Two input one output visible light communication system based on pulse amplitude modulation 

In recent years, new types of services such as mobile Internet and mobile media have been gradually developed and popularized, which has led to an increasing demand for high bandwidth. However, improvements in the existing RF bands to improve spectrum utilization are limited. Therefore, researchers in the wireless field have to expand the communication band to high frequencies,  i.e., from millimeter waveto microwave, to terahertz wave and even further. Wireless spectrum resources have been carefully divided, and its use and distribution are strictly regulated. With the popularization of semiconductor lighting technology, LED-based visible light communication technology has attracted widespread attention and has become a hot spot in the field of wireless optical communication. The visible light communication (VLC) technology uses the on and off of the LED lights to transmit information while achieving illumination. Based on LED lights, VLC enables a combination of LED lighting and communication. The modulation of optical signals is realized by the control of the LED driving current, and the optical signal is converted into an electrical signal by the photodiode during reception, thereby completing the information transmission. Compared with wireless communication, VLC is an emerging communication method that combines lighting and communication. It has many irreplaceable advantages, such as, green security, convenience, security, good performance, no need for authorization of spectrum resources, and many application scenarios.

    The research team of Prof. Chi Nan, Department of Communication Science and Engineering, School of Information Science and Engineering, Fudan University, is dedicated to solving various problems in visible light communication, and proposes various optimization algorithms, codec designs and advanced modulation formats. 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 PAM4 signal is separately modulated on the two LEDs, and the PAM7 signal is generated by superimposing the optical signals during transmission.


Fig. 1  The schematic diagram of PAM7 signals generated by two PAM4 signals superposition.
(a) Unequal probability PAM7 signals; (b) Equal probability signals

    Fig. 1(a) is the schematic diagram of unequal probability PAM7 signals generated by two PAM4 signals superposition. The PAM4 signal has 4 levels and the two signals are superimposed to produce a new 7-level signal. In theory, each PAM4 signal transmits 2 bits of information, and the two signals can transmit 4 bits of information in total, with a total of 16 mapping modes. Since the different level stacking schemes may generate the same new level during the transmission of the optical signal, the new signal generated by the superposition of the two PAM4 signals has only seven levels, namely the PAM7 signal. Since the PAM7 signal only needs 3 bits, the coding efficiency can be improved by mapping 4 bits’ information into 3 bits. The probability of each symbol in the generated PAM7 signal is not equal, and the probability of occurrence of symbol 0 is the largest, which is 1/4. The probability of occurrence of the low-level symbol is large, and the probability of occurrence of a high-level symbol is low, so the average power is reduced and the corresponding PAPR is increased. Devices such as power amplifiers at the receiving end also have nonlinear characteristics, and signals with a large PAPR are susceptible to nonlinearities of the device and it will cause distortion on signals. Therefore, we hope that the PAM7 signal generated at the receiving end is with equal probability. It is necessary to perform inverse coding mapping on the two PAM4 signals at the transmitting end, and adjust the probability of occurrence of each symbol of the two PAM4 signals to obtain a PAM7 signal with equal probability after superposition. Seven points are selected from 16 intersection points as the inverse mapping points for generating the equal probability PAM7 signal. As drawn in Fig. 1(b), seven points with the largest distance from each other are selected as the mapping of the PAM7 signal, so that the average Euclidean distance between the symbols at the transmitting end is the largest, thereby reducing signal inter-symbol interference. The probability of the seven levels is equal, which is 1/7. As a result, the probability of two symbols transmitting at the two transmitters is not equal. Each transmitter only needs to send three different levels of symbols to generate equal probability PAM7 symbols at the receiving end. From Fig. 1(b), it can be found that in the three-level signal, the probability of occurrence of two level symbols having a small interval between adjacent symbols is small and equal, the probability is 2/7. The interval between level with highest probability and its adjacent level is larger, and the highest probability is 3/7. Therefore, this coding method can effectively reduce inter-symbol interference of signals.


Fig. 2  The block diagram and experimental setup of two input one output visible light communication system based on PAM modulation

    At the same time, 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.

Team
The research team of Prof. Chi Nan, Department of Communication Science and Engineering, School of Information Science and Engineering, Fudan University, is mainly engaged in research on high-speed optical communication and high-speed visible light communication, focusing on high-spectrum efficiency multi-dimensional multi-order light. Modulation technology and digital signal processing technologyinclude various advanced modulation formats, codec solutions and various optimization algorithms. The research team has more than 20 Master and PhD students. The team published a total of 412 papers, including 261 papers in journals indexed by SCI, with total citations of 2479 times. In the past 5 years, they published 154 papers in journals indexed by SCI, and were cited 1832 times by others,  4292 times (Google Scholar). The h-index of Prof. Chi Nan is 32. Up to now, he has 43 patents (12 of which have been authorized).


Article
Shi Meng, Zhang Mengjie, Chi Nan. Two input one output visible light communication system based on pulse amplitude modulation[J]. Opto-Electronic Engineering, 2019, 46(5): 180306.
DOI: 10.12086/oee.2019.180306