Research On Adaptive Data Transmission And Frequency Cognition For Hidden Communications

Recently, with exponentially growing demands for high security and reliable data transmission in applications of disaster early warning, military communication, network security, emergency communications, information hiding and interference cancellation, hidden data transmission has attracted a wide range of attention for its great importance, urgent needs and encouraging prospects. As a new field of research in our country, hidden data transmission can provide high performance of communication with reliability, accuracy and security, whereas most of its key technologies are in great need of deep study.With the rapid developments of new concepts and applications, hidden data transmission embraces much more fields of science. To satisfy the critical requirements of practical communications, hidden data transmission is not restricted to the traditional information hiding and should be extended to the hiding of communication manner, transmission mechanism, wireless channel and frequency spectrum, etc. Given the new definition of hidden data transmission in broad sense, the research work in this dissertation carries a rather deep study on the key technologies of adaptive signal processing and frequency recognition for hidden data transmission, and the major achievements and results are outlined as follows.1. Taking the typical hidden communications—meteor burst communication (MBC) for instance, the structure of its transmission data frame is constructed and the mechanism of adaptive data transmission based on channel estimation is proposed. Adopting the collected data of long distance field try from Zhongshan Station to Syowa Station in Antarctic and from Xi'an city of Shaanxi province to Langfan city of HeBei province in China, the characteristic of meteor channel is analyzed which provide theoretical basis for the realization of MBC system. On this basis, a model of signal processing based on the structure of data frame is suggested. There are two distinct modes of operation for signal processing: acquisition and tracking. And the performance of channel estimation for each mode is derived.2. According to the analysis of hidden data transmission mechanism, a receiver based on coefficients-mapped decision feedback equalization (DFE) via channel acquisition is suggested in accordance with the data frame structure. With the channel impulse response being obtained by fast correlation, the relation between channelparameters and equalizer coefficients is derived and the DFE coefficients are directly calculated from the estimated channel parameters, which shows more significantly robust against channel variation and noise than a DFE employing adaptive coefficient tracking does, and therefore makes data receiving stable for adaptive modulation and coding (AMC) of hidden communication system. Computer simulation results are included to support our developments.3. For joint data and channel estimation, per-survivor processing (PSP) provides superior performance and robustness whereas the giant computational complexity limits its application in adaptive hidden data transmission. Here, a suboptimal method called D-PSP is proposed to save the computational time and memory size, which agrees with the slow-fading characteristic of burst channel and makes the MLSD possible for adaptive data transmission. In the meanwhile, an adaptive state reduction of PSP (ASRP) algorithm is presented with only a few states in the trellis diagram being employed, while these states are chosen by the time-varying threshold according to the exponential decay of meteor channel. It is shown that, ASRP can make good tradeoff between performance and computational complexity dynamically, and provides reliable data transmission for adaptive modulation and coding of MBC system.4. In view of the inherent weaknesses of FH sequence families available in various fields, a novel family of frequency hopping sequences based on iterated block cipher is proposed for frequency-hopping multiple-access (FHMA) burst communications. The design offers a class of nonlinear FH codes with high security, large linear span and a uniform spread over the entire frequency bandwidth. Moreover, FH sequences among the family are independent from each other and they perform as well as random patterns in terms of multiple access interference in anti-jamming applications. With the performance of packet error and throughput for FHMA network being derived, the simulation results also show that the proposed sequences have multiple access performance identical to that derived on the basis of random sequence model. Efficiently implemented in field-programmable-gate-arrays (FPGA), the generator prototype of the proposed sequence has been realized and incorporated into fast FH radios.5. High complexity of spread-spectrum (SS)/frequency-hopping (FH) sequence is of great importance for high-security-required multiple-access communications. With the growing developments of the construction of SS/FH sequence in much wider fields, the well-known complexity measures—the linear complexity (LC), the linear complexity profile (LCP) and the k-error linear complexity (k-error LC)—are widelyused but not sufficient to evaluate the complexities of the sequences available, for instance, the cryptographical sequence and the chaotic sequence families. In this paper, a novel complexity metric to evaluate the unpredictability of SS/FH sequence is proposed based on the concept of the approximate entropy (ApEn), which is defined as the maximal randomness of sequences with arbitrary length. The bounds of the ApEn are derived and it is shown that, by appropriately choosing its parameters, the ApEn can effectively discern the changing complexities of SS/FH sequences from a relatively small amount of samples, which is superior to its candidates and can be extended to frequency prediction for cognitive radios. Simulation results and analysis are also included to support our developments.