A New Proposal for a FLL Discriminator Based on Energy

X. Tang, E. Falletti, L. Lo Presti

Abstract:	As the development of Global Navigation Satellite System (GNSS), different satellite systems are coming up, such as Galileo, Compass and modernization of GPS. To improve the performance and compatibility, the signal modulation methods are changed correspondingly. With respect to these changes, we can adjust the traditional receiver solution to adapt to the new trend. In the new GNSS system, there is one significant change which is the period of data. Like in Galileo E1 signal, it is set to 4 ms equal to the period of PRN code. That means in a new GNSS system, the traditional tracking architecture is subject to the phase jump and frequency ambiguity problem caused by frequent data-transition. In [1] both four-quadrant and two-quadrant arctangent discriminators in frequency lock loop (FLL) are analyzed, four-quadrant arctangent discriminator is sensitive to data-transition, though two-quadrant arctangent function can solve the problem but it narrows the pull-in range at the same time. [2] proposed a new discriminator based on energy (the accumulation result of one period of data), but the mathematical expression of the discriminator is too complex which usually involves approximation. Here in this paper, a new FLL discriminator based on energy is also proposed which is different from [2], the mathematical formula of the discriminator is simpler and more accurate. Compared with traditional arctangent discriminators, it has more lager pull-in range which means it can bear more dynamic stress and still can converge to the correct frequency error estimate even with the large initial frequency error. At the same time it is also definitely insensitive to the data-transition. In this paper, based on the cross-ambiguity function (CAF) in [3] and initial work in [4], a new frequency discriminator is derived which is simple and accurate. The new discriminator mainly uses the three different correlator outputs calculated by the correlation of input signal and three different local replicas generated by three different frequencies. Briefly, the structure of the new FLL is very similar with the traditional delay lock loop (DLL). It consists of three channels, namely prompt, left and right. In prompt channel, the frequency of the local generator is f (the current frequency estimate) while in left and right channels, the correlators attempt to correlate the incoming signal with the carrier replicas at frequencies f+fstep (fstep=2/(3*T) ) and f- fstep respectively (T is the integration time), The new frequency discriminator will use the accumulation results from these three channels to estimate the value of frequency error, then as the traditional FLL, the estimate will be passed through loop filter and NCO to update the frequency of local signal generator. Besides that, from the experiments, the advantages of the new FLL with the proposed discriminator can be seen compared with the traditional one. In addition, modern GNSS signals are usually made up of two different components, namely the data and pilot channels. In the presence of a pilot channel, as in the Galileo E1 OS signal, we also can take the advantage of pilot channel of E1 signal, [5, 6] propose the methods on how to combine E1-B and E1-C in the acquisition process, thanks to the property of the new discriminator, we can also apply the idea in the new FLL discriminator to improve the accuracy .Considering the combination of E1-B and E1-C signals, the new FLL will become little complicated, there will be two different code channels, one is for E1-B and the other is for E1-C. So totally there will be six frequency channels connected to the new FLL discriminator which will estimate the frequency error using the input data. Though the calculation loan is increased, but the error will be decreased correspondingly. In conclusion, the new FLL only needs the minimum period every single step (like 4ms in Galileo), it is insensitive to the phase jump and has a larger pull-in range compared with the improved FLL in [1]. The mathematical formula of discriminator is simpler and more accurate than the one in [2]. In the presence of pilot channel in modern GNSS, the accuracy can be also improved by combining the data and pilot channels. The only disadvantage is the increased computation loan. [1] James T. Curran, Gerard Lachapelle, Colin C. Murphy, “Improving the Design of Frequency Lock Loops for GNSS Receivers,” IEEE Transactions on Aerospace and Electronic Systems, vol.48, no. 1, Januray 2012. [2] Jyh-Ching Juang, Yu-Hsuan Chen, “Phase/Frequency Tracking in a GNSS Software Receiver,” IEEE Journal of Selected Topics in Signal Processing, vol. 3, no. 4, August 2009. [3] Letizia Lo Presti, Beatrice Motella, “The Math of Ambiguity,” Inside GNSS, June 2010. [4] X. Tang, E. Falletti, L. Lo Presti, “Fine Doppler Frequency Estimation in GNSS Signal Acquisition Process,” 6th ESA Workshop on Satellite Navigation Technologies and European Workshop on GNSS Signals and Signal Processing, (NAVITEC), 5-7 Dec. 2012. [5] D. Borio, C. O’Driscoll, G. Lachapelle, “Composite GNSS Signal Acquisition over Multiple Code Periods,” IEEE Transactions on Aerospace and Electronic Systems, vol. 46, no. 1, January 2010 [6] T. H. Ta, F. Dovis, D. Margaria, L.Lo Presti, “Comparative study on joint data/pilot strategies for high sensitivity Galileo E1 open service signal acquisition,” IET Radar Sonar and Navigation, 2010, Vol. 4, Iss. 6, pp. 764–779.
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:	3528 - 3535
Cite this article:	Tang, X., Falletti, E., Presti, L. Lo, "A New Proposal for a FLL Discriminator Based on Energy," Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013), Nashville, TN, September 2013, pp. 3528-3535.
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