Fluid Antenna Aided Intra-Cell Pilot Reuse for MIMO Wireless Networks

This paper addresses the problem of pilot contamination in single-cell networks, especially in dense-user areas where intra-cell pilot reuse is inevitable. We utilize fluid antennas (FAs) at the base station (BS) to improve the uplink (UL) channel estimation accuracy in terms of normalized mean square error (NMSE). To this end, we invoke channel spatial correlation as a key metric to characterize the similarity among the spatial structures of users’ channels. Inspired by the fact that the users with low spatial correlation experience less interference, we propose to mitigate pilot contamination by optimizing the FA positions with an innovative objective of minimizing the channel spatial correlation among the pilot-sharing users. To handle the resulting fractional programming problem, we recast it into a more tractable form via establishing upper and lower bounds for the objective function. Subsequently, the reformulated problem is effectively solved through an alternating optimization (AO) framework, where the position of each FA is iteratively optimized exploiting the successive convex approximation (SCA) method. Finally, simulation results validate the effectiveness of the proposed algorithm and demonstrate its remarkable performance in channel estimation and data transmission. In particular, the proposed 16-FA scheme achieves up to 71.2% NMSE reduction over its fixed-position antenna (FPA) counterpart, translating to an approximate 11 dB SNR gain in symbol detection.