Crystallisation and chain conformation of long chain n-alkanes.

Hydrocarbon chains are a basic component in a number of systems as diverse as biological membranes, phospholipids and polymers. A better understanding of the physical properties of n-alkane chains should provide a better understanding of these more complex systems. With this aim, vibrational spectroscopy has been extensively used. This technique, sensitive to molecular details, is the only one able to both identify and quantify conformational disorder present in paraffinic systems. To achieve this, methyl deformations have been widely used as "internal standards" for the normalisation of peak areas. However, in the case of n-alkanes with short chain length, such as n-C[44]H[90] for example, the infrared spectra recorded at liquid nitrogen temperature and reported here show the sensitivity of these latter peaks to the various crystal structures formed. Indeed, the main frequencies of the symmetric methyl bending mode were found between 1384 cm[-1] and 1368 cm[-1] as a function of the crystal form. Changes in the frequency of the first order of the L.A.M. present in the Raman spectra were also observed. At higher temperatures, non all-trans conformers, inferred from different infrared bands present in the wagging mode region, were found to be essentially placed at the end of the n-alkane chains. At the monoclinic phase transition, the concentration of end-gauche conformers, proportional to the area of the infrared band at 1342 cm[-1], increases abruptly. On the contrary, in the spectra recorded at liquid nitrogen temperature no such band is observed. We also studied the degree of disorder in two purely monodisperse long chain n-alkanes, namely n-C[198]H[398] and n-C[246]H[494]. The chain conformation as well as the tilt angle of the chains from the crystal surfaces were determined by means of low frequency Raman spectroscopy and S.A.X.S. measurements on solution-crystallised samples. The increase in the number of end-gauche conformers which was expected to occur with the increase of the tilt angle as a function of the temperature was not detected due to a perfecting of the crystals. Indeed, due to successive heating and cooling to -173°C, the concentration of non all-trans conformers was found to decrease within the crystals. Their numbers were found to be up to six times higher in n-C[198]H[398] crystallised in once folded form than when crystallised in extended form. The C-C stretching mode region of the spectra was used to identify the chain conformation and to estimate the length of the all-trans stem passing through the crystal layers at -173°C. The transition between once folded and extended form crystals was indicated by the presence of additional bands in this region at 1089 cm[-1], 1078 cm[-1] and 1064 cm[-1]. Some of those bands may be related to the fold itself. At the same time, a strong decrease of the intensity of the infrared bands present in the wagging mode region was observed.Finally, the triple layered structure proposed on the basis of X-ray measurements obtained from the crystals of a binary mixture of long chain n-alkanes, namely n-C[162]H[326] and n-C[246]H[494], was confirmed from the study of the C-C stretching mode region of the infrared spectra

Towards fundamental limits of bursty multi-user communications in wireless networks

International audienceConsidering an isolated wireless cell containing a high density of nodes, the fundamental limit can be defined as the maximal number of nodes the associate base station can serve under some system level constraints including maximal rate, reliability, latency and transmission power. This limit can be investigated in the downlink, modeled as a spatial continuum broadast channel (SCBC) as well as in the uplink modeled as a spatial continuum multiple access channel (SCMAC). In this short paper, we summarize the different steps towards the characterization of this fundamental limit, considering four figures of merit: energy efficiency, spectral efficiency, latency, reliabilty

Fundamental Limits of Non Orthogonal Multiple Access (NOMA) for the Massive Gaussian Broadcast Channel in Finite Block-Length

International audienceSuperposition coding (SC) has been known to be capacity achieving for the Gaussian memoryless broadcast channel for more than 30 years. However, SC regained interest in the context of non orthogonal multiple access (NOMA) in 5G. From an information theory point of view, SC is capacity achieving in the broadcast Gaussian channel, even when the number of users tends to infinity. But using SC has two drawbacks: decoders complexity increases drastically with the number of simultaneous receivers, and the latency is unbounded since SC is optimal only in the asymptotic regime. To evaluate these effects quantitatively in terms of fundamental limits, we introduce a finite time transmission constraint imposed at the base station and we evaluate fundamental trade-offs between the maximal number of superposed users, the coding block-length and the block error probability. The energy efficiency loss due to these constraints is evaluated analytically and by simulation. Orthogonal sharing appears to outperform SC for hard delay constraints (equivalent to short block-length) and in low spectral efficiency regime (below one bit per channel use). These results are obtained by the association of stochastic geometry and finite block-length information theory

Saddlepoint Approximations of Cumulative Distribution Functions of Sums of Random Vectors

In this report, a real-valued function that approximates the cumulative distribution function (CDF) of a finite sum of real-valued independent and identically distributed random vectors is presented. The approximation error is upper bounded and thus, as a byproduct, an upper bound and a lower bound on the CDF are obtained. Finally, it is observed that in the case of lattice and absolutely continuous random variables, the proposed approximation is identical to the saddlepoint approximation of the CDF.Dans ce rapport, une fonction qui approxime la fonction de répartition d'une somme de vecteurs aléatoires indépendants et identiquement distribués est présentée. L'erreur d'approximation est majorée, et par consequent, une borne supérieure et une borne inférieure sur la fonction de répartition sont obtenues. Finalement, pour des vecteurs aléatoires absolument continues ou lattices, l'approximation proposée est identique à l'approximation du point de selle de la fonction de répartition

Reduced Complexity MUD-MLSE Receiver for Partially-Overlapping WLAN-Like Interference

