  In some applications of frequency estimation, the frequencies of multiple
sinusoids are required to be estimated from sub-Nyquist sampling sequences. In
this paper, we propose a novel method based on subspace techniques to estimate
the frequencies by using under-sampled samples. We analyze the impact of
under-sampling and demonstrate that three sub-Nyquist sequences are general
enough to estimate the frequencies under some condition. The frequencies
estimated from one sequence are unfolded in frequency domain, and then the
other two sequences are used to pick the correct frequencies from all possible
frequencies. Simulations illustrate the validity of the theory. Numerical
results show that this method is feasible and accurate at quite low sampling
rates.
