New rules allow communication with implanted devices that help restore function to damaged nerves and muscles.

The FCC has authorized the use of 24 megahertz of spectrum in the 413-457 MHz range for wideband devices implanted in the body. Part of the FCC’s MedRadio service, these devices will help treat neurological injuries and disorders by monitoring and activating nerves and muscles to restore sensation and mobility. The action adopts proposals the FCC laid out back in 2009.

The original proponent for the rules, the Alfred Mann Foundation, describes the technology as an “artificial nervous system” that uses electric signals to improve or replace the function of a nervous system impaired by mishap or disease. Patients who might benefit include those suffering from traumatic brain injuries, spinal cord injuries, and various neuromuscular disorders. A person might have several implanted devices that together make up a wireless network, coordinated by an external programmer/controller. Because the devices must transmit a lot of information in short bursts, they require substantial bandwidth, up to 5 MHz. The 400 MHz band is suitable because signals in this range can propagate through body tissues. Authorized power levels are very low, no more than 25 microwatts.

The new devices can operate in four bands: 413-419 MHz, 426-432 MHz, 438-444 MHz, and 451-457 MHz. These, needless to say, are already occupied (as is virtually all of the spectrum below about 50 GHz). Current users include private land mobile, public safety, TV news, and amateur operators. Some of these parties claimed the implanted devices would cause them harmful interference, but the FCC disagreed, based on test results in the record. Some also feared that if their equipment caused interference in the other direction (i.e., interfered with the implanted systems), they would have to turn down power so as to protect the well-being of persons who have the devices installed. The FCC addressed this concern by making the implanted devices “secondary” to other licensed users. In English, this means the new devices may not cause harmful interference to, and must accept all interference from, other licensed services.

Declaring the implanted devices to be secondary solves a legal problem, but does nothing to fix the underlying practical question. The FCC is probably right that a microwatt-level transmitter embedded in body tissue poses little interference threat to a licensed receiver. But we wonder about a person dependent on these devices who lives next door, say, to an amateur licensee operating at the maximum allowed power, which is 1500 watts. Legally, the implantee has no recourse if his internal network malfunctions. He is unlikely even to know that his ham neighbor is responsible for the problem. The rules require the external programmer/controller to monitor all four bands at least once per second and, if necessary, to move the entire network to a quieter band. To cover the case where none of the four bands is usable, the devices must be programmed to shut down in an orderly way. The small risk of that happening, says the FCC, is outweighed by the devices’ potential benefits.

There is also some small risk of interference among multiple people with these devices being congregated in a small area, as could happen in a medical facility. The FCC rejected proposals to require listen-before-talk circuits and contention-based protocols (which require competing systems to take turns). Instead it simply called on manufacturers to cooperate in setting standards for the industry.

Weak though they are, the device transmitters are too powerful to qualify as unlicensed devices. The FCC accordingly lumped them into the “Citizens Band” (CB) category. In theory a CB user requires a license, but need not actually apply for one. Anyone using the service with approved equipment is simply deemed to be licensed. Along with use by truckers (“10-4, Good Buddy”), the CB service also includes a host of other operations whose power is low enough individual licenses are not worth the bother.