Coupled Feedback Loops Research Articles

Digital Finite Impulse Response Equalizer for Nonlinear Frequency Response Compensation in Wireless Communication

Signal distortion can occur when the gain or attenuation of a component changes non-linearly with frequency, which is referred to as nonlinear frequency response. Common communications components such as filters, amplifiers, and mixers can lead to nonlinear frequency responses, which can cause errors in transmitting and receiving. This article outlines the design and demonstration of a static and dynamic finite impulse response (FIR) digital equalizer circuit. Using predistortion topology with a coupled feedback loop, the adaptive Least-Mean Square (LMS) algorithm was implemented. The FIR filter was simulated in MATLAB and Vivado and then implemented onto an Eclypse Z7 Field Programmable Gate Array (FPGA) evaluation board. Simulations showed that the custom RTL module gave the same frequency response that was produced in MATLAB calculations. The filter was able to dynamically equalize the frequency responses of different nonlinear boards that were used as the devices under test (DUT). Measurements showed that the equalizer was able to compensate for system distortion from 0.2 to 0.8 Nyquist frequency. The phase response remained relatively linear across the band of interest, with a group delay flatness less than 10 ns.

Parity–Time-Symmetric Optoelectronic Oscillator Based on Stimulated Brillouin Scattering

Parity–time (PT) symmetry has been intensively exploited in optoelectronic oscillators (OEOs), enabling mode selection in mutually coupled feedback loops. However, two mutually coupled feedback loops are usually established to achieve a spatial PT-symmetric architecture, which increases system complexity and instability. Here, we propose and experimentally prove a PT-symmetric OEO based on stimulated Brillouin scattering (SBS). The PT symmetry is implemented in an overlapping spatial loop in which the gain and loss are realized based on SBS gain and SBS loss, respectively. By adjusting the pump power and the polarization states of the probe and pump lights, the gain and loss can be balanced, thus achieving stable single-mode oscillation. In experiment, a 9.66 GHz microwave signal with a side-mode suppression ratio of 41 dB is generated. At 10 kHz offset frequency, the single-sideband phase noise reaches as low as −120.1 dBc/Hz. This demonstration opens new avenues to enhance mode selectivity in an OEO and can potentially be integrated on a chip.

Analysing a common-source amplifier implemented with floating-gate transistors and coupled feedback loop

ABSTRACT The input stage of an operational or transconductance amplifier could be a common-source amplifier and it should be designed carefully, since it contributes in a significant way to the performance of the entire cell. In this document, we show the results of analysing a basic transconduction cell based on the multiple-input ffioating-gate transistor in a common-source amplifier configuration. A particular feature of the cell that we analyse is that it contains a feedback loop towards one of the coupled gates that serves to improve the linearity. This simple cell could be a input stage in more complex circuits. We present studies of linearity, gain, output resistance and distortion to be compared with an entirely MOS common-source amplifier. Experimental tests were conducted based on an implemented integrated circuit. In our comparison, we note that the cell based on the ffioating-gate transistor has a remarkable linearity in conditions in which the MOS version would not operate properly. In addition, we experimentally observe the phenomena predicted by the theory, such as gain and operation point manipulation, according to the values of the voltages at the other control gates. Thus, it is possible to improve the design of operational amplifiers, and other similar circuits, because they provide greater linearity and less distortion; however, we observed lower transconductance and an increase in the output resistance.

Spontaneous otoacoustic emissions are biomarkers for mice with tectorial membrane defects

