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Adaptive Spectrum Access: Using the Full Spectrum Space

William D. Horne*

The MITRE CorporationMcLean, VA

703-883-6198; whorne@mitre.org

Abstract

Trends in technology are enhancing the future capability of devices to access the electromagneticspectrum using the full range of dimensions associated with the spectrum. This increased capability toaccess the full spectrum “hyperspace” not only improves the ability of systems to use the spectrum butcan also reduce interference and the adverse impact to system performance. This paper presents technicalresearch into advanced adaptive techniques and identifies those parameters or characteristics whereregulation and policy need to be reviewed. Fundamental to these new adaptive techniques is a behavior-based approach to design and, possibly, policy. Consequently, this paper also presents a technical casestudy of one behavior-based approach, Dynamic Frequency Selection, that provides insight into thebenefits and possible limitations of such approaches to cognitive radio design and policy.

1. Introduction

The Federal Communications Commission’s (FCC) Spectrum Policy Task Force (SPTF) report [1]defined three regulatory mechanisms, exclusive use, commons, and command-and-control, for assigningspectrum usage rights; however, each model depends upon recent technological advances to promoteaccess and efficiency, common goals for any spectrum access model. For example, the commons model,even in its most basic form of existing unlicensed bands, essentially requires use of techniques that senseand adapt to the radio environment. In addition, these same adaptive techniques are necessary for theexclusive use model to improve spectrum access through the use of methods such as secondary markets.Consequently, regardless of regulatory model, if flexibility and efficiency are to be promoted, regulationmust enhance or at least allow the use of adaptive techniques.

Many organizations, such as the MITRE Corporation [2-3], are researching and developing the techniquesand technology necessary to enable devices with adaptive spectrum access capability, including the use ofsoftware defined radios (SDRs) [4-5]. This research focuses not only traditional methods to enhanceaccess, such as frequency division multiplexing, but is also exploring ways to adaptively access alldimensions associated with the electromagnetic spectrum. This spectrum “hyperspace” includes not onlythe fundamental parameter, frequency, but also others as well including time, space (both location andsignal directionality), and signal space, which involves power, polarization, coding, and other signalfeatures. Although it is difficult to define the dimensions of the spectrum space [6], the flexibility toaccess spectrum across many of these dimensions provides systems an ability to react and adapt to itsenvironment either in an exploitive, non-cooperative mode or through a coordinated, central controlmethod associated with a networked system. Adaptation is one of the, if not the, fundamental attribute forintelligent wireless systems, that is, systems demonstrating an awareness of its environment and an abilityto automatically react. Recently, such systems are referred to as “cognitive radios” [7-8].

* The views and opinions of the author expressed herein do not necessarily state or reflect those of The MITRECorporation or its clients and sponsors.

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This paper presents research into advances in adaptive spectrum access technology and identifies thoseparameters or characteristics requiring regulation or policy consideration. The paper first discusses thevarious dimensions of spectrum, including the difficulty in actually defining such a spectrum space.Next, the paper reviews the concept and general definition of adaptive spectrum radio along withadvances in technologies enabling this adaptive concept. In addition, demonstrations of technologies andsystems, the subject of the next section of the paper, are important not only for showing the feasibility ofadaptive systems but also for identifying policy considerations. Fundamental to adaptive techniques is anotion of behavior—how does a system react given the conditions of its environment. Consequently, thepaper presents a technical case study of one behavior-based approach, Dynamic Frequency Selection(DFS), to provide insight into the benefits and possible limitations of such approaches to cognitive radiodesign and policy. Finally, drawing from these reviews and case study, the paper concludes with a reviewof policy topics requiring consideration from regulators and other interested parties.

2. The Spectrum Space

When considering adaptive techniques for accessing the spectrum, system designers and policy makersneed to define the parameters over which a system can alter its transmissions. However, identifying thedimensions of spectrum is not clear whether for design purposes or for regulation as one does not want toover-constrain the situation by limiting adaptation to too few parameters while at the same time notexcessively increasing the complexity by defining too many parameters.

One of the concepts discussed in the FCC SPTF Report was expanding regulation beyond the traditionalrealm of frequency and location (space), as epitomized by the International Telecommunication Union(ITU) and US Table of Frequency Allocations that define permitted services according to blocks offrequencies. This limited view of spectrum, as noted by the SPTF Report, prevents the full use of thespectrum and, in many cases, limits the possibilities of technology to improve access to the spectrum.The SPTF Report states:

“The Task Force also analyzed the benefits of parceling out spectrum using variations infrequency, space, power, and time to maximize the use of spectrum. In the past, theCommission has recognized and licensed spectrum primarily by defining spectrum rights interms of the first three dimensions. The Task Force found that new technologicaldevelopments are changing the way in which each of these spectrum dimensions is used. Inaddition, new technology now permit the Commission to increasingly consider the use oftime, in combination with frequency, power, and space, as an added dimension that couldpermit more dynamic allocation and assignment of spectrum usage rights.”

