More cell sites — not more spectrum — is the driving factor when determining maximum data throughput per device at the edge of a cell in a broadband wireless network, such as the one proposed by the FCC in the 700 MHz band for first-responder use.

That’s the message that FCC Chief Technologist Jon Peha wants to make clear as the debate about the proposed network’s configuration, cost and spectrum assets — most notably, whether the D Block should be auctioned for commercial use or reallocated to public safety — continues to get more attention.

In response to claims that mobile public-safety devices would be able to achieve higher data rates for uploading video and other high-bandwidth applications on a broadband network if it had the D Block to couple with the 10 MHz of 700 MHz spectrum licensed to the Public Safety Spectrum Trust (PSST), Peha said that is not the case. The performance of a given device is linked to the signal strength between the device and the cell site, which is determined primarily by the distance the device is from the cell site, he said.

“The rate that you can transmit depends on the output power of the device and how close you are to the tower. It really is not dependent on how much spectrum you have,” Peha said. “If you want to support a high data rate from a video camera to the tower, the quality of the data stream decreases as you move away from the tower, which means you need small cells and a lot more cells, which becomes significantly more expensive.

“If you gave them 500 MHz, but they still had to use the same off-the-shelf devices as everybody else and they still had the same budget to build out the same 44,000 towers, they still could not support 1 Mbps video at the edge of cell.”

Instead of the 44,000 cell sites the FCC projects for the proposed nationwide network, more than 115,000 cell sites would be needed to support the 1.2 Mbps data rates that many in public safety want to support video applications from the scene of an incident, according to the FCC. The FCC models have been built to support video rates between 256 kbps and 384 kbps.

Of course, having 2.7 times as many sites to support the highest throughput rate increases the hardware costs by a similar factor, which is significant. But perhaps even bigger cost factors are that deploying so many cell sites would increase ongoing maintenance costs and dramatically would increase the number of new cell sites needed, instead of being able to leverage existing site locations. And anyone in the wireless industry — public or private — will tell you that securing tens of thousands of new cell-site locations promises to be a very costly and time-consuming endeavor.

While the amount of spectrum does not impact the performance of an individual device, it certainly impacts the aggregate capacity of the network, Peha said. For instance, if a dispatch or command center wanted to send video to responders at an incident site, it would be able to send that video to many more public-safety personnel if it has 20 MHz of spectrum available as opposed to 10 MHz.

Most industry sources believe public safety will have unused spectrum most of the time, even if limited to the 10 MHz block licensed to the PSST. However, when multiple departments and agencies converge at an incident in a given location, many are worried that the capacity within the key cell sector could be overloaded quickly, particularly if multiple video streams — to and from the incident scene — are involved. Other cell sectors in the area may have loads of unused capacity, but the sector serving the incident could be tapped out.

In other words, the amount of total capacity supported by the network needs to be determined, as well as the per-device target throughput rates, Peha said.

“Either we build out enough sites for 1.2 Mbps or we build out enough sites for 512 kbps or we build out enough sites for 256 kbps, and then we should talk about both aggregate and per-device throughput,” he said.

Indeed, it seems clear that such frank discussions are needed. If 1.2 Mbps throughput at the cell edge is determined to be the requirement for public safety, then many more cell sites will be needed, and the proposed $6.5 billion price tag associated with the buildout of the proposed network is not nearly enough.

Meanwhile, if per-cell-sector aggregate capacity needs are determined to be a bigger requirement, then Beltway policy-makers and lawmakers may want to take a harder look at reallocating the D Block to public safety.

As stated in previous commentaries, the key is for the FCC, public safety and Congress to agree on what they want the proposed 700 MHz broadband network to do, so that appropriate policies, laws and funding can be established to make an effective network a reality. Failure to agree on these requirements before such critical decisions are made would almost guarantee that valuable resources — particularly time, spectrum and money — will be wasted in the process.

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