A bit about UHF and RFID

UHF operates, primarily, in 860-960 MHz range allowing for shorter antennas and longer read distances. Reader-Tag communication is implemented using backs-scatter technology. The reader transmits energy, commands and data to the tag which than responds back to the reader by backscattering its identification. In this method, tag communicates with the reader by modulating the received signal and radiating it back to the reader. UHF RFID technology has evolved over the past few years, where the technology works well even in adverse conditions and on metallic products. But even with all the advancements, the performance of UHF RFID has been somewhat limited when it comes to large read and write distances. The tag read and write range depends on several factors such as tag characteristics, propagation environment or path loss and reader’s RF circuit parameters.

UHF RFID Tags

UHF RFID tags consist of an antenna and a microchip and the tags get all the energy for functioning from the electromagnetic radiation emitted by the reader through a RF front end that consists of a rectification circuit, voltage multiplier and a voltage modulator. The tag backscatters the information back to the reader by switching between one of the two states:

State one is matched to the antenna
State two is strongly mismatched

http://blog.odintechnologies.com/odin-rfid-blog/bid/61324/RFID-Long-Distance-Relationship-The-RFID-Tag-and-the-Passive-Reader

The link above shows the picture of the RF communication between the reader and the tag. The forward link suggests the Query signal from the reader to the tag. During the tag response time or the return link, the reader is continuously providing the energy to the tag to wake up and operate by radiating the CW signal. From the above figure we can note that the backscatter response of the tag or the return link is significantly weaker in energy than the forward link signal of the signal.

So what does it takes to create a successful long distance communication between the reader and tag?

RFID Tag Silicon Sensitivity – Silicon sensitivity is defined as the minimum received RF power necessary to switch on RFID tag silicon. This is one of the most important tag limitations that can impact the backscatter signal strength. Lower the tag silicon sensitivity value, longer is the read distance at which the tag can communicate with the reader. There are two factors that can affect the silicon sensitivity value:
- Silicon fabrication process
- RF front end design

Most passive UHF RFID tag silicon sensitivity values lie between -14 to -18 dBm.

Impedance matching between the antenna and tag silicon - Passive UHF RFID tag consists of a microchip attached directly to an antenna. Proper impedance match between the antenna and the chip is crucial in RFID tag design. In order to achieve optimum operating condition, the antenna impedance should be matched correctly to the chip impedance that is known to change with the received power on the chip as well as with frequency (860 to 960 MHz). It directly influences RFID system performance characteristics such as the range of a tag.

Antenna Gain – Antennas are designed to operate best in a specific frequency band. The geometry of the antenna is also dependent to wavelength which is component of frequency. Gain of the antenna is thus dependent on the frequency of operation and geometry. The read range of a tag is maximum in the direction of the antenna gain, where maximum power is available.

Orientation sensitivity – To maximize the read distance, the tag antenna orientation must be matched to that of the reader antenna. Linear polarized reader antennas provide longer read range with linearly polarized tag antenna, but may increase the sensitivity to orientation, whereas circularly polarized reader antennas reduced the orientation sensitivity issue with linearly polarized tag antenna but may reduce the read distance due to the 3dB loss incurred due to the circular polarization.

Tag antenna detuning – Tag detuning occurs when the tag antenna tuning characteristics change when applied to different objects or come is close vicinity to objects. Detuning of the tag antenna result in impedance mismatch which in turn affects the tag read distance.

Path Loss – Path loss is a reduction in the power density of an electromagnetic wave as the wave propagates through space. Path loss may be due to many effects, such as free-space loss, refraction, diffraction, reflection, and absorption. Path loss is largely influenced by environment, propagation medium (dry or moist air), the distance between the transmitter and the receiver, and the height and location of antennas.

Radiated Power from the Reader - UHF RFID makes use of unlicensed bands, example in the North America, maximum allowed EIRP (Equivalent isotropic radiated power) is 4 watts. Most local telecom authorities control the radiated energy to minimize interference with other wireless devices that share the same band or with wireless devices that share adjacent frequencies. This suggests that for a maximum allowed radiated power the read distance is inversely proportionate to the path loss.

Reader sensitivity - Sensitivity in a receiver is normally taken as the minimum input signal required to produce a specified output signal having a specified signal-to-noise (S/N) ratio and is defined as the minimum signal-to-noise ratio times the mean noise power. There are several factors that can affect the readers receive sensitivity, primarily the RF front end design, communication protocol and number of interferes in close vicinity. Most readers today have receive sensitivity of about -80 to -110 dBm.