  In space-air-ground integrated networks (SAGINs), cognitive spectrum sharing
has been regarded as a promising solution to improve spectrum efficiency by
enabling a secondary network to access the spectrum of a primary network.
However, different networks in SAGIN may have different quality of service
(QoS) requirements, which can not be well satisfied with the traditional
cognitive spectrum sharing architecture. For example, the aerial network
typically has high QoS requirements, which however may not be met when it acts
as a secondary network. To address this issue, in this paper, we propose a
hierarchical cognitive spectrum sharing architecture (HCSSA) for SAGINs, where
the secondary networks are divided into a preferential one and an ordinary one.
Specifically, the aerial and terrestrial networks can access the spectrum of
the satellite network under the condition that the caused interference to the
satellite terminal is below a certain threshold. Besides, considering that the
aerial network has a higher priority than the terrestrial network, we aim to
use a rate constraint to ensure the performance of the aerial network. Subject
to these two constraints, we consider a sum-rate maximization for the
terrestrial network by jointly optimizing the transmit beamforming vectors of
the aerial and terrestrial base stations. To solve this non-convex problem, we
propose a penalty-based iterative beamforming (PIBF) scheme that uses the
penalty method and the successive convex approximation technique. Moreover, we
also develop three low-complexity schemes by optimizing the normalized
beamforming vectors and power control. Finally, we provide extensive numerical
simulations to compare the performance of the proposed PIBF scheme and the
low-complexity schemes. The results also demonstrate the advantages of the
proposed HCSSA compared with the traditional cognitive spectrum sharing
architecture.
