The FCC this week asked for comments whether a guard band is needed between the 700 MHz commercial D Block and adjacent public-safety broadband spectrum licensed to the Public Safety Spectrum Trust (PSST), as well as inquiries regarding interoperability and equipment certification.

Under the previous FCC plans, the D Block and the PSST spectrum would be used together to provide a 20 MHz spectral foundation for a proposed nationwide wireless broadband network for first responders. Current FCC plans call for the D Block to be auctioned to a commercial operator that would have no obligation — although it would have the option — to build out its network in conjunction with public safety.

In granting 700 MHz broadband waivers to 21 public-safety entities — many of which geographically cover some of the largest commercial markets in the nation — last week, the FCC approved the waiver jurisdictions using the entire 10 MHz of spectrum in the PSST band with an out-of-band-emission limit that is standard for the commercial wireless industry.

FCC officials note in the comment request that LTE — the technology that public safety and the D Block winner are expected to use — “has requirements to enable co-existence in the same geographical area and co-location between operators on adjacent channels, and that it includes other co-existence and interworking specifications.” The comment request asks whether applying more stringent out-of-band-emission limits on public-safety entities would enable the two networks to co-exist without a guard band and whether their emissions limits should be the same as the limits used by the D Block licensee.

Mobile wireless consultant Andrew Seybold said he believes the FCC’s questions are appropriate but expressed concern about the timing, especially after out-of-band-emission limits were established last week for the 21 waiver jurisdictions.

“The questions they’re asking in this document are questions they should have asked before they released the [national broadband plan],” Seybold said. “The problem is that they’re asking the questions after they’ve let the horse out of the barn. It indicates to me that they just didn’t pay attention.”

A growing number of wireless-industry experts from both the commercial and public-safety sectors in recent months have cited a need for the FCC to include a spectral guard band between the D Block and the PSST spectrum to prevent the two proposed networks from interfering with each other.

FCC officials repeatedly have stated that the agency does not believe a guard band is necessary between the D Block and the PSST spectrum, noting that European spectrum plans for LTE operators do not include guard bands and that there is no guard band between some U.S. 700 MHz commercial-spectrum blocks.

While no regulatory guard bands exist in these situations, Seybold said that the carriers in both the U.S. and Europe plan to have guard bands by not using all the spectrum they have licensed — effectively establishing guard bands internally.

“Ask AT&T what they’re using in a 6 by 6 [MHz spectrum block], and they’ll tell you it’s 5 by 5 [MHz],” Seybold said, noting that the unused 1 MHz of spectrum will be used as a guard band.

If a guard band is needed between the D Block and the PSST spectrum, most industry experts agree that the network absorbing the guard band — whether it is public safety or the D Block — would see a 25% drop in potential network capacity under the current spectrum plan. Proposals to incorporate existing guard-band spectrum licensed to Access Spectrum and Pegasus have been discussed, but many Beltway sources believe pursuing such alternatives would force the FCC to delay the D Block auction beyond the early 2011 date the agency has targeted.

In addition to the guard-band issue, the FCC is asking questions related to public-safety interoperability, public-safety roaming and priority access on commercial networks, network performance and equipment certification. Comments are due to the FCC by June 17.