Power Optimization and ARSs Selection Strategies in Downlink Cell-Free Multi-ARSs Communication Systems

In this study, we examine the downlink cell-free (CF) multiple aerial relay stations (ARSs) system, where ARSs are outfitted with several antennas, distributed randomly in entire responsible areas, and serve a large number of ground users. The application of the CF model is expected to not only improve the performance of the ARS system but also solve the problem of inconsistent throughput between users and the problem of near-far users in the current advanced networks, such as massive multiple-input multiple-output (mMIMO). Moreover, we propose two ARS selection strategies to cut off poor-quality transmission links in the original CF system, thereby reducing the power consumption of ARSs, reducing interference between users, and improving user throughput. We calculate a closed-form formulation for downlink user throughput under the condition of using conjugate beamforming technology. We optimize the pilot and downlink data transmission coefficients using the successive convex approximation (SCA) method and second-order cone programming (SOCP), respectively. The proposed optimization methods mitigate the problems of "pilot contamination" in channel estimation and inter-user interference in downlink data transmission of systems allowing multiple ARS to access the channel simultaneously. Our findings have assessed the downlink system throughput with pilot and data transmission power optimization for superior results based on a variety of selection strategies as well as the CF model (using all ARSs). In addition, the findings also indicate that the proposed novel ARS selection strategies have better user throughput than the CF technique with a specific number of selected ARS. Our proposed system is a promising technology and opens up various practical application scenarios for 6G networks.