阅读说明：本技术 一种用于逆向调制无线光通信的信号判决方法及通信系统 (Signal judgment method and communication system for reverse modulation wireless optical communication ) 是由 徐智勇 方晓东 汪井源 李建华 赵继勇 戚艾林 沈荟萍 于 2021-06-09 设计创作，主要内容包括：本发明公开了无线通信领域的一种用于逆向调制无线光通信的信号判决方法及通信系统,包括：将接收到的逆向调制光信号分聚类窗口处理,逐个窗口通过新型自适应聚类算法进行聚类判决。聚类判决步骤包括：分别计算窗口内所有信号数据点到自适应二聚类算法随机初始化的两个聚类中心的距离；每个信号数据点根据就近原则进行分类,从而得到两个类,并通过计算获取两个类的聚类中心；将当前的两个聚类中心作为初始化的两个聚类中心重复上述步骤,反复进行迭代直至聚类中心最终收敛,输出聚类判决结果。本发明能够通过学习接收到比特的内在特征,并依据其相似性动态自适应地对该窗口内的比特进行分类,从而在一定程度上降低湍流对信号所产生的影响。(The invention discloses a signal judgment method and a communication system for reverse modulation wireless optical communication in the field of wireless communication, comprising the following steps: and (3) clustering the received reverse modulation optical signals by windows, and carrying out clustering judgment on the windows one by one through a novel self-adaptive clustering algorithm. The clustering judgment step comprises the following steps: respectively calculating the distances from all signal data points in the window to two clustering centers randomly initialized by the self-adaptive clustering algorithm; classifying each signal data point according to a proximity principle so as to obtain two classes, and calculating to obtain clustering centers of the two classes; and repeating the steps by taking the current two clustering centers as the initialized two clustering centers, repeating iteration until the clustering centers are finally converged, and outputting a clustering judgment result. The invention can reduce the influence of turbulence on the signal to a certain extent by learning the intrinsic characteristics of the received bits and dynamically and adaptively classifying the bits in the window according to the similarity of the bits.)

1. A signal decision method for reverse modulation wireless optical communication, comprising:

the method comprises the steps of classifying received signals of the wireless optical communication into a window;

performing clustering judgment on the signals in the clustering window one by one through a self-adaptive clustering algorithm;

wherein the clustering decision comprises:

initializing a clustering center:

taking all signal data in each clustering window as a data point set to form a data set;

initializing N clustering centers by taking a data set and a preset clustering number N as input;

outputting clustering judgment:

calculating the Euclidean distance from each data point to each cluster center;

classifying each data point based on Euclidean distance according to a proximity principle to obtain N classes;

recalculating and updating the clustering center of each class by averaging;

repeating the steps based on the updated clustering center until the clustering center is finally converged;

and outputting the clustering judgment result of the corresponding clustering window.

2. The signal decision method according to claim 1, wherein the predetermined number N of clusters is 2.

3. The signal decision method as claimed in claim 1, wherein the received signal of the wireless optical communication is a bit stream signal, and the bit stream signal is a binary signal consisting of 0 and 1.

4. The signal decision method as claimed in claim 3, wherein the clustering window is a bit stream signal with a preset length.

5. The signal decision method for reverse modulation wireless optical communication according to claim 1, wherein the adaptive clustering algorithm employs a K-means algorithm.

6. The signal decision method according to claim 1, wherein the updating the cluster center of each class by averaging recalculation comprises:

wherein m is_jCluster center, n, representing current class j_jIndicates the number of data points in the current class j, y_jRepresenting a data point in the current class j, j ∈ N.

7. The signal decision method for backward modulated wireless optical communication according to claim 6, wherein the final convergence of the cluster center comprises:

when the data points with the highest similarity are classified into a class, the value of the loss function is minimum, and the clustering center is converged; wherein the formula of the loss function is as follows:

wherein the content of the first and second substances,C_jrepresenting class j, n is the number of iterations.

