Cultural Heritage Data from a Humanities Research Perspective: A DARIAH Position

Introduction

Europe faces significant challenges when it comes to the effective and efficient digitisation of our cultural heritage. With over 80% of Europe's cultural heritage remaining to be digitised [START_REF] Nauta | D4.4. Report on ENUMERATE Core Survey 4 , Europeana DSI 2-Access to Digital Resources of European Heritage[END_REF], there is an urgent need to re-think the European digitisation strategy in order to enable Member States to more effectively work together towards the European Commission's 1 ambitious, but hardly achievable, target of all European cultural heritage being digitised by 2025. Furthermore, a large proportion of the 10-20% of Europe's cultural heritage that has already been digitised is rapidly approaching its 'sell-by' date. Being 'first generation' or 'legacy' digitisation, it is no longer of a high enough quality [START_REF] Hill | Quantifying the impact of dirty OCR on historical text analysis: Eighteenth Century Collections Online as a case study[END_REF] for analysis using advanced digital humanities methods.

DARIAH, the Digital Research Infrastructure for the Arts and Humanities , enhances and supports digitally enabled research and teaching across the Arts and Humanities. It develops, maintains and operates an infrastructure that sustains researchers in building, analysing and interpreting digital resources. Digital (digitised and born-digital) cultural heritage is fundamental to our research. Cultural Heritage Data is Humanities Research Data . Without digital cultural heritage, undertaking humanities research with digital methods would be impossible. With an estimated 500,000 humanities researchers in Europe alone (Rossi, 2020), the digital Arts and Humanities community represents a significant community to consider in light of the European Commission's evaluation and possible revision of the Commission Recommendation of 27 October 2011 on Digitisation and Online Accessibility of Cultural Material and Digital Preservation (REC 2011/711/EU). It is for this reason that we would like to share a few additional reflections from the DARIAH Community in this position paper.

Cultural Heritage Data as Humanities Research Data

Even if we only focus on the digitisation needs of the digital arts and humanities research community in Europe, the opportunities offered by digital technologies for the culture heritage sector are manifold. Digitisation for digital humanities research will facilitate the digital transformation in Cultural Heritage Institutions (CHIs) , enabling them to move beyond creating siloed and static digital libraries towards implementing sustainable data curation workflows [START_REF] Padilla | Final Report ---Always Already Computational: Collections as Data[END_REF][START_REF] Candela | Reusing digital collections from GLAM institutions[END_REF]. Through the dynamic extraction and curation of research-driven digital cultural heritage datasets, these 'Collections as Data' can be made accessible online as Findable, Accessible, Interoperable and Reusable (FAIR) humanities research datasets . What is needed now, and what the follow-up to REC 2011/711/EU should include, is a recommendation to conduct a thorough evaluation of the usability of existing digitised cultural heritage collections for specific digital humanities methods [START_REF] Van Strien | Assessing the Impact of OCR Quality on Downstream NLP Tasks[END_REF]. In addition, cultural heritage institutions need guidance on how to improve their digitised collections to meet the needs of 21st-Century digital scholarship including how to navigate the legal challenges related to text and data mining [START_REF] White | Research Libraries: How You Can Support Text and Data Mining[END_REF].

Effective collaboration between the cultural heritage sector, where cultural heritage professionals are recognised as essential partners in research , the humanities research community and data, information and computer scientists is an essential condition for ensuring the possibility of excellent research and innovation in the years to come. This is a policy challenge for the European Commission as well considering that Cultural Heritage and Research are traditionally seen as two separate sectors. Yet it will be essential --and equally beneficial to both sectors --for the Commission to recognise the potential for innovation presented by tighter cross-sectoral cooperation and alignment between Cultural Heritage and Research . Cross-sectoral recommendations and cross-sectoral funding mechanisms should lead the way in reaffirming Europe as a powerful symbolic space in which tradition and innovation are inherently intertwined. Finally, a well-thought-through alignment between a revised REC 2011/711/EU and the flagship initiatives from the Research sector such as the ESFRI Research Infrastructures and the European Open Science Cloud (EOSC) could pave the way towards better interoperability and infrastructural consolidation.

Strengthening Europeana's Role in Research

Europeana, which REC 2011/711/EU recognises as "Europe's digital library, archive and museum," already collaborates with the Arts and Humanities research community. The roots of this collaboration go back to 2013 and the Europeana Cloud project which did extensive investigations around the needs of Humanities and Social Science research communities and looking at the research behaviour and practices (see, for instance, [START_REF] Benardou | From Europeana Cloud to Europeana Research: The challenges of a community-driven platform exploiting Europeana content[END_REF]. Europeana Cloud also served as the prototype infrastructure for Europeana Newspapers , a project that produced a first transnational full-text collection of digitised historic newspapers from Europe, greatly appreciated by the scholarly community2 . The Europeana Cloud project also led to the development of Europeana Research and eventually a Europeana Research Community with the aim of developing a deeper insight into meeting researchers' needs regarding the use of digital cultural heritage.

Cultural Heritage as Open Data

Collaboration with cultural heritage institutions and professionals and establishing as open as possible data exchange protocols are cornerstones of responsible research workflows in the arts and humanities. This cooperation, however, is often constrained by structural, legal and technical challenges. A significant number of collections throughout Europe have been digitised but not made openly accessible due to unclear licensing conditions. To make things worse, some digitised materials are still not openly available to the public -despite being in the public domain (see, for instance Wallace and Euler 2020; and the Passenger Pigeon Manifesto : "a call to public galleries, libraries and museums to liberate our cultural heritage. Trustworthiness, to help all the relevant actors to work together to connect and improve access to heritage data. For this to happen, it is essential for the Commission to address the existing imbalance between the qualitative and the quantitative targets of the European digitisation efforts . In this respect, recognising the diversity of European CHIs in terms of infrastructural and technological maturity and providing support to them to increase their capacities in this respect (for instance, implementing PID policies) is crucial.

We see another major conflict in the current policy landscape that seriously limits the impact of cultural holdings on scholarship and society: the Directive 2003/98/EC on the re-use of public sector information, known as the PSI Directive, is quite permissive regarding the revenue models based on charging fees for the reuse of cultural heritage data . The Directive includes specific provisions which allow museums, archives and libraries to charge higher fees for the re-use of their holdings and allow them to conclude exclusive agreements for the digitisation of their material. This is in sharp conflict with the objectives of both REC 2011/711/EU and the increasingly prevalent European and national open data mandates. While recognising the importance of establishing stable and diversified sources of funding for European CHIs, we recommend that the existing business models be re-evaluated in view of the fundamental question as to whether the generated income from pay-to-access models matches their administrative and implementation costs. In accordance with the general European commitment to open data, the follow-up to REC2011/711/EU should recommend exploring alternative business models, and recognise that public-private partnerships do not always lead to transparent agreements and free, unlimited access to end users.

