| The data link between the air fleet is a complex multi-user spread spectrum communication system, which can support stealth communication, measurement of distance and velocity and time synchronization of whole network between multi-aircraft. It transmits its own position, movement and battlefield command, intelligence, investigative information within the gateway nodes in high-speed transfer rate, which is an important guarantee for the coordinated operations of fleet aircrafts. One direction of modern fleet data link development is to realize stealth communication, and it is urgent for our country to establish such highly reliable, robust inter-cluster stealth data link to fill this gap. Therefore, this dissertation puts forward two types of spread spectrum schemes to cope with the stealth link signal characteristics of fleet:the short code DS system with small transmitting power and the encryption code hopping spread spectrum system.The main work of this dissertation is to research key technologies in the two schemes, namely, the weak signal acquisition in short code DS system and acquisition in code hopping spread spectrum system based on the time synchronization of gateway nodes. Our study focuses on weak signal acquisition, which is one of the most important points in research. The encryption code hopping spread spectrum technology proposed by our project group is new and needed to be improved. Thus, we only take part of this content into research and analysis. This article can be divided into the following aspects:At first, we introduce the principles of DS system and weak signal algorithm, including the theories of DS system to acquire signals, classic acquisition algorithms, the detection performance of each algorithm (the statistical properties of the coherent and non-coherent integration, setting strategies of threshold value) and the comparison of acquisition rate in various algorithms. While the contribution of this paper is given an approach to set acquisition threshold value of full bit under the guidelines of the CFAR(constant false alarm criteria), and also presents a method to calculate the detection probabilities of coherent and non-coherent integration affected by navigation bit modulation and false alarm probability as well. Comparing with the results of actual simulations, we conclude that the theoretical detection probability, which takes navigation bit modulation into consideration, can better reflect the actual results than the classical theory.Secondly, the classic weak signal acquisition algorithms are simulated and improved, including the detection performance of coherent integration, half bit and full bit method in CFAR criterion using Monte Carlo simulation analysis. Besides, we propose an enhanced method to solve time and resource consuming weakness of the full bit and simulate it in MATLAB. The simulation results show that the enhanced full bit algorithm significantly improves the acquisition speed and maintains the original excellent bit detection performance as well.Finally, we analyze the theory of encryption code hopping system to acquire signals and implement its algorithm. The content in this part contains the realization of time synchronization in encryption code hopping system using two-way ranging method. Besides that, we put up with a scheme to acquire signals in encryption code hopping system after time synchronization, namely the FFT＋match filter. In particular, we use MATLAB to simulate the effect of time synchronization for the acquisition detection performance and acquisition speed.Due to the theoretical analysis and simulation results of MATLAB, we conclude that both the short code DS system of small transmitting power using enhanced full bit acquisition algorithm and the encryption code hopping spread spectrum system based on time synchronization FFT＋acquisition algorithm match design requirements of the fleet stealth communication link. |
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