IEEE Communications Magazine • October 201392 0163-6804/13/$25.00 © 2013 IEEE

INTRODUCTION

White spaces are unused spectrum resourcesat specific times and locations that can beexploited through spectrum sharing. TV whitespace (TVWS) exists in the broadcast TVoperating frequencies known as the VHF/UHFband, specifically ranging from 470–790 MHzin Europe [1, 2] and non-continuous 54–698MHz in the United States [3]. The existence ofTVWS enables spectrum sharing among unli-censed white space devices (WSDs) andlicensed protected users of the TVWS band.The TVWS band is currently used by a largevariety of licensed protected services, such asterrestrial TV broadcast services, and programmaking and special event (PMSE) users. Someof the licensed services have resided in thisband for nearly 100 years [4]. Licensing pro-tects the incumbent users of the TVWS bandfrom interference within their service area.Therefore, WSDs operating in the TVWSband are not permitted to interfere with anyprotected incumbent user in their specifiedoperating area.

Propagation characteristics of the TVWSband make it a desirable and convenient spec-trum for many wireless transmission services [5].First, because this band resides under the 1 GHzfrequency, material obstruction is less harmfulthan at higher frequencies, allowing non-line-of-sight coverage [6]. The difference in signal atten-uation between a variety of materials andfrequencies is shown in Table 1 [7], where differ-ences of up to 50 dB are found between 570MHz and 5.7 GHz. Second, the TVWS bandpresents a path loss advantage over unlicensedindustrial, scientific, and medical (ISM) bands(2.4 and 5.7 GHz) due only to operating fre-quency. For example, TV channel 2 (54–60MHz) has 20 dB less path-loss than TV channel30 (566–572 MHz), which itself holds a 20 dBgain over the unlicensed band at 5.7 GHz.

The superior propagation factors of theTVWS band are demonstrated in Fig. 1. Thecapacity and distance components are comparedfor low transmission-power TVWS mobiledevices, 2.4 GHz devices and 5.5 GHz devices,as well as high power TVWS-fixed devices.Wider channels in the high frequency bands,such as the 80 MHz channels used in the 5 GHzISM band, provide higher capacity over a shortrange, but require more infrastructure toachieve wide-area coverage. In contrast, the 6-MHz-wide 4 W white space signal is morerobust and propagates longer distances with asignificant capacity. The calculations for Fig. 1assume free-space propagation using the Friistransmission equation [8] in order to demon-strate relative performance with characteristicsystem parameters.

The excellent propagation characteristics ofthe TVWS band coupled with underutilization inmany locations present desirable potential spec-trum sharing opportunities. To achieve sharingamong WSDs and licensed TV broadcasters andPMSE users, many challenges must be addressedby a common standard. One of the main chal-lenges is guaranteeing the protection of incum-bent users of the TVWS band from interferencein their operating region. WSDs are required tooperate in unoccupied spectrum, which can varyin size, location, and time. This means WSDsmust support different channel widths and beable to learn from an approved geolocationdatabase which channels are available and for

ABSTRACT

Spectrum today is allocated in frequencyblocks that serve either licensed or unlicensedservices. This static spectrum allocation has lim-ited resources to support the exponentialincrease in wireless devices. In this article, wepresent the IEEE 802.11af standard, whichdefines international specifications for spectrumsharing among unlicensed white space devices(WSDs) and licensed services in the TV whitespace band. Spectrum sharing is conductedthrough the regulation of unlicensed WSDs by ageolocation database (GDB), the implementa-tion of which differs among regulatory domains.The main difference between regulatory domainsis the timescale in which WSDs are controlled bythe GDB, resulting in different TVWS availabili-ty and WSD operating parameters. The IEEE802.11af standard provides a common operatingarchitecture and mechanisms for WSDs to satis-fy multiple regulatory domains. This standardopens a new approach to treat spectrum as a sin-gle entity shared seamlessly by heterogeneousservices.

STANDARDS

Adriana B. Flores, Ryan E. Guerra, and Edward W. Knightly, Rice University

Peter Ecclesine and Santosh Pandey, Cisco Systems

IEEE 802.11af: A Standard for TV White Space Spectrum Sharing

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IEEE Communications Magazine • October 2013 93

what time duration. Once operating in an avail-able channel, WSDs are required not to interfere with incumbent devices in neighboringchannels. Finally, WSDs are required to immedi-ately cease transmissions when the databaseinforms them to stop.

To address these challenges, the IEEE802.11af standard provides an internationalframework that adapts to the different WSDoperating parameters and regulatory domainsaround the world. In this article, we present thestandard framework defined by IEEE 802.11af;then we discuss how this framework can beapplied to the two main regulatory approaches.Because the standard is still in the letter ballotdraft process as of June 2013, we focus our dis-cussion on high-level architecture and applica-tions.

STANDARD FRAMEWORKIn this section we describe the primitives andmain mechanisms of the IEEE 802.11af stan-dard. We present the key architecture compo-nents, the communication flow and mechanismsutilized by the standard to satisfy different inter-national regulations and finally we present thephysical layer operation.