Cochlear function depends on the operation of a coupled feedback loop, incorporating outer hair cells (OHCs), and structured to assure that inner hair cells (IHCs) convey frequency specific acoustic information to the brain, even at very low sound levels. Although our knowledge of OHC function and its contribution to cochlear amplification has expanded, the importance of the tectorial membrane (TM) to the processing of mechanical inputs has not been fully elucidated. In addition, there are a surprising number of genetic mutations that affect TM structure and that produce hearing loss in humans. By synthesizing old and new results obtained on several mouse mutants, we learned that animals with abnormal TMs are prone to generate spontaneous otoacoustic emissions (SOAE), which are uncommon in most wildtype laboratory animals. Because SOAEs are not produced in TM mutants or in humans when threshold shifts exceed approximately 25 dB, some degree of cochlear amplification is required. However, amplification by itself is not sufficient because normal mice are rarely spontaneous emitters. Since SOAEs reflect active cochlear operation, TM mutants are valuable for studying the oscillatory nature of the amplification process and the structures associated with its stabilization. Inasmuch as the mouse models were selected to mirror human auditory disorders, using SOAEs as a noninvasive clinical tool may assist the classification of individuals with genetic defects that influence the active mechanisms responsible for sensitivity and frequency selectivity, the hallmarks of mammalian hearing.

A Theoretical Approach to Coupling the Epithelial-Mesenchymal Transition (EMT) to Extracellular Matrix (ECM) Stiffness via LOXL2.

Simple SummaryEpithelial-mesenchymal transition (EMT) is a key process in cancer progression through which cells weaken their cell-cell adhesion and gain mobility and invasive traits. Besides chemical signaling, recent studies have established the connection of EMT to mechanical microenvironment, such as the stiffness of extracellular matrix (ECM). LOXL2 is representative of a family of enzymes that promotes fiber cross-linking in ECM. With increased cross-linking comes increased stiffness, which induces EMT that can, in turn, elevate LOXL2 levels. As such, a positive feedback loop among EMT, LOXL2, and ECM stiffness can be formed. We built a mathematical model on a core biochemical reaction network featuring this feedback loop, and showed how strongly it drives EMT. We also illustrated mechanistically how cross-linking connects with stiffness, using a mechanical model of collagen (a major component of ECM). Using this theoretical framework, we demonstrated the heterogeneity of LOXL2/stiffness and its implications on migrating cancer cells that could seed metastasis, the growth of secondary malignant tumors. This framework can inspire experimental studies of more fine-grained mechanotransduction and biomechanical heterogeneity in cancers.The epithelial-mesenchymal transition (EMT) plays a critical role in cancer progression, being responsible in many cases for the onset of the metastatic cascade and being integral in the ability of cells to resist drug treatment. Most studies of EMT focus on its induction via chemical signals such as TGF-β or Notch ligands, but it has become increasingly clear that biomechanical features of the microenvironment such as extracellular matrix (ECM) stiffness can be equally important. Here, we introduce a coupled feedback loop connecting stiffness to the EMT transcription factor ZEB1, which acts via increasing the secretion of LOXL2 that leads to increased cross-linking of collagen fibers in the ECM. This increased cross-linking can effectively increase ECM stiffness and increase ZEB1 levels, thus setting a positive feedback loop between ZEB1 and ECM stiffness. To investigate the impact of this non-cell-autonomous effect, we introduce a computational approach capable of connecting LOXL2 concentration to increased stiffness and thereby to higher ZEB1 levels. Our results indicate that this positive feedback loop, once activated, can effectively lock the cells in a mesenchymal state. The spatial-temporal heterogeneity of the LOXL2 concentration and thus the mechanical stiffness also has direct implications for migrating cells that attempt to escape the primary tumor.

Coupled Feedback Loops Involving PAGE4, EMT and Notch Signaling Can Give Rise to Non-genetic Heterogeneity in Prostate Cancer Cells.

Non-genetic heterogeneity is emerging as a crucial factor underlying therapy resistance in multiple cancers. However, the design principles of regulatory networks underlying non-genetic heterogeneity in cancer remain poorly understood. Here, we investigate the coupled dynamics of feedback loops involving (a) oscillations in androgen receptor (AR) signaling mediated through an intrinsically disordered protein PAGE4, (b) multistability in epithelial–mesenchymal transition (EMT), and (c) Notch–Delta–Jagged signaling mediated cell-cell communication, each of which can generate non-genetic heterogeneity through multistability and/or oscillations. Our results show how different coupling strengths between AR and EMT signaling can lead to monostability, bistability, or oscillations in the levels of AR, as well as propagation of oscillations to EMT dynamics. These results reveal the emergent dynamics of coupled oscillatory and multi-stable systems and unravel mechanisms by which non-genetic heterogeneity in AR levels can be generated, which can act as a barrier to most existing therapies for prostate cancer patients.

