Faster than Nyquist-an enabler for achieving maximum spectral efficiency in coexistence scenarios?

In the context of spectrum sharing, a loss in spectral efficiency is incurred due to required guard bands needed for a desired co-channel interference level between systems. Our main interest is to show whether FTN signaling can recover the loss of spectral efficiency (SE) due to guard bands by adjusting the symbol duration and tolerating ISI in practical coexistence scenarios. In this paper, the SE of FTN signaling is investigated taking into account a practical multi-access channel, where a single-carrier system based on Nyquist or FTN signaling shares spectrum with a multi-carrier system. A single-user based on OQAM/OFDM system is considered as reference system. Simulation results show that by choosing an appropriate symbol duration, FTN signaling can recover the loss of SE at high SNRs. Furthermore, in coexistence scenarios with small guard bands, using FTN the SE curve is very close to that of the single-user reference system.