8. A communication system for inverse modulation wireless optical communication, the communication system comprising an interrogation terminal, a modulation terminal and an air channel, the interrogation terminal comprising a receiver and a transmitter for signal reception and transmission, the modulation terminal comprising an inverse modulator for signal modulation;

the transmitter transmits a signal to the reverse modulator through the atmosphere channel, and then the signal is transmitted back to the receiver through the atmosphere channel; the receiver performs decision processing on the received signal according to the signal decision method of any one of claims 1 to 7.

Technical Field

The invention relates to a signal judgment method and a communication system for reverse modulation wireless optical communication, belonging to the technical field of wireless optical communication.

Background

Compared with optical fiber communication, the traditional free space optical communication (FSO) has the advantages of flexibility and mobility by using the atmosphere as a transmission medium without laying an optical cable in advance. However, the conventional wireless optical communication needs to symmetrically load the laser transmitting/receiving system at both transmitting and receiving ends and to be equipped with a complicated tracking and aiming system (PTA), which greatly increases the weight, volume, power consumption and technical complexity of the system. The reverse Modulation (MRR) wireless optical communication technology only requires single-ended alignment, so that the wireless optical communication technology has the advantages of light weight, small volume, low energy consumption, high cost efficiency and the like, is favored by communication researchers at home and abroad once being proposed, becomes an important direction for communication field research, and has a wide application prospect.

However, the wireless optical communication channel has a non-stationary characteristic due to the combined effect of energy attenuation of the atmosphere and the effect of atmospheric turbulence. Compared with the traditional optical communication unidirectional link, the laser in the reverse modulation optical communication system passes through the atmosphere channel which returns twice (a forward link and a backward link), the influence on the laser is more serious, the fluctuation of the received signal is larger, and the jitter is more obvious. In addition, due to device limitations and the like, the transmission rate and communication distance of MRR are severely restricted.

Under the condition of no turbulence influence, the judgment can be based on a fixed threshold, but under the condition of atmospheric turbulence, the received signal waveform is seriously distorted due to the influence of turbulence, the detection error of the traditional hard judgment method is higher in probability, and the detection performance based on the fixed threshold is not optimal. In order to optimize the detection performance, the threshold level needs to be varied with the magnitude of incident light irradiance and noise, i.e., to be adaptive; in order to solve the above problem, the present application provides a signal decision method and a communication system for reverse modulation wireless optical communication.

Disclosure of Invention

The invention aims to overcome the defects in the prior art and provides a signal judgment method and a communication system for reverse modulation wireless optical communication, thereby effectively overcoming the influence of atmospheric turbulence on the performance of wireless optical communication and improving the performance of a wireless optical communication system; the technical problem that the waveform of a received signal is seriously distorted under the condition of atmospheric turbulence and under the influence of turbulence is solved.

In order to achieve the purpose, the invention is realized by adopting the following technical scheme:

in a first aspect, the present invention provides a signal decision method for inverse modulation wireless optical communication, including:

the method comprises the steps of classifying received signals of the wireless optical communication into a window;

performing clustering judgment on the signals in the clustering window one by one through a self-adaptive clustering algorithm;

wherein the clustering decision comprises:

initializing a clustering center:

taking all signal data in each clustering window as a data point set to form a data set;

initializing N clustering centers by taking a data set and a preset clustering number N as input;

outputting clustering judgment:

calculating the Euclidean distance from each data point to each cluster center;

classifying each data point based on Euclidean distance according to a proximity principle to obtain N classes;

recalculating and updating the clustering center of each class by averaging;

repeating the steps based on the updated clustering center until the clustering center is finally converged;

and outputting the clustering judgment result of the corresponding clustering window.

Preferably, the preset number N of clusters is 2.

Preferably, the received signal of the wireless optical communication is a bit stream signal, and the bit stream signal is a binary signal composed of 0 and 1.

Preferably, the clustering window is a bitstream signal of a preset length.

Preferably, the adaptive clustering algorithm adopts a K-means algorithm.

