A method for optimizing user received power in highly directional millimeter wave communication systems

Abstract

The present invention belongs to the technical field of wireless communications and specifically relates to a method for optimizing user received power in a highly directional millimeter wave communication system. The method comprises: rotating a reconfigurable smart surface along its horizontal central axis until its downtilt angle is between 0 and a critical value; the highly directional millimeter wave communication system comprises a reconfigurable smart surface, a base station, and a single user, wherein the reconfigurable smart surface is located in a far field near the base station and is vertically positioned; and the critical value corresponds to a specific expression related to communication system structural parameters such as the distance from the base station to the center of the reconfigurable smart surface, the total number of vertically arranged rows in the reconfigurable smart surface, the vertical length of each unit in the reconfigurable smart surface, and the incident and reflection angles of the reconfigurable smart surface in its vertical position before rotation. The method of the present invention can improve the received power of users in a directional millimeter wave communication system.

Description

Technical Field

[0001] The present invention belongs to the technical field related to wireless communications, and more specifically, relates to a method for optimizing user received power in a highly directional millimeter wave communication system. Background Art

[0002] With the acceleration of digital transformation and the continuous increase in the number of terminal devices, bandwidth resources in the current sub-6G frequency band have reached their limits. The millimeter wave band, with its abundant bandwidth resources, can support new mobile application scenarios. However, due to its short wavelength, millimeter wave signals suffer from severe path loss, resulting in significant attenuation of the signal from the base station to the user. Furthermore, users located in base station coverage blind spots also suffer from weak received signals. Traditionally, the method to overcome these limitations has been to use relays to amplify and forward millimeter wave signals. The emergence of reconfigurable smart surface technology offers a new solution to this problem. Its low cost and low power consumption make it a key candidate for 6G wireless communications.

[0003] Most current research on reconfigurable smart surfaces focuses on omnidirectional communication systems in low-frequency bands, assuming that the reconfigurable smart surface is fully illuminated by the transmitted beam. However, in future networks, reconfigurable smart surfaces are expected to be deployed over large areas to simultaneously receive highly directional beams transmitted by different base stations, resulting in only partial illumination. Two approaches can improve receiver performance: optimizing the deployment location of the reconfigurable smart surface, such as at the base station or user end, or by moving the reconfigurable smart surface, but the performance gains from this approach are limited. Another approach is to change the orientation of the reconfigurable smart surface, but this approach requires very high angular accuracy, and a certain error can actually lead to a decrease in system performance. Summary of the Invention

[0004] In response to the above-mentioned defects or improvement needs of the prior art, the present invention provides a method for optimizing the user receiving power in a highly directional millimeter wave communication system, the purpose of which is to establish a rotation optimization scheme for a highly directional millimeter wave communication system assisted by a reconfigurable surface, so as to improve the receiving power of users in the directional millimeter wave communication system.

[0005] To achieve the above objectives, according to one aspect of the present invention, a method for optimizing user received power in a highly directional millimeter wave communication system is provided, comprising: rotating the reconfigurable smart surface along its horizontal central axis until its downtilt angle is between 0 and a critical value;

[0006] The highly directional millimeter wave communication system includes a reconfigurable smart surface, a base station, and a single user, wherein the reconfigurable smart surface is located in a far field close to the base station and is placed vertically; and the critical value is:

[0007]

[0008] Where θ′ zb is the critical value; D tr is the distance from the base station to the center of the reconfigurable smart surface; φ BS is half of the 3dB beamwidth of the highly directional beam emitted by the base station; M is the total number of rows arranged in the vertical direction in the reconfigurable smart surface; S y is the vertical length of each unit in the reconfigurable smart surface; θ t is the incident angle of the reconfigurable smart surface, which is the angle between the incident direction of the highly directional transmission beam and the normal of the reconfigurable smart surface in the vertical state before rotation, θ r is the reflection angle of the reconfigurable smart surface, which is the angle between the reflection direction of the highly directional reflection beam and the normal of the reconfigurable smart surface in the vertical state before rotation.

[0009] Furthermore, the reconfigurable smart surface is rotated to its downtilt angle:

[0010]

[0011] Where θ′ opt is the optimal downtilt angle.

