[STANDARDS]

Ramachandran Ramjee Microsoft Research India Sumit Roy University of Washington USA Krishna Chintalapudi Microsoft Research USA

Editor: Michelle Gong A CRITIQUE OF FCC’S TV WHITE SPACE REGULATIONS

roadband access constitutes the “oxygen” for the Internet Era, to which end, the issue of spectrum management is foundational. In this position paper, we present an overview Bof FCC’s pioneering work framing TV white space rules that allows unlicensed access by secondary users while protecting licensed primary users. The core contribution of this paper is an in-depth analysis of FCC’s TV white space regulations (often treated as a global template). We argue that FCC’s current white space regulations does not achieve the desired balance between effectively promoting unlicensed secondary access and providing adequate protection of the primary. As more countries around the world look at framing white space regulations, we argue for incorporating a more flexible design that can catalyze a white space device ecosystem to flourish, similar to Wi-Fi.

INTRODUCTION network services, based on the operators Broadly speaking, spectrum allocations belief that exclusive rights to spectrum can be classified under two principles/ enables them to offer/tailor desired approaches: services, as well as optimize the network/ delivery mechanisms so as to extract a) granting of exclusive licenses, usually via maximum revenue. The unlicensed model a suitably designed auction mechanism, is used by standards, such as Wi-Fi, to operators for an explicitly identified where the fundamental issue is managing purpose of service (that may not be interference, specifically a) secondary changed) and unlicensed interference to primary b) setting aside “unlicensed bands” for licensed users and b) interference between technology neutral usage, whereby unlicensed secondary users. FCC Part 15 compliant devices do not need a license rules [FCC 01-58] that govern unlicensed to operate (but must nevertheless be bands have mostly focused on “transmit certified), akin to a public “commons.” power” centric rules to ensure limited interference from secondary unlicensed The first scenario conforms to a single users to primary users (e.g. limits on primary network in the licensed band average transmit power over the band, whereas the latter maps to multiple (co- maximum power spectral density and on located) networks. The exclusive license out-of-band emission limit using transmit

model is the operative one for all cellular spectral mask) while encouraging the use bigstockphoto.com Photo,

