David Donovan is the president of the Association for Maximum Service Television
, an industry group which has worked closely with NAB and others on the white space controversy, among other key issues. Donovan recently spoke with HD Notebook:HD Notebook: Why should broadcasters (and consumers) be concerned with the current battle over white spaces, especially when it comes to future HD transmissions?Donovan:
The fundamental issue is interference to new digital television receivers and the government-subsidized digital-to-analog converter boxes. Placing millions of these unlicensed devices on TV channels will result in permanent, irreparable damage to the TV band. With digital, even small amounts of interference can cause a picture to go from perfect to unwatchable. There are a few interference concerns.
The first is co-channel interference caused by an unlicensed device that’s turned on to an occupied TV channel inside the station’s contour. This will occur when the ‘sensing’ device contained in the unlicensed device fails to accurately detect the existence of a TV signals. This is the classic hidden-node problem. Microsoft’s and Philip’s proposed sensing level of -114 db will not be sufficient in these situations. You’ll find many ‘holes’ inside a station’s service area where the device will be fooled, and falsely conclude that no signal is available. As a result, the device turns on to the same channels that consumers are trying to watch. Because this is co-channel interference, the harmful impact from a 100-milliwatt device can extend for kilometers and even miles.
Even if sensing works, placing these devices on first- and even second-adjacent channels will cause interference to DTV receivers. Data from the Communications Research Center Canada and the FCC demonstrates clearly that adjacent-channel interference is a real concern. Depending on the signal strength of the broadcast stations, this adjacent-channel interference can extend for up to 80 meters from a digital TV set. Finally, out-of-band interference has a range of about 75 feet.
The bottom line is that consumers will have no idea what is causing interference to their new digital television sets. The interference could be coming from the apartment next door or down the street. There is not way to police this situation. Once the interfering unlicensed devices are in the hands of consumers, you cannot recall them. The federal government is about to ask consumers to spend billions of dollars in new digital receivers to meet the 2009 transition date. At the same time, Microsoft is saying to the FCC ‘trust me’ and asking the government to let it ‘experiment’ with millions of digital television sets. Nothing could be more damaging to the digital transition than injecting this type of interference into the TV band.”HD Notebook: Yet Microsoft said some of the test results for white-space interference were actually the result of faulty prototype devices used in the testing.Donovan:
I find it ironic that after years of pushing for unlicensed devices, Microsoft delivers a faulty device to the FCC. Imagine what will happen if millions of faulty devices end up in the market. If the FCC engineering team can’t tell if the sensor is broken, what will the average consumer do? The problem is the ‘sensing’ device failed, but the transmitter kept on transmitting. The Philips device was never ‘tested’ outside the lab.
Even if the sensing device ‘worked as advertised,’ it would still cause interference to consumers’ television receivers. The data clearly show that the sensing threshold of -114 dBm is not sufficient to protect TV reception. We understand it’s the best they can do, but it’s not enough. IEEE 802.22 recognized this problem and shifted to a geo-location system—not spectrum sensing—as the primary interference avoidance mechanism. IEEE is correct.
Look at Microsoft’s new data from Los Angeles. Microsoft claims that its device was able to detect a TV signal 100-percent of the time. However, if you look at the data carefully, the device failed to detect a broadcast signal inside the station’s service area at six of 11 sites. In fact, there were 122 instances where the device failed to detect a TV signal at a specific location, but there was a perfectly viewable TV signal nearby. In some cases, the data indicate that signal strength varies significantly from one room to another. Again, this means the device—thinking that the TV channel is vacant—will turn on. The result is co-channel interference to surrounding TV sets. Bottom line, the new Microsoft data proves our point: Sensing will not protect TV receivers from interference.HD Notebook: The FCC has agreed to additional testing, yes?Donovan:
While parties are invited to observe, the FCC has not yet released a test plan [as of Oct. 16]. How many locations will be tested? Will it look at a variety of different terrains or just DC? In fact, the White Space coalition will not be required to provide a complete workable prototype that both senses and transmits. The FCC will just test a sensor. I’m not sure what this proves. We have spectrum analyzers in my office that sense TV signals. Moreover, the key issue is to determine whether there are ‘holes’ inside a station’s service area with signal levels below -114 dBm. We did a quick analysis in Washington and found that there are locations in D.C. that have levels well below -114 dBm. Microsoft’s own data in L.A. proves this fact.
Compounding the problem is the process. In the unlicensed world, the FCC drafts rules that will govern the manufacturer of unlicensed devices. Once rules are drafted, then the FCC has a certification process which insures the devices have been manufactured consistent with the FCC rules. Generally, there is no independent testing in the certification process to determine whether a specific device or model causes interference. Again, the only question is whether the device has been built consistent with the rules. This means you have to get the rules right from the start.
However, if you are not testing actual devices that will be marketed, how do you know how to write the rules? For example, if you are testing incomplete devices that are just sensors and not actual devices that transmit, you may not know enough to write a rule. In short, the FCC will be guessing. This approach may be fine where you are talking about interference from devices to microwave ovens, but it will not work for video. Nearly 50 million DTV sets have been sold so far, and another 60 million will be sold by 2009. The stakes are too high to guess.”