Vincent Ribaud
email: ribaud@univ-brest.fr

Cyrielle Feron

Ashwarya Arora

Sarah Le Bihan

Engineering projects "Savanturiers de l'ingénierie"

The "Savanturiers -School of Research" initiative is a French nationwide educational program that aims to introduce investigative approaches into schools through the creation of science projects. During the course of several Savanturiers projects, primary school pupils struggled with program design, among other difficulties, and we felt the need for methodical, technical and human support. In a computer science degree, the capstone project offers students the possibility of a "learning by doing" approach to software development, from the requirements to the product qualification. To overcome the difficulties mentioned, we have mobilized Bachelor of Computer Science students by giving them the classes involved in the Savanturiers projects as "clients" for their capstone projects. This has reduced the technical scope of the capstone projects but has increased their social utility. Although some Bachelor students did not take advantage of this situation, it was a positive endeavor as attested by the student feedback.

Introduction

The Savanturiers 1 project is an education-through-research project, orchestrated by teachers, led by pupils and accompanied by an academic or engineer mentor. The "Savanturiers -School of Research" initiative is a French nationwide educational program that advocates learning-through-research projects in primary classes. The approach revolves around eight stages, ranging from the establishment of scientific questioning to the restitution of the work accomplished. For projects focusing on engineering, called "Savanturiers de l'ingénierie", the approach is restricted because such projects are intended to respond to a need. Around our university, five classes carried out a "Savanturiers de l'ingénierie" project presented 1 In French, "savant" means "scientist" and "aventuriers" means "adventurers". A possible equivalent in English of "Savanturiers" might be "Adventifics", a contraction of adventurers and scientific.

in Table 1. Classes and mentors experienced some difficulties, such as teaching design, and we felt the need for methodical, technical and human support. We tried to provide this support through the work and help of Bachelor of Computer Science students during their capstone project. This article is an attempt to relate the students' work and pupils' projects mainly through a discussion about the artifacts produced by the students. Section 2 depicts classes and their projects, and difficulties encountered. Section 3 presents the academic context of the capstone project, the artifacts engineered for Savanturiers projects and the students' self-assessment. We conclude in Section 4.

The classes and their projects 2.1. The "Savanturiers de l'ingénierie" initiative

According to the Savanturiers website, education through research refers to both the initiation of pupils to methods and issues of research and the position of the teacher as a pedagogue-researcher. In the particular case of the "Savanturiers de l'ingénierie" projects, our goal is to enable pupils and teachers to learn and use the methods, techniques and know-how of the engineer, in their creative effort to shape tomorrow's world. Unlike an introduction to research, "Savanturiers de l'ingénierie" projects have to follow a logic that responds to a need. For the projects presented in this article, the approach is structured in four steps: 1. statement of work, 2. design, 3. coding and 4. assessment. Project management places each step in time and in space and associates each step with a set of tasks. Comprehension of a task operates on (at least) two orthogonal and complementary dimensions: representational structures and the activities that operate upon them. It is often easier to define the representational structures that are required at the beginning and end of a task rather than to define the task itself. Furthermore, assessment is generally centered more on products than processes, ie focuses on the artifacts produced or consumed by tasks rather than what is going on during the task. Consequently, the assessment step operates on artifacts produced during the project.

A "Savanturiers de l'ingénierie" project

We will illustrate our point with an example of a Savanturiers de l'ingénierie" project. Paragraphs written in italics are related to work products, inputs or outputs of an engineering step. Paragraphs written in normal font discuss steps activities.

Statement of work. A class of 7-9 year old pupils study the language that bees use to transmit information on the foraging areas. When an exploratory bee has found a food source, it is able to convey different information to its fellow bees with a dance called the waggle dance. For the project, we are interested in two pieces of information: the orientation and the distance of the food source in respect to the hive. This information will allow other bees to go to the food source and forage there. The project aims to reproduce the bees' behavior using mbot robots and Scratch programming.

Requirements elicitation activity. We define that the simulation behavior includes four interrelated parts: (a) Representation of the simulation: setting up the hive, robots and the field of flowers; (b) Exploration: the explorer robot looks for some food, finds it and comes back to the hive; (c) Communication: the explorer robot transmits useful information to other robots; (d) Harvesting: the forager robots go to the field of flowers.

Requirements elicitation result. The exploratory bee is simulated using an mbot robot that walks around the classroom to discover a food source such as a field of flowers. The exploratory bee, having arrived at the food source, memorizes or calculates information needed to go from the hive to the source. The exploratory bee transmits the information to other bees. Foraging bees are simulated using mbots which, using information received, go to the food.

Requirements analysis activity. We recognize a typical communication problem between two entities, which requires (at least) three stages: (a) the transmitter synthesizes the information to be exchanged, (b) transmission of a message, (c) the receiver interprets meaningful information from the message received.

Problem decomposition activity.

Once the standard situation is recognized, we can break down the problem and structure its resolution into sub-problems. According to the pupils' familiarity with the problem of foraging bees, the teacher intervenes more or less in the building knowledge process: what is an exploratory bee, what is it looking for, what should it collect as information to be exchanged with its peers, how is the message transmitted, how are messages understood, what are the other bees doing?

Analysis activity regarding the sub-problems and reflection. The teacher and mentor guide the pupils toward the formulation of sub-problems and resolution hypotheses. A broad solving scheme can be prepared by the mentor that might be the following. Simulation of the field: the field of flowers to discover is a sheet of paper placed somewhere in the class. Setting off to explore: a robot explorer leaves the hive, it stops when it has detected the sheet; it measures the angle and the distance towards the hive. Return from mission: after returning to the hive, the robot-explorer transmits the angle and distance to other robots by infrared message. On the way: Forager robots leave in the direction of the "field" and stop on the sheet.

The investigation process starts again for each sub-problem. If we take the example of the search for the "field", this is a typical situation we are faced with in robotics projects: how to explore an unknown space. Once the problem of exploration has been set, it is necessary to break it down into sub-problems: memorize the starting point, criss-cross the class, avoid obstacles, find the "field".

Synthesis activity for possible solutions. The analysis (usually descending) gives possible solutionsfor example, using the distance sensor to recognize the obstacles and the color sensor to recognize the "field"solutions must be assembled to get a working schema.

Design result. The food source is detected using the line follower sensor. The exploratory bee goes through the classroom to discover the color requested. The possible obstacles are detected using the ultrasonic sensor. A compass sensor can be used to measure the direction between the hive and the source. The distance is measured by calibrating the robot speed at the beginning of the session and measuring the time of displacement. The exploratory bee transmits the direction and distance to foraging bees. Foraging bees use the direction and distance information or the course memorized by the explorer to reach the food. Two programs need to be created, for the explorer and for the forager.

Coding activity. This is the development activity of the solution, then the packaging in the form of a set of Scratch programs that can be used to run the simulation.

Coding results. The different programs. The simulation is started and filmed; audio commentary or written comments explain the different parts of the simulation.

Assessment activity. Assessment only makes sense if it helps direct the pupils' attention to the knowledge derived and the skills developed.

Assessment results. Moments focused on meta-cognition, led by the teachers. The aim is for the pupils to realize their evolution and their progress and that they can assign them a certain degree of confidence, thus developing autonomy for subsequent actions.

In practice ... The engineering cycle presented above is an ideal view. In fact, the project failed: globally and at almost each step. Regarding the Statement of Work, pupils had to investigate the domain (bees' language) and formulate a need. Either the domain offered too many study possibilities, or the pupils were too young, or both; but it did not work. The mentor had to impose his Statement of Work artifact to move on to the next step. The mentor envisaged the design activity following a "Divide-and-conquer" approach, but the pupils were unable to divide the problem up correctly. Even when sub-problems were set by the mentor, pupils were unable to see them as part of a whole. Hence the project was divided into three different sub-projects, each of them coded separately. Coding sub-problems was too hard for almost all the pupils. Mathematical notions such as angle and distance are seen at the end of the primary school curriculum and thus were missing. Compass programming in Scratch is difficult and compass calibration is tricky. In the best case, some pupils correctly reproduced the Scratch code provided to solve sub-problems but most of them failed. Savanturiers projects are not intended to be assessed as other school work, but presented and explained to other pupils, parents and teachers. Because it was the first year we had performed Savanturiers projects, we did not organize such a presentation.

The difficulty in learning design

Computer Science in primary school is strongly influenced by constructionism, the theory of learning developed by Papert and Harel, who sees learning as a process where the learner builds his knowledge interacting with the object of study [START_REF] Papert | Constructionism[END_REF], hence the idea that we teach programming through the construction of programs, ie a consciously hands-on approach. It is difficult to convey complex algorithms to children, whereas we can get them to manipulate parts of complex programs. One goal of the Scratch online community, according to Brennan and Resnick, is to support young designers in reusing and remixing, by helping them to come up with ideas and code that will serve as a basis for creating things which are much more complex than they could have created on their own [START_REF] Brennan | New frameworks for studying and assessing the development of computational thinking[END_REF]. If we consider a program as an object with which we can interact, thus an algorithm is an abstraction of this object, and writing an algorithm is a design activity. The main modality of this design activity is the decomposition of a seemingly difficult problem into sub-problems that we know how to solve, which Jeannette Wing states as: "Computational thinking is using abstraction and decomposition when attacking a large complex task or designing a large complex system [START_REF] Wing | Computational thinking[END_REF]." In theory, learning design is performed during a round trip between the questions of the problem space set up by the projects and the elements of the solution space that we are building. In theory, writing a statement of work and eliciting requirements are activities where one highlights the important elements of the information available and the required knowledge. In theory, depending on the pupils' level of knowledge, hierarchically structuring the problem is always led by the project supervisors or delegated to the pupils under the supervisors' control. Moreover, there should be a subsequent period of reflection where pupils apply their knowledge and skills as they would instruments, tools or resources [START_REF] Tardif | Intégrer les nouvelles technologies de l'information: quel cadre pédagogique? ESF[END_REF]. In practice, we found that the pupils knew how to reuse know-how in fairly similar situations, at the same abstraction level. However, almost all the pupils were stuck when they moved between abstraction levels, for instance when they had to design sub-problems or to assemble solution elements.

