Beware Of Unlicensed Operations
By Leslie Taylor
Satellite industry officials need to gird themselves for another battle. Still smarting from the Federal Communications Commission’s (FCC) decision to allow terrestrial multichannel video distribution and data service (MVDDS) in the direct broadcast satellite (DBS) bands, the industry now faces the FCC’s initiative to allow unlicensed operations in the 6525-6700 MHz and 12.75-13.25 GHz bands.
The FCC is letting unlicensed devices operate within large bands of spectrum. Recent examples include allowing Wi-Fi into the 5 GHz band and ultra-wideband (UWB) operations across a large swath of bandwidth. Now, the FCC is proposing that fixed-satellite service (FSS) operators share a potentially unknown amount of spectrum with unlicensed devices.
You can’t fault with the FCC for encouraging innovation, new services and new methods of promoting spectrum sharing. Those goals are laudable. However, the commission should not forget that its primary mission is to ensure that licensed users of spectrum operate without unacceptable interference. That responsibility includes ensuring that satellite systems with primary allocations not face the risk of interference, loss of capacity, or loss of future access to frequency bands because the FCC wants to allow unlicensed operations to use a new “interference metric” to determine whether the feared interference actually would occur.
What is this new “interference metric” and how should satellite operators, manufacturers and users approach the commission’s proposal? And, what specifically is the commission’s proposal?
The commission, in ET Docket No. 03-327, proposes new technical rules that would permit unlicensed devices (for purposes not yet identified) to operate in the 6525-6700 MHz and 12.75-13.25 GHz bands. These bands currently are allocated on a primary basis to FSS. Portions of the bands also are allocated for other services, including radiolocation (used primarily for U.S. government radars), terrestrial fixed services, cable antenna relay systems and space science services. So, there is considerable sharing going on in these bands already. In fact, in the 12.75-13.25 GHz band, the 2003 World Radiocommunication Conference (WRC-03) liberalized the rules applicable to FSS so that greater use of the band could be made for smaller aperture earth stations. Previous international and U.S. domestic rules had restricted operations in this band to large earth stations, thus limiting the band’s use as feeder links. Both the 6525-6700 MHz and the 12.75-13.25 GHz bands are considered the “extended C and Ku-bands” and are subject to a number of restrictions in their use by FSS systems.
WRC-03’s action to liberalize FSS operations in the 12.75-13.25 GHz band followed several years of study to determine what technical rules could best balance the interests of the co-primary spectrum users. Those co-primary users included radiolocation, primarily relied on by the U.S. government for military radars.
New Interference Metric
On the face of it, the FCC’s technical proposal might seem reasonable. Use of new tools to facilitate sharing should be taken seriously. The FCC proposes that the unlicensed devices in the 6525-6700 MHz and 12.75-13.25 GHz bands, both satellite uplink bands, be restricted to operation, on an individual device basis, with EIRP (Effective Isotropic Radiated Power) levels as high as 30 dbm (the number of decibels relative to 1 milliwatt) to 36 dbm. The commission works backwards to get to these numbers. First, it looks at the interference trigger used between FSS systems for coordination, e.g., a Delta T/T (an expression of the amount of interference) of 6 percent. The FCC argues that the cumulative effect of millions of devices, each operating at the indicated EIRP levels, would not exceed a 5 percent Delta T/T. The commission is correct that it is necessary to consider the aggregate interference effect of such devices. The impact of one device on a geostationary (GSO) satellite, in particular, would be impossible to measure. However, millions of devices would raise the noise floor, or temperature, and potentially could impact the FSS uplink operations.
While proposing specific rules, the commission seeks comment on the appropriate interference temperature threshold that will afford “sufficient” protection to licensed satellite operations, as well as comment on the assumptions used in its link budget analysis on which it based its proposed rules.
