光电工程  2019, Vol. 46 Issue (5): 180306      DOI: 10.12086/oee.2019.180306

Two input one output visible light communication system based on pulse amplitude modulation
Shi Meng, Zhang Mengjie, Chi Nan
Key Laboratory of Electromagnetic Wave Information Science, Ministry of Education, Department of Communication Science and Engineering, Fudan University, Shanghai 200433, China
Abstract: To improve the data transmission rate of the conventional point-to-point single input single output (SISO) visible light communication system, a multiple input multiple output (MIMO) visible light communication system is proposed. Considering the complexity of the receiver system, multiple input single output (MISO) visible light communication systems have attracted attention. This paper studies the MISO visible light communication system based on pulse amplitude modulation (PAM), and experimentally proves the advantages of this system in specific scenes. In addition, there are non-linear effects for key devices such as LED light sources and power amplifiers in visible light communication systems. Based on 2×1 MISO visible light communication system, this paper reports a novel equal probability coding mapping scheme for high-order PAM signals with two low-order PAM signals superposition in the optical domain. The system verification is performed through a net data-rate of 700 Mb/s transmission experiment through a red chip of RGB-LED, which proves the feasibility and superiority of this scheme in practice.
Keywords: optical communications    visible light communication    multiple input single output (MISO)    equal probability coding    pulse amplitude modulation (PAM)

1 引言

2 两发一收可见光通信系统中光生PAM7信号的编码映射方案

 图 1 两路PAM4信号叠加生成PAM7信号的编码示意图。(a)不等概率PAM7信号；(b)等概率PAM7信号 Fig. 1 The schematic diagram of PAM7 signals generated by two PAM4 signals superposition. (a) Unequal probability PAM7 signals; (b) Equal probability PAM7 signals

 Signal Equal probability PAM4 Unequal probability PAM4 Equal probability PAM7 Unequal probability PAM7 PAPR 1.80 1.34 2.25 3.6
3 基于PAM调制的两发一收可见光通信系统结构

 图 2 基于PAM调制的两发一收可见光通信系统框图及实验装置图 Fig. 2 The block diagram and experimental setup of two input one output visible light communication system based on PAM modulation

4 单发单收和两发一收PAM7系统实验结果比较

 $H(x) = H = - \sum\nolimits_{x \in \chi } {{P_X}(x)\log _2^{}} {P_X}(x),$ (1)

4.1 单发单收发射端生成不等概率PAM7电信号
 $s(t) = \operatorname{Re} [{A_m}g(t){{\rm{e}}^{{\rm{j}}2{\rm{ \mathsf{ π} }}{f_{\rm{c}}}t}}] = {A_m}g(t)\cos 2{\rm{ \mathsf{ π} }}{f_{\rm{c}}}t,$ (2)
 ${s_{{\rm{Tx}}1}}(t) + {s_{{\rm{Tx}}2}}(t) = ({A_{m1}} + {A_{m2}})g(t)\cos 2{\rm{ \mathsf{ π} }}{f_{\rm{c}}}t,$ (3)
 $P({A_{m1}}) = P({A_{m2}}) = 1/4,$ (4)

 $r(t) = [{s_{{\rm{Tx}}1}}(t) + {s_{{\rm{Tx2}}}}(t)] + {n_0}(t)\\ \;\;\;\;\;= ({A_{m1}} + {A_{m2}})g(t)\cos 2{\rm{ \mathsf{ π} }}{f_{\rm{c}}}t + {n_0}(t)。$ (5)

 $I(t) = |r(t){|^2} + {n_{\rm{r}}}(t) \\ \;\;\;\;\;= {[({A_{m1}} + {A_{m2}})g(t)\cos 2{\rm{ \mathsf{ π} }}{f_{\rm{c}}}t]^2} + {n_0}^2(t) \\ \;\;\;\;\;+ 2{n_0}(t)({A_{m1}} + {A_{m2}})g(t)\cos 2{\rm{ \mathsf{ π} }}{f_{\rm{c}}}t{\rm{ + }}{n_{\rm{r}}}(t) 。$ (6)

 图 3 单发单收发射端不等概率PAM7信号情况下。(a) BER随Vled和Vpp变化关系；(b)不同Vpp下不等概率PAM7信号星座点和BER Fig. 3 Unequal probability PAM7 signal at transmitter in single input single output system. (a) The BER performance with Vled and Vpp; (b) The constellation points and BER performance under different Vpp
4.2 单发单收发射端生成等概率PAM7电信号

 $P({A_{mi}} + {A_{mj}}) = 1/7。$ (7)

 图 4 单发单收发射端等概率PAM7信号情况下。(a) BER随Vled和Vpp变化关系；(b)不同Vpp下等概率PAM7信号星座点和BER Fig. 4 Equal probability PAM7 signal at transmitter in single input single output system. (a) The BER performance with Vled and Vpp; (b) The constellation points and BER performance under different Vpp
4.3 两发一收接收端生成不等概率PAM7光信号

