Research On Multicarrier Modulation For Underwater Wireless Optical Communications

Underwater optical wireless communication(UOWC)is a practical underwater wireless communication means based on the "window characteristics" and high data carring capacity of light in underwater transmission.It is a practical underwater wireless communication scheme,which has received extensive attention.In order to achieve efficient and practical UOWC,transmission distance and communication speed are needed to be taken account of,and ensured that the system has good robustness.Compared with the laser,the light emitting diode(LED)has a simpler structure and a larger scattering angle,which can reduce the failure rate of the system and reduce the requirements for the collimation of the UOWC system.Because the water body has a serious attenuation effect on light,it makes the optical signal arrive at the receiver very weakly.In order to expand the UOWC communication distance,a new type of high sensitivity receiver,single photon avalanche diode(SPAD),has been used as an effective scheme.Orthogonal frequency division multiplexing(OFDM)can be used to improve the communication rate of the UOWC system by assigning different amounts of data and power at different subcarriers to make the system communication performance close to the channel capacity.Therefore,the communication system using the LED as the transmitter,the SPAD as the receiver and the OFDM as the modulation scheme is a kind of system which is suitable for underwater wireless communication.The research on this system is the main work of this paper.The main research content and the research results are summarized as follows:1.In order to improve the communication performance of optical OFDM system,according to the amplitude frequency response characteristic of LED,the minimum Euclidean distance maximization is used as the criterion to optimize the data amount and power distribution of each subcarrier of optialc OFDM.As the channel,the LED has a strong frequency selection characteristics for optical signals.In conventional wireless communication,OFDM adopt proper modulation size and power allocation of each subcarrier,so that the signal has the ability to resist the channel frequency-selectivity,and makes the communication performance of the system close to the channel capacity.Because the traditional wireless channel is a bipolar complex channel,and the optical channel is a unipolar real channel,the method of the data and power allocation of the OFDM subcarriers in the traditional wireless communication can not be directly used for optical communication.According to the amplitude frequency response characteristic of LED,this paper studies the data and power allocation of each subcarrier in OFDM system.With the minimum Euclidean distance maximization as the criterion,the data and power allocation will be reduced to the joint optimization problem,which can be used as the solution model for problems.The optimized scheme effectively resists the frequency-selectivity caused by LED,and gains the BER performance of the system significantly.2.Aiming at the problem that the signal with Poisson noise has no closed-form solution after the FFT,the AR transform is used to improve the optical OFDM with SPAD,and the performance of the system is improved.When SPAD is used as a receiver,the noise of the received signal is transformed from the traditional additive Gauss white noise(AWGN)to additive Poisson noise.The distribution characteristics of the signal containing Poisson noise after FFT has no closed-form yet in mathematics,so a proper receiver can not be designed.Since FFT is an orthogonal transformation,the AWGN after FFT is still satisfied with the Gauss distribution.Using this property,the Anscombe root(AR)transformation employed in the optical OFDM system with SPAD,and the received signal noise is converted from additive Poisson noise to approximate AWGN.After that,the AR inverse transform is added to the transmitter,and an optical OFDM system with SPAD is improved.The traditional Euclidean distance detection scheme is used to compare the improved system with the conventional system,and the improved system has significant performance improvement.3.The communication performance of UOWC system with LED as the transmitter,SPAD as the receiving device and OFDM as the modulation mode is verified,and the underwater voice wireless optical communication system is finally realized.The performance of communication system with LED as transmitter,SPAD as receiver and OFDM as modulation mode is tested offline.Then,combined with the voice chip,the underwater wireless real-time voice communication system is completed,and achieved a high resolution underwater wireless voice communication.