A Guidebook for Mitigating Disruptive WiFi Interference at Airports (2015)

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84 A P P E N D I X C IEEE Family of Standards The Institute of Electrical and Electronics Engineers (IEEE) 802 committee has developed a family of wireless standards that have become dominant in the unlicensed industrial, scientific, and medical (ISM) bands. Equipment designed to one or another of the standards in the IEEE 802 family is the de facto standard for wireless data networks, mobile phone headsets, and a variety of other functions. Although equipment following the IEEE 802 standards does not have exclusive use of any frequency band, it is the most commonly used class of wireless in many of the bands. As Figure C-1 illustrates, IEEE 802 standards have been developed for a wide variety of purposes and operate in a variety of frequency bands. When discussing equipment that follows IEEE 802 standards, there are a range of types. Some equipment strictly follows an IEEE 802 standard and has been tested by an independent authority, such as the WiFi Alli- ance, and certified for its compliance with the standard and interoperability with other certified WiFi equipment. Other devices are designed to the standard but have not been certified, leaving some question about how faithfully the standard was implemented. In other cases, devices are designed to use the IEEE 802 standards but with proprietary differences. An example in this class is a set of devices offered by Ubiquiti, a private vendor, that operate in the 900 MHz ISM band based on 802.11b/g. Similar offerings are available from other vendors. So while the IEEE 802 standards do not support the 900 MHz ISM bands, these companies have adapted them to the band. Perhaps in response, the IEEE 802 committee is now working on 802 standards to operate in this band through the IEEE 802.11ah project. IEEE 802.11 (WiFi) In an article in Electronic Design, “Wi-Fi and Bluetooth Rule the Airwaves” (July 11, 2013, pgs. 28–35), Louis Frenzel provides a very concise technical overview of the different versions of IEEE 802.11: The first generation 11b showed up in 1997 and uses direct sequence spread spectrum (DSSS) to achieve data rates to 11 Mbits/s in a 20-MHz channel in the 2.4 GHz ISM radio band. With the growth of the Internet, that low speed soon became a disadvantage. The second-generation 802.11a appeared in 1999. It was the first to use the 5 GHz ISM band and orthogonal frequency division multiplexing (OFDM) with 64 subcarriers spaced 312.5 kHz apart. Channel bandwidth was 20 MHz, and binary phase-shift keying (BPSK), quadrature phase-shift keying (QPSK), 16-phase quadrature amplitude modulation (16QAM), and 64-state quadrature amplitude modulation (64QAM) types were defined, permitting the data rate to increase to a maximum of 54 Mbits/s. IEEE 802 Standards

IEEE 802 Standards 85 IEEE 802 LAN/MAN IEEE 802.16 Wireless MAN IEEE 802.20 Wireless WMAN IEEE 802.11 Wireless LAN IEEE 802.22 Wireless RAN IEEE 802.15 Wireless PAN IEEE 802.15.1 WPAN IEEE 802.15.3 High-Rate WPAN IEEE 802.15.4 Low-Rate WPAN IEEE 802.15.6 BAN Frequency Band(s) 2.3 GHz 2.5 GHz Uses WiMax Frequency Band(s) 2.4 GHz ISM 3.6 GHz Uses WiFi 5.8 GHz ISM 3.5 GHz 5.8 GHz ISM Frequency Band(s) < 3.5 GHz Licensed Uses Mobile Broadband Frequency Band(s) 54 to 854 MHz Uses WRAN Frequency Band(s) Uses Bluetooth 2.4 GHz ISM Frequency Band(s) 3.1-10.6 GHz Uses UWB Frequency Band(s) 900 MHz ISM Uses Zigbee Frequency Band(s) 402 to 405 MHz MICS Uses WBAN Wireless USB WiMedia WirelessHart 6LowPAN ISA 100.11a 2.4 GHz ISM 608 to 614 MHz WMTS 1395 to 1400 MHz WMTS 1427 to 1429 MHz WMTS 3.1 to 10.6 GHz Figure C-1. IEEE 802 standards organized by field of use and frequency band. While the 802.11a version was more robust because of the OFDM characteristics that mitigated multipath reflections and the 5 GHz assignment meant less interference, the higher frequency still limited the range. This version was never too popular despite its advantages. The big breakthrough came when the 802.11g standard was approved in 2003. It is essentially the same as 11a but operates in the 2.4 GHz band. Using the same OFDM and modulation options, it too can deliver up to 54 Mbits/s. It was immediately popular because the many IC companies competing for the business brought chip prices very low. The current 11n standard is a further improvement over 11a/g. It adds 40-MHz channels and multiple-input, multiple-output (MIMO) features to the OFDM, allowing data rates to increase to as much as 600 Mbits/s.

