  The use of Wake-Up Radio (WUR) in Internet of Things (IoT) networks can
significantly improve their energy efficiency: battery-powered sensors can
remain in a low-power (sleep) mode while listening for wake-up messages using
their WUR and reactivate only when polled, saving energy. However,
polling-based Time Division Multiple Access (TDMA) may significantly increase
data transmission delay if packets are generated sporadically, as nodes with no
information still need to be polled. In this paper, we examine the effect of
multicast polling for WUR-enabled wireless nodes. The idea is to assign nodes
to multicast groups so that all nodes in the same group can be solicited by a
multicast polling message. This may cause collisions, which can be solved by
requesting retransmissions from the involved nodes. We analyze the performance
of different multicast polling and retransmission strategies, showing that the
optimal approach can significantly reduce the delay over TDMA and ALOHA in
low-traffic scenarios while keeping good energy efficiency.
