A Tutorial on Fluid Antenna System for 6G Networks: Encompassing Communication Theory, Optimization Methods and Hardware Designs - New, W.K.; Wong, K.; Xu, H.; Wang, C.; Ghadi, F.R.; Zhang, J.; Rao, J.; Murch, R.; Ramirez-Espinosa, P.; Morales-Jimenez, D.; Chae, C.; Tong, K. (IEEE Communications Surveys & Tutorials, Communications Surveys & Tutorials, IEEE, IEEE Commun. Surv. Tutorials, 2025)