  We consider a multipoint-to-point network in which sensors periodically send
measurements to a gateway. The system uses Long Range (LoRa) communications in
a frequency band with duty-cycle limits. Our aim is to enhance the reliability
of the measurement transmissions.
  In this setting, retransmission protocols do not scale well with the number
of sensors as the duty cycle limit prevents a gateway from acknowledging all
receptions if there are many sensors. We thus intend to improve the reliability
without acknowledgments by transmitting multiple copies of a measurement, so
that the gateway is able to obtain this measurement as long as it receives at
least one copy. Each frame includes the current and a few past measurements.
  We propose a strategy for redundancy allocation that takes into account the
effects of fading and interference to determine the number of measurements to
be included in a frame. Numerical results obtained using the simulation tool
LoRaSim show that the allocation of redundancy provides up to six orders of
magnitude decrease in the outage probability. Compared to a system that blindly
allocates the maximum redundancy possible under duty-cycle and delay
constraints of the gateway and memory constraints of the sensors, our technique
provides up to 30% reduction in the average energy spent to successfully
deliver a measurement to the gateway.
