Uplink Cell-Free Systems with Unmanned Aerial Vehicle Relays: Fairness and Power Optimization

This work investigates an uplink cell-free (CF) system incorporating multiple unmanned aerial vehicles (UAVs) functioning as aerial relay stations (ARSs). The transmitter at ARS employs the decode-and-forward (DF) mechanism to facilitate communication between users and the terrestrial base station (TBS) while operating under the time-division duplex (TDD) protocol. The ARSs, equipped with multiple antennas, collaborate to transmit signals to single-antenna users, with both ARSs and users randomly distributed within a designated area. The communication process is thoroughly examined, encompassing both user-to-ARS and ARS-to-TBS transmission links. Accordingly, we derive closed-form expressions for the data rates of both users and ARSs. We introduce approaches to solving the uplink power optimization problem in the proposed system by employing three distinct strategies: the max–min rate, the max-sum rate, and the max-product rate. To achieve this, we utilize both the bisection search (BS) method and the gray wolf optimizer (GWO) to efficiently determine optimal power allocation. The simulation results demonstrate that the power optimization significantly improves user throughput compared to nonoptimized power allocation across all optimization strategies. Additionally, we reveal that the BS method effectively solves the max–min rate strategy, while the GWO efficiently handles all optimization strategies. Furthermore, the system quality is thoroughly assessed under diverse real-world scenarios, including variations in the number of ARSs, the number of users, the channel estimation duration, and the total power consumption.