Beam adaptive high-speed rail free space optical communication system and coverage performance evaluation method

Abstract

The invention discloses a beam-adaptive high-speed rail free space optical communication system and a coverage performance evaluation method, and the system comprises an optical fiber backbone network, an optical fiber link, a transmitter, a high-voltage cable bearing rod, a receiver, a train internal communication module, a transmitter controller, a receiver controller, an infrared transmitter, an infrared receiver, a transmitter ATP module, and a receiver ATP module. The invention relates to the technical field of high-speed rail mobile communication. According to the beam-adaptive high-speed rail free space optical communication system and the coverage performance evaluation method, the transmitter controller, the infrared transmitter, the receiver controller and the infrared receiver are arranged; in the switching process of the receiver from the source transmitter to the target transmitter, the receiver controller can judge the width of a transmitting beam of the target transmitter according to the intensity condition of a signal transmitted by the target transmitter received by the receiver, so that the system can adaptively adjust the on-off state of the ATP module according to the width of the beam. The device has the advantages of being green, energy-saving, simple in structure and cost-saving.

Description

Technical Field

[0001] The present invention relates to the technical field of high-speed rail mobile communication, and specifically to a beam adaptive high-speed rail free space optical communication system and a coverage performance evaluation method. Background Technique

[0002] High-speed railway, abbreviated as high-speed rail, as a comfortable and fast means of transportation, with its excellent performance, has greatly improved people's travel efficiency, expanded people's living range, and promoted economic development. At the same time, with the rapid growth of the number of passengers, the high-speed rail mobile communication system needs to meet the diverse Internet access needs of passengers. Traditional radio frequency (RF) wireless communication technology, due to its relatively mature technology and ability to cover a large communication area, is currently widely used in high-speed rail mobile communication systems. In addition, technologies such as wireless fidelity Wi-Fi, worldwide interoperability for microwave access WiMAX, and leaky coaxial cables are also used to provide Internet access for high-speed railway mobile communication systems. However, due to reasons such as severe interference, limited bandwidth, and limited rate, these technologies cannot provide high data transmission rates, resulting in problems such as poor user experience and frequent network disconnections.

[0003] In recent years, free space optical (FSO) communication technology has provided a feasible potential solution for high-speed rail mobile communication. FSO communication is a wireless communication technology that uses free space to transmit optical signals and is also known as optical wireless communication. Specifically, it is a line-of-sight technology that uses modulated light to transmit data. Different from traditional optical fiber communication, FSO communication does not require optical fiber as the transmission medium, but transmits optical signals wirelessly through free space. An FSO communication system mainly includes three parts: a transmitter that sends optical signals, a free space transmission channel, and a receiver that obtains the transmitted signals. Among them, the transmitter usually uses light sources such as laser diodes LD or light-emitting diodes LED to emit optical signals, and the receiver receives and restores the signals through optoelectronic conversion devices. Compared with RF communication, FSO communication has advantages such as a larger spectral bandwidth, a faster transmission rate, a lower transmission delay, etc., and does not require a frequency license, is convenient and inexpensive, is less affected by external interference, and does not require the use of cables or optical fibers, saving resources and reducing energy consumption.

[0004] For high-speed rail FSO communication systems, receivers with acquisition-tracking-pointing (ATP) modules have always been used to continuously track and align the transmitted light beam of the transmitter in real time to maintain communication. However, these studies all assume that the transmitter emits a narrow light beam, ignoring the impact of the size of the transmitter's emitted beam on the receiver's reception performance. When the transmitter emits a narrow beam, the line-of-sight link of the FSO communication link is easily affected by train movement, track irregularities, thermal expansion, wind sway, and receiver vibration, resulting in obvious pointing or tracking errors. At this time, the receiver needs to use the ATP module to align in real time to maintain the stability of the communication link. However, when the transmitter emits a wide beam, a large light spot is generated at the receiver, and the size of the light spot is much larger than the light-collecting area of the receiver, that is, the light beam completely covers the receiver. At this time, the receiver can easily capture and receive the signal, thus eliminating the impact of pointing errors. Therefore, when the transmitter emits a wide beam, in order to further reduce energy consumption, the high-speed rail FSO communication system does not need to use the ATP module. Currently, there is a lack of an effective method to substantially determine the beam width. Moreover, for the influencing factors of the FSO channel state, most studies only focus on the impact of atmospheric absorption, scattering, or atmospheric turbulence, rarely involving the width of the emission beam divergence angle and whether it will cause pointing errors and geometric losses, etc. Ignoring the impact of the width of the emission beam divergence angle will inevitably lead to inaccurate analysis of the system's coverage performance.

