Spread Spectrum Research Articles

A High Dynamic Weak Spread Spectrum Signal Acquisition Strategy Based on Iterative Local Search

To solve the acquisition problem of the satellite-ground microwave link ranging of the China Space Station, iterative local search (ILS) is proposed. In this paper, an iterative-local-search-based acquisition strategy is presented, and a Doppler frequency and code phase transfer model is designed, which can accomplish acquisition by multiple rounds of two-dimensional search. The adaptability of the ILS improved traditional acquisition algorithm to a high dynamic and low signal-to-noise ratio (SNR) environment can be significantly improved without relying on prior information. For the classical parallel code phase acquisition algorithm, the performance of the ILS acquisition strategy was experimentally evaluated for high dynamic weak signals, and the results demonstrated the superiority of the proposed strategy. The computational complexity of the ILS improved parallel code phase acquisition algorithm was greatly reduced. The sensitivity of the parallel code phase acquisition algorithm improved by the ILS strategy to the acquisition of weak signals in a dynamic environment was significantly improved.

Localization and tracking of multiple fast moving targets in bistatic MIMO radar

In this paper, we propose a maximum likelihood (ML)-based scheme to accurately localize and track multiple fast-moving targets in multiple-input multiple-output (MIMO) radar systems. For the problem of tracking, the motion of the target will cause a spread spectrum effect, which broadens the beam width and skews the beam steering, rendering accurately localizing and tracking the fast moving targets extremely challenging. To tackle this problem, we use a locally linear model to approximate the real motion states of the target and define a vector to describe its motion state. As such, the motion states of targets at adjacent times can be accurately inferred within a short term based on this approximation model. To incorporate the effect of target motion when performing tracking, we estimate the motion state vector at a reference time from a batch of snapshots by using an ML estimator, instead of estimating the time-varying angular parameters directly as usual. Then the motion state vectors at other times can be inferred based on the estimated one by using the locally linear approximation model. The Cramér-Rao lower bound (CRLB) of the estimated parameters are also derived for performance evaluation. Numerical results validate the performance improvement of the proposed scheme compared to the benchmarks.

LoRa network reconfiguration with Markov Decision Process and Fuzzy C-Means clustering

Long Range (LoRa) is a proprietary modulation technique that uses Chirp Spread Spectrum (CSS) modulation for low power and wide area communications. Despite the advantages of LoRa technology, the reconfiguration of transmission parameters such as Spreading Factor (SF) and Transmission Power (Ptx), remains limited to maximize the uplink traffic. In this paper, we look upon additional parameters such as the Bandwidth (BW) and the Coding Rate (CR). We apply Fuzzy C-Means (FCM) algorithm to acquire knowledge about the quality of each transmission setting. Then, we use this knowledge in Q-learning and Markov Decision Process (MDP) algorithms as a state transition matrix to converge better and faster to the set of transmission settings that maximize the uplink data rate. As the solution should cope with different scenarios, we vary the number of End Devices (EDs), Base Stations (BSs), Packet Sizes (PSs) and Packet Rates (PRs). In addition, we compare our solution with many algorithms such as EXP3, ADR and EXPLoRaTS. Simulation results show that MDP with FCM clustering preprocessing improves better several Quality of Service (QoS) metrics including the Data Rate (DR), Packet Delivery Ratio (PDR), Time on Air (ToA) and Transmission Energy (Etx). Thus, the PDR and the DR were improved by 25%, the ToA was reduced by 40% and Etx was reduced by 20%.

Electromagnetic Fingerprinting of Memory Heartbeats

This paper presents MemScope, a system that fingerprints devices via electromagnetic sensing of their memory heartbeats, i.e., the clock signal that synchronizes memory and memory controller. MemScope leverages the enhanced resolution and security of memory heartbeat fingerprint, which has enriched spectral features thanks to the spread spectrum generation of memory clock, and cannot be concealed as long as the device accesses its memory during computing. MemScope employs signal processing algorithms that allow it to hear the memory heartbeats of devices from a distance, in the presence of noise, and in crowded environments where multiple devices coexist. It then fingerprints memory heartbeats using machine learning tools. Measurements on a set of 65 devices over a month validate the robustness of fingerprint against time variation, and show a high precision and recall. We then use the neural network to build a detector to defend against possible replay attacks. Finally, we further demonstrate the effectiveness of MemScope in two application scenarios, (i) detecting wireless identity spoofing and (ii) identifying and localizing unauthorized hidden cameras.