86 A Guidebook for Mitigating Disruptive WiFi Interference at Airports MIMO uses multiple receivers, transmitters, and antennas to achieve spatial division multi- plexing (SDM). SDM transmits fast multiple data streams concurrently within the same 20- or 40-MHz channel bandwidth. Pre-coding and post-decoding as well as unique path characteris- tics distinguish the data streams. The data rate then can be multiplied by a factor roughly equal to the number of data streams. The 11n MIMO standard permits up to four transmit and four receive channels (4 × 4), although 1 × 2, 2 × 2, and 3 × 3 versions are more widely used. The 600-Mbit/s data rate is achieved using 4 × 4 MIMO with 64QAM in a 40-MHz channel. IEEE 802.15.1 (Bluetooth) Bluetooth’s market dominance and frequent use scenarios that have Bluetooth devices oper- ating close to WiFi devices make it of particular concern when looking at WiFi interference. The Bluetooth community recently introduced a low-power version, named Bluetooth Low Energy (BLE), and Bluetooth is now being called Bluetooth Classic to differentiate it from Bluetooth Low Energy. Louis Frenzel does an excellent job of presenting the technical description of Blue- tooth and Bluetooth Low Energy: BLE still operates in the same ISM license-free 2.4 to 2.483 GHz frequency band as standard Bluetooth, but it uses a different frequency-hopping spread-spectrum (FHSS) scheme. Standard Bluetooth hops at a rate of 1600 hops per second over 79 channels that are 1 MHz wide. BLE FHSS uses 40 channels that are 2 MHz wide to ensure greater reliability over longer distances. Standard Bluetooth offers gross data rates of 1, 2, or 3 Mbits/s. BLE’s maximum rate is 1 Mbit/s with a net throughput of 260 kbits/s. Also, BLE uses Gaussian frequency shift keying (GFSK) modulation. It offers a power out- put of 0 dBm (1 mW) and a typical maximum range of 50 meters. Security is via the 128-bit Advanced Encryption Standard. An adaptive frequency-hopping technique that avoids interfer- ence, a 24-bit cyclic redundancy code (CRC), and a 32-bit message integrity check all improve link reliability. The most common network configurations are point-to-point (P2P) and star. Latency is only 6 ms.17 IEEE 802.15.4 (ZigBee) IEEE standard 802.15.4 was developed for wireless personal area networks. Wireless personal area networks are focused on low-cost, low-speed communication between devices, in contrast to WiFi which is focused on end-users. Sensors and automation systems are the focus for this class of device. Wireless personal area networks intend to provide very low-cost communication between devices with little to no underlying infrastructure. Low power consumption is critical so that battery power is a viable alternative. The target communications range is up to 10-meter. Data transfer rates of up to 250 kbits/s meet the needs of this class of device. IEEE 802.15.4 only addresses the physical and media access layers. It is the basis for: • ZigBee • ISA100.11a 17 Louis Frenzel, “WiFi and Bluetooth Rule the Airwaves,” Electronic Design, July 11, 2013.

IEEE 802 Standards 87 • WirelessHART • MiWi Each of these protocols differentiate themselves in how they develop the upper layers but have in common the lower layers, as defined in IEEE 802.15.4. IEEE 802.15.4 was first released in 2003 with a new version released in 2006 and work continu- ing on future revisions. It supports operation in the following bands: • 868.0–868.6 MHz: Europe • 902–928 MHz: North America • 2400–2483.5 MHz: worldwide use Given its target application and shared frequency bands with WiFi, these devices are likely to be used in airports and add to the RF noise for WiFi. While these devices are mostly low power and intermittent in their communication, the intention to have many of them installed to provide sensing coverage creates an additive effect from their combined operation. History of IEEE 802 Standards This section provides a brief history of the IEEE 802 standards, from an interference view- point. It does not attempt to provide a definitive history of these standards and the equipment that is designed to comply with them. Instead it focuses on the milestones that are relevant to the study of WiFi interference. In 2003, the Federal Communications Commission allocated additional spectrum for un licensed use in the 5 GHz band and established the Unlicensed National Information Infrastructure (UNII) service to facilitate the deployment of competitive wireless broadband services.18 Unlicensed National Information Infrastructure equipment is authorized to oper- ate radio transmitters in the 5.15–5.35 GHz, 5.47–5.725 GHz, and 5.725–5.825 GHz bands on an unlicensed basis, but must comply with technical rules specific to UNII devices to prevent interference. In order to avoid interference to the FAA’s terminal Doppler weather radar (TDWR) installations, the FCC requires that UNII devices operating in the 5.25–5.35 GHz and 5.47–5.725 GHz bands have dynamic frequency selection (DFS) radar detection functionality, which allows them to detect the presence of radar systems and avoid co-channel operations with radar systems.19 However, despite the requirement in the rules that UNII equipment employ DFS capability, the FCC Enforcement Bureau Field Offices continue to encounter instances of interference to TDWR systems caused by UNII devices located in close proximity to TDWR installations.20 The FCC states that such interference “. . . poses a clear hazard to air traffic safety and requires aggressive enforcement.”21 18 See Unlicensed National Information Infrastructure (UNII) Devices in the 5 GHz Band, Report and Order, 18 FCC Rcd 24484 (2003). 19 See 47 C.F.R. § 15.407(h)(2). See also 47 C.F.R. § 15.403(s) (defining U-NII devices as “[i]ntentional radiators operating in the frequency bands 5.15-5.35 GHz and 5.470-5.825 GHz that use wideband digital modulation techniques and provide a wide array of high data rate mobile and fixed communications for individuals, businesses, and institutions.”). 20 See Memorandum from Julius Knapp, Chief, Office of Engineering and Technology, FCC, and P. Michele Ellison, Chief, Enforcement Bureau, FCC, to Manufacturers and Operators of Unlicensed 5 GHz Outdoor Network Equipment Re: Elimina- tion of Interference to Terminal Doppler Weather Radar (TDWR) (dated July 27, 2010), available at http://transition.fcc.gov/ eb/uniitdwr.pdf (last visited April 27, 2013) (OET/EB Memo). 21 Towerstream Corporation, Notice of Apparent Liability for Forfeiture and Order, FCC 13-109 released August 6, 2013.

