| Spread spectrum technique has been widely used in military and civilian communications for its advantages, such as anti-jamming (AJ) and low probability of detection (LPD). However, the traditional spreading codes are usually deterministic pseudo-noise (PN) codes, which can be duplicated, potentially compromising the transmission security. Then the time varying and random spreading codes are expected. In this thesis, we present a novel spread spectrum communication system, Self-Encoded Spread Spectrum (SESS) system, which does not use PN codes, and is unique in that traditional transmit and receive code generators based on maximal-length sequences or chaotic signals are not needed. Then the spreading codes are stochastic and variable.The data recovery at the receiver requires the spreading codes. In SESS system, the receiver doesn't know which spreading code will be used, so first it must de-spread the received signal correctly to regenerate the spreading code. Therefore, fast and reliable synchronization algorithm is one of key technologies in SESS system. The process of synchronization is ordinarily accomplished in two stages, PN acquisition and PN tracking.In this thesis, first, we present the theory of SESS system, the generation of the SESS spreading code and its characteristics and the acquisition theory of conventional spread spectrum communication system. An efficient acquisition scheme based on periodically transmitting the synchronization head, which is composed of binary chaotic codes, using the matched filter and automatic decision threshold-level control based on a so-called constant false alarm criterion for SESS system is present. The acquisition model of SESS system is built and simulated in the AWGN channel, the Raleigh fading channel and imulti-address interfere condition. If choosing the chaotic code with best performance and proper length, proper probability of false alarm and threshold, small acquisition time and large acquisition probability will be achieved. |
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