Tutorial: Part I, Part II, and Part III
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Backscatter Radio Links
Passive and semipassive RFID tags do not use a radio transmitter; instead, they use modulation of the reflected power from the tag antenna. Reflection of radio waves from an object has been a subject of active study since the development of radar began in the 1930s, and the use of backscattered radio for communications since Harry Stockman's work (See Chapter 2) in 1949.

A very simple way to understand backscatter modulation is shown schematically in Figure 14: current flowing on a transmitting antenna leads to a voltage induced on a receiving antenna. If the antenna is connected to a load, which presents little impediment to current flow, it seems reasonable that a current will be induced on the receiving antenna. In the figure, the smallest possible load, a short circuit, is illustrated. This induced current is no different from the current on the transmitting antenna that started things out in the first place: it leads to radiation. (A principle of electromagnetic theory almost always valid in the ordinary world, the principle of reciprocity, says that any structure that receives a wave can also transmit a wave. We shall make use of this principle in discussing antennas in greater detail shortly.) The radiated wave can make its way back to the transmitting antenna, induce a voltage, and therefore, produce a signal that can be detected: a backscattered signal. On the other hand, if instead a load that permits little current to flow--that is, a load with a large impedance--is placed between the antenna and ground, it seems reasonable that little or no induced current will result. In Figure 14, we show the largest possible load, an open circuit (no connection at all). Since it is currents on the antenna that lead to radiation, there will be no backscattered signal in this case. Therefore, the signal on the transmitting antenna is sensitive to the load connected to the receiving antenna.

Figure 14. Simplified Physics of Backscatter Signaling

To construct a practical communications link using this scheme, we can attach a transistor as the antenna load (Figure 15). When the transistor gate contact is held at the appropriate potential to turn the transistor on, current travels readily through the channel, similar to a short circuit. When the gate is turned off, the channel becomes substantially nonconductive. Since the current induced on the antenna, and thus, the backscattered wave received at the reader, depend on the load presented to the antenna, this scheme creates a modulated backscattered wave at the reader. Note that the modulating signal presented to the transistor is a baseband signal at a low frequency of a few hundred kHz at most, even though the reflected signal to the reader may be at 915 MHz. The use of the backscatter link means that the modulation switching circuitry in the tag only needs to operate at modest frequencies comparable to the data, not the carrier frequency, resulting in savings of cost and power. (Real RFID tag ICs are not quite this simple and may use a small change in capacitance to modulate the antenna current instead, for reasons we will discuss in Chapter 5.)

Figure 15. Modulated Backscatter Using a Transistor as a Switch

Note that in order to implement a backscattered scheme, the reader must transmit a signal. In many radio systems, the transmitter turns off when the receiver is trying to acquire a signal; this scheme is known as half-duplex to distinguish it from the case where the transmitter and receiver may operate simultaneously (known as a full-duplex radio). In a passive RFID system, the transmitter does not turn itself off but instead, transmits CW during the time the receiver is listening for the tag signal. RFID radios use specialized components known as circulators or couplers to allow only reflected signals to get to the receiver, which might otherwise be saturated by the huge transmitted signal. However, in a single-antenna system, the transmitted signal from the reader bounces off its own antenna back into the receiver, and the transmitted wave from the antenna bounces off any nearby objects such as desks, tables, people, coffee cups, metal boxes, and all the other junk that real environments are filled with, in addition to the poor little tag antenna we're trying to see (Figure 16). If two antennas are used (one for transmit and one for receive), there is still typically some signal power that leaks directly from one to the other, as well as the aforesaid spurious reflections from objects in the neighborhood.

Figure 16. Realistic Environments Create Many Reflected Waves in Addition to that from the Wanted Tag

The total signal at the receiver is the vector sum of all these contributions, most of which are much larger than the wanted tag signal, with appropriate amplitudes and phases, most of which are unpredictable a priori. Thus, the actual effect of a given change in the load on the tag antenna on the receiver signal is completely unpredictable and uncontrollable. For example, modulating the size of the tag antenna current (amplitude modulation) may not result in the same kind of change in the reader signal. In Figure 17, we show a case where changing the tag reflection from a large amplitude (HI) to a small amplitude (LO) causes the received signal to increase in magnitude without changing phase (the "AM" case). Changing the phase of the tag signal without changing the size of the reflected signal in order to symbolize a LO state may change the amplitude of the reader signal at constant phase (Figure 17, "PSK" case). The only thing we can say with any confidence is that when we make a change in the state of the tag antenna, something about the phase or amplitude of the reader signal will change. In order to make a backscatter link work, we need to choose a way to code the data that can be interpreted based only on these changes and not on their direction or on whether they are changes in phase or amplitude.

Figure 17. The Received Signal is not Simply Correlated to the Tag Signal. The AM Case Assumes the Tag Reduces its Scattered Magnitude Without Changing Phase; the PSK Case Assumes Phase Inversion Without Amplitude Change.

As a consequence, all approaches to coding the tag signal are based on counting the number of changes in tag state in a given time interval, or equivalently on changing the frequency of the tag's state changes. Therefore, all tag codes are variations of frequency-shift keying (FSK). It is important to note that the frequency being referred to here is not the radio carrier frequency of (say) 900 MHz but the tag (baseband) frequency of perhaps 100 or 200 kHz. A binary '1' might be coded by having the tag flip its state 100 times per millisecond, and a binary '0' might have 50 flips per millisecond. Because the frequency being changed is the frequency at which a carrier is being amplitude modulated, techniques like this are sometimes known as subcarrier modulation.

Let's look at one specific example of tag coding, usually known as FM0 (Figure 18). In FM0, the tag state changes at the beginning and end of every symbol. In addition, a binary 0 has an additional state change in the middle of the symbol. Note that, unlike OOK, the actual tag state does not reliably correspond to the binary bit: for example, in the left-hand side of the figure, two of the binary '1' symbols have the tag in the LO state and another '1' symbol has the tag in the HI state. Remember, the reader can't reliably distinguish which state is which but can only count transitions between them. The right side of the figure shows the baseband signal corresponding to a series of identical binary bits to clarify the correspondence of binary '0's with a frequency twice as high as that of binary '1's.

Different tag coding schemes can be used to adjust the offset from the carrier frequency at which the signal from the tags is found. As we will find in Chapter 4, readers have an easier time seeing a tag signal when it is well separated from their own carrier frequency, so higher subcarrier frequencies help improve the ability to read a tag signal. However, if the separation is large compared to the channel size, the tag signal might lie on the signal of another reader in a different channel. Just as with readers, increasing the data rate of a tag signal tends to spread the spectrum out in frequency. To have a flexible choice of tag data rates while minimizing noise, the reader needs to be able to adapt the band of frequencies it tries to receive, adding cost and complexity.

Figure 18. FM0 Encoding of Tag Data.

In real receivers, noise and interference may be present as well as the desired signal. A certain minimum signal-to-noise ratio (S/N) is necessary for each type of modulation in order that it can be reliably decoded by the receiver. The exact (S/N) threshold depends on how accurate you're trying to be and to a lesser extent on the algorithms used for demodulation/decoding. For RFID using FM0, (S/N) of around 10 or better (10 dB or more) is usually sufficient. (Requirements for demodulation of reader symbols, like PIE, in the tag are generally similar.) As we will see in Chapter 8, modern protocols provide alternative modulations that can operate with smaller (S/N) ratios, at the cost of a reduction in the tag data rate.

Next: Link Budgets

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.