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of “etiquettes,” such as listen before talk TV WHITE SPACES & RULES above 512 MHz (above TV channel 20) (LBT) for managing interference between OF “COGNITIVE USAGE” if those available channels are adjacent to unlicensed secondary users.1 After the transition from analog to digital occupied TV channels. Fixed TVBDs can Spectrum license auctions, such as TV broadcasting, TV broadcast in the U.S. utilize an external antenna up to 30 meters those by the FCC, typically generate several is limited to channels 2-51; the unused above the ground and are allowed a higher billion USD as a one-time revenue to spectrum within channels 2-51 at any transmitter power of 1W or up to 4W EIRP the U.S. treasury (e.g. 2006 auction of 90 location is designated as TV White Spaces with a 6 dBi gain antenna. These devices MHz netted USD 14 billion [BULOW]). (TVWS). The FCC, in a series of pioneering are not allowed to be located at a site where However, a multibillion dollar industry notices and regulations starting from 2004, the ground height above the average terrain has also grown in the use of “unlicensed” effectively allowed Part 15 like (unlicensed) (HAAT) exceeds 250 meters, so use in hilly spectrum [THANKI 1]; a recent analysis operation in TV White Space frequencies areas may be restricted. suggests that the value generated from subject to cognitive usage rules for protect- unlicensed spectrum use is $140 billion ing the primary users, i.e. requiring un- Personal/Portable Devices: Personal/ per annum in the U.S. alone [THANKI licensed or secondary TV Band (TVBD) portable device transmissions are restricted 2]. Because users of unlicensed spectrum devices to incorporate ‘‘smart radio’’ features to available channels in the frequency bands enjoy economies of scale and do not pay to protect the primary users [FCC 04-113, 512‐608 MHz (TV channels 21‐36) and for expensive spectrum licenses, unlicensed FCC 08-260, FCC 10-174, FCC 14-144]. 614‐698 MHz (TV channels 38‐51). These networks typically offer a less expensive and devices are limited to a maximum EIRP of more readily deployable form of wireless General Requirements on TV Band 100mW or 40mW if the device is operating service than licensed spectrum – albeit Devices (TVBDs) [FCC 14-144] on a channel adjacent to an occupied TV at a trade-off for quality of service and The unlicensed TVBDs were classified channel. protection from interference. into two functional categories: a) lower Wi-Fi carried about 70% of all mobile power ‘‘personal/portable’’ devices, such as Mode I Personal/Portable Devices: Internet data traffic in 2014 and Wi-Fi’s Wi-Fi–like cards in laptop computers and A Mode I device is not required to use share is expected to rise to 85% by 2018 b) higher power ‘‘fixed’’ devices that could geo‐location or have access to the TV [WI-FI-TRENDS]. The global success of be used to provide services such as 802.22 bands database. Instead, it obtains a list of Wi-Fi even with the relatively small ranges Base station/CPEs providing a backhaul for available channels from either a fixed or of frequency allocated for unlicensed use broadband client access. Mode II personal/portable device. A Mode has led many spectrum advocates to press The personal/portable TVBDs were I device may not initiate a network nor for more unlicensed spectrum, leading to a further sub-divided into Mode I and Mode may it provide a list of available channels more equitably mixed ecosystem of licensed II operations with somewhat different rules to another device. and unlicensed approaches that combines applying as detailed below. Fixed devices are the relative strengths of both. While the intended to operate at a fixed location (e.g. Mode II Personal/Portable Devices: A original notion of (pure) unlicensed usage an outdoor access point) while personal/ Mode II device uses its own geo‐location corresponds to a “commons” scenario portable devices can operate in unspecified capability and access to the TV bands where all networks are treated on-par and locations based on user mobility. A (second- database, either through a direct connection must accept interference from each other, ary) network can only be initiated by a fixed to the Internet or through an indirect rule making can readily accommodate or Mode II TVBD, not a Mode I TVBD. connection by way of a fixed TVBD or provisioning of priority for any sub-set another Mode II TVBD. A Mode II device operating in co- or adjacent channels.2 Fixed Devices: Fixed devices can initiate may select a channel itself and initiate and This is precisely the notion of Cognitive a network by sending enabling signals operate as part of a network of TVBDs, White Spaces (WS), whereby the identified to one or more fixed and/or personal/ transmitting to and receiving from one or primary services are to be protected from portable TVBDs. Fixed devices can use any more fixed or personal/portable TVDBs. unlicensed usage, as we discuss next. available WS frequency but may not operate It may provide a list of available channels to a Mode I device.

1 This is in contrast to EU regulations that mandate LBT use in unlicensed spectrum. 2 For example, in the upper 5 GHz band, Wi-Fi devices share the band with military radar subject to Interference Avoidance Methods: Channel the condition that the Wi-Fi devices are capable of spectrum sensing and if radar is detected, the availability for a TVBD is determined based unlicensed user must immediately vacate the channel. upon geo‐location and database access but

