This article was originally posted in the Urgent Communications IWCE News Letter.
By John Vetter, VP of Business Development
The Federal Communications Commission (FCC) should be commended for its recent unanimous ruling in favor of increased horizontal-axis location accuracy requirements for Wireless Enhanced 911 (E911) systems, and for providing guidance on a path to develop and implement vertical-axis solutions and requirements. The actions taken will better protect the citizenry and let first respond and react more quickly and efficiently to calls for assistance from mobile phones—an important element, considering three out of every four emergency calls are made using wireless devices. While the ruling is an important first measure, subsequent steps need to be taken for the emergency-response system to operate most efficiently and save more lives.
FCC rules for E911 require that confidence and uncertainty data be provided to public-safety answering points (PSAPs). Commercial Mobile Radio Service (CMRS) providers, or wireless carriers, must file their conformance verification procedures with the FCC, and the agency recommends periodic conformance testing.
Location solutions are split into two classes. Network location, which relies entirely on CMRS-provider infrastructure to determine location, can include Received Signal Strength (RSSI), Observed Time Difference of Arrival (Downlink OTDOA), Angle of Arrival (AoA), Multipath Fingerprinting, and combinations of these techniques. Handset location is dependent upon location information provided by, or cooperative analysis with, the user device. It can include techniques based on Global Positioning System (GPS) coordinates, assisted Global Positioning System (A-GPS), Enhanced Observed Time Difference (E-OTD), Enhanced Forward Link Triangulation (E-FLT, CDMA-only), and a combination of the aforementioned.
While market penetration of smartphones increases handset-location use, the continued presence of older GSM and CDMA devices requires that network-location techniques be available. It is estimated that 75% of the mobile phones in the U.S. still use legacy technologies (as of Q2 2014) that do not offer GPS, Wi-Fi, or Bluetooth. Additionally, smartphones with handset-location capability cannot provide location data, if a GPS lock cannot be obtained due to signal-blocking and signal-scattering situations—both of which are quite possible in dense urban areas.
This means that network location often is still needed to service E911 calls. The problem is that network-location techniques rely on accurate placement and alignment of provider infrastructure; unfortunately, that is not always the case.
Recorded location errors of 30–50 meters are not uncommon for tower antennas. Target location errors tend to be approximately twice the tower antenna location error. When the emergency call is made by someone capable of interacting with first responders, an error of up to 100 meters may result in an extended time to contact. If the call is made by a person unable to respond (e.g. a caller who loses consciousness after dialing 911), a 100-meter inaccuracy may be significant.
Improper tower antenna placement results in unfavorable hyperbolic geometries, especially in sparse suburban or rural areas, where tower antennas are distant and located along similar azimuths as observed from the handset. A location error of 15-30 meters could result in a handset location error of 500+ meters. An emergency call from a hiker fallen into a ravine or a car driven down an embankment may not be easily found, resulting in additional first responders being deployed and longer response times.
Because OTDOA is limited, E911 deployment may require additional techniques, such as AoA. When used with OTDOA, AoA can reduce uncertainty and decrease response time to contact. AoA + OTDOA systems require installing specialized antenna arrays. Commonly referred to as “triangulation,” standalone AoA requires at least three tower antenna sites. AoA + OTDOA requires at least two tower antenna sites and works best with three or more.
Target location inaccuracy caused by incorrect tower antenna location data for OTDOA systems and/or inaccurate azimuthal alignment for AoA systems has real consequences beyond failing to meet FCC E911 mandates. A Columbia University study found that, on average, a one-minute reduction in EMS response time equals a 17% decrease in the likelihood of 90-day mortality, the medical standard for measuring effectiveness of treatment or intervention. Approximately 25 million people call for an ambulance each year. Using rough but reasonable assumptions, it can be estimated that improved location accuracy that reduces EMS response time by one minute can save over 10,000 lives annually and have a societal benefit of more than $92 billion.
Using proper alignment tools , such as those outlined in a recently published E911 white paper, that can measure and record latitude and longitude to within 30 centimeters can improve response time and save more lives. When used to align the azimuth of a sectored cellular antenna, ±0.3° RMS can be provided. The appropriate tools can ensure proper azimuth alignment and record more accurate antenna location data for PSAP and RF databases. Benefits include fewer periodic adjustments, and less likelihood of costly and time-consuming system troubleshooting and retesting.
Providing PSAPs with a highly accurate database of tower antenna locations is critical to meeting FCC E911mandates. Accurate azimuthal alignment of antennas is also essential for system performance. Improving antenna position data capture is one of the solutions to improving E911 network-location-based determinations. Achieving these success metrics will create a system that better serves an increasingly mobile community with accuracy and lowered public-safety response times.
John Vetter is the vice president of business development for Sunsight Instruments. John has 20+ years’ experience in RF engineering, wireless-network design, deployment, and post network performance/optimization. As a senior-level manager, he managed the engineering and operations departments of MetroPCS/T-Mobile and Clearwire (now Sprint), working with all major wireless technologies, including 4G. He has worked on the vendor side for Ericsson and Alcatel-Lucent in similar roles.