Feedbacks between a non-Newtonian upper mantle, mantle viscosity structure and mantle dynamics

SUMMARY Previous studies have shown that a low viscosity upper mantle can impact the wavelength of mantle flow and the balance of plate driving to resisting forces. Those studies assumed that mantle viscosity is independent of mantle flow. We explore the potential that mantle flow is not only influenced by viscosity but can also feedback and alter mantle viscosity structure owing to a non-Newtonian upper-mantle rheology. Our results indicate that the average viscosity of the upper mantle, and viscosity variations within it, are affected by the depth to which a non-Newtonian rheology holds. Changes in the wavelength of mantle flow, that occur when upper-mantle viscosity drops below a critical value, alter flow velocities which, in turn, alter mantle viscosity. Those changes also affect flow profiles in the mantle and the degree to which mantle flow drives the motion of a plate analogue above it. Enhanced upper-mantle flow, due to an increasing degree of non-Newtonian behaviour, decreases the ratio of upper- to lower-mantle viscosity. Whole layer mantle convection is maintained but upper- and lower-mantle flow take on different dynamic forms: fast and concentrated upper-mantle flow; slow and diffuse lower-mantle flow. Collectively, mantle viscosity, mantle flow wavelengths, upper- to lower-mantle velocities and the degree to which the mantle can drive plate motions become connected to one another through coupled feedback loops. Under this view of mantle dynamics, depth-variable mantle viscosity is an emergent flow feature that both affects and is affected by the configuration of mantle and plate flow.

Glomerular filtration and podocyte tensional homeostasis: importance of the minor type IV collagen network.

The minor type IV collagen chain, which is a significant component of the glomerular basement membrane in healthy individuals, is known to assemble into large structures (supercoils) that may contribute to the mechanical stability of the collagen network and the glomerular basement membrane as a whole. The absence of the minor chain, as in Alport syndrome, leads to glomerular capillary demise and eventually to kidney failure. An important consideration in this problem is that the glomerular capillary wall must be strong enough to withstand the filtration pressure and porous enough to permit filtration at reasonable pressures. In this work, we propose a coupled feedback loop driven by filtration demand and tensional homeostasis of the podocytes forming the outer portion the glomerular capillary wall. Briefly, the deposition of new collagen increases the stiffness of basement membrane, helping to stress shield the podocytes, but the new collagen also decreases the permeability of the basement membrane, requiring an increase in capillary transmural pressure drop to maintain filtration; the resulting increased pressure outweighs the increased glomerular basement membrane stiffness and puts a net greater stress demand on the podocytes. This idea is explored by developing a multiscale simulation of the capillary wall, in which a macroscopic (μm-scale) continuum model is connected to a set of microscopic (nm-scale) fiber network models representing the collagen network and the podocyte cytoskeleton. The model considers two cases: healthy remodeling, in which the presence of the minor chain allows the collagen volume fraction to be increased by thickening fibers, and Alport syndrome remodeling, in which the absence of the minor chain allows collagen volume fraction to be increased only by adding new fibers to the network. The permeability of the network is calculated based on previous models of flow through a fiber network, and it is updated for different fiber radii and volume fractions. The analysis shows that the minor chain allows a homeostatic balance to be achieved in terms of both filtration and cell tension. Absent the minor chain, there is a fundamental change in the relation between the two effects, and the system becomes unstable. This result suggests that mechanobiological or mechanoregulatory therapies may be possible for Alport syndrome and other minor-chain collagen diseases of the kidney.

