NCTA today filed a letter with the FCC "to focus the commission's attention on the significant potential for harmful interference" that would affect cable services if the current proposals for unlicensed and licensed use of the broadcast "white spaces" are implemented.

While saying that NCTA support the efforts to use the white spaces for delivery of new wireless services, the letter outlined concerns with three primary areas — interference with TV receivers, interference with headends and interference from fixed line use — that it said must first be resolved.

It said: "Of the various proposals and ex partes filed in this proceeding over the past year, none address the implications of TV band devices operating on the same frequencies used by cable. And, in particular, none address the potential disruption of customer viewing of cable programming. This inattention has persevered despite detailed filings by NCTA demonstrating the high likelihood that unlicensed TV band devices, as currently proposed, will interfere with cable service — and despite the commission's own findings in lab tests a year ago confirming the serious risks of 'direct pickup' (DPU) interference to cable-ready DTV receivers." 

In addition to the problem of direct pickup interference to television receivers, the NCTA letter said that the proposed unlicensed TV band devices "pose a significant threat to cable's reception of distant over-the-air television programming at headends. Indeed, cable headends are more susceptible to interference from TV band devices given the fragile nature of the distant broadcast station's signal at the headend receive site. But in every case where interference occurs, the broadcast programming will be wiped out for the entire community served by that cable system."

NCTA also claimed that proposed licensed, fixed uses of the spectrum "present far greater risks of harmful interference to cable customers. FiberTower Corp. and the Rural Telecommunications Group propose installing fixed antennas with an effective transmit power of over 3 kW (+65 dBm) in rural communities. Under this proposal, for example, consumers' cable-ready television sets would be adversely affected from distances as far as three miles away," it said.

Should the FCC proceed with authorizing unlicensed TV band devices, however, NCTA also suggested six requirements in its technical rules to ensure that the devices to do not interfere with delivery of cable services to consumers:

• Restrict the operation of portable devices to a maximum of 10 mW and prohibit transmissions in the VHF channels given the high probability of direct pickup interference to TV receivers.
• Prohibit operations, at a minimum, on channels 2- 4.
• Restrict the operation of fixed devices to at least 400 feet from the external walls of residential buildings.
• Prohibit operation of fixed devices in VHF channels.
• Require spectrum coordination before operation of portable devices on channels adjacent to those being received at headends.

Of the suggested methods by which fixed and portable devices might automatically determine channel availability, it appears that auto-location (GPS or equivalent), combined with regular access to a reliable database containing geographically-indexed lists of available channels, has the potential to provide the flexibility and reliability required to protect headend reception.