Content-Aware User Clustering and Caching in Wireless Small Cell Networks

In this paper, the problem of content-aware user clustering and content caching in wireless small cell networks is studied. In particular, a service delay minimization problem is formulated, aiming at optimally caching contents at the small cell base stations (SCBSs). To solve the optimization problem, we decouple it into two interrelated subproblems. First, a clustering algorithm is proposed grouping users with similar content popularity to associate similar users to the same SCBS, when possible. Second, a reinforcement learning algorithm is proposed to enable each SCBS to learn the popularity distribution of contents requested by its group of users and optimize its caching strategy accordingly. Simulation results show that by correlating the different popularity patterns of different users, the proposed scheme is able to minimize the service delay by 42% and 27%, while achieving a higher offloading gain of up to 280% and 90%, respectively, compared to random caching and unclustered learning schemes.Comment: In the IEEE 11th International Symposium on Wireless Communication Systems (ISWCS) 201

Modèle de lessivage des matières en suspension en milieu urbain

Le présent travail porte sur le développement et la validation du modèle conceptuel RQSM (Runoff Quality Simulation Model). Le modèle RQSM a pour objectif de simuler le lessivage des matières en suspension. Le modèle RQSM considère que l’accumulation est infinie sur les surfaces perméables et imperméables. Il utilise l’énergie cinétique des précipitations pour décrire la mise en suspension des particules solides. Il modélise le transport des matières en suspension à l’aide de la théorie des systèmes linéaires.Les charges de matière en suspension mesurées durant 22 événements pluvieux enregistrés à l’exutoire du sous-bassin (1) de l’arrondissement de Verdun au Canada et à l’exutoire de trois sous-bassins de la ville de Champaign aux États‑Unis ont servi à valider le modèle RQSM. Les charges de matière en suspension simulées par le modèle RQSM ont été comparées aux charges mesurées et aux charges simulées par le modèle de « rating curve » et par le modèle de lessivage exponentiel du logiciel SWMM. Les charges simulées à l’aide du modèle RQSM étaient comparables aux charges mesurées. De plus, la performance du modèle RQSM a été jugée équivalente à celle des modèles exponentiel et « rating curve ». Une analyse de sensibilité menée sur le modèle RQSM a mis en lumière l’influence de chaque paramètre sur la charge simulée.Urban runoff pollution due to suspended particles is of great concern for sewer system managers. Indeed, particles are the main source of urban water pollution because a large amount of chemical elements and organic wastes are associated with particles. Modelling runoff pollution due to particles is essential for the evaluation of its impact on receiving waters and for the adequate design of treatment structures.This paper presents a new conceptual model called RQSM (Runoff Quality Simulation Model) used to simulate the washoff (erosion and transport of particles) of total suspended particles (TSS) at the outlet of urban catchments. Conceptual models can accurately represent complex physical phenomena with a limited number of parameters and a simple mathematical formulation. Moreover, they can be easily calibrated and used at many sites.The build-up of solid particles in the RQSM model is infinite for pervious and impervious areas. The erosion process is related to the rainfall kinetic energy. Two relationships involving two erosion parameters are used, one for the impervious areas and the other for the pervious areas. The transport of suspended particles is described using the linear system theory. A rectangular impulse response function with a duration equal to the time of concentration describes the catchment response to an instantaneous erosion rate. Thereafter, a convolution product between the erosion rate and the rectangular impulse response function is used to compute loads of TSS at the catchment outlet.The mathematical formulation of the RQSM model has three advantages. Firstly, it describes the washoff of TSS by using only rainfall intensity. Secondly, it treats the washoff of pervious and impervious areas separately. Finally, it uses only two parameters to describe the washoff.The RQSM model was validated with the help of 285 TSS data points monitored during 22 rainfall events. Four rainfall events were monitored in the subcatchment (1) of Verdun borough in Canada and 18 rainfall events were monitored on three subcatchments in Champaign city (USA). The catchment areas varied between 6.8 and 177 ha, the catchment slopes between 0.4% and 1%, and the ratio of directly drained impervious area between 0.19 and 0.58. Land use was residential in two catchments and commercial for the other ones.The first step in the RQSM model validation was to calibrate the two erosion parameters. This step was realized using 11 measured TSS pollutographs. The erosion parameter for impervious areas was calibrated first for low intensity rainfall. The erosion parameter for pervious areas was then calibrated for high intensity rainfall.The second validation step was to simulate the TSS load resulting from 11 other rainfalls. The simulated loads of the RQSM model were compared to the measured loads. They were also compared to the simulated loads obtained with the rating curve model and with the SWMM exponential washoff model. Comparisons were made using three performance criteria: the Nash criterion, the ratio of the simulated TSS mass divided by the measured TSS mass (RM), and the peak charge ratio (RP). Reasonable agreement was reached between loads computed with the RQSM model and measured loads. The mean RM was 1.08 and the standard deviation was 35%. Moreover, the Nash criterion was greater than 0.6 for 50% of simulated events. Unfortunately, the number of events was insufficient to detect statistically significant differences in accuracy, between the RQSM model, the rating curve and the exponential model. Nevertheless, a qualitative study based on the value of the three performance criteria showed that the results given by the RQSM model are equivalent to the results given by the SWMM exponential wash off model.Finally, a sensitivity analysis was conducted on the RQSM model. In order to achieve the sensitivity analysis, reference values were attributed to the parameters of the RQSM model. The values of the RQSM parameters were then changed individually and the resulting change in the Nash criterion was noted. The change in the Nash value indicates that for rainfalls of high intensity and short duration, the RQSM method is particularly sensitive to the time of concentration. In the other cases, the erosion parameter of impervious areas was the most sensitive parameter

