Research On Underwater Acoustic OFDM Communication Technology

Compared with the radio communication on the land,the communication rate of the underwater acoustic communication is still very low.The speed of acoustic wave is low in the water,which makes the influence of multiple paths effect seriously to the communication in the underwater acoustic channel.And the inter symbol interference of high speed underwater acoustic communication is very serious.The multiple carriers modulation technology,which is represented by Orthogonal Frequency Division Multiplexing(OFDM),reduces the symbol rate by modulating the data into multiple parallel carriers,which can effectively resist multiple paths interference.The existing underwater acoustic OFDM modulation systems mostly transmit the two way result data of IFFT by I-Q orthogonal modulation.Which makes the communication system expensive and complex.A few systems choose to inserting zeros in frequency data before IFFT,and then only transmit the real way result,which takes up the extra carrier frequency,and causes the transmission power waste.In this paper,by comparing the existing underwater acoustic OFDM modulation solution,proposed the solution by insertion of zero sequence and conjugate symmetric sequence before IFFT processing.Designed the underwater acoustic OFDM communication system and implemented the design on the FPGA chip.In order to accurately seek out the starting position of the OFDM symbol at the receiver,studied the synchronization technology of OFDM underwater acoustic communication,proposed the frame synchronization solution by similarity detection of chaotic angle modulation sequence.Designed and generated the chaotic synchronization sequence.In order to correctly recover the send data,studied the channel estimation and compensation technology of OFDM underwater acoustic communication.And choose the block pilot estimation solution,simulated the estimation results of LS,MMSE,LMMSE.The simulation result proved that LMMSE is more effective.At last,built up a complete OFDM underwater acoustic communication system and carry out communication experiments,the experiments results show that the solution is feasible and reliable.When the transmission distance is 1.3m,the spectral efficiency is 0.74bps/Hz,the error rate is 0.074.