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5 C H A P T E R 1 This Guidebook includes a primer, or tutorial, on radio frequency (RF) interference that focuses on the complex airport terminal WiFi environment. The primer is provided in its entirety in Appendix A. It explains RF interference associated with WiFi and commercial wire- less communications devices in the unlicensed spectrum and the resulting issues for airports and tenants. Written primarily for network engineers, it is a technical guide for that audience. The following is a brief introduction for non-technical personnel. 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 4). 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 âvictimâ receiver (indicated by the blue arrow in Figure 5). The result can be a disruption, degradation, or limitation of the performance to the victim radio receiver circuitry. From the airport terminal operations perspective, there are several interacting mechanisms associated with WiFi that can contribute to interference or what appears to be interference. The user and the airport WiFi network have little control over these. They include: a. Overlapping frequencies in the 2.4 GHz band (assignment of these was done by the FCC). b. WiFi equipment not containing quality interference-resistant designs and components, as typically found in low-cost commercially available equipment. c. Multiple versions of WiFi equipment in use. d. Spontaneous and simultaneous use of WiFi that can cause interference within some local- ized area(s). e. Use of rogue WiFi âhotspotsâ or other such radio transmissions in the airport terminal envi- ronment exceeding FCC spectrum regulations. f. User congestion exceeding network capacity in areas of the airport. The above independent mechanisms are not always intuitively obvious as to their root cause and can lead to other causes often mistaken as interference. For example, the overlapping fre- quencies mask the spectrum interference under network behavior such as lowering the through- put. In addition, there is no clear agreement on what is a level of tolerable interference. WiFi equipment responds to the asynchronous demands of the user, which can be simultaneous and lead to congestion through parts of the network. User WiFi equipment in the airport terminal environment can be designed according to different IEEE standards design and manufactured Radio Frequency Primer
6 A Guidebook for Mitigating Disruptive WiFi Interference at Airports by different vendors. Such equipment can operate in the same spectrum, but have varying and different susceptibility to interference. It is important for the reader to understand that these mechanisms impact interference through the network differently, and manifest themselves in different ways. The details of the Radio Frequency Primer are contained in Appendix A, which covers: ⢠The historical perspective on spectrum regulatory governance, interference mitigation tech- niques, and tolerable interference acceptance evolution. ⢠How the Federal Communications Commission (FCC) manages the non-government spec- trum and is the regulatory agency that provides rules and regulations on the use of the spectrum. ⢠A more in-depth overview of interference and perceived misconceptions. ⢠A process to work through interference incident reports at airports. ⢠The practical side of how to resolve RF interference issues for airport authorities and tenants. ⢠Conclusions and a way forward. Figure 5. Example of multiple channel WiFi interference. Figure 4. Raindrops illustrating concept of interference.