88 A Guidebook for Mitigating Disruptive WiFi Interference at Airports 22 Table from: http://www.intel.com/standards/case/case_802_11.htm Year Version Band Data Rate Purpose 1997 802.11 2.4 2 Mbps The original wireless local area network (WLAN) standard. Supports 1 Mbps to 2 Mbps. Based on a DSSS physical interface. 1999 802.11a 5.8 54 Mbps High speed WLAN standard for 5 GHz band. Supports 54 Mbps. Based on an OFDM physical interface. 1999 802.11b 2.4 11 Mbps WLAN standard for 2.4 GHz band. Supports 11 Mbps. Based on a DSSS physical interface. 802.11d Internaonal roaming — automacally configures devices to meet local RF regulaons. 802.11e Addresses quality of service requirements for all IEEE WLAN radio interfaces. 802.11f Defines inter access point communicaons to facilitate mulple vendor distributed WLAN networks. 2003 802.11g 2.4/5.8 54 Mbps Establishes an addional modulaon technique for 2.4 GHz band. Supports speeds up to 54 Mbps. Implements the physical interface based on OFDM. 2007 802.11 Consolidaon of the 802.11 amendments with the base standard. 802.11h Defines the spectrum management of the 5 GHz band. 802.11k Defines and exposes radio and network informaon to facilitate radio resource management of a mobile WLAN. 2009 802.11n 2.4/5.8 600 Mbps Provides higher throughput improvements. Intended to provide speeds up to 500 Mbps through the use of mulple input, mulple output and higher density modulaons. 2012 802.11 Consolidaon of the 802.11 amendments with the base standard. 802.11s Defines how wireless devices can interconnect to create an ad hoc (mesh) network. 802.11r Provides fast (<50 millisecond), secure and quality of service enabled inter access point roaming protocol for clients. 802.11u Adds features to improve interworking with external (non 802) networks where the user is not pre authorized for access. 802.11v Enhances client manageability, infrastructure assisted roaming management, and filtering services. 2008 802.11y 3.7 Extended 802.11a to the licensed 3.7 GHz band. 802.11z Creates tunnel direct link setup between clients to improve peer peer video throughput. 802.11aa Robust video transport streaming. 2013 802.11ac 5.8 693 Mb/s Extends the IEEE 802.11n in the 5.8 GHz band to achieve higher data rates through wider channel bandwidths, higher density modulaons, and addional mulple input, mulple output streams. 2012 802.11ad 60 7,000 Mbps Uses the 60 GHz band to create very high data rate capability. 2014 802.11af TVWS 26.7 Mbps Operaon in TV White Space frequency band (54790MHz). TBD 802.11ah 0.9 TBD Adds the 900 MHz band to those supported in the WiFi standards. The physical interface is OFDM based. Table C-1. Version and amendments of IEEE 802.11, WiFi.22

IEEE 802 Standards 89 Modulation and Coding Schemes To achieve their higher data rates, the newer versions of the IEEE 802.11 standards support a number of modulation schemes and coding rates. These are defined by the standard and are represented by a modulation and coding scheme (MCS) index value. A unit will vary the MCS used based on the channel conditions.24 WiFi Shipments Shipments in 2013 of WiFi chipsets are projected to be 2.14 billion units, up 20% from 1.78 billion in 2012. Double-digit growth started at least five years ago and is expected to persist at least until 2016. By 2017, WiFi chipset shipments will amount to 3.71 billion units. 23 Wikipedia, “IEEE 802.11n-2009,” Available at: http://en.wikipedia.org/wiki/IEEE_802.11n-2009 24 Wikipedia, “IEEE 802.11n-2009,” Available at: http://en.wikipedia.org/wiki/IEEE_802.11n-2009 Table C-2. MCS index values (GI = guard interval).23

