OFDM Applied In Underwater Acoustic Communication

OFDM is one kind of multi-carrier modulation technique. The main idea of OFDM is to transport high-speed serial data with a plurality of parallel?low-speed orthogonal subcarriers, thus, once the guaranteed bandwidth of each subcarrier is not greater than the coherence bandwidth of the channel, frequency selective fading would be avoided. Cyclic prefix can effectively overcome the impact of multipath effects; partial overlap between sub-carriers, it is possible to improve the spectral efficiency of the system. This thesis studied OFDM using in underwater environment, focusing on channel equalization and compensation of wideband Doppler effects.Firstly, analyzed the characteristics of underwater acoustic channel, then introduced modeling approach of channel, respectively, simulated channel of shallow sea and deep sea; Then, introduced the basic principles of modulation demodulation and several key technologies?the advantages and disadvantages of OFDM, cyclic prefix was also introduced. Then method of setting parameters was introduced.Secondly, analyzed technique of channel equalization in underwater acoustic OFDM system, studied the equalization channel method using VTRM and pilots. Wherein for channel equalization technique based on the pilots, this thesis analyzed different ways of inserting the pilots?different channel estimation criterions and different insertion criterions, compared effect of VTRM using LFM signal, m-sequence and gold-sequence.Then, introduced cause of Doppler frequency shift, studied approach of broadband Doppler estimation and compensation. For broadband Doppler shift, method of time-domain correlation was used to estimate Doppler frequency shift, method of resampling was used to compensate Doppler frequency shift.Finally, summarized main works of this thesis, pointed out the limitations of this article and working directions in the future.