How indoor asset tracking applications are embracing RFID technology

　　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, dating back to the transponder systems used by military aircraft during World War II to distinguish "friend from foe." Since then, RFID has been used in asset identification systems. Recently, it has been launched on the market as a real-time indoor positioning solution. This article briefly outlines how RFID works, how it can be applied to indoor asset tracking, and comparisons with other similar technologies.

　　01How RFID works

　　An RFID system, i.e., radio frequency identification, consists of two parts: a transponder (or tag) containing data that can be read via RF; An interrogator (or reader/writer) can read data from the forwarder.

　　The specific mode of communication between these two parts (known as the "coupling mechanism") determines the system's coverage, complexity, and cost. (Here, "coupling" refers to information transmission between the tag and the reader/writer.) Currently, there are three coupling mechanisms on the market: inductive coupling, capacitive coupling, and backscatter coupling.

　　Inductive coupling

　　Since the development of RFID technology, inductive coupling has existed, with systems at the time including large tags with complex antenna mechanisms, 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 measures radio waves through tags and decodes them into data. The magnetic fields used in these systems drop rapidly, so the effective recognition range of inductive coupling is about 1cm to 1m.

　　Capacitive coupling

　　When large sensing systems became the only option on the market, capacitive coupling systems were developed to reduce the cost and size of RFID. Conductive chip is used on readers and labels to form capacitance and signal data by changing the circuit's capacitance. The range between these systems is very close (1 cm), and patch orientation is important, so typical applications require inserting ID cards into the reader. As inductor circuits shrink, the limited capacitor system market is also shrinking.

　　In fact, most RFID systems currently use some form of capacitive coupling. However, they are still affected by the intensity of the distant magnetic field. To achieve long-distance communication, RFID systems must use higher-frequency signals and rely on electromagnetic signals from electric sources

　　Backscattering coupling

　　Backscatter coupling is a radar principle model. The reader emits UHF or microwave signals, which are reflected upon contact with the target and carry back the target information. This is based on the spatial propagation laws of electromagnetic waves, with a recognition range greater than 1 mm. Of course, whether expanding the recognition range is an advantage or a disadvantage depends on the use case.

　　02Types of RFID tags

　　The RFID market is categorized based on the power supply method of RFID tags. Whether a tag has available onboard power affects its size, price, read range, and whether it can support additional sensors.

　　Passive labels

　　Passive tags do not have internal power supply. They respond by absorbing some information from the reader's signal. This makes them cheap, durable, and noiseless (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 data selection range and require high-power, high-cost readers and writers.

　　Semi-passive labels

　　Semi-passive labels (also known as "semi-active" and "battery assist" labels) have an onboard battery. Like passive tags, they only transmit when there is a signal from the reader and writer. The battery can power both the sensor and the antenna. Semi-passive tags allow more signals to be reflected to the reader/writer, thus providing a longer read/write distance than passive tags. They are larger and more expensive than passive tags, and have limited battery life.

　　Active tags

　　Active tags have a local power source (such as a battery or photovoltaic power) and can broadcast their own signals. Although they are defined as tags, technically they do not rely on receiving and modulating signals from the reader. Instead, they are short-range radios. From an operational perspective, this distinction may not be that important, so we focus on the market and include them here. Compared to passive and semi-passive tags, active tags have a larger recognition range (up to 1 kilometer), increased memory capacity, size, and cost, and can work with weaker readers and writers.

　　03Asset tracking using RFID technology

　　Before evaluating the advantages of RFID as an asset tracking technology, it is necessary to clarify the meaning of "tracking." RFID has been used from the very beginning for asset tracking in the sense of spreadsheets. It makes it easy to identify and record nearby tracked items. If your goal is to ensure that all train carriages passing through Gate A also go through Gate B, or to confirm whether employees are entering the building, then RFID is an excellent solution that has been thoroughly tested and validated.

　　In these use cases, RFID most directly competes with barcodes or QR codes. It has a clear advantage in enabling reading and writing from a distance. Active or semi-passive RFID tags can provide valuable sensor information. On the other hand, readers paired with passive tags are not cheap, while semi-passive tags are very expensive and have limited lifespan.

　　A more challenging type of tracking is learning the real-time location of the tracking asset. Although this is a relatively new RFID use case, there are already many commercial solutions on the market.

　　These programs work in different ways. Some solutions use pure RFID for object identification, while also employing another technology for distance measurement. Almost all those that rely entirely on RFID use active RFID tags. Although some studies have used passive RFID tags, the cost of passive tag readers and the system's limited recognition range have prevented them from being commercially applied

　　Real-time Location Systems (RTLS) using active RFID tags perform better than competing technologies such as Bluetooth, Bluetooth Low Energy (BLE), WiFi, Ultrasonic, and Ultra-Wideband (UWB). RFID is mainly based on the LANDMARC system, which determines location by comparing the RSS of active tags with RSS of reference tags at known locations.

　　RFID has a wider recognition range than BLE. Compared to BLE's 70 meters, it can cover 1 kilometer outdoors. This may be less critical in unobstructed indoor environments (such as walls or floors), but in warehouses or barns, the recognition range of active RFID tags allows businesses to use fewer readers and writers, thereby reducing costs and minimizing potential failures.

　　RFID as an asset tracking solution also has some drawbacks. Like all RF/RSSI-based solutions, it has vulnerabilities. Since RF signals can penetrate walls, it is difficult to determine from which room the signal is coming from. The high bandwidth used by active trackers (especially remote trackers) is highly susceptible to interference. Moreover, compared to BLE, labels and readers are much more expensive.

　　RFID has achieved tremendous success as part of the entire hybrid system. It provides reliable identification and can serve as a complementary technology to positioning information systems that rely on ultrasonic, infrared, or ultra-wideband

　　Conclusion

　　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.