Research On Orthogonal Signal-Division Multiplexing Over Time-Varying Underwater Acoustic Communications

Since the beginning of the 21st century,the status and role of the ocean in territorial sovereignty,national security and economic development has gradually become prominent,with an unprecedented"ocean fever"sweeping the world.As the key technology serving various fields of marine engineering equipment,underwater acoustic communications have entered a high-speed development stage in recent years.Multicarrier modulation with orthogonal frequency-division multiplexing(OFDM)and single-carrier modulation with frequency-domain equalization(SC-FDE)are two popular techniques widely used for underwater acoustic communications.Although both technologies have their own advantages,they also have limitations.To this end,we hope to adopt a new modulation scheme in the physical layer,which can not only retain the advantages of OFDM and SC-FDE,but also avoid their disadvantages as much as possible.At the same time,the signal structure is conducive to achieve higher bandwidth utilization.Orthogonal signal-division multiplexing(OSDM)is a newly emerging modulation scheme which connects OFDM and SC-FDE in a unified framework.It is expected to realize the integration of these two technical camps.This dissertation focuses on channel estimation,channel equalization,diversity processing and other key technologies for OSDM underwater acoustic communications.Meanwhile,the performance of related theoretical methods is verified by numerical simulations and lake experiment.The main contributions of this dissertation are summarized as follows.1.The characteristics of underwater acoustic channels,such as propagation loss,environmental noise,multipath effects and Doppler effects are introduced and analyzed in detail.In order to more accurately represent the characteristics of the time-varying multipath underwater acoustic channel,in this dissertation,the complex exponential basis expansion model(CE-BEM)is used to approximate the doubly-selective channels.2.The low-complexity block and serial OSDM equalization algorithms based on doubly-selective channels is studied.It makes use of the cyclically block-banded structure of the composite channel matrix,and reduces the computational burden through the transformed domain processing.The proposed two equalization algorithms are implemented by block LDL~H factorization algorithm and block iterative matrix inversion algorithm respectively,which can greatly improve the ability of OSDM system to resist inter-vector interference(IVI).Numerical simulations indicate that the proposed OSDM equalization algorithms outperform their OFDM counterparts.At the same time,OSDM could be potentially considered for high-rate underwater acoustic communications over doubly-selective channels.3.To reduce the error introduced by using CE-BEM approximate doubly-selective channels,the OSDM equalization method based on approximate enhancement of diagonal-block-banded(DBB)matrix is proposed.By introducing the time-domain window function which satisfies the minimum band approximation error(MBAE)criterion and the sum-of-exponentials(SOE)constraint,the DBB matrix structure approximation error can be reduced.Furthermore,based on the DBB matrix enhancement and the block LDL~Hfactorization,a low-complexity OSDM equalization algorithm is designed.Numerical simulations indicate that,by using the enhanced DBB approximation,the leakage of IVI and inter-symbol interference(ISI)compared to that using a direct DBB approximation can be significantly reduced,and the proposed OSDM equalization algorithm can mitigate the Doppler-induced performance loss effectively.4.The channel estimation method is designed for OSDM systems,which uses frequency-shifted Chu sequences as pilot vectors.It invokes the property that the Chu sequences and their DFTs are constant-modulus to avoid matrix inversion in channel estimation,which contributes to reducing the computational complexity to a certain extent,and therefore benefit the engineering implementation.Meanwhile,to further simplify the channel estimation,the zero vectors are added around the pilot vectors to achieve decoupling with the data vectors.5.To further explore diversity gain to enable a more reliable transmission compared to standard symbol-rate sampling OSDM,the time-domain oversampled OSDM is proposed.Each vector in time-domain oversampled OSDM is equivalently transmitted over multiple virtual channels,and thus diversity gain can be achieved.Numerical simulations and experimental results indicate that the performance of the OSDM system improves as the oversampling factor increases.