Rodolfo Oliveira

Vehicle and Mission Control of the DELFIM Autonomous Surface Craft

DELFIM is an autonomous surface craft developed at ISR/IST for automatic marine data acquisition and to serve as an acoustic relay between submerged craft and a support vessel. The paper describes the navigation, guidance, and control systems of the vehicle, together with the mission control system that allows end-users to seamlessly program and run scientific missions at sea. Practical results obtained during sea tests in the Atlantic, near Azores islands, are briefly summarized and discussed.

Frequency-domain multipacket detection: a high throughput technique for SC-FDE systems

The traditional approach to cope with collisions is to discard the packets involved in it and to ask for their retransmission. However, since the signal associated to a collision has important information concerning the packets involved, we can efficiently resolve collisions with proper retransmissions. In this paper we consider severely time-dispersive channels and we propose a technique that allows efficient packet separation. We employ SC-FDE schemes (single-carrier with frequency-domain equalization) and we propose an iterative frequency-domain multi-packet detection. Since our technique requires uncorrelated channels for different retransmissions, we also propose an SP technique (Shifted Packets) for retransmissions in fixed channels. Since the total number of transmissions is equal to the number of packets involved in the collision (even when the channel remains fixed for the retransmissions), our technique allows high throughputs. The analysis of our detection technique combined with the NMDA (network-assisted diversity multiple access) MAC (medium access control) protocol shows significant throughput and delay improvements for low Eb/N0 values compared to a TDMA (time division multiple access) approach, where collisions are avoided. This technique is particularly appealing for the uplink of broadband wireless systems, since we consider SC-FDE schemes and the complexity is concentrated in the receiver. By employing the SP scheme we can use the same channel for the retransmissions, with only a small performance degradation.

Review: The influence of broadcast traffic on IEEE 802.11 DCF networks

The performance of the IEEE 802.11 protocol depends on multiple factors. One of them is related with the relative amounts of broadcast and unicast traffic in the total load due to the coexistence of the different transmission schemes applied to each of these types of traffic. This paper presents an analytical model to compute the 802.11 probability of a successful transmission of a frame and the average transmission delay assuming the presence of both unicast and broadcast traffic. Several realistic issues are addressed, as pre- and post-transmission backoffs, variable frame length and finite MAC buffers. Broadcast frame transmission has a swifter algorithm. When compared to broadcast, unicast frame transmission exhibits a more reliable scheme to avoid a frame from being discarded when it collides, but creates a trade off as it can introduce larger transmission delays. Simulation results are presented and compared to the analytical computations validating the models accuracy. Finally, interesting results related with the influence of the amount of broadcast or unicast traffic on the networks performance for non-saturation and saturation operating zones are discussed for different data transmission rates. This is particularly relevant as most of the ad hoc routing algorithms rely heavily on broadcast.

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

Frequency-Domain Multipacket Detection: A High Throughput Technique for SC-FDE Systems

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Analytical BER and PER Performance of Frequency-Domain Diversity Combining, Multipacket Detection and Hybrid Schemes

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 a Realistic Primary Users' Behavior in Single Transceiver Cognitive Networks

Usually, packets involved in a collision are lost, requiring their retransmission. However, the signal associated to collisions has important information concerning the packets involved. In fact, with proper retransmissions we can efficiently resolve collisions. In this paper we propose a frequency-domain multi- packet detection technique for SC-FDE schemes (Single- Carrier with Frequency-Domain Equalization) that allows an efficient packet separation in the presence of successive collisions. This technique allows high throughputs, since the total number of transmissions is equal to the number of packets involved in the collision, even when the channel remains fixed for the retransmissions. Since we consider SC-FDE schemes and the complexity is concentrated in the receiver, this technique particularly appealing for the uplink of broadband wireless systems.

Modelling Delay on IEEE 802.11 MAC Protocol for Unicast and Broadcast Nonsaturated Traffic

Diversity Combining (DC) and Multipacket Detection (MPD) are efficient techniques that cope with packet errors due to severe propagation conditions and/or collisions. To evaluate their network performance on a network simulator, an accurate characterization of the Bit Error Rate (BER) and Packet Error Rate (PER) can successfully replace the behavior of the PHY layer. This choice is better in terms of computational time than implementing a full PHY layer to be jointly simulated with the upper layers. In this paper, an analytical model is proposed to compute the BER and PER of DC, MPD and hybrid techniques, considering a Single-Carrier with Frequency-Domain Equalization (SC-FDE) scenario. The paper also considers two frequency-domain detection methods: a simple linear receiver and a powerful iterative receiver. The proposed model is simple and versatile, since it can support nonhomogeneous transmitting powers, and different channel conditions concerning retransmissions and channel randomization techniques. Several simulations show that the proposed model provides accurate BER and PER results for a wide range of scenarios.

Performance Analysis of the IEEE 802.11 Distributed Coordination Function with Unicast and Broadcast Traffic

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.

Performance of Packet Combining ARQ Error Control in a TDMA SC-FDE System

The existing models for IEEE 802.11 DCF networks only consider unicast frames, ignoring the existence of broadcast traffic. In a real scenario, the stations are most of the times non-saturated and unicast and broadcast frames exist. These specific characteristics influence the service time which consequently affects the queue behaviour. In this paper, the authors model the total frames delay for IEEE 802.11 DCF networks in presence of both unicast and broadcast traffic. Our theoretical analysis proposes a model able to compute the time between the instant when a new frame is inserted on the transmission queue and the instant when its transmission finishes. The time needed to serve a frame (service time) is formally deduced from the IEEE 802.11 transmission procedure, conceptually using the view of one networks node. The authors use an M/M/1/K queue model in order to model each frames queueing delay. The authors validate the total frames delay using several simulations and present some results. These are analysed for different scenarios of broadcast/unicast network loads and different number of nodes.