Section: New Results

Agile Radio Resource Sharing

Wireless Multi-hop Networks

In [6] , we study energy-delay tradeoff in wireless multihop networks with unreliable links. 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. In addition, most of works focus on self-organization protocol design while keeping non-protocol system parameters fixed. While, very few works reveal the relationship between the network performance and these physical parameters, in other words, the best networks performance could be obtained by the physical parameters. This paper investigates the tradeoff between the energy consumption and the latency of communications in a wireless multihop network using a realistic unreliable link model. It provides 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.

Relay and Cooperative Communications

In [16] , we aim at characterizing the gain induced by using relay channels in a linear network under both capacity constraint and realistic energy model. We express a general model based on a convex optimization problem. Then, we use numerical tools to obtain results on the outer and inner bounds of the capacity of the full and half duplex relay channel. We then extend this study with more complex networks based on relay channels, especially networks formed by a linear chain of nodes. We describe the Pareto optimal solutions of the minimization problem with respect to the consumed energy and latency in such a linear network. From the simple case of the linear multi-hop network, we study the gains when implementing a linear chain of relay channels and compare these results to the simpler multi-hop transmission.

In [15] , we present preliminary results on achievables rates in half-duplex cooperative multiple access channels (cmac ).We show that the upper bound on the capacity of the half-duplex cmac can be solved using convex optimization techniques. Under a Gaussian model, we study the maximal achievable rate by every node in the network. We propose a number of scenarios, encompassing existing and theoretical cooperation schemes. Using these hypotheses, we evaluate the performance of both a non-cooperative concurrent access and simple cooperative multi-hop or relaying schemes with respect to the upper bound. The performance is compared for the various scenarios, and we provide analyses of specific cases in order to illustrate how our framework may be used to answer targeted questions about the capacity of cmac s.

In [31] , we aim at obtaining usable bounds on the performance of cmac under a Gaussian model. We first show that the problem can be transformed into a convex optimization problem which makes it easily solvable using numerical tools. We propose, as a line of study, to consider the maximal achievable common rate by every node in the network. We then proceed to express closed-form bounds on the capacity region of the cmac in that common rate scenario. We study simple cooperation schemes based on existing results in relay channels and compare them to other medium sharing approaches. In the end, we show that using the relay-channel based protocols can be efficient for some parameters, but gets less interesting in the Gaussian case if the source-destination links are good enough.

In [30] , we study the optimal power allocations in cmac s, where we aim at maximizing the rate achievable by both sources simultaneously rather than the sum of achievable rates. Separating our study between the coherent and non-coherent case, we obtain closed-form expressions for the optimal power allocations w.r.t. the outer bounds of the capacity region, as well as decode-and-forward and non-cooperative inner bounds. We point out during our resolution that the general cmac model behaves as a multiple access relay channel (MARC), where a ”virtual” relay node is introduced to represent the cooperation between the sources. This equivalent model simplifies the original power allocation problem. We finally show that the general cut-set outer bound on the capacity region of the equivalent MARC matches exactly the tightest known outer bound on the capacity region of the original cmac .

In [17] we address the distributed power adaptation problem on the downlink for wireless cellular networks. As a consequence of uncoordinated local scheduling decisions in classical networks, the base stations produce mutual uncontrolled interference on their co-channel users. This interference is of a variable nature, and is hardly predictable, which leads to suboptimal scheduling and power control decisions. While some works propose to introduce cooperation between base stations, in this work we propose instead to introduce a model of power variations, called trajectories in the powers space, to help each base station to predict the variations of other base stations powers. The trajectories are then updated using a Model Predictive Control (mpc ) to adapt transmit powers according to a trade-off between inertia (to being predictable) and adaptation to fit with capacity needs. A Kalman filter is used for the interference prediction. In addition, the channel gains are also predicted, in order to anticipate channel fading states. This scheme can be seen as a dynamic distributed uncoordinated power control for multichannel transmission that fits the concept of self-optimised and self-organised wireless networks. By using the finite horizon mpc , the transmit powers are smoothly adapted to progressively leave the current trajectory toward the optimal trajectory. We formulate the optimisation problem as the minimisation of the utility function of the difference between the target powers and mpc predicted power values. The presented simulation results show that in dynamic channel conditions, the benefit of our approach is the reduction of the interference fluctuations, and as a consequence a more accurate interference prediction, which can further lead to a more efficient distributed scheduling, as well as the reduction of the overall power consumption.