90 A Guidebook for Mitigating Disruptive WiFi Interference at Airports A total of approximately 18.7 billion WiFi chipset units will be shipped from 2011 to 2017— nearly all of which will belong to the high-performance 802.11n version. By 2015, nearly 1.2 billion handsets out of a total of 1.9 billion cellphones produced that year will include WiFi functionality. Approximately 70% of handsets sold worldwide by then—and well over that figure in North America and Western Europe—are expected to be smartphones with embedded WiFi.25 ABI Research forecasts carrier WiFi access point shipments in 2018 to reach 9.7 million, with the Asia-Pacific region accounting for 70% of that number.26 New IEEE 802.11 Versions The success of the IEEE 802.11 series of standards and WiFi data networks has required and supported an aggressive course of development. In recent years, the IEEE 802.11 committee has averaged a new addition to the standards every year. New frequency bands are being opened for use by WiFi. The data rates are ever increasing. An expanding variety of use cases are being sup- ported. Field problems, not uncommonly the result of the success of WiFi, are a staple on the menu of items receiving attention from the committee. IEEE 802.11ac IEEE 802.11ac was approved and published in January 2014 after three years of development. Products supporting the new version of the standard were already on the market before the standard was published and some projections anticipate 1 billion IEEE 802.11ac devices by 2015. IEEE 802.11ac was developed to support a single link throughput of at least 500 megabits per second (500 Mbits/s) and a total throughput of at least 1 gigabit per second. Concepts intro- duced in IEEE 802.11n are extended to accomplish these data rates, including: • Wider RF bandwidth (up to 160 MHz) • More multiple-input, multiple-output (MIMO) spatial streams (up to eight) • Downlink multi-user MIMO (up to four clients) • High-density modulation (up to 256-QAM) New features provided by IEEE 802.11ac include: • Extended channel binding with 80 MHz channel bandwidth mandatory for stations (vs. 40 MHz maximum in 802.11n), 160 MHz available optionally • Support for up to eight MIMO spatial streams (vs. four in 802.11n) • Downlink multi-user MIMO (allows up to four simultaneous downlink multi-user MIMO clients) • Multiple STAs, each with one or more antennas, transmit or receive independent data streams simultaneously • Space Division Multiple Access: streams not separated by frequency, but instead resolved spatially • Downlink multi-user MIMO (one transmitting device, multiple receiving devices) included as an optional mode 25 HIS iSuppli, “Small Cells with WiFi Set to Reshape Wireless Communications Market,” Microwave Journal, May 15, 2013. Available at: http://www.microwavejournal.com/articles/19858-small-cells-with-wi-fi-set-to-reshape-wireless- communications-market 26 ABI Research, “9.7 Million Carrier WiFi Access Point Shipments in 2018 as Mobile Carriers Jump on the Bandwagon,” Sept. 30, 2013. Available at: https://www.abiresearch.com/press/97-million-carrier-wi-fi-access-point-shipments-in

