A reconfigurable intelligent surface controller

Abstract

This invention provides a Reconfigurable Intelligent Surface, RIS, controller for a wireless telecommunications network, the wireless telecommunications network comprising a transmitter, a first RIS, a second RIS, and a receiver, the RIS controller configured to: configure the first RIS to reconfigure a wireless signal incident at the first RIS to be: convergent, wherein a focal point of the wireless signal in a first dimension is independently configured by the first RIS to substantially align with a coordinate of the receiver in the first dimension, and directed towards the second RIS; and configure the second RIS to reconfigure the wireless signal, as directed from the first RIS and incident at the second RIS, wherein: the focal point of the wireless signal in at least a second dimension is independently configured by the second RIS to substantially align with a coordinate of the receiver in the second dimension.

Description

[0001] A36143

[0002] A RECONFIGURABLE INTELLIGENT SURFACE CONTROLLER

[0003] Field of the Invention

[0004] The present invention relates to a Reconfigurable Intelligent Surface (RIS) controller and

[0005] 5 a method of reconfiguring a wireless signal in a wireless telecommunications network.

[0006] Background

[0007] In wireless telecommunications, a wireless signal being transmitted between a transmitter and receiver generally degrades due to interference from other wireless signals and / or other physical phenomena (e.g. fading and blockage). This has generally been addressed by improving the transmission characteristics (e.g. higher power transmissions or repeaters) or transmission processing techniques (e.g. more robust modulation schemes). An emerging concept in wireless telecommunications is the concept of a reconfigurable propagation environment, or “smart radio environment”,

[0008] 15 which may improve the transmission quality. This may be achieved by use of a surface of electromagnetic material, often known as a Reconfigurable Intelligent Surface (RIS), which may be operated to apply a change to an incident wireless signal so as to improve the transmission quality between the transmitter and the receiver. The RIS may operate in transmissive mode, in which the wireless beam is reconfigured as it passes through

[0009] 20 the RIS, or a reflection mode, in which the wireless signal is reconfigured as it is reflected by the RIS. Changes to the transmitted or reflected wireless signal include changes in direction, shape and distribution of phase and polarisation. Alternative names for the RIS include intelligent reflective surface, large intelligent surface, large intelligent metasurface, programmable metasurface, reconfigurable metasurface, smart reflect-

[0010] 25 arrays, software-defined surface, and passive intelligent surface. The term “reconfigurable” is often used to indicate that one or more electromagnetic properties across the surface of the RIS can be selected to alter one or more properties of the transmitted or reflected wireless signal.

[0011] The RIS comprises a plurality of unit cells, each unit cell typically having a longest dimension that is less than half a wavelength of the signal to be reconfigured. Each unit cell has a controllable electrical property (e.g. surface impedance). A RIS controller provides a control signal to each unit cell by a connection, known as a biasing line. The RIS controller determines a value for the control signal of each unit cell such that a

[0012] 35 distribution of the electrical property across the plurality of unit cells realises a desired A36143 reconfiguration of the incident wireless signal upon transmission or reflection in order to approximate as closely as possible to the desired (transmitted or reflected) properties of the wireless signal (or “goal function”).

[0013] 5 An electromagnetic field can be divided into a near-field region and a far-field region. If a receiver is in the near-field of a RIS, the propagation distances are so short that there are non-negligible amplitude and phase variations across the receiver. In contrast, both the amplitude and phase variations are negligible in the far-field region, in which the amplitude only depends on the propagation distance to the centre of the receiver and the phase variations only depend on the incident angle. The boundary at which the electromagnetic field can be considered near-field or far-field is defined by the Rayleigh distance.

[0014] The determination, by a RIS controller, of the control signal for each unit cell of a RIS is

[0015] 15 more complicated when the UE is positioned in a near-field of the RIS relative to a UE positioned in a far-field of the RIS. That is, for far-field implementations, the goal-function is the required signal amplitude (and possibly phase if required) as a function of the solid angle as seen from the RIS, which can be calculated using Fourier transforms. However, for near-field implementations, the goal-function is the required signal amplitude (and

[0016] 20 possible phase if required) as a function of the UE’s spatial location. This calculation is significantly more complex and is more sensitive to scattering from nearby surfaces. Assuming the RIS controller has knowledge of the real-time spatial location of a UE in the RIS’s near-field, then the control signal of each unit cell for a particular goal-function needs to be calculated for each unit cell of the RIS. The control signals need to be re¬

[0017] 25 calculated each time the UE moves out of focus of the reflected wireless signal. This frequent recalculation of a large number of control signals is time-consuming and energy intensive.

