A black cloud over white spaces

On Nov. 14, 2008, the FCC released a Report and Order (R&O) that allows unlicensed broadband radio transmitters to operate on vacant television channels. The unused television channels are called “white spaces,” and the radio devices are called white-space devices or WSDs. The FCC permits white-space devices to be marketed starting Feb. 18, coinciding with the digital television cutover deadline. The FCC decision is important because it frees up a vast amount of spectrum for Wi-Fi and similar devices; however, it also risks harmful interference to television receivers, wireless microphones, and land mobile radios operating between 470 and 512 MHz.

To better appreciate the FCC white-spaces decision, it helps to understand the digital television (DTV) transition period that started in 1996 and will end on Feb. 17, 2009. After a technology runoff and several field trials, the FCC adopted a mandatory DTV format that achieves high-definition picture quality in a 6 MHz bandwidth. Unlike the NTSC color television standard that was adopted in 1953, the FCC DTV standard is not backward-compatible with analog television. Thus, new spectrum was required to accomplish a DTV transition with minimal disruption to the public.

In 1996, Congress authorized assignment of an additional broadcast channel to each TV station so they could simultaneously operate a digital channel with their analog channel. Later, after several earlier deadlines were abandoned, Congress mandated that Feb. 17 would be the last day for full-power television stations to broadcast in analog format. With only a handful of exceptions, full-power broadcast stations in all U.S. markets currently are broadcasting both analog and digital formats. After Feb. 17, full-power television stations will broadcast digital only, but low-power television stations and television translators may continue to broadcast analog signals for the foreseeable future.

In the U.S., broadcast DTV transmissions must comply with the Advanced Television Standards Committee (ATSC) format, which requires 8-Vestigial Sideband (8-VSB) modulation and a 19.4 Mb/s gross data rate. Today, digital video is transmitted using one or more forms of MPEG 2, and each 6 MHz-wide channel is capable of carrying one 16:9 aspect ratio high-definition television (HDTV) channel or several standard-definition channels. A standard-definition channel employs a 4:3 aspect ratio with resolution that is roughly equivalent to analog television.

Over the past 10 years, Congress and the FCC have made several key spectrum decisions that are affected by the DTV transition. First, the commission decided that after the transition is completed, only Channels 2-51 will be used for broadcast television. Channels 52-59 were auctioned to the highest bidder for various wireless services (including video to cell phones) and Channels 60-69 include spectrum that has been auctioned, spectrum that will be auctioned, and the 700 MHz public-safety radio band. In all cases, existing full-power television stations have priority and will continue operating on Channels 52-69 through Feb. 17.

There are 1738 full power television stations in the U.S., and all but 10 of these stations are simultaneously operating one analog and one digital transmitter. Digital television transmitters are required to attenuate their out-of-channel emissions to a greater extent than analog transmitters, which — along with other technical considerations — allowed the FCC to pack stations more tightly during the digital television transition period.

On Feb. 18 (or sooner in some cases), all 1738 stations will cease analog transmissions and transmit solely on their digital channels. The aggregate spectrum that will become immediately available is 10.428 GHz, but television channels are reused between markets provided there is adequate physical separation between transmitters, so a typical market will see perhaps a dozen new channels (72 MHz). In Denver, for example, there are 14 full-power stations that each will release one channel on Feb. 18.

The amount of new spectrum that will be available is comparable to the 2.4 GHz unlicensed radio band (83.5 MHz), but the frequencies (VHF and UHF) have much lower path losses than 2.4 GHz, making them highly desirable for wireless communications.

Of the decision, FCC Chairman Kevin Martin said that “opening the white spaces will allow for Wi-Fi on steroids.” But not everyone was happy with the decision. For instance, the National Association of Broadcasters (NAB) had been fighting the white-spaces decision for years and issued a statement that said in part that “every American who values interference-free TV should be concerned by today’s commission vote.”

The NAB has argued since 2004 that there is a potential downside to the white-spaces decision. White-space devices operate on vacant television channels, but the particular channels that are available vary by market, and new television services show up quite often. A WSD must be a cognitive radio, meaning that it must scan for authorized television channel emissions and transmit only on those channels that truly are vacant. This requirement presents a serious technical hurdle that has yet to be overcome.

Television channels in the U.S. are used by a variety of services, including full-power television, Class A television stations, low-power television, television translators, television boosters, wireless microphones, land mobile radio in 13 large cities (Channels 14-20 only), and radio astronomy. There is a pecking order among these services, but they all have a higher priority than WSDs, and a WSD is not allowed to cause harmful interference to any of these services.

The bulk of the FCC R&O laid out requirements for preventing interference from WSDs to other services, and stated that the commission “will act promptly to remove any equipment found to be causing harmful interference.” The non-technical requirements are listed in Table 1, while the key technical requirements are outlined in Table 2.

Before the FCC issued its R&O, its Office of Engineering and Technology (OET) conducted extensive testing of five WSD prototypes. These devices failed more than half of the tests, making the NAB, wireless microphone companies and public-safety agencies very nervous. It is not clear from the R&O what specific tests the FCC Laboratory will use to certify WSDs, so it is uncertain how reliable these devices will be in preventing harmful interference.

Some of the things that can go wrong include:

Interference to the WSD

As a consumer-grade device, a WSD’s receiver selectivity may not be sufficient to prevent receiver desensitization from strong television transmitters or land mobile radio repeaters (470-512 MHz). When the WSD is desensitized, it cannot reliably measure transmissions from authorized devices. Similarly, the devices’ out-of-band emissions, including intermodulation products, may be strong enough to cause harmful interference. There does not appear to be any requirement for intermodulation rejection for WSDs.

Interference to TV receivers

The WSD can fail to identify an active channel for several reasons. For example, the WSD may be located in a basement adjacent to a television receiver that employs a rooftop antenna, which is connected to a coaxial cable that may not provide adequate isolation. The WSD won’t detect the over-the-air television signal and may choose to transmit on that channel, causing harmful interference. Similarly, the WSD may create interference to a cable-connected television receiver. Cable television has no white spaces, as all television channels are used on cable.

Interference to wireless microphones

The FCC received numerous comments from wireless microphone users and manufacturers. Like WSDs, wireless microphones operate on vacant television channels, but the R&O requires that the WSD protect the wireless microphone from harmful interference. The key problem is that wireless microphones are portable devices and no database holds useful information on their location or the frequencies in use. Thus, the WSD must rely solely on its ability to detect wireless microphone transmissions, and OET testing showed that the prototype WSDs failed miserably at this task. For example, none of the tested devices could reliably detect the presence of wireless microphones when the WSD was operating on an adjacent channel because the adjacent channel television transmitter desensitized the WSD receiver.

Jay Jacobsmeyer is president of Pericle Communications Co., a consulting engineering firm in Colorado Springs, Colo. He holds BS and MS degrees in electrical engineering from Virginia Tech and Cornell University, respectively, and has more than 25 years of experience as a radio frequency engineer.

TABLE 1
NON-TECHNICAL REQUIREMENTS

TABLE 2
TECHNICAL REQUIREMENTS