A method for beam switching and time slot allocation of high-altitude communication platform high-speed rail communication

Abstract

This invention provides a beam switching and time slot allocation method for high-speed rail communication using a high-altitude communication platform (HAP). A point-to-point communication model is established between the HAP and the high-speed train, including the HAP's antenna model, movement model, and received signal model. Since the HAP acts as an aerial base station, its beam may shift due to its unknown random movement, leading to link quality degradation or even failure. This invention addresses the optimization problem of maximizing data volume over a train's travel distance by optimizing time slot allocation and beam switching. To solve this problem, this invention proposes a heuristic algorithm, designs a beam scanning triggering strategy, and implements a beam scanning algorithm that saves pilot overhead, thereby maximizing data volume.

Description

Technical Field

[0001] This invention relates to the field of high-altitude communication technology, and in particular to a beam switching and time slot allocation method for HAP (High Altitude Platform) high-speed rail communication. Background Technology

[0002] The current railway communication system based on 3GPP (3rd Generation Partnership Project) LTE (Long Term Evolution)-R has many shortcomings. These shortcomings will seriously affect the security, reliability, and stability of railway communication services. First, the coverage and maintenance of LTE-R networks are complex and difficult. The main reason is that LTE-R networks are severely affected by terrain. For example, mountainous areas suffer from severe communication signal congestion, and deployed antennas are easily affected by weather, leading to insufficient coverage. However, LTE-R carries a large number of important services, such as emergency communications, train control, and dispatching. Therefore, maintenance personnel need to obtain network data in a timely manner, locate potential network problems on-site, and handle them. Second, LTE-R has limited bandwidth. With the increase in the number of train communication devices, LTE-R struggles to support communication between a large number of devices and cannot provide effective and stable access for each device, which will lead to the shutdown of some devices. Finally, the coverage of LTE-R wireless network cells is relatively small. With high-speed train operation, communication links between vehicles and between vehicles and the ground require frequent inter-cell handovers, making it difficult to ensure the security of communication links. Although there are mature systems in the field of high-speed railway communication, they still cannot meet the ever-evolving communication demands. Due to network capacity and coverage limitations, terrestrial communication systems alone cannot provide high data rates and reliable wireless access services globally, especially in harsh environments such as oceans and mountains. Therefore, new network architectures must be used in various situations to adapt to different quality-of-service requirements for different services and applications. High Altitude Platforms (HAPs) are aerial systems or platforms operating at altitudes above 20,000 meters, capable of maintaining high altitudes for extended periods. These platforms can remain at high altitudes for days, weeks, or even months, providing various services such as communication, surveillance, reconnaissance, and environmental monitoring. Compared to terrestrial base stations, HAPs can provide a much wider range of communication coverage. The development of HAPs fully leverages the advantages of both terrestrial and satellite systems; therefore, developing high-speed rail air-to-ground networks is an inevitable trend for achieving seamless global coverage and advancing communication networks.

[0003] High-speed rail millimeter-wave communication has attracted widespread attention in academia. Existing technologies include a non-uniform beamforming mechanism that divides the coverage area corresponding to different beams to minimize performance differences between beams. Another approach proposes an adaptive non-uniform hybrid beamforming algorithm aimed at minimizing rate differences caused by location. Although high-speed rail communication has many characteristics that can lead to performance fluctuations, the predictability of its movement trajectory can be leveraged to improve communication quality. A further approach utilizes the regularity of train trajectories to propose a fast beam alignment mechanism for rapid beam switching. Additionally, a solution analyzes the problem of simultaneously providing high-data-rate communication from a high-speed rail access point (HAP) to multiple high-speed trains, studying the estimation of train numbers and arrival directions, as well as algorithms for train tracking, and providing estimation methods for trains in different scenarios such as tunnels and stations. This study investigates the cooperative beamforming problem for simultaneous downlink transmission from multiple high-speed trains to multiple high-speed access points (HAPs). The HAPs are equipped with multiple antennas, while the trains are equipped with single antennas. The channels vary over time, follow a Ricean distribution, and are known. The system performance of the HAPs using three different beamformers—MRT (Maximum Transmission Ratio), ZFBF (Zero Off-Band Forming), and SLNR-MAX (Maximum Signal-to-Interference Ratio)—is analyzed, and the overall achievable throughput is given.

[0004] The disadvantages of the beam switching and time slot allocation methods in the existing HAP high-speed rail communication technologies mentioned above include:

[0005] (1) Much of the existing research on high-speed rail communication focuses on vehicle-to-ground communication, i.e., data transmission between the train and the base station. However, in the vision of integrated air-ground-vehicle communication, communication between airborne base stations and trains is an essential component. Airborne base stations will provide coverage filling outside the ground network coverage area. Fully utilizing the capacity and coverage advantages of HAP, trains can be provided with higher capacity than satellites and coverage of a wider area than the ground. Therefore, HAP-high-speed rail communication still requires further research.

