Papers by Alejandro Martinez-Sala
Resumen-. Los sistemas de localización en interiores son la base de prometedores servicios telemá... more Resumen-. Los sistemas de localización en interiores son la base de prometedores servicios telemáticos conscientes del contexto y entorno y se requiere un control de la posición con un error acotado. La medida de potencia de la señal recibida se puede usar como métrica para determinar la posición a partir de un algoritmo de localización. Se requiere hacer medidas para caracterizar los patrones de la señal recibida en un conjunto de puntos de interés (técnica de fingerprint). El canal en un interior es difícil de modelar y presenta un comportamiento aleatorio. En este trabajo de investigación se usan redes neuronales como herramienta matemática que aprende los patrones del fingerprint y su posición (x,y) asociada y son capaces de estimar una posición. Se evalúan varias arquitecturas del perceptron multicapa y se analiza el error de la estimación de la posición y su influencia con la variabilidad del canal para un escenario de una red WiFi.
This paper presents DIGCOMStudio, a software tool programmed in MATLAB which allows the study of ... more This paper presents DIGCOMStudio, a software tool programmed in MATLAB which allows the study of error rate in low power digital communication devices. These kinds of devices are supposed to be used as transceivers in wireless sensor networks. DIGCOMStudio supposes an excellent tool in order to feed the transmitter and analyze the demodulated signal. It generates base-band digital signals according to the parameters specified by the user. Then it compares them with the received signal and estimates the error rate. It also supposes a didactical scenario easy to use when studying and analyzing the properties of such communications.
Computers in Industry, 2005
Present basis of knowledge management is the efficient share of information. The challenges that ... more Present basis of knowledge management is the efficient share of information. The challenges that modern industrial processes have to face are multimedia information gathering and system integration, through large investments and adopting new technologies. Driven by a notable commercial interest, wireless networks like GSM or IEEE 802.11 are now the focus of industrial attention, because they provide numerous benefits, such as low cost, fast deployment and the ability to develop new applications. However, wireless nets must satisfy industrial requisites: scalability, flexibility, high availability, immunity to interference, security and many others that are crucial in hazardous and noisy environments. This paper presents a thorough survey of all this requirements, reviews the existing wireless solutions, and explores possible matching between industry and the current existing wireless standards. #
In this paper we propose the use of non-persistent CSMA as an anti-collision procedure for RFID a... more In this paper we propose the use of non-persistent CSMA as an anti-collision procedure for RFID active tags. Current proposals for both passive and active tags are based on the framed slotted ALOHA protocol, which does not scale well requiring additional procedures for frame length adaptation. However, active RFID devices already include carrier sense capabilities with no additional cost and, thus, CSMA may be employed seamlessly. Nevertheless, selecting the contention micro-slots of CSMA in the classical way (i.e., with a uniform distribution and an exponential back-off algorithm) does not result in an efficient identification process, as we will demonstrate. Fortunately, better choices can be found. Recently, an optimal distribution for the selection of micro-slots for event-driven sensor networks has been computed, as well as a practical implementation: the Sift distribution. In this work we propose the application of the quasi-optimal Sift distribution along with CSMA for active tag identification. By means of an analytical study, we evaluate the average time needed for identification with this mechanism and compare it with the current ISO 18000-7 and EPC “Gen 2” standard. The results reveal that the Sift-based non-persistent CSMA outperforms both of them. Moreover, it also scales much better, without the need for further adaptation mechanisms.
This document describes a proposal for a simple, low-cost wireless location and identification sy... more This document describes a proposal for a simple, low-cost wireless location and identification system planned to track a significant number of mobile items in an industrial scenario. There are several industrial processes which would be improved just by keeping track of path along the production and distribution chain of a processed item and its modifications. These data would feedback in its turn, the company information and logistics system. The key problems and requirements of such a system are discussed, as well as some suitable technologies. A proposal for a solution based on low-cost, simple reader/writer devices is shown. Since there are not commercial products that meet the requirements demanded, first step in research is the development of a custom device, focusing on the design of efficient access protocols for a large number of devices simultaneously attempting to communicate.
