Delft University of TechnologyMaster’s Thesis in Computer Science

Feasibility Study and Design for WirelessSensor Networks in a Space Environment

Marco Vinicio Alvarez Fernandez

Feasibility Study and Design for Wireless Sensor

Networks in a Space Environment

Master’s Thesis in Computer Science

Embedded Software SectionFaculty of Electrical Engineering, Mathematics and Computer Science

Delft University of TechnologyMekelweg 4, 2628 CD Delft, The Netherlands

Marco Vinicio Alvarez Fernandezmalvarezfernandez@student.tudelft.nl

25th July 2011

AuthorMarco Vinicio Alvarez Fernandez (malvarezfernandez@student.tudelft.nl)

TitleFeasibility Study and Design for Wireless Sensor Networks in a Space Environment

MSc presentation8th August 2011

Graduation CommitteeProf. Dr. Koen Langendoen(chair) Delft University of TechnologyDr. Stefan Dulman Delft University of TechnologyDr. T. Keviczky Delft University of TechnologyMr M. Castera Innovative Solutions In Spaces B.V.

Abstract

Wireless sensor networks is a technology that has been used in a vast numberof applications and environments with successful results in the past. Thereforeincreasing nowadays the number of users of this type of devices and their new ap-plications. In this thesis we worked hand-in-hand with the ISIS B.V. Companyto provide a proper WSN design to be deployed inside a spacecraft structure andsatisfy a group of defined requirements. ISIS or Innovative Solutions in Spaceis a private company founded in January 2006 that focus on the developmentof nanosatellites and several space related services. Therefore this thesis willprovide a detail feasibility study as for a communication and network design,for a wireless sensor network to be deployed in a satellite’s environment. Thestudy will base on the deployment of this type of devices inside the ENVISATstructural model, where a measurements methodology is applied and the finalresults are analyzed. Following this study a communication scheme design isshown and described in detail, explaining the power consumption repercussionof this protocol on the final design. Finally presenting a complete network de-sign providing both hardware and software details of the two main characters ofthis given network, the sensor node and the network’s gateway. The final designis evaluated by a protocol’s simulation which will provide a description on thefinal communication protocol. The results presented in this section will showthe repercussion of the different network parameters on the network behaviorand will provide a final evaluation on the defined final design.

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Preface

I would like to thank the people at ISIS B.V. who made this thesis possible.From ISIS I would like to give special thanks to Maxime Castera and Eddie vanBreukelen who gave me their support through the complete development of thisthesis. I would also like to thank Alex Fitie who help me a great deal on thesimulation part of this thesis. I want also to give special thanks to my supervisorStefan Dulman for the help and guidance since the beginning of the project andfinally I would like to thank my family and friends for their continuous supportover these past 2 years.

Marco Vinicio Alvarez Fernandez

Delft, The Netherlands25th July 2011

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Contents

Preface v

1 Introduction 1

1.1 The Project . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.2 The Problem Statement . . . . . . . . . . . . . . . . . . . . . . . 4

1.3 The Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.4 Thesis Contribution . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.5 Thesis Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2 Related Work 7

2.1 Wireless Sensor Networks signal measurements . . . . . . . . . . 7

2.1.1 Overview on different types of measurements . . . . . . . 8

2.1.2 Signal Strength versus distance . . . . . . . . . . . . . . . 9

2.1.3 Antenna Directionality . . . . . . . . . . . . . . . . . . . . 10

2.1.4 Interference effect . . . . . . . . . . . . . . . . . . . . . . 11

2.2 Existing MAC designs for WSN . . . . . . . . . . . . . . . . . . . 12

2.2.1 IEEE 802.15.4 and the Zigbee protocol . . . . . . . . . . . 13

2.2.2 ALOHA protocol . . . . . . . . . . . . . . . . . . . . . . . 13

2.2.3 Sensor-MAC protocol . . . . . . . . . . . . . . . . . . . . 14

2.2.4 Lightweight Medium Access protocol . . . . . . . . . . . . 17

2.2.5 Crankshaft protocol . . . . . . . . . . . . . . . . . . . . . 18

2.3 Quality of service and Communication Schemes . . . . . . . . . . 20

3 Feasibility Study of wireless sensor networks in a satellite struc-ture 29

3.1 Measurements Devices . . . . . . . . . . . . . . . . . . . . . . . . 29

3.2 Environment and devices configuration . . . . . . . . . . . . . . . 30

3.2.1 Open field environment (Reference) . . . . . . . . . . . . 30

3.2.2 ENVISAT satellite . . . . . . . . . . . . . . . . . . . . . . 32

3.3 Experimental Results . . . . . . . . . . . . . . . . . . . . . . . . . 34

3.3.1 Sky Mote Device . . . . . . . . . . . . . . . . . . . . . . . 35

3.3.2 Gnode Device . . . . . . . . . . . . . . . . . . . . . . . . . 35

3.4 Analysis of Results . . . . . . . . . . . . . . . . . . . . . . . . . . 36

3.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

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4 A WSN design for a space application: System Requirementsand Design Overview 414.1 System Requirements . . . . . . . . . . . . . . . . . . . . . . . . 41

4.1.1 Sensor Node . . . . . . . . . . . . . . . . . . . . . . . . . 414.1.2 Gateway Node . . . . . . . . . . . . . . . . . . . . . . . . 43

4.2 Design overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . 444.2.1 General network overview . . . . . . . . . . . . . . . . . . 45

5 A WSN design for a space application: Communication Proto-col Design 515.1 Data transmission and power consumption . . . . . . . . . . . . . 51

5.1.1 Transmission configuration . . . . . . . . . . . . . . . . . 515.1.2 Conclusion and Quality of Service . . . . . . . . . . . . . 53

5.2 Communication scheme . . . . . . . . . . . . . . . . . . . . . . . 545.2.1 Scheme use cases . . . . . . . . . . . . . . . . . . . . . . . 545.2.2 Frame definition . . . . . . . . . . . . . . . . . . . . . . . 58

6 A WSN design for a space application: Final Network Design 616.1 Sensor Node Design . . . . . . . . . . . . . . . . . . . . . . . . . 61

6.1.1 Core module . . . . . . . . . . . . . . . . . . . . . . . . . 616.1.2 Sensor module . . . . . . . . . . . . . . . . . . . . . . . . 626.1.3 Software Design . . . . . . . . . . . . . . . . . . . . . . . 636.1.4 Hardware design . . . . . . . . . . . . . . . . . . . . . . . 646.1.5 Low Power design . . . . . . . . . . . . . . . . . . . . . . 64

6.2 Gateway Node Design . . . . . . . . . . . . . . . . . . . . . . . . 666.2.1 Data handling module design . . . . . . . . . . . . . . . . 666.2.2 Software design . . . . . . . . . . . . . . . . . . . . . . . . 676.2.3 Redundancy algorithm [2] . . . . . . . . . . . . . . . . . . 676.2.4 Hardware design . . . . . . . . . . . . . . . . . . . . . . . 69

7 A WSN design for a space application: Evaluation 717.1 Communication Protocol Simulation . . . . . . . . . . . . . . . . 71

7.1.1 Simulation Overview . . . . . . . . . . . . . . . . . . . . . 717.1.2 Experimental Results . . . . . . . . . . . . . . . . . . . . 727.1.3 Results Analysis . . . . . . . . . . . . . . . . . . . . . . . 747.1.4 Simulation Conclusions . . . . . . . . . . . . . . . . . . . 76

8 Conclusions and Future Work 778.1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 778.2 Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78

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Chapter 1

Introduction

Wireless sensor networks (WSNs) is a technology that has been used in a vastnumber of applications and environments with successful results in the past.Several studies on medical, commercial and industrial applications have shownsuccessful and promising outcomes, increasing the interest of new users towardsthis type of monitoring solutions.

This type of network is defined as a group of autonomous sensor nodes thatsend their sensed data to a main network gateway by using a radio frequencychannel of communication. These devices are mostly based on a low powerconsumption design making it possible to power the network’s sensor nodeswith their own individual batteries and keep an extended lifetime through evenseveral years of operation without having to restore any of them.

In this thesis we worked hand-in-hand with the ISIS B.V. Company to providea proper WSN design to be deployed inside a spacecraft structure and satisfya group of requirements that will be described further ahead in this thesis.ISIS or Innovative Solutions in Space is a private company founded in January2006 that focus on the development of nanosatellites and several space relatedservices. The company is defined by 4 main branches: ISIS NanoSatellite Mis-sions, www.CubeSatShop.com, Innovative Space Logistics BV and InnovativeData Services BV.

The First branch focuses on providing turnkey space missions for institutional,governmental and commercial customers. The second one is a webshop special-ized in CubeSats and nanosatellites components. The third branch will providea service for the necessary logistics for small satellite launches and finally thefourth branch will provide developing support for NanoSatellites applications.

The client of the project is the European Space Agency (ESA), which is anintergovernmental organisation dedicated to the exploration of space. This wasestablished in 1975 and is currently defined by 18 member states. ESA sciencemissions are based at ESTEC in Noordwijk, Netherlands, Earth Observationmissions at ESRIN in Frascati, Italy, ESA Mission Control (ESOC) is in Darm-stadt, Germany, the European Astronaut Centre (EAC) that trains astronautsfor future missions is situated in Cologne, Germany, and the European SpaceAstronomy Centre is located in Villanueva de la Caada, Spain.

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1.1 The Project

The project to which this thesis gives its contribution has as a main objectiveto demonstrate value of wireless systems for the space domain. The companybelieves wireless communication has several benefits over the wired technology(technology currently used) and can be exploited in order to save time andmoney during a space project, greatly reduce risk of human induced damage(by reducing the human involvement in the assembly and integration stage ofa project), and enhance the capability of certain sub domains in spacecraftengineering.

The current state of the art in spacecraft engineering is that individual equip-ment boxes are interconnected with each other using cable assemblies or har-nessing. A wide number of small sensors, for instance for structural healthmonitoring are distributed throughout the spacecraft and are routed to the on-board computer (OBC) via wired connections. In addition, during ground test,additional sensors are place on the inside or outside of the spacecraft to in-crease the number of data points during testing, these sensors are also wired toelectrical ground support equipment (EGSE).

The integration of cable harness in aircraft and spacecraft is so complex thatseveral companies are actively marketing the capability to do so as a core compe-tence. Companies such as Stork Fokker ELMO (for aircraft) and EADS Astriumare specialized in integrated complex cable harness systems.

Having these aspects in mind the project aims to the advantages the wirelesstechnology can offer over existing solutions, here presenting the following:

• Increased mobility of crew, sensors and instrumented systems by eliminat-ing tethers for data and power routing.

• A strong reduction or even the elimination of cable harnessing, which isprone to workmanship errors and complicates spacecraft configuration andintegration.

• A mass and volume reduction resulting from a reduction in wiring andharness accommodation components.

• Wireless sensors eases retrofit activities as there are fewer physical con-nections with the rest of the spacecraft.

• Wireless sensors allow for a reduction in cost through a more flexibleinfrastructure.

• Wireless communication provides a common network for on board and offboard communications, making it easier to integrate sensors and EGSEand test equipment.

• Wireless communication can circumvent key harnessing bottlenecks suchas rotating mechanisms, deployable booms and inflatable structures.(seeFigure 1.1)

• The use of wireless networks allows for the placement of sensors, with agreat freedom as the impact of the addition or removal of sensor nodes onthe overall configuration is minimal.

• Convenience of integration, disassembly and replacement is a major time-saver when integrating the final spacecraft.

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• Wireless sensor networks can greatly enhance the robustness of an in-terface through mesh networks, without costly complex cross-strappingharnessing.

• With the improvements of sensor and controller miniaturization, smallerform factors are possible for sensor nodes, however, space approved powerand data connectors do not scale down that easily and are currently oftenthe determining factor in a sensors form factor.

Figure 1.1: Harness botleneck inside a spacecraft structure.

The project also specifies WSNs can be used in a host of different spacedomains such as:

• Intravehicle communications,

• Intervehicle networking,

• Planetary surface communications,

• Orbiter relay to surface links

• Assembly, Integration and Test (AIT) activities.

This specific project will focus on the design solutions for the intra-spacecraftand the AIT applications. The domains are described as follows.

Intra-spacecraft (in-orbit) applications

As it was already earlier in this chapter, using wireless networks to connectthese sensors to each other and to the central computer of the spacecraft cangreatly reduce the cable harness required for a mission and thus reduce mass,as well as risk of damages during integration and test. In addition the use ofwireless networks can greatly simplify the use of additional sensor in places thatare hard to reach for wired sensors, such as at the end of external appendages.

AIT applications

During testing and environmental verification of the spacecraft on ground, ad-ditional sensors are placed on the spacecraft’s exterior as well as inside its mainstructure. This is done to increase the number of data points during test, whichis essential to verify that the spacecraft is able to withstand the launch andspace environment it will be subject to during its lifetime. These sensors are

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applied temporarily to the spacecraft and require very expensive wiring to pre-vent any interference on the accurate measurements these sensors are taking.Installing and removing these addition sensors needs to be done with extremecaution, not only is the test article, the spacecraft, of great value, also the testsensors need to be handled with extreme care.

1.2 The Problem Statement

The main goal of this thesis is to contribute to the project described in thischapter by providing a WSN design that will be able to be deployed inside aspacecraft environment and maintain a reasonable operation lifetime (we aimat 5 years) of the system’s application. The feasible application defined in theproject for such technologies was the monitoring of a satellite’s housekeepingsignals (mainly because of their low transmission rates). Therefore the softwareand hardware of each of the parts of the network will provide the necessary so-lution for keeping the monitoring of a temperature sensor signal (housekeepingdata) or a sun sensor signal inside a satellite operating during 5 years minimumfor a maximum of 100 nodes network. The sun sensor works as an AttitudeDetermination and Control System (ADCS) data that because of its low ratemonitoring necessities has been also defined as an interesting sensor option forthis WSN design.

This thesis will answer 3 given research questions, presented as follows:

1. Is it Feasible to deploy a wireless sensor network into a satellite’s structure?

2. What is an efficient communication protocol design for a space applicationenvironment?

3. What is an efficient wireless sensor network design for a space applicationenvironment?

1.3 The Approach

As a solution for this problem, this thesis will provide the necessary designfor both the sensor and gateway nodes of the WSN for the application definedearlier as for an evaluation of the results obtained for such a design. In thissection is important to note that some of the taken decisions on this designwere made before this thesis started. Therefore it is important to emphasizehere five main decisions made in an earlier stage of the project:

1. The Zigbit chip is an SOC device defined by the ATMEGA1281 micro-controller and the AT86RF230 transceiver and will be used as the maincontroller of each of the nodes in the network to be designed.

2. The IEEE 802.15.4 standard is defined as the second layer communicationstandard for the projects final design. In an earlier stage of this projectthe tradeoffs of different communication solutions are analyzed, finallychoosing this standard as the best option for the task.

3. The Zigbee protocol was chosen as the main communication protocol forthe network to be designed. This decision was made based on the followingadvantages:

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• Improved maturity of standard compared to the other (relativelynew) standards.

