Shruthi Koratagere

Anantha Kumar

Gangavarapu Vigneswara

Senior Member, IEEE Sachin Chaudhari

A Survey on Rural Internet Connectivity in India

Keywords: Digital divide, internet connectivity, mobile technologies, shared spectrum, use-cases

Rural connectivity has been a widely researched topic for several years. In India, around 50% of the population have poor or no connectivity to access digital services. Different solutions are being tested and trialed around the world, especially in India. The key driving factor for reducing the digital divide is exploring different technological and economical solutions to lower the cost for the network deployments and improve service adoption rate. This survey aims to study the rural connectivity use-cases, state of the art projects and initiatives, challenges, and technologies to improve digital connectivity in rural parts of India. The strengths and weaknesses of different technologies which are being tested for rural connectivity are analyzed. We also explore the rural use-case of the 6G communication system, which would suit the rural Indian scenario.

I. INTRODUCTION

Although India has already made the giant leap in the digital revolution, there is still a long way to connect the unconnected. As per a report by the various survey companies based on inputs from Telecom Regulatory Authority of India (TRAI), there is a 93% broadband penetration in urban India versus a mere 29.3% in rural India [START_REF]India: monthly ARPU for wireless services[END_REF], [2]. Exacerbated by COVID-19, this digital divide is worsening the existing societal and economic inequalities in the country. Further, there are concerns about widening the divide due to adopting new digital developments such as digital wallets, IoT, and AI.

There is a need to empower the rural community members through reliant connectivity and provide them with the relevant skill sets to enjoy the benefits which are currently spread unevenly across urban and rural India [START_REF]Can wireless technology alone blur the digital di[END_REF]. Leveraged and scaled, the power of information exchange can provide various opportunities such as smart agriculture, education, telemedicine services, ease of transfer of payments through government schemes, e-commerce, awareness on urban developments, and remote job opportunities [START_REF]Enabling 5G in India' -Telecom Regulatory Authority of India[END_REF].

The major challenges that exist when increasing internet penetration in India are unreliable electricity, affordability of the connectivity and user equipment, negligible industry incentives for low-cost connectivity, topographical challenges, cost of infrastructure, backhaul issues, and the performance Shruthi Koratagere Anantha Kumar, is with the StrathSDR Lab, University of Strathclyde, Glasgow, G1 1XQ Scotland, UK (e-mail: k.a.shruthi@strath.ac.uk) Gangavarapu Vigneswara Ihita and Sachin Chaudhari are with the Signal Processing and Communication Research Centre (SPCRC), International Institute of Information Technology, Hyderabad, 500032, India (e-mail: ihita.g@research.iiit.ac.in, sachin.chaudhari@iiit.ac.in) Paventhan Arumugam is Director (R&D) with Education and Research Network (ERNET) India, Indian Institute of Technology, Madras Research Park, Chennai -600113. e-mail: paventhan@eis.ernet.in) trade-offs [START_REF] Harri | 6G white paper on connectivity for remote areas[END_REF], [START_REF] Yaacoub | A key 6g challenge and opportunity-connecting the base of the pyramid: A survey on rural connectivity[END_REF]. Further, the lack of digital awareness in using technology, cultural reservations, and linguistic barriers causing the digital divide is a huge challenge in India [START_REF] Jha | Techno-economic assessment of the potential for LTE based 4G mobile services in rural India[END_REF].

The key solution lies in formulating strategies and initiatives which require inputs from operators, equipment manufacturers, academia, government, and civil bodies to understand the requirements of the rural communities to create sustainable solutions for them to leverage for development [8].

There is extensive research done in solving the digital divide problem around the world for the Indian scenario [START_REF]Can wireless technology alone blur the digital di[END_REF]- [START_REF] Shruthi | Techno-economic study of 5G network slicing to improve rural connectivity in India[END_REF]. In [START_REF]Can wireless technology alone blur the digital di[END_REF], the authors state the need for innovations in the business models for rural areas along with the innovations in the technology. The TRAI in [START_REF]Enabling 5G in India' -Telecom Regulatory Authority of India[END_REF], and the researchers in [START_REF] Harri | 6G white paper on connectivity for remote areas[END_REF], [START_REF] Yaacoub | A key 6g challenge and opportunity-connecting the base of the pyramid: A survey on rural connectivity[END_REF] highlights the different challenges, uses-cases, different solutions and need for innovation in business models for using 5G technology in the Indian setting. In [START_REF] Jha | Techno-economic assessment of the potential for LTE based 4G mobile services in rural India[END_REF], the authors presents the techno-economic feasibility of rural solutions in India. In [8], the Department of Telecommunication (DoT), India presents research on 5G technology and its uses for the Indian market with little focus on rural areas connectivity. A study on role of telecommunication in managing pandemic such as COVID-19 has highlighted the importance of digital connectivity especially in rural areas [START_REF] Jhunjhunwala | Role of telecom network to manage Covid-19 in India: Aarogya setu[END_REF]. The authors in [START_REF] Singh | UHF TVWS operation in Indian scenario utilizing wireless regional area network for rural broadband access[END_REF] explore low cost solutions using TV WhiteSpace (TVWs) in providing rural connectivity in India. 5G encompass combination of various technologies such as 5G, network slicing, TVWS, and WiFi integration, which are being tested to solve rural connectivity challenge [START_REF] Khaturia | Connecting the unconnected: Toward Frugal 5G network architecture and standardization[END_REF]. The need for innovative business models for rural scenarios with MNO and business ecosystem are discussed in [START_REF] Shruthi | Business model for rural connectivity using multi-tenancy 5G network slicing[END_REF] whereas the benefits of using 5G neutral host networks (NHN) in the Indian scenario is explored in [START_REF] Shruthi | Techno-economic study of 5G network slicing to improve rural connectivity in India[END_REF] which shows that it is cost efficient for rural setting. The need for supportive policies for encouraging NHN [START_REF] Lähteenmäki | The evolution paths of neutral host businesses: Antecedents, strategies, and business models[END_REF], and 5G deployments in rural areas with special focus on India scenario is discussed in [START_REF] Oughton | Supportive 5G infrastructure policies are essential for universal 6G: Assessment using an open-source technoeconomic simulation model utilizing remote sensing[END_REF].

