Federico Librino

Cooperation techniques for wireless systems from a networking perspective

The impact of fading and other impairments in wireless channels can be counteracted by leveraging communication diversity and introducing cooperative paradigms, where third-party nodes contribute to assist the communication. In this article we describe and evaluate two possible cooperative approaches, cooperative relaying and coded cooperation. Different from existing works where similar evaluations are mainly performed investigating a single link, we take a network-wide perspective to evaluate the effects of cooperation not only where it is performed but also on other links. We focus on a multiple-input multiple-output ad hoc scenario and show that the improvement brought by cooperative routing and coded cooperation is not always sufficient; in certain cases the former can be ineffective if no proper relay can be selected, and the latter leads to an overall increase of interference, thus worsening the quality of surrounding links. However, we suggest that these two features can be combined in an advantageous manner in order to mutually overcome their problems. Such a joint solution is shown to achieve a significant improvement over the two individual approaches. We conclude by discussing future evolutions of the cooperation paradigm, including both cooperative routing and coded cooperation, and their advanced implementation issues.

Inter-Network Cooperation Exploiting Game Theory and Bayesian Networks

Relay sharing has been recently investigated to increase the performance of coexisting wireless multi-hop networks. In this paper, we analyze a scenario where two wireless ad hoc networks are willing to share some of their nodes, acting as relays, in order to gain benefits in terms of lower packet delivery delay and reduced loss probability. Bayesian network analysis is exploited to compute the probabilistic relationships between local parameters and overall performance, whereas the selection of the nodes to share is made by means of a game theoretic approach. Our results are then validated through the use of a system level simulator, which shows that an accurate selection of the shared nodes can significantly increase the performance gain with respect to a random selection scheme.

Distributed Cooperative Routing and Hybrid ARQ in MIMO-BLAST Ad Hoc Networks

Cooperation has proved to be an effective technique for improving the performance and the efficiency of wireless networks. Most of the existing work on cooperation focuses on the physical layer, with the aim of enhancing the capacity and the quality of a single link. In this paper we propose a cooperative protocol that melds physical, MAC and routing layers to increase the performance of a MIMO-BLAST ad hoc network, where nodes are allowed to transmit simultaneously. Nodes try to resolve in-range delivery of packets with an adaptive distributed Hybrid ARQ scheme to counteract interference from simultaneously active communications, while dynamic route selection is implemented for avoiding transmissions over links with harsh fading conditions. We assess the performance of our scheme through detailed simulations.

Using game theory and Bayesian networks to optimize cooperation in ad hoc wireless networks

Infrastructure sharing has been recently investigated as a viable solution to increase the performance of coexisting wireless networks. In this paper, we analyze a scenario where two wireless networks are willing to share some of their nodes to gain benefits in terms of lower packet delivery delay and reduced loss probability. Bayesian Network analysis is exploited to compute the correlation between local parameters and overall performance, whereas the selection of the nodes to share is made by means of a game theoretic approach. Our results are then validated through use of a system level simulator, which shows that an accurate selection of the shared nodes can significantly increase the performance gain with respect to a random selection scheme.

Multihop Beaconing Forwarding Strategies in Congested IEEE 802.11p Vehicular Networks

Multihop propagation of situational information is a promising technique for improving beaconing performance and increasing the degree of situation awareness onboard vehicles. A possible way of achieving this is by piggybacking information on the beacon packets that are periodically sent by each vehicle in the network, as prescribed by the Dedicated Short-Range Communications standards and the European Telecommunications Standards Institute. However, prescribed limitations on beacon size imply that only information on a very small number of surrounding vehicles can be piggybacked in a beacon packet. In most traffic situations, this number is well below the typical number of vehicles within the transmission range, implying that multihop forwarding strategies must be devised to select which neighboring vehicles information to include in a transmitted beacon. In this paper, we designed different multihop forwarding strategies and assessed their effectiveness in delivering fresh situational information to surrounding vehicles. Effectiveness is estimated in terms of both information age and the probability of experiencing a potentially dangerous situation-awareness blackout. Both metrics are estimated as a function of the hop distance from the transmitting vehicle and in the presence of different levels of radio channel congestion. The investigation is based on extensive simulations whose multihop communication performance is corroborated by real-world measurements. The results show that network-coding-based strategies substantially improve forwarding performance, as compared with a randomized strategy, reducing the average information age by up to 60% and the blackout probability by up to two orders of magnitude. We also consider the effect of multihop propagation of situational information on the reliability of a forward collision warning application and show that network-coding-based propagation yields a factor-3 improvement of reliability with respect to a randomized forwarding strategy and even higher improvements with respect to the case of no propagation.

