David Henri Scheer
email: davscheer@gmail.com

Gilles Chantraine

Intelligence and radicalization in French prisons: Sociological analysis bottom-up

Keywords: France, intelligence, prison, radicalization, radicalization assessment unit, security

In the context of the fight against Islamist radicalization in France, prison intelligence rapidly developed from 2015 through the gradual creation of a dedicated service and a specific corps of professionals. This professionalization of prison intelligence work has deeply transformed the prison administration. This article aims to describe and analyse these transformations on the basis of an ethnographic study conducted in radicalization assessment units (RAUs), which are specific units set up to assess prisoners who have committed or are suspected of committing crimes linked to radical Islam. We shall describe how the guards, probation officers, psychologists and educators participating in assessing the prisoners adapt to the new, encroaching presence of the intelligence mission. We shall analyse the forms of collaboration and competition between this staff and the prison intelligence officers. Lastly, we will examine criticism of the intelligence activity in the RAUs voiced by various professionals. The interpenetration of the assessment work and the intelligence mission -which are formally distinct missions -produces a specific type of knowledge relating to radicalized prisoners: a reproduction of certain representations or 'profiles'.

Introduction

'No democracy sets up a prison intelligence service'. (Minister of Justice, speech to the National Assembly, June 2016) Since the 2015 terrorist attacks in France, prisons have faced criticism, both as potential sites for radicalization and for their feeble capacity to detect threats to national security. In reaction to these critiques, a key measure of the government and prison administration was to develop and officialize prison intelligence activity with the aim of reinforcing the ability to detect and monitor dangerous individuals within prisons. In so doing, French prisons, now more than ever, have become key sites for gathering information for use by national security bodies and the intelligence community. In parallel with the creation of this new intelligence service, by virtue of the fight against radicalization, psychologists and special educators were being hired to reinforce the existing prison staff in the assessment of 'radicalized' or 'potentially radicalized' prisoners. Specialized units were created, known as radicalization assessment units (RAUs).

Although both measures were motivated by the fight against terrorism, intelligence gathering and radicalization assessment are two distinct missions within the prison administration. Officially the RAUs are not places for intelligence gathering, but rather assessment sites. The prisoners assigned to a RAU know why they are there and are invited to participate in their own assessment; the report drawn up at the end of an assessment is shared with the prisoner, who is then assigned to the appropriate facility in line with the results. Prison intelligence, for its part, is a secret mission. In the case of prison intelligence, the prisoners do not or should not know that they are being monitored; classified memos are sent to the service, and intelligence agents act under cover. The official and visible assessment mission thus enters into tension with the secret and partially invisible intelligence mission. The analysis of this tension will serve to illustrate the institutional changes at play as well as the structural biases that underlie anti-terrorism activity in prisons.

This article, based on an ethnographic study in three French prisons, describes and analyses the informal practices of prison intelligence within the RAUs. These are high-security units to which prisoners identified as 'radicalized' are transferred. Prisoners may be assigned to an RAU on the basis of a criminal charge (for an offence of a terrorist nature), suspicion after observing the prisoner's behaviour in detention, reported 'radicalization', ties between the prisoner and other prisoners already identified as radicalized, and so on. Inside these units, the prisoners are assessed over a four-month period in order to gather objective signs of their degree of radicalization and potential to be dangerous. This assessment is conducted jointly by the guards who observe the prisoners daily and by psychologists, specialized educators and probation officers who regularly meet with the prisoners. The whole group of professionals assemble periodically in pluri-disciplinary meetings, often accompanied by a member of the prison management and a prison intelligence officer, as they gradually formulate assessment reports for each prisoner.

This article does not intend to propose a sociology of the assessment activity in these units (see Chantraine and Scheer, 2020a). Rather, it focuses on an essential (and hidden) structuring dimension of the organization and daily functioning of a RAU: prison intelligence. Against the backdrop of the recent, rapid and massive development and professionalization of the prison intelligence services, we start 'from the bottom' to analyse how the activity of these services impacts prison government, through the new relation that comes into place between the intelligence officers and the other professionals working in the RAU. In particular, the purpose of the prison guard function is partially redefined by this new relation. We also analyse the impact of these services' activities on the orientation and classification of prisoners identified as 'radicalized' and the interference of the prison intelligence activity on the very process of prisoner assessment.

This article builds on and contributes to security studies and radicalization studies in the context of the globalization of counter-terrorism [START_REF] Kundnani | The Globalisation of Countering Violent Extremism Policies: Undermining Human Rights[END_REF]. Extending the analyses of banalization of counter-terrorism duties [START_REF] Kundnani | The Globalisation of Countering Violent Extremism Policies: Undermining Human Rights[END_REF][START_REF] Ragazzi | Countering terrorism and radicalization: Securitising social policy?[END_REF], our original empirical data enabled us to see anti-terrorist activity in prisons as a process of self-confirming hypotheses of radicalization and dangerousness. This double movement, between banalization and self-confirmation, fosters the institution's bureaucratic and legal transformation. The structure of this article defends this thesis in three steps: 1) Intelligence gradually came to be incorporated into official missions of the prison administration, in direct reaction to the terrorist attacks committed in France. This new mission, which has taken shape through the creation of a dedicated professional corps of prison intelligence officers, amounts to a significant institutional shift. 2) The other prison professionals must also incorporate certain intelligence missions into their own traditional activity. This incorporation does not come about easily and it raises various reactions, ranging from a sense of new professional importance to professional ethics dilemmas. 3) The prison professionals responsible for assessing the prisoners and the new prison intelligence officers participate in producing a special type of knowledge about the prisoners, the main feature of which is to confirm hypotheses of dangerousness. Thus, although the radicalization assessment job and the intelligence work are completely distinct missions, the tension between these two objectives often leads to validating one's intuitions and retaining only elements that can be incriminating, thus also validating the famous expression 'where there's smoke, there's fire'.

Fields of study, method and emergence of an object of research

This article is based on a sociological study conducted in RAUs in French prisons. The RAU is a temporary unit assembling groups of 'radicalized' prisoners for assessment purposes. Depending on the results and recommendations at the end of the assessment, the prisoners, after their stay in a RAU, may be assigned to 'ordinary detention' in a maximum-security prison, or be placed in solitary confinement or in a radicalization management unit. The study followed an agreement between the prison administration directorate and the National Centre for Scientific Research; the agreement defined a precise deontological, ethical and practical framework, in particular the assurance of protection and anonymization of individuals and data. The research consisted of 100 days of ethnographic observations during immersion in three French prisons and 90 semi-directed interviews with prisoners, prison professionals and prison administration directors.

Although prison intelligence did not appear during the problem definition phase of the research, it quickly emerged in the field as crucial and incontrovertible. The idea that information generated in RAUs on 'radicalized' prisoners would not be used solely for assessment purposes, but also serve as intelligence information, became core to the analysis. Taking into account the professionalization of intelligence activity through the creation of a specialized corps, the efforts of many other professionals (the guards to begin with) to also participate in intelligence missions, and the ponderous effect the intelligence services had on decisions taken in the RAUs, we gradually came to consider that the intelligence activity was at least as important as the formal assessment objective.

The rise and development of prison intelligence in France

In this first section we will discuss the massive development of prison intelligence since 2016, in connection with contemporary transformations in the prison administration culture. This historical overview thus describes the creation of a genuine intelligence service at all levels of the administration (national, regional, local) and the recruitment of trained professionals dedicated to intelligence work. Understanding the origin and specificities of prison intelligence in France is crucial to grasping the extension of intelligence logics and the professional banalization of counter-terrorism in prison.

In France, prison intelligence was set up following the 2004 attacks in Madrid. It was dissolved in 2015, only to be restored and re-christened as the Central Bureau for Prison Intelligence in 2016, in direct response to the Charlie Hebdo and Hyper-Cacher attacks along with the Bataclan attack, which involved terrorists who had become radicalized in prison. Then, in 2019, prison intelligence was designated a national competence service and once again changed its name to the National Service for Prison Intelligence. Prison intelligence thus gradually became an essential link in the French intelligence community. The second version of the French white paper on defence and national security in 2013 refers to prison just once. Nevertheless, the soaring development of prison intelligence has spawned major changes within the prison administration, in terms of relations of power, mission priorities or transformations in the institutional culture. Indeed, as early as 2006, the government's white paper on domestic security against terrorism reflected the concern of public authorities regarding the danger of radicalization in prison: 'Prison has become a place where dangerous proselytization takes place. If we are not careful, this will eventually produce a reservoir of radical activists available to conduct terrorist acts' (General Department of National Defence, 2006: 55). Although contacts between the prison administration and the police were longstanding, they were neither structured nor explicit; the formalization of this relationship has come about through professionalization of intelligence activities. As Manon Veaudor has pointed out, 'promoting cooperation between prison and police services is less a governmental innovation than the consecration of old practices' (Veaudor, 2020: 363). Although the prison was always suspected of being a petri dish for criminal networks and was always a site that produced information on these networks (through the use of informers), it is now the fear of Islamist contagion that provides the administration with justification for these main organizational overhauls. These transformations organize the legal framework for customary practices. In doing so, professionalization of intelligence information inside the prison administration, along with joint development of tools, standards and rules for handling sensitive information in the prison services, reconfigures the relations of cooperation and power among professionals.