Our research is different from the existing research presented in [START_REF]Can wireless technology alone blur the digital di[END_REF]- [START_REF] Shruthi | Techno-economic study of 5G network slicing to improve rural connectivity in India[END_REF]. The authors in [START_REF]Can wireless technology alone blur the digital di[END_REF], [START_REF] Shruthi | Business model for rural connectivity using multi-tenancy 5G network slicing[END_REF] present a generic study on rural digital connectivity and possible solutions but not specific to India. In [START_REF] Yaacoub | A key 6g challenge and opportunity-connecting the base of the pyramid: A survey on rural connectivity[END_REF] the study gives an glimpse of the rural connectivity solutions in India only using fiber backhaul, TVWS and WiFi. In [START_REF] Jha | Techno-economic assessment of the potential for LTE based 4G mobile services in rural India[END_REF], [START_REF] Jhunjhunwala | Role of telecom network to manage Covid-19 in India: Aarogya setu[END_REF], [START_REF] Khaturia | Connecting the unconnected: Toward Frugal 5G network architecture and standardization[END_REF], [START_REF] Shruthi | Techno-economic study of 5G network slicing to improve rural connectivity in India[END_REF] are use of specific solutions rather than an overview of all the possible solutions of rural digital connectivity. In [START_REF]Enabling 5G in India' -Telecom Regulatory Authority of India[END_REF], [START_REF] Harri | 6G white paper on connectivity for remote areas[END_REF], [8], [START_REF] Lähteenmäki | The evolution paths of neutral host businesses: Antecedents, strategies, and business models[END_REF], [START_REF] Oughton | Supportive 5G infrastructure policies are essential for universal 6G: Assessment using an open-source technoeconomic simulation model utilizing remote sensing[END_REF] include different possibilities and policies that could solve the issues and the roles of stakeholders but not an extensive study on suitability to the address the challenges and solutions for the Indian rural scenario.

According to researchers in [START_REF] Jain | Digital india-targeting overall growth: A critical analysis[END_REF], [START_REF]Planning for BharatNet Phase 2[END_REF], lack of awareness of appropriate solutions for digital connectivity in India is a major roadblock that should be addressed. Also, the research performed by [START_REF] Yaacoub | A key 6g challenge and opportunity-connecting the base of the pyramid: A survey on rural connectivity[END_REF] shows that around 70% of surveyed institutions didn't know the existence of BharatNet. The key motivation for this survey is to overcome the lack of awareness among the people regarding the topics such as initiatives accessible for rural digital connectivity, network planning based on the possible use-cases in rural areas rather than a generic urban style network, the key challenges that the deployment needs to address in the rural areas, possible technological solutions, and its potential. In this research, a survey on rural internet connectivity for an Indian scenario is presented. In this paper, we look at the following and try to answer the questions about:

• What are the key use-cases of rural digital connectivity?

• What are the current state-of-the-art initiatives and project undertaken to improve rural connectivity in India? • What are the major challenges that needs to be addressed to improve rural telecommunication? • What are the tests and the trials around the world for improving rural connectivity? Have these solutions been tested in India? • How are 6G solutions going to address rural connectivity needs?

This paper is organised as follows: Section II presents the rural connectivity use-cases and Section III the current stateof-the-art initiatives and projects to improve rural connectivity in India. Section IV presents the main challenges which has to be solved for reducing digital divide. Section V presents a survey on different technologies such as 5G, TVWS, and satellite internet, their role in solving the issues and the trials undertaken around the world. Furthermore, there is brief discussion on 6G solutions which would be focusing on rural connectivity. Finally, conclusions are drawn in Section VI.

II. RURAL CONNECTIVITY USE-CASES

The internet provides immense potential for the development of rural communities. The prominent and emerging usecases for digital connectivity in rural India, as shown in Fig. 1, are explained below: 1) Agriculture: The main occupation of rural communities in India is agriculture. Over the years, a declining employment size of the agricultural sector has been observed. The internet can improve agricultural productivity by facilitating sharing essential information on sowing, crop protection, improving soil fertility and weather. Further, Early-Warning-Systems (EWS) can help prevent significant crop loss by providing information on sporadic events such as floods, drought, or even pest and disease outbreaks. With access to internet connectivity farmers can get access to trusted channels where they can receive best market price of their commodity and other daily updates for their produce to ensuring they receive fair returns. The National Agriculture Market or e-Nam, an electronic trading portal, [18] brings regularisation on how crops are produced and distributed across the country along with quality testing mechanism with government standard and farmer access to a bigger market place. Internet helps farmers manage livestock, and enables the improvement of various aspects of livestock production and education. 2) Healthcare: Monitoring health and remotely accessing healthcare services by rural or isolated communities can be achieved through reliable connectivity. Telehealth includes long-distance clinical health care, patient and professional health-related education, public health, and health administration. It has been used by emergency medical personnel for consultation during natural disasters and in military battle situations. The National Health Portal [19], initiative of the Ministry of Health and Family Welfare, Government of India aims to cover services such as online medical consultation, online medical records, online medicine supply management and pan-India exchange for patient information. Mobile applications for monitoring health such as Pradhan Mantri Surakshit Matritva Abhiyan [START_REF]Pradhan mantri surakshit matritva abhiyan[END_REF] for pregnancy care, NHP Directory Services [START_REF]NHP health: Directory services mobile application[END_REF] for information on hospitals across India and Indradhanush Immunization [START_REF]Mission Indradhanush[END_REF] to track immunization status of children are helping with ease of access of healthcare services, require access to internet. . Further, other pandemic based essential services such as information on the vaccination centre are available on this platform. Connectivity is critical to rural sector to easily avail these e-services. [START_REF] Norp | 5G requirements and key performance indicators[END_REF], [START_REF] Latapu | Bridging the digital divide in Tonga through a sustainable multi-tenancy broadband infrastructure: Are we ready?[END_REF]. 5) Banking: Reliant and secure digital connectivity can strengthen rural access to financial services enabling them to find appropriate insurance, support to manage and mitigate risk. It will assist with better farmer profiling systems for agricultural credit decisions and protection for farmers in times of bad weather or disaster. Governments also use the internet to increase their outreach, improve understanding of policies, and expand credit penetration. Under the jurisdiction of the Ministry of Finance, the National Bank for Agriculture and Rural Development (NABARD) [START_REF][END_REF] provides overall regulation and licensing of regional rural banks. The Pradhan Mantri Jan Dhan Yojana [START_REF] Mantri | [END_REF] aims to expand affordable access to financial services such as bank accounts, remittances, credit, insurance, and pensions. The Government of India scheme, Kisan Credit Card (KCC), also aims to provide credit in activities such as the cultivation of crops, post-harvesting, expenditure, maintenance cost for assets used in agriculture, and investment requirements in agriculture and related activities. Further, the Indian government is promoting e-wallets and has launched many UPI (united payment interface) solutions and BHIM app for a smooth transition to digital payments. 6) E-commerce: With connectivity comes access to regional and global markets. E-commerce opportunities have been crucial in purchasing and selling agro-based products and handicrafts. Rural communities have a platform to showcase diverse cultures and crafts promoting tourism. Indian government initiative called the GeM portal is an e-marketplace from where everyday common consumer goods can be procured. This platform allows onboard products of self-help groups, tribal communities, artisans, weavers, etc., helping them flourish their businesses and maintain market transparency.

