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

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60 A P P E N D I X A One may do Internet searches on “electromagnetic interference” and find many references to the subject. However, they tend to span the range from a Wikipedia explanation of the fundamen- tals to complex circuitry and radio frequency (RF) interference effects. What is missing is a special case tutorial relative to the airport terminal wireless fidelity (WiFi) environment. The airport WiFi environment is increasingly complex and the intent of this primer is to (1) provide some basics on interference phenomena, (2) relate interference to the governing regulatory processes, and (3) pro- vide information on possible ways to manage the WiFi network in the airport environment. The purpose is to explain RF interference associated with WiFi and commercial wireless communica- tions devices in the unlicensed spectrum and the resulting issues for airports and tenants. Introduction Interference is a basic phenomenon described in physics in which two waves superimpose to form a resultant wave of greater or lower amplitude. For example, when two raindrops fall near each other in still water, the rings produced by each travel until they intersect with each other, disrupting the circular ring patterns (Figure A-1). This visible analogy is a simplistic view of how RF interference occurs when a radio transmission or radiation from some other device interacts with another radio. However, in the RF domain the environment can be more complex, with one or multiple interferers at overlapping operating frequencies and at the same or different signal strengths from different access points interacting with the “victim” receiver (indicated by the blue arrow in Figure A-2). The result can be a disruption, degradation, or limitation of the performance to the victim radio receiver circuitry. This is a simplistic explanation of RF interfer- ence, and additional information can be found online. The Federal Communications Commission (FCC) manages the non-government spectrum and is the spectrum regulatory agency that provides rules and regulations on the use of the spectrum. An abbreviated historical perspective of the spectrum regulatory governance evolu- tion and the meaning of “harmful interference” will be provided first, followed by an overview of interference and perceived misconceptions, a process to work through interference incident reports at airports, and the practical side of how to resolve RF interference issues for airport authorities and tenants. Historical Perspective: Spectrum Governance, Interference Mitigation, and Tolerable Interference Evolution This section is intended to provide a brief history of the U.S. regulatory process and the basics of interference and mitigation for readers that may not be familiar with these topics. Then it introduces a potential way forward based on Harm Claim Thresholds. Radio Frequency Primer

Radio Frequency Primer 61 The Communications Act of 1934 established the Federal Communications Commission. The stated purposes of the Communications Act are: “regulating interstate and foreign commerce in communication by wire and radio so as to make available, so far as possible, to all the people of the United States a rapid, efficient, nationwide, and worldwide wire and radio communication service with adequate facilities at reasonable charges, for the purpose of the national defense, and for the purpose of securing a more effective execution of this policy by centralizing authority theretofore granted by law to several agencies and by granting additional authority with respect to interstate and foreign commerce in wire and radio communication, there is hereby created a commission to be known as the Federal Communications Commission, which shall be constituted as hereinafter provided, and which shall execute and enforce the provisions of this Act.” (1) The FCC designed the Communications Act of 1996: “to provide for a pro-competitive, de-regulatory national policy framework designed to accelerate rapidly private sector deployment of advanced information technologies and services to all Americans by opening all telecommunications markets to competition . . .” (2) The Telecommunication Act of 1996 also added and changed some rules to account for the emerging Internet. There are recently proposed amendments (3) to the Communications Act of 1934 to require the FCC to publish on its website and submit to Congress a biennial report on the state of the communications marketplace (4). That report would include an analysis of “the state of compe- tition in the markets for voice, video, and data services, as well as the availability of high-speed and high-quality telecommunications services” in the United States and also “require the FCC to determine whether laws and regulations pose a barrier to entry into communications mar- kets and to include that information in the biennial report” and cancel a number of preexisting requirements for various other reports from the FCC (5). Figure A-1. Raindrops illustrating concept of interference. Figure A-2. Example of multiple channel WiFi interference.