These evolutions took concrete shape through an upheaval of the prison administration's organization chart: the national prison intelligence service was placed under the direct authority of the director of the prison administration. In addition, several professionals from the Ministry of the Interior joined the prison administration in this new service, bringing with them their former professional culture. Certain managers and officers are now accredited and authorized to consult intelligence files and utilize specific information gathering techniques. This growth and professionalization of intelligence within the French prison administration has also entailed an expansion of the legal and technical tools available to enable the prison administration to gather information on prisoners and employees, wiretap private places, capture computer images or data and spy on mobile phones. These new intelligence techniques are an addition to the surveillance apparatus already used by prison staff, including frisking and phone tapping. As well as these formal modifications establishing authorizations and accreditations for prison administration agents, the prison intelligence officers have massively directed their activities towards the fight against terrorism and violent radicalization, especially in connection with the prison administration's formal mission to combat violent radicalization. According to [START_REF] Urvoas | Communiqué de presse du Garde des Sceaux. Inauguration du Bureau Central du Renseignement Pénitentiaire (BCRP[END_REF], the official prison intelligence missions cover three main areas: gathering and analysing information on the security of the prison facilities and prison services (preventing evasions, intrusions and rioting); regular and individualized monitoring of prisoners in relation to underworld activities and organized crime; and information gathering and monitoring in relation to terrorism, henceforth associated with the question of 'radicalization' [START_REF] Brie | Réflexions sur la réappropriation scientifique d'une notion très problématique[END_REF]. The latter area has now clearly taken precedence over the other two, as confirmed by all the prison intelligence actors we met, including this officer:

The question of radicalization, proselytising and terrorism, that must take me . . . [he silently calculates]. I'd say that takes up about 80 to 90% of my work hours. I'm sure that even if we doubled or tripled the number of intelligence officers, it would still take at least 80% of our time. I've had to let my other missions go, but it's a question of priority. (Fred, prison intelligence officer, interview 1) Accordingly, the administrative evolutions and priority given to fighting radicalization go hand in hand with a transformation of the prison institution, which to a greater extent now integrates the intelligence logics and a culture of secrecy coming from the Ministry of the Interior. These institutional transformations have direct consequences on daily practices: those of the intelligence officers, obviously, but also more generally the practices of everyone working in the prison. Among these professionals, the guards are the ones who have seen the meaning of their work deeply altered by the prevalence of intelligence activities. As such, the growing professionalization of prison intelligence (which confirms practices already in existence) as well as the strong encouragement to participate in intelligence efforts, and more globally the fight against terrorism, considerably alters the guards' work -and also, as we shall see, that of the probation officers, the psychologists and the educators -who are led to participate in producing a special type of knowledge about radicalized prisoners.

Prison staff and how they deal with intelligence

In this second section, we will look into the upheavals entailed when prison professionals working in the RAUs integrate the intelligence mission into their work. On one hand, the intelligence activity has brought more prestige to the prison guard profession; on the other hand, integrating intelligence has noticeably changed the habits of other professionals -probation officers, psychologists, educators -making it complicated to reconcile intelligence with their mission to assess the prisoners. These two facets are discussed in more detail in the next two subsections.

The ethnographic study that formed the basis of the present article was spatially limited to the RAUs, even though we interviewed several people in the interregional directorates and the central administration. As such, we cannot claim to analyse the whole circuit for the information collecting, cross-checking and transmission [START_REF] Veaudor | Les 'frontières' de l'ordre carcéral. Affectation, négociation des identités et surveillance en maison d'arrêt [The 'frontiers' of the prison order. Assignment, negotiation of identities and surveillance in prisons[END_REF] that organize intelligence activities. For example, information cross-checking is not done in the prisons, but rather in the interregional offices of prison intelligence. Likewise, we did not have direct access to the exchanges between the national prison intelligence service and political or general intelligence services. From this point of view, therefore, our data remains partial. Through this innovative article we hope to inspire later studies in order to complete the analysis of this major transformation in French prison administration. Despite this inevitable information gap, the analysis enabled us to update our understanding of essential elements regarding the effect this rise in intelligence work has on interprofessional relationships and, more fundamentally, on the way it transforms the work of prison actors. Indeed, in a RAU, whether formally or not and to varying degrees, all the professionals -guards, educators, psychologists and probation officers -are involved in collecting information and interpreting the prisoners' words and actions, primarily for assessment purposes, but also to provide substance for possible intelligence files.

Here, it is important to stress the distinction between assessment work and intelligence work. Even if the tasks themselves occasionally may be similar: gathering, comparing and analysing information in order to compile 'profiles', the purposes are different, something of which the professionals in the field are perfectly aware. The work in the RAU is organized around the assessment mission and all the professionals participate in this official duty. The guards thus add the new assessment mission to their usual tasks of controlling and ensuring order; the probation officers concentrate fully on the new mission; and lastly the most recent professionals, the psychologists and educators, have been explicitly hired to participate in the assessment. Together, these staff members are in contact with the prisoners in order to analyse individual situations and produce synthesis reports so that each prisoner is assigned to the most suitable and adapted path for serving their sentence. The assessment work is carried out with full transparency and the prisoner receives a copy of the assessment report. In addition to this official duty, the same professionals are invited, this time unofficially, to add to the stock of prison intelligence by collecting certain types of sensitive information or spontaneously warning the intelligence services in case of observations or behaviour that raise concerns. More broadly, the professionals are occasionally asked to inform the intelligence services about ties between prisoners in the facility: who spends time with whom; the standing among the prisoners: who is a leader, who tries to convert the younger prisoners; what type of books are found in their cells, and so on. This intelligence mission, which remains invisible and secret, aims to help manage risks, draw up profiles to be monitored and analyse threats based on individuals' potential to be dangerous. The need to, at least partially, integrate the intelligence activity into the assessment process means that the professionals must redefine the sense they give to their job and the actions they take in the prison, which we shall analyse below.

Guards: From security to espionage

In the RAUs, the guards are very carefully selected in view of the new mission entrusted to them: to observe the prisoners and take notes on what they observe. While keeping up their main duty of monitoring and ensuring security, the guards are now also involved in prisoner assessment. The guards' ambitions, however, quickly moved beyond merely participating in the assessments. The association between the two facets of the job -security and assessment -quickly led the guards to feel that they were taking part in intelligence activity. This investment can be explained both by the fact that the guards are familiar with the role of surveillance and analysing possible risks, and by the symbolic enhancement of the guard profession (one that ordinarily has little esteem). The guards continue to do what they know how to do, namely observing and passing on information [START_REF] Chantraine | Ethnography of writings in prison: Professional power struggles surrounding a digital notebook in a prison for minors[END_REF], but this observation work is no longer tainted by the dishonourable and stigmatized job of violating the prisoners' dignity [START_REF] Chantraine | Ordre, pouvoir et domination en détention: les relations surveillants-détenus dans une maison d'arrêt en France [Order, power and domination in detention: The relationship between guards and prisoners in a correctional facility in France[END_REF][START_REF] Chauvenet | La violence carcérale en question[END_REF]. Quite the contrary, it is now held up as an activity essential to the survival of the nation. Thus, the employee who may tend to feel misunderstood and unappreciated by the hierarchy [START_REF] Crawley | Doing Prison Work: The Public and Private Lives of Prison Officers[END_REF] or reduced to the thankless status of 'turnkey' -a vision of the work reduced to merely the opening and closing of doors -becomes a hero who protects their fellow citizens.

It's not the assessment itself that gives meaning to my work, it's the positive consequences behind it. It's preventing attacks, stopping the evil people, saving lives . . . and all that calls for intelligence information. (Cyril,guard,interview 2) If gathering sensitive information is so important to the guards, it is because this activity is conducted in a special context. To explain this state of war, they invoke the number and regularity of terrorist attacks or the risk that the prison poses as a site for radicalization. Therefore, they examine the slightest gestures of the prisoners, perceived as members of a rebellious and dissembling group. The guards' daily work is filled with surveillance scenes like those we observed during our ethnographic study: a guard studies the behaviour of each prisoner during prayer time, noting carefully how long the ablution lasts and how deep someone bows; in the evening a guard removes his boots and belt to move quietly along the corridor, listening to the discussions between one cell and another; in an office a guard photographs all the mail sent and received by the prisoners and files them away while a colleague listens to all the phone calls recorded and transcribes what seem to be the most interesting passages; another guard observes the behaviour of a prisoner during an interview with a female educator: does he shake her hand and look her in the eyes? A guard searches a wastebasket for the scraps of a letter that a prisoner tore up; a guard is interested in the television shows the prisoners watch and writes down who watches spy or war movies; another guard asks the leader of the chess workshop to write down the moves deployed by the prisoners to detect the strategists and the doers, the leaders and followers; one guard meticulously writes down titles of the books in each prisoner's personal effects, with a picture of the cover and notes on the content (inoffensive or dangerous, authorized or prohibited); another guard crouches and crawls along behind a wall to listen to the prisoners' conversations during their walks . . . All these instances of observation and surveillance, unbeknownst to the prisoners, in a logic of providing information useful to security and intelligence, illustrate the guards' commitment to their information gathering mission.

