Design and implementation of a flexible software-based GNSS if signal simulator

Software-based GNSS IF signal simulators provide high flexibility and programmability, while eliminating the uncertainty in hardware-based signal generation. The signals can be reproduced accurately or played back to hardware signal generators. Software-based signal simulators can perform controls at almost all levels of signal generation. The satellite signals generated by payloads, propagated through the wireless channel and received by antennas and RF front-ends of receivers are modeled. Compared to RF signal simulators, they additionally simulate the effect of RF front-end down-conversion, filtering, AGC, sampling and quantization. This paper presents the design and implementation of a software-based IF signal simulator, which enables users to configure most parameters of signal generation process in space, propagation, and user segment. It is intended to meet the requirements of different schemes of new signal structure design. For the space-related simulation, the simulator adopts a general signal model for multiple signal component combination on one carrier. Configurable parameters include carrier frequency, number of signal components and their power allocation, modulation and multiplex methods, spreading code parameters, sub-carrier parameters, ephemeris data and satellite clock error. The spreading code is defined by appropriate chip rate, code type, generation polynomial and initial phase. Alternatively memory codes can be read from code files. Secondary codes are optional as well. Signals can be modulated by sub-carriers (e.g. BOC, MBOC, AltBOC) or not (traditional BPSK/QPSK modulation). Three constant-envelope multiplex techniques are supported - Interplex/CASM, Majority-Vote and Inter-Vote. For the propagation-related simulation, ionospheric error, tropospheric error, multipath and interference are simulated with controllable parameters. For the user-related simulation, the changeable parameters include carrier to noise ratio, frontend bandwidth, intermediate frequency, sampling rate, number of quantization bits and user trajectory. The verification testing of the simulator is carried out at four levels - (1) Typical communication system related parameters of the signal components and the multiplexed signals under various configurations are examined in time, frequency and modulation domain; (2) Navigation performance related parameters are tested using a self-developed GPS/Galileo software receiver, showing that the simulated signals can be acquired and tracked, and the measurement as well as the position solution meet the expected accuracy; (3) The tested multipath envelopes are comparable to the analytical envelopes; (4) The impacts of interference are illustrated by the relation between the estimated C/N0 and the power of simulated interferences.