 $r(t) = {s_{{\rm{Tx}}1}}(t) + {n_{{0_{{\rm{Tx}}1}}}}(t) + {s_{{\rm{Tx2}}}}(t) + {n_{{0_{{\rm{Tx}}2}}}}(t)\\ \;\;\;\;\;= ({A_{m1}}{\rm{ + }}{A_{m2}})g(t)\cos 2{\rm{ \mathsf{ π} }}{f_{\rm{c}}}t + {n_{{0_{{\rm{Tx}}1}}}}(t) + {n_{{0_{{\rm{Tx}}2}}}}(t),$ (8)
 $I(t) = |r(t){|^2} + {n_{\rm{r}}}(t) \\ \;\;\;\;\; {\rm{ = }}{[({A_{m1}} + {A_{m2}})g(t)\cos 2{\rm{ \mathsf{ π} }}{f_{\rm{c}}}t]^2} \\\;\;\;\;\; + {[{n_{{0_{{\rm{Tx}}1}}}}(t) + {n_{{0_{{\rm{Tx}}2}}}}(t)]^2} \\\;\;\;\;\; + 2[{n_{{0_{{\rm{Tx}}1}}}}(t) + {n_{{0_{{\rm{Tx}}2}}}}(t)]({A_{m1}} + {A_{m2}})g(t)\cos 2{\rm{ \mathsf{ π} }}{f_{\rm{c}}}t \\ \;\;\;\;\; {\rm{ = }}{[({A_{m1}} + {A_{m2}})g(t)\cos 2{\rm{ \mathsf{ π} }}{f_{\rm{c}}}t]^2} \\ \;\;\;\;\; + n_{{0_{{\rm{Tx}}1}}}^2(t) + n_{{0_{{\rm{Tx}}2}}}^2(t){\rm{ + 2}}{n_{{0_{{\rm{Tx}}1}}}}(t){n_{{0_{{\rm{Tx}}2}}}}(t) \\ \;\;\;\;\; + 2[{n_{{0_{{\rm{Tx}}1}}}}(t) + {n_{{0_{{\rm{Tx}}2}}}}(t)] \\\;\;\;\;\; \cdot ({A_{m1}} + {A_{m2}})g(t)\cos 2{\rm{ \mathsf{ π} }}{f_{\rm{c}}}t + {n_{\rm{r}}}(t)。$ (9)

 图 5 两发一收接收端不等概率PAM7信号情况下。(a) BER随Vled和Vpp变化关系；(b)不同Vpp下不等概率PAM7信号星座点和BER Fig. 5 Unequal probability PAM7 signal at receiver in two input one output system. (a) The BER performance with Vled and Vpp; (b) The constellation points and BER performance under different Vpp
4.4 两发一收接收端生成等概率PAM7光信号

 图 6 两发一收接收端等概率PAM7信号情况下。(a) BER随Vled和Vpp变化关系；(b)不同Vpp下等概率PAM7信号星座点和BER Fig. 6 Equal probability PAM7 signal at receiver in two input one output system. (a) The BER performance with Vled and Vpp; (b) The constellation points and BER performance under different Vpp

 图 7 系统动态工作范围比较。(a)单发单收发射端生成不等概率PAM7；(b)单发单收发射端生成等概率PAM7；(c)两发一收接收端生成接收不等概率PAM7；(d)两发一收接收端接收生成等概率PAM7 Fig. 7 The comparison of system's dynamic working area. (a) Unequal probability PAM7 signal at transmitter in SISO system; (b) Equal probability PAM7 signal at transmitter in SISO system; (c) Unequal probability PAM7 signal at receiver in two input one output system; (d) Equal probability PAM7 signal at receiver in two input one output system
5 结论

 [1] Zeng L B, O'Brien D C, Le Minh H, et al. High data rate multiple input multiple output (MIMO) optical wireless communications using white LED lighting[J]. IEEE Journal on Selected Areas in Communications, 2009, 27(9): 1654-1662. [Crossref] [2] Mesleh R, Mehmood R, Elgala H, et al. Indoor MIMO optical wireless communication using spatial modulation[C]//Proceedings of 2010 IEEE International Conference on Communications, 2010: 1–5. [3] Yu Z H, Baxley R J, Zhou G T. Multi-user MISO broadcasting for indoor visible light communication[C]//Proceedings of 2013 IEEE International Conference on Acoustics, Speech and Signal Processing, 2013: 4849–4853. [4] Qian H, Yao S J, Cai S Z, et al. Adaptive postdistortion for nonlinear LEDs in visible light communications[J]. IEEE Photonics Journal, 2014. [Crossref] [5] Wang C, Zhou Y J, Chi N. Research of LED's nonlinear distortion compensation algorithm in visible light communications[J]. China Light & Lighting, 2017(7): 9-15, 26. 王灿, 周盈君, 迟楠. 可见光通信中抗非线性方法的比较研究[J]. 中国照明电器, 2017(7): 9-15, 26 [Crossref] [6] Zhang M J, Shi M, Wang F M, et al. 4.05-Gb/s RGB LED-based VLC system utilizing PS-Manchester coded Nyquist PAM-8 modulation and hybrid time-frequency domain equalization[C]//Proceedings of the Optical Fiber Communication Conference, 2017: 1–3.