As discussed in Chapter 1, ultra-wideband technology can deliver a large amount of data with low power spectral density (PSD) due to the modulation of extremely narrow pulses. The brief duration of UWB pulses spreads their energy across a wide range of frequencies, from near DC to several gigahertz. This spread of energy enables UWB signals to share the frequency spectrum with existing radio services and avoids expensive frequency spectrum licensing fees. Although UWB communications offers a promising solution to the problem of an increasingly overcrowded frequency spectrum, the overlay of UWB signals onto existing narrowband systems raises interference concerns for incumbent radio services. Despite the fact that UWB signals have low power spectral densities and reside under the noise floor of most narrowband receivers, the main concern of coexisting spectrum users is that the mass deployment of such devices can potentially raise the noise floor to a harmful level. Figure 4-1 illustrates the overlay of UWB devices with some existing radio services, based on the FCC's preliminary approved emission limits for UWB communications.
Figure 4-1. Overlay of UWB devices with some existing spectrum users
In this chapter, we discuss the potential impact of a UWB mass deployment on other spectrum users, including 802.11a and 802.11b wireless local area networks (WLANs), Global Positioning System (GPS) receivers, and CDMA-based cellular systems. Among these services, GPS is a key concern, because of its applications in life safety and military operations. However, the coexistence of UWB with WLAN devices and cellular systems is equally important to commercial consumer sectors.
The interference discussion presented in this chapter is based on analytical approximations. The analysis involves minimal mathematics and concentrates more on the results obtained from the final interference equation in each case. (We discuss the mathematical derivations of the formulas in great detail in Appendix A.) The discussion here adheres to the emission limits for UWB transmitters in the United States (based on existing regulations by the FCC) and in Europe (based on proposed regulations by the European Telecommunications Standards Institute [ETSI]). We explain the impact of interference from the aggregate effect of UWB devices on each narrowband victim receiver (802.11a/b, GPS, and cellular systems) as a function of UWB transmitter density. As noted, the material presented in this chapter is based on analytical studies; many factors, including the type of modulation scheme, can change the results obtained in real application scenarios.