Handicrafts and paintings from India, such as Kalamkari paintings, Lepakshi handicrafts, and Kondapalli toys, can be displayed and sold in international markets via e-commerce platforms.

III. STATE-OF-THE-ART INITIATIVES FOR IMPROVING RURAL CONNECTIVITY

The government of India is trying to address the issues for rural internet penetration using different projects and initiatives such as BharatNet, PM-WANI, and Digital India [START_REF] Yaacoub | A key 6g challenge and opportunity-connecting the base of the pyramid: A survey on rural connectivity[END_REF], [8], [START_REF] Net | [END_REF] As per the scheme, a decentralized system of public access points will be implemented to increase internet connectivity across the country to address last-mile connectivity at cheap rates. There is no license, no registration, and no fee required for the Public Data Offices (PDOs), which could be small shops or even Common Service Centres as access points [START_REF]The PM-WANI Scheme: An explain[END_REF]. 5) OceanNet: The low cost internet connectivity solution can extend internet signals to fishermen up to 60 km at sea and 45 km beyond the range of cell phone towers [START_REF] Karthik | Study and analysis of OceanNet -marine internet service for fishermen[END_REF], [START_REF] Burugadda | Development of budget friendly wireless access point to use in littoral environments[END_REF]. It is particularly essential for real time alert communications and disaster warnings. 6) Deendayal Upadhyaya Gram Jyoti Yojana: The government of India launched the Deendayal Upadhyaya Gram Jyoti Yojana for village electrification and providing electricity distribution infrastructure in the rural areas.

IV. CHALLENGES SPECIFIC TO INDIAN SETTING

In this section, the different challenges in providing rural internet connectivity is studied. Though the government is encouraging initiatives and projects to improve rural connectivity focusing on middle-mile connectivity, there still exist major challenge in improving last-mile connectivity and expanding the network by the operators [START_REF]Your phone is dead in nearly 28,000 villages of India[END_REF], [START_REF]How COVID-19 deepens the digital education divide in India[END_REF]. In [START_REF]Enabling 5G in India' -Telecom Regulatory Authority of India[END_REF], [START_REF] Yaacoub | A key 6g challenge and opportunity-connecting the base of the pyramid: A survey on rural connectivity[END_REF], [START_REF] Jha | Techno-economic assessment of the potential for LTE based 4G mobile services in rural India[END_REF], [START_REF]Planning for BharatNet Phase 2[END_REF], [START_REF]Your phone is dead in nearly 28,000 villages of India[END_REF], [START_REF]How COVID-19 deepens the digital education divide in India[END_REF], the authors describe different challenges related to rural internet connectivity in India. Hence, the key challenges that needs to be addressed by the operators are:

1) Topography and population distribution: India has diverse topography ranging from plane, plateau to mountain and desserts and extreme climatic conditions. The topography of the village and its population distribution plays a crucial role in internet connectivity. A sparsely populated village is given the lowest priority while deploying networks because of low return on investment. An uneven terrain poses a challenge in providing digital connectivity [START_REF] Yaacoub | A key 6g challenge and opportunity-connecting the base of the pyramid: A survey on rural connectivity[END_REF], [START_REF]GSMA -closing the coverage gap: How innovation can drive rural connectivity -mobile for development[END_REF]. A smooth terrain with fewer obstructions is favorable for providing rural connectivity. Therefore, a unique solution is not possible for rural connectivity. The population of villages in rural parts of India can vary from as low as 100 to more than 10,000 people per village [START_REF]India: monthly ARPU for wireless services[END_REF]. Hence, the population distribution of the village needs to be one of the key factors in demands estimations and network planning. 2) Cost: The main factor which makes rural connectivity difficult is cost of providing internet solutions in rural areas. As stated by authors in [START_REF]Enabling 5G in India' -Telecom Regulatory Authority of India[END_REF], [START_REF] Shruthi | Techno-economic study of 5G network slicing to improve rural connectivity in India[END_REF], [START_REF] Deshmukh | Wireless broadband network on TVWS for rural areas: An Indian perspective[END_REF], the high cost of network is due to various factors such as nonexistence of either backhaul or close by point of presence (PoP), cost of equipment, repair and maintenance, and spectrum licensing in hard to reach locations. By lowering the cost of rural deployments, the scalability of the networks increase. The techno-economic study of different technologies highlight the need for reduced deployment costs as well as operating costs in rural areas for improving rural connectivity [START_REF] Jha | Techno-economic assessment of the potential for LTE based 4G mobile services in rural India[END_REF], [START_REF] Shruthi | Techno-economic study of 5G network slicing to improve rural connectivity in India[END_REF], [START_REF] Oughton | The cost, coverage and rollout implications of 5G infrastructure in Britain[END_REF]- [START_REF] Friedner | 5G infrastructure requirements in the UK[END_REF].

Furthermore, [START_REF]Enabling 5G in India' -Telecom Regulatory Authority of India[END_REF], [START_REF] Yaacoub | A key 6g challenge and opportunity-connecting the base of the pyramid: A survey on rural connectivity[END_REF], [START_REF] Jiang | The road towards 6G: A comprehensive survey[END_REF], [START_REF] Shahraki | A comprehensive survey on 6G networks: Applications, core services, enabling technologies, and future challenges[END_REF] shows that provisioning 2G, 3G, 4G, or 5G in rural regions will be difficult unless the network's cost is reduced. The economic cost modelling used in urban and industrial sectors is not appropriate for rural locations. 3) Per-capita income: Generally, the people living in rural areas have lower per-capita income compared to urban areas. The pricing should be attractive for people living in rural areas to spend on digital applications [START_REF]India: monthly ARPU for wireless services[END_REF], [2], [START_REF]Enabling 5G in India' -Telecom Regulatory Authority of India[END_REF], [START_REF] Yaacoub | A key 6g challenge and opportunity-connecting the base of the pyramid: A survey on rural connectivity[END_REF], [START_REF] Shruthi | Techno-economic study of 5G network slicing to improve rural connectivity in India[END_REF]. There is a need for innovation to lower costs further, and apply new revenue models to attract rural areas customers to enrol for internet services. The pricing of services could be different to attract rural subscribers. 4) Business models: The lack of innovative business models is the major challenge for rural connectivity [2], [START_REF] Yaacoub | A key 6g challenge and opportunity-connecting the base of the pyramid: A survey on rural connectivity[END_REF], [START_REF] Jha | Techno-economic assessment of the potential for LTE based 4G mobile services in rural India[END_REF], [START_REF] Khalil | A comparative study of rural networking solutions for global internet access[END_REF]. The traditional business models are not suitable for rural scenarios as these focuses on high ARPU, high customer base, and high investment duration [START_REF]Can wireless technology alone blur the digital di[END_REF].

