Low Interception Research Articles

Analysis of Machine Learning-Based NLOS Signal Identification Algorithm for UWB Indoor Localization Using CIR Waveform Features

Abstract. Ultra-wideband (UWB) technology stands out among numerous indoor positioning techniques due to its high operating frequency, low interception capability, resistance to multipath effects, and strong penetration. The UWB uses the time-of-arrival (TOA) to estimate the distance between the transmitter and receiver anchors in centimeter accuracy. However, in complex indoor positioning environments, obstacles such as walls, glass windows, metal plates, and wooden doors may block and reflect signals, inevitably causing non-line-of-sight (NLOS) errors that significantly affect positioning accuracy. The NLOS signal has lower signal energy due to the reflections. Thus, the channel impulse responses (CIR) from NLOS and LOS are different. To address the NLOS signal identification issue in UWB positioning, we utilize UWB CIR data collected from various positioning scenarios as the data source. CIR waveform input features are provided for the NLOS signal recognition model, and four machine learning models—Support Vector Machine (SVM), Multi-Layer Perceptron (MLP), K-Nearest Neighbors (KNN), and XGBoost—are trained and optimized for NLOS signal recognition. The aim of the study is to analyze the performance of different machine learning algorithms for NLOS signal recognition in UWB indoor localization using these features. Experimental results indicate that machine learning-based NLOS signal recognition algorithms can achieve an accuracy of approximately 77.46%, precision of 80.46%, and an F1 score of 0.81. Among the four models, the XGBoost model demonstrates generally better recognition performance.

Low‐Interception Waveforms: To Prevent the Recognition of Spectrum Waveform Modulation via Adversarial Examples

AbstractDeep learning is applied to many complex tasks in the field of wireless communication, such as modulation recognition of spectrum waveforms, because of its convenience and efficiency. This leads to the problem of a malicious third party using a deep learning model to easily recognize the modulation format of the transmitted waveform. Some existing works address this problem directly using the concept of adversarial examples in the computer vision field without fully considering the characteristics of the waveform transmission in the physical world. Therefore, we propose two low‐interception waveforms (LIWs) generation methods, the LIW and ULIW algorithms, which can reduce the probability of the modulation being recognized by a third party without affecting the reliable communication of the friendly party. Among them, ULIW improves LIW algorithm by simulating channel noise during training cycle, and substantially reduces the perturbation magnitude while maintaining low interception accuracy. Our LIW and ULIW exhibit significant low‐interception performance in both numerical simulations and hardware experiments.

Picosecond laser-driven coded-source radiography with high resolution and contrast

The X-ray sources for Compton radiography of ICF experiments are generated by using intense picosecond lasers to irradiate wire targets. The wire diameter must be designed thin enough, for example ∼ 10 µm in many published works, to comply a high spatial resolution. This results in a low laser-target interception, which limits the photon yield. We investigated a technique of coded-source radiography based on laser-driven annular sources via Monte Carlo and PIC simulations. The annular X-ray source is formed by laser irradiating tube target in which the effect of electron recirculation plays an important role. We proved that this technique has an increased spatial resolution and contrast than that using the Gaussian source produced by wire targets. Therefore, the diameter of the backlighter target can be significantly increased to uplift laser-target interception without compromising on spatial resolution. This contributes towards a reconciliation between the spatial resolution and photon yield for Compton radiography. The results predict the possibility of improving source photon yield by several times in future experiments.

Study of supramolecular organic frameworks for purification of hydrogen through molecular dynamics simulations

