If you’re in the Internet of Things (IoT) space or wireless in general for that matter, you’ve probably heard about RF sensitivity or receiver sensitivity. But if you’re not an electrical engineer (maybe you’re more on the business side), it can be a confusing term. So that’s the idea behind this article - we want to show you IoT business folks everything you need to know about RF sensitivity, and the role it plays in designing an IoT product for success.
Let’s start by backing up just a step. RF sensitivity is just a piece of the link budget equation, so we’ll start there:
What is a link budget?
Received Power (dBm) = Transmitted Power (dBm) + Gains (dB) − Losses (dB)
A link budget is the accounting of all gains and losses that occur in the wireless signal from the transmitting device as it is passed through various connectors, antennas, open air, etc. on its way to the receiver in a wireless communication system. In looking at the simple equation above, you can see how this is represented. Here’s an important note to keep in mind: dBm is a true measure of power (like a watt). Since dBm is measured on a logarithmic scale, it’s easy to account for in a link budget since all losses are exponential. The small “m” in dBm denotes that it referenced to a milliwatt (so 0dBm = 1 mW, 10 dBm = 10 mW, 20 dBm = 100 mW, 30 dBm = 1 W of RF power).
On the other hand, dB is a relative term only. A 3 dB loss is a loss of 50%, but is not absolute (50% of 10 dBm is 7 dBm, 50% of -110 dBm is -113 dBm).
Now I hope you’re not sensitive, because we’re going to start by comparing a link budget to a family budget. In the comparison, here is what you’d see:
The output of the transmitter = your household income.
The receiver sensitivity = What you need to have leftover at the end of the month for food.
Here are losses that all take some of your energy (like your family expenses that chew into some of your savings):
- Propagation losses = Rent, since it’s usually the biggest loss. This is the spreading out of RF energy as the signal dissipates. Generally this term grows as the square of the distance in “free-space” or as the distance to the power of four along the ground, due to the RF wave interfering with itself as it bounces off the ground.
- Attenuation losses = Car payments, since it may or may not be a big term in the link budget. These are losses associated with propagation through material other than air, like walls. When you’re ready to get serious about link budgets, this paper is an excellent resource on RF attenuation through various building materials.
- Connector losses
- Transmit Antenna losses
- Receiver Antenna losses: Often antennas on small IoT devices are worse than you think. You can easily loss 10 dB or more if your antenna is “electrically small” or less than about ¼ of the wavelength in length. There is a physical limit to the efficiency of such an antenna.
What is receiver or RF sensitivity?
Defined in more typical terms, receiver sensitivity is the minimum magnitude of input signal you need based on a specified signal-to-noise ratio to achieve at minimum error rate. In terms of a family budget, that’s like saying that if you know how much you need to budget for food (sensitivity), then you can calculate how much you’ll need to make in income (the transmitter output), given your path losses (other expenses).
Let’s put this together...
If you transmit +18 dBm and have 150dB of losses (expenses), you are left with -132 dBm of energy at the receiver. If your receiver sensitivity is -140 dBm, you're golden (essentially, you’re a DINK - double income, no kids). If you have a big family to feed (a high data rate, or a lesser radio technology) and your receiver sensitivity is only -120 dBm, you are out of luck.
So that may be an oversimplification of an RF sensitivity equation, but it’s important to keep the broader concept in mind for M2M communication. If you can’t remember anything from this article in a few days, write this down: the greater the sensitivity, the longer the range.
Wondering why we're writing about this? Link Labs’ Wide Area, Low Power technology achieves greater than -140 dBm sensitivity, giving it many times the range of Zigbee, Bluetooth, or WiFi. Learn more about that technology by clicking below...