Algorithms for Network Time Keeping

Abstract — This work describes and evaluates three algorithms for end-to-end time synchronization between a client computer and a server of “true ” time (e.g. a GPS source) using messages transmitted over packet switched networks, such as the Internet. The messages exchanged have the NTP format and the algorithms compared, are performed only at the client side. They are based on adaptive (Kalman) filtering, linear programming and statistical averaging and they are evaluated when the measurements are independent (gaussian case) or when they exhibit long-range dependence (self-similar case). Performance is evaluated according to the estimation error of frequency offset and time offset between client and server clock, the standard deviation of the estimates and the number of packets used for a specific estimation. The algorithms can exploit existing NTP infrastructure and a specific example is presented

Scatter Radio Receivers for Extended Range Environmental Sensing WSNs

Backscatter communication, relying on the reflection principle, constitutes a promising-enabling technology for lowcost, large-scale, ubiquitous sensor networking. This work makes an overview of the state-of-the-art coherent and noncoherent scatter radio receivers that account for the peculiar signal model consisting of several microwave and communication parameters

Sensitive and Nonlinear Far Field RF Energy Harvesting in Wireless Communications

This work studies both limited sensitivity and nonlinearity of far field RF energy harvesting observed in reality and quantifies their effect, attempting to fill a major hole in the simultaneous wireless information and power transfer (SWIPT) literature. RF harvested power is modeled as an arbitrary nonlinear, continuous, and non-decreasing function of received power, taking into account limited sensitivity and saturation effects. RF harvester's sensitivity may be several dBs worse than communications receiver's sensitivity, potentially rendering RF information signals useless for energy harvesting purposes. Given finite number of datapoint pairs of harvested (output) power and corresponding input power, a piecewise linear approximation is applied and the statistics of the harvested power are offered, as a function of the wireless channel fading statistics. Limited number of datapoints are needed and accuracy analysis is also provided. Case studies include duty-cycled (non-continuous), as well as continuous SWIPT, comparing with industry-level, RF harvesting. The proposed approximation, even though simple, offers accurate performance for all studied metrics. On the other hand, linear models or nonlinear-unlimited sensitivity harvesting models deviate from reality, especially in the low input power regime. The proposed methodology can be utilized in current and future SWIPT research

1.1.1 Antenna Sharing and User Cooperation in Wireless Communication 8

This thesis will study the issue of user cooperation and antenna sharing to improve wireless communication, network (autonomous) time keeping and network (autonomous) topology estimation. Content

Nonlinear Energy Harvesting Models in Wireless Information and Power Transfer

This work compares different linear and nonlinear RF energy harvesting models, including limited or unlimited sensitivity, for simultaneous wireless information and power transfer (SWIPT). The probability of successful SWIPT reception under a family of RF harvesting models is rigorously quantified, using state-of-the-art rectifiers in the context of commercial RFIDs. A significant portion of SWIPT literature uses oversimplified models that do not account for limited sensitivity or nonlinearity of the underlying harvesting circuitry. This work demonstrates that communications signals are not always appropriate for simultaneous energy transfer and concludes that for practical SWIPT studies, the inherent non-ideal characteristics of the harvester should be carefully taken into account; specific harvester's modeling methodology is also offered

Multistatic Scatter Radio Sensor Networks for Extended Coverage

Scatter radio, i.e., communication by means of reflection, has been recently proposed as a viable ultra-low power solution for wireless sensor networks (WSNs). This work offers a detailed comparison between monostatic and multistatic scatter radio architectures. In monostatic architecture, the reader consists of both the illuminating transmitter and the receiver of signals scattered back from the sensors. The multistatic architecture includes several ultra-low cost illuminating carrier emitters and a single reader. Maximum-likelihood coherent and noncoherent bit error rate (BER), diversity order, average information and energy outage probability comparison is performed, under dyadic Nakagami fading, filling a gap in the literature. It is found that: (i) diversity order, BER, and tag location-independent performance bounds of multistatic architecture outperform monostatic, (ii) energy outage due to radio frequency (RF) harvesting for passive tags, is less frequent in multistatic than monostatic architecture, and (iii) multistatic coverage is higher than monostatic. Furthermore, a proof-of-concept {digital} multistatic, scatter radio WSN with a single receiver, four low-cost emitters and multiple ambiently-powered, low-bitrate tags, perhaps the first of its kind, is experimentally demonstrated (at $13$ dBm transmission power), covering an area of $3500$ m$^2$. Research findings are applicable in the industries of WSNs, radio frequency identification (RFID), and emerging Internet-of-Things