[0006] (2) Many issues related to beam switching focus on the algorithm design, with different estimation algorithms designed for beam tracking to improve accuracy. However, from a time perspective of the communication process, beam switching is often also related to pilot overhead, i.e., time slot allocation. Time slot allocation involves a trade-off between beam scanning accuracy and transmission rate. The joint time slot allocation and beam tracking process are particularly important for system performance. Therefore, time slot allocation and beam switching in high-speed rail communication still require further research.

[0007] (3) In vehicle-to-ground communication, the base station is fixed on the ground and does not move, thus not affecting the channel. However, since the HAP is located in the stratosphere, it may be affected by factors such as airflow, resulting in disturbances or random movement, which may change the channel at the transmitting and receiving ends. In fact, the train may even fall out of the beam coverage area due to the movement of the HAP. In the case of narrow beams, the disturbance of the HAP may have a significant impact on the system performance, and research on the random movement of the HAP is still limited. Summary of the Invention

[0008] This invention provides a beam switching and time slot allocation method for HAP high-speed rail communication, so as to effectively improve the efficiency of HAP high-speed rail communication.

[0009] To achieve the above objectives, the present invention adopts the following technical solution.

[0010] A method for beam switching and time slot allocation in high-speed rail communication using an aerial communication platform includes:

[0011] Establish a point-to-point communication model between the high-altitude communication platform (HAP) and the high-speed train, including the HAP's antenna model, movement model, and received signal model;

[0012] Based on the point-to-point communication model between HAP and high-speed trains, and the antenna model, movement model, and received signal model of HAP, an optimization problem for maximizing data volume is established.

[0013] By solving the optimization problem of maximizing data volume using an algorithm, the optimal beam direction and time slot allocation results for high-speed rail communication on the high-altitude communication platform are obtained.

[0014] Preferably, the establishment of a point-to-point communication model between the High Altitude Communication Platform (HAP) and the high-speed train includes:

[0015] The HAP system uses directional beamforming and moves randomly. The train's speed v and position are known. The train feeds back the signal and interference-plus-noise ratio (SNR) information to the HAP system via a low-frequency feedback link. The HAP system uses time-division multiplexing, where time is divided into non-overlapping time frames with a frame length of 1ms. Each frame includes 5 subframes, and each subframe has 8 time slots. The train's path loss and received SNR change at set intervals. The number of time slots that remain stationary varies depending on the train's speed.

[0016] Preferably, the establishment of the antenna model, mobility model, and received signal model for the HAP includes:

[0017] Setting up the HAP's antenna model includes: The HAP's directional antenna pattern is as follows:

[0018]

[0019] Where, θ -3db For half-power beamwidth, θ2=3.745θ -3db X = L N +Alog 10 (θ2), L N = -20dB, L F = -30dB, A = 20,

[0020] The antenna gain of the HAP is:

[0021] G = max(G a +G e ,L F )+G p

[0022] G a and G e These represent the antenna gain in the azimuth and elevation directions, respectively. θ a,-3dB and θ e,-3dB These are the half-power beamwidths in the azimuth plane and the half-power beamwidths in the elevation plane, respectively.

[0023] The HAP's movement model is set as follows: the HAP's movement occurs in the xoy plane, and in time slot t, the HAP's position coordinates are q. t =(q x,t ,q y,t ,q z,t The coordinates of HAP in time slot t+1 are:

[0024] q t+1 =q t +v t

[0025] Where v t =(v x,t ,v y,t ,v z,t ), v x,t v y,t and v z,t Let p represent the changes of HAP along the x, y, and z axes, respectively. A Bernoulli distribution is used to describe whether HAP has shifted. HAP has a probability p s Remain stationary with probability 1-p s If a shift occurs, and the shift follows a truncated Gaussian distribution, then:

[0026]

[0027] Where, N T(0,σ 2 (a, b) represents a value with a mean of 0 and a variance of σ. 2 The truncated Gaussian distribution, v x The value of σ must fall within the interval (a, b). 2 =200, (a,b)=(-500,500);

[0028] The HAP's signal reception model is as follows: Assuming a mobile relay (MR) is installed on the exterior of the train's roof, communication between the HAP and the train is equivalent to communication between the HAP and the MR. The position of the MR represents the position of the train. In time slot t, the position coordinates of the MR are m. t =(m x,t ,m y,t ,m z,t The beam direction transmitted by HAP in time slot t is θ. t =(θ a,t ,θ e,t The coordinates of the location of maximum gain on the ground are:

[0029] b t =(b x,t ,b y,t ,b z,t )

[0030] Among them, b x,t =q x,t +Htan(θ e,t cos(θ) a,t ), b y,t =q y,t +Htan(θ e,t sin(θ) a,t ), b z,t =H=20km, the distance from HAP to MR is:

[0031]

[0032] The path loss is calculated using the free space path loss formula:

[0033]

[0034] Where, c = 3 × 10 8 m / s, f = 20 GHz;

[0035] The received SNR of MR in time slot t is:

[0036]

[0037] Among them, P T =35dBm, G T (θ a,t,θ e,t G represents the transmit antenna gain. R =0dBm, N0=-173dBm / Hz, W=200MHz;

[0038] binary variable e t ∈{0,1} represents the allocation of time slot t, e t =0 indicates that time slot t is used for pilot transmission, e t =1 indicates that time slot t is used for data transmission, and the transmission rate of time slot t is:

[0039] R(t) = e t Wlog2(1+γ(t))

[0040] The total amount of data transmitted is:

[0041]

[0042] Where T represents the total number of time slots within a given train travel distance.