[0018] Summary of the Invention

[0019] According to a first aspect of the invention, there is provided a Reconfigurable Intelligent Surface, RIS, controller for a wireless telecommunications network, the wireless telecommunications network comprising a transmitter, a first RIS, a second RIS, and a receiver, the RIS controller configured to: configure the first RIS to reconfigure a wireless signal incident at the first RIS to be: convergent, wherein a focal point of the wireless

[0020] 35 signal in a first dimension is independently configured by the first RIS to substantially A36143 align with a coordinate of the receiver in the first dimension, and directed towards the second R IS ; and configure the second RIS to reconfigure the wireless signal, as directed from the first RIS and incident at the second RIS, wherein: the focal point of the wireless signal in at least a second dimension is independently configured by the second RIS to

[0021] 5 substantially align with a coordinate of the receiver in the second dimension.

[0022] The focal point of the wireless signal in a third dimension may be independently configured by the second RIS to substantially align with a coordinate of the receiver in the third dimension, and the second RIS may further reconfigure the wireless signal to

[0023] 10 be directed towards the receiver.

[0024] The wireless telecommunications network may further comprise a third RIS, the configuration of the second RIS is such that the wireless signal may be directed towards the third RIS, and the RIS controller may be further configured to: configure the third RIS

[0025] 15 to reconfigure the wireless signal, as directed from the second RIS and incident at the third RIS to be: directed towards the receiver, wherein the focal point of the wireless signal in a third dimension is independently configured by the third RIS to substantially align with a coordinate of the receiver in the third dimension.

[0026] 20 The RIS controller may be one of a group comprising: a centralised RIS controller comprising a connection interface for connecting the centralised RIS controller to each RIS, and a set of distributed RIS controller modules, each RIS controller module configuring a particular RIS.

[0027] 25 The RIS controller may configure each RIS based on at least one of a group comprising: a feedback signal from the receiver, the feedback signal identifying a property of the wireless signal as received by the receiver, and a position of the receiver.

[0028] According to a second aspect of the invention, there is provided a wireless

[0029] 30 telecommunications network comprising: the first RIS; the second RIS; and a RIS controller of the first aspect of the invention. The wireless telecommunications network may further comprise the third RIS.

[0030] According to a third aspect of the invention, there is provided a method of reconfiguring

[0031] 35 a wireless signal in a wireless telecommunications network, the wireless A36143 telecommunications network comprising a transmitter, a first Reconfigurable Intelligent Surface, RIS, a second RIS and a receiver, the method comprising the steps of: configuring the first RIS to reconfigure a wireless signal incident at the first RIS to be: convergent, wherein a focal point of the wireless signal in a first dimension is

[0032] 5 independently configured by the first RIS to substantially align with a coordinate of the receiver in the first dimension, and directed towards the second RIS; and configuring the second RIS to reconfigure the wireless signal, as directed from the first RIS and incident at the second RIS, wherein: the focal point of the wireless signal in at least a second dimension is independently configured by the second RIS to substantially align with a coordinate of the receiver in the second dimension.

[0033] The step of configuring the second RIS may be such that: the focal point of the wireless signal in a third dimension is independently configured by the second RIS to substantially align with a coordinate of the receiver in the third dimension, and the wireless signal is

[0034] 15 directed towards the receiver.

[0035] The wireless telecommunications network may comprise a third RIS, and the method may further comprise the steps of: configuring the second RIS such that: the wireless signal is directed towards the third RIS; and configuring the third RIS to reconfigure the

[0036] 20 wireless signal, as directed from the second RIS and incident at the third RIS to be: directed towards the receiver, wherein the focal point of the wireless signal in a third dimension is independently configured by the third RIS to substantially align with a coordinate of the receiver in the third dimension.

[0037] Each configuring step may be based on one of a group comprising: a feedback signal from the receiver, the feedback signal identifying a property of the wireless signal as received by the receiver, and a position of the receiver.

[0038] According to a fourth aspect of the invention, there is provided a computer program

[0039] 30 comprising instructions which, when the program is executed by a computer, cause the computer to carry out the steps of the third aspect of the invention. The computer program may be stored on a computer readable carrier medium. A36143

[0040] Brief Description of the Figures

[0041] In order that the present invention may be better understood, embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings in which:

[0042] 5

[0043] Figure 1 is a schematic diagram illustrating a wireless telecommunications network;

[0044] Figure 2 is a diagram illustrating a first Reconfigurable Intelligent Surface (RIS) of the network of Figure 1 ;

[0045] Figure 3 is a diagram illustrating a second RIS of the network of Figure 1 ;

[0046] Figure 4 is a diagram illustrating a reconfiguration of a wireless signal by the second RIS; Figure 5 is a diagram illustrating a third RIS of the network of Figure 1 ;

[0047] Figure 6 is a diagram illustrating a reconfiguration of the wireless signal by the third RIS; Figure 7 is a flow diagram illustrating a method of reconfiguring a wireless signal; and Figure 8 is a flow diagram illustrating a further method of reconfiguring a wireless signal.

