Assumption Of Synchronization Research Articles

A PAPR correction scheme for LoRa self‐jamming waveforms

AbstractA LoRa self‐jamming scheme that enables secure and covert communications by adding jamming symbols when transmitting is proposed. The scheme has been validated in simulations. The work is continued by evaluating the scheme on real‐world software defined radio (SDR) equipment. Experiments immediately showed that a demodulation is impossible as the received signal is highly distorted. This comes from the fact that adding symbols at transmission actually generates high peak‐to‐average power ratio (PAPR) preventing proper waveform generation by the equipment's hardware. This paper presents a PAPR correction scheme that modifies the waveform. The properties of this modified waveform are analysed and the performances are assessed in simulations with realistic time and frequency synchronization assumption. For the ideal 0 dB PAPR corrected output, the SER performances are close to that of a legacy LoRa communication, while preserving the physical layer security capabilities of the self‐jamming scheme. It is finally shown that our PAPR correction scheme effectively recovers the self‐jamming properties on SDR devices.

Stellar Spin-down in Post-mass-transfer Binary Systems

Motivated by measurements of the rotation speed of accretor stars in post-mass-transfer (post-MT) systems, we investigate how magnetic braking affects the spin-down of individual stars during binary evolution with the MESAbinary module. Unlike the conventional assumption of tidal synchronization coupled with magnetic braking in binaries, we first calculate whether tides are strong enough to synchronize the orbit. Subsequently, this influences the spin-down of stars and the orbital separation. In this study, we apply four magnetic braking prescriptions to reduce the spin angular momentum of the two stars throughout the entire binary evolution simulation. Our findings reveal that despite magnetic braking causing continuous spin-down of the accretor, when the donor begins to transfer material onto the accretor, the accretor can rapidly spin up to its critical rotation rate. After MT, magnetic braking becomes more important in affecting the angular momentum evolution of the stars. Post-MT accretor stars thus serve as a valuable test bed for observing how the magnetic braking prescriptions operate in spinning down stars from their critical rotation, including the saturation regimes of the magnetic braking. The rotation rate of the accretor star, combined with its mass, could provide age information since the cessation of MT. By comparing the models against observations, the magnetic braking prescription by Garraffo et al. is found to better align with the rotation data of post-MT accretors.

A Lack of Mass-gap Compact Object Binaries in APOGEE

Depending principally on mass, the compact object remnant left behind after a star’s life may be a white dwarf, neutron star (NS), or black hole (BH). While we have large samples of each of these remnants, we lack knowledge of the exact conditions separating these outcomes. The boundary between low-mass BHs and massive NSs is particularly poorly understood, as few objects between 2 and 5 M ⊙ are known. To probe this regime, we search the Apache Point Observatory Galactic Evolution Experiment 17th Data Release data set of 657,000 unique stars for binary systems with one stellar component that exhibit large radial velocity shifts over multiple observations. We identify 4751 likely binary systems, and estimate a minimum mass for each system’s “invisible companion” under the assumption of tidal synchronization. Two systems have companion masses ≳2 M ⊙, although we conclude that neither are good candidates for possessing a mass-gap NS or BH companions.

Asynchronous multiuser detection for sparse code multiple access with unknown delays: A compressed sensing approach

SummaryDespite being the subject of a growing body of research, nonorthogonal multiple access has failed to garner sufficient support to be included in modern standards. One of the more promising approaches to nonorthogonal multiple access is sparse code multiple access, which seeks to utilize nonorthogonal, sparse spreading codes to share bandwidth among users more efficiently than traditional orthogonal methods. Nearly all of the studies regarding sparse code multiple access assume synchronization at the receiver, which may not always be a practical assumption. In this work, we aim to bring this promising technology closer to a practical realization by dropping the assumption of synchronization. We therefore propose a compressed sensing‐based delay estimation technique developed specifically for an uplink sparse code multiple access system. The proposed technique can be used with nearly all of the numerous decoding algorithms proposed in the existing literature, including the popular message passing approach. Furthermore, we derive a theoretical bound regarding the recovery performance of the proposed technique and use simulations to demonstrate its viability in a practical uplink system.

