Space/Air Covert Communications:Potentials,Scenarios,and Key Technologies

Space/air communications have been envisioned as an essential part of the next-generation mobile communication networks for providing highquality global connectivity. However, the inherent broadcasting nature of wireless propagation environment and the broad coverage pose severe threats to the protection of private data. Emerging covert communications provides a promising solution to achieve robust communication security. Aiming at facilitating the practical implementation of covert communications in space/air networks, we present a tutorial overview of its potentials, scenarios, and key technologies. Specifically, first, the commonly used covertness constraint model, covert performance metrics, and potential application scenarios are briefly introduced. Then, several efficient methods that introduce uncertainty into the covert system are thoroughly summarized, followed by several critical enabling technologies, including joint resource allocation and deployment/trajectory design, multi-antenna and beamforming techniques, reconfigurable intelligent surface(RIS), and artificial intelligence algorithms. Finally, we highlight some open issues for future investigation.

Communication Resource-Efficient Vehicle Platooning Control With Various Spacing Policies

Platooning represents one of the key features that connected automated vehicles may possess as it allows multiple automated vehicles to be maneuvered cooperatively with small headways on roads. However, a critical challenge in accomplishing automated vehicle platoons is to deal with the effects of intermittent and sporadic vehicle-to-vehicle data transmissions caused by limited wireless communication resources. This paper addresses the co-design problem of dynamic event-triggered communication scheduling and cooperative adaptive cruise control for a convoy of automated vehicles with diverse spacing policies. The central aim is to achieve automated vehicle platooning under various gap references with desired platoon stability and spacing performance requirements, while simultaneously improving communication efficiency. Toward this aim, a dynamic event-triggered scheduling mechanism is developed such that the intervehicle data transmissions are scheduled dynamically and efficiently over time. Then, a tractable co-design criterion on the existence of both the admissible event-driven cooperative adaptive cruise control law and the desired scheduling mechanism is derived. Finally, comparative simulation results are presented to substantiate the effectiveness and merits of the obtained results.

Reliable transmission of consultative committee for space data systems file delivery protocol in deep space communication

In consultative committee for space data systems（CCSDS） file delivery protocol（CFDP） recommendation of reliable transmission,there are no detail transmission procedure and delay calculation of prompted negative acknowledge and asynchronous negative acknowledge models.CFDP is designed to provide data and storage management,story and forward,custody transfer and reliable end-to-end delivery over deep space characterized by huge latency,intermittent link,asymmetric bandwidth and big bit error rate（BER）.Four reliable transmission models are analyzed and an expected file-delivery time is calculated with different trans-mission rates,numbers and sizes of packet data units,BERs and frequencies of external events,etc.By comparison of four CFDP models,the requirement of BER for typical missions in deep space is obtained and rules of choosing CFDP models under different uplink state informations are given,which provides references for protocol models selection,utilization and modification.

Rough communication of crisp concept in fuzzy approximation spaces

To study the problem of knowledge translation in fuzzy approximation spaces, the concept of rough communication of crisp set in fuzzy approximation spaces is proposed. In a rough communication of crisp set in fuzzy approximation spaces, the problem of uncertainty exists, for each agent has a different language and cannot provide precise communication to each other. By means of some concepts, such as CF rough communication cut, which is a bridge between fuzzy concept and crisp concept, cut analysis of CF rough communication is made, and the relation theorem between CF rough communication and rough communication of crisp concept is obtained. Finally, in order to give an intuitive analysis of the relation between CF rough communication and rough communication of crisp concept, an example is given.

An Adaptive Transmission Scheme for Deep Space Communication

Deep space communication is quite different from conventional ground communication due to its time-varying,complexity and large signal delay,which consequently affects communication quality and system efficiency.Adjusting the transmission parameters when the channel environment changes during the communication can guarantee the performance index of the system,and therefore improve communication efficiency. An adaptive transmission scheme of transceiver based on Consultative Committee for Space Data Systems(CCSDS)protocols is proposed in this paper. According to the variation of the deep space channel,the symbol rate of transmission data is adjusted dynamically by estimating the signal-to-noise ratio(SNR) of the receiver in real time and adjusting the channel environment. This scheme can improve the channel utilization and system throughput under the premise of limiting the system bit error rate. Furthermore,this scheme is successfully implemented in Xilinx Virtex-5 FPGA board.

