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How to Quickly Start Low-Power Wireless IoT Sensing with LPWAN RF Modules

By Richard A. Quinnell

Contributed By Digi-Key's North American Editors

When considering wireless connectivity for an Internet of Things (IoT) sensor, it's common for developers to think first of Wi-Fi, Zigbee, or Bluetooth. However, it’s often the case that applications require lower power, longer range, and have a different usage model and lower data rates than those technologies can provide. Rather than design their own wireless interface from scratch and incur the cost, potential delays and rework this approach involves, designers can instead turn to off-the-shelf modules for a range of relatively new low-power, wide-area networks (LPWANs).

These LPWANs, which include Sigfox, LoRaWAN, and the newer Radiocrafts Industrial IoT (RIIoT), are all designed to connect relatively simple sensors operating at modest sample rates, and which are sending short, infrequent bursts of data over long distances—up to and beyond 50 kilometers (km). Such applications often specify extremely tight power constraints to maximize battery life for sensors in remote or inconvenient locations. Ideally, sensors placed in such locations would work reliably off a coin cell or AAA battery for up to 10 years.

This article discusses the design requirements of typical long-range IoT sensing and the characteristics of Sigfox, LoRaWAN, and RIIoT. It then introduces suitable modules from Pi Supply, Sigfox, and Radiocrafts, and shows how to use them.

LPWAN characteristics

The narrow bandwidth of LPWANs are part of their secret sauce for low-power operation (Figure 1). The tenets of information theory state that signal bandwidth and signal-to-noise ratio (SNR) have an intimate relationship with the error rate of information transfers. The greater the SNR, or the narrower the bandwidth, the lower the error rate.

Diagram of narrow bandwidth of LPWANsFigure 1: The narrow bandwidth of LPWANs allows them to operate over longer ranges at lower power. (Image source: Peter R. Egli, via Slideshare)

LPWANs leverage this relationship to obtain highly reliable information transfers over long distances at low output power. By adopting a relatively low data rate, LPWAN systems also reduce their signal bandwidth requirements. The result is that LPWAN systems can communicate over distances measured in kilometers.

A second key element of LPWAN systems is their use of sub-gigahertz (GHz) frequencies in the international unlicensed industrial, scientific and medical (ISM) frequency bands (886 - 906 megahertz (MHz)). Operating at these frequencies (with longer wavelengths) reduces free-space path loss, boosting their effective range for a given transmit power, per Equation 1:

Equation 1 Equation 1

Where:

d = distance

λ = wavelength

At lower frequencies, less radio frequency (RF) energy is absorbed by obstacles such as walls and buildings, giving LPWAN systems excellent penetration capability in urban environments.

ISM-based designs don’t need a license; however, they still need to comply with global power and electromagnetic compatibility regulations for ISM band operation.

LPWAN examples

There are multiple LPWAN options to choose from, but for developers seeking rapid development of sensor-based IoT applications, three clear options are LoRaWAN, Sigfox, and the recently introduced RIIoT. Each of these has the support of pre-configured radio and sensor interface modules that developers can simply drop into their design, as well as development kits that facilitate quick setup and application development.

LoRaWAN is based on open standards managed by the LoRa Alliance and on proprietary spread-spectrum radio technology owned and licensed by Semtech Corp. The network utilizes a star-of-stars topology, allowing individual nodes to communicate with multiple gateways, permitting roaming. It supports bi-directional communications between gateways and nodes, letting gateways relay messages from one node to another, as well as to a cloud-based server.

LoRaWAN allows data rates from 300 bits/s to 50 kbits/s, handles message payloads up to 243 bytes, and uses signal bandwidths of 125 kilohertz (kHz) or 250 kHz. It supports adaptive data rates to maintain signal reliability in changing conditions and can achieve ranges to 5 km in an urban environment and up to 20 km line-of-sight (LoS). Users can develop nodes and tap into commercially operated networks, or establish private networks utilizing their own gateways and backhaul networks.