International audienceThe roll-out density of wireless local area networks (WLANs) has recently witnessed a dramatic increase and is currently reaching saturation levels. The frequency bands designated to WLANs do thus not suffice anymore to provide nonoverlapping, and hence interference-free, communication bands. A large body of research has been dedicated to a wide variety of optimum maximum likelihood sequence estimation (MLSE) and sub-optimum in-band interference mitigation techniques. Our contribution lies in a reduction of the state-space of a MLSE detector in the case of a desired WLAN receiver experiencing delayed interference from some other transmitters operating in partially overlapping spectral bands and over independent frequency-selecting block-fading channels. Based on the formulation of the optimum receiver, we derive a sub-optimum receiver of reduced complexity and demonstrate its satisfactory performance in the context of strong interference

A new application of smart walker for quantitative analysis of human walking

International audienceThis paper presents a new nonintrusive device for everyday gait analysis and health monitoring. The system is a standard rollator equipped with encoders and inertial sensors. The assisted walking of 25 healthy elderly and 23 young adults are compared to develop walking quality index. The subjects were asked to walk on a straight trajectory and an L-shaped trajectory respectively. The walking trajectory, which is missing in other gait analysis methods, is calculated based on the encoder data. The obtained trajectory and steps are compared with the results of a motion capture system. The gait analysis results show that new index obtained by using the walker measurements, and not available otherwise, are very discriminating, e.g., the elderly have larger lateral motion and maneuver area, smaller angular velocity during turning, their walking accuracy is lower and turning ability is weaker although they have almost the same walking velocity as the young people

Unsupervised Log-Likelihood Ratio Estimation for Short Packets in Impulsive Noise

International audiencelmpulsive noise, where large amplitudes arise with a relatively high probability, arises in many communication systems including interference in Low Power Wide Area Networks. A challenge in coping with impulsive noise, particularly alpha­stable models, is that tractable expressions for the log-likelihood ratio (LLR) are not available, which bas a large impact on soft­input decoding schemes, e.g., low-density parity-check (LDPC) packets. On the other band, constraints on packet length also mean that pilot signais are not available resulting in non­trivial approximation and parameter estimation problems for the LLR. In this paper, a new unsupervised parameter estimation algorithm is proposed for LLR approximation. In terms of the frame error rate (FER), this algorithm is shown to significantly outperform existing unsupervised estimation methods for short LDPC packets (on the order of 500 symbols), with nearly the same performance as when the parameters are perfectly known. The performance is also compared with an upper bound on the information-theoretic limit for the FER, which suggests that in impulsive noise further improvements require the use of an alternative code structure other than LDPC

Nouvelle borne atteignable de la probabilité d'erreur pour des transmissions en paquets courts

International audienc

Etude analytique des performances des systèmes radio-mobiles en présence d'évanouissements et d'effet de masque

Cette thèse a pour but l analyse des performances d une communication radio en présence d évanouissements et d effet de masque. Le masquage, du aux obstacles entre l émetteur et le récepteur, fait varier la probabilité d erreur moyenne ("Symbole Error Probability" SEP). L étude de la probabilité de coupure symbole (SEO "Symbol error Outage") définie comme la probabilité que le SEP moyen excède une valeur seuil est donc plus pertinent. La difficulté de son étude réside dans le fait qu une inversion du SEP moyen par rapport au SNR (Signal to Noise Ratio) est nécessaire. Par la méthode de Laplace nous donnons des approximations précises à tout SNR du SEP dans un canal de Nakagami-m et Rice dans le cas mono-antenne. Nous montrons ensuite que ces approximations sont inversibles par rapport au SNR et permettent d exprimer analytiquement le SEO en fonction du SEP cible. Nous intégrons ensuite le codage canal et dérivons des expressions analytiques de la probabilité de coupure paquet pour un codage bloc puis un codage convolutif à entrée décidée. Nous étendons l étude aux systèmes MIMO à codage bloc orthogonaux puis à multiplexage spatial où des formes exactes du SEO sont dérivées. Nous considérons le cas où des interférents sont présents. Nous dérivons une approximation précise et inversible de la probabilité d erreur lorsque les signaux se propagent à travers un canal à évanouissements rapides de Rayleigh. Enfin parce que la prise en compte de l interférence est un moyen avéré d augmenter la capacité des réseaux, nous étudions les performances de la détection multi-utilisateurs à maximum de vraisemblance (MUD-MLSE), dans un contexte WLAN à canaux recouvrant.This thesis focuses on analytical performance studies or radiocommunication systems in shadowing environments. The shadowing implie by the presence of large scale obstacles between transmitter and receiver, induces a change in the average symbol error probabillty (SEP). The symbol error outage (SEO), defined as the probability to observe a given average SEP over a fading channel in a shadowing environment, is hence a more adequate metric. The most challenging issue in the SEO study is that an inversion of the SEP with respect to the signal to noise (SNR) is needed. We have studied the single input single output (SISO) case first. Thanks to the Laplace method, we derive some new tight SEP approximations in Nakagami-m and Ricean fading channels. These new expressions are tight for a large SNR range which improves results available in the open literature. We prove thal our expressions are invertible w.r.t. the SNR, facilitating the SEO to be expressed in closed form as a function of the target SEP. We also derive the packet error outage (PEO) in closed form considering channel block and convolutional codes with hard decision decoding. We then extend this approach to multiple input multiple output (MIMO) systems. We derive closed form-expressions for the·SEO of·MIMO space-time block encoded-as·well as·spatial multiplexing systems. Finally, interfering environments are considered. A SEP invertible expression is derived when the signal of interest and the interfering one experience a Rayleigh fading channel. We also study the performance of an improved multi-user maximum likelihood sequence estimation algorithm in an interfering WLAN context.VILLEURBANNE-DOC'INSA LYON (692662301) / SudocSudocFranceF

Couche physique et antennes

Modélisation du canal radio * Interface radio * Interférences et partage des ressource