US Spectrum Requirements: Projections and Trends - Chapter 7

Chapter 7

Spectrum Sharing

The National Spectrum Resource

In the United States, NTIA and the FCC jointly manage the use of the radio spectrum. The Federal Government uses the spectrum to support missions that include national defense, energy production and distribution, federal law enforcement, protection of national forests and natural resources, space exploration, and other vital government services. Management of spectrum use by Federal agencies is the responsibility of NTIA. The FCC manages spectrum use of the private sector, which includes broadcast entertainment, commercial, industrial, personal uses, and state and local government uses. Although spectrum users have brought higher portions of the spectrum into use through the years, the majority of use still remains below about 20 GHz. This spectrum is a national resource, and must be used to satisfy all demands, current and future, for radiocommunications services for both public and Federal use.

As a basic plan for usage, the radio spectrum is allocated to various radio services in blocks of frequencies. The concept of the block allocation system is that a band of contiguous frequencies is dedicated to one or more radio services, depending on the technical and operational characteristics of the service(s). A block so dedicated is said to be allocated to the radio service(s) associated with that block. Further, within a block the radio services may have a hierarchial structure that grants rights or imposes limitations on the services relative to other services in the same block. The assemblage of these spectrum blocks, along with associated footnotes, is called the National Table of Frequency Allocations,[EN600] and is used for general spectrum planning. The Table also further separates those allocation blocks that are managed by NTIA from those managed by the FCC.

In many cases, an allocation block will be shared by two or more radio services, each with its own allocation status.[EN601] An investigation of the block allocation system was undertaken as part of the NTIA Spectrum Policy Study.[EN602] One of the conclusions from that study stated that "[a]lthough the basic structure of the block allocation system should be retained, NTIA and the FCC should seek to modify it in the next decade to increase flexibility."[EN603] The NTIA Spectrum Policy Study promoted flexibility in allocations, stating that "to the extent that the block allocation system can be modified to permit additional flexibility, users, their customers, and the economy as a whole, will benefit. More rational suballocations will permit users greater opportunities to harness unused spectrum for similar functions. Greater technical flexibility would permit manufacturers to focus on the best standards for their products to meet user needs. And greater user flexibility would permit more private ordering of desirable spectrum uses."[EN604]

There are 40 allocated radio services defined in the Code of Federal Regulations.[EN605] A listing of these services is shown in Table 7-1 below.

TABLE 7-1

RADIO SERVICES

==============================
 Aeronautical Fixed
 Aeronautical Mobile 
 Aeronautical Mobile (R)
 Aeronautical Mobile (OR)
 Aeronautical Mobile-Satellite
 Aeronautical Mobile-satellite (R)
 Aeronautical Mobile-Satellite (OR)
 Aeronautical Multicom
 Aeronautical Radionavigation
 Aeronautical Radionavigation-Satellite
 Amateur
 Amateur-Satellite
 Broadcasting
 Broadcasting-Satellite
 Earth Exploration-Satellite
 Fixed
 Fixed-Satellite
 Inter-Satellite
 Land Mobile
 Land Mobile-Satellite
 Maritime Mobile
 Maritime Mobile-Satellite
 Maritime Radionavigation
 Maritime Radionavigation-Satellite
 Meteorological Aids
 Meteorological-Satellite
 Mobile
 Mobile-Satellite
 Port Operations
 Radio Astronomy
 Radiodetermination
 Radiodetermination-Satellite
 Radiolocation
 Radionavigation
 Radionavigation-Satellite
 Ship Movement 
 Space Operation
 Space Research
 Standard Frequency and Time Signal
 Standard Frequency and Time Signal-Satellite

Flexibility in allocating spectrum would allow additional radio services to share a frequency band according to criteria other than the block allocation. If additional services could be engineered into a band without causing or receiving interference, then the efficiency of that frequency band would increase. Examples of sharing methods are geographical (i.e., distance separation between users), time sharing, class of service (e.g., sharing between fixed and mobile services), and class of user (e.g., military/civil sharing).

One method for increased flexibility in spectrum allocation includes the use of electromagnetic compatibility (EMC) techniques to assess the interference potential between existing systems and new systems to be "engineered in." Some engineering firms support using EMC methods to manage spectrum. EMC techniques allow users to share the spectrum-space among several different radio services. The key for successful use of EMC techniques is for spectrum managers and users to agree to interference criteria that are acceptable to all affected services.[EN606]

With the current capability of personal computers, complex frequency assignment algorithms, using terrain data bases, can now be automated. Commercial software is available for certain radio services, and the Federal Government is developing a system for automation of its frequency selection and assignment procedures. These personal computer-based systems can be used by frequency managers for efficient selection of frequency and radio system parameters.