[0012] Furthermore, the positional relationship among the reconfigurable smart surface, base station, and user in the spatial coordinate system is:

[0013] The coordinates of the reconfigurable smart surface are (0, y RIS , h IRS ), the coordinates of the base station are (0, 0, h T ), the user's coordinates are (x R ,0,h R );

[0014] The azimuth angle of the base station is the angle between the vertical direction of the line connecting the base station and the center of the reconfigurable smart surface and the horizontal reference direction of the reconfigurable smart surface. The azimuth angle is The user's azimuth angle is the angle between the vertical direction of the line connecting the user and the center of the reconfigurable smart surface and the horizontal reference direction of the reconfigurable smart surface. The azimuth angle is

[0015] Furthermore, it also includes:

[0016] Based on the angular deviation of the reflected beam at the current downtilt angle, the spatial range of the user's movement is determined when the received power remains unchanged. The angular deviation of the reflected beam is expressed as:

[0017]

[0018] Where N′ u_h is the number of effective units in the longitudinal direction corresponding to the area illuminated by the transmitted beam during the rotation of the reconfigurable smart surface; λ is the wavelength of the operating frequency band of the reconfigurable smart surface; θ′ t is the incident angle of the current reconfigurable smart surface after rotation, which is the angle between the incident direction of the highly directional transmission beam and the normal of the currently placed reconfigurable smart surface after rotation; θ′ r is the reflection angle of the current reconfigurable smart surface after rotation, which is the angle between the reflection direction of the highly directional reflection beam and the normal of the currently placed reconfigurable smart surface after rotation.

[0019] According to another aspect of the present invention, there is provided a highly directional beam communication device comprising: a reconfigurable smart surface, a base station, and a surface rotation controller;

[0020] When the reconfigurable smart surface is located in the far field close to the base station, the surface rotation controller is used to control the reconfigurable smart surface to rotate along its horizontal central axis so that its downtilt angle is between 0 and a critical value, where the critical value is:

[0021]

[0022] Where θ′ zb is the critical value; D tr is the distance from the base station to the center of the reconfigurable smart surface; φ BS is half of the 3dB beamwidth of the highly directional beam emitted by the base station; M is the total number of rows arranged in the vertical direction in the reconfigurable smart surface; S y is the vertical length of each unit in the reconfigurable smart surface; θ t is the incident angle of the reconfigurable smart surface, which is the angle between the incident direction of the highly directional transmission beam and the normal of the reconfigurable smart surface in the vertical state before rotation, θ r is the reflection angle of the reconfigurable smart surface, which is the angle between the reflection direction of the highly directional reflection beam and the normal of the reconfigurable smart surface in the vertical state before rotation.

[0023] Furthermore, the surface rotation controller is specifically used to control the reconfigurable smart surface to rotate along its horizontal central axis so that its downtilt angle is:

[0024]

[0025] Where θ′ opt is the optimal downtilt angle.

[0026] Furthermore, the surface rotation controller is further configured to determine, based on an angle deviation of the reflected beam at the current downtilt angle, a spatial range of user movement when the received power remains unchanged, wherein the angle deviation of the reflected beam is expressed as:

[0027]

[0028] Where N′ u_h is the number of effective units in the longitudinal direction corresponding to the area illuminated by the transmitted beam during the rotation of the reconfigurable smart surface; λ is the wavelength of the operating frequency band of the reconfigurable smart surface; θ′ t is the incident angle of the current reconfigurable smart surface after rotation, which is the angle between the incident direction of the highly directional transmission beam and the normal of the currently placed reconfigurable smart surface after rotation; θ′ r is the reflection angle of the current reconfigurable smart surface after rotation, which is the angle between the reflection direction of the highly directional reflection beam and the normal of the currently placed reconfigurable smart surface after rotation.

[0029] In general, the above technical solutions conceived by the present invention have the following beneficial effects compared with the prior art:

[0030] 1. This invention proposes a method for optimizing user received power in a highly directional millimeter-wave communication system. The method employs a reconfigurable smart surface to enhance the signal strength of users in the blind spot of a base station transmitting highly directional millimeter-wave signals. The method increases the number of effective reflective elements by varying the downtilt angle of the reconfigurable smart surface. Specifically, the reconfigurable smart surface is rotated along its horizontal central axis until its downtilt angle is between 0 and a critical value. When the reconfigurable smart surface is located in the far field near the base station and is positioned vertically, the critical value is: Where D tr is the distance from the base station to the center of the reconfigurable smart surface; φ BS is half of the 3dB beamwidth of the highly directional beam emitted by the base station; M is the total number of rows arranged in the vertical direction in the reconfigurable smart surface; S y is the vertical length of each unit in the reconfigurable smart surface; θ t is the incident angle of the reconfigurable smart surface, which is the angle between the incident direction of the highly directional transmission beam and the normal of the reconfigurable smart surface in the vertical state before rotation, θ r is the reflection angle of the reconfigurable smart surface, defined as the angle between the reflection direction of the highly directional reflection beam and the normal of the reconfigurable smart surface in its vertical position before rotation. Compared to communication systems assisted by unrotated reconfigurable smart surfaces, the proposed rotation scheme achieves improved performance for all user terminals within the critical downtilt angle range, demonstrating its superior practicality.

