Digital aviation communication system design based on cellular-free network architecture

Abstract

The invention discloses a digital aeronautical communication system design based on a cellular-free network architecture, which belongs to the technical field of aeronautical communication and comprises an aeronautical communication system and a communication service execution module. The aeronautical communication system comprises a network infrastructure and a communication link layer and is responsible for providing underlying physical connection and transmission environment support. The communication service execution module covers a network protocol and information interaction and task execution process; the network infrastructure adopts distributed AP topology planning, airport and route area APs are deployed in a differentiated manner, and parameters are dynamically adjusted in combination with a three-dimensional coverage optimization model; and software and hardware modules comprise ground APs and airborne terminals, so that the problems caused by high-speed movement are solved. The communication link layer carries out multi-dimensional channel modeling, and LDPC code coding and dynamic cluster generation are adopted. The working process comprises the steps of initialization, cluster construction, state monitoring and the like, stable communication in the whole flight process is achieved, and the aviation communication efficiency and safety are improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of aviation communication technology, in particular to a digital aviation communication system design based on a cell-free network architecture. BACKGROUND

[0002] Since the LDACS technology was proposed, the implementation scheme has been continuously improved, but due to its dependence on cellular networks or satellite communication, there are still inherent problems such as frequent cell switching and significant Doppler effect. Although the modern cellular system has a certain frequency offset correction capability, the dramatic and dynamic frequency shift exceeds the conventional design threshold, which will cause the receiver to be difficult to accurately lock and demodulate the signal, the error rate will soar, the effective data throughput will drop sharply, and even cause link step-out interruption, affecting the communication quality.

[0003] At present, the traditional cellular network architecture is generally used in the field of foreign aviation communication to support air-ground information interconnection, and its core is to form a cellular topology through the partition coverage of ground macro base stations, and to realize user access by relying on fixed cell boundary division. However, the cellular architecture has essential limitations in the aviation high-speed scene, and only relies on the single-point transmission mode of the base station to the air, which fails to fully utilize the spatial dimension resources. The frequent cross-zone switching caused by the high-speed movement of the aircraft leads to signaling storm and communication interruption; the vertical coverage capability of the base station antenna is insufficient, and signal holes are prone to occur at high altitudes; the Doppler frequency shift is far beyond the conventional correction threshold. These defects are due to the strong dependence of the cellular architecture on the static ground user model, which cannot adapt to the dynamic three-dimensional continuity requirements of the aviation scene.

[0004] The cell-free network architecture can reconstruct spatial resources through distributed collaboration, and hundreds of low-power access points APs are densely deployed on the ground under the air route to form a dynamic service cluster. All APs and central processing units cooperate in real time to form a "user-centered" virtual large cell. The aircraft is no longer in a specific cell, but is jointly served by the optimal AP cluster, eliminating the switching boundary. The AP can also generate adaptive three-dimensional beams using large-scale antenna arrays, dynamically adjust the beam pointing and beam width through real-time feedback of the aircraft position and velocity vector, and realize high-altitude coverage in the vertical direction. The central processing unit of the cell-free network architecture is responsible for global resource scheduling, channel estimation and cooperative signal processing, and can also consider the priority of aircraft services for joint scheduling in digital aviation communication, improving the efficiency and safety of aviation communication. SUMMARY

[0005] The purpose of the present application is to provide a digital aviation communication system design based on a cell-free network architecture, considering some problems existing in the current aviation communication network architecture, based on a cell-free network architecture, modeling the high-altitude channel, establishing a distributed base station model, and designing a digital aviation communication system to improve the efficiency and safety of aviation communication.

[0006] To achieve the above object, the application provides a digital aviation communication system design based on a cell-free network architecture, comprising an aviation communication system and a communication service execution module, wherein the aviation communication system comprises a network infrastructure and a communication link layer; the aviation communication system provides a bottom-layer physical connection and a transmission environment support, including optimized wireless channel characteristics, stable communication link parameters and basic signal transmission capabilities; the communication service execution module covers network protocols and information interaction and task execution processes, and is responsible for coordination, control and process management of the aviation communication system, and defines and realizes protocol interaction rules between nodes.

[0007] Preferably, the design of the network infrastructure comprises distributed low-power access point (AP) topology planning and software and hardware module design, and specifically comprises the following steps:

[0008] The distributed AP topology planning in the cell-free network is introduced into the aviation communication system by considering the distributed AP distribution in the airport and the air route operation; the aviation communication environment is divided into two types of airport area and air route area according to the differentiated deployment of the use scenarios, the airport area includes the terminal, the runway and the apron, and a high-density AP deployment strategy is adopted to realize three-dimensional coverage; the topology construction of the air route area introduces a dynamic density AP deployment mechanism, the system dynamically adjusts the AP spacing by real-time analysis of the satellite flight path data and the air traffic heat map of the flight schedule; the efficiency of the distributed AP topology planning depends on the regulation and control of the three-dimensional coverage optimization model, the three-dimensional coverage optimization model establishes a ternary coupling relationship among the AP transmission power, the coverage radius and the communication capacity demand; first, the path loss is calculated according to the digital elevation data of the terrain, and the path loss calculation model is as follows:

[0009] PL = 32.4 + 20logf + 20logd (1);

[0010] wherein f represents the signal frequency, d represents the transmission distance, and the AP power is dynamically adjusted according to the mountain shielding condition; secondly, the vertical coverage angle is optimized based on the flight altitude layer distribution to reduce signal airspace waste; finally, the optimal AP density is inversely deduced in combination with the traffic volume prediction to form a closed-loop calibration of the coverage parameters; the AP spacing algorithm is landed through dynamic configuration to improve the spectrum utilization rate under the premise of guaranteeing the QoS;

[0011] The hardware and software module design includes ground AP hardware and software design and airborne terminal design, specifically: investigating air route density, comprehensively considering AP power and coverage range, configuring ground AP spacing according to communication requirements, and integrating large-scale antenna arrays for cooperation with the airborne terminal; the airborne terminal design is the design of airborne hardware for solving the physical layer misalignment problem caused by high-speed movement, the airborne hardware including a Doppler compensation module and a phased array antenna system; the phased array antenna system adopts a four-panel omnidirectional coverage scheme with 64 array elements in each panel, calculates the direction of arrival in real time based on aircraft attitude data and AP position information, and completes beam reconstruction through a gradient descent algorithm to reduce beam pointing error at cruising speed; the Doppler compensation module constructs a two-stage correction mechanism, the radio frequency stage dynamically adjusts the local oscillator frequency through a phase-locked loop to eliminate carrier frequency offset; and the baseband stage adopts an extended orthogonal frequency division multiplexing technology to expand the subcarrier spacing to 75 kHz to resist the Doppler spread effect and reduce the inter-symbol interference power;

[0012] The software-defined control plane is the central nervous system for the cooperation of the hardware and software modules, the dynamic resource scheduling engine optimizes air interface resources in real time according to the AP density parameters of the topology planning, starts non-orthogonal multiple access in high-density areas of the airport to serve multiple users through power domain multiplexing; and switches to large-scale MIMO single-user beam focusing in remote sparse areas to improve edge coverage capability; considering the specially developed air route perception protocol stack, flight plan data is converted into network configuration parameters, and when the aircraft enters an ocean-crossing route, the "energy-saving mode" is automatically triggered to prolong the AP sleep cycle; and in the approach phase, the APs around the runway are pre-activated to form a communication bustling area.

