Backward Channels Research Articles

Predictor‐based switching control for networked systems

SummaryThis paper concerns a networked predictive control (NPC) for linear systems with data time‐varying delays, packet losses and disordering. By utilizing the received time‐delayed state measurement and control input, a predictive algorithm is firstly proposed for approximating the future state prediction. Afterwards, a probability dependent switching control law is proposed. Although the large data transmission delays emerge in the forward and backward channels, the packet disordering in the backward channel can be naturally excluded thus the actuator always receives valid control command during each sampling period with correct time sequence. The proposed NPC not only guarantees the global uniform exponential stability of the overall networked system but also brings merits in tolerating locally unstable sub‐systems. Numerical examples are provided to demonstrate the effectiveness and improvements of the proposed method.

Export Spillovers from Foreign Direct Investment in Kenya's Manufacturing Sector: A Double Hurdle Approach

ABSTRACT: This paper investigates export spillovers from foreign direct investment on the exporting activities of domestic manufacturing firms in Kenya. Specifically, the paper aims at identifying the transmission channels through which foreign direct investment affects the export propensity and export intensity decisions of domestically-owned firms. The study also examines the importance of firm-level heterogeneity on the occurrence and behavior of spillover channels. The study uses a panel data set for manufacturing firms in Kenya, obtained from the World Bank Enterprise Surveys, conducted in three waves covering the period 2007–2018. Employing the Double-Hurdle model approach, and using the maximum likelihood estimation technique, we empirically demonstrate the relevance and significance of distinguishing three channels for intra-industry (horizontal) spillovers – demonstration effects, information effects and competition effects channels and one channel for vertical spillovers, that is, backward linkage channel. Findings of this study shows that domestically-owned firms in Kenya experience significant positive export spillovers on their export participation decisions through the demonstration effects channel and significant negative spillovers via the competition effects channel. The findings also show significant positive export spillover effects on the export intensity decisions of domestically-owned firms via information and competition effects channels and significant negative export spillover effects via the backward linkages channel. The results further provide evidence that firms belonging to industries with low technological levels were unable to absorb spillover benefits from foreign presence via the demonstration effects channel. The results also indicate that small and medium firms were unable to reap export spillovers benefits via information effects and backward linkage channels. The findings have important policy implications for managers of firms and policymakers in Kenya and other developing countries. To enhance export-related spillover benefits, domestic firms need to enhance their absorptive capacities by upgrading their technical capacities and improving quality standards. Governments should facilitate the exchange of information on investor needs, their capacities and skills requirements, thus bridging information gaps between domestic suppliers and foreign investors in addition to supporting domestic market linkages.

How does the productivity of foreign-invested enterprises spill over to domestic firms in Vietnamese manufacturing?

This paper investigates the evidence of productivity spillover from foreign-invested enterprises (FIEs) to local firms through horizontal, backward, and forward spillover channels, using establishment-level data from Vietnam in 2006–2017. The paper also considers the effects of foreign ownership types on the existence and magnitude of the productivity spillover. In addition, the paper examines whether the involvement of domestic firms in global production networks (GPNs) impacts on the nature of the spillover. The findings indicate that productivity from FIEs spills over to local firms through backward and forward channels, but not horizontal channels. Ownership structures of FIEs serve as an important determinant of productivity spillover: joint ventures tend to generate more significant positive productivity spillover to domestic firms than fully owned foreign firms. Lastly, local firms operating within GPNs benefit more from the presence of FIEs compared to those involved in horizontal specialization.

Experimental and numerical study on the flow and heat transfer characteristic of nanofluid in the recirculation zone of backward-facing step microchannels

