FPGA Implementation Issues of a Two-pole Adaptive Notch Filter for GPS/Galileo Receivers

Micaela Troglia Gamba, Emanuela Falletti, Davide Rovelli, Alberto Tuozzi

Abstract:	In the field of satellite navigation technologies, and in particular for Safety-of-Life (SoL) applications, great attention is given to rejection of interference. In particular, continuous wave (CW) signals represent one of the most common types of interference. A properly designed adaptive notch filter should be employed in order to remove it. Such an algorithm has been widely exploited in literature, with many variations. Most of these studies are however mainly focused on the floating point precision model, and only a few of them really consider the potential issues of a real implementation. The objective of this paper is to discuss the issues arising from fixed-point modeling and FPGA implementation of an adaptive notch filter. Previous works showed that in a complex base-band front-end, one CW interfering signal can be eliminated by means of an IIR one-pole adaptive notch filter. For the case of more than one CW, two or more filtering cells can be used in cascade. Here, a two-pole adaptive notch filter is considered. A distortion analysis of the filter output is conducted in the paper by means of Cross-Ambiguity-Function (CAF) and Delay-Locked-Loop (DLL) S-curve evaluation, showing a great distortion of the ranging code after the filtering operation. Thus, to reduce such a distortion, the proposed solution consists in constraining the zeros of the filter at staying on the unit circle in a new and simpler way with respect to existing methods. A run-time change of the algorithm parameters is also performed, improving convergence speed.The VHDL architecture is then discussed. Pipelining and delaying the parameter updating are applied to increase clock frequency. The post Place&Route (P&R) results of the improved version of the two-pole adaptive notch filter on a Virtex 5 show a clock frequency of 47 MHz with an occupied area of 96.4 KGE. Tests with a fully software receiver support the validity of the proposed solution.
Published in:	Proceedings of the 25th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2012) September 17 - 21, 2012 Nashville Convention Center, Nashville, Tennessee Nashville, TN
Pages:	3549 - 3557
Cite this article:	Gamba, Micaela Troglia, Falletti, Emanuela, Rovelli, Davide, Tuozzi, Alberto, "FPGA Implementation Issues of a Two-pole Adaptive Notch Filter for GPS/Galileo Receivers," Proceedings of the 25th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2012), Nashville, TN, September 2012, pp. 3549-3557.
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