Channel Time Delay Research Articles

A tunable spatial coherence imaging with ultrasonic array for defects in composites

ABSTRACT Ultrasonic phased array detection technology is a promising non-destructive testing (NDT) method that has been widely used for defect detection. Due to the high attenuation characteristics of ultrasonic wave propagation in carbon fibre reinforced polymer (CFRP) and the directivity of the array elements, the possibility of false or missed detection for defects that are far from the array elements or located very close together will increase. In addition, the total focusing method (TFM) employs only the defect information contained in the full matrix capture (FMC) dataset while ignoring the spatial information of the array signals, leading to image mixed with artefact and noise. An improved method called p-weighted-TFM (p-WTFM) is proposed. First, this method scales the magnitude of time-delayed channel signal by p-th root while maintaining the phase unchanged. Second, channel summation with energy compensation and directional correction will be realised. Finally, the signal dimensionality is restored by p-th power. Experimental results show that the p-WTFM can use any rational p-value to flexibly adjust image quality. Specially, this method can accurately distinguish adjacent defects spaced 1 mm apart with p value of 2.5, and it can also detect two defects located 10 mm deep with p value of 3.0.

Dynamic Control and Time-Delayed Channel Scheduling Co-Design for Voltage Control in Active Distribution Networks

Dynamic Control and Time-Delayed Channel Scheduling Co-Design for Voltage Control in Active Distribution Networks

Global consensus-based formation control of perturbed nonholonomic mobile robots with time varying delays

This paper proposes a smooth time-variant PID-like control scheme for networks composed of nonholonomic vehicles modeled as differential drive robots affected by unknown input constant disturbances and transmission variable time-delays in the communication channel. Using Barbalat’s Lemma and under the fact that the interconnecting graph is undirected and connected, it is proved that all robots positions asymptotically converge to a position in a given desired formation and they all agree on the center of this formation, while the orientations asymptotically reach a consensus value. Besides, it is proved that given the structure of the proposed new integral action, the external constant disturbances are rejected (exactly estimated) and that velocities converge to zero. Simulation results, with a robot network composed of eight nonholonomic vehicles and considering disturbances subjected to step changes, are presented to illustrate the robustness of the proposed control scheme.

Multipath time delay estimation based on the improved Wigner-ville distribution

The accuracy of underwater multipath channel time delay is crucial in underwater acoustic signal processing. The temporal resolution of the traditional matched filter is restricted by the transmitted signal bandwidth. Wigner-Ville distribution (WVD) which has excellent energy concentration characteristics and effectively avoids the interaction between temporal and frequency resolutions, and is widely used in non-stationary signal processing. However, considering the multipath signal processing, the cross-term in echo WVD would cause inaccurate time delay estimation. Therefore, in this paper, a novel multipath signal time delay estimation method based on the improved WVD is proposed. By utilizing the difference in the continuity of energy changes between auto-term and cross-term in the time-frequency domain, the Seeded Region Growing algorithm is applied and improved to extract auto-term in the WVD. The simulation results show that the method removes cross-term and maintains high concentration characteristics of auto-term, compared with pseudo-WVD and smooth pseudo-WVD, etc. In addition, it improves the accuracy of multipath signal delay estimation and has some anti-noise ability.

A novel control system for synchronizing chaotic systems in the presence of communication channel time delay; case study of Genesio-Tesi and Coullet systems

Time delay is one of the challenges that may occur in the communication channels. The control of chaotic systems due to time delay is a challenging purpose. This paper defines a synchronization system where the Genesio-Tesi is the master, and the Coullet is the slave system. A suitable observer-based sliding mode controller is designed for the goal of synchronization with the communication channel’s time delay. The proposed control system consists of state and disturbance observers and a controller. The state, disturbance observers, and controller are designed to synchronize the chaotic systems. The state observer estimates the master system states, the disturbance observer estimates the model of the uncertainties and external disturbances, and the controller uses the information from the state and disturbance observers in real time. These state and disturbance observers and controller work simultaneously. The proposed controller is based on the Sliding Mode Control (SMC) method and fixed-time stability concepts. This control system is compared with finite-time and asymptotical control systems by defining three performance criteria IAE, ITAE, and ISV. The proposed control method is robust against uncertainties and external disturbances and chattering-free. The MATLAB/Simulink software simulates the control systems and compares their results.