With the development of electronic devices towards high integration and high heat flux, the heat dissipation of electronic products has become one of the bottlenecks restricting the development of electronic devices. Nanofluids, as a flow medium to enhance heat transfer, have been widely concerned and their applications in the field of microelectronic heat transfer have broad prospects. The backward-facing step model is widely used in fields requiring heating or cooling because of its simple and easy processing, the sudden expansion of the flow cross-section and the separation, which shifts the stable flow state to the unstable flow state. In order to study the application characteristics of nanofluids in backward step structure channels, in this paper, the flow and heat transfer characteristics of TiO2-water, SiO2-water and Al2O3-water nanofluids with volume fractions of 0.1%, 0.2% and 0.3% were experimentally and numerically studied under laminar and transitional flow in the backward-facing step microchannel. The height of microchannel steps is 450 μm, 550 μm and 650 μm respectively. The upstream length is 80 mm and the downstream length is 180 mm. The results showed that: (1) nanofluids are more effective in reducing the temperature of the bottom surface downstream of the microchannel, the downstream bottom temperature tends to decrease; (2) the position of the reattachment point moves backward with the increase of Re number and step height, and the Nu number of the bottom of the step increases with the increase of Re as well as the peak value of Nu appears at the reattachment point and increase with step height increasing for laminar flow; (3) For transition flow, when Re is constant, the pressure drop of nanofluid increases with the increased concentration, the flow resistance coefficient increases with the increase of step height and decreases with the increase of Re. The volume fraction of nanofluid is an important factor affecting heat transfer performance. The larger the volume fraction is, the greater the heat transfer coefficient will be. At the same time, increasing the step height can enhance the heat transfer effect, but also increase the flow resistance.

Networked control for linear systems with forward and backward channels in presence of data transmission delays, consecutive packet dropouts and disordering

This paper investigates networked control for discrete-time systems with communication imperfections including data transmission time-varying delays, consecutive packet dropouts and packet disordering. The system states, which are measured by a sensor with periodic sampling period, will be directly transmitted to a sender and further delivered to a remote controller with aperiodic sending time-intervals. Only when the controller receives valid state information, the control signals are calculated and sent to the actuator. Because the data transmission imperfections are simultaneously allowed in the forward and backward communication channels (i.e., sender-to-controller and controller-to-actuator networks), the data transmission procedure from the sensor to the actuator is analyzed in details and the actuator received control command is presented in a time-delay description. Meanwhile, the qualitative relationships between the non-periodical sampling period, time-delay bound, consecutive packet dropout number and disordering are given as well. A delay-independent Lyapunov functional is employed while the sufficient delay-dependent stability condition is obtained. By the proposed method, not only system asymptotical stability of the closed-loop system can be achieved but also the network imperfections can be tolerated. Finally, a numerical example is provided to demonstrate the effectiveness of the proposed method.

Assessing the Impact of Lockdown on Atmospheric Ozone Pollution Amid the First Half of 2020 in Shenyang, China.

During the eruption of COVID-19, a citywide lockdown was executed from 26 January to 23 March 2020, in Shenyang, in which the ozone pollution has recorded significant variations. This paper mainly anatomized the comprehensive characteristics and evolution trends of ozone pollution based on the lockdown period in the first half of 2020. Using the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model and cluster analysis method to establish backward trajectories and channels, the spatial transport process of ozone in the preset period and the causation of typical ozone pollution events were investigated in depth. The results demonstrated that: The ozone concentration pollution in the first half of 2020 was increased than last year. During the lockdown period, the basic pollutants levels were lower than that in pre-lockdown under different proportions, except O3 maximum 8-h moving average (MDA8) was increased by 69.7%, accompanied by the delay of daily peak value, increased pollution days and longer pollution cycle. The typical pollution events were highly consistent with the evolution path of fine aerosol compelled by extreme weather. The ozone concentration and the atmospheric oxidation capacity can be stably maintained at a low level when NO2 concentration remained at 50–70 μg/m3, no matter how much the AQI was. Meanwhile, ozone concentration in the downwind suburban was as low as the central city and soared in few stations amid post-lockdown, simultaneous the correlation between ozone and other pollutants converted from negative to positive. The trajectory indicated that the pollution sources during the lockdown and pre-lockdown were basically Southern Russia, Inner Mongolia, and the three provinces of Northeast China, the pollution from the Bohai Sea provoked ozone pollutants in Shenyang to rebound briskly amid post-lockdown, the pollution of neighboring countries and areas would have a stronger impact on air quality under the effect of lockdown.

