Overview: 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.
In addition, in the VLC system, a series of key devices such as LED light sources and power amplifiers have nonlinear effects, and the nonlinear distortion of the system would damage the system performance. The nonlinearity of the LED modulation curve is the main source of nonlinearity in the VLC system. The visible light signal is mainly modulated in the linear region of the LED to reduce the signal damage caused by nonlinear distortion. However, at this time, the luminous efficiency of the LED is low, and the illumination brightness generally cannot meet the lighting requirements of the actual application scene. What’s more, for high-order modulation signals, the peak-to-average power ratio (PAPR) of the signal is higher due to an increase in the number of signal levels, and is more susceptible to system nonlinearity. Therefore, in the MISO system, multiple LED lights can be used to transmit signals at the same time, low-order signals are modulated on each lamp, and high-order signals are generated by superposition of optical signals to resist nonlinear distortion of the LED at the transmitter. The simultaneous light emission of multiple LED lights can also increase the brightness of the lighting to meet the actual lighting requirements.
This paper studies the MISO visible light communication system based on pulse amplitude modulation (PAM), and proves the advantages of using this system in specific scenes by experiment. Based on 2×1 MISO visible light communication system, a novel equal probability coding mapping scheme for high-order PAM signals with two low-order PAM signals superposition in the optical domain is designed. In the experiment, we investigated and compared the BER performance under different bias tee voltage and Vpp for four systems which are single-input-single-output generation PAM7 signals with equal probability and unequal probability, two-input-one-output generation PAM7 signals with equal probability and unequal probability, respectively. The experimental results show that the scheme we proposed, for generating equal-probability PAM7 signals in 2×1 MISO system with the large average Euclidean distance between symbols and a lower PAPR, can effectively resist the inter-symbol interference and the nonlinear distortion of LEDs and enlarge the dynamic working area of the system. Finally, the system verification is performed through a net data-rate of 700 Mb/s transmission experiment utilizing a red chip of RGB-LED, which proves the feasibility and superiority of this scheme in practice.