62 A Guidebook for Mitigating Disruptive WiFi Interference at Airports A brief summarization of the spectrum interference management governed by the Commu- nications Act of 1934 and technology that influenced the proliferation of small personal com- munications up to the present time is provided in Attachment 1: Spectrum Management in Its Early Days. The regulatory process to place these devices in the spectrum and difficulties they pose to management in an airport environment are discussed in the following sub-sections. Key Points of the FCC Part 15 Unlicensed Spectrum That Apply to WiFi The industrial, scientific, and medical (ISM) radio bands are reserved internationally for the use of RF energy for ISM purposes other than telecommunications such as WiFi. An abbrevi- ated walk-through of the complex background and issues in the WiFi use of unlicensed spec- trum bands in the airport will be provided. The FCC explains in “Part 15—Radio Frequency Devices Rules and Regulations” the regulations under which an intentional, unintentional, or incidental radiator may be operated in unlicensed bands. It also contains the technical speci- fications, administrative requirements, and other conditions relating to the marketing these Part 15 devices. The reader is referred to the complete Part 15 text for all details; only the specifics and difficul- ties that apply to operation of Part 15 devices such as WiFi and other similar technologies in an airport environment will be discussed. Definition of Harmful Interference In FCC Part 15 Section 15.3(m), the definition of harmful interference is stated as: “Any emission, radiation or induction that endangers the functioning of a radio navigation service or of other safety services or seriously degrades, obstructs or repeatedly interrupts a radio communications service operating in accordance with this chapter.” This is an operational definition and uses terms that do not have strict “technical” thresholds associated with the “victim” except for the radio navigation functions and safety services. Radio navigation applies to safety of flight. One commonly associates this with interference affecting radio navigation systems supporting flight, including equipment located near the airport. Mini- mum or no interruption or induced errors to radio navigation is tolerable. Communications radios may fall into this interpretation, particularly if related to safety or security. The frequency bands of communications systems are different from WiFi bands for the most part at the pres- ent time, and interference is highly unlikely to be experienced. The exception is if WiFi is used by airport security or safety functions within or near the airport facility. In this case, it can lead to a dilemma to invoke Part 15 mitigation if security is using the same airport WiFi network as vendors or passengers. Another interesting contentious case would be if some WiFi is used for managing vehicle traffic on the tarmac. Radio frequency WiFi leakage from inside the airport could potentially interfere depending on the airport’s specific layout and building structure. Both systems could be Part 15 but have different levels of criticality of airport service and hence different thresholds for interference. Extensive knowledge of such WiFi implementations at all airports would be needed to provide guidance on whether to strictly use Part 15 or possibly place cases like this at the top priority of “tolerable interference,” discussed next. For the remainder of the systems in the band governed by the FCC Part 15, one could inter- pret the words to infer different thresholds for different victim receivers; i.e., some are very susceptible to interference while others are more interference-tolerant by design, with better components. Priority levels of “tolerable interference” based on criticality of supported airport functions served and passenger priority could be established. At the top level, WiFi uses for

Radio Frequency Primer 63 security, tarmac vehicle control, baggage check-in, and wireless credit card transactions require higher priorities, while conducting business or watching videos in the airport gate area is a lesser priority. These priority levels may be used for airport WiFi distribution or RF isolation of the more critical WiFi uses and the criticality of resolving interference complaints. There are a multitude of examples in each of the above categories that could be given; how- ever, the point to take away is that each interference interaction can have interpretive meaning to the FCC Part 15 operational definition of harmful interference. The equitable mitigation of interference of Part 15 device emissions in an airport depends on understanding interference and what can be done about it. Difficulties Posed by Part 15 to Strictly Enforce in Airport Terminal Environment The important excerpts of the Part 15 regulations pertaining to the use of WiFi and other wire- less systems operating in the 915 MHz, 2.450 GHz, and 5.800 GHz ISM radio bands are as follows: “Persons operating intentional or unintentional radiators shall not be deemed to have any vested or recognizable right to continued use of any given frequency by virtue of prior registration or certification of equipment, or, for power line carrier systems, on the basis of prior notification of use pursuant to § 90.35(g) of this chapter.” “Operation of an intentional, unintentional, or incidental radiator is subject to the conditions that no harmful interference is caused and that interference must be accepted that may be caused by the opera- tion of an authorized radio station, by another intentional or unintentional radiator, by industrial, scien- tific and medical (ISM) equipment, or by an incidental radiator.” “The operator of a radio frequency device shall be required to cease operating the device upon notifica- tion by a Commission representative that the device is causing harmful interference. Operation shall not resume until the condition causing the harmful interference has been corrected.” The quoted words express clear intent. One very important impactful usage of these words is to enforce the cessation of “rogue” hotspot (6) devices that could be brought into the airport environment. Part 15 and Interference Mitigation User Responsibilities In actuality, resolving the responsibility of interference has complexities. In the ideal cases described above and in the Part 15 wording, it is assumed the “blame” is due to an identified RF interference caused by one system. This can be accomplished if the interference is iden- tifiable, emanating from a system with known spectrum characteristics different from WiFi such as a microwave oven in the food court. It is not always easy or straightforward to iden- tify unknown spectrum interference sources. The interference could be from different WiFi networks or multiple systems. Spectrum analyzer traces or equivalent measurement data are needed, accompanied by diagnostic search and identify procedures to uniquely resolve the origi- nating RF interference source. There can also be a “perception” of interference not related to RF. For example, many sys- tem users trying to use the same spectrum at the same time can cause a throughput reduction. Dropped packets and reduced message throughput occur and can portray the illusion that inter- ference is present. The real cause may be a network design issue, the network nearing a user- created demand situation, or, if frequently occurring, the networked system may be approaching the replacement time. There are many similar scenarios, but the point is that reductions in throughput alone do not uniquely indicate interference. As WiFi increases in presence at air- ports, resolution of “perceived interference” vs. RF interference can become an increasing part of the interference mitigation.

