Security and Spectral Efficiency Trade-off for Single Carrier Faster Than Nyquist IoT Systems over Satellite

In narrowband IoT systems over LEO satellite, licensed and unlicensed, it is necessary to take the Doppler rate into account. That is, the variation of the Doppler shift along a frame. In this situation, the use of time-packing techniques reduces the temporal duration of the frames, which benefits to reduce the effect of Doppler rate. Furthermore, the temporal reduction of the transmitted frames is alsobeneficial for increasing the number of users that access to the satellite channel. However, time-packing techniques also have side effects to be considered. For example, the PAPR may limit its utility in terms of spectral efficiency. Note that the overlapping between the consecutive pulse-shapes increases the PAPR of the transmitted frames which reduces the efficiency of power amplifiers. This fact not only depends on the overlapping-degree between pulses but also on the roll-off factor and the pulse-shape that are used [7]. In this regard, we will analyze the PAPR of the FTN in terms of the roll-off and overlapping factors. Nevertheless, it is not only a question of PAPR analysis. At this point it is necessary to remark that the bandwidth of the pulse-shape signals expands linearly with the roll-off factor. Consequently, the gain in the time-domain may be partially reduced or completely lost in the frequency-domain if the desired PAPR is lower or equal than the Nyquist one. In this paper the pulse- shape has been fixed to the well-known square root cosine filter.