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Quantifying TV White Space Capacity; A Geolocation-based Approach

Dimitris Makris, Georgios Gardikis and Anastasios Kourtis

NCSR “Demokritos”, Institute of Informatics and Telecommunications Patr.Gregoriou & Neapoleos str., Ag. Paraskevi, Athens 153 10, Greece

ABSTRACT

The so-called “TV White Spaces” (TVWS) are locally underutilized portions of the terrestrial TV bands and occur as a by-product of the Digital Switchover taking place in most countries across the globe. Thanks to the very good characteristics of the TV bands in terrestrial radio communications, the exploitation of TVWS for local- or regional-area wireless license-exempt networking, under a carefully established regulatory framework, is a very attractive perspective. This paper presents a generic methodology for determining the actual capacity of white spaces using a geolocation-based approach i.e. dynamic assignment of radio resources to TVWS networks according to their geographical location. This methodology is applied in a case study investigating a sample area in southeastern Europe and unveils a significant amount of access capacity which can be unleashed via TVWS exploitation.

INTRODUCTION

Digital Terrestrial Television became a reality in the mid-90’s, when DVB-T and ATSC A/53, the two main standards to enable terrestrial transmission of digitized TV signals, were approved and adopted in Europe and the US, respectively. Since then, various similar standards were developed for use in other countries, mainly in China (DMB-T) and Japan (ISDB-T). Going a step further, in Europe, the second-generation terrestrial standard (DVB-T2), adopted by ETSI in 2009, is offering increased spectral efficiency, greater flexibility and MIMO support. The transition to digital television does not only present benefits to broadcasters and TV viewers, but also introduces –as a by-product– unique opportunities for players of the wireless networking market via the –careful and regulated– exploitation of locally underused portions of the TV bands. This potential benefit is presented and quantified in the following sections.

DIGITAL SWITCHOVER AND WHITE SPACES

The term Digital Switchover (DSO) refers to the replacement of the existing analog TV transmissions around the world with their digital counterparts – a procedure which has been successfully completed in various countries and is in progress in others. Since both analog and digital TV utilize the same frequency bands, DTV standards were designed so that the transmitted digital signal requires exactly the bandwidth of the legacy analog one (8 MHz in Europe, 6 MHz in the US). Therefore, a digital channel entirely replaces an analog one during the switchover procedure. This analog shut-off requires that the viewers must obtain digital receivers to continue viewing the content they used to enjoy with their analog TVs. This restriction has hampered the progress of the DSO even in technologically advanced countries such as the US. Another issue to be taken care of –and which is also due to the re-use of TV bands by DTV

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transmitters– is the co- and adjacent-channel interference between digital and analog transmissions and also between digital ones.

For all the aforementioned reasons, the success of Digital Switchover heavily depends on the careful planning of both the timeline to be followed and the final frequency plan to be used in DTV transmission. The DSO has been an excellent opportunity for countries around the globe to study and adopt a new, sound and well-established frequency plan for TV bands, ensuring optimum usage and eliminating interference between DTV service areas not only within a country but also across neighbouring ones. Probably the most important step towards this goal for a significant part of the globe, including Europe, Africa, Middle East and Russia (ITU Region 1), has been the ITU Regional Radiocommunications Conference (RRC06) held in Geneva, Switzerland, in June 2006. The aim was to regulate the use of a certain portion of the spectrum (174-230 MHz/Band III and 470-862 MHz/Bands IV-V) for digital TV (DVB-T) and digital radio (T-DAB) usage. Bands IV-V were divided into 49 channels with 8 MHz of bandwidth (numbered 21-69) and were devoted to DVB-T usage. The outcome of RRC06 has been the division of ITU Region 1 into geographical allotments (Fig.1a depicts the allotments for Europe and neighbouring countries) and the assignment of a set of channels to each allotment for digital TV transmission. [1] presents an overview of the outcome and decisions of ITU RRC06, as they are detailed in [2].