Effect of DNN Approximation for Channel Estimation and Signal Detection on OFDM Applications

This paper offers a deep learning approximation to realize channel estimation and signal detection that creates the main communication structure skeleton for the orthogonal frequency-division multiplexing (OFDM) system known as an efficient modulation type on 5G. This letter offers an application of deep learning to handle the wireless OFDM channels' end-to-end conduct. First, channel state information (CSI) is predicted explicitly that differs from existing OFDM receivers, then detected the transmitted symbols utilizing the predicted CSI. In the end, CSI is predicted by the suggested deep learning approximation indirectly and transmitted symbols are directly recovered. The structure of the designed receiver occurs of a layer of DNN and soft decisions, which resolves the issues channel estimation error, time delay, and limitation of decoding between users in classic detection techniques. In the simulation results, it is observed that the receiver has powerful stability on the power distribution of user, not only convenient for the linear channel, but also for nonlinear channel when enhancement the number of users, also detection can be well on the receiver. Generally, the efficiency of the modulation system decreases with the features of the multipath channel utilized for transmission. Channel estimation and detection of symbols utilize to reduce the impacts of the channel, which needs high computation and bandwidth conventionally. This paper is used deep neural networks (DNN) for detecting the signal, in this way much effort in detecting the channel is prevented. The proposed method saves priceless bandwidth via used CP in OFDM with a big increase in SNR.

Leader-follower and leaderless pose consensus of robot networks with variable time-delays and without velocity measurements

This paper proposes a novel distributed controller that solves the leader-follower and the leaderless consensus problems in the task space for networks composed of robots that can be kinematically and dynamically different (heterogeneous). In the leader-follower scenario, the controller ensures that all the robots in the network asymptotically reach a given leader pose (position and orientation), provided that at least one follower robot has access to the leader pose. In the leaderless problem, the robots asymptotically reach an agreement pose. The proposed controller is robust to variable time-delays in the communication channel and does not rely on velocity measurements. The controller is dynamic, it cancels-out the gravity effects and it incorporates a proportional to the error term between the robot and the controller virtual position. The controller dynamics consists of a simple proportional scheme plus damping injection through a second-order (virtual) system. The proposed approach employs the singularity free unit-quaternions to represent the orientation of the end-effectors, and the network is represented by an undirected and connected interconnection graph. The application to the control of bilateral teleoperators is described as a special case of the leaderless consensus solution. The paper presents numerical simulations with a network composed of four 6-Degrees-of-Freedom (DoF) and one 7-DoF robot manipulators. Moreover, we also report some experiments with a 6-DoF industrial robot and two 3-DoF haptic devices.

A Multibeam Steerable Parametric Array Loudspeaker for Distinct Audio Content Directing

The parametric acoustic effect has gained much interest in airborne applications as parametric acoustic loudspeakers (PALs) for the unique characteristics of generating self-demodulated low-frequency acoustic waves with high directivity. The development of PAL applications currently requires advanced spatial control of demodulated audio signals for more sophisticated sound field reproduction. Targeting to enhance the control flexibility, this paper proposes an efficient, fully FPGA-based PAL system to provide multiple low-distortion audio signals of different contents to separate directions from a single PAL device. The proposed system can achieve accurate beamsteering by providing channel time delays in nanosecond resolution and can generate independent parametric beams from a time-division-multiplexing (TDM) scheme. The paper presents a compact and scalable FPGA solution for PAL in signal preprocessing, beamsteering, and multi-beam control. The proposed system is verified in simulation and demonstrative experiments for the realization of flexible steering of multiple distinct audio signals into desired directions simultaneously under the TDM control.

