GLRT For Soread Spectrum Communications:Joint Channel Estimation,Equalization And Symbol Detection

Based on signal processing, information theory and propagating physics and aimed at the effective and reliable communication under channel uncertainty, joint channel estimation, equalization and symbol detection, is studied for designing spectrum code division multiple address(CDMA) underwater spatial-temporal array communication system in this thesis under the framework of the generalized likelihood ratio test(GLRT).Spread spectrum is a method of transmission in which the signal occupies a bandwidth in excess of the minimum necessary to send the information; the band spread is accomplished by use of a pseudorandom code which is independent of the data, and a synchronized correlation/matched filter reception with the same code at the receiver is used for dispreading and subsequent data recovery. The redundancy pseudorandom code inherent makes the transmitted signal not only have the property of large time-bandwidth product but also appear similar to random noise, so a multiplicity of benefits such as antijamming/antiinterference, low probability of intercept and so on can be obtained. In multiple-access communication, the transmitted signals can also be distinguished one from another by superimposing a different pseudorandom code, so that multiple users can simultaneously use a common channel for transmission of information. Such a type of communication technique is called code division multiple access(CDMA).Direct sequence and frequency hopping is two common spread spectrum modulation. Direct sequence spread spectrum utilizes the pseudorandom code modulate phase shift keying(PSK) signal to achieve instantaneous spreading of transmission bandwidth. Frequency hopping spread spectrum is different from direct sequence spread spectrum, in each signaling interval, it randomly occupies one or more frequency slots within a large number of contiguous frequency slots in advance according to the output of pseudorandom code generator. Compared to other modulation techniques with no spreading, spread spectrum modulation improves the reliability of system at the expense of widening band. Although CDMA technique enhances spectral efficiency/total capacity per chip relative to single user spread spectrum modulation, the loss in spectral efficiency still exists even for random spreading CDMA, which accurately models the situation where pseudorandom code span many symbol periods, relative to deterministic spreading CDMA and multiple-access with no spreading.Ocean waveguide environment characteristics, especially shallow ocean waveguide environment characteristics, are seriously randomly time-varying delay spread and Doppler spread caused by fluctuations along the boundaries and in the media, both of which can be rapidly time-varying. Therefore, if a signal is transmitted in such channel, the received signal will present time and frequency selective fading. In addition, multiple-user interference, intensions interference and limited bandwidth etc. also affect the performance of underwater acoustic communication system. So if we want to achieve large distance, effective and reliable underwater communication, spatial-temporal array communication must be adopted.The ambiguity function of the large time-bandwidth product waveform has low sidelobe, and "the more disorderly waveform is, the better", which means that the more disorderly wave form has the closer temporal and Doppler spread directivity. So, pseudorandom code is impressed on the transmitted signal at the modulator and removed from the received signal at the demodulator in this thesis.Using the duality of space and time, the ambiguity surface of the large space-bandwidth product array has low sidelobe, and "the more disorderly array is, the better", which means that the more disorderly array has the closer three dimensional directivity. So, the transmitted array and received array are all designed as the "pseudorandom" double spiral linear array (DSLA) in this thesis so as to obtain effective transmission and receiption by transmiting and receiveing beamforming.Based on the geometric representation of signals and signal space analysis, in the case where the messages are equiprobable apriori, the optimal detector that minimizes probability of error in the additive white Gaussian noise channel is maximum-likelihood detector or minimum-distance detector. The resulting receiver is correlation receiver or matched filter. For underwater acoustic communications, there are unknown parameters such as channel response etc. in measured model that describes the propagation and variation of channel, so if the unknown parameters are estimated firstly, hypothesis that has the maximum probability is selected secondly, then the type of detector is GLRT detector, which operates the joint channel estimation, equalization and symbol detection. For tracking the channel, adaptive equalizer/filter such as those based on recursive least square method should be used. There are the properties of additivity in both the aprior information and data information. So we construct state-space model based on the spatial-temporal evolution of channel and data information. In the state-space model, the aprior information is regularly embedded according to sequential Bayes filter algorithm——Kalman/partical filter, and feedback that is used in the algorithm is attributed to improve convergence of iteration process, as a result, robust processing approach is formulated.In order to design sequential GLRT for spatial-temporal array communications system of spread spectrum CDMA with less caculations, we firstly study spatial-temporal matched filter/time reversal and then present cyclosatationary time reversal, time reversal-orthogonal spatial-temporal block code and beamforming of time reversal. Considering that passive time reversal is temporal+spatial processing in fact, we are motivated to adopt the joint spatial and temporal processing such as spatial-temporal minimum mean square error (MMSE) critirion to enhance the performance of system. Then, by combination of time reversal/spatial-temporal MMSE processor with Kalman/partical filter, spatial-temporal array communication systems of spread spectrum CDMA with less caculations are designed for single carrier and multiple carrier underwater communication system. Thus, this thesis is organized as three parts step by step. In the first part source-channel, four-dementionally spatial-temporal array and GLRT for spread-spectrum underwater communications are studied. In the second part GLRT for single carrier spread spectrum communications is studied and in the third part GLRT for multi-carrier spread spectrum communications is studied. In both of last parts,(1) GLRT for spread spectrum communication based on time reversal was studied respectively.(2) GLRT for spread spectrum communication based on spatial-temporal MMSE processor is studied respectively. At last, simulation and experimental data analysis demonstrate the feasibility of the above-mentioned research contents.