Fig. 1(a). Division of Europe into allotments for DTV frequency planning; the marked area is the one investigated in this study (b) Allotments using Channel 21 (Reproduced from [1])

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In order to eliminate interference, DTV planning followed the strategy of frequency reuse, common in cellular network planning, meaning that two adjacent allotments are generally allocated a set of completely different channels. In other words, the use of the same DTV channel in two neighbouring allotments is avoided. This naturally leaves a lot of space where a specific channel is (deliberately) not used. To visualize this, Fig. 1b shows the allotments using Channel 21 in Europe. The remaining space between these allotments, or between the service areas of DTV transmitters using Channel 21, is the so-called white space for this specific channel, which is planned to be unused. The nature of white space can be not only spatial but also temporal; there are cases of DTV transmitters which do not operate around the clock, but only during certain hours.

Compared to cellular network (e.g. 2G/3G) cells, DTV allotments are much more extended, usually covering areas of several hundreds of square kilometers – and so is the white space between them. The perspective which is opened is obvious – can we possibly use this valuable white-space bandwidth for low-power, low-range wireless networking in a strictly localized manner (secondary use) without interfering to licensed DTV transmissions? (primary use) This is the main idea behind the exploitation of TV White Spaces (TVWS) or Interleaved Spectrum, as it is also called.

At this point, it must be mentioned that the so-called “white spaces” are in fact not so “white” – in the sense of completely clean bands - since they naturally suffer from “pollution” due to low-power signals coming from neighbouring DTV allotments. These signals, although they may be too weak to be decoded (i.e. they are unusable), are still a considerable source of interference for TVWS devices [3]. Nevertheless, this “pollution” does not significantly degrade the actual value of white spaces since almost all contemporary radio networking technologies employ adaptive modulation and coding techniques so as to tolerate very high co-channel interference levels. Error-free operation is ensured at most cases, of course at the cost of lower spectral efficiency.

In this context, TV white spaces present a new opportunity for wireless networking in a frequency band which has very good transmission characteristics in the terrestrial environment and at the same time allows for reasonably-sized antennas. As examples of possible applications for secondary TVWS use, one could mention:

• Wireless local networking in TV bands, as an alternative to the highly congested ISM band

• Sensor and ad-hoc/mesh networks

• Regional-area networking, especially suited for providing affordable Internet and integrated-services access to citizens in rural/underdeveloped areas not covered by wireline networks. In this sense, TVWS presents an excellent opportunity for bridging the so-called “broadband divide”

• Femtocell deployment, i.e. TV-band operation of 3G/4G femtocells, eliminating interference to the operator’s cellular network

When it comes to indoor TVWS network deployment, the opportunities are even more appealing, due to the increased isolation with the licensed primary transmissions. Use cases include indoor femtocells and wireless LANs, exhibiting greater coverage and reduced interference compared to ISM-band devices. Furthermore, the lower operating frequencies of indoor TVWS devices can lead to lower energy consumption compared to ISM ones – resulting in extended battery time for handheld/portable use.

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

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The exploitation of white spaces is a challenge not only for manufacturers but also for spectrum regulators. The aspect of secondary spectrum usage of TV bands is quite novel and presents several difficulties when it comes to regulation. Primary users i.e. licensed TV broadcasters must be absolutely protected against potential interference and, at the same time, sufficient freedom must be given to secondary users to exploit the available spectrum. In the US, the FCC has adopted a Second Report and Order for unlicensed operation in TV bands, drafting rules for secondary networks to protect TV broadcasters. In the UK, Ofcom, conducting a public consultation on TVWS use since 2005, has also concluded to certain suggestions [5], including parameters to which secondary networks should comply.