A SURVEY OF DEEP SPACE COMMUNICATIONS

Deep space communications has played an important role in deep space exploration. Compared with common satellite and terrestrial communications, deep space communications faces more challenging environment. The paper investigated the unique features of deep space communica-tions in detail, discussed the key technologies and its development trends for deep space communica-tions.

High-Sensitivity Acquisition of Ultrahigh Dynamic Direct Sequence Spread Spectrum Signals in Space Communications

According to the requirements of the high-sensitivity acquisition of Direct Sequence Spread Spectrum(DSSS) signals under ultrahigh dynamic environments in space communications, a three-dimensional joint search of the phase of Pseudo-Noise-code(PN-code),Doppler frequency and its rate-of-change is presented to achieve high sensitivity in sensing high-frequency dynamics. By eliminating the correlation peak loss caused by ultrahigh Doppler frequency and its rate-of-change offset,the proposed method improves the acquisition sensitivity by increasing the non-coherent accumulation time. The validity of the algorithm is proved by theoretical analysis and simulation results. It is shown that signals with a carrier- to-noise ratio as low as 39 dBHz can be captured with high performance when the Doppler frequency is up to ±1 MHz and its rate-of-change is up to ±200 kHz/s.

Advanced Space Laser Communication Technology on CubeSats

The free space optical communication plays an important role in space-terrestrial integrated network due to its advantages including great improvement of data rate performance,low cost,security enhancement when compared with conventional radio frequency(RF) technology.Meanwhile,CubeSats become popular in low earth orbit(LEO) network because of the low cost,fast response and the possibility of constituting constellations and formations to execute missions that a single large satellite cannot do.However,it is a difficult task to build an optical communication link between the CubeSats.In this paper,the cuttingedge laser technology progress on the CubeSats is reviewed.The characters of laser link on the CubeSat and the key techniques in the laser communication terminal(LCT) design are demonstrated.

A NOVEL APPROACH OF LIMITED-RANDOMNESS FOUNTAIN CODES IN DEEP SPACE COMMUNICATION

Digital fountain is applied into deep space communication for its rateless and non-feedback forward error correction.However,the long code length and encoding overhead are confined factors to guarantee a considerable recovery probability as power and buffer-limited equipment in deep space environment.At the same time,the typical fountain decoding is sub-optimum decoding algorithm.We propose a new approach,Dependent Sequences Compensation Algorithm(DSCA),to improve the encoding efficiency by restricting the randomness in fountain encoding.While decoding algorithm is also optimized by redundant information in stopping set.The results show that the optimized method can obtain a 10-4 decoding failure rate with overhead under 0.20 for code length 500,which indicates the usefulness of the proposed approach in deep space communication.

A NEW ALGORITHM FOR MAIN CARRIER ACQUISITION IN DEEP SPACE COMMUNICATIONS

Compared with common near space satellite Telemetry,Telecommand,and Communication(TT&C),deep space TT&C presents a more challenging environment such as long distance,very low Signal to Noise Ratio(SNR).How to acquire main carrier exactly becomes a hot focus for deep space communications.Already there emerged some main carrier acquisition algorithms,but they all require high SNR and small modulation index.In this paper,we develop a new acquire algorithm.First we use the spectral energy center algorithm to shorten the original sequence,filter out some noise and make the spectral more symmetric.Then we adopt the spectral symmetry algorithm to make full use of the whole spectrum information,and utilize FFT to reduce computation complexity.Simulation results show that our algorithm can acquire main carrier successfully under large modulation index and get good performance with low Carrier to Noise Ratio(CNR).

Design of Public Communication Spaces in Urban Highrises:A Case Study of Xuzhou Hanbang Square Residential Area

Highrise residences with high plot ratio meet housing needs of urban residents, improve living conditions of residents, but also show many unreasonable problems in the design of public communication spaces with a result of interpersonal indifference and lack of sense of identity. This paper took Xuzhou Hanbang Square for example to explore strategies of improving public communication spaces in highrises from the perspectives of entrance lobby, open-floor ground design, ecological garage, and three-level property management. On the one hand, the paper tries to improve quality of residential environment and service quality of public facilities; on the other hand, to enhance sense of neighborhood and identity of residents, and change the current interpersonal indifference.

