Miguel Luis

The Impact of Node's Mobility on Link-Detection Based on Routing Hello Messages

Several routing protocols for Mobile Ad Hoc Networks (MANETS), including the well known Ad hoc On-Demand Distance Vector Routing (AODV) and Optimized Link State Routing (OLSR), propose the use of periodic messages (Hello messages) to detect neighbor nodes. After receiving the first Hello message from one of its neighbors, a node starts the link sensing task by setting up a sensing timer. Each time a new Hello message is received from the same neighbor, the sensing timer is restarted and the link duration is prolonged. If the sensing timer expires, it indicates a long time interval without receiving an Hello message and, consequently, the link is considered broken. The transmission frequency of the Hello messages and the expiration value of the sensing timer truly depends on nodes mobility: if the nodes are moving quickly and the Hello messages are rarely transmitted, the neighbor nodes can be in communication range but they are not detected; in the same scenario, if the expiration value of the sensing timer is too high, a link is sensed broken too late. In this paper, we consider a MANET under the Random Waypoint mobility model. We investigate the relationship between the transmission frequency of the Hello messages and the sensing timer expiration value with the network nodes mobility. We formally deduce the probability of link existence after

Towards a Realistic Primary Users' Behavior in Single Transceiver Cognitive Networks

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Towards Reliable Broadcast in ad hoc Networks

periods of transmission of the Hello message. The probability is later used to define the sensing timer expiration value, considering a given probability that the Hello message transmission fails. Finally, we evaluate our study through both numerically analysis and simulations, which confirms the effectiveness and accuracy of our approach.

Towards the Use of XOR-Based Routing Protocols in Vehicular Ad Hoc Networks

Most of the models intended to describe the throughput of Primary (PUs) and Secondary (SUs) users of Cognitive Radio Networks (CRNs) assume that PUs only change their activity state (ON/OFF) in the beginning of each SUs operation cycle, admitting that PUs are synchronized with SUs operation cycle. This letter characterizes a more realistic scenario where PUs can randomly change their activity state during the SUs operation cycle. We derive an analytical model for the PUs throughput and its validation is assessed through simulation results. The analysis shows that assuming synchronized PUs leads to an undervaluation of the interference caused to PUs, and the interference decreases as more SUs operation cycles are performed per ON/OFF PUs activity state.

Improving routing performance in high mobility and high density ad hoc vehicular networks

This letter presents new results obtained with a novel MAC scheme designed to be reliable and maximize the throughput of a single-hop network. Contrarily to the IEEE 802.11 MAC scheme, all nodes adopt a common optimal contention window size for unicast and broadcast transmissions, which is based on a common view of the channel. In this way, the conditional collision probability is independent of the type of traffic (broadcast or unicast). Simulation results for different amounts of broadcast/unicast traffic indicate that our approach significantly outperforms IEEE 802.11, exhibiting broadcast aggregate throughput gains up to 40%. Moreover, the simulation results indicate that the probability of successful transmitting a broadcast frame can be also improved up to 50% and, as the number of nodes increase, broadcast frames can obtain approximately the same reliability and throughput achieved by IEEE 802.11 unicast frames.

Energy sensing parameterization criteria for cognitive radios

In this paper we present a performance analysis of XOR-based flat routing protocols in high mobility conditions, considering a vehicular ad hoc network (VANET) formed in a highway scenario. First, we describe an XOR-based protocol that incorporates several adaptations of the existing XOR-based routing algorithms for wired networks, in order to cope with the network mobility. Then we propose an improved version of it, XORi, which modifies the protocols information gathering process to accommodate the specific dynamic nature of VANETs topology. Finally, we evaluate the performance of XOR-based protocols with other topology-based routing protocols. Simulation results allow us to characterize the performance of this class of protocols through the comparison of the packet delivery ratio, end-to-end path delay and average number of path hops. When a moderate density of nodes is considered, simulations show that XOR-based algorithms achieve almost the same packet delivery rate as link state algorithms, such as OLSR, while for high density of nodes XOR-based algorithms scale better in terms of delay when compared to source routing algorithms, such as DSR.

Characterization of the Opportunistic Service Time in Cognitive Radio Networks

In ad hoc networks the broadcast nature of the radio channel poses a unique challenge because the wireless links have time-varying characteristics in terms of link capacity and link-error probability. In mobile networks, particularly in vehicular ad hoc networks (VANETs), the topology is highly dynamic due to the movement of the nodes, hence an on-going session suffers frequent path breaks. In this paper we present a method that uses the available knowledge about the networks topology to improve the routing protocols performance through decreasing the probability of path breaks. We propose a scheme to identify long duration links in VANETs, which are preferentially used for routing. This scheme is easily integrated in the existent routing protocols. We describe how to integrate it in the Optimized Link-State Routing Protocol 1. Finally, we evaluate the performance of our method with the original protocol. Simulation results show that our method exhibits better end-to-end path delay (almost one magnitude order lower) and packet delivery ratio (between 25% and 38% higher) than the original protocol. This observation is even more evident when the nodes density increases.

Maximizing throughput-fairness tradeoff in MAC for ad hoc networks

Energy-based sensing (EBS) has been a topic of strong research in the last years due to its importance to Cognitive Radios (CRs). While EBS has been used in several works, different parameterizations have been adopted. In this work we compare different parameterization criteria to determine the decision threshold to the energy detector. We compare their performance through simulations by characterizing the interference caused to primary users and the throughput achieved by the secondary users. We also assess their relative performance.

A Multi-Technology Communication Platform for Urban Mobile Sensing

This paper studies the service time required to transmit a packet in an opportunistic spectrum access scenario, where an unlicensed secondary user (SU) transmits a packet using the radio spectrum licensed to a primary user (PU). Considering a cognitive radio network, it is assumed that during the transmission period of an SU multiple interruptions from PUs may occur, increasing the time needed to transmit a packet. Assuming that the SUs packet length follows a geometric distribution, we start by deriving the probability of an SU transmitting its packet when k > 0 periods of PUs inactivity are observed. As the main contribution of this paper, we derive the characteristic function of the service time, which is further used to approximate its distribution in a real-time estimation process. The proposed methodology is independent of the SUs traffic condition, i.e., both saturated or non-saturated SUs traffic regime is assumed. Our analysis provides a lower bound for the service time of the SUs, which is useful to determine the maximum throughput achievable by the secondary network. Simulation results are used to validate the analysis, which confirm the accuracy of the proposed methodology.

Optimization of a Decentralized Medium Access Control Scheme for Single Radio Cognitive Networks

In the last years, there has been an increasing interest in developing and testing Medium Access Control (MAC) protocols for ad hoc networks based on optimal methods. Several solutions do not take into account the throughput-fairness tradeoff, presenting high throughput performance but under-performing in terms of medium access fairness. This work presents a novel MAC scheme, which is designed to maximize the network throughput-fairness performance 1. In our scheme, all nodes adopt a common optimal contention window, which is based on a common view of the channel. Our proposal improves the access fairness because the contention window is similar to all nodes, and its behavior does not depend on previous MAC states. Departing from the optimal throughput for a saturated network, we devise a scheme to estimate the number of nodes, which is a prime parameter to regulate the medium access control. We show that each node is suitable to estimate the number of competing nodes by using both its own medium access probability and the idle slot probability observed from the channel. Several simulation results evaluate the throughput-fairness performance of our proposal with several state-of-the-art MAC protocols, such as AOB, Idle Sense, GDCF and the well known IEEE 802.11. The results indicate that our approach presents the highest value of medium access fairness among the simulated protocols, meaning that its throughput is closer to the optimal theoretical throughput obtained for a fair network.2