Sinusoidal Amplitude and Phase Estimation
The form of the optimal estimator (5.39) immediately suggests the following generalization for the case of unknown amplitude and phase:That is, is given by the complex coefficient of projection [264] of onto the complex sinusoid at the known frequency . This can be shown by generalizing the previous derivation, but here we will derive it using the more enlightened orthogonality principle [114]. The orthogonality principle for linear least squares estimation states that the projection error must be orthogonal to the model. That is, if is our optimal signal model (viewed now as an vector in ), then we must have [264]
(6.42) 
The optimality of in the least squares sense follows from the leastsquares optimality of orthogonal projection [114,121,252]. From a geometrical point of view, referring to Fig.5.16, we say that the minimum distance from a vector to some lowerdimensional subspace , where (here for one complex sinusoid) may be found by ``dropping a perpendicular'' from to the subspace. The point at the foot of the perpendicular is the point within the subspace closest to in Euclidean distance.

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Sinusoidal Amplitude Estimation