Digital Art as Urban Communication

As mobility and migration become the norm,citizens in modern cities live among people who remain strangers to each other.This creates new opportunities and challenges for urban social life.Public space is a critical forum in which strangers encounter each other and have the opportunity to develop social protocols for coexisting in diversity.New media technologies have huge impact on the form and quality of public space.Digital art can create experimental public spaces in which mediated connections and embodied presence are combined in new ways.Through the practice and research of digital art,we can imagine a communicative city in which urban digital media is less about spectacle,and more about promoting new forms of public speech and social encounter between people.

Application Research on Space Laser Communication in Bistatic Radar System

There exist three synchronizing problems in the bistatic radar system that some signals of the radar receiver must be synchronized with those of the radar transmitter. Several methods realizing data transmission, which are used to complete the synchronization existing in the bistatic radar system, are described. Then a new idea is brought forward that employs space laser communication in the bistatic radar system to realize its data transmission. The theoretic analysis of the idea's usability and its merits are discussed in details. Finally the latest development of space laser communication is introduced, and the utility of the idea is pointed out further.

Outage Probability Analysis of Free Space Communication System Using Diversity Combining Techniques

Recently,free space optical(FSO)communication is gaining much attention towards the research community.The reason for this attention is the promises of high data-rate,license-free deployment,and non-interfering links.It can,however,give rise to major system difficulties concerning alignment and atmospheric turbulence.FSO is the degradation in the signal quality because of atmospheric channel impairments and conditions.The worst effect is due to fog particles.Though,Radio Frequency(RF)links are able to transmit the data in foggy conditions but not in rain.To overcome these issues related to both the FSO and RF links.A free space communication system(FSCS)is proposed,in which the hybrid technology is based on the individual FSO and RF channel.FSCS is a capable solution to overcome the difficulties of the existing systems(FSO and RF)as well as to enhance the overall link reliability and availability.In this paper,FSCS is investigated in terms of performance throughput(i.e.,outage probability and bit-error-rate(BER))by implementing the receive diversity combining techniques.An analysis of the outage probability of the proposed system along with the individual FSO and RF system is developed.Simulation results are presented to support the analysis.It is shown that the proposed system outperforms the individual FSO andRF system and gives a power gain of 3dB over a distinct number of receive antennas.

Modified overlapped partly parallel decode for AR4JA codes in deep space communication

In this paper, according to the AR4JA codes in deep space communication, two kinds of iterative decoding including partly parallel decoding and overlapped partly parallel decoding are analyzed, and the advantages and disadvantages of them are listed. A modified overlapped partly parallel decoding that not only inherits the advantages of the two algorithms, but also overcomes the shortcomings of the two algorithms is proposed. The simulation results show that the three kinds of decoding have the same decoding performance; modified overlapped partly parallel decoding improves the iterative convergence rate and the throughput of system.

The Effects of Power Control on Free-Space Optical Communications during Snowfall and Rainfall

Free-space optical (FSO) communication requires a line-of-sight connection between a transmitter and a receiver in which the information signal is modulated by an optical carrier that propagates in free space. The FSO channel is greatly affected by weather conditions such as fog, rain, and snow. In the literature, several adaptive techniques, such as power control (PC), have been suggested to mitigate channel link degradations. In this paper, we investigate the effects of snow and rain attenuation on the bit error rate (BER) of the FSO system using two types of modulations, the on-off keying (OOK) modulation and the pulse-position modulation (16-PPM). The effect of PC on the performance of FSO communications is also examined in this study. We evaluated the system’s performance with two types of snow, wet snow and dry snow, as well as with different rain regions. Results show that PC improves the BER of the FSO system;a high rate of improvement is found for wet snow and rain. PC has almost no effect with dry snow because of the high attenuation and the limitations on transmitted power. The BER for 16-PPM is better than that for OOK modulation.