Preferably, the updating the cluster center of each class by averaging recalculation includes:

wherein m is_jCluster center, n, representing current class j_jIndicates the number of data points in the current class j, y_jRepresenting a data point in the current class j, j ∈ N.

Preferably, the final convergence of the cluster center includes:

when the data points with the highest similarity are classified into a class, the value of the loss function is minimum, and the clustering center is converged; wherein the formula of the loss function is as follows:

wherein the content of the first and second substances,C_jrepresenting class j, n is the number of iterations.

In a second aspect, the present invention provides a communication system for inverse modulation wireless optical communication, the communication system comprising an interrogation terminal, a modulation terminal and an atmospheric channel, the interrogation terminal comprising a receiver and a transmitter for signal reception and transmission, the modulation terminal comprising an inverse modulator for signal modulation; the transmitter transmits a signal to the reverse modulator through the atmosphere channel, and then the signal is transmitted back to the receiver through the atmosphere channel; the receiver performs decision processing on the received signal according to any one of the signal decision methods.

Compared with the prior art, the invention has the following beneficial effects:

the invention discloses a signal judgment method and a signal judgment system for reverse modulation wireless optical communication, which adopt a self-adaptive clustering algorithm, have self-clustering capability, do not need to give a fixed threshold value, and only need to send received bit streams to the self-adaptive clustering algorithm section by section in a window for clustering judgment. The algorithm can dynamically and adaptively classify the bits in the window according to the similarity by learning the internal characteristics of the received bits, so that the problem that a fixed threshold needs to be given can be effectively solved, and the influence of turbulence on signals is reduced to a certain extent; compared with the existing method, the method has the advantages of simple realization, high cost benefit and the like.

Drawings

Fig. 1 is a received signal comparison diagram of a reverse modulation optical communication link and a unidirectional link when a communication distance is 500 meters according to an embodiment of the present invention;

FIG. 2 is a block diagram of a signal processing flow using an adaptive clustering algorithm according to an embodiment of the present invention;

FIG. 3 is a flow chart of a decision method provided by an embodiment of the invention;

FIG. 4 is a diagram illustrating a simulation of a decision process according to an embodiment of the present invention;

FIG. 5 is a schematic diagram of bit error performance analysis under different turbulence conditions for a conventional decision method and a decision method according to the present embodiment;

fig. 6 is a block diagram of a communication system provided by an embodiment of the present invention.

Detailed Description

The invention is further described below with reference to the accompanying drawings. The following examples are only for illustrating the technical solutions of the present invention more clearly, and the protection scope of the present invention is not limited thereby.

The laser light is transmitted in an unbounded atmospheric space, and random variation of atmospheric temperature, humidity and pressure in a short time and a small range can cause random fluctuation of atmospheric refractive index, thereby causing an atmospheric turbulence effect. Usually by Kolmogorov (Colmorgo)Roff) introduced atmospheric refractive index structural constantThe intensity of the atmospheric turbulence is characterized, and the influence factors are geographical position, altitude, meteorological conditions, season, time and the like. The larger the value, the more turbulent the corresponding. Ranging from weak to strong, typically to a value of 10^-17m^-2/3To 10^-12m^-2/3Typical value is 10^-15m^-2/3. The turbulence has the characteristic of low frequency ramping.

The effects of atmospheric turbulence on laser beams are beam expansion, spot drift, and light intensity flicker. The method is characterized in that the radius of a laser beam transmitted through turbulent atmosphere is increased under the influence of turbulent flow compared with that of a laser beam without turbulent flow, the center of a light spot on a receiving plane rapidly and randomly beats around the center position without the influence of turbulent flow, and the laser beam is independently scattered, diffracted and reflected by small-scale turbulent vortex, so that the mutual interference of multiple beams is caused to redistribute the energy of the beam.

