at least one memory including computer program code;

at least one processor configured to execute the computer program code and cause the the apparatus to perform,

obtaining channel response data comprising a first channel frequency response of a channel over a first frequency spectrum, wherein the first channel frequency response is generated in response to a transmission over the channel or a simulation thereof; and

generating an estimate of a second channel frequency response of the channel over a second frequency spectrum in response to applying the channel response data to a machine-learning, ML, model, wherein the second frequency spectrum is different to the first frequency spectrum.