[0005] Based on the retrieval of the above-mentioned materials, a beam-adaptive high-speed rail free space optical communication system and a coverage performance evaluation method are proposed. It is a high-speed rail FSO communication system that can adaptively determine whether to enable the ATP module according to the width of the transmitted beam of the target transmitter, and on this basis, analyze the coverage performance of the system, providing a theoretical reference for the design of the actual system. Summary of the Invention

[0006] Aiming at the deficiencies of the prior art, the present invention provides a beam-adaptive high-speed rail free space optical communication system and a coverage performance evaluation method, solving the problems proposed in the above background technology.

[0007] To achieve the above objectives, the present invention is realized through the following technical solutions: A beam-adaptive high-speed rail free space optical communication system includes an optical fiber backbone network, an optical fiber link, a transmitter, a high-voltage cable support pole, a receiver, an in-train communication module, a transmitter controller, a receiver controller, an infrared transmitter, an infrared receiver, a transmitter ATP module, and a receiver ATP module;

[0008] The optical fiber backbone network and the high-voltage cable support pole are laid along the railway track. The optical fiber link is arranged inside the high-voltage cable support pole and is connected to the optical fiber backbone network. The optical fiber link is used to connect the transmitter and enable it to exchange information with the service provider;

[0009] The transmitter is deployed on a high-voltage cable supporting pole and transmits the optical signal from the optical fiber link to the receiver through free space;

[0010] The receiver is arranged on the top of the train head, and is used for receiving the optical signal of the transmitter and sending it to the in-train communication module and the receiver controller by wire;

[0011] The receiver controller is used for adaptively determining whether to turn on the receiver ATP module and the infrared emitter according to the optical signal continuously received by the receiver;

[0012] The transmitter controller is respectively connected to the transmitter and the infrared receiver, and is used for turning on the transmitter ATP module after the infrared receiver receives the signal of the infrared emitter, and turning off the transmitter ATP module after the infrared receiver does not receive the signal of the infrared emitter;

[0013] When the transmitter ATP module and the receiver ATP module are turned on simultaneously, they are used to realize the mutual tracking of the light beams between the transmitter and the receiver.

[0014] The present invention is further arranged such that: the transmitter is arranged on the side of the track or directly above the track.

[0015] The present invention is further arranged such that: the transmitter includes a source transmitter, a target transmitter, and a silent transmitter;

[0016] The source transmitter is the transmitter currently communicating with the receiver;

[0017] The target transmitter is the transmitter about to communicate with the receiver;

[0018] The silent transmitter is the remaining transmitter that has not communicated with the receiver.

[0019] The present invention is further arranged such that: the way for the receiver controller to adaptively determine whether to turn on the receiver ATP module and the infrared emitter according to the intensity of the signal transmitted by the target transmitter received by the receiver during the switching process of the receiver from the source transmitter to the target transmitter includes:

[0020] Judge whether the optical signal transmitted by the target transmitter is a wide beam or a narrow beam. When the optical signal is a wide beam, the receiver controller turns off the receiver ATP module and the infrared emitter. When the optical signal is a narrow beam, the receiver controller turns on the receiver ATP module and the infrared emitter.

[0021] The present invention is further arranged such that: the way to judge whether the optical signal transmitted by the target transmitter is a wide beam or a narrow beam is:

[0022]

[0023] wherein, [T 1 , T 2 is the switching time range for the receiver to switch from the source transmitter to the target transmitter, P r is the electric power threshold of the receiver controller, and P rx (t) is the electric power of the signal received by the receiver from the target transmitter at time t.

[0024] The present invention also discloses a method for evaluating the coverage performance of a beam adaptive high-speed rail free space optical communication system, which specifically includes the following steps:

[0025] S1. The signal transmitted by the transmitter adopts on-off keying modulation, and the signal received by the receiver adopts direct detection. It is set that all the ATP modules of the transmitter and the receiver are in the off state. Let [T 1 , T 2 represent the switching time range for the receiver to switch from the source transmitter to the target transmitter. Given the electric power threshold P r of the receiver controller, within the time period of [T 1 , T 2 , if the electric power P rx of the signal received by the receiver from the target transmitter can always be greater than or equal to the electric power threshold P r , it is determined that the target transmitter emits a wide beam, and proceed to S2. Otherwise, if the electric power P rx received by the receiver controller from the target transmitter can always be less than the electric power threshold P r , it is determined that the target transmitter emits a narrow beam, and proceed to S5;

