To clarify why this sort of thing matters in real applications, let's look at a practical example. In the United States, unlicensed readers randomly hop from one frequency to another within the ISM band from 902-928 MHz. Typically, RFID readers use channels that are 500 kHz wide and separated by 500 kHz. When a reader is trying to hear a tag, it transmits a signal of constant amplitude and phase. If reader #1on channel 10 is trying to hear a tag, while reader #2 on channel 11 is producing an emission spectra like those shown in Figure 11, the situation would look something like Figure 12, where the spectrum from reader #2 is scaled for a data rate of about 100 kbps and a distance of about 20 m. In Section 3.5 below, we will find that for typical distances, a tag signal is likely to be 40-90 dB smaller than the CW signal from the reader. The leakage from reader #2 into reader #1s channel is thus comparable to or even larger than the tag signal; it will be difficult to detect the tag when reader #2 is transmitting data. Note, this is happening despite the fact that the tags are only 1-3 m from the reader, much closer than the interfering reader!

Even worse, if one of the readers happened to be near the edge of the ISM band, some of this power may be radiated outside of the allowed frequency range, potentially interfering with users of licensed frequencies, who have often paid for the privilege of exclusive use of said spectrum and get upset when they encounter freeloaders. In the United States, the FCC requires that all radios be tested to ensure that such out-of-band radiation is minimized. Interference and out-of-band emissions represent important limits on how fast data can be transmitted by a reader, and on coding and modulation used, because the speed and method of modulation determine the bandwidth of the resulting signal.

Figure 11. Coding Data as PIE Produces a Strong Narrow Emission Far from the Carrier, as Well as a Higher Average Signal Power Far from the Carrier; the inset Shows How This Band Arises from the '0' Symbol. (The Exact Position of These Features Relative to the Data Rate Varies Depending on the Duration of a Binary '1'.)

Let us pause for a bit of mathematics to clarify the frequency scales of the figures above. An ideal abrupt pulse (an OOK binary '1') of duration t has a spectrum:

Figure 12. Power Far from the Carrier of Reader #2 is in the Channel of Reader #1 if Data Rate is High and Unsmoothed PIE is Used.

This function has some useful special values:

In particular, the first zero of this function is at a frequency of (1/t), where t is the duration of the pulse. When the signal is a modulated carrier wave, the spectrum is centered around the carrier frequency, and the zeros are displaced from the carrier by (1/t) (Figure 13).

A stream of binary pulses--an OOK signal as in Figure 6--is just the sum of a number of these pulses, each with the same spectrum, so the full data stream will also have a spectrum with zero value at the same frequency offset from the carrier. These first zeros determine the width of the main lobe of the signal spectrum and are indicated by the dashed lines in Figure 9. Most of the power in the spectrum is contained within the region about half this wide, that is within a frequency range of (fc -1/(2t)) to (fc +1/(2t)). Thus, the narrowest channel that makes sense for an OOK signal is about twice as wide as the inverse of the data rate; we need 200 kHz to fit in 100 kbps.

Figure 13. A Pulse-modulated Carrier and Corresponding Power Spectrum; Square-root of Power is Shown for Clarity..

PIE is much less efficient because the shortest pulse--the high part of a binary '0', Figure 8--is about 1/3 as long as an OOK pulse for the same data rate, so roughly three times as much spectrum is needed. To fit the main lobe of the spectrum within a 500 kHz channel, we can only use a data rate of around 85 kbps--which, as we will see in Chapter 8, is just about the upper limit on reader data rates in United States operation, using unfiltered PIE-like modulations.

To summarize:

• To convey information on a signal, the signal must be modulated.
• Modulation causes the signal's spectrum to expand, requiring allocation of bandwidth in order to avoid interference.
• The peculiar requirements of passive RFID lead to modulation and coding of binary data that are relatively inefficient in spectral use, limiting reader data rates.

It is important to note that more sophisticated radio systems, such as cellular telephony or IEEE 802.11 (WiFi), use modulation techniques that are substantially more efficient users of spectrum than PIE or OOK. However, these methods generally depend on the ability of the receiver to detect changes in the phase of the high-frequency signal rather than simply determining the power level, which passive RFID tags generally cannot do. As we will discuss in more detail in Chapters 4 and 8, single sideband (SSB) and phase-reversal ASK (PR-ASK) modulations, which use phase information at the reader but require only amplitude detection from the tag, can be used to improve the spectral efficiency.

¹It is worth noting that in this and the next few figures, the spectra are calculated for a series of about 80 random data bits, only a few of which are shown in the upper 'signal' display, in order to keep the diagrams intelligible. If we calculated the frequency spectra over a larger number of bits, they would be smoother, but the spectra shown are reasonably representative of the kind of data actually obtained when the output of a typical frequency-hopping RFID reader is examined over short time scales.

Next: Backscatter Radio Links

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