In the standards domain, the concept of opportunistic spectrum access of TV bands has been the foundation for the IEEE 802.22 spec [6], which defines the PHY and MAC of a regional-access network for TVWS use. At the same time, other technologies such as WiFi and WiMAX are also being adopted to work in the TVWS context in a cognitive/opportunistic manner. IEEE P802.11af is paving the way towards unlicensed 802.11 operation in TV bands (“White-Fi”), while IEEE P802.19.1 studies coexistence issues for devices operating in white spaces. In Europe TVWS standardization is carried out in the ETSI Reconfigurable Radio Systems (RRS) Technical Committee (TC).

OPPORTUNISTIC SPECTRUM ACCESS FOR TVWS EXPLOITATION

A key factor which greatly facilitates TVWS exploitation is the adoption and deployment of mechanisms which safely determine dynamically and in real-time where and when to use TVWS spectrum for secondary use. These mechanisms, employed by the secondary network(s), facilitate the so-called opportunistic spectrum access, which is a feature falling in the more general category of Cognitive Radio (CR). The aim is to provide the secondary wireless network, with the necessary information on: i) which TV channel(s) to occupy and ii) which is the maximum allowed EIRP which can be used. The constraint is the absolute protection of DTV receivers which are tuned to a licensed (primary) DTV transmitter within its declared service area. For this purpose, two main mechanisms have been established; spectrum sensing and geolocation.

The approach of spectrum sensing involves the incorporation of a spectrum scanner in all nodes of the secondary wireless network. These scanners/sensors periodically scan the entire TV band for empty (locally unused) channels. For the network to use a specific TV channel, this channel must be reported empty by the sensors in all nodes of the network. Specific and sophisticated sensing algorithms have been proposed in the literature, which can detect a distant DTV transmission even if the DTV signal is quite below noise level. Sensing methods can be roughly categorized into a) “blind”, i.e. not requiring knowledge of the primary signal (such as simple energy detection or covariance-based detection) and b) “feature-based”, i.e. exploiting characteristics of the primary signal which are known a priori (such as correlation-based or pilot signal detection). Distributed sensing has also been studied, where several terminals collaborate and exchange sensing data so as to increase detection accuracy.

An orthogonal approach, which can be employed with or without spectrum sensing, is geolocation-based spectrum allocation, which constitutes the main focus of this article. Since the map of licensed DTV transmitters and their corresponding allotments/service areas is static, it is also reasonable to assume that one can create a semi-static database with maps of possible locations for TVWS networks, along with allowed power levels and TV channels assigned to each location for secondary use. Given the location of a TVWS network –which can be determined via e.g. a GPS subsystem incorporated in the devices– a comparison against the database map can be used to immediately assign a set of channels and power levels to be safely used by the network, without disturbing licensed DTV allotments. The 802.22 standard

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employs the geolocation approach for channel selection, by mandating that both the Base Station and the associated wireless clients be aware of their geographical location. Using these locations, the 802.22 BS queries the TVWS database to retrieve vacant TV channels in the area and maximum EIRP specified for each available channel.

Towards promoting this capability, the FCC has also already appointed administrators of nation-wide TVWS databases to be exploited by white-space devices. Similarly, Ofcom is also planning the implementation of white-space databases. The aim is to launch geolocation-based TVWS services in the UK early 2013.

If the regulators conclude on the mechanisms to be used along with the associated parameters and thresholds, license-exempt use of secondary TVWS networks could be safely allowed. In this sense, one could deploy and operate a TVWS network anywhere and anytime, without requiring a license (just as WiFi networks are currently deployed and used) and without worrying about possible interference to DTV signals. In a more controlled approach, opportunistic access could be combined with a centralized spectrum broker function –as extensively proposed in the literature for cognitive radio networks– where dynamic, real-time spectrum auctions could grant local use of TVWS spectrum to the party with the highest bid.

ESTIMATING THE ACTUAL WHITE SPACE CAPACITY

In order to determine the actual benefit and potential commercial value of TVWS exploitation, an estimation of the actual white space capacity is essential. In this section, we present a step-by-step methodology which can give a satisfactory approximation on the white space capacity which can be offered. Since the calculations involved are quite complex, we have isolated a restricted region for our case study, as indicated in Fig.1a/b. This region includes Peloponnese and a part of southern continental Greece and its division in allotments is known, along with the channels allocated to each allotment. The selected region is quite representative, since it is uniformly divided into allotments and it also comprises a balanced mix of plain/flat, hilly and mountainous areas.