• Interoperability between different vendors allows for easy develop-ment and manufacturing.

• Possibility of an open-standard Zigbee is underway.

• Possibility to use beacon-enabled mode to allow scheduled transmis-sions.

4. The BitCloud SDK will be used as the main software stack in each of thenetwork nodes. This software is fully compliant with the IEEE 802.15.4standard and the Zigbee protocol.

5. The Star topology with additional gateway nodes is the topology chosenfor the current network design. An assessment on the different types oftopogies was made on the conceptual design of the project and given theapplication and communication to be used, this topology was chosen asthe best choice.

1.4 Thesis Contribution

To solve the problem statement defined earlier in a previous section, this thesismakes the following contributions:

• In this thesis, we make a feasibility study on the deployment of wirelesssensor networks inside a satellite’s structure. Providing the necessaryinformation to make a decision on one of two different communicationfrequencies to be used in the final design to be delivered in this thesis. Itis also important to note, this study will present a first description of thebehavior of this kind of networks inside a satellite’s environment, thereforehelping in the definition fo the final design.

• We propose a communication protocol for the final wireless sensor networkdesign, as for an evaluation of such protocol. Providing the design of acommunication scheme that fulfills the necessary requirements defined bythe project’s client.

• Finally a complete network design is presented providing the currentproject of a hardware and software detailed design for this project’s fi-nal wireless sensor network application.

1.5 Thesis Structure

This document will introduce 6 more chapters were it will be shown the processtowards the design, the final result and an evaluation on this final solution. Thesecond chapter of this thesis will show the related literature that was used asthe basis for the design process and the final result. Even though the Zigbitchip was already chosen in an earlier stage, the decision of what will be thecommunication frequency of this device (868 MHz or 2.4 GHz) was for us todecide. Therefore a group of measurements were made on the ENVISAT engi-neering model, provided by the TU Delft Aerospace Faculty (see Figure 1.2),analyzing both frequencies and making an early assessment on the behavior of

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(a) (b)

Figure 1.2: The ENVISAT structural model.

wireless nodes inside a satellite structure showing their results in chapter 3 ofthis document.

Chapter 4 will present the systems requirements and the already designedoverview for the final wireless sensor network. Next, chapter 5 will describe theconfiguration and decisions made on the communication scheme of the networkand their relation to the power consumption of the design. Chapter 6 shows indetail the design’s decisions for the sensor and the gateway nodes, from bothsoftware and hardware approaches and their final results. Chapter 7 will makean evaluation on the design focusing in the communication scheme of the finalproduct by the use of a communication protocol simulation. Finally concludingwith Chapter 8 which will present the final conclusions of the work made andthe aspects that will be left as future work.

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Chapter 2

Related Work

This chapter will give an overview on the literature used as reference for themain decisions in this thesis. As it was said before, an initial communicationstudy was made to have a better knowledge of the nodes behavior inside asatellite’s structure as for deciding on the frequency to be used in the final designpresented in this document. Therefore, the first section of this chapter will showan overview of several papers presenting the environments and configurationsutilized by researchers on different kinds of WSNs measurements. Followed bya description on the main tests chosen for the final measurements methodologyshown further in master thesis.

In the following chapters it will be shown how one of the main tasks in thisthesis was to reduce the power consumption of the final product design by mak-ing important changes in the chosen communication protocol. Hence, the secondsection of this chapter will give a detail description of several communicationprotocols that were analyzed before, finding a final solution to the given prob-lem. The analysis was focused on the protocols designed for the media accesscontrol (MAC) layer and the advantages and disadvantages they provide to thiskind of solutions.

After this communication scheme was designed to reduce the power consump-tion of the final product, we encountered a new issue. The quality of serviceof the protocol had to be improved without increasing the consumption of thedevice to an undesired level. Therefore the final section of this chapter willmake an assessment of different kinds of researches made on quality of serviceschemes applied to WSNs.

2.1 Wireless Sensor Networks signal measure-ments

The analysis of the link behavior in WSNs has always been an important is-sue among different kinds of studies in the field. In this section an overviewof different types of experimental link measurements will be shown, followedby the description of 3 specific types of measurements and the results obtainedin several research papers. The subsections will present what kind of environ-ments, configurations and applications have been explored in the past and theirimportance to this thesis.

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2.1.1 Overview on different types of measurements

The main idea of testing the communication links of a network is mainly to un-derstand its behavior in a specific application and therefore in the application’senvironment, configuration and respective parameters. Already made studieson WSNs had included environments like shipping containers in [18] where thesensor nodes are positioned on the external faces of a container to localize itscurrent position, or open fields like in [10] where the authors analyzed howthe growth stage of a potato crop affects the signal propagation of a networkdeployed in a potato plantation.

The repercussion of the environment in the communication between nodes ina network is an important issue in the task at hand. Hence, the study madein [16] where the authors compare the behavior of sensor nodes in 3 differentenvironments: an office corridor, a tennis court and an Astroturf field-hockeypitch provides an important contribution to the measurements methodologydefined in this thesis. The paper presents in each of the setups a group ofresults that will provide future users a description of the devices behavior inthese type of backgrounds. The experiments include here are: the interferenceeffect of simultaneous transmissions, the degree of symmetry between nodes andthe use of calibrated RSSI (Received Signal Strength Indicator) measurements.

Other configurations and environments were taken into account for the mea-surements methodology design made for this master thesis. For example in [8]different types of antennas are tested in a football field focusing mainly on thenetwork’s parameters repercussions on the devices signal range. In [4] the mon-itoring of the link between a transmitter and a receiver during several days ona closed office, works as a main setup for analyzing the packet success rate ofsuch configuration. Finally a controlled area is shown in [7] where experimentsare executed in the High Speed Network Research Group Lab in Loughbor-ough University. These experiments were conducted in an indoors area whereother devices were present and therefore producing transmission interferenceand reducing the antenna performance in the area, thus introducing real timeparameters to the experimental results.

For the measurements methodology defined in this thesis, important attentionwas given to the research on environments similar to a satellites structure (aimedsetup for this master thesis). This is the case of the work made in [14], [17]and [25]. In [14] several measurements are done on board and beside a trainstructure, where the structure is defined as a harsh environment and thereforeseveral techniques (electrical and mechanical) are taken into account to protectthe nodes from getting damage during testing time.

In [17] frequency diversity is tested in the unused Aldwych underground rail-way tunnel in London. The measurements here are made over 16 different chan-nels were the transmitter will hop through a frequency range between 2.405 and2.480 GHz to get its final results. As for the last reference presented here, itshows an experimental setup in the Lunar Habitat Wireless Testbed at John-son Space Center in Houston (see Figure 2.1) were the author tests 2 differentcommunication protocols in a spacecraft environment. An important aspect tothis final reference is that one of the protocols tested is the Zigbee communica-tion protocol which as already described before was chosen as the final designprotocol in an earlier stage of the project.

After making an assessment on the papers presented here we decided on 3

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Figure 2.1: Lunar Habitat Wireless Testbed (foreground) at Johnson SpaceCenter in Houston, Texas.

main measurement approaches which will give the desired information on thebehavior of the network nodes inside a satellite’s structure. These are: the sig-nal strength versus the distance between transmitter and receiver, the antennadirectionality assessment and the interference effect of a third node betweentransmitter and receiver. A more detailed description on the research made onthese 3 approaches is described next on this section.

2.1.2 Signal Strength versus distance

This type of test will show the attenuation rate of the signal strength versus theincrease of the distance between a transmitter and a receiver node. In the ex-isting literature there are several configurations and setups for this specific test.For example in [10] it can be seen a configuration for this type of measurementapproach which is based on nodes deployed in a potato field of 154 105 m intwo separated lanes and the distance between nodes is 3m 5 cm (see Figure2.2a). The results of this test can be seen in Figure 2.2b, where the graph showsthe attenuation results for 3 different sizes of the potato plants in the field.

Another example is [16] where the authors use as a configuration a straight lineof nodes which is deployed in three different backgrounds. The measurement isbased on the storage of RSSI (Received Signal Strength Indication) values aftera package reception in each of the nodes in the line. In this section, results ofthe signal attenuation are presented for the tennis court environment (one of thepaper’s setups), where values are compared between uncalibrated RSSI samplesand calibrated ones. Figure 2.3 shows these results, 2.3 (a) shows the resultsobtained with the uncalibrated RSSI values while 2.3 (b) shows the calibratedones.

Nowadays the space domain counts with different sizes of satellites structuresand even though the direction of these technologies is to produce smaller designs,there are still designs of satellite structures as big as 10 meters long. Therefore

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(a) Configuration of sensor nodes (b) Signal Strength versus distance re-sults

Figure 2.2: Radio Wave Propagation in Potato Fields [10].

having this in mind and the fact that the material of such a structure will havean effect in the attenuation of the tranmission signal of a network node, wedecided this specific measurement will contribute to the understanding of thenodes behavior inside the satellite.

2.1.3 Antenna Directionality

The setup chosen for this test was based on the configuration used in [16] wherethey separate the antenna directionality experiment into: sender’s output powerand receiver’s sensitivity. The first one will test the transmitter power outputby rotating the main transmitter in steps of 30 degrees and keeping the receivernodes in a static position. For these measurements a comparison of the resultsand the theoretical radiation pattern of the antenna is made and it can beobserved in Figure 2.4. For the receiver’s sensitivity one is important to havein mind the main setup of the tests in the rest of this paper is a straight line ofthe network nodes. Therefore, for the receivers directionality test the authorsrotate the odd nodes in the line leaving the rest in a static position. The results

Figure 2.3: Results of RSSI values from a straight line of nodes in a tennis courtenvironment from [16].

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of such experiment can be seen in Figure 2.5, where it is shown the results forboth the rotating receivers as the reference of the static ones.

Like it was described before, the satellite’s material, where the measurementsof this thesis will take place, has an important repercussion on the signal’sbehavior. Therefore we decided to introduce this measurement to the plannedtest methodology, having in mind this type of measurement will also contributeto the task at hand.

2.1.4 Interference effect

The interference effect over the links of communication between sensor nodesis an important aspect to take into consideration for a better understandingof a WSN in a specific structure. Therefore the configurations used in [16] foranalyzing these kinds of effects were taken into account for the final methodologydesign used for this thesis.

In [16] it is decided the importance of knowing the behavior of the networkwhen a receiver is in range of more than one transmitter. That is why a specialsetup for such a case is analyzed by setting nodes in an uniform spacing of 30cm apart and programming one of them to transmit a continues known bytethat will represent the interference in the network. The authors will observethe behavior within the radio range of the nodes and will compare 2 differentmodels to the experimental results, trying to decide which of them fits themeasurements results the best.

The first model is the circular model which describes how no communicationis possible given a collision in the link. While the second model, is based onthe knowledge that a correct reception of a message depends on a defined sig-nal strength threshold. This threshold is part of the given formula shown inequations 2.1 and 2.2.

Figure 2.4: The experimental results of a transmitter rotation are shown in (a)while a theoretical radiation pattern is shown in (b). Information taken from[16].

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Figure 2.5: The experimental results of a group of receivers rotation while therest staying in a static position. Information taken from [16].

Signal(distance) = (TX(power))/(1 + distanceβ) (2.1)

Signal(distancetosender)/(Signal(distancetointerferer)+noise) > requiredSNR(2.2)

2.2 Existing MAC designs for WSN

The MAC or Media Access Control sub-layer works as part of the second layer inthe Open Systems Interconnect (OSI) communication model (Data Link Layer)providing each specific node in a network of addressing and channel access con-trol mechanisms. These mechanisms will define the communication protocolbetween the nodes inside a particular network.

As it was said at the begining of this thesis, the decision on the communicationprotocol for the current design was made in an earlier stage of the project. Baseon a detailed assessment on several protocols, the Zigbee protocol (third layer)running over the IEEE 802.15.4 (second layer) standard was chosen as the bestsolution for the problem at hand. By this decision the current design was definedby the advantages and disadvantages this protocol provides.

The following chapters will present how the power consumption of the nodesin transmission mode is one of the main design challenges of this protocol.Therefore, it was decided to make an assessment on the different types of com-munication protocols used on WSNs in the past. Even though we could notchange the decision already made, we could configure the devices to introduceschemes that could improve the given situation.

This subsection will give a description of the different MAC communicationprotocols that were taken into account to improve the behavior of the currentcommunication schemes in our design. Starting by an overview on the alreadydefined protocol and followed by the assessment of 4 different MAC designs, thissubsection will show what has been taken into account in the past by researchersfor their final solutions as for their contribution to our final design.

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2.2.1 IEEE 802.15.4 and the Zigbee protocol

The IEEE 802.15.4 standard provides ultra low power and low bandwidth re-quirements, and primarily aim at WSNs monitoring and control applications. Astandard 802.15.4 network can contain up to 255 nodes (65535 for an extendednetwork) and is most efficient when its transmission duty cycles are low anddata frames are small. Thus, this kind of network does not support isochronousor synchronous data link types. It is also important to note here, the 802.15.4devices are expected to run for months or even years on a primary batterywithout any kind of recharge system.

The Channel access under the 802.15.4 MAC is implemented using a CarrierSense Multiple Access with Collision Avoidance (CSMA-CA) scheme. A nodethat wishes to transmit to a neighbor first listens to the shared channel, and ifit detects the transmission of another node it backs off for a random amountof time before trying again. If no competing transmission is detected, then thenode is free to transmit its message.

This standard does not define a complete protocol stack in the OSI model.Instead, it provides the physical (PHY) layer and medium access control (MAC)sub-layer of the data-link layer in an OSI-type stack. It is up to other protocolsto provide the additional layers, including the network (NWK) and applica-tion (APP) layers. Several standards for these higher-level protocols are stillemerging, and a few have made great progress since the debut of the 802.15.4standard. The first such protocol is ZigBee which is introduced in 2004 andadopts the 802.15.4 stack almost directly.

The Zigbee protocol is able to work on 3 differents modes: idle, sleep andRx/Tx mode; enabling a network’s node to spend most of its time in the lessconsuming mode (sleep) saving an important amount of energy while the net-work is operating. This protocol works with a very small reaction time (node’stime to go from sleep mode to idle mode) and is able to support a data rate upto 250 kbps per channel in a 2.4 GHz band, 40 kbps in the 915 MHz band and20 Kbps in the 868 MHz band.[5]

2.2.2 ALOHA protocol

The ALOHA [12] communication protocol is a contention based protocol de-signed in the 1970s at the University of Hawaii by Norman Abramson. This is avery simple protocol, mainly based on a random channel access scheme, whereany node in the network can request to transmit its data regardless of the statusof the other nodes in the network. The algorithm each node will follow can beseen in the diagram shown in Figure 2.6.

An ALOHA network robustness level is based on the sending of acknowledg-ments from receiving node to transmitting node. In case an acknowledgementis not received when expected, the transmitter will resend the information aftera random value of time. In this section is important to note that this behaviorhas been defined as negative, because if the networks grows to large values thena collision can bring a sequence of collisions and with this an unstable grow ofretransmissions in a specific node in the network.

