Degree of Freedom and Channel Capacity Studies of Near-Field Holographic MIMO

In the sixth generation (6G) wireless communication systems, holographic multiple-input multiple-output (HMIMO) is expected to have great potential in shaping electromagnetic wave, which has stimulated strong research interest. The HMIMO can be represented as a spatially-continuous aperture with its input being current density and output being electric field. To calculate its channel capacity, the degree of freedom (DoF) is introduced from the electromagnetic information theory (EIT), which integrates traditional theories applied in communications, such as electromagnetic theory and information theory. In this paper, the distribution of DoF in the near field and channel capacity are investigated. A boundary between the near field and far field is proposed from the view of DoF. A near-field array response for HMIMO is derived to fully extract DoFs in the near field and it is verified to be applicable in scattering environment with multipath components so that DoFs can be studied in a scattering environment. The effect of mutual coupling (MC) on channel capacity and DoF is studied. The results demonstrate that the proposed array response can fully extract DoFs of the channel and enhance channel capacity.