The efficiency of proton exchange membrane fuel cells (PEMFCs) is heavily dependent on hydrogen purity. The production of hydrogen using methanol releases gases such as CO2 and CO, and hydrogen purification is particularly important. In this study, the performance of hydrogen purification (H2/CO2, H2/CO, and H2/CO2/CO) of three supramolecular organic frameworks (SOF-8, SOF-9a, and SOF-10a) with similar structures and different pore sizes and dimensions (two-dimensional (2D) and three-dimensional (3D)) were comprehensively investigated by molecular dynamics simulations and Grand Canonical Monte Carlo simulations. The monolayer, AA and AB stacked SOFs membranes were investigated. The results show that monolayer membranes exhibited high H2 permeability but low CO2 interception. AA-stacked SOF-10a and AB-stacked SOF-8 and SOF-10a can achieve 100 % separation of H2 from both CO2 and CO, with a minimum H2 permeability of 0.3 × 106 GPU and a maximum of up to 0.66 × 106 GPU in the H2/CO2 separation system, which is nearly 5 orders of magnitude higher than the industry standard. Microscopic analyses indicate that 3D SOFs (SOF-9a and SOF-10a) exhibit stronger interactions with CO2, while still maintain high H2 permeability in different stacking modes.

Model simulation of vegetation canopy precipitation interception in grassland ecosystems on the northeast margin of the Qinghai-Tibet Plateau

Canopy interception plays a crucial role in eco-hydrological processes, and the development of spatiotemporal simulation models of canopy interception is essential for the study of this phenomenon. This study aims to fill the gap in the application of modeling to the study of canopy interception in grassland ecosystems. By using the A.P.J. DE ROO model and the RS-Gash model in conjunction with remote sensing data, we quantitatively simulate and analyze the interception capacity of the grassland vegetation canopy in the study area during the decade. Remote sensing data and measured data are used to assess the simulation accuracy of the models. The model with higher accuracy is used to analyze the spatiotemporal interception of the vegetation canopy in the study area. The results show that the vegetation canopy interception model can be applied not only to forest ecosystems but also to grassland ecosystems; the RS-Gash model has higher simulation accuracy than the A.P.J.DE ROO model; the two models show different simulation accuracy for different grassland types; the vegetation canopy interception rates of different grassland types are different, respectively: alpine meadows is 3.11 %, mountain meadows is 3.82 %, temperate grassland is 2.45 % and temperate desert grassland is 1.13 %; the ratio of vegetation canopy interception area at different levels is roughly low interception area: medium interception area: high interception area = 5:4:1. It can be seen that the application of mechanistic models combined with remote sensing data to simulate canopy interception in grassland ecosystems has great potential. Alpine meadows and mountain meadows are the main contributors to the interception of the grassland vegetation canopy in the study area.

Anti‐interrupted sampling repeater jamming method for random pulse repetition interval and intra‐pulse frequency agile radar

AbstractInterrupted sampling repeater jamming can produce numerous active false targets with high fidelity, which is a significant challenge for coherent pulse radar. This poses a threat to the effectiveness of anti‐jamming technology, such as time‐frequency analysis and frequency agility for conventional constant pulse repetition interval (PRI) radar, which will render the radar inoperable to detect and track the actual target. A random PRI and intra‐pulse frequency agile radar waveform with low interception properties is proposed, which can effectively restrict the acquisition of radar waveform parameters by the jammer, destroy the coherence of jamming signals, and capture the range‐dimensional centroid feature of the echo. Accordingly, a jamming identification method based on the scale constrained Gaussian mixture model and an entity radar target energe recovery algorithm based on the neighbourhood weighted maximum entropy are proposed for target detection. Theoretical analysis and simulation experiments demonstrate the effectiveness of the proposal.

Mitigating airborne microplastics pollution from perspectives of precipitation and underlying surface types.

The issue of airborne microplastics (AMPs) pollution is receiving increasing attention, but effective solutions are still limited. In this study, AMPs were collected in pairs from an open space and under a tree (Ficus virens) in the suburb of Chengdu, southwest China, to investigate the current pollution status. The meteorological factors and underlying surface types were analyzed to investigate whether these factors could influence and mitigate the pollution of AMPs. The results showed that the fibrous AMPs accounted for the vast majority and the dominant polymers were polypropylene-polyethylene (PP-PE) and polypropylene (PP), with an average deposition flux of AMPs of 192 n/m2/d (22.41 µg/m2/d). Rainfall was found to have the prolonged scavenging efficiency for AMPs, which could extend to 8 to 48 hours after the end of rainfall, and this is a new insight into the relationship with meteorological factors. Interactions between the key underlying surface types and AMPs were also studied. The representative tree species (Ficus virens) had a low interception rate of 6.25% for AMPs and retained mainly small-sized AMPs and more fibers. The masses of AMPs loaded into Chengdu rivers could reach 1149 kg annually, with the unit mass load of 13.6 kg/km2 in urban rivers and 8.2 kg/km2 in suburban rivers. The masses intercepted by trees and bushes throughout the city only offset the masses loading in rivers, and open or sparse buildings were found to be sensitive areas for AMPs, which indicated the urgent need to control and mitigate the pollution of AMPs, especially in these sensitive areas. This work comprehensively analyzed the AMPs pollution from various perspectives and attempted to find ways to mitigate this pollution.

