Abdoul Aziz Mbacke
email: aziz.mbacke@inria.fr

Nathalie Mitton
email: nathalie.mitton@inria.fr

Distributed Efficient & Fair Anticollision for RFID protocol

Keywords: RFID Radio Frequency Identification, reader anticollision problem, MAC layer, resource allocation, distributed systems, mobile systems

RFID technology suffers from a recurring issue: the reader-to-reader collision. Numerous protocols have been proposed to attempt to reduce them, but, remaining reading errors still heavily impact the performances and fairness of dense RFID deployments. This paper introduces a new Distributed Efficient & Fair Anticollision for RFID (DEFAR) protocol. It reduces both monochannel and multichannel collisions as well as interference by a factor of almost 90% in comparison with the best state of the art protocols. The fairness of the medium access among the readers is improved to a 99% level. Such improvements are achieved applying a TDMA-based "serverless" approach and assigning different priorities to readers depending on their behavior over precedent rounds. A distributed reservation phase is organized between readers with at least one winning reader afterwards. Then, multiple reading phases occur within a single frame in order to obtain fast coverage and high throughput. The use of different reader priorities based on reading behaviors of previous frames also contributes to improve both fairness and efficiency. Simulation results show the robustness of the proposed solution in terms of different metrics such collision avoidance, fairness and coverage and in comparison with a centralized literature solution.

I. INTRODUCTION

RFID is a technology that allows the contactless and out of sight identification of different goods or individuals through the use of readers and tags associated with the goods or individuals in question. The principle is to emit a radio frequency signal towards tags which in return use the energy issued from the previously received signal to reflect their response [START_REF] Finkenzeller | RFID Handbook: Fundamentals and Applications in Contactless Smart Cards and Identification[END_REF]. This technique is known as back-scattering. RFID technology is nowadays met in numerous applications such as transportation systems, logistics, access control, security, etc. In some installations, one single reader is not enough to cover a specific identification area or the application might require the existence of more than one checking area. For instance, in the case of a warehouse where every good has a tag attached to it, it should be possible to track the different positions or movement of the goods inside the area (entrance/exit of goods, checkpoints, load/unload platforms, ...). In order to overcome the full coverage of the deployment area and propagation dispersion and scattering, dense RFID This work was partially supported by a grant from CPER/FEDER DATA and IPL CityLab@Inria. systems are deployed. In the same instance of the warehouse, a different solution could be envisaged. Instead of relying on static RFID readers deployed on different gates inside the area, these warehouses could rely on human checkers equipped with hand held RFID readers. Equipping these persons with such devices that can be carried around the different parts of the warehouse can facilitate the task. However, such a promiscuity between RFID readers is responsible for recurring collision issues [START_REF] Engels | The reader collision problem[END_REF], [START_REF] Specification | EPC TM radio-frequency identity protocols class-1 generation-2 UHF RFID protocol for communications at 860 mhz -960 mhz version 1[END_REF], which in turn highly impacts the reading performances. As such, the design of an efficient reader anticollision protocol has emerged as one of the most interesting research issues in recent years. The main objective when building an efficient RFID system is to schedule readers' activities to reduce as much as possible interference and collisions in order to improve on throughput, delay and energy consumption. Also, the algorithm design should anticipate a potential deployment of mobile readers.

The literature review broadly shows two main approaches: TDMA-based relying on the use of a central server to add the needed layer of synchronization among the different readers [START_REF] Bueno-Delgado | A geometric distribution reader anti-collision protocol for rfid dense reader environments[END_REF], [START_REF] Ferrero | Fair anti-collision protocol in dense rfid networks[END_REF], [START_REF] Hamouda | Reader anticollision in dense rfid networks with mobile tags[END_REF] and CSMA-based relying on a monitoring of the radio channel and backoff schemes for a given period of time to ensure the medium is unoccupied [START_REF] Birari | Pulse : A mac protocol for rfid networks[END_REF], [START_REF] Amadou | Hamac: High adaptive mac protocol for denserfid reader-to-reader networks[END_REF].

In this paper, we introduce DEFAR, a new anticollision algorithm for RFID. It is, to the best of our knowledge, the first distributed multichannel TDMA anticollision mechanism for RFID. Its main characteristics are as follows: -distributed, local and scalable : each reader runs the same algorithm and relies only on local information sent by its neighbors. Unlike other TDMA-based approaches, DEFAR does not rely on the use of a central entity for synchronization but rather on a loose TDMA approach based on the readers internal clocks and time margins; -multichannel: the four different frequencies standardized by ETSI [START_REF] Etsi | Electromagnetic compatibility and radio spectrum matters erm radio frequency identification equipment operating in the band 865 mhz to 868 mhz with power levels up to 2 w part 2: Harmonized en covering the essential requirements of article 3.2 of the r and tte directive[END_REF] are leveraged in order to reduce collisions; -fair: DEFAR provides the reader with a fair and balanced access to tags, reaching a Jain index of close to 100%; -efficient: DEFAR reduces the number of collisions when compared to the state of the art approaches by a factor of close to 90%, reaching an efficiency level close to 100%; -mobile-ready: the localized approach of the algorithm allows it to handle dynamic scenarios without impacting the different metrics explored.

As every distributed anticollision protocol from the literature, DEFAR assumes 2 main communication channels, one between tags & readers and one between readers. It uses a control channel for the purpose of beacons exchange between readers as well as the four available reading channels for communicating with the tags, making the readers selfaware of their surrounding neighbors as well as their reading behavior. It relies on the use of a beaconing mechanism organized in frames, themselves subdivided in slots. To allow each reader to access the tags in a fair manner, DEFAR introduces a priority scheduling, ensuring no reader spends more than 2 following frames without accessing the medium. Each time a reader accesses the medium, it is allocated a time slot long enough to retrieve the information of all tags laying in its reading range. Through extensive testing, we highlight the following engineering insight, DEFAR is a fair protocol with an equitable access to the medium. Latency is also reduced and maximum coverage of all tags in reading range quickly reached within the first 10 frames.

The rest of this paper is organized as follows, Section II reviews the problem statement and highlights our motivation before Section III explores some state-of-the-art anticollision protocols for RFID. In Section IV we detail the DEFAR algorithm, its performances are evaluated and confronted to the literature protocols in Section V. Section VI prospects the propagation range model and the concept of mobility applied to our proposal. Finally, Section VII concludes by discussing future research directions.

II. PROBLEM STATEMENT A. Collision issues

Different types of collisions can be observed. When one reader accesses multiple tags at the same time, all interrogated tags backscatter their ID simultaneously resulting in a collision and tags failing to be identified. This kind of collisions is referred to as "tag collisions" in the literature and have been studied and solved with ALOHA [START_REF] Khandelwal | Asap: a mac protocol for dense and time-constrained rfid systems[END_REF], tree [START_REF] Myung | Adaptive binary splitting for efficient rfid tag anti-collision[END_REF], and frame-and-tree [START_REF] Eom | An efficient framed-slotted aloha algorithm with pilot frame and binary selection for anti-collision of rfid tags[END_REF] based protocols. This type of collisions is out of the scope of this paper. Another instance of collisions happens when multiple readers access a given tag concurrently. This type of collision is referred to as "reader-to-reader collision". Indeed, aiming at increasing the coverage area and/or improving the total reading delay in RFID systems, one solution was to increase the number of deployed readers. This reader densification unfortunately results in the growth of the number of collisions. In a single channel environment, collisions occur in the cases depicted in Figs. 1a and1b. In Fig. 1a, multiple readers attempt to access a single tag simultaneously. Their reading requests collide on the common channel and the tag cannot be read. In order to avoid this kind of interference, readers should either operate at different times or operate at a distance of at least d = 2 × d CRT with d CRT being the reading range of a reader. In the example of Fig. 1a, only Tags T1 and T3 will be read resp. by readers R1 and R2. Another type of collision is depicted in Fig. 1b: this collision happens when the back-scattered response from a tag collides with the signal of a second neighboring reader. To overcome this problem, readers should either operate at different times or use different frequencies. In Fig. 1b, tags T1 and T3 will successfully be read by readers R1 and R2, while reading of T2 will suffer from the interference of R2.

Following the ETSI EN 302 208-2 regulation [START_REF] Etsi | Electromagnetic compatibility and radio spectrum matters erm radio frequency identification equipment operating in the band 865 mhz to 868 mhz with power levels up to 2 w part 2: Harmonized en covering the essential requirements of article 3.2 of the r and tte directive[END_REF], solutions have been implemented using multichannel property. However, the consideration of a multichannel environment brought to light new issues regarding the interference that might be caused by adjacent channels in the same vicinity. As such, HAMAC (High Adaptive MAC Protocol) [START_REF] Amadou | Hamac: High adaptive mac protocol for denserfid reader-to-reader networks[END_REF] addressed the adjacent channel interference issue and the adjacent channel range, d AC , was introduced. If the distance between neighboring readers is greater than d AC = 3.3 × d CRT , these interference are avoided. As illustrated in Fig. 1c, if readers R1 and R2 operate on different channels, tag T2 will still not be read since it still lies within the adjacent channel interference range of R1 and R2. T1 and T3 will not suffer from the interference range and will respectively be read by R1 and R2. In order to get a reading from all the tags, a configuration as the one shown in Fig. 1d should be considered.