Personal and Ubiquitous Computing, 2008
Wireless sensor networks (WSN) are designed for data gathering and processing, with particular re... more Wireless sensor networks (WSN) are designed for data gathering and processing, with particular requirements: low hardware complexity, low energy consumption, special traffic pattern support, scalability, and in some cases, real-time operation. In this paper we present the virtual TDMA for sensors (VTS) MAC protocol, which intends to support the previous features, focusing particularly on real-time operation. VTS adaptively creates a TDMA arrangement with a number of timeslots equal to the actual number of nodes in range. Thus, VTS achieves an optimal throughput performance compared to TDMA protocols with fixed size of frame. The frame is set up and maintained by a distributed procedure, which allows sensors to asynchronously join and leave the frame. In addition, duty cycle is increased or decreased in order to keep latency constant below a given deadline. Therefore, a major advantage of VTS is that it guarantees a bounded latency, which allows soft real-time applications.
Computers in Industry, 2009
The fresh products supply chain still has room to improve and increase efficiency and even become... more The fresh products supply chain still has room to improve and increase efficiency and even become an intelligent supply chain by means of automating simple but valuable data flows which will be the foundations and first stage for developing new value-added services. ...
IEEE 802.11 is being currently considered to develop telecommunications infrastructures in indust... more IEEE 802.11 is being currently considered to develop telecommunications infrastructures in industrial environments. However, in such environments there may exist a high level of electromagnetic fields (EMF) contamination, due to the machinery in use. Wireless LANs are sensitive to this type of hazard, and therefore the performance of the protocols may be affected inside the industrial facilities. In this paper we conduct a study of the wireless link performance in these industrial scenarios, using personal computing devices. Such equipments will be likely used by employees rather than conventional computers. Since the results do not characterize the environment itself, but the set of environment and hardware, a proper measurement test-bed must include direct measurements with these devices. Besides, due to limitations in these equipments, a specific protocol to measure pairs of packet error probability and latency is also developed. Results reveal a significant linear dependency between both magnitudes, and a large correlation coefficient in the measurements under high EMF activity at mid-large ranges (distances greater than 5 meters)
Computers in Industry
Present basis of knowledge management is the efficient share of information. The challenges that ... more Present basis of knowledge management is the efficient share of information. The challenges that modern industrial processes have to face are multimedia information gathering and system integration, through large investments and adopting new technologies. Driven by a notable commercial interest, wireless networks like GSM or IEEE 802.11 are now the focus of industrial attention, because they provide numerous benefits, such as low cost, fast deployment and the ability to develop new applications. However, wireless nets must satisfy industrial requisites: Scalability, flexibility, high availability, immunity to interference, security and many others that are crucial in hazardous and noisy environments. This paper presents a thorough survey of all this requirements, reviews the existing wireless solutions, and explores possible matching between industry and the current existing wireless standards.
Computer Networks, 2007
In this paper we provide a method to analytically compute the energy saving provided by the use o... more In this paper we provide a method to analytically compute the energy saving provided by the use of Transmission Power Control (TPC) at the MAC layer in Wireless Sensor Networks (WSN). We consider a classical TPC mechanism: data packets are transmitted with the minimum required power to achieve a given packet error probability, whereas the additional MAC control packets are transmitted with the nominal (maximum) power. This scheme has been chosen because it does not modify the network topology, since control packet transmission range does not change. This property also allow us to compute analytically the expected energy savings. Besides, this type of TPC can be implemented in the current sensor hardware, and can be applied directly to several MAC protocols already proposed for WSN. The foundation of our analysis is the evaluation of L ratio, defined as the total energy consumed by the network using the original MAC protocol divided by the total energy consumed if the TPC mechanism is employed. In the L computation we emphasize the basic properties of sensor networks. Namely, the savings are calculated for a network that is active a very long time, and where the number of sensors is supposed to be very large. The nodes position is assumed to be random -for the sake of example a normal bivariate distribution is assumed in the paper-and no node mobility is considered. In the analysis we stress the radio propagation and the distribution of the nodes in the network, that will ultimately determine the performance of the TPC. Under these conditions we compute the mean value of L. Finally, we have applied the method to evaluate the benefits of TPC for TDMA and CSMA with two representative protocols, L-MAC and S-MAC using their implementation reference parameters. The conclusion is that, while S-MAC does not achieve a significant improvement, L-MAC may reach energy savings up to 10-20%.