COMPONENTS OF THEIEEE 802.11AF ARCHITECTURE

In this section we introduce the entities thatform an 802.11af network and present the non-regulatory specific roles these elements execute.

Geolocation Database — The primary ele-ment, and what mainly differentiates the IEEE802.11af operation from other 802.11 standards,is the geolocation database (GDB). The GDB isa database that stores, by geographic location,the permissible frequencies and operating param-eters for WSDs to fulfill regulatory requirements.GDBs are authorized and administrated by regu-latory authorities; therefore, a GDB’s operationdepends on the security and time requirementsof the applied regulatory domain [9].

Registered Location Secure Server — Thenext architectural element in an IEEE 802.11afnetwork is the registered location secure server(RLSS). This entity operates as a local databasethat contains the geographic location and oper-ating parameters for a small number of basicservice sets (BSSs). The RLSS distributes thepermitted operation parameters to the accesspoints (APs) and stations (STAs) within theBSSs under the RLSS’s control [9].

Just as the operation of the GDB depends onthe security and time requirements of regulatorydomains, the role the RLSS plays in the networkvaries across regulatory domains and is explainedin detail later in the regulatory framework.

Geolocation-Database-Dependent Entities— The remainder elements in the IEEE 802.11afnetwork are referenced by the term geolocationdatabase-dependent (GDD), which specifies thattheir operation is controlled by an authorizedGDB, which ensures these satisfy regulationrequirements [9].

Table 1. Received signal magnitude gain in dB (0.0 dB = no attenuation) [7].

Materials 0.57 GHz(dB)

1 GHz(dB)

2 GHz(dB)

5.7 GHz(dB)

0.57 to 5.7GHz (DdB)

Brick 89 mm –1.5 –3.5 –5.4 –15 13.5

Brick 267 mm –4.8 –7 –10.5 –38 33.2

Composite Brick 90 mm/Concrete Wall 102 mm –12 –14 –18 –42 30

Composite Brick 90 mm/Concrete Wall 203 mm –21.5 –25 –33 –71.5 50

Masonry 203 mm –9.5 –11.5 –11 –12.75 3.25

Masonry 610 mm –26.5 –27.5 –30 –46.5 20

Glass 6 mm –0.4 –0.8 –1.4 –1.1 0.7

Glass 19 mm –2.5 –3.1 –3.9 –0.4 –2.1

Plywood (dry) 6 mm –0.15 –0.49 –0.9 –0.1 –0.05

Plywood (dry) 32 mm –0.85 –1.4 –2 –0.9 0.05

Reinforced concrete 203 mm/1% steel –23.5 –27.5 –31 –56.5 33

Reinforced concrete 203 mm/2% steel –27.5 –30 –36.5 –60 32.5

The primary element

and what mainly dif-

ferentiates the IEEE

802.11af operation

to other 802.11

standards is the

GDB. The GDB is a

database that stores

by geographic loca-

tion the permissible

frequencies and

operating parame-

ters for WSDs to ful-

fill regulatory

requirements.

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IEEE Communications Magazine • October 201394

GDD-Enabling Station — The GDD-enablingstation is the equivalent of the entity commonlyknown as the AP. However, in the 802.11af stan-dard this entity controls the operation of theSTAs in its serving BSS. The GDD-enabling STAcan securely access the GDB to attain the operat-ing frequencies and parameters permitted in itscoverage region. With this information the GDD-enabling STA has the authority to enable andcontrol the operation of the STAs under its ser-vice, identified as GDD-dependent STAs. Specifi-cally, the parameters obtained from the GDB arerepresented through a white space map (WSM).The GDD-enabling STA ensures the mainte-nance and distribution of a valid WSM. Addition-ally, the GDD-enabling STA transmits a contactverification signal (CVS) for GDD-dependentSTAs to check validity of the WSM [9].

GDD-Dependent Station — The GDD-depen-dent station can be identified as the STAs in theBSS architecture. However, the 802.11af stan-dard specifies that the operation of the STAs iscontrolled by the serving GDD-enabling STAs.The GDD-dependent STAs obtain the permittedoperating frequencies and parameters in a formof a WSM from either the GDD-enabling STAor RLSS. The validity of the WSM is confirmedthrough the CVS transmitted by the GDD-enabling STA [9].

Registered Location Query Protocol — TheRegistered Location Query Protocol (RLQP)serves as the communication protocol betweenGDD-enabling and GDD-dependent STAs toshare WSM and channel utilization [9]. This pro-tocol enables the operation of the main mecha-nisms used in the IEEE 802.11af standard.Through this communication the STAs can effec-tively select spectrum, power, and bandwidthallowed by their regulation domain.

