Steven J. Crowley, P.E.
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.
Rearden LLC filed an application for experimental license with the FCC on May 24. That caught my eye as as it’s the same company that created a splash in 2011 with its announcement of Distributed-Input-Distributed-Output (DIDO) wireless technology, said to achieve capacity 1000-times the Shannon Limit with sub-millisecond latency. The company issued a white paper authored by company president Steve Perlman and the company’s principal scientist Antonio Forenza, but it didn’t have a lot of details for an engineer. DIDO seemed to be a form of network MIMO, whereby each user is served by all base stations in its vicinity through a complex coordination process. (Various forms of network MIMO will be appearing in successive releases of 3GPP’s LTE Advanced specifications.) Aside from the white paper, there are patents that supported the notion of DIDO as network MIMO.
In the FCC’s incentive spectrum auction proceeding, Information Age Economics (IAE) complains about the reliance of some in the industry (CTIA and Verizon, to name a couple) on a spectrum efficiency metric that simply divides an operator’s nationwide spectrum holdings by the number of subscribers. This results, in one example they give, of Sprint having 3.57 Hz per subscriber and Verizon having 1.05 Hz per subscriber. Some take this as Verizon using spectrum more efficiently, and perhaps being at a disadvantage. As IAE points out, however, spectrum is not partitioned to users this way: cellular infrastructure allows for frequency reuse. If one wants to compare the spectrum efficiency of Sprint and Verizon, don’t look at it nationwide — look at, say, Washington, D.C.
Today the FCC granted Google’s application to conduct a radio experiment in Mountain View, California. When I looked at the application in January, I noted Google withheld some information it felt was confidential, and I took a crack at trying to figure out what was going on based on available information. A couple of days later, the FCC asked Google to provide additional information, and Google responded. Then things sat with no apparent activity for a couple of months.
The experimental license issued today gives Google the authority it sought: use of the 2524-2546 MHz and 2567-2625 MHz bands. In January I noted those bands might be used by Clearwire. In January the FCC asked Google if it had consent from the license holder. Google responded that it “understands that a grant will be conditioned on coordination with affected licensees, and is engaged in discussions to satisfy that obligation.”
Apparently Google hasn’t furnished such consent to the FCC, as the experimental license contains the following “special condition:”
Prior to operation, licensee must successfully coordinate with existing and future Broadband Radio Service/Educational Broadband Service (BRS/EBS) licensees or lessees (as applicable).
So, the FCC is relying on the honor system, which isn’t unusual for experimental authorizations.
Last month I wrote about the Jarvinian Wireless Investment Fund and its application to the FCC to test its proposed terrestrial low-power service (TLPS). TLPS would use both the upper 2.4 GHz unlicensed band and Globalstar’s terrestrial-use spectrum (2473-2495 MHz all in all). That test would take place in Cambridge, Massachusetts.
Jarvinian is back with a second application, similar to the first, but with testing taking place at three locations in Silicon Valley (Sunnyvale and Cupertino, California). The supporting technical exhibit appears essentially the same. Different is the equipment to be used. The Cambridge application specified 50 Linksys WRT54GL access points, 10 Ubiquity UniFi access points, 10 Ubiquity XR2 client cards, and 10 Ubiquity SR-71-12 client cards. The Silicon Valley application specifies 10 TP-LINK TL-WA5110G access points, 20 TP-LINK TL-WR1043ND access points, and 20 Ubiquity SR-71-12 client cards.
This application was received by the FCC on March 6. The Cambridge application was received February 13 and is still pending.
LightSquared has asked the FCC for special temporary authority to conduct four months of tests in support of a potential frequency move. The application, and accompanying exhibit, were received by the FCC on March 5.
As background, to help resolve GPS interference concerns, LightSquared has proposed to conduct a portion of its terrestrial operations in 1670-1680 MHz instead of 1545-1555 MHz. It currently has authority to use half that, 1670-1675 MHz. The 1675-1680 MHz portion, however, is currently allocated for use by meteorological aids such as radiosondes and satellites. The company wants to conduct tests to see if its base stations would be compatible with other services in the 1675-1680 MHz band. A big concern is the radiosondes. Another part of the testing is determining if the radiosondes would be compatible with other services in the 400.15-406 MHz band, if they need to move there to accommodate LightSquared’s needs in 1675-1680 MHz. If a move is needed, the tests would help determine the costs of such a move, and “inform an eventual determination of an appropriate vehicle for meeting these costs” (i.e., who pays).
LightSquared asks to conduct tests across the continental United States. All transmitters would be coordinated with the FCC and NTIA, as needed.
A group launching a search for extraterrestrial intelligence awaits approval of an application it’s filed at the FCC for an experimental radio license. Such searches generally focus on seeking evidence of other life based on what’s received. This one is sending messages. More specifically, “project objectives include the establishment of a communications link capable of being received” at planet outside the solar system.
At its February 20 meeting, the FCC will likely adopt a Notice of Proposed Rulemaking as a first step toward increasing the amount of spectrum available in the 5 GHz band for unlicensed devices. Up to 195 MHz might be made available, which is a 35% increase over the present 555 MHz. Chairman Genachowski announced this initiative at CES in January. A leading application for new spectrum would be IEEE 802.11ac, which could have four instead of the current two 160 MHz-wide channels.
Wireless investment fund seeks FCC authority to test Globalstar’s proposed terrestrial low-power service
Yesterday the FCC received an application for experimental radio license from the Jarvinian Wireless Innovation Fund. Among other activities, the fund is working with Globalstar in its effort to establish a so-called terrestrial low-power service (TLPS). Globalstar petitioned the FCC to create such a service last November. The idea is to take the upper 2.4 GHz unlicensed band that isn’t now available for Wi-Fi, combine it Globalstar’s terrestrial-use spectrum, and create a new service operating on 2473-2495 MHz. This corresponds to IEEE 802.11 (Wi-Fi) channel 14, which can’t be used in the U.S. as it overlaps with Globalstar’s spectrum. Unlike Wi-Fi, TLPS would be managed to control interference.
Here it is: At 600 MHz, interference travels farther than it does at higher frequencies, all else equal.
During the inauguration there were press reports of Jumbotron problems — pixelated video and distorted audio — at the Washington Monument.
Yesterday the Washington Post reported that the problems could have been due to “interference with a microwave signal” that brings video and audio to the Jumbotrons.
Google filed an application at the FCC last week seeking permission to conduct testing of an experimental radio system. Portions of the application and accompanying exhibits have been designated confidential and are thus not available to the public. Even the request for confidential treatment has been redacted. Let’s try to infer what’s happening from the information available.
FCC Chairman Julius Genachowski yesterday announced an initiative to make 195 MHz of more spectrum available for Wi-Fi in the 5 GHz band.
The last time more spectrum for Wi-Fi was made available in this range was in 2003, when an FCC increased the then 300 MHz bandwidth to 555 MHz. As the document in that last link shows, the 5 GHz band is not as neat as the 2.4 GHz band, with other services, such as radar, requiring protection. These coexistence issues will have to be looked at again.