Capital Currents - The LTE Interference Issue
There are no cable immunity standards.
I’ve written about the FCC’s white spaces tests that demonstrated interference to cable TV reception caused by unlicensed transmitters operating on unoccupied TV frequencies. Now even greater risks are on the horizon, namely direct pickup and ingress interference caused by cellular phone companies and LTE mobile technology. It’s a problem because the cable industry has no effective standards for interference immunity in cable modems and set-top boxes.
LTE is the term assigned to the next generation of cell phone technology, also known as 4G. What’s most significant is that this technology will be rolled out by some cell phone companies in the 700 MHz frequency band that was formerly used by TV broadcast stations. Previous generations of cell phone service have been deployed in the 850 and 900 MHz ranges, as well as 1900 and 2100 MHz, higher in frequency than most cable systems use.
With the FCC’s adoption of the digital TV channel plan, broadcasters were forced to give up frequencies above 698 MHz. That corresponds to cable channels 108 and above. Some of these frequencies have been auctioned off, and AT&T and Verizon were two of the big spectrum winners.
There has been much more attention paid to this interference issue in Europe than in the U.S. As early as 2009, Radiocommunications Agency Netherlands (Agentschap Telecom) conducted a set of initial exploratory tests and observed interference in the 790-862 MHz band designated for LTE in Europe. In Europe, these frequencies are mainly used by DOCSIS modems.
A group of European mobile phone operators sponsored a study concluding that, in the long term, interference should be resolved by improved standards for the immunity of cable modems, and perhaps by the inclusion of dynamic channel assignment in the DOCSIS specifications.
Late last year, Ofcom (the British version of our FCC) issued a report of a detailed study it sponsored, finding that seven of the nine tested set-top boxes and all 12 cable modems experienced interference from an LTE phone at a distance of 1 meter. Much of the problem was attributed to plastic cases and holes in metal cases – inadequate shielding.
A joint ETSI/CENELEC European standards committee recommended an immunity standard of 1 V/m for cable boxes in the LTE frequency range, even though LTE phones actually created fields as high as 3 V/m. Most of the Ofcom-tested set-top boxes satisfied that 1 V/m immunity standard, but most of the tested cable modems did not. Ofcom submitted comments to the ETSI/CENELEC committee, urging that the immunity requirement be set at 3 V/m rather than 1 V/m.
Although the Europeans focused on interference from cellular phones rather than cellular base stations, one U.S. engineering consulting company has estimated that an LTE base station could cause interference to cable reception at a distance of 500 feet.
There are two other international bodies that have begun studying the issue – CISPR and ITU. CISPR is the Comité International Spécial des Perturbations Radioélectriques, which means the special international committee on radio interference. It is part of the International Electrotechnical Commission (IEC). For readers familiar with FCC Part 15, CISPR recommendations are the international equivalent of the FCC Part 15 rules that deal with unintentional radio emissions, such as emissions from personal computers. CISPR is planning to develop test procedures and a standardized test signal for testing compliance with the ETSI/CENELEC immunity standard.
The ITU, the Geneva-based International Telecommunications Union, has several “sectors,” including one that develops standards for the use of radio communications (ITU-R) and one that develops standards for wireline communications (ITU-T). Study Group 5 of ITU-T has begun work on a recommendation for mitigating interference from LTE transmissions into cable TV. The goal is to complete this work in 2012.
Cable TV service in the U.S. has no interference rights against licensed cell phone operations. So the obligation falls on cable TV operators to deal with interference from LTE phones.
The FCC has an interference immunity rule, you might say. “Cable ready” consumer electronics devices must be immune from interference in a field of 100 mV/m. But that is less by a factor of 10 compared with the European level of 1 V/m. The FCC’s 100 mV/m requirement is totally inadequate for the case of much higher fields from nearby LTE transmitters.
The SCTE has just created a Sustainability Management Subcommittee. One area of responsibility is electromagnetic compatibility. Let’s see whether they are willing to take on the work to develop an effective interference immunity standard for cable modems and set-top boxes, and whether manufacturers can meet the necessary requirements.
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