Research On Error-resilient Techniques Of Information Intercepting And Processing

Information intercepting is a very special field in electronic informatics, which is an unconventional technique to acquire information from the communication between the transmitter and the receiver as a third party. Different to the conventional information communication, Information intercepting technique have to be faced with the following basic difficult problems: the blind recognition of the parameters of the communication; the blind recognition of the parameters of the signal; how to efficiently process the fading signal which power is under the receiving threshold of the conventional communication; how to demultiplex the mixed information stream with serious errors; and how to determine the value of the information etc. In this thesis we study the main problems above using algebraic theory and statistical method, we present proper modeling description and system theoretic analysis as well as a series innovative fast algorithms to solve the practical problem in information intercepting. In the concrete, we give an in-depth study with some great innovations in the following four aspects.1) Blind recognition of the parameters of the channel coding.How to reversely deduce the channel code in used only from the receiving data stream is an unavoidable difficulty in information intercepting. The famous Berlekamp-Massey (BM) algorithm can fast compute the unknown generator polynomial from a linear recursive sequence (LRS), which is an efficient method to recognize the regular scrambling type in communication. We construct a HomogenousKey Module Equation (HKME) which is described by homogenous ideal of F[x,y],where F is a filed. HKME is a new generalization of the Key Equation (KE) that is very important to depict the BM algorithm. From the aspect of algebraic theory, we generalize the BM algorithm to the problem of computing minimal generators of algebraic module and present a fast algorithm of computing the Grobner basis, then give a fast algorithm of solving the HKME. The new algorithm has some important innovations and properties as following: (1) can solve the problem of fast blindrecognition of convolutional codes; (2) the computational complexity is O(N²),where N is the length of receiving sequence; (3) different to BM algorithm, the new algorithm is error-resilient in a certain extent, then the efficiency of blind recognition is greatly improved.2) Joint source-channel decoding (JSCD) of multiple sources.Strong error-correcting technique is an important issue in information intercepting. To acquire the fading signal which is not designed with error-correcting code or the designed channel coding is not strong enough and improve the gain of signal, we need to investigate a new decoding method different to the current scheme that source decoding and channel decoding is separate. We focus on the unsymmetrical JSCD and present a method on joint decoding of multiplevariable-length encoded sources. By exploiting the a priori redundancy in multiple sources, the decoding performance is greatly improved. Currently most researches on JSCD are concern with single source. However the multiplex of multiple sources is a commonly used technique in modern wide-band wireless communication. Therefore the proposed method is more practical. Our method generalize the conventional Viterbi decoding algorithm, not only the optimal path in channel decoding trellis but also the a priori probability of branches in multiple VLC source trees are considered. The Simulation results show that our new method obtains the gain in SNR of around 1.5dB comparing to the conventional Viterbi algorithm that not exploits the a priori information.3) Fast acquisition of the spread-spectrum pseudonoise (PN) code from the highly noisy channel.Information intercepting from the spread-spectrum signal needs to synchronize the PN code first. We present a novel soft fast correlation attacks (SFCA) algorithm, which is a good generalization and application of hard fast correlation attacks in cryptanalysis field to communication field. With the novel SFCA algorithm, we solve the problem of fast acquisition of PN code in spread-spectrum communication in very low SNR circumstance. The simulation results show that the proposed SFCA algorithm can accomplish the fast acquisition correctly with few chips in very low SNR. In addition, we apply the SFCA to the image data hiding scheme against geometrical distortion, which enhance the ability of resisting geometrical distortion obviously.4) Robust error-resilient demultiplexing of digital signal.Information intercepting from the digital group signal needs to synchronize the information frames and correct demultiplexing is the precondition of information recovery. We study the robust error-resilient digital demultiplexing techniques for information intercepting. We present a frame alignment method based on statistic and a maximum a posteriori (MAP) method for justification prediction based on finite state automaton. The simulation results show that our methods can realize robust demultiplexing with the bit error rate of 2 × 10^-2 and the synchronization maintaining time can reach 15 minutes when the bit error rate up to 5× 10^-2. However with the standard method recommend by ITU, the average time between losses of synchronization is about 1 minute. The computational complexity of our method is very little. Therefore our method can accomplish robust and fast error-resilient information intercepting.