Opportunities for Direct-to-Cell Polarization MIMO in the S-band Downlink of a LEO Satellite Footprint
A single low Earth orbit (LEO) satellite offers almost no spatial degrees of freedom. Its two polarizations, however, can be exploited to carry a second layer to a handset, which 5G New Radio (NR) non-terrestrial networks do not use yet. The gain of this layer in downlink spectral efficiency (SE) is mapped over the footprint of a satellite at 330 km altitude above Munich transmitting at 2 GHz. The channel is modeled as a cascade of Jones factors for the phased-array payload, Faraday rotation, a dual-polarized land mobile satellite (LMS) channel with a line of sight, and a standardized handset antenna element at three attitudes. For a handset with boresight pointing to zenith, ideal precoding selects rank 2 with a probability of at least 0.95 within about 340 km of the nadir point at a reference signal-to-noise ratio (SNR) of 6 dB. With closed-loop precoding, such a handset on the nadir track is in rank 2 during 99% of a pass. At 13 dB, rank selection with closed-loop precoding raises the mean SE over the footprint by 49%, and upright handsets gain about 31%. Their gain is limited to the positions where they face the satellite, since the element gain is low behind the handset. At 3 dB, these gains fall to 15% and 7%.