[0026] S2. The receiver controller keeps the receiver ATP module and the infrared emitter turned off, and the infrared receiver cannot receive the signal emitted by the infrared emitter. The transmitter controller controls the transmitter ATP module to remain in the off state;

[0027] S3. The electrical signal received by the receiver from the target transmitter is:

[0028] y = Rh(L)x + n (1)

[0029] wherein, x ∈ {0, 2P tx} is the optical signal to be transmitted, and the symbols 0 and 2P tx are randomly selected with equal probability from the on-off keying constellation, P tx is the average transmitted optical power, L is the communication distance between the target transmitter and the receiver, R is the photoelectric conversion efficiency of the receiver photodetector, and n is additive white Gaussian noise independent of the input signal with a mean of 0 and a variance of ;

[0030] h(L) is the channel gain, expressed as:

[0031]

[0032] where B is the diameter of the circular detection aperture of the receiver, and θ is the beam divergence angle of the target transmitter.

[0033] γ(λ) is the atmospheric attenuation coefficient, expressed as:

[0034]

[0035] where V is the visibility distance and λ is the wavelength of the light beam;

[0036] q is the size distribution of scattering particles under different weather conditions, expressed as a function of V:

[0037]

[0038] In Equation (2), h a is the atmospheric turbulence that follows the Malaga distribution, and its probability density function is expressed as:

[0039]

[0040] where A and a k are expressed as:

[0041]

[0042] where α is a positive parameter related to the number of effective units in the scattering process, is the Meijer G function, ξ g is the average optical power of the classical scattering component received by the off-axis eddy current coupled to the LOS term, β is a natural number representing the fading parameter, the parameter Ω′ is the average optical power of the coherent contribution, Γ(·) represents the Gamma function, and K v (·) is the modified Bessel function of the second kind of order v;

[0043] The received signal-to-noise ratio is expressed as:

[0044]

[0045] S4. The cell-edge coverage probability is the probability that the signal-to-noise ratio at the maximum communication distance is greater than a given threshold. The cell-edge coverage probability under the wide-beam condition is expressed as:

[0046]

[0047] where r th is the given signal-to-noise ratio threshold, is the equivalent channel gain threshold;

[0048] L max The maximum communication distance between the target transmitter and the receiver, expressed as:

[0049]

[0050] where D is the cell diameter;

[0051] Using the conversion relationship between Equation (5), the second-kind Bessel function and the Meijer G function, and the Meijer G function integral formula, obtain the closed-form expression of the edge coverage probability under wide-beam conditions, and go to S8;

[0052] S5. The transmitter controller turns on the receiver ATP module and the infrared transmitter. The infrared transmitter sends the signal to the infrared receiver, and the transmitter controller turns on the transmitter ATP module to achieve beam alignment and tracking between the target transmitter and the receiver;

[0053] S6. The receiver receives the electrical signal from the target transmitter as in Equation (1). The channel gain h(L) at this time is expressed as:

[0054]

[0055] where h p is the pointing error, and its probability density function is expressed as:

[0056]

[0057] where r is the ratio of the equivalent beam radius at the receiver to the standard deviation of the pointing error displacement at the receiver, w L is the beam waist at distance L;

[0058] From Equation (10) and Equation (11), the probability density function of h(L) is expressed as:

[0059]

[0060] where b k = a k [αβ / (ξ g β + Ω′)] -(α+k) / 2 , and the received signal-to-noise ratio is expressed as:

[0061]

[0062] S7. The cell edge coverage probability under narrow-beam conditions is expressed as:

[0063]

[0064] where is the equivalent channel gain threshold, L max The expression is the same as formula (9). Substitute into formula (16), and further derive it by the Meijer G function integral formula to obtain the closed-form expression of the edge coverage probability in the case of narrow beams;

[0065] S8. Perform simulations based on the closed-form expressions of the edge coverage probability obtained in the case of wide beams or narrow beams to evaluate the coverage performance.

[0066] The present invention is further configured that: the closed-form expression of the edge coverage probability under the wide beam condition is:

[0067]

[0068] The present invention is further configured that: the closed-form expression of the edge coverage probability in the case of narrow beams is:

[0069]

[0070] The present invention provides a beam adaptive free space optical communication system for high-speed railways and a method for evaluating the coverage performance. It has the following beneficial effects:

[0071] (1) By adding a transmitter controller and an infrared transmitter to the transmitter and a receiver controller and an infrared receiver to the receiver in the present invention, the receiver controller determines the width of the beam emitted by the target transmitter according to the intensity of the signal received by the receiver from the target transmitter during the switching process of the receiver from the source transmitter to the target transmitter, enabling the system to adaptively adjust the opening and closing of the ATP module according to the beam width, having the advantages of energy saving, simple structure, and cost saving.

