Simulation of the Code Tracking Performance of Square-Wave Modulated Ranging Codes

Philip A. Dafesh, Andrew Parker, Jack K. Holmes, Srini H. Raghavan

Abstract:	The concept of frequency re-use has been proposed to allow the addition of a new military signal into the same spectral region as the current civilian C/A-Code. The first implementation of this concept was realized by the Tricode-Hexaphase Modulation (THM). In this approach a square-wave modulated PRN code (Manchester formatted) is added onto the same quadrature carrier phase as the current civilian C/A code signal. Subsequent to this approach, a number of similar approaches were proposed that employ square-wave and sine-wave modulated PRN codes to frequency-division multiplex the new military signal on to an offset carrier. This paper describes a realistic simulation of the code tracking performance of the Tricode-Hexaphase GPS waveform using the Signal Processing WorkSystem (SPW) simulation environment. This simulation is representative of the code tracking performance of square-wave modulated PRN codes and sine-wave offset carrier-modulated signals. The simulation results were validated against expected analytical results for C/A and P-code tracking. Similar results were obtained with a previous IF version of the simulation that employed an IF carrier and bandpass correlators rather than a lowpass equivalent signal and the integrate and dump correlators more common to modern digital GPS receivers. The tracking performance results were obtained as a function of loop bandwidth and carrier to noise ratio. The performance was also investigated as a function of the number of samples per chip to enable comparison under practical operating conditions. The effects of receiver filtering of the combined waveforms were also investigated as a function of filter order for a GPS bandwidth allocation of 24 MHz. The results indicate that the Manchester code tracking performance exceeds that of P-code receivers for the same absolute early-late discriminator spacing . The simulation results further indicate that the new Manchester-formatted PRN code has minimal impact on the C/A-Code and P-code tracking, in agreement with previous theoretical predictions of NRZ and Manchester symbol backward-compatibility. After filtering the combined waveform the degradation of the P and M code tracking error was found to increase relative to the unfiltered condition.
Published in:	Proceedings of the 11th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1998) September 15 - 18, 1998 Nashville, TN
Pages:	633 - 641
Cite this article:	Dafesh, Philip A., Parker, Andrew, Holmes, Jack K., Raghavan, Srini H., "Simulation of the Code Tracking Performance of Square-Wave Modulated Ranging Codes," Proceedings of the 11th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1998), Nashville, TN, September 1998, pp. 633-641.
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