64 A Guidebook for Mitigating Disruptive WiFi Interference at Airports Harmful Electromagnetic Interference as Applied to WiFi at Airports Legacy spectrum allocation is built on a hierarchy of licensed users in the spectrum bands, e.g., “primary and secondary.” This hierarchy organizes the interference mitigation process by spectrum allocation priority. On the other hand, all users in the ISM band are equal in priority under the Part 15 governance. There is in general good and not so good equipment in the ISM band depending on the manufacturer’s business model, resulting in high-quality vs. low-cost equipment. Further- more, there is ambiguity in what is meant technically by harmful interference as applied to the vari- ous Part 15 ISM band users. It is unclear what measurements need to be made and compared with established thresholds for declaring whether harmful interference is present or not. This situation is not readily amendable to be enforced by turning off the interferers, as indicated in the FCC rules. Note airport security, medical, or other critical functionality may require some RF interfer- ence protection within the airport if using the same WiFi bands as vendors and other users. However, if using Part 15 equipment, these higher priority systems are not provided any protec- tion in the ISM bands by the strict interpretation of the FCC regulations. What Part 15 Means to WiFi Governance in the Airport Terminal Environment The airport terminal environment will be difficult to manage by simple enforcement of the Part 15 regulations per se. Two alternative solutions/methodology approaches are presented for consideration. Cooperative Approach to Mitigate Interference Within the Terminal The cooperative approach is mainly driven by the FCC ruling that the airport authority cannot control installation and usage of WiFi at airports (7). It is in the best interest within the airport terminal that RF interference mitigation be accom- plished between all concerned. Best interest is defined as a reporting, analysis, and constructive action process to mitigate RF interference that considers the responsibilities of the airport, the business case of the tenants, as well as the responsibilities of the passengers. Passengers are included and have the responsibility of operating equipment that complies with the FCC Part 15 rules and regulations, i.e., do not exceed power limitations. Hotspot devices that are purchased by airport passengers sometimes can exceed the power limits depending on manufacturers. They may be locally beneficial in a home, office, or other spaces where the RF energy emitted by one or a few such devices provides WiFi coverage in dedicated areas. However, these devices brought in number into the airport environment can cause mutual interference to themselves and other WiFi devices such as laptops. If some exceed their specifications, they can make the interference more disruptive to WiFi users. It should be noted that the hotspots may not be the sole source to blame; the network in that locality of the airport may be partially to blame, i.e., the network WiFi adjacent nodes may be operating on the same channel or adja- cent channels in the 2.4 GHz band or their settings could be non-optimal. Note WiFi in the 2.4 MHz band does not have guard bands; rather, the bands can overlap unless deliberate net- work efforts are made to separate local areas by assuring one or two unused frequencies are kept open between adjacent areas. RF hotspots interference is similar in nature, and effects revert to the early days of spectrum management as described in Attachment 1, with the consequence that guard bands were needed between user frequencies to avoid out of band (OOB) emissions on neighboring channels to spill over on simultaneous transmissions. Simultaneous WiFi transmis- sions on the same or adjacent frequencies in a network have the potential to interfere with each other. A more promising candidate solution could be to use the 5 GHz band, where there are

Radio Frequency Primer 65 many more available channels and the channels do not overlap. The topic of passenger hotspots presents potentially severe issues in the WiFi environment that deserve more in-depth analysis. In the following section, a straw man methodology to address interference issues is provided. However, the details need to be further integrated into the spectrum regulatory area before being adopted in the airport environment. Potential Way Forward Based on Harm Claim Thresholds Harm claim thresholds are a method to quantitatively define harmful interference with measur- able, agreed-upon thresholds that apply to equipment in bands of the unlicensed spectrum. The FCC issued a White Paper for comments in January of 2013 regarding interference limits policy (8). The basics of this paper are: “Receivers can be brought into the policy picture with minimal regula- tory intervention by introducing an ‘interference limits’ policy; that is, the establishment of ceilings, called harm claim thresholds, on in-band and out of band interfering signals that must be exceeded before a radio system can claim that it is experiencing harmful interference. Manufacturers and operators are left to determine whether and how to build receivers that can tolerate such interfer- ence, or even determine that they will choose to ignore these limits. Harm claim thresholds thus allow the FCC to provide guidance on the optimization of receiver performance without unduly restrict- ing technical and commercial choice.” Figure A-3 provides a graphical depiction of the approach. Note these harm claim thresholds simply map to levels that the in-band or out of band signal must adhere to preclude harmful interference; however, the manufacturer has the option to implement them or not. The concept has merit in the airport environment and could be imple- mented to control the presence of rogue devices if they exceed the harm claim thresholds, and allow users to select the WiFi equipment that can minimally satisfy the harm claim threshold within the airport environment. Essentially the airport has no say in what equipment is used within its confines. This is in keeping with the spirit of the FCC, which sided with Continental Frequency Space Field strength Assigned frequency Assigned area p x , p t : probability that field strength is exceeded Frequencies for which interference limit is defined Figure A-3. Interference limits policy approach.

