IEEE Communications Magazine • September 2012 1450163-6804/12/$25.00 © 2012 IEEE

INTRODUCTION

Digital terrestrial television became a reality inthe mid-1990s, when DVB-T and ATSC A/53,the two main standards to enable terrestrialtransmission of digitized TV signals, wereapproved and adopted in Europe and the Unit-ed States, respectively. Since then, various sim-ilar standards have been developed for use inother countries, mainly in China (DMB-T) andJapan (ISDB-T). Going a step further, inEurope, the second-generation terrestrial stan-dard (DVB-T2), adopted by the EuropeanTelecommunications Standards Institute(ETSI) in 2009, is offering increased spectralefficiency, greater flexibility, and MIMO sup-port. The transition to digital television notonly presents benefits to broadcasters and TVviewers, but also introduces — as a by-product— unique opportunities for players of the wire-less networking market via the careful and reg-ulated exploitation of locally underusedportions of the TV bands. This potential bene-fit is presented and quantified in the followingsections.

DIGITAL SWITCHOVER ANDWHITE SPACES

The term digital switchover (DSO) refers to thereplacement of the existing analog TV transmis-sions around the world with their digital coun-terparts — a procedure that has beensuccessfully completed in various countries andis in progress in others. Since both analog anddigital TV utilize the same frequency bands,DTV standards were designed so that the trans-mitted digital signal requires exactly the band-width of the legacy analog one (8 MHz inEurope, 6 MHz in the United States). There-fore, a digital channel entirely replaces an ana-log one during the switchover procedure. Thisanalog shut-off requires that viewers must obtaindigital receivers to continue viewing the contentthey used to enjoy with their analog TVs. Thisrestriction has hampered the progress of theDSO even in technologically advanced countriessuch as the United States. Another issue to betaken care of — and which is also due to thereuse of TV bands by DTV transmitters — is theco- and adjacent-channel interference betweendigital and analog transmissions and alsobetween digital ones.

For all the aforementioned reasons, the suc-cess of DSO heavily depends on the carefulplanning of both the timeline to be followed andthe final frequency plan to be used in DTVtransmission. The DSO has been an excellentopportunity for countries around the globe tostudy and adopt a new, sound, and well estab-lished frequency plan for TV bands, ensuringoptimum usage and eliminating interferencebetween DTV service areas not only within acountry but also across neighboring ones. Proba-bly the most important step toward this goal fora significant part of the globe, including Europe,Africa, the Middle East, and Russia (Interna-tional Telecommunication Union [ITU] Region1), has been the ITU Regional Radiocommuni-cations Conference (RRC06) held in Geneva,Switzerland, in June 2006. The aim was to regu-late the use of a certain portion of the spectrum(174–230 MHz/Band III and 470–862 MHz/Bands IV–V) for terrestrial digital TV (DVB-T)

ABSTRACT

The so-called TV white spaces (TVWS) arelocally underutilized portions of the terrestrialTV bands and occur as a by-product of thedigital switchover taking place in most coun-tries across the globe. Thanks to the verygood characteristics of the TV bands in ter-restrial radio communications, the exploita-t ion of TVWS for local- or regional-areawireless license-exempt networking, under acarefully established regulatory framework, isa very attractive prospect. This article pre-sents a generic methodology for determiningthe actual capacity of white spaces using ageolocation-based approach, that is, dynamicassignment of radio resources to TVWS net-works according to their geographical loca-tion. This methodology is applied in a casestudy investigating a sample area in southeast-ern Europe and unveils a significant amountof access capacity that can be unleashed viaTVWS exploitation.

TOPICS IN RADIO COMMUNICATIONS

Dimitris Makris, Georgios Gardikis, and Anastasios Kourtis, National Centre for Scientific Research

“Demokritos”

Quantifying TV White Space Capacity: A Geolocation-Based Approach

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IEEE Communications Magazine • September 2012146

and digital radio (T-DAB) usage. Bands IV–Vwere divided into 49 channels with 8 MHz ofbandwidth (numbered 21–69) and devoted toDVB-T usage. The outcome of RRC06 has beenthe division of ITU Region 1 into geographicalallotments (Fig. 1a depicts the allotments forEurope and neighboring countries) and theassignment of a set of channels to each allot-ment for digital TV transmission. Reference [1]presents an overview of the outcome and deci-sions of ITU RRC06, as they are detailed in [2].

