Asynchronous code division multiple access underwater wireless optical communication system multi-user access method based on optical wireless access point direction self-adaption

Abstract

The invention discloses an asynchronous code division multiple access underwater wireless optical communication system multi-user access method based on optical wireless access point direction self-adaption, and belongs to the technical field of wireless optical communication. According to the method, user azimuth angle estimation and communication are synchronously realized at an OAP end by utilizing a detector array, and a receiving end direction self-adaptive mechanism is implemented based on multi-user detection and arrival angle information so as to maximize the system capacity. And a multi-user access control strategy is formed by further combining a dynamic code word distribution protocol. According to the method, the difficulty in underwater clock synchronization is overcome, the throughput, the number of users and the fairness are improved by optimizing the OAP space orientation, and an effective solution is provided for practical application of UOWC multi-user communication.

Description

Technical Field

[0001] This invention belongs to the field of wireless optical communication technology, specifically relating to a multi-user access method for an asynchronous code division multiple access (CDMA) underwater optical communication (UOWC) system based on optical access point (OAP) directional adaptation. Background Technology

[0002] Underwater wireless optical communication (UOWC) leverages the low attenuation of blue-green light waves in water, offering advantages such as high transmission rates, large bandwidth, low latency, and strong resistance to electromagnetic interference. It is a crucial technology for achieving high-speed, real-time underwater information transmission. This technology has demonstrated immense potential in underwater sensor networks, unmanned underwater vehicle collaboration, and marine environmental monitoring, and is considered a key support for building future underwater Internet of Things (IoT) and intelligent marine communication networks.

[0003] Currently, research and applications of UOWC (Underwater Over-the-Air) communication are mostly focused on point-to-point single-user communication scenarios. However, with the widespread deployment of various underwater devices such as underwater vehicles, sensor nodes, and relay equipment, achieving efficient and reliable multi-user communication and networking among multiple nodes has become an urgent need. Factors such as the strong directionality of optical signals, the significant time-varying and space-varying characteristics of underwater channels, and the difficulty in precise node synchronization pose serious challenges to multi-user access and networking in UOWC. Among these, issues such as interference between users, limited link budgets, and user fairness are particularly prominent.

[0004] To support concurrent multi-user communication, Code Division Multiple Access (CDMA) technology has been introduced into asynchronous Unified Overhead Cable (UOWC) systems due to its strong anti-interference capabilities and lack of strict timing synchronization requirements. Existing research primarily manages interference and optimizes performance through power control at the user end; however, this approach is typically complex and difficult to implement in dynamic underwater environments, lacking flexibility. Furthermore, UOWC optical transceivers generally exhibit strong directionality, and their spatial gain characteristics have not been fully utilized in system-level access control strategies. How to effectively utilize spatial degrees of freedom at the Access Point (OAP) by intelligently adjusting beam or array orientation to suppress interference while serving more users and improving overall system capacity and robustness is a pressing issue that needs to be addressed.

[0005] Therefore, it is necessary to design a cross-layer optimization scheme that can integrate physical layer spatial information and media access control layer strategies to solve the multi-user access problem in asynchronous CDMA-UOWC systems more efficiently and feasiblely, and promote the advancement of UOWC technology towards practical network applications. Summary of the Invention

[0006] To address the aforementioned technical problems, this invention provides a multi-user access method for an asynchronous code division multiple access (OAP) underwater wireless optical communication system based on OAP direction adaptation. This method utilizes a detector array at the OAP end to synchronously estimate user azimuth angles and facilitate communication. Based on multi-user detection and angle-of-arrival information, a receiver-end direction adaptation mechanism is implemented to maximize system capacity. Furthermore, a dynamic codeword allocation protocol is incorporated to form a multi-user access control strategy.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] A multi-user access method for an asynchronous code division multiple access underwater wireless optical communication system based on optical wireless access point direction adaptation, the method comprising:

[0009] Step 1: Detect and parse the access request frames sent by the users, and estimate the angle of arrival of each user signal based on the signals received by the detector array;

[0010] Step 2: Based on the currently received signal strength, the estimated angle of arrival, and the detector's directional gain function, predict the signal-to-interference-plus-noise ratio (SIR) of each user under different spatial orientations at the optical wireless access point.

