  Narrowband IoT (NB-IoT) is the latest IoT connectivity solution presented by
the 3GPP. NB-IoT introduces coverage classes and introduces a significant link
budget improvement by allowing repeated transmissions by nodes that experience
high path loss. However, those repetitions necessarily increase the energy
consumption and the latency in the whole NB-IoT system. The extent to which the
whole system is affected depends on the scheduling of the uplink and downlink
channels. We address this question, not treated previously, by developing a
tractable model of NB-IoT access protocol operation, comprising message
exchanges in random-access, control, and data channels, both in the uplink and
downlink. The model is then used to analyze the impact of channel scheduling as
well as the interaction of coexisting coverage classes, through derivation of
the expected latency and battery lifetime for each coverage class. These
results are subsequently employed in investigation of latency-energy tradeoff
in NB-IoT channel scheduling as well as determining the optimized operation
points. Simulations results show validity of the analysis and confirm that
there is a significant impact of channel scheduling on latency and lifetime
performance of NBIoT devices.