66 A Guidebook for Mitigating Disruptive WiFi Interference at Airports Airlines in a ruling that the Massachusetts Port Authority cannot restrict WiFi use at Boston’s Logan International Airport, but leaves the Port Authority the power to have interference cor- rected to comply with the interference claim thresholds. The bottom line is, in an ideal world, if such an approach were taken with thresholds estab- lished and manufacturers accepted for particular environments such as airports, the mitigation of harmful interference becomes simplified and enforceable in the airport environment. Stricter Governance of Interference to Licensed Systems Near Vicinity of Terminal It is to be noted that there may be a possibility that licensed users are assigned in the ISM bands within or near the airport terminal. This presents a special case where the WiFi system has to mitigate any RF interference to these systems. Perceived Misconceptions, Root Cause Determination, and Interference Incident Reports The intent of this section is to provide an overview of interference, its root cause determina- tion and mitigation techniques, and a method to quantify the magnitude of the RF interference. Interference reporting from airport WiFi users is expected to be mostly from the observ- able perspective, with occasional exceptions of persons who have experienced interference and learned some observable indicators, such as someone in the near vicinity of WiFi equipment using electronic equipment or transmitting on a radio. The issue is that these observable reports do not always correlate with RF interference but may be “perceived” interference related to per- formance of the WiFi equipment in the environment. Perceived Misconceptions Perceived misconceptions are basically a first guess result of observed phenomena relating to the performance degradation of the “victim” WiFi wireless receiver or other commercial commu- nications radio emissions in the ISM bands. Specific examples of performance degradation causes include: hardware or software failures of equipment within network, e.g., routers and radios; dis- ruptions in a particular link or network to the communications receiver; erroneous settings in radio or network; overloading of network during peak usage hours; temporary area RF blockage within the building or caused by local construction; low-cost equipment that has operational limi- tations in a dense usage environment; and other causes. These few examples are meant to illustrate causes that are not related to RF interference but can mask as interference to the WiFi users. Relate Interference Reports to Root Cause The expected growth of WiFi and other commercial communications at airports will inevi- tably produce many interference-related reports. Airport authorities can’t expect to have dedi- cated personnel to resolve each and every report of interference issues. Ideally what is needed is a process that enables airport authorities to resolve the issues within their own personnel resources and capabilities. The process must use a formalism that is simple and adaptable to the complexities of airports, leverages the history of reported interference and past experience of resolving interference mitigation, and adapts to future growth of expected wireless usage at the airport terminal. One plausible way forward would be the development of an “interference inci- dent report” that includes a description of what was observed and a standardized questionnaire.

Radio Frequency Primer 67 These incidence reports can be generated by anyone using or working with the WiFi system and saved in a database format (discussed later in this appendix). Note it would be ideal to leverage the reported and resolved interference experience across all airports. This inter-airport database can enable access to resolutions of past interference issues, saving the expense of individual air- ports rediscovering the mitigation of known solutions, and sharing the technology and interfer- ence mitigation experience not available at all airports. Perceived Interference Unrelated to Electromagnetic Interference The process must also be adaptable to maximize the resolution of perceived interference. One plausible way forward would be the development of procedures or quick check tests that the airport authority and/or impacted user could perform. The results would isolate perceived interference using guidance to be developed, and be reported within the “interference incident report.” If not resolved by the procedures, then the interference is likely to be RF-related. RF-Related Interference The process must also be adaptable to address RF interference at an airport. The airport authority could have access to spectrum test equipment, e.g., a spectrum analyzer; however, affordability may be a hindrance at small airports and a rental or low-cost instrument may be an option. Measured observations of the interference can be recorded and used to confirm if the interference is RF-related. If the source is determined to be RF-related, the steps appearing later in this tutorial could be followed. The Practical Side to Resolving RF Interference The intent is not to itemize RF interference mitigation solutions here but to provide a top- level methodology for airport authorities to address the interference issues when they are found. The FCC ruling that the airport authority cannot control usage of WiFi at airports, combined with the ambiguity of the definition of harmful interference in the regulations, makes it difficult for airport authorities to manage and mitigate WiFi interference. WiFi is important to the func- tioning of an airport if related to a navigation, airport security, or other related safety issue within the airport. Passengers are important since they are the financial input to airports. Tenants and passengers are becoming more dependent on WiFi and other wireless devices. The airport is a business place for its vendors and an extension of its passengers’ office or entertainment space. One reviewer provided a noteworthy comment that characterizes the complexity of the WiFi environment and its enforcement: “There is no question that the FCC reserves control in the context of regulation for itself. However, the over-the-air reception devices (OTARD) rule cited grew from a condominium owner dispute with a landlord (association) over the placement of an RF receiver on their exclusive use/owned space. This was later extended to include RF transmitters in addition to receivers. So control must be considered in the context of the agreement a tenant has. If the agreement is an exclusive lease and use one, the tenant can install and operate RF transmitters in the ISM bands; in those cases, resolution is limited in most cases to reasoning around avoiding ‘mutual destruction’ and achieving mutual benefits, and around cost incentives or dis-incentives that the airport can create. If the agreement is not an exclusive lease and use one, the airport can prohibit the tenant from installing and operating receivers and transmitters. To use an analogy, a condo owner can install a transmitter in their condo but not in the pool or lobby area even though technically they may own a ‘share’ in those areas.” The legal enforcement issues are tangential to interference and they are well-taken. They introduce other methods to preclude interference by the placement of devices that focus on

