Interference hunting is an increasingly important task for the national wireless operators (AT&T Mobility, Verizon Wireless, Sprint and T-Mobile). Fourth generation LTE base stations are broadband and have improved sensitivity, so even weak interfering signals have the potential to cause substantial damage through dropped calls, reduced coverage and degraded network capacity.
Potential interference sources are many and include passive intermodulation (PIM), booster amplifiers, utility power transformers, florescent light ballasts, cable TV set top boxes, cordless phones and any devices operating in the 902-928 MHz license-free band. Occasionally, we run into an interference source we have not seen before.
In 2013, a wireless operator asked us to investigate a particularly difficult interference problem affecting numerous cell sites on the southern California coast. The interference was intermittent and difficult to isolate. When present, it occupied the 1880-1900 MHz band which in the U.S. includes part of the PCS “B” and “C” blocks and all of the “E” and “F” blocks. The interference appeared on several 1.8 MHz wide channels and exhibited the framing characteristics of a time division duplexed system. After observing the interference for two days, our engineer, Mike McGinley, was able to geolocate the source to the San Diego harbor and then specifically to the Vibrant Curiosity super yacht. He contacted the harbormaster and was invited by the yacht’s skipper to tour the yacht which enabled him to isolate the interference source to several DECT cordless phone booster amplifiers.
Digital enhanced cordless telecommunications or DECT for short, is a European standard for cordless phones. It operates in the 1880 to 1900 MHz band using Time Division Multiple Access (TDMA) on one of 10 channels, each 1.728 MHz wide. The system is Time Division Duplexed (TDD) meaning that a channel can support both uplink and downlink transmissions to/from the DECT base station. Peak power is 250 mW, about the same as a modern cell phone. The booster amplifiers extended coverage within the yacht to hard-to-reach locations inside the engine room and below deck, etc.
In hindsight, the problem was understandable — the shipbuilder was authorized to operate DECT in the 1880-1900 MHz band in Europe where the yacht was built, but did not foresee the interference problem when the yacht traveled to other parts of the world.
Mike explained to the skipper that the DECT cordless phones could not legally operate in this band inside the U.S. or in U.S. territorial waters, but the skipper was skeptical and politely declined to turn off the phone system. We then contacted the local FCC field office. The FCC engineer was able to convince the skipper to turn off the system and the interference disappeared.