Section: New Results

Interaction between Algorithms and Architectures

Numerical Accuracy Analysis and Optimization

Participants : Daniel Menard, Karthick Parashar, Olivier Sentieys, Romuald Rocher, Pascal Scalart, Aymen Chakhari, Jean-Charles Naud, Emmanuel Casseau.

Most of analytical methods for numerical accuracy evaluation use perturbation theory to provide the expression of the quantization noise at the output of a system. Existing analytical methods do not consider a correlation between noise sources. This assumption is no longer valid when a unique datum is quantized several times. In [34] , an analytical model of the correlation between quantization noises is provided. The different quantization modes are supported and the number of eliminated bits is taken into account. The expression of the power of the output quantization noise is provided when the correlation between the noise sources is considered. The proposed approach allows improving significantly the estimation of the output quantization noise power compared to the classical approach, with a slight increase of the computation time.

An analytical approach is studied to determine accuracy of systems including unsmooth operators. An unsmooth operator represents a function which is not derivable in all its definition interval (for example the sign operator). The classical model is no valid yet since these operators introduce errors that do not respect the Widrow assumption (their values are often higher than signal power). So an approach based on the distribution of the signal and the noise is proposed. It is applied to the sphere decoding algorithm to determine analytically the error probability due to quantization [53] . We also focus on recursive structures where an error influences future decision. So, the Decision Feedback Equalizer is also considered. In that case, numerical analysis method (as Newton Raphson algorithm) can be used. Moreover, an upper bound of the error probability can be analytically determined. A method to determine the distribution of the noise due to quantization at the output of a system made of smooth operators has been developed [70] . It is based on Generalized Gaussian Distribution and allows take under consideration all possible distributions (uniform, gaussian, laplacian, etc.).

Multi-Antenna Systems

Participants : Olivier Berder, Pascal Scalart, Quoc-Tuong Ngo, Viet-Hoa Nguyen.

Still considering the maximization of the minimum Euclidean distance, we proposed a new linear precoder obtained by observing the SNR-like precoding matrix. An approximation of the minimum distance is derived, and its maximum value was obtained by maximizing the minimum diagonal element of the SNR-like matrix. The precoding matrix is first parameterized as the product of a diagonal power allocation matrix and an input-shaping matrix acting on rotation and scaling of the input symbols on each virtual subchannel. We demonstrated that the minimum diagonal entry of the SNR-like matrix 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 [35] , [36] , [37] . To reduce the decoding complexity of linearly precoded MIMO systems, the sphere decoder was applied instead of maximum likelihood and the performance complexity trade-off was investigated. The sphere decoding (SD) algorithm, proposed as a sub-optimal ML-decoding, just considers a subset of lattice points that drop into the sphere centered by the received point to obtain the decoded solution, thus reducing significantly the complexity. Because the structure of our precoder is complicated and strongly depends on the channel, it exists the case when all power is poured only on the best sub-channel. Some adjustments, therefore, of traditional sphere decoding algorithm were mandatory to adapt to the precoded MIMO systems.

Impact of RF Front-End Nonlinearity on WSN Communications

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

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 [56] . 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.

Cooperative Strategies for Low-Energy Wireless Networks

Participants : Olivier Berder, Olivier Sentieys, Le-Quang-Vinh Tran, Duc-Long Nguyen.

Recently, cooperative relay techniques (e.g. repetition-based or distributed space-time code based (DSTC-based) protocols) are increasingly of interest as one of the advanced techniques to mitigate the fading effects of transmission channel. We proposed a novel cooperative scheme with data exchange between relays before using distributed space-time coding. This fDSTC (full Distributed Space-Time Code) was compared with the conventional distributed space-time coded (cDSTC) protocol. Then, the thorough comparison of the fDSTC and cDSTC protocols in case of non-regenerative relays (NR-relays) and regenerative relays (R-relays) were considered in terms of error performance, outage probability, diversity order and energy consumption via both numerical simulations and mathematical analysis [24] . The previous works consider the energy efficiency of the cooperative relays techniques under the view of ideal medium access control (MAC) protocol. However, MAC protocol is responsible for regulating the shared wireless medium access of the networks, therefore, it has great influences on the total energy consumption of the networks. That lead us to a big motivation to design a cooperative MAC protocol, RIC-MAC (Receiver Initiated Cooperative MAC), by combining preamble sampling and cooperative relay techniques. The analytic results still confirm the interest of using cooperative relay techniques. However, the energy efficiency of the cooperative relay systems may be affected by MAC protocol design, the traffic loads of the networks and the desired latency [24] .