[0013] Preferably, the design of the communication link layer includes multi-dimensional channel modeling, channel coding design, and user-centric dynamic cluster generation, the channel modeling and channel coding design belong to the communication system design level, and the user-centric dynamic cluster generation belongs to the application of the cell-free network technology in the aviation communication system.

[0014] Preferably, the multi-dimensional channel modeling process is as follows:

[0015] A cell-free system is set to contain M distributed APs, K airborne terminal users, each AP is equipped with N AP root antennas, and each airborne terminal user is equipped with N UE root antennas, all APs are connected with a central processing unit (CPU) through a front-haul link for cooperative scheduling and data merging; all links are air-ground links, and three-dimensional propagation, multipath, Doppler, and state changes need to be considered;

[0016]

[0017] wherein, is a MIMO channel matrix between the mth AP and the kth terminal user; β mkis the large-scale fading between the m APs and the kth terminal user, including path loss and shadow fading;G mk is the small-scale fading, and the Rician channel model is used as the channel model, i.e., a Rician channel model;

[0018] Large-scale fading coefficient β mk The energy loss and shadowing effect caused by long-distance propagation are described, and the model is as follows:

[0019]

[0020] where d mk is the three-dimensional spatial distance between the AP and the air terminal, assuming that the coordinates of the mth AP are (x m , y m , z m ), and the coordinates of the kth air terminal are (x k , y k , z k ), and d mk is obtained by the following formula:

[0021]

[0022] In formula (3), PL0 is the reference path loss per unit distance; γ is the path loss factor, and the air-ground direct signal LOS takes 2.1-2.7; is the shadow fading term, which satisfies the lognormal distribution; d0 is the reference distance, which is usually 1 meter;

[0023] The model of the small-scale fading is modeled as follows:

[0024]

[0025] where K mk is the Rician factor, indicating the power ratio of the direct path and the scattered path, and the larger the value, the stronger the direct link, which accounts for the main proportion of the direct link; the smaller the value, the weaker the direct link, and the reflected link accounts for the main proportion, and the multipath effect is obvious;G LOS，mk represents the direct link, which is related to the angle of the antenna placement and determines the angle of arrival and departure of the signal; in a single-antenna scenario, G LOS，mk = 1, and in a multi-antenna communication scenario, the angle of departure of the signal at the sending end and the angle of arrival of the signal at the receiving end are considered, and at this time, G LOS，mk = α T · α R , α T and α R are vector vectors representing angles;G NLOS，mk is the equivalent multipath link caused by reflection, diffraction, scattering, etc. in addition to the direct link, and a plurality of reflected links are superimposed.

[0026] The high-speed movement of the flight platform will cause significant Doppler shift, which will affect the frequency domain orthogonality, especially in the OFDM system, which will be expressed as inter-symbol interference, and which needs to be focused on in channel modeling.

[0027]

[0028] where v k is the instantaneous speed of the air terminal user k; is the carrier wavelength; θ mk is the angle between the movement direction of the air terminal user k and the direction between the user and the AP.

[0029] Preferably, the channel coding adopts a low-density parity-check code, i.e., an LDPC code, and the design process is as follows:

[0030] The LDPC coding, bit interleaving and QAM modulation structure are adopted to achieve a good balance between high throughput and strong error correction performance, and the specific coding design model of the LDPC code is a sparse check matrix H∈{0,1} (n-j)×n defined, which satisfies:

[0031] H·c T = 0 (7);

[0032] where c∈{0,1} n is the coded code word, k is the number of original information bits, n is the number of coded bits, and the code rate The selected numerical values include 1 / 2, 2 / 3 and 3 / 4, the sparse check matrix is obtained through a PE algorithm, the check matrix construction is optimized in combination with the characteristics of the air-ground link, a short frame is used for a high-speed aircraft user to reduce the decoding delay; a local check matrix generated in a distributed manner is used in a multi-AP aviation communication system, and joint checking is performed at the CPU end; joint channel state information feedback is implemented to realize link-level adaptive coding;

[0033] To adapt to a multipath fading channel and improve the anti-interference ability of the system, a bit interleaving coding modulation is used in the aviation communication system.

[0034] Considering the drastic change of the air-ground link channel state, an aviation communication system integrates an adaptive coding modulation mechanism, dynamically selects a coding and modulation combination based on real-time SNR estimation and target QoS constraints, so that the system throughput is maximized and the bit error rate is lower than a threshold.

[0035] Preferably, the user center dynamic cluster is dynamically adjusted in real time, and the cluster update period satisfies the following formula:

[0036]

[0037] where f D is the maximum Doppler shift, and Tcoherence The channel coherence time is 50-200 ms; the cluster structure is checked once every 50-200 ms, and frequent switching is reduced by combining with a prediction mechanism; the cluster update condition is set to be triggered; after the user center dynamic cluster structure is established, a plurality of APs in the cluster jointly complete the functions of beamforming, power control and user scheduling, and a centralized and edge collaborative processing architecture is used to realize cluster service collaborative control.

[0038] Preferably, the communication service execution includes network protocol and information interaction and task execution flow; the information interaction is divided into control signaling interaction and service data interaction; the control signaling interaction is used for link establishment and maintenance, and the service data interaction includes uplink and downlink transmission of the aviation service; the control signaling interaction includes terminal access request, access point response, cluster maintenance command, channel state report and modulation coding feedback; the service data interaction includes task request, task scheduling notification, data fragmentation and recombination mechanism, encrypted transmission and authentication control.

[0039] Preferably, the task execution flow includes four aspects of communication scheduling management, link detection and maintenance, fault tolerance switching and network recovery mechanism.

[0040] The application also provides a design workflow of a digital aviation communication system based on a non-cellular network architecture, characterized by comprising the following steps:

[0041] S1, system initialization and position state acquisition;

[0042] Before communication, start the non-cellular communication terminal carried by the civil aviation aircraft, activate the GNSS positioning module, and acquire the current three-dimensional geographic coordinates (x k ,y k ,z k ), flight speed vector, heading angle and initial wireless channel state in real time; all information is actively sent to the ground central processing unit through the uplink control channel as the basis for subsequent communication strategy judgment;

[0043] S2, region identification and flight path prediction;

[0044] After the central processing unit receives the above information, the region identification module is immediately started, the coordinate matching system built-in airspace division database is used to judge whether the current aircraft is located in an airport region or an air route region; the flight path prediction algorithm is simultaneously run, the path in the next 30 seconds is predicted based on the speed vector and the flight plan, and the AP topology database in the flight path coverage range is searched to construct a candidate access point list, thereby providing prior information for subsequent cluster optimization;

[0045] S3, initial cluster construction and access establishment;

[0046] If the aircraft is in the airport area, the system activates the AP group within a radius of 500 meters according to the preset strategy to form a high-density access cluster, and selects the node with the best channel quality near the tower as the main service AP. The aircraft terminal completes the initial synchronization, frequency offset correction and random access process with the AP to establish a data and control channel connection;

[0047] S4, dynamic cluster adjustment;

[0048] When the aircraft identifies that it has left the airport area and entered the air route space, the central processing unit dynamically selects a number of distributed APs to form a user center cluster according to the deployment rules in formula (1) and the air route traffic heat map. The cluster construction follows the "flight path priority, load balancing" principle, the AP spacing is compressed to 5 kilometers in busy air routes and extended to 50 kilometers in empty areas, and the cluster members are adjusted in real time according to the aircraft heading to maintain the aircraft in a "multi-AP cooperative service" state, realizing continuous coverage without boundaries.

