Appendix B: Examples of Spectrum Sharing in the US
Appendix C: Case Studies - The Role of Receiver Performance In Promoting Efficient Use of the Spectrum
Spectrum Management has generally focused on transmissions. The radio spectrum is allocated among various radio services as reflected in the Table of Frequency Allocations. Transmitters are subject to requirements to ensure they operate within the spectrum allocated for that service and any out-of-band and spurious emissions that might fall outside the spectrum allocated for that service are carefully controlled to minimize the risk of harmful interference to other services. One might assume that receivers abide by the same principles as the transmitters, in other words, that they only receive transmissions within the spectrum allocated for the service in which they are designed to operate. However, often that is not the case and receiver performance can dramatically affect access to and efficient use of the spectrum.20
This Appendix summarizes a number of examples of situations where receiver performance was a significant issue affecting access to the spectrum for new services. It is not intended to parcel responsibility among the various players in each case, but rather to illustrate the nature of the problem in order to develop better ways to prevent or address similar situations in the future. Some of these examples are well-known and long-standing problems that have been dealt with in various ways, such as guard bands or geographic separation, but usually at the expense of some use of the spectrum that would be possible if receiver performance were better.
Many of the potential issues relative to receiver performance and interference between services operating in adjacent spectrum might be avoided through more appropriate placement on the spectrum chart. For example, it is good practice to keep services relying on reception of weak signals in adjacent bands or high-power services in adjacent spectrum. However, services generally cannot be rearranged on the spectrum chart by simply picking them up and moving them to new bands. Often, new services must be placed snuggled into the spectrum space that is available at the time of introduction for the service. This is where the issue of receiver performance that anticipated one type of neighbor and now must deal with a new one comes into play.
The Wireless Communications Service (WCS) was created in 1996 and included 15 MHz spectrum blocks located above and below the Satellite Digital Audio Radio Service (SDARS). The WCS allocation allowed for mobile service, but the technical rules for out-of-band emissions were impractical for mobile devices. After many years of attempting unsuccessfully to deploy a successful business model based on fixed service, the WCS licensees petitioned for rule changes to facilitate mobile service. The performance of the SDARS receivers was one of the critical areas of contention.21 The receivers had been designed assuming only fixed operations in the adjacent bands and therefore did not anticipate a need for strong filtering of signals in nearby adjacent spectrum. As a result, the prospects of overload interference to legacy SDARS receivers from mobile devices required application of strict technical rules and effectively created 5 MHz guard bands on each side of the SDARS spectrum.
The 3650-3700 MHz (50 megahertz) band was re-allocated from Federal government usage (military radars) to non-Federal use in order to meet the requirements of the 1993 budget act. This spectrum was also allocated to the fixed satellite service which had approximately 60 receive sites that could not be relocated to other spectrum. The satellite C-band downlink operates in the upper adjacent spectrum at 3700 – 4200 MHz. An issue that surfaced in the FCC rule making proceeding was that many C-band satellite earth station receivers had front ends extending well into the 3650 – 3700 MHz band. While the FCC deemed the interference risk low due to the directional nature of the satellite service and the anticipated predominant fixed use of this spectrum, the issue risked the possibility of rendering much of this federal transferred spectrum useless.22
Receiver performance was a major area of contention relative to potential use of the AWS-3 spectrum.23 The AWS-3 spectrum is upper adjacent to the AWS-1 spectrum. A petition was filed to use the AWS-3 spectrum for time-division duplex operation. However, the AWS-1 receivers were generally designed to operate across the AWS-3 spectrum consistent with international allocations. Incumbent AWS-1 licensees argued that the receivers were not the issue, but rather that TDD operations must not be permitted to operate adjacent to downlink FDD spectrum without a significant guard band. The potential use of the AWS-3 spectrum remains under consideration.
The AWS-1 downlink spectrum at 2110 – 1155 MHz is upper adjacent to the broadcast auxiliary service (BAS) band at 2025 – 2110 MHz. AWS-1 licensees were required as the newcomers to coordinate with and correct any harmful interference to the BAS operations.24 The AWS-1 band had previously been used for the fixed microwave service and so the BAS equipment had not been designed with sharp filters. As a result, AWS-1 operations were found to cause harmful interference to BAS, requiring the AWS-1 licensees to pay to purchase new filters for the BAS equipment.
TV Receiver performance was a significant issue for the access of unlicensed devices to the TV White Spaces. The roll-off of the TV filters is the dominant factor limiting the amount of energy that a TV White Space device may emit in the white space and therefore the potential applications for the devices.25 The issue is pending under reconsideration.
The performance of analog TV Receivers was a major factor in the creation of white spaces. Certain combinations of channels known as the UHF taboos were not permitted in any given market due to receiver performance issues. Interestingly, the Commission in the early 1970’s contracted with RF Monolithics to develop a TV receiver that would avoid the need for all or most of the taboos.26 However, though the contracted work was successful, no changes came of this project. DTV receivers were assumed to no longer have this need based on the established policies and the Commission did not apply the protections for the UHF taboos. This would be an excellent candidate for an academic case study.