Sigfox is a proprietary protocol developed and managed by Sigfox, which licenses its technology to chip developers and provides users with access to its network through gateway base stations across the globe. By keeping its data rates to 600 bits/s with a signal bandwidth of 100 hertz (Hz), Sigfox is able to maximize range. It can achieve 40 km in LoS conditions and 10 km in urban settings. Its lightweight protocol limits uplink message packets to 26 bytes (with a maximum of 12 bytes of user data) so that transmitters are only powered briefly. Nodes can only send 140 messages per day, and gateways can only send downlink messages to nodes four times a day after receiving an uplink message from them. As a result, nodes spend very little time with their radios active, staying in sleep mode most of the time to minimize their power consumption.

While LPWAN radios are low power, in the real world, low power is a relative term. For example, Radiocrafts has two distinct power options for its Sigfox module offerings. The RC1692HP-SSM high-power sensor interface module communicates with a host microcontroller over a UART connection and offers SPI, I2C, analog, and GPIO ports for connecting to sensors (Figure 2). It operates off a 2.8 to 3.6 volt supply.

Image of Sigfox RC1692HP-SSM radio and sensor interface moduleFigure 2: Full Sigfox radio and sensor interface modules such as the RC1692HP-SSM from Radiocrafts draw as little as 20 microamperes (µA) when not transmitting. (Image source: Radiocrafts)

In sleep mode, the module draws 1 µA. In active mode with sensors attached, it draws less than 20 µA when idle and 292 mA while transmitting.

The lower power RC1682-SSM module targets the European market and draws much less current, only 58 mA when transmitting.

RIIoT is one of the newest LPWAN options for developers to consider. It is built on top of the IEEE 802.15.4g/e physical layer (PHY) standard developed initially for smart metering and process control applications. It adds RF and media access control (MAC) features to support low power consumption, long-range, and advanced security. Communication is bi-directional over a star network, providing predictable network delays of less than 15 ms for near real-time control applications.

RIIoT offers two data rates—5 kbits/s and 50 kbits/s—and two power levels so that developers can optimize the trade-off between battery life, data rate, and range to best fit their needs. At the low-power, high data rate setting, RIIoT networks can achieve a range of 5 km LoS and 200 meters (m) urban, transmitting in bursts of 3.5 milliseconds (ms). With higher output power at lower data rates, their range can reach 60 km LoS and 2 km urban in 45 ms bursts. Sleep current for typical leaf nodes is 0.7 microamps (µA).

Building an RIIoT network involves three main elements: a node, a gateway and network controller software. Individual "leaf" nodes use a module such as the Radiocrafts RC1880CEF-SPR, which integrates an analog-to-digital converter (ADC) along with GPIO, I2C, SPI, and UART interfaces. These nodes communicate wirelessly to a Linux PC that uses either the compatible RC1880CEF-GPR module on a board that can be inserted into an expansion slot, or a USB dongle connected to one of its USB ports.

To fully turn the PC into an RIIoT gateway requires the developer to install a third element—the RIIoT Net Controller middleware. This software not only manages the network, including over-the-air firmware updates to leaf nodes, it also converts data and commands into JSON objects to simplify interfacing to the cloud.

Diagram of complete RIIoT networkFigure 3: A complete RIIoT network contains leaf nodes, a Linux PC hosting a gateway module, and controller software. (Image source: Radiocrafts)

One of the key additions that RIIoT makes to the underlying IEEE202.15.4 standard is an ability to implement end-to-end security on data transmissions. While Sigfox does not support encryption and LoRaWAN supports encryption in its wireless links between node and gateway, RIIoT carries security a step further.

With RIIoT, each node can have a unique security key, allowing the system to keep the message encrypted from the node all the way to the cloud-based application program interacting with it. Gateways can simply pass the encrypted message along; they don't need to access the contents.