[0049] S5, CSI feedback and link state monitoring;

[0050] The aircraft communication terminal and the APs in the cluster continuously perform link state interaction based on channel state information, and the system sets a feedback period of 50-200ms to monitor, including but not limited to, signal-to-noise ratio, bit error rate, throughput rate and delay jitter. If the feedback channel state is stable, the current cluster is maintained and data transmission continues; if the channel quality is found to have decreased below the set threshold, the system triggers the link protection logic.

[0051] S6, fault tolerance switching and cluster reconstruction;

[0052] When link degradation or failure occurs, the central processing unit quickly calls the current backup AP candidate pool, and preferentially selects a new AP cluster with good channel conditions and low node load. The new cluster completes parameter negotiation, synchronization access and beam alignment process with the aircraft terminal through the control channel. The main-standby cooperative mechanism is adopted, the current main AP keeps the task relay uninterrupted, and the standby AP automatically switches to the main service node after initialization. At the same time, the system starts the data buffering and context state synchronization mechanism to ensure business continuity and seamless switching during task migration.

[0053] S7, continuous communication and task closed-loop management;

[0054] The system continuously runs the above-mentioned prediction-monitoring-switching closed-loop control logic throughout the flight process, dynamically adapts to changes in aircraft motion state and environment, and maintains a stable air-ground communication link; after the aircraft enters the destination airport airspace, the system switches to the high-density access mode again, reactivates the airport area AP cluster, and completes the landing phase communication task, including task return, operation report upload, and terminal disconnection; thus, a complete aviation communication task life cycle is completed.

[0055] Therefore, the present application adopts the above-mentioned digital aviation communication system design based on a non-cellular network architecture, introduces non-cellular network communication into the field of aviation communication, designs a distributed base station planning, constructs a time-varying channel model library for aviation environment to improve the modeling accuracy of high-altitude layers, and provides a more efficient design method for global / regional digital aviation communication systems, which not only makes up for the communication vulnerability caused by cell switching in traditional cellular network communication schemes and provides an improvement idea for industry hard problems, but also gives a dynamic cluster base station selection method and a specific scheme of software and hardware module design with higher feasibility and integrity. The task execution process considers various situations of aircraft high-altitude communication in combination with common aviation communication business, can meet the comprehensive demand of aircraft communication, and helps to promote the research, application and development process of aviation communication.

[0056] The technical solutions of the present application will be further described in detail below through the drawings and examples. BRIEF DESCRIPTION OF DRAWINGS

[0057] Figure 1 is the overall implementation scheme of the embodiment of the present application;

[0058] Figure 2 is a network infrastructure schematic diagram of the embodiment of the present application;

[0059] Figure 3 is a communication link layer schematic diagram of the embodiment of the present application;

[0060] Figure 4 is a bit interleaving coding modulation structure diagram of the embodiment of the present application;

[0061] Figure 5 is a communication business execution diagram of the embodiment of the present application;

[0062] Figure 6 is an aircraft communication flowchart of the embodiment of the present application. DETAILED DESCRIPTION

[0063] The technical solutions of the present application will be further described in detail below through the drawings and examples.

[0064] Unless otherwise defined, technical terms or scientific terms used in the present application shall have the ordinary meaning of such terms for a person skilled in the art to which the present application pertains.

[0065] Embodiment one

[0066] The present application provides a digital aviation communication system design based on a cell-free network architecture, the overall implementation scheme is as shown in Figure 1 The aviation communication system includes a network infrastructure and a communication link layer. The aviation communication system provides a bottom physical connection and a transmission environment support, including optimizing the wireless channel characteristics, stabilizing the communication link parameters, and the basic signal transmission capability. The communication service execution module covers the network protocol and information interaction and task execution process, is responsible for the coordination, control and process management of the aviation communication system, defines and realizes the protocol interaction rules between nodes, ensures the collaborative and orderly execution of various tasks, iterates the traditional communication vulnerability problem, and can improve the security and efficiency of communication.

[0067] The design of the network infrastructure includes distributed low-power access point (AP) topology planning and software and hardware module design. The design process is as shown in Figure 2 The specific contents include the following:

[0068] Distributed AP topology planning:

[0069] The distributed AP topology planning is introduced into the aviation communication system considering the distributed AP distribution in the airport and the flight operation. In the design of the digital aviation communication system based on the cell-free network, the distributed AP topology planning constitutes the core architecture of the network infrastructure layer. According to the differentiated deployment of the use scenarios, the aviation communication environment is divided into two types of airport area and air route area. The airport area includes the terminal, runway and apron, adopts a high-density AP deployment strategy, realizes three-dimensional coverage, and closely arranges APs within 500 meters in the present embodiment. This design can support the concurrent communication needs of hundreds of aircrafts, ensure the seamless connection of key services such as tower instructions, flight scheduling and passenger data synchronization in the high-speed mobile scene, and is especially helpful to solve the communication interruption problem caused by frequent switching of traditional cell networks in the airport.

[0070] The topology construction of the route area introduces a dynamic density AP deployment mechanism, the core of which is the flexible resource allocation driven by route density. Through real-time analysis of satellite track data and flight schedule air traffic heat map, the system dynamically adjusts the AP spacing; for example, in transoceanic routes or remote mountainous areas, it is expanded to 20-50 kilometers to save infrastructure costs, while in busy trunk routes such as Beijing-Shanghai and Europe-Asia, it is compressed to 5-10 kilometers to form an ultra-dense network. This "stretchable" deployment mode not only reduces hardware redundancy in remote areas, but also optimizes the number of APs activated 24 hours in advance through route prediction algorithms, achieving dynamic balance between communication capacity and energy consumption. The efficiency of distributed AP topology planning depends on the regulation of the three-dimensional coverage optimization model, which establishes a three-way coupling relationship between AP transmission power, coverage radius, and communication capacity demand. First, the path loss is calculated according to the digital elevation model, and the path loss calculation model is as follows:

[0071] PL = 32.4 + 20logf + 20logd (1);

[0072] where f represents the signal frequency, d represents the transmission distance, and the AP power is dynamically adjusted according to the mountain shielding situation; secondly, based on the flight altitude layer distribution, the vertical coverage angle is optimized to reduce signal airspace waste; finally, combined with traffic volume prediction, the optimal AP density is deduced in reverse to form a closed-loop calibration of coverage parameters; through dynamic configuration of AP spacing algorithm, the spectrum utilization is improved under the premise of ensuring QoS.