[0048] 15

[0049] Detailed Description

[0050] Figure 1 illustrates a wireless telecommunications network 100 comprising a transmitter 1 10, a first Reconfigurable Intelligent Surface (RIS) 120, a second RIS 130, a third RIS 140, a receiver 150 and a RIS controller 160. The RIS controller 160 is connected to the

[0051] 20 first RIS 120 via a first connection 161 , to the second RIS 130 via a second connection 162 and to the third RIS 140 via a third connection 163.

[0052] Figure 1 also illustrates a room comprising a number of surfaces. The transmitter 110 is mounted to a ceiling of the room, the first RIS 120 is positioned on a first wall of the room,

[0053] 25 the second RIS 130 is positioned on a second wall of the room, and the third RIS 140 is positioned on a third wall of the room. A set of axes may be defined by the surfaces of the room, including a first “X” axis, a second “Y” axis, and a third “Z” axis. Each axis of the set of axes is orthogonal to each other axis of the set of axes. As shown in Figure 1 , the X axis extends along the surface of the third wall from the first wall to the second wall, the Y axis extends along the surface of the third wall from a floor of the room to a ceiling of the room, and the Z axis extends along the surface of the first wall.

[0054] The receiver 150 is mobile and is positioned at any point in the room. The position of the receiver 150 may therefore be defined by a coordinate having X, Y and Z-axis

[0055] 35 components. The distances between the receiver 150 and any one of the first, second A36143 and third RIS 120, 130, 140 are such that the receiver 150 is in the near-field of the first, second and third RISs 120, 130, 140 regardless of its position in the room.

[0056] The first RIS 120 comprises a plurality of unit cells 121 . The plurality of unit cells 121 of

[0057] 5 the first RIS 120 comprises sets of unit cells, each set of unit cells defining a ring of unit cells. Figure 2 illustrates the first RIS 120 in more detail showing the sets of unit cells forming concentric annular rings. Figure 2 further illustrates the plurality of unit cells 121 arranged as a grid of 64 unit cells in a first axis and 64 unit cells in a second axis. Each set of unit cells is a grouping of contiguous unit cells of this grid that form a ring or partial

[0058] 10 ring on the first RIS 120. The example shown in Figure 2 comprises a first set of unit cells 121 a forming a ring and a second set of unit cells 121 b forming a partial ring. The plurality of unit cells 121 may therefore also comprise inactive unit cells that are not part of any set of the plurality of unit cells 121 . In other words, the unit cells of each set of unit cells may be considered active unit cells.

[0059] 15

[0060] Each unit cell of a particular ring or partial ring is connected to the RIS controller 160 by the first connection 161 so as to receive an identical control signal. That is, all unit cells of a ring or partial ring apply a reconfiguration in the same manner as a single annular unit cell. As described in more detail below, the control signal applied to the unit cells of

[0061] 20 a ring or partial ring is configured to be a function of the ring’s radius and the control signal supplied by the RIS controller 160.

[0062] The first RIS 120 is controllable so as to reconfigure a wireless signal transmitted by the transmitter 1 10 such that: 1 ) the divergent wireless signal transmitted by the transmitter

[0063] 25 110 is a convergent wireless signal, 2), the convergent wireless signal is communicated to the second RIS 130 coaxially with the X-axis, and 3) the convergent wireless signal has a focal length equal to (or substantially equal to) a distance of a path of the convergent wireless signal between the first RIS 120 and the receiver 150. As described in more detail below, the convergent wireless signal (following its reconfiguration by the

[0064] 30 first RIS 120) follows a path to the receiver 150 via the second and third RIS 130, 140 (that is, it is reflected by the second RIS 130 and by the third RIS 140 on its path between the first RIS 120 and the receiver 150). The focal length of the convergent wireless signal should therefore equal (or substantially equal) the distance of this path such that it focussed (or substantially focussed) on the receiver 150. A36143

[0065] The first RIS 120 is therefore configured as a converging lens for reconfiguring the divergent wireless signal as a convergent wireless signal. That is, the divergent wireless signal of wavelength A radiates from the transmitter 110 at point T, incurs a phase shift as it travels distance t between the transmitter 110 and the first RIS 120, is reflected by

[0066] 5 a point of the first RIS 120 being distance / ' from an origin of the first RIS 120 so as to incur a phase shift being a function of distance r, and incurs a phase shift as it travels distance / between the first RIS 120 and the focal point F. The first RIS 120 must therefore be configured such that the phase shift applied by the first RIS 120 on the incident wireless signal is such that the converging wireless signal is in phase at focal point F. The first RIS 120 is therefore configured such that, for all values of r. phase shift incurred travelling from point T to the first RIS 120 + (r) + phase shift incurred travelling from the first RIS 120 to point F

[0067] 15 = 0

[0068] That is, (r) = — (— )[(t2+ r2)1 / 2+ (f2+ r2)1 / 2] A

[0069] Equation (1 ) defines the ideal annular phase variation across the first RIS 120. The