[0043] Preferably, the step of establishing a maximum data volume optimization problem based on the point-to-point communication model between the HAP and the high-speed train, and the HAP's antenna model, mobility model, and received signal model, includes:

[0044] Based on the point-to-point communication model between HAP and high-speed train, and the antenna model, movement model, and received signal model of HAP, the following formula (8) is used to establish the optimization problem of maximizing data volume:

[0045]

[0046] In the objective function, D represents the total amount of data transmitted by the train over a given travel distance, and e is a binary variable. t This indicates the allocation method of time slot t, used for pilot transmission or data transmission, θ a,t θ represents the antenna azimuth angle of the HAP at time slot t, with a value between 0° and 360°. a,t This represents the antenna elevation angle of the HAP at time slot t, with a value between 0° and 360°. By optimizing the allocation method of each time slot, the antenna elevation angle and azimuth angle are maximized.

[0047] Constraint 1 indicates that each time slot is used either for pilot transmission or user data transmission; Constraint 2 indicates the azimuth range of the beam direction; and Constraint 3 indicates the elevation range of the beam direction.

[0048] Let e=[e1,...,e T ], θ a =[θ a,1 ,...,θa,T ], θ e =[θ e,1 ,...,θ e,T ], where T is the total number of time slots within a given distance.

[0049] Preferably, the step of solving the optimization problem of maximizing data volume through an algorithm to obtain the optimal beam direction and time slot allocation results for high-speed rail communication on the high-altitude communication platform includes:

[0050] Let MR be represented as point M, and the beam direction of MR be represented as (θ). a ,θ e The beamwidths of MR in the azimuth and elevation planes are (BW) a BW e Set the current beam direction θ of the HAP system. t =(θ a,t ,θ e,t ), beam resolution parameter Δθ a ,Δθ e In three-dimensional space, the direction is determined by both the azimuth and elevation angles, denoted by subscripts a and e, respectively. The output data is the optimized beam direction and time slot allocation results.

[0051] In the first time slot, at t=1, the HAP beam is already aligned with the train, and the initial access process is complete. Starting from the second time slot at t=2, the HAP obtains the real-time received SNR value of the train and compares the SNR γ(t) of the current time slot with the average SNR of the previous t-1 time slots. If the SNR of the current time slot drops by more than 0.1 dB compared to the average SNR of the previous t-1 time slots, beam scanning is triggered, i.e., e t =1, allocate the current time slot to pilot transmission; if the SNR of the current time slot decreases by less than 0.1dB compared to the average SNR of the previous t-1 times, then e t =0, data is transmitted in the current time slot, and the beam direction is the same as the beam direction of the previous time slot;

[0052] When e t When = 1, let the angle of the current beam be θ. t =(θ a,t ,θ e,t The beam scanning directions are as follows: 9 directions:

[0053] {(θ a,t -Δθ a ,θ e,t +Δθ e ),(θ a,t ,θ e,t +Δθ e ),(θ a,t +Δθa ,θ e,t +Δθ e ),

[0054] (θ a,t -Δθ a ,θ e,t ),(θ a,t ,θ e,t ),(θ a,t +Δθ a ,θ e,t ),

[0055] (θ a,t -Δθ a ,θ e,t -Δθ e ),(θ a,t ,θ e,t -Δθ e ),(θ a,t +Δθ a ,θ e,t -Δθ e ),}

[0056] Where, Δθ a and Δθ e These represent the beam resolutions in the azimuth and elevation planes, respectively. After sending pilot signals in the above nine directions, the MR selects the beam with the strongest signal quality by comparison and feeds back the direction of the beam with the strongest signal quality to the HAP. The HAP then adjusts the beam direction to the direction of the beam with the strongest signal quality.

[0057] Preferably, the MR selects the beam with the strongest signal quality through comparison, and the HAP adjusts the beam direction to the direction of the beam with the strongest signal quality, including:

[0058] HAP position is q t =(q x,t ,q y,t ,q z,t Assuming the beam direction is (θ) a,t ,θ e,t The coordinates of the location of maximum gain on the ground are:

[0059] b t =(b x,t ,b y,t ,b z,t )

[0060] Among them, b x,t =q x,t +Htan(θ e,t cos(θ) a,t ), b y,t =q y,t+Htan(θ e,t sin(θ) a,t ), b z,t =H=20km;

[0061] The train's position is m t =(m x,t ,m y,t ,m z,t The antenna gain at the train's location is calculated using the antenna pattern. Assuming the train is at point M, the HAP is at point O, and b is the line-of-sight point, first calculate the azimuth deviation angle Δθ between points M and b. a =∠aob and the angle of elevation deviation Δθ e =∠cob, then substitute the two deviation angles into the antenna formula to obtain the antenna gain at point M, which is G = max(G a +G e ,L F )+G p G a =G pattern (Δθ a ), G e =G pattern (Δθ e );

[0062] Calculate reception Different beam directions have different SNRs. Among these 9 beam directions, the beam direction with the largest SNR is selected as the beam direction after HAP adjustment.

