2013 - Strategic Project

Funder

Organizational Unit

Publications

Frame Capture and Reliability Based Decider Implementation in the MiXiM IEEE 802.15.4 Framework

Publication . Borges, Luís M.; Velez, Fernando J.; Barroca, Norberto; Lebres, António

The task of properly modelling the physical (PHY) layer constitutes the most challenging endeavor in wireless networks simulation. Unfortunately, today, the majority of the wireless sensor network (WSN) simulators consider a simple model for the PHY frame reception, which does not account for emerging research on the frame capture (FC) effect. In this paper, we present enhancements for the PHY layer model for the IEEE 802.15.4 standard employed in the MiXiM framework, to account for the FC effect within WSN-based simulations. These improvements are as follows: i) proposal of a formulation for the PHY layer packet reception based on a reliability concept, identified as the Enhanced Reliability Decision Algorithm, which guarantees the delivery of a packet received by the PHY layer to the medium access control (MAC) layer, with a certain value for the reliability (0.9 and 0.99); ii) different frame overlapping scenarios, and iii) different values for the thresholds to decide frame recovery. The work includes the description, implementation and performance evaluation of the proposed decision algorithm, jointly with the FC effect, in the MiXiM framework simulator, for basic MAC and scheduled channel polling (SCP) MAC protocols. Based on the simulation results, the proposed approach can significantly improve simulation accuracy and provide a PHY decision algorithm that guarantees, with a certain reliability, the delivery of frames to the MAC layer.

2014-03Journal article

Open access

Survey on the Characterization and Classification of Wireless Sensor Network Applications

Publication . Borges, Luís M.; Velez, Fernando J.; Lebres, António S.

Nowadays, wireless sensor network (WSN) users are demanding more and more in terms of choice and diversity of applications. Hence, as the diversity of applications is increasing, it is worthwhile to propose a structure for the set of characterization parameters that allows sketching a taxonomy for WSN applications. This taxonomy is established via an application-oriented approach, identifying the specific services offered by each application. In this survey, we fill this gap in the WSN literature by describing the characterization parameters, organized into six different categories. Our taxonomy for application classification is centered on the different sets of parameters that have high impact on a given future WSN application. Typical attributes and values from related research works are considered as a reference, but in this survey, we propose inter- and intra-connections among the considered application groups. Based on these connections, new application groups have been proposed for applications that share common characterization parameters, along with a holistic overview of WSN application taxonomy and the discussion of the three generations of WSNs toward communication between things and the Internet of Things , as well as future trends for the development of WSN applications. Moreover, detailed parameters from different projects and authors in the field of WSNs are joined together for comparison purposes.

2014Journal article

Open access

Dynamic Configuration and Optimization of WiMAX Networks with Relay Power Saving Modes: Measurement-Based Scenario in a Hilly Region

Publication . Robalo, Daniel; Oliveira, João R.; Velez, Fernando J.; Holland, Oliver; Aghvami, A. Hamid

This paper investigates the performances achievable by WiMAX networks deployed in various sectorization configurations, with and without relay stations (RSs). Further, it studies the dynamic adjustment of the configuration to serve traffic loads at different times of the day while maximising the use of opportunistic sleep modes by relays in conjunction with cell zooming, thereby saving energy. The configuration changes and invocation of opportunistic sleep modes also take into account coverage constraints. This paper first reports extensive propagation measurements that have been undertaken in Covilha˜, a hilly area of Portugal which presents a realistic and challenging propagation scenario. Using this scenario as the topographical basis, practical cellular planning results are then obtained and compared, using the dominant path and ray tracing (RT) functionalities of WinpropTM. It is shown that without RSs present, the supported throughput is lower in practice because coverage is not 100 %. Further, for the case with omnidirectional cells, coverage reduces to only approximately 60 % if RSs go into sleep mode, and for the tri-sectored cells case coverage drops from 95.75 to 81.90 % (based on RT calculations) if RSs go into sleep mode. There is, however, still a reasonable economic performance in all cases. Additional results demonstrate that savings typically of 47.6 % in RSs’ average power consumption can be achieved. These savings are shown to result in a financial saving for the operator of 10 % of the combined operational and maintenance cost. However, it is observed that such solutions have to be used cautiously in hilly regions due to challenges in maintaining coverage.

2015-06Journal article

Open access

Transmitted Power Formulation for the Optimization of Spectrum Aggregation in LTE-A over 800 MHz and 2 GHz Frequency Bands

Publication . Acevedo Flores, Jessica Elizabeth; Robalo, Daniel; Velez, Fernando J.

