Research On QAM-OFDM Underwater Wireless Optical Communication System Based On LED Array Light Source

With the deepening of the development and utilization of diverse marine resources,underwater communication technology has received more and more attention.The underwater wireless optical communication can complement the shortages of traditional underwater acoustic communication due to its high transmission speed,large capacity,high bandwidth,low delay,and low power consumption,which is more suitable for the transmission of short or medium ranges and high data rates（above～Mbps）.In this case,underwater wireless optical communication techniques are of broad application prospects.Consequently,designing and developing a robust,low-cost,and miniaturized optical transceiver is key to realizing underwater wireless optical communication.Although LED has several disadvantages,such as narrow modulation bandwidth,weak light intensity,and short transmission distance,it is still quite suitable to act as the transmitter for the optical communication systems within the complex and fickle underwater environment.First,employing a LED transmitter can significantly lower the link alignment requirement due to its large beam dispersion angle.Besides,the data rates of LEDs can reach tens or hundreds of Mbps through high-order modulation.Third,using an array-structured light source,the signal strength can be greatly improved and the transmission distance can be accordingly extended.Last but not least,LED is economical,significantly reducing the deployment cost.On the basis of these points,there is a growing trend to develop LED-based transmitters for underwater wireless optical communication for the purpose of practicality.Therefore,this thesis mainly researches the underwater wireless optical communication system based on LED array light source.The main works are as follows.（1）A free lens for underwater spot shaping is designed.Simulations compare the effect of the lens on spot and light intensity when a single LED and an LED array are respectively applied.At the same time,the light intensity in different water types are compared.Finally,a real object of the lens is produced and its performance is tested.（2）Two optical transmitters and an optical receiver based on LED array light sources are designed.The underwater wireless optical communication system is built and its performance is tested.An experiment is conducted,and the underwater wireless optical transmission is realized.（3）The design and simulation verification of the overall module and each sub-module of QAM-OFDM modulation are completed using FPGA.An m-QAM-OFDM underwater wireless optical communication is set up and and its performance is tested.The experimental results show that the BER is 3.359×10^-3 at the transmission distance of 5m and the maximum transmission rate of 50Mbps,which is lower than the FEC threshold.Additionally,the system BER with/without FFLs are compared,and it is found that BER can be reduced by two orders of magnitude with FFLs.Finally,BERs at the receiver end are measured,and it is verified that the receiving quality is satisfactory within a large distance from the optical axis.