Broadcast to Broadband

Although much public attention has focused on the US digital TV transition -- and the resulting reallocation of analog TV channels by auction to wireless carriers -- the US Federal Communications Commission will decide how to reallocate an even larger swath of prime TV band spectrum this year: the unused “white space” between occupied DTV channels. This reallocation of unused spectrum from broadcasting to broadband permits unlicensed access for both fixed and mobile applications.

In 2002, the FCC’s Spectrum Policy Task Force recommended that spectrum efficiency and innovation could be enhanced by reallocating unused and underutilized spectrum for both flexible-use licenses (by auction) and opportunistic use by smart-radio devices capable of sharing bands on an unlicensed basis.

Shortly after this report, the FCC surprised almost everyone by issuing a Notice of Inquiry asking for comment on the feasibility of reallocating the TV white space for advanced wireless services. In May 2004, under then-chairman Michael Powell, the FCC issued a Notice of Proposed Rule Making (NPRM), tentatively concluding that opening at least some of the vacant channels for unlicensed access would be feasible and beneficial.

Although this policy is strongly supported by high-tech companies and consumer advocates, it’s just as strongly opposed by broadcast licensees and other incumbent users of the TV band.

The opposition is primarily led by the band’s incumbent licensees -- the US National Association of Broadcasters (NAB) and wireless microphone makers and users (such as the sports leagues and entertainment venues that intermittently use white spaces). The NAB refers to white spaces as “interference zones,” and broadcasters agree because they feel mobile broadband devices, even operating at low power, would inevitably interfere with DTV reception and microphone systems. The NAB and its engineering affiliate, the Association for Maximum Service Television (MSTV), have filed studies showing that Wi-Fi-type devices could cause both co-channel interference (if they can’t reliably detect a DTV transmission in a given location) and adjacent-channel interference (the unlicensed device’s transmission could leak and disrupt DTV reception).

In contrast, high-tech firms and consumer groups have argued that a variety of proven technologies are capable of avoiding interference with DTV reception. Intel, Dell, Microsoft, Google, and other members of the high-tech White Spaces Coalition argue in their filings that because proven technologies exist for detecting and avoiding DTV signals, barring mobile devices represents an enormous loss for consumers and potential broadband innovation.

For its part, the FCC has consistently assumed that several technologies are capable of ensuring that white-space devices can identify vacant channels in a local market and operate without interfering with DTV reception on neighboring channels (or in neighboring media markets). In its 2004 NPRM, the FCC’s Office of Engineering and Technology (OET) described three different approaches that white space devices could use to avoid such interference:

• Geolocation / database. In this scheme, the unlicensed device (such as a wireless ISP access point) would use a GPS receiver to cross-check its own location against an online database of licensed TV transmitter locations.
• Beacon permission. An alternative approach, aimed at allowing networks of mobile devices, would require that an unlicensed device not transmit unless it receives a “green light” signal indicating that the specific channel is free to use in that locality.
• “Listen-before-talk” sensing. Another approach strongly favored by the high-tech company coalition would require that each individual device scan and identify unused TV channels before transmitting (with the accuracy enhanced by sharing sensing data with other devices in range)...