The application of ultrawideband technology in future wireless communications and practical exploration

An innovative form of wireless communication technology is called ultra-wideband technology. By directly modulating impulse pulses with extremely rapid rise and fall times, it is possible to create signals with bandwidths of the order of GHz. It has the benefits of being insensitive to channel fading, low power spectral density of the transmitted signal, low interception capability, low system complexity, and the ability to provide several centimeters of positioning accuracy. It also resolves the main propagation issues that have plagued traditional wireless technology for many years. Early high-speed data transmission in close proximity utilised ultra-wideband; more recently, foreign nations have started using its sub-nanosecond ultra-narrow pulse for close-range indoor positioning. Traditional wireless communication modulates signals to various frequency bands using various carrier frequencies, and within a given range, the carrier wave's frequency and power will change. Ultra-wideband wireless communication does not require a carrier wave, but uses pulsed signals to transmit information, where pulsed signals are those that are generated and disappear in a very short time, from a few hundred microseconds to less than a few nanoseconds. Thirdly, it can be positioned with high accuracy and has good resistance to multipath effects in timing. Ultra-wideband technology wireless communication systems generally work at sub-nanosecond pulse widths, and thus have sub-nanosecond time resolution, and its multipath resolution is correspondingly less than 30 cm, the signal will be received in the formation of a 1-nanosecond time reception window, only receive the signal within the reception window, the multipath interference has very good resistance. Fourthly, it has a greater penetration capability. Ultra-wideband pulses have a low frequency, the lowest of any wireless signal with the same absolute bandwidth, so they have greater penetration capability. Fifthly, the structure is simple, and the cost is low. The wireless communication system of ultra wideband technology does not change the traditional carrier wave to realize information transmission. The wireless communication system uses up conversion circuit, down conversion circuit, intermediate frequency circuit and various filters. The system structure is simple, and communication is relatively tolerant and easy. Therefore, the cost of wireless communication using ultra wideband technology is much lower.

In-drain denitrifying woodchip bioreactors for reducing nitrogen runoff from sugarcane

Denitrifying woodchip bioreactors offer an opportunity to intercept and reduce nitrogen loads between agricultural fields and downstream aquatic ecosystems. Here we assessed the performance of three in-drain bioreactor beds installed in open drains receiving runoff and shallow ground water from rain-fed sugarcane production in the Wet Tropics of Australia. Drain nitrate-N concentrations were generally low, with a mean of 0.2 mg L−1 and maximum of 3.3 mg L−1, which may have been partially due to denitrification in the contributing soils. Bioreactors reduced the concentration of nitrate-N in intercepted waters (average 41% reduction). However, removal rates were often limited by nitrate-N availability. Load reduction over the 2018/19 season was just 0.11 kg N ha−1 yr−1. This limited performance was in large part due to the dynamic nature of nitrogen loads in this system. Specifically, a high proportion of the annual nitrogen load occurring during ‘first-flush’ events immediately after fertilization (i.e. 72% during a 10-day period in 2018/19) resulting in considerable bypass flow (i.e. low interception). Our study is the first in the Australian Wet Tropics to assess the annualized performance and total load reductions of in-drain denitrifying bioreactor beds. Although showing their potential, our results highlight the fact that denitrifying bioreactor performance is determined by the hydraulic context of the catchment, and the proportion of the annual nitrate load that can be successfully intercepted.