Such definition of the spectrum space is important as it guides the development of regulation but also,more fundamentally, the rights and responsibilities for accessing spectrum, regardless of the allocationmodel (exclusive use, commons, or command-and-control). While many economists, notably RonaldCoase [9], have noted the importance of defining a bundle of rights, they have not explicitly offered adefinition of rights based on the dimensions of the spectrum space. This definition is important forregulators whether licensing primary users or enabling secondary markets and “underlays” (e.g., low-power operations on non-interference basis). Such a definition remains elusive.

Underlying this difficulty in defining economic or regulatory dimensions is the difficulty in actuallydefining the spectrum space. Several researchers [6] have proposed definitions of the spectrum space.Typically, when defining an n-space, the dimensions are orthogonal, that is, the values uniquely define apoint in the space. However, several of the possible parameters (e.g., coding/modulation), do not

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necessarily define an orthogonal parameter but they can still be used to distinguish signals, albeit with astatistical likelihood.

Without stating the definitive dimensions of the spectrum space, Table 1 lists the dimensions proposed byresearchers. It should be noted that this set, in its entirety, is not necessarily orthogonal. However, it doesprovide an organized set of parameters from which to draw a bundle of rights or to define regulatoryparameters. The current international and national system do, in fact, draw from these parameters.

The purpose here is not to definitively define the complete set of dimensions of the spectrum space;rather, the intent is to indicated that regardless what the minimum set is required for defining rights orregulation, emerging adaptive spectrum systems can adjust its own interaction with the spectrum spaceusing all of these parameters. Advances in technology from digital signal processing to antennas areenabling systems to adapt its access across the entire spectrum space.

3. Adaptive Spectrum Radio: A Notional Definition

Forms of adaptive spectrum access have been implemented in a number of systems in the past includingcurrent and next generation mobile phone standards, but the key difference in the emerging adaptivespectrum radios is the increased flexibility, both in speed and variety of parameters, to alter transmissionsand reception. This section provides a notional definition for adaptive behavior based on two conditions:(1) an opportunistic, non-cooperative situation and (2) a situation in which information is shared in acooperative fashion. This notional definition may also serve to define, in part, the meaning of cognitiveradios.

The key benefits for adaptive spectrum access, regardless of the situation, include:

ß Improved spectrum access and utilization;

ß Maintenance of quality of service in a changing environment; and,

ß Capability to adjust emissions to reduce or maintain levels of interference to other systems.

Table 1: Possible Dimensions of the Spectrum Space

General Class Parameter Units Notes

PowerPower (or fieldstrength)

W (or V/m)Often viewed as theindependent variable of thespectrum space

Frequency Frequency Hz

Time Time sec

Location latitude, longitude, elevation

3 dimensions (can begeneralized to otherreference systems besidesgeocentric)

SpaceSignal direction(transmissiondirection, angle ofarrival)

azimuth, elevation

2 dimensions (note:perspective is importantwhether from transmitter orreceiver)

Polarizationvertical/horizontal(clockwise/counter-clockwise)Signal

Coding/modulation (variable) not necessarily orthogonal

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Rudimentary forms of adaptive spectrum access have been implemented for years in systems operating inunlicensed bands (e.g., cordless phones operating around 45 MHz must use an automatic channelselection mechanism that prevents establishment of a link in an occupied channel). More advancedimplementations of adaptability include the various methods for changing data rate, code rate, andmodulation order implemented by second (2G) and third generation (3G) mobile standards [6-7]. Forexample, Enhanced Data Rates for GSM Evolution (EDGE) adapts the transmission parametersmodulation (GMSK or 8-PSK) and coding rate. This type of adaptability is well suited for packet dataand in situations with slow variations in fading and distance loss over the cell coverage area.

Recent advances in technology, specifically sensing and networking, enable two forms of adaptivespectrum access. The first form, often referred to as opportunistic, responds to a changing radiofrequencyenvironment by automatically adapting its transmissions to zones where no other systems are operating.As shown in Figure 1, the basic approach first senses the spectrum to determine where other systems areoperating and where idle spectrum (“white space”) exists and then synthesizes a waveform to exploit the“white space.” Thus, an adaptive system enables operation with other radio systems in non-cooperativefashion and potentially increases the utilization of the spectrum. Such a system can also share knowledgeof the spectrum environment with other similar devices to ensure other users are not affected by theadaptive system. Besides the inter-operation with other systems in underutilized spectrum, adaptivespectrum systems may also increase spectrum utilization since they do not rely upon static frequencyassignments which, if not required, would remain fallow.