[0011] Step 3: Based on the prediction results of Step 2, determine the feasible solution space and the optimal spatial orientation of the optical wireless access point based on the dual optimization objectives.

[0012] Step 4: Determine whether the predicted signal-to-interference-plus-noise ratio (SINNR) of all connected users and requesting users meets the preset quality of service (QoS) index under the optimal spatial orientation; if it does, adjust the optical wireless access point to the optimal spatial orientation and send an access permission frame and allocate a dedicated communication codeword to the requesting user; otherwise, send an access rejection frame.

[0013] Furthermore, step 1 specifically includes: independently sampling the output signal of each avalanche photodiode channel in the photodetector array using analog-to-digital conversion; performing equal-gain combining of the signals from all channels to complete symbol synchronization and generating corresponding multi-user detector coefficients; based on the detector coefficients, performing asynchronous decorrelation multi-user detection on the data of each channel to obtain the symbol detection quantity for each user in each channel; when an access request frame is detected, using the vector formed by the symbol detection quantities as samples, and employing a multi-signal classification algorithm based on subspace decomposition to estimate the angle of arrival of each user's signal.

[0014] Furthermore, step 2 specifically includes: obtaining the signal amplitude estimate and interference variance estimate of each user from the multi-user detection output of each channel using the maximum likelihood estimation method; constructing a system of linear equations with the interference factor and noise power between users as unknowns based on the amplitude and variance data, and solving it using sparsity constraints to obtain the interference factor matrix and noise power in the current direction; and predicting the received signal strength and corresponding signal-to-interference-plus-noise ratio of each user in the target space orientation by combining the interference factor, noise power, estimated angle of arrival and detector directional gain function.

[0015] Furthermore, in the process of predicting the signal-to-interference-plus-noise ratio (SINR), the prediction of the received signal strength is based on the following model: the received optical power gain is modeled as the product of path loss and detector directional gain; when the optical wireless access point turns, assuming that the path loss remains unchanged, the predicted value of the user signal amplitude output by each channel in the target orientation is calculated by using the signal amplitude in the current direction and the detector directional gain function.

[0016] Furthermore, in step 3, the dual optimization objectives include a first optimization objective, which is mathematically expressed as: traversing all feasible spatial orientations of the optical wireless access point to find the set of orientations that maximizes the number of users whose predicted signal-to-interference-plus-noise ratio is not lower than the service quality threshold, and the set constitutes the feasible solution space.

[0017] Furthermore, in step 3, the dual optimization objective includes a second optimization objective, which is mathematically expressed as: in the feasible solution space, find the direction that maximizes the lowest prediction signal-to-interference-plus-noise ratio among all users, as the optimal spatial orientation.

[0018] Furthermore, in step 4, the physical channel is divided into a common access channel and multiple independent data channels; all users share the access channel and compete to send access requests using a predefined common address code; each user granted access is assigned a dedicated address code and communicates in the designated data channel.

[0019] Furthermore, the method is applicable to scenarios where the user remains stationary during the access process; when the optical wireless access point receives an access request frame, it only executes the angle of arrival estimation, signal-to-interference-plus-noise ratio prediction, and direction optimization process if it has an idle data channel; otherwise, it directly sends an access rejection frame to the user.

[0020] In a second aspect, the present invention provides an electronic device, comprising: one or more processors; and a memory for storing one or more programs; wherein, when the one or more programs are executed by the one or more processors, the one or more processors implement the aforementioned multi-user access method for an asynchronous code division multiple access underwater wireless optical communication system based on optical wireless access point direction adaptation.

[0021] Thirdly, the present invention provides a computer-readable storage medium having executable instructions stored thereon, which, when executed by a processor, enable the processor to implement the aforementioned multi-user access method for an asynchronous code division multiple access underwater wireless optical communication system based on optical wireless access point direction adaptation.

[0022] The beneficial effects of this invention are as follows:

[0023] Increase system capacity and number of users: By utilizing the spatial orientation freedom of the optical wireless access point (OAP) to dynamically adjust the orientation of the receiving array, the system can maximize the number of users served simultaneously while meeting the Quality of Service (QoS), thereby significantly improving the overall system throughput and access capacity.