We're at war with ISIS, don't forget. Here, it's a bit like an observatory of their rear forces. It's in prison where they recruit or sometimes even prepare attacks. Yes, I do spend a lot of time tapping phones or trying to hear what they talk about at night. But my main mission isn't gathering information or spying. I suppose that's just a bonus, you might say. But obviously, I keep my ears open and I hear information. If I think it's useful, I'll pass it on, for sure. It's my duty, you might say. It's not my duty as a guard, but as a French citizen. (Nick,guard,interview 3) In the carceral world, the RAU is a place where conditions for gathering information differ widely from normal prison. To begin with, the number of prisoners in the unit is quite limited and the guard to prisoner ratio is unusual: 12 prisoners for six guards (2:1), compared to over 100 prisoners per guard in normal prisons, characterized by being overcrowded. But they also differ in the time devoted to observation in place of daily management of urgent tasks. While the RAUs are thus privileged sites for observation, we shall see that paradoxically they are often considered to be the wrong devices for intelligence.

Psychologists, educators and probation officers: The intelligence dilemma

To complete the professional panorama in the RAUs, the daily work of some staff members consists solely of prisoner assessment. The probation officers, psychologists and educators speak regularly with the prisoners or observe them in group activities. They progressively build a 'profile' and complete an assessment report that will be used in assigning each prisoner to the most appropriate facility for serving their sentence. The professionals each deploy their own knowledge and techniques: clinical interviews, family inquiries, psychological or actuarial tests (Chantraine and Scheer, 2020b), to analyse each individual situation. Each prisoner is invited to participate in the assessment, which is a transparent process. At the end of this process, the prisoner is given the assessment report. The professionals' recommendations for future handling are explained: assignment to a normal prison or one under a particular security regime; contacts to avoid or pursue; specific programmes to follow with respect to managing violence or de-radicalization, and so on. During the assessment work, on occasion the professionals may obtain troubling information or feel deeply worried about a particular individual's behaviour or words. In such cases they have the ability to transmit this information to the prison intelligence services. Furthermore, a local intelligence agent regularly comes by for informal discussions with the RAU professionals or participates in their meetings. In addition, the prison intelligence services occasionally request information about a prisoner in order to supplement a profile, assess a threat or establish links with people in criminal networks. Although assessment work and intelligence pursue different objectives, there is nevertheless a connection between the assessment activities and the intelligence process. The issue at stake for the professionals, therefore, is to distinguish between information that is relevant solely for the assessment and that which is deemed to be 'sensitive' and thus useful for intelligence.

Prison intelligence do their job and I do mine. I don't need to know their information or their point of view. But we're all in this together. I love my country and want to protect it too. So, when I pass on information, it's always off the record, just to let someone know when I have a major doubt or if, on the contrary, I'm sure that I'm dealing with a dangerous enemy of the Republic. (Omar,educator,interview 4) Some psychologists and educators are quite reluctant to communicate with the intelligence officers. The psychologists, especially, who value their professional independence and the confidentiality of their clinical relationship with the prisoners, regularly adopt techniques to protect themselves from intelligence intrusion into their work, for example by taking notes that are hard to understand and keeping their notebooks under lock and key.

Then there are other professionals (psychologists, educators and probation officers alike) who regularly convey information to the local prison intelligence officers, whether officially in meetings or informally in the hallways or the cafeteria.

I communicate a lot with the intelligence people. Already during meetings, I give them my point of view. We get along well. It's also a question of who I'm dealing with, a lot is informal. It's 'we each have our job to do' but we can work hand in hand to try to avoid the worst [outcome] and we do. Obviously. Even if I do all I can to help [the prisoners] get by, at one time or another you also have to fight them. (Mike,psychologist,interview 5) It is important to note that assessment work itself is at odds with the professionals' usual practices. Their number one task in the RAU is not treatment (the psychologists' habitual role), nor is it educational or criminal case follow-up (the usual job for probation officers). Their job in the RAU is assessment. As such, they have regular meetings with the prisoners, pass on information about them, cross-check their remarks with other information available (police record, contacts with family or friends, observations by colleagues and so on) and provide an interpretation (the 'profile') in an assessment report.

This process -gathering together a stack of information, cross-checking, interpretation -has echoes in the work and data circuits typical of intelligence services. Nevertheless, the distinction between assessment work and intelligence, according to the professionals interviewed, lies in the quality of the information collected. Depending on the degree of reliability or the secret nature of the information, it will either be used for assessment or else transferred to the prison intelligence services. For instance, information deemed reliable and obtained legitimately will usually appear in the context of the assessment, written down and presented to both the prison management and the prisoner as well. On the other hand, elements with a tinge of doubt -such as suspected dissembling, a non-verifiable impression, an isolated element seeming to suggest an individual's potential dangerousness -and information deemed to be sensitive, obtained for example from a protected source, will be transmitted to the local prison intelligence officers.

An effect of this double information circuit in a simultaneous logic of assessment and intelligence, which consists in starting from scratch to create conditions for observation, even if this leads to the persistence of what professionals call 'situations at the limit'. For example, some think that the salience of the intelligence mission pushes them to produce information at any cost, without verification or cross-checking. What is more, intelligence can be perceived as a constraint that complicates the assessment per se, and some professionals try to clearly dissociate their mission from a strictly intelligence-based mission of surveillance and control. Consequently, some take a dim view of the omnipresence of intelligence and occasionally refuse to participate in this 'work in the shadows' (Carla, probation officer).

I don't see any added value in intelligence being part of our mission. I'm a probation officer, not a spy. That's not my job. I personally don't give them a thing. There's even information I will never give out in meetings because I'm afraid it may be used any old way.

-Who by? -By intelligence. They work in the shadows and you don't know what they're going to do with the information we pass on to them. That can lead to sticky situations.

-Like what for example? -[A prisoner] has been polygamous his whole life. I can't tell anyone that. It would only be interpreted negatively. Me, I can talk to the prisoner to see where he's at with this, whether it's cultural or if it's a sign of rejecting democratic values. But if I tell [the] intelligence [service], that's the best way for him to be reported and placed in solitary confinement, and for him to have no chance of an adjusted sentence. (Carla,probation officer,interview 6) If we can thus observe a plurality of stances with respect to intelligence, from refusal to cooperation, it is important to note that in the minds of the RAU staff, the missions of assessment and intelligence information are dissociated. Indeed, the information is used in different ways depending on its very nature or at least what each professional perceives its nature to be. An objective element apt to solidify the assessment will be used for this purpose, while an inkling liable to attract the attention of the surveillance and investigation services will be transmitted to those responsible for prison intelligence. Nonetheless, integrating intelligence into the core assessment work activities leads to new forms of collaboration, competition and conflict, providing fodder for criticism of prison intelligence in the RAUs.

Prison intelligence: Cooperation, jurisdiction conflicts and criticism

In the third section, we examine the concrete production of sensitive information in the RAU, information the intelligence officers then send through their own hierarchy in order to cross-check it with information further up the ladder, especially when it is tied in with police services. We will analyse the problematical cooperation between assessment professionals and intelligence officers: the way that each corps defends its own territories for action, at the risk of trespassing on the other's. Lastly, we will examine a recurring criticism formulated by RAU professionals, whether they are intelligence officers, guards, educators, psychologists or probation officers. They denounce the paradox whereby, even while prison intelligence is omnipresent in the RAU, the specific organization of these units is not suitable for collecting quality information. This last section, combined with the two previous sections, more globally examines the role of intelligence in the process of classifying and orienting the prisoners, as well as the production of a specific form of knowledge relating to 'radicalized' prisoners.

We will describe and analyse the concrete techniques deployed by the intelligence officers to gather information in the RAU. By virtue of organizational particularities in these units, techniques differ from those implemented in ordinary prisons. We will attempt to objectify the intelligence officers' own jurisdiction, in other words the privileged link that this professional group has with the diagnosis, handling or resolution of a specific problem [START_REF] Abbott | The System of Professions: An Essay on the Division of Expert Labor[END_REF]) -in our case that of good information. The intelligence officer's jurisdiction is structured by legal dimensions (as the intelligence officer's investigative power is not the same as that of the other professionals); it is also structured through relations of cooperation and negotiation between different professional corps. Furthermore, this jurisdiction is a territory that other professional groups may call into question. Thus, all the while cooperating and negotiating, the RAU professionals -guards on the front line, but also assessment professionals -may all wish to show that they are actually the real intelligence officers.

The intelligence officers: Pressure from above and dependence on what is below

Since the French Intelligence Act of 24 July 2015, accredited prison agents are authorized to access the automated national file on perpetrators of terrorist offences, 1 thereby including prison administration members in both the consultation and the collection phase for sensitive information on terrorist prisoners. Although the other professionals do not have these same prerogatives, they nevertheless participate in collecting information whether or not they employ the same techniques, and whether they act on the order of their hierarchy, prison intelligence services or on their own initiative. Thus, a high volume of information of all shapes and forms is produced in the RAU. This includes the nature of the relationships among prisoners (whom someone knows or spends time with), contents of communications with the outside (correspondence, telephone conversions), the result of prison cell searches, whether a prisoner practises sports or keeps to themselves, attitude towards prison staff especially female, the type of strategy deployed during a chess workshop, cleanliness of the prison cell, type of reading materials or films watched, respect or lack of for prayer times, and so on. The local prison information officers collect this information through various channels (information from a prisoner or a professional, direct observation, etc.) before sending it on to their interregional hierarchy, which itself reports to the national service. This internal flow of information, which remains within the prison administration and is shared upwards, is less formally structured than the communication with other services, which flows outwards to partnered non-prison agencies, especially the judicial authorities. 2 Furthermore, following attacks carried out in prison -a guard attacked by a radicalized prisoner at the Osny facilities on 4 September 2016; a similar incident at the Condé-sur-Sarthe prison on 5 March 2019 3 -the failure to adequately share information among front-line professionals (guards, assessment staff, etc.) and the prison intelligence services has been regularly criticized.

