Over the past year or two, low power wide area cellular technologies, the likes of LTE-M and NB-IoT, have played a vital role in growing the IoT. By delivering reliable service up to 100 kilometers from the nearest cell tower, connected devices that communicate using LPWA technologies can increase the geographical area within which they stay connected to the cloud. And by offering connectivity for years on a single battery, they can reduce requirements in terms of maintenance and infrastructure, transforming otherwise impractical application into lucrative businesses.
That’s why it’s troubling that some cellular modem manufacturers are choosing to reduce the transmission power of their modems from 23 dBm to 20 dBm. It doesn’t sound like much, but because dBm is a logarithmic unit to measure power, a signal transmitted at 20 dBm is only about half as strong as one transmitted at 23 dBm.
Consider that today’s energy-efficient glazed windows can reduce signal power in common cellular frequencies by 2 to 30 dB, depending on the type of coating used. Reducing transmission power by 3 dBm doubles these values. This compounding effect introduces risk of premature connection loss in more challenging environments.
The introduction of cellular modems with limited transmission power essentially undermines the main selling points of cellular LPWA technologies. It allows companies to trade a lower price point, achieved by reducing their bill of materials, in exchange for reduced availability, coverage, and range of the modem. This trade-off can make certification more demanding and may limit some use cases and applications.
Say you are a device designer looking to cut the cost of your device. Convinced by the low price point, you decide to use an LPWA modem with 20 dBm maximum transmission power. Basically you have two options. You might decide that you and your customers can live with the reduced performance, or maybe you expect your devices to operate only in “friendly” signal environments.
If that isn’t an option, you could add an additional power amplifier to your design to give your device’s transmission an extra boost. This will obviously start to chip away at your initial savings. And it doesn’t stop there. You might also find that your device’s battery is drained faster than expected.
Whichever solution you opt for, ensuring adequate coverage under the 3 dB penalty in output power will inevitably increase demands on antenna efficiency. In small end-devices such as vehicle tracking OBD units that need to provide global connectivity while covering the entire frequency range from 600 MHz to 2100 MHz, this can easily become critical.
And then there are mobile network operator (MNO)-specific complexities. First, several Tier-1 MNOs currently do not let 20 dBm units operate on their networks. This can be a showstopper if your device specifically targets one of the markets these MNOs operate in. And second, the way MNOs assign fielded devices coverage enhancement modes to increase the likelihood that messages are delivered is far from straight forward, and often difficult to predict. They might require weak devices with lower transmission power to repeat each message they send several times, increasing latency, decreasing effective throughput, congesting the airwaves, and, critically, consuming extra power.
Ultimately, as GSMA states in its LTE-M Deployment Guide, the “introduction of lower power output UEs [user equipment] shifts the burden of understanding the intricacies of their chosen UE to the consumer, further complicating an already complex decision process.”
At u-blox, we have decided to stay true to our mission of delivering best-in-class products and services that exceed expectations of our customers and that avoid risking the quality and reliability of their operations. That is why our LPWA module families, including our SARA-R5 module series, support 23 dBm transmission power. With no output power limitations and fully aligned to standard LTE power classes as defined by GSMA, our modules will guarantee that end devices can operate in all signal conditions, with no problems at cell edges, no coverage issues in weak signal conditions, and no unwanted retransmissions.