  Low-power and long-range communication tech- nologies such as LoRa are
becoming popular in IoT applications due to their ability to cover kilometers
range with milliwatt of power consumption. One of the major drawbacks of LoRa
is the data latency and the traffic congestion when the number of devices in
the network increases. Especially, the latency arises due to the extreme duty
cycling of LoRa end-nodes for reducing the overall energy consumption. To
overcome this drawback, we propose a heterogeneous network architecture and an
energy-efficient On-demand TDMA communication scheme improving both the device
lifetime and the data latency of standard LoRa networks. We combine the
capabilities of micro- watt wake-up receivers to achieve ultra-low power states
and pure asynchronous communication together with the long-range connectivity
of LoRa. Experimental results show a data reliability of 100% and a round-trip
latency on the order of milliseconds with end devices dissipating less than 46
mJ when active and 1.83 {\mu}W during periods of inactivity, lasting up to 3
years on a 1200 mAh Lithium battery.