In rural scenarios, the traffic generated is low and it is a loss making business using the traditional deployment methods. This fuels the need to find innovation in business models to convert this profitable. The new business model needs to focus on factors such as low ARPU, low to moderate customer base, and moderate investment duration [START_REF]World internet users statistics and 2020 world population stats[END_REF]. 5) Funding and investment: The MNOs and ISPs build a network with future profits in mind. They obtain funding and investment from big banks, and government agencies [START_REF]Enabling 5G in India' -Telecom Regulatory Authority of India[END_REF]. As the rural telecommunication business is non-profitable for a new entrant or infrastructure providers (InP), the lack of funding allocated for rural last-mile connectivity and its development is another major roadblock [START_REF] Yaacoub | A key 6g challenge and opportunity-connecting the base of the pyramid: A survey on rural connectivity[END_REF]. There is a need for innovations in the possible funding model for local operators in rural areas. The investment requires for setting a telecommunication service is significant and to make it sustainable is key for the scalability of any rural connectivity solutions. 6) Technology: Mobile communication technology needs to be specifically modified for meeting the rural communities' requirements. The key focus of rural networks are on factors such as high coverage (up to 10 km radius), high data rates (100 Mbps or more), high energy efficiency, high spectral efficiency, support all use-cases using single network and smooth roaming [2], [START_REF]Enabling 5G in India' -Telecom Regulatory Authority of India[END_REF], [START_REF] Yaacoub | A key 6g challenge and opportunity-connecting the base of the pyramid: A survey on rural connectivity[END_REF], [START_REF] Shruthi | Techno-economic study of 5G network slicing to improve rural connectivity in India[END_REF], [START_REF] Norp | 5G requirements and key performance indicators[END_REF], [START_REF] Benseny | Value network analysis in a lowcost and affordable internet[END_REF]. There is also a need for local spectrum licensing to encourage micro-operators or InP [START_REF] Shruthi | Business model for rural connectivity using multi-tenancy 5G network slicing[END_REF], [START_REF] Sanguanpuak | Network slicing with mobile edge computing for micro-operator networks in beyond 5G[END_REF]. Open network technologies such as OpenRAN, 5G open-source base stations, and other open source technologies are widely researched around the world, which is expected to significantly reduce the cost of deployments [START_REF] Chih-Lin | 5G: rethink mobile communications for 2020[END_REF]. 7) Electricity: The rural areas have irregular or unreliable power supply issues [START_REF] Yaacoub | A key 6g challenge and opportunity-connecting the base of the pyramid: A survey on rural connectivity[END_REF], [START_REF] Jha | Techno-economic assessment of the potential for LTE based 4G mobile services in rural India[END_REF], [START_REF]GSMA -closing the coverage gap: How innovation can drive rural connectivity -mobile for development[END_REF], [START_REF] Mcguire | Enabling rural broadband via TV white space[END_REF]. The power supply in rural parts of India is inconsistent which needs to addressed [START_REF] Cross | No current: Electricity and disconnection in rural india[END_REF]. The rural telecommunication system stress the need for renewable energy sources and highly power efficient system. The 5G systems are 10x times power efficient compared to 4G systems which is an advantage for rural areas. 8) Digital awareness: Another challenge that needs to be addressed is digital awareness among rural users. People need to be educated on the benefits and comfort offered by digital solutions. As people understand the advantages of rural digital solutions and how they will improve their life, then people would demand better services [START_REF]Enabling 5G in India' -Telecom Regulatory Authority of India[END_REF], [START_REF]GSMA -closing the coverage gap: How innovation can drive rural connectivity -mobile for development[END_REF], [START_REF]GSMA -infrastructure sharing: An overview -future networks[END_REF]. Another factor that needs to be considered is that India is a multi-lingual country. The content should be available in different languages to attract rural subscribers on the network [START_REF] Singhal | Developing information technology solutions in indian languages: Pros and cons[END_REF], [START_REF] Nougarahiya | A review of e-commerce in india: The past, present, and the future[END_REF].

The people living in rural areas would have access to new opportunities, especially in terms of economic growth, and opportunities for the younger generations are unlimited [START_REF]Enabling 5G in India' -Telecom Regulatory Authority of India[END_REF], [START_REF] Norp | 5G requirements and key performance indicators[END_REF].

V. POSSIBLE TECHNOLOGICAL SOLUTIONS FOR INDIAN

SETTINGS

As previously mentioned, rural connectivity objective must overcome numerous obstacles, including innovations in technologies to suit rural needs. Technologies such as 3G and 4G are existing in rural areas [START_REF] Jha | Techno-economic assessment of the potential for LTE based 4G mobile services in rural India[END_REF] but these technologies are expensive to cater to rural conditions. Furthermore, passive sharing does not reduce the cost significantly as active sharing which is supported by 5G and open networks [START_REF] Mahmoud | 6g: A comprehensive survey on technologies, applications, challenges, and research problems[END_REF]. Technologies shown in Fig. 2 such as 5G, hot air balloon base stations, drone base stations, satellite internet and backhaul, long-range WiFi networks, unmanned aerial vehicle, and TVWS are being tested for suitability as rural solutions, and later studying their techno-economic feasibility.

In this section, the various possible technologies which would help in improving rural internet connectivity are discussed. The scope of this research lies in exploring different technologies and understanding their suitability for different rural scenarios.

A. Rural trials around the world 1) 5G and network slicing: 5G is the fifth generation of communication systems supporting three major use-cases such as enhanced mobile broadband (eMBB), ultra-low latency communications (uRLLCs), and massive machine to machine communications (mMTC). To address the issue of rural internet connectivity, researchers began testing the suitability of the design of a 5G communication infrastructure to address the needs of rural use-cases. In [START_REF] Oughton | The cost, coverage and rollout implications of 5G infrastructure in Britain[END_REF], [START_REF] Oughton | Assessing the capacity, coverage and cost of 5G infrastructure strategies: Analysis of the Netherlands[END_REF], [START_REF] Oughton | An open-source techno-economic assessment framework for 5G deployment[END_REF], the research on deploying and operating a nationwide 5G network with a duration 10 years was investigated. It was concluded that policy and technological advancement along with smart innovations in rural telecommunication business models will drive the connectivity. It was also understood that MNOs take a long time to deploy 100% coverage, because the provisioning of telecommunication services is a costly procedure with a low or negative return on investment in rural or remote rural areas. The risk of 5G adoption is also considered, along with the technological and economic viability. The use of spectrum sharing in local 5G networks deployed by the InP is explored in [START_REF] Jeon | Coordinated dynamic spectrum sharing for 5G and beyond cellular networks[END_REF]- [START_REF] Anderson | Evolving Spectrum Sharing Methods, Standards and Trials: TVWS, CBRS, MulteFire and More[END_REF]. The studies highlights the benefits and ease of spectrum sharing in 5G bands especially at the higher frequency bands. In [START_REF] Lun | 5G New Radio for rural broadband: How to achieve long-range coverage on the 3.5 GHz band[END_REF], the authors demonstrates the usage of 3.5 GHz band for long-range coverage to provide broadband services. The results showed the potential usage of 3.5 GHz CBRS bands in providing broadband services especially in rural scenarios.

