Chinese Journal of Lasers, Volume. 45, Issue 3, 306002(2018)
High-Accuracy Visible Light Positioning Method Based on Received Signal Strength Indicator
Based on received signal strength indicator (RSSI), which has wide serviceability, simple and easy-to-construct structure, and good portability, we study a visible light communication positioning method with high accuracy. Due to the interference from background resources, the measurement error of RSSI-based positioning method is difficult to control below centimeters. We use wavelet analysis to significantly reduce the noise mixed with received signal, use the correlation test method to extract the source signal in noise, and use the least square method to estimate location coordinate. We verify the reliability of the method by the simulations under different signal-to-noise ratios (SNRs). It is found that the error of the RSSI-based visible light positioning is less than 1 cm and is insensitive with noise, which is dozens of times better than the theoretical accuracy without processing based on error analysis.
1 引言
得益于可见光通信(VLC)技术的迅速发展,可见光定位(VLP)作为一项新兴定位技术,以其精度高、成本低、可移植性好、系统易于搭建等优势,逐渐跻身于现阶段主流的定位系统行列,成为相关行业的研究热点[
可见光通信技术,是以发光二极管(LED)为信号光源,可见光波为通信载波,自由空间为传输信道的通信技术,具有传输速率高、保密性强、抗电磁干扰性强等特点[
目前的可见光RSSI定位方法的定位精度已达到3 cm[
2 可见光通信与RSSI定位算法
2.1 LED与信道模型
可见光通信的信号光源一般采用白光LED,其发光强度分布通常服从近朗伯光源模型[
式中,
其中
Figure 1.LED radiation and PD reception sketch
在室内可见光通信信道模型中,根据近朗伯光源的辐射特性,在直射链路(LOS)中,接收器所接收到的光功率
式中,
2.2 RSSI定位过程
基于RSSI的室内可见光定位法,是根据检测到的光信号强度获取接收机的室内相对位置的定位方法。该方法通常采用强度调制与直接检测的方法,通过发射端加载不同频率的载波信号来加以区分,同时减小了小区间内信号的相互干扰[
根据LED的光强分布特性,可以得出
Figure 2.Normalized received light intensity distribution on horizontal range
Figure 3.Positioning procedure of RSSI
3 信号处理及定位算法
3.1 相关函数检测
相关函数检测法[
设含噪声的周期信号为
式中,
即
式中
3.2 小波降噪
考虑到强光干扰的随机性,在进行相关函数运算之前有必要对信号进行一定的预处理。本文采用小波分析法来降低噪声干扰,使信号更为规整,便于之后的处理。小波分析是一种多分辨率的时频分析方法[
式中,R为实数域,
Figure 4.Wavelet denoise and cross-correlation detection test results. Waveforms of (a) sampled signal, (b) wavelet denoised signal, and (c) restored signal extracted by correlation method, respectively; frequency spectra of (d) sampled signal, (e) wavelet denoised signal, and (f) restored signal extracted by correlation method, respectively
3.3 最小二乘法估算位置坐标
RSSI定位方法的最后一步是由接收机到各光源的水平距离计算出接收机的位置坐标,常用的算法有三边定位、双曲线定位等算法,本文选用了更为有效的最小二乘法来估测位置坐标。
最小二乘法的原则是使拟合数据与实测数据之间误差的平方和最小[
误差函数可表示为
其中
对误差函数求导,令其等于0得
若
4 仿真及结果
4.1 仿真参数设置
仿真的目的是测试定位方法的定位精度,为此设置多个检测点来评估各个位置的测量误差。
在室内可见光定位系统中,光源分布方式主要有两种[
Figure 5.Two luminaire distribution modes in visible light positioning system. (a) Rectangular distribution mode; (b) triangular distribution mode
Figure 6.District to be measured
Figure 7.Largest horizontal measuring distance of two distribution modes. (a) Rectangular distribution mode; (b) triangular distribution mode
系统中的噪声
其中:
式中,
仿真过程中,认为接收端透镜有足够的视角。其他仿真参数如
|
4.2 理论误差
在参数估计中,任何针对某个参数
式(18)称为Cramer-Rao不等式[
其定义为
式中
若选取距离最近的三个信号源的接收强度来计算,如
4.3 误差仿真结果
在仿真中,每一个检测信号经过小波降噪、信号提取并采用最小二乘法估算位置,得出的矩形与三角形光源分布方式的仿真结果分别如
Figure 8.Simulation result of rectangular distribution mode
Figure 9.Simulation result of triangular distribution mode
对两种排列方式分别做12组类似的仿真测试,并统计每一个位置的定位误差,得到矩形排列方式的均方误差为0.478 cm,三角排列方式的为0.425 cm。在2000余个位置数据中,90%以上的测量误差在1 cm以下。
4.4 噪声强度影响
虽然当信噪比(SNR)为10 dB时仿真结果较好,但还不足以说明该设计能够很好地降低噪声干扰,为此需要测试不同噪声环境下定位误差的变化。保持上述的处理算法和各仿真参数不变,分别在信噪比为20 dB,10 dB,6 dB,3 dB,0 dB条件下进行上述定位精度仿真,并分别采样多组均方误差作对比。
Figure 10.Simulation result of rectangular array positioning with SNR of 20 dB
Figure 11.Simulation result of rectangular array positioning with SNR of 0 dB
Figure 12.Positioning error tendency varied with SNR in triangular distribution
Figure 13.Positioning error tendency varied with SNR in rectangular distribution
由
5 结论
通过分析可见光发射和接收通信链路,采用接收信号强度检测方法,对室内定位方法进行研究,设计了利用小波分析法大幅降噪、互相关检测提取源信号、最小二乘法估算位置坐标的定位方法,利用该方法实现了二维平面定位的仿真,达到了较理想的精度。
由24组位置数据的统计结果可得,本文设计的定位方法在10 dB的信噪比条件下,均方误差基本稳定在0.4~0.5 cm之间;通过CRB进行的误差分析表明,相同参数的系统不经处理直接计算得出的位置坐标理论误差大于13 cm,可见处理方法是非常有效的;在等光源间距的条件下,三角分布方式相比于矩形分布具有更高的稳定性;随着噪声强度的增大,定位误差的增大趋势并不十分明显,说明该方法对于强噪声有更好的抑制效果。
由于具有系统简洁、可移植性强、适用范围广的特点,基于RSSI的可见光定位技术在设备小型化、便携化的科技发展趋势下有很好的应用前景;随着人工智能设备的普及,各相关行业对机器设备的精细操作提出越来越高的要求,高精度的定位方法将为此打下很好的基础。此外,该定位方法对强噪声的抑制作用,使得可见光定位即使在日光通透的室内场所也能够得到有效的应用,甚至有望将其应用领域扩展至公园、广场等户外场所。
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Ye Ziwei, Ye Huiying, Nie Xiangyu, Xi Xiaoyu. High-Accuracy Visible Light Positioning Method Based on Received Signal Strength Indicator[J]. Chinese Journal of Lasers, 2018, 45(3): 306002
Category: Fiber optics and optical communication
Received: Jul. 5, 2017
Accepted: --
Published Online: Mar. 20, 2018
The Author Email: Huiying Ye (iehyye@zzu.edu.cn)