BAN

In [26] we present a simple Body Area Network (BAN) platform that was built to monitor the performance of a marathon athlete all along the race, meeting real-time and QoS constraints, under good transmission conditions. Data collected during the event (packet loss, signal strength) allowed us to obtain a primary knowledge about the behavior of the radio transmissions between the different links in the network. The results of this experiment and ther important disparities observed between the links point out the need to improve the transmission strategy.

Network coding

One of the most powerful ways to achieve transmission reliability over wireless links is to employ efficient coding techniques. In [10] investigates the performance of a transmission over a relay channel where information is protected by two layers of coding. In the first layer, transmission reliability is ensured by fountain coding at the source. The second layer incorporates network coding at the relay node. Thus, fountain coded packets are re-encoded at the relay in order to increase packet diversity and reduce energy consumption. Performance of the transmission is measured by the total number of transmissions needed until the message is successfully decoded at the destination. We show through both analytical derivations and simulations that adding network coding capabilities at the relay optimizes system resource consumption. When the source uses a random linear fountain code, the proposed two layer encoding becomes more powerful as it reduces the transmission rate over the direct link between the source and the destination.

In [27] we study the deployment of fountain codes and network coding in a wireless sensor network (WSN). A WSN is composed of sensor nodes with restricted capacities: memory, energy and computational power. The nodes are usually randomly scattered across the monitored area and the environment may vary. In the presence of fading, outage and node failures, fountain codes are a promising solution to guaranty reliability and improve transmission robustness. The benefits of fountain codes are explored based on an event-driven WSNet simulator considering realistic implementation based on standard IEEE802.15.4. Fountain codes are rateless and capable of adapting their rate to the channel on the fly using a limited feedback channel. In this thesis, we highlight the benefits brought by fountain code in terms of energy consumption and transmission delay. In addition to the traditional transmission with fountain code, we propose in this thesis to study the network coding transmission scheme where nodes are allowed to process the information before forwarding it to their neighbors. By this means, we can say that packet diversity is exploited as each individual packet is unique and contains different representations of binary data. Redundancy is thus optimized since repetitions are avoided and replaced with diversified information. This can further lead to an overall improved performance in cooperative communication where nodes are allowed to assist in relaying packets from the source the destination. We highlight in this thesis the benefits of fountain code combined to network coding and show that it leads to a reduction in transmission delay and energy consumption. The latter is vital to the life duration of any wireless sensor network.

In [9] we tackles the problem of providing end to end reliable transmissions in a randomly deployed wireless sensor network. To this aim, we investigate the simultaneous use of gradient broadcast routing (for its inherent adaptability to any network topology and its changes), fountain codes (for their universal property) and intra-flow network coding (to introduce packet diversity in redundant copies). We present the impact of the proposed strategy on a realistic network. This work permits to highlight that, compared to basic gradient broadcast routing, the strategy not only improves the reliability and the delay in the network but also clearly increases its lifetime.

Vehicular networks

In [22] we study a hybrid propagation model For large-scale variations caused By vehicular traffic in small cells. we present a propagation model generating time series of large-scale power variations for small-cell radio links intersected by vehicular traffic. The model combines stochastic processing and geometric computation. For each road crossing a link, a two-state process parameterized by mobility statistics represents the obstruction status. When the status is set to obstructed, a fluctuation pattern is generated. Based on previously published measurements, both mobility statistics and time series results are validated through the comparison of respectively inter-obstruction duration distributions and outage probabilities. The proposed model avoids resource consuming iterative propagation prediction while providing realistic and frequency adaptive results.

