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

Abstract

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 CN R = 36(dB : 1Hz). In addition, modified PCPSA works in average better than conventional one.

Utilization of multi-rate signal processing for GNSS-SDR receivers

“…Due to high ADC sampling rate, digital decimation filtering before the correlation processing is mandatory in such receivers from the implementation complexity point of view. Especially, in FFT-based processing, high sampling rate would lead to excessive FFT length [10,16]. The digital low-pass filters are designed to suppress the aliasing of out-of-band noise and interference components sufficiently in conjunction with sampling rate reduction.…”

“…However, the acquisition stage can still well operate at a lower sampling frequency as this stage only provides the approximate code phase and Doppler frequency. The previous work by the first author [10] proposed a reduction in sampling frequency for both tracking and acquisition stages which has the drawback of rounding the correlation function's peak and affects the overall performance of the GNSS receivers. In this article, an alternative solution is presented which involves reducing the sampling frequency only for the acquisition stage and uses the original highly oversampled signal for the tracking algorithm.…”

Reduced-complexity FFT-based method for Doppler estimation in GNSS receivers

“…The challenge of long FFTs and IFFTs can be partly relaxed through the use of multi-rate signal processing [12].…”

Frequency Domain Acquisition Method for Dual-System GNSS

“…Since complexity in the number of arithmetic operations can be reduced algorithmically by removing redundant calculations [1], we found DFT can be used to compute the correlation for all the code phases and channels at once at a candidate frequency. Alternatively, DFT can be used for the parallel search in frequency dimension at a candidate code chip phase.…”