COMMUNICATION FLOW BETWEEN ENTITIES

The 802.11af standard defines the communica-tion protocol between the GDD-dependentSTAs, GDD-enabling STAs, and RLSS. Howev-er, the communication flow between the GDBand the high-level entities (RLSS and GDD-enabling STAs) is outside the scope of the802.11af protocol. The standard’s mechanismsare independent of how this communication isperformed, allowing regulators to select thecommunication protocol over the Internet’sinfrastructure.

Figure 2 illustrates two infrastructure BSSscontaining all the components of the IEEE802.11af architecture introduced. As shown inFig. 2, the RLSS and GDD-enabling STAsobtain white space availability through the Inter-net.1 Within the 802.11af scope, the RLSS onlycommunicates with the GDD-enabling STAsthrough infrastructure and operates bidirection-ally. Finally, the GDD-dependent STAs performbidirectional over-the-air communication withGDD-enabling STAs, within either the TVWSband or other ISM bands.

802.11AF MECHANISMSIn this section we present the mechanismsdefined in the 802.11af standard and logicalmessages passed between the architecture enti-ties to satisfy regulatory requirements.

Channel Availability Query — Through thechannel availability query (CAQ) procedure,STAs obtain the available radio frequenciesthat allow operation in their location in theform of a white space map (WSM). In the CAQprocess the RLSS grants the WSM to the CAQrequesting STA. However, in some regulatorydomains the RLSS is required to access theGDB to obtain the channel availability informa-tion. The CAQ request may contain multipledevice locations. The CAQ responding STAmust restrict the WSM validity to either aunique device location or a bounded area ofmultiple locations [9].

The GDD-dependent STA performs a CAQrequest to a GDD enabling STA in three differ-ent cases: first, to remain in the GDD enablestate after enablement times out; second, when achange in channel availability is indicated by theGDD-enabling STA through a CVS; third, if theGDD-dependent STA has moved beyond theregulatory permitted distance [9].

Channel Schedule Management — TheGDD-enabling STAs use the channel schedulemanagement (CSM) procedure to query anRLSS or other GDD-enabling STAs to obtainwhite space channel schedule information. Thechannel schedule indicates a schedule changeand consists of the start and ending times for therequested channels [9].

The GDD-dependent STAs do not performCSM requests. However, the GDD-enablingSTAs can transmit a CSM request to an RLSSor other GDD-enabling STA (with GDB orRLSS access) to query the schedule informationfor white space channels in either TV channelsor WLAN channels.

Figure 1. Capacity vs. distance comparison for different wireless systems calcu-lated with the parameters shown in Table 2.

Range (m)

Capacity vs. distance for different frequency bands

100

Cap

acit

y (M

b/s)

0

200

300

400

500

600

700

800

900

10010 200 300 400 500 600 700 800 900 1000 1100 1200

TVWSTVWS-fixedWLAN-2.4WLAN-5

1 The Google database ofwhite space availability inthe United States appearsat http://www.google.org/spectrum/whitespace/channel/

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IEEE Communications Magazine • October 2013 95

Contact Verification Signal — The contactverification signal (CVS) is sent by a GDD-enabling STA to serve two purposes. First, thetransmission of the CVS establishes which GDD-dependent STAs are within the reception range ofa GDD-enabling STA. Second, the CVS helps theGDD-dependent STAs ensure operation under avalid white space map (WSM) and that it corre-sponds to the serving GDD enabling STA [9].

To validate operation under a correct WSM,the GDD-dependent STAs utilize the map IDfield in the CVS frame. If the map ID value inthe CVS frame is equal to its existing WSM, theGDD dependent STA assumes the operatingWSM is valid and resets its enablement valida-tion timer [9]. However, if the map ID is differ-ent from the existing WSM ID, theGDD-dependent STA transmits a ChannelAvailability Query request to obtain the validWSM in the CAQ response. If the GDD-depen-dent STA does not obtain the valid WSM, itstops transmission after the enablement valida-tion timer is expired [9].

GDD Enablement — The GDD Enablementprocedure allows a GDD-enabling STA to forma network, satisfying regulation requirementsunder the control of a GDB [9]. A GDD-enabling beacon signal is transmitted on avail-able channels in the TVWS band by aGDD-enabling STA to offer GDD enablementservice. A GDD-dependent STA, upon receivingthe GDD-enabling signal, can attempt enable-ment with the GDD Enablement Responseframe. However, some regulatory domainsrequire that, prior to enablement the GDD-enabling STA identifies with a GDB that therequesting GDD dependent STA is authorizedto operate in the location-selected frequencies.

The GDD-dependent STAs have three GDDenablement states: Unenabled, Attempting GDDEnablement, and GDD Enabled. When in unenabledstate, the GDD-dependent STA cannot transmitany frames; instead it passively scans channels foran enabling signal from a GDD-enabling STA tojoin their network. The GDD-dependent STAenters the Attempting GDD Enablement state whenit receives a GDD-enabling signal that allows it toonly transmit the GDD enabling response frameafter GDB authentication, if required by regulation.The GDD Enabled state is reached when the GDD-dependent STA receives a successful GDD Enable-ment Response, which causes the GDD enablementvalidity timer to begin.