Opportunistic Routing

Participants : Olivier Berder, Olivier Sentieys, Ruifeng Zhang.

However, the aforementioned approaches introduce an overhead in terms of information exchange, increasing the complexity of the receivers. A simpler way of exploiting spatial diversity is referred to as opportunistic routing. In this scheme, a cluster of nodes still serves as relay candidates but only a single node in the cluster forwards the packet [80] . Energy efficiency and transmission delay are very important parameters for wireless multihop networks. Numerous works that study energy efficiency and delay are based on the assumption of reliable links. However, the unreliability of channels is inevitable in wireless multihop networks. We investigated the tradeoff between the energy consumption and the latency of communications in a wireless multihop network using a realistic unreliable link model [43] . It provided a closed-form expression of the lower bound of the energy-delay tradeoff and of energy efficiency for different channel models (additive white Gaussian noise, Rayleigh fast fading and Rayleigh block-fading) in a linear network. These analytical results are also verified in 2-dimensional Poisson networks using simulations. The closed-form expression provides a framework to evaluate the energy-delay performance and to optimize the parameters in physical layer, MAC layer and routing layer from the viewpoint of cross-layer design during the planning phase of a network.

Adaptive Techniques for WSN Power Optimization

Participants : Olivier Berder, Daniel Menard, Olivier Sentieys, Mahtab Alam, Trong-Nhan Le.

We proposed a self-organized asynchronous medium access control (MAC) protocol for wireless body area sensor (WBASN). A body sensor network exhibits a wide range of traffic variations based on different physiological data emanating from the monitored patient. In this context, we exploit the traffic characteristics being observed at each sensor node and propose a novel technique for latency-energy optimization at the MAC layer [48] , [26] . The protocol relies on dynamic adaptation of wake-up interval based on a traffic status register bank. The proposed technique allows the wake-up interval to converge to a steady state for variable traffic rates, which results in optimized energy consumption and reduced delay during the communication. The results show that our protocol outperforms the other protocols in terms of energy as well as latency under the variable traffic of WBASN.

System lifetime is the crucial problem of Wireless Sensor Networks (WSNs), and exploiting environmental energy provides a potential solution for this problem. When considering self-powered systems, the Power Manager (PM) plays an important role in energy harvesting WSNs. Instead of minimizing the consumption energy as in the case of battery powered systems, it makes the harvesting node converge to Energy Neutral Operation (ENO) to achieve a theoretically infinite lifetime and maximize the system performance. In [62] , a low complexity PM with a Proportional Integral Derivative (PID) controller is introduced. This PM monitors the buffered energy in the storage device and performs adaptation by changing the wake-up period of the wireless node. This shows the interest of our approach since the impractical monitoring harvested energy as well as consumed energy is not required as it is the case in other previously proposed techniques. Experimental results are performed on a real WSN platform with two solar cells in an indoor environment. The PID controller provides a practical strategy for long-term operations of the node in various environmental conditions.

WSN for Health Monitoring

Participants : Patrice Quinton, Olivier Sentieys.

Applications of wireless sensor devices were also considered in the domain of health monitoring. Together with researchers from CASA team of IRISA-UBS, we investigated the possibility of using ECG-sensors to remotely monitor the cardiac activity of runners during a marathon race, using off-the shelf sensing devices and a limited number of base stations deployed along the marathon route. Preliminary experiments showed that such a scenario is indeed viable, although special attention must be paid to balancing the requirements of ECG monitoring with the constraints of episodic, low-rate transmissions.

The proliferation of private, corporate and community Wi-Fi hotspots in city centers and residential areas opens up new opportunities for the collection of biomedical data produced by sensors carried by mobile non-hospitalized subjects. Using disruption-tolerant networks, it was shown that biomedical data could be recorded using nearby hotspot. A scenario involving a subject wearing an ECG-enabled sensor walking in the streets of a residential area was reported.