LWSBFT: Leaderless weakly synchronous BFT protocol

Asynchronous or partially synchronous Byzantine fault-tolerant (BFT) protocols can tolerate up to one-third Byzantine faults, while synchronous Byzantine fault-tolerant protocols can tolerate up to one-half Byzantine faults. The existing synchronous Byzantine fault-tolerant protocols are leader-based protocols, which may lead to unbalanced load between nodes, and constitute a bottleneck that influences the performance of the whole system. In addition, the synchronous BFT protocols rely on a strong assumption: every message sent by an honest node will arrive at its destination within a known bounded time. In this work, in order to obtain better fairness and allow some degree of asynchrony while tolerating one-half faults, we designed LWSBFT, a leaderless BFT protocol in a weakly synchronous model where the synchronous assumption does not have to hold for all nodes all the time. The leaderless BFT protocol is comprised of the weakly synchronous Byzantine reliable broadcast (RBC) protocol and the weakly synchronous binary Byzantine agreement (BA) protocol, both of which can tolerate one-half faults. The weakly synchronous RBC protocol is used by each node to propose its input, and the weakly synchronous binary BA protocol is used to make a decision for each proposed value. The proposed leaderless BFT protocol can ensure both safety and liveness in a weakly synchronous model. We implement our consensus mechanism in Go, and evaluate the proposed consensus mechanism’s performance in terms of throughput and latency.

Precision, recall, and sensitivity of monitoring partially synchronous distributed programs

Distributed programs are often designed with implicit assumptions about the underlying system. We focus on assumptions related to clock synchronization. When a program written with clock synchronization assumptions is monitored to determine if it satisfies its requirements, the monitor should also account for these assumptions precisely. Otherwise, the monitor will either miss potential bugs (false negatives) or find bugs that are inconsistent with these assumptions (false positives). However, if assumptions made by the program are implicit or change over time and are not immediately available to the monitor, such false positives and/or negatives are unavoidable. This paper characterizes precision (probability that the violation identified by the monitor is valid) and recall (probability that the monitor identifies an actual violation) of the monitor based on the gap between clock synchronization assumptions made by the program/application and the clock synchronization assumptions made by the monitor. Our analysis is based on the development of an analytical model for precision, recall and sensitivity of monitors detecting conjunctive predicates. We validate the model via simulations and experiments on the Amazon Web Services platform.

Assessing the effectiveness of pulsing denial of service attacks under realistic network synchronization assumptions

Abstract Distributed Denial-of-Service (DDoS) is a big threat to the availability of Internet-based services today. Low rate DDoS attacks, especially pulsing attacks, aim to degrade the Quality of Service experienced by users by using only a small amount of attack traffic, unlike conventional volume-based DDoS attacks. To improve the effectiveness and stealthiness, these pulsing attacks assume that the attack packet is concentrated on the server in a very short time (a few milliseconds) using state-of-the-art synchronization techniques. However, even with the most advanced technology in the real world, it is almost impossible to achieve this tight level of synchronization, which means the effectiveness of the pulsing attack can be overestimated based on the exceeded assumption. In this paper, we use the Very Short Intermittent DDoS attack (VSI-DDoS) as an example to measure the practical effectiveness of a pulsing attack in a realistic environment. We found that VSI-DDoSbecame substantially less effective. That is, it lost 85.7% in terms of effectiveness under about 90ms synchronization inaccuracy, which is a very small inaccuracy under normal network conditions.

Standard Condition Number Based Spectrum Sensing Under Asynchronous Primary User Activity

Cognitive radio (CR) is a promising technology which enables the secondary user (SU) to sense and detect the presence or absence of the primary user (PU) in the frequency band of interest. Therefore, high detection probability is needed to ensure that primary user is adequately protected, while low false-alarm probability provides the opportunity of using free channel and a better throughput performance of secondary user. Most of the studies on spectrum sensing techniques were conducted assuming a perfect synchronization between the secondary and the primary users. However, in practice, the synchronous assumption is very hard to satisfy in the context of real cognitive radio networks environment. In this paper, we present a theoretical formulation of the standard condition number (SCN) based spectrum sensing technique under an asynchronous cognitive radio networks. We assume the case where the primary users generate asynchronous slotted traffic. The standard condition number distributions under null and alternative hypotheses are derived where the secondary user is equipped with two receive antennas. Under asynchronous primary user traffic, we establish the existence of a lower limit of the false-alarm probability below which we are unable to derive a decision threshold for the SCN-based detector.