For this specific protocol there exists two different approaches. The first oneis called the unslotted ALOHA which is just based on the behavior describedbefore while the second one or slotted ALOHA will work with transmission and

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Figure 2.6: ALOHA communication protocol node algorithm.

reception windows that are defined thanks to the division in slots of the timeaxis. This protocol can be modeled by the Poission process, where transmissionrequests occur with a rate of G attempts per slot. Therefore the throughputfor successful transmissions in the slotted ALOHA is described by the followingformula, were G represents the attempts of transmission and exp-G representsthe probability of no interference in the communication channel:

S = G · exp−G (2.3)

In the case of the unslotted ALOHA the probability of having interferencein the communication channel is twice the slotted case. Then the throughputformula for this type of communication scheme will be the following:

S = G · exp−2G (2.4)

2.2.3 Sensor-MAC protocol

The sensor-MAC or SMAC [24] is a MAC layer protocol designed explicitlyfor WSN. The protocol mainly aims to reduce the power consumption of thenetwork, but also takes into account a good level of scalability and collisionavoidance capabilities. The authors first identify which are the main sources ofpower consumption in this kind of networks and try to reduce the amount ofenergy consumed in these specific points.

These main sources are:

1. Packet collision: The first source of consumption is the collision of packetswhen the nodes try to access the communication channel. Mainly becauseif there is a collision then the packet has to be discarded and most of the

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time retransmitted. This behavior will increase the energy consumptionof the network when a collision occurs.

2. Overhearing: The next source of consumption is the overhearing effect in anetwork. This means that a node in the network will receive at some pointpackets that are not destined to it, but to another node in the network.Therefore an amount of energy is being consumed but not being effectivelyused.

3. Control Packet Overhead: The third source is the sending of control over-head information which will increase the amount of data being send andtherefore will consume more energy.

4. Idle listening: The last consumption source is the inefficient use of idlelistening which means when a node in the network listens to try to receivea packet that is not being sent. Then again, the node is wasting energythat is not actually being used.

The SMAC is a protocol that as described before, focuses on the search toreduce the power consumption in a WSN by aiming its solution to the 4 con-sumption sources listed before. This solution will consist of 3 major components:a periodic listen and sleep, a collision and overhearing avoidance and finally amessage passing scheme.

Periodic Listen and Sleep

The nodes inside a WSN can usually work on different types of modes. There-fore if the system spends as much time in the mode that will consume the leastenergy (sleep mode), then this will contribute to reduce the power consump-tion of the system itself. The S-MAC uses a periodic listening and sleep modeswitching that will reduce the ineffective listening time and will try to keep thenode awake only when it is actually needed, keeping the waste of energy by nodelistening to a minimum.The base of this scheme is each node in the network will go to sleep a definedtime and will wake up to check if another node in the network wants to commu-nicate with it. This requires synchronization among the closes nodes given theclock drift in each device and that is why the protocol uses 2 different techniquesto make the system robust to this kind of errors. The first one works by givingtimestamps to the measurements that will be relative and not an absolute valueand the second one is keeping the listening value always longer than the clockdrift of the nodes hardware.

The nodes in the network will exchange their schedules by broadcasting theirinformation to the neighboring nodes, this way ensuring the communicationthrough the network. It is also important to note here that if several nodeswant to communicate with a specific node, then they need to compete witheach other for the use of the communication channel (the node should be inlistening mode for the communication to be established). The nodes will useRTS and CTS, the device that first sends the RTS packet will win the control ofthe channel and the node that receives this packet will send back a CTS packet.This way the communication is established and the device will not go to sleepuntil the transmission is finished.

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Figure 2.7: Section description of the listening interval in an S-MAC protocol.

As described before the system needs to be synchronized to prevent a long-time clock drift into the system. That is why the schedule update is achievedby sending a SYNC packet, which is a small packet that contains the addressof the sender node and the time of its next sleep. The listening interval of anetwork node will be then divided in a SYNC receiving section and an RTS re-ceiving section (Diagram showed in Figure 2.7). Each node in the network willbroadcast a SYNC packet periodically allowing new nodes to join the network.

Collision and Overhearing Avoidance

1. Collision avoidanceThe SMAC protocol uses a scheme called virtual carrier sense which worksby keeping a duration field in each transmitted packet. This way if a nodein the network receives a packet which was not meant for, the node willknow how long should go to sleep without trying to use the communi-cation channel. This duration field is saved in a variable called networkallocation vector (NAV) which will also activates an internal timer in thenode. Each node will use the NAV to check if it should try to transmitor keep on waiting for the channel to be free. This is a simple mechanismthat works by decreasing the NAV in a specific node every time the timer(set for this variable) fires. Therefore if the node needs to make use ofthe communication channel it will first check the NAV value, which willbe decrease to 0 by the defined timer, if the NAV value is different from 0this will mean the channel is still being used and the node should go backto sleep.

2. Overhearing avoidanceThis protocol uses as it was described before the RTS and CTS packets toestablish communication between the nodes in a network. These packetsare also used to avoid overhearing by telling the specific node to go tosleep mode after it hears a RTS or CTS packet. Assuming DATA packetsof a transmission are much longer than the control ones then this schemewill prevent the network nodes in the neighborhood of the transmitter tooverhear the DATA packets and their acknowledgments in a transmission.

Message Passing

A message is defined as a group of information that is interrelated to eachother and is constituted by one or several transmission packets. When a messageis transmitted, it needs to be received completely by the receiver before this canperform any kind of processing with the transmitted information. Therefore,as it was said already, the number of packets can introduce a lot of control

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information, increasing the energy consumption as well. The authors of theSMAC based their approach on the fragmentation of these messages into smallpieces and their transmission in one burst. This means keeping only one RTSand one CTS packet for every burst.

This protocol will also work based on communication acknowledgments, sothere will be an acknowledgment for the transmission of every fragment and ifthis fails the time for this transmission will be reserved and the fragment will bere-transmitted immediately. It is important to note the duration field describedbefore is being transmitted in each of the message fragments. Hence if a nearnode hears a RTS or CTS packet, it will go to sleeping mode for the time thattakes to transmit all of the burst fragments.

The components just described will give an important and efficient solutionto the problem of decreasing the amount of energy in a WSN and will providethe opportunity of making trade-offs between energy consumption and commu-nication latency according to the traffic conditions of the network.

2.2.4 Lightweight Medium Access protocol

The Lightweight MAC or LMAC [11] communication protocol bases itself on atime division multiple access (TDMA) scheme. Therefore time is divided intoslots which are assigned to every node in the network. This way each node willhave control on a specific time slot avoiding the possibility of collision and soreducing the amount of energy consumption of the network. The scheme willallow the reuse of slots to noninterfering distances, hence increasing the limit ofthe number of nodes permitted in the network.

Each node in the network will transmit in its defined time slot a messagethat will consist of a control message and a data unit. The first part will bedefined by a fixed size and will carry the information needed by the networkto maintain an efficient behavior of the network. While the second part is ofvariable size and will depend on the application at hand. This control messageis composed by the following information: the ID of the time slot controller,the hops distance to the gateway (routing), the address of the intended receiverand finally the length of the data unit in transmission. It is also important tonote here that the control message also contains the sequence of their time slotin the frame to maintain the synchronization inside the network.

Therefore the nodes in a specific neighborhood will try to receive the controlmessage of their neighbors. If one of the nodes in the network receives a controlmessage that is not addressed to it then this will go directly to sleeping modeand will wake up again at the next time slot. If this is not the case and thenode is addressed in the message, then this particular node will stay listing forthe rest of the message. When the data unit packets are completely receivedby the node then both receiver and transmitter will go to sleeping mode. Aninternal time out timer is set inside the nodes to avoid the waste of energy bylistening in time slots that are not controlled by the current node.

LMAC protocols setup

The way this communication protocol sets the system up, is based on thesynchronization of the nodes of the network. The synchronization is importantbecause, as it was described before, the complete network needs to have the

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knowledge of the time slots division. The gateway will begin controlling a timeslot and will start transmitting control messages to the neighbor nodes (one-hopdistance). These nodes will synchronize their clocks to the gateway clock. Afterone frame of transmission, the nodes close to the gateway will be aware of allthe time slots owned by all the gateways in their reception range.

After this has happened the just synchronized nodes will choose a randomtime slot to control (except the slots already taken) and this is how the nodes inthe network will be able to start transmitting in there own time slot to a specificgateway. The nodes in the network will keep their time slots until their batteryruns out or they are informed that their time slot is colliding with another timeslot in the network.

Because of the chance of this happening then the protocol introduces a schemethat makes the nodes inform their neighbors when a collision has happen be-tween control messages. This scheme will also command the nodes to leave theslots and to choose randomly a new one (free slot). It is also important thenumber of time slots to be larger than the maximum connectivity in the net-work, this way ensuring that every node in the network finds an empty slot tocontrol.

2.2.5 Crankshaft protocol

Crankshaft [6] is a MAC protocol that focuses on saving energy on dense WSNs.This uses node synchronization and offset wake-up schedules to reduce the powerconsumption produced by the overhearing of nodes in a specific neighborhood.The authors of the protocol make an assessment on what are the main problemsin dense WSNs, using this information later on to make decisions on the protocoldesign. These are the following:

1. Overhearing: in a dense network the amount of nodes in a neighborhoodincreases and therefore increases the amount of nodes that can overhearthe information transmitted.

2. Communication grouping: some protocols group communication into ac-tive parts of transmission frames and this increases contention and collisionwhen the amount of node density increases.

3. Neighbor state: there are some existing protocols that save the state oftheir neighbor node, but this create problems in dense networks. Themain problem is that given a dense network, each of the nodes will havemany neighbor nodes and the information of each of them will need alot of RAM memory and if all of them are not saved, this will introducedifferent kinds of problems like the atempt of communicating with theseparticular missing nodes.

communication with these particular missing nodes.

As commented before, Crankshaft works basing itself on offset wake-up sched-ules. This means that different nodes in the network have different offset wake-up times, therefore reducing the amount of nodes overhearing messages in aneighborhood. The protocol divides time into frames and frames into slots.These slots have two types of communication: broadcast and unicast. Thenodes in the network will wake up every time there is a broadcast slot in the

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network making it possible to listen to the message that is going to be trans-mitted for all the nodes in the network.

As for the unicast communication, each node will listen to a specific unicastslot in every frame. A node in the network will be able to send a message toanother particular node because it will know in which specific slot the receiverwill wake up. Is important to note here, the transmitter node will not have aspecific slot control like in a TDMA but the transmission of such message willbe made in a contention based manner.

The authors introduced to the design a message and acknowledgement se-quence for the unicast communication where the system will retransmit a spe-cific message 3 times in the subsequent frames. In the case of a gateway orbase-station the particular node will listen to all of the unicast slots in theframes. The reason is this kind of nodes will probably receive information frommost of the nodes in the network and most likely will be connected to a biggerpower source (Therefore allowing the node to consume more energy than therest of the network).

An important aspect of this system is that in case two neighbor nodes areassigned the same reception slot, the protocol will allow for a node to transmitin its own reception slot. Therefore, if the node contents for the control of thecommunication channel and fails it will return to receiver state for this particularslot.

Synchronization is very important for Crankshaft given that, as describedbefore, the time is divided in frames and these frames in slots. This is also usedto reduce the power consumption by taking into account that nodes doesnt needto be awake during an entire slot to realize if the communication channel is busyor not. Hence, the moment in which the node realizes the state of the channelthis can go back to sleep, in the case of a busy channel or can try to make atransmission in the case of a free one.

Therefore the interval between the start of a slot and the moment the listeningnode turns on its radio is used to resolve contention (contention window). Sothe transmitter node will listen for a short amount of time just before makingits transmission, this way detecting other nodes contention for the same slot. Ifthe slot is free and shortly after the receiving node is known to wake up, thesending node transmits the start symbol and the actual message.

From this summary of protocols we can observe that the main objective ofmost of them is to reduce the power consumption which is also the main concernin our design. Important details were take into account for our final commu-nication scheme. For example, ALOHA provides a concrete functionality witha given simplicity to the system. SMAC makes an assessment on consumptionsources and provides different schemes to avoid them. The LMAC protocolshows another approach to the solution, by instead of working in a contentionbased mechanism like the other protocols showed here. This protocol makesused of a TDMA scheme where the time is divided into slots and therefore col-lisions (which are defined as another power consumption source) are avoidedcompletely by the system. Finally Crankshaft provides us of new schemes thatare useful in the case a network is over populated with nodes (dense WSNs). Inthis thesis we took all of these protocols contributions into account, as for thelimitations the already chosen protocol defined, and designed the best solutionfor the final product of the project.

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2.3 Quality of service and Communication Schemes

In communication networks the term Quality of Service (QoS) is related with thereservation of resources in the network and the different types of mechanismsto control their use. That is why this topic also refers to how robust andtrustworthy a network can be. In this subsection it will be seen the researchthat has been already made on this area and the results obtained on the differenttypes of studies.

In the previous subsection we described the necessity of designing a commu-nication scheme that would reduce the amount of power it was being drawn bythe sensor nodes in the network. Thus, for this task it was necessary not only toassess the existing communication protocols (MAC protocols analyzed before)in the field, but also to take into account the level of QoS the resulting designwill have. This is the reason this thesis presents the research made in the paston QoS and their relation with WSNs.

Some of the literature that is already out there makes an analysis of whichare the main problems that relate with QoS on the WSNs environment. Forexample in [19] and [27] the authors defined the QoS challenges that can befound in a WSN and described them as follows:

1. Bandwith limitation: bandwidth constraint is one of the most commonproblems in networking. Bandwidth in a specific network is not infiniteand so if the load that is being handled by the network exceeds the avail-able resource, then the system will need to start discarding packets anddelaying transmissions.

2. Data redundancy: the duplication of information usually helps with re-liability and robustness, but produces the consumption of extra energyinside the network.

3. Packet categorization: usually in a network there are packets that havea higher priority of being delivered than others, hence there should bemechanisms that will distinguish these categories and use the networkresources accordantly.

4. Multiple traffic types in sensor node with multiple sensing units: the mon-itoring of several types of sensors in the network introduces challenges tothe management of the network. From the QoS point of view the mix-ing of traffic signals in a network will mean the limitations of each signalto one single network. For example un-periodically signals that work byevent detection can be working together in a network with signals thatneed a periodic data monitoring so the network should be design for anunbalanced mixture QoS-constrained traffic.

5. Buffer size limitation: the buffer size in a network will define importantflexibility on the packets movement inside the network. Multi-hop routingof data would typically need long communication sessions and packetsbuffering larger data, especially when the delay jitter is a concern for thenetwork.

6. Resource limitation: in WSN the information from usually a big amountof nodes is being transmitted to a small quantity of gateways. There-fore the amount of resources being used by the network will always be alimitation for the system. Examples of these resources are the following:

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energy, bandwidth, memory, buffer size, processing capability and lim-ited transmission power. Is important to note here that usually for thiskind of networks the main resource issue is the energy consumption of thenetwork.

