The effect of the incoming signal decimation on the performance of the FFT-based acquisition stage in SDR GNSS receivers

In this article the effect of decimation before acquisition stage in conventional Parallel Code Phase Search Acquisition (PCPSA) is discussed. In PCPSA, correlation function of the incoming signal and a locally generated Pseudo Random Noise (PRN) code is extracted by taking the Fast Fourier Transform (FFT) of the incoming signal and multiplying by conjugate FFT of a locally PRN code. Afterward, the IFFT of the resultant signal is carried out and required information is extracted. The incoming signal in acquisition stage inherits a fixed sampling frequency from ADC which causes long FFT and IFFT. Since long FFT and IFFT are not very welcome from implementation point of view one of the solution is using decimation before the acquisition stage. The fact that the most important part of the signal is centralized in a bandwidth of 2Mhz makes it possible to utilize a decimation block before the correlation stage without losing the important information. Decimation catalyzes lower Acquisition Margin (AM) and in order to compensate its effect a coherent integrator is applied at the beginning of the procedure. Thus the peak of correlation function is enhanced and the noise level is attenuated. By comparing AM for conventional PCPSA with modified structure there is an improvement of 2.3848dB in AM when CNR = 36(dB : 1Hz). In addition, modified PCPSA works in average better than conventional one.