Producing artifacts for the Savanturiers projects

Context

The curriculum developed by the ACM (Association for Computing Machinery) states the following terminal requirement for a program in computer science: "Demonstration that each student has integrated the various elements of the undergraduate experience by undertaking, completing, and presenting a capstone project [START_REF] Shackelford | Computing curricula 2005: The overview report[END_REF]." The capstone projects provide students with a "learning-by-doing" approach for the development of software, from requirements to qualification testing of the software product. Indeed, the capstone project progress is supported and supervised by software engineering processes. These processes help students to become aware and to improve what they do, especially when processes are placed in a general perspective and when continuous assessment informs students about the maturity of their practices and the quality of their production. detailed design (green). The detailed design of a system or a sub-system took the component requirements as inputs and broke down, architectured and allocated the requirements to lower-level components, possibly at the programming language level.

School and location

coding and unit testing (blue). This was the coding and testing of the components independently of their integration in higher-level components.

integration and integration tests (indigo). This was the activity that assembled the components and tested them functioning as a whole. The tests organization was written in the tests description.

software qualification (violet). This was the activity of verifying software compliance with its reference framework and in particular that it complied with all the specified requirements.

There were 45 students, divided into three classes of 14, 15 and 16 students. The project was carried out in pairs, or (exceptionally) alone. Inside a class, each pair of students or single student worked on a different project. Available projects were assigned to students on a first-in, first-out basis. Projects were reused from one class to another. During the capstone project, each class had a dedicated room and each student had to be present for at least six hours per day. A teacher was present four hours per day in the classroom, provided guidance and carried out either formative or summative continuous assessment. Students were therefore permanently informed about the progress of their project and the grading related to the different phases and products. At the end of the capstone project, a demonstration allowed students to present and defend their project.

An engineering approach for primary school pupils

Among the authors of this article, two were the teachers of the capstone project. Thanks to a new accreditation established with supervisory authorities, we reoriented the capstone project by introducing two major changes: (i) "customers" of projects were the classes engaged in the Savanturiers projects; (ii) the technical scope was reduced in favor of usability, i.e. that technical aspects -programming educational robots in Scratch -are quite easy for Bachelor of Computer Science students. On the other hand, the educational dimension and the purpose of providing the Savanturiers projects with useful support brought additional and complex questions.

Schools, classes and Savanturiers projects are presented in Table 1.

Two classes belonged to schools located in priority education areas where the socio-economic environment is difficult and where additional means are provided to schools. Three classes belonged to more privileged schools, located in small towns on the coast. Typically, a class was divided into several groups of 4-5 pupils and all groups in a class performed the same Savanturiers project. The five Savanturiers projects were thus proposed to the Bachelor students (the Bee Language project was split into two slightly different projects), and we added two exploratory projects for students carrying out the project alone. Exploratory projects were based on educational robot kits and intended to explore the kits' possibilities for future Savanturiers projects.

The final project documentation had to be uploaded to the community website https://openlab.makeblock.com where mblock users share their projects. Students had the possibility to visit school classes and to train pupils in their products. This activity was rewarded with a bonus mark. Unsurprisingly, the most successful projects were two projects performed by students who played the game through to the end and who went to classes to train pupils. They are the two projects discussed later in this article.

When we designed the new setting for the capstone project, we had in mind the difficulty for students in understanding the purpose, the outcomes and the activities of each life cycle phase.

Because "Savanturiers de l'ingénierie" projects are intended to expose pupils to a straightforward engineering life cycle, we made the assumption that the study and the realization of artifacts intended to support the straightforward life cycle would help students to become aware of a "real" life cycle. A possible drawback -which happened in some student groups -is that students confused "real" phases to be simplified phases. Confusion might also have been due to the fact that we intentionally kept the color scheme in the simplified cycle for pupils: coding (blue). We created programs and eventually we assembled them in a whole.

assessment (violet). We verified that the project worked : approximately, satisfactorily or very well. We also examined the artifacts.

Mission: Pollution

The class from a school located in an education priority area in our town, carried out the research on the environmental conditions around the school and in particular on noise and atmospheric pollution. It was a double level class with 9-11 years old pupils. Most of them were foreign-born and a few may have had difficulties with spoken and written French. The purpose of the Savanturiers project was to collect environmental data using an mbot robot and display data on a computer located in the classroom.

Remember that Bachelor students were divided into three groups. In each group, a student pair chose the Mission : Pollution project, projects results are available at https://openlab.makeblock.com/ A pair was not interested in working with pupils. The two other pairs cooperated in converging towards a common solution that is presented in Table 2.

Student support of the project. The solution was deployed in the classroom during several training sessions.

In the first session, the two pairs of students were made aware of the level of attention and understanding of a class in a priority education area. Students trained pupils to take a single measurement, display a value and compare the value to a threshold. The students also interviewed the class teacher on her project and had a better idea about the project difficulties and what kind of artifacts could work for pupils. The second session was held the following week with a pair of students, where pupils coded a small Scratch program performing noise or gas measurements. They also worked on the concept of threshold and alert, but it was too difficult for them.

The two last sessions took place at the end of the school year with three students. The project was reoriented because pupils wanted to measure the noise during canteen and break times. The pupils learned to control the robot with a tablet app and collected measurements. The measurements series were then studied in class to understand the use of curves. During the last session with the students, the pupils defined a protocol for collecting noise measurements in the canteen. Then the pupils collected measurements over a week, and under the teacher's supervision, they compared data to the initial hypotheses they had made and established conclusions. Choice of Caesar cipher. Tests of the capabilities of mbot robots to send and receive messages using an infrared link.

Design

The project has three related parts: Encryption: programming encryption using a Caesar cipher. Communication: a transmitting robot transmits a secret message to a receiving robot. Decryption: programming decryption using a Caesar cipher.

Decomposition and description of the solution intelligible to a primary school pupil.

Example : For centuries, encryption has stopped messages from being intercepted. One of the best known is the "Caesar cipher". The principle is to replace each letter by another, following a fixed offset that is the same for each letter.

Coding

Development of the solution then "packaging" in the form of a set of Scratch programs that can be assembled by the pupils to carry out the project.

Completed programs : message offset, character look-up, modulo override, encryption, decryption, display.

Assessment

https://openlab.makeblock.com/topic/5ac8ecced441d3dd0d0774ec Description of the project in the shared site On line: documentation, programs.

Table 3. Engineering the project "A machine to cipher and decipher messages"

A machine to cipher and decipher messages

A class from a school by the seaside studied how secret messages can be transmitted from a transmitter to a receiver using cryptography techniques. Pupils were 10-11 years old. The class had a good education level and pupils liked projects and Scratch programming. The project aimed to cipher messages, transmit them using robots, and decipher them. The restitution of the project, under the guidance of the doctoral student in cryptography, mentor of the project, was done during the visit of the class to their future secondary school, during a cryptography workshop jointly organized with a sixth grade class and its mathematics teacher.

Student support of the project. In each of the three student groups, a pair chose this project. The final restitution being complex, several working sessions were held between the teachers and different student pairs. It led students to understand what was required and students also trained teachers to program a Caesar cipher using Scratch and to exchange messages between mbot robots. During a session at the school, a pair of students trained the primary school pupils in the same techniques.

The workshop at the college mobilized the project mentor and four volunteer Mathematics students and required 24 robots. Each primary school pupil was paired with a sixth grade pupil and each child had a robot that allowed him or her to send and receive the encrypted messages. Encryption and decryption were done using Scratch programs made by the pairs of computer science students.

Learning support

The process naturally used by students in computer science is the practice of functional analysis with black boxes, which breaks down functions into simpler functions. "This decomposition allows the complexity to be dominated, especially considering black boxes, that is, components that are not needed, at a certain level of use and understanding, to analyze the inner workings [START_REF] Pair | La technique dans l'école d'aujourd'hui[END_REF]." As pointed out by Bordallo and Ginestet [START_REF] Bordallo | Pour une pédagogie du projet[END_REF], the use of black boxes is pedagogically justified because it preserves the interest of the students by not lengthening the projects excessively and because it corresponds to a systemic functioning of the human mind.

Students also had to train teachers. In a research intended to assess students' accompaniment in primary schools, Lafosse-Marin and Jeanbart state: "The traditional dual situation corresponding to an antisymmetry of competences between teacher-expert and pupils-novices turns into a triadic and multiform situation [START_REF] Lafosse-Marin | L"accompagnement en sciences et technologies á l'école primaire: un enseignement collaboratif pour un meilleur partage des savoirs[END_REF]." In the few cases we are aware of, and especially for teachers with little literacy in digital technologies, it has been observed that teachers joined pupils in a "novice" status and brought students into an expert position, even though they are undergraduate students. We came to the [obvious] observation that the introduction of digital sciences in primary school could not be done without strong support for primary school teachers.

Students feedback

At the beginning of the project, students were asked for their consent for their work to be observed, and for their participation by means of a questionnaire composed of three parts. The first part was about the students' perception of the capstone experience. The second part was a self-assessment of student roles during the capstone project. We used the four roles proposed by Tardif as significant of the learning paradigm [START_REF] Tardif | Intégrer les nouvelles technologies de l'information: quel cadre pédagogique? ESF[END_REF]. The third part allowed students to express their feelings about the objectives of the project.

Table 4 presents the different items in the questionnaire for the first and second parts, on a Likert scale with 5 possible answers: strongly agree (OK), agree (+ OK), neither agree nor disagree (+/-), disagree (-OK), strongly disagree (KO). The last but one column synthesizes all of the answers by ranking the answers as follows: strongly agree (5), agree (4), neither agree nor disagree (3), disagree (2), strongly disagree (1). Because we drastically changed the focus and the content of the capstone project, the last column presents the answers synthesis for the last year. This year, 41 out of 45 students responded to the questionnaire. Last year, 22 students over 25 answered a similar questionnaire.

Table 5 presents the different items of the third part, on the same Likert scale (the third part did not exist last year). The students evaluations were very positive, particularly the objective of making projects for primary schools classes.