B. Centralized vs Distributed

In order to avoid collisions, a coordination among readers should be established. Several solutions have been introduced depending on how readers communicate. Overall, they follow one of the two following paradigms: -centralized: the readers communicate with a superior entity (central server) which organizes reading rounds. These solutions are mainly adopted in TDMA-based approaches. They hardly support dynamics and mobility; -distributed: the readers are able to communicate between each other directly in a peer-to-peer fashion and agree on their reading behaviors to avoid collisions.

While, at first glance, the first kind seems to be more suitable for readers collision issue, connecting every reader in a dense environment to a central server highly impacts the cost of deployment and induces delays that prevent from mobility handling, unlike distributed solutions.

C. Monochannel vs Multichannel

In the first versions of RFID systems, all readers had to communicate with tags using a common channel. This single frequency medium quickly resulted in excessive collisions. Multichannel has thus been introduced in the update of the standard [START_REF] Etsi | Electromagnetic compatibility and radio spectrum matters erm radio frequency identification equipment operating in the band 865 mhz to 868 mhz with power levels up to 2 w part 2: Harmonized en covering the essential requirements of article 3.2 of the r and tte directive[END_REF]. Efficiently assigning different channels to neighboring readers reduces the number of collisions.

DEFAR makes good use of this enhancement by organizing the readings in rounds themselves divided in max slots number of slots, by still taking care of interference that

III. RELATED WORK

This section browses different approaches, although not being exhaustive. We focus on the most significant works for DEFAR, ie five TDMA-based ones: DCS and VDCS also known as Colorwave, GDRA, an evolution of [START_REF] Ferrero | Fair anti-collision protocol in dense rfid networks[END_REF] which implements multichannel, and ACoRAS that inspired the idea in this algorithm for a better efficiency and fairness. We also present CSMA-based algorithms such as Pulse and a more recent algorithm, HAMAC, a multichannel approach that relies on the variation of the contention window.

[13] presented a monochannel distributed algorithm designed with a unique frame size for all readers. Every reader reserves a color (slot) within the frame to access tags and broadcasts the information to its neighbors. In case 2 or more readers collide e.g with the same color, they randomly chose new colors within the same frame size. The issue with such a system is that in dense deployments, the frame size may not be sufficient to contain all readers, resulting in consecutive collisions and throughput drops. Later [START_REF] Waldrop | Colorwave: an anticollision algorithm for the reader collision problem[END_REF] partially solves this issue by introducing thresholds on the number of collisions to allow repetitive colliding readers to expand their frame size as to have more room for available colors. However, having different frame sizes among asynchronous readers can turn out that color reservation may not be shared properly among readers, resulting in multiple collisions and throughput drop. Also having different frame sizes across the system induces a drop in fairness.

Later GDRA [START_REF] Bueno-Delgado | A geometric distribution reader anti-collision protocol for rfid dense reader environments[END_REF], to the best of our knowledge, the only TDMA-based multichannel algorithm of the literature, with a centralized approach, a central server sends on a dedicated channel (or medium) an Arrangement Command containing the number of available slots. Upon reception, every reader randomly chooses a channel among the four available [START_REF] Etsi | Electromagnetic compatibility and radio spectrum matters erm radio frequency identification equipment operating in the band 865 mhz to 868 mhz with power levels up to 2 w part 2: Harmonized en covering the essential requirements of article 3.2 of the r and tte directive[END_REF] frequencies and a slot within the frame size using the Sift distribution [START_REF] Jamieson | Sift: A mac protocol for event-driven wireless sensor networks[END_REF]. Once at the corresponding slot, readers send a beacon to advertise their channel access. In case of multiple readers with the same channel and slot, all contending readers get disabled for the current frame. This algorithm considers the use of bi-static antennas to allow readers to receive other beacons while they are sending their own and also requires a dedicated control channel or medium with a server capable to reach all readers periodically. However, disabling all contending readers in case of a collision induces a throughput drop and efficiency loss. Furthermore, the contention technique used affects the fairness of the system since readers stuck in a denser area can be repeatedly colliding and being disabled while others are accessing the medium. Also, unlike DEFAR, GDRA does not take care of collisions that still exist even with different frequencies.

Hamouda & al. [START_REF] Hamouda | Reader anticollision in dense rfid networks with mobile tags[END_REF] proposed ACoRAS, a TDMA-based single channel centralized algorithm in which readers are assigned colors (slots) by a central server in order to access tags in a first phase. The colors assignment is done following the construction of a Minimum Independent Set. This is done to make sure no two, or more, neighboring readers are assigned the same colors. During a second phase, the number of colors assigned to each reader is reduced according to the number of tags they cover to reduce latency. ACoRAS was an inspiration to our work to ensure no reader spends a defined period of time without accessing the medium. This characteristic taken into account allowed to build a fairer protocol, but the idea of designing a more dynamic protocol prevented us from using a central server for such purposes.

Regarding CSMA-based proposals, several algorithms were suggested. Birari & Iyer proposed Pulse [START_REF] Birari | Pulse : A mac protocol for rfid networks[END_REF], a single channel CSMA-based distributed algorithm. It uses both a control and a data channel for collision avoidance. So while a reader accesses tags on the data channel, it periodically sends beacons to neighboring readers on a dedicated control channel. Such a solution comes at a high cost in terms of energy consumption regarding the number of beacons sent. Also the channel access backoff is not considered, so in case of simultaneous access from multiple readers, collisions may not be detected resulting in a lower throughput. Whereas in our proposal, a channel access mechanism based on a backoff before beaconing is organized.

More recently Amadou & al. designed HAMAC [START_REF] Amadou | Hamac: High adaptive mac protocol for denserfid reader-to-reader networks[END_REF], a CSMA-based, distributed multichannel algorithm. To the best of our knowledge, HAMAC is the only literature approach that manages collisions on tags between different frequencies. It works by defining an initial maximum value for the backoff contention window. This value is then decreased during backoff by each reader depending on the channel state. Although, such a solution may reduce collisions, having different contention frame sizes among readers can result in a drop of fairness among readers as well as an increase of latency for certain readers.

From this observation, we deduce that while centralized solutions seem to be the most performing ones, they are either not scalable for more dynamic deployments [START_REF] Hamouda | Reader anticollision in dense rfid networks with mobile tags[END_REF] or make significant trade-offs regarding throughput and efficiency for the sake of reducing collisions [START_REF] Bueno-Delgado | A geometric distribution reader anti-collision protocol for rfid dense reader environments[END_REF]. CSMA-based approach on the other hand by leveraging on contention window size among readers impact the fairness of access between readers and affects their energy consumption with readers having to either continuously emit a jamming signal or listen to the medium before reading tags [START_REF] Birari | Pulse : A mac protocol for rfid networks[END_REF], [START_REF] Amadou | Hamac: High adaptive mac protocol for denserfid reader-to-reader networks[END_REF]. As such, we propose to focus on a distributed TDMA-based solution that could be better suit dynamic environments without the need of a central server and be able to offer a fair access to the medium to all readers while also being able to reduce collisions and improve throughput so as to be the most efficient possible.

IV. DESIGN OF DEFAR

In regards of the drawbacks expressed in Section III, we designed the Distributed Efficient & Fair Anticollision for RFID algorithm to ensure that at least one reader among the colliding ones would access the medium and read the tags in its vicinity. One of the main motivations when designing DEFAR was to support the deployment of dynamic RFID systems with static/mobile readers and/or tags. We also aimed at creating a distributed algorithm to avoid being dependent to a central server which would preclude our solution from being used in mobile applications and could also make the solution much more expensive to deploy regarding the communication between the readers and the central server. Unlike [START_REF] Bueno-Delgado | A geometric distribution reader anti-collision protocol for rfid dense reader environments[END_REF], our algorithm does not depend on the use of additional hardware. Another issue met in regular algorithms is to ensure that readers deployed in a given area get access to the medium in a fair and balanced manner. We attempt to resolve this issue by allocating different priority levels to readers to access tags.

A. Overview of DEFAR

DEFAR is a distributed multichannel TDMA RFID anticollision protocol. In order for readers to exchange beacons, a dedicated communication channel is used, different from the ones used to access tags. Two ranges are then identified: -the reading range d CRT , previously described in Section II, which is the range within which any laying tag will be successfully interrogated; -the communication range d Com of readers, defined as d Com = 2 × d AC , with d AC = 3.3d CRT being the adjacent channel interference range, in order to overcome both the single channel and adjacent channel collisions presented in Section II. Setting this value for d Com ensures no adjacent channel collision is conceivable since readers closer than d AC (as depicted in Fig 1d) will be able to exchange beacons and organize their readings. Thus, a reader which does not receive any beacon from the area defined by d Com is sure not to collide with any other reader.

