ed in using guard bands between these nar- rowband radio channels to ensure non-interference. In comparisons of the capacity of narrowband radios versus spread-spec- trum radios, the same bandwidth must be used. It seems obvious that nar- rowband radios would provide more overall capacity. Such a judgment, however, considers capacity only as measured by number of users per bandwidth. In fact, in most practical applications, the real criterion is the number of users in a given area. We see from our discussion of Figure 3.1 that, with interference, spread-spectrum radios can operate over a wider area than a corresponding narrowband radio. Using the same bandwidth as a spread-spectrum radio, several narrowband radios can operate in dif- ferent frequency bands. These radios, of course, will have some out-of-band emissions, and their band separation will depend on how effectively this is minimized. In practical applications with complex propagation conditions, it is not clear whether narrowband radios or spread-spectrum radios with a fixed total bandwidth will give the overall highest number of users in a given area. The FCC Part 15 rule has been adopted in part or completely by many other countries. Generally, North, Central, and South American countries have adopted this same rule. Other countries in which European cellular systems occupy parts of the 902-928 MHz band have adopted modified ver- sions of this rule. Most countries worldwide allow some form of unlicensed The Digital Cellular CDMA Standard spread-spectrum radios for the commercial applications mentioned in this chapter. 3.3 THE DIGITAL CELLULAR CDMA STANDARD Based partly on its two-way satellite messaging and position reporting sys- tem OmniTRACS [16], Qualcomm developed a spread-spectrum radio sys- tem for digital cellular phone applications [15, 16, 18]. Unlike traditional multiple-access techniques used with conventional narrowband radio signals, Qualcomm's CDMA system employs spread-spectrum signals. Its develop- ment helps account for the considera