Performance Analysis of an Opportunistic Relay Selection Protocol for Multi-Hop Networks

We analyze the performance of an interference-aware opportunistic relay selection protocol for multi-hop line networks which is based on the following simple rule: a node always transmits if it has a packet, except when its successive node on the line is transmitting. We derive analytically the saturation throughput and the end-to-end delay for two and three hop networks, and present simulation results for higher numbers of hops. In the case of three hops, we determine the throughput-optimal relay positions.

An algorithmic solution for computing circle intersection areas and its applications to wireless communications

The computation of the intersection area of a large number of circles with known centers and radii is a challenging geometric problem. Nevertheless, its resolution finds several applications in the analysis and modeling of wireless networks. Prior literature discusses up to three circles and even in this case there are many possible geometric configurations, each leading to a different involved close-form expression for the intersection area. In this paper, we derive two novel geometric results, that allow the check of the existence and the computation of the area of the intersection regions generated by more than three circles by simple algebraic manipulations of the intersection areas of a smaller number of circles. Based on these results, we construct an iterative algorithm based on a trellis structure that efficiently computes the intersection areas of an arbitrary number of circles. As an example of practical application of our results, we derive the probability that a fixed number of mobiles can be reliably allocated to a set of base stations in code division multiple access-based cellular networks.

Evaluating multi-hop beaconing forwarding strategies for IEEE 802.11p vehicular networks

Multi-hop propagation of situational information is a promising technique for improving beaconing performance and increasing the degree of situational awareness onboard vehicles. However, limitation on beacon size prescribed by standardization bodies implies that only information about 3-4 surrounding vehicles can be piggybacked in a beacon packet. In most traffic situations, the number of vehicles within transmission range is much larger than 3-4, implying that multi-hop forwarding strategies must be devised to select which neighboring vehicles information to include in a transmitted beacon. In this paper, we investigate the effectiveness of different multi-hop forwarding strategies in delivering fresh situational information to surrounding vehicles. Effectiveness is estimated in terms of both average information age and probability of experiencing a situational-awareness blackout of at least 1 sec. Both metrics are estimated as a function of the hop distance from the transmitting vehicle. The investigation is based on extensive simulations whose multi-hop communication performance is corroborated by real-world measurements. The results show that network-coding based strategies substantially improve forwarding performance as compared to a randomized strategy, reducing the average information age of up to 60%, the blackout probability of two orders of magnitude, and providing a performance similar to that of an idealized strategy in which complete situational information is included in the beacon.

D2D Communications in the Uplink: A Context-Aware Approach with Punishment

Device-to-device (D2D) communication is one of the most promising solutions to the dramatic increase of wireless networks traffic load. In D2D communications, mobile nodes can communicate in a semi-autonomous way, with minimal or no control by the base station (BS). In this context, we address the problem of the coexistence of cellular and D2D tiers in the uplink frequencies, where a D2D source is allowed to transmit without direct control of its scheduling by the base station (BS). In order to limit the interference, we add a punishment mechanism triggered by the BS to limit the activity of disturbing terminals. We propose a context- aware channel access mechanism for a D2D source, where the context-awareness is obtained by 1) observing the topology and the wireless transmissions in the proximity of the D2D source, and 2) exploiting the past knowledge learned thanks to a Bayesian network approach. To design the channel access mechanism, we study the tradeoff between maximizing the end-to-end throughput and minimizing the interference to the cellular tier. We then evaluate the performance improvement of the proposed solution, showing the effectiveness of the learning mechanism and the advantages of context awareness.

Performance of Advanced Decoding Schemes for Uplink Relaying in Cellular Networks

In this paper, we analyze the impact of relays on the uplink performance of frequency-division multiple-access cellular networks. We focus our analysis on decode-and-forward techniques, with the aim of measuring the improvements that can be achieved in terms of throughput and energy saving. We apply a stochastic-geometry-based approach to a scenario with intercell interference and full frequency reuse. The first goal of this work is to observe what is the impact of various relay features, such as transmission power, location, and antenna pattern, when a half-duplex constraint is imposed. The second goal is to determine how beneficial relaying can be also for mobiles who are not at the cell edge and who can therefore use a direct link toward the base station. We show that if more refined decoding techniques, such as Successive Interference Cancellation and Superposition Coding, are properly used, considerable gains can be obtained for these mobiles as well.