State estimation and stabilisation of nonlinear networked control systems with mixed time-delay

ABSTRACT In this paper, the problems of time-delay, packet loss and quantisation for network control with global Lipschitz nonlinearity are investigated. The packet losses between the forward and backward channels are considered, and two independent Bernoulli distributions are modelled respectively. An appropriate compensation strategy is proposed to reduce the buffeting and response time of the system. A controller based on differential proportional observer is proposed. The information of estimation errors is used to optimise state estimations. Finally, a simulation example illustrates the feasibility of the method, and the comparisons with existing discrete time-delay system and distributed time-delay system prove the superiority of the method.

Reciprocity Calibration of TDD MIMO Channel for Interference Alignment

Interference alignment (IA) is a promising technique to boost the Multiple-input Multiple-output (MIMO) network throughput. Concurrent data transmission links between multiple pairs of transmitters (TX) and receivers (RX) can be established in the same spectrum via cooperation. To make the channel state information (CSI) available on the transmitter side, channel reciprocity in time division duplex (TDD) networks is assumed by a large number of distributed IA algorithms. The physical channel is reciprocal, but nevertheless the channel considered by IA is not, because it also involves the radio frequency (RF) front ends of the transceiver, which are different circuits for transmit and receive modes individually. The difference between the two circuits yields mismatch between the forward and backward channels. Given the mutual coupling effect of the antenna array, the channel's non-symmetry is further complicated. In this paper, we provide the modeling of the non-reciprocity for the TDD MIMO channels and propose a corresponding calibration method to minimize the mismatch. Experimental validation has been conducted with classic distributed IA algorithms. The results highlight the indispensability of the reciprocity calibration for approaching the network capacity gain promised by IA.

FREQUENCY-INDEPENDENT LIMITS OF VALUES OF SYSTEM PARAMETERS OF CELLULAR COMMUNICATIONS AT MULTIPATH PROPAGATION OF RADIO WAVES IN URBAN AREA

The analysis was carried out in order to clarify the estimates of the expected restrictions on characteristics of new-generation mobile communications (4G, 5G) under the existing restrictions on radiated power of subscriber radio equipment, as well as in conditions where the boundaries of urban sites are outside of the area of free radio waves propagation (RWP) between subscriber and base stations. Analysis was performed using the basic principles and ensemble models of statistical theory of electromagnetic environment, as well as the basic principles of information theory that determines the radio channel capacity in presence of interference. Frequency-independent relationships have been obtained for estimating a number of system parameters of cellular communications under the conditions of multipath RWP in urban canyons and the presence of internal system interference: estimation the required equivalent isotropic radiated power (EIRP) of subscriber stations, the maximum data transmission capacity of the backward radio channel, the maximum distance of qualitative communication, and also the permissible level of internal radio interference at given requirements for communication range, and the information transfer rate of reverse radio channel taking into account the accepted restrictions on EIRP of subscriber radio equipment. The obtained relations allowus to estimate the limits of possible values of these system parameters of modern and future mobile communications; these relations also provide the opportunity to justify the quality requirements for ensuring the intra-system electromagnetic compatibility at the required data rate in backward radio channels of cellular networks and the existing restrictions on EIRP of subscriber stations.

Experimental Mitigation of Atmospheric Turbulence Effect Using Pre-Signal Combining for Uni- and Bi-Directional Free-Space Optical Links With Two 100-Gbit/s OAM-Multiplexed Channels

In this paper, we experimentally demonstrate the mitigation of atmospheric turbulence effects using phase patterns that apply the inverse transmission matrix for pre-compensation in the 200-Gbit/s free-space orbital angular momentum (OAM) multiplexed optical link. In this link, two OAM channels are multiplexed, each carrying a 100-Gbit/s quadrature-phase shift keying (QPSK) signal. In the uni-directional (forward) link, a combination of two OAM modes is generated using a designed phase pattern in each of the two channels. After the two channels are multiplexed, each OAM mode will carry the combination of signals from both channels. Such beams could perform the inverse function of turbulence-induced crosstalk. We explore the link performances including crosstalk and bit error rate (BER) under 6 different turbulence realizations with a Fried parameter of 1-mm. We find the following: (a) the inter-channel crosstalk is reduced by up to 21 dB; (b) the inter-channel crosstalk is below −10 dB for the 6 turbulence realizations when the compensation is applied; (c) a BER below the FEC limit could be realized for the 6 turbulence realizations with an optical signal-to-noise ratio (OSNR) of 16.8 dB for the uni-directional link with the compensation. Moreover, the post-compensation is also investigated for the two 100-Gbit/s OAM-multiplexed backward channels in the bi-directional link. For the backward channels, the phase patterns are used to mitigate turbulence-induced crosstalk at the receiver for backward channels, and the inter-channel crosstalk is reduced by up to 16 dB.

