Cell-Free Optimization Method for Downlink Integrated Satellite-Ground Networks

The integrated satellite-ground network (ISGN) is regarded as a promising technology for future communication systems, which can provide ubiquitous connectivity for various communication scenes, such as remote areas, maritime, and emergency communications. This article presents a novel downlink transmission framework for the cell-free multi-input-multi-output (CFmMIMO) network-based ISGN system. The satellite serves a group of earth stations through multicast technology, while the ground network embraces the cell-free network strategy to serve multiple terrestrial users. Furthermore, the CFmMIMO-based ISGN under consideration operates at millimeter-wave frequencies to satisfy the high data rate demand of the next-generation communication system. In order to enhance the spectral efficiency of the proposed CFmMIMO-based ISGN system, we formulate two joint beamforming design problems, which are denoted as the maximize sum-rate (MSR) problem and the max-min fairness (MMF) problem. Then, to solve this mathematically intractable problem, we initially applied fractional programming, alternating optimization, and successive convex approximation to convert the MSR-based joint precoding problem into an equivalent quadratically constrained quadratic program (QCQP) problem. After that, the QCQP optimization problem is solved using an alternating direction method of multipliers-based algorithm, where we dramatically simplify the algorithm’s computational complexity with several straightforward variable substitutions. Furthermore, we extend our proposed algorithm to address the MMF-based joint beamforming design problem. Finally, the simulation result demonstrates the superiority of the proposed downlink transmission framework as well as the proposed algorithm.