Performance Guidelines and Exclusivity

In the Notice, we asked three questions concerning spectrum sharing. First, we requested comments on standards or performance guidelines regarding sharing between different classes of users (e.g., Federal, non-Federal) to promote sharing and improve spectrum efficiency.

Although there were no specific comments relating to performance guidelines submitted,[EN607] some observations can be made. It was noted that sharing between Federal and non-Federal users would be facilitated if the FCC would adopt transmitter and receiver standards, such as used by Federal Government agencies.[EN608] Although the FCC does not currently set receiver standards, homogeneity of systems and practices within frequency bands would be very desirable if increased sharing between Federal and non-Federal users is to be successful. In particular, in shared fixed service bands, three system parameters are of interest. First, receiver standards should be set to optimize the channel bandwidth.[EN609] Second, a common channelling plan for Federal and non-Federal users would lead to increased spectrum efficiency and reduced costs for equipment.[EN610] Third, a more accurate and comprehensive data base of antenna characteristics used in fixed bands would aid in the analysis of frequency selection for new systems proposed for insertion in a crowded electromagnetic environment.

Generally, interference is a function of transmitter and receiver standards, including the related antenna standards. Frequency/distance separation criteria are dependent on these two main sets of standards. Spectrum efficiency is enhanced when any of these items is optimized. When the bandwidth of a receiver significantly exceeds the necessary bandwidth of the transmitted signal it is to receive, the receiver is susceptible to receiving non-desired signals, and may account for approximately 50 percent of the interference potential of radio systems. For this reason, system performance guidelines must also include receiver standards.

We also asked questions regarding the requirements for worldwide allocations, and the necessity for exclusivity of these allocations in the United States. Several requirements for worldwide allocations were indicated by the comments and by the nature of the radio services. Many frequencies or frequency bands are designated for international and worldwide use. Examples include distress frequencies, radionavigation services, some amateur service bands, HF bands, and various satellite radio services.

Generally, the need for exclusivity is embodied in the need for worldwide allocations, especially for safety services, since commenters have noted that frequency sharing between safety-related services, such as air traffic control and certain radionavigation services, and other services is not currently practicable.[EN611] However, there may be some exceptions. For example, it is planned that air traffic control information will be carried over the same mobile-satellite trunked system as public correspondence, but with over-riding priority and preemption for the safety information. It may also be possible for certain land mobile and international maritime mobile operations to share a common band if the dissimilar users are geographically separated.[EN612] Similarly, certain amateur-satellite bands may be aligned worldwide and yet not require exclusive allocations by individual administrations.

There is also a requirement for national exclusivity for some radio services without concomitant worldwide allocations. One example of this is the exclusive United States national radio astronomy allocation in the 608-614 MHz band, with footnote allocations in ITU Regions 1 and 3 on a secondary basis. Another example is in the amateur service where this service has an exclusive allocation in the 3900-4000 kHz band not aligned internationally with ITU Regions 1 and 3.

Federal/non-Federal Sharing

Many frequency bands have co-equal allocations for both Federal and non-Federal users. Above about 40 GHz, all bands are generally co-equally shared, except for radio services such as broadcasting and amateur, which are allocated for non-Federal use. Below 40 GHz, shared terrestrial service allocations are still abundant, but diminish until in the portion of the allocation table below about 6 GHz most bands are managed either by NTIA for Federal Government use, or the FCC for private sector use.

With the increased use of frequency bands below 6 GHz, services that cannot find relief in the higher bands must turn to sharing with other services, or to sharing between Federal and non-Federal users, to be accommodated.[EN613] The need for sharing seems to be most acute for the mobile and fixed services. However, Federal and non-Federal sharing has been occurring in the 21-23 GHz band for some time. One commenter believes additional sharing may be possible in the 1710-1850 MHz, 3.6-3.7 GHz, and 4.5-4.8 GHz bands.[EN614] Some planning for the use of shared bands has taken place. One commenter noted that sharing between the fixed, mobile, and the fixed-satellite services, as was arranged in the 17.7-19.7 GHz band, is a good example of judicious planning.[EN615]

One area of government and private-sector sharing that deserves immediate attention is that of sharing between different radio services in certain space service bands. While certain space service bands support critical government activities and are not viable sharing options, other bands support TT&C functions, and orbital payload downlinks including environmental sensing information. The expected commercialization of many former government scientific activities would make this sharing highly practical.[EN616]

One private sector commenter stated that "A significant, but largely untapped, source of sharing is between state and local government public safety agencies and the Federal Government."[EN617] The concept of government/private sharing of public safety channels is also endorsed by at least one Federal agency.[EN618] Further, NTIA's Institute for Telecommunication Sciences is closely observing an attempt to establish a shared Federal/state government trunked land mobile system for the State of Colorado.