[0063] As can be seen from the technical solutions provided by the embodiments of the present invention described above, the present invention considers the HAP as an airborne base station for downlink communication with the train. The HAP uses a directional antenna to transmit a narrow beam to increase antenna gain, thereby combating severe path loss. However, due to the random movement of the HAP and the train's movement, the train may fall outside the beam range, requiring beam switching. To address this problem, the present invention designs a time slot allocation algorithm, provides the triggering conditions for beam switching, and designs a low-complexity beam training algorithm to maximize the total amount of data transmitted by the train over a certain distance, while reducing pilot overhead.

[0064] Additional aspects and advantages of the invention will be set forth in part in the description which follows, and will become apparent from the description or may be learned by practice of the invention. Attached Figure Description

[0065] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0066] Figure 1 A flowchart illustrating a beam switching and time slot allocation method for high-speed rail communication on an aerial communication platform, provided as an embodiment of the present invention.

[0067] Figure 2 This is a schematic diagram of a point-to-point communication model between a HAP and a high-speed train, provided as an embodiment of the present invention.

[0068] Figure 3 This is a schematic diagram illustrating how to calculate the transmit antenna gain based on the location of the HAP, the beam direction, and the location of the MR, as a real-time example of the present invention. Detailed Implementation

[0069] Embodiments of the present invention are described in detail below, examples of which are shown in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention.

[0070] Those skilled in the art will understand that, unless specifically stated otherwise, the singular forms “a,” “an,” “the,” and “the” used herein may also include the plural forms. It should be further understood that the term “comprising” as used in this specification means the presence of the stated features, integers, steps, operations, elements, and / or components, but does not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof. It should be understood that when we say an element is “connected” or “coupled” to another element, it can be directly connected or coupled to the other element, or there may be intermediate elements. Furthermore, “connected” or “coupled” as used herein can include wireless connections or couplings. The term “and / or” as used herein includes any and all combinations of one or more of the associated listed items.

[0071] It will be understood by those skilled in the art that, unless otherwise defined, all terms used herein (including technical and scientific terms) have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. It should also be understood that terms such as those defined in general dictionaries should be understood to have the same meaning as in the context of the prior art, and should not be interpreted in an idealized or overly formal sense unless defined as herein.

[0072] To facilitate understanding of the embodiments of the present invention, the following will provide further explanation and description with reference to the accompanying drawings and several specific embodiments. These embodiments do not constitute a limitation on the embodiments of the present invention.

[0073] This invention decomposes the original mixed-integer programming problem of HAP high-speed rail communication into two sub-problems: time slot allocation and beam scanning algorithm. First, the train determines whether to trigger beam scanning based on the received signal-to-noise ratio (SNR). If so, it transmits the information to the HAP via a feedback link. First, when beam scanning is required, time slots are allocated to pilot transmission; the number of time slots depends on the number of beam scanning directions. After beam scanning, the new beam direction is updated, and time slots are allocated to data transmission. Second, during beam scanning, the beam direction transmitted by the HAP is the direction surrounding the beam direction before beam scanning. The train determines the optimal beam based on the signal strength from different directions and feeds this information back to the HAP, which then updates the beam direction.

[0074] The processing flow of a beam switching and time slot allocation method for HAP high-speed rail communication provided in this embodiment of the invention is as follows: Figure 1 As shown, the processing steps include the following:

[0075] Step S10: Establish a point-to-point communication model between HAP and high-speed train, including HAP's antenna model, mobile model, and received signal model.

[0076] A schematic diagram of a point-to-point communication model between a HAP and a high-speed train provided in this embodiment of the invention is shown below. Figure 2As shown. The HAP uses directional beamforming to ensure communication quality. The HAP has a high probability of remaining stationary and a low probability of moving. The movement of the HAP is random and unknown to both the HAP and the train; only the statistical characteristics of the movement are known, not the specific movement process. Due to the movement of the HAP, the coverage area of ​​a given beam will change accordingly. Considering the train's movement, the train is very likely to fall outside the communication range of the beam. The train's speed v and position are known, v = 100–500 km / h. The train feeds back information such as SNR (Signal to Interference plus Noise Ratio) to the HAP via a low-frequency feedback link. The HAP system uses Time-Division Multiplexing (TDMA) mode, where time is divided into non-overlapping time frames, each 1 ms long, with 5 subframes and 8 time slots per subframe. That is, each frame has 40 time slots. Because the distance between the HAP and the train is relatively long, and the HAP has a large coverage area, it is necessary to consider the differences in path loss between the HAP and the train at different locations. Considering the relatively high speed of the train, this invention assumes that the train's position remains constant within 5 meters; therefore, the path loss and received SNR change every 5 meters. The number of time slots remaining stationary also varies depending on the train's speed.

