Section: New Results

Flexible Radio Front-End

Activities in this axis could globally be divided in two main topics: low-power wireless sensors (with applications in wearable devices, guided propagation for ventilation systems, and tag-to-tag RFID), and optimization of waveforms (for wake-up radio receivers and wireless power transfer).

Low-Power WSN

Wearable sensors for health monitoring can enable the early detection of various symptoms, and hence rapid remedial actions may be undertaken. In particular, the monitoring of cardiac events by using such wearable sensors can provide real-time and more relevant diagnosis of cardiac arrhythmia than classical solutions. However, such devices usually use batteries, which require regular recharging to ensure long-term measurements. In the framework of a local collaborative project, we therefore designed and evaluated a connected sensor for the ambulatory monitoring of cardiac events, which can be used as an autonomous device without the need of a battery. Even when using off-the-shelf, low-cost integrated circuits, by optimizing both the hardware and software embedded in the device, we were able to reduce the energy consumption of the entire system to below 0.4 mW while measuring and storing the ECG on a non-volatile memory. Moreover, in this project, a power-management circuit able to store energy collected from the radio communication interface is proposed, able to make the connected sensor fully autonomous. Initial results show that this sensor could be suitable for a truly continuous and long-term monitoring of cardiac events [32].

In collaboration with Atlantic, we have done here a preliminary study [37], [23] of wireless transmissions using the ventilation metallic ducts as waveguides. Starting from the waveguide theory, we deeply studied in simulation the actual attenuation encountered by radiowaves in such a specific medium. This kind of wireless link appears to be really efficient, and therefore highly promising to implement Internet of Things (IoT) in old buildings to make them smarter. This study also expresses a very simple empirical model in order to ease dimensioning a wireless network in such conditions and a specific antenna design enabling both good performance and high robustness to the influence of the environment.

The Spie ICS- INSA Lyon chair on IoT has granted us for a PhD thesis on Scatter Radio and RFID tag-to-tag communications. Some seminal results have shown that it is actually possible to create a communication between two RFID tags, just using ambient radiowaves or a dedicated distant radio source, without the need of generating a signal from the tag itself.

Optimization of waveforms for wake-up radio and energy harvesting

First Filter Bank Multi Carrier (FBMC) signals are employed in order to improve the performance of a quasi-passive wake-up radio receiver (WuRx) for which the addressing is performed by the means of a frequency fingerprint. The feasibility of such kind of WuRx was already demonstrated by using orthogonal frequency-division multiplexing (OFDM) signals to form the identifiers. Together with the main advantage of this approach (i.e. no base band processing needed and consequently a reduced energy consumption), one of the drawbacks is their low sensitivity. Through a set of circuit-system co-simulations, it is shown that by their characteristics, especially high Peak to Average Power Ratio (PAPR) and high out of band attenuation, FBMC signals manage to boost the sensitivity and moreover to enhance the robustness of this kind of WuRx. Moreover, we introduced robust wake-up IDs for quasi-passive wake-up receivers in an Internet of Things context[16]. These IDs can address single devices and are based on the Hadamard codes. Further a novel wake-up threshold is implemented to make the device more sensitive and robust against false wake-ups (FWUs). The wake-up procedure is simulated with a tap delay line (TDL) model for a line of sight (LOS) channel and a non line of sight (NLOS) channel. In both scenarios sufficient wake-up distances are reached with low false wake-up probabilitys (FWUPs). Additionally, the system is tested against the influence of an external bandwidth use. Finally, a recommendation for the global system is given.

In [21], we are proposing a way to maximize the DC power collected in the case of a wireless power transfer (WPT) scenario. Three main aspects are taken into account: the RF (radio frequency) source, the propagation channel and the rectifier as the main part of the energy collecting circuit. This problem is formulated as a convex optimization one. Then, as a first step towards solving this problem, a rectifier circuit was simulated by using Keysight's ADS software and, by using a classical model identification strategy i.e. Vector Fitting algorithm, the state-space model of the passive parts of this rectifier were extracted. In order to verify the extracted model, S11 input reflection coefficients and DC output voltages of the original circuit and the state-space model are compared.

UWB for localization

Ultra Wide Band (UWB) is a wireless communication technology that is characterized, in its impulse radio scheme  [55], by very short duration waveforms called pulses (in the order of few nanoseconds), using a wide band and low power spectral density. Among the many advantages offered by this technology is the fact that the arrival time of a pulse can be determined quite precisely, giving the opportunity to measure the distance between two communicating devices by estimating the flight time of the signal.

Although this technology has been known for a long time, it is only recently that cheap UWB chips have been commercialized for civilian applications. As the UWB technology is sensitive to many parameters, the effective performance of localization systems based on UWB may vary a lot compared to what is announced in datasheets. Some accuracy studies have been performed  [47], [48] but few of them focus on rapid movement of the transceivers.

Indeed, indoor ranging is in itself dependent on many parameter and very difficult to evaluate objectively, but when the transceivers are moving fast (say as if they were attached to dancer's wrists), more parameters are to be taken into account: transceiver calibration, random errors, presence of obstacle, antenna orientation etc.

In [20], we study experimentally the precision of UWB ranging for rapid movements in an indoor environment, based on the technology proposed by Decawave (DW1000  [45]) whose chips have already been integrated in many commercial devices. We show in particular how to improve the precision of the distance measured by averaging the ranging over successive samples.