Modeling and Simulation of Newtonian Fluid Flow through Two-Dimensional Backward-Facing Step Channel with Finite Element�s Technique

Objectives: In this study, the behavior of laminar, incompressible and Newtonian fluid flow through the two dimensional backward facing step channels with the expansion ratio 1:2 is studied. Methods: The governing system of non-linear partial differential equations is worked out with the Galerkin least square finite element method. For computation purpose, the finite element based package COMSOL Multi-Physics 5.0 is used. Findings: The streamline pattern of the velocity field and pressure isobars is conferred at different Reynolds numbers. The minimum and maximum velocity magnitudes as well as pressure at the inlet and outlet including their differences are measured. Moreover, the reattachment length of the vortex organized at the corner of downstream of observed geometry is calculated and contrasted with experimental investigation of Armalyetal and CFD investigation of. Applications: Such fluid dynamics has frequently been used in industries including polymers, petro refineries, paints, biological systems etcetera. Keywords: Finite Element’s Technique, Newtonian Fluid Flow, Non-Linear Partial Differential Equations

Single-End Adaptive Optics Compensation for Emulated Turbulence in a Bi-Directional 10-Mbit/s per Channel Free-Space Quantum Communication Link Using Orbital-Angular-Momentum Encoding

A single-end adaptive-optics (AO) module is experimentally demonstrated to mitigate the emulated atmospheric turbulence effects in a bi-directional quantum communication link, which employs orbital angular momentum (OAM) for data encoding. A classical Gaussian beam is used as a probe to detect the turbulence-induced wavefront distortion in the forward direction of the link. Based on the detected wavefront distortion, an AO system located on one end of the link is used to simultaneously compensate for the forward and backward channels. Specifically, with emulated turbulence and when the probe is turned on, the mode purity of photons carrying OAM l=1 is improved by ~ 21 % with AO mitigation. We also measured the performance when encoding data using OAM {l=-1,+2} and {l=-2,+1} in the forward and backward channels, respectively, at 10 Mbit/s per channel with one photon per pulse on average. For this case, we found that the AO system could reduce the turbulence effects increased quantum-symbol-error-rate (QSER) by ~ 76 % and ~ 74 %, for both channels in the uni-directional and bi-directional cases, respectively. Similar QSER improvement is observed for the opposite direction channels in the bi-directional case.

Single-End Adaptive Optics Compensation for Emulated Turbulence in a Bi-Directional 10-Mbit/s per Channel Free-Space Quantum Communication Link Using Orbital-Angular-Momentum Encoding.

A single-end adaptive-optics (AO) module is experimentally demonstrated to mitigate the emulated atmospheric turbulence effects in a bi-directional quantum communication link, which employs orbital angular momentum (OAM) for data encoding. A classical Gaussian beam is used as a probe to detect the turbulence-induced wavefront distortion in the forward direction of the link. Based on the detected wavefront distortion, an AO system located on one end of the link is used to simultaneously compensate for the forward and backward channels. Specifically, with emulated turbulence and when the probe is turned on, the mode purity of photons carrying OAM ℓ = 1 is improved by ~ 21 % with AO mitigation. We also measured the performance when encoding data using OAM {ℓ = −1, + 2} and {ℓ = −2, + 1} in the forward and backward channels, respectively, at 10 Mbit/s per channel with one photon per pulse on average. For this case, we found that the AO system could reduce the turbulence effects increased quantum-symbol-error-rate (QSER) by ~ 76 % and ~ 74 %, for both channels in the uni-directional and bi-directional cases, respectively. Similar QSER improvement is observed for the opposite direction channels in the bi-directional case.

