Spread Spectrum Research Articles

Research on Detecting Targets’ Accelerations Based on Vortex Electromagnetic Wave in Non-Line-of-Sight Scenario

In this article, the influence of detecting targets’ radial acceleration (RA) and angular acceleration (AA) based on vortex electromagnetic (EM) waves in non-line-of-sight scenarios is investigated. Three typical kinds of cases about obstacles are studied in a simulation model. The spectrum spread of vortex electromagnetic wave with different topological charges under different non-line-of-sight scenarios is analyzed by simulation, and the errors of the targets’ accelerations are calculated. It is found that the detection errors are related to the purities of orbital angular momentum (OAM) modes in spectrum spread after passing through obstacles. The purity of the main mode of a single vortex electromagnetic wave cannot play an absolute role in the detection errors; the component and proportion with higher order modes in OAM spectra spreading have a greater impact on errors.

Spread Spectrum based QoS aware Energy Efficient Clustering Algorithm for Wireless Sensor Networks

Wireless sensor networks (WSNs) are composed of small, resource-constrained sensor nodes that form self-organizing, infrastructure-less, and ad-hoc networks. Many energy-efficient protocols have been developed in the network layer to extend the lifetime and scalability of these networks, but they often do not consider the Quality of Service (QoS) requirements of the data flow, such as delay, data rate, reliability, and throughput. In clustering, the probabilistic and randomized approach for cluster head selection can lead to varying numbers of cluster heads in different rounds of data gathering. This paper presents a new algorithm called "Spread Spectrum based QoS aware Energy Efficient Clustering for Wireless sensor Networks" that uses spread spectrum to limit the formation of clusters and optimize the number of cluster heads in WSNs, improving energy efficiency and QoS for diverse data flows. Simulation results show that the proposed algorithm outperforms classical algorithms in terms of energy efficiency and QoS.

Improved Spread Spectrum Aloha Protocol and Beam-Hopping Approach for Return Channel in Satellite Internet of Things.

This paper examines potential performances of the Spread Spectrum-based random access technique and proposes an Improved Spread Spectrum Aloha (ISSA) protocol for the return channel in satellite Internet of Things (IoT) based on the beam-hopping technique. The key design driver and detailed solution of ISSA protocol are presented in this work and it is shown that the proposed protocol achieves high throughput and low collision probability. To match user/traffic distribution, delay requirement and channel condition with beam allocation better, a low-complexity heuristic beam scheduling algorithm and a more effective Maximum-Weighted Clique (MWC) algorithm have been proposed. The heuristic algorithm considers the user/traffic distribution, inter-beam interference, and fairness primarily. However, the MWC algorithm gives considerations not only on above factors, but also on delay requirement and channel condition (path loss and rain attenuation) to maximize system capacity. The beam angle and interference avoidance threshold are proposed to measure the inter-beam interference, and the link propagation loss and rain attenuation are considered meanwhile in the channel condition. In the MWC algorithm, we construct an auxiliary graph to find the maximum-weighted clique and derive the weighting approach to be applied in different application scenarios. The performance evaluation of our ISSA protocol compared with the SSA protocol is presented, which achieves a gain of 16.7%. The simulation of the ISSA protocol combined with round robin, heuristic, and MWC beam scheduling for the return link in beam-hopping satellite IoTs is also provided. The results indicate that the throughput in nonuniform user distribution is much lower than in the uniform case without the beam scheduling algorithm. Through the application of the scheduling algorithm, the throughput performance can approach the uniform distribution. Finally, the degree of user satisfaction with different scheduling approaches is presented, which validates the effectiveness of heuristic and MWC algorithms.

Low-False-Alarm-Rate Timing and Duration Estimation of Noisy Frequency Agile Signal by Image Homogeneous Detection and Morphological Signature Matching Schemes.

