Multi-Satellite Diversity Through the Use of OTFS

We consider the use of multiple low-Earth orbit (LEO) satellites to improve the spectral efficiency of beyond-5G (B5G) non-terrestrial networks. In these networks, the use of LEO constellations is foreseen to ensure a global coverage and, typically, multiple satellites are in visibility with the user terminal. In a scenario like this, we consider the adoption of the orthogonal time frequency space (OTFS) modulation. In this modulation format, conceived for doubly-selective terrestrial channels, the information symbols are placed in the Doppler-delay (DD) domain, wherein the time-varying channels are quasi-static and sparse. Through the use of proper pilot schemes and channel estimation algorithms, the Doppler-delay pairs and the channel gains of the different scatterers of a terrestrial channel can be estimated and coherent detection can be then performed through the use of proper algorithms. The same channel estimation and detection algorithms also enable the exploitation of diversity, in case of multiple satellites transmitting the same OTFS signal. The different satellites will be, in fact, characterized by different Doppler-delay pairs and channel gains, making the scenario similar to the case of a wireless channels with different scatterers. A cell-free architecture can be thus conceived with a significant improvement with respect to a scenario where each user is served by a single satellite.