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可见光多维无载波幅度相位调制系统

2017-12-16王旭东

电子与信息学报 2017年12期
关键词:王旭东误码频带

王旭东 崔 玉 吴 楠



可见光多维无载波幅度相位调制系统

王旭东 崔 玉*吴 楠

(大连海事大学信息科学技术学院 大连 116026)

针对室内可见光通信调制技术和多用户接入问题,该文基于直流偏置和极性编码信号单极化思想提出两种系统复杂度低、频带利用率高的可见光多维无载波幅度相位(DCO-MCAP, U-MCAP)调制方案。首先利用最大最小优化算法构建无频谱泄漏的频域目标方程,采用完全重建(PR)条件作为非线性约束方程,求取时域正交的多维CAP滤波器组,然后分别采用“添加直流偏置”和“零值位置极性编码”实现信号单极性,以满足可见光通信“强度调制/直接检测”的要求。基于朗伯辐射模型,可见光背景噪声建模为加性高斯白噪声(AWGN),推导了DCO- CAP/MCAP, U-CAP/MCAP 4种调制方案的误符号率理论解,仿真验证了其准确性。在5 m´5 m´3 m的室内场景下,仿真对比分析了2维、3维、4维可见光CAP调制系统的频带利用率、误码率等性能。结果表明,相同星座尺寸时,多维CAP调制的误码性能明显优于传统2维;相同频带利用率时,3种维度系统误码性能相近,可见光多维CAP调制为多用户接入提供可能性。此外,讨论了信道参数对可见光CAP系统的影响,发射机辐射角与收发信机之间距离都与信噪比损失呈现出正相关性。

可见光通信;多维无载波幅度相位调制;完全重建;频带利用率;误符号速率

1 引言

可见光通信(VLC)是一种在可见光谱域利用发光二极管(LED)快速闪灭进行数据通信的新兴无线通信技术,具有安全性高、频谱资源丰富、节省功率以及无电磁干扰等优势[1,2]。LED不仅具有寿命长、成本低、易于小型化等优点,还拥有响应时间短、高速调制等特点,使得VLC成为短距离无线通信领域极具吸引力与竞争力的技术手段,然而LED有限的调制带宽也成为限制VLC系统传输速率的主要因素[3]。无载波幅度相位调制(CAP)技术具备频谱利用率高、复杂度低等优势,是解决这一问题极具潜力的方案之一,近年来广受关注。

2 多维CAP调制原理

传统的2维CAP以QAM调制为基础演变而来,区别在于其不需要载频本振源,取而代之的是一对时域正交的滤波器(同相滤波器、正交滤波器),因此其不仅仅具备QAM调制相同的频带利用率,而且极大地降低了系统复杂度,避免了处理载波同步的问题。式(1)给出传统2维CAP调制同相、正交滤波器有限冲激响应。

假设系统抽样速率为100 MHz,传统2维CAP滚降系数设为0.2,多维CAP滤波器带宽设为最小带宽的1.2倍。图1~图3分别给出了传统2维、3维和4维CAP滤波器组时域与频域波形,若以星座点个数定义星座尺寸,表1给出不同星座尺寸条件下滤波器组采样因子、波特率等指标参数。而图4则给出3维CAP调制的星座图。

图2 3维滤波器时域与频域波形

图3 4维滤波器时域与频域波形

表1 2/3/4D-CAP滤波器设计指标参数

图4 3维CAP星座图

3 系统模型

3.1 信号模型

图5 可见光多维CAP调制系统

3.2 信道模型

VLC系统中,由于传输路径较短,光被吸收以及光扩散引起的衰减非常小。假设发射机LED光源满足朗伯体辐射模式,且使用“强度调制/直接检测”调制形式,则光信号传输链路的直流增益可由式(3)给出[17,18]。这里由于直射信号远大于经障碍物反射的信号,只考虑视距传输(LOS)链路,VLC传输模型如图6所示。

设定5 m´5 m´3 m的室内场景,发射机LED位于室内正中央天花板高度处,接收机PD距离地面0.85 m(大约位于人的腰间口袋)。理想非成像滤波器内部反射系数为1.5,无光带通滤波器,PD的有效面积为1 cm2,其视场角为60o, LED的半功率角为70o。若信号的发射功率为20 W, PD处于该房间的任一位置,其接收到的信号功率分布情况如图7所示。

4 性能分析

4.1 DCO-CAP/MCAP误码性能

其中,表示星座点正确解调的概率,若接收端接收到矢量信号,该信号对应的星座点坐标为, 表达式可由式(5)所示。

图7 接收信号功率

添加直流偏置是可见光系统最常见的信号极性处理方式,操作简单,硬件成本低,不改变系统本身的频带利用率,但功率效率低。由2.1节可知,室内可见光DCO-CAP/MCAP调制系统受信道噪声(AWGN)以及限幅噪声两种噪声干扰,然而当直流偏置足够大时,可去除限幅噪声的影响。接收端光电检测器接收到的信号可以表示为

(9)