Once the GDD-dependent STA is enabledfor operation, its state can be changed to unen-abled by two main causes. First, when the GDDenablement validity timer is expired, consideringthe timer modifications performed by the WSM,CVS, and CAQ procedures. Second, a GDD-dependent STA is required to cease transmissionif it receives from the GDD-enabling STA whoenabled its operation an unexpected GDDenablement response frame with “AuthorizationDeenabled” [9].

Network Channel Control — Network chan-nel control (NCC) is a two-message procedurethat controls the frequency usage in the TVWSband. The NCC requesting STA petitions for

usage of selected frequencies from its WSM byproviding its spectrum mask. The NCC respond-ing STA allows operation in the requested fre-quencies if available by providing the validnetwork channels and related transmit powerconstraints in an NCC response frame. TheNCC procedure is commonly performed betweenthe STAs, the GDD-dependent STA as the NCCrequesting STA and the GDD enabling STA asthe NCC responding STA. It is possible for theNCC responding STA to forward the NCCrequest to the RLSS, which constructs the NCCresponse frame and sends it via the NCCresponding STA. An STA is allowed to performa new NCC request whenever the WSM ischanged.

White Space Map — The WSM is a list ofidentified available white space channels andcorresponding power limitations provided by theGDB.

A GDD-enabling STA is required to obtain

Figure 2. Example TVWS network including all 802.11af architecture entities[9].

Registered locationsecure server

Geo

loca

tion

data

base

GDB1

GDD-enabling STAAP1

GDD-dependent STASTA1

GDD-dependent STASTA2

Geo

loca

tion

data

base

GDB2

GDD-enabling STAAP2

Outside scope of IEEE 802.11af std. Scope of IEEE 802.11af std.

GDD-dependent STASTA3

RLSS

InternetInfrastructureOver the air

Table 2. Calculation parameters assuming free-space propagation.

Parameter TVWS-fixed TVWS WLAN 2.4 WLAN 5

TX power (mW) 4000 40 40 40

Frequency (MHz) 192 518 2437 550

Bandwidth (MHz) 5.33 5.33 20 80

Minimum SNR (dB) 8 8 8 8

TX antenna gain (dBi) 0 0 0 0

RX antenna gain (dBi) 12 –3 0 0

Path loss exponent 4 4 4 4

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IEEE Communications Magazine • October 201396

the permitted frequencies and operating param-eters before it begins transmissions. Based onthe obtained GDB information, the GDDenabling STA generates the WSM to transmit tothe GDD-dependent STAs under its operatingregion. The WSM is transmitted by the GDDenabling STA within the GDD Enablementresponse frame, CAQ response frame and WSMAnnouncement frame. The transmission powerlimitation in a WSM when the channel band-width consists of multiple white space channelsis constrained by the minimum power level foundon the multiple channels [9].

It is important to note that the GDD-depen-dent STAs can only transmit on the availablechannels assigned in their valid WSM. The WSMmay be updated by the GDB as channel avail-ability changes. Whenever a GDD-dependentSTA receives an updated WSM from its GDD-enabling STA, it is obligated to move channels ifoperating in a channel marked as unavailable inthe updated WSM [9].

A final important aspect of the WSM is thatthese are country-specific due to the differencein regulatory operations of GDBs across regula-tory domains. The 802.11af standard provides ageneral format for the WSM element that canbe applied to any regulatory domain. Inside theWSM element the field WSM information, whichspecifies the available channel information forthe TVWS, adapts to the requirements of anyregulation [9].

PHYSICAL LAYERIn the IEEE 802.11af standard the TV highthroughput (TVHT) physical layer (PHY) speci-fication replaces the HT (20 MHz orthogonalfrequency-division multiplex, OFDM) and VHT(20, 40, 80, 80+80, 160 MHz OFDM) PHY spec-ifications in WSD devices when operating inTVWS bands. A TVHT device has support forsingle-channel bandwidths or basic channel unit(BCU) W of 6, 7, and 8 MHz depending on theregulatory domain, as shown in Fig. 3. Addition-

al bonded or non-contiguous bandwidths of 2W,4W, W+W, and 2W+2W are possible, as illus-trated in Fig. 3. Only single-channel bandwidthW and a single spatial stream is mandatory,although multiple-input multiple-output(MIMO) transmissions with 4x space-time blockcoding (STBC) and 4x multi-user (MU) diversityare supported [9].

The TVHT transmission format is similar tothat of a 40 MHz VHT transmission. It currentlydefines 144 OFDM subcarriers for 6 and 8 MHzchannels and 168 for 7 MHz channels, so 6 and7 MHz transmissions are spectrally identical. Forall W, data is sent on subcarrier indices –58to –2 and 2 to 58, with index 0 at DC and 6 pilottones inserted at indices ±11, ±25, and ±53. Inthe case of multiple frequency segments (2W,W+W, etc.), this subcarrier assignment is dupli-cated [9].