[0072] (2) By comparing the electric power with the electric power threshold in the present invention, the judgment of the target transmitter emitting wide beams and narrow beams is realized, and system models in the case of wide beams or narrow beams are respectively established, and the closed-form expressions of the cell edge coverage probability are derived to evaluate the system coverage performance, providing a theoretical basis for evaluating the system coverage performance. Description of the Drawings

[0073] Figure 1 is a schematic diagram of the system composition of the present invention;

[0074] Figure 2 is a flowchart of the method for evaluating the system coverage performance of the present invention;

[0075] Figure 3 is a schematic diagram of the system when the transmitter emits a wide beam and is located directly above the track in the embodiment of the present invention;

[0076] Figure 4Schematic diagram of communication coverage when the transmitter is directly above the orbit in the embodiment of the present invention;

[0077] Figure 5 Schematic diagram of the system when the transmitter in the embodiment of the present invention emits a narrow beam and is directly above the orbit;

[0078] Figure 6 Schematic diagram of the system when the transmitter in the embodiment of the present invention emits a wide beam and is on the side of the orbit;

[0079] Figure 7 Schematic diagram of communication coverage when the transmitter is on the side of the orbit in the embodiment of the present invention;

[0080] Figure 8 Schematic diagram of the system when the transmitter in the embodiment of the present invention emits a narrow beam and is on the side of the orbit;

[0081] Figure 9 Comparison chart of theoretical values and simulation values of coverage performance in the case of the transmitter in the embodiment of the present invention emitting wide beams and narrow beams;

[0082] Figure 10 Comparison chart of theoretical values of coverage performance when the transmitter is in different positions in the embodiment of the present invention. Detailed implementation manners

[0083] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention.

[0084] Please refer to Figures 1-10 , the embodiments of the present invention provide the following technical solutions:

[0085] A beam adaptive high-speed rail free space optical communication system includes an optical fiber backbone network, an optical fiber link, a transmitter, a high-voltage cable support pole, a receiver, an in-train communication module, a transmitter controller, a receiver controller, an infrared transmitter, an infrared receiver, a transmitter ATP module, and a receiver ATP module.

[0086] The optical fiber backbone network and the high-voltage cable support pole are laid along the railway track. The optical fiber link is arranged inside the high-voltage cable support pole and is connected to the optical fiber backbone network. The optical fiber link is used to connect the transmitter and enable it to exchange information with the service provider.

[0087] The transmitter is deployed on the high-voltage cable support pole, and the transmitter is arranged on the side of the orbit or directly above the orbit, and sends the optical signal from the optical fiber link to the receiver through free space. Specifically, the transmitter includes a source transmitter, a target transmitter, and a silent transmitter;

[0088] The source transmitter is the transmitter that is currently communicating with the receiver;

[0089] The target transmitter is the transmitter that is about to communicate with the receiver;

[0090] The silent transmitter is the remaining transmitter that has not communicated with the receiver.

[0091] The receiver is set on the top of the train's head, and is used to receive the optical signal from the transmitter and send it to the in-train communication module and the receiver controller by wire.

[0092] The receiver controller is used to adaptively determine whether to turn on the receiver ATP module and the infrared transmitter according to the intensity of the signal received by the receiver from the target transmitter during the switching process of the receiver from the source transmitter to the target transmitter. Specifically, it judges whether the optical signal emitted by the target transmitter is a wide beam or a narrow beam. When the optical signal is a wide beam, the receiver controller turns off the receiver ATP module and the infrared transmitter. When the optical signal is a narrow beam, the receiver controller turns on the receiver ATP module and the infrared transmitter. Among them, the method for judging whether the optical signal emitted by the target transmitter is a wide beam or a narrow beam is:

[0093]

[0094] wherein, [T 1 , T 2 is the switching time range of the receiver from the source transmitter to the target transmitter, P r is the electric power threshold of the receiver controller, P rx (t) is the electric power of the signal received by the receiver from the target transmitter at time t. The transmitter controller is respectively connected to the transmitter and the infrared receiver, and is used to turn on the transmitter ATP module after the infrared receiver receives the signal from the infrared transmitter, and turn off the transmitter ATP module after the infrared receiver does not receive the signal from the infrared transmitter.

[0095] When the transmitter ATP module and the receiver ATP module are turned on simultaneously, they are used to realize the mutual tracking of the light beams between the transmitter and the receiver.