Accelerate design using modules and kits: RIIoT

Developers looking to implement LPWAN IoT networks can get a quick leg up on their design efforts by using one of the many pre-configured RF and sensor interface modules available for the various networks. Such modules have already solved all the tricky problems of RF design, power minimization, and protocol implementation, making them essentially a drop-in communications device for the host processor. Further, the modules are pre-certified for compliance with the regulatory requirements for the ISM bands. Developers will still need to have their final product certified, but having the radio element already proven makes the final certification considerably easier and more certain.

These modules also help speed design by providing built-in sensor interfaces and control logic. The Radiocrafts RC1880CEF-SPR, for instance, has interfaces for analog input to an ADC, GPIO for switches, I2C and SPI for compatible sensors, and a UART for connecting to the host processor (Figure 4). Developers can drop this module into their design to solve both the wireless communications and sensor interface needs for their system. The module can be programmed to handle the sensor setup, control, and sampling on its own, simplifying the application processor’s task. The sensors and communications simply look like memory reads and writes to the application code.

Diagram of LPWAN systems can include both radios and sensor interfacesFigure 4: Modules for LPWAN systems can include both radios and sensor interfaces, making them simpler to design into IoT sensor systems. (Image source: Radiocrafts)

Development kits, such as the RC1880-RIIOT-DK, can help developers quickly set up a complete end-to-end RIIoT network for experimentation. This kit includes the leaf nodes, gateway modules, and system software for a full network. Software tools for programming the leaf nodes in C to handle the attached sensors are also included.

Modules and dev kits for LoRaWAN and Sigfox

Pre-configured modules for easy IoT system implementation are also readily available for LoRaWAN. A good example is the PIS-1019 RAK811 LoRaWAN module from Pi Supply (Figure 5).

Image of PIS-1019 RAK811 LoRaWAN module from Pi SupplyFigure 5: The PIS-1019 RAK811 LoRaWAN module from Pi Supply has a built-in sensor interface along with a serial port to allow a host microcontroller to control it using standard AT commands. (Image source: Pi Supply)

This device offers a serial port to a host microcontroller that controls the module using standard AT commands. To help set up a complete network, the PIS-1037 development kit for the PIS-1019 contains a gateway concentrator module that can turn a host PCIe controller into a gateway/router access point (Figure 6).

Image of Pi Supply PIS-1037 development kitFigure 6: LoRaWAN users can establish their own network gateway using the resources of the Pi Supply PIS-1037, the development kit for the PIS-1019. (Image source: Pi Supply)

Radiocrafts also has full Sigfox development kits such as the RC1692HP-SSM-DK kit for the RC1692HP-SSM RF module and the RC-1682-SSM DK for the RC1682-SSM RF module. These enable the testing and development of Sigfox radio modules right out of the box. The kits come with temperature and humidity sensors, an accelerometer, and a Hall effect sensor.

Developers using Sigfox don’t have the option of creating their own networks, however. Sigfox operates and maintains the system gateways and backhauls, for which users pay an access fee. The modules come complete with pre-coded IDs and encryption keys, though, and will begin delivering data to the Sigfox cloud with minimal setup, once registered.

Conclusion

For designers looking to connect low data rate sensors to the IoT over a long range at low power, LPWAN solutions such as RIIoT, LoRaWAN, and Sigfox offer compelling alternatives to Wi-Fi, Zigbee, or licensed cellular networks. Each has their respective advantages, but all can address applications ranging from smart meters to smart farming.

Disclaimer: The opinions, beliefs, and viewpoints expressed by the various authors and/or forum participants on this website do not necessarily reflect the opinions, beliefs, and viewpoints of Digi-Key Electronics or official policies of Digi-Key Electronics.

About this author

Richard A. Quinnell

Richard Quinnell has been an engineer and writer for 45 years, covering topics such as microcontrollers, embedded systems, and communications for a variety of publications. Prior to becoming a technical journalist he spent more than a decade as an embedded systems designer and engineering project manager for companies such as the Johns Hopkins University’s Applied Physics Laboratory (JHU/APL). He has degrees in electrical engineering and applied physics, with additional graduate work in communications, computer design, and quantum electronics.

About this publisher

Digi-Key's North American Editors