nologies that enable reconfig- urable system architectures for wireless networks and user terminals. SDR-enabled devices and equipment can be dynamically programmed in software to reconfigure the characteristics of equipment. In other words, a single piece of hardware can perform different functions at different times, depending on the software program- ming. A radio device can become a cellular phone, a GPS receiver, an amateur packet radio, or any other sort of radio transmitting or receiving device. SDR pro- vides an efficient and comparatively inexpensive solution to the problem of build- ing multimode, multiband, multifunctional wireless devices that can be enhanced using software upgrades. SDR architecture is based on a high-level generic model with specific func- tional blocks connected via open interface standards recommendations. The SDR architecture supports three specific domains: handheld, mobile, and base station (or fixed site). The software is implemented by controlling the characteristics of equipment/device subsystems through hierarchical and peer-level modules that support scalability and flexible extensions of applications. The military has successfully demonstrated SDR's abilities and will continue to push this technology to its limits. Commercial wireless products, as well, have Chapter 16 Emerging Wireless Applications slowly evolved toward SDR architectures. The SDR Forum (www.sdrforum.org) has been working closely with global standards bodies and other industry groups, such as Global Radio Standardization Collaboration (GRSC, previously known as RAST; www.rast.etsi.org), the 3G Partnership Project (3GPP; www.3gpp.org), and ANSI (www.ansi.org) to develop standards for bringing SDR to full commercial viability. Cognitive Radio What would be the ultimate handset? To many, it would be a device that could seamlessly switch call and transmission modes to whatever makes the most sense, given the location at a given time-sometimes cellular, other times Wi-Fi, and yet other times Voice over IP (VoI