Frequency-Tunable Parity-Time-Symmetric Optoelectronic Oscillator Using a Polarization-Dependent Sagnac Loop

A widely frequency-tunable parity-time (PT) symmetric optoelectronic oscillator (OEO) implemented based on a single polarization-dependent Sagnac loop is proposed and experimentally demonstrated. Two mutually coupled feedback loops having an identical geometry with balanced gain and loss are implemented to achieve PT-symmetry, which is essential for single-mode selection without using an ultra-narrow passband microwave filter. In the implementation, a Sagnac loop consisting of a polarization beam splitter (PBS) and two polarization controllers (PCs) is employed which functions equivalently as two mutually coupled feedback loops, with the gain, loss and coupling coefficient being independently controllable. Since only a single physical loop is used, the implementation is greatly simplified and the stability is highly improved. The operation of the proposed OEO is experimentally verified. Stable single-mode oscillation with a wide frequency tunable range from 2 to 12 GHz is achieved. The phase noise is −128 dBc/Hz at an offset frequency of 10 kHz.

Maneuver Guidance and Formation Maintenance for Control of Leaderless Space-Robot Teams

A basic problem for a group of space-based robots is how to maintain a rigid formation during orbital maneuvers. To a leaderless team with homogeneous space-based robots, a multirobot controller architecture is proposed in which information flow is included. For each robot, there are two coupled feedback loops with different control objectives. One feedback loop is for guidance and is designed so that each robot can rapidly maneuver and rendezvous at the target point in a neighboring orbit. The other feedback loop is for formation control and is designed so that all robots in the group maintain a desired formation during a maneuver. A modified linear quadratic regulator method is used to design the guidance and control laws, where separate performance indexes are used for the two loops. By analyzing the interaction between the formation control and the maneuver guidance, a new restriction on the communication topology is proposed. An estimator for the state of the formation center is included in this control loop, and to guarantee a consensus on the formation center state estimates among all of the robots, a distributed Kalman filter based on the consensus algorithm is developed. Simulation results demonstrate the approach.

Coupled feedback loops maintain synaptic long-term potentiation: A computational model of PKMzeta synthesis and AMPA receptor trafficking.

In long-term potentiation (LTP), one of the most studied types of neural plasticity, synaptic strength is persistently increased in response to stimulation. Although a number of different proteins have been implicated in the sub-cellular molecular processes underlying induction and maintenance of LTP, the precise mechanisms remain unknown. A particular challenge is to demonstrate that a proposed molecular mechanism can provide the level of stability needed to maintain memories for months or longer, in spite of the fact that many of the participating molecules have much shorter life spans. Here we present a computational model that combines simulations of several biochemical reactions that have been suggested in the LTP literature and show that the resulting system does exhibit the required stability. At the core of the model are two interlinked feedback loops of molecular reactions, one involving the atypical protein kinase PKMζ and its messenger RNA, the other involving PKMζ and GluA2-containing AMPA receptors. We demonstrate that robust bistability–stable equilibria both in the synapse’s potentiated and unpotentiated states–can arise from a set of simple molecular reactions. The model is able to account for a wide range of empirical results, including induction and maintenance of late-phase LTP, cellular memory reconsolidation and the effects of different pharmaceutical interventions.

Association between Unintentional Interpersonal Postural Coordination Produced by Interpersonal Light Touch and the Intensity of Social Relationship.

Interpersonal postural coordination (IPC) produced by interpersonal light touch (ILT), whereby time-series variations in the postural sway between two people unintentionally resemble each other, may be a possible social interaction. From a sociopsychological standpoint, close mutual behavioral coordination is recognized as “social glue,” which represents the closeness of relationships and contributes to the building of a good rapport. Therefore, we hypothesized that if IPC functions as social glue, then IPC produced by ILT also represents a social relationship. Participants were dyadic pairs with a preexisting social relationship (acquaintance, friend, or best-friend), and we assessed the closeness between the partners. Postural sway in two quiet standing conditions—no touch (NT) and ILT (a mutual light touch with <1 N) condition—was concurrently measured with the side-by-side standing position, and the association of IPC with intradyadic closeness (rapport) was analyzed using hierarchical linear modeling. The results showed that unintentional IPC was higher in both axes of the ILT condition than in NT condition. Additionally, IPC in the mediolateral axis (the partner side) of the ILT condition was positively correlated with intradyadic closeness, whereas that in the anteroposterior axis (the non-partner side) showed a negative association. As expected, IPC represented intradyadic closeness (rapport). Results indicate that, in unintentional IPC produced by ILT, the priority of processing sensory feedback for postural control, which is received from the individual and a partner, is modulated depending on the rapport in interactional coupled feedback loops between the two individuals (i.e., good rapport increases the degree of taking in feedback from a partner). Thus, unintentional IPC produced by ILT functions as social glue, and it provides an understanding of the sociopsychological aspect in the human-to-human postural coordination mechanism.

