The circumstances have to be just right, but a new technical report commissioned by U.K. regulatory Office of Communications (Ofcom) finds that Long Term Evolution (LTE) services operating in the 800MHz frequency band can interfere with -- and possibly punch out -- cable services.

The 83-page report (PDF) details the results of field trials conducted in April 2010 undertaken at Virgin Media Inc.'s RF/optical test facility in Birmingham over a seven-week period, and involving nine set-tops and 12 cable modems. The tests checked the technical and regulatory conditions for fixed and mobile communications networks operating in the 790MHz-862MHz band.

The results showed that interference was present in seven of the STBs tested and in all of the modems (the test didn't name the suppliers). The big caveat: Problems occurred when an LTE handset was operating at the maximum permissible transmit power at a separation distance of one meter. So, it tended to occur when one was practically sitting on top of the cable device.

On top of that, picture failure occurred when the LTE signal and cable-TV signal were operating on the same frequency, meaning there's some cause for concern, but not of the Chicken Little variety as mobile service providers around the globe begin to ramp up deployments of LTE. (See CES 2011: Verizon Takes 10 With LTE, CES 2011: AT&T Accelerates LTE Push, Who's Bagged the Biggest LTE Deals? and Where in the World Is LTE?)

Some of the interference issues were tied to product designs, including those that had high numbers of rectangular holes in the cable device metal sheeting that allow unwanted frequencies to seep in and couple with sensitive circuitry.

Although major interference occurred only in limited scenarios, the report, conducted by U.K.-based Cobham Technical Services, also offered some remedies on how cable gear could be improved to be more resistant to LTE signals. Among the suggestions: Operators could insert a 5MHz-wide band between the cable TV and LTE center frequencies, employ additional filtering to the RF circuitry, or simply not use the 832MHz-862MHz band for cable services.

The latter could require some significant work by the cable operator, and the report does acknowledge that the remedies cited do not examine their commercial feasibility.

Concerns about the potential for interference between cable and wireless broadband services aren't all that new. The U.S. cable industry has previously expressed them over previous studies showing that wireless broadband services that tap unlicensed "white spaces" (vacant airwaves between over-the-air TV channels) can interfere with digital cable services and wireless microphones.

The Federal Communications Commission (FCC) approved the use of white spaces last year, setting aside two channels for wireless mics. However, the FCC removed a suggested requirement that white-spaces devices support automated sensing tech that can detect the signals of TV stations and microphones. (See FCC Opens Up TV White Spaces and Cable Worried About 'White Space' Tech.)

We've asked the National Cable & Telecommunications Association (NCTA) for its reaction to the Ofcom test results and if it has any similar concerns over LTE that it had previously with the use of white spaces.

— Jeff Baumgartner, Site Editor, Light Reading Cable