LTE equipment configuration makes guard-band question a key item

Recently, the FCC opened a proceeding that would explore whether a guard band is needed between the commercial D Block in the 700 MHz band and the adjacent 10 MHz swath that is licensed to the Public Safety Spectrum Trust (PSST), which is expected to serve as the spectral foundation for a nationwide wireless broadband for first responders.

As mentioned in this space previously, this is big deal, because taking the guard band out of the commercial spectrum or the PSST spectrum greatly diminishes the capacity of the given network — or increases the costs necessary to maintain the capacity thresholds.

It turns out that the guard-band issue is even bigger than I suspected, according to mobile wireless consultant Andrew Seybold.

For the past few months, my understanding from industry sources has been that LTE equipment operates in spectral increments of 1.25 MHz. So, if a guard band were taken from either the D Block or the PSST block, the usable spectrum would be reduced from 5 x 5MHz to 3.75 x 3.75 MHz — a reduction of 25%. However, Seybold says LTE equipment actually operates in the following bandwidth increments: 1.4 MHz, 3 MHz, 5 MHz, 10 MHz, 15 MHz and 20 MHz.

In other words, if a guard band is taken from either the D Block or the PSST block, the operator would only be able to use 3 x 3 MHz in the network, which represents a 40% decrease in the amount of usable spectrum either side might be anticipating under the current spectrum plan.

For public safety — already arguing that 5 x 5 MHz is not enough to serve its broadband needs while asking Congress to reallocate the D Block to first responders — a 3 x 3 network would constrict the network capacity enough that partnerships with even government entities would be difficult to execute, much less some of the more innovative partnerships proposed with critical-infrastructure entities such as hospitals and utilities.

Of course, the FCC already has told 21 public-safety entities representing some of the most populous areas of the U.S. that they can build networks using the 5 x 5 MHz licensed to the PSST. Could that be changed? Probably, but it would generate a considerable public-safety outcry.

If a guard band is needed and the waiver rules are not changed, the guard band would have to come out of the commercial D Block, which is supposed to be auctioned early next year. However, if the commercial D Block licensee only will be able to use 3 x 3 MHz of spectrum, the amount bidders would be willing to pay for the swath would decrease dramatically.

Currently, Congress is budgeting $3 billion being raised in an auction for a 5 x 5 MHz D Block, and some industry analysts believe that figure is optimistic. If a guard band results in a 40% reduction in usable spectrum, certainly a 40% reduction in the estimated bid price would make sense. But that would mean the U.S. Treasury only getting $1.8 billion in the auction, public safety still would be clamoring for more spectrum and 4 MHz of this prime spectrum would lay fallow — not exactly the type of spectrum utilization that the FCC wants.

Of course, it’s possible that the record in the FCC proceeding determines that no guard band is needed between the PSST spectrum and the D Block, as many in the federal agency have claimed for months. It is important that industry experts with knowledge of LTE networks participate in the proceeding to ensure that the FCC’s record on the matter is as comprehensive as possible.

Whatever the finding, hopefully the commissioners make the correct policy decision. Having a capacity-constricted network is far from ideal, but it would be worse if interference between the two networks undermines a commercial carrier’s business plan or — much worse — results in the loss of life for a first responder or citizen during an emergency incident.

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