Hypertracking Beyond the Nyquist Frequency

We study the problem of tracking or disturbance rejection for signals beyond the Nyquist frequency in the sampled-data context. Given the well-established sampling theorem, one is inclined to infer that it is possible to track or reject only those signals below the Nyquist frequency. However, such a high-frequency signal may be detected as an aliased signal, and this opens a new possibility in tracking and regulation. This article shows that a multirate processing with a suitable choice of a weighting function enables tracking or rejection of high-frequency signals beyond the Nyquist frequency. An example is given to illustrate the result.