  Communication between satellites in low-Earth orbit (LEO) constellations
takes place through inter-satellite links (ISLs). Unlike intra-plane ISLs,
which interconnect satellites belonging to the same orbital plane with fixed
relative distance, inter-plane ISLs experience significant Doppler frequency
shifts, since satellites belonging to different orbital planes exhibit
timevarying relative distance (required, e.g., to minimize the risk of physical
collisions between satellites). In this paper, we consider the problem of
connecting multiple satellites, belonging to a massive LEO constellation, to a
receiving satellite, referred to as the sink. Specifically, we consider a
hybrid multiple access scheme, which employs a combination of non-orthogonal
multiple access (NOMA), where radio-frequency ISLs share the same
time-frequency resource blocks, and orthogonal multiple access (OMA), where
ISLs employ orthogonal resource blocks. The set of satellites transmitting
towards the sink is divided into groups, where NOMA is employed within each
group, whereas OMA is used to separate different groups. Such a scheme subsumes
as special cases both pure-OMA and pure-NOMA. Our study highlights that similar
Doppler frequency shifts have a significant impact on the individual rates of
the satellites in a pure-NOMA scheme, thus reducing the network fairness of
this technique. Motivated by such a fact, we develop design strategies of the
proposed hybrid NOMA-OMA scheme, which exploit inter-plane Doppler frequency
diversity to enhance fairness among the satellites, while ensuring a
significantly higher sumrate capacity compared to the pure-OMA technique.
Numerical results corroborate our theoretical analysis, by demonstrating both
the fairness enhancement of the proposed techniques over the pure-NOMA scheme,
as well as their capacity improvement over the pure-OMA one.