In order to eliminate interference, DTV plan-ning followed the strategy of frequency reuse,common in cellular network planning, meaningthat two adjacent allotments are generally allo-cated a set of completely different channels. Inother words, the use of the same DTV channelin two neighboring allotments is avoided. Thisnaturally leaves a lot of space where a specificchannel is (deliberately) not used. To visualizethis, Fig. 1b shows the allotments using Channel21 in Europe. The remaining space betweenthese allotments, or between the service areas ofDTV transmitters using Channel 21, is the so-

called white space for this specific channel,which is planned to be unused. The nature ofwhite space can be not only spatial but also tem-poral; there are cases of DTV transmitters thatdo not operate around the clock, but only duringcertain hours.

Compared to cellular network (e.g.,second/third generation [2G/3G]) cells, DTVallotments are much more extended, usually cov-ering areas of several hundreds of square kilo-meters; and so is the white space between them.The prospect that is opened is obvious: can wepossibly use this valuable white space bandwidthfor low-power low-range wireless networking ina strictly localized manner (secondary use) with-out interfering with licensed DTV transmissions?(primary use) This is the main idea behind theexploitation of TV white spaces (TVWS) orinterleaved spectrum, as it is also called.

At this point, it must be mentioned that theso-called white spaces are in fact not so “white”— in the sense of completely clean bands —since they naturally suffer from “pollution” dueto low-power signals coming from neighboringDTV allotments. These signals, although theymay be too weak to be decoded (i.e., they areunusable), are still a considerable source ofinterference for TVWS devices [3]. Nevertheless,this “pollution” does not significantly degradethe actual value of white spaces since almost allcontemporary radio networking technologiesemploy adaptive modulation and coding tech-niques so as to tolerate very high co-channelinterference levels. Error-free operation isensured in most cases, of course at the cost oflower spectral efficiency.

In this context, TVWS present a new oppor-tunity for wireless networking in a frequencyband that has very good transmission character-istics in the terrestrial environment and at thesame time allows for reasonably sized antennas.As examples of possible applications for sec-ondary TVWS use, one could mention:• Wireless local networking in TV bands, as

an alternative to the highly congested indus-trial, scientific, and medical (ISM) band.

• Sensor and ad hoc/mesh networks. • Regional-area networking, especially suited

to providing affordable Internet and inte-grated services access to citizens in rural/underdeveloped areas not covered by wire-line networks. In this sense, TVWS presentsan excellent opportunity for bridging theso-called broadband divide.

• Exploitation by cellular networks, comple-menting licensed spectrum usage. In partic-ular, white-space spectrum can be utilizedby 3G/4G femtocells to minimize interfer-ence to macrocells without having to applysophisticated interference mitigation mech-anisms. Moreover, TVWS resources can beacquired on demand (via, e.g., a centralizedspectrum broker) to complement licensedbands in cases where short-term additionalcapacity is needed (at sports events, streetfestivals, etc.).When it comes to indoor TVWS network

deployment, the opportunities are even moreappealing due to increased isolation from thelicensed primary transmissions. Use cases include

Figure 1. a). Division of Europe into allotments for DTV frequency planning;the marked area is the one investigated in this study; b) allotments usingChannel 21 (reproduced from [1]).

a

b

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IEEE Communications Magazine • September 2012 147

indoor femtocells and wireless LANs, exhibitinggreater coverage and reduced interference com-pared to ISM-band devices. Furthermore, thelower operating frequencies of indoor TVWSdevices can lead to lower energy consumptioncompared to ISM ones, resulting in extendedbattery time for handheld/portable use.

A detailed discussion of TVWS exploitationscenarios can be found in [4].

