Laboratory for Computer Communications and Its Applications

Laboratory

Related publications (98)

Beam Selection and Tracking for Amplify-and-Forward Repeaters

Andreas Peter Burg, Ruben Merz, Adrian Schumacher

Mobile network operators constantly have to upgrade their cellular network to satisfy the public's hunger for increasing data capacity. However, regulatory limits regarding allowed electromagnetic field strength on existing cell sites often limit or prevent the installation of new cells on additional frequency bands. Further densifying the radio access infrastructure by means of new cell sites takes time because of the site acquisition, the need for permissions, and the civil construction. To this end, amplify-and-forward repeaters are a cost-efficient method to provide mobile wireless data capacity to concealed areas outdoors, inside buildings, or vehicles, where low signal levels drastically limit the achievable capacity or even prevent communication with distant base stations. Repeaters with high-gain beamforming antennas can be used to decrease the path loss and thereby offer a higher capacity than user equipment otherwise could achieve. Since those highly directional beams must be constantly adjusted, we propose methods to align and track the beam even without having access to in-band beam control mechanisms. Numerical analyses with measurement data show that such non-invasive (system data-agnostic) methods are feasible and only incur a few percent in throughput reduction.

IEEE2022

Network Calculus Made Easy

Jean-Yves Le Boudec

We define general concepts that extend, and simplify, the network calculus developed by Cruz [1,2, 3] for guaranteed quality of service networks. We introduce a definition of service curve, the extended service curve, which is the same for isolated queues and for networks of queues, and makes it possible to model nodes that otherwise do not fit in the calculus of Cruz. We introduce two key technical tools, the

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operators, on arrival and service curves. We show how their systematic use simplifies the derivation of many fundamental results. We obtain a simple characterization of the output flow from a shaper. We show that, if the shaping and arrival curves are concave (which is the case in practice), then the output of a shaper is still constrained by the same arrival curve as the input, in addition to being constrained by the shaper curve. Lastly, we define a general form of deterministic effective bandwidth that is particularly simple and efficient.

EPFL Departement d'Informatique (DI)1996

Increasing Cellular Network Energy Efficiency for Railway Corridors

Andreas Peter Burg, Ruben Merz, Adrian Schumacher

Modern trains act as Faraday cages making it challenging to provide high cellular data capacities to passengers. A solution is the deployment of linear cells along railway tracks, forming a cellular corridor. To provide a sufficiently high data capacity, many cell sites need to be installed at regular distances. However, such cellular corridors with high power sites in short distance intervals are not sustainable due to the infrastructure power consumption. To render railway connectivity more sustainable, we propose to deploy fewer high-power radio units with intermediate low-power support repeater nodes. We show that these repeaters consume only 5% of the energy of a regular cell site and help to maintain the same data capacity in the trains. In a further step, we introduce a sleep mode for the repeater nodes that enables autonomous solar powering and even eases installation because no cables to the relays are needed.

IEEE2022

Security Measures for Grids Against Rank-1 Undetectable Time-Synchronization Attacks

Marguerite Marie Nathalie Delcourt, Jean-Yves Le Boudec

Time-synchronization attacks on phasor measurement units (PMUs) pose a real threat to smart grids; it was shown that they are feasible in practice and that they can have a nonnegligible negative impact on state estimation, without triggering the bad data detection mechanisms. Previous works identified vulnerability conditions when targeted PMUs measure a single phasor. Yet, PMUs are capable of measuring several quantities. We present novel vulnerability conditions in the general case, where PMUs measure any number of phasors and can share the same time reference. One is a sufficient condition that does not depend on the measurement values. We propose a security requirement that prevents it and provide a greedy offline algorithm that enforces it. If this security requirement is satisfied, there is still a possibility that the grid can be attacked, although we conjecture that it is very unlikely. We identify two sufficient and necessary vulnerability conditions, which depend on the measurement values. For each, we provide a metric that shows the distance between the observed and vulnerability conditions. We recommend their monitoring for security. Numerical results on the IEEE-39 bus benchmark with real load profiles show that the measurements of a grid satisfying our security requirement are far from vulnerable.

IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC2022

Experimental Validation of the Real-Time Control of an Electric-Vehicle Charging Station

Sherif Alaa Salaheldin Fahmy, Jean-Yves Le Boudec, Mario Paolone, Roman Rudnik

The high penetration of electric vehicles (EVs) charging stations (CSs), together with the progressive availability of distributed energy resources, increases the risk of grid overloading and power-quality degradation. Real-time control has been advocated by the recent literature as an alternative to costly grid reinforcement. Performance of most of the existing control methods is assessed via simulations, and real-field validation is rarely performed. We present the experimental validation of a recently proposed real-time control method for EVs CS. This method works at sub-second scale and allows the CS to adapt to the rapidly changing state of the grid caused by highly volatile energy resources, such as photo-voltaic (PV) plants. The method fairly allocates the available power, proportional to the EVs needs, while minimizing the EVs battery wear due to frequent charging power variations. It tracks an aggregated power-setpoint that comes from the main grid controller by solving a mixed-integer optimization problem. Our main goal is to show that the method works in the field, i.e., it can control the charge of commercial EVs that are connected to a real grid through a CS. The field validation has two challenges. The first one is to study the real-time capabilities of the method and by analysing how fast it computes the control power-setpoints. The second one refers to the handling of the non-ideal response of EVs to the control power-setpoints due to implementation and reaction delays and inaccuracies. The experimental results demonstrate the performance of the method and show that it can be deployed in the field.

IEEE2021

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The paper contributes to improving the computational performance of controls of distributed energy resources (DERs) in distribution grids for efficient real-time control and short-term scheduling. The considered setting is a distribution grid with heterogeneous DERs controlled with model predictive control (MPC) to track a dispatch plan at its grid connection point (GCP) subject to DERs’ and grid’s constraints. The MPC control is first expressed as a quadratic programming (using linearized grid models) and, then, solved with several state-of-the-art alternating direction optimization methods: AMA, ADMM, AADMM (ADMM with adaptive penalty parameter) and their accelerated variants: FAMA, FAADMM, and FAADMM with restart rule. Performance is tested in terms of computational performance, constraints satisfaction, and optimality against the centralized MPC. The case studies are the CIGRE and IEEE benchmark grid for low- and medium voltage systems hosting different numbers of controllable DERs.

IEEE2021

Deficit Round-Robin (DRR) is a widespread scheduling algorithm that provides fair queueing with variable-length packets. Bounds on worst-case delays for DRR were found by Boyer et al., who used a rigorous network calculus approach and characterized the service obtained by one flow of interest by means of a convex strict service curve. These bounds do not make any assumptions on the interfering traffic hence are pessimistic when the interfering traffic is constrained by some arrival curves. For such cases, two improvements were proposed. The former, by Soni et al., uses a correction term derived from a semi-rigorous heuristic; unfortunately, these bounds are incorrect, as we show by exhibiting a counter-example. The latter, by Bouillard, rigorously derive convex strict service curves for DRR that account for the arrival curve constraints of the interfering traffic. In this paper, we improve on these results in two ways. First, we derive a non-convex strict service curve for DRR that improves on Boyer et al. when there is no arrival constraint on the interfering traffic. Second, we provide an iterative method to improve any strict service curve (including Bouillard's) when there are arrival constraints for the interfering traffic. As of today, our results provide the best-known worst-case delay bounds for DRR. They are obtained by using the method of the pseudo-inverse.

IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC2022

Compilation of real-time control strategies for heterogeneous resources at MV and LV

Plouton Grammatikos, Rahul Kumar Gupta, Mario Paolone, Antonio Zecchino

Description of deliverable and goal 1.1. Executive summary The core of this activity is to provide distribution system operators with tools for the operation of utility-scale distributed battery energy storage systems (BESSs) in order to optimize the integration of stochastic distributed generation. The main goal of this deliverable is to assess two possible strategies for the real-time control of a utility-scale BESS to follow a day-ahead computed dispatch plan. In particular, one solution is based on a grid-aware optimal power flow (OPF)-based control accounting for both grid and BESS operational constraints (thoroughly described in D1.4.4c) [1], whereas the second one is based on the COMMELEC (thoroughly described in D1.2.3c) [2], [3]. The goal of the first method is to achieve the real-time dispatch plan tracking using a grid-aware model predictive control (MPC) to determine the active and reactive power set-points of the BESS so that the aggregated power of all the resources connected to a medium voltage power grid contribution track the dispatch plan while obeying to BESS’s operational constraints as well as the grid’s ones. The grid constraints are modelled using the Augmented Relaxed OPF [4]. COMMELEC is a framework proposed in the literature ([2], [3]) for the real-time control of power grids. It uses a hierarchy of agents to compute explicit active and reactive power setpoints for the resources connected to the grid. Each resource is equipped with a resource agent (RA) whose job is to translate the internal state of the resource into a device-independent format (advertisement). The advertisements are collected by the grid agent (GA), which computes the optimal power setpoints that optimize a global objective. The global objective is the weighted sum of various objectives, including tracking a predetermined dispatch plan at the slack bus, minimizing grid’s nodal voltage deviations from the nominal value, limiting the line currents below the respective ampacities and achieving target internal states for the resources. The proposed control frameworks are validated by dispatching the operation of a 12kV/20MVA MV distribution network in Aigle, Switzerland (i.e. the REeL demonstrator) using a 1.5 MW/2.5 MWh BESS, which is controlled in real-time given the online grid state estimation enabled by the deployed distributed PMU-based sensing infrastructure.

SCCER-FURIES2021

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The goal of this deliverable is to study the operational limits of the electrical distribution feeder of Onnens (medium voltage) and Rolle (low voltage) that are expected to be used as demonstration sites for the Phase II of the SCCER-FURIES. Both networks belong to the assets of Romande Energie and are located in the south-west part of Switzerland in the Vaud canton. We first collect and analyze the load and distributed generation1 time series in both networks. This task is accomplished by integrating coarse data provided by Romande Energie (RE) with refined data available from the photovoltaic installations of the EPFL-DESL. Concerning the methodology, the operational limits of both networks are assessed using a Probabilistic Load Flow developed at the DESL and based on standard methods existing in the literature. As described in the report, the Onnens grid may face nodal over-voltage and lines congestions due to the massive injections coming from a single multi-MW PV plant located at the end of the Onnens medium voltage feeder. On the other hand, the low voltage grids in Rolle are robust against the planned PV installations, i.e., the grids will not suffer any security constraint violations. In what follows, we first briefly describe the methodology adopted to carry-out the study and then the analysis of the steady-state behavior of both grids.

2017

Improving Railway Track Coverage with mmWave Bridges: A Measurement Campaign

Andreas Peter Burg, Ruben Merz, Adrian Schumacher

Bringing cellular capacity into modern trains is challenging because they act as Faraday cages. Building a radio frequency (RF) corridor along the railway tracks ensures a high signal-to-noise ratio and limits handovers. However, building such RF corridors is difficult because of the administrative burden of excessive formalities to obtain construction permissions and costly because of the sheer number of base stations. Our contribution in this paper is an unconventional solution of mmWave fronthauled low-power out-of-band repeater nodes deployed in short intervals on existing masts between high-power macro cell sites. The paper demonstrates the feasibility of the concept with an extensive measurement campaign on a commercial railway line. The benefit of using many low-power nodes with low-gain antennas compared to a baseline with only high-gain macro antennas is discussed, and the coverage improvement is evaluated. Based on the measurement results, a simple path loss model is calibrated. This model allows evaluation of the potential of the mmWave repeater architecture to increase the macro cell inter-site distance and reduce deployment costs.

ASSOC COMPUTING MACHINERY2021

Network Calculus Made Easy

Jean-Yves Le Boudec

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EPFL Departement d'Informatique (DI)1996