Other issues have occurred through the years relative to TV receivers and services operating in adjacent spectrum: Amateur radio service operations at 50 – 54 MHz interfering with TV receiver on channel 2 at 54 – 60 MHz; mutual interference between TV and FM broadcast at the intersection between channel 6 and the FM broadcast band (largely ameliorated by using only a minimal number of DTV channel allotments on channel 6); interference from services operating in the spectrum at 216 – 220 MHz to TV channel 13 at 210 – 216 MHz; land mobile sharing in 11 major cities operating on TV channels 14 – 20; TV channel 51 operations adjacent to 700 MHz A block mobile wireless licensees -- CTIA has filed a petition asking the Commission not to assign any further TV stations to channel 5127; Garage door opener controls operating on Part 15 of the FCC rules on an unlicensed, non-interference basis receiving interference from primary federal land mobile radio systems that could not be remedied easily because the garage door opener controls used super-regenerative receivers with front ends up to 10 MHz wide.28
Receiver performance relative to adjacent channel and intermodulation characteristics was a major element in the issue of rebanding the 800 MHz spectrum to avoid interference between Nextel and Public Safety operations on interleaved channels.29
LightSquared’s proposed deployment of ancillary terrestrial component (ATC) base stations as part of a hybrid terrestrial – satellite service has raised significant concerns about potential harmful interference to the GPS service operating in the upper adjacent spectrum due to the potential for receiver overload.30 GPS receiver performance has been raised as one of the elements in this debate. The FCC has not reached any conclusions on the merits nor made any decision on how to proceed in this matter. The issue of overload interference to Inmarsat from L-band ATC operations was addressed by establishing minimal guard bands for certain safety operations and advising that the Commission does not regulate the susceptibility of receivers to interference from transmissions on nearby frequencies. Rather, the Commission relies on the marketplace – manufacturers and service providers – to decide how much susceptibility to interference will be acceptable to consumers. The Commission noted that it generally does not limit one party’s ability to use the spectrum based on another party’s choice regarding receiver susceptibility.31
It is noted that universities (graduate students) might be a low cost means by which the FCC could obtain an in-depth study on the current spectrum allocation table focusing on the review of the established Guard Bands and the examination of the characteristics of the receivers associated with each of the identified “bands of interest” to determine the acceptable specifications for transmitters given the receiver characteristics in neighboring bands. This study would be enormously valuable in understanding the full scope of the opportunity for spectrum re-allocation, compaction and sharing, and in parallel might serve as an excellent learning vehicle for appropriate graduate programs. As with the rest of the Spectrum Efficiency effort, full Working Group and full TAC input on the development of appropriate company and academic R&D incentives for the creation of ever improving receiver offerings is solicited.
1 CSMAC, Working Group 1 “Definitions of Efficiency In Spectrum Use”, October 1, 2008, p. 2.
2 The CSMAC report also included a category labeled “Cognitive Systems” that addressed “cross application efficiencies” or inter-service efficiencies that could be produced by what it termed “cognitive adaptive spectrum use.”
3 Rec. ITU-R SM.1046-2, "Definition of spectrum use and efficiency of a radio system," 2006.
4 ITU-R SM.1046-2 suggests one possible specific relationship to be SUE = M/U.
5 Spectrum utilization increases with the fraction of time that the spectrum is available. The specific value for T depends upon the system(s) being considered. ITU-R SM.1046-2 suggests that time be ignored (i.e. T = 1.0) for continuously active systems. In other cases, “time” might be set to the fraction of time the system is active.
6 The National Radio Astronomy Observatory, founded in 1956, is a facility of the National Science Foundation that provides state-of-the-art radio telescope facilities for use by the international scientific community. More information is available at http://www.nrao.edu/.
7 Satellite system service types include non-communication systems such as navigation systems (“Global Positioning System”), weather sensors, and imaging systems (used for Google Earth and maps), and a variety of communication systems including television broadcast systems (“DirecTV” and “Dish”) and systems providing point-to-point two-way communication links.
8 The spectrum efficiency will increase with frequency re-use both geographically and via dual polarization, and the proposed spectrum efficiency metric will give credit for both types of frequency re-use.
9 Antenna size refers to physical aperture area for a parabolic reflector or phased array antenna, physical size for a microwave feed horn antenna, and dimensions for a dipole or other omni-directional antenna.
10 The “service field of view” is that portion of the earth’s surface that is visible from, and hence serviceable from, the geostationary orbital slot.
11 As an example, two geostationary satellites co-located at 90° west longitude could each use the same frequency spectrum so that one satellite serves the United States and the other satellite serves Brazil.
12 As an example, if a satellite has the United States as its service area, there will be a portion of southern Canada and a portion of northern Mexico that cannot be served by a co-located satellite operating in the same frequency spectrum.