Coupled feedback loops control the stimulus-dependent dynamics of the yeast transcription factor Msn2

Information about environmental stimuli often can be encoded by the dynamics of signaling molecules or transcription factors. In the yeast Saccharomyces cerevisiae, different types of stresses induce distinct nuclear translocation dynamics of the general stress-responsive transcription factor Msn2, but the underlying mechanisms remain unclear. Using deterministic and stochastic modeling, we reproduced in silico the different dynamic responses of Msn2 to glucose limitation and osmotic stress observed in vivo and found that a positive feedback loop on protein kinase A mediated by the AMP-activated protein kinase Snf1 is coupled with a negative feedback loop to generate the characteristic pulsatile dynamics of Msn2. The model predicted that the stimulus-specific positive feedback loop could be responsible for the difference between Msn2 dynamics induced by glucose limitation and osmotic stress. This prediction was further verified experimentally by time-lapse microscopic examinations of the snf1Δ strain. In this mutant lacking the Snf1-mediated positive feedback loop, Msn2 responds similarly to glucose limitation and osmotic stress, and its pulsatile translocation is largely abrogated. Our combined computational and experimental analysis reveals a regulatory mechanism by which cells can encode information about environmental cues into distinct signaling dynamics through stimulus-specific network architectures.

Maneuvering in Poor Visibility: How Firms Play the Ecosystem Game when Uncertainty is High

Innovation ecosystems are increasingly regarded as important vehicles to create and capture value from complex value propositions. While current literature assumes these value propositions can be known ex-ante and an appropriate ecosystem design derived from them, we focus instead on generative technological innovations that enable an unbounded range of potential value propositions, hence offering no clear guidance to firms. To illustrate our arguments, we inductively study two organizations, each attempting to create two novel ecosystems around new technological enablers deep in their industry architecture. We highlight how ecosystem creation in such conditions is a systemic process driven by coupled feedback loops, which organizations must try to control dynamically: firms first make the switch to creating the ecosystem following an external pull to narrow down the range of potential applications; then need to learn to keep up with ecosystem dynamics by roadmapping and preempting, while simultaneously enacting resonance. Dynamic control further entails counteracting the drifting away of the nascent ecosystem from the firm's idea of future value creation and the sliding of its intended control points for value capture. Our findings shed new light on strategy and control in emerging ecosystems, and provide guidance to managers on playing the ecosystem game.

Multilevel Organizational Adaptation: Scale Invariance in the Scottish Healthcare System

We use the case of a “whole-system” change program in a national healthcare system to empirically examine the multilevel dynamics underlying organizational adaptation. Our analysis demonstrates how the cognitive distance between agents’ causal representations affects opportunities to cooperate in hierarchical systems. Using complexity theory, we identify a scale-invariant causal pathway that can be applied recursively across many organizational levels. At each level, three coupled feedback loops determine how local agents modify their cognitive representations to include uncovered interdependencies and synchronize their adaptive search across organizational boundaries: a “boundary work” loop, a “small wins” loop, and a “parochialism” loop. Our results also point to the scale-dependency of the strength of dissipative processes across levels. These novel results further develop the theory of organizational change and have practical implications for large multilevel organizations, especially regarding the sustainability of improvements.

