Fractional Fourier Domain Hopping Based On Time Reversal ICA For Underwater Acoustic Communications

Underwater acoustic(UWA)frequency hopping(FH)communication is a robust technique in the multipath and low signal-to-noise(SNR)environment.It is an appropriate choice for the severe channel conditions and for the low transmission-rate applications.In traditional FH communications,the frequency hops in the frequency domain.Therefore,the transmission rate is limited by the bandwidth.Considering its inherent merits,we extend the FH technique to different fractional Fourier domains(FrFDs),and realize the FrFD hopping in UWA communications.Since it applies the multiplexing scheme in different FrFDs,the transmission rate,together with the band efficiency,is improved significantly in the limited bandwidth.This multiplexing FrFD hopping is achieved by chirp modulation in the transmitter and fractional Fourier transform(FrFT)in the receiver.In the receiver,chirps with different frequency sweeping rates are used for modulation according to the FrFD hopping patterns.In the receiver,detection is conducted in the corresponding FrFD.Nevertheless,in the multiplexing FrFD hopping technique,there are two problems,namely FrFD delay interference and inter-domain interference(IDI).On one hand,the delay interference is caused by the delay characteristic of FrFT.The FrFD spectral magnitude of a received chirp has an impulse-response-like envelop with the optimal order.That will lead to the FrFD dispersion.On the other hand,the IDI is caused by the FrFD multiplexing.In a specific FrFD,the received chirp in current symbol will be affected by the multipath arrivals of the current and previous chirps with different frequency sweeping rates.The closer the center frequencies and frequency sweeping rates are,the severer IDI will be.However,it is hard to keep the distant center frequencies and frequency sweeping rates in the bandwidth-limited UWA channels.Then IDI cannot be neglected as well.To address these two problems,we propose a multiplexing FrFD hopping technique based on the time reversal(TR)and independent component analysis(ICA).The TR processing can deal with the FrFD delay interference,stacking the multipath arrivals in-phase.That will compress the delay spread and increase the SNR of the received signal.The complex fast ICA(CFast-ICA)can alleviate the FrFD IDI by separating the data signal from the IDI,on the premise of delay spread being compressed.The inherent amplitude ambiguity and order ambiguity can be eliminated by the peak to average power ratio(PAPR)in FrFD.The signal component with the largest PAPR is denoted as the data signal,while the others are denoted IDI.To verify the effectiveness and performance of the TR-ICA based FrFD hopping technique in UWA channels,we conduct a simulation and an experiment.In the simulation,we used 2?4 domains and 2-6 hops for the system.The UWA channels measured in the Xiamen Wuyuan Bay are adopted.The results show that the TR processing can decrease the delay spread in the receiver.When the hopping number is small,the data signal can be separated from the IDI as the mutual information is insignificant.The demodulation result is better than that of the conventional single-channel FrFT demodulation.In the field test,we use the same parameters in the simulation.The results are consistent with those in the simulation.The TR based demodulation outperforms the single-channel FrFT demodulation.When the hopping number is small,the chirps are mutually independent,making the data signal separated from IDI with CFast-ICA.The performance is improved further with the TR-ICA based method.Moreover,the mutual information should be kept below 0.2,and the channel correlation coefficients should be kept below 0.6,to make sure CFast-ICA work well.The contributions of this thesis are listed as follows.1.We explore a multiplexing FrFD hopping technique for UWA communications.The traditional frequency hopping communication is extended to the FrFD hopping communication.With the aid of multiplexing,the bandwidth efficiency increases linearly with the FrFD number.2.We use the TR processing to suppress the inherent FrFD delay interference.By applying the TR processing,the delay spread can be compressed,and the SNR can be improved.Besides,it helps preprocess the signal for the later instantaneous ICA.3.We use the complex fast-ICA algorithm to address the FrFD inter-domain interference.By applying this algorithm,the data signal can be separated from the inter-domain interference.The system performance can be enhanced further.