As mentioned above, there is a meta-cognitive issue with the engineering phases of the projects. Generally speaking, the main goal of a capstone project is to learn-by-doing a simplified cycle of software development through a fairly realistic project. However, in our case, the "customers" of the project do not ask for a solution to their problem, but for guidance and helpful resources. In other words, pupils do not have to be provided with the final product but with pieces of the solution that will help them to carry out the project. For some groups, Bachelor students did not realize (or did not want to take in account) the issue and completed the project instead of the pupils. Obviously, the work was easier than in a typical capstone project and students completed the project in half of the allocated time. For sure, they missed one of the pedagogical objectives we had in mind and they learned probably less than in a typical capstone project. This can be seen in Table 4 by comparing the last but one column (average for this year's capstone project) with the last column (average from the previous year capstone project). On the other hand, these students had fun and enough time to polish their projects and it helped other students struggling with the pedagogical issues. The previous year, the capstone project was a client-server management information system intended to manage personal address books and agendas and a central directory and a social network. The system was implemented using Java, JSF relying on an Oracle RDBMS. Sub-systems interacted through a protocol based on UDP.

The last column of Table 4 shows the average obtained the previous year for questions of the first and second parts of the questionnaire. A comparison of the two last columns synthesizes the pros and cons of the new version of the capstone project. In the first part, we see that the project for primary schools was perceived as less complex, and that compared to the previous year, students felt that they had had more time this year, they had lessened their knowledge and less improved their working methods. In the second part, we also see that there was less investment in the first three roles, probably because the project was less complex this year. On the other hand, there was much more investment in the strategic user role this year, which is probably related to the meta-cognitive issue discussed above and that could mean that students were more sensitive to the quality and usefulness of the resources provided to them or found on the internet.

Without the resources produced by the Bachelor students, the Savanturiers projects presented in Section 3.4 and Section 3.5 would probably have failed. However, there is no guarantee of success, the mentor produced the same kind of resources for the Bee language project (Section 2.2) and the project failed.

Suggestions for improvement

A Scratch-based computational thinking framework Brennan and Resnick developed a computational thinking framework that emerged from their studies of the activities of young Scratch programmers [START_REF] Brennan | New frameworks for studying and assessing the development of computational thinking[END_REF].

The authors defined three key dimensions of the computational thinking framework:

(1) computational concepts, (2) computational practices, (3) computational perspectives.

The concepts that are common to many Scratch projects are transferable to other programming contexts or to other areas: sequences, loops, events, parallelism, conditionals, operators and data [START_REF] Brennan | New frameworks for studying and assessing the development of computational thinking[END_REF]. The 'scratchers' adopted a variety of strategies and practices that the authors grouped into four main sets of practices [START_REF] Brennan | New frameworks for studying and assessing the development of computational thinking[END_REF]: Being incremental and iterative -developing a little, then trying it out, and then developing further; Testing and debugging -be sure that things are working well, find and correct problems; Reusing and remixingbuilding projects based on other people's work to create things that are much more complex than the scratchers could have created on their own; Abstracting and modularizing -building something large by putting together collections of smaller parts. Finally, the authors added a dimension they called perspective [START_REF] Brennan | New frameworks for studying and assessing the development of computational thinking[END_REF]: Expressing -Realize that computation is something that they can use for design and self-expression. Connecting -Be aware of the power and the value of creating with others, and for others. Questioning -Feel empowered to ask questions about and with technology.

Articulating the framework for pupils Our university and National Education representatives for our region signed a 3-year agreement that let our students teach programming (Scratch Jr, Scratch and mblock) in primary school classes. During the academic year 2017-2018, 26 classes benefited from the initiative, including all classes performing a Savanturiers project. The programming course was delivered over 6 consecutive weeks, with a 1-hour lesson each week. The course covered all the computational concepts mentioned above. Pupils coded either using Scratch or mBlock, a Scratch-derived environment for programming educative robots called mbot. According to the teachers and our students, almost all the children loved Scratch programming and acquired a minimal set of programming knowledge. However, we observed that only a few pupils saw programming as an activity they could use on their own. When the programming class was over, most pupils moved on to the next school subject, as they probably did for other subjects.

The dimension of practices is helpful to think about improvements for the future. The goal of a Savanturiers project is to get pupils involved in a rigorous, productive and team-based learning process. We devised the projects presented here using a simplified waterfall cycle, but regarding the dimension of practices presented above, it might not have been appropriate. If we want children to be incremental and iterative, should we switch from a top-down to a bottom-up approach ? In this case, we have to define small problems that pupils solve using iterative design and coding. Then, we present a larger picture where pupils can combine their solutions. Briefly stated, should we replace our "divide-and-conquer" principle by a "tinker-and-assemble" approach? In our opinion, testing and debugging practices are strongly related to the pupils' motivation to achieve a result, a key component in successful learning. It might be the case that a bottom-up approach fosters the pupils' goal to use the piece of code s/he is currently working on, hence developing the pupils' will to fix the code if it does not work as expected. Reusing and remixing support the development of critical code-reading capacities and provoke important questions about ownership and authorship [START_REF] Brennan | New frameworks for studying and assessing the development of computational thinking[END_REF]. Throughout the project, we should encourage conversations with and between pupils about their code and how they can cooperate and give credits to others' work. Abstraction and modularization is an important practice for all design and problem solving and Bachelor students used it intensively when they prepared resources for the pupils' project. The pupils were able to handle a stack of code as a new block, abstracting and modularizing its behavior in a single component. We should develop this ability to promote the "tinker-and-assemble" approach.

Towards a mentoring role In a Savanturiers project, the role of a mentor is to give pupils and their teacher the benefit of learning from an engineers thought process in his/her field of expertise. According to the Savanturiers website, engineers involved with schools are responding to a vital need to develop a spirit of innovation, creativity, critical thinking and a sense of responsibility in todays youth, and helping young people to think about their future in concrete terms. This year, Bachelor students produced resources for the pupils' projects but interacted with pupils only if they volunteered. However, mentors availability is limited and Bachelor students may act as mentor-assistants, the role being part of the capstone experience. We envisage associating Bachelor students with primary classes, their teacher and their mentor at the beginning of the semester when the Savanturiers project starts. Students will accompany the mentor when he or she visits the class and will interact with the class by e-mail or instant messaging, asking for help from the mentor if needed. At the beginning of the capstone project, students will have to report on the class progress and about the simplified engineering cycle the class is following and the artifacts produced. Our goal is to foster a meta-cognitive work on engineering life-cycle, steps and work products. In addition, if students are made aware of pupils progress, we expect that students will produce artifacts that are more suited to the project needs. In terms of outcomes for Bachelor students, they will have the possibility to raise awareness among young people of the engineering-related scientific challenges society faces. They will share their knowledge and engineering skills with teachers and pupils and help them to carry out an engineering project in their classroom.

Conclusion

When carrying out the Savanturiers projects, three difficulties quickly made themselves felt: the teachers' technological maturity, learning about design and availability of support. To overcome these difficulties, we mobilized student capstone projects and gave them the classes engaged in the Savanturiers projects as customers. The technical scope was reduced in favor of usability, meaning that the technical aspectsprogramming educational robots in Scratch -were easily acquired by computer science graduates. However, the educational dimension and the support objectives of Savanturiers projects brought additional and complex issues.

Two projects particularly benefited from this mobilization, but all the students appreciated the concrete and fun aspects of the Savanturiers projects as well as the educational purposes of their capstone projects.

  statement of work (red). The statement of work described the project in a few lines. It established what to do and what materials (e.g. sensors) and software (e.g. mblock) would be used. design (yellow). The design divided the system into different programs and described what each program should do.

Table 1 .

 1 Classes performing a Savanturiers project3.2. OrganizationOur capstone project takes place at the end of the Bachelor of Computer Science curriculum. The two week project related in this article took place in April 2018, between final exams and an internship period. Students were supposed to know and be able to implement the engineering cycle presented below.

				Number of pupils and (age range) Savanturiers project
	J. S.,			14 pupils (7-9)	Biology / Robotics: bee language
	a small (coastal) town			study and simulation with mbot robots
	P. D.,			22 pupils (9-11)	Robotics: reconnaissance and rescue
	our town (priority area)		post-disaster mission
	Q.,			24 pupils (9-11)	Sustainable development / Robotics :
	our town (priority area)		what kind of pollution do we have in our school ?
	J. S.,			22 pupils (10-11)	Mathematics / Programming:
	a small (coastal) town			a machine to cipher and decipher messages
	du P.,			22 pupils (9-11)	Robotics :
	another small (coastal) town		my mbot robot does my work
	We used a colored representation of phases, that
	proved to help students' understanding:
	statement of work (red). The statement of work
	established the project specifications, seen from
	the customer's point of view and expressed them
	in business terms.
	requirements analysis (orange). The	requirements
	analysis formulated the customer requirements
	for the software in the form of functional and
	technical specifications.
	high-level design (yellow). The high-level design
	identified a solution that met the requirements,
	defined	its	component	decomposition,
	architectures	and	allocated	high-level
	requirements to each component. At the end of
	the high-level design, requirements validation
	was organized in the software test plan.

Table 2 .

 2 Engineering the project "Mission: Pollution"

	Phase	Students' work	Artifacts for pupils
		Study of different coding techniques : Fleissner
	Statement of work	Grille, Caesar cipher, Scytale, Knights Templar cipher, Freemason's cipher. Study of the
		communication means between mbot robots.

Table 4 .

 4 Questionnaire about the project and self-assessment of student roles.

	My feeling about the Savanturiers project	OK +OK +/--OK KO	avg
	Requirement elicitation: project topics were suitable for primary school pupils.	12	16	10	3	0	3,90
	Design: project design was suitable for primary school pupils.	5	14	14	7	1	3,37
	Coding: project realization was suitable for primary school pupils.	6	12	14	7	2	3,27
	I appreciate the fact that my project is used by primary school classes.	24	17	0	0	0	4,58
	I appreciate the fact that I can record a video or perform a demonstration of my project for primary school pupils.	15	18	7	1	0	4,15

Table 5 .

 5 Questionnaire on the students' feelings about projects objectives and interest.

Phase

Students' work

Artifacts for pupils

Statement of work

Study of the different actuators, sensors and communication means of mbot robots that will be used for data management.