Network slicing, which encompasses applications ranging from slicing in the radio access network to end-to-end network sharing, is one of the 5G architectures that is being investigated extensively for rural solutions. The network slicing for broadband related applications is explored in [START_REF] Latapu | Bridging the digital divide in Tonga through a sustainable multi-tenancy broadband infrastructure: Are we ready?[END_REF] to improve rural broadband services, which is backed by eMBB services of 5G systems. The research shows the discussions on bridging the digital divide issues and focusing on multi-tenancy sharing of infrastructure to make rural connectivity sustainable. In [START_REF] Noll | 5G network slicing for digital inclusion[END_REF], the authors advocated using a single slice to give basic but free internet connectivity to people who cannot afford it in the Democratic Republic of Congo (DRC), while the rest slices are utilised for commercial operations. The solution educates rural residents about the advantages of having access to the internet. The idea of employing 5G NHN for vertical industries is being tested in a pilot study on the Orkney Islands, United Kingdom, where one of the slices will be utilised for BBC [65] broadcasting applications and other rural industrial verticals. The learning's from the trial is that 5G network slicing is an innovative option to encourage rural connectivity. The business models and value network configuration for 5G NHN in the rural areas is explored in [START_REF] Shruthi | Business model for rural connectivity using multi-tenancy 5G network slicing[END_REF]. The research shows that integration of business ecosystem with rural connectivity solutions to improve digital divide. Furthermore, the technoeconomic feasibility of using 5G NHN in rural parts of the India is studied in [START_REF] Shruthi | Techno-economic study of 5G network slicing to improve rural connectivity in India[END_REF]. The results show that it has a potential to be considered for rural connectivity in India.

2) Satellite internet: In this, low earth orbit satellites are used to provide internet connectivity to hard to reach rural areas and other topography areas which is not provided coverage using terrestrial telecommunication system around the world [START_REF]Can new satellite constellations bridge the digital divide[END_REF], [START_REF] Kodheli | Satellite communications in the new space era: A survey and future challenges[END_REF]. Today, various satellite telecommunication companies such as OneWeb, SapceX, Eutelsat, and Blue origin are launching satellites to provide 5G and internet services for different applications [START_REF] Giordani | Satellite communication at millimeter waves: A key enabler of the 6G era[END_REF]- [START_REF] Giambene | Satellite-5G integration: A network perspective[END_REF]. The pricing is expected to vary depending upon the application served. For example, rural connectivity would be offered at affordable prices whereas internet connectivity for airlines and ships could be offered at higher prices. OneWeb and SpaceX aim to remove the dependency of rural areas on fibre backhaul by providing excellent coverage and data rates using satellite internet.

3) Hot air balloon/UAV/Drone: These technologies are flying antenna system that works as a base station in the The application of hot air balloons/UAV/Drone is studied for digital connectivity in hard to reach areas due to topographical challenges as well as for emergency services in [START_REF] Yaacoub | A key 6g challenge and opportunity-connecting the base of the pyramid: A survey on rural connectivity[END_REF], [START_REF] Sudheesh | Sum-rate analysis for high altitude platform (HAP) drones with tethered balloon relay[END_REF], [START_REF] Hunukumbure | Cost analysis for drone based 5G eMBB provision to emergency services[END_REF]. The cost of the network is highly dependent on the number of drones required to support the use-cases. The trials were performed for drone to acts as temporary base stations during network disruption in [START_REF] Hunukumbure | Cost analysis for drone based 5G eMBB provision to emergency services[END_REF], [START_REF] Han | Emerging drone trends for blockchain-based 5G networks: Open issues and future perspectives[END_REF]. The results were satisfactory in the test trials. The feasibility of drone for a longer duration of usage is yet to be studied. Companies such as Google, Facebook, Amazon, Taara and Walmart are researching on use of these technologies for 5G service provisioning [START_REF] Loon | Expanding internet connectivity with stratospheric balloons[END_REF], [START_REF] Taara | [END_REF].

4) TVWS: TVWS is a telecommunication technology in which information is transmitted using the TV frequency bands. After the digitization of the TV channels, a swathe of frequency became available around the world. In different test trials TVWS is studied for improving rural connectivity. In [START_REF] Singh | UHF TVWS operation in Indian scenario utilizing wireless regional area network for rural broadband access[END_REF], [START_REF] Deshmukh | Wireless broadband network on TVWS for rural areas: An Indian perspective[END_REF], [START_REF] Mcguire | Enabling rural broadband via TV white space[END_REF], [START_REF] Khalil | Feasibility, architecture and cost considerations of using TVWS for rural internet access in 5G[END_REF], the authors study the network by using TVWS technology to provide very long distance wireless backhaul capacity. The TVWS require last-mile connectivity on a different frequency bands such as 2G, 3G, 4G, 5G or unlicensed spectrum, for operations on end-users devices. The frequency bands available for usage varies in different countries around the world. The application of TVWS in combination with WiFi is trialed in India by researchers of IEEE rural 5G broadband standards group [START_REF] Khaturia | Connecting the unconnected: Toward Frugal 5G network architecture and standardization[END_REF]. The authors in [START_REF] Anderson | Evolving Spectrum Sharing Methods, Standards and Trials: TVWS, CBRS, MulteFire and More[END_REF], [START_REF] Zhang | Network slicing based 5G and future mobile networks: Mobility, resource management, and challenges[END_REF] highlight the different use-cases of shared spectrum and other test trials of TVWS in the UK. The results explain the benefits of using shared spectrum bands for providing connectivity and the different frequency sharing technologies around the world.

