Overview

Low-power wide-area network (LPWAN) is a classification of wireless wide-area networks designed specifically for high range and low power consumption. Devices using LPWANs do not maintain constant connection with the application, instead intermittently transmitting small packets of data and returning to sleep modes when not active [1]. These networks are largely used in Internet-of-Things (IoT) applications, where low energy costs and long-distance communications are often desired, especially in remote areas without easy access to other connectivity options (e.g. WiFi, ethernet). For example, temperature, humidity, and CO2 sensors using LPWANs can collect and send hourly data to the application server for environmental monitoring while sustaining on battery power.

Like cellular technologies such as 3G/4G, LPWANs allow broad coverage and mobility compared to LANs. Although LPWANs provide considerably less bandwidth than standard cellular, they offer higher range and much reduced energy and monetary costs [1]. However, the best choice of network remains application-dependent. LPWANs are not suitable for applications requiring low-latency communications and big data transfers, but they excel at performing small, periodic transfers over long distances on only battery power.

Architecture

LPWANs generally use a star topology (i.e. point-to-point links) in which multiple end devices directly connect to a gateway (also called a base station), which connects to the network server over IP, as demonstrated in the figure below. The network server then communicates with the application. LPWANs are mostly intended for uplink transmissions from end devices to applications, but some, including the variants to be discussed in this article, also offer limited downlink transmissions.

LPWAN common topology [2]

Collisions are problematic with this star topology. End devices transmit uplink without checking if other devices are using the same channel, which saves power [2]. However, this may result in collisions at a gateway when it receives simultaneous messages from multiple devices through the same channel [4]. Thus, potential collisions limit the number of nearby end devices a gateway can reliably support.

Licensed and Unlicensed Spectrum

The most popular LPWANs (LTE-M, NB-IoT, LoRaWAN, SigFox) can be categorized by their spectrums:

Licensed Spectrum

Long-term Evolution for Machines (LTE-M) and Narrowband IoT (NB-IoT) are based on the LTE standard. Because they operate on the licensed frequency spectrum, they are guaranteed higher QoS than unlicensed spectrum technologies [4]. They also offer higher bandwidth and more flexible downlink transmissions but consequently have higher power consumption and lower range.

Depending on the provider, both LTE-M and NB-IoT can support Power Save Mode (PSM) and Extended Discontinuous Reception (eDRX) to reduce power usage when idle:

eDRX allows end devices to periodically nap while listening for downlink messages, as the network will buffer downlink messages during eDRX cycles. Thus, depending on the sleep duration, there may be an increase in downlink latency. However, devices can swiftly swap to active state to perform uplink communications [6].

PSM is similar to eDRX but for a longer period of time and a deeper sleep that consumes less energy. In PSM, end devices remain connected while sleeping in-between transmissions for an agreed period with the network operator without listening for paging (new connections) from upstream. Unlike eDRX, both uplink and downlink transmissions are delayed until PSM ends. Because the sleep period is much longer than eDRX cycles, PSM is less suitable for applications requiring low latency [6].

The figure below illustrates PSM and eDRX’s role in-between transmissions. As explained previously, eDRX is periodically broken when checking for paging whereas PSM spans a long duration of time without checking and uses less energy.

PSM and eDRX between transmissions [6]

Unlicensed Spectrum

LoRaWAN and Sigfox, which operate on the unlicensed spectrum, have duty cycles that limit their usage of the industrial, scientific, and medical (ISM) spectrum due to legal restrictions [4]. With their transmission limits and lower bandwidth, these technologies are not suited for applications requiring low latency, but their long range and efficient power consumption make them handy for collecting and periodically sending data in remote areas.

Popular LPWAN Technologies

Here is a brief overview of several popular LPWAN technologies, given in order of decreasing power consumption and bandwidth and consequently increasing range:

Summary of NB-IoT, LoRaWAN, and Sigfox features [2]

LTE-M

LTE-M offers some of the highest bandwidths at 1.08 MHz (vs NB-IoT’s 200 kHz) while maintaining lower cost than 3G and 4G [4]. It can be deployed on existing 4G networks and supports handoff between stations, allowing for mobility. However, it has high power costs compared to other LPWANs.

NB-IoT

Like LTE-M, NB-IoT is LTE-based, but with reduced features to limit its power consumption. It coexists with the LTE and GSM licensed frequency bands [2]. Although messages are restricted to 1600 bytes, unlimited messages are allowed per day. It offers half-duplex at 180kHz and a range of 10 km from a base station in rural areas. Whereas LTE uses orthogonal frequency division multiplexing (OFDM) for uplink and downlink transmissions, NB-IoT uses single carrier frequency division multiple access (SC-FDMA) instead for uplink, since the lower peak-to-average power ratio of SC-FDMA increases power efficiency [3]. However, NB-IoT does not support handoff and carrier aggregation, the latter used in LTE to combine frequency blocks to increase the effective bandwidth [4].

