Spatial Pilot Reassignment in Channel Training Phase of Multi-ARSs Cell-Free System

In this work, we examine the user throughput of a cell-free (CF) system utilizing multiple unmanned aerial vehicles (UAVs) functioning as aerial relay stations (ARSs). Each ARS is equipped with multiple antennas and serves single-antenna users within a specific area. The system employs a decode-and-forward (DF) protocol to connect the terrestrial base station (TBS) with users. Moreover, we consider a small-cell (SC) system relying on the CF architecture, where each user is served by the ARS with the best channel conditions. We analyze the ARSs-user links and ARSs-TBS links to derive expressions for data rates. Additionally, we introduce the Spatial Pilot Reassignment (SPR) algorithm to improve the accuracy of channel estimation. User throughput is evaluated when applying SPR to both CF and SC systems. The results show that at a CDF value of 0.1, the SPR algorithm improves throughput by about 10% compared to the random pilot assignment approach.