Monitoring the Digitisation of European Cultural Heritage

The follow-up to REC 2011/711/EU should make a strong push towards the democratisation of digitisation across Europe . CHIs should be encouraged to move beyond the idea of the digitisation of cultural heritage as a national project: Europe needs cooperative, transnational, multilingual cultural heritage digitisation policies . This is a matter of both principle and practice. As European citizens, we can truly reap the benefits of digitisation only if our digital collections are as diverse as Europe itself. Yet in practical terms, it still remains a challenge to know what has and what has not been digitised, what should be digitised, and by whom. Even within individual Member States, an exclusive focus on nationally recognised masterpieces and canonical works can lead to the suppression of other important regional and local materials, private collections and lesser-known resources in less-resourced languages.

Effective collaboration between Member States cannot be achieved without the participation of transnational research communities such as DARIAH and without a clear understanding of the transnational impact of digitised cultural heritage.

This also means that a future European digitisation strategy will need to be rooted in the assessment of what we have and an examination of what needs to be done to improve both the quantity and quality of digitisation in the future. The current state of digitisation in Europe is still not sufficiently quantified.

To what extent are the current digital collections truly representative of European cultural heritage as a whole? Without reliable statistical data, it will be practically impossible to work towards improving the representation of diverse linguistic and cultural communities in digital collections. In previous years, the Numeric (2007Numeric ( -2009) ) and Enumerate (2011Enumerate ( -2014) ) projects were set up 'to create a reliable baseline of statistical data about digitisation, digital preservation and online access to cultural heritage in Europe'. Europeana subsequently continued this work, with the latest report published in Summer 2017, but this work needs to be continued and expanded. We believe that effective collaboration between Member States in this field can be achieved only if more effort is invested in monitoring digitisation across Europe . The revised REC 2011/711/EU should provide a mechanism for monitoring the actions taken in response to the Recommendation. The revised Recommendation itself should also be reviewed on a regular basis.

2D Digitisation Remains Crucial

Digitisation has an expiry date. Increasingly 'First generation' or 'legacy' digitisation is no longer of a high enough quality for analysis using advanced digital humanities methods. While DARIAH fully supports the need for an increased emphasis on the application of 3D technologies in the area of cultural heritage, it is essential that the revision of REC 2011/711/EU evaluates the current status of 2D digitisation. While significant progress has been made in the digitisation of Europe's historical textual cultural heritage, particularly thanks to advances in Optical Character Recognition (OCR) and Handwritten Text Recognition (HTR), continued investment in improving access to historical texts across the full range of Europe's languages (especially the lesser-resourced languages) is crucial. While the revised recommendation should certainly prioritise 3D technologies, it is essential not to lose sight of the ongoing efforts needed to provide full-text access to Europe's documentary heritage. Finally, we also need to remain critical of equating virtual and physical manifestations and experiences of cultural heritage by retaining the careful balance of being 'digitally-enabled and analogue-aware'.

Advanced Technologies: Artificial Intelligence

The European Commission's White Paper on Artificial Intelligence (February 2020) outlines the importance of a robust European strategy for the development and deployment of AI. However, it fails to recognise the significant innovation potential of AI for Europe's digital cultural heritage. The richness, diversity and multilingual nature of Europe's cultural heritage data is an ideal application area for both the advancement of AI as a research domain, and the acceleration of advanced access to Europe's digital cultural heritage. Major initiatives outside Europe, such as the Library of Congress' Computing Cultural Heritage in the Cloud Project and the UK's Living with Machines project are already demonstrating the potential of such advanced technologies for the Cultural Heritage Domain. For Europe to retain a leading position in the area of AI, the revision of REC 2011/711/EU needs a strong emphasis on cultural heritage as a key application area for the advancement of AI .

Digital Transformation through Capacity Building

The COVID-19 pandemic has clearly demonstrated how essential digital skills are. Within DARIAH, we recognise that our high-level of digital literacy has been a valuable asset for the resilience of our research community. Indeed, the COVID-19 pandemic has emphasised the importance of enabling the cultural heritage sector to seize the opportunities provided by digital technologies, for example, with CHIs extending the remote access to in-copyright digitised materials or developing digitisation-on-demand programmes. However, this digital transformation requires a sustained effort, which should not be underestimated. The revision of REC 2011/711/EU should highlight the need for continued investments in capacity building in the area of digital skills for CHIs, ideally in collaboration with the training and education offerings of digital humanities initiatives such as DARIAH-Campus .

  Any follow-up to REC 2011/711/EU should recognise the importance of research communities for the future development of Europeana and encourage funding opportunities for shared activities with research infrastructures such as DARIAH, CLARIN etc. It should also, in agreement with the 2018 Evaluation of Europeana (COM/2018/612 final), highlight the urgency of continuously improving the quality of aggregated metadata and addressing the lack of actual content in Europeana as a major drawback for both researchers and the general public. The revised REC 2011/711/EU should emphasise both of these aspects with an understanding that metadata, while being crucial for identifying resources, should never be perceived as the ultimate horizon of digitisation. Users need 'actual content' and Europe should lead the way in providing easy access to it.

  ") We recognise the clearing of sharing rights and licensing best practices as an absolute priority to fulfil the promises of REC 2011/711/EU . Institutions should clearly indicate in which cases they serve as the right holders and should follow the principle ' open by default, closed when necessary ' in their licensing policies. In addition, the revised recommendation should take into account the impact on digital cultural heritage of the General Data Protection Regulation (GDPR) which came into force following the publication of REC 2011/711/EU. Several European organisations and consortia (APEF, CLARIN, Europeana, E-RIHS, IPERION) involved in the cultural heritage domain are already working with DARIAH on a Heritage Data Reuse Charter to establish principles and mechanisms for improving the use and re-use of cultural heritage resources by researchers. The follow-up to REC 2011/711/EU should support the creation of a common environment based on the core principles of Reciprocity, Interoperability, Citability, Openness, Stewardship and

https://ec.europa.eu/digital-single-market/en/news/eu-member-states-sign-cooperate-digitising-cultural-heritage

See e.g. https://reviews.history.ac.uk/review/1894 and http://www.europeana-newspapers.eu/category/interviews-with-researchers/


St Md

Jahid Hasan
email: jahid.hasan@fresnel.fr

Mohammad Ali Khalighi
email: ali.khalighi@fresnel.fr

Experimental Implementation of Optical-CDMA for Medical Extra-WBAN Links

Keywords: Body area networks, infrared link, implementation of WBAN, optical wireless communications, optical codedivision multiple access, WBAN prototypes

We consider the use of optical wireless technology based on infrared signals for multi-user wireless body-area networks (WBANs) in medical applications. For the case of uplink extra-WBAN signal transmission, we propose a simple optical code-division multiple access (O-CDMA) signaling scheme to manage the multiple-access channel. We experimentally evaluate the link performance in terms of bit-and frame-error-rates by implementing the proposed scheme on the programmable logic, and show the efficiency of this approach in dealing with multi-user interference, as well as its low-complexity.

I. INTRODUCTION

The recent development of integrated wearable health monitoring sensors and the internet have already enabled e-health solutions to monitor patients from remote locations [START_REF] Wu | An autonomous wireless body area network implementation towards IoT connected healthcare applications[END_REF]. These health sensors communicate through the use of wireless bodyarea networks (WBANs), composed of intra-and extra-WBAN links. Intra-WBAN links allow transmitting data from different sensors to a coordinator node (CN), placed on the patient's body [START_REF] Haddad | Channel characterization and modeling for optical wireless body-area networks[END_REF]. Then, the CN sends the collected data to an access point (AP), via an extra-WBAN link [START_REF] Haddad | Enabling communication technologies for medical wireless body-area networks[END_REF]- [START_REF] Movassaghi | Wireless body area networks: A survey[END_REF].