[0096] The method for evaluating the coverage performance of the beam adaptive high-speed rail free space optical communication system specifically includes the following steps:

[0097] S1. The transmitter uses on-off keying modulation for signal transmission, and the receiver uses direct detection for signal reception. It is set that all transmitter ATP modules and receiver ATP modules are in the off state. Let [T 1 , T 2 represent the switching time range of the receiver from the source transmitter to the target transmitter, and a given electric power threshold P r of the receiver controller is given. In [T 1 , T 2During a time period, if the electric power P of the signal received by the receiver from the target transmitter rx is always greater than or equal to the electric power threshold P r , it is determined that the target transmitter emits a wide beam, and the process proceeds to S2. Otherwise, when the receiver controller receives the electric power P from the target transmitter rx is always less than the electric power threshold P r , it is determined that the target transmitter emits a narrow beam, and the process proceeds to S5. Specifically:

[0098]

[0099] wherein, [T 1 , T 2 is the switching time range for the receiver to switch from the source transmitter to the target transmitter, P r is the electric power threshold of the receiver controller, P rx (t) is the electric power of the optical signal received by the receiver at time t;

[0100] S2. The receiver controller keeps the receiver ATP module and the infrared transmitter closed, and the infrared receiver cannot receive the signal emitted by the infrared transmitter. The transmitter controller controls the transmitter ATP module to remain closed;

[0101] S3. The electrical signal received by the receiver from the target transmitter is:

[0102] y = Rh(L)x + n (1)

[0103] wherein, x ∈ {0, 2P tx} is the optical signal transmitted, and the symbols 0 and 2P are randomly selected with equal probability from the on-off keying constellation tx , P tx is the average transmitted optical power, L is the communication distance between the target transmitter and the receiver, R is the photoelectric conversion efficiency of the receiver photodetector, and n is additive white Gaussian noise independent of the input signal with a mean of 0 and a variance of ;

[0104] h(L) is the channel gain, expressed as:

[0105]

[0106] wherein, B is the diameter of the circular detection aperture of the receiver, θ is the beam divergence angle of the target transmitter,

[0107] γ(λ) (unit: dB / km) is the atmospheric attenuation coefficient, expressed as:

[0108]

[0109] Wherein, V is the visibility distance (unit: km), and λ is the wavelength of the light beam;

[0110] q is the size distribution of scattering particles under different weather conditions, expressed as a function of V:

[0111]

[0112] In Equation (2), h a is the atmospheric turbulence obeying the Malaga distribution, and its probability density function is expressed as:

[0113]

[0114] Wherein, A and a k are expressed as:

[0115]

[0116] Wherein, α is a positive parameter related to the number of effective units of the scattering process, is the Meijer G function, ξ g is the average optical power of the classical scattering component received by the off-axis eddy current coupled to the LOS term, β is a natural number representing the fading parameter quantity, the parameter Ω′ is the average optical power of the coherent contribution, Γ(·) represents the Gamma function, and K v (·) is the modified Bessel function of the second kind of order v;

[0117] The received signal-to-noise ratio is expressed as:

[0118]

[0119] S4. The cell-edge coverage probability is the probability that the signal-to-noise ratio at the maximum communication distance is greater than a given threshold. The cell-edge coverage probability under the wide-beam condition is expressed as:

[0120]

[0121] Wherein, r th is the given signal-to-noise ratio threshold, is the equivalent channel gain threshold;

[0122] L max is the maximum communication distance between the target transmitter and the receiver, expressed as:

[0123]

[0124] Wherein, D is the cell diameter;

[0125] Using the conversion relationship between Equation (5), the second kind of Bessel function and the Meijer G function, and the Meijer G function integral formula, the closed-form expression of the edge coverage probability under the wide beam condition is obtained:

[0126]

[0127] Then go back to S8;

[0128] S5. The transmitter controller turns on the receiver ATP module and the infrared transmitter. The infrared transmitter sends the signal to the infrared receiver, and the transmitter controller turns on the transmitter ATP module to achieve the beam alignment and tracking between the target transmitter and the receiver;

[0129] S6. The receiver receives the electrical signal from the target transmitter as in Equation (1). At this time, the channel gain h(L) is expressed as:

[0130]

[0131] In the formula, h p is the pointing error, and its probability density function is expressed as:

[0132]

[0133] In the formula, r is the ratio of the equivalent beam radius at the receiver to the standard deviation of the pointing error displacement at the receiver, w L is the beam waist at the distance L (the radius calculated at e -2 );

