Consider licensed low power wide area networks (LPWANs), such as NB-IoT, and unlicensed LPWANs, such as LoRaWAN®. LoRaWAN is optimal for long battery lifetime, and for devices that are low cost and only need to send small amounts of data a few times per hour. For its part, NB-IoT is optimal for sending larger packets, guaranteeing downlink latency and guaranteeing a quality of service with the licensed band spectrum. Due to these characteristics, NB-IoT is suited for critical Internet of Things (IoT) use cases, while LoRaWAN is perfect for the volume of IoT (even massive IoT) use cases, because most IoT use cases need low cost devices with a long battery lifetime to achieve an attractive return on . (For a more in-depth comparison of LoRaWAN and NB-IoT, take a moment to read 10 Things About LoRaWAN & NB-IoT.)
Given the expense of the Cellular licensed spectrum, the protocol is synchronous and designed to optimize the spectral usage and throughput of the network. This optimization for spectral utilization comes at the cost of compromised battery life and the cost of the end device. In contrast, with LoRaWAN, the system is designed from the ground up to optimize the device battery lifetime and the cost of the end device; the trade off is that LoRaWAN is less spectrally efficient. This is due to the fact that LoRaWAN uses an asynchronous protocol in unlicensed spectrum.
The effect of asynchronous versus synchronous protocols has profound effects on the battery lifetime of sensors. For example, Semtech conducted a head-to-head comparison test using the T-Mobile NB-IoT network available in San Jose, California or Las Vegas, Nevada as points of reference. The NB-IoT sensor consistently took more than 20 seconds of active time to negotiate a slot to communicate an 11-byte packet. The average current consumption over this 20 second period was 40mA. In comparison, sending the same 11-byte packet over LoRaWAN required an active time of only 1.6 seconds, with an average active current consumption of 6.4mA. This translates into greater than 50 times advantage in battery lifetime for LoRaWAN.
Take a pushbutton for example. Consider a LoRaWAN-enabled pushbutton and an NB-IoT pushbutton that each have a comparable battery size (600mAh). The LoRaWAN-enabled pushbutton can handle more than 70,000 button presses on a single battery, while the NB-IoT pushbutton could handle only about 2,000 button presses on a single battery. So, while there are many good use-cases for NB-IoT, a consumer pushbutton is probably not one of them.
Achieving low cost devices with quality device performance is critical for ensuring the ROI of IoT applications for LPWANs. With the battery of the device typically being the most expensive component in the sensor bill of materials, it is critical to understand what can be achieved with a reasonably-priced battery. A ten-year battery lifetime can always be achieved by using a battery with a massive capacity, but this will come at a cost that is detrimental to most LPWAN business cases. Reasonably priced volume batteries typically have a capacity from 1,000 to 2,000mAh. If a five-year battery life is required with this level of battery capacity, the device will need to be in deep sleep most of the time, with less than 10 seconds of active time per hour of transmit/receive time on air.
Given the synchronous nature of the protocol for Cellular radio access networks, requiring that the sensor stay asleep most of the time to achieve a long battery life is a bad match. What is needed instead is a device-centric asynchronous RAN. The good news is that LoRaWAN is already here and is the standard device-centric RAN for IoT and LPWAN uses.
For more information on LoRaWAN power management, enroll in LoRaWAN Academy™ and review Module 10: Power Consumption.
