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Global Navigation Satellite System (GNSS) has employed various types of Binary Offset Carrier (BOC) Modulation to minimize interference between systems sharing the same band. However, BOC or Binary Phase Shift Keying (BPSK) modulation has limitations, such as occupying a specific band at a high-power level, which reduces interoperability. To address this issue, Stepped Frequency BOC (SFBOC) modulation, a time-varying subcarrier frequency (TVSF) modulation, which changes the BOC subcarrier of the signal to stepwise based on stepped-frequency continuous-wave (SFCW) modulation, is presented as a promising GNSS modulation for future GNSS. Unlike conventional BOC, the output of SFBOC's autocorrelation function (ACF) has a low level in the side lobe and a narrow one at the main lobe, resulting in superior correlation properties. Additionally, SFBOC h as a spectrum close to a uniform distribution in the frequency domain and uses a much wider band than conventional BOC modulation. Compared to conventional TVSF modulation, Generalized BOC (GBOC), SFBOC has lower signal acquisition complexity, requires a smaller bandwidth, and has a similar modulation scheme to Frequency-Hopping BOC, while being robust in a multipath channel environment. In this paper, we analyze the performance of SFBOC signals through diverse parameters (subcarrier frequency, rat e of pseudorandom code) within a limited bandwidth and compare it to conventional GNSS modulations, including BPSK, BOC, CBOC, and TMBOC, which are used in the GNSS L1 and L6 bands. We consider various perspectives of performance evaluation, including correlation properties, spectral properties, interference, distance measurement accuracy, and robustness in Additive White Gaussian Noise (AWGN) and multipath channel environments. Our numerical comparison shows that SFBOC has superior correlation and spectrum properties compared to conventional GNSS modulations and is robust to multipath and noise channel environments. Additionally, SFBOC has high performance in RF compatibility due to low interference between narrow bands and inter-system. We demonstrate the superiority of SFBOC through performance evaluation results compared in this paper and reveal SFCW modulations that were not previously considered as future GNSS signal modulations. |