IEEE 802 Standards 91 • 256-QAM, rate 3/4 and 5/6 modulation added as optional modes (vs. 64-QAM, rate 5/6 maxi- mum in 802.11n) • Beamforming with standardized sounding and feedback for compatibility between vendors (non-standard in 802.11n made it hard for beamforming to work effectively between different vendor products) • Media access control modifications (mostly to support above changes) • Coexistence mechanisms for 20/40/80/160 MHz channels, 11ac and 11a/n devices27 IEEE 802.11ad In May 2007, the IEEE 802 committee started a new study group to investigate “very high throughput” technologies. The study group initially focused on International Mobile Telecommunications-Advanced (IMT-Advanced) operation in the bands < 6 GHz. However, the focus of the study group changed to enhancing 802.11n in the 5 GHz band. In November 2007, the group took up the study of the 60 GHz band. The motivation was that the millimeter wave band could provide for much wider band channels than in the microwave band, enabling single link throughputs greater than 1 Gbps.28 The WiFi Alliance asked to provide usage models to help develop requirements. The general categories of the usage models included wireless display, distribution of high definition TV, rapid upload/download, backhaul, outdoor campus, auditorium, and manufacturing floor. The specific uses that are expected to be most prevalent include compressed video streaming around a house, rapid sync-and-go, and wireless I/O. It is envisioned that TVs and DVRs around the home will have wireless capability and 100+ Mbps aggregate of videos from a DVR can be dis- played wirelessly on TVs in different rooms. With rapid sync-and-go, users can quickly sync movies or pictures between mobile devices such as a phone, a laptop, or a tablet. With a 1 Gbps radio link, a 1 GB video file will take much less than a minute to transfer between devices. Data rates exceeding 1 Gbps will provide the capability for a wireless desktop, with wireless connec- tions between a computer and peripherals such as monitors, printers, and storage devices.29 Network World offered some interesting comments on the new use cases made possible by 802.11ad and the comparison of IEEE 802.11ac and IEEE 802.11ad: • “802.11ac is an extension for pure mainstream WiFi,” said Sean Coffey, Realtek’s director of standards and business development. “It’s evolutionary. . . . You’re not going to see dramati- cally new use cases.” • 802.11ac is a development of the current 802.11n standard, producing improved performance on the same 5 GHz frequency bands. Some routers using the 802.11ac have already been deployed, and the experts on the panel agreed that it will become commonplace by early 2013. • By contrast, 802.11ad adds 60 GHz connectivity to the previously used 2.4 GHz and 5 GHz frequencies, potentially providing multi-gigabit connection speeds and dramatically broaden- ing the number of applications for which wireless can be used. • “There are some unique characteristics about the 60 GHz band that really help in bringing a whole bunch of new use cases,” said Mark Grodzinsky, vice president of marketing for 60 GHz pioneer Wilocity. Some of those uses include wireless docking and uncompressed HD video streaming. 27 Matthew Gast, 802.11ac: A Survival Guide, O’Reilly Media, Inc., 2013. Available at: http://chimera.labs.oreilly.com/ books/1234000001739 28 Eldad Perahia and Michelle X. Gong, “Gigabit Wireless LANs: An Overview of IEEE 802.11ac and 802.11ad,” ACM SIGMOBILE Mobile Computing and Communications Review, Vol. 15, Iss. 3, July 2011. 29 A. Myles and R. de Vegt, “WiFi Alliance (WFA) VHT Study Group Usage Models,” IEEE 802.11-07/2988r4, March 19, 2008. Available at: https://mentor.ieee.org/802.11/dcn/07/11-07-2988-04-0000-liaison-from-wi-fi-alliance-to-802-11-regarding- wfa-vht-study-group-consolidation-of-usage-models.ppt.

92 A Guidebook for Mitigating Disruptive WiFi Interference at Airports • “60 GHz is also highly directional,” he added. “So whereas in 2.4 and 5 [GHz] it’s pretty much an omnidirectional transmission, meaning the antennas just blow energy in all directions, with 60 GHz, it’s very focused.” • However, 802.11ad will still not represent a wholesale shift in the nature of WiFi, according to Coffey. • “When you add in [802.11ad], I would see this as an island of super-high data rate present in a sea of gigabit WiFi. What it does is allow you to do a massive amount of WiFi offload- ing.” The idea is that the localized but high-bandwidth 60 GHz network can be used for specific, highly demanding tasks, keeping the standard 5 GHz frequency free for normal use, he explained. • Devices using the 60 GHz standard could begin to appear in 2014 and become more promi- nent in 2015. This means that the next major transition is still well over a year away—in part because 802.11ac will not be a particularly testing upgrade for most end users.30 In a Microwave Journal article, ABI Research predicts very different deployment patterns for IEEE 802.11ac and IEEE 802.11ad. The researchers at ABI expect fast, widespread adoption of IEEE 802.11ac, while IEEE 802.11ad will see a slower but still impressive adoption rate: • The growth of 802.11ac and 802.11ad will occur in very different ways. 802.11ac will explode into devices, including smartphones, from the start while 802.11ad will see a more modest and staggered growth. 802.11ac is being pushed into smartphones by key carriers’ device requirements that are in sync with 802.11ac hotspot plans for more robust WiFi offload- ing. “The push towards 11ac adoption overpowers the minor additional cost of dual-band 802.11n/802.11ac chipsets that will be used in smartphones,” states research director Philip Solis. “Perhaps surprising even to industry insiders, we will likely see 2X2 802.11ac implemen- tations in smartphones in a few years.” • The proportion of various 802.11ac-enabled products will remain relatively consistent from 2013 to 2018, with smartphones making up 40% of those in 2013 and 46% in 2018, where over 3.5 billion WiFi chipsets with 802.11ac will ship. The WiFi Alliance is just about to start certification of products using the protocol, yet its shipments have started and are already on track to distribute hundreds of millions this year. 802.11ac finally pushes WiFi more towards the 5 GHz spectrum, which is cleaner and permits for the much larger channel sizes that allow for greater speeds and capacity. • 802.11ad will phase from larger to smaller products, starting from peripherals and larger non-handset mobile devices and shifting to smaller and thinner devices over time. 802.11ad will make its way into smartphones in 2015, changing the proportion of 802.11ad-enabled products compared to prior to 2015. Smartphones will account for nearly half of all 802.11ad- enabled products in 2018, though with less than half the volume in smartphones compared to 802.11ac. Even so, over 1.5 billion chipsets with 802.11ad will ship in 2018. 802.11ad pushes WiFi into higher-speed, lower-power personal area networking that will be used simultane- ously with other WiFi protocols. • “As the complexity of WiFi increases, heading towards tri-band 802.11n/802.11ac/802.11ad chipsets, interesting design tradeoffs can be made to optimize for cost, size, and functional- ity,” notes Solis. “Choices can be made around the support of 80 MHz or 160 MHz channel and MIMO configurations based on whether or not 802.11ad is included. Smaller antennae arrays can also be used to save space.”31 30 Jon Gold, “Interop: Don’t Sweat 802.11ac WiFi—Because 802.11ad Will Knock Your Socks Off,” Network World, October 3, 2012. Available at: http://www.networkworld.com/news/2012/100312-interop-80211ad-263036.html 31 ABI Research, “Smartphones Will Account for Nearly Half of Both 802.11ac and 802.11ad Chipset Shipments in 2018,” Microwave Journal, June 11, 2013. Available at: http://www.microwavejournal.com/articles/20086-smartphones-will- account-for-nearly-half-of-both-80211ac-and-80211ad-chipset-shipments-in-2018

