  Holographic multiple-input multiple-output (MIMO) is deemed as a promising
technique beyond massive MIMO, unleashing near-field communications,
localization, and sensing in the next-generation wireless networks.
Semi-continuous surface with densely packed elements brings new opportunities
for increased spatial degrees of freedom (DoFs) and spectrum efficiency (SE)
even in the line-of-sight (LoS) condition. In this paper, we analyze
holographic MIMO performance with disk-shaped large intelligent surfaces (LISs)
according to different precoding designs. Beyond the well-known technique of
orbital angular momentum (OAM) of radio waves, we propose a new design based on
polar Walsh functions. Furthermore, we characterize the performance gap between
the proposed scheme and the optimal case with singular value decomposition
(SVD) alongside perfect channel state information (CSI) as well as other
benchmark schemes in terms of channel capacity. It is verified that the
proposed scheme marginally underperforms the OAM-based approach, while offering
potential perspectives for reducing implementation complexity and expenditure.