Frequency hopping spread spectrum (FHSS) applies widely to communication and radar systems to ensure communication information and channel signal quality by tuning frequency within a wide frequency range in a random sequence. An efficient signal processing scheme to resolve the timing and duration signature from an FHSS signal provides crucial information for signal detection and radio band management purposes. In this research, hopping time was first identified by a two-dimensional temporal correlation function (TCF). The timing information was shown at TCF phase discontinuities. To enhance and resolve the timing signature of TCF in a noisy environment, three stages of signature enhancement and morphological matching processes were applied: first, computing the TCF of the FHSS signal and enhancing discontinuities via wavelet transform; second, a dual-diagonal edge finding scheme to extract the timing pattern signature and eliminate mismatching distortion morphologically; finally, Hough transform resolved the agile frequency timing from purified line segments. A grand-scale Monte Carlo simulation of the FHSS signals with additive white Gaussian noise was carried out in the research. The results demonstrated reliable hopping time estimation obtained in SNR at 0 dB and above, with a minimal false detection rate of 1.79%, while the prior related research had an unattended false detection rate of up to 35.29% in such a noisy environment.

A Superresolution Multipath Estimation Algorithm for DSSS Systems

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.

Iterative double-differential direct-sequence spread spectrum reception in underwater acoustic channel with time-varying Doppler shifts.

Conventional double differential phase-shift keying modulation amplifies the phase noise and performs poorly under the time-varying direct-sequence spread-spectrum (DSSS) communication system. Therefore, the authors propose an iterative reception for DSSS communication in time-varying underwater acoustic channels. First, bit-interleaved coded modulation with iterative decoding integrated with multi-symbol differential detection is used. Second, this paper uses cross correlation method to estimate and track the Doppler shift of each symbol. Based on Doppler estimates, a dynamic linear prediction model is proposed to estimate and track the channel phase variation. Third, an algorithm for adaptive selection of reference signals is utilized to recover the magnitude attenuation of correlation peaks. Numerical simulation results demonstrate that the proposed reception achieves around 9 dB gain compared to conventional differential decision reception under constant acceleration of 0.14 m/s2. During the acoustic communication experiment in Songhua Lake, the proposed reception was tested by using a moving source at a speed of 1-6 knots at 2-m depth and the farthest distance between the transceivers is 2.8 km. The proposed reception achieves only one frame error from a total of 205 frames collected in the lake experiment, and it also achieves error-free communications over 96 frames during a 10 km depth deep-sea experiment.

Remote Phase-Shift LiDAR With Communication

The integrated sensing and communication is emerging as a crucial technology due to reduced hardware costs and broad application scenarios. However, there is a contradiction between ranging and communication in phase-shift light detection and ranging (LiDAR) for the utilization of the subcarrier’s phase. A scheme called phase-shift laser ranging with communication (PLRC) is proposed to enable both ranging and communication using the same transmitted signal in this paper. The message encoded with pseudo-random noise code is modulated into the phase of the subcarrier for laser emission, and thus the received signal carries the data and the phase difference due to the distance. The received signal is subject to correlation, waveform recovery, and phase-shift measurement with the local signal at the LiDAR transceiver, and the distance is derived from the complete phase to avoid range ambiguity. The data is extracted from the spread spectrum signal at the communication receiver. The performance in terms of the measured distance and precision of LiDAR, the bit error rate of communication is obtained for the proposed solution. Meanwhile, the optimized light ratio for ranging and communication is derived. The experimental demonstration is carried out, the range precision is <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3.54\,\,{\times }\,\,10^{-3}$ </tex-math></inline-formula> m, and the communication capability meets 1.43 Mbit/s.

Mitigating Jamming Attack in 5G Heterogeneous Networks: A Federated Deep Reinforcement Learning Approach

Jamming attack is one of the serious security breaches in the upcoming fifth-generation heterogeneous networks (5G HetNets). Most of the existing anti-jamming techniques, such as frequency hopping (FH) and direct sequence spread spectrum (DSSS) lack in self-adaptive capabilities to mitigate the security and privacy issues in highly dynamic 5G HetNet environment. In literature, although reinforcement learning (RL) has been explored a lot in designing various anti-jamming techniques to address the aforementioned problems, but these techniques suffer from the issue of large network resource consumption and slow convergence rate. To mitigate these issues, we propose a federated deep reinforcement learning (DRL) based anti-jamming technique for two-tier 5G HetNets. In the proposal, each femtocell of 5G HetNets is assumed to have multiple single antenna femto users (FUs) and a multi-antenna jammer used to jam the downlink signals from femto base station (FBS) to FUs. Aiming to improve the achievable rate at FUs in the presence of jammers, a joint optimization problem of beamforming and power allocation at FBSs is formulated by considering the quality-of-service (QoS) requirements of FUs. Due to the non-convex nature of the aforementioned optimization problem, we have used the Markov decision process (MDP) to transform the optimization problem into a multi-agent reinforcement learning (MARL) problem. Then, to solve this MDP with large number of states and action spaces, a federated deep reinforcement learning (DRL) scheme is proposed to maximize the achievable rate at FUs. The proposed scheme uses federated learning and dueling architecture of dueling double deep Q network (D3QN) to optimize the beamforming vectors and power allocation jointly at FBSs. The achievable rate performance of the proposed federated DRL scheme is compared with double deep Q network (DDQN) and deep Q network (DQN). Simulation results show that the proposed federated DRL scheme achieves 19.39% and 23.85% better achievable rate in comparison to DDQN and DQN schemes.