[0070] 20 phase shift to be applied by each unit cell of the first RIS 120 is therefore a quantisation (based on the unit cell’s size and possible phase states) to the phase shift calculated by equation (1 ) for a particular focal length and that unit cell’s distance / 'from the origin of the first RIS 120. The first RIS 120 may therefore be calibrated such that the control signal applied to each ring or partial ring of unit cells (having the same distance / ' from the origin of the first RIS 120) results in the incident divergent wireless signal being reconfigured as a convergent wireless signal with a particular focal length. As the angle of incidence and angle of reflection are always the same (as the relative locations of the transmitter 1 10, first RIS 120 and second RIS 130 are static), then this calibration phase determines the control signal applied to each set of unit cells to reconfigure the incident

[0071] 30 divergent wireless signal as a convergent wireless signal with a particular focal length for those angles of incidence and reflection. The relative locations of the first and second RIS 120, 130 are such that the reflected wireless signal is coaxial with the X-axis. In other words, the reflected wireless signal varies in its position on the X-axis - but not on the Y-axis or Z-axis - as it is communicated between the first and second RIS 120, 130. A36143

[0072] The above reconfiguration of the wireless signal by the first RIS 120 is part of an overall reconfiguration to maximise (or at least improve) a property (e.g. the signal strength) of the wireless signal at the receiver 150. As shown in Figure 1 , the onward path of the

[0073] 5 wireless signal following its reconfiguration by the first RIS 120 is via the second and third RIS 130, 140. As described in more detail below, the reconfigurations of the wireless signal by the second and third RIS 130, 140 do not change the X-axis coordinate component of the focal point of the wireless signal. Accordingly, the X-axis coordinate component of the focal point of the wireless signal is determined only as a function of the focal length of the wireless signal as configured by the first RIS 120. That is, the first RIS 120 may implement a reconfiguration so as to increase the focal length of the wireless signal such that the focal point of the wireless signal moves in a positive direction of the X axis, or implement a reconfiguration so as to decrease the focal length of the wireless signal such that the focal point of the wireless signal moves in a negative

[0074] 15 direction of the first axis. The X-axis coordinate component of the focal point of the wireless signal is therefore configured by the first RIS 120 (as described in more detail below) to coincide with the X-axis coordinate component of the receiver 150.

[0075] The second RIS 130 also comprises a plurality of unit cells 131 . The plurality of unit cells

[0076] 20 131 of the second RIS 130 are divided into sets, each set of the plurality of unit cells 131 defining a line along the first axis. Figure 3 illustrates the second RIS 130 in more detail showing the sets of the plurality of unit cells 131 forming rows of lines along the first axis. Figure 3 also illustrates the plurality of unit cells 131 arranged as a grid of 64 unit cells in the first axis and 64 unit cells in the second axis. Each set of the plurality of unit cells

[0077] 25 131 is a grouping of contiguous unit cells of this grid that form a line on the second RIS

[0078] 130 along the first axis. The example shown in Figure 3 comprises a first set of unit cells

[0079] 131 a forming a first line along the first axis and a second set of unit cells 131 b forming a second line along the first axis. The plurality of unit cells 131 may therefore also comprise inactive unit cells that are not part of any set of the plurality of unit cells 131. In other words, the unit cells of each set of unit cells may be considered active unit cells.

[0080] Each unit cell of a particular set of the plurality of unit cells 131 of the second RIS 130 is connected to the RIS controller 160 by the second connection 162. That is, all unit cells of a particular set of the plurality of unit cells 131 share the same second connection so

[0081] 35 as to receive the same control signal, as described in more detail below, such that the A36143 particular set of the plurality of unit cells 131 applies a reconfiguration in the same manner as a single unit cell.

[0082] The second RIS 130 is controllable so as to reconfigure an incident wireless signal (that

[0083] 5 is, the wireless signal transmitted by the transmitter 1 10 and reflected by the first RIS 120) so as to adjust the angle of reflection. As noted above, the second RIS 130 is positioned on the second wall which is co-planar with the Y-Z plane. As shown in more detail in Figure 4, the wireless signal transmitted by the transmitter 110 and reflected by the first RIS 120 is incident at the second RIS 130 at an incident angle. This incident angle is normal to the second RIS 130 (i.e. normal to the Y-Z plane). The second RIS 130 reflects the wireless signal towards the third RIS 140. The wireless signal is reconfigured by the second RIS 130 to set the angle of reflection on the X-Z plane. The angle of reflection on the X-Z plane may be configured as any angle within a range of values on the X-Z plane. The range of values may be between a first angle of reflection

[0084] 15 - such that the wireless signal is reflected towards an end of the third RIS 140 closest to the first wall - and a second angle of reflection - such that the wireless signal is reflected towards an end of the third RIS 140 closest to the second wall. The second RIS 130 reconfigures the wireless signal to configure the angle of reflection of the wireless signal in the X-Z plane only. That is, the angle of reflection of the wireless signal in any other

[0085] 20 plane (e.g. the Y-Z plane) is unaffected by the second RIS 130 in any configuration.