[0031] 2. The present invention proposes an optimal downtilt angle. At the optimal downtilt angle, the receiving power of the user end is maximum.

[0032] 3. The present invention also proposes to provide adjustable signal coverage while ensuring user communication performance based on the angular deviation of the reflected beam after rotation. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 This is an equivalent system model provided by an embodiment of the present invention for a reconfigurable intelligent surface-assisted communication with users in blind areas of a highly directional millimeter wave signal base station;

[0034] Figure 2 Schematic diagram of the downtilt rotation of the reconfigurable smart surface provided by an embodiment of the present invention;

[0035] Figure 3 This is a graph showing how the effective area of ​​a reconfigurable smart surface illuminated by base station transmitting antennas of different apertures changes with downtilt angles, provided by an embodiment of the present invention;

[0036] Figure 4 This is a graph showing how the user-side received power changes with downtilt angles under different aperture base station transmitting antennas for the reconfigurable smart surface provided by an embodiment of the present invention;

[0037] Figure 5 This is a graph showing how the 3 dB beam width of a transmitting beam changes with the downtilt angle under different aperture base station transmitting antennas of the reconfigurable smart surface provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0038] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely for the purpose of explaining the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.

[0039] Example 1

[0040] A method for optimizing user received power in a highly directional millimeter wave communication system comprises: rotating the reconfigurable smart surface along its horizontal central axis until its downtilt angle is between 0 and a critical value;

[0041] The highly directional millimeter wave communication system includes a reconfigurable smart surface, a base station, and a single user. The reconfigurable smart surface is located in a far field close to the base station and is placed vertically. The critical value is:

[0042]

[0043] Where θ′zb is the critical value; D tr is the distance from the base station to the center of the reconfigurable smart surface; φ BS is half of the 3dB beamwidth of the highly directional beam emitted by the base station; M is the total number of rows arranged in the vertical direction in the reconfigurable smart surface; S y is the vertical length of each unit in the reconfigurable smart surface; θ t is the incident angle of the reconfigurable smart surface, which is the angle between the incident direction of the highly directional transmission beam and the normal of the reconfigurable smart surface in the vertical state before rotation, θ r is the reflection angle of the reconfigurable smart surface, which is the angle between the reflection direction of the highly directional reflection beam and the normal of the reconfigurable smart surface in the vertical state before rotation.

[0044] When a user antenna (such as a mobile phone or large communication device) is located in a base station's coverage blind spot, a reconfigurable smart surface is deployed at the base station to increase line-of-sight. The base station antenna emits a highly directional beam; the reconfigurable smart surface is located in the base station's far field, and the illuminated area is smaller than the entire surface area. By rotating the downtilt angle, the number of active elements is increased, thereby improving the received power at the user end.

[0045] The highly directional millimeter wave communication system consists of a highly directional millimeter wave base station antenna, user receiving antennas, and a reconfigurable smart surface. The user antenna is located in a base station coverage blind spot, which can be modeled as a situation where the direct communication link between the base station antenna and the user antenna is blocked. The reconfigurable smart surface is deployed to fill the blind spot.

[0046] Base stations are equipped with large antennas, such as phased array antennas and parabolic antennas, that are much larger than the wavelength. These antennas can emit highly directional beams with very narrow beamwidths. Reconfigurable smart surfaces are located in the far field between the base station and the user, deployed close to the base station with adjustable downtilt angles. The links from the base station to the center of the reconfigurable smart surface, and from the reconfigurable smart surface to the user, are both line-of-sight (LOS) paths. Highly directional electromagnetic waves propagate through free space with negligible delay. By adjusting the phase of each electromagnetic unit on the reconfigurable smart surface, the reflected electromagnetic waves have a constructive effect on the far-field user, providing increased received power at the user end.