Consequently, prison intelligence has to deal with this pressure by providing more information and information of a better quality. However, prison intelligence's different hierarchy levels: national service, interregional agencies and local officers, largely depend on information passed up from the field, thus from the prisons themselves, in other words information sent up via the local prison intelligence officers (but sometimes also directly from the professionals who identified the information). This dependence, described by intelligence officers as the 'need to know' (Oliver, prison intelligence officer, interview 7), underlies the crucial importance of the actors on site. These field actors, moreover, are often in communication with different contacts outside the prison to pass the information on.

Prison intelligence has evolved quickly in recent times, especially in the national and interregional services. It's really taken off. And that was kind of a blow for us, the local officers. The national services envy our privileged relations with outside partners [police services, general intelligence agencies]. We work a lot with the cops, directly or with the gendarmerie and the DGSI [General Directorate for Internal Security]. If we have some information we want to share, we get together for a beer and slip a note under the bar counter. But it's getting more and more complicated. Now we have protocols and all these meetings that ruin the speedy and correct circulation of information. We can't even call up our partners, we're supposed to work with the hierarchy. And those people, all they want is the 'need to know'. A term you hear everywhere now, meaning 'don't tell the others, tell us'. They're aware that we're the ones with information, that's at least something! (Oliver, prison intelligence officer, interview 7)

The mission of the local officers is to gather and compile information about the prisoners in order to identify risk profiles and to also transmit this information to the national services so as to bolster any later investigations and prevent incidents, prison escapes or attacks. The officers responsible for intelligence information are aware that their work, above all, depends on information received directly from the field, thus from the other professionals (guards, psychologists, educators, probation officers or others) or, more rarely, from prisoners. In the words of one officer, 'I have to know about everything, but I don't do a thing' (Mark, prison intelligence officer, interview 8). Here again, the 'need to know' seems to be primordial and always in relation with the staff in direct contact with the prisoners. Yet this does not keep the prison intelligence officers from voicing sharp criticism of the information sent up. For instance, they often describe a bulimia in information collected and documentation of profiles. 'Too much information kills information', John, a local prison intelligence officer explained, on the same occasion noting that he was the only one capable of identifying 'good information'.

The RAU staff collect a lot of useless information. They think they're doing intelligence work, but they're

The impermeability of the RAU has the opposite effect: a lack of information. We get snatches of information from phone taps or listening at night, but it's pretty poor. In fact, that's not true . . . we have tonnes of information! But we have neither the time to sort it out, nor the . . . I'm not convinced I'm going to find anything interesting. In all that comes our way, there are only commonplace things, unfounded unchecked elements, isolated snatches of information. Impossible to do intelligence work with that. . . . The aim is just to fill up. They don't try to have anything pertinent, just fill up. Fill, write, fatten up the files.

(John, prison information officer, interview 9).

As the RAUs are the sites for assessment and, as such, devices for intense observation of the prisoners, their ability to produce intelligence information is thus open to serious criticism. Occasionally considered as bad information-collecting tools, they are mainly symptomatic of how hard it is to reconcile detention management, assessment work and the intelligence mission.

The RAUs: Sites for intense observation, but criticized

While the professionals regularly invoke the objective of national security to justify their work in the RAU, local security is also at stake: measures to prevent proselytizing inside prison walls, steps to avoid aggressions or attacks inside prisons, identifying and confiscating writings that are illegal or of a jihadist nature and so on. All these cases raise the question of specific information production methods inside the RAUs.

One of the rare analyses of the functioning of prison intelligence in ordinary detention settings recalled the essential role of the informer, this 'stool pigeon' with whom the guard or intelligence officer attempts to enter into direct contact (Veaudor, 2020: 355). In the RAU, however, the informer plays a lesser role. Unlike ordinary detention, where the prisoners have more room for manoeuvre and where their actions are potentially significant for intelligence, this margin is drastically reduced in the RAU, leading some professionals to take risks willingly in order to produce information despite the odds. For example, during a meeting in a RAU to which an officer of the national prison intelligence service had been invited, the question of taking risks to produce information was debated:

In a meeting, the troubling case of one prisoner, Akhmad, was evoked, especially because he held a lot of sway over a young prisoner also assigned to the RAU.

-Guard: 'It's interesting to see the evolution. [The young prisoner] has ordered the same books as Akhmad. He only goes out for the daily walk when Akhmad goes, without the others. We changed the exercise groups and let them be, to observe. You need to watch how the groups form'.

-Probation officer: 'You're laying a trap. You'll have to remove it one day or another . . .'.

-Guard: 'That's the tool of our trade. Let us keep on working. It's the way we can get information too'.

-Prison intelligence officer: 'Well, I've got 450 guys to monitor, I need information. I want information on Akhmad. I want info on that guy! In my view, he's extremely dangerous'.

-Guard: 'I'm observing him using the only tool I have. I accept his requests to change prison cell. I let him talk to the other highly radicalized people and recruiters. I watch how he approaches the most fragile ones. I enter into his game to understand the strings he pulls'.

(Field notes)

This issue -exactly how far one can go to produce information -is not the only critique addressed to RAUs on the way they produce information. Indeed, the intelligence officers raise one essential question: are the RAUs the right place for observation to produce quality intelligence information? The officers we spoke to invariably replied that it was not, for a variety of reasons. To begin with, the duration of time spent in the RAUs -16 weeks, the period foreseen for the assessment workseems far too short for the prisoners to adopt natural behaviour. Furthermore, the prisoners, knowing they are being assessed, always try to present themselves in the best light possible. Then, the officers doubt the RAU's efficiency from an intelligence point of view, by virtue of the simple idea that security and control run counter to intelligence.

If you want to have good intelligence information in a RAU, you absolutely have to have less security, let them have their mobile phones, let them live. In the RAU, it's just guard duty. And security kills intelligence. (Nordine,prison intelligence officer,interview 10) Despite these critiques, collecting and compiling observation and interview notes are important tasks in the RAUs and they produce a large volume of data. Even if, according to the intelligence officers, the specific knowledge produced by and in the RAUs tends to be vacuous and marked by speculation and inconsistency, it must nevertheless be handed in, if only as protection against any accusation of professional misconduct. Prison intelligence thus becomes an important facet of the RAU professionals' daily work. For example, in a guard office, all the furniture and the work itself is organized around information gathering, meticulously written down or entered into the computer. We observed how carefully the guards recorded the relations between prisoners, behaviour observed and activities: visitors are listed, incoming and outgoing mail photographed and filed, gestures and interactions during walks noted and so on. A file of daily and weekly observations is thus shared among the guard team (who add to it continuously), with the assessment staff and also with the directors and prison intelligence officers of their local prison. This office even has a desk specially set aside for this task: a guard listens to phone taps of all the communications with prisoners in the RAU and transcribes the passages he deems to be relevant, or he lists all the books in the prisoners' possession (adding, after consulting an imam, the notations 'authorized', 'prohibited' or else 'unsuitable contents'). Not only is the office a strategic place for cooperation among the guards and the intelligence officers, it is also the spatial expression of the territorial conflict between these two professional groups.

I'd readily say that we do a job that's more targeted than intelligence is. We're in contact with these guys, we know them, observe them and see how they change. We're not sitting at our computers. They [prison intelligence officers] base themselves on hearsay, nothing but hearsay. And hearsay gets deformed. (Thomas,guard,interview 11) This jurisdiction conflict also affects the other professionals in charge of assessing the prisoners. For instance, the psychologists, educators and probation officers often explain that their job, in parallel with the assessment work per se, consists of real investigation -that's how they describe their mission -whereas the intelligence officers simply wait for information to be sent to them, at the same time showing no reciprocity in circulating information:

Every now and then I've given information to the prison intelligence services. I did my own investigation of one guy. I interrogated his wife in detention, I called his family and colleagues of mine who had dealt with him in other prisons. I really went in depth with this file. I transmitted everything to prison intelligence. Then . . . nothing. I don't really understand what help they can give us. We feed them data more than the other way around. There's no exchange. And what they do have to say is often so simplistic that I don't even take it into account. (Martine,educator,interview 12) By virtue of this one-way information flow, associated with the image of an information-producing machine, the professionals often consider that the portrait of a prisoner, gradually compiled by the intelligence services, is nothing more than caricature.

