Low Earth Orbit (LEO) satellites undergo a period of rapid development driven by ever-increasing user demands, reduced costs, and technological progress. Since there is a paucity of literature on the security and reliability issues of LEO Satellite Communication Systems (SCSs), we aim to fill this knowledge gap. Specifically, we critically appraise the inherent characteristics of LEO SCSs and elaborate on their security and reliability requirements. In light of this, we further discuss their vulnerabilities, including potential security attacks launched against them and reliability risks, followed by outlining the associated lessons learned. Subsequently, we discuss the corresponding security and reliability enhancement solutions, unveil a range of trade-offs, and summarize the lessons gleaned. Furthermore, we shed light on several promising future research directions for enhancing the security and reliability of LEO SCSs, such as integrated sensing and communication, computer vision aided communications, as well as challenges brought about by mega-constellation and commercialization. Finally, we summarize the lessons inferred and crystallize the take-away messages in our design guidelines.

Low Earth Orbit Satellite Security and Reliability: Issues, Solutions, and the Road Ahead

The Internet of Remote Things (IoRT) supported by low-Earth orbit (LEO) satellites is becoming indispensable for remote sensing and it will play an important role in the forthcoming sixth-generation (6G) communication network. In exploring its applications in remote mining and smart grid, etc., it is found that the implementation of IoRT faces challenges, including limited energy supplies, high-mobility, and security vulnerabilities. To address these challenges, we propose employing collaborative LEO satellites to enable the implementation of secure and green IoRT. By combining the uplink signals received at collaborative LEO satellites, the signal-to-noise ratio (SNR) can be significantly improved, so that relieving the transmit power requirement of the energy-limited terminal. Aiming at constructing a collaborative LEO satellite-based IoRT network, this article introduces the system design principles regarding to frequency planning, waveform selection, collaboration strategies, and terminal design. In order to obtain optimal collaboration performance, we propose a signal coherent combining scheme to compensate Doppler shift, propagation delay, and phase differences. Furthermore, we propose a modified SUMPLE algorithm to estimate and compensate phase differences among satellites, which is applicable to direct-sequence spread spectrum (DSSS) signal scheme. Simulation results demonstrate that our proposed algorithm outperforms the traditional SUMPLE algorithm in combining gain.

Collaborative LEO Satellites for Secure and Green Internet of Remote Things

A direct-sequence spread-spectrum (DSSS) signal has many advantages, such as low SNR and strong anti-jamming ability; thus, it is widely used in various military and civil communication systems. However, hiding and high-data-rate transmission technologies have become the key factors affecting the future applications of the existing DSSS. To solve these problems, an incremental-data stealth-transmission (IDST) method is proposed, which stealthily transmits incremental data through variations in the spread spectrum function and alternation of the weighted fractional Fourier transform modulation order. The incremental data are not transmitted through a real channel but through the function relationship to achieve index transmission. Without affecting the spread spectrum gain and SNR tolerance of the original data, the proposed method greatly improves the amount of information transmitted and enhances the concealment of incremental data.

Incremental-data stealth-transmission method in DSSS

The invention discloses a high-dynamic pseudo code continuous wave radar long-time accumulation method, which comprises the following steps of: firstly, dynamically reducing the order of a target through an adjacent cross-correlation method, correcting code phase drift caused by high dynamic and high maneuverability of the target, and reducing the Doppler diffusion order; correcting Doppler migration through Lucker transform, and realizing phase-coherent accumulation of signal energy; and finally, estimating a target parameter through peak detection and first-order search The problems of codephase drift and Doppler migration caused by the high-speed and high-maneuverability target in the pseudo code continuous wave radar are effectively solved, and long-time accumulation and high-precision parameter estimation are realized; code phase drift of the target is corrected through adjacent cross correlation, and compared with a traditional KT method, influence of velocity ambiguity on correction is effectively avoided; a non-search Lucker transform method is adopted to carry out phase-coherent accumulation on signals after code phase drift correction; and compared with a traditional multi-dimensional search GRFT method, the operation complexity is greatly reduced

High-dynamic pseudo code continuous wave radar long-time accumulation method

In order to provide high data rates for future wireless communication systems and high-quality communication systems, many new methods have been developed. In recent years, the use of these devices has exploded. In the near future, this technology would require very efficient bandwidth features, such as bandwidth selection, frequency diversity, and high spectral accuracy. The networks are growing because current capacity is small, and further demand for a higher data rate is also expected. Furthermore, 4G (Fourth Generation) networks must be pleased with downlink mobile multimedia facilities of up to 20 - 100 Mbps and cellular uplink connectivity of 2 - 20 Mbps. To support high data rate transfer, a Multiple Access Wireless Modulation System, referred to as the Multi-Carrier Code Division Multiple Access (MC CDMA), has recently been proposed. Previous studies did not discuss transmitting the image data in the mc-cdma system on the rayleigh fading channel due to the large amount of noise that occurs as a result of the multipath fading channel, but by using a orthogonal spreading code (Orthogonal Variable Spreading Factor (OVSF)) the noise can be reduced and ofdm modulation reduces intersymbol interference (ISI). This paper discusses the bit error rate (BER) of theAdditive white Gaussian noise (AWGN) channel and rayleigh channel output of the MC-CDMA system using OVSF code. The relationship between the bit error rate and the signal to noise ratio is discussed when image transmited over AWGN and rayleigh channel using different modulation and then Comparison is made between the two channels. 