[0047] Establish an equivalent system model based on the actual communication system, such as Figure 1 As shown. The coordinates of the base station are (0, 0, h T ), the coordinates of the reconfigurable smart surface are (x RIS ,y RIS , h RIS ), the user's coordinates are (x R ,0,h R ). The optimal horizontal position of the reconfigurable smart surface is That is, it is placed at the base station end, and without loss of generality, the height of the base station is greater than the height of the reconfigurable smart surface. As an optimal option, the azimuth angle of the base station is The user's azimuth is The reconfigurable smart surface array consists of M rows and N columns of units, and the length and width of the unit are S x and S y , the incident angle of the reconfigurable smart surface (the angle between the incident beam and the surface normal) is θ t , the reflection angle (the angle between the surface normal and the reflection direction) is θ r In the case of optimal beamforming, the phase of the mth row and nth column element is It can be expressed as:

[0048]

[0049] Where λ is the operating wavelength and m is the row where the corresponding unit is located.

[0050] D tr and D rr are the distances from the base station to the center of the reconfigurable smart surface and from the center of the reconfigurable smart surface to the user, respectively. The expressions are as follows:

[0051]

[0052] The incident angle of the reconfigurable smart surface is θ t , the reflection angle is θ r , and according to the geometric relationship, θ t =y RIS / D tr ,θ r =y RIS / D rr Under far-field conditions, the user's received power is obtained according to the classic path loss model:

[0053]

[0054] Among them, P t is the transmission power of the base station, P r is the transmission power of the base station. t and G r are the power gains of the base station transmitting antenna and the user antenna respectively, and the amplitude of each unit of the reconfigurable smart surface is A.

[0055] Specifically, such as Figure 2 As shown, half of the 3dB beamwidth of the base station transmitting a highly directional beam is defined as φ BS , is 2φ BS ≈1.22λ / R BS , where R BSis the aperture of the base station antenna. The (projected) angle between the transmitting beam and the reconfigurable smart surface is θ s , then according to the geometric relationship, θ s =π / 2-θ t Due to the existence of 3D deviation, the shape of the base station main beam projected onto the reconfigurable smart surface area is irregular, and is actually an asymmetric ellipse. In order to facilitate analysis, it is approximated as an ellipse, and the horizontal and vertical axes are respectively taken as the average values ​​of the corresponding actual projection axes, which are defined as n e and m e According to the sine theorem, they can be expressed as

[0056]

[0057] are the lengths of the axes of the asymmetric ellipse projected onto the reconfigurable smart surface. The area of ​​the ellipse-like surface is represented by S i =πn e m e , where n e is the horizontal axis of the ellipse (half the sum of n1 and n2), m e is the vertical axis of the ellipse. When the reconfigurable smart surface is not rotated, the received power at the user end can be re-expressed as

[0058]

[0059] The received power at the user end can be increased by increasing the area of ​​the illuminated area of ​​the reconfigurable smart surface.

[0060] Figure 3 The curve of the area illuminated by the reconfigurable smart surface changes with the downtilt angle under different base station antenna apertures. The system setting parameters include, for example: the operating frequency of the reconfigurable smart surface is 27GHz, the number of units is 30 rows and 30 columns, a total of 900 units, and the length and width of the unit are both half a wavelength. The amplitude A of all units m，n = 1. The height of the base station is h BS =12m. The distance between the user and the base station is x R =10m. The transmission power of the base station antenna is P t =0.1W, the gain of the base station antenna and the user antenna are both 109.6. The incident angle and reflection angle of the reconfigurable smart surface are calculated to obtain θ t =21.8° and θ r =52°. As can be seen from the figure, the larger the aperture of the base station antenna, the smaller the effective area of ​​the reconfigurable smart surface, and a larger downtilt angle is required to achieve a constant area. Without loss of generality, the height of the base station antenna is greater than the height of the reconfigurable smart surface. In addition, to avoid occlusion of the transmitted signal due to rotation, the range of the downtilt angle is set to θ′∈(0,π / 2-θ t), it should be noted that, Figures 3 to 5 The "rotation angle" described at the horizontal axis is the meaning of the downtilt angle mentioned above. After the downtilt angle of the reconfigurable smart surface is rotated by θ', the horizontal axis of the ellipse of the illuminated area does not change, and the vertical axis continues to increase until the RIS in the vertical direction is completely illuminated. That is, the effective area of ​​the highly directional beam emitted by the base station projected onto the rotated reconfigurable smart surface is a quasi-ellipse, and during the rotation process, the short axis of the quasi-ellipse remains unchanged, and the vertical axis continues to increase. Therefore, the illuminated area of ​​the reconfigurable smart surface continues to increase during the rotation process until the length of the vertical axis of the quasi-ellipse is equal to the longitudinal length (vertical direction) of the reconfigurable smart surface, and the area of ​​the quasi-ellipse during the rotation process is expressed as S' i =πn e m′ e , where n e is half of the horizontal axis of the ellipse, m′ e It is half of the longitudinal axis of the ellipse.

