Consequences of MBOC Approximation by BOC Modulation

Previous research results conclude that the MBOC(6,1) modulation has advantages in terms of acquisition and tracking performance compared to the BOC(1,1) proposed in the first design of the Galileo L1 and GPS L1C signals. However, a higher sampling rate is necessary to acquire and track the full MBOC signal and consequently increases the processing time and power requirements. This increase in processing can become an issue for commercial receivers; particularly those that require minimal power consumption. To address this problem, one solution is to assume that the incoming signal is modulated only by a BOC(1,1), thereby ignoring the BOC(6,1) component and allowing for a smaller frontend bandwidth (with a commensurate reduction in sampling rate). With this in mind, the goals of this paper are to study the impact of this approximation on acquisition and tracking performance, to quantify the tracking performance relative to the cases where (i) the full MBOC signal is tracked and (ii) only the BOC(1,1) portion of the signal is tracked. Since information is ignored when only the BOC(1,1) is considered, the performance is inferior to the case of MBOC. This degradation is quantified to assess the performance of the BOC(1,1) compared to the MBOC in presence of interference and multipath. To assess performance in the presence of multipath, a simulation study is performed. Multipath is simulated at various delays relative to the direct signal and a normalized Early-Late processing is realized for both the BOC(1,1) and the MBOC. In terms of interference, two kinds of interference signals are simulated at different offsets from the L1 central frequency: a continuous wave interference and a narrow band interference. In both cases, the effect of the interference is greater on the BOC(1,1) when the interference is centered in one of the main lobes of the BOC(1,1) for acquisition and tracking. Moreover, the improvement brought by the MBOC modulation increases with the power of the interference.