List of electronic modules with their features to use : sound sensor and gas sensor.

Design

The project includes several aspects: Mobile measurement: On demand, the robot goes to the measuring point, collects the environmental data and returns to its base.

Fixed measurement:

On a regular and programmed basis, the robot collects the environment data. Recording: Fixed or mobile measurements made by the robot are transmitted and stored persistently.

Processing:

from the collected data and according to the parameters chosen by the user (time interval, spatial zone, type of measurement), the data are displayed in the form of curves and diagrams.

Diagram of the decomposition of the solution as well as the description of the different elements of the solution that would be intelligible to a primary pupil. Example : The program is divided into three parts. The first part (initialize block) allows the program to be started properly. Then, we use an indefinite repeat loop that contains two other parts. The second part (robot control block) moves the robot and takes measurements. The third part (display measurement block) displays the measurements graphically.

Coding

Development of the solution then "packaging" in the form of a set of Scratch programs that can be assembled by the pupils to carry out the project.

Completed programs : initialize, drive the robot, take the measurements, display the measurements in a chart, display the measurements in a curve.

Assessment

https://openlab.makeblock.com/topic/5ac5ecb9d441d3dd0d06f9eb Description of the project on the shared site Online: videos, photos, programs.


Olivier Alévêque

Eric Levillain
email: eric.levillain@univ-angers.fr

Lionel Sanguinet

Spectroelectrochemistry on electroactive self-assembled monolayers: cyclic voltammetry coupled to spectrophotometry

Keywords:

come     Spectroelectrochemistry on electroactive self-assembled monolayers: Cyclic voltammetry coupled to spectrophotometry

Introduction

The frantic race towards nanotechnologies has led to the development of modified electrodes having smaller and smaller thicknesses and the molecular scale has been reached by a simple and reproducible way: the self-assembled monolayers (SAMs) [START_REF] Love | Self-assembled monolayers of thiolates on metals as a form of nanotechnology[END_REF].

The nanometric dimensionality has never been a problem for the electrochemical characterization of SAMs with usual techniques (cyclic voltammetry, I-E measurements…) because the limiting factor is not the thickness of the layer, but the number of redox species immobilized on the electrode surface. Despite a global, and not local, response of the electrode, some investigations showed that the profile of the electrochemical responses could be a salient tool in order to describe the nanometric organization of the SAMs [START_REF] Aleveque | Electroactive mixed self-assembled monolayers: Lateral interactions model updated to interactions between redox and non-redox species[END_REF].

The nanometric scale of the electroactive layer limits the possibility to characterize SAMs via usual absorption and fluorescence spectroscopic methods because 1) their optical densities are very weak, 2) signal-to-noise ratio is very low according to classical spectroscopic instruments and 3) no turnkey system is available especially for these nanometric systems. However, some studies showed that it was possible to estimate the surface coverage and to set up molecular engineering rules. More specific techniques, such as near-field spectroscopy (AFM, STM…) [START_REF] Chen | Controlled chemical and structural properties of mixed selfassembled monolayers of alkanethiols on Au(111)[END_REF][START_REF] Kim | Adsorption behavior of binary mixed alkanethiol molecules on Au: Scanning tunneling microscope and linear-scan voltammetry investigation[END_REF], ellipsometry [START_REF] Atre | Chain-Length Dependence of the Structure and Wetting Properties in Binary Composition Monolayers of Oh-Terminated and Ch3-Terminated Alkanethiolates on Gold[END_REF], IRRAS [START_REF] Bertilsson | Infrared Study of Thiol Monolayer Assemblies on Gold -Preparation, Characterization, and Functionalization of Mixed Monolayers[END_REF], SERS [START_REF] Paxton | Surface-Enhanced Raman Spectroelectrochemistry of TTF-Modified Self-Assembled Monolayers[END_REF], allowed to obtain some structural informations about the SAM, but generally the experimental conditions demand to operate on a "non-wet" surface.

How about spectroelectrochemistry on SAMs? A literature survey shows that this domain is not really studied. To the best of our knowledge, less than a dozen publications report experimental results involving an electrochemical/spectroscopic coupling dedicated to monolayers, essentially in potentiostatic conditions (i.e. fluorescence [START_REF] Rivera-Gandia | In-situ fluorescence spectroscopy of self-assembled monolayers of HS-(CH2)(n)-fluorescein and HS-(CH2)(6)-poly(dT)(18)-fluorescein at gold electrodes under cyclic voltammetric conditions[END_REF], PM-IRRAS [START_REF] Tagliazucchi | PM IRRAS spectroelectrochemistry of layer-by-layer self-assembled polyelectrolyte multilayers[END_REF], SERS [START_REF] Paxton | Surface-Enhanced Raman Spectroelectrochemistry of TTF-Modified Self-Assembled Monolayers[END_REF], UV-visible absorption [START_REF] Ashur | Transparent Gold as a Platform for Adsorbed Protein Spectroelectrochemistry: Investigation of Cytochrome c and Azurin[END_REF][START_REF] Ivashenko | Electrochemical Write and Read Functionality through Oxidative Dimerization of Spiropyran Self-Assembled Mono layers on Gold[END_REF], fluorescence microscopy [START_REF] Shepherd | Selective reductive desorption of a SAM-coated gold electrode revealed using fluorescence microscopy[END_REF] …). This work clearly show that spectroelectrochemical coupling provides excellent tools to establish detailed structure-property relationships.

*c. Manuscript Click here to view linked References

Here, we present a time-resolved spectroelectrochemical measurement bench which allows to finely follow the absorption spectrum evolution of a self-assembled monolayer as function of the applied redox potential.

In order to demonstrate the ability of this new tool, we validated the bench with electroactive SAMs prepared on Au substrates from a disubstituted bithiophene derivative (compound 1) and confronted the data on SAMs to those obtained on the parent compound in solution (i.e. thin layer conditions).

Experimental

Chemicals

The synthesis of the compound 1 was adapted from the parent thiol derivative described elsewhere [START_REF] Sanguinet | Desorption/ionization on self-assembled monolayer surfaces (DIAMS)[END_REF] and is depicted on Figure 1A.

Au substrate and SAM preparation

Au substrates were prepared by deposition of ca. 5 nm of chromium followed by ca. 100 nm of gold onto a glass substrate through a shadow mask (MECACHIMIQUE / France) using physical vapor deposition system (PVD ME300 PLASSYS / France) and were made immediately before use. This protocol provides reproducible Au(111) surfaces with high crystallographic quality, low roughness (Ra less than 2 nm) and with a defined geometry. They do not undergo post-treatment after completion and that is an advantage.

SAMs were prepared on fresh Au substrates (0.2 cm 2 ) by immersing for 15 min in 1 mM solution of 1 in methylene chloride at 293 K [START_REF] Sanguinet | Desorption/ionization on self-assembled monolayer surfaces (DIAMS)[END_REF].

Spectroelectrochemical experiment

Electrochemistry and time-resolved spectroelectrochemistry in solution were performed using the already described home self-made cell [START_REF] Gaillard | Visible Time-Resolved Spectroelectrochemistry -Application to Study of the Reduction of Sulfur (S-8) in Dimethylformamide[END_REF][START_REF] Dias | Electrochemistry coupled to fluorescence spectroscopy: a new versatile approach[END_REF]. Electrochemical measurements were carried out using a platinum wire counter electrode and a silver wire as a quasi-reference electrode with a Biologic SP-150 potentiostat driven by the EC-Lab software including ohmic drop compensation. Experiments were recorded in dry HPLCgrade acetonitrile with tetrabutylammonium hexafluorophosphate (Bu 4 NPF 6 , electrochemical grade, Fluka) as supporting electrolyte. All solutions were prepared and transferred into the spectroelectrochemical cell in a glove box containing dry, oxygen-free (<1 ppm) argon, at room temperature.

In order to use commercially available thermostated cell holders, the spectroelectrochemical cell dedicated to SAMs is dimensionally close to the conventional quartz cuvette (outer dimensions = 12.5 mm x 12.5 mm x 45 mm). The inner part of the cuvette (Hellma ® Analytics) has been specially redesigned, in order to insert, parallel to the quartz windows, a home self-made interdigitated three Au electrodes with high precision (Figure 2). Spectrophotometric measurements were carried out in direct reflexing mode on the working electrode (i.e. Pt or Au) with a homemade bench composed of different Princeton Instruments modules (light sources, fibers, monochromators, spectroscopy camera and software). The connection between the light source, the cell and the spectrophotometer is ensured through a "Y-shaped" optical fiber bundle: 18 fibers guide the light to the cell, and 19 fibers collect the reflected light from the cell to visible (320-1080 nm / maximum acquisition frequency 2 MHz) and IR (900-1700 nm / maximum acquisition frequency 8 MHz) CCD detectors. The sensitivity of the spectroscopic measurement (< 3 e -at 100 kHz and < 13 e -at 2 MHz between 320 and 1080 nm; 400 e -(high gain) and 5000 électrons (low gain) between 900 nm and 1700 nm) allows performing a spectroelectrochemistry experiment under the usual conditions of electrochemistry.

Results and discussion

Spectoelectrochemistry in solution

As expected [START_REF] Sanguinet | Desorption/ionization on self-assembled monolayer surfaces (DIAMS)[END_REF], compound 1 presents a characteristic optical adsorption band with a lambda max of 340 nm (~17 000 M -1 cm -1 per redox unit) (Figure 1B) and exhibits a fully reversible one-electron process in positive direction (+0.49 V vs. Fc + /Fc) in 0.1 M Bu 4 NPF 6 /CH 2 Cl 2 (Figure 1C and3B).

The oxidation of 1 is known to produce a reversible dimerization of the radical cation (i.e. EC Dim process) leading to a π or σ-dimer. Spectroelectrochemical experiments (Figure 3C and 3D), performed to probe the oxidation process between 350 and 1700 nm, allowed characterizing two strong absorption bands (570 and 900 nm) and two shoulders (510 and 815 nm), as well as a weaker band close to 670 nm (Figure 3E).

