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area coverage. Low power consumption directly descends
from the need to avoid further load on power supply, since the
sensor node must operate in an " always on " condition due to
the specific involved measurands.
From an IoT point of view, two main candidates, namely
SigFox by Sigfox S.A. and LoRaWAN by LoRa Alliance,
stand out. SigFox is a private data transmission service available in about 67 countries in the world. Service subscribers
are allowed to transmit every day 140 uplink messages per
node, each consisting of a maximum of 12 bytes, and only
four downlink messages, consisting of a maximum of eight
bytes. According to the procedure described above, SigFox
service would be feasible in the monitoring state of Geigerbased nodes; on the contrary, if an anomalous radiation level
is detected, the maximum daily number of uplink messages
will be rapidly reached, thus making the service unavailable.
Moreover, the dimension of data exchanged by the nodes in
identification state for a single energy spectrum acquisition exceeds the 1680 bytes offered in the whole day.
Because of this limitation, the authors chose to use LoRaWAN, a communication protocol characterized by the
availability of either licensed/proprietary or personal network infrastructure. LoRaWAN is based on an optimization
of the Chirp spread spectrum modulation that, combined with
proper configuration parameters, is capable of assuring sensitivity as low as -130 dBm. It is able to grant efficient, secure
and reliable transmission up to 15 km in open space and about
two km in an urban environment. The local sensor network
has a star topology, with individual nodes that can register
themselves and communicate with one or more LoRaWAN
Gateways, while remaining independent from one another.
The modules for LoRaWAN communication have been
realized by means of suitable devices provided by STMicroelectronics, namely a STM32 Nucleo F401RE board equipped
with a Semtech SX1272 LoRa Shield.
The module has been programmed through the integrated
development environment (IDE) Mbed in such a way to act as
a serial LoRa modem for the sensor node. In particular, a traditional asynchronous receiver/transmitter bus was adopted
to transfer data from the node to the communication module
and configure commands from the module to the node. Data
received from the sensor are encapsulated in a proper structure according to the so-called " key-value " paradigm and
transmitted to the LoRa Gateway. Configuration and monitoring messages, exchanged between nodes and the IoT
platform through the Gateway, present a simple structure and
a frequency of repetition (tens of minutes during regular monitoring operation) which does not require ad hoc solutions.
In regards to long range communication, a MultiConnect®
ConduitTM by MultiTech was adopted; the considered LoRa
Gateway exchanges data with network nodes through LoRaWAN protocol and communicates with the open-source IoT
platform by means of either mobile or Ethernet connection. In
particular, a publisher/subscriber model is exploited for data
transmission to and reception from the platform thanks to a
Message Queue Telemetry Transport (MQTT, also known as
38	

ISO/IEC PRF 20922) broker. MQTT is a lightweight application protocol that allows devices to communicate effectively
and asynchronously. The broker is a messaging server that
matches interests between subscribers and publishers with
subsequent messages dispatching. In other words, a publisher
sends messages on a specific topic to the broker. A subscriber
(i.e., a consumer who subscribes to messages posted on the
same topic) receives them from the broker when available.
This communication model greatly simplifies the protocol and
turns out to be one of the main application layer protocols for
IoT applications.
More specifically, data exchanged with sensor nodes are
published on a topic whose structure is " lora/device_lora_address/packet_recv " and the payload of the message contains
the encrypted measured values. Once decrypted, the data
are sent to the MQTT broker built in the IoT platform with a
dedicated topic. A similar approach is followed when configurations are sent from the platform; the messages are
dispatched to the device whose LoRa address corresponds to
that of the received MQTT topic.
Finally, the open-source IoT platform, namely ThingsBoard, was adopted to store and visualize monitoring data.
ThingsBoard allows straightforward set up of an IoT platform,
thanks to offering the possibility of: adding nodes whose security is assured by a specific token required to exchange data
with the platform; profiling user access to limit the visibility of
the results according to their identity; and providing a suitable
set of gauges and graphs for the visualization of the measurement results. This way, it is possible to:
◗◗ view the node status (active, inactive, or alarmed)
◗◗ view historical trends of measurements carried out by the
active sensors
◗◗ update name and physical location of nodes
◗◗ update acquisition parameters (e.g., characteristic times,
storage frequency), and
◗◗ update calibration parameters (e.g., conversion factors,
threshold limits).

Radiation Sensor Nodes
As described, the hardware architecture of the sensor network
node consists of three main components: the embedded control system, the electronic interface, and the sensor. According
to the specific measurand, different solutions have been designed and implemented; however, all of the nodes share the
same embedded system, namely BeagleBone Black, a Linuxbased prototyping board whose functionality are suitably
tailored to the specific sensor. The embedded system mainly
processes the measures provided by the sensor interface sensor and deals with the communication with the LoRaWAN
module. In [6], the authors have already presented the network node (whose current implementation is given in Fig. 3a),
equipped with a Geiger detector and used to point out anomalous radiation conditions, making the platform move from
monitoring to identification states.
To suitably detect the specific radioactive pollutant, the
network is now equipped with two different sensor nodes

IEEE Instrumentation & Measurement Magazine	

December 2020



Instrumentation & Measurement Magazine 23-9

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