Detection of Direct Sequence Spread Spectrum Signals Based on Deep Learning

Direct spread spectrum communication has the advantages of strong anti-jamming ability and low probability of interception, which plays an essential role in both civil and military communications. The detection of direct sequence spread spectrum (DSSS) signal becomes very difficult because of its low power spectral density. In this paper, the detection of DSSS signals under non-cooperative conditions is carried out based on the classification technology of deep learning. Firstly, a detection scheme based on convolutional neural network (CNN) is proposed, in which the neural network is used to learn the features of DSSS signal and noise automatically without extracting the features in advance. In addition, we also propose a hybrid CNN-correlation (CORR)-based detection scheme in order to reduce the computational complexity. In this scheme, the autocorrelation of the received signal is truncated and used as the input of the neural network for training and inference. A large number of simulation results show that the detection performance of the proposed two schemes is significantly better than the traditional autocorrelation-based detection method in various scenarios.

Non-Binary PRN-Chirp Modulation: A GNSS Fast Acquisition Signal Waveform

In this letter, we propose a new non-binary modulation which allows both Global Navigation Satellite Systems (GNSS) synchronization and the demodulation of non-binary symbols, without the need of a pilot signal, with the aim to provide a fast first position, velocity and time fix. The waveform is constructed as the product of i) a pseudo-random noise sequence with good auto-correlation and cross-correlation properties, and ii) a chirp spread spectrum family, which allows to demodulate non-binary symbols even if the signal phase is unknown. In order to demodulate the data, a bank of non-coherent matched filters is proposed. Because of the particular modulation structure, the receiver is capable to demodulate the navigation message faster while allowing the basic GNSS signal processing functionalities. Illustrative results are provided to support the discussion.

Dual-Mode Chirp Spread Spectrum Modulation

In this letter, we propose dual-mode chirp spread spectrum (DM-CSS) modulation for low-power wide-area networks. DM-CSS is capable of achieving a higher spectral efficiency (SE) relative to its counterparts, such as Long Range (LoRa) modulation. Considering the same symbol period, the SE in DM-CSS are augmented by: (i) simultaneously multiplexing even and odd chirp signals; (ii) using phase shifts of 0 and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\pi $ </tex-math></inline-formula> radians for both even and odd chirp signals; and (iii) using either up-chirp or down-chirp signal. The SE increases by up to 116.66% for the same bandwidth and spreading factor relative to LoRa. We present a complete transceiver architecture along with non-coherent detection process. Simulation results reveal that DM-CSS is not only more spectral efficient but also more energy efficient than most classical counterparts. It is also demonstrated that DM-CSS is robust to phase and frequency offsets.

Horizontal Wavenumber Spectra of Vertical Vorticity and Horizontal Divergence of Mesoscale Dynamics in the Mesosphere and Lower Thermosphere Using Multistatic Specular Meteor Radar Observations

AbstractSpecular meteor radars (SMRs) have significantly contributed to the understanding of wind dynamics in the mesosphere and lower thermosphere (MLT). We present a method to estimate horizontal correlations of vertical vorticity (Qzz) and horizontal divergence (P) in the MLT, using line‐of‐sight multistatic SMRs velocities, that consists of three steps. First, we estimate 2D, zonal, and meridional correlation functions of wind fluctuations (with periods less than 4 hr and vertical wavelengths smaller than 4 km) using the wind field correlation function inversion (WCFI) technique. Then, the WCFI's statistical estimates are converted into longitudinal and transverse components. The conversion relation is obtained by considering the rotation about the vertical direction of two velocity vectors, from an east‐north‐up system to a meteor‐pair‐dependent cylindrical system. Finally, following a procedure previously applied in the upper troposphere and lower stratosphere to airborne wind measurements, the longitudinal and transverse spatial correlations are fitted, from which Qzz, P, and their spectra are directly estimated. The method is applied to a special Spread spectrum Interferometric Multistatic meteor radar Observing Network data set, obtained over northern Germany for seven days in November 2018. The results show that in a quasi‐axisymmetric scenario, P was more than five times larger than Qzz for the horizontal wavelengths range given by ∼50–400 km, indicating a predominance of internal gravity waves over vortical modes of motion as a possible explanation for the MLT mesoscale dynamics during this campaign.

