These products can simplify the design of battery-operated IoT devices with low-cost connection to cellular services for worldwide roaming operation

By Richard Quinnell, editor-in-chief

Many
devices in the Internet of Things use Wi-Fi as their link to the cloud, but
there are a host of applications needing both mobility and wide-area
connectivity that Wi-Fi cannot offer. For such applications, a variety of
low-power wide-area network (LPWAN) technologies are on offer, but most require
creation of a proprietary service network to provide extensive coverage. Now,
cellular phone networks are implementing a variation on the LTE services that they
already have in place that will meet the IoT’s LPWAN needs, and pre-certified
modules are the key for designers seeking to quickly enter such application
spaces.

The
ever-popular Wi-Fi works well for a broad range of IoT applications as long as
devices remain within the network’s typical 50-m range. It’s even possible to
extend the range to a kilometer or so. But there are numerous existing and
potential IoT applications that need much more freedom in their location and
movement than Wi-Fi can offer. Most of these must also be battery-powered, which
is a second strike against the power-hungry Wi-Fi. What these applications need
is a low-power, wide-area network (LPWAN).

To
let battery-powered IoT devices operate far from an access point and freely
roam over vast — even
intercontinental —
distances, many alternate LPWAN technologies have
arisen. These include such options as LoRa, Sigfox, and Weightless. But all
have the same drawback: Their use depends on first establishing a network of
access points that cover the geographic area in which the IoT device is to
operate. For the most part, these LPWAN alternatives have only built up
networks with very limited coverage. And many of the alternatives do not allow
devices to readily roam from one access point to another.

LTE-M_Guide_Android

Cellular technology is evolving to
improve worldwide support of IoT applications. Image source: Pixabay.

There
is one type of wireless network, however, that covers vast areas of the world
and supports roaming among access points without losing their connection:
cellular telephony. The freedoms that cellular connectivity supports have led
to many different attempts at leveraging the network for IoT communications. Some
applications have used the approach of piggybacking on a user’s cellphone to
connect to the cloud, but that has many limitations, not the least of which is
the requirement that an active phone needs to always be nearby. Others have
simply built cellphone functionality into the IoT device, but this approach has
its own limitations. The cellular network was not originally designed to
support data traffic, so power requirements, call setup complexity, and
connection costs were all issues.

The
cellular industry has been moving to address these issues, though, especially
with its adoption of 3GPP Release 13 in 2016, which set
the stage for bringing the IoT into the cellular industry’s long-term evolution
(LTE) programs. With Release 13 came two
approaches
to providing IoT-optimized connectivity on LTE networks: LTE
Category-M1, also called LTE CAT-M and LTE-M, and LTE Category-M2, also known
as CAT-NB1 or simply narrowband IoT (NB-IoT). These are variations on existing
cellular technologies that support devices needing relatively infrequent,
block-oriented data transfers rather than the continuous streaming that
characterizes smartphone data. They require much less handshaking and overhead
when establishing a connection and much less average energy expenditure than
previous cellular IoT options. In fact, the design goal for the standards
included allowing a device to achieve 10-year operating lifetime on a battery.
These LTE options also enjoy a much lower cost connection to service providers
than other cellular IoT.

Network
support for CAT-M and NB-IoT is currently building, with CAT-M the dominant
form in North America and NB-IoT growing in popularity elsewhere in the world.
Already in the U.S., cellular service provider Verizon is growing CAT-M coverage
on its networks, with AT&T and T-Mobile not far behind. By 2020, IoT
developers will have access to near continental coverage using this approach.

But
getting a design to market that leverages this opportunity presents a major
challenge. Anyone who has worked through FCC certification for their wireless
device already knows that considerable RF expertise is needed to ensure that a
product passes all of its compliance tests. The problem is doubled when working
with cellular connectivity. Cellular service providers also require that a
design be certified to work with their network before they will allow devices
to connect, and their tests differ from FCC certification. Furthermore, service
providers all have slightly different certification requirements that pertain
specifically to their networks.

LTE CAT-M modules simplify cellular IoT
design

By
far the easiest, least expensive, and speediest approach to meeting all of these
certifications is to build the device around a CAT-M modem module that has
already been FCC- and carrier-certified. Fortunately, many such modems are
already available, with more coming. Finding the right module, though, requires
developers to carefully consider several key parameters.

LTE-M_Guide_Gemalto_EMS31

Modules offer drop-in
cellular connectivity pre-certified by both regulatory agencies and carriers. Image
source: Gemalto.

One,
of course, is power requirements, which, in turn, define battery life. But
power requirements are difficult to spec without knowing application
characteristics such as the required radio range, data processing requirements,
amount of data to exchange per message, data rate (which affects the time that it
takes to send a message), and how often messages are to be sent. The modem can
offer power management capabilities such as Release 13’s power-saving mode
(PSM) and the extended discontinuous reception (eDRX) feature that allows for
long sleep periods without a device losing its link to the network.

To
get a sense of the design’s battery life, then, developers will need to look at
those system factors as well as module attributes like sleep-mode current,
active-receiver current, active-transmission current, time spent transitioning
from sleep to active modes, and the like. A need for geolocation information
may also require a power evaluation of any global navigation satellite system
(GNSS) receiver that the module might include.

In
addition to calculating average power consumption for their design, developers
need to pay attention to peak power demand and plan their system accordingly. A
battery that provides long operating life based on average power usage may not
perform well supplying current surges when the transmitter is active. A
supplemental energy-storage system such as a supercapacitor may be needed in
the device’s power design.

Then
there are the radio requirements to consider. The channels on which the radio
will operate as well as carrier certification are essential to ensuring that
the module will function in the product’s target geographic areas. For those
seeking global operation, a module’s
ability to support NB-IoT as well as CAT-M will be important. Factors such as
maximum transmit power and receiver sensitivity feed into link budget
calculations that determine the module’s useful range from cell towers, as well.

Developers
should also examine the type of CAT-M modem that their system requires. Some
are simply transceivers that serve as an IO peripheral to a host application
processor. Others have the processing and IO resources to serve both as modem
and application processor. A check of the interfaces that a modem offers will
help reveal the ways that a module can serve, and module size will show if it
will meet the product’s space restrictions.

LTE-M_Guide_Fig-3

To get you started on finding the right
module for your application, EP has prepared a selection guide. You can
download the complete version using the link below.

There
are a number of carrier-certified modules available that developers can
essentially drop into their product design, as well as some chipsets with which
the more ambitious development teams can create their own LTE CAT-M modems. To
get you started on your search, EP has prepared a summary chart of
representative module offerings that you can download using the link at the end
of this article (EP registration required). Developers can also check out the
websites of carriers offering LTE CAT-M services, such as AT&T or Verizon, for lists of modules they
have certified for their networks.

Be
aware, though, that cellular IoT is a moving target. Release 13 defined the
basic operations that a system requires, but additions are being made
continually. Some modules now support voice-over-LTE (VoLTE), for instance,
which would allow the IoT device to include a voice channel for their users.
Module maker Sequans Communications is working with software company PoLTE
Corporation
to embed
location technology that does not depend on GNSS. And more enhancements are
undoubtedly on the way.

But now is the time to get started if you’re
going to catch the initial wave of long-range, mobile IoT devices that LTE CAT-M
modules will empower. Fortunately, many of the module vendors as well as others offer development
kits that allow experimentation with the technology as the networks finish
building out. Cellular connectivity will not be necessary for every mobile IoT
application, but where it is needed, these LTE CAT-M modules can help
accelerate a design’s arrival in the market.