Section: New Results

Agile radio resource sharing

Participants : Jean-Marie Gorce, Claire Goursaud, Katia Jaffrès-Runser, Nikolaï Lebedev, Guillaume Villemaud, Paul Ferrand, Philippe Mary.

This section presents our recent results concerning the realistic modeling of wireless links to develop realistic models and efficient simulations. This work include theoretical developements like symbol error outage modeling, but also some applications in the context of LTE multi-cells association, or oportunistic relaying in the context of wireless sensor networks. Other contributions about resource sharing are presented in next sections below, in the section 'network optimization' and the section 'network coding'.

In[28] , we addressed the problem of finding a tractable expression for the symbol error outage (SEO) in flat Nakagami-m fading and shadowing channels. We deal with M-ary phase shift keying (M-PSK) and quadrature amplitude modulation (M-QAM) which extends our previous results on BPSK signaling. We propose a new tight approximation of the symbol error probability (SEP) holding for M-PSK and M-QAM signals which is accurate over all signal to noise ratios (SNRs) of interest. We derive a new generic expression for the inverse SEP which facilitates the derivation of a tight approximation of the SEO in a lognormal shadowing environment.

In [44] , we consider on-body BAN nodes transmitting information towards a common sink, in a star topology (Body Area Networks (BAN) offer amazing perspectives to instrument and support humans in many aspects of their lives). While this setup is usual in wireless networks, the high instability of the BAN radio channel and the proximity of the body make classical communication protocols inefficient. These networks are further constrained by the low transmission power required for both battery life and health concerns. Opportunistic cooperation techniques are of great interest in such environment to ensure reliable communications. In previous works, we studied simple opportunistic relaying schemes under independent BAN links, using a packet error outage criterion. In this paper, we introduce a more realistic case where shadowing variations around the body are now assumed strongly correlated. Generally speaking, there is a lack of definitive measurements and models for the shadowing correlation in multi-hop networks, while it can play a crucial role at the higher layers. Based on the measurement and simulation results of the French BANET project, we use the BAN context as an illustrative example to exhibit how shadowing correlations have a strong impact on relaying approaches performance.

Opportunistic networking aims at exploiting sporadic radio links to improve the connectivity of a multi-hop network and to foster data transmissions. Broadcast nature of the wireless channel is an important feature that can be exploited to improve transmissions by using several potential receivers. Opportunistic relaying is thus the first brick for opportunistic networking. However, the advantage of opportunistic relaying may be balanced by energy increase related to having simultaneous active receivers. In [32] , we proposed a thorough analysis of opportunistic relaying efficiency under different realistic radio channel conditions. The study aims at finding the best trade-off between two objectives: energy and latency minimizations, under a hard reliability constraint. We derive an optimal bound, namely, the Pareto front of the related optimization problem, which offers a good insight into the benefits of opportunistic routings compared with classical multi-hop routing schemes. Meanwhile, the lower bound provides a framework to optimize the parameters in physical layer, MAC layer and routing layer from the viewpoint of cross layer during the design or planning phase of a network.

This work has been extended in In [70] for relay channels. The gain induced by using relay channels in a linear network under both a capacity constraint and a realistic energy model is evaluated. We express a general model based on a convex optimization problem, allowing us to use numerical tools to obtain similar results for outer and inner bounds to the capacity of the full and half duplex relay channel. We then further the 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 for 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. This work will be published in 2012 in IEEE WCNC.

In [82] we extended this formalims derived for a linear network to a more general case: the problem of deriving fundamental trade-off bounds for wireless ad hoc networks when multiple performance criteria are of interest. It proposes a MultiObjective (MO) performance evaluation framework composed of a broadcast and interference-limited network model, a steady state performance metric derivation inspired by a discrete Markov chain formalism and formulates the associated MO optimization problem. Pareto optimal performance bounds between end-to-end delay and energy for a capacity-achieving network are given for the 1-relay and 2-relay networks and assessed through simulations.