Existing WBAN solutions based on radio-frequency (RF) technology suffer from electromagnetic interference (EMI), due to using the unlicensed band. Therefore, optical wireless communications (OWCs) using visible-light or eye-safe infrared (IR) have become an interesting solution, in particular, thanks to their inherent security features and the license-free noncongested optical spectrum [START_REF] Ghassemlooy | Visible Light Communications: Theory and Applications[END_REF].

In practical use cases of medical WBANs, several patients may need to transmit their vital signs simultaneously through a shared network, which can be enabled by employing multiple access (MA) schemes. The optical code-division MA (O-CDMA) technique was proposed for medical WBANs in [START_REF] Barbot | Performance of a mobile wireless optical CDMA monitoring system[END_REF], [START_REF] Chevalier | Wireless optical technology based body area network for health monitoring application[END_REF], due to its robustness, low implementation complexity, low data-rate requirement (less than 100 Kbps), and enhanced security features. Unlike time-division MA, O-CDMA allows asynchronous data transmission by several users by assigning an optical orthogonal code (OOC) to each user. Note that frequency-division-based MA techniques suffer from implementation complexity and relatively low energy efficiency. This is crucial for extra-WBANs as the CN requires a relatively long battery lifetime (typically one week [START_REF] Patel | Applications, challenges, and prospective in emerging body area networking technologies[END_REF]).

O-CDMA was first introduced for use in optical fiber communications in [START_REF] Salehi | Code division multiple-access techniques in optical fiber networks -part I: Fundamental principles[END_REF]- [START_REF] Shalaby | Chip-level detection in optical code division multiple access[END_REF]. There have been also several theoretical studies to investigate the performance of O-CDMA for OWC in indoor environments [START_REF] Zahedi | A photon counting approach to the performance analysis of indoors wireless infrared CDMA networks[END_REF]- [START_REF] Aminzadeh-Gohari | Analysis of power control for indoor wireless infrared CDMA communication[END_REF]. Different receiver (Rx) structure of O-CDMA for wireless local area networks was studied from a digital design perspective in [START_REF] Ghaffari | Wireless optical CDMA LAN: digital design concepts[END_REF], and experimentally investigated in [START_REF] Ghaffari | Wireless optical CDMA LAN: digital implementation analysis[END_REF]. Also, in the WBAN context, the effect of patients' mobility on the performance of O-CDMA inside a hospital room was analyzed through simulations for intra-and extra-WBANs [START_REF] Barbot | Performance of a mobile wireless optical CDMA monitoring system[END_REF], [START_REF] Chevalier | Wireless optical technology based body area network for health monitoring application[END_REF].

To the best of our knowledge, the practical design and implementation of O-CDMA for medical WBANs have not been explored in detail so far. Indeed, O-CDMA implementation can be challenging, as its performance depends on the quality of the algorithms developed for synchronization between a transmitter (Tx) and a Rx [START_REF] Yang | Performance analysis for synchronization and system on CDMA optical fiber networks[END_REF]. The performance may also be severely impaired by MA interference (MAI) due to the well-known near-far problem in asynchronous mode [START_REF] Cameron | Performance analysis of CDMA with imperfect power control[END_REF], [START_REF] Barry | Digital Communications[END_REF].

This paper investigates the implementation of an O-CDMA system for extra-WBAN in medical applications based on IR signals. Since extra-WBAN links are mainly used for transmitting data from a CN placed on the patient body to an AP, we focus here on the uplink. First, we compare different possible O-CDMA Rx architecture to select the most suitable one in our application, given the requirements of low implementation complexity, and the constraints imposed by near-far problem and the MAI effect. Then, we implement the selected O-CDMA Rx structure on a field-programmable gate array (FPGA) and propose an analog front-end (AFE) for the transmission and reception of the optical signals. Finally, we evaluate the performance of the implemented system in managing the MAI in terms of bit-error-rate (BER) and frame-error-rate (FER). We show that our Rx structure can operate with a BER lower than 10 -3 without any error correcting algorithm up to 20 cm for the average 16 mW transmit power.

The remainder of the paper is organized as follows. In Section II, we describe O-CDMA based systems, including system architecture and digital design concepts. Next, Section III presents the implementation details of the FPGA prototypes. Experimental results are presented in Section IV. Lastly, the main conclusions of the work are drawn in Section V.

II. SYSTEM DESCRIPTION

A. General Assumption

We consider the case of four patients in a hospital ward, sending their health monitoring data via their respective CN to a single AP placed in the center of the ceiling. Each CN, placed on the patient's shoulder to minimize the probability of line-of-sight (LOS) blocking, transmits its OOC-coded data asynchronously to the AP. In other words, transmission takes place without any synchronization between different CNs.

We assume that the data-rate requirement of each CN is less than 100 Kbps [START_REF] Haddad | Enabling communication technologies for medical wireless body-area networks[END_REF], [START_REF] Movassaghi | Wireless body area networks: A survey[END_REF]. Given this relatively low data-rate, we use the simple on-off-keying (OOK) modulation. Also, we assume that there is always a LOS link between the Tx and the Rx, and hence, neglect the effect of multiple reflections.

B. General Principles of OOC-Based O-CDMA

In O-CDMA signaling scheme, at the Tx side, each user (i.e., CN) is at first assigned a signature code (SC) such as an OOC. Each bit from the user data stream is multiplied by this OOC and transmitted through the optical channel. The signals transmitted by all users are received on the Rx's PD. To recover the data of a specific user, the received signals are at first correlated with the corresponding assigned OOC. The output of each correlator is then compared with a predefined threshold C Th . The resulting signal is then demodulated to extract the transmitted bits of each user.

An OOC is a sequence of binary values called chips, denoted by OOC (L, W, ζ a , ζ b ), where L is the OOC length, W is the number of ones in the sequence called weight, and ζ a and ζ b stand for the auto-and cross-correlation constraints [START_REF] Chung | Optical orthogonal codes: Design, analysis and applications[END_REF]. By definition, strict orthogonality between such binary sequences can be achieved when ζ a = ζ b = 0. However, our signaling here is based on intensity modulation and direct detection that relies on real and positive signals. Therefore, OOCs satisfy instead ζ a = ζ b = 1, which provides the best trade-off between ζ a and ζ b . For OOC (L, W, 1, 1) codes, the maximum number of users N is upper bounded by [START_REF] Chung | Optical orthogonal codes: Design, analysis and applications[END_REF]:

N ≤ L -1 W (W -1)
.

