DMR Association Responds to TDMA Interference Allegations
Wednesday, July 11, 2012 | Comments
Photo courtesy Hytera Communications
The Digital Mobile Radio (DMR) Association responded to allegations of TDMA interference issues in the VHF spectrum, saying the problem isn’t technology based but rather a spectrum and system design problem.
“DMR is a TDMA technology which offers the principle benefit of two simultaneous and independent talk paths in one single 12.5 KHz channel. DMR also provides comprehensive data and application support. Combining these two benefits results in many DMR deployments providing voice and data simultaneously in one RF channel,” said Monique Princen, chair of the DMR Association marketing working group. “In analog systems, the simultaneous transmission of voice and data impacts voice priority and/or voice quality. Many analog users have abandoned or significantly reduced the use of data applications such as vehicle location reporting on their radio network because of these negative side effects. A migration to DMR has made it possible to support voice and data on the same channel, yielding greater spectrum efficiency and improved quality.”
In May, 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. An FCC ruling on the issue is expected in coming weeks, according to industry sources.
"While DMR systems are deployed in multiple bands, the interference cases reported center in the U.S. VHF public-safety shared spectrum environment and comprise only very few instances of the DMR public-safety system deployments in that band. The issues mentioned in the PSCC letter have been observed exclusively in the U.S. VHF band where multiple networks use the same frequencies on a coordinated 'best case' but non-exclusive basis," said a letter from the DMR Association. "In this particular band, there is no systematic pairing of transmit and receive frequencies, and as such a base station can be configured to transmit on the receive frequency of a second base station. This peculiarity represents a major challenge for system planning, regardless of the technology employed.
"U.S. DMR users are taking advantage of the benefits resulting from the standard’s data communications features, including GPS and AVL applications. In the past, only about 10 percent of mission-critical communications network traffic might be data transmissions; in case of unrestrained use of data service, 70 to 80 percent of the network traffic on DMR systems can go for data. For this very reason, DMR data services can be configured to limit the access to the channel in accordance to the resources available," said Princen.
John Rayfield Jr., president of Rayfield Communications in Springfield, Mo., said his customers are implementing the technology’s data features so there is more traffic than before. “Very few agencies or businesses previously used AVL around here unless they had a stand-alone data system,” Rayfield said. “When it comes free with the radio and you just add the software, a lot more people are using it.”
The PSCC coordinators said they will 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.
"We believe what is happening with the 3 dB reduction is an unfair treatment of DMR that penalizes system coverage,” Princen said. “If we would have a technical problem, we would understand it, but this is a design problem. If the system is designed properly, you can avoid this.
"The solution to any particular issue that a user may be experiencing should be resolved by working cooperatively with their respective system supplier and applicable frequency advisory committee as needed." Princen said. "In the DMR Association’s view, the few cases of interference reported compared to the multitude of DMR systems deployed indicate that resolution of any issues needs to be applied on a case-by-case basis instead of by broad restrictions that would negatively impact all DMR users."  
Motorola Solutions has the largest installed base of DMR systems in the United States.
“For a user looking to migrate to digital, the design of the system must take into account that the system runs on shared frequencies,” Princen said. “DMR has all the features to be called a polite system.”
The DMR Association clarification document is available here.
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