[0077] Setting up the HAP's antenna model includes: The HAP's directional antenna pattern is as follows:

[0078]

[0079] Where, θ -3db For half-power beamwidth, θ2=3.745θ -3db X = L N +Alog 10 (θ2), L N = -20dB, L F = -30dB, A = 20.

[0080] The antenna gain of the HAP is:

[0081] G = max(G a +G e ,L F )+G p

[0082] G a and G e The antenna gain in the azimuth and elevation directions, respectively, G a and G e Use the above formula to calculate. θ a,-3dB and θ e,-3dB These are the half-power beamwidths in the azimuth plane and the half-power beamwidths in the elevation plane, respectively.

[0083] The HAP mobility model is as follows:

[0084] The movement of the HAP is described by a Gaussian-Bernoulli distribution. The HAP's movement occurs only in the xoy plane, and its height remains unchanged. In time slot t, the HAP's position coordinates are q. t =(q x,t ,q y,t ,q z,t The coordinates of HAP in time slot t+1 are:

[0085] q t+1 =q t +v t

[0086] Where v t =(v x,t ,v y,t ,v z,t ), v x,t v y,t and v z,t These represent the variations of HAP along the x, y, and z axes, respectively. These three variations are independent across different time slots; for simplicity, the subscript 't' will be omitted in the following description. Since the statistical characteristics of the variations of HAP along the x, y, and z axes are consistent, only the x-axis will be described below, i.e., describing v... x The distribution of HAP. First, the Bernoulli distribution is used to describe whether HAP moves, with HAP moving with probability p. s Remain stationary with probability 1-p s A shift occurs. If a shift occurs, the shift follows a truncated Gaussian distribution. That is:

[0087]

[0088] Where, N T (0,σ 2 (a, b) represents a value with a mean of 0 and a variance of σ. 2 The truncated Gaussian distribution, v x The value of σ must fall within the interval (a, b). 2 =200, (a,b)=(-500,500). Since this invention only considers the movement of HAP in the xoy plane, therefore v z =0.

[0089] The HAP's signal reception model is as follows: It is assumed that only one Mobile Relay (MR) is located outside the train's roof. Communication between the HAP and the train is equivalent to communication between the HAP and the MR; the position of the MR represents the position of the train. In time slot t, the position coordinates of the MR are m. t =(m x,t ,m y,t ,m z,t The beam direction of the HAP transmission in time slot t is θ. t =(θ a,t ,θ e,t Then the coordinates of the location of maximum gain on the ground are:

[0090] b t =(b x,t ,b y,t ,b z,t )

[0091] Among them, b x,t =q x,t +Htan(θ e,t cos(θ) a,t ), b y,t =q y,t +Htan(θ e,t sin(θ) a,t ), b z,t =H=20km. At this time, the distance from HAP to MR is:

[0092]

[0093] The path loss is calculated using the free space path loss formula:

[0094]

[0095] Where, c = 3 × 10 8 m / s, f = 20 GHz.

[0096] The received SNR of MR in time slot t is:

[0097]

[0098] Among them, P T =35dBm, G T (θ a,t ,θ e,t G represents the transmit antenna gain, calculated from the antenna model. R =0dBm. N0=-173dBm / Hz, W=200MHz.

[0099] binary variable e t∈{0,1} represents the allocation of time slot t. t =0 indicates that time slot t is used for pilot transmission, i.e., beam training, e t =1 indicates that time slot t is used for data transmission. Therefore, the transmission rate of time slot t is...

[0100] R(t) = e t Wlog2(1+γ(t))

[0101] The total amount of data transmitted is:

[0102]

[0103] Where T represents the total number of time slots within a given train travel distance. The travel distance is set to 30km.

[0104] Step S20: Based on the point-to-point communication model between HAP and high-speed train, and the antenna model, movement model, and received signal model of HAP, establish an optimization problem to maximize the amount of data.

[0105] P1:

[0106] st

[0107]

[0108]

[0109] In the objective function, D represents the total amount of data transmitted by the train over a given travel distance, and e is a binary variable. t This indicates the allocation method of time slot t, used for pilot transmission or data transmission, θ a,t θ represents the antenna azimuth angle of the HAP at time slot t, with a value between 0° and 360°. a,t The antenna elevation angle of the HAP at time slot t is represented, ranging from 0° to 360°. The antenna elevation and azimuth angles are optimized by allocating each time slot to maximize D. Constraint 1 indicates that each time slot is used either for pilot transmission or user data transmission; constraint 2 indicates the azimuth angle range in the beam direction; and constraint 3 indicates the elevation angle range in the beam direction.

[0110] For convenience, let e = [e1,...,e T ], θ a =[θ a,1 ,...,θ a,T ], θ e =[θ e,1 ,...,θ e,T ], where T is the total number of time slots within a given distance.