While several attempts to quantify white space capacity can be found in the literature, the differentiation of the methodology and results presented in this article, can be identified in three main aspects:

First, we have adapted our methodology to the European case and in general to all countries having adopted DVB-T and following the ITU RRC06 frequency plan (ITU Region 1, including Russia, Middle East and Africa). Indeed, while various studies exist on the availability of TVWS in the United States, there is little evidence of what can be expected in the rest of the world. US-related studies [3] cannot be directly extrapolated e.g. to the European case, due to significant differences in DTV standards used, channel bandwidth, protection ratios, deployment scenarios of TVWS networks, regulatory aspects and even terrain morphology. Very few studies exist for outside the US, such as [7] which attempts to quantify TVWS capacity in the UK in a limited area using the spectrum sensing approach.

A second innovative aspect is the inclusion of the actual terrain morphology in all propagation calculations. Most existing TVWS studies employ fixed, general-use propagation curves such as the ones of the ITU-R P.1546-2 model used in RRC06 (Annex 2.2 of [2]). However, in areas with significant variations in terrain morphology, such as the area examined, there is a considerable added value in TVWS exploitation which cannot be overlooked; the irregular terrain itself becomes a physical barrier which can

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greatly improve the isolation between the secondary (opportunistic TVWS) and the primary (licensed DTV) network, thus increasing flexibility in local spectrum usage.

Last, the methodology presented follows a more regulatory rationale i.e. tries to protect licensed allotments regardless of the location of the existing DTV transmitters within them. Most research efforts on TVWS capacity solely compute the coverage areas of already deployed DTV transmission sites and exclude them from TVWS use. However, this approach depends on the current DTV deployment configurations (which may change over time, and may not coincide with the licensed coverage area) and also disregards the use e.g. of DTV repeaters/gap-fillers which may be additionally deployed in order to better serve the licensed allotment.

It must be also mentioned that, while several studies assume spectrum sensing as the main mechanism for opportunistic spectrum usage [3], our methodology leaves sensing aside and assumes the mere employment of a centralized geolocation-based mechanism. Indeed, in a scenario with fixed DTV transmitters and allotments, whose geographical location is known and unchanged, it is reasonable to assume that the calculation and employment of a set of fixed maps with allowable TVWS network locations for every channel is a well-established and fully justified approach. This is in-line with recent FCC recommendations, which mandate that fixed TVWS devices rely on geolocation to determine their operating channels. That is, they determine their position and query a nation-wide white space database in order to retrieve the vacant channels for the specific area. What is more, as explained in [8], the spectrum sensing approach, using extremely low detection thresholds, usually results in a heavy under-estimation of the actual white space area. Very weak DTV signals coming from distant transmitters are detected and the associated channels are ruled out, although practically no DTV receiver in the area can actually demodulate and decode them. The authors in [8] explain why spectrum sensing can lead to an over-estimation of protected areas of even 3.5x in comparison to a well-laid out geolocation mechanism; this is a considerable drawback, leading to waste of valuable spectrum. On the other hand, the geolocation approach not only provides greater flexibility for TVWS networking, but also seems safer for licensed TV broadcasters which should be relieved to see that their service area is geographically “fenced”. It is thus of no surprise that Ofcom has concluded that “the most important mechanism in the short to medium term will be geolocation” [5].

In this context, we assume for this study outdoor TVWS networks, which solely use the geolocation method. The local TVWS network is assumed to comprise 100 terminals simultaneously transmitting at the entire 8MHz bandwidth of the channel (a quite pessimistic assumption). We also assume that these terminals operate at maximum transmit EIRP of 4 Watts [7], in compliance with the 802.22 scenario and also with the FCC recommendation.

