3.2 THE FCC PART 15 RULES
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 S/J, 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,S/J, between 10 and 20 dB, depend- ing on whether voice or data is being transmitted. For a good narrowband radio operating with a minimum S/Jof 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 requiredS/J⫽10 dB of a narrowband radio becomes the required S/J⫽ 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 free space. Using an antenna with additional gain, the intended receiver will dramatically increase
Figure 3.1.Jamming power versus processing gain.
the region of operation in this interference environment. The operation region is defined by the S/Jmeasured at the antenna output of the receiver.
This S/J depends on the antenna gain in the direction of the transmitter (determinesS) and the antenna side lobe attenuation of the receiver antenna in the direction of the jammer (determines J). Here, the region of operation will be a complex region determined by the antenna pattern of the receiver.
If the transmitter and jammer also have directional antennas, then this region will be complicated by all three antenna patterns.
For fixed locations for the radios, antenna polarization can be used to separate signals. Normally, for narrowband signals, antenna polarization may not provide adequate isolation when radios are located in the same area. Since spread-spectrum signals have additional protection against interference when different codewords are used, antenna polarization can be used very effectively to increase capacity. Spread-spectrum radios allow for a much more flexible application of antenna techniques for increasing capacity.
It is clear that the determination of the number of radios that can work effectively in a given area is a very complex matter. Spread-spectrum radios are less sensitive to interference and can operate with smaller S/Jthan con- ventional narrowband signals. These signals, however, require more band- width. Narrowband radios may be able to use this same band by creating many non-interfering frequency channels. Typically, up to 20 percent of this frequency band may be wasted 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 practicalapplications, 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
spread-spectrum radios for the commercial applications mentioned in this chapter.