In an O-CDMA system with OOC codes, error-free data recovery is possible despite MAI, if C Th and W satisfy the following conditions [START_REF] Salehi | Code division multiple-access techniques in optical fiber networks -part I: Fundamental principles[END_REF], [START_REF] Chung | Optical orthogonal codes: Design, analysis and applications[END_REF]:

C Th > N -1 and W > N -1. (2)

C. O-CDMA Rx Structure

O-CDMA's performance highly depends on the type of Rx structure used to detect and retrieve data. The three most viable Rx structures for OWCs are correlator, chip-level detector (CLD), and correlator with a hard-limiter (CHL) [START_REF] Ghaffari | Wireless optical CDMA LAN: digital design concepts[END_REF].

A correlator or correlation Rx is the simplest Rx structure, see Fig. 1(a). It consists of a low pass filter and an analog-todigital (A/D) converter with a sampling rate at least equal to the chip time. The sampled chips are correlated with the desired user's OOC code, and the output is then compared with an optimal threshold C Th to detect a bit '0' or '1'. Correlation Rx suffers from the MAI and the near-far problem [START_REF] Ghaffari | Wireless optical CDMA LAN: digital design concepts[END_REF].

The CLD is based on the power of each chip collected by the PD, which is compared to an optimal threshold [START_REF] Shalaby | Chip-level detection in optical code division multiple access[END_REF]. It also consists of a filter and an A/D converter as shown in Fig. 1(b). Each arm in the figure is used to detect one pulsed (i.e., ON) chip. Finally, all the values from each arm are added and compared with the weight W to detect the transmit bits. The performance of the CLD is better than that of the correlation Rx [START_REF] Shalaby | Chip-level detection in optical code division multiple access[END_REF]. However, it also suffers from near-far problem [START_REF] Shalaby | Chip-level detection in optical code division multiple access[END_REF].

In a CHL Rx, a hard-limiter (HL) is added before the correlator [START_REF] Ghaffari | Wireless optical CDMA LAN: digital design concepts[END_REF], which clips the received signals at a defined level of threshold voltage, and hence converts the signals into binary sequences. Sampling takes place right after the HL with a locally generated clock with frequency at least equal to the chip rate, see Fig. 1(c). Therefore, the sampler of a CHL Rx does not require an A/D and can be directly implemented on an FPGA, which significantly reduces its implementation complexity. The main interest of CHL is that it minimizes the MAI effect by restricting the voltage at the same level for all users [START_REF] Salehi | Code division multiple-access techniques in optical fiber networks -part II: Systems performance analysis[END_REF], [START_REF] Mashhadi | Code-division multiple-access techniques in optical fiber networks -part III: Optical AND logic gate receiver structure with generalized optical orthogonal codes[END_REF]. Furthermore, the near-far problem is reduced because the defined threshold voltage ensures reception of balanced signals from near and far users. Here, due to its lower implementation complexity and its resilience to MAI and the near-far problem, we consider to use CHL.

D. Digital Design Concept on Tx/Rx Synchronization

As stated in Section II-A, CNs transmit their OOC-coded data to the AP without any synchronization between them. However, at the AP side, it is crucial to have bit-level synchronization between the corresponding Tx-Rx pairs in order to recover the data of each user properly. This bit-level syn- chronization relies on mechanisms implemented on both the Tx and the Rx sides as explained in the following.

1) Tx: On the Tx side, training patterns are added before transmitting OOC-coded information bits, enabling the Rx to synchronize its code with the corresponding Tx, provided that the Rx has full knowledge of the training pattern sent. The length of the training pattern depends on the type of synchronization algorithm used by the Rx [START_REF] Ghaffari | Wireless optical CDMA LAN: digital design concepts[END_REF].

2) Rx: The synchronization process in an O-CDMA Rx is realized by an acquisition and tracking (AT) circuit, as in RFbased CDMA Rxs [START_REF] Proakis | Digital Communications[END_REF], [START_REF] Keshavarzian | Optical orthogonal code acquisition in fiber-optic CDMA systems via the simple serial-search method[END_REF]. Initial phase synchronization is done at the acquisition stage with a precision better than half of the chip duration T c . Three kinds of acquisition algorithms are proposed for use in OWC, namely simple serial search, multiple shift, and matched filter [START_REF] Ghaffari | Wireless optical CDMA LAN: digital design concepts[END_REF]. The tracking circuit is the second step of the synchronization process. It is used for fine tuning, based on the generation of half-chip early and late replicas of the data. Correlation is then done with each replica and compared with a threshold C Th to ensure synchronization.

In practice, it is shown that the matched filter algorithm is preferable to the other approaches as it requires a smaller number of training patterns and a shorter mean acquisition time [START_REF] Ghaffari | Wireless optical CDMA LAN: digital design concepts[END_REF]. Thus, we consider this solution in our design.

III. IMPLEMENTATION

A. Overall Architecture of System

As explained in Section II-A, our O-CDMA based extra-WBAN system consists of four patients, each with a CN attached on the shoulder, and an AP at the center of the room ceiling. The architectures of a CN and the AP are shown in Figs. 2(a) and 2(b), respectively. Both the CN and AP are composed of two main parts: the digital hardware (HW) for generation or recovery of the O-CDMA data stream, and the AFE for transmission or reception of the optical signals. In the subsequent subsections, we discuss the implementation details of the AFE and the digital HWs.

B. AFE Design

On the CN side, the role of the AFE is to transmit the OOC-coded data stream generated by the digital HW circuitry through the use of a light-emitting diode (LED). Since this data stream is a binary sequence, we use a simple LED driver based on a MOSFET switch, see Fig. 2(a).

On the AP side, as represented in Fig. 2(b), the AFE consists of a PD followed by a trans-impedance amplifier (TIA) to convert the received photo-current into a voltage. The resulting signal is then passed through a first band-pass filter to filter out noises generated by the ambient light and the TIA (i.e., the thermal noise), followed by a voltage amplifier and a second band-pass filter to center the signal around 0 V. Finally, we employ a comparator as a zero-crossing detector, rather than a conventional HL, in order to boost the positive part of the signal to a fixed level (here, to 3.2 V) while setting the negative part of the signal to 0 V. This way, the AFE output signal meets the input/output standard LVCMOS 3.2 V of our FPGA board.

In practice, both the transmission and reception AFEs are implemented on a single board, as represented on Fig. 3. The LED operates at 940 nm with a semi-angle at half power of 60 • . The Tx 3 dB modulation bandwidth (BW) is 9 MHz. On the Rx side, the 3 dB BW is 15 MHz and the overall Rx gain considering TIA and amplifier is 62 dB. The PD has a 26.4 mm 2 active surface, a 0.6 A/W sensitivity at 940 nm and a full angle at half-sensitivity of 120 • .

C. CN Digital HW Design

In the CN digital HW, the serial data stream is encoded with the corresponding OOC code. Here, without loss of generality, we consider L = 8 data bits per transmitted packet in addition to L p = 3 training bits (which are set to one) as preamble for synchronization purpose. The corresponding block diagram is shown in Fig. 4.

First, the serial data stream is passed through a serial-toparallel shift register ("SIPO" block in Fig. 4) in order to form the payload of the O-CDMA data frame. The SIPO block is followed by a dual clock first-input first-output (FIFO) memory block. Note that in digital system design, FIFOs are used to transfer data between two different clock domains [START_REF] Apperson | A scalable dual-clock FIFO for data transfers between arbitrary and haltable clock domains[END_REF]. Here, the FIFO is used to transfer data from the local clock domain (i.e., clock for sensors data) to the chip clock domain.