Efficiently All-Digital Code Tracking for Band-Limited DSSS Systems

In this letter, we have studied the all-digital code tracking problem for band-limited direct sequence spread spectrum (DSSS) systems. Band-limited DSSS systems have many advantages over rectangular-pulse DSSS systems. However, the current all-digital code tracking methods for band-limited DSSS systems have more computational complexity than the latter. Therefore, we propose an efficiently all-digital code tracking method for band-limited DSSS systems where we implement despreading in the frequency domain with others and take advantage of the narrow time-width characteristic of the signal autocorrelation to achieve a lower complexity and constant tracking performance, i.e., the constant steady-state time-delay error variance at the same time. We also give a closed-form expression of the steady-state time-delay error variance of the proposed method in this letter, which can be used to analyze the performance of the proposed method theoretically and design proper band-limited DSSS systems. The theoretical analysis and simulation results show that the proposed method has lower computational complexity and constant tracking performance.

Thorough Study of Multi-Switching-Frequency-Based Spread-Spectrum Technique for Suppression of Conducted Emissions from Wireless Battery Chargers

The multi-switching-frequency technique is one of the spread-spectrum techniques for suppression of conducted emissions generated by wireless battery chargers. Its advantage is a relatively easy implementation with a microcontroller. In this paper, an original thorough experimental study of the effect of the multi-switching-frequency-based spread spectrum technique parameters (e.g., combinations of number of pulses, frequency order, etc.) on the performance characteristics (conducted emissions levels, efficiency, etc.) of an inductive-resonant wireless battery charger with a closed loop control is presented. It is shown that combinations of a number of pulses and frequency order have a noticeable impact on the performance characteristics of the wireless chargers. The suppression of the conducted emissions can be improved significantly by using optimized parameters of the technique. Moreover, it is proved experimentally that a relatively inexpensive microcontroller with a transceiver can be used to implement both closed-loop control of the wireless charger and the multi-switching-frequency technique.

Fixed-Switching-Frequency Modulated Model Predictive Control for Islanded AC Microgrid Applications

In this paper, a fixed-switching-frequency modulated model predictive control (M2PC) is established for a two-level three-phase voltage source inverter (VSI) working in an islanded AC microgrid. These small-scale power systems are composed by two or more VSIs which interface DGs, controlling the voltage amplitude and frequency in the system, and simultaneously sharing the load active and reactive power. Generally, these operational characteristics are achieved using hierarchical linear control loops, but with challenging limitations such as slow transient reaction to disturbances and high proneness to be affected by parameter modifications. Model predictive control may solve these issues. Nevertheless, the most used and developed predictive control scheme, the finite-set model predictive control (FS-MPC), presents the drawback of having the harmonic spectrum spread over all the frequencies. This brings issues with coupling between the different hierarchical control levels of the whole microgrid system, and eventually, when designing the filters for main-grid connection. This paper aims to solve these issues by developing the fixed-switching-frequency M2PC working with higher-level control loops for operation in an islanded AC microgrid. These advantages are proved in an AC microgrid configuration where methodology for paralleling multiple M2PC-regulated VSIs is described, with rapid transient response, inherent stability, and fully decentralised operation of individual VSIs, achieving proper load power sharing, eliminating circular currents, and proper waveforms for output currents and capacitor voltages. All these achievements have been confirmed via simulation and experimental verification.

