Optimization method for user receiving power in highly directional millimeter wave communication system

Abstract

The invention belongs to the related technical field of wireless communication, and particularly relates to a method for optimizing user receiving power in a highly directional millimeter wave communication system, which comprises the following steps of: rotating a reconfigurable intelligent surface along a transverse horizontal central axis of the reconfigurable intelligent surface until a downward inclination angle of the reconfigurable intelligent surface is between 0 and a critical value; the high-orientation millimeter wave communication system comprises a reconfigurable intelligent surface, a base station and a single user, and the reconfigurable intelligent surface is located in a far field close to the base station and is vertically placed; the critical value corresponds to a certain expression; specifically, the distance between a base station and the center position of the reconfigurable intelligent surface, the total number of rows arranged in the vertical direction in the reconfigurable intelligent surface, the vertical length of each unit in the reconfigurable intelligent surface, and the incident angle and the reflection angle of the reconfigurable intelligent surface in the vertical placement state before rotation are related to communication system structure parameters. According to the method, the receiving power of the user in the directional millimeter wave communication system can be improved.

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, the number of terminal devices continues to increase, resulting in the bandwidth resources of the current sub-6G frequency band reaching their limits. The millimeter wave band can support new mobile application scenarios because of its abundant bandwidth resources. However, due to its short wavelength, millimeter wave signals have serious path loss, resulting in severe attenuation of the signal from the base station to the user. In addition, users in the coverage blind spot of the base station will also cause the user to receive a weak signal. The traditional way to overcome such limitations is to use relays to amplify and forward millimeter wave signals. The emergence of reconfigurable smart surface technology provides a new solution to this problem, and it has the advantages of low cost and low power consumption, becoming one of the key candidate technologies for 6G wireless communications.

[0003] At present, most of the research on reconfigurable smart surfaces focuses on omnidirectional communication systems in low-frequency bands, and assumes that the reconfigurable smart surface can be fully illuminated by the transmitting beam. In future networks, reconfigurable smart surfaces are expected to be deployed over a large area to simultaneously receive highly directional beams transmitted by different base stations, so only part of the area is illuminated. There are two solutions to improve the performance of the receiving end. One is to optimize the deployment position of the reconfigurable smart surface, such as deploying it at the base station end or the user end, or by moving the position of the reconfigurable smart surface, but the performance gain brought by this solution is limited. Another solution is to change the orientation of the reconfigurable smart surface, but this solution has high requirements for the angle accuracy, and a certain error will cause the system performance to decline. Summary of the invention

[0004] In view of the above 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 user receiving power in the directional millimeter wave communication system.

[0005] To achieve the above object, 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 the lateral horizontal center axis of the reconfigurable smart surface 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] In the formula, 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 3 dB beam width 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] Further, the reconfigurable smart surface is rotated to its downward tilt angle:

[0010]

[0011] In the formula, θ′ opt is the optimal downtilt angle.

[0012] Furthermore, the positional relationship among the reconfigurable intelligent 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 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 projected onto the reconfigurable smart surface and the horizontal reference direction of the reconfigurable smart surface. The azimuth is The user's azimuth 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 is

[0015] Furthermore, it also includes:

[0016] According to the angle deviation of the reflected beam at the current downtilt angle, the spatial range of user movement is determined when the received power remains unchanged, where the angle 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 working 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, and is taken as the angle between the reflection direction of the highly directional reflection beam and the normal of the reconfigurable smart surface currently placed 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, and the critical value is:

[0021]

[0022] In the formula, 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 3 dB beam width 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 lateral horizontal central axis so that its downtilt angle is:

[0024]

[0025] In the formula, θ′ opt is the optimal downtilt angle.

[0026] Further, the surface rotation controller is also used to: determine the spatial range of user movement when the received power remains unchanged according to the angle deviation of the reflected beam at the current downtilt angle, 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 working 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, and is taken as the angle between the reflection direction of the highly directional reflection beam and the normal of the reconfigurable smart surface currently placed after rotation.

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

[0030] 1. The present invention proposes a method for optimizing user received power in a highly directional millimeter wave communication system, which uses a reconfigurable smart surface to enhance the signal of a user in a blind area of ​​a base station transmitting a highly directional millimeter wave signal, and increases the number of effective reflection units by changing the downtilt angle of the reconfigurable smart surface. Specifically, the reconfigurable smart surface is rotated along the horizontal central axis of the reconfigurable smart surface until its downtilt angle is between 0 and a critical value. When the reconfigurable smart surface is located in the far field close to the base station and is placed 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 3 dB beam width 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. Compared with the communication system assisted by the unrotated reconfigurable smart surface, the rotation scheme proposed in the present invention rotates the reconfigurable smart surface within the critical downtilt angle range, and the user end will obtain performance improvement, which has strong practicality.

