| Ultra wideband (UWB) chaotic signal has a broad spectrum, so it has lowprobability of interception, high communication rate, high security and strong anti-multipath characteristics in communications. It is an ideal signal to resolve the rate, security and interference problems for communication system. Focus on the problems of UWB chaotic signal's generation, UWB chaotic communication system's construction, modulation mode and multiple-access mode, some key techniques in the UWB chaotic communications were discussed in this dissertation. Main contents of the dissertation include following aspects:1. The characteristics, classifications and research methods of the chaotic systems, the description of chaotic signal were discussed. The discrete, continuous chaotic generating method and chaotic synchronization were emphasized. Aiming at the difficulty of UWB chaotic signal generation in video frequency, a chaotic signal generation circuit using cascading Colpitts circuits with digital chaotic driving was improved. The generated chaotic signal has a broad and flat frequency spectrum and is satisfied with the definition of UWB of FCC. The robust synchronization method was validated.2. The common coherent and incoherent demodulation chaotic communication system was researched. The emphases were put on Differential Chaos-Shift-Keying (DCSK) and Frequency Modulated Differential Chaos-Shift-Keying (FM-DCSK). Considering that the noise in demodulated template has an important influence on the FM-DCSK UWB system's performance, an improved scheme was put forward. In AWGN channel the system's bit error rate (BER) was obtained in theory. The result shows that compared to unimproved system, the improved system has lower standard for synchronization and can effectively reduce the noise in demodulation template and symbol.3. The common multiple-access schemes for DCSK have two disadvantages. Firstly, all users share the same slot, the only way to decrease multiple-access interference is utilizing the low correlativity of chaotic signal. Secondly, the transmitted signals for reference and symbol are not same or reverse, so the strong anti-multipath character is destroyed. Some multiple-access schemes for DCSK system were researched. A novel time-hopping multiple-access scheme for DCSK system was put forward. It assigns different PN time-hopping codes to different users. The signals for reference and symbol in a frame have the same time-hopping value. In AWGN channel the system's bit error rate was obtained in theory. Analysis and simulation show that time-hopping multiple-access scheme can overcome above two disadvantages and has lower bit error rate by adjusting parameter.4. Using Time-Hopping pulse position modulation (TH-PPM), an UWB system has some disadvantages that the discrete power spectral density (PSD) is centralized and may arouse multiple-access interference. An UWB system utilizing chaotic pulse position modulation (CPPM) based on Tent map was introduced. The PSD of CPPM signal was deduced in theory. Analysis and simulation show that, for UWB system, CPPM can obtain more even PSD, its discrete PSD is smaller than TH-PPM's. Therefore it has less interference on other communication systems, lower probability of detection and higher security.5. For increasing chaotic communication rate, based on the low cross-correlation of wide band chaotic signal, a multiplexing ultra-wide band system based on generated UWB chaotic carrier was put forward. Simulations show that the system has low BER in high communication rate.6. The frame and signal formation of the UWB FM-DCSK system and UWB many bits FM-DCSK system were analyzed. In the multipath channel the two systems' BER were obtained in theory. The result shows that the two systems have good performance in the mutipath channel. Simulations on UWB FM-DCSK system were made in UWB channel model. An UWB FDM-DCSK system was proposed. In AWGN channel the system's BER was obtained in theory. Simulations show that compared with DCSK system, in the same BER it can decline signal-to-noise ratio (SNR) about 3 to 4dB and reach higher communication rate.
|
PDF Full Text Request