REGULATORY FRAMEWORKIn this section we present the regulatory essen-tials for spectrum sharing adopted by IEEE802.11af. We introduce the different control andmonitoring operations used by the two mainapproaches to GDB implementation and howthe 802.11af standard adapts to different regula-tory time requirements.

INCUMBENT PROTECTIONWSDs’ spectrum access in the TVWS band isbased on the regulatory requirement of non-interference to protected licensed devices.Incumbent users include both broadcast servicessuch as digital terrestrial television (DTT) andPMSE users that include wireless microphones,among other services [1, 10]. To avoid interfer-ing with protected devices, a WSD is required tobe aware of the operating frequency and regionof all protected devices. A WSD has limitedcapabilities and therefore obtains this informa-tion from a regulated GDB. This approach isadopted by most regulators because it guaran-tees reliable and precise information from a cen-tralized, secure, and verified entity [1, 10].

A detailed interference analysis showing howregulations on device protection impact whitespace availability is presented by Webb in [10].However, in this section we present the currentlydeployed regulations and how the 802.11af stan-dard adapts to these.

REGULATORY IMPLEMENTATION OF THE GDBThe two main methods of operating with theGDB are reflected in the operation constraintseach approach enforces on their unlicensedwhite space users.

Open-Loop GDD

GDB Operation — The first approach is anopen-loop GDD system, in which the databasegrants operation to unlicensed devices on a dailybasis on channels indicated as available in thattimeframe [3]. The main drawback to thisapproach is that interference is treated as abinary event. Instead, interference should betreated as a function of the devices and theiremissions footprint.

Figure 3. TVHT PHY channel configurations: TVHT W, 2W, W+W, and2W+2W

f[MHz]

Primary TVHT W Secondary TVHT W

TVHT W+W

1 BCU

f[MHz]

1 BCU 1 BCU

1 BCU

Primary TVHT 2W Secondary TVHT 2W

TVHT 2W+2W

1 BCU 1 BCU

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IEEE Communications Magazine • October 2013 97

Usage — An open-loop GDD system is imple-mented by the United States regulator, the Fed-eral Communications Commission (FCC).Under this regulation, TVWS operation isallowed in 6 MHz channels within the frequen-cies 54–698 MHz in TV channels 2, 5, 6, 14–35,and 38–51 [3]. The WSDs follow an up to 48-hour schedule that provides the list of availablechannels in this time period, under which a stat-ic set of maximum transmit power rules are fol-lowed [3].

Under this regulation WSDs have a flexibleoperating region because these authenticate withthe GDB once a day. However, fixed and con-servative transmit power is required due to thelarge timescale of the feedback, leading to rigidor binary operation of channel availability.

Parameter Regulation — In the open-loop sys-tem, to ensure protection of incumbent users themaximum permitted transmission power is con-servative, especially at neighboring channels. Forportable WSDs, the FCC allows a maximumeffective isotropic radiated power (EIRP) of 100mW (20 dBm) per 6 MHz bandwidth on unoccu-pied channels. And only a maximum EIRP of 40mW (16 dBm) for the first channel adjacent toany primary user. Fixed WSDs are limited to amaximum power delivered to the antenna of 1W (30 dBm) and no greater than 4 W (36 dBm)EIRP from any attached antenna per each 6MHz channel. These power regulations can begeneralized as 12.6 dBm EIRP permitted perany 100 kHz [3].

The downside of such rigid power regulationis that 80 percent of the potential white spacechannels become unavailable, limiting whitespace implementation to rural areas and mini-mizing the development of spectrum-sharingtechnologies [11].

Closed-Loop GDD

GDB Operation — The second approach is aclosed-loop GDD, in which frequent interactionbetween the GDB and the WSDs allows flexibleoperating parameters that apply to a specificdevice characteristics and location. In thismethod, the unlicensed user is under tight con-trol by the database, a command-and-controlsystem that is possible through constant feed-back [1, 2].

Usage — A closed-loop GDD system is fol-lowed by the European (European Telecommu-nications Standards Institute, ETSI) and UnitedKingdom (Ofcom) regulators. WSDs’ operationis permitted in 8 MHz channels within the fre-quencies 470–790 MHz [1, 2].

The WSD is required to perform a GDBdiscovery procedure in a time interval relevantto its operating location [2], commonly everytwo hours. Once an approved GDB is located,the WSD requests the operational parameters ofits specific operating region, and in responseprovides the GDB with its device parameters.The operational parameters sent to the WSDhave time validity and only apply to the specif-ic reported location. Upon time expiration or50 m movement from the last reported posi-

tion, the WSD must request new operationparameters [2].

The operational parameters transmitted by aGDB contain the upper and lower DTT channelfrequency, the maximum power spectral densityper 100 kHz within allowed channels, and thetime validity of such operation parameters andsensing levels for PMSE and DTT for futureimplementation of spectrum sensing [1]. TheWSD sends to the GDB the intended channelusage parameters, which include the intended fre-quencies and the corresponding transmissionpower for each 100 kHz. These parameters arerequired to be acknowledged by the GDB beforea WSD operates in the TVWS band [1].