Fig. 1 is a comparison graph of reverse modulation optical communication link (MRR) and unidirectional link (FSO) received signals at a communication distance of 500 meters, and when a transmitting end transmits a direct current optical signal with stable intensity, a receiving end receives levels through the reverse modulation optical communication link and a conventional optical communication unidirectional link. It can be seen that turbulence can cause random fluctuations in the received signal level, which can be particularly severe for a reverse-modulated optical communications link.

Among beam spread, spot drift, and intensity flicker, intensity flicker is a major factor affecting the performance of atmospheric laser communication systems and is typically expressed in terms of an intensity flicker index. The light intensity flicker may cause random intensity fluctuations in the received signal, which may seriously degrade the performance of the atmospheric communication system. The light intensity flicker of the unidirectional link and the reverse modulation link is detected and compared through experiments, and the light intensity flicker of the reverse modulation optical communication link is far greater than that of the unidirectional link.

Assuming that a transmission signal is x, the reverse modulation wireless optical communication adopts an intensity modulation/direct detection mode, generally adopts OOK modulation, and x is a binary bit stream composed of 0 and 1. Through the channel with the influence of turbulence, the receiving end signal can be expressed as:

y＝ηI_cx+n

where η represents the effective photoelectric conversion ratio of the photoelectric receiver, N ~ (0, N)₀) Representing white Gaussian noise, I_cIndicating channel fading.

The expectation of the received signal is:

E(y)＝E(ηI_cx+n)＝ηE(I_c)E(x)+E(n)

defining the received signal to noise ratio as: SNR ═ eta I_c)²/2N₀；

The essence of the turbulence is that the received signal y generates a low frequency random fluctuation on the basis of the transmitted signal x.

The decision of the signal is to recover 0 and 1 in the transmitted signal x by the received signal y, and is a classified process in nature. Since the effect of turbulence is slowly changing, the transmitting end still has similarity when a transmitted 0 (or 1) arrives at the receiving end within a certain time. Similar values are gathered together and divided into two different categories, then the two categories are labeled, the core idea of the self-adaptive clustering algorithm is to divide a set of a plurality of sample data into 2 categories, and the distance from a point in each cluster to the center of the cluster is the minimum.

As shown in fig. 2, a signal processing flow diagram of the adaptive clustering algorithm is adopted.

Given a vector containing n 1-dimensions (magnitude in dimension) Y ═ Y₁,y₂,...,y_n}. Adaptive clustering algorithm divides n samples into 2 different classes C₁,C₂And satisfyC₁∪C₂Y. In clustering, distance is used as a measure of similarity, and the square of a sample amplitude difference value is used as the distance:

d(y_i,y_j)＝||y_i-y_j||²＝(y_i-y_j)²

loss function J_CAs an optimization objective:

in the formula

m_jIs of the class C_jMean of middle samples, n_jIs of the class C_jThe number of samples in the cluster, it can be seen that the value of the loss function is the smallest when the sample with the highest similarity is classified into a subclass.

The algorithm flow is as follows:

first, a data set Y and a cluster number of 2 are input, then 2 cluster centers are initialized from the data set, let T be 0,respectively calculating the distance from each point in the data set to 2 cluster centersTo obtainDividing the data points into the clusters with the closest distance, after all objects are distributed, recalculating the mean value of each cluster to obtain a new cluster centerAnd (4) sequentially iterating, once iteration converges (assuming that T is T), the clustering center does not change, the clustering is considered to be finished, and a clustering result is output, otherwise, T is T +1, and iteration is continued until convergence.

Let the communication rate of the communication system be R_bThe frequency range of turbulence is (0, f) in bps, and in Hz, defining the ratio of γ, usually γ ≧ 1000. Conveying applianceAt a certain input bit gamma, the effect of turbulence is substantially constant and the received light intensity is independent and stable. Based on such a premise, the conventional hard decision method still has a poor effect because a fixed decision threshold value cannot be given in consideration of the whole transmission process on the premise of random fluctuation.