Secure Optical Communication Using a New 5D Chaotic Stream Segmentation

According to its complex properties like ergodicity, unpredictability, and sensitivity to its initial states, chaotic systems are ‎attracting more and more attention and are widely used for security purposes. Moreover, ‎the chaotic signals are considered suitable for spread spectrum modulation due to their wideband properties. It is able to reduce the peak to average ‎power ratio (PAPA). This paper presents a new Dynamic Diffeo-Difference Multi-Dimensional (DDD-MD) system. It is used as ‎a key for a new cryptosystem designed based on the chaotic stream segmentation (CSS) method. The proposed ‎system provides the best trade-off between efficiency, robustness, and high data rate transmission. The behavior of the ‎proposed chaotic system is evaluated numerically by analyzing the Lyapunov exponent spectrum, complexity, and attractor ‎phase diagram. Besides, it is practically assessed based on the principle of system ‎circuit design; the circuit diagram of the system is prepared and simulated by Multisim, which shows high consistency with ‎the numerical simulation. These evaluations show that the proposed system has a rich dynamics behavior to be realized in ‎an encryption system and provides a foundation for many engineering and physical applications‎.

An Improved Spread-Spectrum Technique for Reduction of Electromagnetic Emissions of Wireless Power Transfer Systems

The application of conventional spread spectrum techniques for conducted electromagnetic emission (EME) reduction in inductive-resonant wireless power transfer (WPT) systems may not reduce conducted EME enough due to specific frequency characteristics of the resonant systems and it can lead to some “adverse effects”, mainly in terms of decreased efficiency. Therefore, in this paper, an improved spread spectrum approach, multi-switching frequency and multi-duty cycle (MFMD) technique, is proposed. The proposed approach can give a considerably better conducted EME reduction along with a better efficiency than the conventional spread spectrum techniques based on a multi-switching frequency scheme. In the proposed approach, the inductive-resonant WPT system can operate at multiple switching frequencies (e.g., three different frequencies) and for a part of a control signal with a specific switching frequency, there is a specific duty cycle. The technique can be implemented in a simple way using an inexpensive 8-bit microcontroller. The effect of the MFMD scheme on the conducted EME and efficiency of the WPT system is studied in detail. The WPT system conducted EME and the efficiency are studied experimentally with a designed laboratory prototype. The performance characteristics of the WPT system with the MFMD scheme are compared to those with the multi-switching frequency scheme and without the spread spectrum. The WPT system with the proposed spread spectrum technique has a better performance than that with the conventional spread spectrum technique.

Research on EMI suppression of high frequency isolate quasi-Z-source inverter based on chaotic frequency modulation

As a new type of topology inverter, the isolated quasi-Z-source inverter is suitable for photovoltaic power generation systems because of its high efficiency in power conversion, high boost ratio and electrical isolation. By increasing the operating frequency of the inverter, the inductance and capacitance volume of the inverter can be reduced, so that the inverter has higher power density. However, high-frequency switching actions will produce a large number of current and voltage sudden changes, leading to electromagnetic interference (EMI), affecting other components or the entire system and the stable operation of the power grid. Chaotic frequency modulation is a method to reduce the electromagnetic interference level of switching converters from the source. This method using the 10-scroll chaotic signal of the even-symmetric segmented Chen system as a spread spectrum signal to broaden the electromagnetic interference noise energy spikes to the surrounding continuous frequency bands, thereby reducing the spectrum peaks and making the system has better electromagnetic compatibility performance. In this paper, the high frequency isolated quasi Z-source photovoltaic grid-connected micro-inverter is studied, and the chaotic frequency modulation technology is used to suppress the electromagnetic interference of the inverter.

Design of Distance Teaching System for Interior Design Course Based on Internet of Things and Education Platform

Distance education system plays an important role in promoting the integration of professional characteristics and application practice. In order to solve the problem of network throughput decline caused by communication channel data conflict, a distance teaching system of interior design course is designed based on Internet of Things and education platform. In the hardware part, the audio acquisition circuit and video acquisition circuit are designed to ensure the quality of signal transmission. In the software part, the communication architecture is set based on the Internet of Things technology, and a course data anticollision algorithm is proposed, which uses the spread spectrum code to separate the corresponding data to avoid data collision. Based on the education platform, the function structure of distance education system is designed to complete the course management and maintenance. The experimental results show that the maximum throughput of the distance education system designed in this paper is 3.54 MB/s, which is 0.86 MB/s and 0.97 MB/s more than the system based on big data technology and artificial intelligence, and it can effectively avoid the problem of data conflict.

