Massive Spatial Multiplexing in 6G: What kind of antenna arrays are needed?

Multi-antenna communication technology can, in theory, provide great bit rates between a transmitter and receiver through spatial multiplexing; that is, sending different spatial layers over different propagation paths. Unfortunately, traditional systems operate in the far field where there is often at most one strong propagation path. This might change in 6G. By increasing the carrier frequency, adding more antennas, and densifying the network infrastructure, we will enter a paradigm where communications mostly happen in the radiative near-field. In this keynote, we will revisit the fundamentals of multiple-input multiple-output (MIMO) communications and explore the new features that arise in the near field. The relation between spatial modes, spherical wave fronts, and array geometries will be described and illustrated. Is massive spatial multiplexing the next untapped signal dimension that can sustain the capacity growth in future networks?