Comparison of Correlation-Based OFDM Radar Receivers

Various correlation-based receivers have been proposed in passive bistatic and active monostatic radar exploiting information-bearing orthogonal frequency-division multiplexing (OFDM) transmissions, but too little has been dedicated to establishing their relations and advantages over each other. Accordingly, this paper provides an analytical comparison of the most commonly encountered filters, along with a performance analysis regarding three criteria: computational complexity, signal-to-interference-plus-noise-ratio and resilience to ground clutter. The last two especially assess the possible detrimental effects of the random sidelobes (or pedestal) induced by the data symbols in the range-Doppler map. Although simulations show that none of the filters performs unanimously better, the ones employing circular correlations globally evidence attractive results

Clairvoyant Clutter Mitigation in a Symbol-Based OFDM Radar Receiver

This paper investigates clutter rejection techniques in an OFDM symbol-based radar receiver. Two rejection filters that assume known the clutter covariance matrix are proposed. These aim at mitigating not only the clutter main peak but also its noise-like pedestal that leads to target masking issues. Performance is assessed with synthetic data on filters outputs and in terms of signal-to-clutter-plus-noise-ratio. Results show that the proposed methods succeed, to some extent, in uncovering exo-clutter targets. Rejecting clutter within the symbol-based architecture (instead of prior to) is advantageous for slowly-moving targets

Correlation-Based Radar Receivers with Pulse-Shaped OFDM Signals

In waveform sharing scenarios, various radar receivers have been developed for orthogonal frequency-division multiplexing (OFDM) signals. More general waveforms, such as pulse-shaped multicarrier modulations received little attention so far, despite their increased robustness to high-Doppler scatterers. In this paper, we compare the performance of two correlation-based radar receivers, namely the matched filter and the symbol-based technique, when used with different pulse-shaped multicarrier waveforms. We express the signal-to-interference-plus-noise-ratio in the range-Doppler map, taking into account the pedestal (or random sidelobes) induced by the symbols. Benefits of pulse shaping is further illustrated in a realistic vehicular scenario, in presence of multiple targets and ground clutter. In this context, the symbol-based approach outperforms the matched filter while enjoying a low-computational complexity. More generally, our results reveal the multicarrier pulse shape as a relevant degree of freedom in waveform co-design approaches (e.g., cognitive radar/communication systems)

Effect of Nitrogen on Properties of Na2O-CaO-SrO-ZnO-SiO2 Glasses

Glasses in the Na2O-CaO-SrO-ZnO-SiO2 system have previously been investigated for suitability as a reagent in Al-free glass polyalkenoate cements (GPCs). These materials have many properties that offer potential in orthopedics. However, their applicability has been limited, to date, because of their poor strength. This study was undertaken with the aim of increasing the mechanical properties of a series of these Zn-based GPC glasses by doping with nitrogen to give overall compositions of: 10Na2O-10CaO-20SrO-20ZnO-(40-3x)SiO2-xSi3N4 (x is the no. of moles of Si3N4). The density, glass-transition temperature, hardness, and elastic modulus of each glass were found to increase fairly linearly with nitrogen content. Indentation fracture resistance also increases with nitrogen content according to a power law relationship. These increases are consistent with the incorporation of N into the glass structure in threefold coordination with silicon resulting in extra cross-linking of the glass network. This was confirmed using 29Si MAS-NMR which showed that an increasing number of Q2 units and some Q3 units with extra bridging anions are formed as nitrogen content increases at the expense of Q1 units. A small proportion of Zn ions are found to be in tetrahedral coordination in the base oxide glass and the proportion of these increases with the presence of nitrogen

Mechanical properties, structure, bioactivity and cytotoxicity of bioactive Na-Ca-Si-PO-(N) glasses