Teleoperación robusta de sistemas mecánicos basada en la estructura de control con compensación activa de perturbaciones

<p>Se presenta una estrategia de control para garantizar la estabilidad de sistemas de teleoperación formados por mecanismos de n grados de libertad (nGDL), con incertidumbres paramétricas, perturbaciones externas, medición parcial de los vectores de estado y sin el uso de sensores de fuerza. Se asume que los mecanismos se encuentran lo suficientemente cercanos uno del otro, de tal forma que el problema de retardos ocasionados por el medio de comunicación es despreciable. La estrategia se basa en la aplicación de la estructura de control con compensación activa de perturbaciones, la cual incorpora observadores de estado discontinuos y filtros paso bajo que permiten la estimación de las variables de estado y otras señales no medidas, así como los términos de perturbación presentes en ambos sistemas, que permiten la implementación de los controladores. El desempeño de la estrategia de teleoperación se ilustra a través de experimentos con mecanismos de uno y dos grados de libertad.</p>

Predictive control compensation for networked control system with time-delay

This study proposes a novel predictive control compensation method to deal with random time-delay in networked control system. Different from other compensation strategies, this study adopts different compensation strategies for input time-delay and output time-delay to improve the control effect. First, for the input channel time-delay, the corresponding buffer is set by the time-stamp in the packet. Combining the historical output value and the control variable, the fast implicit generalized predictive control algorithm is adopted to design the predictive controller to compensate for the input channel time-delay. Second, for the output channel time-delay, the controller cannot measure it. A control compensator is designed by adding a feedback loop. The output control variable of the predictive controller is adjusted by predicting the error between the actual control signal and the controller output at the historical sampling time, so as to compensate for the time-delay of the output channel. In addition, the stability of the proposed time-delay compensation method is analyzed. Finally, simulation and experimental results show that the proposed method has good control and compensation effect, and improves the output performance of the system. The networked control system with random time-delay is stable. Furthermore, the simulation results also show that this time-delay compensation method needs less computing time and is more suitable for the practical applications.

Design of a wide-area damping controller to tolerate permanent communication failure and time delay uncertainties

The wide-area damping controllers showed to be effective in improving the damping ratio of the low-frequency oscillation modes. This controller requires remotes signals sent by Phasor Measurement Units located in different positions of the power system and, then, these signals are highly susceptible to communication time delays, failures and cyber-attacks that may compromise controller performance. This paper presents a procedure based on particle swarm optimization to tune the parameters of the central controller. Considering a set of operating conditions, the proposed procedure will search the central controller parameters that maximize the damping ratio of all eigenvalues of the linear model. A strategy to deal with permanent communication channel failures and time delay uncertainties in a given interval will be included in the proposed model and procedure. Then, the resulting central controller will present robustness to multiple operation conditions of the power system, to time delay uncertainties and to permanent failure of the channels of the controller. The proposed procedure was carried out in the IEEE 68-bus system and the performance of the designed controller was evaluated by modal analysis and time-domain nonlinear simulation.

Finite Time Chaos Synchronization in Time-Delay Channel and Its Application to Satellite Image Encryption in OFDM Communication Systems

This paper proposes a finite time chaos synchronization approach for the secure communication of satellite imaging. To this end, chaotic oscillators are considered in both the transmitter and receiver ends to generate the chaotic encryption/decryption keys. To mitigate the non-negligible channel time-delay between the receiver and transmitter, we propose a robust controller design. The proposed approach is designed based on the Lyapunov stability theory and the finite-time synchronization concept to attain finite time synchronization in a time-delayed channel. By using synchronized chaotic keys, a physical-layer chaotic encryption scheme for transmitting the satellite images is designed in orthogonal frequency-division multiplexing wireless network. The proposed chaotic-based satellite image encryption/decryption system is validated using a numerical simulation study. Additionally, to analyse the robustness and demonstrate the efficiency of the proposed chaotic encryption structure, a set of security analysis tools such as histogram analysis, key space analysis, correlation test, information entropy and other statistical analysis were performed.

Haptic Tele-Driving of Wheeled Mobile Robots Under Nonideal Wheel Rolling, Kinematic Control and Communication Time Delay

The increasing application of wheeled mobile robots (WMR) in many fields has brought new challenges on its control and teleoperation, two of which are induced by contact slippage phenomenon between wheel and terrain as well as time delays in the master-slave communication channel. In the WMR bilateral tele-driving system, in this paper, the linear velocity of the slave mobile robot follows the position command from the haptic master robot while the slippage-induced velocity error is fed back as a haptic force felt by the human operator. To cope with the slippage-induced nonpassivity and constant time delays, this paper proposes three methods to design the WMR bilateral teleoperation system’s controller. An experiment system is set up with Phantom Premium 1.5A haptic device as the master robot and a simulation platform of WMR as the slave robot. Experiments with the proposed methods demonstrate that they can result in a stable WMR bilateral tele-driving system under wheel’s slippage and constant time-delays.