[0111] Step S30: Solve the above optimization problem P1 of maximizing data volume using an algorithm to obtain the optimal beam direction and time slot allocation results for high-speed rail communication on the high-altitude communication platform.

[0112] Figure 3 This is a schematic diagram illustrating how to calculate the transmit antenna gain based on the location of the HAP, the beam direction, and the location of the MR, as a real-time example of the present invention. It is assumed that the HAP is currently located at the origin of the coordinate system shown in the diagram, the MR is represented by point M, and the beam direction is (θ). a ,θ e The beamwidths in the azimuth and elevation planes are (BW) respectively. a BW e The point where the beam has maximum gain on the ground is point b. The azimuth angle of point M relative to the aiming direction is ∠aob, and the elevation angle relative to the aiming direction is ∠cob. The value of the offset angle is then calculated to obtain the antenna gain.

[0113] The objective function of this invention is to maximize the amount of data, but there is a contradiction: time slot allocation. If time slots are allocated to pilot signals, a better beam can be searched, but at the cost of data transmission, since pilot transmission does not transmit data, so the amount of data is zero. If time slots are allocated to data transmission (i.e., without searching for the direction where the antenna gain is maximum, i.e., not allocating to pilot signals), more data can be obtained, but at the cost of the antenna gain not being ideal, and therefore the amount of data may not be maximized.

[0114] This invention assumes that a transmission link has been established between the HAP and the train in the initial stage. However, since the directional beam is emitted from the HAP, if the beam direction remains unchanged but the position of the HAP changes, the coverage area of ​​the beam on the ground will also change, and the antenna gain obtained at a given location on the ground will also differ due to the movement of the HAP. The HAP is unaware of its own random movement, thus leading to the optimization problem of this invention.

[0115] The HAP is unaware of its own location, but it can obtain the train's real-time received signal-to-noise ratio (SNR). This is achieved by establishing a low-frequency feedback link between the train and the HAP, which does not interfere with the transmission link. The HAP dynamically adjusts the direction of its transmit beam based on the train's received SNR, while simultaneously allocating time slots.

[0116] The input data for the time slot allocation and beam switching algorithms in this invention is: the current beam direction θ of the HAP system. t =(θ a,t ,θ e,t ), beam resolution parameter Δθ a ,Δθe In three-dimensional space, the direction is determined by both the azimuth and elevation angles, denoted by subscripts a and e, respectively. The output data is the optimized beam direction and time slot allocation results.

[0117] In the first time slot, at t=1, the HAP beam is already aligned with the train, and the initial access process is complete. Within each time slot, the HAP obtains the real-time received SNR value of the train. Starting from t=2, as the HAP moves, the beam may deviate, leading to a decrease in the received SNR. Since the transmit power is fixed, the received SNR depends on the transmit antenna gain and path loss. Therefore, SNR can be considered a measure of antenna gain. The HAP needs to determine whether each time slot is allocated to pilot or data based on the proposed algorithm, and simultaneously determine the beam direction within that time slot. The time slot allocation is determined according to the following criteria: the SNR γ(t) of the current time slot is compared with the average SNR of the previous t-1 time slots. If the SNR of the current time slot decreases by more than 0.1 dB compared to the average SNR of the previous t-1 time slots, beam scanning is triggered, i.e., e t =1, allocate the current time slot to pilot transmission. If the SNR of the current time slot decreases by less than 0.1dB compared to the average SNR of the previous t-1 time slots, then e t =0, data transmission continues in the current time slot. If data transmission occurs in a time slot, the beam direction remains the same as the previous time slot, and no change in beam direction is required. Changing the beam direction requires the pilot transmission to be performed.

[0118] If e t =1 indicates a large beam direction deviation, resulting in a significant drop in SNR. A rescan is needed to update the beam direction, with one direction scanned per time slot. While scanning the entire space would yield the beam direction with the highest SNR, it would require many time slots, making it potentially less optimal from the perspective of maximizing total data volume. Considering the train's continuous trajectory, the optimal beam is likely located around the current beam direction. Therefore, the current beam angle is θ. t =(θ a,t ,θ e,t The beam scanning directions designed in this invention are as follows: 9 directions:

[0119] {(θ a,t -Δθ a ,θ e,t +Δθ e ),(θ a,t ,θ e,t +Δθ e ),(θ a,t +Δθ a ,θ e,t +Δθe ),

[0120] (θ a,t -Δθ a ,θ e,t ),(θ a,t ,θ e,t ),(θ a,t +Δθ a ,θ e,t ),

[0121] (θ a,t -Δθ a ,θ e,t -Δθ e ),(θ a,t ,θ e,t -Δθ e ),(θ a,t +Δθ a ,θ e,t -Δθ e ),}

[0122] Where, Δθ a and Δθ e These represent the beam resolution in the azimuth and elevation planes, respectively. After sending pilot signals in the above nine directions, the MR selects the beam with the strongest signal quality by comparison and feeds this information back to the HAP. Then, the HAP adjusts the beam direction.