Stabilization of Linear Systems Over Networks With Limited Communication Capacity

In this paper, we discuss the modeling and control problem for networked systems with access constraints and packet dropouts. Due to the limitation of communication capacity, only the limited number of sensors and actuators are allowed to gain access to network medium according to a stochastic access protocol, and packet dropouts may happen in both backward and forward channels, which can be described by i.i.d Bernoulli processes. Under consideration of the access constraints and packet dropouts, the networked control system can be modelled as a Markov jump system. In such a framework, to solve the controller synthesis problem, the time-varying Kalman filter is first designed. Then, by using the theory of Markovian jump systems and dynamic programming, an optimal controller is designed such that the networked system is exponentially mean-square stable while minimizing the quadratic cost. Finally, two illustrative examples are given to demonstrate the effectiveness of the proposed results.

Improved Lower Bounds on Mutual Information Accounting for Nonlinear Signal–Noise Interaction

In fiber-optic communications, evaluation of mutual information (MI) is still an open issue due to the unavailability of an exact and mathematically tractable channel model. Traditionally, lower bounds on MI are computed by approximating the (original) channel with an auxiliary forward channel. In this paper, lower bounds are computed using an auxiliary backward channel, which has not been previously considered in the context of fiber-optic communications. Distributions obtained through two variations of the stochastic digital backpropagation (SDBP) algorithm are used as auxiliary backward channels and these bounds are compared with bounds obtained through the conventional digital backpropagation (DBP). Through simulations, higher information rates were achieved with SDBP, {which can be explained by the ability of SDBP to account for nonlinear signal--noise interactions

Wave-optics simulation of the double-pass beam propagation in modulating retro-reflector FSO systems using a corner cube reflector.

Free-space optical (FSO) communication utilizing a modulating retro-reflector (MRR) is an innovative way to convey information between the traditional optical transceiver and the semi-passive MRR unit that reflects optical signals. The reflected signals experience turbulence-induced fading in the double-pass channel, which is very different from that in the traditional single-pass FSO channel. In this paper, we consider the corner cube reflector (CCR) as the retro-reflective device in the MRR. A general geometrical model of the CCR is established based on the ray tracing method to describe the ray trajectory inside the CCR. This ray tracing model could treat the general case that the optical beam is obliquely incident on the hypotenuse surface of the CCR with the dihedral angle error and surface nonflatness. Then, we integrate this general CCR model into the wave-optics (WO) simulation to construct the double-pass beam propagation simulation. This double-pass simulation contains the forward propagation from the transceiver to the MRR through the atmosphere, the retro-reflection of the CCR, and the backward propagation from the MRR to the transceiver, which can be realized by a single-pass WO simulation, the ray tracing CCR model, and another single-pass WO simulation, respectively. To verify the proposed CCR model and double-pass WO simulation, the effective reflection area, the incremental phase, and the reflected beam spot on the transceiver plane of the CCR are analyzed, and the numerical results are in agreement with the previously published results. Finally, we use the double-pass WO simulation to investigate the double-pass channel in the MRR FSO systems. The histograms of the turbulence-induced fading in the forward and backward channels are obtained from the simulation data and are fitted by gamma-gamma (ΓΓ) distributions. As the two opposite channels are highly correlated, we model the double-pass channel fading by the product of two correlated ΓΓ random variables (RVs).

Learning in the machine: The symmetries of the deep learning channel

In a physical neural system, learning rules must be local both in space and time. In order for learning to occur, non-local information must be communicated to the deep synapses through a communication channel, the deep learning channel. We identify several possible architectures for this learning channel (Bidirectional, Conjoined, Twin, Distinct) and six symmetry challenges: (1) symmetry of architectures; (2) symmetry of weights; (3) symmetry of neurons; (4) symmetry of derivatives; (5) symmetry of processing; and (6) symmetry of learning rules. Random backpropagation (RBP) addresses the second and third symmetry, and some of its variations, such as skipped RBP (SRBP) address the first and the fourth symmetry. Here we address the last two desirable symmetries showing through simulations that they can be achieved and that the learning channel is particularly robust to symmetry variations. Specifically, random backpropagation and its variations can be performed with the same non-linear neurons used in the main input–output forward channel, and the connections in the learning channel can be adapted using the same algorithm used in the forward channel, removing the need for any specialized hardware in the learning channel. Finally, we provide mathematical results in simple cases showing that the learning equations in the forward and backward channels converge to fixed points, for almost any initial conditions. In symmetric architectures, if the weights in both channels are small at initialization, adaptation in both channels leads to weights that are essentially symmetric during and after learning. Biological connections are discussed.