However, the adaptive clustering algorithm has the self-clustering capability, and only a sample value point clustering window obtained by sampling (a certain number of sample value points are taken as a clustering window w) needs to be handed to the adaptive clustering algorithm for clustering judgment without giving a fixed threshold. To take advantage of the algorithm, the clustering window should be smaller than γ. The algorithm can dynamically and adaptively classify the sample value points in the window by learning the internal characteristics of the received sample value points according to the similarity of the sample value points, so that the problem that a fixed threshold needs to be given can be effectively solved, and the influence of turbulence on signal judgment is reduced to a certain extent. Compared with the existing method, the method provided by the invention has the advantages of simple implementation, high cost effectiveness and the like.

And (4) subjecting the received sample set Y to a self-adaptive clustering algorithm for processing window by window. Fig. 3-4 are specific processes for classifying data within a window using an adaptive clustering algorithm. Fig. 3 is a flow chart of a clustering decision method, and fig. 4 is a simulation diagram of a clustering decision process.

The adaptive clustering algorithm first randomly initializes 2 cluster centers (denoted by + sign in the figure),

then the distances from all the data points to 2 clustering centers are respectively calculated, each data point is classified according to the principle of proximity, so that two classes (one class is represented as a hollow circle, and the other class is represented as a solid point) can be obtained,

on this basis, the centroid (cluster center) of the current class is recalculated.

And then recalculating the distance from each data point to a new clustering center, reclassifying, repeating iteration until the clustering center is finally converged and the target function is minimum, and outputting a clustering result. At this time, the data is divided into two types, one type is 0 and the other type is 1.

As shown in fig. 6, this embodiment further provides a communication system for inverse modulation wireless optical communication, including an interrogation terminal, a modulation terminal and an air channel, where the interrogation terminal includes a receiver and a transmitter for receiving and transmitting signals, and the modulation terminal includes an inverse modulator for modulating signals; the transmitter transmits a signal to the reverse modulator through the atmosphere channel, and then the signal is transmitted back to the receiver through the atmosphere channel; the above signal judgment method of the receiver carries out judgment processing on the received signal.

The present application considers that under conditions of weak turbulence, the forward and backward links are uncorrelated (ρ)_X0) and no energy loss is generated in both forward and backward links, when mu is-1/2 sigma². The size of the turbulence can be represented by the value of σ, which is 0, 0.1, 0.2, 0.3, respectively. Considering that 0 and 1 are equi-probability, e (x) is 0.5. N to (0, N)₀) Therefore, e (n) is 0.η is constant (typically 0.8), so that the fixed decision threshold G ═ e (y) of the conventional hard decision can be obtained as 0.4. Generation 10 of text⁷The size w of the clustering window is 300 and gamma is 10 for x data points⁴Magnitude, and the obtained simulation result are shown in fig. 5, and the two judgment methods are used for bit error performance analysis under different turbulence.

Compared with the bit error rate of the traditional hard decision method and the adaptive clustering algorithm under the conditions of no turbulence and turbulence, the two methods have equivalent performance when no turbulence exists. When there is turbulence, the turbulence has a great influence on the hard decision method of a given fixed threshold, so that the bit error rate is rapidly increased. In contrast, the performance of the adaptive clustering algorithm is significantly better. Sigma is 0.1, error rate is 10^-6And the signal-to-noise ratio requirement of the self-adaptive clustering algorithm is reduced by about 2dB compared with the traditional hard decision. With the improvement of the signal-to-noise ratio, at the moment, the channel is mainly influenced by turbulence, and the adaptive clustering algorithm and the traditional method finally tend to stable values respectively, which also reasonably explains that the adaptive clustering algorithm can improve the system performance but cannot completely eliminate the influence of the turbulence on signal transmission.

The method is based on solving the problem that turbulence influences signals, and provides a self-adaptive clustering algorithm on the basis of traditional hard decision. Through simulation analysis, the performance of the algorithm on judgment is improved to a certain extent.

The above description is only a preferred embodiment of the present invention, and it should be noted that, for those skilled in the art, several modifications and variations can be made without departing from the technical principle of the present invention, and these modifications and variations should also be regarded as the protection scope of the present invention.