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spectrum sensing can also be used. For fixed a handful of small startups [TVBD]. The and Mode II devices, geographic coordinates integration of embedded WS devices into need to be determined to an accuracy of consumer portable/handheld devices has +/‐ 50 meters by utilizing incorporated not yet occurred, stalling the onset of any geo‐location capability. Fixed devices that virtuous market cycle based on increasing are professionally installed do not need adoption volume. As a result, the future geo‐location capability if the coordinates of WS technology under the regulations are provided at time of installation. All proposed by FCC looks bleak. devices must register their coordinates with In this section, building upon some the database and must re‐establish their preliminary observations in [TCCN 2015], position each time it is activated from a we provide an in-depth assessment for this power‐off condition. Fixed devices shall state-of-affairs and suggest alternate designs access and update their database at least that could allow a large white space device once every day and personal/portable device FIGURE 1. One of the first FCC-approved white ecosystem to develop, similar to that of must check its location at least once every spaces device is a 1.5-lb radio from KTS Wireless. Wi-Fi. 60 seconds. Operation on a channel must cease immediately if the database indicates ISSUES WITH FCC TVWS RULINGS the channel is no longer available. Mode 1. Database showing little or no white II personal/portable devices also must incumbents [SHELL 11]. TVBDs that meet space availability in dense metro areas: re‐check its position if it changes location the spectrum sensing requirements for The WS predictions in the database are during operation by more than 100 meters. personal/portable devices are limited to a based on assumed radio propagation (path A Mode II device can also establish a maximum EIRP of 50mW. All fixed and loss) models that are used to determine the bounded area in which a channel is available personal/ portable TVBDs must be capable coverage or service region of any primary at all locations within the area and operate of detecting ATSC digital TV, NTSC analog (DTV) broadcast source and an additional on a mobile basis within that area. TV and wireless microphone signals at the no-talk region for protecting the TV following detection thresholds: receivers from interference. Needless to TV Bands Database: A TV bands database say, even the best RF propagation models maintains records for all authorized ➢ ATSC digital TV signals: ‐114 dBm, accurately fit only a specific set of scenarios, (primary) services that operate in the averaged over a 6 MHz bandwidth and no one model can fit the wide variety TV bands and contains the following ➢ NTSC analog TV signals: ‐114 dBm, of topological conditions (urban, sub- information: transmitter coordinates, averaged over a 100 kHz bandwidth urban, rural; open, obstructed, indoor, effective radiated power (EIRP), antenna ➢ Low power auxiliary & wireless etc.). It has been demonstrated that current height above terrain (HAAT), antenna microphone signals: ‐107 dBm, database predictions of available WS are pattern, beam tilt, channel number and call averaged over a 200 kHz bandwidth. almost always conservative, erring on the sign. A TV bands database administrator side of protecting the primary incumbents (DBA) may charge a fee for the provision FCC TVWS RULES: A CRITIQUE [CHAKRABORY]. For example, the of lists of available channels and other As with any new rule making to promote database predicts zero or very little WS associated information and for registering the commercial use of TV White Spaces, availability in several U.S. metro areas fixed TVBDs. However, the FCC OET the FCC sought to achieve the reasonable as of this writing (e.g., Figure 2). At the has stipulated that all DBAs in any region “median” of balancing the incentives/ national level, the availability of white must provide exactly the same output (WS opportunities for proponents seeking to spaces in the U.S. decreases significantly channel listing etc.) in response to a TVBD develop viable new unlicensed networks with increasing density of users as shown client query. In effect, this is achieved by based on TV band devices (TVBDs) with in Figure 3 (see [ROY15]). This situation requiring all DBAs to use exactly the same measures necessary to ensure that any severely limits the market size for potential input formation, i.e., the official and publicly co-located licensed or primary services are manufacturers, resulting in the reluctance available CDBS database of broadcast protected. A clear measure of the success of big manufacturers to enter the white stations along with a common FCC OET of any policy is whether vendors move space device market. specified algorithm TV coverage area into the market subsequently with WS estimation. transceivers and the rate of uptake into A two-tier architecture that integrates consumer devices. To date, more than ten targeted spectrum sensing along with a Spectrum Sensing: Spectrum sensing years after the first FCC notice for proposed Database is necessary to improve the may be used in lieu of geo‐location and ruling making in TV white spaces [FCC accuracy and availability of WS channels, database access but devices must submit an 04-113], the only WS devices available on especially in densely populated areas application for certification and demonstrate the market resemble those in Figure 1, i.e., where few white space channels are with a high degree of confidence that they intended for high power, point-to-point available. For example, a recent study will not cause harmful interference to backhaul segment and manufactured by shows that, in the New York City Metro

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FIGURE 2. Google white space spectrum database showing zero white FIGURE 3. Available white spaces, occupied TV channels and space channels available in Manhattan, NY, USA. unused TV channels versus population density in the U.S.

