Parallel Acquisition of GNSS Signal Based on Combined Code

L. Xu, K. Chen, R. Ying, P. Liu, W. Yu

Abstract:	In Global Navigation Satellite Systems (GNSS), acquisition is the process of searching all possible combination candidates of code-phase and carrier Doppler for one satellite. Conventional match filter (MF) acquisition algorithm searches all possible Doppler candidates simultaneously for one satellite and one phase. Computation complexity explodes if numerous satellites and code-phases have to be searched simultaneously. In this study, we propose a new acquisition algorithm based on conventional MF to reduce the search time and computation complexity. The proposed algorithm searches different PRN codes (pseudo-random noise code, one pseudo-random noise code corresponds for one satellite) or different code-phases of one PRN code simultaneously by constructing a new combined code. The main different between the new method and the conventional MF is that it replaces ordinary PRN code by a new combined code. Without considering effects of navigation bits and carrier Doppler, parallel PRN search (different satellites simultaneously), referred as P-PRN search, can achieve a significant reduction of search time and computation complexity to 94%-70% and 47%-35% respectively; parallel code-phase search (different code-phases from one satellite simultaneously), referred as P-CP search, can achieve a significant reduction of search time and computation complexity to about 70% and 35% respectively. In this study, we average two different PRN codes to generate a new combined code. According to the auto-correlation and cross-correlation properties of PRN codes, the generated combined code can also be used to search two PRN codes simultaneously, at the cost of some performance degradation. Based on our observation, about half of the values of the new combined code are zeros. So the computation complexity can be halved by ignoring zero values. But combined codes degrade the signal to noise ratio (SNR) by imposing more noise to correlation results. To achieve same SNR as conventional MF, we extend the coherent integration time by 0.44 times longer. We also consider the extra cost of confirming time needed and the cross-correlation between two different PRN codes. In terms of search time and computation complexity, results show that P-PRN search achieve different level of performance improvements to 94%-70% and 47%-35% respectively, according to three different situations: two satellite signals exist, one signal exists and no signal exists. Finally, we investigate the effect of the selection of different PRN codes. We conclude that combining the PRN codes from the satellites that have higher visibility probability first can achieve better overall acquisition performance in practice. Additionally, based on this observation, we design an optimal acquisition strategy integrated with existing satellite search algorithms and get a significant improvement on acquisition performance. In order to search two code-phases of one PRN code simultaneously, we generate a new combined code by averaging two PRN codes with different code-phases. Similar with the case of P-PRN search, about half of the values of the new combined code are zeros, which leads to a significant reduction of computation complexity. Additionally, we need to extend the coherent integration time and consider the extra cost of confirming time needed and auto-correlation of the PRN code. Results show that P-CP search reduces the search time and computation complexity to 70% and 35% respectively. The results above shows that P-CP search achieve better performance than P-PRN search. Yet the degree of parallelism (DOP) is not limited to 2; we can also generalize the two methods by increase DOP. For instance, we increase DOP of P-CP search to 4. About 3/8 elements of the mixed are zeros; the others are positive ones, negative ones, positive twos and negative twos. Moreover correlation between signal and local replica can be achieved by shifting operation. Though we need to extend the coherent integration time further (roughly double) to compensate the degradation of SNR and consider the extra cost of confirming time, the search time and computation complexity achieve a more significant reduction compared to the case when DOP is 2. Another aspect we need to consider is the carrier Doppler. The conclusion above is derived from the assumption that carrier Doppler is zero. SNR will decreases with the extension of coherent integration time at the presence of carrier Doppler. To compensate SNR degradation caused by carrier Doppler, we have to narrow the carrier Doppler search bin step, which means to search more carrier Doppler candidates when carrier Doppler search space is fixed. Additionally, we gain some carrier Doppler resolution benefits. However, the search time and computation complexity increased. For P-PRN search when two signals exist, the search time and computation complexity are reduced to 94% and 47% respectively when the number of possible carrier Doppler candidates remains the same; otherwise, the search time and computation complexity is 130% and 66% respectively when the number of possible carrier Doppler candidates increases to compensate the SNR degradation due to carrier Doppler. Similar results hold for P-CP search. When DOP increases, additional extension of coherent integration time will leads narrower carrier Doppler search bin step due to carrier Doppler. Another observation is that the number of zero values of combined code decrease with the increase of DOP. In sum, to get a better trade-off between benefits from larger DOP and degradation by carrier Doppler, we suggest selecting an appropriate DOP and therefore gaining the best overall acquisition performance. In this study, a new parallel acquisition algorithm, including P-PRN search and P-CP search used in a MF by constructing new combined code is analyzed. Both of two parallel search algorithms reduce the search time and computation complexity even considering the effect of carrier Doppler. Analysis and results show that P-CP search outperform P-PRN search and selecting an appropriate DOP will achieve significant performance improvements compared to conventional MF method.
Published in:	Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013) September 16 - 20, 2013 Nashville Convention Center, Nashville, Tennessee Nashville, TN
Pages:	3607 - 3614
Cite this article:	Xu, L., Chen, K., Ying, R., Liu, P., Yu, W., "Parallel Acquisition of GNSS Signal Based on Combined Code," Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013), Nashville, TN, September 2013, pp. 3607-3614.
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