Each data bit then has to be multiplied by the OOC. By considering a maximum of four users, we deduce from Equations ( 1) and ( 2) that the OOC length has to be at least F = 49. Since the input data rate is assumed to be 100 Kbps, see Section II, the chip clock rate at which the OOC-coded data have to be sent is set to 5 MHz. In practice, the multiplication with the SC is done by an AND logic gate. Therefore, each bit of the payload is first duplicated F times by the "Reshape" block and then passed through an AND gate to apply the OOC. Note that the training bits are also OOC-coded to avoid MAI from other users (see Fig. 4). These L p sequences are added to the beginning of the frame by the aid of a multiplexer (the "MUX" block), controlled by the "Control Sequence" block. Finally, a parallel-to-serial shift register ("PISO" block in Fig. 4) converts the OOC-coded data frames to serial spread which are then transferred to the Tx AFE.

D. AP Digital HW Design

The architecture of the AP digital HW for detecting the data of a given user is represented in Fig. 5. The same architecture is used for every user. As it can be seen, the data are first sampled ("Sampler" block in Fig. 5). The sampling process is done in the digital domain with a locally generated sampling clock CK(t) of frequency 5 MHz. From a digital design point of view, synchronization failure is very common when sampling an asynchronous input with respect to a clock, that is when the asynchronous input (i.e., spread data) changes during the set up time of the clock [START_REF] Chaney | Anomalous behavior of synchronizer and arbiter circuits[END_REF]. To deal with this problem, we used three cascaded flip-flops (FFs) inside the sampler.

After sampling, at first, acquisition is performed in the "AV" block. This latter searches the training patterns by matched filter algorithm, as described in [START_REF] Ghaffari | Wireless optical CDMA LAN: digital design concepts[END_REF], and obtains initial time reference of the received spread data with a precision better than half of the chip duration T c . Then the received spread data is sent to the verification stage in the "AV" block to detect false alarms (due to MAI and/or noise) by comparing with a predefined threshold. If a false alarm is detected, then the "AV" block starts performing acquisition again. Otherwise, the Acquisition Performed flag is set to high (see Fig. 5). This latter, in fact, activates acquisition or tracking process of the de-multiplexer ("DEMUX" block).

Afterwards, the tracking circuit starts more precise synchronization using the so-called early-late technique. To do this, the "ST" block generates early and late versions of the data, sampled by the shifted clocks CK(t-T c /2) and CK(t+T c /2), respectively. These replicas of the data are correlated with the OOC code, and the outputs of the correlators are then compared with the main data (i.e. the data that is sampled with the clock CK(t)) to obtain tracked spreading data. Meanwhile, if the outputs of the correlators are less than the threshold C Th , a synchronization error signal is sent to the "AV" block to start a new acquisition.

IV. EXPERIMENTAL RESULTS

To implement our O-CDMA digital HW, we consider the case of two CNs (Txs), transmitting their data simultaneously to a single AP (Rx). We have assigned the codewords {0, 1, 4, 14} and {0, 2, 7, 26} to the CNs, where each number represents the position of '1' in the sequence of the OOC (49, 4, 1, 1) code. We then implemented the full digital HWs described in Section III, on Xilinx ® Zynq series 7z020 FPGAs. We have summarized the design synthesis results from the Xilinx ® Vivado design suite in Table I. As expected, the Tx remains quite simple to implement, with an estimated on-chip power consumption of 0.752 W. Note that when implemented on an ASIC (Application-Specific Integrated Circuit), the power consumption can be considerably reduced [START_REF] Kuon | Measuring the gap between FPGAs and ASICs[END_REF].

We have carried out the experimental validation of the proposed O-CDMA system in two steps. First, we verified that the digital HW algorithms were functional, which is described in Subsection IV-A. Then, we experimentally analyzed the performance of the whole system, as explained in Subsection IV-B.

A. Validation of Digital Design

A design implemented on an FPGA is usually validated functionally by performing a post-implementation simulation on the EDA (Electronic Design Automation) simulator. This, however, does not guarantee the behavior of the design in the real HW. Therefore, a built-in IP (intellectual property) core called integrated logic analyzer (ILA) is used to analyze and debug the system in real time after HW implementation.

Here, both Tx digital HWs are implemented on the same FPGA board, and the Rx digital HW is implemented on another FPGA board. In order to transmit OOC-coded data streams from both Txs with a single FPGA, we simply generated the two streams independently and then added them with an OR gate. This way, on the CN side, the FPGA had a single data output containing the signals from both users. This output was connected with a wire to the input of the second FPGA embedding the AP digital HW. Consequently, the transmission channel between the CN and the AP could be considered as very low noise and thus not being the source of performance degradation. This allowed us to evaluate the performance of the digital HW algorithms.

The real-time timing diagrams in Fig. 6 are captured from FPGA's ILA through a JTAG cable. We transferred known pattern of frames from Tx1 (i.e., 33 16 and cc 16 ) and Tx2 (i.e., 0f 16 and 3c 16 ) repeatedly to compare the recovered data after In Fig. 6(a), the transmitted data sequences from Tx1 and Tx2 are denoted by sp out tx1 and sp out tx2, respectively. These spread data generated from Txs are passed through the OR gate and transferred by the port sp out comb.

The data received by the AP, denoted by spc in, are sampled by a locally generated clock, as seen in Fig. 6(b). The frames recovered by the Rx are defined as dout rec1 and dout rec2, where frames are valid if and only if dout valid 1 and dout valid 2 are set to high level for Tx1 and Tx2, respectively. We observed that under the considered "ideal" conditions (i.e., without any noise) the transmitted data bits were recovered from both users without any MAI error.

B. Validation of O-CDMA System

In order to validate the whole O-CDMA system, we replaced the OR gate and wired channel by the AFEs. Figure 7 shows the experimental setup, where the main parameters are given in Table II. Both Tx AFEs are placed at a distance of 20 cm from each other and driven by the same FPGA. The measured average transmit power of the IR LED is about 16 mW, that is well bellow the eye-safety requirement [START_REF] Kahn | Wireless infrared communications[END_REF]. Note that, we have W = 4 ON pulse for the SC length L = 49.

First, we verified the behavior of the O-CDMA system at short distance. Figure 8 shows the frames recovered at the Rx when it is placed at a distance of 20 cm from the Txs. As in Fig. 6(a), we can see that the data being sent by both Txs are recovered completely. Then, we varied the distance Z between the Txs and the Rx from 20 cm to 1 m and processed the data captured from the ILA with MATLAB ® in order to extract the BER and FER with respect to the distance, as shown in Fig. 9. We notice that, logically, as the distance Z increases, the BER increases due to decreased received power. Remember from Fig. 2(b) that the comparator acts as a zero-crossing detector, which outputs a 3.2 V level if the signal is positive, and 0 V otherwise. Therefore, when the signal is weak and noisy, there might be unexpected zero-crossings leading to an unstable output signal and, eventually, to bit errors.