We begin our approach with the regulatory constraint that all DTV receivers within a licensed allotment must be absolutely protected against interference coming from the TVWS secondary network. Then, we try to determine the areas in which the TVWS network should operate in order to satisfy this constraint. This procedure is carried out on channel-by-channel basis. Let’s for example concentrate on channel 33 (566 – 574 MHz). Fig. 2a shows the licensed allotments for channel 33. The area of all licensed allotments is naturally excluded from TVWS operation at the specific channel. Thus, we try to find candidate locations for safe TVWS usage outside the allotments.

The methodology is as follows:

1. On the boundaries of each licensed allotment, we consider 30 test (reference) points carefully selected taking in mind the terrain morphology. Increasing the number of reference test points increases accuracy but linearly extends simulation time.

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2. At each boundary test point, we assume an outdoor DTV receiver which –as a worst-case- employs omnidirectional antennas and receives at the minimum decodable signal level, as defined in [2]. DTV transmissions are assumed to comply to the DVB-T standard, using 16-QAM constellation and convolutional code rate of 3/4.

3. We divide the region outside the licensed allotments into area elements of 450x450m.

4. We select an arbitrary area element and we consider a TVWS network operating at that specific location.

5. We calculate the co-channel interference caused by the TVWS network to each of the assumed DTV receivers at the boundary test points of the surrounding licensed allotments. The propagation loss required for interference calculation is estimated taking into account the actual terrain profile, using the point-to-point Longley-Rice Irregular Terrain Model (ITM) [9] and publicly available elevation data. A Matlab-based simulation platform was especially developed from scratch for this purpose.

6. Only if the interference caused to all assumed DTV receivers at the reference points is below the allowed levels (i.e. the protection ratios adopted by ITU [2] for DTV planning) then the specific location is considered “safe” and marked as candidate for TVWS secondary use.

7. Steps 4-6 are repeated for all 450x450m area elements outside the licensed allotments which reside on the land (the procedure could be also extended to the sea in order to cover maritime use cases).

The results are visualized in Fig.2a, where the locations where TVWS operation should be allowed have been marked. The contribution of the mountainous areas to the physical isolation between TVWS networks and DTV licensed areas is clearly visible. Note that the upper part of the southern continental area along with the Aegean islands were excluded from the scanning procedure, due to possible interference to allotments outside the examined area.

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Fig. 2. (a) DTV Allotments allocated to TV channel 33 and allowed locations for TVWS devices operating in this channel (b) A morphological opening procedure exposes candidate wide-area cells for TVWS cellular

networks operating at Ch 33.

The geolocation approach involves the comparison of the physical location of the TVWS network (as reported by a GPS device) to the calculated map. If the location has been marked as “white”, then the network is allowed to operate at the specific channel. However, the map of Fig 2(a) includes several locations which are isolated. It would be unrealistic to assume that a regional-area TVWS network of several tens of terminals would be restricted in a 450x450m area. In this sense, assuming wider secondary network cells of 3.2kmx3.2km, we need to locate in Fig.2a the portions of the “white” areas where a 3.2kmx3.2km cell would fit. This is achieved by an image-processing procedure known as morphological opening (i.e. erosion followed by dilation) on the white space image, where the structural element is a disc having a diameter of 3.2km. The results are shown in Fig.2b, which shows the allowed locations for deploying a wide-area TVWS wireless network operating at Channel 33 (566 – 574 MHz) with the aforementioned characteristics without causing interference to licensed DTV allotments.

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In order to compute the total available TVWS spectrum in each location, we repeat the aforementioned procedure for all 49 Band III/IV channels (from 21 to 69) and superimpose the results in a final image. Simulations have been quite demanding and merely their execution -for the studied area and resolution mentioned - lasted about one month, in parallel sessions hosted by four dual-core workstations. The results are visualized in Fig. 3. Fig. 4. shows the statistical distribution of the available TVWS spectrum.

