S atellite-terrestrial integrated(STI)systems represent the right solution to meet complex requirements of several services and sharing of the limited spectral resources between satellite systems and terrestrial ones must be considered to optimize performance.Network architectures and traffic demand are different for the satellite component and for the terrestrial 5G/6G one，so that the requirements of spectral resources for satellite and terrestrial systems are expected to vary dynamically in a significant range.

Then，over the overlapped coverage area of satellite and terrestrial systems inter-system interference may arise but also jamming，which affects every wireless system(either terrestrial or satellite)，is an issue.For this reason，it is necessary to define suitable interference and self-interference models and to evaluate the impact of interference on the system capacity.

Moreover，safety，security and reliability represent particularly serious issues in the depicted scenario due to its intrinsic complexity.

Definitively，it is of paramount important to exploit deep research activities on spectrum management，spectrum sensing，spectrum access and spectrum sharing and then to share the outcomes.

Last but not least，Unmanned Aerial Vehicles(UAVs)play an important role in space-air-ground integrated networks，having been applied in many fields(target search，regional patrol，signal detection)for its advantages in terms of high efficiency，low cost and flexibility but improvements are desirable in terms of deployment，flight trajectory planning and security.

The research results presented in this special issue are helpful to realize the object of spectrum plan coordination，intelligent interference sensing and decision，interference management，anti-interference and physical-layer security enhancement.

The call for paper achieved a meaningful success，having stimulated a large number of contributions focused on from spectrum sensing，dynamic spectrum access，spectrum sharing，energy-efficient UAVs deployment，anti-jamming trajectory plan for unmanned aerial vehicles(UAVs)，secure transmission in Satellite-UAV integrated system，multiple access，blind source separation for OTFS modulated signal，blind interference reduction and signal recovery for MIMO IoT，blockchain consensus mechanism used in LEO satellite systems，multiple frequency-hopping(FH)signal parameters estimation，multi-channel adaptive filter，etc.

This special topic issue focuses on improving the performance of the satellite-terrestrial integrated(STI)systems.Based on the submissions and careful peer review，twelve papers have been finally accepted by the guest editors.Among the accepted papers，three of them are focused on spectrum management and other three papers on optimization of UAV’s deployment，flight path and secure transmission.Two of them are focused on interference suppression using blind source separation.One paper is related to multiple access in the STI system.One paper is related to blockchain technology used in LEO systems.The last two papers approach parameters estimation in frequency-hopping(FH)system and design multi-channel adaptive filter.We regret to not having been able to accept more excellent papers within this Special Issue due to the limited space.

The first paper，“Performance analysis of Spectrum

Sensing based on Distributed Satellite Clusters under Perturbation，” discusses the spectrum sensing performance of a distributed satellite clusters(DSC)under perturbation，aiming to enhance the sensing ability of weak signals in the coexistence of strong and weak signals.

The second paper，“Dynamic Spectrum Access based on Prior Knowledge enabled Reinforcement Learning with Double Actions in Complex Electromagnetic Environment，” takes into account the fast-changing dynamic interference spectrum environment，and propose a spectrum access scheme based on the prior knowledge，which can adjust the access channel online according to historical information and obtain the optimal access policy.

The third paper，“Improving SINR via Joint Beam and Power Management for GEO and LEO Spectrum-Sharing Satellite Communication Systems，”discusses the spectrum sharing problem between GEO and LEO satellite communications systems，and propose a joint beam-management and power-allocation(JBMPA)scheme to maximize signal-to-interference plus noise ratio(SINR)，while maintaining the ongoing wireless links spanning from LEO satellites to their corresponding users.

The fourth paper，“Anti-jamming Trajectory Design for UAV-Enabled Wireless Sensor Networks(SNs)Using Communication Flight Corridor，” investigates the anti-jamming trajectory to guarantee the effective data collection and aims to minimize the UAV’s flight energy consumption in a finite task period by jointly optimizing SNs collection sequence and UAV flight trajectory.

The fifth paper，“Secure Transmission in Satellite-UAV Integrated System Against Eavesdropping and Jamming:A Two-Level Stackelberg Game Model，” pays attention to the physical layer security transmission problem existing in the information backhaul link of the satellite-UAV integrated network，and proposes a two-layer Stackelberg game model to resist full-duplex eavesdropping and jamming attacks.

The sixth paper，“An Energy-Efficient UAV Deployment Scheme for Emergency Communications in Air-ground Networks with Joint Trajectory and Power Optimization，” discusses the deployment problem of unmanned aerial vehicles(UAVs)and proposes a UAV broadcast coverage strategy that can maximize energy efficiency under terrestrial users’

requirements.To solve the non-convexity problem，an iterative algorithm by jointly optimizing trajectory and power allocation is proposed.

The seventh paper，“Independent Vector Analysis Based Blind Interference Reduction and Signal Recovery for MIMO IoT Green Communications，”considers the inherent problem in frequency domain independent component analysis(FDICA)assisted multiple-input multiple-output orthogonal frequency-division multiplexing(MIMO-OFDM)based Internet of Thing(IoT)systems and proposes a new detection mechanism to reduce inter-carrier interference and multiple access interference.

The eighth paper，“Intelligent Blind Source Separation Technology Based on OTFS Modulation for LEO Satellite Communication，” discusses the problem of the significant Doppler effect caused by the high-speed movement of the satellite relative to the ground and a novel blind separation algorithm is proposed for LEO satellite communication system，which performs OTFS modulation in the Delay-Doppler domain.Simulation results show that the proposed algorithm performs better than the conventional ones and the performance of mixed signal separation in Delay-Doppler domain is better than that in Time-Frequency domain.

The ninth paper，“Deep Unfolding for Cooperative Rate Splitting Multiple Access in Hybrid Satellite Terrestrial Networks，” considers a two-user system in hybrid satellite terrestrial network(HSTN)where one of them is heavily shadowed and the other uses cooperative rate splitting multiple access(RSMA)to improve the transmission quality.

The tenth paper，“A Public Blockchain Consensus Mechanism for Fault Tolerant Distributed Computing in LEO Satellite Communications，” considers that the dynamically changing network environment affects the results of distributed computing，and propose a novel public blockchain consensus mechanism that applies a distributed computing architecture in a public network to improve the fault tolerance rate.

The eleventh paper，“Design of Multi-channel Adaptive Filter by Constructing Multi-dimensional Wiener-Hopf Equation，” considers signal self-interference problems of symbols in the same channel for the satellite-to-ground high-speed data transmission link，and a multi-channel adaptive filter is designed，and the influence of five channel non-ideal factors is suppressed to improve the BER performance.

The last paper，“Parameter Estimation of Multiple Frequency-Hopping Signals Based on Space-Time-Frequency Analysis by Atomic Norm Soft Thresholding with Missing Observations，”addresses the problem of multiple frequency-hopping(FH)signal parameters estimation in the presence of random missing observations，and exploits the inherent incomplete data processing capability of atomic norm soft threshold to analyze the space-time matrix and complete the accurate estimation of the hopping time and frequency of the received FH signals.

The guest editors take advantage of this opportunity to thank all the authors for their contributions and also thank the reviewers for their diligent work，professional comments and for the time and effort handsomely dedicated.Their professional opinions are the key to ensure this Special Topic Issue in high quality.