## Section: New Results

### Agile Radio Resource Sharing

This axis addresses the challenges relative to the network perspective of software radio. While the two other axes have their focus on the design of the software radio nodes, axis 2 deals with coexistence and cooperation in a multi-user communications perspective.

A first research direction concerns theoretical limits of different reference scenarios where trade-offs between spectral efficiency, energy efficiency, stability and/or fairness are analyzed. This work exploits multi-users information theory, game theory and stochastic geometry. This year, a particular focus has been put on the interference channel with feedback and on dense wireless networks. New problems have been also investigated with the simultaneous energy and information transmission problem for energy harvesting and some specific attacks in smart grids.

In parallel our research activities are also driven by applicative frameworks. Concerning 4G RAN, a new interference alignment scheme has been proposed, simulated and implemented on CortexLab. This work has been presented as one of the promising technologies proposed by Greentouch. IoT has been identified as a new challenge for 5G with the objective of serving a very large number of nodes per cell, in a connectionless manner and with very small packets. The original transmission technology using ultra narrow band modulation and proposed by Sigfox for large area of IoT nodes has been investigated. A multiband CSMA strategy has been also evaluated in collaboration with CEA-Leti for dense Wifi like IoT access networks. Body area networks (BANs) represent also a very challenging applicative framework, with strong dynamics, interference environments, and low energy requirements. In partnership with Euromedia and Hikob, our studies focused on dynamic algorithms for information gathering in a sport event broadcast system. Additionnally, localization capabilities at the body scale may offer interesting perspetives but require specific MAC protocols.

#### Fundamental Limits

##### Energy efficiency - Spectral Efficiency (EE-SE) Tradeoffs in Wireless RANs

The spectral and energy efficiency (SE-EE) trade-off in cellular networks has attracted significant recent interest in the wireless community [1] . The work in [7] studies this fundamental limit with a simple and effective method. The proposed theoretical framework is based on an optimal radio resource allocation of transmit power and bandwidth for the downlink direction, applicable for an orthogonal cellular network. The analysis is initially focused on a single cell scenario, for which in addition to the solution of the main SE-EE optimization problem, it is proved that a traffic repartition scheme can also be adopted as a way to simplify this approach. By exploiting this interesting result along with properties of stochastic geometry, this work is extended to a more challenging multi-cell environment, where interference is shown to play an essential role and for this reason several interference reduction techniques are investigated. Special attention is also given to the case of low signal to noise ratio (SNR) and a way to evaluate the upper bound of EE in this regime is provided. This methodology leads to tractable analytical results under certain common channel properties, and thus allows the study of various models without the need for demanding system level simulations.

##### Interference Channels with Feedback

The capacity region of the two-user linear deterministic (LD) interference channel with noisy output feedback (IC-NOF) is fully characterized in [35] , [26] . This result allows the identification of several asymmetric scenarios in which implementing channel-output feedback in only one of the transmitter-receiver pairs is as beneficial as implementing it in both links, in terms of achievable individual rate and sum-rate improvements w.r.t. the case without feedback. In other scenarios, the use of channel-output feedback in any of the transmitter-receiver pairs benefits only one of the two pairs in terms of achievable individual rate improvements or simply, it turns out to be useless, i.e., the capacity regions with and without feedback turn out to be identical even in the full absence of noise in the feedback links. As a byproduct, the exact conditions on the signal to noise ratios on the feedback links to observe an improvement on either a single rate, both single rates, or the sum-rate capacity, for any IC-NOF are also fully described in [41] .

##### Simultaneous Energy and Information Transmission

The fundamental limits of simultaneous information and energy transmission in the two-user Gaussian multiple access channel (G-MAC) with and without feedback are fully characterized in [33] , [9] . All the achievable information and energy transmission rates (in bits per channel use and energy-units per channel use respectively) are identified. Thus, the information-energy capacity region is defined in both cases. In the case without feedback, an achievability scheme based on power-splitting and successive interference cancelation is shown to be optimal. Alternatively, in the case with feedback (G-MAC-F), a simple yet optimal achievability scheme based on power-splitting and Ozarow’s capacity achieving scheme is presented. Three of the most important observations in this work are: (a) The capacity-energy region of the G-MAC without feedback is a proper subset of the capacity-energy region of the G-MAC-F; (b) Feedback can at most double the energy rate for a fixed information rate; and (c) Time-sharing with power control is strictly suboptimal in terms of sum-rate in the G-MAC without feedback.