LoRa and IoT tools usage possibility in the agricultural sectors

In this paper, I present a LoRa spread spectrum networking device that provides significant efficiency improvements over previously used techniques through digital techniques. Subsequently, the LoRaWAN network is presented, which can communicate with the network server available on the Internet through gateways. Their data rate and transmission power can be optimised depending on the distance from the base stations and the time required for communication can be measured in tens of seconds.&#x0D; In the second major part of the article, Internet of Things devices that also communicate with each other using the LoRa network will be presented. It allows smart devices used in our everyday life to communicate, which are supported by sensors and transmitters to record states and perform actions.&#x0D; IoT devices are also becoming increasingly common in agriculture. Farmers who are already using the technology can better monitor their livestock and crops. IoT can also be used effectively in many areas of agriculture, including dairy, vegetable, fruit and greenhouse operations.

Direct Sequence Spread Spectrum Signal’s Demodulator Acquisition Implementation Based on Fast Fourier Transform. Part 1. Problem Statement and Solution Approach

The article is dedicated to direct sequence spread spectrum signals acquisition. Communication channels that use direct sequence spread spectrum signals are characterized by significant delay and carrier frequency shift as a rule. That’s why the acquisition is most significant problem in receive equipment design. Suggested approach to acquisition problem base on the Fast Fourier Transform and discussed modeling suggested algorithm’s results.

Acquisition Time and Probabilities of Detection and False Alarm in Direct Sequence Code Division Multiple Access Systems

Multiple access interference (MAI) is one of the most important issues to consider in the design, implementation and operation of 5G systems using code division multiple access (CDMA) technique based on spread spectrum that offers a more flexible solution, compared to other techniques. Especially, it permits a much higher user bit rate, due to the bandwidth allocated to the emitted signal; thus, allowing very attractive multimedia services. In this paper, we develop the expressions of the detection probability, false alarm probability and the average acquisition time - in a direct sequence CDMA (DS-CDMA) transmission system - using a serial search and taking into consideration MAI. The evolution of these parameters according to the signal- to-noise ratio (SNR) and the decision threshold leads to study performances of the system in terms of detection and average acquisition time. The obtained results reveal that the acquisition time decreases when the SNR increases. Additionally, it is found that the probability of detection increases when the threshold decreases, and increasing SNR it reaches a constant value for an SNR/chip around 0 dB for all thresholds.

Time-space united coding spread spectrum single photon counting imaging method

In this paper, we demonstrate a new imaging architecture called time-space united coding spread spectrum single photon counting imaging technique by combining the space coding based single-pixel imaging technology and spread spectrum time coding based scanning imaging technology. This method has the advantages of range ambiguity-free and large time-bandwidth product. Under the interference of noise, this method can accurately restore depth images. In this work, the time-space united correlation nonlinear detection model based on single photon detection, forward imaging model and signal-to-noise ratio model is derived, and the depth image is restored by convex optimization inversion algorithm. The theoretical model and simulation experiments show that compared with the traditional single pixel imaging method based on spatial coding, this method improves the quality of scene reconstruction. Using m-sequence as time coding, imaging has higher noise robustness. In addition, compared with the traditional space coding single pixel imaging technology, the imaging mean square error of the proposed method is reduced by 4/5 and the imaging mean squared error is reduced by 9/10 after introducing the second correlated method. The proposed imaging architecture in this paper may provide a new path for non-scanning lidar imaging 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.

Multicode Signaling in a Filter Bank Multicarrier Spread Spectrum System and Its Application to HF Communications

The use of filter banks for implementing multicarrier spread spectrum systems leads to a class of effective waveforms that are highly resilient to partial-band interferers. Such waveforms can be also designed to keep the peak-to-average power ratio (PAPR) of the resulting signal at a minimum level. The use of multiple spreading gain vectors (known as multicodes), on the other hand, is an effective method for increasing the data rate in spread spectrum systems, in general. This paper presents a detailed analysis of a class of filter bank multicarrier spread spectrum (FBMC-SS) waveforms and demonstrates an effective receiver implementation of them when multicodes are applied. Application of the developed multicode waveform for communications over high-frequency (HF) skywave channels is also explored, and the benefits that it provides are studied both numerically, through computer simulations, and experimentally, by examining the receivers performance over a variety of skywave links.

