r (up to 1 W) unlicensed spread-spectrum radios that have essentially the same power density (power per bandwidth) as unlicensed narrowband radios. Since spread-spectrum radios are less sensitive to interference (antijamming characteristic), it was felt that such signals might not interfere with each other any more than unlicensed nar- rowband signals with less power. Thanks to Marcus's efforts, in 1985 the FCC ruled to allow high-power spread-spectrum radios in ISM bands (Table 3.1). Frequency Hopping The maximum dwell time during each hop is 0.4 sec, forcing a minimum hop rate of 2.5 hops per second. Differences in the maximum instantaneous band- The FCC Part 15 Rules Table 3.1. FCC Part 15 rules. Carrier Frequency Total Bandwidth 902-928 MHz 26 MHz 2.4000-2.4835 GHz 83.5 MHz 5.725-5.850 GHz 125 MHz width and minimum number of hop channels for frequency hopping radios in each of the bands include the following: Minimum Number ISM Band Maximum Bandwidth of Hops 902-928 MHz 500 kHz 2.4000-2.4835 GHz 1.0 MHz 5.725-5.850 GHz 1.0 MHz Direct Sequence Direct-sequence radios have a minimum spreading bandwidth of 500 kHz, with a minimum processing gain of 10 dB, implying a minimum of 10 chips per data bit. The Part 15 rule was unusual in that it came from within the FCC rather than as a response to a request from outside the FCC. With a higher power limit and no FCC licensing required, commercial companies began to introduce a wide range of innovative spread-spectrum radios capable of communicating over greater distances than the earlier low- power narrowband unlicensed radios. Some spread-spectrum wireless appli- cations in these ISM bands are discussed in the following sections. 3.2.1 Indoor Applications Commercial spread-spectrum radios that took advantage of this FCC Part 15 rule were first used in indoor office applications. These range from sim- ple radios that link office equipment, e.g., several personal computers with a shared printer, to more complex wireless local area networks (WLANs). Spread-spectrum cordle