Parameter Regulation — Unlike the staticparameter regulation of the open-loop system,the closed-loop approach has granular parame-ter regulations that apply to a specific device andlocation. This allows WSDs to have flexible per-mitted transmission power dependent on loca-tion, frequency, and time. This translates intohigher average transmission power when theWSD is at a greater distance from the incum-bent user, and further apart in frequency toavoid adjacent channel interference.

The tight feedback and granular parameterspecification allow the GDB to modify the WSDoperation at any instant. A closed-loop systemhas the advantage of being able to enforce time-sensitive regulations. For example, the GDDenabling STA (AP) is required to stop transmis-sion within 60 s when instructed by the GDB.Similarly, a GDD-dependent STA is required tostop transmissions within 1 s when requested bythe GDD-enabling STA and within 5 s if com-munication is broken [1, 2].

802.11AF AND REGULATORY DOMAINS

Architecture Roles — The roles of the archi-tecture entities presented earlier depend on theapplied regulatory domain, and their securityand timeline constraints [9]. In Table 3 we pre-sent the main differences these entities perform.

The WSDs or GDD-dependent STAs operateunder different transmit power limitations. Underan open-loop system the WSD has fixed powerlimitations for in-channel and adjacent channeltransmissions; however, under a closed-loopdomain the WSDs have flexible power limits thatdepend on the operating frequency, location,and time. Next, the GDB licensing period in aclosed-loop system is non-interactive and oper-ates WSM on a timescale of one to two days [9].However, in a closed-loop system the GDB isable to control the WSDs typically in a two-hourlicensing period or by defining a specific WSMvalidation time; closed-loop licensing methodsrequire the intended location and emission foot-print of the WSD [9]. The RLSS in an open-loop domain only performs as an informativeentity that forwards the WSM from the GDB tothe GDD-enabling STAs. On the contrary, theRLSS in a closed-loop system performs authori-tative commands where it can calculate WSMfor the BSSs under its control. Finally, and mostimportant, the tight control of a closed-loopdomain allows the system to stop WSD transmis-

Under a regulation

WSDs have a flexible

operating region

because these

authenticate with

the GDB once a day.

However, fixed and

conservative transmit

power is required

due to the large time

scale of the

feedback, leading

to rigid or binary

operation of channel

availability.

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IEEE Communications Magazine • October 201398

sions in a short time span due to the appearanceof a licensed service.

Mechanisms — The use of the 802.11af mecha-nisms presented depends on the timescalerequirements of the applied regulatory domain[9]. In Table 4 we present how these mecha-nisms are used to satisfy the different regulatorytime requirements in a day, hour, and minutetime span [9].

The CAQ is the procedure in which WSDsrequest the available radio frequencies foroperation. This is an informative mechanismthat applies to both daily and hourly consulta-tion, used by open and closed-loop systems,respectively. Similarly, the CSM is an informa-tive procedure in both daily (open-loop) andhourly (closed-loop) consultation that is usedby WSD to obtain white space channel sched-ule information. In the same way, the NCCprocedure controls the frequency usage inboth daily and hourly consultation. None ofthese mechanisms are applied in a minuteresponsiveness for either open or closed-loopregulators.

Next, the CVS procedure is used to ensureWSD operation in available frequencies. TheCVS usage in a daily consultation by open-loopregulators is required to be secure becauseWSMs are broadcast to all WSDs in a definedarea. However, CVS is not required to be securein an hourly consultation because the closed-loop regulators use a unique WSM per client orgroup of clients. The minute time span is used ifa WSD misses consecutive signals from theGDD-enabling STA, and indicates that a changein channel frequency has occurred.

Finally, GDD enablement is used by GDD-dependent STAs to join a network, and there-fore is required in a daily, hourly and minutetime span, but used in the feedback time ofthe applied regulatory domain. Similarly, theWSM is required in al l t imescales by theGDD-enabling STAs to ensure that GDD-dependent STAs (under its control) satisfyregulations.

POSSIBLE FUTURE DIRECTIONS FORSPECTRUM SHARING

A proposal for spectrum sharing has recentlybeen published by the President’s Council ofAdvisors on Science and Technology (PCAST)[12]. In this report, the authors highlight thepotential for rapid economic stimulus and growthby making underutilized government-held spec-trum available for secondary users via spectrumsharing. The PCAST presents spectrum as aroad analogy, where they propose usage of spec-trum like a “wide multi-lane superhighway,where lanes are continuously shared by manycars, trucks, and other vehicles” [12]. Essentially,different wireless services share spectrum andmove across lanes, depending on their usagerequirements. Meeting the vision of efficientspectrum sharing will require a number of addi-tional innovations discussed in the report andsolutions to the following open problems.

