  Intelligent reflecting surfaces (IRSs) are being studied as possible low-cost
energy-efficient alternatives to active relays, with the goal of solving the
coverage issues of millimeter wave (mmWave) and terahertz (THz) network
deployments. In the literature, these surfaces are often studied by idealizing
their characteristics. Notably, it is often assumed that IRSs can tune with
arbitrary frequency the phase-shifts induced by their elements, thanks to a
wire-like control channel to the next generation node base (gNB). Instead, in
this work we investigate an IRS-aided time division multiple access (TDMA)
cellular network, where the reconfiguration of the IRS may entail an energy or
communication cost, and we aim at limiting the number of reconfigurations over
time. We develop a clustering-based heuristic scheduling, which optimizes the
system sum-rate subject to a given number of reconfigurations within the TDMA
frame. To such end, we first cluster user equipments (UEs) with a similar
optimal IRS configuration. Then, we compute an overall IRS cluster
configuration, which can be thus kept constant while scheduling the whole UEs
cluster. Numerical results show that our approach is effective in supporting
IRSs-aided systems with practical constraints, achieving up to 85% of the
throughput obtained by an ideal deployment, while providing a 50% reduction in
the number of IRS reconfigurations.