IEEE 802 Standards 93 IEEE 802.11af IEEE 802.11af, approved in February 2014, allows wireless local area network operation in TV white space spectrum in the VHF and UHF bands between 54 and 790 MHz. Because of its operation in the TV white space, meaning on locally unused TV channels, the standard is often referred to as White-Fi and Super WiFi. The FCC requires special measures to be implemented to limit interference to primary users of these bands, such as analog or digital TV and wireless microphones. The low frequencies used have excellent propagation characteristics, making these devices particularly appealing for covering larger distances or penetrating buildings and walls. The standard supports 9 MCS standards and speeds up to 35.6 Mbps. IEEE 802.11ah IEEE 802.11ah is being developed to support operation in the sub-gigahertz spectrum, par- ticularly the 900 MHz ISM band (902–928 MHz) in the U.S. and the 868.0–868.6 MHz band in Europe. The standard is anticipated to be released in May 2016. New Frequency Bands With two new frequency bands, the 60 GHz and TVWS bands, being added in the past 3 years and the IEEE 802.11ah, sub-gigahertz version anticipated in the next two, WiFi is dramatically expanding the bands and operating frequencies used. In addition, the FCC is working on adding 195 MHz of new spectrum to the UNII band, where WiFi already operates. WiFi initially just operated in the 2.4 GHz ISM band, which still holds a majority of the WiFi traffic. For general use, operation in the UNII and ISM bands between 5–6 GHz was added. Specialized applications are supported in the 3.6 and 4.9 GHz bands. It is unclear how these expanding operating frequencies will impact airport operations or devices travelers bring into airports. However, each development is undertaken for good rea- sons. Some are targeted at specific uses such as the limitation of the 4.9 GHz band to public safety applications. Others, like the traditional 2.4 and 5 GHz bands, are for general use. The new frequency bands create the opportunity to separate services in frequency, with the resulting decrease in interference and congestion. However, the increasingly complex landscape of operating frequencies brings new problems such as increased risk of intermodulation interfer- ence and system interoperability challenges. With skillful implementation, the new frequency bands will greatly reduce interference and congestion. However, it would be naive to assume that intermodulation and other problems will be entirely avoided. Band Fragmentation For both the WiFi and cellular networks, band fragmentation is a strong trend. More fre- quency bands are being opened for use by both WiFi and cellular systems. This is being driven by the tremendous popularity and benefits of these wireless networks. The picture is complicated further by the continuing technological development of both WiFi and cellular. These forces create a complex set of RF protocols and frequency bands in which they operate. The picture is made more complex by different countries, usually for very good local reasons, making different bands available or applying different operating rules to the same bands.

94 A Guidebook for Mitigating Disruptive WiFi Interference at Airports Positively, the trend toward band fragmentation makes available more spectrum in which to operate. However, it also creates a complex matrix of possible interference problems. For equipment designers and network operators, the need to support multiple operating modes is a growing problem. According to a February 2013 Qualcomm press release: “Band fragmentation is the biggest obstacle to designing today’s global LTE devices, with 40 cellular radio bands worldwide . . . multiband, multimode mobile devices supporting all seven cellular modes, including LTE-FDD, LTE-TDD, WCDMA, EV-DO, CDMA 1x, TD-SCDMA and GSM/EDGE.”33 As Figure C-2 illustrates, to fully enable international operation, user equipment (UE) is required to support as many as 40 bands and 7 operating modes. Even operation in a single country and on a single carrier requires that multiple frequency bands and operating modes be supported. WiFi has its own challenges with a growing number of bands and modes in use and more being planned. While operation in the 2.4 GHz band is dominant, followed by the 5.8 GHz band, other bands are available or potentially becoming available, e.g., 900 MHz, 3.6 GHz, and 60 GHz. The newer versions of IEEE 802.11 introduce spatial division multiplexing (SDM). SDM transmits multiple data streams concurrently within the same channel. The data rate is then multiplied by roughly the number of data streams being used. A further refinement is the use of a variety of modulation and coding schemes (MCS). Different MCS levels can be used, based on the channel characteristics, capabilities of the transmitter and receiver, and signal quality. Then there is the ability to vary the guard interval. A guard interval is a period of time between symbol transmissions that allows reflections from multipath to settle before the next symbol is sent. The rate at which data can be sent becomes a combination of the Fast Fourier Transform (FFT) time 32 Sunil Patil, “LTE Band Fragmentation,” CTIA 2013, Las Vegas, NV, May 22, 2013. 33 Qualcomm, “Qualcomm RF360 Front End Solution Enables Single, Global LTE Design for Next-Generation Mobile Devices,” February 21, 2013. Available at: http://www.qualcomm.com/media/releases/2013/02/21/qualcomm-rf360-front- end-solution-enables-single-global-lte-design-next Figure C-2. A mobile device must support over 40 RF bands to be fully international.32