In agreement with the work of Hapiot et al. in acetonitrile [START_REF] Neudeck | pi-dimer from bithiophene radical cations. Investigation of equilibrium constants as a function of substituent size and supporting electrolyte using fast conversion electrochemical cells[END_REF], the absorption bands close to 570, 815 and 900 nm (i.e. 490, 560, 800 and 880 nm in acetonitrile) are assigned to the radical cation and the absorption band close to 510 and 670 nm (i.e. 450-490 and 660 nm in acetonitrile) is typical to the dimer at 293 K.

Electrochemistry and spectroelectrochemistry on SAMs

The main experimental difficulty is connected with the dimensionality of the studied material. Indeed, the intensity of the spectroscopic signatures depends on the amount of patterns immobilized on the metal surface (, mol.cm -2

): the surface coverage is typically around 10 -10 mol.cm -2

!

In the case of an absorption measurement by reflexion (imposed by the spectroelectrochemical setup), Beer's law in a SAM can be expressed as follows:

           -1 -1 -2
, M cm Absorbance = 2 1000 with , mol cm

A simple numerical calculation shows that the absorbance on a monolayer is very low, around 0.002 for  = 10 -10 mol.cm -2 and  =10 000 M -1

.cm -1

. This limit imposes some constraints: (1) the molar extinction coefficients of the immobilized molecules must be high (> 10 000 M -

), (2) the surface coverages must be high (> 10 -10 mol.cm ). As expected, the surface coverage () reaches a value of 2 10 -10 mol cm -2 at 293 K.

Spectroelectrochemical experiments, probing the oxidation process between 350 and 1700 nm, were performed during two successive cycles (Figure 4C). First, the absorbance values are very low (i.e. some 10 -), in agreement with equation 1. Second, the absorbance profile is different from the one obtained in solution, suggesting a change in the environment of the redox sites: three absorption bands emerge at around 515, 680 and 910 nm, suggesting only the dimer in attendance (Figure 4E) and no optical band was observed between 1000 and 1700 nm. Third, the absorption bands arise at each oxidation process with same values, showing the stability of the measurement. At Last, the good signal-to-noise ratio allows extracting, with the help of 2D digital filters [START_REF] Alévêque | Electroactive mixed self-assembled monolayers: A numerical overview of phase segregations[END_REF] performed with SCILAB © , the derivative of the absorbance vs. time. For instance, the variation in the derivative of the absorbance at 680 nm vs. time fits with the one of the current vs. time during the two cycles (Figure 4D).

Discussion and conclusion

Based on this work, it appears possible to probe the spectroelectrochemical behavior of SAMs in visible and NIR ranges and, according to the signal-to-noise ratio, to extract voltabsorptograms. It is important to note that spectroelectrochemical measurements on SAMs are reliable, accurate and reproducible but they mainly depends on the sensitivity and the noise of the detectors, the quality of the calibrations and the maintenance of the bench.

This study shows that the confinement of redox sites in the material favors the presence of dimer in the case of an EC Dim process, suggesting that a SAM could be viewed as a high concentrated solution. This interesting result will have to be confirmed by further work on other usual electrochemical mechanisms and on mixed SAMs [START_REF] Aleveque | Electrocatalytic activity of nitroxyl mixed self-assembled monolayers: combined effects of the nanoscale organization and the composition[END_REF][START_REF] Flink | Sensor functionalities in self-assembled monolayers[END_REF].

To conclude, we hope this new spectroelectrochemical bench, also dedicated to fluorescence measurements, will provide tools to assist in establishing detailed structure-reactivity relationships for interfacial reactions on SAMs and on mixed SAMs and, will help to explain why the reactivity of some reactions on the surface are amplified or inhibited.

- 2 )

 2 and (3) the high sensitivity of the spectroscopic detectors and the resulting signalto-noise ratio must the recording of voltabsorptograms possible.The electrochemical response of the SAMs of 1 in 0.1 M Bu 4 NPF 6 /CH 2 Cl 2 (Figure4A and 4B) is consistent with a reversible redox system (+0.49 V (vs Fc + /Fc)) confined at the Au electrode, as shown by the linearity of peak currents with the scan rate (up to 50 V s -1

Acknowledgments

The authors thank Hellma ® Analytics and Princeton Instruments companies for the quality of their products and services. This work was supported by the Contrat Plan État Région 2007-2013 (Pays de la Loire -France). The authors thank Flavy Alévêque for her critical reading of the manuscript.


P Pajusco

P Pagani

FREQUENCY DEPENDENCE OF THE UWB INDOOR PROPAGATION CHANNEL

Keywords: Ultra-wideband, radio wave propagation, channel characterization, frequency dependence

Ultra wideband (UWB) is seen as a major technology for future high rate, short range radio links. For the development of UWB-based communication systems for indoors, an accurate knowledge of the propagation channel is necessary. However, most of the available studies regarding the UWB radio channel concentrate on the lower frequencies of the FCC-defined spectrum mask, and experimental data is generally missing for the evaluation of the frequency dependence of the channel parameters. In this paper, an extensive sounding campaign is presented, where the UWB channel is probed from 3.1 GHz to 11.1 GHz in a typical indoor environment. Following a thorough analysis of the effect of the antennas on the measured data, a characterisation of the main propagation parameters is presented, with a specific interest on their frequency dependence. A frequency power decay close to the theoretical loss of 20 dB per decade is observed. Other typical parameters, such as the environment dependent path loss exponent, the delay spread or the power delay profile decay exponent show limited variations with increasing frequency. This behaviour may be explained by the characteristics of the main building materials constituting indoor environments, which present a relative stability in the frequency range of interest.

Introduction

UWB is a promising technology for short range telecommunication systems, consisting of transmitting signals over a wide frequency band, typically in the order of 500 MHz to several GHz [START_REF] Yang | Ultra-wideband communications: an idea whose time has come[END_REF]. In 2002, the Federal Communications Commission (FCC) released its first Report and Order, allowing the use of UWB signals in the 3.1 GHz -10.6 GHz band [START_REF] Fcc | First report and order, revision of Part 15 of the Commission's rules regarding ultra-wideband transmission systems[END_REF]. A number of applications are currently envisioned, providing high data rates for data transfer using low cost terminals ( [START_REF] Yang | Ultra-wideband communications: an idea whose time has come[END_REF], [START_REF] Porcino | Ultra-wideband radio technology: potential and challenges ahead[END_REF]). A large amount of research on the physical layer has thus been done for a few years. In order to develop efficient transmission systems, one of the main issues is to establish a realistic propagation channel model. Such a model is expected to accurately reproduce the key effects of an UWB radio link, such as the temporal selectivity, the spatial and temporal fading, and the effect of people moving in the vicinity of the terminal. UWB channel modelling is generally based on the analysis of experimental data, allowing for the characterization of the main channel parameters [START_REF] Molisch | Channel models for ultrawideband personal area networks[END_REF]. Accordingly, a number of UWB propagation experiments have been carried out since the appearing of this technology in the field of telecommunication ( [START_REF] Irahhauten | An overview of ultra wide band indoor channel measurements and modeling[END_REF], [START_REF] Molisch | IEEE 802.15.4a channel model -final report[END_REF]). Nevertheless, these propagation experiments are usually restricted to the lower part of the FCC-defined band. Only a few analyses focus on the whole UWB band extending from 3.1 GHz to 10.6 GHz ( [START_REF] Kunisch | Measurement results and modeling aspects for the UWB radio channel[END_REF]- [START_REF] Cassioli | The role of path loss on the selection of the operating bands of UWB systems[END_REF]). Thus, further investigations on the UWB channel are required, especially regarding the impact of the frequency. This paper presents an experimental analysis of the influence of the frequency on the different characteristics of the UWB propagation channel. For this purpose, we studied the indoor radio channel over the global 3.1 GHz -10.6 GHz band, and on 7 partial bands of 528 MHz each, which corresponds to the bandwidth selected for the UWB systems standard ECMA-368 [START_REF]High Rate Ultra Wideband PHY and MAC Standards[END_REF]. Section 2 describes the experiment performed in a typical indoor environment. The influence of the antenna on the measurement results is then analysed in Section 3. The two next sections are respectively dedicated to propagation loss results (Section 4) and to the study of large scale parameters in the delay domain (Section 5).

Experiment description 2.1 General measurement setup

Propagation channel measurement can be performed in either temporal or frequency domains. Due to the ease and advantages of its implementation, we chose a sounding technique in the frequency domain using a Vector Network Analyser (VNA) HP8510C. The Channel Transfer Function (CTF) H(f) was thus obtained using standard S 21 parameter measurements. The propagation channel was sampled over 4005 equally spaced frequencies between 3.1 and 11.1 GHz. Hence, the whole FCC-defined frequency band for indoor UWB applications was probed. This setup allowed a maximum excess delay of about 500 ns and a temporal resolution of about 125 ps. Conical Monopole Antennas operating in the 1 GHz -18 GHz band (CMA 118) were used at each side of the radio link. The transmitting (Tx) antenna was mounted on a trolley to simplify the displacement to the different measurement locations. Depending on the configuration between the transmitter and the receiver, we used up to three Low Noise Amplifiers (LNA), with respective gains of 20 dB, 28 dB and 38 dB. The three LNA present a constant gain over the measured frequency band, and their transfer function was subtracted from the channel measurement at the calibration stage. A step motor enabled the displacement of the Tx antenna along a circle of 40 cm in diameter. For each measurement, 90 complex CTF were measured at regular intervals along the circular path. This setup enabled evaluating the local variations of the UWB propagation channel. The receiving (Rx) antenna was settled on a wall at a height of about 2 meters above the floor. Due to the asymmetry of the antenna pattern, the antenna was mounted upside down. This antenna was placed next to the VNA to minimise the loss of the received signal. The other port of the VNA was connected to the Tx antenna with a long and low loss cable. The maximum Tx-Rx distance was limited to 20 m. The whole measurement process (calibration, antenna rotation, video capture and data saving) was automated using a specifically developed Matlab TM software. The schematic diagram of our measurements is depicted in Fig. 1.

