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Publication# Modeling and predicting mobility in wireless ad hoc networks

Abstract

Wireless Ad Hoc Networks are a particular paradigm where wireless devices communicate in a decentralized fashion, without any centralized infrastructure or decision. In order to avoid a situation where nodes chaotically try to communicate, distributed and localized structures (graphs, trees, etc.) need to be built. Mobility brings challenging issues to the maintenance and to the optimality of such structures. In conventional approaches, structures are adapted to the current topology by each node periodically sending beacon messages, which is a significant waste of network resources. If each node can obtain some a priori knowledge of future topology configurations, it could decide to send maintenance messages only when a change in the topology effectively requires updating the structure. In this Doctoral Thesis, we investigate this approach and define the Kinetic Graphs, a novel paradigm regrouping mobility predictions for a kinetic mobility management, and localized and distributed graph protocols to insure a high scalability. The Kinetic Graph framework is able to naturally capture the dynamics of mobile structures, and is composed of four steps: (i) a representation of the trajectories, (ii) a common message format for the posting of those trajectories, (iii) a time varying weight for building the kinetic structures, (iv) an aperiodic neighborhood maintenance. By following this framework, we show that any structure-based ad-hoc protocol may benefit from the kinetic approach. A significant challenge of Kinetic Graphs comes from prediction errors. In order to analyze them, we illustrate the relationship between the prediction model and the mobility model. We decompose the prediction errors into three metrics: the adequacy between the prediction and the mobility models, the predicability of the mobility model, and the mobility model's realism. Following the framework, we define a kinetic model for the modeling of the trajectories and then analyze the extents of the effects of each error metric and develop solutions in order to reduce them. We finally adapt the Multipoint Relaying (MPR) protocol, used by the Optimized Link State Routing protocol (OLSR), and show the significant improvements that may be obtained by using the Kinetic Graph Framework, even on the very challenging vehicular networks.

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Suhas Diggavi, Matthias Grossglauser, Dominique Florian Tschopp

Dynamic networks are those where the topology changes over time and therefore efficient routes need to be maintained by frequent updates. Such updates could be costly in terms of consuming throughput available for data transmission, which is a precious resource in wireless networks. In this paper, we ask the question whether there exist low-overhead schemes for dynamic wireless networks, that could produce routes that are within a small constant factor (stretch) of the optimal route-length. This is studied by using the underlying geometric properties of the connectivity graph in wireless networks. For a class of models for mobile wireless network that fulfill some mild conditions on the connectivity and on mobility over the time of interest, we can design distributed routing algorithm that maintains the routes over a changing topology. This scheme needs only node identities and therefore integrates location service along with routing, therefore accounting for the complete overhead. We analyze the worst-case (conservative) overhead and route-quality (stretch) performance of this algorithm for the aforementioned class of wireless network connectivity and mobility models. In particular for these models, we show that our algorithm allows constant stretch routing with a network wide control traffic overhead of $O(n\log^2 n)$ bits per mobility time step (time-scale of topology change) translating to $O(\log^2 n)$ overhead per node (with high probability for wireless networks with such mobility model). Additionally, we can reduce the maximum overhead per node by using a load-balancing technique at the cost of a slightly higher average overhead. We also demonstrate through numerics that these worst-case bounds are quite conservative in terms of the constants derived theoretically.

