  Six-dimensional movable antenna (6DMA) is an effective solution for enhancing
wireless network capacity through the adjustment of both 3D positions and 3D
rotations of distributed antennas/antenna surfaces. Although freely
positioning/rotating 6DMA surfaces offers the greatest flexibility and thus
highest capacity improvement, its implementation may be challenging in practice
due to the drastic architecture change required for existing base stations
(BSs), which predominantly adopt fixed-position antenna (FPA) arrays (e.g.,
sector antenna arrays). Thus, we introduce in this letter a new BS architecture
called hybrid fixed and movable antennas (HFMA), which consists of both
conventional FPA arrays and position/rotation-adjustable 6DMA surfaces. For
ease of implementation, we consider that all 6DMA surfaces can rotate along a
circular track above the FPA arrays. We aim to maximize the network capacity
via optimizing the rotation angles of all 6DMA surfaces based on the users'
spatial distribution. Since this problem is combinatorial and its optimal
solution requires prohibitively high computational complexity via exhaustive
search, we propose an alternative adaptive Markov Chain Monte Carlo based
method to solve it more efficiently. Finally, we present simulation results
that show significant performance gains achieved by our proposed design over
various benchmark schemes.
