Mobile-satellite tower operations would share part of the same band used for Bluetooth, Wi-Fi, and many more unlicensed devices.

My colleague Don Evans ably posted an earlier report on the FCC’s proposal to allow Mobile-Satellite Service (MSS) tower-based operations to move into the adjacent 2.4 GHz unlicensed band – home to Bluetooth, the most-used forms of Wi-Fi, and countless other kinds of devices.

This follow-up post drills down on how the proposal might affect unlicensed users.

Globalstar has long been licensed for MSS operations at 2483.5-2495 MHz, immediately above the unlicensed band at 2400-2483.5 MHz. See the diagram. Until now, Globalstar’s license has allowed only satellite downlinks.

As Don explained, the FCC now proposes two changes. It would allow Globalstar to implement an Ancillary Terrestrial Component (ATC), which is FCC-speak for “towers.” Globalstar could communicate directly from towers to handsets – no satellite needed. And, for this application, it would expand the MSS/ATC band down into unlicensed spectrum by 10.5 MHz, to 2473 MHz. Unlicensed users could continue to operate alongside ATC.

Besides millions of unlicensed transmitters, the newly proposed ATC segment at 2473-2483.5 MHz also houses industrial, scientific and medical (ISM) equipment, radar, fixed point-to-point microwave, broadcast auxiliary, and private land mobile two-way radios. You would think the FCC might hesitate at trying to squeeze in yet another service. But the shortage of spectrum suitable for mobile data is so severe that the FCC cannot pass up any opportunity.

The diagram depicts the three most-used U.S. Wi-Fi channels, numbered 1, 6, and 11. Eight other channels overlap these (and each other), but see much less use. Note that U.S. Wi-Fi tops out at 2473 MHz. Globalstar cited this fact in requesting use of the band above 2473 MHz, suggesting that the above-and-below separation would limit the interference it causes to Wi-Fi.

The reason why Wi-Fi stays below 2473 MHz is relevant here. It turns on the out-of-band limits for the 2.4 GHz band. These are asymmetrical – relatively relaxed below 2400 MHz, and much more stringent above 2483.5 MHz. Wi-Fi transmitters can thus operate close to the lower edge of the band, but must stay well clear of the upper edge, in order to meet the tighter limits there. In practice the useful maximum frequency is around 2473 MHz. (One could design a Wi-Fi transmitter that operates higher in the band and still meets the out-of-band limits, but it would cost more and have a shorter battery life.)

Why the stringent limits above 2483.5 MHz? To protect the weak signals at 2483.5-2495 MHz coming down from Globalstar’s satellites! Are you following this? Wi-Fi avoids 2473-2483.5 MHz to protect Globalstar; then Globalstar says to the FCC, Hey, since Wi-Fi is not using that segment, why not give it to us?

Not to be outdone, Wi-Fi interests have made the complementary request: to relax the upper-edge out-of-band limits so that Wi-Fi can operate all the way up to 2483 MHz.

Bluetooth is another major 2.4 GHz unlicensed user. A Bluetooth transmitter “hops” among 80 possible channels ranging from 2400 up to 2480 MHz, each of them about 1 MHz wide. It has the capability of avoiding occupied frequencies, but FCC rules require the transmitter to always hop among at least 15 channels. In Wi-Fi-infested areas, Bluetooth – which operates at much lower power – does much of its hopping in the Wi-Fi-free zone above 2473 MHz. The prospect of ATC moving into those frequencies could spell trouble for Bluetooth users.

There is no requirement that unlicensed 2.4 GHz applications use the Wi-Fi or Bluetooth protocols. Countless thousands of consumer, commercial, and industrial devices use all manner of other modulations and waveforms, each with its own pattern of frequency occupancy, and its own potential susceptibility to the proposed ATC operations. Devices deliberately designed for the upper end of the band, so as to avoid interference from Wi-Fi, may be particularly vulnerable.

The FCC proposes a power limit for ATC that is identical to that for most 2.4 GHz unlicensed operations: 4 watts effective isotropic radiated power. The idea is to put ATC on an equal footing with unlicensed devices when competing to get a signal through. But it may not work that way in practice. Many unlicensed units operate at much lower power levels, often just a few tens of milliwatts. A full-power ATC facility may overpower nearby devices.

Worse, out-of-band emissions from ATC operations could threaten unlicensed operation far below 2473 MHz. The FCC proposes the same out-of-band limits for ATC that it applies to unlicensed-device emissions below 2400 MHz: namely, 20 dB below the highest in-band emission. This might seem even-handed. In practice, though, an ATC transmitter operating at the maximum allowable power can have out-of-band emissions comparable to the in-band power typically used by mobile Wi-Fi devices. The proposal to keep ATC frequencies separate from those used by Wi-Fi may not afford as much protection as Globalstar suggests.

Still, the proposed technical rules represent a creative effort on the FCC’s part. Rather than just open another band under MSS/ATC rules, the FCC is trying for a hybrid approach, requiring that ATC operate in the unlicensed band much like an unlicensed device. This particular band already hosts a massive and technologically sophisticated ecosystem. The FCC’s aim is to fold in another service with minimum disruption to those already in place.

The FCC even proposes to set aside a long-standing rule that prohibits unlicensed devices from causing interference to a licensed service. ATC is licensed. Yet an ATC handset user would have no interference protection from a nearby Wi-Fi device. We can hear gasps of disbelief from the cognoscenti. The doctrine that protects licensed users from unlicensed devices has been a mainstay of FCC regulation for decades. For the FCC to suggest anything different is about like the U.S. Congress condemning both the flag and motherhood. But the rule would be difficult to enforce, in this context, and its proposed inapplicability here will doubtless soften objections from unlicensed users.

The overall success of the FCC’s approach in practice will depend in large part on the power levels Globalstar chooses to deploy. With so much of our personal and business communication now dependent on Wi-Fi, Bluetooth, and other 2.4 GHz unlicensed technologies, there could be significant risks in going forward. We trust the FCC has evaluated them carefully, and we commend the agency for proposing a novel set of solutions.

To tell the FCC what you think, go to its electronic comment filing site and enter Proceeding Number 13-213. Comments are due on May 5, 2014, and replies on June 4.