Underlying this form of opportunistic adaptive access is the assumption that spectrum is not always usedacross the dimensions of the spectrum space, especially frequency and time. Depending upon thefrequencies that the adaptive system can access, the amount of available spectrum may vary. Severalefforts underway [10] indicate that the spectrum is not fully used across all its dimensions. One keyprogram, discussed later in this paper, is DARPA’s next Generation (XG) program that is explicitlyexploiting this known “white space” in an opportunistic approach.

Another form of adaptive spectrum access exploits shared information or operations to cooperativelydetermine access to the spectrum. Such an operation is inherent, albeit in a limited manner, in existingsystems like trunked radio systems that share channels between users and cellular systems that assignchannels to users (either time slots or CDMA codes). In a more expansive fashion, emerging meshnetworking systems exhibit adaptive and dynamic behavior. Unlike opportunistic approaches, sharinginformation can greatly improve spectrum access utilization by coordinating access across the variousspectrum dimensions. Several methods may be employed to distribute the spectrum knowledge andcontrol whether through beacons or control channels or by sharing databases of existing users.

Future systems may employ either or both forms of adaptive spectrum access, so they should not beviewed as mutually exclusive. In addition, other taxonomies for adaptive systems are possible includingcentralized versus distributed control of adaptation. The key characteristics or behaviors of adaptivespectrum systems are their ability to exploit knowledge of the electromagnetic environment to adapt itsoperations and access to the spectrum. The knowledge may be gained through measuring or sensing thespectrum or through sharing information between systems. Key challenges remain for technologydevelopers including the need to demonstrate the feasibility of adaptive techniques across the variousdimensions of spectrum. This will require working with the policy and regulatory community to developappropriate policies and procedures.

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Figure 1: Adaptive Spectrum Access (Opportunistic)

4. Technology Advances and Demonstration Projects Enabling Adaptability

Enabling this automated, cognitive behavior is a wide-range of advances in technology along with avariety of targeted government and industrial research and development projects. Digital signalprocessing-based radios, typically referred to as Software Defined Radios (SDR), provide the platform forthese dynamic radio systems. However, a variety of technologies contribute to the increasing capabilityto access the spectrum across all of its dimensions. In addition to a variety of activities advancing SDRsand underlying technologies, several targeted projects are investigating adaptive techniques for spectrumaccess.

Technology Advances

While there are different ways to categorize technology, Table 2 provides a general overview oftechnologies fundamental to adaptive access of the spectrum. The intent of Table 2 is only to highlightunderlying technology advances and not meant as an exhaustive review. For those desiring additionaldetails, technical publications of the Institute of Electronic and Electrical Engineers (IEEE) providedetailed overviews of various technologies while academic papers, such as [11], provide good general-audience overviews of the technologies and policies.

Create waveform(s) to use idle spectrum

Determine which channelsare busy

Repeat steps 1 & 2 to monitor spectrumand adapt when environment changes

Radio Frequency (MHz)

Radio Frequency (MHz)

Waveform adaptable tovariable, non-contiguousspectrum

Tim

e (s

ec)

Tim

e (s

ec)

Step 1

Step 2

Step 3

Occupancy informationmay be shared withother radio node

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Table 2: Technology Developments Enabling Adaptability

TechnologyCategories

Elements Trends Limitations

Analog-to-Digital and Digital-to-Analog Converters

Increasing sampling rateand resolution

Advances notcommensurate toadvances in processingelements

Application-SpecificIntegrated Circuit (ASIC)

Lower power, greaterprocessing

Non-programmability

Field Programmable GateArray (FPGA)

Processing speed not asgreat as ASICs

Digital Signal Processor(DSP)

Processing speed not asgreat as ASICs/FPGAs

Digital SignalProcessing

Micro-processors

Lower power, greaterprocessing withprogrammability

Power consumptionhigher than otherprocessing devices

Antennas

Adaptive techniquesusing arrays of elementsprovide- Dynamic beam steering

- Interference cancellation- Multiple antenna/channel techniques

Power Amplifiers, Low NoiseAmplifiers

Increasing bandwidth butlimited advances

RadiofrequencyDevices

Filters, etc. Increasing bandwidth butlimited advances

Performance bandwidthlimits broadbandcapabilities

Data representation(Extensible MarkupLanguage (XML), ResourceDescription Framework(RDF), etc.)