[0024] Enhancing user fairness and communication robustness: A two-stage optimization strategy is adopted (first maximizing the number of users that meet QoS, and then optimizing the signal-to-interference-plus-noise ratio of the worst user in the solution space using the maximum-minimum criterion), which effectively guarantees the communication quality of all access users and improves the fairness and anti-interference capability of the system in dynamic underwater environments.

[0025] Reducing system implementation complexity: Traditional solutions rely on power control at the user end to suppress multi-user interference, which is complex and limited by the power consumption and processing capabilities of the underwater node. This invention shifts the optimization focus to the OAP (Output Assistive Terminal) end, utilizing its strong signal processing and beam control capabilities to reduce the burden on the user end and the overall system control complexity.

[0026] Achieving cross-layer optimization and efficient access control: This paper proposes a complete cross-layer access control strategy by deeply integrating user azimuth estimation and signal-to-interference-plus-noise ratio (SIR / NOT) prediction from the physical layer with dynamic codeword allocation and access decision from the media access control layer. This strategy supports user contention for access and intelligent OAP scheduling without requiring global clock synchronization, thereby improving channel utilization and access efficiency.

[0027] Improving technical feasibility and application potential: The proposed method is based on the actual array detector structure and asynchronous CDMA detection mechanism. It achieves direction estimation and interference management through digital signal processing, which enhances the feasibility and engineering application value of the scheme in actual underwater scenarios and is of great significance for promoting the networked deployment of UOWC. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of a multi-user access method for an asynchronous code division multiple access underwater wireless optical communication system based on optical wireless access point direction adaptation, according to the present invention.

[0029] Figure 2 This is a schematic diagram of a photodetector array structure and coordinate system according to an embodiment of the present invention, wherein (a) is a top view of the array structure, (b) is a left view of the array structure, and (c) is the coordinate system;

[0030] Figure 3 This is a block diagram of the digital signal processing proposed in this invention;

[0031] Figure 4 This is the symbol detection matrix obtained by performing signal detection on each APD output in this invention;

[0032] Figure 5 This is a flowchart illustrating the access behavior of the user terminal and the optical wireless access point proposed in this invention. Detailed Implementation

[0033] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0034] like Figure 1 As shown, this invention provides a multi-user access method for an asynchronous code division multiple access underwater wireless optical communication system based on optical wireless access point direction adaptation. The method includes:

[0035] Step 1: Detect and parse the access request frames sent by the user, and estimate the angle of arrival (AoA) of each user signal based on the signals received by the detector array.

[0036] Step 2: Based on the currently received signal strength, the estimated angle of arrival, and the detector directional gain function, predict the signal-to-interference-plus-noise ratio (SINR) of each user under different spatial orientations of the optical wireless access point (OAP).

[0037] Step 3: Based on the prediction results of Step 2, determine the feasible solution space with the first optimization objective of maximizing the number of users that meet the preset Quality of Service (QoS) indicators; then, within the feasible solution space, determine the optimal spatial orientation of the optical wireless access point with the second optimization objective of maximizing the worst signal-to-interference-plus-noise ratio among all users.

[0038] Step 4: Determine whether the predicted signal-to-interference-plus-noise ratio (SINNR) of all connected users and requesting users meets the preset quality of service (QoS) index under the optimal spatial orientation; if it does, adjust the optical wireless access point to the optimal spatial orientation and send an access permission frame and allocate a dedicated communication codeword to the requesting user; otherwise, send an access rejection frame.

[0039] In step 1, such as Figure 2 As shown, under this array geometry, there are a total of M=5 avalanche photodiode (APD) detection planes with different tilt angles. Figure 2 (a) and (b) are the top view and left view of the structure, respectively. The following diagram shows how the structure is constructed. Figure 2 The coordinate system is shown in (c). Based on the array structure, each APD channel is sampled using a separate ADC, so one APD corresponds to one channel.