The exploitation of white spaces is a chal-lenge not only for manufacturers but also forspectrum regulators. The aspect of secondaryspectrum usage of TV bands is quite novel andpresents several difficulties when it comes toregulation. Primary users (i.e., licensed TVbroadcasters) must be absolutely protectedagainst potential interference; at the same time,sufficient freedom must be given to secondaryusers to exploit the available spectrum. In theUnited States, the FCC has adopted a SecondReport and Order for unlicensed operation inTV bands, drafting rules for secondary networksto protect TV broadcasters. In the United King-dom, Ofcom, conducting a public consultationon TVWS use since 2005, has also concluded oncertain suggestions [5], including parameters towhich secondary networks should comply.

In the standards domain, the concept ofopportunistic spectrum access of TV bands hasbeen the foundation for the IEEE 802.22 specifi-cation [6], which defines the physical (PHY) andmedium access control (MAC) layers of aregional access network for TVWS use. At thesame time, other technologies such as WiFi andWiMAX are also being adopted to work in theTVWS context in a cognitive/opportunistic man-ner. IEEE P802.11af is paving the way towardunlicensed 802.11 operation in TV bands, whileIEEE P802.19.1 [7] studies coexistence issues fordevices operating in white spaces. In EuropeTVWS standardization is carried out in theETSI Reconfigurable Radio Systems (RRS)Technical Committee (TC).

OPPORTUNISTIC SPECTRUM ACCESSFOR TVWS EXPLOITATION

A key factor that greatly facilitates TVWSexploitation is the adoption and deployment ofmechanisms that safely determine dynamicallyand in real time where and when to use TVWSspectrum for secondary use. These mechanisms,employed by the secondary network(s), facilitatethe so-called opportunistic spectrum access,which is a feature falling in the more generalcategory of cognitive radio (CR). The aim is toprovide the secondary wireless network, withnecessary information on:• Which TV channel(s) to occupy• The maximum allowed effective isotropic

radiated power (EIRP) that can be usedThe constraint is the absolute protection ofDTV receivers that are tuned to a licensed (pri-mary) DTV transmitter within its declared ser-vice area. For this purpose, two mainmechanisms have been established: spectrumsensing and geolocation.

The approach of spectrum sensing involvesthe incorporation of a spectrum scanner in all

nodes of the secondary wireless network. Thesescanners/sensors periodically scan the entire TVband for empty (locally unused) channels. Forthe network to use a specific TV channel, thischannel must be reported empty by the sensorsin all nodes of the network. Specific and sophis-ticated sensing algorithms have been proposedin the literature, which can detect a distant DTVtransmission even if the DTV signal is far belowthe noise level. Sensing methods can be roughlycategorized into blind, that is, not requiringknowledge of the primary signal (e.g., simpleenergy detection or covariance-based detection)and feature-based, that is, exploiting characteris-tics of the primary signal which are known a pri-ori (e.g., correlation-based or pilot signaldetection). Distributed sensing has also beenstudied, where several terminals collaborate andexchange sensing data to increase detectionaccuracy.

An orthogonal approach, which can beemployed with or without spectrum sensing, isgeolocation-based spectrum allocation, whichconstitutes the main focus of this article. Sincethe map of licensed DTV transmitters and theircorresponding allotments/service areas is static,it is also reasonable to assume that one can cre-ate a semi-static database with maps of possiblelocations for TVWS networks, along withallowed power levels and TV channels assignedto each location for secondary use. Given thelocation of a TVWS network, which can bedetermined via, say, a GPS subsystem incorpo-rated in the devices, a comparison against thedatabase map can be used to immediately assigna set of channels and power levels to be safelyused by the network without disturbing licensedDTV allotments. The 802.22 standard employsthe geolocation approach for channel selection,by mandating that both the base station (BS)and the associated wireless clients be aware oftheir geographical location. Using these loca-tions, the 802.22 BS queries the TVWS databaseto retrieve vacant TV channels in the area andthe maximum EIRP specified for each availablechannel.

Toward promoting this capability, the FCChas also already appointed administrators ofnationwide TVWS databases to be exploited bywhite space devices. Similarly, Ofcom is alsoplanning the implementation of white spacedatabases. The aim is to launch geolocation-based TVWS services in the United Kingdom inearly 2013.