Although sharing between Federal and non-Federal users is increasing, there is no comprehensive, integrated program for future spectrum sharing. Significant regulatory barriers exist that impede frequency band sharing between Federal and non-Federal users. It was stated that a more effective spectrum management approach would include developing innovative programs to encourage the sharing of government spectrum by the private sector, and government use of commercial services in order to help alleviate the shortage of spectrum.[EN619] Two commenters have suggested that NTIA establish a spectrum sharing committee to promote and coordinate spectrum sharing among spectrum user groups.[EN620]

The NTIA Organization Act required NTIA to submit a plan to Congress addressing improvement in the spectral efficiency of the Federal mobile services.[EN621] Specifically, Congress required Federal land mobile systems to be as spectrum-efficient and as cost-effective as commercial systems. In response, NTIA prepared the Land Mobile Spectrum Efficiency report[EN622] containing an analysis of the current Federal land mobile infrastructure with respect to spectrum efficiency and cost effectiveness.

The report included a summary of Federal land mobile requirements and technologies, and a plan to re-channelize Federal land mobile bands to narrower bandwidths. Further, NTIA established the framework for implementing procedures for increased Federal use of commercial radiocommunication service offerings.

Other Sharing Issues

Although the Notice requested comments regarding performance guidelines and standards for increased sharing, a number of commenters provided additional thoughts on inter-service sharing. While the following is not meant to be a complete evaluation of sharing issues, it reflects the main comments regarding the probability of success in certain inter-service sharing situations.


Mobile

In the search for additional spectrum, the concept of mobiles sharing spectrum allocated to other user classes is being intensely studied. Three distinct categories of mobile systems can be considered in the context for sharing. The first is the classic vehicular or hand-held unit, with transmitter power ranging from one to over one hundred Watts, operating in simplex or half-duplex, or in a demand-access mode such as trunking. Platforms can be on land, at sea, or airborne. The second category is the transportable system, such as electronic news gathering, using high-gain antennas and transmitting for a limited time from any one location. The third category is broadly termed "wireless systems" and include low-power PCS, wireless PBX's, and wireless radio local area networks.

Sharing between the mobile service and the space services in the 2025-2110 MHz and 2200-2290 MHz bands is considered to be a tenuous situation.[EN623] However, low-density mobile systems, such as electronic news gathering systems, have been sharing successfully with the space services in the 2025-2110 MHz band. Similarly, sharing between mobile systems and LEO mobile-satellite systems in the 148-149.9 MHz range, concerned parties have agreed to. Low-density mobiles, along with stations in the fixed and fixed-satellite services can also share in limited degrees with other space systems, such as passive sensors in the earth exploration-satellite service. The degree of sharing increases with the frequency bands above 20 GHz.[EN624]

The advent of PCS may provide opportunities for frequency sharing between PCS systems and other systems, such as the fixed service and possibly analog cellular systems. It has been stated that the use of wide-band CDMA could be overlaid on existing fixed or cellular systems without any degradation to transmission quality.[EN625]


Radio Astronomy

The radio astronomy service is characterized by a relatively small number of sites using very sensitive receivers and high-gain antennas. Further, certain observations can be made only on given frequencies or bands, since natural phenomena govern the existence of the extra-terrestrial transmissions and their spectral positions. Radio astronomers cannot be relocated to other frequencies, but given the small number of sites nationally, sharing the allocated radio astronomy bands is possible under certain conditions. For example, radio astronomy and other passive services can equally share any passive frequency band, since by definition no emissions are allowed in these bands.[EN626] However, line-of-sight sharing between radio astronomy stations and stations in the active services is not possible due to the high sensitivity of the radio astronomy receivers and the extremely low-level signals to be detected.[EN627] NSF states that geographical sharing between the radio astronomy service and some active services is possible if the separation distances are large enough, but sharing with the mobile service is possible only under other special circumstances.[EN628]

The fixed service, however, is a particularly good candidate for sharing and is best able among other services to share with radio astronomy.[EN629] This sharing is feasible, in part, due to the predictability of the fixed service. Consequently, the activities of the service can be coordinated, usually based on geographical separation, or occasionally on some other arrangement. Geographical sharing is also possible with the fixed-satellite service in the frequency bands limited to Earth-to-space transmissions. It is not possible, however, for radio astronomy to share a band with unlicensed devices because of their unpredictable and ubiquitous nature.[EN630] Other active services, such as aeronautical mobile, fixed- and mobile-satellite downlinks, and radionavigation-satellite services have potential for causing interference to radio astronomy stations. Of particular concern to the radio astronomers is the emergence of ultra-wideband systems, also known as "impulse" systems. These systems may have operational bandwidths up to several hundred megahertz, and traverse some radio astronomy bands. These systems could cause a rise in the level of background noise, or corrupt received data. However, it was stated that impulse radios can share spectrum with conventional radio transmissions (including spread spectrum) without interfering with them or being interfered with by them.[EN631]