7. Energy and delay trade-off: the power consumption of radio transmissionsis proportional to the distance squared and this consumption is increasedif there is noise or other kind of signal corruption in the network environ-ment. The way the network reduces this consumption is by routing theinformation through several nodes in the network. This is a helpful solu-tion but will have important repercussion in the delay of the data throughthe network. Since packet queuing delays control propagation delays, theexpected QoS routing of sensor data would need to make a trade-off be-tween energy efficiency and delivery deadlines. It is also important to takeinto account that the energy load must be evenly distributed among allsensor nodes so that the energy at a single sensor node will not be drainedout very soon and a long-live time of the network can be achieved.

8. Multiple gateways: finding the correct amount of gateways in a WSN canbecome an important challenge in a particular network. The increase oftheir number can introduce more resources to the network but can alsointroduce more requirements to it.

9. Network dynamics: WSNs are very variable networks where topologychanges due to node mobility, failure and addition can affect greatly thebehavior of the network. This kind of networks can also have unreliableactions when circumstances like a large scale of nodes in network canproduce non-deterministic results.

Having these challenges in mind, it is important to also observe how theauthors of [27] make a qualification of which are the actual requirements for thedifferent layers in an OSI model for a WSN. The paper divides the requirementsper layer and describes them in the following manner:

1. The Application layer: these will be the requirements defined by the userand therefore will depend on the final goal of a specific application. Inthis paper are defined:

(a) System Lifetime: this requirement is defined by the time when thesystem is deployed until the moment when it stops satisfying theusers requirements.

(b) Response Time: this refers to the time it takes for a query made bythe user to be answer. Applications with hard time constraints needto assure a specific response time for its user.

(c) Data novelty: this term refers to the time between an event is de-tected by a sensor in a specific node in the network to the momentwhen this data is received in the gateway of such network. Againthis is an important point for time constraint applications.

(d) Detection Probability: this term refers to the probability of detectinga real phenomenon and transmitting such value to a network gateway.

(e) Data reliability: is a term used for describing how representative ofthe real word phenomenon is the data obtained.

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(f) Data Resolution: this refers to temporal and spatial granularity ofthe data obtained by the sensor.

2. Transport Layer: for this layer the points the authors defined as require-ments for WSNs are:

(a) Reliability: this term is used to describe the percentage of packetssuccessfully received in reference to those that were actually trans-mitted by the different sources.

(b) Bandwidth: this is a very common term in the communication fieldand determines the number of packets received per unit time fromthe different sources in the network.

(c) Latency: this refers to the shortest total delay at the intermediatenode in transmitting a unique packet from all different sources in thenetwork to a specific destination.

(d) Cost: this term defines the number of transmissions needed to re-trieve a packet from all sources in the network.

3. Network Layer: the QoS requirements for this layer are:

(a) Path Latency: this is used to define the number of hops in a networkbetween source and destination.

(b) Routing Maintenance: this term refers to the energy consumptionrate to maintain routes between source and destination.

(c) Congestion Probability: this means the probability of the traffic loadin the network to exceed the capacity of all the links on the path.

(d) Routing Robustness: this term refers to the maximal rate of packetloss a routing scheme must sustain.

(e) Energy Efficiency: refers to the amount of energy consumed to trans-mit a packet from source to its destination.

4. Connectivity Maintenance Layer: the QoS requirements for this layer are:

(a) Network Diameter: this refers to the maximal transmission latencybetween source and destination.

(b) Network Capacity: this refers to the number of packets that can betransmitted concurrently in the network.

(c) Average Path Cost: this value defines the average amount of energyconsumed to transmit one packet between all source destination pairs.

(d) Connectivity Robustness: this term defines the maximal allowednumber of failed links the network must sustain.

(e) Connectivity Maintenance: this value measures the energy consump-tion rate to maintain a connected network topology.

5. Coverage Maintenance Layer: the QoS requirements for this layer are:

(a) Coverage Percentage: defines the percentage of area monitored by atleast one sensor in the system.

(b) Coverage Reliability: defines the minimal allowed sensing probability.

(c) Coverage Robustness: defines the minimal number of sensors moni-toring the same location in the monitored environment.

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(d) Coverage Maintenance: refers to the number of messages exchangedto provide network coverage.

6. MAC Layer: the QoS requirements for this layer are:

(a) Communication Range: this term refers to the maximal distancebetween one-hop data transmission. Therefore the distance betweena transmitter node and its destination without having intermediatenodes in its path.

(b) Throughput: this term refers to the maximal number of data framesthat can be transmitted successfully per unit time.

(c) Transmission Reliability: this term defines the percentage of success-fully transmitted frames in the system.

(d) Energy Efficiency: this term refers to the amount of energy consumedto successfully transmit one frame of data within one-hop in thenetwork.

7. Physical Layer: for this last layer the paper describes which hardwarecapabilities are and how they affect the requirements on the layers of theOSI model in the system. The authors separate these capabilities in 3areas:

(a) Unit capabilities:

i. Channel Speed: this aspect will have an impact on the Through-put defined before in the MAC Layer.

ii. Coding: this will impact the Throughput and Transmission Re-liability defined before in the MAC Layer.

iii. RF Power: this aspect affects Communication Range, Transmis-sion Reliability and Energy Efficiency defined in the MAC Layer.

(b) Processor capabilities:

i. Location: this affects Location Accuracyii. Timing: : this affects Timing Accuracy

iii. Processing Speed: this aspect affects the Processing Latency inthe data management layer.

iv. Computation Power: this aspect will have an impact Compu-tation Cost, Data Abstraction and Data Accuracy in the datamanagement layer, as well as Energy Consumption in the loca-tion/time service layer.

(c) Sensing unit capabilities:

i. Measurement Accuracy: this aspect will impact Coverage Re-liability and Coverage Robustness in the coverage maintenancelayer.

ii. Sensing Range: this will impact the Coverage Percentage in thecoverage maintenance layer.

iii. Sensing Power: this aspect will have an impact on all the re-quirements in the coverage maintenance layer.

Before going into design and tool solutions on QoS in WSNs, it was also usefulto observe what the authors in [23] provide as a new method of classification ofWSNs that extends the traditional approach to include QoS.

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Figure 2.8: WSNs application scenarios classification.

This paper makes a classification of WSNs applications by scenarios. As itcan be seen in Figure 2.8 each level in the tree goes deeper and deeper into thenetwork applications constraints, giving a new point of approach to determinethe WSNs requirements.

The tree shows a first level based on the deployment dynamics, which canbe static, mobile or a mixed of both types. Next it goes to the strategy whereit can go into random or deterministic. Where random deployment means adeployment where there is no control over the nodes location.

The third level shows the control scheme used in the application, this canbe a closed-loop or an open-loop. The first case refers to an application whichuses a feedback loop to control the response of the system and therefore uses aboth ways communication between sensor node and gateway. While the secondcase means a one way communication without using any kind of feedback onthe system.

The next level goes into the data delivery model where these are classifiedinto: event-driven, query-driven and continuous. The first one is used when a apredefined event triggers the sending of data by a sensor node. The second onerepresents when applications in a node respond only to specific queries made bythe gateway or other networks. As for the last one, it represents the applicationswhere the sensor nodes report periodically to the gateway of the network.

The last level of the tree refers to the time constraints of the application andthis is classified in either time constrained or no-time constrained. An appli-cation which is time constrained demands a strict time boundaries to receivedata from the nodes in the network and eventually send feedback to sensors oractuators. If this is not the case then the application is defined as a no timeconstrained application.

After the description of the tree shown in Figure 2.8 is made, the authorsgive a final classification that separates the WSN requirements into two mainbranches: the information processing and the communication processing. In thispaper the authors defined what they called a taxonomic tree that encapsulatesthe application and network parameters of the system, aiming mainly to providea suitable target to classify and evaluate the services given by the WSN to theapplication (The tree is shown in Figure 2.9).

As it can be seen the information processing section will be divided into:information gain, information efficiency and the application specific metrics.The first dimension specifies the amount and quality of the information received

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Figure 2.9: Taxonomy of WSNs (with QoS) requirements.

from the nodes in the network. The second one specifies energy and resourceefficiency in obtaining the information from the nodes. As for the last one refersto parameters like localization resolution and data storage of the application.

The next section, communication processing, is divide into: network dynam-ics, security and privacy, performance, deployment and coverage. The firstdimension refers to mobility associated to the devices and subjects within thenetwork like nodes, gateway and user mobility. The second one specifies aspectslike authentication, confidentiality, integrity, access control and accountability.The next one refers to parameters like connectivity, delivery rate, latency andbandwidth. Finally the coverage dimension specifies the network parameters ofspace and time coverage.

The literature shows different kinds of tools and design solutions for severalof the challenges and requirements mention on the papers described above.An interesting tool is used in [9] where the authors make a design of a QoSframework for WSNs basing themselves on the service-oriented computationapproach used in web services. This approach includes characteristics such asautonomy, loose coupling, reuse-oriented, contract-oriented which are very muchapplicable on WSN.

The paper uses features like loosely coupled dynamics and the wrapping ofresources to encapsulate the lower level network infrastructure and node re-sources. This is how the authors manage to accomplish QoS-driven resourceallocation and scheduling, and implement service cooperation by relying onService-oriented middleware.

The authors design a QoS guarantee framework shown in Figure 2.10 whichis divided into three main layers. The first layer is defined as the layer of nodefacility that consists of nodes and embedded software deployments on WSNs.The second layer is denominated the layer of logical resources which encapsulatesnodes and logic resource and provides the resources description. (”ResourceWS in Figure 2.10”). The last layer is called the layer of QoS services; thisencapsulates QoS requirements of the different applications and maps them intolayers of logical resources by dynamic negotiation among services (”QoS-WS”in Figure 2.10).

In [19] an algorithm is proposed for a dual radio transmission called DualRadio Transmission Protocol or DRTP. The nodes for such algorithm are de-fined by two sensing units and two transmitters. This has been designed forapplications where there is a rate guarantee service for the transmission of data

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Figure 2.10: QoS guarantee framework for wireless sensor network based onservice-oriented middleware.

of one sensor while it is a best effort service for the data of the other sensor ina node. The nodes coordinate the transmission of the data by a specific timingand achieve load balancing among the sensors network. This protocol algorithmovercomes the problem related to event capturing and real time data transferin a WSN.

The behavior of the algorithm is quite simple. A specific node in the networkwill have two different sensors S1 and S2. S1 will be defined as the best effortsensor data and S2 will be defined as the guarantee service data. The networkswill be divided in node clusters within the network and each cluster will beprovided of a called cluster head (CH) which will communicate with other clusterheads and the main gateway. So if a node in the cluster wants to transmit data,this node will have to transmit its data to the cluster head node and this willtransmit to the gateway (sink) of the network.

If a present reading is different from a previous transmitted one, then the valuemost be transmitted. The data need it to be send from S1 will be transmittedby a low power radio in the node, while the data need it to be send from S2will be transmitted by the high power radio in the node. The transmission ofthe node will depend on parameter called α, which is an application specificvalue and can be adjusted according to the application. The value of α, canbe adjusted according to the application to ensure a minimum or maximumrate guaranteed for different sensor readings. Figure 2.11. Shows model of suchnetwork behavior.

In [26] an admission control scheme with multi-constrained QoS (ACMQ) ispropose for WSN. The scheme offers an admission decision for real-time datatraffic, but also provides a sending rate for the non-real-time one. The real-timetraffic will have the highest priority of the system and will not be affected byany kind of adjustment to all the sending rates of ongoing non-real-time dataflows.

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Figure 2.11: Dual Radio Transmission Protocol system Model.

The authors based themselves in the concept that keeping the communica-tion channel busy ratio to a certain threshold will help maximize the networkthroughput and provide a QoS for the network users. Therefore, the total trafficin the network its regulated through call admission control for real-time trafficand fairness-aware rate control for the non-real-time traffic.

To be able to fully use the bandwidth of the network and keep it unsaturated,the authors set a threshold to an 87% of its maximum value, leaving a 3% spacein case of saturation. They set an upper bound for the bandwidth consumed bythe real-time traffic to an 80% of the threshold bandwidth. These two thresholdswill be adaptable to the traffic configuration of the specific network, this wayleaving at least 20% of the threshold bandwidth to the non-real-time data traffic.Also keeping in mind real-time packets have the highest priority in the outgoingqueue, which means they will always be put on the top of the queue.

As said before, the design uses also a fairness-aware control scheme for thenon-real-time data traffic at each node which in order to keep the networkunsaturated sets an initial sending rate for such data traffic. This scheme willbase itself on the total available bandwidth for non-real-time traffic and thenumber of active non-real-time flows and adjusts the sending rate of this typeof traffic to a certain appropriate level.

Another application that we took into account is [21] which focuses it work onreliability, availability and serviceability (RAS) as means of providing securityintegrity to WSNs. In this paper it is described how by trying to achieve QoS inthe network it is easy to detect malicious actions when certain parts of the net-work do not fulfilled the requirements for such behavior. The authors evaluatedmodels to show how QoS can be linked to a secure WSN and by using networknodes they experimentally assed how by finding problems on the satisfaction ofQoS constraints it is possible to notify malicious actions in network in the longrun.

Finally in [13] the authors implement a performance monitoring sensor net-work with an end-to-end QoS support. They provide a practical system imple-mentation of an end-to-end QoS in an integrated sensor-WiFi using off-the shelfhardware and software solutions. Figure 2.12 shows a diagram of the systemarchitecture.

From the work presented in this subsection we were able to make an assess-

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Figure 2.12: Software architecture.

ment on the network needs, challenges and requirements depending on severalareas. Thus, designing a communication scheme with the highest level of QoSpossible, given the limitations of the already made decisions.

The information obtained from the challenges and requirements showed beforeenable us to define a better scope for our problem. Therefore, defining whatwas necessary for our final product, what was not possible to obtained and whatsimply did not applied to the task at hand.

For example, from the challenges defined before is clear that in our casewe will focus on the challenges ”Energy and delay trade off” and ”Multiplegateways”. As from the requirements defined for each of the OSI-model layersimportant attetion was given to ”Systems Lifetime” on the application layer,”Energy Efficiency” and ”Transmission reliability” in the Mac layer. We alsoencountered several requirements that were out of the scope of our thesis orsimply already defined in an earlier stage. Such examples are the requirementspresented for the Network and Physical layers which are defined by the choicestaken on topology and hardware before this thesis started.

Other types of classifications scenarios and tools like the taxonomic tree from[21] helped as new approaches to a better definition of our main scope. Finallythe applications shown in [17], [24], [19] and [11] presented how the design ofQoS solutions used on WSNs can be applied to real-life applications.

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Chapter 3

Feasibility Study of wirelesssensor networks in asatellite structure

At the beginning of this thesis we stated the necessity of making a feasibilitystudy on the behavior of WSNs inside a satellite’s structure. The main objectiveof this task is to focus on making a final decision on the frequency the Zigbitchip will used as the communication frequency of its radio transceiver.

The study was also made with the idea of making an early assessment on howthe final design will behave given these specific surroundings. Therefore, a mea-surements methodology was defined as it was described in the previous chapterand two different settings were chosen as test environments. This methodol-ogy was applied to a defined configuration of nodes deployed in the ENVISATstructure and an open field setup which was utilized as a final results testbench.