Signaling at Crossroads: The Dialogue between PDEs and PKA is Spoken in Multiple Languages

Signaling at Crossroads: The Dialogue between PDEs and PKA is Spoken in Multiple Languages

Birhythmicity and Hard Excitation from Coupled Synthetic Feedback Loops

Synthetic biology opens up the possibility of creating circuits that would not survive in the natural world and studying their behaviors in living cells, expanding our notion of biology. Based on this, we analyze on a synthetic biological system the effect of coupling between two instability-generating mechanisms. The systems considered are two topologically equivalent synthetic networks. In addition to simple periodic oscillations and stable steady state, the system can exhibit a variety of new modes of dynamic behavior: coexistence between two stable periodic regimes (birhythmicity) and coexistence of a stable periodic regime with a stable steady state (hard excitation). Birhythmicity and hard excitation have been proved to exist in biochemical networks. Through bifurcation analysis on these two synthetic cellular networks, we analyze the function of network structure for the collapse and revival of birhythmicity and hard excitation with the variation of parameters. The results have illustrated that the bifurcation space can be divided into four subspaces for which the dynamical behaviors of the system are generically distinct. Our analysis corroborates the results obtained by numerical simulation of the dynamics.

Accurate timekeeping is controlled by a cycling activator in Arabidopsis

Transcriptional feedback loops are key to circadian clock function in many organisms. Current models of the Arabidopsis circadian network consist of several coupled feedback loops composed almost exclusively of transcriptional repressors. Indeed, a central regulatory mechanism is the repression of evening-phased clock genes via the binding of morning-phased Myb-like repressors to evening element (EE) promoter motifs. We now demonstrate that a related Myb-like protein, REVEILLE8 (RVE8), is a direct transcriptional activator of EE-containing clock and output genes. Loss of RVE8 and its close homologs causes a delay and reduction in levels of evening-phased clock gene transcripts and significant lengthening of clock pace. Our data suggest a substantially revised model of the circadian oscillator, with a clock-regulated activator essential both for clock progression and control of clock outputs. Further, our work suggests that the plant clock consists of a highly interconnected, complex regulatory network rather than of coupled morning and evening feedback loops. DOI:http://dx.doi.org/10.7554/eLife.00473.001.

Stability and Hopf Bifurcation Analysis of Coupled Optoelectronic Feedback Loops

The dynamics of a coupled optoelectronic feedback loops are investigated. Depending on the coupling parameters and the feedback strength, the system exhibits synchronized asymptotically stable equilibrium and Hopf bifurcation. Employing the center manifold theorem and normal form method introduced by Hassard et al. (1981), we give an algorithm for determining the Hopf bifurcation properties.

Noise propagation in gene regulation networks involving interlinked positive and negative feedback loops.

It is well known that noise is inevitable in gene regulatory networks due to the low-copy numbers of molecules and local environmental fluctuations. The prediction of noise effects is a key issue in ensuring reliable transmission of information. Interlinked positive and negative feedback loops are essential signal transduction motifs in biological networks. Positive feedback loops are generally believed to induce a switch-like behavior, whereas negative feedback loops are thought to suppress noise effects. Here, by using the signal sensitivity (susceptibility) and noise amplification to quantify noise propagation, we analyze an abstract model of the Myc/E2F/MiR-17-92 network that is composed of a coupling between the E2F/Myc positive feedback loop and the E2F/Myc/miR-17-92 negative feedback loop. The role of the feedback loop on noise effects is found to depend on the dynamic properties of the system. When the system is in monostability or bistability with high protein concentrations, noise is consistently suppressed. However, the negative feedback loop reduces this suppression ability (or improves the noise propagation) and enhances signal sensitivity. In the case of excitability, bistability, or monostability, noise is enhanced at low protein concentrations. The negative feedback loop reduces this noise enhancement as well as the signal sensitivity. In all cases, the positive feedback loop acts contrary to the negative feedback loop. We also found that increasing the time scale of the protein module or decreasing the noise autocorrelation time can enhance noise suppression; however, the systems sensitivity remains unchanged. Taken together, our results suggest that the negative/positive feedback mechanisms in coupled feedback loop dynamically buffer noise effects rather than only suppressing or amplifying the noise.