Throughout this year, FCC officials have maintained that public safety does not need 700 MHz D Block spectrum except in disaster-type scenarios such as 9/11 or Hurricane Katrina, a position supported by an agency white paper released in the spring.

Indeed, the 10 MHz of spectrum licensed to the Public Safety Spectrum Trust (PSST) represents a nice chunk of airwaves that can support the transmission of considerable data via LTE, the 4G technology chosen by public safety and the FCC for the band. And, in all but the most extreme cases, it’s doubtful that first-responder usage will exceed the total bandwidth available throughout a given network that’s utilizing just the 10 MHz of PSST spectrum.

Unfortunately, that’s not really the issue for public safety. First responders need to know that they will have access to the throughput they need when responding to an emergency, especially when numerous public-safety users — often from multiple departments, if not disparate jurisdictions — are arriving at the incident scene. That reality means that the most relevant question is: Is there enough broadband capacity in a single cell sector to serve the needs of first responders at an incident scene?

Motorola recently talked with a member of the firefighting community to project the needs of first responders at a scene under three different scenarios, including a gas-tanker spill with 29 vehicles responding and a refinery explosion with 65 vehicles at the scene, Motorola CTO Paul Steinberg said.

“The bottom line is that a lot of folks and a lot of vehicles show up,” he said. “We modeled both the uplink and the downlink wireless data demands that we would expect.”

Not surprisingly, the primary bandwidth demand in each of the scenarios was video. Using the LTE public-safety “sweet spot” ratio of 70% of capacity dedicated to downlink and 30% dedicated to uplink, the 10 MHz of PSST spectrum would not allow enough uplink throughput under any of the scenarios and would enable enough downlink throughput in only the gas-tanker spill, the least-demanding of the scenarios considered. In such cases, public safety would need to roam onto a commercial carrier’s network, which is the FCC’s model.

However, with 20 MHz of spectrum — the amount of spectrum that would be available if Congress reallocated the D Block to first responders — the same throughput demands could be reached in all but the most demanding scenarios, which would greatly reduce the need for first-responder agencies to roam onto commercial networks.

For the exercise, Motorola assumed that fixed cameras at an incident would use 1.2 Mbps data rates, while handheld cameras would consume 384 kbps, Steinberg said. In the gas-tanker scenario, using just two fixed cameras and one handheld camera at the scene would exceed the uplink capacity provided by10 MHz of spectrum.

FCC officials have noted that uplink throughputs largely are based on signal strength, so the uplink capabilities often are a product of the number of cell sites in a given geographic region, not the amount of spectrum that is available. However, Steinberg said that the signal strength for fixed cameras — often linked to steady power sources and able to utilize antennas that are better than those available on handheld devices — is sufficient to reach the 1.2 Mbps threshold, which provides the resolution that many first-responder agencies want when using video to make strategic decisions with lives on the line.

Steinberg noted that there are variables to be considered. Most notably, the throughputs included in the model were based on average signal strength within a cell sector. If the incident occurred at the cell edge, the throughputs would be lower; if the incident occurred near a cell site, the data rates would be greater, he said.

Meanwhile, regardless of how much spectrum is allocated for first-responder broadband services, managing and prioritizing bandwidth utilizations are going to be important, Steinberg said. Eventually, public safety would benefit from dynamic bandwidth allocation by commanders, based on which users have needs that are most critical to achieving the goals of the mission.

“We’ll get there,” Steinberg said. “Honestly, I believe that will be possible. But commercial technology doesn’t quite support that case, so we’re going to have to do some things to adapt it that way.”

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