[0031] 2. The present invention proposes an optimal downtilt angle. At the optimal rotation 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 It is an equivalent system model provided by an embodiment of the present invention for a reconfigurable intelligent surface to assist communication with users in a blind area of ​​a highly directional millimeter wave signal base station;

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

[0035] Figure 3 is a curve diagram showing changes in the effective area of ​​the reconfigurable smart surface illuminated by base station transmitting antennas of different apertures as a function of the downtilt angle provided by an embodiment of the present invention;

[0036] Figure 4 It is a curve diagram of the change of the received power of the user end with the downtilt angle under the base station transmitting antennas with different calibers of the reconfigurable smart surface provided by the embodiment of the present invention;

[0037] Figure 5 It is a curve diagram showing the variation of the 3 dB beam width of the transmitting beam 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 purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain 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 can be combined with each other as long as they do not conflict with each other.

[0039] Embodiment 1

[0040] A method for optimizing user received power in a highly directional millimeter wave communication system comprises: rotating the reconfigurable smart surface along the lateral horizontal center axis of the reconfigurable smart surface 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, wherein 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] In the formula, 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 3 dB beam width 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 the user antenna (such as a mobile phone or large communication device) is in the coverage blind spot of the base station, a reconfigurable smart surface is deployed at the base station to increase the visual link. The base station antenna transmits a highly directional beam; the reconfigurable smart surface is located in the far field of the base station and the illuminated area is smaller than the area of ​​the entire surface. The number of effective units is increased by rotating the downtilt angle, thereby improving the receiving power at the user end.

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

[0046] The base station is equipped with large antennas that are much larger than the wavelength, such as phased array antennas and parabolic antennas, which can emit highly directional beams with very narrow beam widths. The reconfigurable smart surface is located in the far field between the base station and the user, deployed close to the base station, and has an adjustable downtilt angle. 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 paths (LOS paths). Highly directional electromagnetic waves propagate through free space, and the delay is negligible. By adjusting the phase of each electromagnetic unit of the reconfigurable smart surface, the reflected electromagnetic waves can play a constructive role at the far-field user end, providing the receiving 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 a preferred embodiment, 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] Among them, 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 Then, 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, 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. For the convenience of 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 longitudinal 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 region of the reconfigurable smart surface.

[0060] Figure 3 The curve of the area illuminated by the reconfigurable smart surface with the downtilt angle under different base station antenna apertures. The system setting parameters, for example, include: 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 is 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 blocking 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, during the rotation process, the illuminated area of ​​the reconfigurable smart surface continues to increase 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 longitudinal 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 , ignoring 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. 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 classical 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 The curve of the user end receiving power changing with the downtilt angle during the rotation of the reconfigurable smart surface proposed in this embodiment. During the rotation of the reconfigurable smart surface, the user end receiving power first increases, and the receiving power is maximum when the optimal downtilt angle is reached. After that, the effective area illuminated remains unchanged, and the receiving power of the surface starts to decay as the rotation continues, but it is still greater than the performance before the surface rotates within a certain rotation range. The optimal downtilt angle of the reconfigurable smart surface is:

[0072]

[0073] Among them, φ BS is half of the 3 dB beam width of the highly directional beam emitted 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, the emission angle and the 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 definition 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 units in the vertical direction 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 =1.5, 2 and 2.5m, the best downtilt angles of RIS are 32.47°, 42.22° and 47.69° respectively. In addition, compared with the unrotated scheme, the received power is increased by 7.7%, 9.6% and 10.6% at the best downtilt angle 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 ], even if there is a certain rotation error, it does 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 is equal to 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 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 the performance of the system, while excessive rotation will lead to the deterioration of system performance.

[0100] In general, the millimeter wave frequency band has abundant spectrum resources. Usually, the millimeter wave signal is a highly directional narrow beam. Therefore, only some units of the reconfigurable smart surface are illuminated. The rotation scheme proposed in this embodiment increases the number of effective reflection units. Key technical means: Compared with the communication system assisted by the non-rotated reconfigurable smart surface, the reconfigurable smart surface constructed in this application can achieve a significant improvement in the receiving power performance of the user end under the optimal downtilt angle, and the receiving power of the user end can be improved before rotating to the critical downtilt angle.

