  Developing low-power wide-area network (LPWAN) solutions that are efficient
to adopt, deploy and maintain are vital for smart cities. The poor
quality-of-service of unlicensed LPWAN, and the high service cost of
LTE-M/NB-IoT are key disadvantages of these technologies. Concatenating
unlicensed with licensed LPWANs can overcome these limitations and harness
their benefits. However, a concatenated LPWAN architecture will inevitably
result in excess latency which may impact users' quality-of-experience (QoE).
To evaluate the real-life feasibility of this system, we first propose a
concatenated LPWAN architecture and experimentally measure the statistics of
end-to-end (E2E) latencies. The concatenated delay margin is determined by
benchmarking the latencies with different LPWAN architecture schemes, namely
with unlicensed IoT (standalone LoRa), cellular IoT (standalone LTE-M), and
concatenated IoT (LoRa interfaced with LTE-M). Through extensive experimental
measurement campaigns of 30,000 data points of E2E latencies, we show that the
excess delay due to LPWAN interfacing introduces on average less than 300
milliseconds. The proof-of-concept results suggest that the latency for
concatenating unlicensed LPWAN with cellular IoT is negligible for smart city
use cases where human perception and decision making is in the loop.