2007We consider a cross-layer design of wireless ad-hoc networks. Traditional networking approaches optimize separately each of the three layers: physical layer, medium access and routing. This may lead to largely suboptimal network designs. In this work, we propose a jointly optimal design of the three layers, and we show a significant performance improvement over the conventional approach. In the first part of this thesis, our goal is to select appropriate performance metrics for the joint optimization problem. To that respect, we analyze several existing rate-maximization performance metrics for wireless ad-hoc networks: maximizing the sum of rates, max-min fairness and proportional fairness. We first show with several examples that it is not clear if and how max-min fairness can be defined on each one of the examples. We give a formal proof that the max-min fair rate allocation exists on a large class of sets of feasible rates, among which are the feasible-rate sets of all known ad-hoc networking examples. We also give a centralized algorithm to compute the max-min fair allocation whenever it exists. Next, we compare the three metrics for ad-hoc scenarios in terms of efficiency and fairness. We prove that, similar to wired networking,maximizing the sum of rates leads to gross unfairness and starvation of all but the flows with the best channel conditions. We also prove that, contrary to wired networking, max-min fairness yields all flows having the same rate, thus causing large inefficiencies. These findings offer theoretical explanations to the inefficiency and unfairness phenomena previously observed in the contexts of 802.11 and UWB networks. Finally, we show that proportional fairness achieves a good trade-off between efficiency and fairness and is a good candidate for a rate-based performance metric in wireless ad-hoc settings. Having shown that the proportional fairness is an appropriate optimization objective for our problem, in the second part of the thesis we consider a joint optimization of rates, transmission powers, medium access (scheduling) and routing, where the goal of the optimization is to achieve proportional fairness. We first analyze networks built on physical layers that have a rate which is a linear function of SNR at the receiver (such as UWB or low-gain CDMA systems). We find that the optimal solution is characterized by the following principles: (1)Whenever a node transmits, it has to transmit with the maximum power; otherwise it has to remain silent (0 - PMAX power control). (2) Whenever data is being sent over a link, it is optimal to have an exclusion region around the destination, in which all nodes remain silent during transmission, whereas nodes outside of this region can transmit in parallel, regardless of the interference they produce at the destination. (3) When a source transmits, it adapts its transmission rate according to the level of interference at the destination due to sources transmitting in parallel. (4) The optimal size of this exclusion region depends only on the transmission power of the source of the link, and not on the length of the link nor on positions of nodes in its vicinity. As for the routing, we restrict ourselves to a subset of routes where on each successive hop we decrease the distance toward the destination. We also show that (5) relaying along a minimum energy and loss route is better than using longer hops or sending directly, which is not obvious since we optimize rate and not power consumption. Finally (6), the design of the optimal MAC protocol is independent of the choice of the routing protocol. We present a theoretical proof of optimality of 0 - PMAX power control, and the remaining findings we show numerically on a large number of random network topologies. Next, we consider narrow-band networks, where rate function is a strictly concave function of SNR. There, previous findings do not always hold. We show that in some cases, the size of the exclusion region and the optimal routing depend on transmission powers, and that the optimal MAC design depends on the choice of routing. Nevertheless, as we show with the example of 802.11 networks, a significant improvement over the existing 802.11 MAC can be achieved even with simpler, suboptimal strategies. Although this result is shown by simulations on a simplified model, it still gives further directions on how to improve the performance of RTS/CTS based protocols.

Dario Floreano, Grégoire Hilaire Marie Heitz, Karol Jacek Kruzelecki, Bixio Rimoldi, Stefano Rosati

This paper reports experimental results on self-organizing wireless networks carried by small flying robots. Flying ad hoc networks (FANETs) composed of small unmanned aerial vehicles (UAVs) are flexible, inexpensive and fast to deploy. This makes them a very attractive technology for many civilian and military applications. Due to the high mobility of the nodes, maintaining a communication link between the UAVs is a challenging task. The topology of these networks is more dynamic than that of typical mobile ad hoc networks (MANETs) and of typical vehicle ad hoc networks (VANETs). As a consequence, the existing routing protocols designed for MANETs partly fail in tracking network topology changes. In this work, we compare two different routing algorithms for ad hoc networks: optimized link-state routing (OLSR), and predictive-OLSR (P-OLSR). The latter is an OLSR extension that we designed for FANETs; it takes advantage of the GPS information available on board. To the best of our knowledge, P-OLSR is currently the only FANET-specific routing technique that has an available Linux implementation. We present results obtained by both Media Access Control (MAC) layer emulations and real-world experiments. In the experiments, we used a testbed composed of two autonomous fixed-wing UAVs and a node on the ground. Our experiments evaluate the link performance and the communication range, as well as the routing performance. Our emulation and experimental results show that P-OLSR significantly outperforms OLSR in routing in the presence of frequent network topology changes.