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RFID technology is not the only solution for indoor positioning applications. However, as part of a multi-technology system, it is a very effective solution.
RFID tags have a long history, which can be traced back to the transponder system of military aircraft that distinguished “friends and enemies” during World War II. Since then, RFID has been used in asset identification systems. Recently, it was introduced to the market as a real-time indoor positioning solution. This article briefly outlines how RFID works, how it is applied to indoor asset tracking, and its comparison with other similar technologies.
01How RFID works
The RFID system, or radio frequency identification, consists of two parts: a transponder (or tag), which contains data that can be read by RF; and an interrogator (or reader/writer), which can read the data of the transponder.
The specific way of communication between these two parts (called “coupling mechanism”) determines the coverage, complexity and cost of the system. (The “coupling” here refers to the information transmission between the tag and the reader.) At present, there are three coupling mechanisms on the market: inductive coupling, capacitive coupling and backscatter coupling.
Inductive coupling
Since the development of radio frequency identification (RFID) technology, inductive coupling has always existed. The system at that time included large tags with complex antenna mechanisms, which were mainly used to track large objects (such as cars and cattle). Inductively coupled tags extract information from the magnetic field generated by the reader and modulate it. The reader/writer measures the electric wave through the tag and decodes it into data. The magnetic field used in these systems will drop rapidly, making the effective identification range of inductive coupling about 1 cm to 1 m.
Capacitive coupling
When large-scale sensing systems became the only option on the market, capacitive coupling systems were developed to reduce the cost and size of RFID. Use conductive patches on readers and tags to form capacitance and signal data by changing the capacitance of the circuit. The range between these systems is very close (1 cm), and the orientation of the patch is very important, so a typical application is an ID card that must be inserted into the reader. With the shrinking of inductive circuits, the market for limited capacitive systems is also shrinking.
In fact, most current RFID systems use some form of capacitive coupling. However, they are still affected by the strength of the magnetic field at a distance. In order to achieve long-distance communication, radio frequency identification systems must use higher frequency signals and rely on electric electromagnetic signals.
Backscatter coupling
Backscatter coupling is a radar principle model. The reader emits UHF or microwave signals. These signals are reflected when they hit the target and carry back target information. Based on the law of electromagnetic wave propagation, the recognition distance is greater than lm. Of course, whether expanding the scope of recognition is an advantage or a disadvantage depends on the use case.
02RFID tag type
The RFID market is divided into types according to the power supply mode of RFID tags. Whether the tag has an available on-board power supply will affect its size, price, reading range, and whether it can support additional sensors.
Passive tags
Passive tags have no internal power source. They react by absorbing some information in the reader’s signal. This makes them cheap, durable, and noise-free (in the radio spectrum). Due to the lack of consistent data, they cannot be used to write and store sensor data. They have a lower range of data selection, and need to be equipped with high-power, high-cost readers.
Semi-passive tags
Semi-passive tags (also known as “semi-active” and “battery assist” tags) have an onboard battery. Just like passive tags, they are only transmitted when there is a signal from the reader. The battery can power the sensor or the antenna. Semi-passive tags allow more signals to be reflected to the reader, and thus have a farther reading and writing distance than passive tags. They are larger, more expensive, and have limited battery life than passive tags.
Active tag
Active tags have a local power source (such as batteries, photovoltaics) and can broadcast their own signals. Although they are defined as tags, in terms of technology, they do not rely on receiving and modulating the signal from the reader. Instead, they are short-range radios. From an operational point of view, this distinction may not be that important, so we focus on the market and include them here. Compared with passive tags and semi-passive tags, active tags have a larger recognition range (up to 1 km), memory capacity, size and cost will also increase, and can work with weaker readers .
03Use RFID technology to track assets
Before evaluating the advantages of RFID as an asset tracking technology, the meaning of “tracking” needs to be clarified. RFID has been used for asset tracking in the sense of a spreadsheet from the beginning. It makes it easy to identify and record nearby tracking items. If your goal is to ensure that all train cars that pass through gate A also pass through gate B, or to confirm whether there are employees entering the building, then RFID is an excellent solution that has been fully tested and verified.
Of these use cases, RFID most directly competes with barcodes or QR codes. It has an obvious advantage of being able to read and write from a distance. Active or semi-passive RFID tags can provide valuable sensor information. On the other hand, the price of readers matching passive tags is not cheap, while semi-passive tags are very expensive and have a limited service life.
A more challenging type of tracking is to learn the real-time location of tracked assets. Although this is a relatively new RFID use case, there are already many commercial solutions on the market.
These programs work differently. Some programs purely use RFID for object recognition, while using another technology for ranging. Almost all those who rely entirely on RFID use active RFID tags.Although there are some studies on the use of passive RFID tags, the cost of passive tag readers and the low identification range of their systems make them not commercially applicable.
The performance of the real-time location system (RTLS) using active RFID tags is compared to competing technologies such as Bluetooth, Bluetooth Low Energy (BLE), WiFi, ultrasound, and ultra-wideband (UWB). RFID is mainly based on the LANDMARC system, which determines the location by comparing the received signal strength (RSS) of an active tag with the RSS of a reference tag at a known location.
RFID has a larger recognition range than BLE. Compared with the 70 meters of BLE, it can cover 1 km outdoors. This may not be so important in indoor environments with no obstacles (such as walls or floors), but in warehouses or barns, the recognition range of active RFID tags allows companies to use fewer readers, thereby reducing costs and Reduce potential failures.
RFID as a solution for tracking assets also has some disadvantages. Like all RF/RSSI-based solutions, it also has vulnerabilities. Because radio frequency signals can penetrate walls, it is difficult to determine which room the signal originated from. The high bandwidth used by active trackers (especially long-range trackers) is very susceptible to interference. Moreover, compared with BLE, the cost of tags and readers are very expensive.
RFID has achieved great success as part of the entire hybrid system. It provides reliable identification and can exist as a supplementary technology for positioning information systems that rely on ultrasound, infrared or ultra-wideband.
Concluding remarks
Currently, RFID technology cannot provide an independent solution for indoor positioning. In this regard, it is not the only option. However, as part of a multi-technology system, RFID has brought decades of reliable identification history to indoor positioning.
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