Environment description

Measurements were performed in a typical indoor office environment, as depicted on Fig. 2. Two indoor office configurations were investigated. In the first scenario, the "access point" was located in a meeting room at a height of 2.19 m. In the second one, the "access point" was located in a corridor at a height of 2.45 m. In both cases, Line-Of-Sight (LOS) and Non Line-Of-Sight (NLOS) measurements were performed. The distance between transmitter and receiver varied from 1 m to 20 m. The trolley supporting the Tx antenna was placed at more than 120 locations, of which one third corresponded to a LOS configuration and two thirds to a NLOS configuration. At each location, 90 measurements were performed using the rotating arm, leading to a total of over 10 000 UWB CTF.

Data processing

During the propagation experiment, raw data from the VNA were stored in files. The measurement data was then calibrated with respect to a reference measurement where the emitter and the receiver were directly cable-connected. For measurements involving an LNA, the LNA transfer function was subtracted from the measurement at this stage. Several parameters were then extracted, by considering different bandwidths and central frequencies. First, the set of parameters was computed over the whole FCC-defined band (3.1 GHz -10.6 GHz). Then, the same set of parameters was computed over 7 partial bands of 528 MHz each, which corresponds to the bandwidth specified by the UWB systems standard ECMA-368 [START_REF] Cheung | A Path Loss Comparison Between the 5 GHz UNII Band (802.11a) and the 2[END_REF]. The central frequencies of the selected bands extended from 4 GHz to 10 GHz with a 1 GHz step. As a result, it was possible to analyse the effect of the central frequency on the characteristics of the UWB propagation channel. Most of computed parameters are recapped hereafter. Each measurement being performed in the frequency domain, the measured quantity is the CTF H(f). Note that the CTF represents the joint effects of the channel and of the antennas, the influence of the sounding equipment being removed through the calibration procedure. How the antenna effect is treated will be explained in details in Section 3. The average Power Transfer Function (PTF) was obtained by averaging the power of different co-located CTF along the measurement circular path:

( ) ( ) ∑ = = M m m f H M f PTF 1 2 1 (1)
where M = 90 represents the number of co-located measurements, m represents the measurement index and H m (f) is the corresponding CTF. Due to the CTF calibration, the PTF merely represents the average channel power attenuation for each measured frequency. The PTF was then used to compute the UWB Propagation Loss (PL) for a given central frequency f c in the measured band and a given observation bandwidth BW, according to the following formula.

( ) ( )

∑ + < - > = 2 2 1 BW c n BW c n f f f f n c BW f PTF N f PL (2)
where N represents the number of measured frequencies f n in the considered bandwidth. To compute time domain parameters, the Channel Impulse Response (CIR) h(τ) is obtained from the CTF using an inverse Fourier transform.. A Hanning window was applied at this stage to reduce the side lobe level. The Power Delay Profile (PDP) was deduced using a power average, as follows:

( ) ( ) ∑ = = M m m h M PDP 1 2 1 τ τ (3)
where the notations M and m are the same as in equation ( 1). The decreasing slope of the PDP was fitted to the following exponential approximation, using a least squares method:

γ τ τ - ≅ e B PDP . ) ( 2 (4)
where γ represents the PDP decay exponent, and B is a normalising constant. The mean delay τ m and the delay spread τ RMS were finally computed according to the following equations:

( ) ( ) ∫ ∫ ∞ + ∞ - +∞ ∞ - ⋅ = τ τ τ τ τ τ d PDP d PDP m (5)

Antenna analysis 3.1 General description

When the radio channel is sounded over a frequency range representing more than an octave, the antenna may severely affect the measurements. The power received via each multipath impinging the Rx antenna depends directly on the antenna radiation pattern. Thus, if the antenna pattern is modified, the apparent channel properties may fluctuate accordingly. For instance, a narrow beam antenna focuses the transmitted energy on the direct path and the effect of the multipath components will therefore be attenuated. To minimize the influence of the antenna, we selected the CMA 118 antenna which presents an omni-directional radiation pattern and is vertically polarized. A picture of the antenna and its orientation are presented in Fig. 5. In order to precisely assess the effect of the antennas used for the experiment, both Tx and Rx antennas were characterized in 3 dimensions (3D) in an anechoic chamber. The radiation patterns of the two antennas are quite similar. However, in spite of the high quality of this product in terms of gain flatness over the measurement bandwidth, we observed a noticeable variation of the antenna characteristics as the frequency increases. As an example, Fig. 6 depicts the 3D antenna pattern measured at three selected frequencies within the FCC-defined frequency band. We can notice that the radiation pattern becomes less and less omni-directional in the elevation plane as the frequency increases. To evaluate the omni-directional behaviour of the antenna in the horizontal plane, we computed the minimum, the median and the maximum antenna gain in azimuth for each elevation. Figure 7 depicts the results obtained for the three frequencies taken as examples in Fig. 6. One can observe that the radiation pattern is relatively constant in azimuth for a given elevation and a given frequency.

Influence of the antennas on path loss a) Distance domain

To evaluate the influence of the antenna on the distance dependent path loss, we consider a typical LOS configuration, as depicted in Fig. 8. In a typical sounding experiment, we normally expect a more severe channel attenuation when the Tx-Rx distance increases. However, an increasing distance also leads to a decrease of the elevation angle θ. Hence, the observed attenuation is not only due to changes in the length of the direct path, but also to variations of the antenna pattern with elevation. In our experiment, measurements were performed for distances ∆x varying from 1 m to 20 m. With a height difference ∆y of about 90 cm, the elevation θ varied between 3° and 42°. Figure 7 shows for three demonstrative frequencies that in this region, the radiation pattern presents small, but not negligible fluctuations. Depending on the Tx-Rx distance and on the frequency, the global variation of the direct path power due to the Tx and Rx antennas may be significant, in the order of 10 dB. Thus, the effect of the nonideal antenna needs to be removed from the channel measurement. Otherwise, the antennas may introduce apparent fluctuations in the path loss parameters.

b) Frequency domain

In an ideal, free-space configuration, the propagation loss PL in dB between the transmitted and received power is given by Friis' formula:

( ) ( ) ( ) f G f G c d f d f PL Rx Tx - -       ⋅ ⋅ ⋅ = π 4 log 20 , ( 7
)
where f represents the frequency in Hz, d represents the Tx -Rx distance in m, G Tx and G Rx represent the Tx and Rx antenna gains in dBi, and c is the speed of light. Equation [START_REF] Kunisch | Measurement results and modeling aspects for the UWB radio channel[END_REF] implies that for a given distance d, the channel average PTF (in linear scale) should be proportional to f -2 . Noteworthy frequency power decay has been observed on previous UWB channel measurements ([8, 9, 13]). In [START_REF] Alvarez | New channel impulse response model for UWB indoor system simulations[END_REF], the channel power attenuation was observed to be proportional to f k f -, with k f varying between 1.6 and 2.8. Reference [START_REF] Buehrer | Characterization of the ultra-wideband channel[END_REF] proposed a rather different approach, by fitting the measured PTF in dB to an exponential law. When considering frequency power decay, one should keep in mind that the frequency term ( )

f log 20 ⋅
in Friis' formula is related to the effective aperture of an ideal, isotropic antenna, while the characteristics of the real antennas are taken into account in the gains G Tx and G Rx . PTF measurements performed with non-ideal antennas are hence highly sensitive to the antenna patterns. Figure 9 illustrates this dependency by representing a PTF measured in a LOS configuration, along with the theoretical path loss computed from equation [START_REF] Kunisch | Measurement results and modeling aspects for the UWB radio channel[END_REF], where the gains G Tx and G Rx were measured in an anechoic chamber at an elevation corresponding to the measurement situation. One can observe the predominant effect of the antennas, as the general shape of the measured PTF faithfully follows the variations of the antenna gains. Again, this result shows that the effects of non-ideal antennas should be taken into account prior to the computation of the channel characteristics.

Propagation loss results

Frequency dependent path loss

In order to assess the frequency dependent power decay linked to a channel using theoretical isotropic antennas, we studied the experimental values of the channel path loss at regularly spaced frequencies between 4 GHz and 10 GHz. For each measurement location, the pathloss ( ) d f PL , was extracted at the frequencies of interest from the measured PTF. In order to remove the distance dependency, each PTF was initially normalised, by arbitrarily setting the attenuation of the total received power over the whole measurement bandwidth to 0 dB. The resulting normalised path loss ( ) f PL norm could then be fitted to a linear approximation of the type:

( ) ( ) ( ) f S f f k f PL f PL f norm norm +         ⋅ ⋅ + = 0 0 log 10 (8)
where k f represents the frequency power decay exponent, f 0 is the central frequency and S(f) accounts for the fluctuations of the experimental data around the linear approximation. The normalised path loss averaged over all measurement locations is represented against frequency in Fig. 10. At first, no antenna correction was applied (black dots). One may note an increase of the path loss with increasing frequency, but there is no way to distinguish between the frequency dependent power decay and variations due to the antenna pattern. The computed power decay exponent is k f = 2.60, leading to a standard deviation of the measured data around the linear approximation of 1.2 dB. Secondly, the measured antenna gains G Tx (f) and G Rx (f) were subtracted from the measured PTF at each frequency of interest prior to path loss calculation (white dots). The antenna gain was selected by taking the direction of the direct Tx-Rx path into account. This approach may seem simplistic, as the total power is not received via the direct path only, but it allows a sensible compensation of the antenna effect, leading to satisfactory results: in this case, the dispersion of the measured plots around the linear approximation is reduced to a standard deviation of 0.6 dB. The frequency dependent power decay k f falls to 2.28. With respect to the error level inherent to the measurement and computation methods, this value may be considered close to the theory. Hence, we recommend using the theoretical frequency loss of ( )

f log 20 ⋅
in the modelling of UWB channels, which correspond to the theoretical effect of ideal, isotropic antennas.