NB-IoT can operate in 3 modes for its frequency band:

• Stand-alone: having its own frequency band.
• Guard-band: using frequency bands between LTE resource blocks (the guard band). Interference from adjacent channels can harm performance [7].
• In-band: using LTE resource blocks. Both LTE and NB-IoT performance consequently decrease.

LoRaWAN

LoRaWAN is a MAC layer protocol built on the LoRa physical layer protocol, owned by the LoRa Alliance. It has higher interference immunity than NB-IoT and consequently higher range in rural areas (20 km). However, it has a reduced payload of 243 bytes and maximum bitrate of 50 kbps. Like NB-IoT and LTE-M, there are no restrictions on the number of messages that can be transmitted per day, but LoRaWAN imposes a up-link duty cycle restriction, which may vary with local regulations and the chosen frequency band [4].

LoRaWAN uses chirp spread spectrum modulation (CSS), which enables low signal degradation and strong resistance to interference, allowing for low power usage. Furthermore, LoRaWAN’s adaptive data rate (ADR) mechanism can survey network conditions to dynamically alter the spreading factor, bandwidth, code rate, and transmission power to maximize throughput while minimizing power consumption [5].

6 different orthogonal spreading factors are offered to operate as virtual channels, increasing the number of allowed simultaneous transmissions. High spreading factors increase transmission distance, but the low data rate increases collision probability of transmissions from multiple devices because messages consequently stay in the air longer. Long transmission times are especially problematic because LoRaWAN uses ALOHA without carrier sensing [4]: devices save power by not checking for ongoing transmissions but are required to retransmit in the resulting collision. Slotted MAC protocols could be used in place of pure ALOHA to reduce collision rates, but enforcing synchronization between end devices to send messages at specific intervals may be difficult with LoRaWAN’s duty cycle restrictions and long transmission times. Furthermore, LoRaWAN’s large coverage would complicate detecting for in-flight transmissions (i.e. carrier sensing) to a gateway [4].

It is half-duplex. End devices can be divided into 3 classes based on their downlink behavior:

• Class A: Devices wait for two receiving windows after each uplink transmission (i.e. devices initiate all downlink transmissions).
• Class B: The gateway transmits beacons to synchronize devices in order for Class B devices to periodically open receive windows. Class B devices also maintain the behavior of Class A devices.
• Class C: Devices maintain open receive windows, requiring high power consumption [5].

Because LoRaWAN-connected devices broadcast to all nearby gateways, communication reliability is improved. In addition to forwarding messages to the application, the network server filters duplicate messages and sends acknowledgements back. Furthermore, handoff is supported between different gateways [2].

SigFox

Of the popular LPWANs, SigFox offers the most range and power efficiency, at the cost of transmission frequency, bandwidth, and payload size. Uplink packets can store 12 data bytes, downlink 8 bytes, over 40 km of range. Similar to LoRaWAN, downlink transmissions are initiated by the uplink. Sigfox uses a 100 Hz ultra-narrowband frequency, which reduces required transmission power by increasing the signal-to-noise ratio [3].

With a 1% duty cycle restriction for fairness and traffic regulation, devices can use the channel for at most 36 seconds per hour, or 6 messages per hour [4]. Because uplink transmission cannot always be acknowledged, devices broadcast a message 3 times via different frequency channels to ensure transmission reliability [2].

Exploring LPWAN Supported Hardware

While LPWAN offers many advantages, particularly for IoT, it does require specific hardware. Choosing which LPWAN-supported hardware to use often depends on the project’s needs. Therefore, we will explore both standalone transceivers and systems on a chip (SoCs) with LPWAN integration for LoRaWAN and Sigfox.

Transceivers

For projects that want to add LPWAN capabilities without swapping out existing microcontrollers, one can choose to add an LPWAN transceiver. Below, we explore one option for both LoRa and Sigfox.

LoRa: Semtech SX127x

The Semtech SX127x accomplishes all the goals of LPWAN offering a low current consumption of 9.9mA combined with an operating range of up to 10000 meters. This is accomplished via its built in +20dBm amplifier as well as a -148dBm sensitivity. To add this to an existing project the Semtech SX127x interfaces to a microcontroller via SPI. The wide range and low power draw makes it ideal for projects like long-range irrigation systems.

Sigfox: STMicroelectronics S2-LP

The S2-LP offers similar performance to the Sx127x while working using Sigfox as opposed to LoRa. With a max current consumption of 8.6 mA during reception and a standby consumption of 500 nA, the S2-LP has a slightly lower power draw than the SX127x while having a slightly worse sensitivity of -130dBm. It also integrates using a 4 pin SPI interface, making it easier to add to existing projects.