5) WiFi long range:

Long range WiFi is the technology in which WiFi is used for long distance wide-area communication. This technology is being trailed as well as tested by different researchers and is easier to setup [START_REF] Yaacoub | A key 6g challenge and opportunity-connecting the base of the pyramid: A survey on rural connectivity[END_REF], [START_REF] Burugadda | Development of budget friendly wireless access point to use in littoral environments[END_REF], [START_REF] Anderson | Evolving Spectrum Sharing Methods, Standards and Trials: TVWS, CBRS, MulteFire and More[END_REF], [START_REF] Lun | 5G New Radio for rural broadband: How to achieve long-range coverage on the 3.5 GHz band[END_REF]. The key advantages of long range WiFi is low cost and quick deployment but the challenges lies in the maximum permissible operational power levels as it is operates in unli-censed spectrum bands [START_REF] Ab-Hamid | Self-sustainable energy efficient long range WiFi network for rural communities[END_REF]. This technology also uses MIMO systems to provide long range coverage in rural areas and adhere to the usage policy of unlicensed spectrum bands. The recent study in IEEE 802.11ah shows that it has a potential for long range backhaul and long outdoor network [START_REF] Aust | Outdoor long-range WLANs: A lesson for ieee 802.11ah[END_REF] with power restrictions. The paper discussed the strengths and weakness of the technology and provide a survey of research from 2002 -2014. This concept is extended further and researched to the networks in 2021 for topics such as fair allocation of resources, co-existence with other networks and use-case suitability.

B. Overview of possible technologies for rural connectivity

In Table I, various rural technologies and their strengths as well as weaknesses are summarized. The challenge with 5G networks for rural solutions that needs to be addressed are local spectrum licensing, ease of new entrants in local markets, subscription of services, performance of network slicing. Furthermore, in [START_REF] Yaacoub | A key 6g challenge and opportunity-connecting the base of the pyramid: A survey on rural connectivity[END_REF], [START_REF] Jiang | The road towards 6G: A comprehensive survey[END_REF], [START_REF] Mcguire | Enabling rural broadband via TV white space[END_REF], the importance of renewable energy for rural connectivity was investigated, with the issue of connection to an alternate energy source in the absence of renewable sources. Similarly, though TVWS communication has a long range, but parameter limits exist for deployment in terms of factors such as power levels, maximum allowable interference levels and spectrum licensing [START_REF] Yaacoub | A key 6g challenge and opportunity-connecting the base of the pyramid: A survey on rural connectivity[END_REF], [START_REF] Singh | UHF TVWS operation in Indian scenario utilizing wireless regional area network for rural broadband access[END_REF], [START_REF] Jiang | The road towards 6G: A comprehensive survey[END_REF], [START_REF] Mcguire | Enabling rural broadband via TV white space[END_REF]. The price and reliability issues associated with solutions such as hot air balloon base stations, drone base stations, satellite backhaul, long range WiFi, and unmanned aerial vehicles must be solved for these solutions to become scalable, according to the rural connectivity study [START_REF] Yaacoub | A key 6g challenge and opportunity-connecting the base of the pyramid: A survey on rural connectivity[END_REF], [START_REF] Jiang | The road towards 6G: A comprehensive survey[END_REF]. These solutions are not yet totally appropriate for use in rural areas at the current stage for a wide-area network. Different rural technologies are compared and their viability is assessed in terms of network architecture, performance, important characteristics, and deployment concerns in the [START_REF] Khalil | A comparative study of rural networking solutions for global internet access[END_REF].

C. Overview of rural trials in the Indian scenario

This section presents a short discussion of the tests and trials of the above-discussed technologies in the Indian scenario. The need for sustainable internet business models and possible

Technology

References Strengths Weakness 5G [START_REF] Shruthi | Business model for rural connectivity using multi-tenancy 5G network slicing[END_REF], [START_REF] Shruthi | Techno-economic study of 5G network slicing to improve rural connectivity in India[END_REF], [START_REF] Oughton | The cost, coverage and rollout implications of 5G infrastructure in Britain[END_REF], [START_REF] Al-Dulaimi | 5G networks: Fundamental requirements, enabling technologies, and operations management[END_REF] Very high speed, versatile applications, reasonable pricing, remote maintenance, software defined networking, shared spectrum, supports multiple frequencies, works well up to 500 kmph, energy efficient High TCO, difficulty obtaining spectrum licence, requires legal approvals and network planning, revenue depends on the subscription rates Satellite internet [START_REF] Yaacoub | A key 6g challenge and opportunity-connecting the base of the pyramid: A survey on rural connectivity[END_REF], [START_REF]Can new satellite constellations bridge the digital divide[END_REF], [START_REF] Giordani | Satellite communication at millimeter waves: A key enabler of the 6G era[END_REF], [START_REF] Giambene | Satellite-5G integration: A network perspective[END_REF] Provide coverage anywhere on the earth, high speed data rates, easy setup, supports OTT, smooth handover for mobile devices, low latency High subscription fees, customer premise equipment (CPE), higher cost, dependent on weather, no roaming for in-house antenna, lower data capacity limits compared to terrestrial networks Hot air balloon/ Unmanned aerial vehicles/ Drone [START_REF] Yaacoub | A key 6g challenge and opportunity-connecting the base of the pyramid: A survey on rural connectivity[END_REF], [START_REF] Sudheesh | Sum-rate analysis for high altitude platform (HAP) drones with tethered balloon relay[END_REF], [START_REF] Hunukumbure | Cost analysis for drone based 5G eMBB provision to emergency services[END_REF], [START_REF] Taara | [END_REF] Wireless infrastructure, long distance coverage, relatively easy to setup anywhere, short requirements for base station Techno-economic feasibility is not fully studied, weather related challenges, legal approvals, interference planning, fuel issues, roaming agreements, continuous service requirement is not feasible TVWS [START_REF] Singh | UHF TVWS operation in Indian scenario utilizing wireless regional area network for rural broadband access[END_REF], [START_REF] Deshmukh | Wireless broadband network on TVWS for rural areas: An Indian perspective[END_REF], [START_REF] Mcguire | Enabling rural broadband via TV white space[END_REF], [START_REF] Anderson | Evolving Spectrum Sharing Methods, Standards and Trials: TVWS, CBRS, MulteFire and More[END_REF], [START_REF] Khalil | Feasibility, architecture and cost considerations of using TVWS for rural internet access in 5G[END_REF], [START_REF]Connected nations 2020 -ofcom[END_REF] Very good coverage, supports LTE, 5G small cells and internet, long distance transmission, wireless backhaul, long transmission ranges, lower cost Spectrum licence, DSA technology, licence duration, middle-mile technology, user-devices do not operate on these bands, power restrictions, interference management, dynamic change in frequency of operation WiFi long range [START_REF] Lun | 5G New Radio for rural broadband: How to achieve long-range coverage on the 3.5 GHz band[END_REF], [START_REF] Singh | Wireless for communities: Empowering communities through wireless connectivity[END_REF] Easy to setup, unlicensed bands, devices support WiFi bands, plug and play devices, in-house or local areas networks, very high data rates Limited power supply, interference prone, customised for rural requirements, handover for mobile networks technologies is discussed in [START_REF] Thakur | Sustainable wireless internet connectivity for rural areas[END_REF]. The rural network will only become sustainable when the business model suits the demand and capacity available for that village. The suitability and challenges of using long-range WiFi, 2G/3G, LTE, and a few other technologies have been discussed in [START_REF] Anusha | Comparative analysis of wireless technology options for rural connectivity[END_REF]. The research shows that WiFi involves lower cost and easy setup but prone to interference, whereas the LTE is suitable to support roaming for rural customers though it is expensive. In [START_REF] Pentland | DakNet: rethinking connectivity in developing nations[END_REF], the authors present the 'DakNet' using long-range WiFi to connect rural parts of India. The usage of LTE for rural connectivity is discussed in [START_REF] Jha | Techno-economic assessment of the potential for LTE based 4G mobile services in rural India[END_REF]. The author highlights the need for cheaper CAPEX costs and low spectrum licensing costs. The researchers in [START_REF] Burugadda | Development of budget friendly wireless access point to use in littoral environments[END_REF] show the usage for long-range WiFi for affordable connectivity for fishermen using the concept of network hops. The network provides coverage up to 100 km using the multi-hop network concepts. In [START_REF] Khaturia | Connecting the unconnected: Toward Frugal 5G network architecture and standardization[END_REF], the authors discuss the technology and architecture related to using 5G IEEE standard network slicing for rural connectivity using WiFi as last-mile network. This network also uses TVWS as a backhaul solution. All these researchers show the use of WiFi routers in the village for end-users to access internet. There is a need to make mobile technologies easily accessible and available to increase mobile services' uptake. TVWS trials were performed for connecting schools in rural parts of Srikakulam, Andhra Pradesh, India [86]. In [START_REF] Oughton | Supportive 5G infrastructure policies are essential for universal 6G: Assessment using an open-source technoeconomic simulation model utilizing remote sensing[END_REF], the author highlights the need to eliminate spectrum fees as a supportive policy to encourage 5G deployments in the India scenario and in future, enable 6G networks in the rural areas, especially in India. The Free Space Optical Communication (FSOC) Technology is trialed for providing connectivity in hard to reach places in Kohima, Nagaland by ERNET [START_REF] Taara | [END_REF], [87].

