For the present purposes, we shall suggest a plausible and convenient lower limit of around -75 dBm (0.03 nW), deferring justification of this value until later. With the reader's indulgence, we shall proceed to use these unjustified assumptions to construct a diagram of the reverse link power in the same fashion as that previously constructed for the forward link; the result is depicted in Figure 23. We construct a second line like the first but starting at 5 dB less than the tag received power. Note that in this case, as the line descends, we are physically moving back towards the reader. If we move back 3 m (to intercept the dotted vertical line labeled 'forward-link-limited range'), we find the reader receives about -55 dBm, about 20 dB in excess of the power required by the reader's receiver. In fact, a receiver could be an additional 29 m away before the signal would fall so low as to fail to be received for this threshold value.
Figure 23. Forward- and Reverse Link Budget Calculation for Passive Tag, United States Operation.
While the details of our simplified calculations are hardly authoritative, the observation that passive tags are forward-link-limited has historically been generally correct. The reason is that tag IC power requirements of tens or hundred of microwatts are actually monstrously large compared to the tiny signal powers that can be detected by a good-quality radio receiver. However, as the required power delivered to the IC is decreased with continued progress in IC technology, this may change.
To understand why, we need to understand how the power returned to the reader scales with tag-reader distance. Note in Figure 23 that the starting power for the tag scales with the received power. If we double the distance to the tag, the power the tag receives falls by a factor of 4, and thus, the transmit power associated with the tag (the reverse link power) also falls by a factor of 4. But this power has to travel twice as far to get back to the reader, so the received power at the reader falls by an additional factor of 4. The net result for a doubling of the distance is a 16-fold (24) decrease in the received power at the reader. The received power from a backscatter link falls as the inverse fourth power of the distance:
In the case of a power-hungry passive tag, this scaling is rendered moot by the need to provide a fixed forward power to the tag. However, when the tag power is reduced by (say) 10 times, the forward-link-limited range increases by a factor of about 3. The received signal thus decreases by 20 dB, placing it at the threshold for this example receiver: the tag becomes reverse-link-limited (at least for this receiver). As we will see in Chapter 5, reader sensitivity is dependent on several design choices, particularly, antenna configuration, and will become more important as tag IC power is scaled to lower values. For a semipassive tag, the forward-link requirement is much more lenient since the received power must only be decoded not exploited, and inverse-fourth-power scaling is very important in determining the range of the tag.
2EPC global discourages the use of the terms forward and reverse link for readers and tags, but these terms are widely used in other areas of wireless networking, where an asymmetric link is under consideration, and seem perfectly applicable to RFID.
Next: Effect of Antenna Gain and Polarization on Range
About the Author
Daniel Dobkin is an RFID consultant, writer and teacher. He holds six patents as inventor or co-inventor. He is the author of such books as: Principles of Chemical Vapor Deposition and RF Engineering for Wireless Networks, and The RF in RFID. Additionally, he is a published author of 25 technical publications. He has taught RFID courses internationally in Singapore for the SMa/RFID Focus; and domestically at SDForum, Mitre Corporate University, and San Jose State University. Daniel is a Stanford University PhD in Applied Physics. He has MS and BS degrees from CalTech.
Printed with permission from Newnes, a Division of Elsevier. Copyright 2007. "The RF in RFID: Passive UHF RFID in Practice by Daniel M. Dobkin. ISBN-10: 0750682094For more information about this title and other similar books, please visit www.elsevierdirect.com.