Standardization enablinggreater sharing of data

Data/KnowledgeTools

Knowledge (semantic tools,Web Ontology Language,etc.)

Standardization andobject-orientedapproaches allowing“knowledge-based”operations

Early stages ofstandardization andlimited experience inradiocommunicationsystems

The general categories denoted in Table 2 should not be viewed as stand-alone technologies in isolationof the others. The convergence and simultaneous development in several areas can combine to createpowerful new applications or techniques. For example, the increased signal processing capabilities ofintegrated circuits in conjunction with developments in antenna design combine to form powerfulapplications like adaptive arrays that can dynamically steer the antenna beam or cancel unwanted signals.In addition, this conjunction also enables new techniques like multiple-input multiple-output (MIMO)systems that exploit multiple signal paths to increase the capacity that a channel can carry.

The principal area enabling adaptability is advances in digital signal processing. These advances includeincreased capabilities of microprocessors and other programmable, processing components as listed Table2. System architectures (i.e., SDRs) using these programmable devices need to consider the trade-offbetween flexibility and performance. Flexible signal processing, favoring an architecture centered onprogrammable devices (DSP, FPGA), enables a system to evolve and be changed even after deployment.Alternatively, the need for processing wideband signals generally requires application specific solutions.

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As an example SDR architecture, Figure 2 depicts an architecture that blends the speed of ASICs withthe flexibility of DSPs. The benefits of SDRs using these processing elements include increasedflexibility (upgradeability, customization, faster-time-to-market, and adaptability) and lower costs(economies of scale and increased modularity).

The next major category includes a variety of elements including antennas, amplifiers, filters, and otherrelated electronic components. The key advances and limitations for these elements are primarilyassociated with the bandwidth over which the elements perform. For example, the performance ofantennas is typically limited to a defined frequency band with significant decreasing performance asbandwidth increases. These limitations inhibit adaptive systems to adjust its transmission across thefundamental dimension of frequency. In addition, as noted previously, advances in processing along withnew antenna concepts like adaptive arrays are enabling greater capability to adapt.

A new emerging area of technology enabling adaptability includes data and knowledge tools that providefor standardized approaches to defining, sharing, and interpreting data. By standardizing data, adaptivesystems can readily define operations based on shared information. For example, significant programsdeveloping SDR-based systems, such as the Joint Tactical Radio System (JTRS), have specifiedExtensible Markup Language (XML) for use in defining shared data. XML and related standards such asResource Description Framework (RDF), are cross-platform, software and hardware independent tools forstructuring, storing, and transmitting data. Future advances will not only define data but also allowinferences and operations to be extracted using automated tools. Such emerging “knowledge” tools (e.g.,the World Wide Web Consortium’s Web Ontology Language (OWL)) will exploit defined relationshipsbetween objects, sometimes referred to as “ontologies,” in a manner that allows a machine to makeinferences and determine actions.

Research & Development Projects

In addition to developing the underlying technologies, system research and development projects arerequired to demonstrate the technical principles for dynamic spectrum access. Recent recommendationsfor testbeds for wireless system development [12] further promote the need for technology developers todemonstrate capabilities and limitations in order to assist both end-users and the policy community togain confidence in new adaptive wireless approaches and applications.

r(t)

s(t)

FPGAs

DSPs

FPGAs

ASICs

ASICs

Balancing Performance versus Flexibility

- Highest Performance

- Lowest Flexibility

- Lowest Performance

- Highest Flexibility

A/D

DAC

Figure 2: Example Software Defined Radio Architecture

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Principal objectives for demonstrations include:

ß Feasibility;

ß Identification of system operations concepts;

ß Definition of architectural approaches; and,

ß Exploration of policy considerations.

Demonstrations are not intended as final indicators of future applications or design, but they do provide ameans to explore operations concepts and policy options. Two example research projects provideillustrations of adaptive spectrum access systems.

The MITRE Corporation has developed a testbed, referred to as the Adaptive Spectrum Radio (ASR), todemonstrate feasibility of the adaptive spectrum radio concept with the intention of providing four basiccapabilities: (1) periodic estimation of a channel’s occupancy state; (2) periodic adaptation of a time-limited waveform in response to occupancy state estimates; (3) periodic "joint occupancy vector”negotiation (see Figure 3) with subsequent burst data transfer; and (4) measurement of impairments toprimary users. The testbed uses Commercial-Off-The-Shelf (COTS) products with a processingarchitecture similar to that shown in Figure 2 and a functional architecture as shown in Figure 4. Thetestbed consists of two chassis that operate as two independent radios along with signal generators forsimulating the radiofrequency environment. To date, the MITRE testbed has been used to demonstratethe essential feasibility of adaptive spectrum access and to identify design and policy considerations.