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

[0102] According to the angle deviation of the reflected beam at the current downtilt angle, the spatial range of user movement is determined when the received power remains unchanged, where the angle 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 working 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, and is taken as the angle between the reflection direction of the highly directional reflection beam and the normal of the reconfigurable smart surface currently placed after rotation.

[0105] By adjusting the downtilt angle of the reconfigurable smart surface, the 3dB beam width of the reflected beam can be adjusted while ensuring the received power at the user end, so that the reflected signal has a wider coverage range. Therefore, the 3dB beam width of the reflected beam through the reconfigurable smart surface is adjustable, which can provide adjustable signal coverage while ensuring the user's communication performance. Theoretically, it is explained as follows:

[0106] Figure 5 The following is the curve of the 3dB beam width of the beam reflected by the reconfigurable smart surface during the rotation process under different antenna apertures. When the reconfigurable smart surface's downtilt angle rotates, the change in the illuminated area will affect the number of longitudinal effective units, and thus affect the 3dB beam width of the reflected beam. The number of longitudinal units 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 angle range corresponding to the decrease of the received power from the peak value to half of the maximum value, that is, let |Ξ| 2 =1 / 2.

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

[0120]

[0121] Using Taylor expansion of 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 ), then we can get:

[0124]

[0125] Simplifying, we 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 units in the longitudinal direction tends to a constant value, while the cosine value of the incident angle will become smaller, which will cause the 3dB beamwidth of the reflected beam to gradually become wider. This means that while ensuring system performance, the reflected beam can be adjusted by rotating the RIS, providing adjustable signal coverage while ensuring the user's communication performance.

[0129] Therefore, during the rotation of the reconfigurable smart surface, 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 general, the present invention provides a method for optimizing the user received power in a highly directional millimeter wave communication system, which is used to characterize the rotation optimization scheme of a reconfigurable smart surface-assisted highly directional millimeter wave communication system. When the base station transmits a highly directional beam, the reconfigurable smart surface is rotated to the optimal downtilt angle to maximize the number of effective reflection units, and all rotations within the critical downtilt angle range are effective, which can improve the received power of the user end. In addition, the 3dB beam width of the reflected signal passing through the reconfigurable smart surface is adjustable, so that the user communication performance can be guaranteed while providing adjustable signal coverage.

[0131] Embodiment 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, and the critical value is:

[0134]

[0135] In the formula, 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 3 dB beam width 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 implementation, 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] In the formula, θ′ opt is the optimal downtilt angle.

[0139] As a preferred implementation, the surface rotation controller is further used to: determine the spatial range of user movement when the received power remains unchanged according to the angle deviation of the reflected beam at the current downtilt angle, wherein the angle 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 working 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, and is taken as the angle between the reflection direction of the highly directional reflection beam and the normal of the reconfigurable smart surface currently placed after rotation.

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

[0143] The relevant technical solution is the same as that in Embodiment 1 and will not be described in detail here.

[0144] It will be easily understood by those skilled in the art that the above description is only 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 protection scope 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 the lateral horizontal center axis of the reconfigurable smart surface until the downtilt angle thereof 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: In the formula, 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 3 dB beam width 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, characterized in that: Rotate the reconfigurable smart surface to its downward tilt angle: In the formula, θ′ opt is the optimal downtilt angle.

3. The optimization method according to claim 1, characterized in that: The positional relationship among the reconfigurable intelligent 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 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 projected onto the reconfigurable smart surface and the horizontal reference direction of the reconfigurable smart surface. The azimuth is The user's azimuth 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 is 4. The optimization method according to claim 1, characterized in that: Also includes: According to the angle deviation of the reflected beam at the current downtilt angle, the spatial range of user movement is determined when the received power remains unchanged, where 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 working 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, and is taken as the angle between the reflection direction of the highly directional reflection beam and the normal of the reconfigurable smart surface currently placed 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, and the critical value is: In the formula, 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 3 dB beam width 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. A 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 downward tilt angle is: In the formula, θ′ opt is the optimal downtilt angle.

7. A highly directional beam communication device according to claim 5, characterized in that: The surface rotation controller is further used to determine the spatial range of user movement when the received power remains unchanged according to the angle deviation of the reflected beam at the current downtilt angle, 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 working 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, and is taken as the angle between the reflection direction of the highly directional reflection beam and the normal of the reconfigurable smart surface currently placed after rotation.

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