The medium access is divided into different time frames which are themselves again divided into max slots. Each slot can be accessed by up to 4 readers in the same vicinity using one of the 4 channels [START_REF] Etsi | Electromagnetic compatibility and radio spectrum matters erm radio frequency identification equipment operating in the band 865 mhz to 868 mhz with power levels up to 2 w part 2: Harmonized en covering the essential requirements of article 3.2 of the r and tte directive[END_REF]. This permits to have up to N = max slots × 4 contending readers (as explained in Section II-C) in a defined vicinity against just N = max slots in a single channel environment. This reduces collisions by a 4 factor while improving the throughput by the same factor. The slots are organized into two different phases : -First, a beaconing phase for each reader to discover its neighbors and ensure it can access the medium. During this phase each reader observes a random backoff period before sending its beacon. The way we designed the number of backoff slots allows an insubstantial number of eventual beacon collisions. A large range of beaconing slots are available (see Fig 2) for readers to randomly chose from and only readers at the current ongoing reading slot are awaken thus making beacon collisions very unlikely to happen.

-Second is a reading phase for readers to access tags for a defined period of time. Regarding the previous beacons exchange between readers and the disabling of colliding readers, no collision is then conceivable when reading tags, since on each slot and frequency, there cant be more than one reader trying to read tags : the corresponding competition is based on the information gathered during the beaconing phase and solved as follows.

Beacon Tag interrogation Beacon Tag interrogation . . . The readers are able to follow the flow of frames with beaconing and reading phases thanks to a loose TDMA approach based on internal clocks and time margins. To overcome the general disabling of all readers in case of a collision, we introduce an ID comparison to allow at least one of the contending readers to access the tags. Also, in order to avoid the same readers to constantly access the tags, we create different priority levels to increase the chances of previously failing readers, while decreasing the ones for previously winning ones. We define 3 different priority levels for the readers. NEUTRAL is the priority of all readers at start. LAZY is the lowest priority it is given to readers which have successfully read the tags in their range on the previous frame. PUMPED UP, the highest priority, is given to readers that have failed accessing the medium, thus giving them a higher probability of accessing tags during the following frame (see Fig 3).

At the system start, all the readers are in their NEUTRAL priority and wait for their chosen slot to come up. Each slot has a duration of T slot . This duration is split into two parts, T beacon [START_REF] Etsi | Electromagnetic compatibility and radio spectrum matters erm radio frequency identification equipment operating in the band 865 mhz to 868 mhz with power levels up to 2 w part 2: Harmonized en covering the essential requirements of article 3.2 of the r and tte directive[END_REF] used by readers to send beacons at the start of their slot and T CRT [START_REF] Ferrero | Fair anti-collision protocol in dense rfid networks[END_REF], [START_REF] Bueno-Delgado | A geometric distribution reader anti-collision protocol for rfid dense reader environments[END_REF], [START_REF] Amadou | Hamac: High adaptive mac protocol for denserfid reader-to-reader networks[END_REF], [START_REF] Amadou | How to improve csma-based mac protocol for dense rfid reader-to-reader networks?[END_REF] used to access and read tags. A frame can then be defined as T f rame = max slots * T slot with T slot = T beacon + T CRT . At the beginning of the first frame, all readers randomly select a slot between [0; max slots[ and a channel among the available ones standardized by [START_REF] Etsi | Electromagnetic compatibility and radio spectrum matters erm radio frequency identification equipment operating in the band 865 mhz to 868 mhz with power levels up to 2 w part 2: Harmonized en covering the essential requirements of article 3.2 of the r and tte directive[END_REF]. Knowing the length of a slot (as described in the previous subsection) readers then wait for their corresponding slot for a duration of t = chosen slot * T slot with the default NEUTRAL priority. Once at the corresponding chosen slot, a reader R i observes a random backoff period before sending its beacon containing its ID and token on the dedicated communication channel. Two scenarii can occur (Algo. 1). Scenario 1: R i does not receive any beacon during T beacon . R i is thus the only reader with this corresponding token in its vicinity. It can then access the medium to read tags during T CRT and switches its priority to LAZY (l. 5-6). Scenario 2: R i receives one or more beacon(s) with corresponding tokens during T beacon . R i thus compares the IDs contained in the received beacons with its own: -Either ID of R i is the smallest. R i then accesses to the medium for T CRT and switches to LAZY priority (l. 8-9); -or ID of R i is greater than any of the IDs received. R i leaves the contention and switches its priority to PUMPED UP (l. 10). It loses the previously chosen token and randomly picks another set of channel and slot for the next frame. This process is kept throughout the entire first frame.

During the following frames, we keep the same collision resolving idea with a twist accounting the previously introduced priorities of the readers. Readers once again randomly select a slot and channel among the available ranges. Once the corresponding slot to the chosen token is up, reader R i observes a random backoff period and sends its beacon with not only its ID and token but also its priority level. The following can happen (Algo. 2): Scenario 1: R i is LAZY: -no collision: there are no concurring readers in the vicinity of R i , it sends its beacon but does not receive any beacon from its neighbors. It then accesses the tags for T CRT and remains in LAZY priority (l. 5-6); -collision: there is at least one concurring reader in the vicinity of reader R i which receives at least one corresponding beacon from one of its neighbors (l. 7): * R i matches the priority levels of the readers that sent their beacons, if any of them is a PUMPED UP one, R i shuts off and waits for the the next frame with a PUMPED UP priority (l. 9-10); * if all the readers that have sent beacons are LAZY as well, R i then matches its ID with the readers that sent their beacons. It resolves the contention with respect to the IDs as done in the first round (l. 12-14). Scenario 2: R i is PUMPED UP: -no collision: there are no concurring readers in the vicinity of R i , it sends its beacon but does not receive any beacon from its neighboring readers. It then accesses the tags for T CRT and switches its priority to LAZY (l. 5-7); -collision: there is at least one concurring reader in the vicinity of reader R i which receives at least one corresponding beacon from one of its neighbors (l. 7):

* R i compares the priority levels of the readers that have sent their beacons during phase 1, if all of them are LAZY ones, R i accesses the medium to read tags for T CRT and then switches to LAZY priority (l. end if 13: end if

V. PERFORMANCE EVALUATION

In order to highlight the benefit brought by our algorithm, we implemented both DEFAR and GDRA using WSNet 1 to fairly evaluate their performances in various scenarii. WSNet is an event-driven simulator for large scale wireless networks. We considered a dense RFID system where 100 Algorithm 2 Next frames for a reader R i

1: slot i ← (int)random[0; max colors] 2: channel i ← (int)random[1; f max ] 3: if current slot == slot i then 4:
Send beacon if R i has the lowest ID then R i wins.

13:

Read tags; priority i ← LAZY 14:

else priority i ← P U M P EDU P end if 28: end if to 500 readers are randomly deployed uniformly on a 1000 × 1000 meters square area with 2000 tags arranged. With such a deployment, we have a density of at least 1 colliding reader for each reader and a coverage of at least 3 tags with 100 readers deployed. With 500 readers deployed, each has at least 3 other concurring readers and a coverage of 4 tags. According to the reader maximum transmission power defined in [START_REF] Etsi | Electromagnetic compatibility and radio spectrum matters erm radio frequency identification equipment operating in the band 865 mhz to 868 mhz with power levels up to 2 w part 2: Harmonized en covering the essential requirements of article 3.2 of the r and tte directive[END_REF], we set the reader to tag communication range to d CRT = 10m while the interference range is set to

d AC = 3.3 × d CRT and d Com = 2 × d AC .
For each scenario 100 different simulations lasting 400 seconds each are run. The results are presented with 95% confidence intervals. In order to conduct our simulations, we used a value of max slots = 4. This value has been defined after multiple tests that revealed it was the best value for a compromise between high throughput and latency. In fact, increasing max slots would improve the throughput of the system and lower the number of collisions since readers would have more available slots to chose from. But in return, the latency would be increased as well since an incremented number of slots would also be unoccupied, uselessly widening the length of the frame. After numerous tests a value of max slots = 4 was chosen. The values of T beacon = 5ms and T CRT = 460ms are also defined according to [START_REF] Etsi | Electromagnetic compatibility and radio spectrum matters erm radio frequency identification equipment operating in the band 865 mhz to 868 mhz with power levels up to 2 w part 2: Harmonized en covering the essential requirements of article 3.2 of the r and tte directive[END_REF], [START_REF] Ferrero | Fair anti-collision protocol in dense rfid networks[END_REF], [START_REF] Bueno-Delgado | A geometric distribution reader anti-collision protocol for rfid dense reader environments[END_REF], [START_REF] Amadou | Hamac: High adaptive mac protocol for denserfid reader-to-reader networks[END_REF], [START_REF] Amadou | How to improve csma-based mac protocol for dense rfid reader-to-reader networks?[END_REF]. Performance metrics for RFID system algorithms used for the characterization of our proposal are clearly introduced in [START_REF] Gandino | Evaluation criteria for reader-to-reader anti-collision protocols[END_REF].