The most significant project exploring adaptive spectrum access is DARPA’s next Generation (XG)project that is developing a prototype adaptive spectrum system and is defining a set of abstract behaviorsand associated protocols to guide the operations according to spectrum management policy [13]. Thisambitious project plans to demonstrate both the feasibility and operations in realistic radiofrequencyenvironments. Most notably, the XG project is defining a set of behaviors and protocols that willtranslate spectrum management policy into clearly defined operations of the radio. These concepts andother associated approaches are discussed in Section 6.

Sp

ec

tru

m @

Ts

Frequency

ASR 1

Sp

ec

tru

m @

Ts

ASR 2

Frequency

Negotiate Common Passband

Negotiate Common Passband

Frequency

Dis

join

t P

as

sb

an

d

Adaptive Waveform @Ts

Figure 3: MITRE Testbed Frequency Occupancy Negotiation

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Figure 4: MITRE Testbed Functional Architecture

Other projects exploring adaptive spectrum access techniques include a variety of projects developing adhoc and mesh networking systems. These projects include both government, primarily DoD projects likethe JTRS Wideband Networking Waveform, and industry projects, such as IEEE 802 standards workexploring possible ad hoc networking enhancements to the baseline wireless networking standards.Although the primary work in these projects focuses on cooperative networking approaches, extensionsmay be developed that allow additional adaptive techniques for opportunistic operations as well.

5. Case Study: 5 GHz Sharing and Dynamic Frequency Selection (DFS)

While the previous sections have defined the spectrum space and the increasing capability to dynamicallyaccess the spectrum across this multi-dimensional space, the actual benefits and implementations of“cognitive” behavior needs to be understood both for future development of systems and the need forpolicy to define appropriate boundaries. One way to attempt an understanding is through the case studyof an emerging technology that is being implemented in future radio local area networks (RLAN).

Background

During the late 1990s, wireless local area networks , notably the IEEE 802.11 family of standards, knownas WiFi, and the European HIPERLAN system, became popular as broadband wireless systems enablingInternet connections in office, commercial, and home situations. Based on projected needs for additionalspectrum beyond the previously defined bands in the 2.4 GHz band and several bands near 5 GHz (seeTable 3), proponents requested additional spectrum in the 5 GHz region. However, opening this spectrumto RLANs would require sharing with other systems, notably government radar systems.

SpectrumAnalyzer

Test SignalSource

OptionalSingle Freq

Tx

SpectrumEstimator

AdaptiveSystem

Controller

Transmitter /Modulator

ReceiveSubsystem

Tx/Rx ChassisAssy

Combined Rxspectrumenvironment

Controlsoftware& Policy

agent

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Table 3: 5 GHz ITU Allocations & Selected Country Allocations

Allocation to services

(specific allocations in selected regions/countries)

Frequency(MHz)

Region 1(Europe/Africa) Region 2 (Americas) Region 3 (Asia)

MOBILE** except aeronautical mobile (selected footnotes: 5.BD02)

5150-5250* Licensed Exempt(Europe) (HIPERLAN)(200 mW e.i.r.p. limit)

U-NII*** (United States)(50 mW tx power. limit)

Hi-speed wireless access (Japan)

RADIOLOCATION****

MOBILE** except aeronautical mobile (selected footnotes: 5.BD02 5.BD04)5250-5350*

Licensed Exempt(Europe) (HIPERLAN)(200 mW e.i.r.p. limit)

U-NII*** (United States)(250 mW tx power limit)

5350-5470 No mobile (e.g., WiFi, HIPERLAN) systems

RADIOLOCATION****

MOBILE** except aeronautical mobile (selected footnotes: 5.BD02 5.BD07)5470-5725*

Licensed Exempt(Europe) (HIPERLAN)(200 mW e.i.r.p. limit)

Proposed U-NII*** (UnitedStates)(1 W tx power limit)

RADIOLOCATION

5725-5825* U-NII*** (United States)(1 W tx power limit)

ITU Footnotes

5.BD02 The use of the bands 5150-5350 MHz and 5470-5725 MHz by the stations in the mobile service shall be inaccordance with Resolution [COM5/16] (WRC-03). (WRC-03)