Wireless Sensor Networks (WSN) are formed by a large number of sensing nodes at the ground level.... more Wireless Sensor Networks (WSN) are formed by a large number of sensing nodes at the ground level. These devices are monitoring and measuring physical parameters from the environment. Simulation is used to study WSN, since deploying test-beds supposes a huge effort. However simulation results rely on physical layer assumptions, which are not usually accurate enough to capture the real behaviour of WSN. In this work several measurement campaigns are performed in three different scenarios: an open quasi-ideal area, a university yard and a park. The main contribution of this work is that a two slopes lognormal path-loss near ground outdoor channel model at 868 MHz is validated, and compared to the widely used one slope model. This model is useful for simulations because its computational cost is low.
Wireless Sensor Networks (WSN) are designed for data gathering and processing, with particular re... more Wireless Sensor Networks (WSN) are designed for data gathering and processing, with particular requirements and constraints: low hardware complexity, low energy consumption, special traffic pattern support, scalability, and in some cases, real-time operation. In this paper we present the Virtual TDMA for Sensors (VTS) MAC protocol, which intends to support the previous features, focusing particularly on real-time operation. VTS adaptively creates a TDMA arrangement with a number of timeslots equal to the actual number of nodes in range. Thus, VTS achieves an optimal throughput performance compared to TDMA protocols with fixed size of frame. The TDMA frame is set up and maintained by a distributed procedure, which allows sensors to asynchronously join and leave the frame. A major advantage of VTS is that it guarantees a bounded latency, which allows soft real-time applications. An expression for the upper latency bound is also provided in this paper. VTS performance is evaluated by simulation. Results show less power consumption than other proposals in the field. We also introduce a novel multi-hop operation by coordinated sleep/awake cycles among clusters.
Radio Frequency Identification (RFID) systems are one of the enabling technologies for the ubiqui... more Radio Frequency Identification (RFID) systems are one of the enabling technologies for the ubiquitous computing paradigm. At the moment, the EPCglobal organization leads the development of industry-driven standards for this field and has settled the EPC "Gen 2" as a reference standard. In this paper, we analyze the anti-collision procedure of EPC "Gen 2" to find the time needed to identify a population of tags, by means of the finite Markov-chain of the system. In addition, a UHF multi-reader prototype based on time division multiple access (TDMA) scheme is evaluated in this work. In this TDMA scheme, the reader timeslot duration is allocated according to the computations obtained from our analytical study. The main conclusions derived from this implementation are summarized in this work.
IEEE Communications Magazine, 2006
The formidable growth of WSN research has opened challenging issues about their performance evalu... more The formidable growth of WSN research has opened challenging issues about their performance evaluation. Despite the steady increase in mathematical analysis and experimental deployments, most of the community has chosen simulation for their study. Although it seems straightforward, this approach becomes a quite delicate matter. Complexity is caused by several issues. First, the large number of nodes heavily impacts simulation performance and scalability. Second, credible results demand an accurate characterization of the sensor radio channel. New aspects, inherent in WSN, must be included in simulators, e.g. a physical environment and an energy model, leading to different degrees of accuracy versus performance. Moreover, many necessary models are in the continuous-time domain (e.g. heat transmission, battery discharge), being complex to integrate into discrete event network simulators. These issues result in an exponential growth of the overall network state information. Through this survey we review these problems both quantitatively and qualitatively while depicting a common suitable simulation model. We also briefly describe the most significant simulation frameworks available.
Wireless Sensor Networks (WSN) are formed by a large number of networked sensing nodes. It is rat... more Wireless Sensor Networks (WSN) are formed by a large number of networked sensing nodes. It is rather complex, or even unfeasible, to model analytically a WSN and it usually leads to oversimplified analysis with limited confidence. Besides, deploying test-beds supposes a huge effort. Therefore, simulation is essential to study WSN. However, it requires a suitable model based on solid assumptions and an appropriate framework to ease implementation. In addition, simulation results rely on the particular scenario under study (environment), hardware and physical layer assumptions, which are not usually accurate enough to capture the real behavior of a WSN, thus, jeopardizing the credibility of results. However, detailed models yields to scalability and performance issues, due to the large number of nodes, that depending on application, have to be simulated. Therefore, the tradeoff between scalability and accuracy becomes a major issue when simulating WSN.
Uploads
Papers by Alejandro Martinez-Sala