[0040] Figure 3 This invention demonstrates the digital signal processing procedure from receiving optical signals to obtaining communication data and user AoA. M-channel APDs receive optical signals, which are then sampled by an ADC to obtain M data streams. These streams are then combined using equal-gain combining to obtain a combined signal. Symbol synchronization is performed on the combined signal to obtain the relative time delay of user symbols. Multi-User Detector (MUD) coefficients are generated using the symbol time delay, and decorrelation multi-user detection is performed on the output signal of each APD to obtain... Figure 4 The detection matrix shown, in which Let be the symbol detection quantity for user k in the m-th channel. This is a vector composed of the detection values ​​of user k in each channel. The Multiple Signal Classification (MUSIC) algorithm is used to classify... As a sample vector, AoA estimation can be performed for user k. In intensity-based detection, the response of the received optical power is:

[0041] ,

[0042] in, Let m be the direction vector from the light source of user k to the APD numbered m. Let m be the normal vector of the APD receiving plane. The superscript T indicates transpose, using N consecutive... Construct the sampling covariance matrix and decompose it into signal subspaces. and noise subspace The spatial spectral function can be expressed as:

[0043] ,

[0044] in, The guide vector is used. Discrete sampling is performed on the spatial region of interest to obtain candidate direction vectors. Using the spatial spectral function, traversing the candidate direction vectors yields the spatial distribution of spectral values, where the spectral peaks correspond to... Let AoA be the value of user k.

[0045] In step 2, the purpose of SINR prediction is to predict the magnitude of the SINR of the OAP in the target direction. In asynchronous CDMA decorrelation detection, the expression for SINR is:

[0046] ,

[0047] in, The amplitude of the signal output by user j from MUD channel m. Let j be the interference factor generated by user j on user k. The noise power of the APD. To select a vector, This is the inverse of the user correlation matrix constructed for the local spreading code. To predict the user's SINR under the new OAP direction, the problem is transformed into estimating the interference factor, noise power, and signal amplitude.

[0048] The interference factor and noise power estimation are performed as follows: First, maximum likelihood estimation is performed for all users and all channels to obtain the mean of the MUD output. and variance These are respectively used as the signal term and the interference term. The following system of linear equations is constructed:

[0049] ,

[0050] Among them, the observation vector The interference vector of each user on each channel The vector to be determined is formed by stacking. The interference factor vector of user k is given by System matrix It has a blocky structure:

[0051] ,

[0052] The (m,j)th element is Column vector The (j)th element is When the difference in received power among users is small, most interference can be effectively eliminated, therefore most elements of the vector to be found are close to 0. Modeling this problem as a LASSO problem and using a standard quadratic programming algorithm can yield an estimate of the vector to be found. .

[0053] Ignoring scattering, the received optical power gain Modeled as the product of path loss and APD directional gain:

[0054] ,

[0055] in, For path loss, The directional gain of the APD. Let be the angle between the direction vector of user k and the normal vector of APD numbered m. The amplitude of the received signal output by channel m to user k is given in the current direction. When the array rotates, since the optical path remains constant, the amplitude of the received signal output by APD m in the new direction is predicted. for:

[0056] ,

[0057] At this point, the interference factor estimate will be... Noise estimates and Substituting the values ​​into the SINR expression can predict the SINR value in the new direction.

[0058] In step 3: assuming the use of Figure 2 The APD labeled 1 communicates to find the optimal direction of OAP with the goal of maximizing total throughput. In a CDMA system with fixed rate and codeword length, this optimization objective is equivalent to maximizing the number of users that meet QoS metrics. In this system, the SINR threshold is used as the QoS metric. To enhance robustness, user number optimization is performed first, followed by SINR optimization.

[0059] Among them, user number optimization: maximizing the satisfaction The number of users, i.e.:

[0060] ,

[0061] in, for Figure 2 The OAP direction in the coordinate system is SINR of user k at time The SINR threshold, This represents the feasible solution space consisting of all OAP directions that can support the maximum number of users. Further filtering is performed to avoid instability caused by the optimal solution being taken at the boundary of the set.

