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Connecting people with emergency services reached a pinnacle of simplicity when 911 was rolled out as an emergency number across the United States. With one number, people in distress could get the help they needed dispatched as soon as possible. As increasing numbers of households have cut the cord on their traditional landline telephones, 80 percent of 911 calls are now placed via cell phone. Emergency calls need to connect 100 percent of the time, which makes mobile coverage, first and foremost, a public safety issue.
Understanding which buildings fall short of providing adequate service can assist local governments in working with building owners and mobile operators to make needed improvements. This falls into two broad categories: First Responder Push to Talk systems, and Commercial Mobile Services used by both Public Safety Agencies and the general public.
Poor indoor coverage impacts public safety
If someone is in distress and unable to place an outgoing call, first responders will not be aware there is an emergency that requires their response. For this reason, the Safer Buildings Coalition defines three pillars of in-building safety communications:
- Mobile 911 Calls Must Get Out with Location Accuracy
- Mobile Mass Notifications Must Get In
- First Responder Communications Must Work
If a building cannot deliver these basic characteristics, the environment puts the occupants and the property itself at risk.
Determining a precise location can be a significant challenge if the device does not have an unobstructed view of the sky. As more GPS satellites can “see” the device, the more accurate the location the system can provide. Work is underway by industry leaders and public safety agencies to improve indoor location, but since it is a complex issue unto itself, this article will focus solely on indoor wireless network coverage.
Why indoor coverage is challenging
Anyone who’s ever tried to place a call from an elevator is not surprised that indoor coverage can be much worse than outdoor coverage. And the deeper into a building you go, the worse the signal typically gets. Penetrating walls is difficult for a cellular signal, though some of the spectrum blocks that mobile companies have licenced are better for this task than others. Low band (longer wavelengths) spectrum tends to be much better at penetrating concrete and brick than high band (shorter wavelength) spectrum.
Low-e glass can inhibit signals
Another factor in poor indoor signal strength is often windows. The introduction of low-e glass has provided huge energy savings for building owners and is positive for the environment. However, the unintended effect is a negative impact on wireless communications.
How glass compares to other building materials in shielding the interior from wireless signals depends upon the type of glass. The chart below offers some surprising comparisons. The attenuation column represents the reduction in the amplitude of the signal. For this example we use 900 MHz, a common low-band spectrum used throughout most of the world and considered to be better at building penetration than higher band frequencies.
The more energy efficient the glass, the more the signal level is reduced. Consider that for each 3 dB attenuation (loss), the signal strength is reduced by half. A 6 dB attenuation means a 75 percent loss in signal; at 9 dB, an 87.5 percent loss. As this reduction is exponential, the double glazing low-e glass, near the bottom of the chart, represents a signal reduction of 99.9 percent.
It’s not uncommon to see someone who is struggling to maintain a call walk toward the window in an attempt to improve their reception. If a building has installed energy efficient glass, most of the available signal may well be coming through the walls. If this person is trying to connect to emergency services, the results could be tragic.
How first responders get coverage
After an initial investment by the federal government, billions of dollars are currently being spent by AT&T to build the new FirstNet network, bringing prioritized broadband telecommunications to the nation’s first responders. State and local governments are also investing to upgrade equipment. This new network is using a dedicated spectrum band (Band 14, also known as the Upper 700 MHz D-block) and also provides prioritized access to the AT&T commercial bands as needed during an emergency.
With fewer users compared to a commercial network, the FirstNet network will experience less congestion and, therefore, a higher signal quality than those serving hundreds of millions of users and devices.
With the addition of High-Power User Equipment (HPUE) Power Class 1, the FirstNet devices can transmit on Band 14 at up 31 dBm. This is a significant increase from the standard 23 dBm (Power Class 3). This can improve FirstNet coverage in fringe areas by up to 80 percent Specifically, the ability for the cell site to better “hear” the user equipment can be the difference between a dropped or completed VoLTE call, delivered text message, or the transfer of mission critical data.
While FirstNet is being built into the robust system that has been promised, first responders still use their proprietary Land Mobile Radio (LMR) networks as their primary means of voice communication. Portable cell sites are also available in some circumstances to supplement wireless coverage where needed.
As the cost to deploy indoor coverage technology declines, public safety officials within each local government should be considering how to implement code changes that will improve access to emergency communications. This process will take many years, so it is important to have empirical data to help prioritize which structures are most at risk. This may be a national issue, but it will be solved at the local level, one building at a time.
By Bryan Darr, EVP, Smart Cities, Ookla
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