An important fact that has been lost in the debate about opening unused television channels (TV white spaces) to white space devices is that the Pentagon has already approved the unlicensed sharing of military spectrum in the 5 GHz band for devices utilizing similar spectrum sensing technology proposed for white space devices in the TV band. In the upper part of the 5 GHz band, wireless devices are required to dynamically scan the airwaves and avoid operation on spectrum utilized by radar systems. The experience of the 5 GHz band clearly demonstrates the ability of the FCC, device manufacturers, and primary users to develop technological standards and requirements that allow spectrum to be shared on an unlicensed basis without causing interference to critical uses such as military radar.

Unlicensed Access in U.S. Military Radar Spectrum (5 GHz Band)

When considering opening the upper part of the 5 GHz band (5470 5725) for use by wireless network devices, it was imperative to the National Telecommunication Information Administration (NTIA) and the Pentagon that products entering the new band would protect both fixed and mobile U.S. military radar systems.

• Importance of Preventing Interference. Radar is highly susceptible to interference even from a wireless device operating at very low power. A radar system transmits a powerful set of pulses into an environment. When these pulses strike an object, their energy is scattered and a small amount of that energy is bounced back towards the transmitting station. The radars receiver then measures the time difference and the frequency shift information to calculate distance and other information about the object it struck. A wireless device operating on the same frequency as the radar or nearby to a radar antenna would show up on the radars display, with the devices wireless signal producing streaks and other irregularities that severely inhibit the ability of the military user to accurately assess the environment.

• Developing Technical Standards. In 2003, the FCC opened a proceeding to allow unlicensed wireless local area network (WLAN) devices to operate in the military radar band (5470 5725). Given the increased security threats after the 9-11 attacks, the Pentagon required devices to protect sensitive radar systems that could pick up small boats or planes (typically smaller and less reflective targets) out of background clutter as well as detect stealth aircraft that can hide from most conventional radar systems. Over the next three years, working together with the Pentagon, NTIA and the wireless networking industry, the FCC developed an extended set of technical requirements that would ensure wireless equipment protect radar systems operating in the band. In June 2006, the FCC released the final set of technical standards required for device certification.

• Listen before Talk, Dynamic Frequency Selection (DFS). The FCCs technical standards require a device operating in the military radar band to utilize dynamic frequency selection (DFS) to detect radar systems and avoid utilizing the radars operating frequency. Wireless devices operating at a power level of 200 mW (twice the proposed power level of 100 mW for mobile white space devices in the TV band) must be able to detect radar signals of -62 dBm and higher power devices must be able to detect radar signals at -64 dBm. If a device senses a radar signal it must leave the frequency it has detected the signal on. Within 10 seconds, the device must cease all transmissions on that frequency.