Manipulating Multi-Path Channel Asymptotic Orthogonality by Adjusting Antenna Spacing

In conventional downlink multi-user communication systems, each user will experience severe inter-user interference due to the channel correlations, which has a limitation on the rate performance. To overcome this difficulty, the beamforming techniques are exploited to mitigate the inter-user interference (e.g., the zero-forcing (ZF) beamforming). While it always yields a sub-optimal performance, due to the inherent channel correlations caused by the fixed-position antennas (FPAs). However, this key issue can be efficiently addressed by applying the movable antenna (MA)/fluid antenna (FA) technology. In this paper, we investigate an MA-assisted multi-user wireless communication systems, where a linear array is equipped with multiple MAs to serve multiple users each with a single-FPA. We conduct theoretical analyses and unveil that the asymptotic orthogonality can be achieved for the multipath channels in both far-field and near-field by adjusting the adjacent antenna spacing. Numerical results are provided to validate the efficacy of our analyses and demonstrate that the channel correlations with the MA can be significantly decreased as compared to the conventional FPAs.