##### Multiple Access Channel and Broadcast Channel with Linear Feedback Schemes

In [11] , it is shown that for the two-user Gaussian broadcast channel with correlated noises and perfect feedback the largest region that can be achieved by linear-feedback schemes equals the largest region that can be achieved over a dual multi-access channel when in this latter the channel inputs are subject to a "non-standard" sum-power constraint that depends on the BC-noise correlation. Combining this new duality result with Ozarow's MAC-scheme gives an elegant achievable region for the Gaussian BC with correlated noises. A constructive iterative coding scheme is then presented for the non-symmetric Gaussian BC with uncorrelated noises that is sum-rate optimal among all linear-feedback schemes. This coding scheme shows that the connection between the MAC and the BC optimal schemes is tighter than what is suggested by our duality result on achievable rates. In fact, it is linear-feedback sum-rate optimal to use Ozarow MAC-encoders and MAC-decoders— rearranged—to code over the BC.

#### Low Complexity Receivers for Massive MIMO Systems

In wireless communications, Multi-user massive MIMO network is a scenario that has been recently proposed, where many mobile terminals are served by a Base Station (BS) equipped with a very high number of antennas. In such a scenario, the detection in the uplink remains a challenge, since the BS is required to detect signals transmitted from all users while trying to exploit full received diversity. The optimal detection criterion that fulfills the diversity requirement is the Maximum-Likelihood (ML) joint detection which has been proposed to detect jointly the transmitted signals. However, such a criterion is not applicable to the addressed multi-user massive MIMO scenario due to its computational complexity that increases exponentially with the number of signals to be detected. In our work paper, we have proposed a relaxed ML detector based on an iterative decoding strategy that reduces the computational cost. We exploit the fact that the transmit constellation is discrete, and remodel the channel as a MIMO channel with sparse input belonging to the binary $\{0,1\}$. The sparsity property allows us to relax the ML problem as a quadratic minimization under linear and $\ell $1-norm constraint. We then prove the equivalence of the relaxed problem to a convex optimization problem solvable in polynomial time. Simulation results illustrate the efficiency of the low-complexity proposed detector compared to other existing ones in very large and massive MIMO context.

#### Distributed Radio Resource Management

##### Interference Alignment in Cellular Networks with no-Explicit Coordination [32]

Current networks aim to support high data rates for end users by increasing the spectral efficiency in bits-per-Hertz, at the expense of the energy efficiency of the network. Indeed, an important part of the energy consumption of mobile networks is proportional to the radiated energy, which relies on the frequency bandwidth and the transmission power. Any energy efficient transmission scheme should exploits the whole system bandwidth by allocating the entire available spectrum to each base station. Such an approach, however, leads to significant interference increase and performance degradation for mobiles located at the cell edges. The key challenge is to balance interference avoidance and spectrum use to reach an optimal spectral efficiency – energy efficiency (EE-SE) trade-off. The work achieved in the framework of Greentouch collaboration is based on the non classical interference alignement scheme proposed by Suh and Tse in dowlink mode. The key contribution relies on users scheduling with a unique criteria based as well on QoS priorities and orthogonality of precoding directions. The spectral efficiency is improved by a factor 2 for edge users and a energy saving of about $30\%$ is made possible. This scheme has been evaluated on simulation scenarios as defined by Greentouch partners and a simplified version has been implemented on FIT/CorteXlab and demontrastrated during the final event of Greentouch (New-York, June 2015).

#### RANs for IoT : Dense and Connectionless Solutions

Internet of Things (IoT) is going to take a major place in the telecommunications market as announced in technical and public medias. The paradigm of IoT relies on the deployment of billions of objects having the capability of transmitting information about their context and environment and to create a real-time, secured and efficient interaction between the real and the virtual worlds, pushing them to evolve from the state of cousins to the state of Siamese twins. IoT revealed to be a key technology for solving societal issues such as digital cities, intelligent transportation, green environment monitoring or medical care and elderly person monitoring.

IoT has strong connections with machine-to-machine (M2M), and sometimes in literature, both terms refer to the same idea. From our point of view, IoT covers a broader scope including as well the technology and the applications. On the opposite, M2M refers to the technologies that allow machines or objects to communicate.

In any case, from the technical point of view, the main challenge of this new paradigm is to let a huge number of machine type devices (MTDs) be connected to the Internet at a low cost, with a limited infrastructure and featuring a very long life time with very small battery or energy needs [4] .

In this global picture, we may consider different technical issues. M2M has first been defined to connect MTDs in their vicinity. The proposed solutions extensively rely on the research results produced over the last 20 years for ad-hoc and wireless sensor networks. Initiated 20 years ago from theoretical concepts, this very active research area has gone up to the definition of full standards ($802.15.4$, $802.15.6$, Zigbee, Bluetooth) which have already found a market.

More recently, the IoT paradigm has been extended to the problem of connecting all these MTDs to the Internet, and through Internet to anyone or anything. The massive connection of objects spread over the world is a challenge that has some similarities with the paradigm of cellular networks which aimed at connecting people. This similarity attracted the interest of mobile network providers, to exploit such attractive potential market and IoT has been identified as a target for the future 5G.