If the regulators concur on the mechanismsto be used along with the associated parametersand thresholds, license-exempt use of secondaryTVWS networks could be safely allowed. In thissense, one could deploy and operate a TVWSnetwork anywhere and anytime, without requir-ing a license (just as WiFi networks are currentlydeployed and used) and without worrying aboutpossible interference to DTV signals. In a morecontrolled approach, opportunistic access couldbe combined with a centralized spectrum brokerfunction,as extensively proposed in the literaturefor CR networks, where dynamic, real-time spec-trum auctions could grant local use of TVWSspectrum to the party with the highest bid. Themechanisms for dynamic assignment of radio

A key factor that

greatly facilitates

TVWS exploitation is

the adoption and

deployment of

mechanisms that

safely determine

dynamically and in

real time where and

when to use TVWS

spectrum for sec-

ondary use. These

mechanisms facilitate

the so-called oppor-

tunistic spectrum

access.

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resources constitute a vast research field, whichis out of the scope of this article.

ESTIMATING THEACTUAL WHITE SPACE CAPACITY

In order to determine the actual benefit andpotential commercial value of TVWS exploita-tion, an estimation of the actual white spacecapacity is essential. In this section, we present astep-by-step methodology that can give a satis-factory approximation on the white space capaci-ty which can be offered. Since the calculationsinvolved are quite complex, we have isolated arestricted region for our case study, as indicatedin Fig. 1a/b. This region includes Peloponneseand a part of southern continental Greece, andits division in allotments is known, along withthe channels allocated to each allotment. Theselected region is quite representative, since it isuniformly divided into allotments, and it alsocomprises a balanced mix of plain/flat, hilly, andmountainous areas.

While several attempts to quantify whitespace capacity can be found in the literature, thedifferentiation of the methodology and resultspresented in this article can be identified inthree main aspects.

First, we have adapted our methodology tothe European case and in general to all countrieshaving adopted DVB-T and following the ITURRC06 frequency plan (ITU Region 1, includingRussia, the Middle East, and Africa). Indeed,while various studies exist on the availability ofTVWS in the United States, there is little evi-dence of what can be expected in the rest of theworld. For example, U.S.-related studies [3] can-not be directly extrapolated to the European casedue to significant differences in DTV standardsused, channel bandwidth, protection ratios,deployment scenarios of TVWS networks, regu-latory aspects, and even terrain morphology.Very few studies exist for outside the UnitedStates, such as [8], which attempts to quantifyTVWS capacity in the United Kingdom in a lim-ited area using the spectrum sensing approach.

A second innovative aspect is the inclusion ofthe actual terrain morphology in all propagationcalculations. Most existing TVWS studies employfixed general-use propagation curves such as theones of the ITU Radiocommunication Sector(ITU-R) P.1546-2 model used in RRC06 (Annex2.2 of [2]). However, in areas with significantvariations in terrain morphology, such as the areaexamined, there is considerable added value inTVWS exploitation that cannot be overlooked;the irregular terrain itself becomes a physicalbarrier which can greatly improve the isolationbetween the secondary (opportunistic TVWS)and the primary (licensed DTV) network, thusincreasing flexibility in local spectrum usage.

Last, the methodology presented follows amore regulatory rationale; that is, it tries to protectlicensed allotments regardless of the location ofthe existing DTV transmitters within them. Mostresearch efforts on TVWS capacity only computethe coverage areas of already deployed DTV trans-mission sites and exclude them from TVWS use.However, this approach depends on the current

DTV deployment configurations (which maychange over time, and may not coincide with thelicensed coverage area) and also disregards theuse, for example, of DTV repeaters/gap fillers thatmay be additionally deployed in order to betterserve the licensed allotment.

