Simulcast saturates Missouri county
Apr 1, 2005 12:00 PM, By Jim Barthold
Jefferson County, Mo., has a good claim on the title of “unique,” at least as far as the type of calls received by its sheriff's department are concerned.
The county's northern edge butts up against St. Louis, both the city and the county, where “officers respond to anything you would see in any inner city. We have gangs; we have robberies and rapes and murders and all that stuff you see in an inner city,” said Sheriff Glenn Boyer.
The southern end of the 660-square-mile county is a little less urban.
“We still respond to cattle rustling charges,” Boyer said.
Two-thirds of the county's 215,000 residents reside in unincorporated areas “and rely on the Sheriff's Department for services and the local fire districts and ambulance districts rather than a municipal fire department,” Boyer said.
The county is unique in other ways. The terrain is hilly and laden with mineral deposits that make traditional radio transmissions challenging. Until recently, transmissions from the county's two antenna towers were a hit-and-miss proposition for radio dispatchers.
“The signals just were not penetrating down into the valleys. We had officers that were in fairly precarious situations,” Boyer said. “They would have no radio communications with our dispatchers — sometimes even in the cars off a 200 W car base. We were not able to talk to our dispatchers.”
A state-sanctioned 911 government entity, overseen by an elected board of governors — of which Boyer is a charter member —operates public safety communications in Jefferson County. Tired of worrying about the safety of first responders because of sporadic communications, the board decided to deal with the transmission problem by saturating the county via simulcasting.
The county gave the design and engineering contract to Midwest Radio Systems, a local company that Doug Bell, Midwest's lead technician, described as a “little fish in a big pond in the St. Louis metro region.”
Bell proposed a seven-site receive simulcast system to cover the entire county using five channels: a dispatch channel, a fire channel, an EMS channel and two sheriff repeater channels. The county approved it.
However, there was a problem as the the initial vendor claimed to have simulcast capabilities but didn't measure up to the task — literally. “We have to measure the round trip delay of how long it takes a signal to get into the system, go up to the transmitters and come back to the receiver,” Bell said. “We found that the muxes they supplied weren't holding time. They would jump around on us.”
So Bell, under the county's close scrutiny, chose Harris Corp. instead.
“The only company that could make this work was Harris with their SynchroCast [simulcast] option built into their multiplexers,” Bell said. “If it hadn't been for that, I'd probably be in jail right now. The sheriff would probably lock me up because his three-quarter-of-a-million dollar system wasn't working.”
Harris had worked with simulcast before and knew how to fill the Jefferson County dead spots with live radio signals. Using simulcast, the county transmits the same signal from multiple transmitters at the same time so that it doesn't matter which transmitter the receiver hears.
While simulcast works for Jefferson County, it's not the only way to solve a dead zone problem. Other communities have blasted more power from a signal transmitter to cover the space, which, while a viable option in some instances, may overpower the approved transmission zone in others. Another method is to use smaller transmitters and less power but send signals over multiple channels; however, channels are tough to come by, and getting more is often impossible.
Simulcast, though, enabled Jefferson County to “go with lower-powered transmitters scattered throughout the area and get better in-building penetration in those local areas,” said Bob Brown, business development manager for Intraplex products in Harris's networking and government solutions group.
The simulcast system in Jefferson County started with three sites and expanded, so “now they can use a lower power and maybe directional antennas to cover geographic areas they really need to cover,” Brown said.
With simulcast, he added, timing is everything.
“You tie all these transmitters together via some type of data network like a T-1 or microwave so you can launch all the audio from all the transmitters at the same time — under 50 microseconds difference,” Brown said.
To make that happen, the system must sync up the signals, no matter what type of transport is used.
“It's not always the same,” Brown said. “A T-1 system from the telephone company does not always have the same delay based on the routing of the T-1, and there are some microwave systems that do not have a stable delay through them.”
Harris's system addresses the challenge by using a global positioning system to automatically time the delays and adjust for any network changes. The biggest problems occur in overlap zones where two transmitters with about equal signal strength could confound the receiving radios, Brown said. That's where the SynchroCast technology comes into play.
“When you're in an overlap zone, where either one of the transmitters has that much higher power over the other, your receiver is listening to both, flopping back and forth, going from one transmitter to the other. As long as the audio is lined up, and the frequency is lined up, the receiver doesn't really tell the difference,” he said.
Harris's close relationship with M/A-COM also helped save the county money. The incumbent radio provider was prepared for simulcast, and, even more important, for the way the Harris system works.
“We put some special software in the radios to make them function better in overlap,” said Dave Brown, M/A-COM's senior principal engineer. “We put that in all our radios, not just radios that are going to be used on simulcast systems.”
The software handles a dual problem: simulcast overlap and its sibling, multipath interference.
“An overlap region looks a lot like multipath, where you have a signal from a tower and a signal bouncing off a mountain,” Brown said. “Simulcast gives you a manmade version of multipath in known locations, and the key to getting a good simulcast design is controlling where those overlap regions are and minimizing the size and time difference in them.”
So while the county spent nearly a million dollars for transmission and tower equipment, it didn't spend a penny on new radios to tie together nearly 30 countywide agencies and central dispatch.
“Everything married up to this system,” Boyer said. “It was not a matter of having to dump everything that we previously had and install new radios in patrol cars and issue new walkies or anything like that. We kept all of our old equipment, and that married right up to the additional towers and the simulcast system.”
Harris's Brown said that's part of what makes the SynchroCast system unique.
“We don't care what kind of radio system is there,” he said. “It allows a police department with existing equipment to then simulcast as opposed to going out and buying a brand new radio system from a major manufacturer to install it as a simulcast.”
While the system is not new, the way it's being used by Jefferson County is unique.
“To do so many channels and do so many sites … it's not unprecedented for a lot of big areas, but it's a pretty good sized project for a county that's a medium-sized county, even in Missouri,” Bell said.
Best of all, said Boyer, it works.
“We now have officers that can talk on a hand-held radio that couldn't talk on a mobile unit from inside the patrol car prior to the implementation of this new system,” Boyer said.
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