This chapter initially describes the environment’s setup and the devices usedfor the task at hand. Followed by the actual results of both frequencies (2.4GHz and 868 MHz) in the two environments. Finalizing with a results analysisand the conclusions drawn from it.

3.1 Measurements Devices

The devices used for these measurements were provided by the Embedded Soft-ware Department. These are two off the shelf devices with the necessary charac-teristics to reach the main goals defined earlier in this chapter and are presentedas follows:

1. Sky mote: is a low power wireless module for WSNs applications. Thisdevice uses the Texas Instrument MSP430 F1611 microcontroller and theChipcon 2.4 GHz CC2420 transceiver. This transceiver is fully compliantwith the IEEE 802.15.4 standard and works to a speed of 250kps.[15]

2. Gnode G301: is a low power wireless module for WSNs applications. Thisdevice uses the Texas Instrument MSP430 F2418 microcontroller and theTexas Instruments 868 GHz CC1101 transceiver. Where the latter worksto a speed of 500kps.[20]

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Both devices were programmed with the same application over the TinyOsoperating system set to work under the same transmission power and speed.The transmitter application will transmit a continuous byte as payload and thereceiver will storage the Received Signal Strength Indication (RSSI) value ofevery packet received.

3.2 Environment and devices configuration

This section shows each of the test environments where the study was made andpresents a detail description on measurements methodology utilized on each ofthem. Also showing the configuration of the devices used in each of the settings.

3.2.1 Open field environment (Reference)

In this environment the node devices where set 100 cm above the ground withplastic tubes and separated 50 cm from each other. Figure 3.1 shows a smallsketch of the positioning and surroundings of the devices in the field. The figures3.2a and 3.2b show some examples of the settings of both devices.

Open field measurements methodology

For this configuration and background 3 types of measurements were applied:

1. Signal Strength vs Distance: In this test a node is transmitting continu-ously while the receiver is positioned on different distances of 50 cm steps(the RSSI values are save for each of the steps). As Figure 3.1 shows, thenode positions are on a straight line and from 0 to 300 cm with a distanceof 50 cm between each position.

2. Signal Strength vs Rotation of the Nodes: In this test the transmitternode is rotated in steps of 45 degrees while the receiver stays in a staticposition (the RSSI values are save for each of the steps). Next, the sametest is made but in this case the transmitter is set in a static position

Figure 3.1: Sketch of the positioning and surroundings of the devices in the testenvironment.

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(a) Sky Motes devices. (b) Gnodes devices.

Figure 3.2: Test devices in the open field reference environment .

and the receiver node is the rotated one. Note: the distance between thedevices is of 100 cm in a straight line as the test described previously.Figure 3.4a shows a sketch on the direction of the rotation of both thetransmitter and the receiver.

3. Signal Strength vs Transmitter interference: a main transmitter and areceiver are set in a straight line with a distance of 100 cm between ea-chother. An interference transmitter is set exactly in the middle of the

Figure 3.3: Example of the positioning of the nodes in the open field setup.

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(a) Open field environment. (b) ENVISAT environment.

Figure 3.4: Rotation of the sensor nodes on the two different environments.

distance (50 cm). Both transmitters are set with an ID so that it is possibleto check the RSSI value of the receptions from the main transmitter.

3.2.2 ENVISAT satellite

In this environment the devices where strapped to the inside wall of the satelliteand separated by 50 cm from each other. Figure 3.5a shows how this structuralmodel still has its original wiring, therefore helping to create a more represen-tative environment for the given tests. While Figures 3.5b, 3.6a and 3.6b showhow the open areas of the satellite are closed and covered with cardboard andaluminum foil to simulate the real structure. Figure 3.7 gives an overview ofthe materials used and the positioning of the devices inside the structure.

(a) Original wiring inside the ENVISATcavities.

(b) Aluminum foil used to cover openareas.

Figure 3.5: Test environment ENVISAT satellite

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(a) (b)

Figure 3.6: External view of the cardboard and aluminum foil cover.

ENVISAT measurements methodology

For this configuration and background 5 types of measurements were applied.For this environment two extra measurements were included, aiming to a betterunderstanding of the devices communication in this particular environment.The methodology is defined as follows:

1. Signal Strength vs Distance: In this test a node is transmitting continu-ously while the receiver is positioned on different distances of 50 cm steps(the RSSI values are save for each of the steps). As figure 3.7 showed, thenode positions are on a straight line. Note: the satellite is separated incompartments of 165 cm long. So the measurements of 50 and 100 cm aremade inside the first compartment while the measurements for 150 and200 cm are made on a second one.

2. Signal Strength vs Rotation of the Nodes: In this test the transmitter nodeis rotated in steps of 90 degrees while the receiver stays in a static position(the RSSI values are save for each step). Next, the same test is made butin this case the transmitter is set in a static position and the receiver nodeis the one being rotated. Note: the distance between the devices is of 100cm in a straight line as the test described previously (because the distanceis 100 cm both nodes are on the same compartment). Figure 3.4b showsa sketch on the direction of the rotation of both the transmitter and thereceiver.

3. Signal Strength vs Transmitter interference: a main transmitter and areceiver are set in a straight line with a distance of 100 cm between ea-chother. An interference transmitter is set exactly in the middle of thedistance (50 cm). Both transmitters are set with an ID so that it is possi-ble to check the RSSI value of the receptions from the main transmitter.Note: All nodes are positioned on the same compartment.

4. Close positioning through an internal wall: In this test 2 nodes are posi-

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Figure 3.7: The nodes are strap to the internal wall of the satellite with 50 cmdistance between them.

tioned one on each side of an internal wall of the satellite (2.5 cm) and theRSSI values of the communication are saved. Figure 3.8 shows a simplesketch of the positioning of the sensor nodes for this specific test.

5. Close positioning with distances of 10 and 25 cm: In this test 2 nodesare position with a distance of 10 and 25 cm between them and the RSSIvalues of the communication are saved (Always in a straight line as in thefirst configuration).

3.3 Experimental Results

The experimental results are presented in the following graphs. In Figures 3.9aand 3.12a we can see the results obtained in the Signal Strength vs Distancemeasurement. The Figures show the attenuation of the devices signals in boththe ENVISAT and the reference field. The results show how the attenuationis almost linear with a small slope in the case of the reference field. While

Figure 3.8: Sketch of the sensor nodes positioning for the ”Close positioningthrough an internal wall” test

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the results inside the ENVISAT show a more drastic change for each of themeasurements. It is important to note here the measurements taken from 50and 100 cm are in the first cavity of the structure while the 150 and 200 cm areon a second one. Also an interesting fact is the measurements taken in the firstcavity for the Sky Mote devices are above the reference values while the onestaken for the Gnode device are always below.

Figures 3.9b and 3.12b show the results from the Signal Strength vs DistanceInterference measurement. The results show in both cases an almost steadystrength reception and a higher value of signal strength inside the satellite’sstructure for both devices.

The results obtained for the rotation measurements are shown in two differ-ent figures for each of the devices. Figures 3.9c and Figures 3.9d show boththe rotation of transmitter and receiver in Cartesian coordinates for the SkyMote device. While Figures 3.12c and Figures 3.12d show the same informationfor the Gnode device. In this figures a comparison is made between both thereference field and the ENVISAT structure showing most of the time higher sig-nal strength inside the satellite’s structure than in the reference field for bothdevices.

Another set of figures is defined for the rotation test, we thought this willbring a better appreciation of the measurements results. Therefore, Figures3.10a and 3.10b show the rotation results for the Sky Mote device, and Figures3.13a and 3.13b show the results for the Gnode device. These are presentedin polar coordinates and in this case the comparison of the results is not madebetween each of the fields but is made between transmitter and receiver rotationresults.

Finally as it was described earlier in this chapter, two more tests were includedinside the satellite’s structure. The transmission through an internal wall ispresented by Figures 3.11a and 3.14a for both Sky Mote and Gnode respectively.While the Close positioning measurements are shown by Figures 3.11b and 3.14bfor both devices.

3.3.1 Sky Mote Device

The measurements made with the Sky Mote device are values read from theRSSI register in the CC2420 radio in the device. The RSSI register value isa reference to the power of reception at the RF pins by using the followingequation:

P = RSSI V AL+RSSI OFFSET [dBm] (3.1)

The offset defined here should be found in an empirical way during the systemdevelopment, but for these results the value used is -45. This value was takenfrom the CC2420 datasheet [3], which is given as an approximation of suchconstant.

3.3.2 Gnode Device

The measurements made with the Gnode device are values read from the RSSIregister from the CC1101 [22] radio in the device. The values are representedin dBs with a 0.5 dB resolution.

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(a) Signal Strength vs Distance (b) Signal Strength vs Transmitter Interfer-ence

(c) Signal Strength vs Transmitter’s Rotation (d) Signal Strength vs Receiver’s Rotation

Figure 3.9: Results obtained on both the open field and the ENVISAT environ-ment (Sky Mote device).

3.4 Analysis of Results

The interpretation of the results shown previously is presented in this sectionas follows:

1. The Signal Strength vs Distance: these results show how measurementsmade on the same cavity of the ENVISAT satellite have higher signalstrength values than the ones made in the Open field environment for theSky Mote devices. Figure 3.9a shows an average attenuation at the 200cm distance of 8,29 dBs difference between the ENVISAT and Referenceenvironment transmitter signals. While Figure 3.12a shows an averagevalue of 23,76 dBs differece at the same distance.

2. Signal Strength vs Rotation of the Nodes: both Figures 3.10a and 3.13ashow how for both devices the signal strength is not affected in a rep-resentative way in the open field environment. While Figures 3.10b and3.13b show how important is the antenna direction inside the ENVISATsatellite structure.

3. Signal Strength vs Transmitter interference: these results show a stablevalue of power reception in both environments and in both devices. Show-ing there is still signal reception even with the interferece generated by

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(a) Open field environment (b) ENVISAT environment

Figure 3.10: Results obtained from the rotation of sensor nodes shown in polarcoordinates (Sky Mote device).

(a) Transmission through internal wall (b) Transmitter and Receiver at 10 and 25 cmdistance

Figure 3.11: Raw results obtained from tests in ENVISAT environment (SkyMote device).

the intruding transmitter.

4. Close positioning through an internal wall: the power reception shownin Figures 3.14a and 3.11a is stable for both devices. Showing there isstill signal reception even with the nodes positioned in the configurationpresented in Figure 3.8.

5. Close positioning with distances of 10 and 25 cm: the signal strength ishigher at 25 cm than at 10 cm distance for both devices. But there is stillreception in both cases of the data transmitted.

3.5 Conclusion

The main objective of these measurements was to define the best option betweentwo different frequencies and also to observe the behavior of both frequenciesinside a satellite as a deployment environment. As it is shown in Table 3.1this goal is reached by comparing the results of several types of measurements

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(a) Signal Strength vs Distance (b) Signal Strength vs Transmitter Interfer-ence

(c) Signal Strength vs Transmitter’s Rotation (d) Signal Strength vs Receiver’s Rotation

Figure 3.12: Results obtained on both the open field and the ENVISAT envi-ronment (Gnode device).

inside the ENVISAT satellite and the reference values obtained on an open fieldenvironment for both frequencies (2.4 GHz and 868 MHz).

To have a clear comparison of the results obtained, this section shows a sum-mary comparison table (Table 3.1) that describes the main results for eachtest and frequency. In this section it can be seen that the behavior of bothfrequencies is quite similar. For example in the test call ”Signal Strength vsRotation of the Nodes”, both frequencies show the position of the antenna isvery important for the final power result. Also the table shows that for thetest ”Close positioning with distances of 10 and 25 cm” both frequencies showthe power value obtained is higher for the 25 cm distance than for the 10 cm one.

The only important difference in these results was found on the ”Signal Strengthvs Distance” test. In this section Table 3.1 shows the power values obtainedfor the 2.4 GHz frequency have less attenuation of the transmission signal ata longer distance between the reference values (Open field) and the ENVISATresults, as for also better results on the distances inside the same cavity (EN-VISAT environment). Therefore we conclude the best option for this particularenvironment from both frequencies is the 2.4 GHz one.

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(a) Open field environment (b) ENVISAT environment

Figure 3.13: Results obtained from the rotation of sensor nodes shown in polarcoordinates (Gnode device).

(a) Transmission through internal wall (b) Transmitter and Receiver at 10 and 25 cmdistance

Figure 3.14: Raw results obtained from tests in ENVISAT environment (Gnodedevice).

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Test Frequency 2.4 GHz Frequency 868 MHz

Signal Strength vs Distance Measurements made on the samecavity (ENVISAT) have highervalues than the reference (Openfield) measurements. Averageattenuation at 2 m of the trans-mitter is 8,29 dBs

Measurements made on the samecavity (ENVISAT) have lowervalues than the reference (Openfield) measurements. Averageattenuation at 2 m of the trans-mitter is 23,67 dBs

Signal Strength vs Rotation ofthe Nodes

Is very clear the position of theantenna changes in a significantway the power reception (spe-cially inside the satellite)

Is very clear the position of theantenna changes in a significantway the power reception (spe-cially inside the satellite)

Signal Strength vs Transmit-ter interference

The power reception has a stablevalue (more stable in an Openfield)

The power reception has a sta-ble value (more stable inside thesatellite)

Close positioning through aninternal wall

The power reception has a stablevalue with a standard deviationof 1,02

The power reception has a stablevalue with a standard deviationof 1,18

Close positioning with dis-tances of 10 and 25 cm

The power reception at a dis-tance of 25 cm is higher than theone at 10 cm. The average ofboth measurements is very closeto each other with an average dif-ference of 1.27 dBs

The power reception at a dis-tance of 25 cm is higher than theone at 10 cm. The average ofboth measurements is very closeto each other with an average dif-ference of 2.84 dBs

Table 3.1: Frequencies Comparison Table

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Chapter 4

A WSN design for a spaceapplication: SystemRequirements and DesignOverview

Given the results obtained in the previous chapter and the decisions made in anealier stage of the current project, this chapter will present the design’s systemrequirements as for a general overview of the WSN final design. The systemrequirements section will focus on the main characters of the network, thereforeclassifying these specifications into sensor node and gateway node requirements.While the design overview section will describe the two use case solutions forthe current project and will present a general description of the network’s func-tionality.

4.1 System Requirements

This section will present the requirements for both sensor and gateway nodes ofthe final WSN design for this master thesis. These requirements were definedgiven the decisions made in the previous chapter and the needs defined by theclient at the beginning of the current project.

4.1.1 Sensor Node

Independently from the power system used and therefore independent from thelocation of the sensor inside or outside the spacecraft cavities, the system blocksare described in Figure 4.1.

The temperature will be sensed at 3 locations using long wired temperaturesensors, coupled to their own channel of the ”conditioning” microcontrollersADC. Digital signal will be acquired by the conditioning microcontroller on-board the sensor module, which will perform the following tasks:

• Sensor readout via I2C.

