A brief overview of RFID technology and testing challenges for complex operating environments where throughput and communication issues may be caused by multiple readers, dense-mode environments and pre-existing non-RFID signals
The applications of radio-frequency identification (RFID) are growing rapidly as equipment prices drop and global markets expand. The use of embedded RFID is increasing. Coordinating bodies such as the Ubiquitous ID Center and the T-Engine Forum have been formed. And the GSM Association now supports embedding of RFID-based near-field communications capability into cellular phones.
However, a big challenge in RFID is optimising throughput (data reading speed) in complex, or even harsh, RF environments. Passive RFID tags may respond to any reader or readers in range. Protocols exist to work with this behaviour, but the result is a complex communications behaviour that can be difficult to test without the right equipment. In addition, embedded RFID systems may need to function, when integrated into the same device, with cellular, WLAN, Bluetooth or Zigbee technologies. Finally, interference from other users in the same band needs to be considered.
The result is a need to simulate complex RF environments and analyse the performance of the RFID system under these conditions before deployment. The pulsed nature of RFID and the typical interference sources make this task all the more difficult.
RFID technology overview
An RFID system at its simplest consists of a tag, which may be passive, and a reader. Architecturally, reading passive tags is somewhat different from the traditional full-duplex data link. Unlike traditional active data links, passive tags rely on the received RF energy to power themselves. These also do not generate their own transmit carrier signal. Rather, they modulate some of the energy being transmitted by the interrogator to the tag in a process known as backscattering.
By changing the loading of the tag’s antenna from absorptive to reflective, a continuous-wave (CW) signal from the interrogator can be modulated. This process is very similar to using a mirror and the sun to signal someone at a distance. It also eliminates the need for precision frequency sources and power-hungry transmitters in the tag. Since the reader and the tag share the same frequency, these must take turns sending information. Backscattering restricts communications between the reader and the tag to a half-duplex system.
Since the uplink from tag T to reader R (denoted by T→R) is modulated from the interrogator’s CW signal, it is possible to use spread-spectrum techniques such as frequency hopping. Any spreading or hopping on the interrogator’s signal is automatically removed in the homodyne down-conversion of the receiver, as it shares the same local oscillator signal.
This simple system turns complex when multiple tags, multiple readers and interference are present. Let’s take a look at two RFID design challenges that come from these deployment issues.
Multiple-reader and dense-mode environments
The broadband nature of passive RFID tag presents some challenges for dense (multiple) reader sites. Since the tag reader sets the system’s frequency of operation, and the tag is a broadband device that responds to any reader, the tag has limited ability to respond to a specific reader. Passive tags may try to respond to all readers that are interrogating them.
Many RFID systems are implemented in a multiple-reader or dense-mode environment. Here are some definitions:
Single-reader environment: A single reader is operating in an environment.
Multiple-reader environment: The number of simultaneously operating readers is less than the available number of channels.
Dense-reader mode: It’s the most challenging environment, where the number of readers is greater than the number of channels.
Reader and tag interference can take place within the operating environment—the zone within which the reader’s RF signal is attenuated by less than 90 dBc (a radius of approximately 1 km in free space). Consequently, many readers end up operating in a dense-mode environment whether by design or due to neighbouring RFID readers.
A warehouse application with fixed readers and accurate spectrum planning is likely to have minimal interference from neighbours within 1 km. However, a mobile RFID device should expect a dense-reader mode environment due the lack of control over safe mitigation distances. In this case, it becomes critical to discover what signals may be present in the environment where the RFID system will be, or is, deployed and understand the behaviour of the reader and tags in the presence of interference.
To handle this environment, ISO18000-6C readers that have been certified for dense environments often switch to Miller-modulated sub-carrier (MMS) encoding. This elaborate encoding provides more transitions per bit and is therefore easier to decode in the presence of noise. But it is slower for the same tag’s backscatter-link frequency (BLF).