[0061] m′ e is the vertical axis of the rotated ellipse, expressed as:

[0062]

[0063] Further, let:

[0064]

[0065] Taking the derivative of the above formula, we can get:

[0066]

[0067] Since in a highly directional antenna, φ BS Very small. Assuming that the base station uses a phased array antenna to generate an ultra-narrow, highly directional beam with a 3dB beamwidth of less than 1°, it can be considered that θ t >>φ BS , ignore φ in the denominator of the above formula BS , then by simple derivation we can know that m′ e As θ′ increases, it increases monotonically until the longitudinal axis of the reconfigurable smart surface is fully illuminated, and the area of ​​the illuminated area remains unchanged. Therefore, during the rotation process, the effective area of ​​the reconfigurable smart surface continues to increase before the area becomes constant.

[0068] The received power at the user end can be expressed as follows according to the classic received power expression:

[0069]

[0070] Among them, P′ r is the received power of the user end under the current downtilt angle of the reconfigurable smart surface, Pt is the transmission power of the base station (preset value). t and G r are the power gains of the base station transmitting antenna and the user receiving antenna (preset), and the amplitude of each unit of the reconfigurable smart surface is A.

[0071] Figure 4 This is a curve showing how the user-end received power changes with the downtilt angle during the rotation of the reconfigurable smart surface proposed in this embodiment. During rotation, the user-end received power initially increases, reaching its maximum when the optimal downtilt angle is reached. Thereafter, the illuminated effective area remains unchanged, and the received power begins to decrease with continued rotation, but remains greater than the pre-rotation performance within a certain range. The optimal downtilt angle for the reconfigurable smart surface is:

[0072]

[0073] Among them, φ BS is half of the 3dB beamwidth of the highly directional beam transmitted by the base station (preset value), M is the total number of rows arranged in the vertical direction in the reconfigurable smart surface, S y is the vertical length of the unit.

[0074] During the rotation process, the received power at the user end can be rewritten as:

[0075]

[0076] Among them, when the base station, reconfigurable intelligent surface (RIS) and user positions are determined, α is a constant and β is a variable related to the incident angle, emission angle and downtilt angle.

[0077] Next, we analyze the effect of downtilt angle on received power. Similarly, due to φ BS is so small that it can be ignored, and β can be rewritten as:

[0078]

[0079] Taking the derivative of the above formula, we can get:

[0080]

[0081] Since θ t ,θ r ∈(0,π / 2), it can be found that β(θ′) is always greater than 0 in the entire domain, that is, the received power increases with the increase of the downtilt angle θ′. However, when the illuminated area of ​​RIS does not change, that is, all the vertical units of RIS are illuminated, the vertical axis of the illuminated elliptical area is: m′ e =MS y / 2. Then the received power at this time is:

[0082]

[0083] In the above formula, κ is a constant, let:

[0084]

[0085] but When θ′=(θ r -θ t ) / 2, When θ r >θ t , then 0<(θ r -θ t ) / 2<π / 2-θ t . Therefore, when θ′∈[0,(θ r -θ t ) / 2], When θ′∈[θ r -θ t ) / 2,π / 2-θ t ],

[0086] Further:

[0087]

[0088] Similarly, ignoring φ in the denominator BS The optimal downtilt angle can be obtained by adjusting the size of .

[0089] When the base station antenna diameters are R BS When the RIS is rotated to 1.5, 2, and 2.5 meters, the optimal downtilt angles are 32.47°, 42.22°, and 47.69°, respectively. Furthermore, compared to the unrotated scheme, the received power at the optimal downtilt angles is increased by 7.7%, 9.6%, and 10.6%, respectively.

[0090] After the optimal downtilt angle, the received power at the user end begins to decrease. The downtilt angle corresponding to the intersection of the received power in the decreasing stage and the curve of the reconfigurable smart surface without rotation is defined as the critical downtilt angle θ′ zb , this downtilt angle is the critical point for improving the receiving power performance, in [0,θ′ zb ] range, even if there is a certain rotation error, it will not affect the performance improvement. The critical downtilt angle of the reconfigurable smart surface is:

[0091]

[0092] The received power of the RIS without rotation equals the received power of the RIS with rotation only when the received power starts to decrease due to rotation. Therefore:

[0093]

[0094] In addition, at this stage, m′ e =MS y / 2, then further:

[0095]

[0096] According to the formula of sum and difference of cosine, we can get:

[0097]

[0098] The critical downtilt angle can be obtained according to the inverse trigonometric function.