La prévision en temps réel des charges de polluants dans un réseau d'assainissement urbain

L'objectif principal du présent travail est la prévision en temps réel des charges de polluants dans un réseau d'assainissement urbain. La méthodologie préconisée dans cette étude se base sur deux outils. En premier lieu, le modèle de la courbe de tarage a été utilisé afin d'exprimer la corrélation entre les charges de polluants et les débits de ruissellement. Ce modèle a été sélectionné en raison de sa simplicité et de la disponibilité des paramètres nécessaires pour sa mise en œuvre. L'hypothèse de synchronisme systématique entre les pointes de l'hydrogramme et du pollutogramme dans ce modèle constitue une des faiblesses que nous proposons de surmonter dans le cadre du présent travail. Ainsi, le modèle de la courbe de tarage a été modifié par l'introduction d'un terme de déphasage qu'on identifie en temps réel. D'autre part, la constance des paramètres mis en jeu dans le modèle classique de la courbe de tarage constitue un autre obstacle pour la reproductibilité des phénomènes au cours du même événement et d'un événement à l'autre. Afin de surmonter cette deuxième faiblesse, le filtre de Kalman a été utilisé pour identifier les paramètres d'un modèle dynamique en fonction des erreurs de prévision constatées à chaque pas de temps. La méthodologie a été testée avec succès sur le secteur I de la ville de Verdun du Québec. Le modèle établi a été validé à l'aide de trois critères de performance, à savoir, le coefficient de Nash, le rapport des pointes mesurées/prévues et leur déphasage. Selon ces critères, les résultats trouvés par le modèle dynamique concordent bien avec les mesures.It is normally unrealistic to send the total combined water volume generated during a rainfall event to a wastewater treatment plant and this approach is not retained as a viable solution when physical and economic constraints need to be accounted for. It becomes therefore pertinent to reduce the pollution from a given area by limiting water treatment to the most polluted portion of the runoff volume. For this purpose, various municipalities have expressed an urgent need for an automated system that could dynamically manage all the hydraulic components of their urban drainage basins. However, such a system of management in real time requires short-term forecasting of the water quality in the drainage basins. The main object of this work is the development of tools for the real-time forecasting of pollutant loads in an urban sewer network. The method used in this study is based on two tools: the rating curve model and the Kalman filter.The rating curve model is used to explain the correlation between pollutant loads and runoff. This model was selected because of its simplicity and the availability of the parameters necessary for its implementation. The rating curve model has several important characteristics. First of all, the formulation of the model is independent of the accumulation phase and the load accumulated over the basin is assumed to be unlimited. A second characteristic consists in the normalized form in which runoff is present in the model as a flow rate, so that the rating curve model can integrate the quantitative and qualitative aspects of urban runoff in a simple formulation, which requires parameters available in real time.The assumption of systematic overlap between the hydrograph and pollutograph peaks constitutes the main weakness of this model, which we propose to overcome within the framework of this work. Thus, the rating curve model was modified by the introduction of