This "hot trend" is called "Passive Internet of Things" (IoT).
What exactly is passive IoT? How does it differ from existing IoT technologies? Is it truly based on cutting-edge technology, or is it just another marketing gimmick?
I. What is Passive Internet of Things (IoT)?
Passive Internet of Things (IoT), as the name suggests, is IoT without a "source".
"Source" refers to power source or energy source.
Those who have worked on transport networks are certainly very familiar with the concept of "passive". Passive Optical Network (PON) and Passive Wavelength Division (WDM) are both related to "passive".
Simply put, passive means it doesn't connect to an external power source and doesn't have a battery.
Passive Internet of Things (IoT) does not mean the network is passive, but rather that the terminal nodes are passive.
In other words, the network remains unchanged, and the terminal node devices connected to the network do not have power cords or built-in batteries.
So, here's the problem. End-node devices have sensors that need to generate data, chips that need to perform calculations, and modules that need to transmit and receive signals. Without a power source, who will provide the energy to do all this?
In fact, the so-called "passive" does not mean that the terminal node does not use electricity, but rather that it obtains electricity (energy) in a different way.
As you might quickly recall, solar energy is a common way to obtain electricity (energy).
Solar energy is converted into electricity through photovoltaic panels and stored for later use.
In addition to solar energy, there is also kinetic energy and thermal energy.
For example, some shared bicycles have built-in power generation modules. When you pedal, they charge the smart lock.
Some remote controls use a push-button power generation method. When you press it, mechanical force causes material deformation, converting kinetic energy into electrical energy to drive the device.
Note! The solar, kinetic, and thermal energy mentioned above are not the focus of our discussion on "passive Internet of Things" today.
The “passive Internet of Things” that everyone is talking about now mainly refers to the Internet of Things based on radio electromagnetic energy capture technology.
In other words, it refers to the Internet of Things (IoT) technology that uses IoT terminals to capture and collect energy by collecting radio waves emitted from the network side.
What came to mind?
I think many people must have thought of RFID, which stands for Radio Frequency Identification.
The electronic tags we commonly refer to use RFID technology.
The principle of RFID is simple: when a tag approaches a reader, it receives the radio frequency signal emitted by the reader, generating an induced current and thus gaining energy. Using this energy, the tag sends information, enabling communication with the reader.
Besides RFID, another technology came to mind: Xiaomi's wireless charging technology launched last year.
It was incredibly popular at the time, offering 5W wireless charging for a single device within a radius of several meters.
In fact, RFID is a passive Internet of Things.
Now, based on RFID, passive IoT hopes to extend further and expand passive interconnection based on Wi-Fi, Bluetooth, 3G, 4G and even 5G communication technologies.
II. Technical Challenges of Passive Internet of Things
As everyone knows, RFID is a very mature technology. The key to the proper functioning of RFID is the close proximity of the tag and the reader.
The greater the distance, the lower the density of electromagnetic energy, and the more difficult it is to obtain energy.
RFID is an inductive coupling technology. The antenna is shaped like a coil, and the transmission of electromagnetic energy is completed within the inductive field area over a very short distance.
Wi-Fi and Bluetooth have a much longer operating range than RFID, and 3G/4G/5G have an even longer range. This is not a sensing field, but a radiation field.
The antenna technology used in the radiation field is mainly dipole antennas or microstrip antennas. It is extremely difficult to transmit electromagnetic energy within the radiation field using these antennas.
microstrip array antenna
Previously, this was unimaginable. Now, with advancements in semiconductor technology, the power consumption of terminal chips has been reduced to the mW level or even lower. Coupled with continuous upgrades in energy conversion technology, energy capture and utilization in long-distance communication technology have become a possibility.
Passive Internet of Things (IoT) also has several other typical characteristics:
Energy oscillation
The energy of passive IoT nodes will no longer follow a static trend of high to low. Since the energy comes from the environment, it will exhibit a dynamic state of high and low.
Node imbalance
Energy acquisition in passive IoT nodes is random and unstable, and the energy distribution in the entire network may not be uniform, which can also lead to differences between each node.
Energy constraints
Passive IoT acquires energy in a different way, and the ambient energy it collects is very weak, generally in the range of nanowatts (nW) to microwatts (μW), and is affected by the energy storage capacity of the nodes.
Connection vulnerability
The network connectivity of passive IoT is directly affected by the energy of each node. When the energy of some nodes falls below a certain level, these nodes become isolated, causing the network to become disconnected. Due to the oscillating nature of energy, network connectivity is fragile, intermittent, and difficult to maintain continuous connectivity.
As everyone can see, these are not desirable features, but rather drawbacks. These drawbacks limit the application of passive IoT.
III. Research Progress of Passive Internet of Things
The success of RFID has given rise to high hopes for the passive Internet of Things.
However, we must still note that the passive IoT applications we see in the media are still in their early stages.
The emerging passive IoT star companies and their research results this year are mainly based on NFC and Bluetooth.
Regarding Wi-Fi, I've seen a report about the "University of Washington's School of Electrical Engineering".
Their researchers proposed that passive devices can be powered and transmit data by using reflection modulation technology of radio frequency signals. They are developing passive technology in addition to Passive WiFi and further applying it to LoRa to achieve passive node transmission over distances of hundreds of meters.
As for passive IoT using 4G/5G, it seems there are no applications to be seen.
Last month, Wang Tao, Huawei’s executive director and president of ICT products and solutions, proposed a passive Internet of Things (IoT) concept for 5.5G at the 5G-Advanced Innovation Industry Summit, hoping that 5G networks could enable passive IoT.
However, there seems to be no clear news yet as to whether 3GPP will include passive IoT in R18.
IV. The Significance of Passive Internet of Things
Why is the passive Internet of Things (IoT) so popular?
To put it bluntly, it's because of the enormous market value behind it.
As I mentioned before, the application scenarios of cellular IoT can be divided into high-speed, medium-speed, and low-speed.
The mainstream technologies for low-speed IoT are NB-IoT, LoRa, etc.
In the original development plan, NB-IoT was already considered the most "low-end" cellular IoT technology. It has the lowest speed, the lowest power consumption, the lowest cost, and a battery standby time of up to 10 years.
As a result, it was found that NB-IoT was still insufficient to achieve hundreds of billions of IoT connections.
NB-IoT's reliance on energy (batteries) increases its cost and limits its wider adoption.
For example, we can install NB-IoT modules on all electricity meters and water meters in China, but can we install NB-IoT modules on all clothes, all goods, and all express parcels? No.
Therefore, people proposed the concept of "passive Internet of Things" to expand the Internet of Things network.
Another level added
The biggest advantage of passive IoT is that it does not require batteries at all.
NB-IoT requires a battery replacement every 10 years (ideally), while passive IoT does not. This not only reduces the labor costs associated with battery replacement but also reduces the cost of battery components.
Currently, general-purpose UHF RFID tags can be priced at 2-3 cents. NB-IoT modules, on the other hand, cost around ten yuan, a difference of several tens of times.
Secondly, eliminating the need for batteries is environmentally friendly. Although a single battery is small, the sheer number of billions of them means their environmental impact cannot be underestimated.
Third, without batteries, the size of the terminal can be further reduced. For example, like RFID, it can be a small patch, which will greatly benefit terminal design.
In conclusion, passive IoT is a very promising development approach. However, it may take a long time before we can truly make this path a success.
