OVERCOMING FADING WITH ANTENNA DIVERSITY
RJ Burkholder, Research Professor of Electromagnetics and RF at The Ohio State University
In the last several blogs I maintained that the antenna is the most important part of a UHF RFID system (see figure below). This is because the computer software, reader, and RFID tag are fairly optimized now, and often outside the control of the system designer anyway. Performance improvements must come from the proper selection of the antenna and its deployment.
A typical UHF RFID inventory system
Before getting into the fine art of antenna deployment, it was first necessary to understand the basic principles of how antennas work and how the electromagnetic field radiated by an antenna fills and penetrates a given space. The main issues are polarization, fading, gain, EIRP (effective isotropic radiated power) and diversity. A good understanding of these issues will aid the designer in selecting the type and number of antennas, and where to put them for optimum performance.
To review, we first explored the most basic characteristic of an antenna, namely, its gain. This number is defined as the directional amplification of the antenna compared with an antenna that radiates equally in all directions (isotropic). Gain is closely related to Effective Isotropic Radiated Power (EIRP). EIRP is defined as the amount of power that a theoretical isotropic antenna would emit to produce the peak power density observed in the direction of maximum antenna gain. EIRP is limited by the FCC to 36 dBm (decibels relative to a milliwatt) of RF power. As we saw, because of this limit, it does not always help to use a high-gain antenna because the EIRP will likely be exceeded unless the RFID reader power is reduced accordingly.
In the last blog we learned about the important characteristic of antenna polarization. Polarization defines the predominant direction of the electric field radiated or received by an antenna. It was illustrated that using a single patch-type reader antenna in a static scenario is likely to miss a significant percentage of tags simply because the polarization is misaligned. Even a circularly polarized antenna, which can detect a tag in any orientation transverse to the radiation direction, will miss tags that are oriented along the radiation direction as shown below.
Two antennas provide polarization diversity for reading an RFID tag that a single antenna is not able to read due to the tag orientation.
This introduced the concept of antenna diversity, which means using more than one antenna to cover a given region in order to overcome the limitations of a single antenna. This brings us to the present topic of mitigating fading with antenna diversity. Polarization mismatch is not the only limitation of using a single reader antenna. Any single antenna has nulls where the radiated field is very low. Typically, an antenna is designed so that the nulls are towards the back and sides, and the main beam is free from nulls when the antenna radiates in empty space. However, in a realistic RFID environment there is always multipath, as illustrated in the figure below.
Overcoming fading in a static enviroment with antenna diversity. Multiple RFID antennas provide spatial diversity for reading a tag that a single antenna is not able to read due to fading caused by multipath interference.
When multipath is present, due to reflections from the floor, ceiling, walls, and large objects, the electric field associated with each path can form an interference pattern. At points where the fields add in phase, constructive interference occurs and the pattern has a strong peak. Conversely, where the fields are out of phase, destructive interference occurs and the pattern has a null. If you’ve ever been at a stop light and notice the radio reception has gone out, you are probably in a null. The reception comes back when your car moves out of the null sometimes just by moving a few feet. Wireless communications systems always have to deal with this issue. One solution is to use a different frequency because the interference pattern is highly frequency dependent. Changing the frequency moves the nulls. This is one reason why UHF RFID in North America covers a relatively large 26 MHz frequency band from 902 to 928 MHz. Readers can frequency-hop within this band to avoid interfering with other nearby readers, but also to help mitigate the effects of fading and improve the read rate.
Another example you have probably seen is a wireless router. They usually have at least two antennas. Some of the higher capacity routers have three or four antennas. The primary purpose of these antennas is to provide diversity in polarization and space. The interference pattern and nulls of an antenna are very sensitive to its placement and orientation. Another antenna placed just a few inches away will have a completely different pattern. This allows two or more antennas to have far better coverage of a multipath rich environment than a single antenna.
The same principles of antenna diversity apply to RFID readers. That’s why most UHF readers have more than one antenna port. A general misunderstanding in RFID is that more antennas let you cover more area. That is true to a certain extent, but if you only have one antenna covering a given area, the read rate will suffer. For the best coverage, at least two antennas should cover any given point in the region of interest. This is illustrated below.
Diversity-rich RFID coverage zone using for patch antennas on opposite sides. Overlapping beams from different directions provide spatial diversity as well as polarization diversity.
The example above shows an effective way to cover a fairly large area using four patch antennas (up to 20 feet). The antennas can be connected to the same 4-port RFID reader. This arrangement provides polarization diversity as well by tilting the beams. 4-port readers can be easily extended to 16 antennas using a multiplexing switch. There are many possibilities for antenna placement, but this basic concept of exploiting diversity will greatly improve tag reading performance.
In this blog we have extended the concept of antenna diversity to include spatial diversity. The next blog will show how the Wave® antenna is ideally suited for zone coverage with a minimum number of antennas. Each antenna uniquely provides multiple overlapping beams in the zone surrounding the antenna.
Next blog: The patented Wave® Antenna – the only antenna designed specifically for item-level RFID.
The opinions expressed on this webpage are the author’s and do not necessarily represent the opinions of The Ohio State University.