Channel Nonstationarity and Consistency for Beyond 5G and 6G: A Survey

As the fifth-generation (5G) wireless communication networks are at the stage of commercial deployment, beyond 5G (B5G)/sixth-generation (6G) wireless communication networks have also been under extensive research. In the B5G/6G era, the vision of the wireless communication network is the so-termed space-air-ground-sea integrated network (SAGSIN), which will focus on more various and dynamic communication scenarios, including vehicle-to-vehicle (V2V), high-speed train (HST), unmanned aerial vehicle (UAV), satellite, and maritime communications. Meanwhile, B5G/6G communication systems will also employ two potential technologies, i.e., millimeter wave (mmWave)-terahertz (THz) and ultra-massive multiple-input multiple-output (MIMO), and have a new development trend, i.e., integrated sensing and communication (ISAC) systems. For the successful design of B5G/6G communication systems, accurate and easy-to-use channel models, which can fully mimic the underlying characteristics and features of B5G/6G channels, are indispensable. However, more diverse communication scenarios, higher frequency band, larger-scale antenna array, and the emergence of ISAC systems in B5G/6G will bring two significant points of concern for wireless channels, i.e., channel non-stationarity and channel consistency. Channel non-stationarity is a typical channel characteristic, whereas channel consistency is an inherent channel physical feature. To capture those, extensive works have been carried out, but have not yet been adequately summarized, compared, and analyzed. This paper first provides the definitions of channel non-stationarity and channel consistency from mathematical and physical perspectives, and then discusses the necessity of capturing them for various wireless applications. Recent advances in the topic of capturing channel non-stationarity and channel consistency are further elaborated and investigated. Additionally, simulation results concerning them are provided and analyzed. Finally, promising and meaningful future research directions for this topic are outlined.