On the Trade-off Between Accuracy and Delay in Cooperative UWB Navigation

In ultra-wide bandwidth (UWB) cooperative navigation, nodes estimate their position by means of shared information. Such sharing has a direct impact on the position accuracy and medium access control (MAC) delay, which needs to be considered when designing UWB navigation systems. We investigate the interplay between UWB position accuracy and MAC delay for cooperative scenarios. We quantify this relation through fundamental lower bounds on position accuracy and MAC delay for arbitrary finite networks. Results show that the traditional ways to increase accuracy (e.g., increasing the number of anchors or the transmission power) as well as inter-node cooperation may lead to large MAC delays. We evaluate one method to mitigate these delays

On the Trade-Off Between Accuracy and Delay in Cooperative UWB Localization: Performance Bounds and Scaling Laws

Ultra-wide bandwidth (UWB) systems allow for accurate positioning in environments where global navigation satellite systems may fail, especially when complemented with cooperative processing. While cooperative UWB has led to centimeter-level accuracies, the communication overhead is often neglected. We quantify how accuracy and delay trade off in a wide variety of operation conditions. We also derive the asymptotic scaling of accuracy and delay, indicating that, in some conditions, standard cooperation offers the worst possible tradeoff. Both avenues lead to the same conclusion: indiscriminately targeting increased accuracy incurs a significant delay penalty. Simple countermeasures can be taken to reduce this penalty and obtain a meaningful accuracy/delay trade-off

Accuracy and Delay: An Inherent Trade-off in Cooperative UWB Navigation

Location-aware applications and wireless sensor networks are becoming essential in our daily lives from a commercial, and public perspectives. The need of localization information to drive the applications is a key requirement. New technologies have emerged to tackle the problem of the limitations of the Global Positioning System (GPS) solutions. Ultra-wideband (UWB) is one of those emerging RF-technologies. The thrive in search for better accuracy involves improved ranging algorithms, higher transmission powers, and the use of cooperation among nodes. The goal of this thesis is to investigate the trade-off between medium access control (MAC) delay and accuracy for UWB systems based on hands-on experience and practical implementation with state-of-the-art equipment, based on two-way-ranging and a spatial time division multiple access scheme (STDMA).Paper A investigates the connection between accuracy and MAC delay for noncooperative scenarios. We quantify, by means of lower bounds how traditional methods to improve accuracy such as increased number of anchors, and increased communication range comes at a significant cost in terms of delay. Techniques such as selective ranging and eavesdropping help alleviate the trade-off and reduce the MAC delay in favor of mobile networks with tolerable accuracies. Paper B extends the work for cooperative scenarios, where nodes cooperate with each other by means of shared information. This sharing has an impact not only on the position accuracy but also on the MAC delay which we quantify by means of lower bounds, both for the accuracy and MAC delay. Once again, selective ranging is evaluated to reduce the MAC delay for finite cooperative networks. We show how indiscriminate cooperation leads to large MAC delays, which has a direct impact on the update rate for high mobility scenarios. Finally, Paper C unifies all findings by including derivations of the accuracy and MAC delay lower bounds for noncooperative and cooperative networks, evaluating selective ranging and eavesdropping to cope with the tradeoff in different conditions. Numerical evaluations are included for several distinct operations. Furthermore, we characterize the trade-off behavior for dense-location aware networks for both noncooperative and cooperative cases by means of scaling laws. We conclude by introducing a delay/accuracy parameter which can uniquely quantify the trade off between accuracy and MAC delay as a function of the agent and anchor density. Noncooperative eavesdropping shows to outperform cooperative networks in terms of accuracy with reasonable delays. Finally, in terms of scaling, we found that, under certain conditions, standard cooperative positioning exhibits the worst possible trade-off among the considered strategies

