  The space-air-ground integrated network (SAGIN) is a pivotal architecture to
support ubiquitous connectivity in the upcoming 6G era. Inter-operator resource
and service sharing is a promising way to realize such a huge network,
utilizing resources efficiently and reducing construction costs. Given the
rationality of operators, the configuration of resources and services in SAGIN
should focus on both the overall system performance and individual benefits of
operators. Motivated by emerging symbiotic communication facilitating mutual
benefits across different radio systems, we investigate the resource and
service sharing in SAGIN from a symbiotic communication perspective in this
paper. In particular, we consider a SAGIN consisting of a ground network
operator (GNO) and a satellite network operator (SNO). Specifically, we aim to
maximize the weighted sum rate (WSR) of the whole SAGIN by jointly optimizing
the user association, resource allocation, and beamforming. Besides, we
introduce a sharing coefficient to characterize the revenue of operators.
Operators may suffer revenue loss when only focusing on maximizing the WSR. In
pursuit of mutual benefits, we propose a mutual benefit constraint (MBC) to
ensure that each operator obtains revenue gains. Then, we develop a centralized
algorithm based on the successive convex approximation (SCA) method.
Considering that the centralized algorithm is difficult to implement, we
propose a distributed algorithm based on Lagrangian dual decomposition and the
consensus alternating direction method of multipliers (ADMM). Finally, we
provide extensive numerical simulations to demonstrate the effectiveness of the
two proposed algorithms, and the distributed optimization algorithm can
approach the performance of the centralized one.