Moreover, we see from Fig. 9 that the performances of the two Txs are not exactly the same. This is because in our set-up Txs were not exactly at the same distance from the Rx and did not have exactly the same angle of emission with respect to the Rx. This could be considered as a slight form of the near-far problem, which is not mitigated here, as expected for a CHL Rx since we use a comparator instead of a true HL. Indeed, the comparator output is sensitive to the received power level.

V. CONCLUSIONS

We proposed digital HW architectures for the CN and the AP for the case of extra-WBAN uplinks. The O-CDMA technique was considered to address the multi-user feature of such applications. We also considered AFE design mainly based on CHL while accounting for ease of integration of the AFE part with the digital HW circuitry. Moreover, we implemented the low-complexity digital HW part on the programmable logic. For the case of two users (i.e., two Txs), firstly, we verified the error-free signal reception on a high signal-to-noise ratio (SNR) system (i.e., using a wired link), and then studied the performance of the system with wireless links in terms of BER and FER for different SNR values (i.e., varied link distance). However, the performance of the system can be improved using error-correcting codes. The future direction of this work would be to investigate further the near-far problem and evaluate the robustness of the proposed Rx architecture.

Fig. 1 :

 1 Fig. 1: Different types of Rx structures (a) Correlation Rx: Based on a simple correlator, (b) CLD: The correlation is done based on the received power of each chip, (c) CHL: An HL is added before a simple correlator.

Fig. 2 :

 2 Fig. 2: Overall architectures of (a) a CN (Tx) and (b) the AP (Rx) with a focus on the main AFE blocks.

Fig. 3 :

 3 Fig. 3: The off-the-shelf AFE board for IR optical transmission and reception.

Fig. 4 :

 4 Fig. 4: Conceptual block diagram of CN digital HW based on O-CDMA for extra-WBAN links. SIPO, PISO, and MUX stand for serialin parallel-out, parallel-in serial-out shift register, and multiplexer, respectively.

Fig. 5 :

 5 Fig. 5: Conceptual block diagram of AP digital HW based on O-CDMA corresponding to one user. Here, AV stands for acquisition and verification. The "ST" block generates early and late replicas of the spread data.

Fig. 6 :

 6 Fig. 6: Txs and Rx timing diagrams captured through ILA for the case of wired transmission link: (a) Txs send spread data through an OR logic by the port sp out comb, (b) transmitted data of both Txs are recovered by the Rx.

Fig. 7 :

 7 Fig. 7: Experimental setup. Here, Txs are placed at a distance of 1 m from the Rx.

Fig. 8 :

 8 Fig. 8: Recovered data from the two Txs which are positioned at a distance of 20 cm from the Rx. The data were read through the built-in ILA of FPGA via a JTAG port.

Fig. 9 :

 9 Fig. 9: Measured BER and FER from experiments, where distance Z between the Txs and the Rx was varied from 20 cm to 1 m.

TABLE I :

 I Logic Resources Required for at CN. LUT and and BUFGCTRL stand for look-up table and global clock buffer.

	Logic	Number of Resources
	LUTs	95
	LUTs as Memory	22
	LUTs as Logic	73
	FFs	255
	BUFGCTRL	2

TABLE II :

 II Parameters Used in the Experiment.

	Parameter	Value
	OOC code	(49,4,1,1)
	Number of users	2
	Threshold	4
	CN1 OOC code	{0,1,4,14}
	CN2 OOC code	{0,2,7,26}
	Tx 3 dB modulation BW	9 MHz,
	Rx 3 dB reception BW	15 MHz,
	O-CDMA required BW	5 MHz
	Rx maximum gain	62 dB
	PD active area	26.4 mm 2
	PD responsivity	0.6 A/W
	Rx full angle at half-sensitivity 120 •
	LED wavelength	940 nm
	LED average transmit power	16 mW
	LED semiangle at half power	60 •

ACKNOWLEDGMENT

This work has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 764461 (VisIoN).


Oussama Haddad
email: oussama.haddad@fresnel.fr

M Ali Khalighi
email: ali.khalighi@fresnel.fr

Stanislav Zvanovec
email: xzvanove@fel.cvut.cz

Performance Analysis of Optical Intra-WBAN Links

Keywords: Wireless body-area networks, Optical wireless communications, Link performance, Outage probability

We evaluate the performance of wireless body area networks (WBANs) in medical applications based on optical wireless transmission. We focus on the intra-WBAN links, where a set of medical sensors transmit the measured data to a coordinator node, placed on the patient's body. Based on a Monte Carlo raytracing approach and considering the effects of body shadowing and mobility, we propose statistical models for the channel DC gain based on kernel density estimation. Then, we investigate the outage probability performance of the different intra-WBAN links and discuss the impact of user mobility and node placement on the patient's body.

I. INTRODUCTION

Wireless body area networks (WBANs) are short-range networks, designed to connect wearables in the vicinity of the human body. The use of WBANs has been motivated by technological advances made in many fields of engineering, such as sensor miniaturization, and the development of low power wireless communication technologies. A WBAN consists of a set of sensor nodes (SN), sending their collected data to a coordinator node (CN), placed often on the users body. The CN then forwards these data to an access point (AP) [START_REF] Movassaghi | Wireless body area networks: A survey[END_REF]. Typical applications of these networks include medical environments where they can be used for vital signs monitoring and therapeutic stimulations to improve the quality of life of patients and reduce the treatment costs [START_REF] Botsis | Current status and future perspectives in telecare for elderly people suffering from chronic diseases[END_REF].

Current WBAN solutions use generally unlicensed frequency bands in the radio-frequency (RF) domain. However, due to the considerations of electromagnetic interference, data security, and concerns on the impact of RF waves on the human tissues, optical wireless communications have recently received increasing interest, as they can offer an alternative/complementary solution within this context [START_REF] Ghassemlooy | Visible Light Communications: Theory and Applications[END_REF], [START_REF] Haddad | Enabling communication technologies for medical wireless body-area networks[END_REF].

An important point in studying the performance of opticalbased WBANs is to accurately model the user mobility. In fact, the changes in the geometry between the SN and the CN, may cause occasional beam shadowing. A few studies have considered this such as [START_REF] Julien-Vergonjanne | Optical Wireless Communications: An Emerging Technology[END_REF], which studied optical intra-WBAN links, i.e., between SNs and the CN, based on Monte Carlo ray-tracing (MCRT) simulations. User mobility was considered based on uniformly distributed random user positions and orientations of the sources/detectors, while the patients body was modeled by a simple rectangular surface. Other works such as [START_REF] Hoang | Optical wireless network design for off-body-sensor based monitoring[END_REF]- [START_REF] Haddad | Channel characterization for optical extra-WBAN links considering local and global user mobility[END_REF] studied the case of extra-WBAN links, i.e., between the CN and an AP.

This paper aims to study the performances of the intra-WBAN links. In contrast to the previous works, we take into consideration both local and global movements of the body, where we model the body parts movement by using a 3D animation of a walk cycle and random trajectories based on a modified random waypoint (RWP) mobility model. We consider two different placements at the shoulder and the hip. Based on channel models obtained through MCRT simulations, we provide statistical channel models using the kernel density estimation (KDE) method. We then evaluate the link performance for a simple on-off keying (OOK) modulation scheme based on the outage probability criterion, given the random time-varying nature of the channel.

