A Fast Acquisition Method of DSSS Signal Based on Double FFT Layers

Fast Fourier transform (FFT) searches code phase and carrier frequency of DSSS signal in frequency domain at the same time. The use of FFT for fast acquisition has gained its popularity in receiver technology especially under high dynamic situation. Accuracy of initial acquainted carrier frequency is very important for later carrier tracking loop. A coarse carrier frequency may lead to a long transitory time or even signal losing. In order to get a more accurate signal carrier frequency, FFT length should be increased. It will cause an expensive computational load and occupy more hardware sources. In this paper, a fast acquisition method was put forward for high dynamic DSSS signal catching based on double layers of short FFT. The first FFT layer was used for quick acquisition of code and carrier. It can only give a coarse carrier frequency. The second FFT layer was used for accurate frequency calculation based on the result of the first FFT layer. The PLL can get into the better tracking state quickly with the help of the accurate carrier frequency. The NFLL will be never used for carrier frequency traction any more. For the reason that the operation of the two layers FFT was occurred sequentially, the hardware sources could be shared between them. This method was verified with the real signal and the result was given in detail.