[0073] In actual work, to cope with the special challenges of the aviation environment, the topology planning deeply integrates anti-interference and reliability enhancement mechanisms. In high-density areas of the airport, beamforming technology is used to compress the AP horizontal lobe angle to 30°, so that the runway AP and the terminal building AP work in orthogonal spatial resources, completely avoiding co-frequency interference; in the dynamic deployment area of the route, a "AP cooperative cluster" architecture is constructed, when the aircraft passes through the coverage edge of adjacent APs, multi-point joint transmission is automatically triggered, and signal coherence combining is used to eliminate switching interruption. There is a strong coupling relationship between this topology and hardware design, the large-scale antenna array of ground AP provides spatial reuse capability, so that a single AP can serve 8 parallel flights; while the airborne phased array antenna system realizes millisecond-level beam tracking, cooperates with the Doppler compensation module to pre-correct the frequency offset, and together solves the problem of signal distortion caused by high-speed movement, laying the physical foundation for seamless global coverage.

[0074] Software and hardware module design:

[0075] In the architecture of the digital aviation communication system, the software and hardware module design is the physical basis for supporting the distributed AP topology landing, and its core lies in the coordination of ground AP and airborne terminal. The software and hardware module design includes ground AP software and hardware design and airborne terminal design, specifically: investigating the air route density, comprehensively considering the power and coverage range of AP, configuring the spacing of ground AP according to the communication demand, integrating large-scale antenna array for coordination with airborne terminal. In this embodiment, the ground AP adopts a dual-engine architecture of "large-scale antenna array + high-precision synchronization", deploys a 256TRx-level millimeter wave phased array antenna, generates 128 independent controllable beams through hybrid beamforming technology, realizes spatial reuse in three-dimensional space, and assumes that each flight is allocated a bandwidth of 100MHz, so that a single AP can simultaneously serve 8 flights; at the same time, based on GPS / Beidou dual-mode timing and optical fiber transmission delay compensation, a nanosecond-level time synchronization module is integrated to ensure that the distributed AP units achieve signal coherence when cooperating in transmission and reduce the time delay jitter of multi-AP joint transmission. This design directly enables high-density deployment in topology planning, and when the spacing between APs in the airport area is compressed to 500 meters, the same frequency interference can be avoided through beam isolation, which is an improvement over traditional cellular networks.

[0076] The airborne terminal design is the design of airborne hardware for solving the problem of physical layer misalignment caused by high-speed movement. The airborne hardware includes a Doppler compensation module and a phased array antenna system. The phased array antenna system adopts a four-panel omnidirectional coverage scheme with 64 elements per panel, calculates the direction of arrival based on the aircraft attitude data and AP position information in real time, completes beam reconstruction through the gradient descent algorithm to reduce beam pointing error at cruising speed; the Doppler compensation module constructs a two-stage correction mechanism, the radio frequency stage dynamically adjusts the local oscillator frequency through the phase-locked loop to eliminate the carrier frequency offset; the baseband stage adopts an extended orthogonal frequency division multiplexing technology to expand the subcarrier spacing to 75kHz to resist the Doppler spread effect and reduce the inter-symbol interference power. This combined scheme of hardware pre-correction and algorithm compensation can greatly reduce the bit error rate in high-speed scenarios.

[0077] The software-defined control plane is the central nervous system of the software and hardware module coordination, and the dynamic resource scheduling engine optimizes the air interface resources in real time according to the AP density parameters of the topology planning, starts non-orthogonal multiple access in the high-density area of the airport to serve multiple users through power domain multiplexing; in the remote sparse area, it switches to large-scale MIMO single-user beam focusing to improve the edge coverage capability; considering the special development of the air route perception protocol stack, the flight plan data is converted into network configuration parameters, and when the aircraft enters the transoceanic route, the "energy saving mode" is automatically triggered to prolong the AP sleep cycle; and in the approach phase, the APs around the runway are pre-activated to form a communication bustling area. This software and hardware cooperative design can make the system achieve high business prediction accuracy on typical routes, and improve the reliability by two orders of magnitude compared with traditional aviation communication systems.

[0078] In the design of digital aeronautical communication system based on the non-cellular network, the design of communication link layer plays a key role in connecting the network infrastructure and the specific communication service implementation. The design of communication link layer can be carried out from three aspects of multi-dimensional channel modeling, channel coding design and user-centered dynamic cluster generation. The channel modeling and coding design belong to the level of communication system design, and the user-centered dynamic cluster generation belongs to the application of non-cellular network technology in aeronautical communication system. The specific design process is as shown in Figure 3

[0079] The process of multi-dimensional channel modeling is as follows:

[0080] The digital aeronautical communication channel has the characteristics of strong time variation, high dynamicity and multi-path fading, and the communication terminal moves at high speed in the three-dimensional space, so that the traditional static or semi-static channel model cannot accurately depict the real performance of the system. In the non-cellular network, multiple distributed access points cooperate to provide services for all users, and channel modeling, as the basis of link layer design, aims to accurately describe the wireless transmission characteristics between users and multiple APs to support key mechanisms such as beamforming, joint reception and scheduling. Therefore, a set of multi-dimensional dynamic channel modeling mechanism needs to be constructed to meet the modeling requirements in the environment of distributed access point cooperation, non-orthogonal link superposition and dynamic scheduling.

[0081] Suppose that a non-cellular system contains M distributed APs, K air terminal users, each AP is equipped with N AP root antennas, and each air terminal user is equipped with N UE root antennas, all APs are connected with the central processing unit (CPU) through the front link for cooperative scheduling and data merging; all links are air-ground links, which need to consider three-dimensional propagation, multipath, Doppler and state change;

[0082]

[0083] wherein, is the MIMO channel matrix between the mth AP and the kth terminal user; β mk is the large-scale fading between the m APs and the kth terminal user, including path loss and shadow fading; G mk is the small-scale fading, and the Rician channel model is used as the channel model, i.e. the Rician channel model. In this embodiment, the air-ground link signal usually has a line-of-sight main path, so the Rician channel model is used. In the process of signal propagation, the amplitude and phase of the signal change randomly due to the multipath effect, and this channel model assumes that the amplitude of the signal obeys the Rician distribution.

[0084] The large-scale fading coefficient β mk describes the energy loss and shadowing effect caused by long-distance propagation, and is modeled as: ​

[0085]

[0086] wherein d mk is the three-dimensional space distance between the AP and the air terminal, assuming the coordinates of the mth AP are (x m , y m , z m ) and the coordinates of the kth air terminal are (x k , y k , z k ), d mk is obtained from the following formula:

[0087]

[0088] In formula (3), PL0 is the reference path loss per unit distance; γ is the path loss factor, and the air-ground direct signal LOS takes 2.1-2.7; is the shadow fading term, which satisfies the lognormal distribution; d0 is the reference distance, usually 1 meter;

[0089] The model of small-scale fading is modeled as follows:

[0090]

[0091] wherein K mk is the Rician factor, representing the power ratio of the direct path and the scattered path, and the greater the value, the greater the direct link strength, which accounts for the main proportion of the direct link; the smaller the value, the weaker the direct link strength, and the reflected link accounts for the main proportion, and the multipath effect is obvious; G LOS，mk represents the direct link, which is related to the angle of the antenna placement and determines the angle of arrival and departure of the signal, and in the single antenna scene, G LOS，mk = 1, and in the multi-antenna communication scene, the signal departure angle of the sending end and the signal arrival angle of the receiving end are considered, and at this time G LOS，mk = α T · α R , α T and α R are the vector vectors representing the angle; G NLOS，mk is the equivalent multipath link caused by reflection, diffraction, scattering and other reasons in addition to the direct link, and the equivalent multipath link after superposition of multiple reflected links;

[0092] The high-speed motion of the flight platform will produce significant Doppler shift, which affects the frequency domain orthogonality, especially in the OFDM system, which is manifested as inter-symbol interference, and needs to be focused on in channel modeling,

[0093]

[0094] wherein v kis the instantaneous speed of the air terminal user k; is the carrier wavelength; θ mk is the angle between the moving direction of the air terminal user k and the direction between it and the AP.

