Design And Implementation Of Equalizer For Multicarrier Underwater Communication System

With the rapid development of utilizing and developing the resource of the ocean,underwater communication becomes an essential technology for military and civilian purposes. Unlike the air medium, acoustic signal is the only medium propagate long distance effectively under the water in the lakes and ocean. OFDM technology can adapt itself to narrow bandwidth and strong interference of multipath in underwater acoustic channel. We will design effective channel estimator and equalizer to avoid inter-symbol interference, as well as improve the performance of the underwater acoustic communication system and resistance of frequency- selective fading.Here we studied algorithms of channel estimator and equalizer based on the typical underwater acoustic communications system. Based on the performance of the receiver in different algorithm structure and by simulation in different channel environment,combined with the limitation of FPGA, we designed receiver and equalizer which give satisfactory performance in implementation of the hardware. The main contents were as follows: Firstly, the characteristics, basic principles, and development status of OFDM underwater acoustic communication system were outlined in this paper. The present research and development status of channel estimator and equalizer were expounded and summarized. The advantages of OFDM in underwater acoustic communication system and the importance of channel estimator and equalizer in elevation of performance of underwater acoustic communication were described in this paper.Secondly, according to the characteristics of narrow bandwidth, strong interference of multipath and frequency- selective fading in underwater acoustic channel, we studied the principle of the algorithm in channel estimation and equalization and summarized the advantages and disadvantages of typical algorithms. Several algorithms and structures applied to OFDM underwater acoustic communication system were proposed and confirmed as the key subject in the research. Thirdly, the entire model of receiver in OFDM underwater acoustic communication system, along with the channel estimation and equalization module, was established in Matlab. As the basis of simulation of different environment by adjusting channel parameters, corresponding algorithms and improved design were simulated in Matlab. We presented the optimal algorithm forimplement in FPGA hardware system by comparing error rates in various signal-to-noise ratios. Fourthly, equalizer applied in FPGA platform and corresponding algorithms of channel estimation and equalization were designed after simulation and debugged within the whole receiver system. Data buffers between the FPGA and upper PC were developed by analyzing hardware resource and computing complexity for testing and application. At last, we experimented in bucket and lake with equalizers,transmitters and receivers and conducted underwater acoustic and directional transmission by parametric array transmitter, which validated the resistance of strong interference of multipath and frequency-selective fading, and ensured favorable error code rates.Adding the equalizer can significantly improve the performance of avoiding the inter-symbol interference. The BER reaches 10-3 at 25 dB SNR. And the application on FPGA also increases the value of the equalizer.