  This paper studies the large-scale cell-free networks where dense distributed
access points (APs) serve many users. As a promising next-generation network
architecture, cell-free networks enable ultra-reliable connections and minimal
fading/blockage, which are much favorable to the millimeter wave and Terahertz
transmissions. However, conventional beam management with large phased arrays
in a cell is very time-consuming in the higher-frequencies, and could be
worsened when deploying a large number of coordinated APs in the cell-free
systems. To tackle this challenge, the spatial-spectral cell-free networks with
the leaky-wave antennas are established by coupling the propagation angles with
frequencies. The beam training overhead in this direction can be significantly
reduced through exploiting such spatial-spectral coupling effects. In the
considered large-scale spatial-spectral cell-free networks, a novel subchannel
allocation solution at sub-terahertz bands is proposed by leveraging the
relationship between cross-entropy method and mixture model. Since initial
access and AP clustering play a key role in achieving scalable large-scale
cell-free networks, a hierarchical AP clustering solution is proposed to make
the joint initial access and cluster formation, which is adaptive and has no
need to initialize the number of AP clusters. After AP clustering, a subchannel
allocation solution is devised to manage the interference between AP clusters.
Numerical results are presented to confirm the efficiency of the proposed
solutions and indicate that besides subchannel allocation, AP clustering can
also have a big impact on the large-scale cell-free network performance at
sub-terahertz bands.