[0095] Through the above systematic multi-dimensional channel modeling, not only the wireless propagation characteristics between the user and the distributed access point in the air-ground communication environment are accurately described, but also the key channel factors such as path loss, shadow fading, small-scale fading, Doppler shift and LOS / NLOS state change are effectively reflected. This channel model provides a theoretical basis and simulation basis for constructing a stable, robust and low-latency communication link, and is especially suitable for high-dynamic aviation scenarios under the architecture of a cell-free network. Relying on the model, the system can realize accurate estimation and prediction of the link state, thereby providing a solid support for key technology modules such as protocol development, link scheduling, resource allocation, beamforming and interference management of the digital aviation communication system based on the distributed AP topology. At the same time, the modeling framework can also be extended to subsequent simulation verification, system-level performance evaluation, air-ground integrated network optimization and cross-platform integrated communication research, and has good universality and expandability.

[0096] The channel coding design process is as follows:

[0097] The air terminal of the aircraft and the unmanned aerial vehicle moves at a high speed, the channel changes frequently, and is affected by complex environmental factors such as multipath fading, path loss, and non-fully-los propagation. Therefore, it is of key significance to design an efficient, low-latency, and strong error correction capability channel coding scheme to ensure the stability and data integrity of the communication link. The system adopts the Cell-Free architecture of distributed access point cooperative service, and the terminal can simultaneously receive multiple AP signals, so the coding scheme should also have good joint decoding capability at the receiving end and link adaptability. The channel coding design needs to meet the following goals: improve the error performance under low signal-to-noise ratio; support high-speed transmission and real-time data communication; match the air-ground channel model, have anti-multipath and anti-burst error capability; adapt to distributed cooperative modulation or multiple access point joint transmission; and flexibly combine with the modulation scheme to support adaptive coding and modulation mechanism.

[0098] For the air-ground communication scenario, several channel coding schemes have good performance, which are LDPC code, Turbo code, Polar code and BCH / RS code. LDPC code is based on sparse check matrix, and has quasi-linear decoding complexity. The advantages are close to the Shannon limit, support parallel processing, and are suitable for high-speed air-ground link. The embodiment adopts LDPC coding, bit interleaving and QAM modulation structure, which can realize a good balance between high throughput and strong error correction performance. The specific LDPC coding design model is a sparse check matrix H∈{0,1} (n-k)×n Definition, which satisfies:

[0099] H·c T = 0 (7) ;

[0100] where c e {0, 1} n is the encoded codeword, k is the number of original information bits, n is the number of encoded bits, code rate The values that can be selected include 1 / 2, 2 / 3, 3 / 4, the sparse check matrix is obtained by the PE algorithm, the check matrix construction is optimized in combination with the characteristics of the air-ground link, for high-speed aircraft users, a short frame is used to reduce the decoding delay; in the multi-AP aviation communication system, a locally generated check matrix is used, and joint checking is performed at the CPU end; joint channel state information feedback is used to realize link-level adaptive coding;

[0101] To adapt to the multipath fading channel and improve the system anti-interference ability, the aviation communication system adopts bit interleaved coding modulation; the specific structure is as shown in Figure 4 .

[0102] Considering the severe change of the air-ground link channel state, the aviation communication system integrates an adaptive coding modulation mechanism, based on real-time SNR estimation and target QoS constraint, dynamically selects the coding and modulation combination, so that the system throughput is maximized and the error rate is lower than the threshold.

[0103] By introducing a strong error correction coding technology suitable for air-ground scenarios, modulation interleaving and link adaptation mechanism, the system can effectively improve the data transmission reliability and communication efficiency under high-speed mobile and dynamic channel conditions, and lay a technical foundation for building a strong robust, scalable and low-latency link layer communication capability for digital aviation communication systems based on non-cellular network architecture.

[0104] The user-centered dynamic cluster process is as follows:

[0105] In this embodiment, the communication architecture takes large-scale distributed access points to cooperatively serve user terminals as the core, and the non-cellular network emphasizes the user-centered design idea, each user is cooperatively served by a group of dynamically selected access points, so as to improve the channel gain, reduce the interference and improve the service fairness. In the high-speed air-ground communication environment, the user position and channel state change constantly, if the fixed cluster scheme is still used, it will cause unstable link quality, waste of system resources and other problems. Therefore, building a dynamic and self-adaptive user-centered cluster generation mechanism has become a key problem in the design of the aviation communication system under the non-cellular architecture.

[0106] The core goal of the user-centered dynamic cluster generation mechanism is to select a group of best AP clusters for each user to perform downlink or uplink communication; the cluster can be dynamically adjusted over time to match user movement and channel fluctuations; supports AP resource sharing and cooperative scheduling to improve spectrum and energy efficiency; can reduce the total system interference and improve the service quality of edge users.

[0107] Aircraft usually move at a speed of 200m / s or more, and in order to adapt to the high-speed moving scene, the cluster needs to be dynamically adjusted in real time or quasi-real time, and the cluster update period satisfies the following formula:

[0108]

[0109] Where, f D is the maximum Doppler shift, T coherence is the channel coherence time; the cluster structure is checked every 50-200ms, and the frequent switching is reduced by combining with the prediction mechanism; the cluster update condition is set to trigger type; after the user center dynamic cluster structure is established, a number of APs in the cluster jointly complete the functions of beamforming, power control and user scheduling, and a centralized and edge collaborative processing architecture is used to realize the collaborative control of cluster service.

[0110] After the communication link layer is designed, the system enters the communication service execution phase. This phase is responsible for realizing the data transmission, protocol interaction and task scheduling of each specific service in the digital aviation communication system, and ensuring the stability, real-time performance and reliability of the communication system in the air-ground high-speed environment. The communication service execution relies on the distributed access point and the user center dynamic cluster structure, and has high adaptability and task collaboration ability. The communication service execution includes the design of network protocol and information interaction mechanism and the design of task execution process, and the specific design Figure 5 As shown.

[0111] The network protocol and information interaction are as follows:

[0112] The network protocol system needs to be compatible with the characteristics of high dynamic, low delay and multi-user collaboration, and the protocol stack design aims at lightweight, modularization and scalability. The information interaction should support the following functions:

[0113] 1) Link initialization and identity authentication: including user access authentication, distributed AP confirmation and virtual cluster synchronization establishment;

[0114] 2) Resource request and allocation negotiation: the terminal sends a service resource request to the cluster AP, and the CPU uniformly schedules the spectrum, time and power resources;

[0115] 3) Real-time control and feedback channel: a low-delay feedback mechanism is set in the data interaction process;

[0116] 4) Cross-node collaboration mechanism: realizing the coordination of data forwarding, user switching and task cooperation between different APs to ensure continuous coverage of users;

[0117] 5) Integration interface with upper layer service: providing a data abstraction interface for the application layer, supporting multiple types of services such as flight data, images, control instructions, etc.