I systematically transmit [my remarks] to the intelligence [service]. I never hear a 'thanks' or even a 'received' in return. I write down my doubts about the quality of the information, but the intelligence people delete those remarks. To be honest, I don't think they do a good job. They settle for interpreting observations shared on the common files. And what's more, they're under extreme pressure from the hierarchy who want to read about dangerous [prisoners]. (Victor,guard,interview 13) This is a regular criticism of profiling in the RAUs: that the lack of well-balanced exchange between different services, the amount of information produced and the multiple filters inevitably lead to compiling profiles that confirm the hypothesis of dangerousness. This criticism once again raises questions about the quality of the RAUs with respect to the production of intelligence information.

Conclusion

The prison institution is regularly presented in terms of inertia and self-perpetuation [START_REF] Chantraine | Les temps des prisons[END_REF][START_REF] Scheer | Conceptions architecturales et pratiques spatiales en prison: De l'investissement à l'effritement, de la reproduction à la réappropriation[END_REF]. From the angle of the sociology of the prison, the analysis we propose in this article is a significant contribution because the development of prison intelligence is undoubtedly one of the most important transformations in the recent history of the French prison administration -an area in which France is also a pioneer. However, it has never been the object of an in-depth analysis. The development of these services enshrines an evolution characterized both by increasing participation of the prison administration in the homeland security effort (Mbanzoulou et al., 2013) and the reinforcement of police work in the prisons [START_REF] Quinquis | Le plan 'Urvoas' : un pas de plus dans la transformation policière de l'administration pénitentiaire[END_REF]. Nonetheless, only a few works discuss the recent development of intelligence services within the prison administration [START_REF] Delarue | Le renseignement pénitentiaire [Prison intelligence[END_REF][START_REF] Scotto | Le renseignement pénitentiaire [Prison intelligence[END_REF], stressing how these services are fixated on violent Islamist-inspired radicalization [START_REF] Warusfel | Justice et renseignement dans la lutte contre la radicalization violente [Justice and intelligence in the fight against violent radicalization[END_REF] or else, depending on the correctional establishment, just the opposite: on more traditional forms of criminality [START_REF] Veaudor | Les 'frontières' de l'ordre carcéral. Affectation, négociation des identités et surveillance en maison d'arrêt [The 'frontiers' of the prison order. Assignment, negotiation of identities and surveillance in prisons[END_REF]. The sociology of prison intelligence, therefore, should be seen in the very historicity of the prison, between heritage and novelty: it is founded on old penitentiary practices (observation, the heart of the intelligence trade) at the same time as it reconfigures them.

Thus, this article has assumed the difficult task of narrating the dramatic reforms to carceral governance with regard to radicalization, then putting these transformations into a narrative that explains competing and contradictory intelligence practices from the perspectives of guards, assessors and the mediating intelligence officers. If the study can undeniably provide input to the sociology of the prison, the same can be said in the realm of intelligence studies. Indeed, the development of intelligence studies in both France and the United States was originally marked by the prevalence of a historical focus [START_REF] Dewerpe | Espion. Une anthropologie historique du secret d'Etat contemporain[END_REF][START_REF] Holeindre | La ruse et la force : une autre histoire de la stratégie [Guile and Strength: Another History of Strategy[END_REF][START_REF] Jackson | Intelligence and the state: An emerging 'French school' of intelligence studies[END_REF][START_REF] Scott | The study of intelligence in theory and practice[END_REF]Warusfel, 2007). Historical studies on intelligence were undertaken from the 1990s, first in the fields of military and political history, later in the field of international relations [START_REF] Forcade | Objets, approches et problématiques d'une histoire française du renseignement : un champ historiographique en construction [Objects, approaches and problematics of a French history of intelligence: A historiographical field under construction[END_REF]. Intelligence then developed gradually as a field of study of its own, especially when analysing the emergence of liberal democracy and the growth of state apparatus [START_REF] Laurent | Pour une autre histoire de l'État. Le secret, l'information politique et le renseignement [For another history of the state: Secrecy, political information and intelligence[END_REF]. This reference to history can be explained primarily through the archive approach. Consulting archives that contain declassified data helps reveal the hidden face of the state, its grey areas (Laurent, 2003). In the European context, and more specifically in France, studies on intelligence have become more diversified [START_REF] Ocqueteau | Introduction. Champ pénal/Penal field (17)[END_REF], whether in their approach or their disciplinary fields, and whether or not they claim to fall under intelligence studies [START_REF] Lyon | Surveillance Studies: An Overview[END_REF]. Although, in France, the functionalist orientation prevails less than in the United States, most writings nevertheless place security -whether as a contemporary state imperative, as legitimating surveillance practices or in a fragile balance with liberties -at the centre of their discussions [START_REF] Balzacq | Théories de la sécurité : les approches critiques [Security Theories: Critical Approaches[END_REF][START_REF] Bauman | Repenser l'impact de la surveillance après l'affaire Snowden : sécurité nationale, droits de l'homme, démocratie, subjectivité et obéissance [Rethinking the impact of surveillance after the Snowden affair: National security, human rights, democracy, subjectivity and obedience[END_REF][START_REF] Bigo | L'évolution du paysage européen en matière de liberté et de sécurité : rapport à mi-parcours sur le projet Challenge [The changing European landscape in terms of freedom and security: Mid-term report on the challenge project[END_REF][START_REF] Chopin | Effective but inefficient: Understanding the costs of counterterrorism[END_REF]. In general, studies on intelligence focus on spying and international politics, or else the effects of intelligence information or services on foreign policy (in particular see [START_REF] Baud | Encyclopédie du renseignement et des services secrets[END_REF][START_REF] Cécile | Le renseignement français à l'aube du XXIe Siècle [French Intelligence at the Dawn of the 21st Century[END_REF]. Little research has been done on domestic intelligence, and even less on the percolation of intelligence logics into other state administrations. Along these lines, by discussing the development of domestic intelligence inside the French prison administration itself, this article advances topics as yet lightly touched on in sociology of intelligence and in intelligence studies, even though these topics are quite timely and primordial for analysing transformations in contemporary intelligence .

Along these lines, it is interesting to note how intelligence logics have entered the prison culture: the sharp rise in prison intelligence services, staff imported from the general intelligence agencies, designation of officers in charge of intelligence among prison guards and granting of accreditation to consult files with sensitive information, among others. are all mutations that bring upheaval to prison culture. It can be seen that the integration of counter-terrorism missions has become virtually commonplace, as has occurred in other institutions [START_REF] Awan | Terrorism in the Classroom: Security, Surveillance and a Public Duty to Act[END_REF][START_REF] Strausz | The banality of counterterrorism 'after, after 9/11'? Perspectives on the prevent duty from the UK health care sector[END_REF][START_REF] Kundnani | The Globalisation of Countering Violent Extremism Policies: Undermining Human Rights[END_REF][START_REF] Martin | Identifying potential terrorists: Visuality, security and the channel project[END_REF][START_REF] Ragazzi | Countering terrorism and radicalization: Securitising social policy?[END_REF]. At the very least, the prison professionals (guards, probation officers, psychologists, educators, religious mediators, directors) are all instructed to be vigilant and to warn the hierarchy or specialized intelligence services should they have strong suspicions or if they have observed something troubling. In certain specific sites that house prisoners already identified as potentially dangerous (in the RAUs first and foremost), this vigilance and participation in detecting threats to security take up a large portion of the staff's informal work. The guards and assessment professionals, in daily contact with terrorist or 'radicalized' prisoners, participate in collecting sensitive information liable to be of interest to the intelligence services and security forces.

We ourselves observed a clear banalization of counter-terrorism in prison, with staff members who invest a lot of effort in these tasks (the guards), and others who integrate this new mission into their daily work with varying degrees of difficulty (the psychologists, educators and probation officers). From its origins, the prison has been a security institution: its main function is to neutralize individuals. In the case at hand, however, this role is seen in its extensive form: the prison professionals directly or indirectly assist in identifying threats and measuring how dangerous individuals may be; in other words, safeguarding national security, beyond that of the prison. In any case, this shift -whereby staff, who are not trained in intelligence, directly participate in its production -generates a specific knowledge related to 'radicalized' prisoners and more broadly raises questions regarding detecting of dangerousness and threats to security.

The objective here is not to measure the objective quality of the information extracted and compiled, in other words the effectiveness and efficiency [START_REF] Chopin | Effective but inefficient: Understanding the costs of counterterrorism[END_REF] of this contribution to counter-terrorism. Rather, our aim is to understand the nature of the knowledge produced: what types of information are generated through the involvement of all the prison staff in identifying and measuring threats with respect to radicalization and terrorism?

To begin with, we notice a concentration on the notion of dangerousness. In fact, unlike the assessment process that intends to analyse all the facets of an individual -both positive and negative, thus both risk and protection factors-the information that the prison professionals transmit to the intelligence services are characterized by the concentration of inculpating elements. Only observations that potentially reflect a security risk are passed on. In the end, this observation is fairly trivial: in order to guarantee national security, the intelligence agencies are entrusted to measure and qualify threats; this role is even more primordial in the realm of counter-terrorism [START_REF] Boer | Counter-terrorism, security and intelligence in the EU: Governance challenges for collection, exchange and analysis[END_REF][START_REF] Hughbank | Intelligence and its role in protecting against terrorism[END_REF], and it eclipses other intelligence missions such as diplomatic influence, espionage technology or providing background for state decisionmaking.