68 A Guidebook for Mitigating Disruptive WiFi Interference at Airports building codes, overall safety of employees and passengers, and installation standards for wiring and security. They also introduce questions relating to interference that need to be considered. For example, are there communications or operations uses of WiFi in the same or a nearby spectrum that could be impacted by interference for safety, emergency response, or security? Any one of these listed considerations could limit or preclude antenna location installation at the airport. If the installation proceeds forward, the FCC Part 15 compliance as applied to the use of WiFi equipment must be addressed. The only way to stop use of any system transmitting is to show that the user is not transmitting an acceptable quality signal by using the FCC Part 15 regulations, i.e., the device is over the power limit or violating some other restriction. Note compliance with FCC Part 15 can be invoked with the hotspot equipment and hotspots brought into the airport by passengers. If the hotspot complies with the Part 15 rules, it is essen- tially authorized for use. However, to enforce this in a crowded airport, with the potential of many hotspots in a localized area, could be impractical or nearly impossible. The security issue raised by passengers’ hotspot devices may need further investigation to determine if it is some- thing airports can leverage (6). One suggestion to alleviate concentration of hotspots in an area would be to post notices or other enticements to please turn off personal hotspots. This primer is not the appropriate vehicle to completely address the complex issues airports face. However, the following recommendations provide guidance to mitigate and manage WiFi interference before it becomes a serious issue. Keep in Place What Works Many airports have lounges, smoking rooms, or dedicated lounge areas for business and regu- lar travelers. They may have Internet using cable connection or WiFi. If the usage is acceptable with cable, keep it in place. The lowest cost to the airport is to keep these areas in operation and explore other such areas for use. If WiFi is to be installed, use one or two separated channels for the room, particularly in the 2.4 GHz band. Structure a Cooperative Governance Approach to Establish Interference Root Cause and Quantify Interference The first steps to defining the interference mitigation structure are meant to organize the effort. The structure needs to adapt to individual airports or groups of small, medium, and large airports since WiFi interference can depend on size and density of users. This tutorial will outline this approach, but the details need to be worked out in the future before implementation. It is suggested if not already in place that each airport establish a governance board composed of the airport authority, vendors, and other users of WiFi equipment within or near the termi- nal. The structure should be simple but provide a forum where WiFi and wireless interference reports are submitted and mitigated. It should be emphasized that this way forward begins with a cooperative approach whereby resolution of WiFi and other Part 15 wireless communications interference is accomplished in a beneficial way for all. Establish the Governance and Enforcement Framework That WiFi Users and Airport Authorities Will Follow A general document should be prepared for governance and adapted for different sizes of airports. The bounds within which the airport authority and WiFi users can manage and operate must be made clear. Note the airport authority cannot tell a user to shut down a device unless the