[0086] The second RIS 130 is therefore configured as a tilted mirror for changing the angle of reflection in the X-Z plane only. This is achieved, in this example, by configuring each set of unit cells of the plurality of unit cells 131 of the second RIS 130 to apply a phase shift as a function of distance along the Z-axis. The phase shift may vary as a linear function of distance along the Z-axis, i.e. (z) = kz (2)

[0087] Equation (2) defines the ideal annular phase variation across the second RIS 130. The

[0088] 30 phase shift to be applied by each unit cell of the second RIS 130 is therefore a quantisation (based on the unit cell’s size and possible phase states) to the phase shift calculated by equation (2) for that unit cell’s distance z from the origin of the second RIS 130. The second RIS 130 may therefore be calibrated such that a control signal applied to each set of unit cells (having the same distance z from the origin of the second RIS A36143

[0089] 130) results in the incident wireless signal being reconfigured with a particular angle of reflection in the X-Z plane.

[0090] Figure 1 illustrates the onward path of the wireless signal towards the third RIS 130. The

[0091] 5 wireless signal is reflected by the third RIS 140 towards the receiver 150. As described in more detail below, the reconfiguration of the wireless signal by the third RIS 140 configures the angle of reflection of the wireless signal in the Y-Z plane only such that the angle of reflection of the wireless signal in the X-Z plane is unaffected by the third RIS 140 in any configuration. Accordingly, the reconfiguration applied to the wireless signal by the second RIS 130 is the only configuration of the wireless signal in the X-Z plane. Therefore, the wireless signal has a focal point at a particular coordinate within the room in which the Z-axis coordinate component is determined only as a function of the angle of reflection on the X-Z plane as configured by the second RIS 130. This Z- axis coordinate component of the focal point of the wireless signal is therefore configured

[0092] 15 by the second RIS 130 (as described in more detail below) to coincide with the Z-axis coordinate component of the receiver 150.

[0093] The third RIS 140 also comprises a plurality of unit cells 141. The plurality of unit cells 141 of the third RIS 140 are divided into sets, each set of the plurality of unit cells 141

[0094] 20 defining a line along a second axis. Figure 5 illustrates the third RIS 140 in more detail showing the sets of the plurality of unit cells 141 forming rows of lines along the second axis. Figure 5 also illustrates the plurality of unit cells 141 arranged as a grid of 64 unit cells in the first axis and 64 unit cells in the second axis. Each set of the plurality of unit cells 141 is a grouping of contiguous unit cells of this grid that form a line on the third RIS 140. The example of Figure 5 shows a first set of unit cells 141 a forming a first line along the second axis and a second set of unit cells 141 b forming a second line along the second axis. The plurality of unit cells 141 may therefore also comprise inactive unit cells that are not part of any set of the plurality of unit cells 141. In other words, the unit cells of each set of unit cells may be considered active unit cells.

[0095] 30

[0096] Each unit cell of a particular set of the plurality of unit cells 141 of the third RIS 140 is connected to the RIS controller 160 by the third connection 163. That is, all unit cells of a particular set of the plurality of unit cells 141 share the same third connection so as to receive the same control signal, as described in more detail below, such that the A36143 particular set of the plurality of unit cells 141 applies a reconfiguration in the same manner as a single unit cell.

[0097] The third RIS 140 is controllable so as to reconfigure an incident wireless signal (that is,

[0098] 5 the wireless signal transmitted by the transmitter 1 10 and reflected by the first and second RISs 120, 130) so as to adjust the angle of reflection. As noted above, the third RIS 140 is positioned on the third wall which is co-planar with the X-Y plane. As shown in more detail in Figure 6, the wireless signal transmitted by the transmitter 1 10 and reflected by the first and second RISs 120, 130 is incident at the third RIS 140 at an incident angle on the Y-Z plane. The third RIS 140 reflects the wireless signal towards the receiver 150. The wireless signal is reconfigured by the third RIS 140 to set the angle of reflection on the Y-Z plane. The angle of reflection on the Y-Z plane may be configured as any angle within a range of values on the Y-Z plane. The range of values may be between a first angle of reflection - such that the wireless signal is reflected along a

[0099] 15 negative direction of the Y-axis towards the floor of room - and a second angle of reflection - such that the wireless signal is reflected along a positive direction of the Y- axis towards the ceiling of the room. The third RIS 140 reconfigures the wireless signal to configure the angle of reflection of the wireless signal in the Y-Z plane only. That is, the angle of reflection of the wireless signal in any other plane (e.g. the X-Z plane, as

[0100] 20 noted above) is unaffected by the third RIS 140 in any configuration.