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

Fig. 4. Statistical distribution of available TVWS spectrum

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It can be calculated that an average of 125 MHz of TVWS spectrum is available in every location, a quite significant amount, if we consider that it is located in a relatively low –and also very valuable- part of the RF spectrum. It should be mentioned that calculations have included the worst-case assumption that all TV channels allocated to each allotment have been licensed to and occupied by DTV broadcasters; since this scenario is quite unlikely, the amount of TVWS spectrum can be even higher. It must be also noted that the results of the present study are comparable with outcomes of other studies such as [4], which calculates an average white-space capacity of 150MHz per location in the UK.

With certain assumptions, these figures can be translated to per-capita wireless capacity. Assuming a typical spectral efficiency of 2 bits/sec/Hz (a conservative estimation, when compared to the performance of emerging technologies such as LTE-Advanced [10]), it can be deduced that an aggregate capacity of 250 Mbps can be offered in a 3.2kmx3.2km location. Considering that the average population density in Europe is 70 inhabitants/km2 and assuming a contention ratio of 1:40, we can calculate that TVWS exploitation alone can provide a rough average of almost 14 Mbps of aggregate wireless access capacity to each citizen – a quite attractive value.

OTHER CONSIDERATIONS

When a geolocation approach as the one aforementioned is to be brought to commercial use, there are a number of additional issues to be taken into consideration.

The first one is related to the spectrum usage flexibility of the radio technology to be used in the secondary network. The previous section presented the calculation of the overall amount of TVWS spectrum available in each location. It is clear that this spectrum will be mostly fragmented i.e. will comprise several non-contiguous TV channels of 8 MHz each. Emerging technologies, such as LTE-Advanced, are capable of exploiting this fragmented spectrum as a whole, thanks to state-of-the-art carrier aggregation techniques [10]. However, currently widespread technologies such as WiFi and WiMAX - which could be theoretically directly applicable for white-space networking via adjustment in their RF front-end in order to work in the TV bands - require a considerable amount of contiguous spectrum. For example, white-space operation of an 20-MHz IEEE 802.11g network requires three consecutive 8-MHz channels to be available at a given location.

In this context, to accommodate for this requirement, we carry out a post-processing of the aforementioned simulation results in order to derive the maximum amount of contiguous TVWS spectrum available at each location. The results are depicted in Fig.5. In this case, it is calculated that, on average, the maximum contiguous TVWS spectrum block per location is 27MHz.

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Fig. 5. Map of maximum contiguous (non-fragmented) TVWS spectrum (in MHz) available at each location

A second concern which should be noted is more of regulatory nature and refers to adjacent channel interference (ACI) issues. It is true that any radio apparatus, including TVWS devices, emits signals with non-ideal spectrum masks, i.e. suffers from power “leakage” to adjacent channels. It is thus possible that a high-power TVWS device operating at channel N could cause interference to nearby TV sets tuned to DTV signals at channels N±1. For this purpose and in order to protect primary users in a more absolute manner, the local Regulator could demand that, at any given location, for the secondary network to work in channel N, not only this specific channel (N) but also its adjacent (N+1, N-1) must be locally unused. This restriction –which has been recommended by both FCC and Ofcom [7] – greatly reduces the amount of available TVWS spectrum. In order to demonstrate the effect of this restriction, Fig.6 shows a re-calculation of the results of Fig.3, having excluded the use of channels adjacent to licensed ones. The average TVWS spectrum available per location falls to 30 MHz.

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Fig. 6. Map of available TVWS spectrum (in MHz), excluding use of channels adjacent to licensed ones

It should be noted, however, that the adjacent-channel restriction could be safely avoided for secondary networks with very strict spectrum masks and/or with signal bandwidth considerably narrower than the 8 MHz of the TV channel. For example, in a 5-MHz LTE deployment, one could assume that, under normal conditions, power leakage to adjacent TV channels could be ignored. Another factor, which could help eliminate this restriction, could be the establishment of a minimum distance between a DTV set/DTV receiver and a transmitting TVWS device so as to avoid potential interference.