Radio Frequency Primer 69 device is interfering with a flight navigation or support role, e.g., a repeater, or it is operating outside the Part 15 specifications. The future Airport Network and Location Equipment (ANLE) system (real-time aircraft posi- tion reporting on the airport surface) is performing a preliminary investigation on the appli- cability of wireless local area network and cellular network technologies, for use by the airport wireless system in C-band. The technology considered IEEE 802.11a, IEEE 802.11b, CDMA2000, and Wideband Code Division Multiple Access (WCDMA) (9). It is mentioned here as an indi- cation of future considerations that could impact the use of WiFi systems inside the airport depending on individual airport design and frequency bands. However, this system may require protection from other WiFi interference sources within the airport. (Note that the propagation of higher frequencies can penetrate through some building walls with seemingly small openings. This potential futuristic case is one reason for including the FCC Part 15 methodology, since the ANLE system could follow its process or rules similar to protecting radio navigation devices.) Interference Incidence Reports An interference incident reporting system needs to be established to gather the users’ observa- tions of the interference. The basic input elements will consist of a description of the interference observed and answers to a set of questions filled out by the observer. It should also contain the originator information; the date, time, and location observed; a description of the environment in which the issue occurred; accompanying information on any actions taken to identify the source; and other pertinent information. The report information should be made available to the airport authority, all users, and others for independent inputs observing the same problem or working with interference mitigation issues. This questionnaire can be generic or tailored to specific airports. Analysis of Interference Incidence Reports An analysis of the interference incidence report and actions taken to determine the root cause of the reported interference should be cooperatively executed to determine if the report is in the perceived category or in the electromagnetic category. Access to all previous incident reports either by questionnaire or online database (described in the next section) will prove useful for determining root cause and potential mitigation techniques. The intent is to identify other reports of the same type and possible work-arounds or existing resolutions. The details of the analysis may consist of reviewing previous incidence reports and reso- lutions, or determining if the problem is in a perceived category involving the hardware or network, or is in the RF domain. Once the RF domain is determined, spectrum measurements should be done to identify the source of interference and quantify the severity of the interference. All analyses, actions, and findings should be recorded and placed in the interference incidence reporting system. If the report is within the perceived interference category, it can transition to the closed status once the root cause is fixed and verified. Once the source is quantified, the final resolution may be easy for interference to licensed users because the resolution actions are the responsibility of the interferer. However, resolution is probably the most difficult part in a Part 15 environment. Database for Interference Incidence Reports and Resolutions for Future Use It is recommended that a database be used to keep track of the reports, share resolutions, and assist airports in quantifying the magnitude of the interference issue. It will contain the

70 A Guidebook for Mitigating Disruptive WiFi Interference at Airports originator, report identification with accompanying information on date opened, status of report (opened/closed), severity of report, actions taken, and other pertinent information. The database information should be available to the airport authority, all users, and others working with the interference mitigation issue. There are many off-the-shelf database products, some of which may be already implemented in the airport environment for leveraging. Implementation Approach to Interference Resolution This section focuses on the resolution of RF interference, which will likely be the most difficult issue for the airport and users. The suggestions offered in this section are not all-inclusive but are representative concepts for considerations. In addition, there may be a more optimum solution for specific airports and environments. Future work may be needed to develop a candidate list of such solutions. First-Order Simple Solutions Some interference can be mitigated if it is caused by a single device having simple remedies, like changing the interferer transmit antenna location, moving a non-WiFi device (e.g., micro- wave oven), lowering the transmit power of the interferer if the equipment has power control options, or other such changes. These solutions should be explored first for cost and expediency. If a piece of equipment is found faulty, it is advisable to seek repair or replacement. Resolution Impacting Changes to Equipment The mitigation of RF interference usually belongs within the interferer responsibility; how- ever, sometimes it may be more cost-effective or expedient if one or a small number of “victims” are involved. Upgrades to software should be more palatable to users over hardware changes including replacement of their installed system. Interference may be resolved by upgrading the WiFi equipment. Some or all may not choose to make the change depending on the cost. The first attempt towards resolution could be to have a discussion with owners of the WiFi equipment as to whether the changes would benefit all or a majority of users. If the cost were offset and linked to increased business, the changes could be agreed to over a time period. However, an issue may occur where the root cause interference affects a large number of the WiFi users within the airport, and the hardware or software fix would alleviate a widespread RF interference problem within the terminal. It may be good business practice as a last resort for the airport to resolve the issue by offering incentives to these users to make the changes. This should not be interpreted as a direction for the airport to follow but a suggestion to consider. Each equipment upgrade has its own operating budget and business model. The FCC regulations may be invoked if the change brings the systems into compliance with the regulations. However, enforcement is not always accomplished by this mechanism. Resolution Involving Equipment Moving Resolution involving WiFi equipment repositioning may be done at little or no cost to any- one if the movement is within the business area of the interferer. Moving the interferer source further away from the victim takes advantage of the increased distance dependency of path loss to weaken the interferer signal below a level that produces “tolerable” interference. The new location needs to be tested so that the interference issue isn’t passed on to others. However, if the movement is outside of one or both user spaces, it may become impractical. A poten- tial solution is adding a link (optical) from the interferer user’s area to a new location of the

