Spatial And Temporal Turbo Communication System Over Doubly Spread Underwater Acoustic Channel

This thesis focus on efficient and reliable communications over channel uncertainties. With the information theory, signal processing and propagation physics as the basis, we construct the optimum transient observer in communication problem, which is the Turbo communication system that can approach the Shannon channel capacity bound. Combined with propagation physics, we study the deconvolution image formation problem of dy-namic sparse time delay and Doppler doubly spread channel, which is an underdetermined inverse problem. After we derive the doubly spread channel estimation from image for-mation, we combine it with Turbo communication system which approaches the Shannon channel capacity bound to get the product:a novel Turbo communication system over doubly spread channel. Because of the spatial-temporal array can bring tremendous per-formance gain of channel capacity and communication error ratio by multiplexing and di-versity, we naturally combine the optimal array processor, that is, the general sidelobe can-celler (GSC) with Turbo communication system. The product is a novel spatial-temporal-frequency GSC. The communication speed and reliability of communication system can be improved tremendously by diversity in spatial, time delay and frequency dimensions. Fi-nally, with the combination of spatial-temporal-frequency GSC and Turbo communication system, we get the spatial and temporal Turbo communication system over doubly spread channel.Turbo communication system is an optimum transient observer that can approach the Shannon channel capacity bound. The Turbo principle can be formulized as follow:per-form sequential Bayesian estimation of symbols and codes with iteratively updated extrin-sic information as a priori information. The Turbo principle is in essence composed of three basic elements:(1) the state-space model;(2)the innovations process and (3)feedback. So it is the optimum transient observer of communication problem. In order to study the ef-ficiency of Turbo communication system, at first we need to answer the question about communication performance bound, that is, the bound for reliable communication speed under some channel environment. Channel coding theory implies that there exist commu-nication systems that can reach channel capacity bound. Moreover, the bound is tight. For those reasons, channel capacity can be used as a standard for the quantative judgement of the efficiency of Turbo communication system. In the example of this thesis, the difference between the Turbo communication system and the minimum input signal-to-noise-ratio for reliable communication is1.4dB, which shows the efficiency of Turbo communication system.In order to deploy the Turbo communication system in doubly spread channel, we need the help of propagation physics. Based on the wave equation, this thesis analyzes the physical formation of sparse time delay and Doppler doubly spread channel. With the wavenumber integration method, we derive the description function of wave field caused by moving source in the waveguide. Because the depth seperated Green function only ex-ists nonzero values in a few horizontal wavenumbers, the sparsity of doubly spread chan-nel is promised in the physics sense. The deconvolution image formation of time delay and Doppler doubly spread channel is an underdetermined inverse problem, so the con-ventional overdetermined least square method can not be used to solve that problem. The channel sparsity priority, which is derived from propagation physics, is exploited by the constrained optimization method to introduce constraint on the solution space. That makes the underdetermined inverse problem solvable. The performance of constrained optimiza-tion method is better than that of the cross ambiguity function method, which solve the inverse problem from the forward direction. That is because the cross ambiguity function method does not utilize the physical priority. Because of the impacts of sea surface wave and internal wave and so on, the time delay and Doppler doubly spread channel also exhibit the dynamic property. The sequential image formation of sparse channel should be devel-oped. Actually, it can be found out that the constrained optimization problem is equivalent to find the intersection point of all the constrain sets. Every sample time, the received data will cause one new constrained set, keep projecting on the constrained set will finally lead us to the true solution of the channel.Since the dynamic sparse time delay and Doppler doubly spread channel image for-mation problem has been solved, we combine the time delay and Doppler doubly spread channel estimation with the Turbo communication system that approaches the channel ca-pacity bound. Traditionally, the time delay and Doppler doubly spread channel has been recognized as one of the biggest challenges of underwater acoustic communication. The traditional coherent communication technique which is based on the Doppler frequency shift compensation can not achieve reliable communication over such channel. The first combination tries to treat such channel as a chance and get processing gain from it. The