Amplify-and-Forward Gateway With Zero-Forcing in Multiuser Environments

In this paper, we investigate and analyze the performance of amplify-and-forward (AF) gateways employing zero-forcing (ZF) beamforming techniques in a space division multiple access (SDMA) scenario. A random number of users are isolated from their destination(s), and the only means to communicate with their destination(s) is through the AF gateway equipped with multiple antennas and employing the ZF beamforming technique. All forward and backward channels experience small- and large-scale fading. New exact analytical expressions are derived for the overall spectral efficiency in the collaborative and noncollaborative SDMA scenarios. In the latter case, ZF is implemented at the gateway only, whereas with regard to the collaborative scenario, ZF is applied at both the sources and the destinations. The new results are used to investigate the impact of different system parameters on the overall efficiency of ZF AF gateways. The accuracy of the new results is confirmed through Monte Carlo simulations.

Bilateral Control of Functional Electrical Stimulation and Robotics-based Telerehabilitation.

Currently a telerehabilitation system includes a therapist and a patient where the therapist interacts with the patient, typically via a verbal and visual communication, for assessment and supervision of rehabilitation interventions. This mechanism often fails to provide physical assistance, which is a modus operandi during physical therapy or occupational therapy. Incorporating an actuation modality such as functional electrical stimulation (FES) or a robot at the patient's end that can be controlled by a therapist remotely, to provide therapy or to assess and measure rehabilitation outcomes can significantly transform current telerehabilitation technology. In this paper, a position-synchronization controller is derived for FES-based telerehabilitation to provide physical assistance that can be controlled remotely. The newly derived controller synchronizes an FES-driven human limb with a remote physical therapist's robotic manipulator despite constant bilateral communication delays. The control design overcomes a major stability analysis challenge: the unknown and unstructured nonlinearities in the FES-driven musculoskeletal dynamics. To address this challenge, the nonlinear muscle model was estimated through two neural networks functions that approximated unstructured nonlinearities and an adaptive control law for structured nonlinearities with online update laws. A Lyapunov-based stability analysis was used to prove the globally uniformly ultimately bounded tracking performance. The performance of the state synchronization controller was validated through experiments on an able-bodied subject. Specifically, we demonstrated bilateral control of FES-elicited leg extension and a human operated robotic manipulator. The controller was shown to effectively synchronize the system despite unknown and different delays in the forward and backward channels.

Secret-Key Agreement Over Non-Coherent Block-Fading Channels With Public Discussion

Motivated by recent interest in physical-layer secret-key generation over wireless fading channels, we study the non-coherent secret-key generation capacity of a block-fading wireless channel with channel reciprocity and bi-directional (two-way) communication. We assume a non-coherent main channel, i.e., the realization of channel gains on the main channel is not known to any terminal. The eavesdropper is assumed to have both perfect Channel State Information of its own channel and orthogonal observations from the forward and backward channels. As our main result, we establish new upper and lower bounds on the secret-key generation capacity with public discussion, which are structurally similar. The upper bound can be expressed as a sum of three terms—one of the terms arises due to channel reciprocity, while the other two terms correspond to communication on the forward and backward channels, respectively. In the limit of long coherence period, the contribution from channel reciprocity vanishes to zero, whereas the other terms prevail. The lower bound is based on a separation based scheme. In each coherence block, we use the first symbol for training while the remainder of the coherence block is devoted to source emulation, i.e., to generate correlated sources between the terminals. The lower bound expression also consists of three terms and admits an interpretation similar to the upper bound expression. For Rayleigh fading channels, in the high signal-to-noise ratio (SNR) regime, the gap between the upper and lower bounds is shown to vanish inversely with the coherence period. Numerical results indicate significant performance gains over training-only schemes even for moderate values of SNR and small coherence periods.