Section: New Results

Interaction between Algorithms and Architectures

Design Methodologies for Software Defined Radios

Participants : Matthieu Gautier, Olivier Sentieys, Emmanuel Casseau, Arnaud Carer, Ganda-Stéphane Ouedraogo, Mai-Thanh Tran, Vaibhav Bhatnagar.

Software Defined Radio (SDR) is a flexible signal processing architecture with reconfiguration capabilities that can adapt itself to various air interfaces. It was first introduced by Joseph Mitola as an underlying structure for Cognitive Radio (CR). The FPGA (Field Programmable Gate Array) technology is expected to play a key role in the development of SDR platforms. FPGA-based SDR is a quite old paradigm and we are fronting this challenge while leveraging the nascent High Level Synthesis tools and languages.

Actually, our goal is to propose methods and tools for rapid implementation of new waveforms in the stringent flexibility paradigm. We proposed a novel design flow for FPGA-based SDR applications [38] [70] . This flow relies upon HLS principles and its entry point is a Domain-Specific Language (DSL) which partly handles the complexity of programming an FPGA and integrates SDR features.

Adaptive Precision under Performance Constraints in OFDM Wireless Receivers

Participants : Olivier Sentieys, Matthieu Gautier, Fernando Cladera [Master's Student] .

To cope with rapid variations of channel parameters, wireless receivers are designed with a significant performance margin to reach a given Bit Error Rate (BER), even for worst-case channel conditions. Significant energy savings come from varying at run time processing bit-width, based on estimation of channel conditions, without compromising BER constraints. To validate the energy savings, the energy consumption of basic operators has been obtained from real measurements for different bit-widths on an FPGA and an ARM processor using soft SIMD. Results show that up to 66% of the dynamic energy consumption can be saved using this adaptive technique.

MIMO Systems and Cooperative Strategies for Low-Energy Wireless Networks

Participants : Olivier Berder, Olivier Sentieys, Pascal Scalart, Matthieu Gautier, Le-Quang-Vinh Tran, Duc-Long Nguyen [Master's Student] , Ruifeng Zhang, Viet-Hoa Nguyen.

Since a couple of years, the CAIRN team has reached a significant expertise in multi-antenna systems, especially in linear precoding. In order to obtain an efficient, simple and general form of precoders, we considered an SNR-like matrix to approximate the minimum distance. The precoding matrix is first parameterized as the product of a diagonal power allocation matrix and an input-shaping matrix and demonstrated that the minimum diagonal entry of the latter is obtained when the input-shaping matrix is a DFT-matrix. The major advantage of this design is that the solution can be available for all rectangular QAM-modulations and for any number of datastreams [28] . On the other hand the sphere decoder was applied at the receiver side instead of maximum likelihood and the performance complexity trade-off was investigated. Some adjustments of traditional sphere decoding algorithm were mandatory to adapt to the precoded MIMO systems [55] .

Another way to exploit the MIMO diversity, especially in WSN where only one antenna can be supported by limited size devices, is to use space-time codes in a distributed manner. In this context, a new protocol, called fully distributed space-time coded (FDSTC) protocol having information exchange between relays, was proposed and compared with the conventional distributed space-time coded (DSTC) protocol using non-regenerative relays (NR-relays) and regenerative relays (R-relays). At the same spectral efficiency, FDSTC has better performance in terms of outage probability in high SNR regions. In terms of energy efficiency, the FDSTC protocol is shown to outperform DSTC for long-range transmissions [32] . As very few dedicated MAC protocols exist, we investigated a novel low-latency MAC protocol (ARQ-CRI) for low-power cooperative wireless sensor networks WSNs, while preserving (in high traffic mode) or even increasing (in low traffic mode) energy-efficiency [54] . An energy efficient opportunistic MAC protocol with the mechanisms of reservation and a relay candidate coordination were also proposed, and the multi-relay transmission probability was analyzed. Simulation and experiment results on a real wireless sensor network platform in different channels demonstrated the proposed scheme greatly reduces the multi-relay transmission probability and achieves about 84% improvement of energy efficiency compared with the traditional opportunistic MAC schemes [66] .

Energy Harvesting and Adaptive Wireless Sensor Networks

Participants : Olivier Berder, Olivier Sentieys, Arnaud Carer, Mahtab Alam, Ruifeng Zhang, Trong-Nhan Le.

As tiny sensor nodes are equipped with limited battery, the optimization of the power consumption of these devices is extremely vital. In typical WSN platforms, the radio transceiver consumes major proportion of the energy. Major concerns are therefore to decrease both the transmit power and radio activity. We designed an adaptive transmit power optimization technique that is applied under varying channel to reduce the energy per successful transmitted bit. Each node locally adapts its output power according to the signal-to-noise ratio (SNR) variations (for all the neighbor nodes). It is found that by dynamically adapting the transmit power on average can help to reduce the energy consumption by a factor of two [36] .

