On Tuesday, March 15, the ARRL filed a Petition for Rulemaking and a Request for Temporary Waiver  to authorize the use of single-time-slot Time Division Multiple Access (TDMA) emissions in the amateur bands at and above 50 MHz, wherever multiple-time-slot TDMA is authorized. (See below for copies of these two documents.)

The ARRL -- which called its Petition “very narrow in scope” -- seeks to facilitate the use of and experimentation by radio amateurs with existing narrowband spectrum-efficient digital voice and data technology. “Such technology is now in regular and increasing use in the private land mobile radio services, but its use in the Amateur Radio Service is now apparently unintentionally precluded by two specific Commission rules,” the ARRL’s Petition stated.

The Petition asks the FCC to allow those amateurs who are presently using a Motorola narrowband (12.5 kHz) digital land mobile system -- commercially marketed as MotoTRBO -- to be used legally. Because of some restrictions in the Part 97 rules, the TDMA repeaters (which are multiple-time-slot devices) are legal, but the mobiles and portables are not because the emissions used (single-time-slot TDMA) are not authorized anywhere, due to the emission designator.

The legality of the use of these systems, however, was drawn into question only recently. The ARRL called it “urgent to allow these existing systems to continue to operate and to allow the sponsors of them to avoid losing their investment in them.” Therefore, contemporaneously with its Petition, the ARRL submitted a Request for Temporary Waiver of the same rules sought to be modified in the Petition. This would, if granted, permit these systems to continue to operate (on a non-interference basis), while the Petition is under review and subject to its outcome.

The use of TDMA digital emissions in certain frequency bands in the Amateur Service is on the increase, the ARRL noted. There are numerous narrowband UHF repeater facilities now operating that use multiple slot TDMA repeaters and single slot TDMA handheld digital transceivers, principally in the 70 cm band. These systems have been installed primarily in the western part of the US and in the New York City area, but also in several midwestern states.