A Narrowband GPS Receiver for Multipath Mitigation

Chaithanya Viswa, H. Howard Fan

Abstract:	Multipath mitigation is an on-going research topic for GPS receivers. Large time-delay multipaths, i.e., time delays of one chip time (1 microsecond) or more, are easily separated by correlators in a GPS receiver due to easy separation of correlation peaks. Medium and small time-delay multipaths are more difficult to detect and separate since the correlation peaks are not separated, but rather distorted from the shape of that of a single path. Many solutions have been proposed, almost all of them require faster sampling than the Nyquist rate, some require up to ten or more times a sampling frequency than the Nyquist frequency. This in turn requires a large bandwidth on the RF frontend, i.e., a large pre-correlation bandwidth. A large bandwidth RF frontend admits more noise, and is also more prone to interference. Therefore a narrowband solution would be more desirable. However, with a narrowband RF frontend and with the Nyquist rate sampling, not enough samples are contained in a correlation peak, making it very difficult if not impossible to detect and mitigate multipaths. In this paper we present a novel method to reconstruct high-sampling rate (super-Nyquist rate) correlation functions from low-sampling rate (Nyquist-rate) correlation functions using the new signal processing paradigm of compressive sampling or compressive sensing (CS). Since the correlation function of GPS signals is sparse, with only a few high peaks, it is suitable to apply the CS theory so that the above reconstruction can be achieved with almost probability one. We use a narrowband RF frontend, e.g., of a 2 MHz bandwidth as in a typical C/A code GPS receiver. With the aid of the CS, we are able to reconstruct super-Nyquist rate correlation functions of, e.g., 16 MSPS with a CS reconstruction ratio of 8. Subsequent curve fitting enables us to separate multipaths with time separations of only one or two such super-Nyquist rate samples, i.e., at 0.1 chip time or less. This method can be used in the acquisition stage, which will in turn benefit the tracking stage. Simulations confirm the advantages of this method: It works for well below the lowest SNR level of not only a conventional narrowband receiver, but also a conventional super-Nyquist rate (wide bandwidth RF frontend) receiver.
Published in:	Proceedings of the 2015 International Technical Meeting of The Institute of Navigation January 26 - 28, 2015 Laguna Cliffs Marriott Dana Point, California
Pages:	849 - 859
Cite this article:	Viswa, Chaithanya, Fan, H. Howard, "A Narrowband GPS Receiver for Multipath Mitigation," Proceedings of the 2015 International Technical Meeting of The Institute of Navigation, Dana Point, California, January 2015, pp. 849-859.
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