[0134] From Equation (10) and Equation (11), the probability density function of h(L) is expressed as:

[0135]

[0136] In the formula, b k = a k [αβ / (ξ g β + Ω′)] -(α+k) / 2 , and the received signal-to-noise ratio is expressed as:

[0137]

[0138] S7. The cell edge coverage probability under the narrow beam condition is expressed as:

[0139]

[0140] In the formula, is the equivalent channel gain threshold, and the expression of L max is the same as Equation (9). Substitute f h(Lmax)(h) Substitute into Equation (16), and further derive using the Meijer G - function integral formula to obtain the closed - form expression of the edge coverage probability in the case of a narrow beam:

[0141]

[0142] S8. Perform simulations based on the closed - form expressions of the edge coverage probability obtained in the case of wide beams or narrow beams, and evaluate the coverage performance.

[0143] Example 1

[0144] Multiple transmitters are installed on the high - voltage cable support pole at a certain interval, located directly above the track, and the divergence angle of the transmitter is a wide beam.

[0145] As shown in the appendix Figure 3 As shown, the transmitters are fixedly installed on the high - voltage cable support pole at a certain interval, located directly above the track, and point the laser at the receiver. Considering the case where the divergence angle of the target transmitter is a wide beam, as shown in the appendix Figure 4 As shown, the positions of the transmitters are represented by black dots, and the receiver is located on the dotted line. The cell diameter is defined as D.

[0146] At this time, the corresponding edge coverage probability is:

[0147]

[0148] Example 2

[0149] The transmitters are installed on the high - voltage cable support pole at a certain interval, located directly above the track, and the divergence angle of the transmitter is a narrow beam.

[0150] As shown in the appendix Figure 5 As shown, the transmitters are fixedly installed on the high - voltage cable support pole at a certain interval, located directly above the track, and point the laser at the receiver. Considering the case where the divergence angle of the target transmitter is a narrow beam, the cell coverage schematic diagram of this communication system is the same as that in Example 1.

[0151] At this time, the corresponding edge coverage probability is:

[0152]

[0153] Example 3

[0154] The transmitters are installed on the high - voltage cable support pole at a certain interval, located on the side of the track, and the divergence angle of the transmitter is a wide beam.

[0155] As shown in the appendix Figure 6 As shown, the transmitters are fixedly installed on the high - voltage cable support pole at a certain interval, located on the side of the track, and point the laser at the receiver. Considering the case where the divergence angle of the target transmitter is a wide beam, the cell coverage schematic diagram of this communication system is as shown in the appendixFigure 7 As shown in the figure, when the transmitter covers the same range, the coverage performance of the transmitter in two positions is compared. Let its divergence angle be θ, and let D 1 = D / (2sin(θ / 2)).

[0156] At this time, the corresponding edge coverage probability is as follows:

[0157]

[0158] Embodiment 4

[0159] The transmitters are installed on the high-voltage cable support poles at regular intervals and are located on the side of the track, and the divergence angle of the transmitters is a narrow beam.

[0160] As shown in the appendix Figure 8 As shown, the transmitters are fixedly installed on the high-voltage cable support poles at regular intervals and are located on the side of the track, pointing the laser at the receiver. Considering the case where the divergence angle of the target transmitter is a narrow beam. The cell coverage schematic diagram of this communication system is the same as that of Embodiment 3. When the transmitter covers the same range, the coverage performance of the transmitter in two positions is compared.

[0161] At this time, the corresponding edge coverage probability is as follows:

[0162]

[0163] Simulation verification

[0164] As shown in the appendix Figure 9 As shown, it shows the edge coverage probabilities of the wide beam and the narrow beam varying with the transmit power P max when the position of the transmitter is fixed under different maximum communication distances L tx . It can be seen that the simulation values corresponding to the wide beam and the narrow beam are in good agreement with the theoretical values, verifying the accuracy of the derived theoretical expressions. In addition, when the transmit power P tx is fixed, as L max decreases, the edge coverage probabilities corresponding to the wide beam and the narrow beam gradually approach each other.

[0165] At the same time, although in the case of the wide beam, there is no need to turn on the ATP module to perform beam tracking between the transmitter and the receiver to save costs, but as L max increases, the received power attenuation caused by geometric loss also increases significantly, and the coverage performance is getting worse compared with the narrow beam. Therefore, in practical applications, it is necessary to make a trade-off to adjust the width of the transmitter's transmitting beam, so as to achieve the purpose of minimizing costs while ensuring the communication performance requirements.