a lag term identified in real time. In order to define the time lag parameter in real time, a mobile window has been programmed to scan the two observation vectors of flow rates and loads. Theoretically speaking, the time lag corresponds to the maximum of the cross correlation function between flow rate and load vectors observed in real time. Three cases are therefore possible. In the first case, an increase of the pollutograph precedes that of the hydrograph and the time lag is positive. In this case and in a context of real-time management, loads are determined using a forecast model for flow rates. Measured flow rates are considered in this work as forecasted flow rates. If the hydrograph precedes the pollutograph, the time lag "d" is negative and the loads are related to the flow rate measured at an instant that precedes forecast time by "d" times the time step. When, finally, the two curves are perfectly synchronous, the "d" parameter is equal to zero and the flow rates are forecasted on the basis of the flow rates measured at the time of forecasting. The model is thus sufficiently flexible and adapted to the various foreseeable conditions.In addition, the constancy of the parameters concerned in the classic rating curve model constitutes another weakness with respect to the reproducibility of the phenomena during the same event and from one event to another. In order to overcome this second weakness, the Kalman filter was used to identify the parameters of a dynamic model according to the forecast errors noted with each time step. Use of the Kalman filter also allowed us to eliminate the calibration procedure required by the static model. With this filter, the dynamic model continuously readjusts its parameters to satisfy the non-stationary behaviour of hydrological phenomena.The methodology was tested successfully on the sector I of the town of Verdun (Quebec). The established model was validated using three performance criteria, namely, the Nash coefficient, the peak ratio and the lag between measured and forecasted values. According to these criteria, the results obtained with the dynamic model agree well with measurements

Electro-optical properties of an orthoconic liquid crystal mixture (W-182) and its molecular dynamics

We observed that the perfect dark state problem could be solved by using orthoconic antiferroelectric liquid crystal (OAFLC) instead of normal AFLC by comparing the properties of isocontrast and dispersion chromaticity of W-182 OAFLC and normal AFLC CS-4001. We electro-optically observed that several subphases such as SmCγ*, SmC*β, SmC*α and antiferroelectric SmI*A phases exist in W-182 OAFLC. We dielectrically observed in 4 μm thin cell that during heating, several new phases appeared. In the high temperature antiferroelectric region, a higher order than SmC* phase could be detected dielectrically, in the temperature range of 91–98 °C, behaving similar to SmCγ* and also, another phase below SmC* region could be dielectrically detected in the temperature range of 103–1100 °C, behaving similar to SmCα*, and an antiferroelectric, similar to SmIA* phase, was observed in the lower temperature region of the antiferroelectric phase; those are definitely arising due to surface force and interfacial charges interactions. We observed both PH and PL relaxation modes in both cells, although they differed in their strength and relaxation frequency. We studied extensively our observations of PH and PL modes in the antiferroelectric region, a Goldstone mode in the ferroelectric region and a soft mode in the ferroelectric region and SmA* phases