考虑多维CAP高斯信道下的误符号率以及可见光直射链路情况,可得DCO-CAP/MCAP系统误符号率理论表达式为

4.2 U-CAP/MCAP误码性能

U-CAP/MCAP系统较DCO-CAP/MCAP来说,有效降低了功率效率,但是其利用“0”值代表符号使系统本身的频带利用率增加1倍,并且极性恢复过程中需要比较信号大小,系统实现复杂度增加。背景噪声为AWGN的可见光U-CAP/MCAP系统性能除去受信道衰减影响外,还将受到极性恢复过程中信号极性误判的影响。这一判决信号极性的过程为非线性过程,其高样本数的输入信号同样满足高斯分布,因此可通过Bussgang定理分析极性判决对系统信噪比的影响。

5 仿真结果

本节选取3.2节的应用场景,信道背景噪声建模为AWGN,在可识别的信噪比范围内,对基于传统2维CAP的DCO-CAP, U-CAP调制方案以及基于3维、4维CAP的DCO-MCAP(添加17 dB直流偏置)、U-MCAP调制方案的误码性能进行了仿真分析。此外,讨论了信道参数对可见光CAP调制系统的影响。

图8给出了星座尺寸保持一致时DCO-CAP, U-CAP与3维DCO-MCAP, U-MCAP的仿真性能对比结果,为了验证文中4.1节以及4.2节误码性能分析的准确性,同时给出相同条件下4种调制的理论解析性能。而图9则同样在星座尺寸一致时,给出DCO-CAP, U-CAP与4维DCO-MCAP, U-MCAP的仿真曲线和理论曲线。从两图可以看出,4种调制算法的仿真结果与理论结果吻合良好,验证了文中给出的闭式解。同时还可以看出星座尺寸相同时,可见光多维CAP调制系统的误码性能明显优于可见光传统2维CAP调制系统,参考表1可知其原因在于多维CAP系统牺牲了频带利用率。进一步分析可知,当对系统误码率有固定要求时,可见光CAP系统可通过消耗频带的方式,增加系统吞吐量。此外,U-CAP/MCAP调制算法相较于添加17 dB直流偏置的DCO-CAP/MCAP调制算法可靠性要高,这是由于U-CAP/MCAP调制算法牺牲了频带利用率以及系统复杂度。

图8 星座尺寸相同时可见光CAP系统误码性能比较(3维与传统2维)

图9 星座尺寸相同时可见光CAP系统误码性能比较(4维与传统2维)

6 结论

图10 频带利用率相同时可见光CAP调制系统误码性能比较

图11 发射机辐射角对误码性能的影响

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王旭东: 男,1967 年生,博士,教授,主要研究方向为MIMO无线通信、空间调制、光无线通信.

崔 玉: 女,1993年生,硕士生,研究方向为可见光通信与可见光定位.

吴 楠: 男,1979 年生,博士,副教授,主要研究方向为现代移动无线通信系统、可见光通信系统(包括MIMO、OFDM、信道编码、协作通信、自组织网络)等.

Multi-dimensional Carrierless Amplitude and Phase Modulationfor Visible Light Communication System

WANG Xudong CUI Yu WU Nan

(,,116026,)

Considering the modulation technique and multiple-access topology for indoor Visible Light Communication (VLC), two Multi-dimensional Carrierless Amplitude and Phase (Multi-CAP) modulation schemes are proposed in this paper, namely, DC biased Optical Multi-CAP (DCO-MCAP) and Unipolar optical Multi-CAP (U-MCAP). The design object is modeled as a mini-max optimization problem with a non-constraint condition named Perfect Reconstruction (PR), which can be solved by the quadratic programming algorithm. Only real and unipolar signals can be intensity modulated, therefore several modifications are performed in CAP VLC systems accordingly. A DC bias added to the signal and a pair of new samples including a zero padding to mark signal polarity are adopted to obtain unipolar signal respectively.According to Lambertian reflection model and considering light of sight link corrupted by Additive White Gaussian Noise (AWGN), the theoretical Symbol Error Rate (SER) of DCO-CAP/MCAP and U-CAP/MCAP are derived and corroborated by Monte Carlo simulations. Meanwhile, the performance of three kinds of dimensional (2D, 3D, 4D) optical CAP systems are analyzed and compared in the room in size of 5 m´5 m´3 m. The numerical results show that the error performance of optical Multi-CAP is superior to that of traditional two dimensional optical CAP with the same constellation, and the multi-CAP (3D or 4D) scheme can not only achieve approximate SER performance with respect to the same bandwidth efficiency, but also enable multiple access in VLC system. In addition, the effects of channel parameters on the system performance are evaluated which shows that the loss of SNR obviously presents positive relation with the increase of the radiation angle or the distance between the transmitter and the receiver.

Visible Light Communication (VLC); Multi-dimensional Carrierless Amplitude and Phase (Multi-CAP) modulation; Perfect Reconstruction (PR); Bandwidth efficiency; Symbol Error Rate (SER)

TN929.1

A

1009-5896(2017)12-3004-09

10.11999/JEIT170276

2017-03-31;

2017-09-11;

2017-11-01

通信作者:崔玉 yucui@dlmu.edu.cn

国家自然科学基金(61371091)

The National Natural Science Foundation of China (61371091)

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