These researches, combined with new sensor devices developed by the BOWI project, open up a large range of applications where high-performance sensor devices would allow health monitoring, or sport events organization.

Reconfigurable Video Coding

Participants : Emmanuel Casseau, Hervé Yviquel.

In the field of multimedia coding, standardization recommendations are always evolving. To reduce design time taking benefit of available SW and HW designs, Reconfigurable Video Coding (RVC) standard allows defining new codec algorithms. The application is represented by a network of interconnected components (so called actors) defined in a modular library and the behaviour of each actor is described in the specific RVC-CAL language. Dataflow programming, such as RVC applications, express explicit parallelism within an application. However general purpose processors cannot cope with both high performance and low power consumption requirements embedded systems have to face. Hence we are investigating the mapping of RVC specifications on hardware accelerators or on many tiny core platforms. Actually, our goal is to propose an automated co-design flow based on the Reconfigurable Video Coding framework. The designer provides the application description in the RVC-CAL dataflow language, after which the co-design flow automatically generates a network of processors that can be synthesized on FPGA platforms. We are currently focussing on a many-core platform based on the TTA processor (Very Long Instruction Word -style processor). Hervé Yviquel did a 4-months stay (Spring 2012) at Tampere University of Technology, Finland, in the group of Jarmo Takala who is developing a co-design toolset for TTA processor automated generation. Such a methodology permits the rapid design of a many-core signal processing system which can take advantage of all levels of parallelism. This work is done in collaboration with Mickael Raulet from IETR INSA Rennes and has been implemented in the Orcc open-source compiler. At present time the mapping of the RVC-CAL actor network is straightforward: every actor is mapped on a TTA processor based on our collaboration with Jani Boutellier from the University of Oulu (Finland). To reduce the area of the platform, TTA processor usage rate has to be improved, i.e. several actors are to be mapped onto a single processor. Work in progress is about this. It requires an actor partitioning step to define the set of actors that will be executed on the same processor. Due to the dynamic behaviour of the application, we expect we will be able to use profiling to get some feedbacks for the partitioning.

A Low-Complexity Synchronization Method for OFDM Systems

Participants : Pramod P. Udupa, Olivier Sentieys, Pascal Scalart.

A new hierarchical synchronization method was proposed for initial timing synchronization in orthogonal frequency-division multiplexing (OFDM) systems. Based on the proposal of new training symbol, a threshold based timing metric is designed for accurate estimation of start of OFDM symbol in a frequency selective channel. Threshold is defined in terms of noise distributions and false alarm which makes it applicable independent of type of channel it is applied. Frequency offset estimation is also done for the proposed training symbol. The performance of the proposed timing metric is evaluated using simulation results. The proposed method achieves low mean squared error (MSE) in timing offset estimation at five times lower computational complexity compared to cross-correlation based method in a frequency selective channel. It is also computationally efficient compared to hybrid approaches for OFDM timing synchronization.

Flexible hardware accelerators for biocomputing applications

Participants : Steven Derrien, Naeem Abbas, Patrice Quinton.

It is widely acknowledged that FPGA-based hardware acceleration of compute intensive bioinformatics applications can be a viable alternative to cluster (or grid) based approach as they offer very interesting MIPS/watt figure of merits. One of the issues with this technology is that it remains somewhat difficult to use and to maintain (one is rather designing a circuit rather than programming a machine). Even though there exists C-to-hardware compilation tools (Catapult-C, Impulse-C, etc.), a common belief is that they do not generally offer good enough performance to justify the use of such reconfigurable technology. As a matter of fact, successful hardware implementations of bio-computing algorithms are manually designed at RT-level and are usually targeted to a specific system, with little if any performance portability among reconfigurable platforms. This research work, funded by the ANR BioWic project, aims at providing a framework for helping semi-automatic generation of high-performance hardware accelerators. This research work builds upon the Cairn research group expertise on automatic parallelization for application specific hardware accelerators and has been targeting mainstream bioinformatics applications (HMMER, ClustalW and BLAST). The Biowic project ended in early 2012. Naeems Abbas, a PhD student funded by the project defended his PhD in May 2012.