IEEE 802 Standards 95 and the guard interval. In IEEE 802.11 a/g OFDM, the data or symbol rate is 250 kHz, or 4 µS. The 4 µS time is a combination of a 3.2 µS FFT time and 0.8 µS guard interval. However, IEEE 802.11n defines both the 0.8 µS and a short guard interval (SGI) of 0.4 µS. When channel condi- tions allow use of the SGI, the data rates can increase by 11%. IEEE 802.11n defines 31 MCS states, two channel bandwidths, and two guard intervals. That makes for 124 possible combinations and a very wide range of potential data rates. IEEE 802.11ac defines 39 MCS states, four channel bandwidths, and two guard intervals. This increases the number of possible combinations from 124 to 242, and an even greater spread in the achievable data rates, as shown in Figure C-3. RF interference can have any of several different impacts on a WiFi network. In most cases, RF noise will degrade the signal quality, lowering the achievable data rate. RF noise is not the only fac- tor that impacts the data rate. The amount of signal reflections, multipath, in the environment will also have a very significant impact on the data rate. RF interference can also cause packet errors, decreasing the data rate because more packets must be retransmitted. An example of this is when an interferer’s transmission only overlaps with a WiFi transmission some of the time. The WiFi sig- nal might be experiencing excellent signal conditions, but periodically the interferer’s transmission coincides with it, causing a packet error and the resulting need to retransmit the packet. In these cases, the impact of the interference is not a loss of connection but a slower data rate. If the user is doing something that doesn’t need a high data rate, this might not even be noticeable. However, if a high data rate is needed because a lot of data is being sent or if the wireless local area network is already congested with a lot of concurrent users, then the impact can be very significant. In extreme cases, RF interference can totally block a WiFi signal, at least on some channels or even in an entire band. Here again, if the interference only blocks some channels, the WiFi clear channel assessment (CCA) may simply identify that and move to a different channel, perhaps in a different band. The user may not even notice. However, under crowded conditions the loss of a channel or band could be catastrophic. In addition, IEEE 802.11 has six different operating modes that a device can operate in: • Master (acting as an access point) • Managed (client, also known as station) • Ad-hoc • Mesh • Repeater • Monitor Depending on the mode, the device will respond to network traffic differently and will pass different information to the operating system and higher-level applications. Depending on the operating mode, it may be difficult or even impossible for a user to know that they are expe- riencing interference. This also creates the very real possibility that network managers will not get the information they need or misunderstand the data they are getting. They may think that they are being given an accurate picture of the activity on their network, but the sources of their information may be screening off critical information and only giving them part of the picture. It is vital that network managers understand how their network data is coming to them and what information their tools may be filtering out. Filtering of data works both ways. When data is filtered in the right way for the problem being worked on, it helps the network manager to quickly spot where the problem is. But when the filter is incorrectly matched to the current need, it can blind the network manager to the true problem. Network fragmentation creates a vast array of possible situations. When the fragmentation of the WiFi and cellular networks is brought together, an incredible number of possible combi- nations are created. The potential is that in any specific situation a number of combinations of

96 A Guidebook for Mitigating Disruptive WiFi Interference at Airports Figure C-3. Modulation and coding schemes used by IEEE 802.11 n/ac.34 WiFi and cellular signaling combinations may operate in close proximity without any problem. However, other combinations may be very problematic. If CCA and other interference manage- ment methods work well, then the conflicting combinations may be automatically identified and avoided, without the user ever being aware of it or needing to take any action. However, in other situations these mechanisms either may not be capable of dealing with the situation they face or not have options available to avoid the interference. 34 Figure C-3 is from the front panel of an HTML5 reference tool developed by Wireless Training & Solutions (WiTS), http:// www.wirelesstrainingsolutions.com/. It is available at: http://www.aerohive.com/pdfs/Blog/MCS_Chart_802.11ac_v.06.html