[0101] The third RIS 140 is therefore configured as a tilted mirror for changing the angle of reflection in the Y-Z plane only. This is achieved, in this example, by configuring each set of unit cells of the plurality of unit cells 141 of the third RIS 140 to apply a phase shift as a function of distance along the Y-axis. The phase shift may vary as a linear function of distance along the Y-axis, i.e. (y) = ky (3)

[0102] Equation (3) defines the ideal annular phase variation across the third RIS 140. The

[0103] 30 phase shift to be applied by each unit cell of the third RIS 140 is therefore a quantisation (based on the unit cell’s size and possible phase states) to the phase shift calculated by equation (3) for that unit cell’s distance y from the origin of the third RIS 140. The third RIS 140 may therefore be calibrated such that a control signal applied to each set of unit cells (having the same distance y from the origin of the third RIS 140) results in the A36143 incident wireless signal being reconfigured with a particular angle of reflection in the Y-Z plane.

[0104] Figure 1 illustrates the onward path of the wireless signal towards the receiver 150. As

[0105] 5 noted above, the reconfiguration of the wireless signal by the second RIS 130 configures the angle of reflection of the wireless signal in the X-Z plane only such that the angle of reflection of the wireless signal in the Y-Z plane is unaffected by the second RIS 130 in any configuration. Accordingly, the reconfiguration applied to the wireless signal by the third RIS 140 is the only configuration of the wireless signal in the Y-Z plane. The

[0106] 10 wireless signal has a focal point at a particular coordinate within the room in which the Y-axis coordinate component is determined only as a function of the angle of reflection in the Y-Z plane as configured by the third RIS 140. This Y-axis coordinate component of the focal point of the wireless signal is therefore configured by the third RIS 140 (as described in more detail below) to coincide with the Y-axis coordinate component of the

[0107] 15 receiver 150.

[0108] The overall configuration of the wireless signal is therefore a combination of reconfigurations by the first, second and third RISs 120, 130, 140 so as to independently configure the X-axis component of the wireless signal’s focal point by the first RIS 120,

[0109] 20 independently configure the Y-axis component of the wireless signal’s focal point by the second RIS 130, and independently configure the Z-axis component of the wireless signal’s focal point by the third RIS 140. The first, second and third RIS 120, 130, 140 are therefore configured such that the focal point of the wireless signal has the same (or substantially the same) coordinates in the X-axis, Y-axis and Z-axis as the receiver 150.

[0110] 25 This maximises the signal strength of the wireless signal at the receiver 150 (or at least improves the signal strength relative to a scenario in which the first, second and / or third RIS 120, 130, 140 do not apply a reconfiguration to the wireless signal).

[0111] The receiver 150 is configured to measure a property of the wireless signal, such as a

[0112] 30 signal strength, and send a report to the transmitter 110 comprising the measured property. This feedback loop is used to determine the configurations of the first, second and third RIS 120, 130, 140, as will now be described.

[0113] A method of reconfiguring the wireless signal will now be described with reference to

[0114] 35 Figure 7. The transmitter 110 is in a transmitting state and therefore transmits a wireless A36143 signal. In a first step (S101 ) , the controller 160 controls the first RIS 120 so as to apply a set of reconfigurations to the wireless signal, each reconfiguration causing the wireless signal’s focal point to have a different X-axis component. The set of reconfigurations may monotonically increase or monotonically decrease the X-axis component of the

[0115] 5 wireless signal’s focal point. Following each reconfiguration of the set of reconfigurations, in step S103, the controller 160 receives, from the receiver 150, a measurement of the property of the wireless signal. In this example, the property is the signal strength of the wireless signal. For completeness, it is noted that the controller 160 controls the second and third RIS 120, 130 during steps S101 and S103 so as to

[0116] 10 apply the same reconfiguration to the wireless signal. Therefore, only the first RIS 120 changes its reconfiguration of the wireless signal during steps S101 and S103.

[0117] In step S105, the controller 160 controls the first RIS 120 to use the reconfiguration of the set of reconfigurations that maximised the signal strength of the wireless signal at

[0118] 15 the receiver 150 as determined in step S103.

[0119] Steps S101 to S105 therefore define a first stage of reconfiguring the wireless signal so as to identify an X-axis coordinate of the wireless signal that is equal (or substantially equal) to the X-axis coordinate of the receiver 150.

[0120] 20

[0121] In step S107, the controller 160 controls the second RIS 130 so as to apply a set of reconfigurations to the wireless signal, each reconfiguration causing the wireless signal’s focal point to have a different Z-axis component. The set of reconfigurations may monotonically increase or monotonically decrease the Z-axis component of the wireless

[0122] 25 signal’s focal point. Following each reconfiguration of the set of reconfigurations, in step S109, the controller 160 receives, from the receiver 150, a measurement of the property of the wireless signal. In this example, the property is the signal strength of the wireless signal. For completeness, it is noted that the controller 160 controls the first and third RIS 1 10, 130 during steps S107 and S109 so as to apply the same reconfiguration to

[0123] 30 the wireless signal. Therefore, only the second RIS 130 changes its reconfiguration of the wireless signal during steps S107 and S109.