It must be also mentioned that, while severalstudies assume spectrum sensing as the mainmechanism for opportunistic spectrum usage [3],our methodology leaves sensing aside andassumes the mere employment of a centralizedgeolocation-based mechanism. Indeed, in a sce-nario with fixed DTV transmitters and allot-ments, the geographical location of which isknown and unchanged, it is reasonable to assumethat the calculation and employment of a set offixed maps with allowable TVWS network loca-tions for every channel is a well established andfully justified approach. This is in line withrecent FCC recommendations, which mandatethat fixed TVWS devices rely on geolocation todetermine their operating channels. That is, theydetermine their position and query a nationwidewhite space database in order to retrieve thevacant channels for the specific area. What ismore, as explained in [9], the spectrum sensingapproach, using extremely low detection thresh-olds (established to protect any licensed devicearound the secondary network), usually resultsin heavy underestimation of the actual whitespace area. Very weak DTV signals coming fromdistant transmitters are detected, and the associ-ated channels are ruled out, although practicallyno DTV receiver in the area can actually demod-ulate and decode them. Things become evenmore complicated if the secondary networks usethe same radio technology as the primary ones;the sensing mechanism cannot identify thelicensed signal in this case. The authors in [9]explain why spectrum sensing can lead to overes-timation of protected areas of even 3.5x in com-parison to a well-laid-out geolocationmechanism; this is a considerable drawback,leading to waste of valuable spectrum. More-over, sensing increases complexity, cost, andenergy consumption at the secondary networkterminals since intelligent sensing algorithmsrequire sophisticated circuitry. On the otherhand, the geolocation approach not only pro-vides greater flexibility for TVWS networking,but also seems safer for licensed TV broadcast-ers, which should be relieved to see that theirservice area is geographically “fenced.” It is thusno surprise that Ofcom has concluded that “themost important mechanism in the short to medi-um term will be geolocation” [5].

In this context, we assume for this study out-door TVWS networks that only use the geoloca-tion method. The local TVWS network isassumed to comprise 100 terminals simultane-ously transmitting over the entire 8 MHz band-width of the channel (a very pessimisticassumption). We also assume that these termi-nals operate at maximum transmit EIRP of 4 W[8], in compliance with the 802.22 scenario andalso with the FCC recommendation. If low-power personal/portable devices are consideredinstead, the candidate usage area is significantlyexpanded.

We begin our approach with the regulatory

In a more controlled

approach, oppor-

tunistic access could

be combined with a

centralized spectrum

broker function –as

extensively proposed

in the literature for

cognitive radio net-

works– where

dynamic, real-time

spectrum auctions

could grant local use

of TVWS spectrum

to the party with the

highest bid.

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constraint that all DTV receivers within alicensed allotment must be absolutely protectedagainst interference coming from the TVWS sec-ondary network. Then we try to determine theareas in which the TVWS network should oper-ate in order to satisfy this constraint. This proce-dure is carried out on a channel-by-channelbasis. For example, let us concentrate on chan-nel 33 (566–574 MHz). Figure 2a shows thelicensed allotments for channel 33. The area ofall licensed allotments is naturally excluded fromTVWS operation at the specific channel. Thus,we try to find candidate locations for safe TVWSusage outside the allotments.

The methodology is as follows:1. On the boundaries of each licensed allot-

ment, we consider 30 test (reference) pointscarefully selected taking in mind the terrainmorphology. Increasing the number of ref-erence test points increases accuracy butlinearly extends simulation time.

2. At each boundary test point, we assume anoutdoor DTV receiver that — as a worstcase — employs omnidirectional antennasand receives at the minimum decodable sig-nal level, as defined in [2]. DTV transmis-sions are assumed to comply to the DVB-Tstandard, using 16-quadrature amplitudemodulation (QAM) constellation and con-volutional code rate of 3/4.

3. We divide the region outside the licensedallotments into area elements of 450 ¥ 450m.

4. We select an arbitrary area element andconsider a TVWS network operating at thatspecific location.

5. We calculate the co-channel interferencecaused by the TVWS network to each ofthe assumed DTV receivers at the bound-ary test points of the surrounding licensedallotments. The average long-term propaga-tion loss required for interference calcula-tion is estimated taking into account theactual terrain profile, using the point-to-point Longley-Rice irregular terrain model(ITM) [10] and publicly available elevationdata. A Matlab-based simulation platformwas especially developed from scratch forthis purpose. The ITM calculations assumed10-m-high antennas with vertical polariza-tion.

