A non-line-of-sight visible light communication method using step-by-step partitioned wavefront shaping

Abstract

The present invention belongs to the technical field of visible light communication, and specifically is a non-line-of-sight visible light communication method using step-by-step partitioned wavefront shaping. The core idea of ​​the method is to perform different partition optimizations on a spatial light modulator. First, the spatial light modulator is divided into grid units with a larger area, and a preliminary and rapid optimization is performed to obtain an initial phase diagram; then, the spatial light modulator is divided into units with a smaller area and a denser grid for more precise optimization. An APD photodetector is used as a receiver to calculate a feedback signal, and an iterative optimization algorithm is used to control the spatial light modulator to modulate the phase of the incident light, thereby realizing the transmission control of light after passing through a scattering medium more quickly and efficiently. The present invention can realize the focusing of light after passing through a scattering medium, and on this basis, realize non-line-of-sight visible light communication. A higher light intensity gain can be obtained with fewer iterations.

Description

Technical Field

[0001] The present invention belongs to the technical field of visible light communication, and in particular relates to a non-line-of-sight visible light communication method. Background Art

[0002] Indoor visible light communication is a scenario with great application potential. However, when light hits the scattering surface in the room (such as walls and object surfaces), the light will propagate in different forms in all directions, and only a small part can reach the receiving end, resulting in a large loss of light energy at the receiving end, which greatly affects the communication performance. The spatial light modulator can modulate the phase of light to achieve focusing of light after it passes through the scattering surface.

[0003] There are many methods to control the transmission of light through scattering media. The feedback-based random partition wavefront shaping algorithm is a very common one. Its advantages are that the device is easy to implement and the optimization step size is adjustable. By setting the number of sub-regions N , each iteration randomly selects M The optimization result is finally obtained by gradually shifting the phase of each sub-region. The choice of the step size needs to be balanced between the shaping gain and the convergence speed. If the step size is large, the convergence speed is fast, but the shaping gain is not enough; if the step size is small, the convergence speed is slow, and it may even fail to converge. Summary of the invention

[0004] The purpose of the present invention is to propose a non-line-of-sight visible light communication method using step-by-step partitioned wavefront shaping, which can quickly and efficiently achieve focusing of light after passing through a scattering medium, thereby realizing high-speed visible light communication at non-real distance.

[0005] The present invention proposes a non-line-of-sight visible light communication method using step-by-step partition wavefront shaping, which is based on a non-line-of-sight visible light communication system. Figure 2 As shown, it includes: a laser light source, a beam expander, a spatial light modulator, a scattering medium, and an APD photodetector. Its optical path is: the light emitted by the laser light source passes through the beam expander, enters the spatial light modulator, is modulated, is reflected by the scattering medium, and is received by the APD photodetector.

[0006] The core idea of ​​the method of the present invention is to adopt a step-by-step partition wavefront shaping method, that is, to perform different partition optimizations on the spatial light modulator in the non-line-of-sight visible light communication system, and to perform phase modulation of the incident light from fast to slow and from coarse to fine, so as to focus the light more quickly and effectively after passing through the scattering medium, thereby realizing non-line-of-sight high-speed visible light communication.

[0007] The non-line-of-sight visible light communication method using step-by-step partitioned wavefront shaping provided by the present invention is based on a non-line-of-sight visible light communication system, and the specific steps are:

[0008] Step 1: For a spatial light modulator in a non-line-of-sight visible light communication system, an initial number of sub-regions is set, and pixels on the spatial light modulator are divided into regular regions according to the number of sub-regions. The signal strength collected by the APD photodetector is used as feedback, and an iterative optimization algorithm is used to control the spatial light modulator to modulate the phase of the incident light. After preliminary iterations, the received signal strength is continuously improved to obtain an initial phase diagram.

[0009] Here, the regular area division is carried out to Figure 3 For example, the step-by-step regional partitioning is divided into three steps, namely 2*2 partitioning, 3*3 partitioning and 6*3 partitioning, which are optimized in turn;

[0010] Step 2: By continuously increasing the number of sub-regions, the density of sub-region division is improved, and the area of ​​the sub-region is reduced; the number of iterations is increased to achieve more refined phase modulation, so that the intensity of the received signal is further improved, and the intermediate incident light phase diagram is obtained;

[0011] Step 3: Repeat step 2, and after multiple rounds of partition iterative optimization, obtain the final incident light phase diagram, thereby obtaining the optimized optical path;

[0012] Step 4: Based on the optimized optical path obtained in step 3, use DMT + bit-loading technology (please attach references) [1,2], and allocate different bit loading numbers to each subcarrier according to the signal-to-noise ratio and forward error correction threshold of the subcarrier, under the premise of power budget, to achieve non-line-of-sight high-speed visible light communication.