[0124] In step S11 1 , the controller 160 controls the second RIS 130 to use the reconfiguration of the set of reconfigurations that maximised the signal strength of the wireless signal at

[0125] 35 the receiver 150 as determined in step S109. A36143

[0126] Steps S107 to S11 1 therefore define a second stage of reconfiguring the wireless signal so as to identify a Z-axis coordinate of the wireless signal that is equal to (or substantially equal to) the Z-axis coordinate of the receiver 150.

[0127] 5

[0128] In step S113, the controller 160 controls the third RIS 140 so as to apply a set of reconfigurations to the wireless signal, each reconfiguration causing the wireless signal’s focal point to have a different Y-axis component. The set of reconfigurations may monotonically increase or monotonically decrease the Y-axis component of the wireless

[0129] 10 signal’s focal point. Following each reconfiguration of the set of reconfigurations, in step S115, the controller 160 receives, from the receiver 150, a measurement of the property of the wireless signal. In this example, the property is the signal strength of the wireless signal. For completeness, it is noted that the controller 160 controls the first and second RIS 120, 130 during steps S1 13 and S115 so as to apply the same reconfiguration to

[0130] 15 the wireless signal. Therefore, only the third RIS 140 changes its reconfiguration of the wireless signal during steps S1 13 and S1 15.

[0131] In step S117, the controller 160 controls the third RIS 140 to use the reconfiguration of the set of reconfigurations that maximised the signal strength of the wireless signal at

[0132] 20 the receiver 150 as determined in step S115.

[0133] Steps S1 13 to S1 17 therefore define a third stage of reconfiguring the wireless signal so as to identify a Y-axis coordinate of the wireless signal that is equal to (or substantially equal to) the Y-axis coordinate of the receiver 150.

[0134] 25

[0135] Once the first, second and third stages are complete, the first, second and third RISs 120, 130, 140 thereafter apply an overall configuration to the wireless signal transmitted by the transmitter 110 that is focussed on the receiver 150. The signal strength and energy of the wireless signal is therefore maximised (or at least improved) at the receiver

[0136] 30 150.

[0137] As noted in the Background section, a conventional method of reconfiguring a wireless signal using a single RIS requires significant computational resources to calculate the control signal of each unit cell to correctly reconfigure the wireless signal in each

[0138] 35 dimension. This is further complicated when considering near-field propagation. The wireless telecommunications network 100 described above requires fewer computational resources by implementing a plurality of RIS, wherein only the first RIS reconfigures the divergent wireless signal to a convergent wireless signal, and each RIS independently configures the focal point of the convergent wireless signal in a particular dimension.

[0139] 5 That is, in the conventional RIS, the number of distinct control signals to be calculated scales with n2(where n2is the number of unit cells of the single RIS, in which n equals the number of unit cells on each axis). As each set of unit cells of the first, second and third RIS 120, 130, 140 only require a common control signal, the number of distinct control signals to be calculated in the wireless telecommunications network 100 scales

[0140] 10 as 3n.

[0141] Furthermore, the focal length of the wireless signal between the first RIS 120 and the receiver 150 is longer relative to the focal length between the single RIS and the receiver in the conventional method. This has an additional benefit in that the reconfigured

[0142] 15 wireless signal has fewer sidelobes as there is less deviation between the incident and reconfigured wireless signals. There is therefore less power lost to these sidelobes.

[0143] The method may be defined by (as illustrated in Figure 8): configuring the first RIS to reconfigure a wireless signal incident at the first RIS to be: convergent, wherein a focal

[0144] 20 point of the wireless signal in a first dimension is independently configured by the first RIS to substantially align with a coordinate of the receiver in the first dimension, and directed towards the second RIS (step S201 ); configuring the second RIS to reconfigure the wireless signal, as directed from the first RIS and incident at the second RIS, wherein: the focal point of the wireless signal in at least a second dimension is independently

[0145] 25 configured by the second RIS to substantially align with a coordinate of the receiver in the second dimension (step S203).

[0146] The skilled person will understand that the steps of the above method involving the receiver 150 providing feedback to the controller 160 are non-essential. Instead, the

[0147] 30 controller 160 may obtain position information of the receiver 150 and identify a reconfiguration of each RIS that results in the wireless signal being focussed at that position. The controller 160 may therefore implement a calibration phase so as to determine the corresponding coordinate on a particular axis for each reconfiguration of a set of reconfigurations of each RIS.

[0148] 35 A36143

[0149] The skilled person will also understand that it is non-essential for each RIS to be a reflective RIS. That is, the RISs may be all transmissive or a mixture of transmissive and reflective.

