The emergence of FM broadcast interference to 700 MHz LTE

The law firm of Fletcher, Heald & Hildreth has a post that’s unsettling to engineers such as I who don’t expect very different radio technologies on very different frequencies to interfere with one another.

It seems that 700 MHz base station receivers have become so sensitive they’re susceptible to 8th-order harmonic interference from FM broadcast stations. This is the case even though the FM transmitters meet FCC emission requirements. LTE receivers have become better than the FCC’s Rules.

The lawyers make several good arguments in support of the broadcasters. Unfortunately for the FM stations, Section 73.317(a) of the FCC’s Rules, which governs FM emissions, includes this provision: “. . . should harmful interference to other authorized stations occur, the licensee shall correct the problem promptly or cease operation.” We’re not lawyers, but that seems to be an overarching broad requirement that, until now, hasn’t been much of a concern.

These cases of interference are now being handled on an ad hoc basis, with some encouraging cooperation between broadcasters and the mobile industry. As mobile broadband receivers continue to improve and become even more sensitive, however, they will be even more susceptible to interference from FM harmonics. This should be looked at more formally by the FCC, perhaps in an inquiry or in a rulemaking proceeding.


One Response to “The emergence of FM broadcast interference to 700 MHz LTE”

  1. Baruch Cyzs says:

    Dear Mr. Crawley
    I read your interest year old article about FM harmonics and took the liberty to tell you that our company has found ultimate solution to the problem.
    I would like to introduce you to Ubiqam product that you may find great interest.
    Ubiqam has developed an add on product by the name UBIFIX that is an ‘add on’ box with RF interface that is connected between the antennas and the WCDMA/LTE base station.
    This box is able to cancel any broad/narrow band interference such as channel 51 DTV, FM harmonics, GSM interference to WCDMA/LTE, wireless mikes, SMR radios, spark-gap interference, intentional jammers and any interference you may encounter as long it is received by both antennas. The product is deployed in many Israeli sites and had gone a successful trial with AT&T in US site.
    If you may find interest I will be glad to send you any additional material.
    Thank You,
    Baruch Cyzs
    UBIQAM chief scientist

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