[0013] In the present invention, as the iterative optimization algorithm is continuously performed, the number of divided sub-regions gradually increases, the precision of the algorithm is continuously improved, and the gain of optical power can be increased to the greatest extent.

[0014] In the present invention, the optimization of the incident light phase is performed step by step and in different regions. The initial iteration speed is fast but the precision is poor. The subsequent iteration speed is slow but the precision is good. The combination of the two can shorten the time required for optimization as much as possible while ensuring the gain.

[0015] In the present invention, when setting the number of sub-regions, the number can be set arbitrarily. In principle, the upper limit of the number of sub-regions is the number of pixels included in the airborne light modulator.

[0016] In the present invention, the position of the APD photodetector can be placed randomly in principle, and the optimization algorithm has strong adaptability and can achieve focusing of scattered light at any position.

[0017] In the present invention, after the optimization is completed, the light will be focused on the APD photodetector after passing through the scattering medium. The use of DMT + bit-loading technology can avoid the problem of single carrier being susceptible to interference, and at the same time, according to the signal-to-noise ratio of the subcarrier and the forward error correction threshold, the highest spectrum efficiency and transmission rate can be obtained under the premise of power budget.

[0018] The present invention has the following advantages:

[0019] The present invention achieves rapid convergence in the early stage of the iterative optimization algorithm and achieves precise phase modulation in the later stage of the algorithm by setting different partition sizes. The operation method is relatively simple. It only needs to set different partition sizes, the proportion of sub-regions taken in each iteration, and different iteration times to complete the focusing of light after passing through the scattering medium. Compared with the traditional wavefront shaping algorithm, this method has the advantages of fast convergence speed and high shaping gain. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is a flow chart of the present invention.

[0021] Figure 2 Diagram of the non-line-of-sight visible light communication system designed for the present invention.

[0022] Figure 3 This is an example of the regular area division performed in step (1) of the present invention.

[0023] Figure 4 It is a diagram of the experimental process of the present invention.

[0024] Figure 5 This is the graph of algorithm convergence results.

[0025] Figure 6 The comparison of communication rates before and after shaping. (a) shows the change of communication rate with receiving angle, and (b) shows the change of communication rate with receiving distance.

[0026] In the figure, 1 is a laser light source, 2 is a beam expander, 3 is a spatial light modulator, 4 is a scattering medium, and 5 is an APD photodetector. Implementation

[0027] The present invention is further described below in conjunction with the accompanying drawings.

[0028] The present invention is a non-line-of-sight visible light communication method based on a step-by-step partition wavefront shaping algorithm. Figure 1As shown, first, it is necessary to set the parameters of the iterative optimization algorithm, including the number of sub-regions in each round of iteration, the proportion of sub-regions taken in the iteration, and the number of iterations in each round; then use the optimization algorithm to optimize the optical path; after the optimization is completed, use DMT+ bit-loading technology to achieve non-line-of-sight high-speed visible light communication.

[0029] Figure 2 The example shown in the figure shows a non-line-of-sight visible light communication system based on the step-by-step partition wavefront shaping algorithm proposed by the present invention. The system includes a laser light source module 1, a beam expander 2, a spatial light modulator 3, a scattering medium 4, and an APD photodetector 5. The basic process of non-line-of-sight visible light communication based on the step-by-step partition wavefront shaping algorithm is as follows:

[0030] Step 1: The laser light source module 1 emits a laser, and the laser beam is enlarged to a size suitable for the incident surface of the spatial light modulator 3 through the beam expander 2;

[0031] Step 2, the spatial light modulator 3 modulates the phase of the incident light, and then projects the modulated laser onto the scattering medium 4;

[0032] Step 3: After being scattered, the light is received by the APD photodetector 5 to obtain the signal strength at the current moment;

[0033] Step 4: Using the signal detected by the APD photodetector 5 as feedback, executing a step-by-step partition wavefront shaping algorithm, controlling the spatial light modulator 3, and modulating the phase of the incident light to achieve focusing of the scattered light;

[0034] Step 5: After the algorithm is executed, DMT + bit-loading technology is used to achieve high-speed visible light communication on the optimized non-line-of-sight optical path.

[0035] For specific experimental examples, see Figure 4 As shown:

[0036] Step 1: The laser light source module (LD) emits laser light, and the laser beam is enlarged to the maximum size that can utilize the incident surface of the spatial light modulator through a beam expander.

[0037] Step 2: Divide the spatial light modulator into 4*4 areas, iterate 20 times, take 1 / 16 of the area in each iteration, and perform eight-step phase shift; use the signal detected by the APD as feedback, optimize, and obtain a preliminary phase diagram;

[0038] Step 3: Divide the spatial light modulator into 8*8 areas, iterate 30 times, take 1 / 8 of the area in each iteration, and perform eight-step phase shift; use the signal detected by the APD as feedback, optimize, and obtain a preliminary phase diagram;

[0039] Step 4: Divide the spatial light modulator into 16*16 areas, iterate 40 times, take 1 / 4 of the area in each iteration, and perform eight-step phase shift; use the signal detected by the APD as feedback, optimize, and obtain a preliminary phase diagram;

[0040] Step 5: Use DMT + bit-loading technology to achieve high-speed visible light communication on the optimized non-line-of-sight link.

