Robust Channel Estimation Techniques For Underwater Acoustic Modulation Technologies

Recently,underwater wireless communication is becoming very essential and necessary for applications such as Countries Sea areas security,defence,and ocean science.Underwater communications have four main communication technologies namely underwater optical communication,underwater electromagnetic communication,underwater acoustic(UWA)communication and underwater magnetic induction communication.But the UWA communication is the most suitable one for long distance communications.The underwater acoustic(UWA)channel has many difficulties like time variability,long delay spread,Doppler spreading factor,attenuation,and bandwidth limitation.Because of these problems of the UWA channel,the reliability of communication system has been reduced and achieving high data becomes challenging.To overcome these issues,many modulation technologies have been proposed for UWA communication.For proper and exact recovery of transmitted data the perfect channel state information(P-CSI)needs to be known at the receiving side.Practically,the prior information of the channel is not often available to the receiver in practical.So,channel estimation is very critical and crucial for researchers.In this thesis,we have focused on channel estimation for different modulation technologies in UWA communication.Firstly,Filter bank multicarrier(FBMC)modulation technology is well-thought-out as a promising contestant for communication systems,which may be replacement of OFDM in terrestrial wireless communication and UWA communication.The channel is estimated by coding and auxiliary pilot for underwater FBMC system.Here,we have considered multipath UWA channel with additive white Gaussian noise(AWGN).Secondly,time domain synchronization orthogonal frequency division multiplexing(TDS-OFDM)attracts scientists/researchers due to efficient spectral efficiency,energy efficiency and fast synchronization in comparison to zero padding orthogonal frequency division multiplexing(ZP-OFDM)and conventional cyclic prefix orthogonal frequency division multiplexing(CP-OFDM).Moreover,its performance is also degraded by inter-block interference(IBI)over multipath channels due to its long delay spread.To elude IBI,dual pseudo-random noise(PN)sequence is developed,unfortunately,spectral and energy efficiency has been effected.As we know that the underwater communication is battery based and channel is bandwidth limited,due to the sparse nature of UWA channels compressive sensing(CS)has been exploited to obtain the time-varying CSI by spending a small redundancy.Here,IBI free-region proposes to estimate accurate UWA channel impulse response and mitigate its interference.For underwater TDS-OFDM system in a time-varying real sparse multipath channel,look-ahead backtracking orthogonal matching pursuit(LABOMP)based sparse channel estimation technique is proposed.Moreover,for UWA channel,the Doppler shift is estimated and is also compensated by the PN sequence.Finally,Spatial Modulation Technologies(SMTs)are very incredible techniques which lessen the inter-channel interference,inter-carrier interference(ICI),inter-antenna synchronization(IAS)and the system complexity for multiple-input multiple-output(MIMO)communication systems.However,SMTs have been attracted towards UWA MIMO communication systems due to high energy and spectral efficiency.Our research is based on the spatial modulation(SM)scheme,generalized spatial modulation(GSM)scheme and fully generalized spatial modulation(FGSM)scheme.All these schemes have one/multiple antennas active for data symbol transmission for UWA communication system in any defined time interval.P-CSI is acquired by the receiver to exact data detection in SMTs.Moreover,the perfect channel knowledge is not available in practical at the receiver end.The estimation of the channel is very critical to attain CSI.In SMTs,the pilot based recursive least square(RLS)adaptive channel estimation scheme has been proposed over the underwater time-varying MIMO channel.Furthermore,maximum likelihood(ML)based decoder is utilized to detect the antenna indices and transmitted data from the signal which is received at the receiver and also UWA MIMO channel is estimated.The proposed techniques performance has been evaluated and demonstrated through numerical computation of bit error rate(BER)and mean square error(MSE)using Monte Carlo iterations.Spectral and energy efficiencies have also been calculated in our work.Moreover,we have achieved higher spectral and energy efficiency as compared to conventional methods.Furthermore,we have also tested TDS-OFDM in underwater,where the experiments were performed at Wuyuan Bay,Xiamen,China.