[0062] SINR optimization: in the feasible solution space of the first-level optimization To further optimize SINR and enhance system robustness, the Max-Min criterion is adopted, namely:

[0063]

[0064] Through this two-stage optimization strategy, the system first ensures the maximization of access capacity and selects the most reliable solution, reserving the largest fading margin for channel fluctuations, thereby significantly enhancing the stability and anti-interference capability of the link.

[0065] In step 4: In asynchronous CDMA-UOWC, the channel is divided using spreading codes. In this invention, the physical channel is divided into an access channel and a data channel. All users share a single access channel, using a predefined common address code to send access requests. The data channel ensures that users can access different channels without conflict. This invention is applicable to scenarios where users remain stationary during the access process. Figure 5 This invention demonstrates the access behavior of users and the OAP designed in this invention. Users initiate access requests on the access channel through contention. The OAP needs to make an admission decision based on the availability of the data channel and the SINR prediction result, and sends a dedicated address code on the downlink channel via a feedback frame. When a user has access needs, it enters a request state, constructs an access frame, sends the request, and waits for feedback. If a timeout occurs, it retransmits. After receiving the feedback frame, the user extracts the allocated address code, switches to the corresponding data channel to upload data, until it actively releases the request or is terminated by the OAP. The OAP simultaneously listens to all channels. When the access channel detects an access frame, if there is currently an idle physical channel, the OAP will estimate the AoA of the user seeking access and the users already connected, and predict the optimal direction and the corresponding SINR; otherwise, it will directly send a rejection feedback. This invention ensures that the addition of a user seeking access does not affect the communication quality of already connected users. Let there be a total of Ka connected users. When the predicted SINR satisfies:

[0066] ,

[0067] The OAP rotates to the optimal orientation and sends a confirmation. Otherwise, the OAP will reject the user's request for access.

[0068] In summary, the asynchronous CDMA underwater wireless optical communication multi-user access control method proposed in this invention, based on optical wireless access point orientation adaptation, deeply integrates physical layer spatial resource utilization with MAC layer access control through a cross-layer optimization strategy. This effectively solves the problems of strong inter-user interference, limited access capacity, and poor fairness in traditional underwater optical communication multi-user systems. This method utilizes a receiver detector array to achieve high-precision estimation of user orientation and intelligently adjusts the OAP spatial orientation based on this. Without requiring strict clock synchronization, it maximizes system throughput and the number of access users while ensuring communication fairness and link robustness among users. Compared to traditional schemes that rely on user-end power control, this invention concentrates complexity on the more capable access point side, making it more feasible and practical in engineering.

[0069] In a second aspect, the present invention provides an electronic device, comprising: one or more processors; and a memory for storing one or more programs; wherein, when the one or more programs are executed by the one or more processors, the one or more processors implement the aforementioned multi-user access method for an asynchronous code division multiple access underwater wireless optical communication system based on optical wireless access point direction adaptation.

[0070] Thirdly, the present invention provides a computer-readable storage medium having executable instructions stored thereon, which, when executed by a processor, enable the processor to implement the aforementioned multi-user access method for an asynchronous code division multiple access underwater wireless optical communication system based on optical wireless access point direction adaptation.

[0071] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above descriptions are merely specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A multi-user access method for an asynchronous code division multiple access underwater wireless optical communication system based on optical wireless access point direction adaptation, characterized in that, The method includes: Step 1: Detect and parse the access request frames sent by the users, and estimate the angle of arrival of each user signal based on the signals received by the detector array; Step 2: Based on the currently received signal strength, estimated angle of arrival, and detector directional gain function, predict the signal-to-interference-plus-noise ratio (SIR) of each user under different spatial orientations at the optical wireless access point; Step 3: Based on the prediction results of Step 2, determine the feasible solution space and the optimal spatial orientation of the optical wireless access point based on the dual optimization objective; Step 4: Determine whether the predicted signal-to-interference-plus-noise ratio (SINNR) of all connected users and requesting users meets the preset quality of service (QoS) index under the optimal spatial orientation; if it does, adjust the optical wireless access point to the optimal spatial orientation and send an access permission frame and allocate a dedicated communication codeword to the requesting user; otherwise, send an access rejection frame.