Distance dependent path loss over the FCC band

In this paragraph, we analyse the propagation loss parameters on the global FCC-defined frequency band extending from 3.1 GHz to 10.6 GHz. As observed in Section 4.1, a frequency loss of ( ) f log 20 ⋅ accounting for ideal, isotropic antennas may be assumed. Section 3 also pointed out that the real antennas used during the experiment could adversely affect propagation loss analyses in both frequency and distance domains. Hence, the collected data was corrected to the possible extent, by removing the gain of both antennas, measured at the central frequency of the analysed band, with an angular elevation corresponding to the direction of the Tx-Rx path. Note that for the global UWB frequency band, the measurement performed at each frequency was corrected using the nearest measured radiation pattern. The resulting path loss in dB was fitted to the general formula in the form:

( ) ( ) ( ) d S d d k f f d f PL d f PL d +         ⋅ +         ⋅ + = 0 0 0 0 log 10 log 20 , , (8)
where k d is the environment dependent path loss exponent, d 0 is an arbitrary distance of 1 m, f 0 is the central frequency of 6.85 GHz, and S is the shadowing in dB, with zero mean and standard deviation σ S . Figure 11 depicts the path loss results for the global UWB frequency band, i.e. 3.1 GHz -10.6 GHz. The measurement plots mainly follow a linear decay in log-scale, which corresponds to an exponential decay of the received power with respect to the distance. The parameters k d , PL(f 0 , d 0 ) and σ S were computed by linear regression using a least squares method. In the LOS case, a path loss exponent k d = 1.62 was recorded, with a standard deviation σ S = 1.7 dB. In the NLOS case, the measurement plots are somewhat more dispersed, leading to a value of k d = 3.22, with a standard deviation σ S = 5.7 dB. These values are slightly lower but in the range of the UWB path loss exponents computed from other measurements in an indoor environment [START_REF] Molisch | IEEE 802.15.4a channel model -final report[END_REF][START_REF] Kunisch | Measurement results and modeling aspects for the UWB radio channel[END_REF]. The value of PL(f 0 , d 0 ) was respectively evaluated at 53.7 dB in the LOS case and 59.4 dB in the NLOS case.

Frequency dependence of the path loss parameters

A few results only are available in the literature about the frequency dependence of the path loss exponent. An increase of k d with frequency is suggested in the NLOS case in [START_REF] Lao | Transmission coefficients measurement of building materials for UWB systems in 3 -10 GHz[END_REF].

Reference [START_REF]High Rate Ultra Wideband PHY and MAC Standards[END_REF] reports a marked increase of the path loss exponent in the LOS case, while NLOS measurements exhibit the opposite trend. Hence, more experimental analysis was required on this subject. Table 1 summarizes the values obtained for k d , σ S and PL(f 0 , d 0 ) when derived from the seven partial bands of 528 MHz each. The path loss exponent k d varies between 1.44 and 1.92 in the LOS case and between 2.82 and 4.21 in the NLOS case. As a global observation, the path loss exponent is relatively stable. Slightly higher values may be detected at high frequencies (8 GHz to 10 GHz). This small increase may arise from a variation in the characteristics of the building material in the vicinity of the radio link, but no regular trend is observable. The parameters σ S and PL(f 0 , d 0 ) do not appear to be affected by the central frequency of the observed band. Measurement results presented in [START_REF] Muqaibel | Ultra wideband material characterization for indoor propagation[END_REF] indicate that the complex permittivity of concrete and plasterboard are relatively stable over the whole band defined for UWB communications. Similarly, reference [START_REF] Pagani | A Study of the Ultra-Wide Band Indoor Channel: Propagation Experiment and Measurement Results[END_REF] reports reasonably low variations of the transmission loss measured for concrete and wood between the lower and upper frequencies of the UWB spectrum (less than 2 dB). These building materials constitute most of the walls in our measured environment. It is thus not surprising to observe a relative stability in the propagation properties of the UWB radio channel with respect to the frequency. Furthermore, one should note that all measurements were performed on a single floor. Inter-floor measurements involve possible wave propagation through reinforced concrete slabs, where the regular metallic netting could introduce frequency dependencies in the received signal. This was not the case in our experiment.

5 Large-scale delay domain results

Delay spread

The delay spread τ RMS was computed from each collected PDP, over the whole FCC-defined band (3.1 GHz -10.6 GHz). A threshold of 20 dB below the maximum value of the PDP was used to select the significant delays only and avoid the effect of noise. Over all measurement locations in a LOS configuration, the average value of the delay spread was τ RMS = 4.1 ns with a standard deviation σ τ = 2.7 ns. In the NLOS configuration, the average delay spread was τ RMS = 9.9 ns with a standard deviation σ τ = 5.0 ns. These figures agree with the values generally reported in the literature on UWB measurements [START_REF] Molisch | IEEE 802.15.4a channel model -final report[END_REF].

In order to observe the evolution of these parameters with frequency, the average value of τ RMS computed for seven partial bands of 528 MHz each are plotted against central frequency in Fig. 12. For each band, the value of σ τ is represented by the length of the vertical line. No frequency dependence of the RMS delay spread is noticeable. The values of τ RMS and σ τ obtained in each 528 MHz-wide frequency band are relatively close to the values observed for the global band extending from 3.1 GHz to 10.6 GHz.

Power Delay Profile

Figures 3 and4 present typical PDPs in LOS and NLOS configurations. Note that for the convenience of path interpretation, the delay on the horizontal axis has been converted in path length in meters. In both LOS and NLOS cases, one may observe one or several clusters, corresponding to a main path followed by an exponential decay of diffuse power. In the LOS case, walls or pieces of furniture in the vicinity of the radio link produce strong reflected or diffracted echoes, which explains the presence of peaks in the PDP. The global shape of the PDP is generally smoother in the NLOS case, however multiple clusters may occasionally appear. The slope of the PDP exponential decay has been computed for each measurement location as explained in section 2.3. The decay exponent γ was separately computed for the whole FCC-defined band and for the seven partial bands of 528 MHz width each. Over the global band ranging from 3.1 GHz to 10.6 GHz, the average value of the decay exponent was γ = 15.1 ns with a standard deviation of σ γ = 3.8 ns. From the wide range of experimentally observed values [START_REF] Molisch | IEEE 802.15.4a channel model -final report[END_REF][START_REF] Kunisch | Measurement results and modeling aspects for the UWB radio channel[END_REF], this parameter seems highly environment dependent, and more research should be concentrated on this issue. Figure 13 presents the average values of the PDP decay exponent obtained for seven bands of 528 MHz each, with different central frequencies. The corresponding standard deviations are represented by the length of the vertical lines.

Clearly, there is no dependence of the decay exponent on the central frequency of each band, as the observed values are similar for all considered partial bands.

Conclusion

In this paper, we presented an exhaustive measurement campaign designed in the 3.1 GHz -11.1 GHz frequency band to characterise the UWB indoor propagation channel.

The main purpose was to evaluate the impact of frequency on the channel main parameters. Using a VNA and two antennas CMA 118, over 10 000 channel transfer functions were collected in both LOS and NLOS configurations. Despite the high quality of the antenna specifications in terms of gain flatness across the band, we demonstrated that the non-ideal antenna pattern could adversely affect the measured data in both frequency and distance domains. Hence, the antenna characteristics need to be precisely measured and removed to the possible extent from the raw collected data. This technique was validated through the computation of the frequency dependent power decay: after correction of the antenna effects, the channel frequency loss was shown to approach the theoretical loss of 20 dB per decade. On a modelling point of view, this corresponds to using ideal, isotropic antennas. The effect of the practical antennas with respect to this theoretical case should be modelled separately, and added in the simulation chain for a realistic reproduction of the global radio link.

The main channel parameters were extracted over the global FCC-defined frequency band, extending from 3.1 GHz to 10.6 GHz. A decrease of the received power with increasing distance was observed, with an average path loss exponent of 1.62 in the LOS case and 3.22 in the NLOS case. The measured PDP generally presented a clustering of the multipath components, with respective average delay spreads of 4.1 ns and 9.9 ns in the LOS and NLOS cases, and an average decay exponent of 15.1 ns. When computed from several partial bands of 528 MHz each at different central frequencies, these parameters showed no or little impact of the frequency. A slight increasing trend of the path loss exponent with increasing frequency could be observed, but this topic needs further research for validation. As a general observation, a constant behaviour of the channel may be assumed over the whole UWB frequency band, in addition to the abovementioned frequency power decay. Using the adequate frequency dependent path loss formula, models based on measurements performed in the lower part of the UWB spectrum could thus be easily generalised to higher central frequencies. This conclusion comes in line with published results on the transmission coefficients of the main building material in indoor environments, which presented a relative stability across the FCC-defined frequency spectrum for UWB devices.

Figure 1 :Figures 3

 13 Figure 1: Schematic diagram of the measurement device.

Figure 3 :

 3 Figure 3: Example of average Power Delay Profile in LOS configuration.

Figure 4 :Figure 5 :

 45 Figure 4: Example of average Power Delay Profile in NLOS configuration.

Figure 6 :

 6 Figure 6: Examples of measured radiation patterns of the antenna CMA 118 at 3 GHz (a), 7 GHz (b) and 10 GHz (c).

Figure 8 :

 8 Figure 8: Antennas configuration during the experiment. ∆x, ∆y and d respectively represent the horizontal, vertical and effective distances between the antennas, and θ is the elevation angle of the direct path.

Figure 9 :

 9 Figure 9: Measured PTF compared to Friis' formula.

Figure 10 :

 10 Figure 10: Average normalised path loss vs. frequency.Figure11: Scatter plot of the power attenuation vs. distance.

Figure 12 :

 12 Figure 12: Average RMS delay spread and standard deviation vs. central frequency.

Table 1 :

 1 Distance dependent path loss parameters for different frequency bands.

	Band-	Center		LOS			NLOS
	width (GHz)	freq. (GHz)	k d	σ S (dB)	PL 0 (dB)	k d	σ S (dB)	PL 0 (dB)
	0.528	4	1.44	2.2	54.9 2.93	5.5	60.9
		5	1.63	2.6	52.3 3.12	5.7	58.5
		6	1.50	2.8	55.1 2.82	6.1	62.9
		7	1.51	2.6	53.2 2.93	5.7	61.2
		8	1.81	3.6	51.2 3.43	6.2	58.1
		9	1.92	2.3	53.5 4.21	6.9	55.9
		10	1.91	2.1	52.3 3.46	6.3	57.9
	7.5	6.85	1.62	1.7	53.7 3.22	5.7	59.4


G Tesserault

N Malhouroux
email: nadine.malhouroux@orange-ftgroup.com

P Pajusco

MULTI-FREQUENCIES CHARACTERIZATION OF BUILDINGS MATERIALS: ANGULAR AND POLARIZATION ANALYSIS

Keywords:

The engineering optimisation of wireless system, such as WIFI, WIMAX, and UWB that work at many different frequencies is based on propagation modelling. So, the purpose of this paper was to model the electromagnetic properties of various building materials over a large frequency range. A measurement campaign was done at different frequencies, incidence angles and polarization in an anechoic chamber. Results of measurements on typical construction materials are presented and discussed for horizontal and vertical polarization.

