  Cell-free massive multi-input multi-output (MIMO) has recently gained much
attention for its potential in shaping the landscape of sixth-generation (6G)
wireless systems. This paper proposes a hierarchical network architecture
tailored for cell-free massive MIMO, seamlessly integrating co-located and
distributed antennas. A central base station (CBS), equipped with an antenna
array, positions itself near the center of the coverage area, complemented by
distributed access points spanning the periphery. The proposed architecture
remarkably outperforms conventional cell-free networks, demonstrating superior
sum throughput while maintaining a comparable worst-case per-user spectral
efficiency. Meanwhile, the implementation cost associated with the fronthaul
network is substantially diminished.