[0118] The information interaction is divided into control signaling interaction and service data interaction. The control signaling interaction is used for link establishment and maintenance, and the service data interaction includes uplink and downlink transmission of various aviation services. The control signaling interaction includes terminal access request, access point response, cluster maintenance command, channel state report and modulation coding feedback. The terminal access request content includes UE identification, position estimation, speed information and initial channel state. The access point response allocates an initial cluster, specifies a main service AP and a resource time slot. The cluster maintenance command dynamically reconstructs the cluster and indicates to add or release an AP. The channel state report reports the link quality of each AP in the current cluster. The modulation coding feedback is the feedback of relevant information from the air terminal to the CPU, which is used for adaptive adjustment. Finally, the confirmation feedback step should be included to quickly feed back whether the packet is successfully received or not, which is used for retransmission determination. The service data interaction includes task request, task scheduling notification, data fragmentation and recombination mechanism, encrypted transmission and authentication control steps. The task request is initiated by the flight platform, such as image upload and trajectory report. The task scheduling notification is the scheduling decision issued by the system end. The data fragmentation and recombination mechanism ensures reliable transmission of large data packets. The encrypted transmission and authentication control ensure data security.

[0119] In this embodiment, the network layer protocol uses IPv6, which supports dynamic allocation of address pool cluster virtual IP. The transmission layer protocol uses two types of protocol combinations of UDP protocol and QUIC protocol, which are used for access control, cluster synchronization, CSI feedback control type message, and are flexibly selected according to the business scenario.

[0120] The task execution process is as follows:

[0121] In the digital aviation communication system based on the non-cellular network architecture, during the system operation, complex dynamic factors such as high-speed movement, multi-user concurrency, air-ground link rapid fading and sudden events are faced. In order to ensure the stable and reliable execution of the system communication task, a perfect task execution process needs to be designed. From the aspects of communication scheduling management, link detection and maintenance, fault tolerance switching and network recovery mechanism, the overall task execution framework is constructed to realize the ordered process of task level.

[0122] Task execution depends on the judgment and scheduling of the system on the current business type and resource state. First, the task sent by the terminal is identified and classified, which can be set as control type, real-time type, periodic type and emergency type. Each type of task sets the scheduling strategy according to parameters such as time delay, bandwidth, reliability and priority. The distributed scheduling strategy is to dynamically allocate time slots, spectrum and power resources according to task priority, user location and link state under the coordination of the central control unit. The scheduling adopts the cluster dynamic matching mechanism of "user center", which preferentially guarantees the transmission of high-priority tasks.

[0123] During the task execution, the system needs to monitor the communication link quality in real time to ensure the stability of task transmission. In the real-time link state detection mechanism, the user terminal periodically reports the channel state information, and the AP end monitors the signal reception quality. In the link poor early warning mechanism, the link quality threshold needs to be set, such as the signal-to-noise ratio and the packet loss rate requirement. Once the index is abnormal, the system enters the link protection state. The link quality feedback optimization mechanism needs to adjust the transmission power or switch the modulation mode in real time.

[0124] Fault tolerance and switching strategy should deal with sudden link interruption, node failure or abnormal interference. The system needs to have an efficient fault tolerance mechanism and switching strategy. The access point fast switching mechanism needs to be set. When the current main AP channel quality decreases, the system can enable the standby AP in advance. Each user terminal maintains a list of candidate APs, and tries to re-access in priority order. The cluster-level fault tolerance mechanism also needs to be set. When a cluster AP fails, the CPU triggers the "cluster reconstruction process" to quickly rebuild a small-scale cooperative cluster with adjacent APs to ensure uninterrupted task.

[0125] Network recovery mechanism is when the communication is interrupted or the network topology is abnormal, the system should be able to automatically identify and execute the network recovery process. The link or node failure is determined by the timeout of the periodic signal exchange between the node and the CPU or the civil aircraft. The reconnection strategy is that the local node performs fast link scanning, the CPU allocates a new access point list, and assists the UE to reconnect.

[0126] The design of the digital aviation communication system based on the non-cellular network architecture described in the embodiment has a working process as shown in Figure 6 Assuming that a civil aircraft carrying a non-cellular communication terminal takes off from an airport area with high-density AP deployment and cruises on a remote route with sparse APs, the specific working process includes the following steps:

[0127] S1, system initialization and position state acquisition;

[0128] Before communication, start the non-cellular communication terminal carried by the civil aircraft, activate the GNSS positioning module, and obtain the current three-dimensional geographic coordinates (x k ,y k ,z k ), flight speed vector, heading angle and initial wireless channel state in real time. All information is actively sent to the ground central processing unit via the uplink control channel as the basis for subsequent communication strategy judgment;

[0129] S2, region identification and flight path prediction;

[0130] The central processing unit receives the above information, immediately starts the area identification module, and judges the position of the current aircraft as an airport area or a route area according to the built-in airspace division database of the coordinate matching system; simultaneously, the track prediction algorithm is run, the path in the next 30 seconds is predicted based on the speed vector and the flight plan, and the AP topology database in the track coverage range is searched to construct a candidate access point list to provide prior information for subsequent cluster optimization;

[0131] S3, initial cluster construction and access establishment;

[0132] If the aircraft is in the airport area, the system activates the APs within a radius of 500 meters according to the preset strategy to form a high-density access cluster, and selects the node with the optimal channel quality near the tower as the main service AP. The aircraft terminal completes the initial synchronization, frequency offset correction and random access process with the AP, and establishes the data and control channel connection;

[0133] S4, dynamic cluster adjustment;

[0134] When the aircraft identifies that it has left the airport area and entered the route airspace, the central processing unit dynamically selects a number of distributed APs to form a user center cluster according to the route traffic heat map and the deployment rules in formula (1). The cluster construction follows the principles of "track priority and load balancing". The distance between APs is compressed to 5 kilometers in busy routes and extended to 50 kilometers in empty areas. The cluster members are adjusted in real time according to the heading of the aircraft to maintain the aircraft in a "multi-AP cooperative service" state and realize continuous coverage without boundaries.

[0135] S5, CSI feedback and link state monitoring;

[0136] The aircraft communication terminal and the APs in the cluster continuously perform link state interaction based on channel state information. The system sets a feedback period of 50-200 ms to monitor, including but not limited to, signal-to-noise ratio, bit error rate, throughput rate and delay jitter. If the feedback channel state is stable, the current cluster is maintained and data transmission continues. If the channel quality is found to have decreased below the set threshold, the system triggers the link protection logic.

[0137] S6, fault tolerance switching and cluster reconstruction;

[0138] When a link degradation or failure event occurs, the central processing unit quickly calls the current backup AP candidate pool and preferentially selects a new AP cluster with good channel conditions and low node load. The new cluster completes parameter negotiation, synchronous access and beam alignment process with the aircraft terminal through the control channel; adopts a master-backup cooperative mechanism, the current master AP keeps the task relay uninterrupted, and the backup AP automatically switches to the main service node after initialization; at the same time, the system starts the data buffering and context state synchronization mechanism to ensure business continuity and seamless switching during task migration.