5.BD04 In the band 5250-5350 MHz, stations in the mobile service shall not claim protection from the radiolocationservice, the Earth exploration-satellite service (active) and the space research service (active). These services shall not imposeon the mobile service more stringent protection criteria, based on system characteristics and interference criteria, than thosestated in Recommendations ITU-R M.1638 and ITU-R SA.1632. (WRC-03)5.BD07In the band 5470-5725 MHz, stations in the mobile service shall not claim protection from radiodeterminationservices. Radiodetermination services shall not impose on the mobile service more stringent protection criteria, based onsystem characteristics and interference criteria, than those stated in Recommendation ITU-R M.1638. (WRC-03)

Resolution [COM5/16] (WRC-03)

Resolution requires, inter alia, the following:- Dynamic Frequency Selection (DFS) required for 5250-5 350 MHz and 5470-5725 MHz bands- Indoor use only with maximum 200 mW e.i.r.p. in 5150-5250 MHz band- In 5250-5350 MHz, limited e.i.r.p. (250 mW/1 W with other constraints) and indoor use should be promoted

Footnotes* Other services allocated in several sub-bands including EARTH EXPLORATION-SATELLITE** Mobile service includes WiFi, HIPERLAN, and other similar Radio Local Area Networks (RLANs)*** Unlicensed National Information Infrastructure (U-NII)**** Radiolocation service includes radar systems

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Consequently, the ITU initiated a series of studies to explore the possibility for sharing and to define, ifpossible, technical restrictions necessary to allow sharing. During the resulting studies in the ITURadiocommunication Sector (ITU-R) study groups, several recommendations were prepared that definethe operations of RLANs and the general characteristic of radar systems. In addition, based on extensivesimulations of projected deployments of RLANs and radar system operations, the ITU-R prepared arecommendation [14] that defines the necessary conditions for RLANs to share with radar systems. Thisrecommendation was spurred by a US Government and industry agreement that formally defined severalkey parameters. At the 2003 ITU World Radiocommunication Conference in Geneva, the memberadministrations of the ITU agreed to allow the sharing of the 5 GHz bands as it provided a primaryallocation to both the radiolocation (e.g., radar systems) and the mobile services (e.g., WiFi systems).The new allocations are shown in Table 3. In the United States, the FCC has initiated proceedings [15] toupgrade to the allocations and to proscribe DFS for RLAN systems. In addition, an ongoing informaleffort of US government and industry representatives, led by the National Telecommunications andInformation Administration (NTIA), plan to test DFS-enabled systems to verify operations.

Dynamic Frequency Selection (DFS)

Unlike most technical restrictions of the ITU or regulatory agencies like the FCC that define simplepower limits or similar constraints, the recommendation defined a behavior-based mechanism, DFS. Thismechanism requires a rudimentary “cognitive” ability to understand the radiofrequency environment andto adapt its operations.

The operations achieve the required protection of radar systems by avoiding the use of, or vacating, achannel identified as being occupied by radar equipment based on detection of radar signals. Theparameters indicated in Table 4 define the performance of a set of proscribed DFS behaviors (illustratedin Figure 5):

ß Channel Availability Check. Before initiating transmissions on a channel, a RLAN must monitor fora defined period (Channel availability check time) the radio channel to determine whether a radar orother signal is present based on a set threshold (DFS detection threshold) and averaging period (1 µs).

ß In-Service Monitoring. The RLAN must continually monitor the channel by searching for radarsignals in-between normal transmissions. This requires “quiet” periods between bursts of data.

ß Channel Abdication. Upon detection of a signal above the threshold (DFS detection threshold), theRLAN must cease all transmission within a defined period (Channel move time). Normal datatransmissions may occur for a maximum of 200 ms with all control signaling ending by the definedperiod (Channel move time).

ß Channel Non-Occupancy. Once a channel has been determined to contain a radar or other systemsignal, the RLAN must not utilize the channel for a defined period of time (Non-occupancy time).

Essentially, the RLAN implementation of DFS provides a “listen-before-talk” operation and allows radaror other systems to essentially shut down the operations of nearby RLANs through its own nominal orsystematic transmissions. The DFS mechanism requirements, however, do not specify the actualmethodology for channel reassignment or the technique used to detect radar or other signals—theseremain the province of the system designer.