6. Only if the interference caused to allassumed DTV receivers at the referencepoints is below the allowed levels (i.e., theprotection ratios adopted by ITU [2] forDTV planning) is the specific location isconsidered “safe” and marked as a candi-date for TVWS secondary use.

7. Steps 4–6 are repeated for all 450 ¥ 450 marea elements outside the licensed allot-ments that reside on the land (the proce-dure could also be extended to the sea inorder to cover maritime use cases).The results are visualized in Fig. 2a, where

the locations where TVWS operation should beallowed have been marked. The contribution ofthe mountainous areas to the physical isolationbetween TVWS networks and DTV licensedareas is clearly visible. Note that the upper partof the southern continental area along with the

Aegean islands were excluded from the scanningprocedure, due to possible interference to allot-ments outside the examined area.

The geolocation approach involves compari-son of the physical location of the TVWS net-work (as reported by a GPS device) to thecalculated map. If the location has been markedas “white,” the network is allowed to operate atthe specific channel. However, the map of Fig.2a includes several locations that are isolated. Itwould be unrealistic to assume that a regional-area TVWS network of several tens of terminalswould be restricted to a 450 ¥ 450 m area. Inthis sense, assuming a wider secondary networkcells of 3.2 km ¥ 3.2 km, we need to locate inFig. 2a the portions of the white areas where a3.2 km ¥ 3.2 km cell would fit. This is achievedby an image processing procedure known asmorphological opening (i.e., erosion followed bydilation) on the white space image, where thestructural element is a disc having a diameter of3.2 km. The results are shown in Fig. 2b, whichshows the allowed locations for deploying awide-area TVWS wireless network operating onChannel 33 (566–574 MHz) with the aforemen-

Figure 2. a) DTV allotments allocated to TV channel 33 and allowed locationsfor TVWS devices operating in this channel; b) a morphological opening pro-cedure exposes candidate wide-area cells for TVWS cellular networks operat-ing at Channel 33.

a

b

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tioned characteristics without causing interfer-ence to licensed DTV allotments.

In order to compute the total available TVWSspectrum in each location, we repeat the afore-mentioned procedure for all 49 Band III/IVchannels (from 21 to 69) and superimpose theresults in a final image. Simulations have beenquite demanding and merely their execution —for the studied area and resolution mentioned— lasted about one month, in parallel sessionshosted by four dual-core workstations. Theresults are visualized in Fig. 3. Figure 4 showsthe statistical distribution of the available TVWSspectrum.

It can be calculated that an average of 125MHz of TVWS spectrum is available in everylocation, a quite significant amount if we consid-er that it is located in a relatively low — andalso very valuable — part of the radio frequency(RF) spectrum. It should be mentioned that cal-culations have included the worst case assump-tion that all TV channels allocated to each

allotment have been licensed to and occupied byDTV broadcasters; since this scenario is quiteunlikely, the amount of TVWS spectrum can beeven higher. It must also be noted that theresults of the present study are comparable withoutcomes of other studies such as [4], which cal-culates an average white space capacity of 150MHz per location in the UK.

With certain assumptions, these figures canbe translated to per-capita wireless capacity.Assuming a typical average spectral efficiency of2 b/s/Hz (feasible with emerging technologiessuch as LTE-Advanced [11]), it can be deducedthat an aggregate capacity of 250 Mb/s can beoffered in a 3.2 km ¥ 3.2 km location. Consider-ing that the average population density inEurope is 70 inhabitants/km2 and assuming acontention ratio of 1:40, we can calculate thatTVWS exploitation alone can provide a roughaverage of almost 14 Mb/s of aggregate wirelessaccess capacity to each citizen — a quite attrac-tive value. For a more precise estimation, theresults depicted in Fig. 3 can be combined witha population density map so as to derive theactual available spectrum per capita in eachlocation.