[0099] When the base station antenna diameters are R BS =1.5, 2, and 2.5m, the critical downtilt angles of the RIS are 55.11°, 60.81°, and 63.44°, respectively. Furthermore, when the downtilt angle exceeds the critical downtilt angle, the received power at the user end begins to decrease, indicating that appropriate rotation of the reconfigurable smart surface can improve system performance, while excessive rotation can lead to deterioration of system performance.

[0100] In general, the millimeter wave band has abundant spectrum resources. Millimeter wave signals are typically highly directional, narrow beams. Therefore, only a portion of the units on the reconfigurable smart surface are illuminated. The rotation scheme proposed in this embodiment increases the number of effective reflective units. Key technical means: Compared to communication systems assisted by non-rotated reconfigurable smart surfaces, the reconfigurable smart surface constructed in this application can significantly improve the user end's received power performance at the optimal downtilt angle, and can improve the user end's received power before rotating to the critical downtilt angle.

[0101] As a preferred embodiment, the optimization method further includes:

[0102] Based on the angular deviation of the reflected beam at the current downtilt angle, the spatial range of the user's movement is determined when the received power remains unchanged. The angular deviation of the reflected beam is expressed as:

[0103]

[0104] Where N′ u_h is the number of effective units in the longitudinal direction corresponding to the area illuminated by the transmitted beam during the rotation of the reconfigurable smart surface; λ is the wavelength of the operating frequency band of the reconfigurable smart surface; θ′ tis the incident angle of the current reconfigurable smart surface after rotation, which is the angle between the incident direction of the highly directional transmission beam and the normal of the currently placed reconfigurable smart surface after rotation; θ′ r is the reflection angle of the current reconfigurable smart surface after rotation, which is the angle between the reflection direction of the highly directional reflection beam and the normal of the currently placed reconfigurable smart surface after rotation.

[0105] By adjusting the downtilt angle of the reconfigurable smart surface, the 3dB beamwidth of the reflected beam can be adjusted while ensuring the received power at the user end, giving the reflected signal a wider coverage area. Therefore, the 3dB beamwidth of the reflected beam passing through the reconfigurable smart surface is adjustable, providing adjustable signal coverage while ensuring user communication performance. Theoretical explanation is as follows:

[0106] Figure 5 Figure 3 shows the 3dB beamwidth of the beam reflected by the reconfigurable smart surface during rotation, as a function of the downtilt angle for different transmitting antenna apertures. As the reconfigurable smart surface's downtilt angle rotates, the change in the illuminated area affects the number of effective longitudinal elements, which in turn affects the 3dB beamwidth of the reflected beam. The number of longitudinal elements can be equivalently expressed as:

[0107]

[0108] The 3dB beamwidth is twice the deviation between the reflection angle and the incident angle.

[0109] The angular deviation of the reflected beam is expressed as:

[0110]

[0111] Among them, θ′ t =θ t -θ′ is the incident angle of the reconfigurable smart surface after rotation, θ′ r =θ r -θ′ is the reflection angle.

[0112] Assuming that the reflection coefficients of all RIS units are the same, the received power of the receiving antenna is:

[0113]

[0114] in:

[0115]

[0116] Since the azimuth angles of the transmitting antenna and the receiving antenna are π / 2 and 3π / 2 respectively, the above formula can be simplified to:

[0117]

[0118] Furthermore, the half-power beamwidth is the angular range corresponding to the time when the received power drops from the peak to half of the maximum value, that is, let |Ξ| 2 =1 / 2.

[0119] The sinc(·) function is an even function, and the beamwidth of the reflected beam is related to the angular deviation, so Ξ can be rewritten as:

[0120]

[0121] Using Taylor expansion of the cosine function and trigonometric identity transformation, we can get:

[0122] sinθ′ r -sinθ′ t =(θ′ r -θ′ t )cosθ′ t +O((θ′ r -θ′ t ) 2 )

[0123] Furthermore, sinc 2 (x) = 1 - x 2 / 3+O(x 3 ), we can get:

[0124]

[0125] Simplifying, we can get:

[0126]

[0127] Since (N′ u_h ) 2 >>1 / 2, [(N′ u_h ) 2 -1 / 2]≈(N′ u_h ) 2 , substituting into the original formula, we can prove the following conclusion:

[0128] As the RIS rotates, the number of elements along the longitudinal axis approaches a constant value, while the cosine of the incident angle decreases, causing the 3dB beamwidth of the reflected beam to gradually widen. This means that while maintaining system performance, the reflected beam can be adjusted by rotating the RIS, providing adjustable signal coverage while maintaining user communication performance.

