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在可见光信道中,用信道损耗或者信道直流增益来描述信道的衰减特性[7],信道的直流增益H(0)定义为[8]:
$ H\left( 0 \right) = \frac{{{P_{\rm{t}}}}}{{{P_{\rm{r}}}}} $
(1) 式中, Pt为发射端平均功率;Pr为接收端光平均功率。图 1所示为可见光信道通信模式[9-10]。图中,φ为发射光与光源法线之间的夹角,θ为入射光与接收端法线的夹角,A是接收器的物理面积,D为发射端与接收端之间的距离。
光接收功率为[11]:
$ {P_{\rm{r}}} = {P_{\rm{t}}}\frac{{\left( {m + 1} \right){{\cos }^m}\varphi }}{{2{\rm{ \mathsf{ π} }}{D^2}}}AT\left( \theta \right)g\left( \theta \right)\cos \theta $
(2) 系统直流增益为:
$ H\left( 0 \right) = \frac{{\left( {m + 1} \right){{\cos }^m}\varphi }}{{2{\rm{ \mathsf{ π} }}{D^2}}}AT\left( \theta \right)g\left( \theta \right)\cos \theta $
(3) 式中,m为LED朗伯辐射模型的辐射指数,T(θ)是光滤光器的增益,g(θ)是光集中器的增益。
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基于小波变换的OFDM系统是利用多分辨率分析的思想[12],将传统OFDM中的傅里叶变换替换为小波变换,用小波变换中的低频信息和高频信息表示信号源信号[13],选择正交小波基作为子载波,经小波逆变换后合成DWT-OFDM信号,在接收端通过小波变换恢复出原始信号,起到调制解调的作用。其系统框图如图 2所示[14]。图中,series/parallel(S/P)表示串并转换,parallel/series(P/S)表示并串转换,cyclic prefix(CP)表示循环前缀,visible light communication(VLC)表示可见光通信。
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Haar小波是Daubechies家族中唯一一个同时具有紧支撑和对称性的正交小波[15-16],Haar小波的定义如下:
$ \psi (t) = \left\{ {\begin{array}{*{20}{l}} {1,( - 0.5 < t < 0)}\\ { - 1,(0 < t < 0.5)}\\ {0,({\rm{other}})} \end{array}} \right. $
(4) 式中,t表示时间变量。
其频域形式为:
$ \psi (\omega ) = {\rm{i}}\frac{4}{\omega }{\sin ^2}\left( {\frac{\omega }{4}} \right){{\rm{e}}^{ - \frac{{{\rm{i}}\omega }}{2}}} $
(5) 式中,ω表示频率变量,且满足正交条件[17]:
$ \left\langle {\psi \left( t \right),\psi \left( {{2^j}t} \right)} \right\rangle = 0 $
(6) 当小波经过伸缩平移后,可以得到一个小波序列[18]:
$ {\psi _{\left( {a,b} \right)}}\left( t \right) = \frac{1}{{\sqrt {\left| a \right|} }}\psi \left( {\frac{{t - b}}{a}} \right) $
(7) 式中, a, b∈R, R表示实数集; a≠0, a为尺度系数,b为平移系数。在离散化小波中,对a和b进行离散化得:a=a0j,b=ka0jb0,k, j∈Z, Z表示整数集。a0≠1, 且a0>1表示扩展步长, 为固定值。此时对应的离散小波函数ψj, k(t)可以表示为[19-20]:
$ {\psi _{j,k}}\left( t \right) = a_0^{ - j/2}\psi \left( {a_0^{ - j}t - k{b_0}} \right) $
(8) 离散小波变换系数可以表示为:
$ {C_{j,k}} = \int_{ - \infty }^{ + \infty } f (t)\psi _{j,k}^ * (t){\rm{d}}t = \left\langle {f,{\psi _{j,k}}} \right\rangle $
(9) 式中,f表示f(t)变换后的小波系数,ψ*表示ψ的复共轭。小波重构公式为:
$ f\left( t \right) = C\sum\limits_{ - \infty }^{ + \infty } {\sum\limits_{ - \infty }^{ + \infty } {{C_{j,k}}} } {\psi _{j,k}}(t) $
(10) 式中,C为与变换系数f无关的常数。
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由于双极性的复数OFDM时域信号不适用于IM/DD的可见光通信系统中,而在DWT-OFDM系统中,不存在复值信号,采用实值小波变换,得到信号的幅值信息[21]。如图 3所示,为快速傅里叶变换OFDM(fast Fourier transformation OFDM, FFT-OFDM)与DWT-OFDM信号的实虚部波形对比图,根据对Haar小波的仿真,说明了小波函数可直接应用于IM/DD系统,降低了系统实现的复杂性。
根据Haar小波重构原理,本文中定义一个DWT-OFDM信号的帧结构,如图 4所示。当信号在传输时,被分为低频信息与高频信息两部分进行传输,MAC表示低频信息矩阵,MDC表示高频信息矩阵。
根据一层Haar小波系数的分配方式,在OFDM信号帧结构当中,对数据信息进行小波系数分配,将数据信息分为低频信息MAC与高频信息MDC ,对小波进行重构,实现离散小波逆变换。
当小波变换的点数为N时,在正交Haar小波中,分解算法得到的低频信息与高频信息的矩阵模块相同。所以一层小波系数分配过程为:将原矩阵X的第1行~第N/2行矩阵中的信息作为低频信息MAC,MAC为N/2×N/2的矩阵;第(N/2+1)行~第N行矩阵中的信息作为高频信息MDC,MDC为N/2×N/2的矩阵;MAC1和MDC1是MAC中分配的低频信息矩阵与高频信息矩阵,如图 5所示。
基于小波变换的可见光OFDM通信系统性能优化
Performance optimization of visible light OFDM communication system based on wavelet transform
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摘要: 在可见光通信中, 由于信道的多径效应和信道衰减在传统正交频分复用(OFDM)系统会产生符号间干扰, 从而降低系统的可靠性。为了保障通信质量, 采用带有循环前缀的OFDM系统来抵抗多径效应引起的符号干扰; 为了降低系统的误比特率和峰均比, 采用Haar小波来实现系统有效性、可靠性、峰均比等参量的性能优化, 并采用蒙特卡洛法进行了仿真验证。结果表明, 当系统的误比特率为10-4时, 离散小波变换OFDM系统较快速傅里叶变换OFDM(FFT-OFDM)系统的误码性能大约提高了5dB, 通信效率提高了大约11%;当系统的峰均比为5dB时, FFT-OFDM系统的互补累计分布函数(CCDF)值接近10-2, DWT-OFDM系统的CCDF值为0。该研究为可见光小波变换OFDM通信提供了参考。Abstract: In visible light communications, multipath effects and channel attenuation in traditional orthogonal frequency division multiplexing (OFDM) systems can generate intersymbol interference and reduce system reliability. In order to guarantee the quality of communication, OFDM system with cyclic prefix was used to resist the symbolic interference caused by multipath effect. In order to reduce bit error rate(BER) and peak-to-average power ratio(PAPR) of the system, Haar wavelet was used to optimize the performance of system parameters such as validity, reliability and PAPR. Monte Carlo method was used to verify the simulation results. The results show that, when bit error rate of the system is 10-4, BER performance of discrete wavelet transform OFDM system is about 5dB higher than that of fast Fourier transform OFDM (FFT-OFDM) system. Communication efficiency has been improved by about 11%. When PAPR of the system is 5dB, complementary cumulative distribution function (CCDF) of FFT-OFDM system is close to 10-2. CCDF value of DWT-OFDM system is 0. This study provides a reference for OFDM communication based on visible light wavelet transform.
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