D. 6G -integration of different technologies

The latest generation of telecommunication systems being researched is 6G. Tetra-Hertz frequency band communication is supported by 6G and a minimum data speed of 1 Gpbs speeds per user. Furthermore, 6G is expected to offers an extremely high data rates, extraordinary features and customised network as per the design requirements. In [START_REF] Yaacoub | A key 6g challenge and opportunity-connecting the base of the pyramid: A survey on rural connectivity[END_REF], [START_REF] Jiang | The road towards 6G: A comprehensive survey[END_REF], [START_REF] Shahraki | A comprehensive survey on 6G networks: Applications, core services, enabling technologies, and future challenges[END_REF], a survey on delivering 6G connectivity in rural areas, challenges, different technologies and viable solutions is discussed. According to the authors, 6G is expected to be a novel solution which would address rural connectivity issues around the world providing the policies are supportive for network upgradation and spectrum licensing. There are different research groups which are working on 6G research ideas to tackle digital divide issues. The 6G includes rural connectivity as one of the key use-cases unlike 5G [START_REF] Yaacoub | A key 6g challenge and opportunity-connecting the base of the pyramid: A survey on rural connectivity[END_REF]. A brief comparison of 5G and 6G communication systems is given in Table II. The key rural connectivity aims of 6G as stated in [START_REF] Harri | 6G white paper on connectivity for remote areas[END_REF], [START_REF] Yaacoub | A key 6g challenge and opportunity-connecting the base of the pyramid: A survey on rural connectivity[END_REF], [START_REF] Oughton | Supportive 5G infrastructure policies are essential for universal 6G: Assessment using an open-source technoeconomic simulation model utilizing remote sensing[END_REF], [START_REF] Jiang | The road towards 6G: A comprehensive survey[END_REF], [START_REF] Mahmoud | 6g: A comprehensive survey on technologies, applications, challenges, and research problems[END_REF] are as summarised below:

• Low-cost rural connectivity technology, backhaul solutions to reduce the overall cost, remote monitoring and maintenance of the network. • Efficient spectrum usage in the spectrum sharing bands, energy consumption to address unreliable energy supply, resource allocation to support multiple applications as well as utilisation. The network supports guaranteed QoS and high reliability to encourage MNO investments for the rural connectivity ecosystem. • Smart infrastructure for telecommunication services in rural areas.

The 6G rural connectivity solutions would encourage the business for micro-operators or InP, as well as the MNOs, internet service provider (ISP), temporary service provider who can lease services from the InP in rural areas for providing services to their end-customers. The demand and supply should be proportional to keep the cost reasonable. The proposed research ideas include use-cases for various stakeholders such standardisation bodies, national and international regulations, equipment manufacturers, MNOs, ISP, schools, hospitals, councils, indoor-building service providers, satellite service providers, vehicular connectivity providers and hospitals. The 6G rural solutions could also be extended to the Indian scenario.

VI. CONCLUSION

This research paper provides a survey on rural connectivity challenges and possible solutions in India. The problem of providing high-speed connectivity to more than 50% population of India is serious and needs to be addressed. Thus, in this paper, the use-cases, the various government initiatives and projects, challenges, and technologies required for rural connectivity in the rural Indian scenario are explored, listed, and analyzed. The typical technological solutions, and its strengths, as well as weakness, is listed. The lesson learnt from the survey is that no single solution is suitable for all villages. The challenges, initiatives and technologies needs to be considered and weighted according to the suitability to find a solution to cater to the use-cases and demand of the rural areas. Finally, this paper presents an overview of possible rural solutions using the 6G technologies.

Fig. 1 .

 1 Fig. 1. Rural connectivity use cases

Fig. 2 .