[0123] A total of nine time slots are required for beam scanning. For the train, each beam direction corresponds to a different antenna gain, which is reflected in the train's received SNR. The HAP position is q. t =(q x,t ,q y,t ,q z,t Assuming the beam direction is (θ) a,t ,θ e,t The coordinates of the maximum gain position (line-of-sight point) on the ground are:

[0124] b t =(b x,t ,b y,t ,b z,t )

[0125] Among them, b x,t =q x,t +Htan(θ e,t cos(θ) a,t ), b y,t =q y,t +Htan(θ e,t sin(θ) a,t ), b z,t =H=20km.

[0126] The train's position is m t =(m x,t ,m y,t ,m z,t The antenna gain at the train's location is calculated using the antenna pattern. Figure 3 For example, assuming the train is at point M, the HAP is at point O, and b is the line-of-sight point, first calculate the azimuth deviation angle Δθ between points M and b. a =∠aob and the angle of elevation deviation Δθ e =∠cob. Then, substitute the two deviation angles into the antenna formula to obtain the antenna gain at point M, which is G = max(G a +G e ,L F )+G p G a =G pattern (Δθ a ), G e =G pattern (Δθ e ).

[0127] Then calculate the received data. The calculation of PL has been given above. Different beam directions have different SNRs. Among these 9 beam directions, the direction with the largest SNR is selected as the adjusted beam direction. The scanning process for the 9 beam directions takes a total of 9 time slots, so the beam direction will be adjusted at the (t+10)th time slot. The complete allocation and scanning process is then finished.

[0128] The algorithm is as follows:

[0129]

[0130]

[0131] In summary, the embodiments of this invention aim to improve the problem of frequent beam switching caused by rapid movement in high-speed rail communication. To reduce the number of beam switching, this invention utilizes a high-altitude communication platform (HAP) as an airborne base station to provide services to the high-speed rail, proposing a point-to-point system model of HAP-high-speed rail. To combat path loss, the HAP uses a directional antenna to provide services to the high-speed rail. Considering the movement of the HAP, the beam may become misaligned, leading to a decrease in communication quality or even communication interruption. In this case, the HAP needs to perform beam scanning to update the beam direction. Since the movement status of the HAP cannot be accurately obtained, this invention proposes a beam scanning scheme that does not require channel state, taking advantage of the continuity of the movement trajectory. A time slot allocation algorithm is also proposed. By allocating time slots for transmission and switching beam directions, the system's data transmission capacity is increased while reducing the system's pilot overhead.

[0132] Those skilled in the art will understand that the accompanying drawings are merely schematic diagrams of one embodiment, and the modules or processes shown in the drawings are not necessarily essential for implementing the present invention.

[0133] As can be seen from the above description of the embodiments, those skilled in the art can clearly understand that the present invention can be implemented by means of software plus necessary general-purpose hardware platforms. Based on this understanding, the technical solution of the present invention, or the part that contributes to the prior art, can be embodied in the form of a software product. This computer software product can be stored in a storage medium, such as ROM / RAM, magnetic disk, optical disk, etc., and includes several instructions to cause a computer device (which may be a personal computer, server, or network device, etc.) to execute the methods described in various embodiments or some parts of the embodiments of the present invention.

[0134] The various embodiments in this specification are described in a progressive manner. Similar or identical parts between embodiments can be referred to mutually. Each embodiment focuses on describing the differences from other embodiments. In particular, for apparatus or system embodiments, since they are basically similar to method embodiments, the description is relatively simple; relevant parts can be referred to the descriptions in the method embodiments. The apparatus and system embodiments described above are merely illustrative. The units described as separate components may or may not be physically separate. The components shown as units may or may not be physical units; that is, they may be located in one place or distributed across multiple network units. Some or all of the modules can be selected to achieve the purpose of this embodiment according to actual needs. Those skilled in the art can understand and implement this without creative effort.

