Optimization of Tracking Loops for Signals of Opportunity in Mobile Fading Environments

C. Yang, T. Nguyen

Abstract:	As signals of opportunity (SOOP), terrestrial wireless communication and broadcast signals are promising for positioning and navigation in urban environments where GPS signals are challenged. Indeed, such radio signals of opportunity with strategically placed transmitters of high power are mostly available in populated areas and are designed primarily for indoor reception where GPS often fails to function. In a paper presented at 2012 JNC (Colorado Spring, CO), we addressed a critical issue for positioning with signals of opportunity, that is, the lack of a sufficient number of signal sources of the same kind in a region, which leads to an unfavorable geometry dilution of precision (GDOP), and the use of mixed signals of opportunity was recommended among possible remedies. Experimental results were presented for mobile positioning with digital television (DTV) signals and cellular phone signals (CDMA2000). In this paper, we address another challenge of using SOOP for positioning, which concerns with multipath and non line-of-sight (NLOS) signals. For a mobile user, the multipath in an urban environment creates severe Rayleigh fading, which critically affects the quality of communications and, even worse, the performance of timing and ranging measurements. Indeed, fast fading creates “holes” in data streams that cannot be corrected by conventional coding schemes, thus preventing continuous reception of signals by mobile users. The problem is further compounded by the fact that the DTV standard used in the US today, namely, ATSC-8VSB, is not designed for mobile but fixed reception. Only one out of 313 segments per data field (24 ms) contains a pseudorandom (PN) code that can be used for timing or ranging. Such a low duty cycle (0.3%) requires special design of correlators and code tracking loops for mobile users particularly when low quality clocks are used in transmitters and receivers. It is well-known that narrowly spaced correlators provide superior performance against multipath for a stationary or slow-moving user. However, it is not the case for mobile users in a severe Rayleigh fading environment. High dynamics requires a correlator structure commensurate with the variable delay experienced by the receiver with tracking loop parameters to balance the noise performance. In addition, special acquisition and reacquisition schemes are necessary so as to coast through the “holes” with instantaneous recovery after complete signal losses. In this paper, we will present a hardware prototype with 7 radio channels, tuned to different DTV stations and cellular towers, describe the baseband signal processing algorithms, and present the mobile test platform and test scenarios. Field test data are used to optimize tracking loops and the field test results will be presented to show the effects of fading environments and the functionality and performance of the optimized design. Research supported in part under AFRL/RYWN SBIR Phase II Contract No. F8650-10-C-1735.
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:	479 - 487
Cite this article:	Yang, C., Nguyen, T., "Optimization of Tracking Loops for Signals of Opportunity in Mobile Fading Environments," Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013), Nashville, TN, September 2013, pp. 479-487.
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