13In satellite systems, response time is also determined by the latency resulting from propagation delay of the radio signal as it traverses the distance between the satellite and the earth terminals. This latency is influenced by whether the satellite is in a high, medium, or low altitude earth orbit.
14 Later versions of the table will include spectrum efficiency metrics proposed for other systems.
15 The metric proposed above for personal communications systems (“Information bits per second per Hz of allocated (licensed) spectrum per square Km of service area”) is equivalent to a bits per unit area measure.
16 For example, mission-critical public safety systems generally have at least 95% coverage reliability requirements with less than 3% bit error rates over those areas.
17 For example, public safety systems generally require at least a 15:1 peak to average communications capacity (i.e., having 15 times the user capacity available for emergency situations compared to average communications needs) with less than 1% call/access blocking probability.
18 “Spectrum Utilization Efficiency” is defined in section 2 of Annex 1 of ITU Recommendation ITU-R SM.1046-2
19 “Spectrum Utilization Factor” is defined in section 2 of Annex 1 of ITU Recommendation ITU-R SM.1046-2.
20 For purposes of this discussion, receiver performance refers to the characteristics that affect the ability to reject harmful interference such as front-end filtering and not to characteristics that are effectively addressed in the marketplace such as voice or picture quality, data throughput, reliability, etc.
21See Report and Order and Second Report and Order in WT Docket No. 97-293, IB Docket No. 95-91, and GEN Docket No. 90-35, In the Matter of Amendment of Part 27 of the Commission’s Rules to Govern the Operation of Wireless Communications Services in the 2.3 GHz Band and Establishment of Rules and Policies for the Digital Audio Radio Satellite Service in the 2310-2360 MHz Frequency Band at http://hraunfoss.fcc.gov/edocs_public/attachmatch/FCC-10-82A1.doc
22See Memorandum Opinion and Order in ET Docket No. 04-151, WT Docket No. 05-96 and ET Docket 02-380,In the Matter of Wireless Operations in the 3650 – 3700 MHz band, at paras.56-60, http://hraunfoss.fcc.gov/edocs_public/attachmatch/FCC-07-99A1.doc. See also NTIA Report 94-313Analysis of Electromagnetic Compatibility between Radar Stations and 4 GHz Fixed Satellite Earth Stations discussing solution of satellite receiver overload through use of filtering, http://www.its.bldrdoc.gov/pub/ntia-rpt/94-313/94-313.pdf
23See Notice of Proposed Rule Making in WT Docket No. 07-195 In the Matter of Service Rules for the 2155 – 2175 MHz band, at pars. 61 – 63, http://hraunfoss.fcc.gov/edocs_public/attachmatch/FCC-07-164A1.doc
24See Report and Order in WT Docket no.02-153 In the Matter of Service Rules for Advanced Wireless Services in the 1.7 GHz and 2.1 GHz bands at http://hraunfoss.fcc.gov/edocs_public/attachmatch/FCC-03-251A1.doc at paras. 127 - 130.
25See generally Second Memorandum opinion and Order in ET Docket No. 04-186 and ET Docket No. 02-380 In the Matter of Unlicensed Operation in the TV Broadcast Bands and Additional Spectrum for Unlicensed Devices below 900 MHz and in the 3 GHz Band, http://hraunfoss.fcc.gov/edocs_public/attachmatch/FCC-10-174A1.doc
27 On March 15, 2011, CTIA - the Wireless Association (CTIA) and the Rural Cellular Association (RCA) submitted a “Petition For Rulemaking and Request for Licensing Freezes” RM-11626, wherein they requested certain actions to limit TV broadcasting on channel 51. The Commission on August 22, 2011 issued a public notice announcing a freeze on the filing and processing of applications for operation on TV channel 51. See http://transition.fcc.gov/Daily_Releases/Daily_Business/2011/db0822/DA-11-1428A1.doc
28See Consumers May Experience Interference to Their Garage Door Openers Near Military Bases, February 15, 2005, http://fjallfoss.fcc.gov/edocs_public/attachmatch/DA-05-424A1.doc
29See Report and Order, Fifth Report and Order, Fourth Memorandum Opinion and Order and Order in WT Docket No. 02-55, ET Docket No. 00-258, In the Matter of Improving Public Safety Communications in the 800 MHz band, http://www.800ta.org/content/fccguidance/FCC_04-168_08.06.04.pdf
31See Memorandum Opinion and Order and Second Order on Reconsideration in IB Docket No. 01-185, In the Matter of Flexibility for Delivery of Communications by Mobile Satellite Service Providers in the 2 GHz band, the L-band, and the 1.6/2.4 GHz bands, at paras. 51-59, http://hraunfoss.fcc.gov/edocs_public/attachmatch/FCC-05-30A1.pdf
*Sharing Work Group contributors to this White Paper were Peter Bloom, John Chapin, Richard Currier (current editor), Brian Daly, Dick Green, Dale Hatfield (former editor), Julie Knapp, John Leibovitz, Geoffrey Mendenhall, Dan Reed, Dennis Roberson, Jesse Russell, and Paul Steinberg.