##### Performance of Ultra-NarrowBand Techniques

The Ultra-narrow-band technology is an appealing solution for the low throughput wireless sensor networks (10b/s - 1 kb/s). It is complementary to the classical cellular networks thanks to its low energy consumption and very long range communication (up to 50 km in free-space) [4] . This technology has already been deployed and is proved to be ultra-efficient for point-to-point communications in Sigfoxs’ network. Nodes are transmitting at a random time and random frequency carrier (random frequency division multiple access schemes : R-FDMA), so the uplink is exposed to interference. In our approach, we have proposed to model this interference for the UNB network when taking into account the path-loss and Rayleigh effects, with stochastic geometry tools. The obtained model allows us to estimate the system performance, and its capacity in terms of maximum number average of simultaneous nodes in a unique cell [37] . We have also considered the replication mechanism, and identified the optimum number of replications.

##### Multiband CSMA for Dense Wireless Networks in Uplink

In this approach, the objective is to mitigate the degradation of the throughput and delay performance in wireless local area networks (WLAN) that employ carrier sense multiple access collision avoidance (CSMA/CA) protocol with request to send and clear to send (RTS/CTS) mechanism, when a large number of IoT like nodes are deployed. In our approach, the overhead is reduced with a modified handshake mechanism. The medium access control (MAC) overhead caused by the RTS and CTS messages is high comparing to the total duration of successful transmission. In order to reduce the MAC overhead we propose in this work a new strategy to serve many users successively. This strategy consists on sending many RTS in parallel by different stations on different frequency sub-bands. Once the RTS messages do not collide with each other, there will be no need to resend the RTS and wait for a CTS to gain the channel access [21] .

#### Algorithms and Protocols for BANs

##### Information Gathering in a Group of Mobile Users

Distributed decisions within any group of agents, is a very active research area and theoretical results as well as efficient algorithms have already been proposed but in the context of wireless networks, the task is made harder due to possible transmission errors, channel asymmetry, dynamic behaviour of the channel and node mobility. In this work, we consider a group of mobile agents moving roughly in a common direction. We study different algoritimic solutions allowing each agent to periodically discover its neighbours: one-hop neighbours as well as multi-hop neighbors. The reference scenario is a bike race, during which groups are susceptible to split or merge. The objective is a live gathering of information about who is present in a group for live TV broadcasting. For that, we need a fully distributed approach allowing every agent to discover with a consensus algorithm the list of neighbours participating to the same pack. This study may be of interest for various other applications such as group navigation support in crowded environments, autonomous navigation of a fleet of robots. . . This problem exhibits some similarities with a clustering problem. However, a clustering problem aims at exploiting the structure of a graph and to form some subgroups to ensure a good structure of the network for further communications while our objective is rather to estimate the groups naturally formed in the real world. Hence, we have focused on distributed decision algorithms, which are widely present in the literature. Max-consensus problem has been much less studied than average consensus. The proposed algorithms are based on the N-dimension generalization of the Random Broadcast Max-Consensus algorithm, allowing each agent to build and share the list of its muli-hop neighbors. We extend this approach to a dynamic context where the group information needs to be updated according to possible group merge or split. Experimental validation has been done in the context of a cycling race with 10 agents, equipping each bicycle with a wireless sensor node to assess the interactions between the racers and to provide a live monitoring of the dynamic evolution of the cyclists groups that form during the race.

##### MAC Protocols and Algorithms for Localization at the Body Scale

The purpose of this work is to evaluate the impact of the node speed on the ranging estimation for location applications with Wireless Body Area Networks (WBAN). While estimated with the 3-Way ranging protocol (3-WR), this distance between two nodes placed on the body can be affected by the human movements [30] , [17] . Thus, we study theoretically the ranging error with the 3-WR, based on a perfect channel, a MAC layer based on TDMA using two scheduling strategies (Single node localization (P2P-B) and Aggregated & Broadcast (A&B)) and a PHY layer based on Ultra Wideband (IR-UWB) [31] . We demonstrate the accuracy of the model, and show that the distance error is highly correlated with the speed of nodes [16] , while the associated mobility model has an impact on the design of MAC strategies by simulation [18] .

#### Other Topics

##### Data Injection Attacks in Smart Grids

Multiple attacker data injection attack construction in electricity grids with minimum-mean-square-error (MMSE) state estimation is studied for centralized and decentralized scenarios in [34] . A performance analysis of the trade-off between the maximum distortion that an attack can in- troduce and the probability of the attack being detected by the network operator is considered. Within this setting, optimal centralized attack construction strategies are studied. The decentralized case is examined in a game-theoretic setting. A novel utility function is proposed to model this trade-off and it is shown that the resulting game is a potential game. The existence and cardinality of the corresponding set of Nash Equilibria (NE) in the game is analyzed. For the particular case of two attackers, numerical results based on IEEE test systems are presented. These results suggest that attackers perform better when they seize control of power flow measurements instead of power injection measurements.