Low Power Wide Area (LPWA) technologies: a real-world comparison

The world of wireless technologies is expanding to meet the needs of the emerging Internet of Things phenomenon. While cellular has its place in this new world of IoT, particularly in a backhaul role, it is limited by cost, complexity and high power requirements.

Instead, other technologies designed specifically to meet the needs of IoT continue to make tracks across the globe such as the open standard technology LoRaWAN and proprietary technology Sigfox.

At the same time 3GPP, the global body responsible for cellular mobile communications standards, has developed two variants of its LTE technology designed to meet the needs of IoT: Narrowband-IoT (NB-IoT) and CAT-M1.

With Telstra now offering Cat-M1 throughout its mobile network and Vodafone trialling NB-IoT in preparation for the launch of commercial services anyone wanting a communications technology for an IoT application will have an increasingly wide choice.

However each of these technologies has different characteristics and the most suitable technology will depend to a large extent on the application. So how do they compare?

Shared versus exclusive spectrum

The most important distinction between low-powered wide area network (LPWAN) technologies such as LoRaWAN and Sigfox and the LTE variants is the regulatory regime governing the frequencies at which they operate.

LPWAN technologies operate at frequencies with minimal regulation – in Australia they are subject to class licences. This means so long as the equipment meets certain performance criteria, such as total output power, anyone can install and operate base stations or devices that operate at these frequencies anywhere they wish.

With such a regime interference is inevitable and while the various LPWAN technologies have techniques to overcome this, it does mean they are not suitable for mission-critical communications such as SCADA, because communication can be interrupted by interference from nearby equipment.

NB-IoT and Cat-M1 operate in the same frequencies as the mobile telephone networks – frequencies for which mobile operators have paid millions of dollars to gain exclusive access. The operators have total control of the devices that use their spectrum. They design and build their networks to ensure that transmitters operating at the same frequency are located with separation sufficient to avoid interference.

However the use of free shared spectrum comes at a cost.

Broadcast or unicast

A most important differentiator between LoRaWAN and cellular technologies is multicasting: with LoRaWAN a single message can be addressed to multiple devices, with NB IoT and CAT-M1 each devices must messaged individually. This makes LoRaWAN ideal for applications where a large number of devices need to be controlled in sync, for example to turn on sprinklers in a smart agriculture application, or streetlights in a smart city.

While Cat-M1 and NB IoT have been designed to work with devices that can run for years on an AA battery, CAT-M1 is inherently more power hungry than LoRaWAN, Sigfox or even NB-IoT.

With Cat-M1 there is no provision for endpoint context to be retained by the network between transmission so every time a CAT-M1 endpoint wakes up to send data it must re-register with the network – a process that consumes a significant amount of power.

NB-IoT solves the problem of maintaining contact after the device shuts down: the network maintains the context and when the device wakes up it can start sending data almost immediately.

Cat-M1 is however better suited to applications where data volumes are large, such as video cameras. Cat-M1 can deliver hundred kilobits/second compared to a maximum of 230 bytes for a typical LoRaWAN network.

Because LoRaWAN is an open standard IoT application developers have much greater flexibility when it comes to developing devices combining IoT and communications functions. With CaT-M1 and NB-IoT they are limited to interfacing to a closed module providing the communications functions.

With LoRaWAN it is possible to build a tiny, integrated, ultra low power IoT device that combines communications functionality and application on a single CPU by taking a transceiver chip and combining it with a microcontroller.

Comparing LoRaWAN and Sigfox

While the application might make the choice between a cellular technology such as Cat-M1 or NB-IoT and a LPWAN technology like LoRaWAN or Sigfox fairly easy, the differences between LoRaWAN and Sigfox are rather more subtle.

They have many similarities and their simultaneous rise has generated much debate as to their relative merits and fitness for IoT applications. However this is not an apples-to-apples comparison. It’s an apples-to-oranges comparison: both are edible fruits that grow on trees, but that’s where the similarities end.

What LoRaWAN and Sigfox have in common is that both have been developed specifically for the needs of IoT and both operate in what, in Australia, is called class-licensed spectrum (between 915MHz and 928MHz).

That’s really where the similarities end. How they operate within these frequencies determines their immunity from interference and their potential to cause interference. It determines their ability to communicate data upstream and downstream, and it determines how much battery power they need to send or receive the same amount of data.

A question of interference

To increase its immunity from interference, Sigfox transmits the same information on three pseudo randomly selected frequencies. This increases transmission time, and power consumption.

In LoRaWAN information is encoded in the radio transmission in a pseudo random way across a broad frequency range, 125kHz, 250kHz or, potentially 500kHz. To any receiver other than a LoRaWAN receiver, which ‘knows where to look’ for the encoded information, the signal looks like noise.

This means it is more resistant to interference and less likely to cause interference. And because the information is transmitted only once, LoRaWAN uses less power than Sigfox to transmit the same amount of information.

Open versus proprietary

More importantly, LoRaWAN is an open standard: anyone can manufacture LoRaWAN base stations and devices and anyone can install and operate a LoRaWAN network. Sigfox is a proprietary technology owned by the French company of the same name. Only organisations authorised by Sigfox can operate a Sigfox network.

Sigfox is also an end-to-end service where the functions and features available are those determined by Sigfox.

In the case of LoRaWAN, a network operator such as NNNCo can add functionality to the underlying standard to meet specific use cases so long as these do not violate the standard and maintain compatibility with LoRaWAN devices.

Not all applications require sophisticated functions and features: there are many IoT use cases that simply require the base station to periodically receive small amounts of data from a device that can operate for years on a small battery and require zero or very little data to be sent to the device. LoRaWAN and Sigfox are equally well suited to such applications.

However when there is a requirement for significant data uplink and for even a small amount of downlink data, LoRaWAN is superior.

The ups and downs of data transfer

A LoRaWAN network can send between 59 and 230 bytes per uplink. Sigfox is limited to 12 bytes per uplink. This is fine for communicating simple information, such as a switch turning on or off, or the level of water in a tank, but insufficient for even modest amounts of additional information.

Also it does not allow for encryption of the data. If that is needed it must be done at the application level whereas data over a LoRaWAN network can be encrypted at the network level.

Sigfox devices are also limited to eight bytes per downlink transmission compared with 59 to 250 with LoRaWAN and the way Sigfox receives downlink data consumes more power shortening battery life.

After making an uplink transmission a LoRaWAN device will shut down for one second then wake up for one second to listen for any data from the base station. A Sigfox device remains active for 20 seconds after an uplink and can only receive one eight byte packet of data within that period.

There is never one answer for every problem. We see LoRaWAN as complimentary to cellular technologies and continuing to grow as a global force in IoT. Watch this space…