Outage Probability versus Carrier Frequency in GeoSurf Satellite Constellations with Radio-Links Faded by Rain

For sites located in different climatic regions, we estimated the relationship between the annual average probability distributionof exceeding a fixed rain attenuation, and the carrier frequency in the range 16 to 100 GHz, in the zenith paths of GeoSurf satellite constellations. In these constellations rain attenuation is independent of the altitude and number of satellites. Rain attenuation iss calculated with the Synthetic Storm Technique, a reliable prediction method, by using on-site measured rain-rate time series. A suitably defined outage probability factor shows that the outage probability, for fixed power margin, tends to saturate as frequency increases. In wideband radio-links, such as in spread spectrum design, there is very likely a long-term distortion due to the in-band outage probability. The results are oriented to design systems faded by rain attenuation whose value is also the total power margin available due to a mixture of coding and hardware technology, whose combination is not of concern here.

Enhancing diversity of OFDM with joint spread spectrum and subcarrier index modulations

This paper proposes a novel spread spectrum and sub-carrier index modulation (SS-SIM) scheme, which is integrated to orthogonal frequency division multiplexing (OFDM) framework to enhance the diversity over the conventional IM schemes. Particularly, the resulting scheme, called SS-SIM-OFDM, jointly employs both spread spectrum and sub-carrier index modulations to form a precoding vector which is then used to spread an M-ary complex symbol across all active sub-carriers. As a result, the proposed scheme enables a novel transmission of three signal domains: SS and sub-carrier indices, and a single M-ary symbol. For practical implementations, two reduced-complexity near-optimal detectors are proposed, which have complexities less depending on the M-ary modulation size. Then, the bit error probability and its upper bound are analyzed to gain an insight into the diversity gain, which is shown to be strongly affected by the order of sub-carrier indices. Based on this observation, we propose two novel sub-carrier index mapping methods, which significantly increase the diversity gain of SS-SIM-OFDM. Finally, simulation results show that our scheme achieves better error performance than the benchmarks at the cost of lower spectral efficiency compared to classical OFDM and OFDM-IM, which can carry multiple M-ary symbols.

Secret sharing scheme based on spread spectrum ghost imaging.

(n,n) secret sharing is a procedure for splitting the key into several pieces so that no subset is sufficient to read the encrypted message except the entire one. In this paper, we propose a novel, to the best of our knowledge, secret sharing scheme based on spread spectrum ghost imaging (SSGI). In the scheme, a dealer divides a secret key, which is obtained by a modulo 2 addition over n sequences from a Hadamard matrix, into n pieces to n participants, and then the dealer uses the secret key as a direct sequence code to encrypt an image using the SSGI method. The resultant bucket signal, as the ciphertext, is then shared with n participants, and the modulated speckle patterns in SSGI are also transmitted to nparticipants by private channels for preventing eavesdropping. An independent signal is used as a protective signal to prevent the information leakage in SSGI. In the decryption process, only if all participants are honest and cooperative, the secret key can be obtained; thus, the ciphertext can be decrypted to the correct image using the second-order correlation algorithm. Otherwise, the image information cannot be extracted. The theoretical analysis shows that the probability of finding out others' pieces for successfully reconstructing the image for one dishonest participant is (1/2N)n-1, where N is the length of the secret key, and n is the number of participants. The simulation and experimental results demonstrate that the proposed scheme has a stronger data security capacity in optical information encryption.

Performance-Enhanced Optical Non-Orthogonal Multiple Access Enabled by Orthogonal Chirp Division Multiplexing

