LPWAN Basics: What Enables a Long Battery Life
We all know an intriguing quality of Low Power Wide Area Networks (LPWANs) is its ultra-low power consumption. Most LPWAN technologies claim that they can sustain a battery life of more than 10 years – making them the first “go-to” connectivity type when it comes to battery-operated IoT sensor networks.
But how can LPWANs achieve such a long battery life? In this blog, we'll cover 3 main approaches.
1. Sleep Mode
LPWAN end nodes are programmed to be active only when a message needs to be transmitted. Outside this time, the transceivers are turned off and fall into deep sleep mode (“idle” time) whereby very minimal power is consumed. Assumed that a node is required to send only few messages (uplink) a day, power usage remains significantly low.
In bi-directional communication, end nodes have to also be awake to listen for downlink messages sent from the base station as well. A listening schedule can then be set up so that nodes only wake up at predefined times to receive downlink messages. Alternatively, nodes and base stations can be coordinated so that a downlink message is sent shortly after an uplink arrives. This helps reduce the time a node needs to be “on” for data reception.
2. Asynchronous Communication
Most LPWANs operating in the unlicensed spectrum employ asynchronous communication with lightweight Medium Access Control (MAC) protocols. For example, ALOHA random access protocol is commonly used. In ALOHA systems, a node accesses the channel and sends a message anytime without signaling the base station for permission or sensing current transmission by other nodes for coordination.
A major advantage of such random access protocols is that no complex control overhead is required. This drastically reduces power consumption and simplifies transceiver design. On the downside, asynchronous communication threatens to greatly hamper scalability. This is because data transmission is uncoordinated among nodes, which increases the chance of packet collision and data loss.
3. Star Topology
Thanks to their long physical range, LPWAN can be deployed in star topology while still effectively covering geographically vast areas. As explained in a previous blog, one-hop star topology saves more energy than the mesh topology of short-range wireless networks by orders of magnitude.
Is Battery Life the Same Among Different LPWAN Technologies?
The answer is definitely no. In fact, power consumption and the resulted battery life can vary significantly not only among different LPWAN technologies, but also among different deployment modes of the same technology. Below we look at 2 major attributable factors.
First, “on-air” radio time – a main indicator of power consumption during transmission – greatly differs across LPWAN systems. To be clear, transmission is the most energy-intensive activity of end nodes. On-air time is the total time a message travels from a node to the base station. Other things being equal, the shorter the on-air time, the lower the power consumption. If the same message is sent 3 times for redundancy, its total on-air time and power consumption triple.
Second, not all LPWANs adopt a combination of all 3 approaches discussed above. For example, to enhance Quality-of-Service (QoS), cellular LPWANs employ a synchronous protocol whereby end nodes have to signal the base station for permission to send a message (i.e. handshake). Besides imposing higher energy requirements due to excessive overhead, this process makes power consumption of each transmission and total battery life unpredictable. This is because it is difficult to predict how many times handshakes need to be performed until a message is allowed to be sent.
Recognized by ETSI, Telegram Splitting introduces a unique transmission method to minimize on-air time while resolving the trade-off between QoS and power consumption.
As a final note, 10 or even 20 years are actually a very long time for a battery lifespan, but to be realized a multitude of factors need to be thoroughly considered. Besides general conditions like message frequency and the type of battery used (ideally ones with low self-discharge rates), at the end of the day, choosing the right LPWAN technology really matters.
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