[0139] S7, continue communication and task closed-loop management;

[0140] The system runs the above-mentioned prediction-monitoring-switching closed-loop control logic throughout the flight process, dynamically adapts to changes in aircraft motion state and environment, and maintains a stable air-ground communication link; after the aircraft enters the destination airport airspace, the system switches to the high-density access mode again, reactivates the airport area AP cluster, and completes the landing phase communication task, including task backhaul, operation report upload, and terminal disconnection; thus, a complete aviation communication task life cycle is completed.

[0141] Therefore, the present application adopts the above-mentioned digital aviation communication system design based on a non-cellular network architecture, introduces non-cellular network communication into the field of aviation communication, designs a distributed base station plan, constructs a time-varying channel model library for the aviation environment to improve the modeling accuracy of the high-altitude layer, and provides a more efficient design method for global / regional digital aviation communication systems, which not only makes up for the communication vulnerability caused by cell switching in the traditional cellular network communication scheme and provides an improvement idea for the industry's hard problem, but also gives a dynamic cluster base station selection method and a specific scheme for the design of software and hardware modules with higher feasibility and integrity. The task execution process considers various situations of aircraft high-altitude communication in combination with common aviation communication business, can meet the comprehensive demand of aircraft communication, and helps to promote the research, application and development process of aviation communication.

[0142] Finally, it should be noted that: the above examples are only used to illustrate the technical solutions of the present application and not to limit it, although the present application has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that: it can still modify or equivalently replace the technical solutions of the present application, and these modifications or equivalent replacements also cannot make the modified technical solutions deviate from the spirit and scope of the technical solutions of the present application.

Claims

1. A digital air communication system design based on a cell-free network architecture, characterized by: The application relates to an aviation communication system and a communication service execution module, wherein the aviation communication system comprises a network infrastructure and a communication link layer; the aviation communication system provides a bottom physical connection and a transmission environment support, including optimizing wireless channel characteristics, stabilizing communication link parameters and basic signal transmission capacity; the communication service execution module covers network protocol and information interaction and task execution processes, is responsible for coordination, control and process management of the aviation communication system, and defines and realizes protocol interaction rules between nodes.

2. The design of a digital aeronautical communication system based on a cell-free network architecture according to claim 1, characterized in that: The design of the network infrastructure comprises distributed low-power access point (AP) topology planning and software and hardware module design, and specifically comprises the following steps: The distributed AP topology planning in the cell-free network is introduced into the aviation communication system by considering the distributed AP distribution in the airport and air route operation; the aviation communication environment is divided into two types of airport area and air route area according to different deployment scenes, the airport area comprises a terminal building, a runway and a parking apron, a high-density AP deployment strategy is adopted to realize three-dimensional coverage; a dynamic density AP deployment mechanism is introduced into the topology construction of the air route area, the system dynamically adjusts the AP spacing by analyzing satellite flight path data and an aviation flow heat map of a flight schedule in real time; the efficiency of the distributed AP topology planning depends on the regulation and control of a three-dimensional coverage optimization model, the three-dimensional coverage optimization model establishes a three-element coupling relationship of AP transmission power, coverage radius and communication capacity demand; firstly, the path loss is calculated according to the terrain digital elevation data, and the path loss calculation model is as follows: PL = 32.4 + 20logf + 20logd (1); wherein f represents a signal frequency, d represents a transmission distance, and the AP power is dynamically adjusted according to the mountain shelter condition; secondly, the vertical coverage angle is optimized based on the flight altitude layer distribution to reduce signal space waste; finally, the optimal AP density is deduced reversely in combination with the traffic volume prediction to form a coverage parameter closed-loop calibration; the AP spacing algorithm is landed through dynamic configuration, and the spectrum utilization is improved under the premise of guaranteeing QoS; The software and hardware module design comprises ground AP software and hardware design and airborne terminal design, specifically as follows: the ground AP spacing is configured according to the communication demand by investigating the air route density, integrating the power and coverage range of the AP and integrating a large-scale antenna array for cooperation with the airborne terminal; the airborne terminal design is the design of airborne hardware for solving the physical layer misalignment problem caused by high-speed movement, the airborne hardware comprises a Doppler compensation module and a phased array antenna system; the phased array antenna system adopts a four-panel omnidirectional coverage scheme containing 64 array elements per panel, the direction of arrival is calculated in real time based on the aircraft attitude data and the AP position information, the beam reconstruction is completed through a gradient descent algorithm, and the beam pointing error is reduced at the cruising speed; the Doppler compensation module constructs a two-stage correction mechanism, the local oscillator frequency is dynamically adjusted through a phase-locked loop at the radio frequency stage to eliminate the carrier frequency offset; at the baseband stage, an extended orthogonal frequency division multiplexing technology is adopted to expand the subcarrier spacing to 75 kHz to resist the Doppler spread effect and reduce the inter-symbol interference power. The software-defined control plane is the central nerve of the cooperation of software and hardware modules, and the dynamic resource scheduling engine optimizes air interface resources in real time according to the AP density parameter of the topology planning, starts non-orthogonal multiple access in the high-density area of the airport, and serves multiple users through power domain multiplexing; in the remote sparse area, it is switched to large-scale MIMO single-user beam focusing to improve the edge coverage capability; considering the special development of the route sensing protocol stack, the flight plan data is converted into network configuration parameters, and when the aircraft enters the transoceanic route, the "energy saving mode" is automatically triggered to prolong the AP sleep cycle; and in the approach stage, the APs around the runway are pre-activated to form a communication bustling area.

3. The design of a digital aeronautical communication system based on a cell-free network architecture according to claim 1, characterized in that: The design of the communication link layer includes multi-dimensional channel modeling, channel coding design, and user-centered dynamic cluster generation. The channel modeling and channel coding design belong to the communication system design level, and the user-centered dynamic cluster generation belongs to the application of the cell-free network technology in the aviation communication system.