This work starts by proposing a formulation to calculate the transmitter power needed to cover cells of different sizes, whilst maintaining the average signal to interference-plus-noise ratio constant, and near the maximum, for two Long Term Evolution (LTE) systems operating over non-contiguous frequency bands, 800 MHz and 2 GHz, with an integrated common radio resource management (iCRRM) entity. In the context of spectrum aggregation (SA), iCRRM is able to switch users between the two LTE-Advanced scenarios to facilitate the best user allocation and maximize the total network throughput in these LTE systems. We address a formulation based on the computation of the average received power and average co-channel interference in cellular topologies with frequency reuse pattern K = 3, keeping the presence of coverage holes insignificant, whilst considering the COST-231 Hata path loss model. We have verified how the normalized power increases as the cell radius increases. The objective of applying this formulation in the dimensioning process is to save power for the shortest coverage distances. It has been found that without SA the maximum average cell throughput is observed in the presence of 80 simultaneous users within the cell (40 for each LTE system, operating in different frequency bands). We have considered traced-based video sessions with a (video) bit rate of 128 kbps. In this scenario, through extensive simulations cell average supported throughput of approximately 6,800, 8,500 and 9,500 kbps have been obtained for the cases without SA (considering the sum of the 800 MHz and 2 GHz systems capacities), with a simple CRRM and with iCRRM, respectively. It was also found that when the peak throughput is achieved with 80 users, the average cell packet loss ratio without SA, with CRRM and iCRRM present values of 22, 11 and 7 %. The average cell delay with both CRRM and iCRRM entities is 22 ms, whereas without SA is equal to 32 ms. Finally, the cost/revenue tradeoff is analysed from the operator/service provider’s point of view, whose main goal is obtain the maximum profit from his business. It was found that CRRM increases the total profit in percentage, compared to a simple allocation, without SA. Nevertheless, the profit growth with iCRRM is even larger, from 253 to 296 % for 1,000 m and a price of 0.010 €/MByte. Therefore, our proposal for SA is convenient not only in terms of technical features and QoS, as loss and delay have been obtained within a range of reasonable values, but also in terms of economic aspects.

2014Journal article

Open access

Performance assessment of mobility solutions for IPv6-based healthcare wireless sensor networks

Publication . Caldeira, João Manuel Leitão Pires; Rodrigues, Joel José Puga Coelho; Lorenz, Pascal

This thesis focuses on the study of mobile wireless sensor networks applied to healthcare scenarios. The promotion of better quality-of-life for hospitalized patients is addressed in this research work with a solution that can help these patients to keep their mobility (if possible). The solution proposed allows remote monitoring and control of patients’ health in real-time and without interruptions. Small sensor nodes able to collect and send wirelessly the health parameters allow for the control of the patients' health condition. A network infrastructure, composed by several access points, allows the connection of the sensor nodes (carried by the patients) to remote healthcare providers. To ensure continuous access to sensor nodes special attention should be dedicated to manage the transition of these sensor nodes between different access points’ coverage areas. The process of changing an access point attachment of a sensor node is called handover. In that context, this thesis proposes a new handover mechanism that can ensure continuous connection to mobile sensor nodes in a healthcare wireless sensor network. Due to the limitations of sensor nodes’ resources, namely available energy (these sensor nodes are typically powered by small batteries), the proposed mechanism pays a special attention in the optimization of energy consumption. To achieve this optimization, part of this work is dedicated to the construction of a small sensor node. The handover mechanism proposed in this work is called Hand4MAC (handover mechanism for MAC layer). This mechanism is compared with other mechanisms commonly used in handover management. The Hand4MAC mechanism is deployed and validated through by simulation and in a real testbed. The scenarios used for the validation reproduces a hospital ward. The performance evaluation is focused in the percentage of time that senor nodes are accessible to the network while traveling across several access points’ coverage areas and the energy expenditures in handover processes. The experiments performed take into account various parameters that are the following: number of sent messages, number of received messages, multicast message usage, energy consumption, number of sensor nodes present in the scenario, velocity of sensor nodes, and time-to-live value. In both simulation and real testbed, the Hand4MAC mechanism is shown to perform better than all the other handover mechanisms tested. In this comparison it was only considered the most promising handover mechanisms proposed in the literature.

2013-10Doctoral thesis

Open access