The remainder of the paper is organized as follows. In Section II, we present the main assumptions, the considered scenario, and the simulation methodology. Next, we describe in Section III statistical channel modeling and the performance analysis approach. Numerical results including the outage probability for different ambient lighting conditions are presented in Section IV. Finally, Section V concludes the paper.

II. ASSUMPTIONS AND SIMULATION METHODOLOGY

Let us consider the scenario of a patient wearing an intra-WBAN and walking inside a hospital room. We consider the network to have 10 SNs connected to the CN in a star network topology. As it was defined in Section I, both SNs and CN are supposed to be placed on the body. To study the effects of body parts on the link performance, we model the patients body by a detailed 3D mesh that gives an accurate representation of the body parts. We distribute the SNs and CNs over the body as it is shown in Fig. 1. The considered SN positions cover the most likely placements for potential medical applications. Also, we study the effect of CN location by placing it on the shoulder or on the hip, as suggested by [START_REF] Julien-Vergonjanne | Optical Wireless Communications: An Emerging Technology[END_REF], [START_REF] Hoang | Optical wireless network design for off-body-sensor based monitoring[END_REF].

We consider a hospital room of dimensions (5 × 5 × 3) m 3 , which is, for the sake of simplicity, considered here as empty and without furniture. The room has plaster walls and a pinewood floor, where their reflectivity values are given in Table I [START_REF] Miramirkhani | Channel modeling and characterization for visible light communications[END_REF]. To model the patient mobility, we use a 3D animation of a walk cycle to model the relative movement of body parts using Blender [START_REF]Blender[END_REF]. In addition, we generate a random trajectory based on a modified RWP model, where a cubic spline interpolation across the waypoints is performed to get smooth rotations on the user trajectory. By combining these two models, we obtain a set of mobility configurations for our performance analysis. We then use the MCRT-based Opticstudio software of Zemax [START_REF]Opticstudio[END_REF] to get the channel impulse response (CIR) h(t) for each mobility configuration. To ensure user comfort, we choose to work in the infrared (IR) spectrum. We place at each SN an IR LED of a central wavelength λ 0 = 850 nm and ∆λ = 30 nm. The radiation pattern is considered as Lambertian with order m = 1, corresponding to a half angle of 60 • [START_REF] Barry | Simulation of multipath impulse response for indoor wireless optical channels[END_REF]. At the CN, we consider two types of photo-detectors (PD), i.e., PIN and APD, both considered to have an active area of 1 cm 2 . Such a large active area is justified by the typical low data rates (< 1 Mbps) used in medical WBANs [START_REF] Movassaghi | Wireless body area networks: A survey[END_REF]. To suppress the visible ambient light, we consider a long-pass absorption filter of 800 nm cut-on frequency. The receiver (Rx) field-of-view (FOV) is considered as 60 • . Table I summarizes the parameters used in our simulations. Lastly, to quantify the link performance, we will consider the outage probability P out , which is defined as the probability that the biterror-rate (BER) for a given channel realization is higher than a given threshold BER th .

III. PERFORMANCE ANALYSIS

A. Statistical Channel Modeling

We focus here on the channel DC gain H 0 , defined as H 0 = h(t) dt. We generate a random trajectory based on the modified RWP and calculate the corresponding H 0 for each point. This results in N = 516 realizations of H 0 for each SN and CN link, based on which we propose a statistical model for H 0 based on non-parametric KDE method. This avoids us to do time-consuming MCRT simulations for calculating P out . For a given set of N realizations x j of a random variable x with a probability density function (PDF) f X (x), the KDE estimate of the PDF is given by [START_REF] Epanechnikov | Non-parametric estimation of a multivariate probability density[END_REF]: where K is the kernel, considered here to be Gaussian, and δ is the smoothing parameter, calculated from δ = 1.06 σ N -1/5 , where σ is the estimated standard deviation of log(x) [START_REF] Silverman | Density Estimation for Statistics and Data Analysis[END_REF].

fX (x) = 1 N δ N j=1 K x -x j δ , (1)
For instance, we have presented in Figs. 2(a) and 2(b) the histograms of the simulated H 0 together with the fitted PDF based on KDE, for the links between SN3, and CN1 and CN2, respectively. The corresponding δ is 0.06 and 0.11, for the cases of CN1 and CN2, respectively. We notice a good fit between the histograms and the fitted PDF. Meanwhile, we note that the histogram for the case of CN2 is more spread compared to CN1, which is due to shadowing caused by the swing of the right arm during the walk cycle. Also, the mean H 0 is higher for the case of CN2 because the link from SN3 to CN2 experiences firstorder reflections, whereas the link to CN1 involves more often second-order reflections.

B. Link Performance Analysis

Using the statistical models obtained in the previous subsection for every SN-CN link, we randomly generated 10 6 values for H 0 . Then, for each channel realization, we calculated the BER for an uncoded non-return to zero (NRZ) OOK-based link for given data-rate R b and transmit power P t . Assuming optimal maximum likelihood (ML) detection, we have [START_REF] Dabiri | FSO channel estimation for OOK modulation with APD receiver over atmospheric turbulence and pointing errors[END_REF]:

BER = 1 4 erfc γ th -αI 1 2(ασ 2 s,1 + σ 2 0 ) + 1 4 erfc I 1 -γ th 2(σ 2 s,1 + σ 2 0 ) , (2)
where erfc

(x) = 2 √ π ∞ x exp(-u 2
)du is the complementary error function, α = 0.2 is the LED extinction ratio, and γ th is the optimal ML threshold that minimizes the BER. Also, I 1 = RGP t H 0 is the received current for symbol '1', where R = 0.65 A/W, stands for the PD responsivity and G is the photo-detector gain (G = 1 for PIN and G = 50 for APD). the average photo-current generated by a PIN PD due to the background noise [START_REF] Gfeller | Wireless in-house data communication via diffuse infrared radiation[END_REF]. Lastly, σ 2 s,1 = 2eGF BI 1 in ( 2) is the variance of the signal shot noise. We neglect the dark current noise compared with background and thermal noises.

For the case of APD, γ th is calculated as follows [START_REF] Dabiri | FSO channel estimation for OOK modulation with APD receiver over atmospheric turbulence and pointing errors[END_REF]:

γ th,APD = - I 1 σ 2 0 σ 2 s,1 + I 2 1 σ 4 0 σ 4 s,1 + αI 2 1 σ 2 s1 + (1 + α)I 2 1 σ 2 0 σ 2 s1 + (σ 2 s,1 + σ 2 0 )(ασ 2 s,1 + σ 2 0 ) (α -1)σ 2 s,1 ln ασ 2 s,1 + σ 2 0 σ 2 s,1 + σ 2 0 0.5 . (3)
In the case of using a PIN photo-detector, given the negligible signal shot noise [START_REF] Xu | Impact of different noise sources on the performance of PIN-and APD-based FSO receivers[END_REF], we have

γ th,PIN = (α + 1)I 1 2 . (4)
IV. NUMERICAL RESULTS For the considered system, we present here numerical results to study the performance of the intra-WBAN links in terms of outage probability. For P out calculation, we consider BER th = 10 -3 . Note that given that P t is the transmit power for sending an "On" OOK symbol, the average transmit power equals (1 + α) P t /2, with α the extinction ratio.