Area, about 40-75% of white space TVBDs. This architecture is consistent existence problem may include LBT, spectrum is “lost” due to the conservative with a) optimizing channel allocation perhaps augmented with additional database, most of which can be recovered to various secondary users and b) database support. by targeted sensing augmentation promoting better secondary co-existence [CHAKRABORTY]. Thus,local spectrum (discussed next). 3. Lack of alternatives for Indoor information generated by client-side Operations: Per the Second Report and (targeted) sensing can be effectively fused 2. No consideration of secondary-to- Order (that allowed for two classes of with prior information (such as predictive secondary coexistence: In our opinion, TV bands devices – fixed and personal/ primary coverage regions) to obtain more another oversight is the complete omission portable), Mode II personal/portable accurate WS prediction accuracy. In other of any consideration of secondary co- devices must incorporate a geo-location words, channel access by TVBDs should existence in FCC TVWS rulings that capability that self-locates device always require the kind of authorization exclusively concentrates on protection of coordinates to within +/- 50 meters as in current TVWS database the primary. As mentioned earlier, unlike accuracy. With current state-of-art of architecture, but assisted by client-side the EU, FCC typically does not mandate indoor location technology where GPS does information. techniques like LBT for secondary co- not work (especially in urban areas where Such an architecture also addresses existence in unlicensed spectrum. However, concrete buildings are common), this level another deficiency in today’s white space secondary co-existence will be a bigger of accuracy cannot be achieved reliably. As market, namely, the inability of database challenge in TV white spaces compared a result, a significant class of mass market providers to differentiate themselves. to 2.4GHz and higher frequency ISM Mode II devices are precluded from indoor Today, all WS database providers report bands given that propagation and thus use, dealing a significant blow to their identical channel availability information interference is spread over larger distances market uptake. The FCC initially did not (since this information is computed in TV UHF spectrum. While IEEE 802.19 consider potential alternatives tailored to based on the FCC authorized models). defines standards for co-existence between indoor use – a significant market in dense Allowing the database to be augmented different secondary networks, most TV metro areas where the need for an alternate with local sensing information can white space products in the market today to the crowded Wi-Fi spectrum is severe. incentivize different database providers to do not support the standard. As a result, A recent update to the white space place sensing nodes at various locations TVWS rulings are unable to provide rules by the FCC in 2015 has resulted in (e.g., metros) in order to provide a an assurance that TVBD networks can the inclusion of an option of specifying richer, more accurate channel availability scale effectively, wherein the secondary- the location uncertainty by devices to the information, thereby differentiating their to-secondary interference becomes the database [FCC 15-99]. This addresses the database product, while simultaneously fundamental performance limiter. An strict location accuracy requirements of increasing the available capacity for effective solution to the secondary co- the previous regulations and could perhaps

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FIGURE 4. Spectrum masks (scaled) for Wi-Fi, LTE FIGURE 5. Spectrum masks (scaled) for FCC and and FCC white space device. ETSI Class 1 through Class 5.

spur the development of indoor white space 4G cellular technologies, such as LTE strict mandate, incorporating flexibility products. and results in equipment costs for fixed in system parameters, such as spectrum However, the propagation models customer premises equipment that are masks can enable a variety of new that are utilized by FCC are still based on 65% higher compared to comparable applications for white space devices. outdoor propagation and do not consider equipment operating in other frequency major factors, such as building penetration bands. As a result, expensive and b. Spectrum Sensing System Parameters: loss. Measurements have shown that indoor complex analog filtering must be added Portable TVBDs that incorporate white space channel availability can be to device front-end to meet the out-of- spectrum sensing are limited to a significantly higher than outdoors [YING]. band emission limits, and essentially maximum EIRP of 50 mW (lower than Given the lack of white space channels rules out a low-cost, lower power solution the 100 mW maximum for devices that in dense metro areas, taking building necessary for integration into client-side consult a database), while being required penetration loss into account will result in portable devices. to detect DTV signals at a sensitivity the availability of a few channels for indoor Besides the negative impact on device of -114 dBm. Fixing these values use, providing an impetus to the indoor cost, the above exemplifies another key without considering any DTV receiver white space device market. issue with FCC TVWS rule making characteristics3 results in further sub- – specifying fixed parameter values optimality of secondary network designs. 4. Inflexible Settings for Various System that negatively impact the potential of Spectrum sensing at a sensitivity of Parameters secondary (WS) networks. -114 dBm is not only overly conservative a) Transmit Spectrum Mask/Out-of- An alternative to the FCC approach is [MISHRA] but also very challenging. Band Emissions: The transmit mask the approach adopted by U.K.’s Ofcom for For a 6 MHz TV channel, noise floor is adopted by the FCC for TVBDs sets white space device regulations [ECMA around -100 dBm considering a 6 dB extremely stringent limits on out-of-band 392, ECC 2011]. Instead of insisting noise figure; this translates to an SNR emissions. The allowed channel emission that all white space devices implement of -14 dB. For a portable device with limit for each of the four maximum a single sharp spectrum mask, Ofcom smaller antenna size, effective SNR will power levels at which TV bands devices allows white space devices the flexibility be in the range of -20 to -18 dB. Signature can operate is simply the maximum of choosing from a variety of spectrum detection techniques become unreliable power minus 72.8 dB. The rules allow masks, for example, ETSI class 1 to and energy detection methods are TVBDs to operate at the maximum class 5 [ETSI 301 598] (Figure 5) while fundamentally flawed at such low SNRs, permissible power in a bandwidth of 5.5 controlling their maximum transmit MHz (instead of 6 MHz), with 250 KHz power. Thus, a less stringent ETSI class 5 for roll-off for in-band signal. As shown mask device will only be able to transmit 3 FCC rule making is entirely focused on transmit- in Figure 4, this emission mask for TV at a much lower power than an ETSI class ters and does not incorporate any receiver-side recommendations. This represents another missed band devices is 35 dB more stringent 1 mask device, which may very well be opportunity since pushing for improved DTV than the industry standards for Wi-Fi fine if the former is a low-cost Internet receiver design would inevitably also improve and about 20 dB more stringent than of Things (IoT) device. Thus, instead of a capacity (and market size) of TVBD devices.