so obtained new processor can be seen as the optimum transient observer over underwa-ter acoustic doubly spread channel. From the structure of the processor, we know that the equalizer part achieve receiver diversity in both temporal and spatial dimension. It can op-erate reliably under time delay and Doppler doubly spread channel. The innovation process iteration between the equalizer and decoder can reduce the symbol error ratio and bit error ratio tremendously, which make the system to approach the Shannon channel capacity and obtain communication efficiency.After we have achieved the first combination between the doubly spread channel es-timation and the Turbo system, we turn our attention to a larger field. That is, to combine our previous developed system with the spatial and temporal array processing technique. At first we need to answer why we should pursue such technique? How much process-ing gain can we get from it? This thesis answer such problems from several viewpoints. At first we derive the pairwise error probability of communication systems operating in fading channel as (2). P (error)～Gc·SNR-Gd (2) where Gc represents coding gain and Gd represents diversity gain provided by spatial and temporal array communication system. The diversity gain Gd can make the pairwise error probability to decrease exponentially as the signal-noise-ratio increase. Obviously the di- versity gain can provide more considerable performance improvement than the coding gain. For the spatial and temporal array Turbo communication system over dynamic time delay and Doppler spread channel, Gd∝nrnt(M+1)L, which means that one can improve the system performance by diversity in transmitting, receiving, multipath and frequency dimension. So from the viewpoint of reliability, which is the control of pairwise error probability, the development of spatial and temporal Turbo array communication system will lead to considerable advantages. From the Shannon channel capacity viewpoint, the spatial and temporal array communication system can make the channel capacity increase linearly with the effective channel number by multiplexing. In the case of fading channel, the spatial and temporal array system can decrease the interrupt probability exponentially as the diversity order at a fixed interrupt capacity. So the spatial and temporal array com-munication system can improve the efficiency and reliability remarkably by multiplexing and diversity. So it is necessary to achieve the combination of Turbo communication sys-tem over doubly spread channel with the spatial and temporal array processing technique. The result is the spatial and temporal Turbo communication system over doubly spread channel.In order to make the optimum array processing system to operate reliably over doubly spread channel, we combine the equivalent counterpart of optimum array processor, that is, the GSC with the doubly spread channel estimation naturally. From the combination we derive a novel spatial-temporal-frequency GSC. Communication system can improve the communication speed and reliability tremendously by diversity in spatial, time delay and frequency dimensions. Finally, with the combination of spatial-temporal-frequency GSC and Turbo communication system, we get the spatial and temporal Turbo communication system over doubly spread channel. The advantages of the processor is summarized as follows.1. The time delay and Doppler doubly spread channel estimation is incorporated into the processor explicitly. The spatial-temporal-frequency GSC can improve the sig-nal quality and suppress inter-symbol interferences and cross channel interferences from coherent diversity in spatial, temporal and frequency dimensions. Since the jamming problem caused by cross channel interferences has been solved properly, we can improve the communication speed of the system with multiplex technique tremendously. So the communication efficiency is guaranteed.2. Turbo communication system is the optimum transient observer which can approach the Shannon channel capacity bound. By incorporating the Turbo principle, the reli-ability of system can be guaranteed by the knowledge of the coding structure in the transmitted signal.3. The replica vector of the spatial-temporal-frequency GSC is the functional of channel image formation result. By iteration of Turbo system, the channel image formation result will become better, that in return improve the performance of array processing. All in all, the novelties of this thesis can be summarized as follows.1. the dynamic sparse time delay and Doppler doubly spread channel image formation;2. the combination of time delay and Doppler doubly spread channel estimation and the Turbo communication system which can approach the Shannon channel capacity bound;3. the combination of the Turbo communication system over doubly spread channel and the spatial-temporal array processing technique.The remainder of this thesis is organized as follows. Chap.1is the introduction of the thesis background. Chap.2,3and4correspond to the first, second and third novelties of this thesis. Chap.5presents a sea experiment to verify the theory in this thesis. Finally, the thesis is concluded and future research directions are given in Chap.6.