[0041] Figure 5 To compare the convergence results of the algorithms, the three methods are compared. The step-by-step partitioning wavefront shaping method of the present invention has better focusing performance and higher optimization efficiency than the wavefront shaping algorithm using 4*4 partitions. When the 4*4 partition wavefront shaping algorithm tends to converge, the algorithm of the present invention can still achieve further focusing effects. Compared with the 16*16 partition wavefront shaping algorithm, the algorithm of the present invention has the characteristics of faster convergence speed, avoiding the slowness of the early search, and adopting the idea of ​​step-by-step partitioning, from large to small, from coarse to fine, while taking into account the optimization speed, it maximizes the efficiency of wavefront shaping.

[0042] Finally, the optical communication rate before and after wavefront shaping is compared, and the results are as follows: Figure 6 As shown. Figure 6 It can be seen that the wavefront shaping algorithm proposed in the present invention can compensate for the diffuse reflection of light when it passes through the scattering medium to a large extent, thereby realizing the focusing of light after passing through the scattering medium, and finally realizing the improvement of the communication rate. We studied the influence of different receiving angles and receiving distances on the communication rate. It can be seen that when the receiving angle is equal to the reflection angle, the wavefront shaping effect is the best. As the angle shifts, the effect of the shaping algorithm decreases. As the receiving distance increases, the shaping effect will also decrease.

[0043] [1] RA Shafik, MS Rahman, and AR Islam, “On the extended relationships among EVM, BER and SNR as performance metrics,” inInternational Conference on ICECE (IEEE, 2006), pp. 408–411.

[0044] [2] Zhou Y, Zhu X, Fangchen HU, et al. Common-anode LED on a Sisubstrate for beyond 15 Gbit / s underwater visible light communication[J]. Photonics Research: English Edition, 2019(009):000.

Claims

1. A non-line-of-sight visible light communication method using step-by-step partitioned wavefront shaping is based on a non-line-of-sight visible light communication system, the non-line-of-sight visible light communication system comprising: Laser light source, beam expander, spatial light modulator, scattering medium, APD photodetector; The optical path is as follows: the light emitted by the laser light source passes through the beam expander, enters the spatial light modulator, is modulated, is reflected by the scattering medium, and is received by the APD photodetector; it is characterized by adopting step-by-step partition wavefront shaping, that is, performing different partition optimizations on the spatial light modulator in the non-line-of-sight visible light communication system, and performing phase modulation of the incident light from fast to slow and from coarse to fine, so as to focus the light after passing through the scattering medium more quickly and effectively, thereby realizing non-line-of-sight high-speed visible light communication. The specific steps are as follows: Step 1: For a spatial light modulator in a non-line-of-sight visible light communication system, an initial number of sub-regions is set, and pixels on the spatial light modulator are divided into regular regions according to the number of sub-regions, the signal intensity collected by the APD photodetector is used as feedback, and an iterative optimization algorithm is used to control the spatial light modulator to modulate the phase of the incident light; After initial iterations, the received signal strength is continuously improved, and an initial phase diagram is obtained; Step 2: By continuously increasing the number of sub-regions, the density of sub-region division is improved, and the area of ​​the sub-region is reduced; the number of iterations is increased to achieve more refined phase modulation, so that the intensity of the received signal is further improved, and the intermediate incident light phase diagram is obtained; Step 3: Repeat step 2, and after multiple rounds of partition iterative optimization, obtain the final incident light phase diagram, thereby obtaining the optimized optical path; Step 4: Based on the optimized optical path obtained in step 3, use DMT + bit-loading technology, and allocate different bit loading numbers to each subcarrier according to the signal-to-noise ratio and forward error correction threshold of the subcarrier, under the premise of power budget, to achieve non-line-of-sight high-speed visible light communication.

2. The non-line-of-sight visible light communication method according to claim 1, characterized in that: As the iterative optimization algorithm continues, the number of divided sub-regions gradually increases, and the precision of the algorithm continues to improve, thereby maximizing the gain of optical power.

3. The non-line-of-sight visible light communication method according to claim 1, characterized in that: The optimization of the incident light phase is carried out step by step and in different regions. The initial iteration speed is fast but the precision is poor. The subsequent iteration speed is slow but the precision is good. The combination of the two can shorten the optimization time as much as possible while ensuring the gain.

4. The non-line-of-sight visible light communication method according to claim 1, characterized in that: When setting the number of sub-regions, the upper limit of the number of sub-regions is the number of pixels included in the spatial light modulator.

Images