Underwater Wireless Visible Light Communication System Based On MIMO-OFDM

Multiple-input multiple-output orthogonal frequency division multiplexing(MIMO-OFDM)is a hotspot in the research of wireless visible light communication.It can increase the reliability and bandwidth utilization.In this paper,underwater visible light communication based on MIMO-OFDM technology is discussed.On the one hand,the influence of seawater on the channel fading caused by absorption and scattering of light is mitigated.On the other hand,optical link collimation is reduced and the area of effective communication is increased.In order to thoroughly understand the OFDM technology,we first study the single-input single-output OFDM(SISO-OFDM)underwater wireless optical communication(UWOC)using a red-light laser.Although the red light suffers from large absorption,it has the advantage of small scattering effect.In addition,as red-light laser diodes(LDs)enjoy very mature manufacturing technologies,high power red-light LDs or LD array are widely available with higher responsivity for red light.The experimental results demonstrate that the modulation characteristics of the red-light LD is superior.A transmission rate close to 5 Gb/s and a transmission distance of 6-m are achieved by using OFDM together with power loading.We have numerically demonstrated that red light outperforms the blue-green light in highly turbid water in terms of both extinction coefficient and channel bandwidth.Since the reliability of UWOC in highly turbid water is a hard nut to crack,we envision that the directly modulated red-light LD-based scheme is a strong candidate for the future broadband UWOC systems that are robust to different water types.MIMO technology can achieve the spatial diversity gain,leading to effectively improved reliability.In this work,a complete MIMO-OFDM UWOC system using blue light-emitting diodes(LEDs)is established.We discuss the key technologies of the MIMO-OFDM system,such as synchronization,channel estimation,and space-time-frequency code.The system achieves a net rate of 27.082 Mb/s and a communication distance of 2-m in clear water.The 2x2 MO-OFDM UWOC experiments are carried out in different types of turbid water.The performance of repetition coding OFDM(RC-OFDM),space time block coding OFDM(STBC-OFDM)and space frequency block coding(SFBC-OFDM)systems are compared and analyzed.The BER performance of a multiple-input single-output OFDM(MISO-OFDM)signal was also measured as a benchmark for system performance comparison.Compared with the MISO system,the performance of the MIMO system is improved due to the received diversity gain.The delay of the received signal caused by the spatial diversity has a great impact on the system using the repetitive code.And for the STBC system,the received signals can be demodulated by combing two consecutive received signals in a specific way,such the delay of the receiving signal has no influence on the system.So MIMO-OFDM system with orthogonal space-time block code will have better performance than the system with repetitive code.