Interference blocks communications on county transit authority radio
Interference on the digital, trunked radio communications system operated by the Orange County Transportation Authority, Orange, CA, sometimes makes it impossible for the agency to communicate with its workers within 300 feet of some wireless telephone antenna sites operated by Nextel Communications, Reston, VA.
OCTA has responsibility to provide public transportation in the 797 square miles of Orange County, including 34 cities, unincorporated areas and 2.8 million residents. OCTA provides local, rail feeder and express bus transportation and “paratransit” transportation service. OCTA also provides motorist services including freeway call boxes, towing services and abandoned vehicle abatement. (“Paratransit” service is door-to-door transportation for handicapped or disabled passengers.)
The agency operates an OpenSky time-division, multiple-access radio system made by M/A-Com, Lowell, MA. The system supports voice and data communications, including mobile data terminals.
OCTA filed a report that describes the interference as causing a “total loss of communications” near the cell sites. OCTA and Nextel first conducted a field investigation in March 2000, after which Nextel told the agency that it had made frequency and combiner modifications.
The report, written by Ian Telfer, vice president of engineering for Sunset Beach, CA-based Cinergy Innovations, OCTA’s system assurance contractor, describes the improvement resulting from the modifications as “negligible.”
The OCTA report said that Nextel had offered what it described as a “clear channel” at 851.0875MHz (FCC channel 4) for additional testing. But in its interference report, OCTA said that because Nextel was operating on both the upper and lower adjacent channels, using channel 4 for testing would be “futile.”
Representatives of Nextel, M/A-Com and Cinergy Innovations were not available for comment.
Ed Muncy, OCTA’s project manager, said that the agency has used its OpenSky radio system from M/A-Com in regular daily operations in its bus system since September 2000.
“The Nextel interference problem is an annoyance and needs to be addressed,” he said. “It’s not such a local matter that it affects us, but it’s a matter of national interest because the Nextel issue interferes with taking this technology forward, and this type of technology [digital, voice-over-Internet Protocol, TDMA trunked radio] is an important development in the RF business. So we need to address this interference problem in order not to stifle the proliferation of these types of radio systems.”
David Sikora, a representative of Orbital Transportation Management Systems Group, the prime contractor for OCTA’s 800MHz system, said that the M/A-Com system replaced an earlier Motorola system.
“The new system originally had two sites, and an additional three sites were called for to fill in coverage gaps. We’ve completed two of those additional sites, with one to go,” Sikora said.
Sikora said that Orbital finished installing the mobile units covered under the contract in August 2000, at which time 479 buses were equipped. He said that OCTA since has installed additional units. He said that the entire system has reached the stage of “conditional acceptance” by OCTA.
