[From the last episode: We saw how energy harvestingTechnology that can grab energy from the environment for local use with low-power devices. lets devices pick up energy from their local environment to extend battery life or even eliminate a battery.]
As our last entry in this series on energy-related topics, let’s talk about the radio. I know, the radio is the thing you listen to in your car. But within electronic engineering, a radio is anything that can transmit or receive radio waves. And radio waves are electromagnetic signals sent over radio frequencies (RF).
As a reminder, electromagnetic waves include radio, microwave, infraredLight below the frequency of (or with longer wavelength than) visible light. The wavelength range is roughly from 800 nm to 1 mm. Near infrared is the range near the visible part of the spectrum, from 800 nm to 2500 nm., visible light, and on up to X-rays and gamma rays. It’s just a matter of what their frequencyThe rate of change of a signal, specified in hertz, or cycles per second. is. So you can think of radio waves as just really low-frequency light.
Your WiFiA common type of wireless network used to connect computers and phones to each other and the internet.? RF. Cell phone? RF. Anything that talks wirelessly does it over radio frequencies, and the circuits that transmit and receive the signals are called the radio. And, depending on the kind of signaling you’re doing, the radio can chew up a chunk of energy. That’s an important consideration for wireless IoTThe Internet of Things. A broad term covering many different applications where "things" are interconnected through the internet. devices.
What’s Worse: Sending or Receiving?
In some situations, transmitting a message can use more powerThe rate of energy consumption. For electricity, it’s measured in watts (W).. That’s because it has to send a signal strong enough to get to… the tower or wherever the other end of the link is. That tower is probably running on wired power, not a battery, so it has no problem generating a signal that an IoT device can hear. But when the IoT device answers back, it has to rely on its meager energy supply.
In other ways, however, receiving can be an energy issue. But that depends on the protocol or “language” being used for the electronic conversation. That’s because radios have to turn on and off to save power.
It would be nice if a radio receiver could just sit there on all the time – kind of the way it is with a cell phone. But that would burn too much energy, and so an important way of saving energy in IoT devices is to keep the radio on for no longer than necessary. Got a short message to send? Power up, send it, and shut everything back down.
That works great when sending a message, because the IoT device itself is in control. It knows that it wants to send something, and so there’s little uncertainty about powering things up and down.
When to Listen?
It’s different with receiving, however. If you can’t just listen all the time, how do you know when to listen? How do you know that you’re not going to miss a message due to napping?
There are different ways of dealing with this. One way could be to have a tiny part of a circuit listening and waking up the rest of the circuit when there’s a message. But even just listening with everything else off uses a lot of energy. So that’s not likely to work in tough circumstances.
Another option that some protocolsAn agreed way of doing something. Like a convention, except that protocols tend to be related to processes. use is that, if an IoT sends a message, then it listens for some period of time right afterwards in case there’s a response. Once that window closes, then it doesn’t listen again until the next message is sent. But that’s pretty limiting, depending on the application.
Yet another simple-sounding solution is to say, “Well, I’ll wake up and listen every 10 minutes to see if a message is waiting.” Sounds simple enough, but there’s a catch. Yeah, the receiver can wake up at the appointed time, but the sender has to send the message at that same time. If the receiver wakes up too late or goes back to sleepThe state of a machine where as many non-essential electronic elements as possible are shut down, without powering the machine completely down. too early, it might miss part or all of the message.
That means that the receiver and the sender have to synchronize their clocks so they both know when that time window opens and closes. And, here again, there’s a catch.
Synchronizing Watches
The two clocks in the sender and receiver aren’t connected. In theory, you could use something like GPSGlobal Positioning System. A satellite system that sends signals to earth. GPS receivers can get those and figure out where on the earth you are. This is specifically a US-based system, an example of the more generic term “GNSS.” for precise timing, but that can use too much power. So the sender and receiver are going to be running on separate clocks. They can synchronize at some time – like when the IoT device sends a message, but, after that, those clocks are going to subtly, slowly diverge. One will be slower than the other, guaranteed.
The clocks have specs governing how accurate they have to be, which is helpful, but physical reality is that they won’t be the same. So when the receiver wakes up, it has to take into account that its clock is likely going to be different from the sender’s clock. It doesn’t know which one is faster or slower, so it has to wake up a bit earlier… just in case.
You might think that you could save more energy by having more time go by in between wake-ups. To some extent that’s true, but with the situation we’re talking about, that just means more time for the clocks to get out of synch. Which means that the IoT receiver has to wake up even earlier.
In this situation, the longer between wake-ups, the wider the window for the receiver has to be, and so, at some point, it’s on long enough that you may not be saving as much energy anymore.
Engineers are well aware of these issues, and they design around them. But I bring it up because it’s kind of an unexpected way that attempting to save energy may not succeed as much as might be hoped. There’s always a catch!
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