Radio Frequency Primer 71 WiFi equipment, which could be an option at some expense. Once a satisfactory location is determined for the above cases, a first attempt towards resolution could be to have a discus- sion as to whether the changes would benefit the owner(s) from the cost perspective. If the RF interference is sufficiently severe to affect business, the cost of the changes may be offset by the return to the previous or greater business level and the implementation may be amortized over a time period. Incentives for All Concerned The interference incidents resolution will likely incur some costs to implement. It is also anticipated that vendors will be hesitant to bear the associated costs, despite the wording of the FCC regulations to turn off the interference under the stated conditions. The airport does not have authority to enforce RF interference mitigation for Part 15 devices except where there is interference to navigation systems. In such an environment, the airport authority can offer cost-sharing or other types of incen- tives in cases where the parties do agree to an interference incident resolution but stall in imple- menting the resolution. However, this is at the prerogative of each airport authority, and further research and brainstorming are needed to accomplish the intent of interference incident resolu- tion in an equitable way without unreasonable and unnecessary cost burden to the airport. Some Possible New Technologies That Offer Promise The spectrum is becoming crowded with the new technologies brought into the market- place to meet growing communications needs, including personal, civil authorities, emergency responders, law enforcement, military, air and space. The crowding of spectrum is a natural consequence of these needs, and interference can be experienced between users in the bands. Interference topics often portray a bleak picture, but there are technology solutions that can potentially aid in avoiding interference. Research on making RF communications spectrally effi- cient and interference tolerant and resistant has introduced interference mitigation techniques to be implemented into radios. As time progresses, the cost of implementing these techniques diminishes to where they are plausible for low-cost radios such as WiFi. Some of these are cited below for consideration. “Carrier sense multiple access (CSMA),” “Collision Avoidance,” and “Request to Send/Clear to Send (RTS/CTS)” are becoming commonplace and readily implemented functions in soft- ware radios for precluding interference between radios and within networks for voice and data. The simple concept of each of these techniques is to listen for a clear channel before transmis- sion. Such techniques aid to avoid interference; however, they may not always be an adequate answer in heavily used and spatially dense WiFi networked environments such as a crowded airport terminal waiting area. Multiple-input and multiple-output (MIMO) is a promising technology that can beneficially contribute to the airport environment. MIMO uses multiple antennas at both the transmitter and receiver to transfer more data at the same time to improve communication performance. It offers significant increases in data throughput and link range without additional bandwidth or increased transmit power by spreading the same total transmit power over the antennas to improve spectral efficiency (more bits per second per hertz of bandwidth) and link reliability. MIMO is part of modern wireless communication standards such as WiFi. Some non-WiFi systems have built-in interference avoidance capabilities to choose a fre- quency not in use by such techniques as “Dynamic Spectrum Access.” This is emerging from the research stage and is quite promising.

72 A Guidebook for Mitigating Disruptive WiFi Interference at Airports Conclusions and Way Forward Interference is a complex issue. The determination of the root cause of the interference has to separate the “perceived” from the RF in order to apply the appropriate mitigation. The airport WiFi and wireless environment poses a challenge to mitigate the interference from Part 15 devices at airports. There is a responsibility to mitigate any WiFi RF interference to radio navigation, safety on the tarmac, or security on airport ground systems if they occupy the same or adjacent spectrum bands. Note these cases could be caused by RF leakage from inside or external to the airport terminal. There could be other systems that are unlicensed at airports that deal with ground traffic, and these will also likely require protection from interference. The enforcement can become complex if all the systems use unlicensed WiFi. However, the solution should be relatively simple, with systems focusing on safety having the priority. The mitigation of interference between WiFi and other Part 15 devices within the airport terminal presents issues and challenges due to the absence of a regulatory process for resolu- tion. A cooperative interference resolution is suggested; however, additional details need to be developed. The harm claim threshold approach would ideally work in an airport environment, but this approach needs further adaptation to the airport environment. An interference incident report database methodology is suggested to assist the mitigation process, preserve records, and make available the resolutions to others. It provides a way to measure the magnitude of the interference with time, and the data collected can be leveraged by all airports, used for lessons learned and long-range planning for WiFi installation upgrades. This primer focused on RF interference in an unlicensed spectrum and highlighted some resulting issues for airports and tenants. It should not be considered all-inclusive but provides an overview of the issues.

