In fixing the problem with transmitted power by replacing OOK with PIE, weve made another problem worse. Radio waves travel everywhere, so in some sense the radio medium is shared between various users. For example, I would like to be able to read tags on packages in my storeroom despite the fact that the storeroom is also illuminated by the local broadcast radio and television stations, cellular phone basestations, the radio link from the taxi across the street, and the satellite downlink to the neighborhood cable TV system. Using a single medium for many signals is known as multiplexing. The most common form of multiplexing in radio, in use for almost a century, is frequency-division multiple access (FDMA): different users transmit using different carrier frequencies, and receivers are adapted to capture only the frequency of interest. (Signals can also be multiplexed in time and in coding. In RFID, time multiplexing is used when a reader uses an anticollision algorithm to poll tags one at a time; see Chapter 8 for more details.) We will discuss the means used to filter the desired frequencies from the received signal in more detail in Chapter 4; for the present, it suffices to know that this operation can be accomplished. An RFID reader transmits on a frequency within the band at 902-928 MHz (in the United States), and listens to responses only within that band, rejecting the AM radio broadcast at 1 MHz, the television transmission at 52 MHz, the cellular transmission at 874 MHz, and so on.

This scheme would seem to allow an unlimited number of users to share the electromagnetic spectrum. However, recall that a signal must be modulated in order to convey information. When we modulate the signal, we increase the signal bandwidth. We saw an indication that this would be so in examining analog sinusoidal modulation of a signal (Figure 5). A modulated signal occupies a finite region of frequency, and neighbors must be separated by something like that amount in frequency to avoid interference.

Furthermore, choices we make in modulation affect how much bandwidth we use. For example, if we modulate the signal faster by making the individual symbols take less time--that is, if we increase the data rate--we use more bandwidth. This phenomenon is illustrated in Figure 9¹, where we show the power spectrum of a modulated signal, and we have made use of the dB notation for spectral power introduced in Section 3.2 above. The spectrum has its largest power near the carrier frequency fc, but a considerable amount of power is transmitted at frequencies rather far from the carrier, as we might have suspected from Figure 3.5 above. The distance from the carrier frequency to the first major 'dip' in the spectrum is inversely proportional to the symbol time t that is, it is the same as the data rate R = 1/t for OOK. The shorter the symbol time, the faster we can send data, but the more bandwidth we use.

How we send symbols also matters. An abrupt step at the edge of each symbol gives more power far from the carrier than a smooth transition between low and high power states, as depicted in Figure 10. (Note that the residual power shown far from the carrier for the smooth symbols in this figure is affected by the specific method of smoothing the symbol and the accuracy of the numerical model.) Of course, the ability to smooth the transitions is limited by the duration of the symbols: at some point, changes happen so slowly that fully on or fully off states are never reached, causing the transmitted power to fall (and become data dependent). Smoothing the signals also makes the receivers problem harder. It doesn't really matter when you test the voltage of a signal like that in left side of Figure 10 as long as you are within the symbol, but the smoothed signal on the right side is best sampled exactly at the center of the symbol, where the power is either at its maximum value or nearly zero. Sampling at any other times will result in more power for a nominal 0 or less power for a nominal '1': that is, the measured modulation depth is reduced. Thus, the receiver needs to do a better job of synchronizing with the incoming signal if that signal is smoothed.

Finally, the way we code the signal also matters. By examination of Figure 6 and Figure 8, we can see that pulse interval encoding will result in shorter pulses than OOK for the same data rate, so from Figure 9, it seems likely that PIE would have a wider spectrum than OOK for the same data rate. This expectation is confirmed in Figure 11: substituting a stream of PIE symbols at the same average data rate for OOK symbols results in reduced power very near the carrier, but more power far from the carrier. In particular, a strong, narrow emission is seen at a frequency which turns out to correspond to (1/duration of a binary '0'); as depicted by the inset in the figure, the strong resemblance of a '0' symbol to a sine function results in a concentration of power at the corresponding frequency. The more diffuse band at half this offset results from the binary '1' symbol.