MINIMUM RECEIVER PERFORMANCEREQUIREMENTS

Interference in adjacent channels can be mitigat-ed at the transmitter, the receiver, or both. Cur-rent regulation limits interference at thetransmitter by imposing power constraints, guardbands, and physical separation requirements.However, a receiver’s design also strongly affectsthe interference impact of a given secondarytransmitter [12]. Current DTV receiver hard-ware costs have been reduced by using lower-quality digital filters, making them more sensitiveto energy in neighboring bands. Secondary trans-mitters run the risk of saturating adjacent-bandreceiver analog-to-digital converters (ADCs)even in the case of perfect transmission filtering.For example, the U.S. nationwide Long TermEvolution (LTE) network planned by LightSquared was recently prevented from beingdeployed because GPS receivers are notdesigned to tolerate any adjacent-spectrum inter-ference. There is no standard for characterizing

Table 3. Key operating distinctions between the open and closed loop systems.

Open loop GDD system (FCC) Closed loop GDD system (ETSI, Ofcom)

WSD Tx powerlimits

• 1 W for fixed devicesPortable devices:• 100 mW per 6 MHz• 40 mW adjacent channel

Flexible, dependent on distance in frequencyand location from incumbent

GDB licensingperiod

• Daily with a 48-hour schedule• Non-interactive

• WSM validity defined by GDB• Typically two-hour period• Demand and control• Tight feedback

RLSS role • Informative• Forward WSM to WSDs

• Normative• Authoritative operation• Calculation of operating parameters

Time response tolicensed serviceappearance

• Not definedStop transmissions:• AP WSD within 60 s• STA WSD within 1 s

Current regulation

limits interference at

the transmitter by

imposing power con-

straints, guard bands

and physical separa-

tion requirements.

However, a receiver’s

design also strongly

affects the interfer-

ence impact of a

given secondary

transmitter.

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IEEE Communications Magazine • October 2013 99

and specifying receiver behavior across spectrumsharing device classes, limiting spectrum alloca-tion and making adjacent-channel interference amajor cause of spectrum scarcity. This results ina severe reduction of the number of channelsavailable and the allowable transmit power ofsecondary devices. Therefore, regulations tocharacterize and guarantee minimum receiverperformance in the presence of interference arerequired to enable efficient utilization of thespectrum band.

OPERATIONAL FEEDBACK GRANULARITYSome of the largest unresolved issues with spec-trum sharing depend on the space, time, fre-quency, and power granularity devices aregranted spectrum access. The margin of protec-tion for primary users will limit the spectrumavailability for secondary users. Tighter sensingand reporting feedback loops could reduce thosemargins while maintaining protection. In addi-tion, the mobility of the primary and secondarydevices also influences these decisions. A 40 mWWSD that is mobile may be allowed to use thesame channels as a 4 W fixed device under FCCregulations, but the transmit power restrictioncould be eased if it provided more timely geolo-cation updates.

While geolocation databases were initiallyproposed to solve the problem of hidden termi-nals in cognitive radio networks, it is expectedthat a centralized system could contain fullknowledge of the location of all radio devicesusing the band and their characteristics. Thisallows several key networking issues to beresolved, such as congestion and frequency plan-ning on large timescales, or even transmissionscheduling over short timescales. A centralizedcontroller can be used to assign services toappropriate frequency bands and adapt to theirever changing usage. For example, spectrum thatis not available for high-power secondary usecan be provided to low-power secondary radiosto form short-range or low-rate networks [12].

The 802.11af standard does not directlyaddress these issues, as they remain to be settled

by regulatory authorities. However, the mecha-nisms presented in the standard are designed tobe flexible and adapt to changing spectrum shar-ing approaches.

CONCLUSIONSpectrum sharing illustrates the potential ofunderutilized spectrum, enabling increased per-formance across a broad range of devices andservices. We present the 802.11af standard, whichpermits white space devices to harmoniouslyshare the TV white space band with incumbentservices, such as TV broadcast and PMSEdevices. Spectrum sharing in the TVWS band isachieved by the use of a geolocation databasethat contains full knowledge of licensed and unli-censed usage of the band. We discuss and com-pare the two approaches to implement the GDB:open and closed-loop systems. These approachesmainly differ in the timescale in which monitor-ing operations occur, which leads to large differ-ences in white space availability and WSDoperation. The IEEE 802.11af standard providesa common architecture, a communicationscheme, and a control structure that allows stan-dardization across both open and closed-loopapproaches. Finally, we present how receiver tol-erance to interference and the control feedbackgranularity influence the innovation and perfor-mance of spectrum sharing technologies.

REFERENCES[1] Ofcom, “Regulatory Requirements for White Space

Devices in the UHF TV band”; http://www.cept.org/Doc-uments/se-43/6161/, July 4, 2012; retrieved Sept. 2013.

[2] ETSI, “EN 301 598 White Space Devices (WSD); WirelessAccess Systems Operating in the 470 MHz to 790 MHzFrequency Band,” Oct. 2012.

[3] Electronic Code of Federal Regs., Title 47, part 15, sub-part H, Television Band Devices; http://www.ecfr.gov/;retrieved Sept. 2013 from GPO.

