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 paper 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. At the last, the simulation result demonstrates the superiority of the proposed downlink transmission framework as well as the proposed algorithm.