Television Broadcasting

Sharing the UHF television broadcast spectrum with other services, particularly mobile, has been an issue for many years. It was noted in the comments that in most cases, this sharing was in fact a local reallocation of the broadcast spectrum to the mobile service.[EN632] One commenter stated that broadcast television is allocated spectrum that could be used for land mobile applications. Lack of selectivity in TV receivers eliminates 2-4 times the channel width from other use. Stricter television receiver standards could make hundreds of channels available for land mobile use.[EN633] Since broadcast receivers are pervasive in our society, co-channel sharing with television services is very difficult, and indeed probably not feasible. The introduction of advanced television systems, with the requirement for additional use of currently allocated UHF television channels, would make the local reallocation technique less viable. However, with the advent of digital modulation, ancillary services carried by the UHF television transmissions appears to be technically feasible.


Amateur

The amateur service operates in many of its bands on a secondary basis. Several amateur bands are shared successfully with Federal (military) radiolocation. The sharing is successful primarily because the peacetime use of military radars does not affect amateur operation in large areas of the United States. However, there are significant coordination areas surrounding some radars in the 420-450 MHz band.[EN634] Amateur operations also share spectrum with industrial, scientific, and medical (ISM) and LMS systems. Interference to amateur communications is accepted in these bands on the basis that some useful operation is better than exclusion from the band. Amateur operation in the 7100-7300 kHz band, while primary in the United States, suffers interference from international broadcasters operating on a primary basis in Regions 1 and 3. Amateurs have requested an alignment of the band internationally, but broadcasters are also requesting more HF broadcasting spectrum.


Television Receive Only

The deregulation of private satellite dishes in the early 1980's has led to a burgeoning industry termed "home satellite dish" (HSD). The success of this industry is dependent on the sharing between the fixed service in the 3.7-4.2 GHz band and the downlinks of the fixed-satellite service that carry television and video programming for residential HSD consumers. Techniques have been worked out to prevent most of the interference to HSD systems from fixed links. However, the HSD systems are not licensed, thus not protected. Additional systems introduced into the band will increase the probability of interference.

If the 3.7-4.2 GHz band is to be used for the accommodation of fixed service users moved from the 2 GHz bands, then the sharing problem may be severe in many areas. Microwave users would prefer to use other bands, as they do not want to interfere with even the non-licensed television receive-only (TVRO) systems.[EN635] The HSD industry will be further impacted by the advent of digital satellite transmissions, which will be more vulnerable to interference from fixed links than the present analog satellite signals. Sharing between these users, while now affordable and practicable, may in the future be impossible.[EN636]

Military and Civil Sharing

Recent downsizing of DOD components, coupled with current demands for global mobility of our military forces, places greater demands on military spectrum planning than ever before. To exacerbate the problem, there is a potential for the military services to lose some spectrum for which they have primary control due to spectrum legislation. An aid to military spectrum planning may be found in increased spectrum sharing between the military and other spectrum users in non-military frequency bands. Exclusive military frequency bands may not be required for those systems that are used only for training in the United States, and are deployed elsewhere. Increased access to non-military bands on a coordinated basis for this type of training would increase the flexibility of military spectrum use in the United States. However, certain military spectrum requirements prevent spectrum sharing in some instances.

A significant amount of sharing currently exists. The Army uses many frequencies in the 30-88 MHz for tactical and training purposes on a coordinated basis with private sector users. The Navy shares the 928-941 MHz band with private sector land mobile operations, on a non-interference basis. Further, the military gained access to the 420-450 MHz band for radiolocation use on a shared basis. The DOD often leases satellite time from commercial providers, using Federally-owned earth stations. The USAF has stated that if DOD should be granted the same protected status as other users of commercial satellite services, the DOD's satellite communications ability would be greatly enhanced, and commercial use could increase.[EN637]

However, additional sharing is expected to be required for the military to maintain its radiocommunications infrastructure in view of the possible loss of frequencies to the private sector. Military-civil sharing is generally of two forms; long-term and short-term. One form of a long-term sharing arrangement is the military/amateur sharing in certain radiolocation bands. A form of short-term sharing is the coordination of certain frequencies for military tests and training exercises. Examples of this are Army tactical exercises using frequencies in the 30-88 MHz band coordinated with the FCC, and certain JTIDS testing requiring military coordination with the FAA. The DOD has suggested coordinated sharing with the radio astronomy service as one way to have available spectrum for tactical and training purposes.[EN638]

Conclusions


Return to Report Table of Contents.

Proceed to Part 2, Technologies Affecting Spectrum Requirements.