### Parametric Techniques for BOC Signals  
The second class of multipath mitigation techniques is the parametric ones. Theoretically, parametric techniques can thoroughly eliminate the impact of multipath signals by estimating the characteristics (time delay, amplitude and relative carrier phase) of both the line of sight (LOS) and the reflected signals. The maximum likelihood approach, such as multipath estimating delay locked loop (MEDLL) [17], is among the most popular parametric techniques. Techniques of this type outperform others since they have the potential to sweep away multipath signals more thoroughly but at the cost of relatively high computational load. Besides, they also need a-priori assumptions about the number of extra paths. When this priori information is not consistent with the fact, considerable estimation errors may be introduced. So, they cannot work properly when the number of paths is very large or unknown [18]. Another kind of parametric methods is the so-called subspace estimation techniques, namely the multiple signal classification (MUSIC) [19], and its modified method, estimation of signal parameters via rotational invariance technique (ESPRIT) [20; 21]. This class of method can estimate not only the characteristics of multipath signals but also the number, so it does not need any a priori information. The weakness of this method lies in its bad capacity of estimating the amplitude of multipath components and high sensitivity to the noise. Besides, because of large-scale matrix transformations and singular value decomposition, its computational burden is usually too high for practical application [18]. Anti-multipath techniques in the frequency domain is also a promising category of parametric methods. It is different from most conventional techniques implemented in the time domain. Reference [22] implemented multipath detection by monitoring the abnormal energy in the frequency domain, but no mitigation is performed. The satellite whose LOS signal is severely affected by reflected paths is excluded from the navigation solution. Actually, it is not a parametric technique. Reference [23] presented a frequency domain processing method by nulling out the multipath frequency spectrum. But, this technique requires satellite elevation and antenna height as a-priori information to estimate the static multipath parameters before multipath mitigation. It may be effective for a fixed receiver, such as the monitoring receivers used in the Wide Area Augmentation System (WAAS), but infeasible for a moving receiver with changing surroundings. Another interesting kind of frequency domain processing method is the FFT-based method presented by [24; 25]. Mathematically, the superposition of multipath signals in the time-domain is equivalent to the spectrum multiplication of the line of sight (LOS) and a multipath channel transfer function (CTF) in the frequency-domain. This principle can be employed to achieve anti-multipath purpose in the frequency domain. As this FFT-based method obtains the power spectra by directly taking FFT without statistical averaging, we just call it direct-FFT method in this paper. The direct-FFT method is not sensitive to different modulation types, as well as the number and amplitudes of multipath signals, showing high feasibility for different scenarios. Nevertheless, this method is still suboptimal for its high computation complexity and vulnerability to the noise. To sum up, though the parametric techniques show better multipath mitigation performance on BOC signals compared with the non-parametric techniques, they are generally limited by the excessive algorithm complexity and sensitivity to the noise. Basically, if we have a carrier to noise ratio (C/N\({}_{0}\)) higher than 40 dB-Hz, we are likely in strong LOS scenario, thus without multipaths [26]. It is important to study such algorithms below 40 dB-Hz. Besides, the adoption and promotion of BOC-modulations also make multipath mitigation tougher.