This passive security strategies have hysteresis,depend on specific environment,channel conditions,and eavesdropping methods,and are not robust enough.To realize robust secure transmission,active security transmission schemes based on integrated sensing and communication(ISAC)platform and UAV collaborative jamming are designed for the security and covertness requirements of OTFS transmission,respectively.In the active security scheme,legitimate transmitters use the ISAC platform to sense the state information of the legitimate channel in delay-Doppler domain.Then,based on this sensed information and the inter symbol interference of OTFS signals to construct the OTFS signal-pilot joint transmission frame with differential channel estimation function,which can effectively deteriorate the receiver performance of illegal users and enhance the security rate of legal users.