Public Safety Advocate: FirstNet Network Capacity

Thu Apr 20 16:06:50 2017

Before FirstNet, those who wanted to auction the 5 MHz by 5 MHz portion of the 700-MHz spectrum known as the D block, which was adjacent to the 5 MHz by 5 MHz portion of the spectrum already licensed to the Public Safety Spectrum Trust (PSST) for Public Safety broadband contended that Public Safety would never need more than the 10 MHz of spectrum already allocated, therefore the D block should be auctioned with no restrictions to a network operator and used for commercial services.

The Public Safety community prevailed and FirstNet was born having full priority access on 10 MHz X 10 MHz of spectrum now known as the FirstNet Spectrum or band 14. FirstNet is licensed for the full 20 MHz of spectrum in the 700-MHz band that will provide full-priority access for Public Safety while permitting AT&T to make use of the excess spectrum for its own customers. This was designed in the law to be a full-up public/private partnership with the private partner footing most of the cost of the network build as well as operation of the network. The payback for the partner is the use of the spectrum when it is not needed by Public Safety. AT&T won the award and not only does Public Safety gain a band 14 network, it gains access to all of AT&T’s existing network spectrum on a priority or pre-emptive basis. Considering that there was still concern about major incidents running out of spectrum, this new network design should mitigate most if not all of the congestion problems.

To understand why this is true it is necessary to return to the results of the first real-world band 14 capacity testing that took place in 2011. The results of these tests were only one of the many reasons the Public Safety community won the battle. It was in 2011 that my team was hired by the East Bay Regional Communications Authority (EBRCSA) to run capacity tests on the first 5 MHz X 5 MHz band 14 system in operation. At that time only two cell sites were up and operational when we set about running some real-world tests. The project was called Project Cornerstone and the tests were run in Alameda County using sites built by Motorola, which had obtained a BTOP grant for the region from the NTIA prior to FirstNet.

The timing could not have been better since we were faced with several capacity studies provided to the FCC and Congress by two wireless network operators that were pushing for the D block to be auctioned. There was also a report from Dr. Peha, a visiting academic to the FCC from, I believe, Carnegie Melon University. It was his report that the FCC and members of Congress placed the most validity on simply, it appeared, because of his credentials. His main argument was that 5X5 MHz of spectrum was all we needed for Public Safety. He cited the 3GPP standards body test for LTE capacity, which was to calculate the number of users that could be served within 19 cell sites (57 cell sectors) with a common amount of interference. On its face this paper seemed to prove concisely that Public Safety would have more than enough spectrum.

My rebuttal was that you cannot measure capacity for Public Safety as you would for a commercial system. Instead, you need to measure capacity on a per-cell-sector basis because most Public Safety incidents take place within a single cell sector or only a few sectors, therefore it is important to know how much data and video can be transmitted in both directions within a single cell sector where there are multiple Public Safety responders.

The report can be found Here . The tests took place at three different locations (near the center of the cell site, at the mid-point of coverage, and at the cell edge) and Panasonic graciously provided us with a dozen Toughbooks for the tests. We used unity gain antennas in a MIMO (2 antenna per device) configuration and had a server located at the Alameda County EOC. One of my folks who is very good at coding wrote a series of tasks that could be executed on the server remotely to send text and video clips down to the units and to send multiple videos and data streams back up to the server. The results were captured for review after the tests were complete. We also were fortunate that Anritsu provided us with test engineers and its LTE test sets to view the spectrum in real time. The results we obtained after we ran each set of tests multiple times proved how much data and video would be sent and received in a single cell sector. Please remember that these tests were done in 2011 and that LTE has developed well beyond the capacity restraints we found in those tests.

The Network Configuration is depicted here:
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We conferred with the Public Safety community and defined three types of incidents and the number of Public Safety vehicles and personnel that would be assigned to each incident. We had a bank robbery with a potential hostage situation, a multi-story building fire, and a multi-vehicle accident with multiple injuries and extensive damage to the vehicles. The results are shown below as well as our scaling of the capacity tests up to 10 MHz X 10 MHz to show the difference. I am happy to say that the results were widely distributed to the FCC, to Congressional staffers, and to the news media, and we have been told that real-world testing did make a difference in the outcome in Congress.

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Actual Network capacity versus required capacity (5X5 MHz) :
 

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Extrapolating to a 10 MHz X 10 MHz Network:
 

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Those working toward obtaining the full 20 MHz of spectrum were successful and FirstNet was born in early 2012. There was still a concern, at least on my part, whether even 20 MHz of spectrum would be enough when an incident required many Public Safety responders and was only covered by one or two cell sectors. Now that the RFP award winner AT&T has stated it will provide access to all of its LTE network holdings, having enough capacity is most likely no longer an issue. I say most likely because there could be instances where demand for spectrum access between Public Safety and AT&T’s commercial customers might still put a real strain on the network but unlike with Land Mobile Radio systems where capacity capability is measured across the entire network, with LTE each cell sector and each cell has its own capacity.

If one cell is blocked and there is a neighboring cell that also covers into the same area, then the capacity is increased. Most importantly, just a cell site away from the incident, where the traffic is normalized, the network will continue to operate and provide all of that cell site’s capacity to those in that cell. Further, today’s LTE network technology enables cell site coverage to be trimmed down and also expanded as needed to handle peak loading. A lot of this is done automatically but it is also watched closely by those in the network operations center. Further, as AT&T upgrades its commercial network FirstNet will receive the same level of upgrades including small cell deployment, which will also result in even more network capacity and faster speeds.

I believe what AT&T has done will become a big deal for Public Safety, but I have to also wonder how much of the total available capacity in a given area AT&T will let Public Safety use at the expense of its commercial customers. AT&T, as with the other network operators, is very good at managing network capacity across cell sectors and sites. FirstNet and its new ability to make use of not only band 14 but all of AT&T’s LTE assets will mean programs and people who allocate capacity on a real-time basis will have to add a new variable to their formulas and criteria.

It is also incumbent on FirstNet, AT&T, and the Public Safety community to train those in the field so they understand they will have better access to more network capacity but even that has a cap to it. The biggest issue I believe will be how to make the best use of this new-found capacity without harming AT&T’s existing customer base. I am sure those who study the network traffic models have taken all of this into account, but there will also be a learning curve and more adjustments as more Public Safety users come onboard and make more use of the network. The end result should be that FirstNet has scored a real advantage for the Public Safety community with AT&T’s pledge to provide more capacity as needed. Now we all have to be careful to use it wisely.

Andrew M. Seybold

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