Spatial variability of throughfall in heavily fogged old-growth Fagus orientalis forests is controlled by fog precipitation and stand structural characteristics

Ecological impacts of the spatial heterogeneity of throughfall (TF) in forest ecosystems are recognized in ecohydrology. The objective of this study was to evaluate the spatial variability of TF and to quantify the effects of forest cover, fog, and gross rainfall (GR) characteristics on TF. Data were collected from May 2016 to November 2017 on an event basis in an intact, old-growth oriental beech (Fagus orientalis L.) stand in northern Iran. Over the measurement time period, 22 rainfall events were observed (total rain = 648 mm). Because the region experienced considerable fog, TF was greater than 100% of GR in 14 rainfall events with negative interception loss (total = -24.1 mm), causing low interception values, and 8 rainfall events were found with positive I (total = 25.5 mm). The low TF spatial variability and increasing TF depth observed in this study were related to increases in rainfall depth and intensity. TF was significantly and positively correlated with LAI and significantly and negatively correlated with canopy openness for rainfall events of more than 50 mm. The high amount of TF depth highlighted the role of LAI to enhance fog interception and the role of fog precipitation to increase TF in the study site. Spatial continuity at an effective range of 3-8 m was observed by variogram analyses. The increasing spatial continuity of TF with increasing GR observed in this study, along with the larger effective range for GR >50 mm and correspondingly similar LAI range, suggest an effect of LAI on the spatial patterning of TF for larger GR. Therefore, this heavily fogged ecosystem received considerable TF due to fog interception, and higher LAI increased TF for larger rainfall events. Our findings can optimize forest management practices by accounting for the effect of Fagus orientalis trees on the local hydrological processes of the Hyrcanian mountain forests.

Passive Localization for Frequency Hopping Signal Emitter Based on Synthetic Aperture Principle

The frequency hopping (FH) signal has received much research interest due to its low interception probability. In the FH signal localization, the variation of signal frequency introduces error into localization methods involving phase or frequency information. In order to deal with the problem of positioning measurement estimation for unknown FH signal emitters, this article proposes a synthetic aperture passive positioning method. Baseband modulation of received signals is compensated by the time difference method. Then, the de-chirp method is introduced for carrier frequency estimation. The Doppler frequency of each pulse is compensated by a Doppler frequency compensation matrix, and the cost function related to the emitter position is constructed by 2-D focus results of the received signal at all frequencies. The emitter position is obtained through a gird search. Simulation and experimental data show that the proposed method is superior to several existing positioning methods especially when the signal-to-noise ratio (SNR) is low.

Design of Low Probability Detection Signal with Application to Physical Layer Security

In this work, we mainly focus on low probability detection (LPD) and low probability interception (LPI) wireless communication in cyber-physical systems. An LPD signal waveform based on multi-carrier modulation and an under-sampling method for signal detection is introduced. The application of the proposed LPD signal for physical layer security is discussed in a typical wireless-tap channel model, which consists of a transmitter (Alice), an intended receiver (Bob), and an eavesdropper (Eve). Since the under-sampling method at Bob’s end depends very sensitively on accurate sampling clock and channel state information (CSI), which can hardly be obtained by Eve, the security transmission is initialized as Bob transmits a pilot for Alice to perform channel sounding and clock synchronization by invoking the channel reciprocal principle. Then, Alice sends a multi-carrier information-bearing signal constructed according to Bob’s actual sampling clock and the CSI between the two. Consequently, Bob can coherently combine the sub-band signals after sampling, while Eve can only obtain a destructive combination. Finally, we derived the closed-form expressions of detection probability at Bob’s and Eve’s ends when the energy detector is employed. Simulation results show that the bit error rate (BER) at Alice’s end is gradually decreased with the increase in the signal-to-noise ratio (SNR) in both the AWGN and fading channels. Meanwhile, the BER at Eve’s end is always unacceptably high no matter how the SNR changes.

