“…In this section, we provide a brief overview of the existing work in three relevant domains: cluster-based networks [ 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 ], traditional algorithm-based UAV trajectory planning [ 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 , 37 ], and DRL-based UAV trajectory planning [ 38 , 39 , 40 , 41 , 42 , 43 ].…”