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Figure 4.1: Functional Block Diagram Sensor Node.

• Applying sensor data reduction schemes, to make sure only relevant infor-mation is sent.

While the Zigbit Controller (MCU) will look after the following tasks:

• Interaction pattern setup (sample rates, events, etc.)

• Scheduling/Configuration setup

• Energy control for the connected power source, like shutdown, sleepingmodes, etc.

If the information meets the criteria of the data reduction scheme algorithm,the Zigbit controller will be triggered out of sleep mode and the informationwill be transmitted to the gateway. The transceiver will go back to sleep afterthis operation has been performed.

Software requirements

The software functionality is split over the 2 microcontrollers, the Core Micro-controller and the Conditioning Microcontroller. The software application shallcover the following functionalities:

1. Perform the necessary readout and transmission (Radio Frequency) fromthe sensor value to the defined gateway.

2. Have the necessary scheduling configuration for the relation between thesensor sample and the value to be transmitted.

3. There should be a sensor dependent energy control for the connected powersource (shutdown, sleeping modes, etc.).

4. The software shall allow for a data reduction scheme on the sensor side.

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Hardware requirements

The necessary hardware requirements for the sensor node application are thefollowing:

1. Zigbit Microcontroller with the capacity of supporting the desired com-munication protocol stack (Zigbee protocol) and the necessary interfacesto support the following:

(a) Radio Transceiver

(b) I2C communication capabilities

2. Conditioning microcontroller supporting the following:

(a) Low power sleep mode.

(b) 10 bits resolution ADC and more than 3 channels.

(c) Capability to run a data reduction algorithm.

3. Radio Transceiver that supports the IEEE 802.15.4 standard and the Zig-bee protocol. This with a communication frequency of 2.4 GHz. TheRadio transceiver has to support an interface to communicate with a mi-crocontroller as the one described before. An antenna is required as well.

4. Sensor. The sensor type depends on the application. The sensor shall becompatible with the resolution of the ADC available on the conditioningmicrocontroller. Perform the necessary readout and transmission (RadioFrequency) from the sensor value to the defined gateway.

4.1.2 Gateway Node

The gateway node is quite similar to the sensor node without the sensor module.To accommodate for more reliability on the overall sensor network and reducethe risk of gateway failure, a gateway redundancy approach has been followed,therefore several additional components have been added such as a real-timeclock used for synchronizing the 2 redundant gateways.(see Figure 4.2)

Software requirements

The software application to be run in the gateways microcontroller will re-quire certain characteristics to be able to accomplish the most efficient behavior.These characteristics are the following:

1. Initial network parameters are required for operation after a reset, toreturn to a nominal (or default) network state. The gateway should makesure this operation takes place when the network is powered.

2. When a new node wants to join the network the gateway should give anID and introduce it to the system.

3. There should be a task that saves the time stamp of the received data.

4. There should be a task that performs a redundancy management, when asecond gateway is installed next to the primary gateway.

5. There should be a task that performs a protocol conversion for communi-cation with a PC/ on-board computer.

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Figure 4.2: Functional Block Diagram Gateway.

Hardware requirements

The gateway is an important element of the overall network therefore its designshall match with the following requirements to provide optimized performances:

1. The MCU shall support the desired communication protocol stack (Zig-bee protocol) and the necessary interfaces to support the following com-ponents:

(a) Real Time Clock. Required by the gateway redundancy mechanism.

(b) Radio Transceiver. Required to receive the radio signal from thenode.

(c) Non volatile memory. Required to log the data acquired by the gate-way.

(d) Communication with a second Microcontroller of the same kind. Re-quired by the gateway redundancy mechanism.

2. Radio Transceiver shall support the IEEE 802.15.4 standard and the Zig-bee protocol. The chosen transceiver shall support a frequency of 2.4GHz. The Radio transceiver shall support a communication interface witha microcontroller responding to the previously defined requirements. Theradio shall include an antenna with the proper power range defined forthe current application.

3. Real Time Clock. The RTC is required for the synchronization betweenthe hot gateway and the cold gateway. It will be powered by an externalflat battery and will communicate with both main and redundant gate-ways.

4.2 Design overview

In this section it will be given a general overview of the existing system as forsome of the decisions already made by the moment this thesis was started. First

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Figure 4.3: System’s topology description.

an overview of the network’s general design decisions as for a description of bothuse cases solutions will be shown, followed by an explanation of the completesystem functional flow.

4.2.1 General network overview

The overall system ranges from the sensor itself to its data transmission towardsthe spacecraft data bus or the AIT computer used for testing purposes. Thesystem being modular and the spacecraft bus considered as black box - no specialinterfaces will be designed between the bus and the system gateway.

The final design goal of this project is to be able to accommodate for commu-nication within a satellite or during AIT activities on the ground. These two usecases are similar design wise, they only differ in terms of power systems, topol-ogy and system boundaries. While the data collected by the gateway would bemade available to the OBC as housekeeping data and ADCS data during orbitcase, those data would simply be collected by a test PC in the AIT use case.

A gateway redundancy algorithm is also designed specifically for the in-orbitconfiguration, and is not expected in an AIT configuration, where assistance

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and refurbishment can easily be provided. (See Figure 4.3)

Overview In-orbit communication

The communication between the gateway and the spacecraft is here left outsideof the system boundaries as it could be of several types: SpaceWire, CAN, etc.

Data handling:

In the in-orbit communication use case, the data will be collected from 3 sensors(such as Temperature sensors) and be requested via a polling mechanism by theprocessing unit of the node, before being transmitted via the TRX module to-wards the gateway.

Power module:

2 types of power sources will be used for in-orbit communication:

1. Primary battery only: In this case, a primary battery is powering thewhole sensor nodes including its MCU, TRX, conditioning microcontrollerand sensors. The challenge clearly lies in the duration of the mission andthe system is being designed in the assumption of 5 years in-orbit activity.The primary battery alternative is preferred for the sensors needed insidethe cavities of the spacecraft away from any solar power possibilities.

2. Combination secondary battery + solar cells: In this case, a secondarybattery is powering the whole sensor node including its MCU, TRX, andconditioning microcontroller and sensors. This power combination makesuse of a Solar cell to recharge and condition a secondary which will providepower at all time (including eclipse phases). This alternative is preferredfor the sensing of hard to reach locations such as antenna booms, magne-tometers booms etc. The main drawback of this option is accommodatingfor enough power during eclipse and the limited operational thermal rangeassociated with batteries, coupled with a location that is both exposed tocold space and to direct sunlight, which implies a wide temperature range.

Topology:

The chosen topology is the Star topology with additional gateway nodes whichmight be required for the design, since distances between nodes and gatewaybecome too high, or obstructions cause interference in transmissions. For exam-ple, ’north’ side will have its own (dual-redundant) gateway, ’south’ side of thespacecraft, and internal (near instruments) sensors will have another gateway.(See Figure 4.4)

Overview AIT communication

While the in-orbit configuration is fixed before launch and cannot be servicedor changed on-flight, the AIT configuration of the network is reusable recon-figurable and shall be designed as such. The centralized approach allows for

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Figure 4.4: For large size satellites, multiple gateways (each redundant) areused.

configuration by a single PC and centralized control of the different sub net-works. The communication between the gateway(s) and the AIT computer isdone via USB and therefore can easily be serviced or reprogrammed.

When a new node wants to join to the network, it reaches to the default gate-way, the PC Configurator receives notification of a new node and may assign itto a certain (location-dependent) gateway through the use of a sub network ID.In hardware, each sub network consists of a gateway and USB connection.

Data handling:

In the AIT communication use case, the data will be collected from the sen-sors (such as Temperature sensors) and be requested via a polling mechanismby the conditioning microcontroller of the node, before being transmitted viathe Zigbit controller to the TRX module towards the gateway. The gatewaywill then communicate the data towards the AIT computer for collection andinterpretation. It is intended that the AIT computer can request data from thegateway at any given time.

Power module:

2 types of power sources will be used for AIT operations:

1. Wired power: The gateway(s) is directly connected to the AIT computer.Most likely this connection will be done via USB.

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Figure 4.5: Star topology.

2. Primary battery only: In this case, a primary battery is powering thesensor node. The drawbacks from the in-orbit configuration do not applyhere as the primary battery does not require to be used for long duration.It can be refurbished before starting a new AIT campaign, making a highlymodular test point.

Topology:

The chosen topology is here the Star topology divided in clusters of sensorfamilies. (See Figure 4.5)

System functional flow

As seen in Figure 4.6, a temperature is sensed on the sensor node side of thesystem at specific time intervals, here at a frequency of 1Hz. The sensed analogsignal will be conditioned for each sensor into a digital signal processed by theconditioning microcontroller. A simple data reduction algorithm will be appliedon the sensed data to determine whether the data is relevant or not, regardinga certain difference threshold with the previously sensed value. If the data isrelevant the microcontroller onboard the Zigbit chip will be triggered out ofsleep mode and the signal will be transmitted towards the gateway.(see Figure4.6).

At the gateway side the data will be processed and transmitted to and onrequest by the bus master, either SC-bus or AIT computer. The followingfunctional block diagram presented in Figure 4.7 details the different functionalblocks of the system and their interactions. As seen in this diagram, the gatewaymodule and the sensor node module have rather similar components.

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Figure 4.6: Overall system flow diagram.

Figure 4.7: System functional block diagram.

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Chapter 5

A WSN design for a spaceapplication:Communication ProtocolDesign

In chapter 1 of this thesis it was shown how the Zigbee communication protocoland the BitCloud SDK are already made decisions, defined in an earlier stageof the current project. Having these two choices as the base of our design’scommunication protocol, this chapter presents the relation between the datatransmission and the power consumption of the network’s sensor node. Finallyshowing a communication scheme to be used in the final prototype of the project.

5.1 Data transmission and power consumption

Given that BitCloud SDK is fully compliant with the Zigbee protocol. Theonly way to transmit any information from the gateway to the sensor node isby using an already implemented polling mechanism that can be enabled ordisabled by the existing API. Therefore this section will describe the differenttypes of configurations that can be used to transmit 1 byte of payload from aparticular sensor node to the gateway of the network. The main objective of suchexplanation is to show not only the different configuration but the repercussionof each configuration on the power consumption of the device.

5.1.1 Transmission configuration

Transmission of 1 byte of payload with the use of a communicationapplication layer acknowledgment and polling mechanism enable.

This configuration is initially the default configuration of the BitCloud SDK.This means there will be an acknowledgment at the end of the transmissionand there will also be in between several polling requests from the sensor nodeto the networks gateway. It should be noted, two important variables already

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Figure 5.1: Transmission with application ACK enable and polling mechanismenable.

defined in the current API are taken in account to define the system behavior:the CS AUTO POLL and CS INDIRECT POLL RATE. The first one is a flagthat will disable or enable the polling mechanism to be started automaticallyevery time a transmission is requested. The second one is the time the sensornode will take to poll the information from the gateway. This configuration willshow the following sequence:

1. The node will wake up and spend approximately 40 ms in Tx/Rx mode(approx. 20 mA current consumption).

2. Next will go to run mode for approximately 900 ms (approx. 14 mAcurrent consumption).

3. Finishing in again 40 ms of the Tx/Rx mode (approx. 20 mA currentconsumption).

An important note here is that for this configuration the best result obtainedwas to reduce the CS INDIRECT POLL RATE to the smallest value possible(minimum value is 10), getting as a result a transmission sequence that juststays for 100 ms in Tx/Rx Mode (approx. 20 mA current consumption) andgoes back to sleep.(see Figure 5.1)

Transmission of 1 byte payload without the use of a communicationapplication layer acknowledgment and polling mechanism enable.

For this configuration the transmission sequence will look like a 40 ms step sig-nal. Therefore showing how the node will wake up spend approximately 40 msin Tx/Rx mode (approx. 20 mA current consumption) and will go back to sleep.When the acknowledgment is disable the variable CS INDIRECT POLL RATEis not taken any more in account.(see Figure 5.2)

Transmission of 1 byte payload without the use of a communicationapplication layer acknowledgment and polling mechanism disable.

For this last type of configuration the CS AUTO POLL flag is set to false (dis-abling the polling mechanism from every transmission) as well as the applicationlayer acknowledgment. Therefore the final result will be a transmission peak of

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Figure 5.2: Transmission with application ACK disable and polling mechanismenable.

just 6-8 ms in Tx/Rx mode (approx. 20 mA current consumption).(see Figure5.3)

5.1.2 Conclusion and Quality of Service

The results obtained and described in this section show the power consumptionof each of the possible configurations to transmit a package from a sensor node toa gateway node in the current wireless sensor network. As it is shown here, theuse of the application layer acknowledgment will generate the highest currentconsumption with a minimum transmission duty cycle of approximately 100 ms.While the last configuration shown, where both the acknowledgment and thepolling mechanism are disable, will result in a transmission of approximately6-8 ms of duty cycle.

Therefore if the current system needs to reduce its power consumption to theminimum but also enable a polling mechanism in the communication to be ableto send commands from the gateway to the sensor node. Then the best solutionfor the current system will be to use as much as possible the last configurationgiven here (6-8 ms) and every given time (this time will depend on the appli-cation) enable the polling mechanism to receive the necessary commands fromthe network gateway (therefore approximately 40 ms duty cycle).

Figure 5.3: Transmission with both application ACK and polling mechanismdisable.

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Another important point to conclude here is, given the results obtained withthe application layer acknowledgment; if the systems main objective is the powerconsumption then new communication schemes should be introduced to keep thebest quality of service level possible in the system. The next section will givea description of the scheme used in the current design and will show how itwill improve the systems functionality and how it will keep the best quality ofservice that can be obtained with the current network tools.

5.2 Communication scheme

5.2.1 Scheme use cases

This section shows 4 different use cases that describe the communication schemeto be used in the current application. The objective of these functionalities isto maintain a low level of power consumption while keeping a simple algorithmthat will improve the quality of service of the system.

This algorithm will use a reception counter (RX count) and a transmissionrequest counter (Tx Count) to make a decision on 3 main parameters on eachof the network nodes. The reception counter will be kept at the gateway nodebeing incremented every time a frame is received from a specific node. Whilethe transmission request counter value will be transmitted inside every frame aparticular sensor node transmits to the gateway.

These 3 main parameters that will be affected by the decision of the describedalgorithm will be: Node Th, Number reTx and Polling Th. The first one willdefine the transmission resolution from a particular node. The second one willdefine if the current node should perform a number of retransmissions every timea request is generated. The last one will decide the amount of transmissions tobe made by the sensor node before polling a command from the gateway.

The decision to increase or reduce the values of these parameters will be basedon the quality of the link between the sensor node and the gateway. These willbe computed by comparing the reception counter (RX count) and the transmis-sion request one (Tx Count). Therefore if the algorithm defines a link to beweak, this will try to increase the number of retransmissions (Number reTx) orreduce the resolution of the signal in a particular node for example.

Description of systems parameters:

• Mode: This value will be used to let the gateway know if the informationthat has been transmitted from a particular sensor node is a data frameor other kind of information. In the current application this will be usedto define a data mode and a heartbeat mode.