However, this fixation with dangerousness is coupled with another element: the self-confirmation that is at play in gathering, filtering and redistributing information on 'radicalized' individuals. Indeed, the most prominent common denominator we found in our analysis of the information transmitted to intelligence -but also, to a lesser extent, information retained at the end of the prisoner assessment process (Chantraine and Scheer, 2020a) -is the fact that this information systematically confirms the initial hypotheses. Stated more simply: if individuals are assigned to radicalization assessment units, they are certainly radicalized; if this individual is under surveillance, they are certainly dangerous. It appears impossible to invalidate the initial hypothesis. The self-confirmation at play in the process of knowledge production regarding radicalization is what confirms a label that has already been conferred on the subjects. When we analysed the trajectories of the prisoners assigned to and assessed by the RAUs, we noticed a form of tautology. The prisoners transferred to a RAU are selected on the basis of their penal profile, completed by a series of information mainly issuing from intelligence (both in and outside prison) and indicating 'radicalization to assess'. The RAUs are thus populated partially by virtue of intelligence information. Then, in the RAU, the joint set of the professionals' observations and the constant effort to bolster intelligence files always implies the risk of staging a dangerous individual, when the only elements retained are those that can be interpreted as confirming the main hypothesis. Lastly, the prison intelligence services participate actively in decisions regarding transfer and assignment of the prisoners following the assessment. In the mind of our interviewees, the fact that prisoners assigned to the RAU had already been selected for their supposed 'radicalization', coupled with the fact that the objective of the RAU apparatus is precisely to assess the degree of radicalization, increases the risk of a more inculpating overinterpretation. The focus on radicalization, violence and danger thus entails the potential risk of directing the nature of the information and interpretation that end up on the desks of the intelligence services.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the French prison administration (Direction de l'administration pénitentiaire), within the framework of a research convention between the administration and the National Centre for Scientific Research (CNRS, France).

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C Pan
email: c.pan@qmul.ac.uk

J Kolb
email: j.f.kolb@qmul.ac.uk

M Elkashlan
email: maged.elkashlan@qmul.ac.uk

Y Hao Are With Queen

H Ren
email: hren@seu.edu.cn

M Chen

M Di

A L Swindlehurst

Abstract-Reconfigurable intelligent surfaces (RISs) or intelligent reflecting surfaces (IRSs), are regarded as one of the most promising and revolutionizing techniques for enhancing the spectrum and/or energy efficiency of wireless systems. These devices are capable of reconfiguring the wireless propagation environment by carefully tuning the phase shifts of a large number of low-cost passive reflecting elements. In this article, we aim for answering four fundmental questions: 1) Why do we need RISs? 2) What is an RIS? 3) What are RIS's applications? 4) What are the relevant challenges and future research directions? In response, eight promising research directions are pointed out.

Index Terms-Reconfigurable Intelligent Surface (RIS), Intelligent Reflecting Surface (IRS), Large Intelligent Surface (LIS).

I. WHY DO WE NEED RIS?

Although fifth-generation (5G) wireless networks are being rolled out worldwide, the key physical layer technology therein is massive multiple-input multiple-output (MIMO) operating in the sub-6 GHz bands, while millimeter wave (mmWave) communication, originally envisioned as one of three pivotal technologies in 5G networks, has not been widely adopted.

The key impediments of mmWave communication include its sensitivity to blockages, limited coverage, and severe path loss. However, some innovative applications, such as immersive virtual reality, high-fidelity holographic projections, digital twins, connected robotics and autonomous systems, industrial internet of things, intelligent transportation system and braincomputer interfaces, are expected to be supported by 6G-andbeyond communications [START_REF] Tariq | A speculative study on 6G[END_REF]. These applications entail high quality-of-service (QoS) requirements such as extremely high data rates, ultra-high reliability, and ultra-low latency, which cannot be readily supported by the existing systems. Given the large amount of available bandwidth at higher frequencies, communications in the mmWave and even Terahertz bands will be an inevitable trend. The array gain of massive MIMO techniques at the base stations (BSs) mitigates the path loss at high frequencies, but fails to solve the blockage problem. More densely deployed BSs can help eliminate blockages and fill coverage holes, but this is a costly solution both in terms of its infrastructure (and backhaul requirements) and power consumption. Hence, new cost-effective and power-efficient technologies are needed to solve these problems.

Recently, reconfigurable intelligent surfaces (RISs), have been envisioned as a key enabling technology to circumvent the above-mentioned issues. RISs can be installed on large flat surfaces (e.g., walls or ceilings indoors, buildings or signage outdoors) in order to reflect radio-frequency (RF) energy around obstacles and create a virtual line-of-sight (LoS) propagation path between a mmWave source and the destination. An RIS is a planar surface consisting of an array of passive reflecting elements, each of which can independently impose the required phase shift on the incoming signal [START_REF] Wu | Towards smart and reconfigurable environment: Intelligent reflecting surface aided wireless network[END_REF], [START_REF] Basar | Wireless communications through reconfigurable intelligent surfaces[END_REF]. Based on the specific materials of the reflecting elements, the RIS can be classified into antenna-array-based [START_REF] Tan | Increasing indoor spectrum sharing capacity using smart reflect-array[END_REF] and metasurfacebased structures [START_REF] Cui | Coding metamaterials, digital metamaterials and programmable metamaterials[END_REF]. By carefully adjusting the phase shifts of all the reflecting elements, the reflected signals can be reconfigured to propagate towards their desired directions. Due to rapid developments in metamaterials, the reflection coefficient of each element can be reconfigured in real time to adapt to the dynamically fluctuating wireless propagation environment.

II. WHAT IS AN RIS?

As shown in Fig. 1-(a), a typical RIS architecture fabricated with metamaterials mainly consists of a planar surface and a controller. The planar surface may be made of a single or, in general, multiple layers. In [START_REF] Wu | Towards smart and reconfigurable environment: Intelligent reflecting surface aided wireless network[END_REF], for example, a threelayer planar surface is designed. The outer layer has a large number of reflecting elements printed on a dielectric substrate to directly act on the incident signals. The middle layer is a copper panel to avoid signal/energy leakage. The last layer is a circuit board that is used for tuning the reflection coefficients of the RIS elements, which is operated by a smart controller such as a field-programmable gate array (FPGA). In a typical scenario envisioned for their operation, the optimal reflection coefficients of the RIS are calculated at the BS, and then sent to the RIS's controller through a dedicated feedback link. The design of the reflection coefficients depends on the channel state information (CSI), which is only updated when the CSI changes, on a much longer time scale than the data symbol duration. Fig. 1-(a) shows the structure of each reflecting element, in which a positive-intrinsic negative (PIN) diode is embedded. By controlling the voltage through the biasing line, the PIN can switch between 'On' and 'Off' modes as shown in the equivalent circuit of Fig. 1-(b), which can realize a phase shift difference of π in radians [START_REF] Wu | Towards smart and reconfigurable environment: Intelligent reflecting surface aided wireless network[END_REF], [START_REF] Cui | Coding metamaterials, digital metamaterials and programmable metamaterials[END_REF]. To increase the number of phase shift levels, more PINs have to be integrated in each element.

RISs also have important advantages for practical implementations. For example, the RIS reflecting elements only passively reflect the incoming signals without any sophisticated signal processing (SP) operations that require RF transceiver hardware. Hence, compared to conventional active transmitters, RISs can operate with much lower cost in terms of hardware and power consumption [START_REF] Wu | Towards smart and reconfigurable environment: Intelligent reflecting surface aided wireless network[END_REF], [START_REF] Di Renzo | Smart radio environments empowered by reconfigurable intelligent surfaces: How it works, state of research, and the road ahead[END_REF]. Additionally, due to the passive nature of the reflecting elements, RISs can be fabricated with light weight and limited layer thickness, hence they can be readily installed on walls, ceilings, signage, street lamps, etc. Furthermore, an RIS naturally operates in full-duplex (FD) mode without self-interference or introducing thermal noise. Therefore, they achieve higher spectral efficiency than active half-duplex (HD) relays, despite their lower signal processing complexity than that of active FD relays requiring sophisticated self-interference cancelation. In Table I, we compare RISs to various kinds of relays, since they both serve to create alternative transmission links. The acronyms of AF and DF in Table I refer to amplify-and-forward and decode-and-forward, respectively.

III. WHAT ARE THE RIS'S APPLICATIONS?

By judiciously tuning the phase shifts of the reflecting elements of the RIS, the reflected signals can be constructively superimposed with those from the direct paths for enhancing the desired signal power, or destructively combined for mitigating deleterious effects of multiuser interference. Hence, RISs provide additional degrees of freedom to further improve the system performance. In the following, we list some typical RIS applications in various emerging systems.

A. RIS-aided mmWave Systems

MmWave techniques have the potential of supporting high data rates given their high bandwidth. However, communication at mmWave frequency also has some drawbacks, such as its severe pathloss. Fortunately, this can be mitigated by its huge array gain provided by a large antenna array within a compact space, given its short wavelength. Another impediment is that it is vulnerable to blockages by cars, pedestrians, and trees. The readily penetration loss is also high, which cannot be readily addressed by using a large antenna array. Instead, RISs can be deployed to construct an auxiliary transmission link even when the direct link is blocked.

