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10.17.2008 12:00AM
FCC Report Reveals Likely White Space Device Interference Problems

(From Doug Lung's RF Report)

The FCC Office of Engineering and Technology released its Evaluation of Prototype TV-Band White Space Devices – Phase II earlier this week.

Don't judge the results of the testing by the first paragraph of the Executive Summary, which states: "At this juncture, we believe that the burden of 'proof of concept' has been met. We are satisfied that spectrum sensing in combination with geolocation and database access techniques can be used to authorize equipment today under appropriate technical standards and that issues regarding future development and approval of any additional devices, including devices relying on sensing alone, can be addressed."

A careful reading of the Report, especially the field tests, indicates there are significant problems to be addressed with devices that rely on sensing alone and that even operation on channels not being used for broadcasting or wireless microphones could cause problems for people viewing cable TV, where there are no white spaces.

The last paragraph in the Executive Summary states:

"Anecdotal tests were performed at two field sites to assess the interference potential from WSD transmitters to cable television reception via direct pick-up of signals by cable system components. These tests showed that under certain circumstances, when the transmit antenna was placed in close proximity to a cable connected TV, direct pick-up interference was observed. The direct pick-up interference potential appears to be highly dependent on the interconnection among various system components (e.g., cable amplifiers, splitters and set-top boxes) being used."

Curing the interference may be easy for white space device users—they can turn it off or move it to channels they don't watch. Of course, if they are in a condominium or apartment building that won't help their neighbors that do want to watch that channel. The problem will become worse as more white space devices are turned on.

I've outlined the problems with detection of TV channels and wireless microphones in previous articles. The Executive Summary confirms this problem:

"Wireless microphone sensing tests were performed using the I²R and Philips devices at two field locations. The tests were conducted first with microphones off, and then turned on, in pre-determined channels to determine if the devices could sense the presence of wireless microphones. At both sites and all the test locations, the Philips device reported all the channels on which the microphones were designated to transmit as occupied whether the microphone was transmitting or not. The I²R device indicated several channels as available even when the microphones were on."

Motorola's devices using geolocation and a database to determine the channels available for use worked much better. Provided the database is kept up to date, the devices can't be modified to transmit on channels blocked by the database, and if the geolocation is accurate, the Motorola system appears to adequately protect over-the-air TV viewers and wireless microphone users. The power level of devices likely to be used in the home or near cable TV set-top boxes or wiring will have to be limited to prevent them from causing interference to cable TV reception.

How bad is the cable TV interference?

At Field Test Site 3 (residential), interference was noticed when the Adaptrum white space device antenna was placed near the TV or wiring. The report includes this description:

"The prototype transmitter was tuned to channel 21 and activated at full power (no external attenuation). The picture quality on the cable channel was observed to be significantly degraded. The Adaptrum prototype provides for a variable transmit bandwidth. Using that feature, the transmit bandwidth was reset from 4.5 MHz to 6.0 MHz and the demonstration repeated. Interference to the TV became noticeably worse.

The cable service signal path was changed to include the existing cable routing (including the distribution amplifier, etc., but without the converter box) and the test was repeated. Again, interference was observed on the TV receiver and was also noted on an analog TV in an adjacent room, which was tuned to Channel 73, in the form of a complete loss of picture."

Adding an attenuator to drop the power of the WSD to 7 dBm EIRP reduced the interference to "barely perceptible" on either TV.

Digital cable reception was more severely impacted:

"The cable converter (set-top) box was inserted into the signal path and connected to the DTV's tuner input. The set-top box was tuned to virtual Channel 220 (cable Channel 77) and a good quality digital picture was observed. The Adaptrum transmitter was tuned to Channel 26 and activated at full power (no external attenuation). Interference was immediately observed in the form of a complete loss of picture.

The variable attenuator was incrementally increased to reduce the radiated power in 1 dB steps. At a reduction of 18 dB (4 dBm EIRP) the picture reappeared. The attenuation in the cable signal path was removed and the test repeated. Picture loss was again observed at a transmit attenuation setting of 12 dB (10 dBm EIRP)."

The FCC engineers were able to solve the interference problem by replacing all of the existing routing from the cable service wall outlet to the converter box (including amplifier, splitters and cables) with a "laboratory-grade patch connecting the wall outlet and the converter box input." Even with the Adaptrum transmitter operating at full power, when the laboratory grade cable was used no interference was observed until the transmit antenna moved within 0.3 meters of the converter box.

Cable interference was also observed at another field test site.

Interference to off-air reception depends on many factors, but the report states that testing showed "a DTV receive system tuned to a weak DTV channel can experience interference at significant separation distances (data extrapolation indicates up to 1.2 km) from the WSD transmitter when it is radiating a signal at approximately 150 mW EIRP."

The FCC has an order—actually a Second Report and Order and Memorandum Opinion and Order—on white spaces on the agenda for a vote on its Election Day open commission meeting.

My interpretation of the results in the Report is that any WSDs, unless operated at very low power, are likely to cause problems with cable TV reception, especially in multiple-tenant dwellings. Based on the Report, devices that depend on sensing clearly don't work and it isn't clear, given the physics involved, that they can ever work. A strictly controlled WSD system using geolocation and an accurate database should be able to protect over the air TV reception and channels designated for wireless microphones, but that may not help cable TV viewers. To protect reception at cable TV headends and TV translator sites, the database will have to include the distant signals being received at these locations.



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