• Batt ChLevel: This variable will be used to transmit the charge levelvalue of the battery in the node that is currently communicating with thegateway.

• Tx Count: This counter variable is increased every time a new transmis-sion is initiated in the respective sensor node.

• Rx Count: This counter variable is kept in the gateway side and is in-creased every time a transmission is received from a specific node.

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• Node Th: This variable will keep the value of the transmission thresholdfor each specific node. It is important to make emphasis that if the valueis transmitted by the sensor node then this is the current value being usedin the node, but if the value is being transmitted by the gateway then thegateway is ordering the node to update the value.

• Val (1-3): This will represent the sensed values from each of the sensorsconnected to the sensor node.

• Number reTx: This value will keep the amount of retransmissions thecurrent node will do every time is woken up to produce a transmission.This will increase the probability of reception in the case there is a poorlink quality between gateway and a specific node.

• Current TxReqCount: This counter variable will keep the current numberof transmissions made by the node without producing a polling request tothe gateway.

• Polling Th: This threshold variable will define the amount of transmis-sions to be made by the node before polling a command from the gateway.

Use Case 1. Data transmission: The sensor node wakes up, transmitsa data frame and goes back to sleep.

Use Case Description: A sensed value reaches the already defined nodes thresh-old. The sensor node will wake up transmit one Data Frame and will go backto sleep. The gateway receives the Data Frame and goes to the process callData Reading in the diagram. The retransmissions variable is set to 0 and thetransmissions request counter has not reach the polling threshold.(see Figure5.4)

Data Reading: This process will update the values received in the Data Frameand will increase the Rx Count. This last variable is a reception value kept inthe gateway node as information to compute a quality value of the link.

Parameters values:

• Number reTx = 0

• Current TxReqCount < Polling Th

Use Case 2. Polling Request and Data Transmission: The sensornode wakes up, polls for a command frame, updates internal values,transmits a data frame and goes back to sleep.

Use Case Description: A sensed value reaches the already defined nodes thresh-old. The sensor node will wake up and will start the polling of commands fromthe gateway node. The Cmd Decisions process will take place before any kindof command frame is generated. After receiving the command frame (If there isany being generated) from the gateway the sensor node will transmit its DataFrame to the gateway and will go back to sleep. The gateway receives the DataFrame and goes to the process call Data Reading in the diagram.(see Figure 5.5)

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Figure 5.4: Sequential Diagram of Use Case 1.

Cmd Decision: This process will decide on the over the air commands to betransmitted. These commands are the update of three main variables: Num-ber reTx, Polling Th, Node Th. Therefore the gateway will decided if there isneed for a change in the number of retransmission, in the polling mechanismthreshold or in the node threshold value for a particular node in the network.As it was already described this decision will be based on the results of theTx Count and Rx Count variables.

Data Reading: This process will update the values received in the Data Frameand will increase the Rx Count.

Parameters values:

• Number reTx = NA

• Current TxReqCount >= Polling Th

If the variable Current TxReqCount is equal or greater than the Polling Thone, then one polling request and only one data transmission will be producedwithout taking in account the variable that decides the number of retransmis-sions in the system (Number reTx)

Use Case 3. Data Retransmissions: The sensor node wakes up, trans-mits a data frame with several retransmissions (maximum 2 retrans-missions) and goes back to sleep.

Use Case Description: A sensed value reaches the already defined nodes thresh-old. The sensor node will wake up transmit several times (the number of timesis defined by the variable Number reTx) the Data Frame and will go back tosleep. The gateway receives the Data Frame and goes to the process call DataReading in the diagram. The retransmissions variable is set to a value greaterthan 0 and the transmissions request counter has not reach the polling thresholdone. It is important to note that each of the retransmissions of the Data Framewill keep the same Tx Count value. Therefore the gateway will only receive oneof the transmissions made and will increase the Rx Count only once as well.(seeFigure 5.6)

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Figure 5.5: Sequential Diagram of Use Case 2.

Data Reading: This process will update the values received in the Data Frameand will increase the Rx Count.

Parameters values:

• Number reTx > 0 (Maximum value is 2)

• Current TxReqCount < Polling Th

The worst case scenario is the transmission of 3 Data Frames, therefore atransmission duty cycle of 24 ms.

Figure 5.6: Sequential Diagram of Use Case 3.

Use case 4. Heartbeat Transmission: The sensor node wakes up,transmits a Heartbeat Frame and goes back to sleep.

Use Case Description: The internal Heartbeat counter in the sensor node reachesa defined threshold. This generates the transmission of the Heartbeat frame andgoes back to sleep. The gateway receives the frame going directly to the Data

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Reading process.(see Figure 5.7)

Data Reading: This process will update the values received in the HeartbeatFrame and will increase the Rx Count.

Important note: For this specific use case the parameters of retransmission(Number reTx) and polling (Polling Th) are not taken into account. The mainidea of such frame is to keep the gateway informed of the status of a sensornode in case no transmission has been generated. Therefore inside every sensornode there will be a Heartbeat timer that will be cleared every time a requestof transmission is made. If this is not the case and a long time passes (valueto be defined by the application) without any transmission, then the timer willgenerated a Heartbeat transmission keeping the network informed of its currentcondition.

Figure 5.7: Sequential Diagram of Use Case 4.

5.2.2 Frame definition

In this section of the document a definition of the transmission frame is given.This is a compilation of the already defined frame for the Zigbit chip by Atmeland the frame that would be used in the current application. The last one beinga gathering of data needed from each of the nodes in the network. Table 5.1

Zigbit Frame

Bytes Number 4 1 1 Variable 2Description Preamble SDF Payload

LengthPayload CRC

Table 5.1: Structure of the Zigbit frame.

shows the structure of the already given Zigbit frame were it can be seen thatthere is at least 8 bytes between header and footer that will be always part of thedata transmitted. Also is important to note here that the BitCloud User Guide[1] document gives a limitation to the size of the Payload section of this frameto 84 bytes maximum size. Table 5.2 shows the structure of the data frame

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Data Frame

Bytes Number 1 1 1 1 1 1 1Description Mode Batt ChLevel Tx Count Node Th Val1 Val2 Val3

Table 5.2: Structure of the Data frame.

that is going to be used for the transmission of each of the sensor nodes in thenetwork. The data frame is included in the Payload section of the Zigbit frameshown in table 5.1. The frames previously described apply to the transmissions

Heartbeat Frame

Bytes Number 1 1 1 1Description Mode Tx Count Batt ChLevel Node Th

Table 5.3: Structure of the Heartbeat frame.

made from the sensor node to the gateway of the network. But there it is stilla last frame that shows the structure of the data from the gateway to a specificsensor node in the network. This will be defined by the 3 different variablesthe gateway will be able to update over the air. These variables were describedalready in section 3.7.1 and the structure of the frame is shown in table 6 below.

Cmd Frame

Bytes Number 1 1 1Description Number reTx Polling Th Node Th

Table 5.4: Structure of the Command frame.

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Chapter 6

A WSN design for a spaceapplication: Final NetworkDesign

This chapter describes thoroughly the WSN final design for the project presentedin this thesis. The result will be shown and clasified into sensor and gatewaynode design as the requirements previously shown in chapter 4 of this document.The details on each of the clasifications will be described and the final solutionwill be presented.

6.1 Sensor Node Design

As chapter 4 described previously there is a defined modularity to the finalnetwork design. This section will introduce both the core and sensor modulesof the Sensor Node design, therefore providing the final solution for this givendesign.

6.1.1 Core module

Microcontroller

The microcontroller selected for this project is the ATMega1281, it is an 8 bitAVR with a maximum clock frequency of 16MHz. It has been selected for boththe gateway and the sensor node as part of the Atmel Zigbit module. Thetemperature will be sensed at 3 locations using long wired temperature sensors,coupled to their own ADCs.

Digital signal will be acquired by the conditioning microcontroller on-boardthe sensor module, which will perform the following tasks:

• Sensor readout via analog-digital converter (ADC) ports.

• Applying sensor data reduction schemes, to make sure only relevant infor-mation is sent.

While the Zigbit Controller (MCU) will look after the following tasks:

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• Interaction pattern setup (sample rates, events, etc.)

• Scheduling/Configuration setup

• Energy control for the connected power source, like shutdown, sleepingmodes, etc.

If the information meets the criteria of the data reduction scheme algorithm,it will be transmitted over the wireless link (after wake up of the transceiver)towards its corresponding gateway. The transceiver will go back to sleep afterthis operation has been performed. The microcontroller will as well go back tosleep while its ADC sampling will stay on. The microcontroller is responsiblefor the energy control, including its own.

RF-Transceiver

The radio transceiver used for this system is the AT86RF231, a low power2.4Ghz using the 802.15.4 protocol. It is part of the Zigbit chip together withthe microcontroller. The result of the preliminary test campaigns on both clearfield and satellite structural model shown in chapter 3 of this tesis, was used todecided on the communication frequency of such transceiver.

6.1.2 Sensor module

The sensor module is composed of the sensors and conditioning for each sensors.

Conditioning microcontroller

The conversion from analog to digital will be performed by the ADC onboardthe Conditioning Microcontroller. The microcontroller to be used is a low powerPIC18F4680, which has so far flown 3 years in Low Earth Orbit and on somemore ISIS subsystems. Each sensor has its own channel on the onboard ADC ofthe microcontroller and the data are sampled every time T, for instance at 1Hzfor the temperature sensors. A data reduction scheme is applied on the acquireddata by the conditioning microcontroller to wake up the Zigbit microcontrolleronly for relevant data and therefore keep the power consumption of the wholenode to a minimum. The PIC controller only uses 1mA to read out data fromits ADC compare to 14.5mA on the Zigbit microcontroller which would makeit a power bottleneck of the system if used for data polling.

Sensors

Temperature Sensors

The main housekeeping sensor which will be used in this project is the tem-perature sensor, as low data rate is required to retrieve useful and accurateinformation from it.

Sun sensor

Coarse sun sensors are currently being investigated for the project. For thecurrent design it will be used a photodiode as a coarse sun sensor application.

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This coarse sun sensor is being looked at for the purpose of the prototype. Typ-ical GEO missions would make use of a different and more power hungry typeof coarse sun sensor.(see Figure 6.1)

Figure 6.1: Coarse sun sensor.

6.1.3 Software Design

The purpose of the mechanism shown in Figure 6.2 is to reduce the usage ofthe transmitter and of the Zigbit controller which are the components using themost power.

Figure 6.2: Sensor node software flow.

Sequence description:

1. Step 0: The conditioning controller is in idle state for 1s.

2. Step 1: After a 1s period the conditioning controller starts poling the datafrom the sensor via its ADC.

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3. Step 2: The conditioning controller analyzes whether the sensed value isequal to the previously sensed value or within a specific range around theprevious value (threshold).

4. Step 3: If the present value differs from previously sensed value (threshold)the Zigbit controller is triggered and the data is transmitted towards thegateway.

5. Back to Step 1: If the present value equals the previously sensed valuethen the data is NOT transmitted towards the gateway and the controllergoes back to waiting mode.

6.1.4 Hardware design

Figure 6.3 shows the schematic diagram for the sensor node board. This schematicshows the ports and components that are planning to be used in this specificboard. It is important to note here that the JTAG and the FT232R are portsthat will be included only on the development board but will not be includedin the final product.

Figure 6.3: Sensor node schematic.

6.1.5 Low Power design

There are 3 types of power being considered for this prototype, 2 based on full-wireless approach and the third one bringing a wired power alternative. The

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following block diagrams describe the power alternatives within the wirelessnode architecture:

Primary battery powered node

For the solution inside the spacecraft cavities, a primary power source is oneof the design options and therefore was investigated. Duration of 5 years inorbit requires a large amount of available power if a decent sampling needs tobe achieved.(see Figure 6.4)

Figure 6.4: Primary battery powered node.

Secondary battery + cell powered node

The following configuration is meant to be used for sensors outside of the space-craft structure, for instance for antenna booms, solar array or magnetometerbooms. Such a configuration is necessary for sensors needing to be on at alltime including during the eclipse phases of the orbit.(see Figure 6.5)

The setup is made of 3 main components:

• The cell

• The BCR (Battery Charge Regulator)

• The secondary battery itself

Figure 6.5: Secondary battery + cell powered node.

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Figure 6.6: Logical Architecture of the gateway side.

Power saving design

The purpose here is to show the power consumption in simplified use cases anddraw conclusions from them for the design.

The following cases were considered:

1. Event driven temperature sensing with 3 sensors connected:

• Read cycle temperature sensor node: The temperature measurementis compared to the previous measurement, but does not meet thecriteria for waking up the transmitter and sending it over the wirelessnetwork.

• Transmit cycle temperature sensor node: The temperature measure-ment meets the criteria for sending and it is transmitted

2. Higher data rate sensing transmitting every measurement.

• Read and transmit continuously as a coarse Sun sensor node: Readout a photodiode used as a coarse Sun sensor and transmit at 10 Hzupdate frequency.

6.2 Gateway Node Design

This section will describe the final design for the WSNs Gateway Node. Forthis design is necessary to defined the data handling of the device as for thesoftware and hardware decisions taken for the task at hand. The last two topicwill provide the final solution for the requirements presented in chapter 4 of thisthesis.

6.2.1 Data handling module design

The BitCloud SDK comprise of a HAL (Hardware Abstraction Layer) and var-ious APIs for the network stack (i.e. ZDO, the Zigbee Device Object APIs).

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Figure 6.7: Gateway node software flow.

The highest level is an event-based task handler that can execute user andapplication code.

The initial network parameters are required for operation after a reset, toreturn to a ”nominal” (or default) network state, that sensor nodes can join to.In the reconfiguration parameters area, various sensor interaction pattern datais stored for the network (including the nodes connected to that gateway), aswell as network oriented parameters, like beacon enabled mode, CSMA mode,GTS mode, etc.

At the PC side, a GUI will take care of network setup, application setup andanalyzing of network status during operation. (see Figure 6.6)

6.2.2 Software design

The flow diagram shown in Figure 6.7 describes the behaviour of the gatewaynode. The device waits for a message from any of the nodes in the currentnetwork. If the message is received then this application will check the datastructure received and read the ID of the Node from where it was transmitted.When the ID is read the application will search for an specific data FIFO thatwill be link to the ID of each of the nodes of the network and will write thenew data in the FIFO. Once the writing has been completed the applicationwill check an internal timer and after a specific time will create a log file on inthe Flash memory of the controller.

6.2.3 Redundancy algorithm [2]

During the conceptual design of the project, the concept of gateway redundancywas introduced to provide higher reliability at the data collection point. Thecontroller part will be in control of switchovers and will decide on check pointingnotifications. A wrapper creates the required check-pointing data as command-data and sends it to the standby gateway, where this gateway unwraps this data

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Figure 6.8: ZigBit based Dual Redundant Gateway Architecture.

and then stores data for eventual switchover.(see Figure 6.8)Real-Time Clock

For time stamping, the RTC is directly connected to the gateways I2C interface,and provides an interrupt pin for periodic update of the internal system clock.The internal clock source runs on a 32-kHz crystal, however due to tempera-ture variations the clock might easily drift, which is why an update from theexternal RTC(DS232) is needed. The DS3232 has a high-accuracy, temperaturecompensated, built in clock source and stores date and time. Together with theinternal system clock a resolution of 1ms can be achieved for time stamping thedata.

