The universe as a quantum gravity condensate
Publication date: Available online 24 March 2017 Source:Comptes Rendus Physique Author(s): Daniele Oriti This is an introduction to the approach to the extraction of cosmological dynamics from full quantum gravity based on group field theory condensates. We outline its general perspective, which sees cosmology as the hydrodynamics of the fundamental quantum gravity degrees of freedom, as well as its concrete implementation within the group field theory formalism. We summarize recent work showing the emergence of a bouncing cosmological dynamics from a fundamental group field theory model, and provide a brief but complet...
Source: Comptes Rendus Physique - March 25, 2017 Category: Physics Source Type: research
Testing loop quantum cosmology
Publication date: Available online 16 March 2017 Source:Comptes Rendus Physique Author(s): Edward Wilson-Ewing Loop quantum cosmology predicts that quantum gravity effects resolve the big-bang singularity and replace it by a cosmic bounce. Furthermore, loop quantum cosmology can also modify the form of primordial cosmological perturbations, for example by reducing power at large scales in inflationary models or by suppressing the tensor-to-scalar ratio in the matter bounce scenario; these two effects are potential observational tests for loop quantum cosmology. In this article, I review these predictions and others, and...
Source: Comptes Rendus Physique - March 16, 2017 Category: Physics Source Type: research
Do we really understand the cosmos?
Publication date: Available online 24 February 2017 Source:Comptes Rendus Physique Author(s): Thanu Padmanabhan Our knowledge about the universe has increased tremendously in the last three decades or so — thanks to the progress in observations — but our understanding has improved very little. There are several fundamental questions about our universe for which we have no answers within the current, operationally very successful, approach to cosmology. Worse still, we do not even know how to address some of these issues within the conventional approach to cosmology. This fact suggests that we are missing some import...
Source: Comptes Rendus Physique - February 24, 2017 Category: Physics Source Type: research
Wavelet modulation: An alternative modulation with low energy consumption
Publication date: February 2017 Source:Comptes Rendus Physique, Volume 18, Issue 2 Author(s): Marwa Chafii, Jacques Palicot, Rémi Gribonval This paper presents wavelet modulation, based on the discrete wavelet transform, as an alternative modulation with low energy consumption. The transmitted signal has low envelope variations, which induces a good efficiency for the power amplifier. Wavelet modulation is analyzed and compared for different wavelet families with orthogonal frequency division multiplexing (OFDM) in terms of peak-to-average power ratio (PAPR), power spectral density (PSD) properties, and the impact of...
Source: Comptes Rendus Physique - January 19, 2017 Category: Physics Source Type: research
Energy and angular momentum transfers from an electromagnetic wave to a copper ring in the UHF band
Publication date: Available online 23 December 2016 Source:Comptes Rendus Physique Author(s): Olivier Émile, Christian Brousseau, Janine Émile, Kouroch Mahdjoubi Electromagnetic waves could carry orbital angular momentum. Such momentum can be transferred to macroscopic objects and can make them rotate under a constant torque. Based on experimental observations, we investigate the origin of orbital angular momentum and energy transfer. Due to angular momentum and energy conservation, we show that angular momentum transfer is due to the change in the sign of angular momentum upon reflection. This leads to a rotationa...
Source: Comptes Rendus Physique - December 23, 2016 Category: Physics Source Type: research
Electrostatic vibration energy harvester with 2.4-GHz Cockcroft –Walton rectenna start-up
Publication date: Available online 23 December 2016 Source:Comptes Rendus Physique Author(s): Hakim Takhedmit, Zied Saddi, Armine Karami, Philippe Basset, Laurent Cirio In this paper, we propose the design, fabrication and experiments of a macro-scale electrostatic vibration energy harvester (e-VEH), pre-charged wirelessly for the first time with a 2.4-GHz Cockcroft–Walton rectenna. The rectenna is designed and optimized to operate at low power densities and provide high voltage levels: 0.5 V at 0.76 μW/cm2 and 1 V at 1.53 μW/cm2. The e-VEH uses a Bennet doubler as a conditioning circuit. Experiments show a 23-V...
Source: Comptes Rendus Physique - December 23, 2016 Category: Physics Source Type: research
On energy harvesting for augmented tags
Publication date: Available online 23 December 2016 Source:Comptes Rendus Physique Author(s): Dahmane Allane, Yvan Duroc, Gianfranco Andia Vera, Rachida Touhami, Smail Tedjini In this paper, the harmonic signals generated by UHF RFID chips, usually considered as spurious effects and unused, are exploited. Indeed, the harmonic signals are harvested to feed a supplementary circuitry associated with a passive RFID tag. Two approaches are presented and compared. In the first one, the third-harmonic signal is combined with an external 2.45-GHz Wi-Fi signal. The integration is done in such a way that the composite signal ...
Source: Comptes Rendus Physique - December 23, 2016 Category: Physics Source Type: research
Multiband rectenna for microwave applications
Publication date: Available online 21 December 2016 Source:Comptes Rendus Physique Author(s): Abderrahim Okba, Alexandru Takacs, Hervé Aubert, Samuel Charlot, Pierre-François Calmon This paper reports a multiband rectenna (rectifier + antenna) suitable for the electromagnetic energy harvesting of the spill-over loss of microwave antennas placed on board of geostationary satellites. Such rectenna is used for powering autonomous wireless sensors for satellite health monitoring. The topology of the rectenna is presented. The experimental results demonstrate that the proposed compact rectenna can harvest efficiently t...
