Design and Implementation of a Multi-Signal-Structure GNSS Signal Simulator Using Low IF Digital Quadrature Modulation

Yanhong Kou, Bo Zhang, Yun Zhao

Abstract:	Signal simulators play an important role in research and design validation of Global Navigation Satellite System (GNSS), especially of user equipments. They simulate GNSS signals in various conditions in order to provide signal sources for debug and test of receivers and their key components. They can also serve to test and assess the performance of new signal structure before the corresponding new satellites are deployed. This paper presents a Software Defined Radio (SDR) architecture using a low-IF digital quadrature modulation method to accommodate multi-signal-structure, multi-channel and multi-level-output on a common hardware platform. The simulator can work at four kinds of output mode: (1) the navigation data and measurement data; (2) the digital IF signals; (3) the analog IF signals; (4) the analog RF signals of all visible satellites after combining several user-specified carrier frequencies. The signal model, system implementation, frequency plan and power control plan are addressed in detail. Realtime digital IF signal synthesis is performed in a high-capacity FPGA. The first-stage quadrature modulation is carried out at a low- IF to ensure the signal quality and easy processing with limited clock rate. Then the combined plural signals for all visible satellite for each carrier frequency are digitally up-converted before being sent to the DAC, where the output IF can be configured by the interpolating factor and the second LO frequency. The RF LO signals are generated by a PLL frequency synthesizer with a programmable frequency dividing ratio. The prototype of the simulator is developed and tested with a commercial GPS receiver and a self-developed software receiver. All the four-level outputs of the simulator can use RINEX files as ephemeris sources to enable true position solution. The accuracy and quality of simulated signals are verified in the spectrum, correlation, measurement, and position domains.
Published in:	Proceedings of the 21st International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2008) September 16 - 19, 2008 Savannah International Convention Center Savannah, GA
Pages:	2677 - 2691
Cite this article:	Kou, Yanhong, Zhang, Bo, Zhao, Yun, "Design and Implementation of a Multi-Signal-Structure GNSS Signal Simulator Using Low IF Digital Quadrature Modulation," Proceedings of the 21st International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2008), Savannah, GA, September 2008, pp. 2677-2691.
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