Hierarchical Byzantine fault-tolerance protocol for permissioned blockchain systems

Emerging blockchain technology has introduced a new challenge to the distributed system research: Can Byzantine fault-tolerance protocols scale up to, for example, hundreds of nodes? In this work, we introduce HiBFT, a hierarchical Byzantine fault-tolerance protocol to address the problem. The core idea is to divide replicas into groups and exchange consensus messages among groups, thus avoiding the necessity of message broadcasting. The motivation for such approach bases on the hierarchical property of network architecture in permissioned block chains, our target deployments. HiBFT works very much in the same way as the classical Practical Byzantine Fault-Tolerance protocol. However, it replaces the concept of physical replica with a logical one that represents a replica group. As such, protocol message complexity can be reduced from $$O(N^2)$$ to $$O(s^2)$$ where N and s are the total number of replicas and the number of groups. Additionally, using threshold signature scheme for representing a logical group results in two important improvements: The cost of signature verification is significantly reduced at each replica; blocks can be secured more effectively in terms of signature size. Our protocol guarantees safety and liveness under partially synchronous assumption with a correctness proof. Our experiment results show that the protocol can scale up to hundred of nodes.

TDA-MAC: TDMA Without Clock Synchronization in Underwater Acoustic Networks

This paper investigates the application of underwater acoustic sensor networks for large scale monitoring of the ocean environment. The low propagation speed of acoustic signals presents a fundamental challenge in coordinating the access to the shared communication medium in such networks. In this paper, we propose two medium access control (MAC) protocols, namely, Transmit Delay Allocation MAC (TDA-MAC) and Accelerated TDA-MAC, that are capable of providing time division multiple access (TDMA) to sensor nodes without the need for centralized clock synchronization. A comprehensive simulation study of a network deployed on the sea bed shows that the proposed protocols are capable of closely matching the throughput and packet delay performance of ideal synchronized TDMA. The TDA-MAC protocols also significantly outperform T-Lohi, a classical contention-based MAC protocol for underwater acoustic networks, in terms of network throughput and, in many cases, end-to-end packet delay. Furthermore, the assumption of no clock synchronization among different devices in the network is a major advantage of TDA-MAC over other TDMA-based MAC protocols in the literature. Therefore, it is a feasible networking solution for real-world underwater sensor network deployments.

From Latency-Insensitive Design to Communication-Based System-Level Design

By the end of the 20th century, the continuous progress of the semiconductor industry brought a major transformation in the design of integrated circuits: as the speed of global wires could not keep up with the speed of ever-smaller transistors, the digital chip became a distributed system. This fact broke the synchronous paradigm assumption, i.e., the foundation of those computer-aided design (CAD) flows which had made possible three decades of unique technology progress: from chips with thousands of transistors to systems on chips (SoCs) with over a billion transistors. Latency-insensitive design (LID) is a correct-by-construction design methodology that was originally developed to address this challenge while preserving as much as possible the synchronous assumption. A broad new approach that transforms the fundamentals of how complex digital systems are assembled, LID introduces the protocols and shells paradigm, which offers several main benefits: modularity (by reconciling the synchronous paradigm with the dominant impact of global interconnect delays that characterizes nanometer technologies), scalability (by making key properties of the design be correct by construction through interface synthesis), flexibility (by simplifying the design and validation of a system through the separation of communication from computation), and efficiency (by enabling the reuse of predesigned components, thus reducing the overall design time). This paper overviews the principles and practice of LID, offers a retrospective on related research over the past decade, and looks ahead in proposing the protocols and shells paradigm as the foundation to bridge the gap between system-level and logic/physical design, a requisite to cope with the complexity of engineering future SoC platforms.