Variability in Snowpack Isotopic Composition between Open and Forested Areas in the West Siberian Forest Steppe

The stable water isotopes in snow (primarily 18O and 2H) are widely used for tracing hydrological and ecological processes. However, isotopic signatures of snow can be significantly modified by topography and land cover. This study assesses spatial and temporal variability of the bulk snowpack isotopic composition (δ18O, δ2H, d-excess) between forested (pine and birch) and open areas in the West Siberian forest steppes. Isotopic samples were collected over the peak snow accumulation in 2017–2019. The snow isotopic composition within forested areas differed from open steppes, mainly in reducing d-excess (1.6‰ on average). We did not find a significant effect of canopy interception on snow enrichment in heavier isotopes. Snowpack in the pine forests was even lighter by 3.6‰ for δ2H compared to open areas, probably, due to low energy inputs and interception capacity. Additionally, snow depth significantly influenced the isotopic composition spatial variability. As snow depth increased, δ18O and δ2H values decreased due to conservation within the snowpack and less influence of sublimation and moisture exchange with the soil. However, this pattern was only evident in winters with below-average snow depth. Therefore, taking into account snow depth spatial and seasonal variability is advisable when applying the isotopic methods.

Interception Probability Versus Capacity in Wideband Systems: The Benefits of Peaky Signaling

Spread-spectrum techniques have found extensive use in broadband communications, both in military and commercial applications, for their low interception probability. However, it has been shown that these techniques prove ineffective on non-coherent fading channels with very large bandwidths since its capacity decreases with the increase of the bandwidth, eventually going to zero as the bandwidth goes to infinity. Peaky frequency-shift keying is a promising alternative modulation technique expected to achieve high data rates while maintaining a low interception probability. The current study explores such modulation techniques and outlines the conditions for which the reliability and capacity of the system increase while reducing the probability of interception compared to typical spread spectrum schemes. We show that peaky frequency-shift keying achieves a considerably higher transmission rate than non-peaky signaling for any given target level of interception probability, especially for outdoor fading scenarios in the low signal-to-noise ratio regime.

EVALUATION OF THE STEREOSCOPIC DOSE DISTRIBUTION IN ENDOSCOPIC RETROGRADE CHOLANGIOPANCREATOGRAPHY ROOM USING N1 PHANTOM AND TLD TECHNIQUE

This study envisaged the evaluation of the stereoscopic dose distribution and the corresponding exposure dose for the medical staff in endoscopic retrograde cholangiopancreatography (ERCP) room using the spatial dose distribution method and thermo-luminescent dosimetry (TLD) technique. Feasibility screening and calibration of 135 new TLD-100H chips were performed and sealed into 45 packages with 3 TLDs per pack. Subsequently, the 45 TLD packs were allocated in a matrix form in the ERCP room ([Formula: see text][Formula: see text]cm2) with various heights of 85, 90, 130, 150, and 180[Formula: see text]cm above the ground. In addition, three human-sized dolls with lead vast were located at the position of the medical staff to simulate the ambient exposure dose. The TLDs were held for 2-minute exposure (max, min, average) [Formula: see text] Gy dose as 85[Formula: see text]cm (520.9, 14.9, 31.1), 90[Formula: see text]cm (428.6, 15.7, 28.6), 130[Formula: see text]cm (40.7, NA, 28.0), 150[Formula: see text]cm (1650.3, NA, 1650.3) and 180[Formula: see text]cm (17.6, 15.0, 16.4), respectively. The minimum detectable limit (MDL) of the TLD chips was 6.85[Formula: see text]nC, and the TLD dose calibration curve was revised by using a weighting factor according to lean backward scenario to reach low interception. It was found that the maximal exposure dose of the annual workload (12[Formula: see text]m/d*5[Formula: see text]d/wk*50[Formula: see text]wk/yr=3000[Formula: see text]m) was overestimated as 14.7, 14.5, and 14.1[Formula: see text]mGy, respectively for the medical doctors, radiologists and nurses from the viewpoint of health physics.

Spatial Information-Theoretic Optimal LPI Radar Waveform Design.