On the trade-off between uncertainty and delay in UWB and 5G localization

Location-aware technologies in combination with emerging wireless communication systems\ua0have revolutionized many aspects of our daily lives by means of applications within\ua0the commercial, public and military sectors. Ultra-wideband (UWB) and 5G stand\ua0out as emerging radio frequency (RF) based technologies that tackle the limitations of\ua0Global Positioning System solutions. The thrive in search for better accuracy involves\ua0improved ranging algorithms, higher transmission powers, network densification, larger\ua0bandwidths, and the use of cooperation among nodes in the network. However, practical\ua0implementations introduce communication related constraints. In this thesis, we study\ua0the trade-off between localization accuracy and communication constraints in terms of\ua0delay. This trade-off is investigated and quantified for two of the most rapidly growing\ua0RF technologies for high precision positioning: UWB and 5G.In UWB, we investigate the trade-off between medium access control (MAC) delay and\ua0accuracy based on a two-way-ranging and a spatial time division multiple access scheme.\ua0We quantify this relationship by deriving lower bounds on localization accuracy and MAC\ua0delay during the measurements phase, which is often neglected in the analyses. We find\ua0that the traditional means to improve accuracy such as increased number of anchors,\ua0increased communication range, and cooperation among nodes, come at a significant cost\ua0in terms of delay, which can be mitigated by means of techniques such as selective ranging\ua0and eavesdropping. We summarize and generalize our findings by characterizing the\ua0position error and delay lower bounds by deriving asymptotic scaling laws. These scaling\ua0laws are presented for dense noncooperative and cooperative networks in combination\ua0with delay mitigation techniques. Moreover, we introduce a delay/accuracy trade-off\ua0parameter, which can uniquely quantify the trade-off as a function of the agent and\ua0anchor density. Finally, we consider the problem of fast link scheduling and propose an\ua0optimization strategy to perform robust ranging scheduling with localization constraints.\ua0We propose two MAC-aware link selection heuristic approximation approaches which\ua0show similar performance as the optimal solution, but alleviate the problem complexity.In 5G, we analyze the interplay between communication and positioning within the initial\ua0access procedure between a transmitter and a receiver in a millimeter-wave multipleinput\ua0multiple-output system. We exploit the ability of the receiver to determine its\ua0location during the beam selection process and thus, improve the subsequent selection\ua0of beams within initial access. First, assuming that only the transmitter has beamforming\ua0capabilities, we propose an in-band position-aided transmitter beam selection\ua0protocol for scenarios with direct line-of-sight and scattering. Then, we extend the work\ua0and propose an in-band position-aided beam selection protocol where we also allow for\ua0the receiver to perform beamforming in scenarios with line-of-sight, reflected paths, and\ua0possible beam alignment errors. Both protocols show similar performance compared to\ua0their conventional counterparts in terms of final achieved signal-to-noise ratio, but they\ua0are significantly faster and can additionally provide the position and orientation of the\ua0devices in an accurate manner

From the Characterization of Ranging Error to the Enhancement of Nodes Localization for Group of Wireless Body Area Networks

International audienceTime-based localization in Wireless Body Area Networks (WBANs), has attracted growing research interest for the last past years. Nodes positions can be estimated based on peer-to-peer radio transactions between devices. Indeed, the accuracy of the localization process could be highly affected by different factors , such as the WBAN channels where the signal is propagating through, as well as the nodes mobility that bias the peer-to-peer range estimation, and thus, the final achieved localization accuracy. The goal of this paper consists in characterizing the impact of mobility and WBAN channel on the ranging and localization estimation, based on real mobility traces acquired through a motion capture system. More specifically, the ranging error is evaluated over all the WBANs links (i.e. on-body, off-body and body-to-body links), while an impulse Radio Ultra-Wideband (IR-UWB) physical layer, as well as a TDMA-based Medium Access Control (MAC) are playing on. The simulation results show that the range measurement error can be modeled as a Gaussian distribution. To deal with the gaus-sianity observation of ranging error and to provide high positioning accuracy, an adjustable extended Kalman Filter (EKF) is proposed

Optimization of positioning capabilities in wireless sensor networks : from power efficiency to medium access

