More Ports, More Overhead? A Diversity–Estimation Trade-Off Perspective for Efficient Fluid Antenna Configuration

Fluid Antenna Systems (FAS) offer a promising solution for enhancing spatial diversity and link reliability in 6G wireless networks by dynamically selecting the optimal port from reconfigurable antenna positions. However, in block-fading channels, increasing the number of ports enhances spatial diversity and improves the average transmission rate, but also incurs higher channel estimation (CE) overhead, thereby reducing the effective transmission time. This paper addresses the trade-off between diversity gain and CE cost by optimizing port configuration to maximize energy efficiency (EE) in a downlink system with a multi-port fluid antenna user. Under a block-correlated channel model, we analyze the effects of port correlation and CE overhead, and derive closed-form expressions for expected transmission rate, average transmitted data, and EE. The EE is shown to be unimodal with respect to the number of ports. Accordingly, a low-complexity three-point search algorithm is proposed to efficiently identify the optimal port number. Simulation results demonstrate that the proposed method achieves significant EE improvements and remains robust under various fluid antenna scaling factors.