| In recent years,with the widespread application of light-emitting diodes(LEDs)and its increasingly perfect functions,it has brought earth-shaking changes to human society,and its lighting application has penetrated into all fields of people's daily life.At the same time,due to the electro-optical modulation characteristics of LEDs,LED-based indoor visible light communication(VLC)technology is gradually becoming a research hotspot in the field of broadband optical communication access.In addition,the rapid increase in mobile smart terminals with multimedia and Internet connection functions in recent years,as well as the promotion of various concepts such as big data services,cloud computing,and Internet+,have led to explosive growth in mobile data traffic.VLC has the advantages of large modulation bandwidth,no electromagnetic radiation and supporting high-speed data transmission.It supports and realizes a wireless access network with a centralized communication architecture for full-band applications between multiple bands such as radio frequency communication and millimeter wave communication to meet the current high-speed and large-bandwidth requirements of indoor big data access and transmission.Therefore,it is technically feasible and necessary to study the modulation and demodulation technology of multi-color multiplexed visible light signals for indoor big data access.In this paper,the basic structure and principle of indoor VLC access system are analyzed,and its transceiver mode and channel model are modeled and analyzed.The principle of modulation and demodulation of on-off keying(OOK),pulse amplitude modulation(PAM),quadrature amplitude modulation(QAM)and orthogonal frequency division multiplexing(OFDM)are introduced.The transmission characteristics of the two multi-color multiplexing VLC system are analyzed by numerical simulation and experiments.The application of various modulation and demodulation techniques in indoor VLC access system is analyzed theoretically and simulated experimentally.The research focus of this paper is to design an indoor four transmitter and one receiver VLC system based on PAM modulation and multiple-input multiple-output(MIMO)technology.The light intensity distribution and received optical power distribution on the receiving plane of the system are studied and compared with two optical transmitters.The transceiving performance of 4PAM signal and OOK-NRZ signal in indoor 4-transmitter-1-receiver MISO system and 1-transmitter-1-receiver SISO system is compared and analyzed.The research results show that four LED optical transmitters have better light intensity distribution characteristics and received light power improvement advantage;in the two systems of 1×1 SISO-VLC and 4×1MISO-VLC,with the increase of SNR,the receiving BER performance of the two signals is better,and MISO system performs better than SISO system.The research shows,the transceiver performance of the 4PAM signal after gray encoding is stronger than that of the 4PAM signal without Gray encoding.This paper also improves the traditional OFDM system and designs a DC biased optical OFDM(DCO-OFDM)access system to adapt to the transmission characteristics of VLC system.The transmission performance of QAM signal in this system is studied,and the BER performance of four QAM signals with different modulation orders under different bias current is compared and analyzed.At the same time,the BER versus SNR curves of 64QAM signals with different bias currents in DCO-OFDM system are analyzed.The results show that with the increase of bias current,the BER performance of QAM signals with different orders is better,and when it reaches a certain degree,the BER performance decreases.Under the same receiving SNR,with the decrease of bias current,the receiving BER performance is better,and under certain conditions,the BER can be lower than 10-3;When the BER of the received signal is 10-3,the 64QAM-OFDM signal has a clear constellation.Finally,a 100km underwater optical communication system is designed,and the transmission performance of 10Gbit/s 4PAM signal in the system is studied.The experimental results show that the system has good BER performance,and 4PAM signal has higher spectral efficiency than traditional NRZ signal. |
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