Check pointing data

There are two different classes of information that needs to be check pointed:

• Static parameters: The following parameters are considered as static pa-rameters, selected topology, maximum network depth and buffer size amounts.

• Dynamic parameters: Parameters such as operating channels might changefrequently during operation of the network, and are therefore consideredas dynamic parameters.

Due the self-healing and adaptability nature of Zigbee, not many dynamicparameters should be used for dynamic check pointing. The proposed schemeallows for a fast and easy recovery of the gateway and for the usage of different

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files programmed for each gateway having, therefore each of them having itsown parameters. These are based on OS-like file system functionality and leavecheck pointing invisible to the application task. Also, these are meant to run ona single device (e.g. acting as a single node failure recovery) and do not supportother hardware devices than the tested ones. As the main goal is increasingavailability and increase robustness of the network, so adding hardware redun-dancy requires an external communication scheme.

Heart beat as node fail detection

The standby gateway uses the heartbeat pulse to monitor the health of theactive gateway. In the scheme above, simple heartbeat pulses of 10ms havebeen used to detect a failure of the master device. Connecting this heartbeatdirectly to an Interrupt pin on the microcontroller will make the Hot gatewayable to detect a malfunction of the Cold gateway with simple software counters.This also allows for hardware based heartbeats like PWM pulses.

6.2.4 Hardware design

Figure 6.9 shows the schematic diagram for the gateway board. This schematicshows the ports and components that are going to use in this specific board. Itis important to note here that the JTAG and the FT232R are ports that willbe included only on the development board but will not be included in the finalproduct. Also here it is shown that there is a redundancy circuit that appearsto be all in one board but actually will be separated into two different boardsconnecting the Zigbit chips by two external cables for check pointing.

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Figure 6.9: Gateway schematics.

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Chapter 7

A WSN design for a spaceapplication: Evaluation

In chapters 5 and 6 of this thesis a final WSN design was provided as a solutionfor the requirements shown in an earlier stage of this document. Therefore,as a final step of this design an evaluation is presented with the objective ofproviding the necessary conclusions on the quality of this final solution.

This chapter will present a protocol simulation to evaluate the communicationprotocol defined by this thesis. The simulation will model an operating WSN ina specfic environment which will recreate a real-life space application and willprovide enough information to defined the necessary conclusions for the currentsystem design.

7.1 Communication Protocol Simulation

As it was described earlier, we will present here the simulation made over thecommunication scheme developed and presented in chapter 5 of this document.This section will start by making a complete description on the background andnetwork parameters the simulation will take into account, as for an overview onthe information that will be analyzed in this protocol’s evaluation.

Followed by a group of experimental results that will be shown as the outcomeof the different parameter combinations and simulated scenarios that will be de-scribed here. Finalizing with the data analysis made on the given experimentalresults and the conclusions obtained from these given results.

7.1.1 Simulation Overview

A WSN is represented in this section by 1 gateway, 1 communication channeland a growing amount of sensor nodes. Each node in the network will beconnected to 3 different temperature sensors that will be sensing a satellite’stemperature signal. This signal is described by the formula shown in 7.1, whereφ refers to the phase of the profile signal and will be defined by a random valuewith the objective of recreating a more realistic value on each of the sensorsmeasurements.

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Temp(t) = sin((2π) · t/(6000) + φ) (7.1)

The measurement made by each of the sensors in a network node will beupdated with a sample frequency of 1 Hz, while the transmission of the valueto the network’s gateway will depend on a sensor node’s threshold. This valuewill defined the necessary magnitude of signal’s variation to generate a datatransmission from a sensor node to the network’s gateway.

For the scenarios evaluated in this simulation this threshold will be set to 1degree, this means a sensed value will be transmitted if the result between thenew measurement taken and the last value transmitted is 1 degree difference.

Another important aspect that will be taken into account in this simulationis the Heartbeat transmission. Therefore each of the nodes in the network willbe provided of an internal timer that will wait 5 seconds to transmit a Hearbeatframe to the network’s gateway. This means that if during the lapse of 5 secondsthere has not been any kind of transmission request by the node, the timer willgenerate a Heartbeat transmission informing the gateway of its current status.

In this section three different scenarios will be analyzed as described earlierand are presented as follows:

1. Scenario 1: The first scenario evaluated in this simulation is based on UseCase 1 presented in chapter 5 of this document. Therefore every nodein the network will be defined by a number of 0 retransmissions and apolling mechanism threshold of 5. These parameters will define how eachnode in the simulated network will make only one transmission every timeit transmits a Data Frame and it will request a Cmd Frame from thenetworks gateway every 5 Data Frames transmitted.

2. Scenario 2: The second scenario evaluated in this simulation is based onUse Case 3 presented in chapter 5 of this document. Therefore everynode in the network will be defined by a number of 2 retransmissions anda polling mechanism threshold of 5. These parameters will define howeach node in the simulated network will make 3 transmissions every timeit transmits a Data Frame and it will request a Cmd Frame from thenetworks gateway every 5 Data Frames transmitted.

3. Scenario 3: The third scenario evaluated in this simulation is based onUse Case 1 presented in chapter 5 with the difference that the pollingmechanism of this evaluation will be made to a lower value. Thereforeevery node in the network will be defined by a number of 0 retransmissionsand a polling mechanism threshold of 3. These parameters will define howeach node in the simulated network will make 3 transmissions every timeit transmits a Data Frame and it will request a Cmd Frame from thenetworks gateway every 5 Data Frames transmitted.

7.1.2 Experimental Results

The experimental results obtained by simulating the behavior of the three sce-narios defined earlier in this chapter are described as follows:

• Figures 7.1a, 7.1b and 7.1c presents a percentage of the number of trans-missions requested versus the increment of the nodes in the network. This

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Figure shows two different results, the first curve represents the amountof successful packets transmitted after each transmission request and thesecond one the amount of dropped packets for each of the transmissionsrequested.

• Figures 7.2a, 7.2b, 7.2c presents the average time between transmission(successful ones) per each node in the network versus the increment of theamount of nodes in the network.

• Figures 7.3a, 7.3b and 7.3c presents the power consumption per nodeversus the increment of the amount of nodes in the network. The Figureshows the power consumption generated by five different events:

1. Successful message transmissions.

2. Successful heartbeat transmissions.

3. Successful command polling transmissions.

4. Unsuccessful transmission requests.

• Based on the results presented in Figures 7.3a, 7.3b and 7.3c a new groupof results is calculated. Figures 7.4a, 7.4b and 7.4c will show these resultsby providing the energy consumption over 5 years of operation versus theincrement of the nodes in network. These Figures will present the samefive events shown in the power consumption group of results.

(a) Scenario 1. Experimental Results.

(b) Scenario 2. Experimental Results. (c) Scenario 3. Experimental Results.

Figure 7.1: Percentage Message Transmission versus Number of Network Nodes.

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(a) Scenario 1. Experimental Results.

(b) Scenario 2. Experimental Results. (c) Scenario 3. Experimental Results.

Figure 7.2: Average Time Between Transmissions versus Number of NetworkNodes.

7.1.3 Results Analysis

From the experimental results obtain in this chapter we can described the fol-lowing:

From Figures 7.1a, 7.1b and 7.1c we can observe how in all three of them thenumber of successful packets starts to decrease when the amount of nodes inthe network starts to increase. This behavior makes sense if we think that byincreasing the amount of nodes in the network then we will increase the amountof channel contenders. Therefore the amount of dropped packets will start toincrease when the number of nodes in the network starts growing.

Another important aspect that can be seen here is how the variation of theretransmissions and the polling threshold input parameters will increase theamount of dropped packets, therefore reducing the amount of successful trans-missions made by the nodes in the network. Given that a requirement of theproject made in an earlier stage is to prove the successful operation of the net-work around a maximum of 100 nodes. In this section we can observe that theworst case in this simulation presents a 90% of successful transmissions in a nodeinside a 100 nodes network which we have defined as a satisfactory transmissionpercentage for the given design.

From Figures 7.2a, 7.2b and 7.2c we can observe how the average time be-tween transmission (successful ones) is around 11 seconds for all of the scenariosevaluated and maintains constant through the increment of the networks nodes.An important note to define here, is the change of the input parameters in eachof the scenarios does not affect in a representative way the final results of this

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(a) Scenario 1. Experimental Results.

(b) Scenario 2. Experimental Results. (c) Scenario 3. Experimental Results.

Figure 7.3: Power Consumption per node versus Number of Network Nodes.

evaluation.

As it was described during the earlier chapters in this thesis, an importantaspect of the given design is to be able to keep a reduce amount of powerconsumption of the system. The objective of this task is to increase the life-time of the system by reducing the amount of energy drawn from the primarybattery that will power it. Therefore the amount of energy drawn by a singlenode in the network during a 5 years range should be evaluated.

That is the reason why Figures 7.3a, 7.3b and 7.3c present the average powerconsumption per node in the network versus the increment amount of nodesin the network. These Figures show how the amount of packet being droppedduring the networks operation influence the average power a node in the networkconsumes. In this Figures can also be seen how the second scenario has thebiggest amount of total power consumption followed by the third evaluation.

An interesting fact in these results is how the consumption generated by theregular transmissions is over the other measurements in the second scenario butis the last value on the other two evaluations. Therefore, it is clear how thenumber of retransmissions in a node has more repercussion on its total powerconsumption that the change on the polling mechanism threshold.

As final results the average energy consumption of a given node in the networkduring a time range of 5 years is presented in Figures 7.4a,7.4b and 7.4c. Theseresults show how for a life-time of a 5 years time range will need a battery thatprovides over 14 Wh for the worst case presented in these results (Figure 7.4b).It is also important to note here, that if we reduce the amount of nodes in thenetwork and define a scenario that mix scenarios 1 and 2 the resulting energyvalue can be reduced greatly. This final results also show how the Heartbeat

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(a) Scenario 1. Experimental Results.

(b) Scenario 2. Experimental Results. (c) Scenario 3. Experimental Results.

Figure 7.4: Energy Consumption during 5 years versus Number of NetworkNodes.

transmission have an important repercussion on the final energy consumption,therefore by increasing timer to a value higher than the 5 seconds defined forthis simulation the final power consume can be reduce in a greatly manner.Finally if we focus again the results for the network in 100 nodes of size, thisvalue can be reduce to about 50% of less energy consumption presented in theseresults.

7.1.4 Simulation Conclusions

The conclusions obtained by this system evaluation are as follows:

1. The percentage of successful transmission for a network of 100 nodes is over90% of the packets transmitted in the worst case of the communicationscheme parameters and over 95% for the best case. Therefore the networksbehavior is feasible for the given application.

2. The amount of energy consumed over a 5 years’ time range in the worstcase is of 14 Wh, but there exist several ways to reduce this final value toeven 50% its magnitude. Given the battery results of an ISIS provider thisvalue is possible to be designed. Therefore the life-time goal is reachable.

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Chapter 8

Conclusions and FutureWork

This thesis provides a feasibility study as for a complete design of a wirelesssensor network for a space application environment. In here we defined anefficient communication protocol as for a complete design on both sensor andgateway nodes that describe the network for this given application. The resultsin for this design are not only presented in detail but also evaluated and thereforeproved to satisfy the project’s requirements. In the following section we willpresent the obtained conclusions on this thesis as for future work that canimprove the work presented in this document.

8.1 Conclusions

As it was already said in the introduction of this thesis, the main idea of theproject was to give an assessment of the wireless sensor networks technologyas for a final prototype to be deployed in a space environment. In this thesiswe showed the steps to this final goal. First by giving a description of thefirst measurements made (chapter 3) on this type of structure from where weobtained the next conclusions:

1. The position of the nodes antenna have an important repercussion onthe strength of the signal received and therefore on the communicationbetween the nodes of the network inside the satellite’s structure.

2. The frequency signal of 2.4 GHz will have less signal attenuation thanthe 868 MHz inside the satellite’s structure. This was concluded after thecomparison results made in Table 3.1 in chapter 3.

Followed by the implementation network design from where we draw the follow-ing conclusions:

1. The use of BitCloud introduces several features to the system but is im-portant to configurate it correctly to reduce as much as possible the powerconsumption.

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2. The use of retransmissions and polling mechanism control can improvethe quality of service of the communication without losing the low powerconsumption required by the design.

Finalizing with the evaluation made in chapter 7 of this document which focuseson the communication scheme provided in this thesis as a solution to the task athand. The experimental results and the described analysis define the followingconclusions:

1. The percentage of successful transmission for a network of 100 nodes is over90% of the packets transmitted in the worst case of the communicationscheme parameters and over 95% for the best case. Therefore the networksbehavior is feasible for the given application.

2. The amount of energy consumed over a 5 years’ time range in the worstcase is of 14 Wh, but there exist several ways to reduce this final value toeven 50% its magnitude. Given the battery results of an ISIS provider thisvalue is possible to be designed. Therefore the life-time goal is reachable.

Therefore the main research questions presented at the beginning of this thesisare answer in the following manner:

1. The results obtained and presented in chapter 3 show it is feasible todeploy a wireless sensor network inside a satellite’s structure.

2. The final communication design presented in chapter 5 and evaluated inchapter 7 provides an efficient communication scheme that satisfies theproject’s requirements.

3. Chapter 6 of this thesis presents a detailed design on both hardware andsoftware components on a wireless sensor network for a space applicationenvironment.

8.2 Future Work

Given time constrains and project deadlines some details were left out of thescope of the current thesis and should be taken into account in future work.Starting for the measurements made in chapter 3 some aspects can be foundinteresting for future research. For this thesis the analysis of the RSSI values onthe reception nodes was the only measurement use to analyze the behavior ofthe nodes frequencies inside the satellite structure. Our objective was to have anoverview of the network behavior and for this main objective this measurementswere enough. But for future work can be found interesting to not focus only onthe RSSI value but also on measurements like the following:

1. The analysis of the delay on the packet reception can be used to under-stand if the signals inside the satellite structure behaved in a constructivemanner.

2. The analysis of the percentage packet lost inside the satellite can also giveimportant information about the networks behavior.

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For the design part of this thesis several hardware and software decisions weremade before having the actual transmission and power consumption results ofthe devices. Therefore now that we have a better knowledge of the devices, wethink the following aspects should be taken into account for future WSNs spacedesigns:

1. A new assessment on the node’s hardware can be done. This way tryingto analyze the communication protocols used on the devices and their ap-plicability to the space settings. The objective here will be to reduce evenmore the size of the node by eliminating the ’conditioning’ microcontrollerdescribed in chapter 6.

2. Another aspect that can be interesting to pursue is to make a deeperanalysis on network topologies to be deployed on this type of structures. Adeeper analysis on network topologies was out of the scope of this projectand therefore a more detail study on this topic can improve the finalnetwork design.

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