4. The design of a digital aeronautical communication system based on a cell-free network architecture according to claim 3, characterized in that: The multi-dimensional channel modeling process is as follows: Setting a cellular system contains M distributed AP, K air terminal users, each AP equipped with N AP Root antenna, air terminal users equipped with N UE Root antenna, all APs are connected with central processing unit CPU through front link, collaborative scheduling and data merging; all links are air-ground links, which need to consider three-dimensional propagation, multipath, Doppler and state changes; wherein, Hm k is a MIMO channel matrix between the mth AP and the kth end user; β mk is a large-scale fading between m APs and the kth end user, including path loss and shadow fading; G mk is a small-scale fading, using a Rician fading channel as a channel model, i.e., a Rician channel model; Large scale fading coefficient β mk The energy loss and shadowing effects due to the propagation at long distances are described and modeled as: where d mk is the three-dimensional spatial distance between the AP and the aerial terminal, assuming the coordinates of the mth AP are (x m , y m , z m ) and the coordinates of the kth aerial terminal are (x k , y k , z k ), d mk is given by the following equation: In formula (3), PL0 is a reference path loss per unit distance; γ is a path loss factor, and the value of the direct signal from the satellite to the terminal (LOS) is 2.1-2.7; is a shadow fading term, satisfying a lognormal distribution; d0 is a reference distance, usually 1 meter; The model of small-scale fading is modeled as follows: where K mk is the Rician factor, representing the ratio of direct path and scattered path power, the greater the value represents the direct link strength, the main proportion of the direct link; the smaller the value, the direct link strength is weak, the main proportion of the reflected link, the multipath effect is obvious; G LOS，mk represents the direct link, which is related to the angle of the antenna placement, determines the signal angle of arrival and departure, and in the single antenna scene, G LOS，mk = 1, in the multi-antenna communication scene, the signal departure angle of the sending end and the signal arrival angle of the receiving end are considered, at this time G LOS，mk = α T · α R , α T and α R are the vector vectors representing the angle; G NLOS，mk is the equivalent multipath link caused by reflection, diffraction, scattering and other reasons in addition to the direct link, and the equivalent multipath link after superposition of multiple reflected links; The high-speed movement of the flight platform will produce significant Doppler shift, affecting the frequency domain orthogonality, especially in OFDM systems, which manifests as inter-symbol interference, which needs to be paid special attention to in channel modeling, where v k is the instantaneous speed of air terminal user k; is the carrier wavelength; θ mk is the angle between the direction of motion of air terminal user k and the direction between it and the AP.

5. The design of a digital aeronautical communication system based on a cell-free network architecture according to claim 3, characterized in that: The channel coding adopts low-density parity-check code, i.e. LDPC code, and the design process is as follows: Adopting LDPC encoding, bit interleaving, QAM modulation structure, realizing high throughput and strong error correction performance good balance, LDPC specific encoding design model is by sparse check matrix H belongs to {0,1} (n-k)×n Definition, satisfy: H·c T = 0 (7); where c∈{0,1} n is the encoded codeword, k is the number of original information bits, n is the number of encoded bits, code rate The selected values include 1 / 2, 2 / 3, 3 / 4, the sparse check matrix is obtained through the PE algorithm, the check matrix construction is optimized in combination with the characteristics of the air-ground link, and short frames are used for high-speed aircraft users to reduce decoding delay. In the multi-AP aviation communication system, a locally generated partial check matrix is used for joint checking at the CPU end; joint channel state information feedback is used to realize link-level adaptive coding; In order to adapt to the multipath fading channel and improve the system anti-interference ability, the aviation communication system adopts bit interleaved coding modulation; Considering the drastic change of the air-ground link channel state, the aviation communication system integrates an adaptive coding and modulation mechanism, based on real-time SNR estimation and target QoS constraints, dynamically selects the coding and modulation combination to maximize the system throughput and keep the error rate below the threshold.

6. The design of a digital aeronautical communication system based on a cell-free network architecture according to claim 3, characterized in that: The user-centered dynamic cluster is adjusted in real time, and the cluster update period satisfies the following formula: where f D is the maximum Doppler shift, T coherence is the channel coherence time; the cluster structure is checked every 50-200 ms, and the frequent handover is reduced by combining with the prediction mechanism; the cluster update condition is set to be triggered; after the dynamic cluster structure of the user center is established, a number of APs in the cluster jointly complete the functions of beamforming, power control, and user scheduling, and a centralized and edge collaborative processing architecture is used to realize the collaborative control of cluster services.

7. The design of a digital aeronautical communication system based on a cell-free network architecture according to claim 1, characterized in that: The communication service execution includes network protocol and information interaction as well as task execution process; information interaction is divided into control signaling interaction and service data interaction; control signaling interaction is used for link establishment and maintenance, and service data interaction includes uplink and downlink transmission of aviation services; control signaling interaction includes terminal access request, access point response, cluster maintenance command, channel state report and modulation coding feedback; service data interaction includes task request, task scheduling notification, data fragmentation and recombination mechanism, encrypted transmission and authentication control.

8. The design of a digital aeronautical communication system based on a cell-free network architecture according to claim 7, characterized in that: The task execution process includes communication scheduling management, link detection and maintenance, fault tolerance switching and network recovery mechanism.

9. A digital aeronautical communication system design workflow based on a cell-free network architecture according to any one of claims 1-8, characterized in that, The steps include: S1, system initialization and position state acquisition; Before communication, the non-cellular communication terminal carried by the civil aviation passenger plane is started, the GNSS positioning module is activated, the current three-dimensional geographic coordinates (x k ,y k ,z k ), flight speed vector, heading angle and initial wireless channel state are obtained in real time, all information is actively sent to the ground central processing unit via the uplink control channel as the basis for subsequent communication strategy judgment; S2, region identification and flight path prediction; After receiving the above information, the central processing unit immediately starts the region identification module, matches the built-in airspace division database according to the coordinates, judges the current position of the aircraft as an airport area or a route area, synchronously runs the flight path prediction algorithm, estimates the path in the next 30 seconds based on the speed vector and flight plan, and retrieves the AP topology database in the flight path coverage range to construct a candidate access point list, providing prior information for subsequent cluster optimization; S3, initial cluster construction and access establishment; If the aircraft is in the airport area, the system activates the APs within a radius of 500 meters to form a high-density access cluster according to the preset strategy, and selects the node with the best channel quality near the tower as the main service AP. The aircraft terminal completes the initial synchronization, frequency offset correction, and random access process with the AP to establish data and control channel connection. S4, dynamic cluster adjustment; When the aircraft identifies that it has left the airport area and entered the air route space, the central processing unit dynamically selects a number of distributed APs to form a user center cluster according to the air route traffic heat map and the deployment rules in formula (1). The cluster construction follows the principles of "flight path priority and load balancing". The AP spacing is compressed to 5 kilometers on busy routes and extended to 50 kilometers in open areas. The cluster members are adjusted in real time according to the aircraft's heading to maintain the aircraft in a "multi-AP coordinated service" state and achieve continuous coverage without boundaries. S5, CSI feedback and link state monitoring; The aircraft communication terminal and the APs in the cluster continuously interact based on channel state information link state. The system sets a feedback period of 50-200 ms to monitor signal-to-noise ratio, bit error rate, throughput rate, and delay jitter. If the feedback channel state is stable, the current cluster is maintained and data transmission continues. If the channel quality is found to have dropped below the set threshold, the system triggers the link protection logic. S6, fault tolerance switching and cluster reconstruction; When link degradation or failure occurs, the central processing unit quickly calls the current backup AP candidate pool and preferentially selects a new AP cluster with good channel conditions and low node load. The new cluster completes parameter negotiation, synchronization access, and beam alignment with the aircraft terminal through the control channel. The current main AP remains uninterrupted, and the standby AP automatically switches to the main service node after initialization. At the same time, the system starts data buffering and context state synchronization mechanism to ensure business continuity and seamless switching during task migration. S7, continuous communication and task closed-loop management; The system continuously runs the above-mentioned prediction-monitoring-switching closed-loop control logic throughout the flight process, dynamically adapts to changes in aircraft motion state and environment, and maintains a stable air-ground communication link. When the aircraft enters the destination airport airspace, the system switches to the high-density access mode again, reactivates the airport area AP cluster, and completes the landing phase communication tasks, including task return, operation report upload, and terminal disconnection.

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