A last (but not trivial) issue is related to the implementation and storage cost of the geolocation database. There are two scenarios regarding the access to the database. The first one assumes that the database is maintained in a centralized manner (e.g. at the national Regulator) and all TVWS devices perform requests to this database to acquire information on locally available channels. In this case, the geolocation maps can be of virtually unlimited resolution and complexity. The second scenario involves a version of the geolocation maps statically stored in each TVWS device and manually updated at specific intervals e.g. via a firmware update procedure. With this approach, the TVWS network, upon acquisition of its natural position via e.g. embedded GPS receivers, immediately knows which channels to use. This eliminates the need for external communication; however poses limitations on the size and resolution of the geolocation maps. For example, the maps created in this study involve, as aforementioned, rectangular area elements of 450x450 meters. At this resolution, it can be calculated that spectrum usage maps covering the entire Europe, containing the allowable channels and EIRP levels for each location, require approximately 2.4 GB of local storage. To reduce this size, apart from data compression techniques, one can employ a down-sampling procedure, as described in [8], however excluding an amount of useful TVWS areas.

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CONCLUSIONS

The study presented in this paper, in line with similar results presented in the literature, unveils a significant amount of wireless access capacity, which can be unleashed via the exploitation of TV White Spaces; the possible applications are virtually innumerable. Prompt and coordinated actions are necessary from the side of manufacturers, standardization bodies and especially national spectrum regulators in order to establish a sound technical and legal framework for such exploitation. Instead of feeling threatened by the “invasion” in an area of the spectrum traditionally devoted to them, broadcasters should see TVWS as a new area for novel and unprecedented business opportunities. Not only the license-exempt scenario but especially the approach of spectrum auctions and spectrum brokers for the use of the TVWS spectrum is bound to open new business cases for all players in the wireless and broadcasting market. Significant advances in this field should be expected in the near future.

REFERENCES

[1] T. O’Leary, E. Puigrefagut, W. Sami, “GE06 – Overview of the second session (RRC-06) and the main features for broadcasters”, EBU Tech. Rev., Oct. 2006

[2] International Telecommunication Union, “Final Acts of the Regional Radiocommunication Conference for planning of the digital terrestrial broadcasting service in parts of Regions 1 and 3, in the frequency bands 174-230 MHz and 470-862 MHz (RRC-06)”, Geneva, 2006

[3] K. Harrison, S. Mishra, A. Sahai, “How much white-space capacity is there?”, in Proc. IEEE Symposium on New Frontiers in Dynamic Spectrum, Singapore, 6-9 April 2010, pp. 1-10

[4] M. Nekovee, “Cognitive Radio Access to TV White Spaces: Spectrum Opportunities, Commercial Applications and Remaining Technology Challenges”, in Proc. IEEE Symposium on New Frontiers in Dynamic Spectrum, Singapore, 6-9 April 2010

[5] Ofcom statement, “Digital Dividend: Cognitive access”, Online at http://stakeholders.ofcom.org.uk/consultations/cognitive/statement/, July 1, 2009

[6] IEEE 802.22 Working Group on Wireless Regional Area Networks, http://www.ieee802.org/22/

[7] M. Nekovee, “Quantifying the Availability of TV White Spaces for Cognitive Radio Operation in the UK”, in Proc. IEEE ICC 2009, Workshop on Cognitive Wireless Networks and Systems, Dresden , Germany, June 2009, pp. 1-5

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[9] A. G. Longley and P. L. Rice, “Prediction of tropospheric radio transmission loss over irregular terrain. A computer method”, NTIS ESSA Tech. Rep. ERL 79-ITS 67, July 1968

[10] M. Iwamura, K. Etemad, R. Mo-Han Fong, R. Love, “Carrier aggregation framework in 3GPP LTE-advanced”, IEEE Comm. Mag., 48(8), Issue: 8, August 2010, pp. 60-67