To further extend the system lifetime of WSN, energy harvesting techniques have been considered as potential solutions for long-term operations. Instead of minimizing the consumed energy as for the case of battery-powered systems, the harvesting node is adapted to Energy Neutral Operation (ENO) to achieve a theoretically infinite lifetime. Several types of energy sources can be used, as light, motion or heat [51] . We even investigated the possibility for a single sediment-microbial fuel cell (MFC) to power a wireless sensor network [31] . Through experiments conducted on the PowWow platform, it was shown that the energy harvesting device adapts to the intermittent power supplied by the MFC, and the radio-transmitter is able to switch from a continuous to degraded mode. Given the harvesting capability, we then tried to design power managers (PM) able to optimize the quality of service of WSN while maintaining ENO. Our PM adapts the duty cycle of the node according to the estimation of harvested energy and the consumed energy provided by a simple energy monitor for a super capacitor based WSN to achieve the ENO [52] . When possible, as is sometimes the case for solar or wind energy, it is also of prime interest to benefit from an accurate energy predictor to estimate the energy that can be harvested in the near future, therefore we proposed a low complexity energy predictor using adaptive filter [53] . Finally, with colleagues from University College of Cork, we recently investigated the possibility to combine energy harvesting platforms with low power wake-up radios. A nano-watt wake-up radio receiver (WUR) was used cooperatively with the main transceiver in order to reduce the wasted energy of idle listening in asynchronous MAC protocols, while still maintaining the same reactivity [50] .

Impact of RF Front-End Nonlinearity on WSN Communications.

Participants : Amine Didioui, Olivier Sentieys, Carolynn Bernier [CEA Leti] .

In the context of a collaboration with CEA Leti, we studied the impact of RF front-end non-linearity on the performance of wireless sensor networks (WSN). More specifically, we investigated the problem of interference caused by intermodulation between in-band interferers. We analyzed this problem using an enhanced model of signal-to-interference-and-noise ratio (SINR) that includes an interference term due to intermodulation. Using a WSN simulator and the selectivity and the third-order input intercept point (IIP3) specifications of a radio transceiver, we have shown that the new SINR model provides helpful information for the analysis of intermodulation problems caused by in-band signals in IEEE 802.15.4 WSNs. In [45] , we presented a reconfigurable receiver model whose purpose is to enable the study of reconfiguration strategies for future energy-aware and adaptive transceivers. This model is based on Figure of Merits of measured circuits. To account for real-life RF interference mechanisms, a link quality estimator is also provided. We show that adapting the receiver performance to the channel conditions can lead to considerable power saving. The models proposed can easily be implemented in a wireless network simulation in order to validate the value of a reconfigurable architecture in real-world deployment scenarios.

HarvWSNet: A Co-Simulation Framework for Energy Harvesting Wireless Sensor Networks.

Participants : Amine Didioui, Olivier Sentieys, Carolynn Bernier [CEA Leti] .

Recent advances in energy harvesting (EH) technologies now allow wireless sensor networks (WSNs) to extend their lifetime by scavenging the energy available in their environment. While simulation is the most widely used method to design and evaluate network protocols for WSNs is simulation, existing network simulators are not adapted to the simulation of EH-WSNs and most of them provide only a simple linear battery model. To overcome these issues, we have proposed HarvWSNet, a co-simulation framework based on WSNet and Matlab that provides adequate tools for evaluating EH-WSN lifetime [44] . Indeed, the framework allows for the simulation of multi-node network scenarios while including a detailed description of each node's energy harvesting and management subsystem and its time-varying environmental parameters. A case study based on a temperature monitoring application has demonstrated HarvWSNet's ability to predict network lifetime while minimally penalizing simulation time [40] .

Synchronisation Algorithms and Parallel Architecture for Wireless and High-Rate Optical OFDM Systems

Participants : Pramod Udupa, Olivier Sentieys, Arnaud Carer, Pascal Scalart.

Multi-band Coherent Optical OFDM (MB CO-OFDM) is widely predicted to be one of the technologies which will empower 100 Gigabit Ethernet (100GbE) networks. CO-OFDM uses coherent technology and advanced digital signal processing (DSP) to achieve net data rate of 10 Gbps in a single band. This strict throughput requirement puts a constraint on the kind of signal processing algorithms and architectures used for building the system. In [72] , a scalable parallel architecture using radix-2${}^2$ for IFFT was proposed. The second proposal consists of a scalable parallel timing synchronization algorithm which can support very high input rates at the receiver. MOPS count as well as area versus throughput for the synchronization algorithm are provided for the OFDM transceiver to show the improvements due to proposed architecture. Architecture exploration was performed using a leading-edge high-level synthesis (HLS) tool.

A novel low complexity parallel algorithm and its associated architecture were proposed for initial synchronization in orthogonal frequency division multiplexing (OFDM) systems. The method is hierarchical and uses auto-correlation for the first step and cross-correlation for the second step [60] . The main advantage of the proposed approach is that it reduces the computational complexity by a factor of five (80%), while achieving similar mean square error (MSE) as cross-correlation based methods. The method uses block-level parallelism for auto-correlation step, which speeds up the computation significantly. After fixed-point analysis, a parallel architecture is proposed to accelerate both coarse and fine synchronization steps. This parallel architecture is scalable and provides speed-up proportional to number of parallel blocks [59] .