A. Collisions

We distinguish two types of collisions: channel access collision and reading collisions. Channel access collision are observed in the case where multiple readers chose the same beaconing slot or when different readers in the same vicinity fail to chose different slots and channels. Reading collisions are detected when multiple readers access the tags in their intersecting surroundings at the same time. When the former occurs we consider an unsuccessful query section happened and tags were not read. The latter ones are not considered since according to our work and the one used to compare they cannot happen since readers in that situation are disabled by the respective algorithms. Fig. 4a shows the number of collisions according to the density of readers deployed. Here we can witness that GDRA records considerably more collisions than DEFAR algorithm. This can be explained by the creation of an independent set setup process in DEFAR, during which after the case of a collision we maintain at least one reader enabled. Also within the span of a frame we have max slots different reading sequences in DEFAR against a single one in GDRA. For a token [chosen slot,chosen channel], we may only have one single reader for a whole frame in its surrounding in GDRA since all readers on the same channel, regardless of their slots, get disabled. While in DEFAR, we can have as many readers as there are available slots in the design, thus cutting the number of collisions.

B. Efficiency

Efficiency defines the ratio of the number of successful query sections (SQS) over number of attempted query sections (AQS) as follows: Ef f = SQS AQS . Such a metric assesses how well protocols avoid collisions. Fig. 4b demonstrates the efficiency of DEFAR and GDRA. Previous results shown in Fig. 4a explain the shapes of the efficiency plots. Since DEFAR has less collisions than GDRA, it induces a better efficiency with DEFAR outperforming GDRA. The smaller number of collisions result in a better efficiency for DEFAR making it a more suitable protocol for dense environments.

C. Jain's fairness index

Jain's fairness index [START_REF] Ferrero | A fair and high throughput reader-to-reader anticollision protocol in dense rfid networks[END_REF] allows the evaluation on how fairly the throughput is distributed among the different readers in the system. It is computed as :

I Jain = | n i=1 xi| 2 n× n i=1 x 2 i
where x i is the throughput of the i-th reader and n is the number of readers. When the throughput is equal for all readers, I Jain = 1 and in the worst case I Jain = 1 n . Fig. 4c displays the Jain's Fairness Index. We can first witness that both protocols have a really high fairness index never dropping under 90%. But DEFAR is fairer since it favors previously failed readers to access the channel, balancing the queries between concurring readers.

D. Coverage delay

Coverage delay presents the minimal time needed to read, at least once, all tags that are in reading range of the readers in the system. The smaller the value of coverage delay, the faster and more efficient the protocol is. Fig. 4d shows the evolution of the number of read tags throughout the frames. In the case of GDRA, we can see that it can take up to 23 frames in order to successfully read all covered tags. While in the case of DEFAR, all covered tags can be read within less than 10 frames. The higher efficiency as well as the better collisions management and fairness of DEFAR makes it more versatile for dense environments.

VI. DISCUSSIONS A. Propagation model

Throughout the modeling of our algorithm and the extensive testing conducted, we used an isotropic propagation model. Such a model is usually considered for simplicity purposes since the proposals are regarding collisions detection mechanisms. As seen in Figures 1a, 1b and1c, 1d the reading and interference ranges of readers are represented as concentric circles around the considered reader.

In future works, we plan to extend our analysis using more realistic propagation models. First, the dedicated communication channel used for reader-to-reader coordination has to take into account the deployment environment. In urban or industrial settings, there might be readers that are not neighboring on this channel but generating reading collisions. Similarly, different types of RFID beam antennas are found in practical systems, which can modify the patterns of competing readers. We believe that DEFAR will still be the most efficient and fair approach, though collisions have to be evaluated under these models. The trade-off between reading efficiency and energy consumption is also interesting to investigate, in particular if the communication power on the coordination channel is a parameter.

B. Mobility

In order to explore the behavior of our proposed algorithm in different environments, we simulated different configurations considering static/moving readers and/or tags. This allowed us to witness the incidence of mobility on our proposal regarding main metrics such as collisions and fairness. For general purposes we considered a random waypoint mobility model, in future works more precise mobility models could be tested to check their incidence. Simulations were run with the same parameters as in Section V.

Fig. 5a displays the number of collisions in the system according to the number of readers deployed using different mobility environments. The number of collisions increases with the number of deployed readers as well. The number of colliding readers is not affected by the mobility factor, similar results are obtained with the different settings.

Fig. 5b represents the efficiency of the proposal over the settings tested. Regarding the previous results on collisions we can explain the similar results obtained with efficiency observed here. Despite the shape of the curves, the efficiency still remains high with a lowpoint of 95.21% with the most dense deployments.

Fig. 5c shows the Jain's Fairness Index of our proposal with different mobility schemes considered. We observe that the fairness slowly decreases with a raising number of readers deployed but still remains with performances close to 100%. It is interesting to see that we have two different shapes here, one for the case where the readers are moving and another where the readers are static. In the case of static readers, the performances are slowly diminishing whereas in the case of mobile readers the drop is more fair. This means that our proposal actually shows better performances regarding the fairness in a mobile scheme.

Simulation results show that DEFAR is not really affected by the mobility factor. It even performs better in terms of fairness in a mobile environment. Collisions performances are not altered and remain very satisfying. Such results prove that our proposal is relevant in uses cases that require the deployment of a dense and mobile RFID system as the ones presented in the introduction regarding the monitoring of warehouses using mobile readers hand-held or mounted on autonomous robots. This paper presents DEFAR RFID anticollision protocol, a TDMA-based distributed multichannel protocol intended to reduce the collisions in RFID systems while maintaining a high level of fairness and efficiency. DEFAR leverages on the possible use of multiple frequencies to access the tags and on the ability to neighboring readers to communicate together. DEFAR is compliant with ETSI EN 302 208 [START_REF] Etsi | Electromagnetic compatibility and radio spectrum matters erm radio frequency identification equipment operating in the band 865 mhz to 868 mhz with power levels up to 2 w part 2: Harmonized en covering the essential requirements of article 3.2 of the r and tte directive[END_REF] standards, allowing it to take advantage of the multichannel characteristic and being versatile for its use in different environments. As shown, DEFAR outperforms state of the art protocols. DEFAR manages the medium access by ensuring that in case of a collision at least one reader accesses the channel thus reducing the number of colliding readers and improving coverage speed. Also thanks to the definition of different priority levels, DEFAR improves the fairness among readers. Extensive simulations highlighted the compelling advantages of DEFAR over GDRA which is presented as the best anticollision protocol for RFID.

Other simulations made with mobile readers and/or tags also proved the high level of performance in these different environments. These simulations also show that the performance metrics levels are very similar whether in a mobile or static environment.

The next steps of this work will enhance DEFAR to leverage on the specificity of RFID collisions and also investigate the energy consumption of DEFAR protocol with different RFID materials in order to setup an autonomous energy harvesting RFID system. Experiments over a real RFID testbed are also considered in order to evaluate the physical layer interference and variations on our proposed algorithm. Finally, we will focus on the integration of RFID systems in a larger multihop wireless network.

Figure 1 :

 1 Figure 1: Collisions

Figure 2 :

 2 Figure 2: Frame design

Figure 3 :

 3 Figure 3: Priority levels diagram

Figure 4 :

 4 Figure 4: Performance evaluation results

Figure 5 :

 5 Figure 5: Mobility performances evaluation

  [START_REF] Ferrero | A fair and high throughput reader-to-reader anticollision protocol in dense rfid networks[END_REF][19]; * if any of the readers that have sent a beacon is in a PUMPED UP priority as well, R i then compares the IDs and resolves the contention with respect to the IDs as done in the first round (l. 21-23).

	Algorithm 1 First frame for a reader R i
	4:	Send beacon
	5:	if no beacon received then	No collision
	6:	Read tags; priority i ← LAZY
	7:	else	At least one other reader has chosen the
		same token
	8:	if R i has the lowest ID then	R i wins.
	9:	Read tags; priority i ← LAZY
	10:	else priority i ← P U M P EDU P
	11:	end if
	12:

1: slot i ← (int)random[0; max colors] 2: priority i ← N EU T RAL 3: if current slot == slot i then

http://wsnet.gforge.inria.fr/


François Nicolas
email: francois.nicolas@onera.fr

Francis Micheli

David Donjat

Aurélien Plyer

Frédéric Champagnat

Guy Le Besnerais

F Champagnat

Fréderic Champagnat

3D reconstruction of compressible flow by synchronized multi camera BOS

Keywords: BOS, Tomography, Underexpanded Jet

Introduction

Background Oriented Schlieren is a simple optical technique aiming at measuring density gradients. A textured background is recorded by a camera, firstly without flow. Then a second picture is taken with a phase object in between the camera and the background. The light rays deflections caused by refractive index variations leads to virtual displacements of the background pattern on the camera sensor. Those displacements can be computed using image correlation methods, such as PIV software, and the deviation maps can be derived from the geometry of the experiment.