M-Sequences and Sliding Window Based Audio Watermarking Robust Against Large-Scale Cropping Attacks

Large-scale cropping (LSC) is one of the mostly-used operations in desynchronization attacks and can easily destroy the watermark information by deleting continuous audio slices from the watermarked audio. In this work, we propose a spread spectrum (SS) based audio watermarking scheme to resist against LSC attacks more robustly from both theoretical and empirical perspectives. Specifically, we first perform discrete wavelet transform (DWT), graph-based transform (GBT), and singular value decomposition (SVD) on the host audio signal to produce transform coefficients. Next, we embed the chaotic encrypted watermark into DWT-GBT-SVD coefficients by the SS technique. Then, we combine m-sequences with the encrypted watermark to generate the watermarking key, which can theoretically guarantee the self-restoration of the cropped watermark based on the periodicity of m-sequences. Additionally, we develop an effective sliding window (SW) strategy to extract the fragmentary watermark slices from DWT-GBT-SVD coefficients and restore the integral watermark by the watermarking key. Finally, the proposed audio watermarking scheme, named m-SW-LSC, is compared with the state-of-the-art audio watermarking methods on audio signals with different genres and lengths under various attacks with different ratios. Experimental results demonstrate that our m-SW-LSC has a superior performance in restoring the complete watermark and exhibits significantly high robustness against LSC attacks.

Estimating the Secret Key of Spread Spectrum Watermarking Based on Equivalent Keys

The security of spread spectrum (SS) watermarking largely depends on the difficulty of estimating its secret key. Some estimators have been proposed to estimate the secret key in the known-message attack (KMA) scenario. However, the estimation accuracies of existing estimators are not satisfactory when the number of observations is not large enough. Currently, it is still a challenging and open problem to design more effective estimators. In this paper, we propose an equivalent keys (EK)-based estimator to estimate the secret key for both the traditional and more secure SS watermarking methods. Equivalent keys form an equivalent region, which is the intersection of a unit hypersphere and a hypercone. According to the Monte Carlo simulation, we find that the secret key can be estimated by adding up the equivalent keys uniformly sampled from the equivalent region. Thus, the proposed estimator selects equivalent keys from randomly-generated vectors by exploiting the pairs of watermarked signals and their embedded messages. A theoretical analysis is performed for the proposed estimator to evaluate the estimation accuracy. Experimental results verify the theoretical analysis and show the superiority of the proposed estimator over existing estimation methods. Furthermore, this paper also shows the insecurity of the more secure SS watermarking methods in the KMA scenario from a practical perspective for the first time.

Rogue Waves in the Southern North Sea—The Role of Modulational Instability

Abstract The role of the modulational instability for rogue wave generation in the ocean is still under debate. We investigated a continuous dataset, consisting of buoy and radar wave elevation data of different frequency resolutions, from eight measurement stations in the southern North Sea. For periods with rogue waves, we evaluated the presence of conditions for the modulational instability to work, that is, a narrow-banded wave spectrum in both frequency and angular direction. We found rogue waves that exceed twice the significant wave height indeed occur at slightly lower frequency bandwidths than usual. For rogue waves that are defined only by high crests, this was, however, not the case. The results were dependent on the measurement frequency. The directional spreading of the buoy spectra yielded no information on the presence of a rogue wave. In general, all spectra estimated from the dataset were found to be broad in frequency and angular direction, while the Benjamin–Feir index yielded no indication on a high nonlinearity of the sea states. These are unfavorable conditions for the evolution of a rogue wave through modulational instability. We conclude that the modulational instability did not play a substantial role in the formation of the rogue waves identified in our dataset from the southern North Sea. Significance Statement This work investigates whether rogue waves measured in the southern North Sea may have been generated by a modulational instability. The latter is a nonlinear mechanism of wave energy focusing that has been proven mathematically and confirmed in laboratory experiments. However, it is still unclear whether this mechanism is responsible for rogue wave generation under realistic ocean conditions. The modulational instability primarily arises when waves have similar frequencies and directions. In our data, these conditions were not satisfied. This finding leads to the insight that the modulational instability is not the most probable mechanism to generate rogue waves in our dataset.