inually increasing demands on limited spectrum resources, coupled with the observation that, even in those parts of the RF spectrum that are fully allocated, the bandwidth is typically in use only 10-20% of the time. Following on from this review, the FCC issued a so-called notice of proposed rulemaking (NPRM) in May 2004, proposing to open up the 76-698 MHz portion of the TV spectrum in the US for unlicensed usage. This ruling would allow wireless networks to make unlicensed use of the unused "white space" in these licensed TV broadcast bands. Two types of devices are permitted to make unlicensed use of these bands under this ruling, as shown in Table 14-4. This opportunity led to the start up of the IEEE 802.22 Working Group in October 2004, which aims to develop MAC and PHY layer specifications for wireless regional area networks (WRAN) to operate in unused VHF/UHF TV bands between 54 and 862 MHz. Spectrum agile radio is the main enabling technology for the 802.22 work. Table 14-4: FCC Permitted Devices for Unlicensed Operation in TV Bands Device type Characteristics Fixed devices Maximum transmit power of 1W Must be either professionally installed to operate in locally unused channels or fitted with GPS and a means of determining which channels are locally vacant. This could be an Internet connection to a Spectrum Policy Server which provides location base spectrum coordination. Mobile devices Maximum transmit power of 100 mW Must receive a control signal from a device that determines which channels are vacant. Leading Edge Wireless Networking Technologies Spectrum Sharing Approaches and Challenges Approaches to spectrum sharing are broadly categorised as either vertical or horizontal. Vertical, also known as primary, sharing occurs when a channel is identified that is not being used - a so-called "white space" - while horizontal, also known as secondary, sharing is the efficient coexistence of two or more networks simultaneously operating in the same spectrum. Horizontal sharing relies either on rule based c