From deviation maps recorded on the same flow from various viewpoints, the 3D density field can be reconstructed [START_REF] Atcheson | An evaluation of optical flow algorithms for background oriented schlieren imaging[END_REF] [6] [START_REF] Ota | Computed-tomographic density measurement of supersonic flow field by colored-grid background oriented schlieren (CGBOS) technique[END_REF]. Recently, we have proposed a novel one-step approach for such 3D reconstruction [START_REF] Nicolas | A direct approach for instantaneous 3D density field reconstruction from Background Oriented Schlieren (BOS) measurements[END_REF]. In the line of [START_REF] Atcheson | An evaluation of optical flow algorithms for background oriented schlieren imaging[END_REF], our technique is based on simultaneous acquisitions of the flow by several cameras. In [START_REF] Nicolas | A direct approach for instantaneous 3D density field reconstruction from Background Oriented Schlieren (BOS) measurements[END_REF] good results were obtained with 12 cameras on aerothermal test cases where weak density gradients conditions ensure the validity of the paraxial hypothesis for ray trajectories. Moreover, for such incompressible flows, the 3D temperature field can be 18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 4 -7, 2016 derived from the density providing a way to validate the reconstruction from independent temperature measurements [START_REF] Donjat | Study of a co-flowing hot jet: an application of direct 3DBOS technique in research wind tunnel[END_REF].

While traditional schlieren has been extensively used for the study of transonic and supersonic flows, providing precise visualizations of shock waves, the extension of 3DBOS reconstruction is not straightforward and is limited by two difficulties. The first issue is related to the blurring effect associated with strong gradients in BOS images, which degrades the accuracy of estimated deviations. In [START_REF] Leopold | Reconstruction of the Unsteady Supersonic Flow around a Spike Using the Colored Background Oriented Schlieren Technique[END_REF] a solution based on correlation of color pattern is proposed. In the present work, we rely on classical graylevel images but we show that, with a careful choice of experimental conditions, deviation maps with high resolution, comparable to traditional schlieren visualization, can be obtained. The second issue is related to the paraxial approximation which is not valid anymore when shock waves are involved. Our goal is to measure the ability of the 3DBOS algorithm presented in [START_REF] Nicolas | A direct approach for instantaneous 3D density field reconstruction from Background Oriented Schlieren (BOS) measurements[END_REF] to deal with strong density gradients.

Investigation of compressible effects on a BOS experiment

Shock waves and more generally strong density gradients induce severe light ray deviations. As a consequence, astigmatism is generated causing some blurring on the image as it can be seen on The blurring effect is damageable for image correlation methods: the correlation score decreases, and for severely blurred areas the estimation fails. Moreover, such missing deviations, even on restricted parts of the imaged field, can severely degrade the reconstruction of 3D density field.

In this section, we study the effect of experimental choices both on the sensitivity and reliability of deviation estimation, and on local blur for the study of an underexpanded jet. Our facility consisted in a 22mm diameter convergent nozzle supplied by 9 bar pressurized air tank (see Fig. 2). Different Nozzle Pressure Ratios (NPR= jet total pressure/ambient static pressure) are investigated, ranging from 2 to 5. The air is regulated at a constant temperature equals to 20°C via a 570kW heater in order to prevent from condensation. The shape of the nozzle ensures a straight sonic condition on the jet exit. A JAI BM-500GE camera, equipped with a 70 mm lens, is placed at Zc = 1m from the nozzle whereas the background is moved between 18cm and 2.9m. For each position (Zb= 0.18m, 0.4m, 1.3m, 2.9m), the background is designed in order to get 3-4 pixels dots on the camera image. The background is lighted with a Quantel Twins BSL double pulsed laser. Both pulses fire during the camera exposure time which leads to a better illumination. The effective exposure time is then equal to the delay between the two laser pulses (100ns).

Fig. 3 BOS principle.

Two parameters have been studied: the influence of the background position and the effect of the camera aperture. As seen on the diagram presented in Fig. 3, the pixel displacement measured on the camera is proportional to the distance from the background to the flow. Moving the background closer to the experiment reduces the effect of density gradients thus reduces the blurring effect.

Unfortunately, if a lower flow-background distance allows a better capture of shock waves, it also leads to a loss of sensitivity of the measurement process, at the risk of mitigation, or even complete elimination, of the small density gradients. We have tested various background positions and conducted an evaluation of the deviations estimation. From the four cases (Zb= 2.9, 1.4, 0.4 and 0.18) we have selected the position Zb= 400mm as a good trade-off between the capture of small density variations such as acoustic waves and the ability of resolving high gradients associated to shock waves.

We will now focus on the aperture parameter influence. For each f-number (7.1, 8, 11, 16 and 22), the laser intensity has been adjusted to get the right amount of light. As the f-number increased, the astigmatism is reduced because the rays suffering from too large deviations do not enter the camera anymore. On Fig. 4, one can see the strong influence of the aperture: it not only allows a better correlation but reveals finer details on the flow topology which were hidden by the blurring effect. In this case, f#=16 represents the best choice. For larger f-numbers, diffraction impacts the resolution of the background, spreading the dots intensity and increasing correlation noise on BOS images. Displacement norm in pixels (scale from 0 to 10).

In conclusion of this 2D study, by choosing wisely its set up, one can acquire very good quality BOS images, with a spatial resolution comparable to traditional schlieren visualization, as demonstrated on Fig. 5.

Underexpanded jet 3D density field acquisition

We build our 3D BOS experiment based on the empirical study conducted for 2D BOS in the previous Section. Four background plates covered with 0.2mm dots are placed at a distance Zc=400mm of the jet. For the study of jets, placing the cameras on half a ring is the configuration chosen by most previous references ([7] [START_REF] Atcheson | An evaluation of optical flow algorithms for background oriented schlieren imaging[END_REF]). We follow this line by setting up our 12 cameras on a hexagonal bench at a distance Zb=1000mm of the jet (see Fig. 6). The range of NPR condition is defined from 2 to 5. A PIV laser is still used as light source, but now we split the beam into four before spreading each one on the backgrounds. This solution enables to cover the 4 background patterns as a whole. As in the previous 2D case, the two laser pulses are used during the exposure time, both 12 cameras and laser are synchronized with a TTL pulse generators. Unfortunately, the amount of light remaining for each camera does not allow us to use the optimal f-number of 16 and we will operate the cameras at f#=8. This choice leads to some filtering of fine scale structures, but as we are interested here in the reconstruction of mean density field, this loss is less critical as it can be seen on the mean field comparison on Fig. 4. Currently, ongoing work focuses on the setup improvement in order to increase the amount of light recorded by the cameras and to work at higher f-numbers. This enhancement will be very relevant for the reconstruction of instantaneous flow field, especially at high NPR.

During the acquisition, images are transferred to the RAM memory via an Enterasys switch and then moved to SSD disks. This solution enables us to acquire images at 10Hz. Finally, the multi cameras system is calibrated as explained in [START_REF] Sant | Multi-camera calibration for 3DBOS[END_REF] by moving a 2D white calibration plate cover with black dots into the common volume of the cameras.

Results

In the current section we present both an average and instantaneous reconstruction for a moderately underexpanded jet (NPR=2) and the mean field of a strongly underexpanded jet (NPR=5). The NPR=5 mean field contains very high gradients. It has a well-defined topology and is less complex than the instantaneous one. Moreover, it can be compared with numerous previous works and with CFD simulations. With the actual set up, strong astigmatism effects prevent us to reconstruct satisfactory results on the instantaneous case. Average 3D reconstructions are performed from the mean displacement field of each camera resulting of the average of 900 samples. The calculation is done over a volume of 10x7x7 cm3 using between 40 and 100 million of 0.2 mm side voxels. A 3D mask is also used to help the reconstruction. As explained in [START_REF] Nicolas | A direct approach for instantaneous 3D density field reconstruction from Background Oriented Schlieren (BOS) measurements[END_REF], the solution to this inverse problem is defined as the minimizer of a compound criterion made of a least square data term penalized with a L2 criterion on density gradients. The regularization parameter λ is chosen accordingly to the Lcurve strategy [START_REF] Hansen | Analysis of discrete ill-posed problems by means of the L-curve[END_REF]. Compared to aerothermal cases which were considered in [START_REF] Nicolas | A direct approach for instantaneous 3D density field reconstruction from Background Oriented Schlieren (BOS) measurements[END_REF], the selected parameter is much lower here, (λ=10 -6 compared to λ=10 -4 in [START_REF] Nicolas | A direct approach for instantaneous 3D density field reconstruction from Background Oriented Schlieren (BOS) measurements[END_REF]) preventing from smoothing strong density gradients.