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due to uncertainty of noise power. Thus, b) allowing flexibility in choice of white space and Applied Probability in 1988. Presently he sensing is not implemented by white device spectrum masks and other system is Integrated Systems Professor of Electrical space devices in the market today. parameters, to better promote development Engineering, Univ. of Washington, where his research interests include fundamental and growth of a diverse and vibrant white analysis/design of wireless communication and c. Height above Average Terrain Limit: space device ecosystem while still preserving sensor network systems spanning a diversity The range at which a TVBD can cause protection of the primary licensed user. Q of technologies and system application areas: interference to licensed users increases next-gen wireless LANs and beyond 4G cellular with the height of the device’s antenna. Ramachandran Ramjee received his B.Tech networks, heterogeneous network coexistence, Thus, FCC adopted a maximum antenna in Computer Science from IIT Madras, and his spectrum sharing, white space networking and height limit of 30 meters above ground M.S./Ph.D. in Computer Science from University software defined radio platforms, vehicular and of Massachusetts, Amherst. He is currently a underwater networks, smart grids and RFID level (AGL) for fixed devices. However Principal Researcher at Microsoft Research, India. sensor networking. in scenarios where a TVBD is located His research interests include network protocols at high point, the relevant parameter to and architecture, wireless networking and Krishna Kant Chintalapudi is a researcher in measuring interference to the primary is mobile computing. He is an ACM Distinguished the Mobility and Networks Research group at the antenna height above average terrain Scientist and a Fellow of the IEEE. Microsoft Research, Redmond. He graduated from the University of Southern California (HAAT). Thus, while the AGL limit is Sumit Roy received a B. Tech. degree from with a PhD in Computer Science, and his kept at 30 m, a HAAT limit of 250 m was the Indian Institute of Technology (Kanpur) in research interests broadly lie in the area of added. Unfortunately, this decision has 1983, and the M. S. and Ph. D. degrees from wireless networking systems. He has served the side-effect of unnecessarily limiting the University of California (Santa Barbara), in the program committees of several top-tier TVBD coverage for several scenarios in all in Electrical Engineering in 1985 and 1988 conferences and is currently an associate editor hilly areas, notably for a high-elevation respectively, as well as an M. A. in Statistics for the IEEE Transactions on Mobile Computing. transmitter and low-elevation receiver. Again, a more flexible regulation, such as setting TVBD transmit power limits as a function of key scenario parameters REFERENCES (AGL, HAAT) or setting (AGL, HAAT) [ECC 159] “Technical & Operational Requirements [FCC 15-99] “NPRM In the Matter of Amendment parameters as a function of location for Possible Operation of Cognitive Radio of Part 15 of Commissions Rules for Unlicensed of the TVBD would have been a more Systems in the White Spaces of the Frequency Operations in the Television Bands, Repurposed Band 470-790 MHz,” Cardiff, Jan 2011. 600 MHz bands …” FCC 15-99, Aug. 2015. desirable approach. [ECC 2011] “Technical and Operational [TVBD] “Consumer White Space Gadget Chipsets Requirements for the possible operation of in the Works,” http://onforb.es/25NJ4xz In summary, perhaps as a result of being Cognitive Radio Systems in the White Spaces of [WI-FI-TRENDS] Wi-Fi Wireless Trends 2015: a pioneer in crafting TVWS regulations, we the Frequency Band 470-790 MHz,” Electronic The New Network. http://bit.ly/1YfCz4k believe that FCC has been conservative and Communications Comm. Rpt. 159, Jan. 2011. [BULOW] J “Winning Play in Spectrum Auctions,” [ETSI EN 300 328] “Electromagnetic compatibility Bulow, J. Levin and P. Milgrom, Feb. 2009. rigid in their regulations. 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