Source: Comptes Rendus Physique - December 21, 2016 Category: Physics Source Type: research
Analysis of the faster-than-Nyquist optimal linear multicarrier system
Publication date: Available online 19 December 2016 Source:Comptes Rendus Physique Author(s): Alexandre Marquet, Cyrille Siclet, Damien Roque Faster-than-Nyquist signalization enables a better spectral efficiency at the expense of an increased computational complexity. Regarding multicarrier communications, previous work mainly relied on the study of non-linear systems exploiting coding and/or equalization techniques, with no particular optimization of the linear part of the system. In this article, we analyze the performance of the optimal linear multicarrier system when used together with non-linear receiving struct...
Source: Comptes Rendus Physique - December 19, 2016 Category: Physics Source Type: research
Fundamental energetic limits of radio communication systems
Publication date: Available online 15 December 2016 Source:Comptes Rendus Physique Author(s): Jean-Yves Baudais The evaluation of the energy consumption of a radiocommunication requires to analyse the life cycle of the elements used. However, this analysis does not specify the energetic limits. Theoretical approaches allow one to draw these limits, which are known in multiple cases of information transmission. However, the answers are not always satisfactory, in particular in the case of time-varying channels. After a brief presentation of the notion of energetic limits of a radiocommunication, and beginning with a glob...
Source: Comptes Rendus Physique - December 15, 2016 Category: Physics Source Type: research
Performance of low-power RFID tags based on modulated backscattering
Publication date: Available online 13 December 2016 Source:Comptes Rendus Physique Author(s): Zeinab Mhanna, Alain Sibille, Richard Contreras Ultra Wideband (UWB) modulated backscattering (MBS) passive Radio-Frequency IDentification (RFID) systems provide a promising solution to overcome many limitations of current narrowband RFID devices. This work addresses the performance of such systems from the point of view of the radio channel between the readers and the tags. Such systems will likely combine several readers, in order to provide both the detection and localization of tags operating in MBS. Two successive measur...
Source: Comptes Rendus Physique - December 13, 2016 Category: Physics Source Type: research
Turbo-FSK, a physical layer for low-power wide-area networks: Analysis and optimization
Publication date: Available online 13 December 2016 Source:Comptes Rendus Physique Author(s): Yoann Roth, Jean-Baptiste Doré, Laurent Ros, Vincent Berg As the Internet-of-Things is becoming a reality, the need for a new Low-Power Wide-Area (LPWA) network emerged in the last few years. Numerous low-cost devices will be connected, and this requires an optimization of the link budget: the physical layer needs to be designed highly energy efficient. The combination of M-ary orthogonal Frequency-Shift-Keying (M-FSK) modulation and coding in the same process has been shown to be a promising candidate when associated with ...
Source: Comptes Rendus Physique - December 13, 2016 Category: Physics Source Type: research
Wireless energy transfer: Dielectric lens antennas for beam shaping in wireless power-transfer applications
Publication date: Available online 9 December 2016 Source:Comptes Rendus Physique Author(s): Ricardo Gonçalves, Nuno B. Carvalho, Pedro Pinho In the current contest of wireless systems, the last frontier remains the cut of the power cord. In that sense, the interest over wireless energy transfer technologies in the past years has grown exponentially. However, there are still many challenges to be overcome in order to enable wireless energy transfer full potential. One of the focus in the development of such systems is the design of very-high-gain, highly efficient, antennas that can compensate for the propagation los...
Source: Comptes Rendus Physique - December 8, 2016 Category: Physics Source Type: research
Theoretical and practical limits of superdirective antenna arrays
Publication date: Available online 5 December 2016 Source:Comptes Rendus Physique Author(s): Abdullah Haskou, Ala Sharaiha, Sylvain Collardey Some applications as Wireless Power Transfer (WPT) require compact and directive antennas. However, Electrically Small Antennas (ESAs) have low efficiencies and quasi-isotropic radiation patterns. Superdirective ESA arrays can be an interesting solution to cope with both constraints (the compactness and the directivity). In this paper, the theoretical and practical limits of superdirective antennas will be presented. These limits can be summarized by the directivity sensitivity ...
Source: Comptes Rendus Physique - December 5, 2016 Category: Physics Source Type: research
Revisiting thermal radiation in the near field
Publication date: Available online 28 November 2016 Source:Comptes Rendus Physique Author(s): Jean-Jacques Greffet Thermal radiation is generally assumed to be both spatially and temporally incoherent. In this paper, we challenge this idea. It is possible to design incandescent sources that are directional and spectrally selective by taking advantage of surface waves. We also report the discovery of the enhancement by several orders of magnitude of the energy density close to an interface at a particular frequency as well as the enhancement of the radiative flux between two interfaces when surface phonon polaritons can ...
Source: Comptes Rendus Physique - November 28, 2016 Category: Physics Source Type: research
