PSCC to Certify TDMA for Voice Only Following Interference Reports
After nearly three months of research, the Public Safety Communications Council (PSCC) advised the FCC that the public-safety certified frequency advisory committees will certify TDMA coordinations only for voice emissions because of interference reports in VHF spectrum.
“The issues we are receiving reports on can be caused by many things, one of which is the data carried on the radios,” said William Brownlow, telecommunications Manager for American Association of State Highway and Transportation Officials (AASHTO). “The GPS and AVL type of information is supposed to be secondary but because it’s superimposed on the voice stream, you can’t separate the two of them. It’s the loading of the channel with all the data that’s one cause of the interference being reported. So until we can figure out how to separate the data and voice, we can’t certify the system coordination except for voice at present.”
The coordinators will also only certify TDMA coordinations at power levels that are 3 dB or more below the currently licensed analog effective radiated power (ERP), or at least 3 dB or more below the values indicated in 90.205, the safe harbor tables.
The interference is affecting only Digital Mobile Radio (DMR) deployments in VHF frequencies between 150 and 174 MHz, said Brownlow. “It’s only noticed in the VHF frequencies because they aren’t paired,” he said. “Some of the interference has been reported from stations 140 kilometers apart.”
The PSCC said in February it received reports of harmful interference from users around the country from TDMA systems. One instance was a channel successfully shared when the use averaged around 10 percent and now approaches or is at a 100 percent duty cycle. Another source of interference is false triggering of certain continuous tone-coded squelch system (CTCSS) tones in analog receivers, presumably by harmonics of the TDMA framing rate, which fall on or close to CTCSS frequencies, opening receivers and causing constant noise in affected channels. The February letter from PSCC to the FCC is available here.
Monique Princen, chair of the DMR Association marketing working group, said the association is aware of the reports. “Based on our member’s know-how and experience with customers using DMR and analog equipment on shared frequencies worldwide, both the reduced available duty cycle for analog users in shared systems as well as false CTCSS triggering issues can be prevented,” Princen said.
She said the reported instances of false CTCSS triggering are extremely low. “Users looking to migrate to digital and to share a channel with analogue terminals should have few concerns regarding this issue but should discuss it with their supplier to determine if there exists the need to conduct tests to detect any problem.”
The PSCC, a consortium of the four public-safety frequency advisory committees, also recommended that a limitation be placed on licenses, advising affected operators that only voice operations and incidental signaling, such as embedded unit identification and emergency status indication, which do not appreciably extend channel occupancy, are allowed. In February, PSCC agreed to limit VHF TDMA systems to a maximum 10 watts ERP.
The FCC hasn’t yet responded to the PSCC initiative. PSCC also asked the Telecommunications Industry Association (TIA) to conduct testing of all available TDMA hardware operating in the VHF band to assess their impact on existing equipment. TIA hasn’t provided a formal response, Brownlow said.
PSCC is composed of AASHTO, Association of Public-Safety Communications Officials (APCO) International, the Forestry Conservation Communications Association (FCCA) and the International Municipal Signal Association (IMSA).