[0135] The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for beam switching and time slot allocation for high-altitude platform-high-speed train communication, characterized in that, include: Establish a point-to-point communication model between the high-altitude communication platform (HAP) and the high-speed train, including the HAP's antenna model, movement model, and received signal model; Based on the point-to-point communication model between HAP and high-speed trains, and the antenna model, movement model, and received signal model of HAP, an optimization problem for maximizing data volume is established. The optimal beam direction and time slot allocation for high-speed rail communication on the high-altitude communication platform are obtained by solving the optimization problem of maximizing data volume through an algorithm. The establishment of the antenna model, mobility model, and received signal model for HAP includes: Setting up the HAP's antenna model includes: The HAP's directional antenna pattern is as follows: wherein is the half-power beamwidth, , , , , , , , The antenna gain of the HAP is: and G(0) and G(90) are the antenna gains in the azimuth and elevation directions, respectively, , and are the half-power beamwidths in the azimuth and elevation planes, respectively. The movement model of the HAP is set as: the movement of the HAP occurs in a plane, in the first time slot, the position coordinates of the HAP are , and the coordinates of the HAP in the time slot are: where , , and respectively represent the amount of change in the axis, axis and axis, the HAP is described by a Bernoulli distribution whether the HAP moves or not, the HAP remains stationary with probability and moves with probability , if it moves, the movement is subject to a truncated Gaussian distribution, i.e.: in, The mean is The variance is The truncated Gaussian distribution, The value must fall within the interval Inside, , ; The HAP's signal reception model is as follows: Assuming a mobile relay (MR) is installed on the exterior of the train's roof, communication between the HAP and the train is equivalent to communication between the HAP and the MR. The position of the MR represents the position of the train. Within the time slot, the position coordinates of MR are HAP in The beam direction transmitted within the time slot is The coordinates of the location of maximum gain on the ground are: in, , , The distance from HAP to MR is: The path loss is calculated using the free space path loss formula: in, , ; MR in The received SNR of the time slot is: in, , For transmit antenna gain, , , ; binary variable Indicates time slot The allocation, Indicates time slot Used for pilot transmission, Indicates time slot Used for data transmission, time slots The transmission rate is: The total amount of data transmitted is: in, The total number of time slots within a given train travel distance; The aforementioned algorithmic solution to the data volume maximization optimization problem, yielding the optimal beam direction and time slot allocation results for high-speed rail communication on the high-altitude communication platform, includes: Represent MR as a point The beam direction of MR is represented as The beamwidths of the MR in the azimuth and elevation planes are respectively Set the current beam direction of the HAP system. Beam resolution parameters In three-dimensional space, the direction is determined by the azimuth and elevation angles, denoted by subscripts a and e respectively. The output data is the optimized beam direction and time slot allocation results. In the first time slot, At that time, the HAP beam was already aligned with the train, and the initial access process was completed. Starting from the second time slot, the HAP obtains the real-time received SNR value of the train and sets the SNR of the current time slot. and before The average SNR of each time slot is compared. If the SNR of the current time slot is lower than that of the previous time slot, the comparison is made. If the average SNR drops by more than 0.1 dB, beam scanning is triggered. Allocate the current time slot to pilot transmission; if the SNR of the current time slot is lower than that of the previous time slot... If the average decrease in SNR is less than 0.1 dB, then The current time slot transmits data, and the beam direction is the same as the beam direction of the previous time slot. when At that time, let the current beam angle be... The beam scanning direction is in the following 9 directions: in, and These represent the beam resolutions in the azimuth and elevation planes, respectively. After sending pilot signals in the above nine directions, the MR selects the beam with the strongest signal quality by comparison and feeds back the direction of the beam with the strongest signal quality to the HAP. The HAP then adjusts the beam direction to the direction of the beam with the strongest signal quality.

2. The method according to claim 1, characterized in that, The establishment of a point-to-point communication model between the High Altitude Communication Platform (HAP) and high-speed trains includes: The HAP system uses directional beamforming, and the HAP moves randomly, affecting the train's speed. The location is known. The train feeds back the signal and the interference plus noise ratio (SNR) information to the HAP system through the low-frequency feedback link. The HAP system adopts a time-division multiplexing mode, and time is divided into non-overlapping time frames with a frame length of 1ms. Each frame includes 5 subframes, and each subframe has 8 time slots. The train's path loss and received SNR change at each set distance. The number of time slots that remain stationary also varies at different train speeds.

3. The method according to claim 1, characterized in that, The aforementioned optimization problem for maximizing data volume, based on the point-to-point communication model between HAP and high-speed trains, and the antenna model, mobility model, and received signal model of HAP, includes: Based on the point-to-point communication model between HAP and high-speed train, and the antenna model, movement model, and received signal model of HAP, the following formula (8) is used to establish the optimization problem of maximizing data volume: Among them, in the objective function The total amount of data transmitted by the train over a given travel distance is represented by a binary variable. Indicates time slot The allocation method is used for pilot transmission or data transmission. Indicates HAP in time slot The antenna azimuth angle at that time is between 0° and 360°. Indicates HAP in time slot The antenna elevation angle, ranging from 0° to 360°, is optimized by adjusting the allocation of each time slot, resulting in... maximize; Constraint 1 indicates that each time slot is used either for pilot transmission or user data transmission; Constraint 2 indicates the azimuth range of the beam direction; and Constraint 3 indicates the elevation range of the beam direction. make , , , This represents the total number of time slots within a given distance.

4. The method according to claim 1, characterized in that, The MR selects the beam with the strongest signal quality through comparison, and the HAP adjusts the beam direction to the direction of the beam with the strongest signal quality, including: HAP location is Assuming the beam direction is The coordinates of the location of maximum gain on the ground are: in, , , ; The train's location is The antenna gain at the train's location is calculated using the antenna pattern. Assuming the train is at point M, the HAP is at point O, and b is the line-of-sight point, first calculate the azimuth deviation angle between points M and b. and elevation angle deviation angle Then, substituting the two deviation angles into the antenna formula, we obtain the antenna gain at point M, which is... ,in , ; Calculate Received SNR Different beam directions have different SNRs. Among these 9 beam directions, the beam direction with the largest SNR is selected as the beam direction after HAP adjustment.

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