DoF analysis for 6G near field communication at FR3 and FR-THz with spherical wavefronts

In this work, we investigate the multiplexing gain or effective degrees of freedom (EDoF) of line-of-sight MIMO channels in the light of the extremely large-scale antenna arrays (ELAA) at terahertz (THz) and frequency range 3 (FR3) bands. This represents array and spectrum combinations set to be used for 6G communication systems operating in the near field (NF) and that exploits the spherical wave propagation model. We re-examine several useful metrics that characterise the NF channels with respect to range, EDoF, channel rank and condition number. The results show the relationships among EDoF, DoF and the theoretical multiplexing upper bound. We then propose practical spatial multiplexing based on the EDoF, EDoF/DoF ratio, and the EDoF-based channel condition number. These revised channel metrics reflect the channel singular values’ spreads and strengths better in the light of ELAA. The results will aid practical spatial multiplexing in 6G networks in enhancing the capacity of communication systems using FR3 and THz-based ELAA and operating in the NF.