In this paper, the design of low probability of intercept (LPI) radar waveforms considers not only the performance of passive interception systems (PISs), but also radar detection and resolution performance. Waveform design is an important considerations for the LPI ability of radar. Since information theory has a powerful performance-bound description ability from the perspective of information flow, LPI waveforms are designed in this paper within the constraints of the detection performance metrics of radar and PISs, both of which are measured by the Kullback-Leibler divergence, and the resolution performance metric, which is measured by joint entropy. The designed optimization model of LPI waveforms can be solved using the sequential quadratic programming (SQP) method. Simulation results verify that the designed LPI waveforms not only have satisfactory target-detecting and resolution performance, but also have a superior low interception performance against PISs.

Power control and beamforming of phased-MIMO radars to achieve a low probability interception

In this paper, the optimum detector of co-located Phased-Mimo Radars (PMRs) with signal diversity is designed and its detection probability and false alarm rate are obtained as performance metrics. Then, the Low Probability Intercept (LPI) optimization of radar is considered while the desired detection performance and power budget limit are preserved. The LPI optimizations design power allocations, beamforming weights deal with the phased array property, and orthogonal signals selection deals with the MIMO property. The complexities arising from the joint optimization of power allocations and beamformings have been handled by some proposed relaxations and lemmas. The proposed solutions' effectiveness is analyzed by numerical simulations. Simulation results verify the superiority of optimized solutions in comparison to the conventional benchmarks in terms of LPI property, detection probability, and power consumption.

Radar Emitter Identification Based on Spectral Resemblance Coefficient

The traditional radar emitter identification method based on the five parameters of the pulse is difficult to achieve the desired effect in today’s abnormally dense electromagnetic environment. Modern radars often impose intentional modulation in the signal to improve the anti-jamming and low interception performance. Therefore, a radar emitter identification method based on the spectrum resemblance coefficient is proposed in this paper, which reflects the approximation degree between the signal spectrum and various basis functions by calculating the resemblance coefficients between them. Simulation results show that the method proposed in this paper has strong anti-interference capability and can effectively distinguish different types of intra-pulse modulated radar signals.

Sparse processing with range migration compensation capability for a frequency agile orthogonal frequency division multiplexing waveform

Digital-to-analogue and analogue-to-digital in a digital array radar limits the transmission and reception of instantaneous broadband signals (IBSs), which makes it a technical bottleneck for digital array radar to obtain an instantaneous broadband high-resolution range profile (HRRP). A frequency-agile orthogonal frequency division multiplexing (OFDM) waveform with a high repetition frequency pulse train is proposed, which uses multiple narrowband OFDM to realise large bandwidth so that the digital array radar can easily realise the broadband working mode. This frequency hopping signal makes the digital array radar have the characteristics of broadband HRRP formation and low interception simultaneously. To solve the problem of range migration caused by high-speed moving targets, a two-stage sparse processing algorithm with range migration compensation capability (FS-RMCSP) in slow time and fast time domains is proposed. By constructing a sparse processing dictionary matrix with range compensation operator according to the phase offset of the moving target echo, the proposed algorithm can complete the compensation of target range migration with high resolution, accurately recover the range-Doppler parameters of target, and realise the coherent accumulation of target echo. The simulation results show that this waveform can obtain the range-Doppler high-resolution characteristics equivalent to the IBS in the radar tracking working mode. Compared with the conventional sparse signal processing algorithm, the FS-RMCSP algorithm significantly improves the performance of target reconstruction in the scene of high-speed moving targets.

Power control and beamforming in passive phased array radars for low probability of interception

In this paper, we investigate low probability interception (LPI) in passive phased array radars. Power control and transmit beamforming (BF) are used to minimize the received power by the interceptor. Meanwhile, desired target detection performances and the limited power budget of radar are guaranteed. The LPI problem is solved analytically and some theorems are derived for the design of optimum BF weights. Two LPI optimization problems from different perspectives are formulated and solved analytically in preparing closed-form solutions for designing transmit BF weights. LPI performance and computation time of the proposed algorithms are compared with that of the benchmark algorithms. Simulation results show that proposed algorithms not only minimize the interception probability of radar in comparison with benchmark algorithms but also satisfy desired detection performance and power budget constraints simultaneously. The proposed algorithms outperform the standard numerical Barrier algorithm in complexity and optimality.