A. Effect of PD Type and Data-Rate

Let us first compare the effect of link data-rate on the performances of PIN-and APD-based Rxs. For this, concerning the background noise, we set I b = 200µA [START_REF] Gfeller | Wireless in-house data communication via diffuse infrared radiation[END_REF]. We have shown in Fig. 3 plots of P out versus P t for R b ranging between 1 kbps and 50 Mbps. Here, for the sake of brevity, we show only the link from SN3 to CN1. We reasonably get a lower P out for increased P t , while a higher P out is obtained for increased R b due to increased thermal noise variance. As expected, APD provides a better performance due to its internal gain: for instance, for R b = 200 kbps and a target P out of 10 -4 , the required P t is about 41 and 29 mW for the cases of PIN-and APD-based Rxs, respectively.

B. Comparison of Intra-WBAN Links for Low Background Noise Level

Let us now compare the performances of the different links from SNs to CNs. We consider only the case of R b = 100 kbps for both the PIN-and the APD-based Rxs, for the sake of brevity. First, we consider the case of negligible background noise with I b = 2 µA (corresponding to background noise from fluorescent lights [START_REF] Moreira | Characterisation and modelling of artificial light interference in optical wireless communication systems[END_REF]), which could correspond to nighttime conditions. We have presented in Fig. 4 plots of P out versus P t for different SN links and for a PIN-based Rx. Note that the P out values for the APD case are too low for the considered R b and are not shown here. We notice that, generally, CN1 experiences a better link performance for SNs placed on the upper part of the body. For instance, to achieve a target P out = 10 -4 , the required P t for SN7 are 13.5 and 28 mW, for the cases of CN1 and CN2, respectively. Meanwhile, CN2 experiences a better performance for SNs placed on the lower part of the body. As another important result, we notice that, for the SNs located on the upper part of the body, the P out slopes in the case of CN2 are smaller compared to the case of CN1, which means that CN1 is less affected by intensity fluctuations, or in other words, by channel fading. The inverse holds for the SNs on the lower part of the body, e.g., SN6, SN8 and SN9. Based on these observations, we can deduce that the choice of CN placement is application dependent. For instance, for EEG applications, most of SNs will be placed on the head, where CN1 is a better choice. On the other hand, for ECG applications, SNs are placed on the heart, arms, and legs, where CN2 position should be preferred. Lastly, note that we can reduce the thermal noise by using a larger R L , which is feasible given the low data rates needed in our context.

Comparing the results for different SNs, we can deduce the effect of mobility and shadowing. For instance, we notice from P out plots of SN3 and SN5 (placed on the chest and left arm, respectively), that the former link achieves a better performance. We observe the same trend between SN2 and SN4 links. As expected, local mobility results in a degradation of the link performance. Also, from P out plots in Fig. 4 SN10, we notice a better performance for the former for the case of CN1, whereas SN10 achieves a better performance for the case of CN2 from Fig. 4(b). We can explain this by the higher probability of shadowing for CN2, due to the swing of the right arm during the walk.

C. Comparison of Intra-WBAN Links for Relatively High Background Noise Level

Consider now the case of relatively high background noise level with I b = 200 µA, that can be considered as corresponding to daytime conditions [START_REF] Gfeller | Wireless in-house data communication via diffuse infrared radiation[END_REF]. Plots of P out versus P t for this case are shown in Fig. 5. We notice the same trends between SN-CN links as in the previous subsection with a degradation of the link performances with increase in the background noise level, as expected. For instance, for the case of the PIN-based Rx, the required P t to achieve the same performances as in the previous case increases for all SNs by approximately 2 to 5 mW. Considering the link form SN5 to CN1, for a target P out of 10 -4 , the required P t is 43 mW from Fig. 5(a), while it is 38 mW from Fig. 4(a). Notice that the performance degradation is more significant in the case of the APD-based Rx. This was expected due to the internal gain of the APD.

Comparing PIN-and APD-based Rxs, we observe the same trend between the different SN links, with the APD-based Rx still having a better performance (i.e., requiring a lower P t to achieve a target P out ). For instance, for the link between SN10 and CN2, the required P t to achieve P out = 10 -4 is 54 mW for the case of PIN-based Rx from Fig. 5(b), while it is only 37.5 mW when using an APD-based Rx from Fig. 5(d).

V. CONCLUSIONS

In this work, we studied the performance of optical intra-WBAN links for medical applications. Taking into account the effects of patient's local and global mobility in a room, we proposed accurate statistical channel modeling based on nonparametric KDE. Based on this, we investigated the outage probability performance of different SN-CN links for the two cases of PIN-and APD-based Rxs. We showed the impact on the link reliability of CN positioning, which is related to the effect of link shadowing caused by the body parts movements. We showed that link shadowing resulting from local mobility can cause a significant degradation of the link performance. Therefore, the optimal placement of the CN should depend on the targeted application. We further studied the impact of the background noise depending on the PD type on the required transmit power to ensure a required outrage probability. Overall, given the generally low levels of required P t , which remain in the eye-safe region, we can conclude the feasibility of implementing optical WBAN links in practice, in both daytime and nighttime conditions.

Fig. 1 .

 1 Fig. 1. Considered placements of the CNs and SNs.

Furthermore, σ 2 0 = σ 2 th + σ 2 b , where σ 2 b = 2eG 2 FFig. 2 .

 222222 Fig. 2. Histogram of H0 for 518 channel realizations of SN3, for the case of (a) CN1 and (b) CN2, and the corresponding estimated PDF by KDE with Gaussian kernel.

  (a) of SN3 and

Fig. 3 .Fig. 4 .

 34 Fig. 3. Pout versus peak transmit power Pt for different data-rates R b for the case of SN3-to-CN1 link using (a) a PIN (b) an APD. Uncoded NRZ-OOK modulation.

Fig. 5 .

 5 Fig. 5. Pout versus the peak transmit power Pt for different SNs for the cases of (a) CN1 with PIN PD, (b) CN2 with PIN PD, (c) CN1 with APD, and (d) CN2 with APD. Relatively high background noise level with I b = 200 µA.

TABLE I SIMULATION

 I PARAMETERS.

		m	1
	Tx (SN)	λ 0	850 nm
		∆λ	30 nm
		FOV	60 •
		Active Area	1 cm 2
	Rx (CN) [14]	R APD Gain	0.65 A/W 50
		APD Excess Noise Factor	3
		TIA Load Resistor	50 Ω
		I b	200 µA
		Dimensions	(5 × 5 × 3) m 3
	Room [10]	Walls Reflectivity Floor Reflectivity	0.83 (Plaster) 0.87 (Pinewood)
		Ceiling Reflectivity	0.83 (Plaster)
	Body	Dimensions Reflectivity	(1.7 × 0.3 × 0.2) m 3 0 (absorbing)

ACKNOWLEDGMENT

This work has been supported by VisIoN, a European project funded by the European Union's Horizon 2020 research and innovation program under the Marie Skłodowska-Curie Grant Agreement No. 764461.