WSN – IoT
porting both is often only marginally higher
than that for a WiFi-only transceiver.
This can be leveraged to ease tasks such
as installation. First, a simple Bluetooth
connection to an app hosted by a mobile
device can be used to set up the device.
Once configured, it can switch to using
the WiFi protocol for data transfers.
A further option that has emerged
recently is DECT Ultra Low Energy
(ULE). It has the advantage over many
of the IoT short-range protocols of having
dedicated RF spectrum instead of
shared access to the 2.4 GHz ISM band.
DECT ULE’s range can extend as far
as 300 m outdoors and 50 m indoors.
The DECT protocol lets multiple gateways
cooperate to extend the range of
a single network much further than the
core 300 m. Although DECT was originally
developed for wireless telephony,
the ULE version provides low-power
communication for IoT sensor nodes.
In the short-range environment,
the gateway is normally managed by
the user. In the low-power wide-area
networking (LPWAN) environment, the
gateway can be privately owned but
access can also be through public networks.
A protocol that offers the choice
of either is LoRA.
Based on a transceiver design by
semiconductor supplier Semtech, LoRA
employs unlicensed spectrum and
provides users with the option to deploy
their own gateways or have their devices
communicate with third-party networks.
Some cities have deployed networks
based on LoRA that are free to access
and service providers have appeared
that rent access to their gateways.
To avoid interference problems from
other users on the same RF band,
LoRA uses a spread-spectrum modulation
scheme supporting datarates from
300 b/s to 50 kb/s. The range can be
up to 10 km and the use of comparatively
low frequencies makes it possible
to reach devices buried below ground,
such as water meters.
Sigfox uses ultra-narrowband transmission
to extend its range to as much
as 50 km in rural areas. Whereas LoRA
is designed to support bidirectional
communication, Sigfox is optimised for
low-datarate transfers in one direction
– usually from the sensor node to the
server. Datarates range from 10 b/s to
1 kb/s. Sigfox is not completely unidirectional:
the protocol supports acknowledgement
packets so the sensor
node can determine whether a communication
has been received, supporting
applications such as security alarms.
Figure 2: BLE modules like the MBN52832 device can support the most
demanding Bluetooth low energy IoT applications.
One advantage of Sigfox’s focus on
one-way data transfers is that it can
help preserve power on the sensor
node, thus extending battery life. If the
node only has to wait for acknowledgements,
which are received very quickly
after transmission, there is no need for
the node to wake on a regular cycle to
listen for downlinks from the gateway.
Whereas LoRA provides the option for
users to operate their own gateways, all
communications on Sigfox pass through
the company’s own gateways. Although
it has less operational flexibility this
has the benefit of providing users with
a single supplier that provides network
support in a large number of countries.
Cellular connectivity is already widely
used for machine-to-machine applications.
In recent years, the industry has
augmented the basic GPRS offerings with
a variety of protocols that support either
higher datarates or lower-power operation.
A key advantage of cellular connectivity
is that operators are able to manage
congestion and interference much more
readily than is possible with unlicensed
spectrum, which improves long-term
reliability. The open nature of the protocols
themselves provides a rich array of
compatible silicon and RF modules.
The first change came with Enhanced
Coverage GSM, which improves the ability
of cellular signals to reach more distant
nodes or connect to buried sensor
nodes. EC-GSM can handle signals that
are 20 dB weaker than standard GPRS
and supports datarates up to 10 kb/s.
The arrival of Long Term Evolution
(LTE) has brought with it several options
for IoT connectivity, thanks to
the 4G protocol’s more efficient use of
RF spectrum. The first to arrive was
Cat-M, which supports 1Mb/s datarates
for both the uplink and downlink using
half-duplex communication. Cat-M also
provide energy-saving enhancements.
Compared to the core LTE protocol
used by mobile phones, Cat-M can
operate with fewer updates from the
base station. The frequency of updates
can be reduced to the point where the
sensor node only has to wake up every
ten minutes or so, which can greatly
preserve battery life for devices that
monitor slow-changing conditions, such
as soil moisture.
Narrowband-IoT (NB-IoT) provides
further enhancements to energy efficiency.
NB-IoT uses a much narrower
transmission band than full LTE:1.4 MHz
rather than 20 MHz. This is accompanied
by a reduction in transmit power to
further improve battery life. In an ongoing
process of enhancements, Release
14 of the LTE standard by 3GPP, has
further improved efficiency by supporting
techniques to allow nodes to
disconnect rapidly after a transmission
to reduce leakage power. Datarates of
50 kb/s on the downlink and 20 kb/s
on the uplink are possible, extending to
50 kb/s if multi-tone signalling is employed
for the uplink.
Thanks to the rich selection of
protocols suitable for IoT use, whether
operating in a short-range or wide-range
scenario, developers and integrators
can be sure to find one that fits the
application. Independent module suppliers
such as Murata can advise on
which makes sense for each situation
and provide solutions based on the best
available silicon on the market.
18 MW November - December 2018 www.mwee.com