On the mean field of the moderately underexpanded jet, the train of shock wave is clearly visible in the potential core region. This pattern results from the successive reflection of expansion fan and shock wave on the jet boundary. The result being limited to the intersection The instantaneous reconstruction presented in Fig. 8, shows much more complexity. If the first two cells are quite stable (as we compare with the mean field) this is not the case for the following ones. One can also notice the instability of the second part of the jet where shock cells aspect is completely distorted. Some over-smoothing on the reconstruction also impacts the quality of the result and the capture of finer scales. A good way to finer analyze our algorithm performance is to apply our direct operator on the reconstructed density field and compare the resulting deviation maps to the input ones. As it can be seen on Fig. 9, there is good agreement between synthesized deviations and measured ones. We also compared in Fig. 11 the position of the MD and his diameter with both schlieren measurements realized previously on the same underexpanded jet and [START_REF] Addy | Effects of axisymmetric sonic nozzle geometry on Mach disk characteristics[END_REF]. Although the geometry of the Mach disk is over-smoothed by the reconstruction process, the location and size are quite consistent with this experimental database. Finally, we can also notice that the subsonic region behind the MD is well captured by our technique whereas the Abel inversion and schlieren CT of Takano [START_REF] Takano | Three-dimensional rainbow schlieren measurements in underexpanded sonic jets from axisymmetric convergent nozzles[END_REF] show some difficulties in this particular region. Moreover, our reconstruction highlights the ring shape of the reflected shock.

Conclusion

This paper focuses on 3D reconstructions of a compressible jet by BOS. Our contribution is twofold. First, we have demonstrated that using an optimized set-up and assuming that illumination is sufficient to choose a large aperture number, good quality BOS image can be recorded, leading to estimated deviation of resolution close to schlieren techniques. Second, we have built a 3D BOS experiment involving 12 cameras mounted around an underexpanded jet so as to conduct a preliminary evaluation of the 3DBOS methods of [START_REF] Nicolas | A direct approach for instantaneous 3D density field reconstruction from Background Oriented Schlieren (BOS) measurements[END_REF] in compressible conditions. The results are very promising. The estimated flow is slightly oversmoothed, as the result of various choices, both in the deviation estimation step and in the reconstruction step. Getting more accurate estimation of deviation is mainly a matter of further improving the experimental set-up, but modification of the correlation method, and in particular use of pixelwise optical flow methods developed in Computer Vision could also bring further gains. However, on-going work focuses on the reconstruction part in order to circumvent the paraxial hypothesis and avoid oversmoothing of the density gradients by the regularization term.

To conclude, measurement of 3D density fields for compressible applications is very challenging.

It is also a great interest for comparison with CFD computation and might be a way to better understand light distortion through shock waves. Particle position errors and velocity errors in supersonic PIV could benefit from such measurements and might be corrected with BOS [START_REF] Elsinga | Evaluation of aero-optical distortion effects in PIV[END_REF].

Fig. 1 .

 1 Fig. 1.

Fig. 1

 1 Fig. 1 Example of astigmatism effect induced by an underexpanded shock cell.

Fig. 2

 2 Fig. 2 Schematic view of the convergent nozzle generating our underexpanded jet.

18th

  International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 4 -7, 2016

Fig. 4

 4 Fig.[START_REF] Elsinga | Evaluation of aero-optical distortion effects in PIV[END_REF] Influence of the camera aperture on instantaneous BOS field (top) and mean field (bottom).

Fig. 5

 5 Fig. 5 Comparison between BOS image (top) and schlieren image (bottom). NPR=5.

Fig. 6

 6 Fig. 6 3D BOS bench and supersonic hot jet facility.

18th

  International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 4 -7, 2016volume of all cameras, the very beginning of the jet is not reconstructed. The 3D mask edges visible on Fig.7might denote a lack of iterations in the reconstruction process to allow the regularization to smooth those artifacts. If our code perform well in this case, it is important to observe the existence of artifacts in the first two shock plane.

Fig. 7

 7 Fig. 7 Mean density field reconstruction for NPR=2 case. Slice at y=0.

Fig. 8

 8 Fig. 8 Instantaneous density field reconstruction for NPR=2 case. Slice at y=0 on the left and isodensity on the right.

18th

  International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 4 -7, 2016

Fig. 9

 9 Fig. 9 Deviation map comparisons for εx map at the top and εz map at the bottom.

Fig. 10

 10 Fig. 10 Mean density field reconstruction for NPR=5 case on the left and Density gradient magnitude on the right. Slice at y=0. 3D BOS reconstruction on the upper part and RANS simulation on the lower part.

Fig. 11

 11 Fig. 11 Evolution of the Mach disk position and diameter with respect to the NPR from [8].Comparison with the RANS simulation and our schlieren and 3D BOS measurement.


Beatrice Pradarelli
email: beatrice.pradarelli@lirmm.fr

Pascal Nouet

Laurent Latorre

Per Peers Learning Education Approach to Teach Industrial Test to Undergraduate Students

Keywords: per peers learning, education, quiz, course video, evaluation, skills, competencies, industrial test, ATE programming, I

This paper presents a return of experience relative to per peers learning approach to teach industrial test of integrated circuits (IC) to undergraduate students from Polytech Montpellier, using the test platform of the CNFM. The objectives of this innovative approach are double: technical and educational. The students have to acquire knowledge and competencies about IC testing (test methods, test program development, debugging, diagnosis, test results and graph analysis technics) in a way they are the protagonists of their own learning and the one of their peers. A specific course organization has been put in place as well as dedicated training materials combining quizzes and video tutorials have been developed to guide the students achieving this new role. A course evaluation form has been created, based on the F2A Louvain University one, to help the students providing classifiable feedback about this new educational approach. The analysis of the students' exam results is presented.

INTRODUCTION

From 2008 to 2014, a group of 4 th year students from the Electronics department of the engineering school Polytech Montpellier, were trained to the industrial test of integrated circuits (IC) through a project of 28h they selected. This project oriented pedagogical approach was organized to have the students being the protagonists of their learning [START_REF] Pradarelli | Une pédagogie par projet pour des étudiants acteurs et auteurs de leur apprentissage[END_REF], [START_REF] Pradarelli | Industrial Test Project Oriented Education[END_REF]. It allowed teaching IC testing fundamentals and practice to 10 up to 18 students (in binomial groups) without any usage limitations of the operational resources of the CNFM (Coordination Nationale pour la Formation en Microelectronique) [3]. The CNFM is a French public organization that federates academic and industrial partners for the purpose of education in Micro and Nano-electronics. CNFM focuses on making heavy educational resources such as professional CAD tools, clean rooms, or industrial test equipment available for common use, by all French universities and industrial partners. The IC testing course used in particular the industrial test equipment of CNFM center in Montpellier [4]. In 2014-2015, the syllabus was modified and two major changes impacted the industrial test course. First, the reduction of hours: 18 instead of 28, forcing the project oriented pedagogical approach to move to another format keeping the objective of putting the students in the center of learning. Secondly, the course concerned all the 4 th year students of the Electronics department, i.e., 28 students instead of an average of 14, generating a bottleneck for the students' practice of the test equipment. This paper describes the new approach developed to continue teaching industrial test keeping the same educative and technical goals. The section 2 is dedicated to the description of the context of mass production testing, the CNFM test course environment and the test equipment requirements for students' good practice. Section 3 presents the educational approach itself: organization, calendar and dedicated training material, students' and approach evaluation. The analysis of both the students and the pedagogical approach evaluations are provided in section 4. Section 5 is the conclusion of this return of per peers learning experience.

II. INDUSTRIAL TEST COURSE

A. Industrial test of integrated circuits

Mass production test of Integrated Circuits consists in verifying using automatic test equipment (ATE) a big quality of a product with respect to its specifications. The objective is to detect the defective devices as soon as possible in the device manufacturing cycle in order to decrease the production test cost.

The production test has no other purpose than verifying that manufactured circuits meet the datasheet. Basically, a production test consists in a flow-chart implementation of elementary tests (Test Flow), each having a simple Pass/Fail output (see Fig. 1). Doing so, it is possible to distinguish the failure origin and to perform a sorting (binning) for both good and bad circuits. For instance, this approach is used to sort processors by operating frequencies. The production test is considered right after foundry at the wafer level (wafer sort) and after packaging (final test). Test flow example with two "good bins" to sort devices depending on their performance.

B. CNFM TEST PLATFORM 1) Tester hardware

The industrial tester is a V93K from Verigy® recently bought by Advantest®, one of the four major test equipment manufacturers in the world. The V93K platform targets SoC (System-on-Chip) products (i.e. Digital, Mixed Signal, RF, embedded memory, etc.) testing. The basic elements that compose the Automatic Test Equipment (ATE) are illustrated in Fig. 2. The main part is the testhead. It can host up to 18 boards (Pin Electronics) for a maximum number of 512 digital pins (or channels) based on existing 32 channels boards. Programming is performed using a regular computer running dedicated software under Linux. Fiber optics is used to allow fast communication between the tester and the CPU. The software/hardware interaction is built upon the concept of "live-machine". When programming through software, data in the tester memory is continuously updated. The counterpart of this interaction scheme is that the software needs the physical tester to operate. For parallel use in the context of a classroom, the hardware is not divisible and it would lead to a dead end without the availability of an "offline" mode.