IEEE 802 Standards 97 A potential for interference that appears particularly concerning is created by the approaching use by LTE of the bands adjacent to the 2.4 GHz ISM band, which WiFi uses so heavily. Mobile phones and other LTE UE devices operating either below or above the 2.4 GHz ISM band have the potential for interfering with WiFi (Figure C-4). LTE Band 40 operates below the 2.4 GHz ISM band, using TDD between 2.3–2.4 GHz. LTE Band 41, 2496–2690 MHz, also operates in TDD mode but just above the 2.4 GHz ISM band. With these channels there is the potential for LTE to WiFi or WiFi to LTE interference. LTE Band 7 operates on an FDD basis with the UE transmitting (uplink) between 2500–2570 MHz, and the base (downlink) between 2620–2690 MHz. With LTE Band 7, the risk is only for LTE to WiFi interference. In North America, LTE Band 7 is used by Bell and Rogers in Canada. Sprint used LTE band 41 in the U.S. nTelos is in trials using LTE band 41 in the U.S. Aeronet is planning on using the band in Puerto Rico. This suggests that interference between LTE and WiFi devices may become a problem in the areas where LTE uses bands 7, 40, or 41. Component manufacturers work closely with their leading customers to support their needs and have the components available for the next generation of technology. Freescale is a leading provider of RF components, particularly power amplifiers, and a good example of how new components show the next steps their customers will be taking with their products. The newest Freescale Airfast LDMOS transistors are designed specifically for TD-LTE base- stations at the 2.3/2.6 GHz frequency bands (Figure C-5). These transistors span a broad range of power points, from 50 W to 200 W. The AFT26HW050S/GS targets metrocell basestation applications in the 2496- to 2690-MHz band. In an asymmetrical Doherty configuration, it delivers 47.4 dBm of peak power.36 Figure C-4. Developing LTE-WiFi interference.35 Figure C-5. The newest Freescale Airfast LDMOS transistor. 35 Thomas Lindner, “LTE Band Fragmentation—Challenges for Cellular Platform Designs,” LTE World Summit, Barcelona, May 2012, Day 2, T11. 36 Louis Frenzel, “Freescale’s Ritu Favre Discusses Today’s RF Technologies,” Electronic Design, October 3, 2013.

98 A Guidebook for Mitigating Disruptive WiFi Interference at Airports Freescale would not be making the significant investment that developing a new product requires if their customers didn’t need it. Clearly Freescale is hearing that customers are plan- ning on building mobile network basestations for the 2496–2690 MHz band, just above the 2.4 GHz ISM band where so much WiFi operates. The issue of band fragmentation is complex enough if equipment can be relied on to correctly comply with local regulatory requirements. However, in today’s highly mobile world, it is com- mon for equipment designed to operate under one country’s frequency allocations and service rules to be used in another. A wide array of wireless devices can be bought in one country but used in another, where they may be operating in violation of local regulations.

Abbreviations and acronyms used without definitions in TRB publications: A4A Airlines for America AAAE American Association of Airport Executives AASHO American Association of State Highway Officials AASHTO American Association of State Highway and Transportation Officials ACI–NA Airports Council International–North America ACRP Airport Cooperative Research Program ADA Americans with Disabilities Act APTA American Public Transportation Association ASCE American Society of Civil Engineers ASME American Society of Mechanical Engineers ASTM American Society for Testing and Materials ATA American Trucking Associations CTAA Community Transportation Association of America CTBSSP Commercial Truck and Bus Safety Synthesis Program DHS Department of Homeland Security DOE Department of Energy EPA Environmental Protection Agency FAA Federal Aviation Administration FHWA Federal Highway Administration FMCSA Federal Motor Carrier Safety Administration FRA Federal Railroad Administration FTA Federal Transit Administration HMCRP Hazardous Materials Cooperative Research Program IEEE Institute of Electrical and Electronics Engineers ISTEA Intermodal Surface Transportation Efficiency Act of 1991 ITE Institute of Transportation Engineers MAP-21 Moving Ahead for Progress in the 21st Century Act (2012) NASA National Aeronautics and Space Administration NASAO National Association of State Aviation Officials NCFRP National Cooperative Freight Research Program NCHRP National Cooperative Highway Research Program NHTSA National Highway Traffic Safety Administration NTSB National Transportation Safety Board PHMSA Pipeline and Hazardous Materials Safety Administration RITA Research and Innovative Technology Administration SAE Society of Automotive Engineers SAFETEA-LU Safe, Accountable, Flexible, Efficient Transportation Equity Act: A Legacy for Users (2005) TCRP Transit Cooperative Research Program TEA-21 Transportation Equity Act for the 21st Century (1998) TRB Transportation Research Board TSA Transportation Security Administration U.S.DOT United States Department of Transportation