OTHER CONSIDERATIONSWhen a geolocation approach such as the afore-mentioned one is to be brought to commercialuse, there are a number of additional issues tobe taken into consideration.

The first one is related to the spectrum usageflexibility of the radio technology to be used inthe secondary network. The previous sectionpresented the calculation of the overall amountof TVWS spectrum available in each location. Itis clear that this spectrum will be mostly frag-mented; that is, it will comprise several non-con-tiguous TV channels of 8 MHz each. Emergingtechnologies, such as Long Term EvolutionAdvanced (LTE-Advanced), are capable ofexploiting this fragmented spectrum as a whole,thanks to state-of-the-art carrier aggregationtechniques [11]. However, currently widespreadtechnologies such as WiFi and WiMAX, whichcould be theoretically directly applicable towhite-space networking via adjustment in theirRF front-end in order to work in the TV bands,require a considerable amount of contiguousspectrum. For example, white-space operation ofa 20 MHz IEEE 802.11g network requires threeconsecutive 8 MHz channels to be available at agiven location.

In this context, to accommodate this require-ment, we carry out post-processing of the afore-mentioned simulation results in order to derivethe maximum amount of contiguous TVWSspectrum available at each location. The resultsare depicted in Fig. 5. In this case, it is calculat-ed that, on average, the maximum contiguousTVWS spectrum block per location is 27 MHz.

It must be noted, however, that the require-ments for contiguous spectrum are relaxed byemerging wireless networking standards, such asthe proposed 802.11af, which currently foreseesa flexible channelization model supporting band-widths of 5, 10, 20, and 40 MHz to adapt to vari-ous white spectrum slots.Figure 4. Statistical distribution of available TVWS spectrum.

TVWS spectrum available in each location (MHz)50

1

0

Perc

ent

of lo

cati

ons

2

3

4

5

6

7

0 100 150 200 250 300

Figure 3. Map of available TVWS spectrum (in MHz) at each location.

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IEEE Communications Magazine • September 2012 151

A second concern that should be noted ismore of a regulatory nature and refers to adja-cent channel interference (ACI) issues. It is truethat any radio apparatus, including a TVWSdevice, emits signals with non-ideal spectrummasks (i.e., suffers from power “leakage” toadjacent channels). It is thus possible that ahigh-power TVWS device operating at channelN could cause interference to nearby TV setstuned to DTV signals at channels N ± 1. Forthis purpose and in order to protect primaryusers in a more absolute manner, the local regu-lator could demand that, at any given location,for the secondary network to work in channel N,not only this specific channel (N) but also itsadjacent channels (N + 1, N – 1) must be locallyunused. This restriction, which has been recom-mended by both FCC and Ofcom [8], greatlyreduces the amount of available TVWS spec-trum. In order to demonstrate the effect of thisrestriction, Fig. 6 shows a recalculation of theresults of Fig. 3, having excluded the use ofchannels adjacent to licensed ones. The averageTVWS spectrum available per location falls to30 MHz.

It should be noted, however, that the adja-cent channel restriction could be safely avoidedfor secondary networks with very strict spectrummasks and/or signal bandwidth considerably nar-rower than the 8 MHz of the TV channel. Forexample, in a 5 MHz 802.11af deployment, onecould assume that, under normal conditions,power leakage to adjacent TV channels could beignored. Another factor that could help elimi-nate this restriction could be the establishmentof a minimum distance between a DTV set/DTVreceiver and a transmitting TVWS device so asto avoid potential interference.

A last (but not trivial) issue is related to theimplementation and storage cost of the geoloca-tion database. There are two scenarios regard-ing the access to the database. The first oneassumes that the database is maintained in acentralized manner (e.g., at the national regula-tor), and all TVWS devices perform requests tothis database to acquire information on locallyavailable channels. In this case, the geolocationmaps can be of virtually unlimited resolutionand complexity. The second scenario involves aversion of the geolocation maps statically storedin each TVWS device and manually updated atspecific intervals (e.g., via a firmware updateprocedure). With this approach, the TVWS net-work, upon acquisition of its natural positionvia, say, embedded GPS receivers, immediatelyknows which channels to use. This eliminatesthe need for external communication; however,it poses limitations on the size and resolution ofthe geolocation maps. For example, the mapscreated in this study involve, as aforementioned,rectangular area elements of 450 ¥ 450 m. Atthis resolution, it can be calculated that spec-trum usage maps covering all of Europe, con-taining the allowable channels and EIRP levelsfor each location, require approximately 2.4Gbytes of local storage. To reduce this size,apart from data compression techniques, onecan employ a down-sampling procedure, asdescribed in [9], but excluding an amount ofuseful TVWS areas.