Choosing Between SX127x and S2-LP

When deciding between the SX127x and S2-LP, one should consider which protocol best suits their needs:

• LoRaWAN offers a better range of data rates and larger data payloads, making it more flexible, but it is more prone to interference and has slightly higher power consumption.
• Sigfox has a more rigid data structure but lower power consumption and better resistance to interference.

LPWAN Integrated SoCs

For new projects without existing microcontroller hardware, one might consider using an SoC with a built in LPWAN transceiver. This offers the benefit of making it easier to interface via LPWAN due to built in optimizations as well as better library support.

LoRa: STMicroelectronics STM32WL5x

The STM32WL5x is an integrated SoC that offers LoRa support equipped with a 32-bit Arm Cortex -M4/M0 processor. Using an internal SPI interface, the built in RF module communicates with the dual core processor allowing for flexible division of processing power. Being compatible with all Arm software and development tools, the STM32WL5x offers an ideal platform to build LoRa enabled IoT devices. One of the main advantages of using an integrated SoC is they are often catered to applications that require LPWAN, notably low power consumption. The STM32WL5x only consumes <72 µA/MHz when active and 360 nA in standby. Overall, this package is great for those familiar with ARM architecture that need LoRa support. The STM32WL line also offers support for other LPWAN protocols making it easier to adapt.

Sigfox: Uplynx XS8001-T-NG48NRYhttps://www.st.com/content/ccc/fragment/product_related/rpn_information/recommended_cards_logos/group0/13/bf/7e/b3/2d/39/4c/04/NUCLEO-WL55JC/files/NUCLEO-WL55JC.jpg/jcr:content/translations/en.NUCLEO-WL55JC.jpg)

The Uplynx XS8001-T-NG48NRY offers slightly different architecture compared to the STM32WL5x. Rather than using an ARM based microcontroller, it uses an AndesCore 32bit MCU. The SoC boasts a 10-bit ADC as well GPIO pins that one can interface to via UART. While this chip offers fantastic performance, development may prove to be more difficult than with the STM based boards due to AndesCore MCUs having less community support.

Conclusion

Overall, LPWAN is supported on a variety of hardware that can come in all in one package like the STM32WL5x or as a transceiver which one has to interface with themselves. When choosing hardware one should consider whether they are adding to an existing project or building a new project from scratch as well as consider which MCU architecture best suits their needs.

References

[1] Wolbert, Derrick. “What is LPWAN? (low-power wide-area network)?” Hologram. Accessed: Nov. 1, 2024. [Online.] Available: https://www.hologram.io/blog/what-is-lpwan/

[2] K. Mekki, E. Bajic, F. Chaxel and F. Meyer, "Overview of Cellular LPWAN Technologies for IoT Deployment: Sigfox, LoRaWAN, and NB-IoT," 2018 IEEE International Conference on Pervasive Computing and Communications Workshops (PerCom Workshops), Athens, Greece, 2018, pp. 197-202, doi: 10.1109/PERCOMW.2018.8480255.

[3] Prauzek, Michal et al. “IoT Sensor Challenges for Geothermal Energy Installations Monitoring: A Survey.” Sensors (Basel, Switzerland), vol. 23,12 5577. 14 Jun. 2023, doi:10.3390/s23125577

[4] M. Jouhari, N. Saeed, M. -S. Alouini and E. M. Amhoud, "A Survey on Scalable LoRaWAN for Massive IoT: Recent Advances, Potentials, and Challenges," in IEEE Communications Surveys & Tutorials, vol. 25, no. 3, pp. 1841-1876, thirdquarter 2023, doi: 10.1109/COMST.2023.3274934.

[5] A. Maleki, H. H. Nguyen, E. Bedeer and R. Barton, "A Tutorial on Chirp Spread Spectrum Modulation for LoRaWAN: Basics and Key Advances," in IEEE Open Journal of the Communications Society, vol. 5, pp. 4578-4612, 2024, doi: 10.1109/OJCOMS.2024.3433502.

[6] Rosende, Jazz. “eDRX and PSM for IoT: Battery-saving Features Explained.“ Onomondo. Accessed: Nov. 1, 2024. [Online.] Available: https://onomondo.com/blog/edrx-and-psm-for-lte-low-power-iot/

[7] Blackman, James. “Three NB-IoT deployment models – and why NB-IoT performance is not what it should be.” RCR Wireless. Accessed: Nov. 1, 2024. [Online.] Available: https://www.rcrwireless.com/20190718/carriers/three-nb-iot-deployment-models

Extension References

• Semtech SX1276 Transceiver
• STMicroelectronics S2-LP
• STMicroelectronics STM32WL5x
• Uplynx XS8001-T SoC Datasheet