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Table 4: DFS Performance Requirements

Parameter Value

DFS detection threshold(normalized to the output of a 0 dBi antenna)

–62 dBm for devices with a maximum e.i.r.p.of < 200 mW and

–64 dBm for devices with a maximum e.i.r.p.of 200 mW to 1 W averaged over 1 µs

Channel availability check time 60 s

Non-occupancy period 30 min

Channel move time £ 10 s

Figure 5: Dynamic Frequency Selection (DFS) Operations

Design and Policy Implications

Although DFS-enabled RLANs provide only a rudimentary level of adaptive spectrum capabilities, itdoes set precedence for “cognitive” systems. A preliminary review reveals several key observationsconcerning the process leading to the agreements and the specification of the DFS:

ß The ITU agreements define a set of behaviors required for a system and specifically defines theassociated functional and performance requirements;

ß The required parameters were determined through models of existing systems and statisticalsimulations of projected deployments;

Channel AvailabilityCheck (60 sec)

Radar pulses

Non-occupancy period(30 min)

Radar Signal(channel x)

RLANTransmissions(channel x)

Case 1) RLAN Detects Radar Signal on Channel x Prior to Operations

Channel AvailabilityCheck (60 sec)

Radar pulse

Non-occupancyperiod (30 min)

Radar Signal(channel x)

RLANTransmissions(channel x)

Case 2) RLAN Detects Radar Signal on Channel x After Operations Initiated

Data bursts

Radar pulse detected

Channel move time (10 sec)

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ß The agreement essentially defines a primary service (radiolocation) and a subordinate user (mobile),even though both services are nominal given “primary” allocation status; and,

ß Certification or other verification testing requires a test program to not only measure specificparameters, as is classically performed for certification, but also to certify behavior in a specifiedradiofrequency environment--expanding the normal testing required for regulatory certification.

Although the DFS mechanism is only proscribed for a limited set of systems (RLANs operating inspecific 5 GHz bands), it does serve as a case study for cognitive behavior that can be used to raise policyquestions including:

ß When defining a set of behaviors and associated parameters, should they be defined based on existingsystems or should another means be used to consider possible future systems that may promoteinnovation and introduction of new systems?

ß Can more extensive behaviors, such as interactive mechanisms requiring negotiations (e.g., secondarymarkets) or identification of specific systems, be defined rather than purely reactive mechanisms?

ß Can behaviors be defined that do not necessitate primary and subordinate systems?

ß Can a set of well-defined, high-level behaviors be defined that can be used for defining appropriate“cognitive” functions?

ß Can a regularized set of test procedures be defined for any behavior-based regulation?

The following section will explore these and other considerations.

6. Design and Policy Considerations

The increased capability of emerging adaptive spectrum radios to access the spectrum across all itsdimensions, on a cursory level, appears to challenge notions of fixed frequency assignments and rigidcertification–pillars of the current spectrum regime. However, the introduction of behavior-basedregulation may provide a means to define rights within the existing regulatory framework and regulatorymodels. As noted earlier, adaptive capabilities are essentially fundamental to improving spectrumutilization regardless of the underlying model (command & control, commons, or exclusive use).

Development of the prototype adaptive systems along with the DFS case study reveal several possiblemechanisms and parameters for adaptability that can be employed by system designers and, possibly, forpolicy. Behaviors may be either internal self-directed or external, observable operations, with theexternal behaviors of most interest for regulation. Table 5 provides a high-level set of behaviors that candefine operations for adaptive systems. These high-level categories, however, do not specify the sets ofparameters or details that may be necessary to enable sharing or other desired outcomes. As an example,the DFS mechanism is essentially a non-cooperative, casual behavior with specific parameters (i.e.,threshold detection levels, etc.) defined based on existing systems and simulations to enable sharingbetween dissimilar systems (radars and RLANs). Another possible casual mechanism, “interferencetemperature,” was identified by the FCC Spectrum Policy Task Force report. This mechanism allows thesystem to transmit as long as certain defined thresholds (“interference temperature”) are not exceededwithin a given region as defined by the dimensions of the spectrum space. Of course, notions like“interference temperature” may require additional knowledge of other systems within the defined spaceso it may require an interactive approach.

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Table 5: High-Level Adaptive Behaviors

High-Level Behavior (external) Mechanism/Parameter Considerations

Prohibited

Limit access through definedspectrum space regions(frequency band, time, location,directionality, etc.)