Delay Resistant Transport Protocol for Deep Space Communication

The throughput of conventional transport protocols suffers significant degradation with the increased Round Trip Time (RTT) typically seen in deep space communication. This paper proposes a Delay Resistant Transport Protocol (DR-TCP) for point-to-point communication in deep space exploration missions. The issues related to deep space communication protocol design and the areas where modifications are necessary are investigated, and a protocol is designed that can provide good throughput to the applications using a deep space link. The proposed protocol uses a cross layer based approach to find the allocated bandwidth and avoids initial bandwidth estimation. A novel timeout algorithm estimates the timeout duration with an objective to maximize throughput and avoid spurious timeout events. The protocol is evaluated through extensive simulations in ns2 considering high RTT values typically seen in Lunar and Mars Exploration Networks under different conditions of packet error rates. DR-TCP provides a significant increase in the throughput as compared to traditional transport protocols under the same conditions. A novel adaptive redundant retransmission algorithm is also presented to take care of the high PER in deep space links. The effect of the Retransmission Frequency has been critically analyzed considering both Lunar and Deep Space scenarios under different levels of PER. The results are very encouraging even in high error conditions. The protocol exhibits a RTT independent behavior in throughput, which is the most desirable quality of a protocol for deep space communication.

Acquisition,Pointing and Tracking System for Shipborne Space Laser Communication without Prior Information

A space laser communication acquisition,pointing and tracking(APT)system based on the beacon laser is designed without prior information.And then,a new target scanning method and a pointing and tracking algorithm are proposed.The target scanning mode is the round-trip triangular wave scanning,and it means that scanning track of the PAN-TILT platform follows the triangular wave repeatedly.For the pointing and tracking algorithm,the beacon laser is used as the auxiliary aiming light source.The position of the beacon laser in the viewfield of the complementary metal oxide semiconductor(CMOS)camera is calculated by the centroid algorithm.In order to realize the target tracking,the joint control method of the angle control and the angular velocity control is used.The simulation and experimental results show that the APT system can achieve full coverage scanning in the scanning area and capture the target in one scanning cycle successfully.After capturing the PAN-TILT platform,the pointing and tracking algorithm can track the PAN-TILT platform quickly and accurately,and the tracking accuracy is up to 0.22 mrad.

Developments of Space-Time, Complete Special Relativity, and Superluminal Entangled Communication

Time and space are two very important basic concepts in philosophy and nature.In special relativity we proved that there are necessarily two symmetrical topological structures separated by the light-cone,which includes the generalized Lorentz transformation(GLT)for the spacelike interval,in which phase velocity is superluminal.Based on quantum entanglement as new fifth interaction,we research a simple superluminal entangled communication,whose key is to establish two mutually entangled particles or devices Alice(A)and Bob(B).We observe and control the information of A position,and then can know the corresponding results of the other B.This is not to send directly information each other.It may be superluminal,and should agree and test GLT.Moreover,we research some new possible developments of time and space,such as the fractal dimension extended to the complex dimension,the higher dimensional time,and the arrow of time.A generalized Noether’s theorem is proposed.In quantum theory,we search the higher dimensional complex space in supersymmetry,and the space-time operators.

Acquisition and Tracking Technology for Space Laser Communication

The divergence angle of laser beam used in space laser communication is usually no more than 100μrad.Using laser beam with small divergence angle to achieve acquisition and tracking for space laser link has always been a difficult problem.In addition,the random nature of the atmosphere will affect the satellite-ground laser link,which increases the difficulty of the acquisition and stable tracking for the laser link.Thus,taking into account the above challenges for satellite-ground laser communication,an acquisition and tracking scheme of using both beacon beam and signal beam was designed for the Laser Communication Terminal(LCT)of Shijian 20 satellite.In-orbit test results indicated that under the condition of moderate atmospheric turbulence(atmospheric coherence length r0≈3 cm),the process of acquisition and tracking for the satellite-ground laser link can be completed within 1 s after the initial pointing between the LCT and Optical Ground Station(OGS)is performed,and the tracking error was less than 1μrad(3σ).In addition,the laser link can be re-established quickly once being interrupted by unsteady atmospheric turbulence,and can be maintained for a long time under moderate twurbulence conditions,which lays a foundation for future application of satellite-ground laser communication.