Definition
RFID (Radio Frequency Identification) identifies targets through radio signals. Active RFID has a built-in battery and actively transmits signals, allowing for long transmission distances, and is used for RTLS asset tracking; passive RFID has no battery and is powered by the reader to activate, commonly used for access control and logistics tags.
Technology Classification
Passive RFID: the reader emits radio waves, and the tag obtains energy through electromagnetic induction and reflects its ID. Working distance is usually a few centimeters to a few meters. It is low-cost and used for access control, logistics, and retail; Active RFID: the tag has a built-in battery and actively transmits signals with a range of up to hundreds of meters, used for asset tracking and RTLS positioning. Frequency bands include LF (125kHz), HF (13.56MHz), UHF (860–960MHz), etc., with different characteristics for each band.
Use Cases
Applications include access control and attendance, logistics tracking, retail loss prevention, asset management, vehicle identification, production traceability. Combined with RTLS, indoor asset positioning can be achieved; compared with BLE Beacon, RFID is lower in cost and does not require a battery (passive), but positioning accuracy and real-time performance are generally moderate. Selection should consider recognition distance, multi-tag anti-collision, and environmental adaptability.
Coupling Method and Inventory Slots
Passive tags take power from the field; active tags transmit on their own
What radio-frequency identification shares is an exchange of identity by radio. What differs is where the energy comes from. A passive tag has no battery. The reader's alternating magnetic field, or its radio wave, induces energy in the tag antenna, and the tag modulates its identity back. Distance is limited by field strength, usually to the scale of a doorway or a gate. An active tag carries its own battery and transmits itself. The reader only receives. Distance can be much greater, but the battery has to be changed and the transmit period has to be managed. A semi-active tag sits between the two: the battery only keeps the chip ready, and communication is still excited by the reader. Decide the energy source before talking about the antenna, or range figures are not comparable.
The band decides the coupling. Low frequency and high frequency rely more on the magnetic near field. How they cope with metal and liquids depends heavily on antenna size. Ultra-high frequency relies more on a propagating wave. Read range is longer, but metal reflection and liquid absorption break the field into pieces. An anti-collision protocol lets many tags take turns answering in slots. How many tags one inventory round can collect depends on the number of slots, how fast the tags are moving, and how long they stay in the field.
Metal and liquids change the tuning
A portal reader fits places where tags move in a fixed direction and stay long enough to finish one inventory round, such as a warehouse door or a station on a line. A handheld reader fits a small count where the angle can be adjusted to catch a miss. Metal shelving, products that contain water, and a tag stuck to a metal housing all detune the antenna. A range measured on a wooden bench in the lab shrinks until a receipt into stock is impossible. Use a metal-mount tag matched to the surface, and measure again on the real rack. Do not make up the difference by raising reader power. Too much power saturates nearby tags, wakes distant tags only just, and also misreads the next aisle.
It does not provide a continuous coordinate. One successful read only says the tag was inside the antenna coverage at that moment. After it leaves there is no track, unless read points are added or a positioning system is used instead. When a room-level location is needed, design the read points as doors and stations. Do not expect one omnidirectional antenna to cover a whole warehouse.
Multi-tag collisions and reader power
A missed inventory is the most common incident. Tags stacked together, oriented across the antenna polarization, or moving through too fast do not get a slot in which to answer. If software writes "not read" as "already left the warehouse," stock disappears on paper. Separate "never entered" from "entered and was missed." Where needed, put an antenna on each side of the portal and record how many complete rounds an inventory took. Another error is a repeated read. A tag lingering in the doorway is booked as an entry or an exit every time it is seen, and the count is inflated. Add dwell time and a direction check.
Writing is easy to overlook. At the edge of the field a passive tag has just enough energy to be read and not enough to be written. The write reports success, and the next read still returns the old value. Finish writes in the near field and verify by reading back. When several readers cover the same area at once, they drown each other on the same channel and no tag answers anyone. Plan channel, duty cycle, and antenna direction together. Finally, an identity is only a number. If the business system has not bound that number to the asset register, what was read is a string of digits that cannot be acted on.
Set tag encryption and lock state before the tags are issued. If a read-only identity can still be rewritten by anyone, a reader beside the portal can change an asset number, and the inventory system cannot tell a real asset change from a rewritten tag. Turn write protection on after the register is bound. Retire a tag with a new number. Do not recycle the old one. Recycling an old number makes historical entry and exit records point at a different item later.