What’s wrong with 6.25 kHz migration?
In March 2007, the FCC released an order that extended the deadline for vendors to manufacture equipment capable of operating on 6.25 kHz-wide channels to Jan. 1, 2011, but declined to set a deadline for licensees to transition to such channels.
At least 80% of this nation’s public safety agencies operate conventional radio systems below 512 MHz (VHF/UHF). There are many reasons for this, chief among them equipment costs and favorable propagation characteristics below 512 MHz. These systems typically utilize frequency division multiple access, or FDMA, technology.
The Project 25 Phase II standard has been identified as the migration path to 6.25 kHz equivalency. Technologies based on this standard use time division multiple access, or TDMA, in which a 12.5 kHz channel is divided into two time slots. TDMA was chosen because of developmental obstacles associated with FDMA, such as power consumption, size and device design. But TDMA also requires continuous interconnection between base station and subscriber units, a capability that is not present in the majority of today’s systems operating below 512 MHz.
While it is not anticipated that a Project 25 narrowband standard based on FDMA technology will be completed by Jan. 1, 2011 — nor is one currently being pursued — FDMA products with 6.25 kHz equivalency exist today in the marketplace. Companies such as Kenwood and ICOM have been able to develop products with both 12.5 kHz and 6.25 kHz modes that, while not Project 25 certified, meet the FCC’s 6.25 KHz equipment certification requirements and the commission’s long-term migration plan.
That said, there are a couple of significant problems with the FCC’s order. Licensees operating below 512 MHz, as things stand now, will have two options after Jan. 1, 2001: a TDMA-based device that can be used only if the licensee is operating an interconnected network, or an FDMA-based radio that isn’t standards-based.
Neither option makes much sense for the licensee. The former would require the licensee to migrate to an interconnected system in order to leverage the device’s TDMA capabilities, or spend more — compared with an FDMA-based device — to purchase a radio with a mode that it can’t use — due to the FCC’s certification rules — just to achieve 6.25 kHz equivalency. In either event, the licensee’s equipment costs would be increased significantly and unnecessarily. In the latter option, the licensee would be stuck with a proprietary product that likely won’t be interoperable — which would be a shame given the progress that has been made in that area.
Indeed, should the implementation of 6.25 KHz technologies cause interoperability gaps — because manufacturers working without benefit of an industry standard took differing approaches regarding how best to meet the FCC’s narrowbanding mandate, which in turn created incompatibility between radios operating in the same band — years of progress in migrating from analog radios to digital radios will be undone. This would be a huge step backward for public safety, the standards process and land mobile radio in general.
Stephen T. Devine is a public safety services manager for technology consultancy Bearingpoint. Previously, he served in operational and technical roles in the Missouri State Highway Patrol’s communications division for 22 years. He can be reached at [email protected].