In Wireless Sensor Networks (WSN), the ability of sensor nodes to know its position is an enabler for a wide variety of applications for monitoring, control, and automation. Often, sensor data is meaningful only if its position can be determined. Many WSN are deployed indoors or in areas where Global Navigation Satellite System (GNSS) signal coverage is not available, and thus GNSS positioning cannot be guaranteed. In these scenarios, WSN may be relied upon to achieve a satisfactory degree of positioning accuracy. Typically, batteries power sensor nodes in WSN. These batteries are costly to replace. Therefore, power consumption is an important aspect, being performance and lifetime of WSN strongly relying on the ability to reduce it. It is crucial to design effective strategies to maximize battery lifetime. Optimization of power consumption can be made at different layers. For example, at the physical layer, power control and resource optimization may play an important role, as well as at higher layers through network topology and MAC protocols. The objective of this Thesis is to study the optimization of resources in WSN that are employed for positioning purposes, with the ultimate goal being the minimization of power consumption. We focus on anchor-based positioning, where a subset of the WSN nodes know their location (anchors) and send ranging signals to nodes with unknown position (targets) to assist them in estimating it through distance-related measurements. Two well known of such measurements are received signal strength (RSS) and time of arrival (TOA), in which this Thesis focuses. In order to minimize power consumption while providing a certain quality of positioning service, in this dissertation we research on the problems of power control and node selection. Aiming at a distributed implementation of the proposed techniques, we resort to the tools of non-cooperative game theory. First, transmit power allocation is addressed for RSS based ranging. Using game theory formulation, we develop a potential game leading to an iterated best response algorithm with sure convergence. As a performance metric, we introduce the geometric dilution of precision (GDOP), which is shown to help achieving a suitable geometry of the selected anchor nodes. The proposed scheme and relative distributed algorithms provide good equilibrium performance in both static and dynamic scenarios. Moreover, we present a distributed, low complexity implementation and analyze it in terms of computational complexity. Results show that performance close to that of exhaustive search is possible. We then address the transmit power allocation problem for TOA based ranging, also resorting to a game theoretic formulation. In this setup, and also considering GDOP as performance metric, a supermodular game formulation is proposed, along with a distributed algorithm with guaranteed convergence to a unique solution, based on iterated best response. We analyze the proposed algorithm in terms of the price of anarchy (PoA), that is, compared to a centralized optimum solution, and shown to have a moderate performance loss. Finally, this dissertation addresses the effect of different MAC protocols and topologies in the positioning performance. In this direction, we study the performance of mesh and cluster-tree topologies defined in WSN standards. Different topologies place different constraints in network connectivity, having a substantial impact on the performance of positioning algorithms. While mesh topology allows high connectivity with large energy consumption, cluster-tree topologies are more energy efficient but suffer from reduced connectivity and poor positioning performance. In order to improve the performance of cluster-tree topologies, we propose a cluster formation algorithm. It significantly improves connectivity with anchor nodes, achieving vastly improved positioning performance.En les xarxes de sensors sense fils (WSN), l'habilitat dels nodes sensors per conèixer la seva posició facilita una gran varietat d'aplicacions per la monitorització, el control i l'automatització. Així, les dades que proporciona un sensor tenen sentit només si la posició pot ésser determinada. Moltes WSN són desplegades en interiors o en àrees on la senyal de sistemes globals de navegació per satèl.lit (GNSS) no té prou cobertura, i per tant, el posicionament basat en GNSS no pot ésser garantitzat. En aquests escenaris, les WSN poden proporcionar una bona precisió en posicionament. Normalment, en WSN els nodes són alimentats amb bateries. Aquestes bateries són difícils de reemplaçar. Per tant, el consum de potència és un aspecte important i és crucial dissenyar estratègies efectives per maximitzar el temps de vida de la bateria. L'optimització del consum de potència pot ser fet a diferents capes del protocol. Per exemple, en la capa física, el control de potència i l'optimització dels recursos juguen un rol important, igualment que la topologia de xarxa i els protocols MAC en les capes més altes. L'objectiu d'aquesta tesi és estudiar l¿optimització de recursos en WSN que s'utilitzen per fer posicionament, amb el propòsit de minimitzar el consum de potència. Ens focalitzem en el posicionament basat en àncora, en el qual un conjunt de nodes coneixen la seva localització (nodes àncora) i envien missatges als nodes que no saben la seva posició per ajudar-los a estimar les seves coordenades amb mesures de distància. Dues classes de mesures són la potència de la senyal rebuda (RSS) i el temps d'arribada (TOA) en les quals aquesta tesi està focalitzada. Per minimitzar el consum de potència mentre que es proporciona suficient qualitat en el posicionament, en aquesta tesi estudiem els problemes de control de potència i selecció de nodes. Tenint en compte una implementació distribuïda de les tècniques proposades, utilitzem eïnes de teoria de jocs no cooperatius. Primer, l'assignació de potència transmesa és abordada pel càlcul de la distància amb RSS. Utilitzant la teoria de jocs, desenvolupem un joc potencial que convergeix amb un algoritme iteratiu basat en millor resposta (best response). Com a mètrica d'error, introduïm la dilució de la precisió geomètrica (GDOP) que mostra quant d'apropiada és la geometria dels nodes àncora seleccionats. L'esquema proposat i els algoritmes distribuïts proporcionen una bona resolució de l'equilibri en l'escenari estàtic i dinàmic. Altrament, presentem una implementació distribuïda i analitzem la seva complexitat computacional. Els resultats obtinguts són similars als obtinguts amb un algoritme de cerca exhaustiva. El problema d'assignació de la potència transmesa en el càlcul de la distància basat en TOA, també és tractat amb teoria de jocs. En aquest cas, considerant el GDOP com a mètrica d'error, proposem un joc supermodular juntament amb un algoritme distribuït basat en millor resposta amb convergència garantida cap a una única solució. Analitzem la solució proposada amb el preu de l'anarquia (PoA), és a dir, es compara la nostra solució amb una solució òptima centralitzada mostrant que les pèrdues són moderades. Finalment, aquesta tesi tracta l'efecte que causen diferents protocols MAC i topologies en el posicionament. En aquesta direcció, estudiem les topologies de malla i arbre formant clusters (cluster-tree) que estan definides als estàndards de les WSN. La diferència entre les topologies crea diferents restriccions en la connectivitat de la xarxa, afectant els resultats de posicionament. La topologia de malla permet una elevada connectivitat entre els nodes amb gran consum d'energia, mentre que les topologies d'arbre són més energèticament eficients però amb baixa connectivitat entre els nodes i baix rendiment pel posicionament. Per millorar la qualitat del posicionament en les topologies d'arbre, proposem un algoritme de formació de clústers.Postprint (published version