What Are White Spaces?

White spaces, in the context of radio frequency spectrum, are portions of licensed broadcast spectrum that are unused or unoccupied in a given geographic area and at a given time. The term refers most specifically to the television broadcast bands in the VHF and UHF range (roughly 54 to 862 MHz), where spectrum that was formerly occupied by analog television stations has become available following the digital television transition. Because television channels are not licensed everywhere simultaneously, substantial swaths of spectrum sit idle in rural and suburban areas, and even in densely licensed cities there are gaps between occupied channels. Spectrum regulators and the wireless engineering community have devoted significant effort to enabling unlicensed secondary use of these bands without causing interference to incumbent broadcasters.

TV White Spaces and Spectrum Availability

The digital television transition, completed in the United States in 2009, dramatically increased the amount of available spectrum in television bands by compressing the same number of broadcast services into a smaller set of channels. The remaining unoccupied channels, the TV white spaces (TVWS), constitute a geographically variable resource: a rural area near no broadcast transmitters may have dozens of vacant channels, while a dense urban market may have only two or three. The Federal Communications Commission's white space rules allow unlicensed devices to operate in these bands provided they do not cause interference to licensed incumbents. To achieve this, devices must use geo-location databases that specify which channels are available at a given coordinate, and must query those databases before transmitting. The FCC maintains a list of accredited database administrators who provide this spectrum availability information in real time.

Cognitive Radio and Dynamic Spectrum Access

The underlying technology enabling white space exploitation is cognitive radio: a radio system capable of sensing its spectral environment and adapting its operating parameters, including frequency, power, and modulation, in response to what it detects. Spectrum sensing allows a cognitive radio to identify whether a TV channel is occupied by either a licensed transmitter or a co-channel unlicensed device before attempting to use it. The KNOWS (Cognitive Radio Networks Over White Spaces) research program, published in IEEE conference proceedings, was among the early demonstrations of cognitive radio operation across TV white space channels, addressing the key engineering challenges of sensing threshold, sensing time, and the hidden terminal problem. Dynamic spectrum access based on cognitive radio extends beyond TVWS to other partially occupied bands, including the CBRS band at 3.5 GHz, where a tiered authorization framework similarly uses database management to coordinate spectrum sharing.

IEEE 802.22 and Regulatory Framework

The IEEE 802.22 standard defines an air interface for Wireless Regional Area Networks (WRANs) operating in TV white space spectrum. The standard, developed by an IEEE working group beginning in 2004, specifies a physical layer based on orthogonal frequency-division multiple access (OFDMA) and a medium access control layer with provisions for spectrum sensing, dynamic frequency selection, and coexistence with other white space users. A base station complying with IEEE 802.22 can serve customer premises equipment at distances up to 100 kilometers using propagation characteristics favorable in the VHF and UHF bands, making it a candidate technology for broadband access in rural areas where fiber and cable are uneconomical. Research reviewed in the survey of cognitive radio access to TV white spaces evaluates the standard's sensing performance requirements and the regulatory frameworks adopted in multiple jurisdictions, noting that both the United States and the United Kingdom moved to conditional authorization well ahead of formal IEEE standardization.

Applications

White spaces have applications in a range of fields, including:

• Rural broadband connectivity in areas lacking cable, fiber, or cellular infrastructure
• Internet of Things (IoT) networks benefiting from the long range and building penetration of sub-1 GHz bands
• Smart agriculture, enabling sensor and control networks over large agricultural properties
• Disaster recovery communications when licensed infrastructure is damaged or unavailable
• Research testbeds for dynamic spectrum access algorithms and cognitive radio protocols