[4] Washington Gov. Printing Office, An Act to RegulateRadio Communication; http://earlyradiohistory.us/1912act.htm; Aug. 13, 1912, retrieved Sept. 2013.

[5] Ofcom, Digital Dividend Review; http://stakeholders.ofcom.org.uk/consultations/ddr/, Dec. 19, 2006;retrieved Sept. 2013.

Table 4. GDD mechanisms and applied timescales [9].

Mechanism Daily consultationrequired (FCC)

Hourly consultationrequired (ETSI, OFCOM)

Minute responsiveness(NONE)

Channel availabilityquery (CAQ) Informative Informative Not applicable

Channel schedulemanagement (CSM) Informative Informative Not applicable

Network channelcontrol (NCC) Informative Informative Not applicable

Contact verificationsignal (CVS) Required to be secure May be secure Loss of consecutive

signals requires action

GDD enablement Required Required Required

White space map(WSM) Required for enabling STA, might be translated for dependent STA

Some of the largest

unresolved issues

with spectrum

sharing depend on

the space, time,

frequency, and

power granularity

devices are granted

spectrum access.

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IEEE Communications Magazine • October 2013100

[6] R. Thanki, “The Economic Significance of License-Exempt Spectrum to the Future of the Internet,” June2012.

[7] W. C. Stone, “NIST Construction Automation ProgramReport No. 3: Electromagnetic Signal Attenuation inConstruction Materials,” NIST, tech. rep., Oct. 1997.

[8] H. T. Friis, “A Note on A Simple Transmission Formula.”Proc. IRE 34, no. 5, 1946, 254–56.

[9] IEEE 802.11 Working Group, “IEEE 802.11af Draft 5.0,Amendment 5: TV White Spaces Operation,” June 2013.

[10] W. Webb, “On Using White Space Spectrum,” IEEECommun. Mag., vol. 50, no. 8, Aug. 2012, pp. 145–51.

[11] eComm “Spectrum 2.0 — What’s Really Happening;http://fora.tv/2009/03/05/Spectrum_20_-_Whats_Real-ly_Happening, 2009; retrieved Sept. 2013.

[12] Executive Office of PCAST, “Realizing the Full Potentialof Government-Held Spectrum to Spur EconomicGrowth,” tech. rep., Jul. 2012.

BIOGRAPHIESADRIANA B. FLORES (a.flores@rice.edu) is a Ph.D. candidate inthe Department of Electrical and Computer Engineering atRice University, Houston, Texas. In 2013, she received herM.S. in electrical engineering from Rice University. In 2009,she received her B.S. in electrical engineering from ITESMin Monterrey, Mexico. She joined the Rice Networks Groupin 2011, where she works under the guidance of Dr.Edward Knightly and holds a Texas Instruments Distin-guished Fellowship. Her research focuses on the design ofmedium access control protocols for efficient channelusage in wireless networks.

RYAN E. GUERRA (war@rice.edu) received his B.S. and M.S.E.in electrical engineering from Johns Hopkins University ofBaltimore, Maryland in 2008. He is currently a Ph.D. stu-dent in electrical engineering at Rice University, developingfrequency-agile software-defined radio hardware and sys-

tems. His research focus is on wireless networks planning,deployment, and management in addition to cross-layerimplementation and design.

EDWARD W. KNIGHTLY (knightly@rice.edu) is a professor ofelectrical and computer engineering at Rice University. Hereceived his Ph.D. and M.S. from the University of Californiaat Berkeley, and his B.S. from Auburn University. He is aSloan Fellow and a recipient of the NSF CAREER Award. Hisgroup’s current projects include deployment, operation, andmanagement of a large-scale urban wireless network in anunder-resourced community in Houston. The network is thefirst to provide residential access in frequencies spanningfrom unused UHF DTV bands to WiFi bands, and employscustom-built programmable and observable access points.

PETER ECCLESINE (pecclesi@cisco.com) has been working onwireless in Cisco’s Business Development organization since1996, within the Corporate Development TechnologyGroup developing new markets and new technologies. Hewas an electrical engineer for 29 years, and co-foundedtwo companies. He is part of the IEEE 802.11 WorkingGroup as the Vice Chair and Technical Editor for the IEEE802.11af standard and editor of IEEE 802.11j for 4.9 and 5GHz operation in Japan. Additionally, he has worked onunlicensed wireless at 60/90 GHz and licensed wireless at70/80 GHz.

SANTOSH PANDEY (sanpande@cisco.com) received his B.S. inelectrical engineering from the University of Mumbai in2002, and his M.S. and Ph.D. from Auburn University in2007. He joined Cisco Systems in 2007 as a system engi-neer, where he has primarily worked on location algo-rithms in wireless networks. He has additionally worked onhandoff and handover algorithms and simulations for fixedmobile convergence. He actively participates in IEEE 802.11Task Group AE, which aims to prioritize managementframes in 802.11 networks.

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