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The FCC, after nearly two years, has affirmed its plan to allow overlapping uses of the 3.650 GHz band: The agency has reserved 50 MHz for a special kind of licensed use that will make it possible to run reliable high-signal-strength systems from fixed base stations using WiMax or—an import or—any other standard or technology that conforms to what sound like fairly broad rules. The band will allow 20 watts of transmit power versus a maximum of 1 watt for an omnidirectional antenna with Wi-Fi and like systems in 2.4 GHz and some of 5 GHz.
The licensing require is just a bare minimum. There will be no restriction on the number of licensees, and licensees will be required to register themselves and how they’re using the band in their area. The users of the band will have to coordinate among themselves to avoid interference and system problems. Failing that, the FCC could step in. But no licensee under this regime will have any priority rights over any other.
It’s a significant departure from the Part 15 rules that govern unlicensed Wi-Fi and other devices in 2.4 GHz and 5 GHz, in which every device is required to accept interference without complaint, but not generate too much interference. The 2.4 GHz band in particular is a problem because there are many competing uses of the band, including licensed users that do have priority (amateur radio operators among others).
The band isn’t completely clear, however. Earth receiving stations for satellite systems will have priority over new uses, and they will have broad exclusion zones that will prevent this band from being broadly used in urban areas, especially on the coasts. Wireless ISPs and others who can, however, negotiate with earth station operators and come up with solutions that let them operate in the band.
Initially, therefore, rural areas will see much simpler use—with potentially no overlapping wireless ISPs, even—and, Harold Feld argues, so will municipalities. The band is perfect for backhaul and network links for a muni network, and cities are in good positions to negotiate with the existing licenseholders—they’re on the hook if something goes wrong.
The lower 25 MHz of the band will require scheduling of time slots among competing devices in the same airspace; the upper 25 MHz entire band will use open contention rules similar to those in Wi-Fi (listen to see if anyone’s talking, and then start talking if they’re not). That’s WiMax rules for the lower band, more or less, and Wi-Fi rules for the whole thing. (Note: I had originally stated that only the 25 MHz top half of the band would allow contention rules; not the case. See the comment from Harold Feld below.)
I’ve linked to Harold Feld’s analysis. Feld works for the Media Access Project, a non-profit that’s interested in expanding the number of voices that get heard by influencing media ownership, telecom, and spectrum policy. So he has a horse in this race. But his description is detailed and tells you who has won and who has lost (no one, really) in the FCC’s decision.
Meanwhile, Ubiquiti already has a piece of gear ready to go—it’s designed for either 3.5 GHz (a more broadly available worldwide chunk of spectrum) and 3.65 GHz with contention.
Posted by Glennf at June 11, 2007 2:19 PM
Thanks for the link. Just one minor correction. While the FCC restricted WiMax-type restricted contention-based protocols to the lower 25 MHz, unrestricted wifi-type contention based protocols can be used anywhere in the 50 MHz. They just won't work as well in the bottom 25 MHz if there are users with restricted wimax-type devices.
Posted by: Harold Feld at June 12, 2007 8:02 PM
How do WiMAX base stations share "time slots" in the lower half of the band? WiMAX modems require a preamble and downlink map at the beginning of each frame. So who gets the beginning of the frame? In addition, what ISP would be willing to pay full price for WiMAX base stations and risk only getting to load them up with half or less the channel capacity worth of customers?
I don't think the FCC has even half a clue on how continuous coverage wide area wireless networks are designed. It's all about planning for efficient frequency reuse and without some kind of property rights to a chunk of spectrum one can't both control interference and aggressively use the same chunk of spectrum over and over in a cellular reuse pattern.
Posted by: George B at June 11, 2007 11:44 PM