Kinematically coupled spacecraft relative motion without attitude synchronization assumption

Abstract A kinematically coupled spacecraft relative translational and rotational motion model without attitude synchronization assumption is derived. A kinematically coupled relative motion is described by combining the relative translational and rotational dynamics of arbitrarily selected points on the spacecraft. This kinematically coupled relative motion model is derived by extending the existing nonlinear relative translational dynamics to include the kinematic coupling effect due to feature points which are not the centers of the mass. The kinematically coupled relative motion is illustrated by two numerical simulation scenarios: spacecraft formation flying and final phase of rendezvous.

Adaptive RAKE receiver in chaos based pilot-added DS-CDMA system

This paper presents the theoretical analysis of adaptive multiuser RAKE receiver scheme in frequency selective fading channel for direct-sequence code division multiple access (DS-CDMA) system. Least mean square (LMS) algorithm is used to estimate the channel coefficients. Chaotic sequences are used as spreading sequence and corresponding bit error rate (BER) in closed form is derived for imperfect channel estimation conditions. Performances of chaotic sequences are compared with pseudorandom noise (PN) sequences. Under perfect synchronization assumption, various simulation results are shown to investigate the performance of the proposed system.

Performance Analysis of Adaptive Chaos Based CDMA System with Antenna Diversity in Frequency Selective Channel

This paper presents the theoretical analysis of the adaptive multiuser DS-CDMA system with antenna diversity in frequency selective channel environment. Pilot symbols are added to users' data to estimate the complex fading coefficients. Least mean square algorithm is used for estimating the channel coefficients. Performance of receiver antenna diversity using maximal ratio combining is investigated. Effect of channel estimation and intersymbol interference is studied analytically and probability of error in closed form is derived. Under the perfect synchronization assumption, simulation results show that the performance of system deteriorates with increase in number of multipaths components and can be improved using antenna diversity.

Cognitive radio networks with asynchronous spectrum sensing and access

Cognitive radio technology can significantly improve spectrum utilization efficiency by enabling SUs to access the licensed spectrum dynamically without causing excessive interference to PUs. Currently, most of the existing research on cognitive radio networks has been conducted based on an assumption that SUs can perfectly synchronize with PUs. However, when SUs are working with unknown behaviors of PUs, the synchronous assumption becomes invalid. To tackle this issue, we introduce several approaches for asynchronous spectrum sensing and asynchronous spectrum access, respectively, with which SUs can access the primary channels dynamically even without achieving synchronization with the PUs. In this article, we discuss the key techniques, corresponding solutions, and potential application scenarios of the asynchronous spectrum sensing and access schemes. Specifically, we present two asynchronous spectrum sensing schemes under non-cooperative and cooperative scenarios, respectively. Meanwhile, a distributed asynchronous spectrum access algorithm is also illustrated. Moreover, the proposed schemes are compared with existing works under the same framework, which enables readers to understand the motivation, problem formulation, methodology, and results of the asynchronous spectrum sensing and spectrum access schemes.

Jitter-Robust Spectral Shaping in OFDM

Timing jitter in OFDM systems results in non-ideal sampling and pulse-shaped transmission times and thus, is an important limiting factor for practical OFDM systems. In this paper, the effect of sampling jitter on the spectrum of a transmitted OFDM signal is considered. This effect in particular can lead to a significant degradation in the performance of spectral shaping techniques in OFDM, e.g., out-of-band radiation reduction techniques, that are mostly based on the assumption of ideal sampling times and accurate synchronization. In this work, we first evaluate the effect of random sampling time jitter on the performance of the active interference cancellation (AIC) technique, a well-known out-of-band radiation reduction technique for OFDM. Then, using a minimax approach, we propose a robust scheme that takes the jitter effect into consideration and therefore, makes the sidelobe suppression technique robust against worst-case random jitter. Numerical simulations show a performance improvement of almost 3 dB in interference reduction for our proposed robust scheme in the presence of timing jitter.