2. The multi-user access method for an asynchronous code division multiple access underwater wireless optical communication system based on optical wireless access point direction adaptation as described in claim 1, characterized in that, Step 1 specifically includes: performing independent analog-to-digital conversion sampling on the output signal of each avalanche photodiode channel in the photodetector array; performing equal-gain combining on the signals of all channels to complete symbol synchronization and generating corresponding multi-user detector coefficients; based on the detector coefficients, performing asynchronous decorrelation multi-user detection on the data of each channel to obtain the symbol detection quantity of each user in each channel; when an access request frame is detected, using the vector formed by the symbol detection quantities as samples, and employing a multi-signal classification algorithm based on subspace decomposition to estimate the angle of arrival of each user's signal.

3. The multi-user access method for an asynchronous code division multiple access underwater wireless optical communication system based on optical wireless access point direction adaptation as described in claim 2, characterized in that, Step 2 specifically includes: obtaining the signal amplitude estimate and interference variance estimate of each user from the multi-user detection output of each channel using the maximum likelihood estimation method; constructing a system of linear equations with the interference factor and noise power between users as unknowns based on the amplitude and variance data, and solving it using sparsity constraints to obtain the interference factor matrix and noise power in the current direction; and predicting the received signal strength and corresponding signal-to-interference-plus-noise ratio of each user in the target space orientation by combining the interference factor, noise power, estimated angle of arrival and detector directional gain function.

4. The multi-user access method for an asynchronous code division multiple access underwater wireless optical communication system based on optical wireless access point direction adaptation as described in claim 3, characterized in that, In the process of predicting the signal-to-interference-plus-noise ratio (SINR), the prediction of the received signal strength is based on the following model: the received optical power gain is modeled as the product of path loss and detector directional gain; when the optical wireless access point turns, assuming that the path loss remains unchanged, the predicted value of the user signal amplitude output by each channel in the target orientation is calculated by using the signal amplitude in the current direction and the detector directional gain function.

5. A multi-user access method for an asynchronous code division multiple access underwater wireless optical communication system based on optical wireless access point direction adaptation, as described in claim 1, is characterized in that... In step 3, the dual optimization objectives include a first optimization objective, which is mathematically expressed as: traversing all feasible spatial orientations of the optical wireless access point to find the set of orientations that maximizes the number of users whose predicted signal-to-interference-plus-noise ratio is not lower than the service quality threshold, and the set constitutes the feasible solution space.

6. The multi-user access method for an asynchronous code division multiple access underwater wireless optical communication system based on optical wireless access point direction adaptation as described in claim 1, characterized in that, In step 3, the dual optimization objective includes a second optimization objective, which is mathematically expressed as: in the feasible solution space, find the direction that maximizes the lowest prediction signal-to-interference-plus-noise ratio among all users, as the optimal spatial orientation.

7. A multi-user access method for an asynchronous code division multiple access underwater wireless optical communication system based on optical wireless access point direction adaptation, as described in claim 1, is characterized in that... In step 4, the physical channel is divided into a common access channel and multiple independent data channels; all users share the access channel and compete to send access requests using a predefined common address code; each user who is granted access is assigned a dedicated address code and communicates in the designated data channel.

8. A multi-user access method for an asynchronous code division multiple access underwater wireless optical communication system based on optical wireless access point direction adaptation, as described in claim 1, is characterized in that... The method is applicable to scenarios where the user remains stationary during the access process; when the optical wireless access point receives an access request frame, the angle of arrival estimation, signal-to-interference-plus-noise ratio prediction, and direction optimization process are only executed if there is an idle data channel. Otherwise, directly notify the user that the access frame is rejected.

9. An electronic device, characterized in that, include: One or more processors; Memory, used to store one or more programs; When one or more programs are executed by the one or more processors, the one or more processors implement the multi-user access method for an asynchronous code division multiple access underwater wireless optical communication system based on optical wireless access point direction adaptation as described in any one of claims 1-8.

10. A computer-readable storage medium, characterized in that, It stores executable instructions that, when executed by a processor, enable the processor to implement the multi-user access method for an asynchronous code division multiple access underwater wireless optical communication system based on optical wireless access point direction adaptation, as described in any one of claims 1-8.

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