Research On Digital Signal Processing Applied In High Speed Underwater Optical Wireless Communication System

As the increasingly ongoing expansion of human activities in ocean environments,such as marine mineral resources exploration,oceanographic research,marine biological exploration,and marine environment monitoring,etc.,the exchange of large volumes of marine data between underwater equipments is urgently required and indispensable.The rapid harvesting these massive data puts forward a higher requirement to the communication capability of underwater equipments.Compared with conventional submarine cable communication,underwater acoustic communication,and radio frequency communication,cooperation with underwater mobile platform and seafloor observatory network,UOWC featuring high communication rate,low time latency,cost-effectiveness,and good maneuverability,will pave a way for fast and real-time harvesting huge volumes of marine data.UOWC is an important communication technology,which will be integrated into the space-sky-ground-ocean wireless communication network in the future.Based on main seawater optical properties,an UOWC channel model in seawater is established by means of Monte Carlo simulation.The distribution of light intensity and channel impulse response are analyzed by numerical simulation in different turbidity of water and various transmission distances when LEDs or LDs are employed.LED-based UOWC system is suitable for short-distance underwater transmission application due to its characteristic of omnidirectional coverage,while LD-based UOWC system,fits well in long-distance high-data rate underwater communication application with the merits of high bandwidth and excellent beam direction.In terms of single-carrier modulation,pulse amplitude modulation(PAM)UOWC system based on decision feedback equalization(DFE)is proposed,and the effectiveness and robustness of DFE are experimentally demonstrated in different water environments.The experimental results show that data rate of 10.20Gbps(OOK),16.94Gbps(PAM4),and 17.69Gbps(PAM6)are achieved over a 5-m underwater transmission.Conventional linear equalizer can not compensate the nonlinear impairment and distortion induced by the nonlinear effect.The effectiveness of single-carrier nonlinear equalization is experimentally verified in the LED-based single polarization/polarization multiplexing UOWC systems and the results show that compared with linear equalization,3.44dB signal-to-noise ratio(SNR)gain and>2.5dB received sensitivity improvement can be achieved by the nonlinear equalization in the single polarization UOWC system.Compared with the single polarization system,the transmission rate approximately doubles in the polarization multiplexing UOWC system.In order to lower the peak-to-average power ratio(PAPR)of multi-carrier UOWC system,DFT-S OFDM modulation is proposed to improve the system performance.The effectiveness and practicability of DFT-S OFDM are experimentally demonstrated in different water environments.The experimental results show that compared with discrete multi-tone(DMT)modulation,>3dB received sensitivity and>17%capacity enhancement can be achieved by the DFT-S OFDM.In order to compensate the nonlinear impairment and distortion in the multi-carrier system,time-domain feed forward nonlinear equalization(TD-FFE)and time-domain iterative decision feedback nonlinear equalization(TD-DFE)based on Volterra model are adopted.The experimental results show that nonlinear equalization can effectively combat the nonlinear interference.Compared with the linear equalization,4.16dB SNR gain and?1 Gbps capacity enhancement can be obtained by the TD-DFE.To further reduce the number of equalizer taps in nonlinear channel equalization,compressed sensing is utilized to deal with the UOWC channel.The experimental results show that UOWC channel is deemed to be sparse,and the Orthogonal Matching Pursuit(OMP)algorithm can reduce the number of estimated parameters by 76.19%,while the performance degradation is negligible.Finally,probabilistic constellation shaping(PCS)is utilized to obtain shaping gain to further approach the channel capacity limit in the UOWC system.PCS-DMT technique can experimentally achieve a substantial capacity improvement compared to the widely used bit-power loading scheme.With LD modulation bandwidth of 4GHz,a net data rate of 19.96Gbps over 5-m,19.43Gbps over 15-m,and 19.02Gbps over 25-m underwater transmission is realized.Note that 19.96Gbps is a new record for the highest net data rate in a single LD-based UOWC system.To alleviate high frequency fading of the PCS-DMT system,precoding is introduced to improve the system performance.The feasibility of precoding PCS technique in the UOWC system is experimentally demonstrated.The experimental results show that received sensitivity can be improved by 2.39dB owing to the precoding PCS-64QAM.