Introduction

Wireless systems, such as WIFI, WIMAX and UWB, among others, are expanding rapidly due to the increasing demand on mobile services. The deployment of these networks requires the study of the propagation environment. In indoor context, the received power is attenuated due to the interaction of electromagnetic waves with surrounding walls and furniture. It is necessary to know the reflection and transmission properties of the construction structures to simulate wave propagation in buildings. These properties are different from one material to another and depend on different parameters, such as composition, incident angle and polarisation. The polarization influence on building materials has not been investigated very often although there is much literature on building materials at different frequencies [START_REF] Sagnard | Characterization of construction materials for the prediction of UWB propagation: frequency and temporal responses[END_REF][START_REF] Correia | Estimation of materials characteristics from power measurements at 60 GHz[END_REF]. Some reported analysis [START_REF] Davis | Complex Permittivity of Planar Building Materials Measured With an Ultra-Wideband Free-Field Antenna Measurement System[END_REF][START_REF] Stavrou | Review of constitutive parameters of building materials[END_REF] show different results on dielectric constants that can be explained by different experimental configurations such as antennas, radio links or materials. The characterization of building materials over a large frequency band [2-16 GHz] using the same experimental conditions will allow an objective analysis of the frequency dependence of the materials. A measurement campaign was performed at the Orange Labs centre in Belfort, on several typical construction materials, glass, plasterboard, door and cinder-block using verticalvertical (VV) and horizontal-horizontal (HH) polarisation. The objective of this study is to estimate the dielectric parameter value of various building materials by comparing the measured transmission coefficients with predictions obtained from a theoretical model as a function of the polarization, frequency and incident angle.

Theoretical modelling

The literature includes various methods that can be used to determine the dielectric properties [START_REF] Kong | Electromagnetic Wave Theory[END_REF]. If the assumption can be made that the sample is a homogeneous dielectric, the transmission coefficient T is expressed by:

( ) ( ) 2 0 2 1 exp 1 exp 2 R j k d T R j δ δ ′   - - -   = ′ - - (1)
where

2 2 sin d r π δ ε θ λ = - , 0 2 k π λ =
, λ is the wavelength in free space, d is the thickness of the sample, r ε is the relative complex permittivity of the sample, and θ is the incident angle. R′ is substituted by the Fresnel's reflection coefficient in the cases where the field is in horizontal or vertical polarization.

If the sample has a layered structure that can be modelled by a dielectric N-layer, the transmission coefficient is given by:

0,1 total T T ′ = (2) ( ) ( ) , 1 1, 2 1 , 1 1, 2 1 , 1 , 1 exp 0 1 1 exp 2 i i i i i i i i i i i i N N T T j i N R R j T T i N δ + + + + + + + + + + ′  -∆ ≤ ≤ -  ′ ′ + - =   =  (3) Where 2 2 sin i i i d r π δ ε θ λ = - , 0 cos i i i k d δ θ ∆ = -
, λ is the wavelength in free space, d i is the thickness of the i-th layer sample, i r ε is the relative complex permittivity of the i-th layer, and θ is the incident angle. , 1

i i R + and , 1 i i T + are the
Fresnel reflection and transmission coefficients for the interface between the i-th layer and the i+1-th layer, in the cases where the field is in horizontal or vertical polarization .

Measurement setup

Measurements were carried out inside an anechoic chamber where the material was fixed vertically on a rotating arm. The angle of incidence with respect to the normal to the material was varied by turning the material in 5° increments. Measurements were made for angles of incidence between -65° to +65°. The angles of incidence were limited to this range in order to comply with the width of the first Fresnel ellipsoid [START_REF] Sagnard | In situ measurements of the complex permittivity of materials using reflection ellipsometry in the microwave band: experiments (part II)[END_REF]. Two identical standard horn antennas (ETS Lindgren 3117) that covered the frequency range from 1 to 18 GHz were used as transmitting and receiving antennas. The measured material was placed at the midpoint between both antennas (Figure 1). The distance between antennas was 4.60m such that the material sample object was in the far field of each antenna. With this configuration, the incident electromagnetic field that arrives on the material is essentially a plane wave. A schematic sketch of the free space measurement system is presented in Figure 1. The measurement procedure consists in a frequency-swept method, performed with a Vector Network Analyzer (ZVB 20). By connecting antennas to the two ports of the analyzer and by measuring the S 21 parameter, the frequency response of the channel was obtained. S 21 was measured at 6000 stepped frequency points in the range of 2 GHz to 16 GHz. A time domain gating function was used to filter out edge diffraction effects and reflections from other parts of the system such as cables, connectors and analyzer, to mention but a few, and guaranteed that only the direct signal was measured. The following steps were applied for the measurement. Firstly, measuring the channel without material between the antennas performed the free space calibration with the effect of antennas. Secondly, the measurement was realized with the material between the antennas. Measurements were repeated for the two combinations of polarizations (HH, VV). The measured S 21 with and without material gives the transmission coefficient from which the relative complex permittivity ( )

r r r j ε ε ε ′ ′′ = - is extracted.

Data analysis method

The data analysis method consisted in comparing the experimental transmission coefficient to an internal multilayer model for transmission and reflection coefficients. The comparison result made the reconstruction of the dielectric properties of the sample studied easier. A least-squares criterion is used and the optimal couple ( r ε ′ , r ε ′′ ), which minimize the function error S, is researched:

( ) ( ) 2 exp 1 1 1 , , , , N M theo j i j i j i S T f T f NM θ ε ε θ = = ′ ′′ = - ∑ ∑ ( 4
)
where N is the number of measurement points in frequency domain, M is the number of measurement points in angular domain, T theo is the theoretical transmission coefficient (single layer or multilayer), and T exp is the measured transmission coefficient. The value of this function gives the corresponding RMS modelling error and indicates the accuracy between the measured data and the analytical data

Experimental and theoretical results

The transmission coefficient of a number of materials was measured using HH and VV polarisation of the two antennas.

Glass

Cinder Block

A panel of cinder block wall was measured. The cinder block wall was more complex than the others sample (Figure 13). As does the door, Cinder block was represented by a 3layered model, 2 layers for the block structure and one for air. Determination of the thickness of each layer was also included in the optimisation. Figure 8 shows the measured transmission coefficient and the predicted one from the 3layer and the 1-layer model. with an RMS of 0.211 for VV polarization. The thickness obtained from the optimization was 1.33 cm for block structure and 1.07 cm for the air gap in the HH polarization. Values for VV polarization was 1.27 cm for block structure and 1.08 cm for the air gap.

It follows from the figure 12 and from results obtained that the 1 layer model and the 3 layer model were not appropriate for the estimation of the dielectric parameters. More advanced studies on periodic structure may be undertaken to validate a theoretical model [START_REF] Honcharenko | Transmission and Reflection Characteristics at Concrete Block Walls in the UHF Bands Proposed for Future PCS[END_REF].

Conclusion

The theoretical model, based on homogeneous material or multilayered material was used to estimate the dielectric parameters by comparing measured and predicted results. Experimental results of transmission measurements for three types of building materials using VV and HH polarisation were presented. The results showed agreement between the predicted and the measured values for a wide frequency band and wide range of incident angle. The dielectric parameters showed no significant variation with the polarization. Furthermore, the dielectric parameters presented no variation with the frequencies for low loss materials. A single value could be used to represent the dielectric parameters for 2 to 16 GHz. As a result, the values obtained may be used to improve the prediction quality of ray-tracing models in indoor environments. Further work is needed to investigate the propagation in multilayer structure.

Figure 1 :

 1 Figure 1: Illustration of the measurement setup

Figure 2 :

 2 Figure 2: Transmission coefficient of glass from measurement and the theoretical model The dielectric constant was firstly estimated for glass. The values obtained from measured transmissivity were found to

Figure 2

 2 Figure 2 represents the measured and computed values obtained at normal incidence. The results obtained show small variations between the model and the experimental value. The 0.5 dB difference in the 2-8 GHz band for the vertical polarization may be explained by negligible variations in r

  Figure 12: Transmission coefficient of cinder block from measurement and the theoretical model The results obtained were 9.7792 * 0.708 r

Figure 13 :

 13 Figure 13: Structure of cinder block

Table 1 :

 1 Table 1 lists the selected samples and their dimensions. Material dimension

	Materials Thickness	Dimension
		(cm)	(cm×cm)
	Glass	0.5	127.5×84.5
	Plasterboard	1.3	100×120
	Door	4.5	200×83
	Cinder Block	10	100×100

ε estimations for this band. Yet, the dielectric parameters are close for the two polarizations. The approach used is validated by plotting the transmission coefficient for the angle range analyzed (Figure 3 and Figure 4).

Plasterboard

Measurements were repeated for the plasterboard sample. The results obtained were 2.25 * 0.0175 r j ε = with an RMS error (the single layer model was used) of 0.0741 for HH polarization and 2.41 * 0.0258

with an RMS of 0.085 for VV polarization. The curve fitting obtained for measurements is shown in Figure 5. Fallowing the glass, a small dissimilarity exists between experimental and model value but it is negligible (0.1 dB).

Door

A door was measured. Figure 8 shows the measured transmission coefficient and that predicted from the 3-layer and the 1-layer model. The single layer model does not give a satisfactory result for the door sample. with an RMS of 0.061 for VV polarization. The thickness estimation obtained from the optimization was 1.24 cm for the chipboard layer and 1.93 cm for the air gap in the HH polarization. Values for VV polarization was 1.42 cm for chipboard layer and 1.87 cm for the air gap. The RMS for the 1 layer model was 0.139 for the HH polarization and 0.154 for the VV polarization.