Ultrasound Baseband Delay-Multiply-and-Sum (BB-DMAS) nonlinear beamforming

Compared to conventional Delay-and-Sum (DAS) beamforming, Delay-Multiply-and-Sum (DMAS) imaging uses multiplicative coupling of channel pairs for spatial coherence of receiving aperture to improve image resolution and contrast. However, present DMAS imaging is based on the radio-frequency (RF) channel signals (RF-DMAS) and thus requires large oversampling to avoid aliasing and switching of band-pass filtering to isolate the corresponding spectral components for imaging. Baseband DMAS (BB-DMAS) beamforming in this study is based on the demodulated channel signals to provide similar results but with simplified signal processing. The BB-DMAS beamforming scales the magnitude of time-delayed channel signal by p-th root while maintaining the phase. After channel sum, the output dimensionality is restored by p-th power. The multiplicative coupling in BB-DMAS always renders baseband signal and thus the need for oversampling is eliminated. Besides, the BB-DMAS can use any rational p values to provide flexible image quality and an explicit relation between BB-DMAS beamforming and channel-domain phase coherence exists. Our results show that the image characteristics between BB-DMAS and RF-DMAS are similar. The suppression of lateral side lobe level, grating lobe level and uncorrelated random noises gradually increases with the rational p value in BB-DMAS beamforming. The image contrast improves from −24.8 dB in DAS to −34.3 dB, −43.0 dB and −51.4 dB in BB-DMAS, respectively with p value of 1.5, 2.0 and 2.5. In conclusion, BB-DMAS beamforming provides flexible manipulation of image quality by introducing baseband spatial coherence in the ultrasonic imaging.

Research and Development of a Highly Reconfigurable OFDM MODEM for Shallow Water Acoustic Communication

Orthogonal frequency division multiplexing (OFDM) provides an effective approach to combat multipath with high bandwidth efficiency for underwater acoustic communication, thus design and implementation of acoustic OFDM MODEMs draw more and more attention. Extensive work has been performed to mitigate multipath, Doppler shift, and peak average power ratio from the viewpoint of theoretical analysis. Meanwhile, design and implementation of a parameter-configurable OFDM to adapt to highly diverse underwater channels and various missions become more and more important. Traditional MODEM with fixed parameters cannot achieve the best tradeoff between data rate and bit error rate. In order to obtain the best communication performance under long time delay channels, Doppler channels, and frequency-selective channels, in this paper, we provide the design and implementation of a parameter-configurable underwater acoustic OFDM MODEM with the purpose to achieve flexibility. Specifically, the reconfigurable parameters of the OFDM MODEM include the number of receivers, the number of null subcarriers, the number of pilot subcarriers, as well as the type of channel estimation algorithms. The number of pilot subcarriers and type of channel estimation methods are determined based on channel characteristics, the number of null subcarriers is determined based on the Doppler shift, and the number of the received channels is based on fading channel. Moreover, in this paper, the Doppler estimation performance under different number of null subcarriers, and the channel estimation performance under different number of pilot subcarriers and under different channel estimation algorithms, are analyzed in terms of bit error rate. Finally, the effectiveness of the proposed highly configurable OFDM MODEM is demonstrated by two sea trials corresponding to different communication scenarios.

Effect of underwater suspended particles on the transmission characteristics of polarized lasers

The complex problem related to the transmission channel of underwater polarized lasers is caused by underwater suspended particles. In order to study the effect of suspended particles on underwater optical communication links and laser polarization characteristics, a method based on a combination of Mie scattering theory and Monte Carlo numerical simulation is used to establish the transmission model of underwater photons. This method is applied to analyze the effect of suspended particles on normalized received energy and channel delay. We also investigate the effects of particle and channel length on the polarization characteristics of four different types of polarized lasers. Theoretical analysis and simulation results indicate that the optical coefficients of particles increase with increasing particle size. Thus, when the transmission channel length is the same, the received normalized energy and light intensity will decrease, and the channel time delay will increase. Meanwhile, a depolarization phenomenon with respect to the laser will be generated with increasing particle size, and particle size has a greater effect on linearly polarized light than circularly polarized light. Therefore, circularly polarized light maintains good polarization characteristics in the underwater laser transmission process.

