  In the future, urban regions will encounter a massive number of
capacity-hungry devices. Relying solely on terrestrial networks for serving all
UEs will be a cost-ineffective approach. Consequently, with the anticipated
supply and demand mismatch, several UEs will be unsupported. To offer service
to the left-out UEs, we employ an energy-efficient and cost-effective
beyond-cell communications approach, which uses reconfigurable intelligent
surfaces (RIS) on a high-altitude platform station (HAPS). Particularly,
unsupported UEs will be connected to a dedicated control station (CS) through
RIS-mounted HAPS. A novel resource-efficient optimization problem is formulated
that maximizes the number of connected UEs, while minimizing the total power
consumed by the CS and RIS. Since the resulting problem is a mixed-integer
nonlinear program (MINLP), a low-complexity two-stage algorithm is developed.
Numerical results demonstrate that the proposed algorithm outperforms the
benchmark approach in terms of the percentage of connected UEs and the
resource-efficiency (RE). Also, the results show that the number of connected
UEs is more sensitive to transmit power at the CS than the HAPS size.
