Analytical Channel Modeling: From MIMO to Extra Large-Scale MIMO

Multiple antenna technologies, from traditional multiple-input multiple-output (MIMO) to massive MIMO and the emerging extra large-scale MIMO, have consistently played a pivotal role in enhancing transmission rates by increasing the number of antennas. To guide the design of transmission strategies, channel models, especially analytical ones, are always significant tools, which can also reveal the performance improvements brought about by multiple antenna technologies. Analytical channel models have enjoyed significant success in traditional MIMO and massive MIMO systems. Nevertheless, due to the extended size of the array in an extra large-scale MIMO system, the distance between the receiver and the transmitter decreases and new channel properties, which did not manifest in massive MIMO systems, begin to kick in. To model the channel tailored for extra large-scale MIMO systems analytically, it is crucial to conduct a comprehensive review of traditional analytical MIMO channel models, which serves as a foundational step in understanding the fundamental characteristics of multi-antenna channels. In this paper, we first provide a survey on the state-of-the-art analytical MIMO channel models from the perspective of spatial correlation and signal propagation. Subsequently, we summarize the new properties of extra large-scale MIMO systems, i.e., near-field properties and spatial non-stationarities, and their influences on analytical channel modeling. Our objective is to elucidate how these novel properties affect the analytical MIMO channel models, and ultimately facilitate the development of precise analytical channel models well-suited to the extra large-scale MIMO systems.