May be used to protect existingsystems during introduction ofadaptive systems or forprotecting critical systems

Given location in the spectrumspace (e.g., physical location andfrequency), access may bepermitted with specificconstraints

May require mandated existenceand availability of databases

Given measured environment,access may be permitted withspecific constraints

- Requires system to measureenvironment and reactaccordingly (e.g., DFS)- May also include limits basedon the overall environment (e.g.,“interference temperature”)

Casual (if-then)

Given signal identification,access may be permitted withspecific constraints

Use of both databases andmeasured environment todetermine actions

Interactive (negotiated or sharedcontrol)

Negotiated access usingspectrum space dimensions(e.g., request access to x bandduring y timeframe in location z)

- Such behavior enablesregulatory constructs likesecondary markets- Less interactive mechanisms,such as beacons, can also beused to define access

Dynamic

Rather than pre-defined actions,systems can automaticallydetermine what criteria andactions are needed based onmeasurements, sharedinformation, and requirements(e.g., interference levels)

Enables innovative conceptsbased on dynamic system needsand conditions

Such attempts for defining high-level behaviors and parameters, however, may neglect the need todetermine the highest level of spectrum access principles. Rather than specifying detailed sets ofbehaviors and parameters regulators and system designers may wish to consider ways to define theoverarching principles for achieving the goals of spectrum management, centered on spectrum access.Fundamental to any high-level principle is a set of clearly defined access rights based on the full multi-dimensional spectrum space. If achievable, such principles may allow systems to adapt behavior basedon the rights rather than highly defined mechanisms like DFS.

Finally, these issues will be further explored as part of FCC proceedings on cognitive radios and othertopics resulting from the Spectrum Policy Task Force report, including likely Notice of Inquiries (NOIs)in the Fall of 2003. Another opportunity for involvement is the DARPA XG program’s open “Requestfor Comment (RFC)” process soliciting comments on its proposed vision and protocols [13]. This “RFC”effort plans an extensive review of possible adaptive behaviors, protocols, and policy.

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7. Conclusion

Enabled by advances in technology and targeted research and development programs, emerging systemscan adaptively access the spectrum across a wide-range of dimensions. This increased capability raises anumber of questions for system designers and policy makers. As a first step to defining future adaptivespectrum access, this paper has identified, through the use of existing demonstration efforts and a casestudy, key considerations and questions for system designers and regulators to consider, includingpossible high-level sets of behaviors. Although no definitive proposals are recommended, this study alsoidentified several opportunities in which researcher may wish to engage to further explore and define thefuture of adaptive spectrum access.

References

[1] Spectrum Policy Task Force Report, FCC, ET Docket No. 02- 135, Nov 2002

[2] D. Schaefer, “Wide Area Adaptive Spectrum Applications,” MILCOM 2001, Vienna, VA, Oct 2001

[3] W. Horne, P. Weed, & D. Schaefer, “Adaptive Spectrum Radio: A Feasibility Platform On The PathTo Dynamic Spectrum Access,” International Symposium On Advanced Radio Technologies, Boulder,CO, 4-7 March 2003

[4] R. Berezdivin, R.Breinig, & R. Topp, “Next Generation Wireless Concepts, Communications andTechnologies,” IEEE Commun. Mag., Mar 2002

[5] S. Nanda, K. Balachandran, & S. Kumar, “Adaptation Techniques in Wireless Packet Data Services,”IEEE Commun. Mag., Jan 2000

[6] R. J. Matheson, “The Electrospace Model as a Frequency Management Tool,” InternationalSymposium On Advanced Radio Technologies, Boulder, CO, 4-7 March 2003

[7] FCC, Workshop on Cognitive Radio Technologies, 19 May 2003 (see TBD web site for presentations)

[8] J. Mitola, Cognitive Radio: An Integrated Agent Architecture for Software Defined Radio,Dissertation, KTH, June 2000

[9] R. Coase, “The Federal Communications Commission,” 2 J. L. & ECON. (1959), 1

[10] P. Marshall, “Spectrum Management Protocols in the DARPA Next Generation CommunicationsProgram,” SDR’02 Tech Conf, San Diego, Nov 2002

[11] W. Lehr, F. Merino, S. Eisner Gillett, “Software Radio: Implications for Wireless Services, IndustryStructure, and Public Policy,” 2002 Telecommunications Policy Research Conference, Alexandria, VA,Sep 2002

[12] Defense Science Board, “Wideband Radio Frequency Modulation: Dynamic Access to MobileInformation Networks,” Office of the Under Secretary of Defense for Acquisition, Technology, andLogistics, Washington, DC, July 2003

[13] DARPA XG Program web site: http://www.darpa.mil/ato/programs/XG/

[14] Recommendation ITU-R M.1652, “Dynamic frequency selection (DFS) in wireless access systemsincluding radio local area networks for the purpose of protecting the radiodetermination service in the 5GHz band,” (2003)

[15] FCC, Notice of Proposed Rulemaking, “Revision of Parts 2 and 15 of the Commission’s Rules toPermit Unlicensed National Information Infrastructure (U NII) devices in the 5 GHz band,” ET DocketNo. 03 122, 2003