3.2.5 Results of the Part 15 Rule In response to the FCC Part 15 ruling allowing unlicensed operation in selected radio bands, many new wireless applications have emerged for spread-spectrum radio. Without licensing, however, there is no protection against interference from other unlicensed users. Nevertheless, spread- spectrum radios inherently possess a certain degree of protection against interference (10 dB is the minimum processing gain). As Dr. Marcus pre- dicted, the FCC Part 15 rule has been a catalyst for innovative wireless applications and has stimulated the development of many new forms of low- cost spread-spectrum radios. Companies can offer wireless products that can be used by purchasers immediately, without their having to wait for a license. Perhaps the best protection for spread-spectrum radios is their inherent robustness against interference and large multipath delays. Figure 3.1 illus- trates this by showing a transmitter and an interfering (jamming) radio in the same band transmitting with the same power. The bit error rate at a receiver depends on the signal-to-jammer ratio SIJ, which depends on the location of the receiver relative to the transmitter and the interfering sig- nal. Typically, for a good narrowband radio, acceptable bit error rates can be achieved with a signal-to-jammer ratio, SIJ, between 10 and 20 dB, depend- ing on whether voice or data is being transmitted. For a good narrowband radio operating with a minimum SIJ of 10 dB, the receiver can operate at the region shown in the shaded area of Figure 3.1. The FCC requires that direct-sequence spread-spectrum radios have a minimum processing gain of 10 dB. With a 10-dB processing gain, the required SIJ = 10 dB of a narrowband radio becomes the required SIJ = 0 for the spread-spectrum radio. Such a spread-spectrum radio can receive signals as shown in the left halfplane of Figure 3.1. With more pro- cessing gain, this area of reception can be increased with additional band- width. This figure assumes omnidirectional antennas in