Inference-Based Distributed Channel Allocation in Wireless Sensor Networks

Interference-aware resource allocation of time slots and frequency channels in single-antenna, halfduplex radio wireless sensor networks (WSN) is challenging. Devising distributed algorithms for such task further complicates the problem. This work studiesWSN joint time and frequency channel allocation for a given routing tree, such that: a) allocation is performed in a fully distributed way, i.e., information exchange is only performed among neighboring WSN terminals, within communication up to two hops, and b) detection of potential interfering terminals is simplified and can be practically realized. The algorithm imprints space, time, frequency and radio hardware constraints into a loopy factor graph and performs iterative message passing/ loopy belief propagation (BP) with randomized initial priors. Sufficient conditions for convergence to a valid solution are offered, for the first time in the literature, exploiting the structure of the proposed factor graph. Based on theoretical findings, modifications of BP are devised that i) accelerate convergence to a valid solution and ii) reduce computation cost. Simulations reveal promising throughput results of the proposed distributed algorithm, even though it utilizes simplified interfering terminals set detection. Future work could modify the constraints such that other disruptive wireless technologies (e.g., full-duplex radios or network coding) could be accommodated within the same inference framework

Multi-Antenna Channels Capacity Estimation Made Easy Towards the non-Rayleigh Channels Case

We attempt to discover closed form expressions to numerically evaluate the capacity of multi-antenna wireless channels. We start from known results for the Gaussian (Rayleigh) channel where the numerical evaluation of the capacity can be simplified by recent advances of random matrices theory and involves the probability distribution of a Wishart matrix. Our goal is to explore different than Rayleigh channels which as a result lead to non-Wishart capacity formulas. Tools based on zonal polynomials and their limitations will be explored

Inference-Based Resource Allocation for Multi-Cell Backscatter Sensor Networks

This work studies inference-based resource allocation in ultra low-power, large-scale backscatter sensor networks (BSNs). Several ultra-low cost and power sensor devices (tags) are illuminated by a carrier and reflect the measured information towards a wireless core that uses conventional Marconi radio technology. The development of multi-cell BSNs requires few multi-antenna cores and several low-cost scatter radio devices, targeting at maximum possible coverage. The average signal-to-interference-plus-noise ratio (SINR) of maximum-ratio combining (MRC) and zero-forcing (ZF) linear detectors is found and harnessed for frequency sub-channel allocation at tags, exploiting long-term SINR information. The resource allocation problem is formulated as an integer programming optimization problem and solved through the Max-Sum message-passing algorithm. The proposed algorithm is fully parallelizable and adheres to simple message-passing update rules, requiring mainly addition and comparison operations. In addition, the convergence to the optimal solution is attained within very few iteration steps. Judicious simulation study reveals that ZF detector is more suitable for large scale BSNs, capable to cancel out the intra-cell interference. It is also found that the proposed algorithm offers at least an order of magnitude decrease in execution time compared to conventional convex optimization methods

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Cooperative diversity has been recently proposed as a way to form virtual antenna arrays that provide dramatic gains in slow fading wireless environments. However, most of the proposed solutions require simultaneous relay transmissions at the same frequency bands, using distributed space-time coding algorithms. Careful design of distributed space-time coding for the relay channel is usually based on global knowledge of some network parameters or is usually left for future investigation, if there is more than one cooperative relay. We propose a novel scheme that eliminates the need for space-time coding and provides diversity gains on the order of the number of relays in the network. Our scheme first selects the best relay from a set of M available relays and then uses this ”best ” relay for cooperation between the source and the destination. Information theoretic analysis of outage probability shows that our scheme achieves the same diversity-multiplexing gain tradeoff as achieved by more complex protocols, where coordination and distributed spacetime coding for M relay nodes is required. Additionally, the proposed scheme increases the outage and ergodic capacity, compared to non-cooperative communication with increasin