[0166] As shown in the appendix Figure 10 As shown, it shows the edge coverage probabilities in the above four typical cases varying with the transmit power Ptx For the change, assuming that the coverage range of the transmitter is the same, i.e., D 1 = D / (2sin(θ / 2)). Without loss of generality, take D = 0.1 km. In the third embodiment, for P e find the maximum value. At this time, θ = 0.4 rad, and the optimal position of the transmitter on the side of the orbit is determined. As can be seen from the appendix Figure 10 In the case where the transmission power P tx is constant, when the transmitter is on the side of the orbit, the edge coverage probabilities corresponding to the wide beam and the narrow beam are both less than the case when the transmitter is directly above the orbit. In addition, in the case of the narrow beam, the edge coverage probability when the transmitter is on the side of the orbit is slightly less than the edge coverage probability when the transmitter is directly above the orbit. However, in the case of the wide beam, the edge coverage probability when the transmitter is on the side of the orbit is significantly less than the edge coverage probability when the transmitter is directly above the orbit. Therefore, in scenarios with high communication performance requirements, it is not suitable to transmit a wide beam when the transmitter is on the side of the orbit.

[0167] It should be noted that in this article, relational terms such as first and second are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the term "comprising", "including" or any other variant thereof is intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements not only includes those elements, but also includes other elements not expressly listed, or also includes elements inherent to such process, method, article or device.

[0168] Although the embodiments of the present invention have been shown and described, it will be understood by those of ordinary skill in the art that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the present invention, and the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. Beam adaptive high-speed rail free space optical communication system, characterized by: It includes an optical fiber backbone network, an optical fiber link, a transmitter, a high-voltage cable bearing rod, a receiver, a train internal communication module, a transmitter controller, a receiver controller, an infrared transmitter, an infrared receiver, a transmitter ATP module and a receiver ATP module; The optical fiber backbone network and the high-voltage cable bearing rod are laid along the railway track, the optical fiber link is arranged inside the high-voltage cable bearing rod and connected to the optical fiber backbone network, and the optical fiber link is used to connect the transmitter and exchange information with the service provider; The transmitter is deployed on a high voltage cable carrying pole and transmits the optical signal from the optical fiber link to the receiver through free space; The receiver is arranged on the top of the train head, and is used to receive the optical signal of the transmitter, and send it to the train internal communication module and the receiver controller via wired mode; The receiver controller is used to adaptively determine whether to turn on the receiver ATP module and the infrared transmitter according to the strength of the signal transmitted by the target transmitter received by the receiver during the switching process of the receiver from the source transmitter to the target transmitter; The transmitter controller is connected to the transmitter and the infrared receiver respectively, and is used to turn on the transmitter ATP module after the infrared receiver receives the signal from the infrared transmitter, and turn off the transmitter ATP module after the infrared receiver does not receive the signal from the infrared transmitter; When the transmitter ATP module and the receiver ATP module are turned on at the same time, they are used to achieve mutual tracking of light beams between the transmitter and the receiver.

2. The beam adaptive high-speed rail free space optical communication system according to claim 1, characterized in that: The transmitter is arranged on the side of the track or directly above the track.

3. The beam adaptive high-speed rail free space optical communication system according to claim 2 is characterized in that: The transmitters include a source transmitter, a target transmitter and a silent transmitter; The source transmitter is a transmitter currently communicating with the receiver; The target transmitter is a transmitter that is about to communicate with the receiver; The silent transmitters are the remaining transmitters that are not communicating with the receiver.

4. The beam adaptive high-speed railway free space optical communication system according to claim 1, characterized in that: The receiver controller is used to adaptively determine whether to turn on the receiver ATP module and the infrared transmitter according to the strength of the signal transmitted by the target transmitter received by the receiver during the switching process of the receiver from the source transmitter to the target transmitter, including: It is determined whether the optical signal emitted by the target transmitter is a wide beam or a narrow beam. When the optical signal is a wide beam, the receiver controller turns off the receiver ATP module and the infrared transmitter. When the optical signal is a narrow beam, the receiver controller turns on the receiver ATP module and the infrared transmitter.

5. The beam adaptive high-speed railway free space optical communication system according to claim 4, characterized in that: The method of determining whether the optical signal transmitted by the target transmitter is a wide beam or a narrow beam is: Where [T1, T2] is the switching time range of the receiver from the source transmitter to the target transmitter, P r is the electric power threshold of the receiver controller, P rx (t) is the electrical power of the target transmitter signal received by the receiver at time t.