[0129] Therefore, as the reconfigurable smart surface rotates, the width of the reflected beam will become wider, which can ensure the user's receiving power while tolerating the user's movement in a small range, and the communication quality will not be affected.

[0130] In summary, this invention provides a method for optimizing user received power in a highly directional millimeter-wave communication system, characterizing a rotation optimization scheme for a reconfigurable smart surface-assisted highly directional millimeter-wave communication system. When a base station transmits a highly directional beam, the reconfigurable smart surface is rotated to an optimal downtilt angle to maximize the number of effective reflective elements, with effective rotation occurring within the critical downtilt angle range. This improves user-side received power. Furthermore, the 3dB beamwidth of the reflected signal from the reconfigurable smart surface is adjustable, thereby ensuring user communication performance while providing adjustable signal coverage.

[0131] Example 2

[0132] A highly directional beam communication device comprising: a reconfigurable smart surface, a base station, and a surface rotation controller;

[0133] When the reconfigurable smart surface is located in the far field close to the base station, the surface rotation controller is used to control the reconfigurable smart surface to rotate along its horizontal central axis so that its downtilt angle is between 0 and a critical value, where the critical value is:

[0134]

[0135] Where θ′ zb is the critical value; D tr is the distance from the base station to the center of the reconfigurable smart surface; φ BS is half of the 3dB beamwidth of the highly directional beam emitted by the base station; M is the total number of rows arranged in the vertical direction in the reconfigurable smart surface; S y is the vertical length of each unit in the reconfigurable smart surface; θ t is the incident angle of the reconfigurable smart surface, which is the angle between the incident direction of the highly directional transmission beam and the normal of the reconfigurable smart surface in the vertical state before rotation, θ r is the reflection angle of the reconfigurable smart surface, which is the angle between the reflection direction of the highly directional reflection beam and the normal of the reconfigurable smart surface in the vertical state before rotation.

[0136] As a preferred embodiment, the surface rotation controller is specifically used to control the reconfigurable smart surface to rotate along its horizontal central axis so that its downtilt angle is:

[0137]

[0138] Where θ′ opt is the optimal downtilt angle.

[0139] As a preferred embodiment, the surface rotation controller is further configured to determine the spatial range of user movement under the condition of unchanged received power based on the angular deviation of the reflected beam at the current downtilt angle, wherein the angular deviation of the reflected beam is expressed as:

[0140]

[0141] Where N′ u_h is the number of effective units in the longitudinal direction corresponding to the area illuminated by the transmitted beam during the rotation of the reconfigurable smart surface; λ is the wavelength of the operating frequency band of the reconfigurable smart surface; θ′ t is the incident angle of the current reconfigurable smart surface after rotation, which is the angle between the incident direction of the highly directional transmission beam and the normal of the currently placed reconfigurable smart surface after rotation; θ′ r is the reflection angle of the current reconfigurable smart surface after rotation, which is the angle between the reflection direction of the highly directional reflection beam and the normal of the currently placed reconfigurable smart surface after rotation.

[0142] The rotation optimization method proposed in this embodiment can increase the spatial coverage of the reflected beam while ensuring user receive power performance and accommodating user mobility. It should be noted that the surface rotation controller generally also includes a controller for the reconfigurable intelligent unit, which is used to control the reconfigurable intelligent unit to perform beamforming and achieve conventional functions.

[0143] The relevant technical solutions are the same as those in Example 1 and will not be described again here.