CONCLUSIONS

The study presented in this article, in line withsimilar results presented in the literature,unveils a significant amount of wireless accesscapacity that can be unleashed via the exploita-tion of TV White Spaces; the possible applica-tions are virtually innumerable. Prompt andcoordinated actions are necessary from theside of manufacturers, standardization bodies,and especially national spectrum regulators inorder to establish a sound technical and legalframework for such exploitation. Instead offeeling threatened by the “invasion” in an areaof the spectrum traditionally devoted to them,broadcasters should see TVWS as a new areafor novel and unprecedented business oppor-tunities. Not only the license-exempt scenariobut especially the approach of spectrum auc-tions and spectrum brokers for the use of theTVWS spectrum are bound to open new busi-ness cases for all players in the wireless and

Figure 6. Map of available TVWS spectrum (in MHz), excluding use of chan-nels adjacent to licensed ones.

Figure 5. Map of maximum contiguous (non-fragmented) TVWS spectrum (inMHz) available at each location.

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broadcasting market. Significant advances inthis f ield should be expected in the nearfuture.

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BIOGRAPHIESDIMITRIS MAKRIS received his B.Sc. degree in electronics in2010 from the Technological Educational Institute (TEI) ofAthens. He is an M.Sc. candidate in electronics and radiocommunications, following a course supported by theDepartment of Physics and Informatics & Telecommunica-tions, University of Athens. He was with the Media Net-works Laboratory of NCSR ‘’Demokritos’’ for his practicaltraining, developing software in Matlab for estimatingDVB-T radio coverage. He then joined the WiCeAR (WirelessCommunications & e-Applications Research Group) Labora-tory in the Department of Electronics at TEI. Currently, heis participating in the Next Generation Millimeter WaveBackhaul Radio (NexGenMiliWave) research project in theframework of the Corallia Clusters Initiative, focusing onthe design and simulation of a diplexer and antenna at the60 GHz band.

GEORGIOS GARDIKIS [M’02] received his Diploma in electricaland computer engineering from the National TechnicalUniversity of Athens in 2000 and his Ph.D. from the sameuniversity in 2004. His expertise lies in the fields of digitalterrestrial and satellite broadcasting, distribution networksfor multimedia service provisioning, quality of experienceassessment, and novel mechanisms for application/networkcoupling. He has participated in several national and EU-funded R&D projects, including FP6/IST IMOSAN, in whichhe served as Technical Coordinator. He has collaboratedwith the University of the Aegean, conducting a nationalresearch project regarding experimental DVB-H deploy-ment, and with the Ministry of Transport and Communica-tions as consultant on digital broadcasting issues. Atpresent, he is an associate researcher at NCSR “Demokri-tos,” Media Networks Laboratory and also a visiting assis-tant professor at the Department of Applied Informaticsand Multimedia, TEI of Crete. He is a member of the IEEECommunications and Broadcast Technology Societies,W3C/”Web and TV” Interest Group, and Technical Chamberof Greece since 2001. He has more than 40 publications ininternational journals and refereed conferences/workshops.

ANASTASIOS KOURTIS is a research director and currentlydeputy director of the Institute of Informatics and Telecom-munications of NCSR “Demokritos.” He has extensive expe-rience in EU funded projects and has been Project Managerin four of them. His technical and research activities havebeen in the areas of wired and wireless broadband net-work infrastructures, quality of service, data encryptiontechniques, network management, and network virtualiza-tion. He has supervised a number of Ph.D.s and is theauthor or co-author of more than 100 scientific papers.

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