Image transmission on MC-CDMA system over Rayleigh and AWGN channel using OVSF spreading code

A novel coherent integration method for high dynamics direct sequences spread spectrum (DSSS) signal is presented. The presented method is based on adjacent cross-correlation function (ACCF) and Lv's distribution (LVD). ACCF is firstly used to compensate the code phase drift and reduce the Doppler frequency migration order. After that, LVD is used to accomplish the integration and obtain the parameter estimates. The presented method can estimate motion parameters without any searching procedure. Compared with the existing methods, the presented method can achieve a good balance between the computational cost and the detection and estimation performance. Numerical simulations are provided to verify the effectiveness of the proposed method.

Long-time coherent integration for high dynamic DSSS signal

In direct sequence spread spectrum systems, the multipath interference is known to be one of the most important factors in degrading the measurement accuracy. To resolve this problem, we propose a superresolution multipath estimation method based on waveform replacement and adaptive-filtering. In this method, the continuous pseudorandom noise code autocorrelation waveform of the matched filtering output is first replaced by a designed triangular pulse waveform. Subsequently, a reiterative minimum mean-square error adaptive filtering approach is applied to realize superresolution estimation of multipath parameters. The proposed method can achieve accurate multipath estimation even in complex multipath scenarios containing short-delay multipaths and make a good balance between the computational complexity and the estimation performance of multipath parameters. Simulation results verify the effectiveness of the proposed method.

A Superresolution Multipath Estimation Algorithm for DSSS Systems

The invention discloses a high-dynamic carrier phase tracking method for echoes of a pseudo code modulated continuous wave radar. According to the principle of the method, a frequency locking ring isconnected with a phase locking ring in parallel and is used for tracking the carrier frequency in digital intermediate frequency signals of the echoes of the pseudo code modulated continuous wave radar, so as to eliminate high-order dynamic components in high-dynamic carrier signals; and the frequency locking ring is subjected to locking determination, the phase locking ring is subjected to locking determination by virtue of a locking determination result of the frequency locking ring when the frequency locking ring is locked and then can work at a relatively low dynamic state, so as to realize the high-precision tracking of the carrier phase in the digital intermediate frequency signals of the echoes of the pseudo code modulated continuous wave radar. According to the method, the good dynamic tracking capability of the frequency locking ring and the high carrier phase tracking precision of the phase locking ring are adequately exerted, and the precise tracking problem of high-dynamicand non-steady carrier signals under a serious non-steady state of a detection signal caused by high-maneuvering attitude adjustment and orbit changing of a carrier can be effectively solved.

High-dynamic carrier phase tracking method for echoes of pseudo code modulated continuous wave radar

The invention discloses a continuous wave radar phase interference angle measuring method based on receiving channel multiplexing, wherein high-precision phase interference angle measurement of low-signal-to-noise-ratio radar echo can be realized on the condition of relatively low resource cost. An antenna array of a continuous wave radar is composed of L angle measuring antennae. An antenna switching control signal is set. A continuous wave signal which is received by each receiving channel is caused to pass through a multiple-to-one switch, thereby generating a time division multiplexing single-channel angle measuring signal. Delay processing which matches a receiving channel delay is performed on the antenna switching control signal, thereby generating a sorting control signal. After signal processing is performed on the time division multiplexing single-channel angle measuring signal, a complex-baseband signal which comprises a same-phase branch and an orthogonal branch is obtained. Through controlling by the sorting control signal, sorting is performed on the multiplex-baseband signal, thereby obtaining L independent pulse modulating receiving signals. After segmented integration, the signals are recovered to continuous wave signals, thereby obtaining L independent measuring signals. Phase locked tracking is performed for extracting carrier phase information and finishingphase interference angle measurement.

Continuous wave radar phase interference angle measuring method based on receiving channel multiplexing

The problems of range cell migration (RCM) and Doppler frequency migration (DFM) occurred during the long time integration may severely affect the integration performance for maneuvering targets. In this paper, the RCM is corrected through dividing the range frequency spectrum into several segments, which decreases the range resolution based on the relationship between the size of the range cell and the signal bandwidth. After that, a well-focused image of a maneuvering target with third-order motion is obtained via segmental dual-scaled correlation transform (SDSCT), and three motion parameters, i.e., velocity, acceleration, and acceleration rate, are effectively estimated. The proposed method can detect multiple maneuvering targets with third-order motion with low computational complexity. Compared with several representative methods, the proposed method achieves a good balance between the computational cost and the parameter estimation performance. Finally, simulated results are used to validate the effectiveness of the proposed method.

Fengyu Wang

Papers

Long-time coherent integration for high dynamic DSSS signal

A Superresolution Multipath Estimation Algorithm for DSSS Systems

High-dynamic carrier phase tracking method for echoes of pseudo code modulated continuous wave radar

Continuous wave radar phase interference angle measuring method based on receiving channel multiplexing

Efficient Motion Parameter Estimation for Maneuvering Target based on Segment Processing