peer-reviewedBioactive glasses are able to bond to bone through formation of carbonated hydroxyapatite in body fluids. However, because of their poor strength their use is restricted to non-load-bearing applications. The effects of nitrogen addition on the physical and mechanical properties and structure of bioactive oxynitride glasses in the system Na–Ca–Si–P–O–N have been studied. Glasses with compositions (mol.%): 29Na2O–13.5CaO–2.5P2O5–(55 −3x)SiO2–xSi3N4 (x is the no. of moles of Si3N4) were synthesised with up to 1.5 at% P and 4.1 at% N. A novel 3-step process was used for addition of P and N and this proved successful in minimising weight losses and producing homogeneous glasses with such high SiO2 contents. The substitution of 4.12 at% N for oxygen results in linear increases in density (1.6%), glass transition temperature (6%), hardness (18%) and Young’s modulus (74%). Vickers Indentation Fracture (VIF) resistance (Kifr) was calculated from various relationships depending on the load, indent diagonal, crack lengths and Young’s modulus to hardness (E/H) ratio. Firstly, Meyer’s index n is calculated from the slope of the logarithmic plot of load versus indent diagonal. Then by comparing the experimental slopes of the logarithmic plots of crack lengths versus load it is concluded that the cracking mode is Radial Median type. The substitution of 4.12 at% N for oxygen results in an increase in Kifr of 40%. These increases in properties are consistent with the incorporation of N into the glass structure in three-fold coordination with silicon which results in extra cross-linking of the glass network. The structure of these bioactive oxynitride glasses was investigated by solid state nuclear magnetic resonance (MAS NMR) of 31P and 29Si. The structure reveals that all the N atoms are bonded to Si atoms with the formation of SiO3N, SiO2N2 and Q4 structural units with extra bridging anions at the expense of Q3 units. The bioactivity of the glasses has been evaluated by soaking them in simulated body fluid (SBF) and results confirm that all these oxynitride glasses are bioactive. Cytotoxicity tests based on different concentrations of these bioactive glass powders in a cell growth environment have also shown that they are not cytotoxic

Apport de la RMN dans les matériaux vitreux: les bioverres et les verres de confinement, et dans les matériaux cimentaires

My research work concerns 2 main themes: i) the doping of different glasses and their structural analysis – either by phosphorus in this case the glasses are used as bone substitutes called bioglasses belonging to the SiO2-Na2O-CaO-P2O5 system to improve the bioactivity or by nitrogen to increase their mechanical properties, or used as containment glass in order to stabilize certain radioactive waste containing significant quantities of phosphorus and/or molybdenum, ii) structural analysis of cementitious materials also used for conditioning short and medium-lived radioactive waste. The structural analysis for these two materials was carried out thanks to nuclear magnetic resonance of the solid (NMR) which was an essential tool to characterize them given that they are in one case disorganized materials, and in the second case the products resulting from hydration are poorly crystallized compounds in the form of a gel. Thus, with regard to bioglasses, the role of phosphorus has been studied to understand the mechanism of bioactivity as well as the role of nitrogen in increasing physico-chemical properties. As for cementitious materials, two issues were studied : the impact of the increase in temperature on the chemical structure and therefore on certain physico-chemical properties, and the existence of interactions between the cementitious materials and the waste or adjuvants.Mes travaux de recherche concernent 2 thèmes principaux : i) le dopage de différents verres et leur analyse structurale – soit par le phosphore dans ce cas les verres sont utilisés comme substituts osseux appelés bioverres appartenant au système SiO2-Na2O-CaO-P2O5 pour améliorer la bioactivité ou par l’azote pour augmenter leurs propriétés mécaniques, soit utilisés comme verre de confinement afin de stabiliser certains déchets radioactifs contenant des quantités importantes de phosphore et/ou de molybdène, ii) l’analyse structurale de matériaux cimentaires utilisés aussi pour le conditionnement de déchets radioactifs à vie courte et moyenne. L’analyse structurale pour ces deux matériaux s’est effectuée grâce à la résonance magnétique nucléaire du solide (RMN) qui fut un outil indispensable pour les caractériser étant donné que ce sont dans un cas des matériaux désorganisés, et dans le second cas les produits issus de l’hydratation sont des composés mal cristallisés se présentant sous forme de gel. Ainsi à propos des bioverres le rôle du phosphore a été étudié pour comprendre le mécanisme de bioactivité ainsi que le rôle de l’azote dans l’augmentation des propriétés physico-chimiques. Quant aux matériaux cimentaires, deux problématiques ont été abordées : l’impact de l’augmentation de la température sur la structure chimique et donc sur certaines propriétés physico-chimiques, et l’existence d’interactions entre les matériaux cimentaires et les déchets ou les adjuvants

Réjection clairvoyante du fouillis de sol dans l'espace sous-bande d'un récepteur radar OFDM

Cette étude porte sur l'annulation du fouillis de sol au sein d'un récepteur radar OFDM monostatique symbol-based. Un filtre de réjection, qui suppose la matrice de covariance du fouillis connue, est mis en place dans l'espace sous-bande. Il vise à atténuer non seulement le pic principal du fouillis mais aussi son auto-interférence associée, qui affecte uniformément la carte distance-Doppler et masque possiblement des cibles. Nous simulons ici plusieurs scénarios de transmission et comparons les rapports signal sur fouillis-plus-bruit en sortie. L'impulsion localisée temps-fréquence est, par opposition à l'impulsion rectangulaire avec préfixe cyclique, plus appropriée si les cibles sont proches et rapides

Dense granules of the infectious stages of Toxoplasma gondii: Their central role in the host/parasite relationship.In The biology of Toxoplasma gondii,

(in press