[0144] It will be easily understood by those skilled in the art that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for optimizing user received power in a highly directional millimeter wave communication system, characterized in that: include: Rotating the reconfigurable smart surface along its horizontal central axis until its downtilt angle is between 0 and a critical value; The highly directional millimeter wave communication system includes a reconfigurable smart surface, a base station, and a single user, wherein the reconfigurable smart surface is located in a far field close to the base station and is placed vertically; and the critical value is: Where θ′ zb is the critical value; D tr is the distance from the base station to the center of the reconfigurable smart surface; φ BS is half of the 3dB beamwidth of the highly directional beam emitted by the base station; M is the total number of rows arranged in the vertical direction in the reconfigurable smart surface; S y is the vertical length of each unit in the reconfigurable smart surface; θ t is the incident angle of the reconfigurable smart surface, which is the angle between the incident direction of the highly directional transmission beam and the normal of the reconfigurable smart surface in the vertical state before rotation, θ r is the reflection angle of the reconfigurable smart surface, which is the angle between the reflection direction of the highly directional reflection beam and the normal of the reconfigurable smart surface in the vertical state before rotation.

2. The optimization method according to claim 1, wherein: Rotate the reconfigurable smart surface to its downtilt angle: Where θ′ opt is the optimal downtilt angle.

3. The optimization method according to claim 1, wherein: The positional relationship among the reconfigurable smart surface, base station, and user in the spatial coordinate system is: The coordinates of the reconfigurable smart surface are (0, y RIS , h RIS ), the coordinates of the base station are (0, 0, h T ), the user's coordinates are (x R ,0,h R ); The azimuth angle of the base station is the angle between the vertical direction of the line connecting the base station and the center of the reconfigurable smart surface and the horizontal reference direction of the reconfigurable smart surface. The azimuth angle is The user's azimuth angle is the angle between the vertical direction of the line connecting the user and the center of the reconfigurable smart surface and the horizontal reference direction of the reconfigurable smart surface. The azimuth angle is 4. The optimization method according to claim 1, wherein: Also includes: Based on the angular deviation of the reflected beam at the current downtilt angle, the spatial range of the user's movement is determined when the received power remains unchanged. The angular deviation of the reflected beam is expressed as: Where N′ u_h is the number of effective units in the longitudinal direction corresponding to the area illuminated by the transmitted beam during the rotation of the reconfigurable smart surface; λ is the wavelength of the operating frequency band of the reconfigurable smart surface; θ′ t is the incident angle of the current reconfigurable smart surface after rotation, which is the angle between the incident direction of the highly directional transmission beam and the normal of the currently placed reconfigurable smart surface after rotation; θ′ r is the reflection angle of the current reconfigurable smart surface after rotation, which is the angle between the reflection direction of the highly directional reflection beam and the normal of the currently placed reconfigurable smart surface after rotation.

5. A highly directional beam communication device, characterized in that: include: Reconfigurable smart surfaces, base stations, and surface rotation controllers; When the reconfigurable smart surface is located in the far field close to the base station, the surface rotation controller is used to control the reconfigurable smart surface to rotate along its horizontal central axis so that its downtilt angle is between 0 and a critical value, where the critical value is: Where θ′ zb is the critical value; D tr is the distance from the base station to the center of the reconfigurable smart surface; φ BS is half of the 3dB beamwidth of the highly directional beam emitted by the base station; M is the total number of rows arranged in the vertical direction in the reconfigurable smart surface; S y is the vertical length of each unit in the reconfigurable smart surface; θ t is the incident angle of the reconfigurable smart surface, which is the angle between the incident direction of the highly directional transmission beam and the normal of the reconfigurable smart surface in the vertical state before rotation, θ r is the reflection angle of the reconfigurable smart surface, which is the angle between the reflection direction of the highly directional reflection beam and the normal of the reconfigurable smart surface in the vertical state before rotation.

6. The highly directional beam communication device according to claim 5, characterized in that: The surface rotation controller is specifically used to control the reconfigurable smart surface to rotate along its horizontal central axis so that its downtilt angle is: Where θ′ opt is the optimal downtilt angle.

7. The highly directional beam communication device according to claim 5, characterized in that: The surface rotation controller is further configured to determine, based on the angle deviation of the reflected beam at the current downtilt angle, the spatial range of user movement when the received power remains unchanged, wherein the angle deviation of the reflected beam is expressed as: Where N′ u_h is the number of effective units in the longitudinal direction corresponding to the area illuminated by the transmitted beam during the rotation of the reconfigurable smart surface; λ is the wavelength of the operating frequency band of the reconfigurable smart surface; θ′ t is the incident angle of the current reconfigurable smart surface after rotation, which is the angle between the incident direction of the highly directional transmission beam and the normal of the currently placed reconfigurable smart surface after rotation; θ′ r is the reflection angle of the current reconfigurable smart surface after rotation, which is the angle between the reflection direction of the highly directional reflection beam and the normal of the currently placed reconfigurable smart surface after rotation.

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