 2 Fig. 2. Different rural technologies

TABLE I COMPARISON

 I OF DIFFERENT RURAL CONNECTIVITY SOLUTIONS

ACKNOWLEDGEMENT


domain: spi phys

text:
Benjamin Leclaire

Ivan Mary

Cédric Liauzun

Stéphanie Péron

Andrea Sciacchitano

Andreas Schröder

Philippe Cornic

Frédéric Champagnat

First challenge on Lagrangian Particle Tracking and Data Assimilation: datasets description and planned evolution to an open online benchmark

1: ONERA, Department of Aerodynamics, Aeroelasticity and Acoustics, Meudon, France 2: ONERA, Department of Aerodynamics, Aeroelasticity and Acoustics, Châtillon, France 3: Delft University of Technology, Faculty of Aerospace Engineering, Delft, The Netherlands 4: DLR, Institute of Aerodynamics and Flow Technology, Göttingen, Germany 5: ONERA, Department of Information Processing and Modelling, Palaiseau, France *corresponding author: benjamin.leclaire@onera.fr

In the last decade, Lagrangian Particle Tracking (LPT) has emerged as one of the leading measurement techniques for the quantitative determination of fluid flows in three-dimensional domains (see e.g. [START_REF] Schanz | Shake-The-Box: Lagrangian particle tracking at high particle image densities[END_REF], due to its accuracy in reconstructing particles velocities and material accelerations. Due to the scattered nature of the obtained result, at the particles positions only, significant research efforts have also been placed in the development of dedicated Data Assimilation (DA) techniques, aiming at finally reconstructing full 3D velocity and pressure fields on regular Cartesian grids (see, e.g., [START_REF] Schneiders | Dense velocity reconstruction from tomographic PTV with material derivatives[END_REF].

Within the framework of the Horizon 2020 project HOMER (Holistic Optical Metrology for Aero-Elastic Research), research groups at DLR, ONERA and TU Delft have jointly organized the 3 rd Workshop and 1 st Challenge on DA & CFD Processing for PIV and LPT. The challenge purpose was to assess the state-of-the-art on LPT and DA algorithms in a situation reflecting a typical experimental campaign. The turbulent wall-bounded flow in the wake of a cylinder was chosen, being a situation with important turbulent fluctuations due to the wake, both in velocity and pressure. Diameter = 0.01 and gap from the wall = 0.01 of the cylinder were chosen in comparison with the targeted boundary layer thickness ≈ 60 , and with reference to studies in literature on vortex shedding with ground effect (see e.g. [START_REF] Wang | Near-wake flow characteristics of a circular cylinder close to a wall[END_REF], in order to obtain standard shedding behaviour while maintaining large enough wall pressure fluctuations. Monotone Integrated Large Eddy Simulations (MILES) were performed with the ONERA HPC multi-block structured aerodynamic solver FASTS, using second-order finite volume spatial discretization and second-order implicit time integration. The computational domain size is of 1.8 × 0.1 × 1.0 in the streamwise ( ), spanwise ( ) and wallnormal ( ) directions, with mesh cell number of 1367 × 500 × 247 = 168,824,500 cells. The flow zone used for the datasets is of 0.1 × 0.05 × 0.03 ; it is centred in span, with its upstream end located 0.035 downstream of the cylinder centre, and extending from the wall to 0.03 above the wall. Typical cell size in this region was Δ = 0.4

, Δ = 0.2 and 0.0165 ≤ Δ ≤ 0.47 . The method of [START_REF] Lund | Generation of turbulent inflow data for spatially-developing boundary layer simulations[END_REF] was used to obtain a developed turbulent boundary layer, and periodicity was imposed in the spanwise direction. Free-stream Reynolds number per unit length was set to , = 665,000, leading to a boundary layer momentum thickness Reynolds number of = 4,150 ten diameters upstream of the cylinder. From these simulations, similarity was used to transpose the initial air flow situation towards a water flow with similar dimensions, thereby leading to equivalent free-stream velocity = 0.667 . ! . Figures 1 and2 respectively present a sample flow snapshot, and the time-and span-averaged mean and fluctuating streamwise velocity component, illustrating the intensity of the turbulent fluctuations, largely due to the shed vortices and their secondary structures. As shown in particular by Figure 2, the flow zone of the dataset corresponds to a progressive wake recovery. Simulations were run with embedded propagation of synthetic pointwise tracer particles, whose positions were initially chosen randomly, using interpolation of the velocity field and a 3 rd order Adams-Bashforth time scheme. Overall, 22 sets of particle trajectories of different lengths, each corresponding to different starting flow instants, were generated in order to build all datasets, consisting of two-pulse (TP), four-pulse (FP) or time-resolved (TR) data for the LPT challenge, and of TR data for the DA challenge. Analysis of the trajectories (not shown here) indicated that significant curvatures and accelerations logically coincided mostly with shed vortices, and in a lesser extent with near-wall turbulent structures. Synthetic particle images for the LPT cases were obtained by considering four virtual cameras with a sensor size of 1920×1200 pixels and 10 µm pitch, using pinhole projection with no Scheimpflug and no distortions, camera locations and focal lengths leading to an equivalent voxel size of 60 μ . Calibration data (list of points and projections), with no error, were provided to the participants. Particles were given a polydisperse intensity distribution, then spread to form the images using a Gaussian (# = 0.6) point-spread function, modelling diffraction-limited imaging. Thermal and shot noise were also added in the images. Datasets were generated corresponding to 0.005, 0.025, 0.05, 0.08, 0.12, 0.16 particles per pixel (ppp), for the TP and FP cases, and also to 0.2 ppp for the TR case. The three DA cases (corresponding to 0.005, 0.025 and 0.16 ppp) were generated by adding a random Gaussian position noise (with # = 0.1 voxel) to the raw trajectories. These datasets were publicly released for download on March 9 th , 2020, see https://w3.onera.fr/first_lpt_and_da_challenge/. Participants were then requested to upload their processed results by July 17 th , 2020, consisting of particles positions and velocities (and possibly accelerations depending on the cases) for the LPT challenge, and of velocity, velocity gradient and pressure fields on a prescribed 3D grid for the DA challenge. Analysis of the participants results will be presented in two separate communications also submitted to the ISPIV2021 symposium (Sciacchitano et al. and Sciacchitano et al.). Future steps of this work, of which some are underway, are to complete the challenge data portal with an automatic assessment capability, with error and performance metrics computed and communicated to the participant upon data upload, in a similar idea as widely done in the computer vision community (see, e.g. [START_REF] Geiger | Are we ready for autonomous driving? the kitti vision benchmark suite[END_REF]. Additionally to a full presentation of the flow simulation and final datasets of the LPT and DA challenges, this communication will present the first development steps and principles of this future open online benchmark, which is foreseen to be available to any team interested in development of LPT and DA methods, and conceived to be, in the long run, progressively completed with further datasets relevant to the field.

Figure 1 -

 1 Figure 1-Sample flow snapshot, including Q-criterion iso-surfaces color-coded by streamwise velocity u, isocontours of pressure p at the lower and side walls.

Figure 2 -

 2 Figure 2-Streamwise mean and fluctuating velocity components, with averaging performed in time and span. Profiles on the right correspond to streamwise locations depicted by the vertical dashed lines in the left figures.