LTE over satellite
IoT is one of the current economic motors at global level. Initially, they were devised for short range terrestrial communications (i.e WIFI, Bluetooth). However, with the introduction of Sigfox and Lora –based systems, the range of their communications was extended a lot. In this sense, LEO systems appear as an attractive strategy for providing service to IoT devices with large range capabilities. Similarly, LTE-based systems are also designed for terrestrial communications. The large spectrum demand and reduced latencies, LEO systems appear as an attractive solutions for providing service to cellular LTE systems.
Unfortunately, LEO channel is affected by Doppler effect which degrades the performance of the classical terrestrial IoT/LTE devices. For this reason, AMSP department has modelled the Doppler in a satellite swath and designing IoT/LTE receivers robust to it. Figure 1 shows the Doppler shift that suffers a carrier of f=900MHz in terms of the altitude of the satellite. Note as the larger the altitude of the satellite, the lower the Doppler shift is. On the contrary, the latency of LEO channels is quite reduced respect to GEO ones. Specifically, LEO satellites have a maximum latency around 10-30ms whereas GEO ones have a latency ranged between 0.25-0.3 seconds (See Figure 2). Consequently, LEO systems result a quite interesting air-interface for providing 5G communications. At this point, it is necessary to remark that the visibility time of a LEO satellite is quite reduced. For a satellite at 1000Km of altitude its visibility period is around 10 minutes (Figure 3). As a result, hand over strategies are crucial for providing global coverage using LEO satellites.
Figure 1. Doppler shift in terms of the satellite altitude.
Figure 2. Latency of a satellite link in terms of its altitude.
Figure 3. Visibility time of a satellite in terms of its altitude.