In [21] , we performed measurements of large-scale variations caused by vehicular traffic in small-cell. This paper presents and characterizes large-scale variations of received power generated by vehicular traffic crossing a radio link. Measurements in the 2 GHz band for several small-cell configurations involve various transmitter heights, link distances and urban densities. Observations showed that stronger losses up to 30 dB are due to medium to high vehicles. Lower vehicles have a smaller impact in links perpendicular to traffic, but amplitude variations and duration can reach larger values when the receiver is at cell radius limits.

security

In [18] we study Security Embedding on ultra wideband impulse radio(uwb -ir ) Physical Layer. The main goal of this work is to incorporate security in an existing ultra wideband (UWB) network. We present an embedding method where a tag is added at the physical layer and superimposed to the uwb -impulse radio signal. The tag should be added in a transparent way so that guaranteeing compatibility with existing receivers ignoring the presence of the tag. We discuss technical details of the new embedding method. In addition, we discuss embedding strength and we analyze robustness performance. We demonstrate that the proposed embedding technique meets all the system design constraints.

In [11] we study Jamming in time-hopping ultrawide band (th-uwb ) Radio. With the great expansion of wireless communications, jamming becomes a real threat.We propose a new model to evaluate the robustness of a communication system to jamming. The model results in more scenarios to be considered ranging from the favorable case to the worst case. The model is applied to a th-uwb radio. The performance of such a radio in presence of the different jamming scenarios is analyzed. We introduce a mitigation solution based on stream cipher that restricts the jamming problem of the TH-UWB communication to the more favorable case while preserving confidentiality.

Network Information Theory

Fundamental performance limits of multi-hop wireless transmissions are being investigated in [33] from a multiobjective perspective where transmission decisions (i.e. relay selection, scheduling or routing decision) modify the trade-off between capacity, reliability, end-to-end delay or network-wide energy consumption. In our previous work presented in the Inria research report RR-7799, Pareto-optimal performance bounds and network parameters have been derived for a 1-relay and 2-relay network within a MultiObjective(MO) performance evaluation framework. We show in this report that these bounds are tight since they can be reached by simple practical coding strategies performed by the source and the relays. Such strategies constitute achievable lower MO performance bounds on the real MO performance limits. More precisely, we adopt a coding strategy where the source transmits a random linear fountain code which is coupled to a network coding strategy performed by the relays. Two different network coding strategies are investigated. Practical performance bounds for both strategies are compared to the theoretical bound. We show that the theoretical bound is tight: generational distance between the practical and theoretical bound for the best strategy is only of 0.0042

In [19] we revisit the problem of non-cooperativ association of mobiles to access points using game theory. We consider in this paper games related to the association problem of mobiles to an access point. It consists of deciding to which access point to connect. We consider the choice between two access points or more, where the access decisions may depend on the number of mobiles connected to each one of the access points. We obtain new results using elementary tools in congestion and crowding games.

In [23] we study stochastic analysis of energy savings with sleep mode in Orthogonal Frequency-Division Multiple Access (ofdma ) wireless networks. The issue of energy efficiency in ofdma wireless networks is discussed in this paper. Our interest is focused on the promising concept of base station sleep mode, introduced recently as a key feature in order to dramatically reduce network energy consumption. The proposed technical approach fully exploits the properties of stochastic geometry, where the number of active cells is reduced in a way that the outage probability, or equivalently the signal to interference plus noise distribution, remains the same. The optimal energy efficiency gains are then specified with the help of a simplified but yet realistic base station power consumption model. Furthermore, the authors extend their initial work by studying a non-singular path loss model in order to verify the validity of the analysis and finally, the impact on the achieved user capacity is investigated. In this context, the significant contribution of this paper is the evaluation of the theoretically optimal energy savings of sleep mode, with respect to the decisive role that the base station power profile plays.