The Study Of Weak Signals In Dsss System With Long Coherent Accumulation Pn Code Acquisition Technology

With the development of science and technology ,and the rising requirement of living standards,the requirements of communication technology is increasing . People still want to enjoy high quality and probability communication in very difficult circumstances. Especially in the field of deep space exploration, indoor and shelter conditions, it puts forward higher requirements of communication technology to achieve the desired effect. Since the direct sequence spread spectrum (DSSS) system take advantages of anti-interference ability, low intercepted probability, good multiple access capability, anti-multipath and good performance of security ,it usually be used in the wireless communication system, which works in those weak signal environments. To complete the code acquisition in this working environment is the most important part.It is also the premise of decoding and other next steps .So ultimate goal of this article is to complete the acquisition of weak signals .There are two major problems in code acquisition, delay and frequency offset. Thus derived a range of issues, such as data transition, coherent accumulation length, spectral resolution, Doppler changes, code offset and so on. This paper thoroughly studied these issues. Analyzed the existing solutions .Improved the solution of major problems .And designed a weak signal PN code acquisition system.In this paper for the detection of weak signal problem, using FFT techniques to instead of directly coherent accumulation, to achieve the parallel searching for phase delay, to improve the acquisition time and to save hardware resources .In order to improve the gain of weak signals acquisition system, proposed a method of using delayed data blocks take separately coherent accumulation to avoid the effects of data transition. Improve the ability of the system to capture weak signals. At the same time, because of increasing coherent accumulation length, system is extremely sensitive to Doppler frequency offset. Therefore, proposed an improved frequency offset estimation algorithm. Through the first estimate can make the system residual offset error achieve coherent cumulative accuracy. Determined by simulation to use the adaptive PAPR threshold detection method for determining. Analysis PN code acquisition strategy under the high dynamic environment, which Doppler frequency shift is changing rapidly, and other problems in practical applications. Finally, verifying the feasibility of the system by MATLAB .Then, in order to implement the system on FPGA, write VHDL code of key modules.