Radio Frequency Primer 73 Attachment 1: Spectrum Management in Its Early Days Spectrum interference management in its infancy state, governed by the Communications Act of 1934, provided “empty frequency space” known as guard bands (Figure A1-1) for adjacent channel protection on one or both sides of the assigned frequencies. This was done to “preclude out of band (OOB)” radio emission interference due to the vacuum tube radios not being able to suppress these RF signals to acceptable levels in the immediate adjacent bands. Safety of flight was the primary concern of spectrum usage outside airport facilities, and any potential radio frequency (RF) interference emanating from inside was kept out of frequency bands for flight systems. Interference management was relatively simple. This simple technique was adequate for decades with rather large and bulky vacuum tube technology, as compared with modern hand-held devices, to preclude interference since the guard bands avoided RF spill-over into the two neighboring frequency assigned channels. Spec- trum use was relatively sparse compared with the present. The broadcasters and other users were fewer in number than today. Spectrum assignments done this way precluded interference for the most part, with exceptions caused by anomalous propagation and equipment shortfalls that did not adequately preclude OOB emissions in the adjacent channels. Early materials research since the invention of transistors provided solid state technology to gradually allow movement past the vacuum tube era. The radios gradually evolved to smaller sizes, where today the electronics are on a chip and the bulk is in the earphone or speaker. New applications of RF systems permitted automation and an explosion in numbers of devices to enable control and communicate to other devices. Laptops, tablets, computer devices, and hands-free interfaces to these devices enable tasks to be performed with greater business effi- ciency away from the office. The micro-electro-mechanical systems (MEMs) and nano-electro- mechanical systems (NEMs) improvements to the solid state technology permit RF radios and devices to be designed as implants. This technological revolution had profound impacts on the RF spectrum. The following is a synopsis of the evolution that caused spectrum reallocation specifically pertaining to WiFi in airports: • The number of low-cost RF devices in the environment quickly saturated spectrum avail- ability in the past few decades, leading to the reallocation and sale of spectrum. Note there are dedicated bands that serve critical functions (e.g., radio navigation, radio astronomy) that are mostly immune to this spectrum issue. • Unlicensed bands such as the 2.4 GHz industrial, scientific, and medical (ISM) band were created to permit usage of the devices without spectrum regulatory frequency assignments for individual use. Some key impacts are: – Adjacent channel guard bands would no longer be assigned as a rule for OOB interference protection. Figure A1-1. Guard band example.

74 A Guidebook for Mitigating Disruptive WiFi Interference at Airports – Spectrum use specifications made resolution of interference part of non-regulatory gover- nance (i.e., users work out the interference issues), with some exceptions in the unlicensed bands. Simply put, the vague definition of “harmful interference” was passively replaced with loose bounds of “tolerable interference” (tolerable is interpretive/subjective to indi- vidual cases) and left to the users to resolve disputes. – It was left to the device manufacturers to clean up their OOB emissions or operate in the environment. Low-cost components to preclude or filter OOB emissions are available on the market, but not all manufacturers implement them in their devices to make their de- vices competitive in the commercial marketplace. – Unlicensed bands change the frequency management procedures in bands like 2.4 GHz, with the users bearing the burden of handling interference. • Low-cost RF devices proliferate in the marketplace, including WiFi. Heavy competition usu- ally means the devices are manufactured with less robustness against interference and radio propagation effects. Examples include dropped cell phone calls; intermittent disruption of WiFi and cell phone coverage at various locations or while in motion; and over the air disrup- tions in urban, dense foliage, or hilly locations. The public has become somewhat tolerant to dropped calls and less than optimum performance of networks. To the user, the cause of the disruptions is not directly perceivable, but some of these short time disruptions could be due to interference. As a direct consequence, intermittent WiFi interference disruptions or less than peak perfor- mance in crowded places such as airports may not always be objectionable but dependent on the specific circumstances. This is a key point on how to prudently manage both throughput and interference of the WiFi network with the increasing passenger demand for WiFi services in the airport environment. However, the airport WiFi installations will expose a unique intersection between “licensed” and “unlicensed” environments. Radio frequency systems from both environments may interact and simultaneously cause “tolerable” and what is known as “harmful” interference depending on the viewpoint of the owners of the systems.

Radio Frequency Primer 75 References 1. Public Law Number 416, Act of June 19, 1934, ch. 652, 48 Stat. 1064, by the 73rd Congress, signed by President Franklin D. Roosevelt, codified as Chapter 5 of Title 47 of the United States Code, 47 U.S.C. § 151 et seq. 2. Conference Report, Telecommunications Act of 1996, House of Representatives, 104th Congress, 2d Session, H.Rept. 104-458, at p. 1. 3. Federal Communications Commission Consolidated Reporting Act of 2013 (H.R. 2844; 113th Congress). 4. “H.R. 2844—Summary,” United States Congress. 5. “CBO—H.R. 2844,” Congressional Budget Office. 6. “Security Experts Warn of Dangers of Rogue WiFi Hotspots,” Hilary Whiteman, CNN (2009). 7. On November 1, 2006, the Federal Communications Commission (FCC) sided with Continental Airlines in a ruling that Massachusetts Port Authority cannot restrict WiFi use at Boston’s Logan International Airport. 8. “Interference Limits Policy: The Use of Harm Claim Thresholds to Improve the Interference Tolerance of Wireless Systems,” White Paper, Receivers and Spectrum Working Group, FCC Technological Advisory Coun- cil Version 1.0 (2013). 9. Airport Wireless Local Area Network: Technology Survey and Simulation Tool Development MITRE TECHNICAL REPORT MTR 03W0000051 August 2003. Izabela Gheorghisor, Dr. Yan-Shek Hoh, Minh Nguyen (2013).