6. The beam adaptive high-speed railway free space optical communication system coverage performance evaluation method applied to claim 5 is characterized by: The specific steps include: S1. The transmitter transmits signals using on-off keying modulation, and the receiver receives signals using direct detection. All transmitter ATP modules and receiver ATP modules are set to be in the off state. Let [T1, T2] represent the switching time range of the receiver from the source transmitter to the target transmitter. Given the electric power threshold P of the receiver controller r , in the time period [T1, T2], if the receiver receives the electric power P of the signal from the target transmitter rx Can always be greater than or equal to the electric power threshold P r , it is determined that the target transmitter transmits a wide beam and goes to S2. Otherwise, the receiver controller receives the electric power P from the target transmitter. rx Can always be less than the electric power threshold P r , it is determined that the target transmitter transmits a narrow beam, and the process goes to S5; S2, the receiver controller keeps the receiver ATP module and the infrared transmitter turned off, the infrared receiver cannot receive the signal sent by the infrared transmitter, and the transmitter controller controls the transmitter ATP module to remain turned off; S3. The electrical signal received by the receiver from the target transmitter is: y=Rh(L)x+n (1) Where x∈{0,2P tx } is the transmitted optical signal, and the symbols 0 and 2P are selected with equal probability from the on-off keying constellation. tx , P tx is the average transmitted optical power, L is the communication distance between the target transmitter and the receiver, R is the photoelectric conversion efficiency of the receiver photodetector, and n is independent of the input signal with a mean of 0 and a variance of Additive Gaussian white noise; h(L) is the channel gain, expressed as: Where B is the diameter of the circular detection aperture of the receiver, θ is the beam divergence angle of the target transmitter, γ(λ) is the atmospheric attenuation coefficient, expressed as: Where V is the visibility distance and λ is the wavelength of the light beam; q is the size distribution of scattered particles under different weather conditions, expressed as a function of V: In formula (2), h a For atmospheric turbulence that obeys the Malaga distribution, its probability density function is expressed as: In the formula, A and a k It is expressed as: Where α is a positive parameter related to the number of effective units in the scattering process, is the Meijer G function, ξ g is the average optical power of the classical scattered component received by the off-axis eddy current coupled to the LOS term, β is a natural number representing the fading parameter, parameter Ω′ is the average optical power of the coherent contribution, Γ(·) represents the Gamma function, K v (·) is the v-order modified Bessel function of the second kind; The received signal-to-noise ratio is expressed as: S4. The cell edge coverage probability is the probability that the signal-to-noise ratio at the maximum communication distance is greater than a given threshold value. The cell edge coverage probability under wide beam conditions is expressed as: In the formula, r th For a given signal-to-noise ratio threshold, is the equivalent channel gain threshold; L max is the maximum communication distance between the target transmitter and the receiver, expressed as: In the formula, D is the cell diameter; Using equation (5), the conversion relationship between the second-kind Bessel function and the Meijer G function and the integral formula of the Meijer G function, the closed expression of the edge coverage probability under the wide beam condition is obtained, and then go to S8; S5, the transmitter controller turns on the receiver ATP module and the infrared transmitter, the infrared transmitter sends a signal to the infrared receiver, and the transmitter controller turns on the transmitter ATP module to achieve beam alignment and tracking between the target transmitter and the receiver; S6. The receiver receives the electrical signal from the target transmitter in the same manner as in equation (1). The channel gain h(L) is expressed as: In the formula, h p is the pointing error, and its probability density function is expressed as: Where r is the ratio of the equivalent beam radius at the receiver to the standard deviation of the pointing error displacement at the receiver, w L is the waist at distance L; According to equations (10) and (11), the probability density function of h(L) is expressed as: Where b k =a k [αβ / (ξ g β+Ω′)] -(α+k) / 2 , the received signal-to-noise ratio is expressed as: S7. The cell edge coverage probability under narrow beam conditions is expressed as: In the formula, is the equivalent channel gain threshold, L max The expression is the same as (9), Substituting into equation (16), the Meijer G function integral formula is further derived to obtain the closed expression of edge coverage probability in the case of narrow beam; S8. Perform simulation based on the closed-form expression of edge coverage probability obtained in wide beam or narrow beam conditions to evaluate coverage performance.

7. The coverage performance evaluation method of the beam adaptive high-speed railway free space optical communication system according to claim 6 is characterized in that: The closed expression of edge coverage probability under the wide beam condition is:

8. The coverage performance evaluation method of the beam adaptive high-speed railway free space optical communication system according to claim 6 is characterized by: The closed expression of edge coverage probability in the narrow beam case is:

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