Near-Field Line-of-Sight MIMO Precoding with Distributed Antennas

The validity of the far-field assumption is being challenged by emerging 6 G technical trends characterized by higher carrier frequencies, larger antenna apertures, and denser networking. This paper investigates near-field line-ofsight multiple-input-multiple-output (MIMO) with a focus on the practically desirable implementation using distributed antennas. Firstly, the feasibility conditions are addressed taking into account the impact of spatial aliasing. The aliasing-free and the partially aliased cases are distinguished, where the achievable spatial degrees of freedom are shown to be subject to array apertures and antenna spacings, respectively. Secondly, a codebook-based precoding for channel feedback is proposed exploiting the inherent structure of the channel. Finally, numerical simulations show that the proposed codebook achieves a performance comparable to the standardized high resolution codebook in 5G NR, but with only one-tenth of the feedback overhead.