Channel Simulation And Transmission Experiment Of Faster-than-Nyquist Underwater Blue-green Laser Communication

The ocean covers more than 70 percent of the global area and contains abundant resources.One of the necessary conditions for marine exploration is the capability of underwater communication.Underwater acoustic communication is a mature means of commun ication at present,but due to the influence of multi-path interference and Doppler Effect,the transmission bandwidth of underwater communication is narrow and the delay is high,which cannot meet the requirement of high-speed transmission.Underwater Wireless Optical Communications(UWOC)provides large bandwidth transmission with low latency and high security.However,the scattering of light from seawater results in temporal dispersion,which limits the channel bandwidth.In addition,photoelectric modulation and receiver limit the bandwidth.Bandwidth limitations of two aspects affect transmission performance and lack of effective compensation methods.In this paper,we studied the bandwidth limitation of underwater channel and transceiver by simulation and experiment respectively,and adopted the the maximum likelihood sequence estimation(MLSE)equalization algorithm to compensated the bandwidth in order to improve the system performance.The innovation and main work of this paper include the following contents:First,based on the Monte Carlo algorithm combined with Fournier Forand volume scattering phase function,we establish the simulation model of photon motion in water,analyze the channel impulse response of UWOC system in the conditions of different transmission distances and different fields of view(FOVs)of the harbor water,and calculate the corresponding channel bandwidth.The simulation results show that the temporal dispersion produces intersymbol interference(ISI),which causes the severe channel bandwidth limitation in turbidity waters.For the harbor waters,when the transmission distance increases from 8 meters to 16 meters,the channel bandwidth decreases from 0.5GHz to 0.1 GHz.Second,based on the channel impulse response obtained by Monte Carlo simulation,we simulated the bandwidth limitation and compensation scheme of UWOC system in the conditions of different transmission rates and different transmission distances,and verified the performance improvement effect of MLSE equalization algorithm.Simulation results show that,based on MLSE equalization detection and comparing the symbol-by-symbol(SbS)detection scheme,the maximum bit rate that meets 7%forward error correction code(FEC)is increased 1from 0.4 Gbps to 0.8 Gbps within 12 meters.When the signal is transmitted within 10 meters at a rate of 0.8 Gb/s,the power consumption is reduced by 4dBm.Third,we built an underwater faster-than-Nyquist transmission experimental platform and studied the transceiver side's bandwidth limitation in the UWOC system.Based on MLSE equalization detection,0.9Gb/s and 1.1 Gb/s on-off keying(OOK)transmission within 1 meter can be achieved under the condition that the system bandwidth is only 167 MHz,which is more than twice as high as the data rate achieved by SbS detection.