B. RIS-aided Multicell Networks

To maximize spectrum efficiency (SE), multiple BSs in different cells reuse the same scarce frequency resources, which leads to inter-cell interference, especially for the celledge users. Specifically, the desired signal power received by the cell-edge user from its serving BS is comparable to the interference received from its neighbouring cells. Hence, the cell-edge users suffer from a low signal-to-interference-plusnoise ratio (SINR). To address this issue, the authors of [START_REF] Pan | Multicell MIMO communications relying on intelligent reflecting surfaces[END_REF] proposed to deploy an RIS at the cell boundary as shown in Fig. 2. In such a setting, the RIS is able to simultaneously enhance the signal gleaned from the serving BS, and mitigate the interference from the other. The simulation results of [START_REF] Pan | Multicell MIMO communications relying on intelligent reflecting surfaces[END_REF] showed that the sum rate achieved by an RIS-aided system having 80 reflecting elements may double that without an RIS.

C. RIS-aided Simultaneous Wireless Information and Power Transfer (SWIPT) Networks

SWIPT is a promising technique of providing cost-effective power delivery to energy-limited internet of things (IoT) networks, where a BS with constant power supply broadcasts wireless signal to information receivers (IRs) and energy receivers (ERs) simultaneously. The key challenge in SWIPT systems is that the ERs and IRs operate under different power supply requirements. Explicitly, ERs require a received power on the order much higher than IRs. As a result, ERs should be deployed in closer proximity to the BS than IRs to harvest sufficient power, since the signal attenuation limits the ERs' practical operational range. To deal with this problem, the authors of [START_REF] Pan | Intelligent reflecting surface aided MIMO broadcasting for simultaneous wireless information and power transfer[END_REF] proposed to deploy an RIS in the proximity of the ERs, as shown in Fig. 2. The simulation results of [START_REF] Pan | Intelligent reflecting surface aided MIMO broadcasting for simultaneous wireless information and power transfer[END_REF] revealed that to ensure a minimum harvested power of 0.2 mW, the operational range of the ERs can be extended from 5.5 meters to 9 meters, when the RIS is equipped with 40 reflecting elements.

D. RIS-aided Mobile Edge Computing (MEC) Networks

In novel future applications such as virtual reality (VR), computation-intensive image and video processing tasks must be executed in real time. However, due to the limited power supply and hardware capabilities of typical VR devices, these tasks cannot be accomplished locally. To tackle this issue, these computationally intensive tasks can be offloaded to powerful computing nodes that are usually deployed at the edge of the network. However, for some special cases where these devices are far from the MEC node, they can suffer from a low data offloading rate due to the severe path loss, which leads to excessive offloading delays. To overcome this impediment, a novel RIS-aided MEC framework was proposed in [START_REF] Bai | Latency minimization for intelligent reflecting surface aided mobile edge computing[END_REF], as shown in Fig. 2. The simulation results of [START_REF] Bai | Latency minimization for intelligent reflecting surface aided mobile edge computing[END_REF] showed that the overall task latency can be reduced from 115 ms to 65ms, if a 100-element RIS is employed.

E. RIS-aided Non-orthogonal Multiple Access (NOMA)

NOMA constitutes a promising future multiple access technique in future wireless networks, in which each orthogonal resource block is shared by multiple simultaneous users. This significantly enhances the spectral efficiency of conventional orthogonal multiple access (OMA). However, in some special cases when the users' channel vectors are orthogonal to each other, NOMA may not be a good option. The ideal implementation scenario for NOMA is when all the users' channel vectors represent the same angular direction. To broaden the application of NOMA, RIS can be introduced into the system for beneficially manipulating the wireless channel vectors of all users, so that one user's channel vector can be aligned with the other one's [START_REF] Ding | A simple design of irs-noma transmission[END_REF].

F. RIS-aided Multicast Networks

Multicast transmission based on content reuse has attracted wide research attention, since it is capable of mitigating the tele-traffic, hence it will play a pivotal role in future wireless networks. Some typical examples using multicast transmission include video conferencing, video gaming and TV broadcast.

In multi-group multicast communications, identical content is shared within each group, and each group's data rate is limited by the user with the weakest channel gain. To deal with this issue, an RIS-aided multicast architecture was proposed in [START_REF] Zhou | Intelligent reflecting surface aided multigroup multicast MISO communication systems[END_REF] as shown in Fig. 2. By carefully tuning the RIS phase shifts, the channel conditions of the weakest link can be enhanced.

G. RIS-aided Physical Layer Security (PLS) Networks

Due to the broadcast nature of wireless transmission, wireless links are prone to security threats such as jamming attacks or secure information leakage. Recently, PLS techniques have received extensive research attention, since they can avoid complex key exchange protocols, and are suitable for latencysensitive applications. In order to maximize the rate of a secure communication link, both artificial noise and multiple antennas have been proposed. However, when both the legitimate users and eavesdroppers have correlated channels or when the eavesdroppers are closer to the BS than the legitimate users, the achievable secure rate remains limited. To tackle this issue, in [START_REF] Cui | Secure wireless communication via intelligent reflecting surface[END_REF], an RIS was deployed in a network operating in the presence of an eavesdropper as shown in Fig. 2, for mitigating the information leakage to the eavesdroppers, while simultaneously increasing the received signal power at the legitimate users.

H. RIS-aided Cognitive Radio (CR) Networks

CRs are capable of enhancing the SE by allowing secondary users (SUs) to reuse the same spectrum with primary users (PUs) while controlling the interference inflicted by the SU transmitters (SU-TXs) on the PU receivers (PU-RXs). A standard approach is to use beamforming for maximizing the sum-rate of the SUs, while ensuring that the interference power at the PU-RXs remains below the interference temperature (IT) limit. However, the beamforming gain is limited, when the SU-TX to SU-RX link is weak, and the channel gain between the SU-TX and PU-RX is much higher. To handle this issue, an RIS can be deployed in the vicinity of the PU-RXs, as shown in Fig. 2. The RIS is used for mitigating the interference towards the PU-RXs, while improving the signal power at the SU-RXs.

IV. CASE STUDY

In this section, we present a case-study to show the benefits of deploying an RIS in multicell networks. Specifically, we consider the RIS-aided two-cell network shown in Fig. 3. Each cell has two users that are randomly positioned in a circle with radius of 20 m. The path-loss exponent between the BS and the users is α BU = 3.75. The path-loss exponents of the BS-RIS link and of the RIS-user link are set to α BI = α IU ∆ = α RIS = 2.2. The small-scale fading is assumed to be Rayleigh fading. All other parameters are given in [START_REF] Pan | Multicell MIMO communications relying on intelligent reflecting surfaces[END_REF].

Explicitly, in [START_REF] Pan | Multicell MIMO communications relying on intelligent reflecting surfaces[END_REF], an iterative algorithm was proposed for jointly optimizing the transmit precoding at the BS and the passive beamforming at the RIS for maximizing the weighted sum rate (WSR). In the following, we compare it (termed as 'RIS-aided') to the following two benchmark schemes:

1) RandPhase: The phase shifts of the RIS are randomly generated from [0, 2π]. The transmit precoding at the BS is designed based on the method of [START_REF] Pan | Multicell MIMO communications relying on intelligent reflecting surfaces[END_REF].

2) No-RIS:

There is no RIS in the system. Firstly, we study the impact of the RIS-related path loss exponent α RIS on the WSR performance in Fig. 4. It can be observed from this figure that the WSR achieved by the 'RIS-aided' algorithm decreases with the increase of α RIS and finally converges to the WSR of the 'no-RIS' scheme. The reason for this is that by increasing α RIS , the RIS-related links suffer from severe signal attenuation, hence the signal reflected by the RIS becomes too weak. However, when α RIS is small, significant performance gains can be achieved by an RIS over the scenariooperating without RIS. This provides important design insights indicating that the location of RIS should be carefully chosen for avoiding obstacles both in the BS-IRS link and IRS-user link.

Then, the impact of RIS location X RIS on the WSR performance is evaluated in Fig. 5, where the RIS moves from X IRS = 50 m (Cell center) to X IRS = 300 m (cell edge). The 'RIS-aided' algorithm is shown to achieve superior performance over the other two benchmark schemes. It is interesting to observe that the WSR obtained by the 'RISaided' algorithm first decreases for 50 m< X IRS < 150 m and then increases with X IRS . The minimum WSR value is achieved when the RIS is located mid-way between BS 1 and the RIS. This is in contrast to the conventional relaying scheme, where the best performance is achieved when the relay is halfway between the transmitter and the receiver. Furthermore, the WSR achieved when the RIS is at the cell edge is much higher than in the case when the RIS is at the cell center. This is because positioning the RIS near BS 1 is only beneficial for users in cell 1, while all users benefit from the RIS, when it is at the cell edge.

V. WHAT ARE THE RELEVANT CHALLENGES AND RESEARCH DIRECTIONS?

Although RISs are indeed appealing for the above applications, their implementation poses several challenges as well. In the following, we list eight promising future research directions.

Direction 1: Channel Estimation

To reap the full benefits of RIS-aided wireless networks, the CSI should be estimated in support of the phase shift design. We may consider near-instantaneous CSI and long-term CSI.

