Information, Modulation, and Multiplexing
A periodic signal that persists indefinitely, without changing its amplitude, frequency, or Phase--a continuous wave (CW) signal--carries no information other than the fact that it is present. In order to convey data, a signal needs to change. We normally think of this change as a relatively slowly changing variation--modulation--imposed on the periodic signal, for example:

The function m(t) is said to contain the baseband information, and the relatively high frequency cosine function is the carrier. When the function m(t) is another sine or cosine (presumably of much lower frequency), we can make use of trigonometric identities (see Appendix 2) to rewrite the signal in a revealing fashion:

A sinusoidal modulation splits the carrier wave into two signals called sidebands, one above and one below the carrier, each displaced by the modulating frequency (Figure 5). While a continuous sinusoidal modulation is hardly more interesting or useful than a CW signal, this result suggests that when a signal is modulated, the resulting frequency spectrum becomes wider.

Signals of interest for RFID are generally digitally modulated. A digitally modulated signal is a stream of distinct symbols. A simple example with substantial relevance for RFID is on-off keying (OOK). The signal power is kept large (m = 1) to indicate a binary 1 and small or zero (m = 0) to represent a binary '0.' An example is shown in Figure 3.6. In OOK, each symbol is a period of fixed duration in which the signal power is either high or low. Each OOK symbol represents one binary bit, though other types of symbols can convey more than one bit each. Any circuit that can change the output power, such as a simple switch, can be used to create an OOK signal, and any circuit that can detect power levels can demodulate (extract the data from) the signal. For example, a diode--an electrical component that passes electrical current only in one direction and blocks current flow in the opposite direction--can rectify a high-frequency signal, turning it into pulses of DC. These pulses can be smoothed with a storage capacitor to produce an output signal that looks very much like the baseband signal m(t) (see Figure 17 in Chapter 2). If the diode responds rapidly, it can be used at very high frequencies. Modern diodes can operate up to over 1 GHz, allowing passive RFID tags to demodulate a reader signal using only a diode and capacitor.

Unmodified OOK is admirably simple and seems promising as a method of modulating a reader signal. However, there is a problem with OOK for passive RFID. As we noted in Chapter 2, a passive RFID tag depends on power obtained from the reader to run its circuitry.

If that power is interrupted, the tag cannot operate. However, imagine the case of an OOK signal containing a long string of binary 0s: in this case, m = 0 for as long as the data remains 0. The tag will receive no power during this time. If the data remains 0 for too long, the tag will power off and need to be restarted, a situation not likely to be conducive to reliable operation. Even when some binary 1s are present, the power level delivered to the tag is strongly data dependent, an undesirable trait.

A common solution to the power problem is to code the binary data prior to modulation. One RFID coding approach is known as pulse-interval encoding (PIE). A binary '1' is coded as a short power-off pulse following a long full-power interval, and a binary '0' is coded as a shorter full-power interval with the same power-off pulse (Figure 7). The resulting coded baseband signal m(t) is then used to modulate the carrier (Figure 8). PIE using equal low and high pulses for a 0 ensures that at least 50% of the maximum power is delivered to the tag even when the data being transmitted contains long strings of zeros, and if the high is three times as long for a '1', a random stream of equally mixed binary data will provide about 63% of peak power. Note that in this case, the data rate becomes dependent on the data: a stream of binary 0s will be transmitted more rapidly than a stream of binary 1s. A single symbol has two features--the off-time and on-time--but still conveys only one binary bit. (This scheme is used in EPCglobal Class 1 Generation 2 readers. Other passive RFID standards use slightly different coding schemes, all generally characterized by the desire to have the reader power on as much as possible to power the tag.)