In "offline" mode, the tester hardware is emulated in software so that SmarTest® operates just the same as in online mode as long as no real test result is expected. In this mode the students develop their test program and one after the other they move to the "online" mode to verify and correct their test program testing a device. 20 "offline" licenses are available but as Smartest consumes a lot of memory, only 9 of them are run at the same time during students' practice. Figure 4 represents the network organization of the test platform. This approach allows all students of a class to share their test session, increasing the interactivity between them and the teacher who can help a student having trouble and guide her/him to solve her/his issue.

III. INNOVATIVE PEDAGOGICAL APPROACH PRESENTATION

A. Choice of the teaching approach

The main objectives of the course were technical and pedagogical. First, teaching industrial test required lessons to explain the fundamentals, i.e., mass production test philosophy, test method concepts and technics to allow students understanding how the functionalities and performances of integrated circuits are verified. It also required practice (labs) using the test equipment (tester hardware and tester software interface). A solution to solve the mismatch between the number of students (28) and the number of "offline" licenses available for the labs (9) was to gather the students by 3 or 4: 8 groups of 3 students and 1 group of 4. The educational target was to guide the students to be actors of their own learning. With this group configuration (3 or 4 students per group), the challenge was to motivate all group members to fairly contribute to the labs. The solution was to

Classroom with Terminals

Account Server select the "per peers learning" approach, in which students teach each other and interact with other students to attain educational goals. This approach was only used to the part of the course needing online access to the tester. It concerned the learning of functional and characterization tests. Faults detection and diagnosis methods were taught to the whole classroom.

B. Educational approach organization

The idea of using the "per peers learning" technic was to teach one part of the course (lesson and labs) to 18 students (2 students of the 8 trinomial groups and 2 of the group of 4) and a different part of the course to the 10 remaining students (1 student of the 8 trinomial groups and 2 of the group of 4) during 2 sequences of 4 hours for each group. During the next 4 hours (after the 8 first ones), students returned to their original groups, transferred their learning to their peers and all together shared their recent knowledge and know-how to perform labs. The difficulty of this approach was to verify that each student acquired all the technical competencies, meaning the ones taught by the teacher and the ones taught by the peers.

The teacher had to anticipate the progress of the course, the potential blocking points in order to schedule a specific calendar that left time for reviews and brainstorming sessions. This innovative approach also required developing dedicated training materials, like quiz and video tutorials, to help the students successfully achieving the "per peers learning" phase.

C. Course progress 1) Step 1: Course presentation to students

The first two hours were dedicated to present:

• The context of mass production test in order to give the students a clear vision of the challenges of testing integrated circuits in the semi-conductors industry. • The learning objectives, the educational approach, the course organization and calendar (table I), the expected acquired competencies at the end of the course, the students' competencies evaluation mode and the approach evaluation by the students. It was particularly important to clearly explain the "per peers learning" approach to the students who understood that their peers' learning acquisition of one part of the course depended of their own learning motivation, investment and quality. At the end of the first step, all the groups were built. Each student knew what she/he will learn from the teacher and will learn from their peers, what she/he will have to teach to peers, the course schedule, the competencies on which he will be tested and that the course will end with a cake party.

2)

Step 2: Classic learning: teacher to both groups of students During two sessions of 4 hours, the teacher taught to group n°1 composed of 18 students (grouped by 2) and to group n°2 composed of 10 students (standalone student) different aspects of the industrial test of integrated circuits. Group n°1 learnt the basics of functional testing: study of the device under test, initiation to tester hardware and software interface (Smartest®), learning of the functional test method, the test flow concept and their implementation on tester. Group n°2 learnt the basics of characterization: parametric tests to verify the AC and DC parameters of a device data sheet, test results analysis and margins calculation, shmoo plots technics and graph interpretation to identify any parameter dependency, any malfunctioning at uncovered test conditions during mass production. At the beginning of the second session, a short brainstorming was proposed to each group in order to status on their knowledge, know-how and gaps. To ease the communication, colored post-it papers have been distributed to all students. They had to write down the misunderstood and put them on a white board. The teacher gathered the post-it papers by item and answered them once he was sure to have understood the request. May be the funny aspect of the post-it papers and the anonymous process contributed to motivate the students to actively participate. Any gap was explained. At the end of the 8 hours, both groups answered to a quiz whose objective was to help them facing the level of their acquired competencies before teaching their peers. Once again any misunderstanding was explained again.

3) Step 3: Per peers learning Unfortunately, more than a month had passed by since the last session with group n°2. To guaranty the success of per peers learning it was crucial that the topics learnt by the students from both groups were fresh in their mind. So, the first ½ hour of the 4 hours session was dedicated to the review of both groups learning acquisitions. Two classrooms were booked to ease this step. The teacher provided to group n° 1 and n° 2, the list of skills all students from group n° 1 must be able to teach the students from group n°2 and vice-versa. Because the tester software interface SmarTest® offers a lot of possibilities and is complex to apprehend when not used every day, the teacher provided to the students from group n°1 a series of videos created with Camtasia® software, in order to help them remembering the usage of SmarTest®. Each video was a record of a command of the tester interface needed to create a test flow or a single test, completed with visual comments describing the different steps of the usage. The teacher provided a guideline to group 2 remembering how to proceed to analyze shmoo plots (variation of 2 test parameters at the same time). Then the original 8 trinomial groups and the group of 4 students gathered in the same classroom and the per peers learning process started following the organization and schedule:

• 4 trinomial groups and the group of 4 students worked on teaching the functional test methods and practicing using the offline mode of the test equipment while 4 trinomial groups concentrated on teaching characterization test skills during 1h30. • 15 minutes break • 4 trinomial groups and the group of 4 students worked on teaching characterization skills while 4 trinomial groups concentrated on teaching the functional test methods and practicing using the offline mode of the test equipment during 1h30. • 15 minutes debriefing at the end of the course The teacher did not give the students any guideline to teach their peers to respect their own process and rhythm but paid attention to the way each group worked.

4)

Step 4: Classic learning: teacher to all students During the 2 hours of the next session, all students were initiated to fault detection method and tools and, following a given diagnosis procedure they learnt how to proceed to identify the root cause of a defect. The main objective was to raise the students' awareness to physical defects that can affect digital devices. The students had to diagnose 5 test reports each representing the detection of one defect coming from the introduction of one stuck-at fault on one pin of the device under test. Students had to share their answers before the teacher randomly selected students to explain the solution of the exercise.

5) Students' learning skills evaluation

The students were evaluated on the test concepts and technics, tester interface commands, shmoo plot graph analysis and faults diagnosis. The exam was a 1-hour quiz (10 questions) and 3 exercises (shmoo plot graph analysis and stuck-at fault diagnosis). Almost all questions had been treated during the course or during the review phase through the quiz. It was the same approach for shmoo plot graphs analysis and faults diagnosis exercises. This choice of exam mode was based on a remark made by Antoine de la Garanderie on p48 of his book [START_REF] De | Réussir ça s'apprend[END_REF]: "We have the past (knowledge and know how) only as much as we take care of registering it in the future (when we learn)". This author studied the learning methods used by very good students and concluded that they learnt projecting their new learning in the context they would need it, like an exam, avoiding stress and knowing exactly where to search for it in their brain memory. In our case, the objective was to put back the students in a context they knew (the time when they learnt in-group or when they performed the quiz during the review phase) to limit the emotions coming from the stress of the exam and to allow their brain concentrating on reactivating their competencies. In order to avoid the students to copy each other, 3 versions of the exam were created.

6) Educational approach evaluation by the students As students from group n°1 learnt the basics of functional testing from the teacher and the characterization competencies from group n° 2 peers and students from group n°2 learnt characterization competencies from the teacher and the basics of functional testing from group n° 1 peers, it was important to write a dedicated evaluation form that allowed identifying and measuring any skills level mismatch resulting from a same topic taught by the teacher and peers. A second aspect of the evaluation form concerned the students' feedback about the learning in-group method. The overall document was based on an evaluation form from FA2L Louvain teachers [8].

• The document was organized as follow:

1. Student self-evaluation on her/his learning with the teacher and the teaching approach itself 2. Student self-evaluation on her/his learning with peers and the per peers learning approach For these 2 self-evaluations, the students had to fill tables in which they had to rank their level of knowledge and know-how on topics taught by the teacher and their peers. To evaluate both teaching approaches, they had to provide their level of satisfaction concerning the course organization, the allocated time for lessons, practice and per peers learning and, the relevance of the quiz and Camtasia® videos usage. 3. Trinomial group and group of 4 students' evaluation of the per peers learning approach using a spider diagram (see figure 5) 4. Students' self-feedback about the advantages and disadvantages of working in-group to learn skills.