Time-Varying Communication Channel High Altitude Platform Station Link Budget and Channel Modeling

Because of the high BER (Bit Error Rate), low time delay and low channel transmission efficiency of HAPS (High Altitude Platform Station) in the near space. The link budget of HAPS and channel model are proposed in this paper. According to the channel characteristic, the channel model is set up, combined with different CNR (Carrier Noise Ratio), elevation angle, coding situations of wireless communication link by using Hamming code, PSK (Pulse Shift Keying) and Golay code respectively, then the situations of link quality and BER are analyzed. The simulation results show that the established model of the link budget and channel are suitable for the theoretical analysis results. The elevation of the HAPS communication link is smaller while the BER is increasing. The case of channel in the coding is better than in the un-coded situation. When every bit power and thermal noise power spectral density is larger, the BER of the HAPS communication link is becoming smaller.

Direct and Two-Step Positioning in Visible Light Systems

Visible light positioning (VLP) systems based on light emitting diodes can facilitate high accuracy localization services for indoor scenarios. In this paper, direct and two-step positioning approaches are investigated for both synchronous and asynchronous VLP systems. First, the Cramer–Rao lower bound (CRLB) and the direct positioning-based maximum likelihood estimator are derived for 3-D localization of a visible light communication receiver in a synchronous scenario by utilizing information from both time delay parameters and channel attenuation factors. Then, a two-step position estimator is designed for synchronous VLP systems by exploiting the asymptotic properties of time-of-arrival and received signal strength estimates. The proposed two-step estimator is shown to be asymptotically optimal, i.e., converges to the direct estimator at high signal-to-noise ratios. In addition, the CRLB and the direct and two-step estimators are obtained for positioning in asynchronous VLP systems. It is proved that the two-step position estimation is optimal in asynchronous VLP systems for practical pulse shapes. Various numerical examples are provided to illustrate the improved performance of the proposed estimators with respect to the current state-of-the-art and to investigate their robustness against model uncertainties in VLP systems.

Developing the architecture of integrated 5G mobile network based on the adaptation of LTE technology

The issues of constructing integrated fifth-generation mobile access networks based on the further development and modification of the LTE technology were studied. The LTE frame structure and features of using the LTE technology in real-time systems with packet delay control were analyzed. It was concluded that it is possible to reduce the time delays in the LTE radio channel by modifying the frame structure and the method of dynamic allocation of the time-frequency resource. A method was proposed for reducing delays in a radio channel by forming a dedicated logical channel in the ad hoc mode in which a channel allocation unit is formed based on the subframes. Moreover, combined synchronous and asynchronous multiplexing of real-time data into a synchronous sequence of packet transport modules was performed in the ad hoc channel. As a result of the studies, architecture and basic principles of the fifth-generation mobile access network based on modification of the LTE technology were developed. This will enable connection to the network of various sensor-equipped devices with individually specified requirements to the intensity of the data flow and allowable packet delays.

Pinning lag synchronization of complex dynamical networks with known state time-delay and unknown channel time-delay

In this paper, a linear and adaptive feedback pinning strategy is used to study the lag synchronization of complex dynamical networks with known state time-delay and unknown channel time-delay. Firstly, based on the Lyapunov stability theory, a novel Lyapunov functional, which involves the estimated error $$\hat{e}_{i}(t)$$ rather than the general synchronization error $$e_{i}(t)$$ , is constructed. Secondly, in view of the unknown information of the channel time-delay, two available pinning controllers are designed such that the considered networks achieve lag synchronization. Furthermore, by a proper adaptation mechanism, we estimate the unknown channel time-delay successfully under the case that the initial value of the estimated channel time-delay is larger than true channel time-delay, i.e., $$\hat{\tau }(0)>\tau $$ and the another case $$\hat{\tau }(0)<\tau $$ . Finally, the effectiveness and correctness of the lag synchronization criteria are verified through two simulation experiments.