Symbiotic FAS Strategies for 6G UAVs Assisted Backscatter Networks
This paper investigates a fluid antenna system (FAS) enabled symbiotic radio (SR) network featuring a UAV mounted FAS communicating through a wireless power transfer (WPT) signal with a remote cluster head and an ambient tag. To evaluate system reliability, we derive the upper (UB) and lower (LB) bounds for both the composite and backscatter (BcS) outage probabilities, formulate the coexistence outage probability (COP), and present an asymptotic analysis that explicitly characterizes the system's spatial diversity and coding gains. We propose three novel symbiotic strategies; maximum backscatter selection (MBS), joint balanced selection (JBS) and threshold aware priority selection (TAPS) and compare them with the conventional maximum composite gain selection (MCGS) and random selection methods. Under a joint optimization framework, the macroscopic UAV 2D spatial placement and microscopic realization - level port selection were formulated. Since the joint outage Pareto frontier is highly non-convex, the computationally expensive epsilon-constraint method identified the optimal knee-point. While the symbiotic novel TAPS strategy demonstrated identical performance in a single step with linear O(MN) complexity. Moreover, an asymptotic closed form mobility analysis under Jakes' fading model proves that the symbiotic FAS tracking protocol consumes under 10% of the channel coherence time. Simulation results and COP heat maps validate the optimal coexistence performance at the ideal UAV coordinates with 60% reduction in the required transmit power.