On the Capacity of Faster Than Nyquist Signaling

This letter considers the capacity computations of faster-than-Nyquist (FTN) signaling. It calculates the theoretical capacity of FTN signaling which is obtained by a correlated input. The capacity-achieving power spectral density (PSD) is derived and its superiority over the independent input is shown. The practical issue imposed by the capacity-achieving PSD, i.e., out-of-band (OOB) emission, is shown. To solve this issue, an upper-bound is introduced for the input PSD to limit the OOB emission. The new optimization problem is solved and the constrained PSD is obtained. The introduced PSD captures the trade-off between the obtained capacity and the OOB emission