In this paper, we propose a novel optical non-orthogonal multiple access (NOMA) scheme based on orthogonal chirp division multiplexing (OCDM) in a low-cost intensity modulation and direct detection system for passive optical networks (PON). The proposed OCDM-NOMA scheme can effectively improve spectral efficiency and serve more users by multiplexing spectrally overlaid OCDM signals in power domain. Under the advantages of the chirp spread spectrum, the proposed OCDM-NOMA scheme can also enhance the bit error rate (BER) performance compared to NOMA scheme based on orthogonal frequency division multiplexing (OFDM). A 17.8 Gb/s OCDM-NOMA PON transmission over 25 km standard single-mode fiber is experimentally demonstrated. The results show that, when compared to OFDM-NOMA and precoding OFDM-NOMA, OCDM-NOMA can achieve receiver sensitivity gains of 1.3 dB and 0.5 dB for the high-power branch signal, and 1.4 dB and 0.6 dB for the low-power branch signal at a BER of 3.8×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−3</sup> . Furthermore, we investigate the feasibility of using higher-order modulation in NOMA technology for the first time to further improve data throughput and spectral efficiency. In a 22.2 Gb/s transmission, the optimal power division ratio is determined, and the transmission performance is investigated. According to the experimental results, OCDM-NOMA with higher-order modulation can be a promising candidate for next-generation PONs.

Millimeter-Precision Ultrasonic DSSS Positioning Technique With Geometric Triangle Constraint

In this paper, we present a 3-D ultrasonic measurement technique that uses direct sequence spread spectrum (DSSS) with simultaneous multiple code-division multiple access (CDMA) signals. This technique can be applied in precise indoor locating systems. By placing three microphones close to one another separated by distances on the order of the ultrasound wavelength and imposing the constraint that one transducer and two from the three microphones form three triangles in the space (i.e., the triangle constraint) for 3-D ultrasonic DSSS trilateration measurement, millimeter 3-D positioning precision is achieved. The experimental results for a positioning system comprising a set of three microphones and four ultrasound transducers, each of them transmitting its respective 256-bit DSSS code, demonstrate that the proposed system can measure the spatial 3-D location of the receiver with a standard deviation of less than 1.21 mm. The proposed technique is suitable for potential future indoor positioning system (IPS) applications that require sub-centimeter 3-D positioning.

Secure Spread Spectrum Communication Using Super-Orthogonal Optical Chaos Signals

This paper proposes an analog spread spectrum secure communication system by designing a super-orthogonal two-dimensional electro-optic time delayed chaotic system. Two negatively correlated (called &#x201C;super-orthogonal&#x201D;) broadband optical chaotic signals can be generated. According to the binary symbol to be transmitted, the two signals are coupled into channel in turn. Benefiting from the wide spectrum and super-orthogonal characteristics of the two optical chaotic signals, the scheme shows excellent anti-noise performance which is verified by numerical simulations. Moreover, by introducing the modulation signal time delay feedback, the security of the system can be improved. The system is capable of resisting time delay signature extraction and return map attack, which are inherent vulnerabilities of time-delayed chaotic system and chaotic shift keying mechanism.

Spread Spectrum Technique With Random-Linear Modulation for EMI Mitigation and Audible Noise Elimination in IH Appliances

In this letter, a random-linear modulated spread spectrum technique for the induction heating (IH) appliances is proposed. The proposed method can effectively mitigate electromagnetic interference (EMI) emissions and can eliminate audible noises. Besides, it can suppress the total harmonic distortion (THD) compared with other modulations. The proposed random-linear modulation is evaluated by using a 1.8-kW IH prototype system. Experimental results verify the performance of EMI mitigation, audible noise elimination, and low THD using the proposed random-linear modulation compared with those of the conventional periodic and random modulations.

Spread Spectrum Coded Radar for R2R Interference Mitigation in Autonomous Vehicles

Autonomous Vehicles (AVs) rely on a set of radar sensors operated on frequency modulated waveforms. Due to the large bandwidth requirement of frequency modulated radars, only a limited number of proximate vehicles can be allowed for a concurrent transmission within the available spectrum. However, with the explosive growth of AVs, the available spectrum may soon reach its capacity. As a results, the coexistence of multiple AVs working on same resource, may lead to the problem of Radar-to-Radar (R2R) interference, also known as radar blindness. In this paper, we propose the notion of coded waveforms to minimize the R2R interference among the vehicles operating on the same resource. Specifically, the spread spectrum codes have been used as another degree of freedom (i.e., along with time-frequency resources) to spread the inter-vehicular radar interference over the wider spectrum, which enable to orthogonalize more number of AVs over the available range of spectrum. In addition, we have formulated a Spread Spectrum-based Radar Transmission Scheme (SS-RTS), and described the transmission and reception through SS-RTS. Also, the SS-RTS has been compared with the existing Graph-based Resource Allocation (GRA) scheme. Further, simulation results verified that SS-RTS significantly reduces the inter-vehicular R2R interference and outperforms GRA in terms of blind probability.