MSS Feeder Link Analogy
At WRC-03, and at the FCC, substantial spectrum has been made available for unlicensed services (primarily Wi-Fi) in the 5 GHz band. Developing the technical rules, which are embodied in the International Radio Regulations and the FCC’s regulations, was a very complex process, involving much technical study over a period of years. Several ITU Radiocommunication Sector (ITU-R) recommendations were developed to address the operation of wireless local area networks (LANs) in the 5150-5250 MHz band. Within that band operate non-GSO mobile satellite service (MSS) feeder links used primarily by Globalstar and ICO. Those interested in the interference noise proceeding should take a look at these ITU-R recommendations that include M. 1454, S. 1426 and S. 1427. These recommendations provide methodologies for analyzing aggregate interference into the MSS feeder links from terrestrial unlicensed devices and provide for limitations on the EIRP of the individual unlicensed devices. When this work initially was undertaken, there was concern that even developing such rules would undermine the concept of primary allocations, and by the back door, confer status to unlicensed devices that they do not warrant. Historically, unlicensed devices operate on a non-interference basis with respect to licensed devices.
However, now the FCC proposes a great leap forward. Not only will Wi-Fi devices be accommodated in the 5 GHz and 2.4 GHz bands, but 175 MHz (in the C-band) and 500 MHz (in the Ku-band) also will be made available for unlicensed operations. The commission states that its “preliminary analysis” shows that over 53 million devices in the C-band and over 369 million devices in the Ku-band can operate without exceeding the 5 percent Delta T/T into an individual spacecraft receiver. This analysis, however, ignores several factors. First, what is the impact if devices are clustered in small geographical locations, thus intensifying the increase in the interference temperature into the spacecraft receiver? Second, what is the impact of taking account of these devices in the design, construction, cost and coordination of FSS systems with other FSS systems, while these systems already must be engineered to share with other co-primary service operations? Third, how does the commission take into account the interference impact to satellite systems licensed by other administrations? Fourth, does this proposal pre-empt the international process because it would surely have multinational implications in practice? Fifth, what happens if the projected numbers (assuming they are correct) are exceeded? How does the FCC address the enforcement of aggregate interference limits by unlicensed devices? The commission already has experience with the difficulty of changing direction once consumer devices are widely adopted.
With this proceeding, the commission has the potential to create a huge constituency for unlicensed devices – the manufacturers, value-added resellers and users. Such constituencies can be expected to wield considerable political and economic clout. If satellite systems experience interference, it is doubtful the commission could or would act to require reduction in the power or number of the unlicensed devices. For the FCC to cite the 5150-5250 MHz band as an example that such sharing is feasible ignores the fact that it is far too soon to assess the aggregate impact of millions of devices on satellite receivers. Very few Wi-Fi devices are in operation in that band today. Moreover, the MSS operators have faced considerable opposition within the U.S. in putting forth technical papers into the international process proposing mechanisms for measuring such aggregate interference. The unlicensed community’s objection to such papers is what would a country do if the interference were too great, so why would we want a method for measuring and identifying the source of the interference?
What is the impetus for the commission’s proposal? The satellite bands are the only ones in the interference noise proceeding for which the commission proposes specific rules. The rest of the proceeding is at a much more exploratory level. A Notice of Inquiry asks for comments about many questions involving the operation of unlicensed devices in licensed bands. There was no petition for rulemaking that spurred the commission’s action. One question is, what unlicensed devices are being proposed for use in these bands? Another is, if these devices have exhausted other available spectrum?
The satellite community should take the commission’s proposal very seriously and respond with technical analysis of the FCC’s proposed rules and underlying assumptions. In addition, the industry should address the commission’s policy arguments concerning the introduction of unlicensed devices into additional frequency bands prior to the operation of unlicensed devices in the 5 GHz band. The U.S. government should address the impact on its operations in these bands, particularly the 12.75-13.25 GHz band. The U.S. Defense Department (DoD) worked hard to craft rules to permit Wi-Fi into additional spectrum in the 5 GHz band. However, the technical means adopted, including dynamic frequency selection (DFS), has yet to be implemented, yet alone evaluated in practice.
Spectrum sharing brings benefits to industry and users. However, spectrum sharing, particularly with unlicensed devices, should be introduced only after careful study, identification of the need for spectrum by the devices, and showing that existing spectrum available for unlicensed devices is not adequate to meet important public needs. Spectrum sharing also should come only after careful consideration of the impact of a new interference approach on licensed services operating pursuant to primary allocations.
Leslie Taylor is a telecommunications consultant in Bethesda, Md. She can be reached by phone at 301/229-9410 or by e-mail at LTAYLOR@LTA.COM .