[0150] 5 The skilled person will also understand that the wireless telecommunications network 100 may comprise the first RIS for reconfiguring the divergent wireless signal as a convergent wireless signal, and one further RIS for steering the convergent wireless signal in two dimensions. Although this requires more computational resources compared to the above method and reduces the focal length between the first RIS 120

[0151] 10 and the receiver 150, it is still more computationally efficient that using a single RIS.

[0152] The skilled person will also understand that it is non-essential for the wireless telecommunications network to use a single RIS controller 160. Each RIS may alternatively be controlled by a dedicated controller module.

[0153] The skilled person will understand that any combination of features is possible within the scope of the invention, as claimed.

[0154] 20

Claims

A36143CLAIMS1 . A Reconfigurable Intelligent Surface, RIS, controller for a wireless telecommunications network, the wireless telecommunications network comprising a5 transmitter, a first RIS, a second RIS, and a receiver, the RIS controller configured to: configure the first RIS to reconfigure a wireless signal incident at the first RIS to be: convergent, wherein a focal point of the wireless signal in a first dimension is independently configured by the first RIS to substantially align with10 a coordinate of the receiver in the first dimension, and directed towards the second RIS; and configure the second RIS to reconfigure the wireless signal, as directed from the first RIS and incident at the second RIS, wherein: the focal point of the wireless signal in at least a second dimension is15 independently configured by the second RIS to substantially align with a coordinate of the receiver in the second dimension.

2. A RIS controller as claimed in Claim 1 , wherein the focal point of the wireless signal in a third dimension is independently configured by the second RIS to20 substantially align with a coordinate of the receiver in the third dimension, and the second RIS further reconfigures the wireless signal to be directed towards the receiver.

3. A RIS controller as claimed in Claim 1 , wherein: the wireless telecommunications network further comprises a third RIS,25 the configuration of the second RIS is such that the wireless signal is directed towards the third RIS, and the RIS controller is further configured to: configure the third RIS to reconfigure the wireless signal, as directed from the second RIS and incident at the third RIS to be:30 directed towards the receiver, wherein the focal point of the wireless signal in a third dimension is independently configured by the third RIS to substantially align with a coordinate of the receiver in the third dimension.

4. A RIS controller as claimed in any one of preceding claims, wherein the RIS35 controller is one of a group comprising:A36143 a centralised RIS controller comprising a connection interface for connecting the centralised RIS controller to each RIS, and a set of distributed RIS controller modules, each RIS controller module configuring a particular RIS.

55. A RIS controller as claimed in any one of the preceding claims, wherein the RIS controller configures each RIS based on at least one of a group comprising: a feedback signal from the receiver, the feedback signal identifying a property of the wireless signal as received by the receiver, and a position of the receiver.

6. A wireless telecommunications network comprising: the first RIS; the second RIS; and15 a RIS controller as claimed in any one of the preceding claims.

7. A wireless telecommunications network comprising the RIS controller as claimed in Claim 3 or either Claim 4 or Claim 5 when dependent on Claim 3, further comprising the third RIS.

208. A method of reconfiguring a wireless signal in a wireless telecommunications network, the wireless telecommunications network comprising a transmitter, a first Reconfigurable Intelligent Surface, RIS, a second RIS and a receiver, the method comprising the steps of:25 configuring the first RIS to reconfigure a wireless signal incident at the first RIS to be: convergent, wherein a focal point of the wireless signal in a first dimension is independently configured by the first RIS to substantially align with a coordinate of the receiver in the first dimension, and directed towards the second RIS; and configuring the second RIS to reconfigure the wireless signal, as directed from the first RIS and incident at the second RIS, wherein: the focal point of the wireless signal in at least a second dimension is independently configured by the second RIS to substantially align with a35 coordinate of the receiver in the second dimension.19A361439. A method as claimed in Claim 8, wherein the step of configuring the second RIS is such that: the focal point of the wireless signal in a third dimension is independently5 configured by the second RIS to substantially align with a coordinate of the receiver in the third dimension, and the wireless signal is directed towards the receiver.

10. A method as claimed in Claim 8, wherein the wireless telecommunications network comprises a third RIS, and the method further comprises the steps of: configuring the second RIS such that: the wireless signal is directed towards the third RIS; and configuring the third RIS to reconfigure the wireless signal, as directed from the second RIS and incident at the third RIS to be:15 directed towards the receiver, wherein the focal point of the wireless signal in a third dimension is independently configured by the third RIS to substantially align with a coordinate of the receiver in the third dimension.1 1. A method as claimed in any one of Claims 8 to 10, wherein each configuring20 step is based on one of a group comprising: a feedback signal from the receiver, the feedback signal identifying a property of the wireless signal as received by the receiver, and a position of the receiver.

12. A computer program comprising instructions which, when the program is executed by a computer, cause the computer to carry out the steps of any one of Claims 8 to 11 .

13. A computer readable carrier medium comprising the computer program of Claim

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