  Intelligent reflecting surface (IRS) is regarded as a revolutionary paradigm
that can reconfigure the wireless propagation environment for enhancing the
desired signal and/or weakening the interference, and thus improving the
quality of service (QoS) for communication systems. In this paper, we propose
an IRS-aided sectorized BS design where the IRS is mounted in front of a
transmitter (TX) and reflects/reconfigures signal towards the desired user
equipment (UE). Unlike prior works that address link-level
analysis/optimization of IRS-aided systems, we focus on the system-level
three-dimensional (3D) coverage performance in both single-/multiple-cell
scenarios. To this end, a distance/angle-dependent 3D channel model is
considered for UEs in the 3D space, as well as the non-isotropic TX beam
pattern and IRS element radiation pattern (ERP), both of which affect the
average channel power as well as the multi-path fading statistics. Based on the
above, a general formula of received signal power in our design is obtained,
along with derived power scaling laws and upper/lower bounds on the mean
signal/interference power under IRS passive beamforming or random scattering.
Numerical results validate our analysis and demonstrate that our proposed
design outperforms the benchmark schemes with fixed BS antenna patterns or
active 3D beamforming. In particular, for aerial UEs that suffer from strong
inter-cell interference, the IRS-aided BS design provides much better QoS in
terms of the ergodic throughput performance compared with benchmarks, thanks to
the IRS-inherent double pathloss effect that helps weaken the interference.