Wormhole Attack Detection Algorithms in Wireless Network Coding Systems

Network coding has been shown to be an effective approach to improve the wireless system performance. However, many security issues impede its wide deployment in practice. Besides the well-studied pollution attacks, there is another severe threat, that of wormhole attacks, which undermines the performance gain of network coding. Since the underlying characteristics of network coding systems are distinctly different from traditional wireless networks, the impact of wormhole attacks and countermeasures are generally unknown. In this paper, we quantify wormholes' devastating harmful impact on network coding system performance through experiments. We first propose a centralized algorithm to detect wormholes and show its correctness rigorously. For the distributed wireless network, we propose DAWN, a Distributed detection Algorithm against Wormhole in wireless Network coding systems, by exploring the change of the flow directions of the innovative packets caused by wormholes. We rigorously prove that DAWN guarantees a good lower bound of successful detection rate. We perform analysis on the resistance of DAWN against collusion attacks. We find that the robustness depends on the node density in the network, and prove a necessary condition to achieve collusion-resistance. DAWN does not rely on any location information, global synchronization assumptions or special hardware/middleware. It is only based on the local information that can be obtained from regular network coding protocols, and thus the overhead of our algorithms is tolerable. Extensive experimental results have verified the effectiveness and the efficiency of DAWN.

An Asynchronous Channel Hopping Sequence for Rendezvous Establishment in Self Organized Cognitive Radio Networks

A cognitive radio performs spectrum sensing, so that the device is able to self-organize appropriate networking functions. Rendezvous is the process by which the nodes explore the available channels to establish a link when they want to communicate with each other. Although using a dedicated control channel can be a solution to find the common control channel, it may result in a bottleneck and create a single point of failure. In this paper, we propose an asynchronous channel hopping sequences to establish a link in self organizing cognitive radio networks without the assumption of global clock synchronization. Our scheme provides successful communication rendezvous within a bounded time interval. Theoretical analysis and simulation results showed that our scheme outperforms the existing asynchronous schemes used for similar purpose in terms of many performance criteria.

Reevaluation of McCulloch–Pitts–von Neumann’s clock

In neuroscience research, a firing pattern expression, called a firing snapshot, defined by firing counts of multiple neurons in discrete time-bin is frequently used. However, the firing pattern expression over multiple neurons requires synchronization commonly known as a "central clock" in computation circuits to externally control or observe multi-neuron firing. Since it is not appropriate to hypothesize the central clock in the brain, we have to reconsider the synchronization assumption. We found the historical origins of central clock synchronization in the works of McCulloch, Pitts and von Neumann and propose an alternative perspective to study neural processing without the central clock. In the "First draft of a report on the EDVAC" known as the first proposal of store-program type computer, von Neumann incorporated McCulloch and Pitts' arbitrary assumption of constant synaptic delay into the logical circuit modeling. He introduced a constant pulse delay stabilized by a central clock to synchronize multiple pulse lines. This constant delay carried over to automaton theory by von Neumann and then theoretical neuroscience as an integral part of distributed representation and firing pattern analysis. Instead, we propose a rethinking of neuronal processing by focusing on a variable synaptic delay. We review neuron-type dependent differences and point out that the maximum inter-stimulus interval which can affect the last stimulus response is longer than the minimum inter-spike interval, indicating the possibility of asynchronous input summation without a firing rate level clock. This fundamental baseline contributes to the investigation of time consuming processes: e.g., the compression of appositional synaptic inputs with duration into a single firing.

Channel Hopping Sequences for Rendezvous Establishment in Cognitive Radio Sensor Networks

In Cognitive Radio sensor networks, a pair of Cognitive Radio (CR) users wishing to communicate should first agree on a specific channel called control channel. Communication rendezvous is used to establish a control channel between CR users. Channel Hopping (CH) protocol is a sequence-based approach that provides an effective method for implementing rendezvous without relying on a Common Control Channel (CCC). In this approach, all CR users hop to the same rendezvous channel and hence this channel will serve as a control channel between them. In this paper, we propose two Channel Hopping Sequences (CHS) to establish a link in CR sensor networks with and without the assumption of global clock synchronization. Our schemes provide successful communication rendezvous within a bounded time interval. Theoretical analysis and simulation results showed that both schemes outperform the existing schemes used for similar purpose in terms of many performance criteria.