IV. EDUCATIONAL APPROACH OUTCOMES

A. Analysis of students' exam results

The final mark of the exam was the sum of the quiz marks and the ones from the 3 exercises:

• 10 points for the 10 questions quiz • 3 points for the shmoo plot analysis • 3 points for the stuck-at fault identification • 4 points to insert a stuck-at fault on a device and indicate in a test vector the faulty cycles and outputs Over the 28 students who attended the industrial test course, 13 of them got a mark between 16,2/20 and 20; 8 of them got a mark between 15/20 and 16,14/20 (average value) and 7 of them got a from 11,5/20 up to 14,5/20. The details of the marks are listed in the table II. As a reminder, group n°1 learnt functional test skills from the teacher and the characterization ones from their peers; group n°2 acquired characterization competencies from the teacher and functional test ones from their peers. The columns called "Quiz marks" and " Shmoo Marks" were relative to "per peers learning". The columns called "Diag Marks" and "Fault Marks" concerned lessons given by the teacher to the whole class.

Looking at the 2 nd , 3 rd and 4 th columns of the table II, questions appeared about the influence of the teaching approach (teacher versus peers reflected by the 2 groups) and/or the quiz subject number even if the 3 versions of the quiz were randomly distributed to the students.

The extraction of the marks versus the quiz version is presented in the table III as well as the group number: 1purple, 2-yellow. The average and standard deviation values of each quiz version were calculated in order to have an idea of the marks distribution around the average value. The number of students from both groups who answered to quiz version 1 was fairly the same. The average value of this quiz is quite high and its standard deviation is the biggest with almost 3 points. It was random but it appeared that quiz version 2 had been only answered by group n°1 students which got the lowest average value but not the biggest standard deviation. Finally, quiz version 3 which had fairly the same number of students from both groups scored the best with the lowest standard deviation. As there was no obvious evidence, we concluded that the quiz version did not influence the marks. Focusing on table IV, it appeared again that both average and standard deviation values of each group were similar, showing no obvious evidence of the influence of the teaching methods.

In order to go further, the details of the quiz marks and the shmoo plot exercise were studied per group number. The shmoo plot lesson was given by the teacher to group n° 2 students and they taught their peers from group n°1. The average values resulting from the 18 marks of group n°1 and 10 ones from group n°2 shown 0,12% difference, which was irrelevant.

Focusing on the quiz questions, we studied the influence of the questions relative to the competencies given to both groups directly from the teacher and the ones from the "per peers learning" approach. In Table V, Quiz Q1 column represents the questions relative to competencies taught by the teacher to group n°1, so students from group n°2 learnt these skills from their peers. Consequently, Quiz Q2 column represents the opposite. This time, a small difference between the skill levels appeared pointing out that the students who learnt skills from the teacher had slightly higher marks. In fact, this difference did not result from the teaching method itself but rather from the time allocated to each one: both groups spent 8 hours learning from the teacher and only 6 hours from their peers. In addition, group n° 1 students had the challenging mission to explain group n°2 students how to use the tester interface tool SmarTest® which was complex task.

B. Analysis of the students'teaching approach evaluation

The collection of the students' answers about their learning experiences with the teacher and peers and their level of acquired skills were first analyzed. The objective was to point out any major skill levels difference between both teaching methods on a same topic based on students' feedback and not marks. The analysis shown that students who considered having a lower acquisition level on a topic with the per peers approach mentioned that it was the consequence of the lack of time allocated to this innovative teaching method and not due to the approach itself. In general, the students liked the "per peers learning" approach because it gave them a new role: being the actor of their peers' learning. It meant that they had to take written notes and be pro-active during the course session with the teacher in order to understand, ask questions if necessary and memorize the concepts, technics and practice. Most of the students realized that they were not enough concentrated during the lessons and met reminding what they learnt. They also realized that their learning was not complete only when they started teaching their peers and had difficulties to explain some topics. They mentioned that not enough time was dedicated to per peer learning session and they would have preferred to follow a procedure to teach their colleagues. The students considered that teacher's choice not to provide them a guideline during the transfer phase to respect their own teaching rhythm was not appropriated. It influenced the acquired knowledge and know-how levels. Students would have liked to have a review session after per peer learning one to be sure to have acquired all necessary skills and competencies. Students would have liked to have more time to practice on the tester to get familiar with its usage and the complexity of its interface. They also mentioned a lack of time allocated to shmoo plots explanation and practice. They liked having Camtasia® videos to review the tester interface usage and they also appreciated the quiz, during the course, to evaluate their own knowledge.

They considered that there was too much time between to 2 test courses that did not ease remembering their learning.

C. Improvements

The major improvements to apply to this innovative education approach concern the "per peers learning" part:

• To write a dedicated procedure to guide the students (who want to use it) teaching their peers. • To give a quiz to the students at the end of the "per peers learning session" to verify all students have acquired all the required skills and competencies. The procedure would to have each student answering the question alone, then, the responses would be shared and discussed with peers, before questioning the teacher. A last improvement would be to have only a week delay between each test course.

V. CONCLUSION

This paper presented a return of experience relative to a "per peers learning" approach to teach industrial test of integrated circuits (IC) to 4 th year students from Polytech Montpellier, using the test platform of the CNFM. From pedagogical point of view, it was an interesting experience because students get quickly motivated and ready to succeed their "teaching mission". For the teacher, it was very time consuming, as it was regularly necessary to adjust the course organization to fit the calendar and learning objectives. It also required a lot of efforts to create appropriated training materials. Nevertheless, most of them are reusable with other students or teachers, which is a real benefit. Some improvements will be applied to the "per peers learning" approach to guaranty no skill level mismatch between competencies learnt from the teacher and the peers. This innovative teaching method has been put in place to solve technical issues (tester licenses number) but it clearly appeared that it could be apply to other courses.

Fig. 1 .

 1 Fig. 1.Test flow example with two "good bins" to sort devices depending

Fig. 2 .

 2 Fig. 2. CNFM test platform main components 2) Tester software and tester virtualization The test program is developed into an environment called SmarTest®, which is based on the open platform Eclipse® shown in figure 3.

Fig. 3 .

 3 Fig. 3. Smartest® programming environment based on Eclipse® IDE

Fig. 4 .

 4 Fig. 4. CNFM test platform environment VNC technology (Virtual Network Connection) [3] has been used to implement the concept of virtual classroom.This approach allows all students of a class to share their test session, increasing the interactivity between them and the teacher who can help a student having trouble and guide her/him to solve her/his issue.

Fig. 5 .

 5 Fig. 5.Spider diagram for the "per peers learning" approah evaluation by the groups of students 7) Cake party The teacher proposed a cake party to celebrate the end of the course. All students brought food and beverages and shared them as shown on the figure 6.

Fig

  Fig. 6. Polytech MEA4 students -April 2015

LINUX WORKSTATION DEVICE UNDER TEST TESTHEAD with all the pin electronics

	VERIGY_OFF2
	Linux
	Optical Link
	VNC
	Remote
	Login
	PIN ELECTRONICS
	• Supply
	• Digital
	• Analog
	• RF

Tester SmarTest® "Offline Mode" SSH & VNC servers VERIGY_ON Linux SmarTest® Online & Offline Mode SSH & VNC servers

TABLE I .

 I

INDUSTRIAL TEST COURSE CALENDAR

TABLE II

 II

			.		STUDENTS' MARK DETAILS
					Quiz		Diag	Fault
	Stude	Grou	Mark	Quiz	Marks	Shmoo	Marks	Marks
	nt #	p #	s	#	(10)	Marks (3)	(3)	(4)
	1	1	19	1	10	3	3	3
	2	1	13,5	1	7,5	2	1	3
	3	1	18,5	1	9	2,5	3	4
	4	1	17,5	1	9	1,5	3	4
	5	1	18	1	9	2	3	4
	6	1	15,75	2	8,25	1,5	2	4
	7	1	17,5	2	9,25	2,75	2	3,5
	8	1	11,75	2	8,75	2	1	0
	9	1	18,5	2	10	2,5	2	4
	10	1	11,5	2	8	1,5	0	2
	11	1	19	2	10	3	2	4
	12	1	15,5	2	9	2,5	1	3
	13	1	15,5	2	9	1,5	2	3
	14	1	15	2	8	0	3	4
	15	1	15	3	8,5	3	1	2,5
	16	1	20	3	10	3	3	4
	17	1	17	3	10	3	1	3
	18	1	15,75	3	9	2,75	2	2
	19	2	15	3	9	3	0,5	2,5
	20	2	17,5	3	10	2	2	3,5
	21	2	16,5	3	9	3	1	3,5
	22	2	14,25	3	8,5	2,75	2	1
	23	2	15,5	3	9,5	0	3	3
	24	2	19	3	10	3	2	4
	25	2	13,5	1	10	0	2,5	1
	26	2	14,5	1	5,5	3	3	3
	27	2	12	1	8	2,5	0,5	1
	28	2	20	1	10	3	3	4

TABLE V

 V

	.	COMPARISON OF TEACHING APPROACHES
		Quiz Q1	Quiz Q2
	Average marks/gp 1	5,75	3,333333333
	Average marks/gp 2	5,45	3,65
	Ratio average gp/gp %	5,217391304	8,675799087