  Recently, the concept of holographic multiple-input multiple-output (MIMO) is
emerging as one of the promising technologies beyond massive MIMO. Many
challenges need to be addressed to bring this novel idea into practice,
including electromagnetic (EM)-compliant channel modeling and accurate
performance evaluation. In this paper, an EM-compliant channel model is
proposed for the holographic MIMO systems, which is able to model both the
characteristics of the propagation channel and the non-ideal factors caused by
mutual coupling at the transceivers, including the antenna pattern distortion
and the decrease of antenna efficiency. Based on the proposed channel model, a
more realistic performance evaluation is conducted to show the performance of
the holographic MIMO system in both the single-user and the multi-user
scenarios. Key challenges and future research directions are further provided
based on the theoretical analyses and numerical results.
