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Spatial Frequencies and Degrees of Freedom in Near-Field Communications

This video explores the fundamental differences and similarities between near-field and far-field wireless communications, focusing on spatial frequencies and the degrees of freedom (DoF) available in multiple antenna systems. It begins by analyzing how wireless signals interact with antenna arrays through spatial frequency components, transitioning from traditional far-field approximations to more complex near-field behavior. Key concepts such as the spatial discrete Fourier transform (DFT), spatial bandwidth, and their dependence on propagation distance and angle are introduced. The presentation demonstrates that even in the radiative near-field—where spherical wavefronts dominate—the channel can still be represented using spatial frequency expansions, albeit with a richer structure. The impact of these characteristics on multi-user MIMO communications is discussed, showing that near-field conditions can offer greater chances for user separability without increasing the spatial DoF. Theoretical modeling approaches and practical implications for the rank of spatial correlation matrices in fading environments are also covered, providing insights into how to harness the potential of near-field channels in next-generation wireless systems. The presenter is Prof. Emil Björnson, KTH Royal Institute of Technology. You can learn more by reading the following magazine paper: Alva Kosasih, Özlem Tuğfe Demir, Nikolaos Kolomvakis, Emil Björnson, “Spatial Frequencies and Degrees of Freedom: Their roles in near-field communications,” IEEE Signal Processing Magazine, vol. 42, no. 1, pp. 33-44, January 2025. Preprint: https://arxiv.org/pdf/2407.07448 Download the slides: https://github.com/emilbjornson/prese... The video was recorded for the International Workshop on 6G Channel Measurements and Modeling (6GCMM 2025), held in Nanjing on May 15-16, 2025. https://www.awitec-cmm.com/en/ #wireless #6g #MIMO #frequencies