Concerning the near-instantaneous CSI estimation, we consider a typical RIS-aided wireless system where a multiantenna BS serves a single-antenna user with the aid of an RIS. Let us denote the channel spanning from the BS to the RIS and that from the RIS to the user by H and h r , respectively. In most situations having only the cascaded CSI of the two hops defined as G = diag(h H r )H is sufficient. However, since the number of reflecting elements is usually very large, the cascaded channel G contains a large number of channel coefficients. Hence, their estimation requires a large number of pilots, which is proportional to the number of reflecting elements. How to reduce the channel estimation overhead remains an open problem.

To facilitate the phase shift design based on long-term CSI, the angle or location information should be available at the BS. Unfortunately, conventional angle/location estimation algorithms may not be applicable for RIS-aided networks, when the direct channel spanning from the BS to the user is blocked. This is because a conventional BS can transmit pilot beams for tracking the users, while the RIS is passive, hence cannot send pilot signals. Thus, low-complexity yet highperformance angle/location algorithms have to be conceived for RIS-aided networks.

Direction 2: Passive Beamforming Design Based on Imperfect Parameters

Given the estimated CSI, the phase shifts have to be jointly designed together with the BS's active beamforming for achieving the desired objectives. However, most of the existing contributions are based on the assumption of perfect instantaneous CSI, which is unrealistic in practice. For estimating the cascaded CSI, one should first estimate the direct BS-user channel by switching off the RIS, and then estimate the overall channel by switching on the RIS. The cascaded CSI can be calculated by subtracting the direct BSuser channel response from the overall channel response. Since the direct BS-user channel cannot be perfectly estimated, the subtraction operation will further contaminate the cascaded CSI, similar to the error propagation of successive interference cancellation. Thus, the cascaded CSI error is sizeable and should be taken into account. An initial attempt was devoted to a simple RIS-aided multiple-user downlink scenario in [START_REF] Zhou | A framework of robust transmission design for IRS-aided MISO communications with imperfect cascaded channels[END_REF], and more emerging application scenarios are imperative to be investigated.

Furthermore, in addition to imperfect CSI, we encounter realistic transceiver hardware impairments (HWI), caused by non-linear amplifiers, low-resolution analog-to-digital converters (ADCs) and imperfect oscillators. To reduce the hardware cost and power consumption, a limited number of quantized phase-shifts may be used. This will impose quantization noise on the phase shifts of the RIS elements. Additionally, the popularly assumed phase-only reflection model is not accurate in practice, since the reflection amplitude tends to depend on the value of the phase shift itself. Compared to conventional systems operating without RIS, the impact of HWIs in RISaided systems is complex due to the presence of quantized phase at the RIS. Hence, robust transmission design is needed that takes into account the HWIs at both the transceivers and the RIS.

On the other hand, acquiring the near-instantaneous CSI may be challenging for the following reasons. Firstly, the training overhead is excessive when the number of reflecting elements is large. Secondly, the BS has to compute its beamforming weights as well as the phase shifts at the RIS, which entails solving a large-scale optimization problem. Thirdly, when the number of RIS elements is large and the channel is rapidly varying, the required capacity of the feedback link from the BS to the RIS also increases, which imposes a high overhead and high cost. To address these challenges, it is appealing to design the phase shifts based on the longterm CSI relying on both angular and location information, which changes much more slowly. Unfortunately, only very few contributions were devoted to this research area [START_REF] Han | Large intelligent surface-assisted wireless communication exploiting statistical csi[END_REF].

Direction 3: Distributed Algorithms with Low Overhead Exchange

In the RIS-aided multicell scenario of [START_REF] Pan | Multicell MIMO communications relying on intelligent reflecting surfaces[END_REF], the transmission design is centralized. In particular, the algorithm proposed requires a central processing unit (CPU) for collecting all the complex-valued channel matrices over the network. The CPU computes all the active beamforming weights and phase shifts, and then sends them back to the corresponding nodes. However, these centralized algorithms suffer from a heavy feedback overhead and high computational complexity, which is an impediment. Note that compared to conventional RISfree systems, the large-dimensional cascaded channel matrix must additionally be fed back to the CPU.

Hence, it is imperative to design distributed algorithms, where each BS can make transmission decisions based on its local CSI and limited information exchange with other BSs. Distributed algorithms have appealing advantages over centralized algorithms, such as low information exchange overhead, reduced computational complexity and increased scalability.

Direction 4: System Design for RIS-aided Frequencydivision Duplex (FDD) systems

Most existing contributions related to RIS have considered channel estimation for time-division duplex (TDD)-based implementations due to the appealing feature of channel reciprocity. However, recent results in [START_REF] Chen | Angledependent phase shifter model for reconfigurable intelligent surfaces: Does the angle-reciprocity hold?[END_REF] revealed that the RIS phase-shift model depends on the incident electromagnetic angles, which implies that the assumption of channel reciprocity in TDD systems may not hold in practice. Hence, it is imperative to study channel estimation and transmission design for FDD RIS systems. Due to a large number of reflecting elements at the RIS, large-dimensional channel matrices have to be fed back to the BS in FDD RIS systems, which incurs high feedback overhead.

Direction 5: Application of RIS in Terahertz Communications Compared to mmWave communications, Terahertz (THz) communications can provide more abundant bandwidth and higher data rates. Despite its abundant bandwidth, THz frequencies suffer from strong atmospheric attenuation, molecular absorption and extremely severe path loss, which limits its operational range. Moreover, such high-frequency signals are very prone to blockage effects, and thus they are unable to support reliable communication links. These drawbacks impair its practical implementation. The use of RISs is a promising remedy to address these issues due to their ability to create alternative signal paths. In the THz band, path-loss peaks appear in different frequency bands, and thus the total bandwidth has to be divided into several sub-bands having different bandwidths. Furthermore, THz electronic components can be made compact, and thus the RIS can accommodate a massive number of tiny reflecting elements, allowing us to realize a holographic array having a near-continuous aperture. Hence, channel estimation and beam pattern design for RIS-aided THz communications is an exciting area for future study.

Direction 6: Mobility Management Mobility management is a challenging problem for RISaided wireless networks. Due to the rapid movements of users, the BS may lose its connection with them, unless agile mobility management schemes are used. Since RISs are passive, they cannot send pilot signals to track the movement of the users. Hence, it is much more challenging to track roaming users, especially when the direct links between the BS and users are blocked.

Direction 7: Deployment Issues The deployment strategy for RIS reflecting elements has a significant impact on the generation of the RIS-related channel coefficients and thus also on the system performance limit. From a practical implementation perspective, RIS deployments have to take into account the hardware cost, location availability, user distributions, and the requested services. There is a fundamental question to be answered: Given a total number of reflection elements, is centralized deployment better than distributed deployment? To answer this question, we have to address the following two research issues:

1) For centralized deployment, where to deploy the RIS?

In the vicinity of the BS or the users, or where else between the BS and the user? 2) For distributed deployment, how many smaller-size groups of RISs should the total number of reflecting elements be partitioned into? Where to deploy these smaller-size RISs? Direction 8: AI-driven Design and Optimization The RIS phase shift matrices have to be optimized for enhancing the system performance. In practical deployments, each RIS is equipped with hundreds of reflecting elements. Most of the existing contributions on phase shift design tend to rely on model-based optimization methods, which require a large number of iterations to find a near-optimal solution, which is due to the non-convexity of phase shift constraints and owing to the non-convex nature of the objective function. Thus, the existing methods will incur high computational complexity, which are not suitable for real-time applications. To address this issue, artificial intelligence (AI)-based method is an appealing data-driven scheme which can extract system features without a specific mathematical model. Once trained, the optimal solution can be found by simple algebraic calculations. Additionally, the trained model can be quite robust both against imperfect CSI and hardware impairments.

VI. CONCLUSIONS

In this article, we have answered four critical questions associated with RIS. Additionally, we have demonstrated that they are capable of mitigating the challenging blockage and coverage issues of mmWave or THz communications. We briefly introduced the basic RIS hardware architecture and its main advantages over relays. We also discussed their potential integration into emerging wireless applications, including multicell, SWIPT, MEC, NOMA, multicast, PLS, CR systems, mmWave. Finally, to provide useful guidance and spark additional research interests, we also formulated eight promising research directions.

Fig. 1 .

 1 Fig. 1. Architecture of an RIS.

Fig. 2 .

 2 Fig. 2. Typical applications of RIS in various emerging Sub-6 GHz systems.

Fig. 3 .

 3 Fig. 3. Simulation setup.

Fig. 4 .

 4 Fig. 4. Weighted sum rate versus the path loss exponent α RIS .

Fig. 5 .

 5 Fig. 5. Weighted sum rate versus the RIS location X RIS .

  Reconfigurable Intelligent Surfaces for 6G Systems: Principles, Applications, and Research Directions Cunhua Pan, Hong Ren, Kezhi Wang, Jonas Florentin Kolb, Maged Elkashlan, Ming Chen, Marco Di Renzo, Fellow, IEEE, Yang Hao, Fellow, IEEE, Jiangzhou Wang, Fellow, IEEE, A. Lee Swindlehurst, Fellow, IEEE, Xiaohu You, Fellow, IEEE and Lajos Hanzo, Fellow, IEEE