Simulcast rebuild
The revamping of a 30-year-old legacy public safety network requires upgrades in interconnects, controllers, radios and clear communications about software
Reworking a public safety radio communications system while it remains in service is a tough task. Here is a little story about a walk in the “100-acre wood,” [well, in San Luis Obispo County, it’s really about 3,300 square miles] and how we are upgrading and rebuilding our simulcast public safety radio networks to provide better service.
An oxymoron? Some statements are self contradictory. I’m sure you have heard jokes about “jumbo shrimp” or “military intelligence” as examples of oxymorons. Ten years ago, “simulcast solutions,” would have been considered an oxymoron. The ideas were good, but the implementations were difficult, and the results were often poor. Today, many good solutions are possible, and it is no longer so difficult. Today, you can purchase completely packaged simulcast systems or stitch together your own from a variety of suppliers.
The legacy system The County of San Luis Obispo, CA, has three public safety networks and one general business simulcast radio network constructed from a mix of technology. Our networks have been in service since 1972, when the hardware was less robust than the engineering designs. The county implemented a system with tube-type transmitters, quartz high-stability oscillators (HSO) and bulk delay-line time correction with an analog microwave radio interconnect.
In 1972, the model was the Orange County, CA, public safety simulcast radio system, and the original and successful work of Gary Gray. For our own implementation, the concept worked better in theory than in practice because we have such rugged topography.
Over the years our system was upgraded various times, as technological improvements became available. Now we’re upgrading again. We are stitching it together with a variety of existing equipment and new suppliers.
Project goals “Reconfigure, improve and maintain service” are the project goals. The plan to accomplish these goals includes reconfiguring our microwave interconnect system from a loop into a hub-and-spoke “star” layout to simplify simulcast issues such as timing. This also shortens some paths and provides a better interconnect system.
Improvement of the quality and coverage area is important because the county is such a rugged and difficult area to cover. We must accept that coverage of 90% of the area, 90% of the time (90/90) may never be accomplished with a reasonably priced system. We plan for as close to 90/90 in the main population areas as we can accomplish.
Maintenance of service during this rebuild project is essential because our business is public safety radio support. We must keep the existing systems running except for brief outages.
Project description – Replace microwave radio interconnect – Replace, replace, replace. The first item to replace is the microwave radio interconnect, changing from an analog looped system to a Harris Constellation digital system. Digital gives greater control over the timing and phase of the connected radios. The Constellation is a new, software-defined and software-operated radio with the necessary monitored hot standby and redundancy features for carrying public safety radio traffic. We cut traffic over to the system in January. – Replace radio sets – Successful simulcasting requires that all radio sets be of the same type, so we are replacing many units. When new Ericsson Mastr-III replacement radio sets are received, they are tagged for system and site placement to keep the same production and version all together in a given system. The fewer variations within a system, the better the simulcast results. Soon all the systems will have matched radio sets. Installations are difficult because of facility space constraints. – Replace HSO with GPS – The schedule for replacing our not-so-stable HSO with GPS-disciplined oscillators is on target. Over the years, the “high stability” in the HSOs has been a problem. At 460MHz, a little drift in the channel oscillator creates a big problem. With the advent of GPS for time and frequency lock, stability is superb and constant. GPS is a godsend for simulcast. San Luis Obispo now operates Zyfer Odetics Telecom Accusync GPS-disciplined reference oscillators at all the simulcast sites. On some older radios, we install external synthesizers to allow use of GPS reference. -Replace time correction with DSM-II – The last necessary task is to replace “bucket-brigade,” manually adjusted, time correction with Motorola DSM-II channel and time-delay adjusting cards. This technology takes care of all the time-delay correction, with the correction tied to GPS. The technology is self-correcting for system variations and reasonable amounts of route switching. These cards are specific to the microwave radio Integrated Multiple Access Channel System (IMACS) channel banks by Premisys (Motorola Tensr). IMACS channel banks, with conventional four-wire E&M and two-wire FXS channel cards and DSM-II channel cards, are now installed at all simulcast site locations.
Figure 1 on page 44 shows the locations for the major radio sites. (Some receiving and fill-in sites are not shown.) The county’s southeast corner is sparsely populated because the San Andreas fault defines the east county line. It is not a nice place to live. It also lacks good radio system coverage.
Main site scheme The San Luis Obispo main terminal site contains all necessary hardware and systems to initiate, control and test our simulcast system. In the main site diagram in Figure 2 at the right, you may notice that we left out the ordinary details of receive audio via voter panels, and just showed the essential simulcast information. In our rebuild, the Motorola Conventional Simulcast Controller Interface (CSCI) does much of the “heavy lifting” to make the system work. The CSCI accepts the outgoing audio stream, adds CTCSS (if desired), adds reverse burst (if desired), performs pre-emphasis, takes care of limiting and passes along the push-to-talk (PTT). The Motorola Simulcast Distribution Amplifier (SDA) takes this signal and splits it up for as many as 10 channel feeds. This scheme does one-point audio and CTCSS processing to modulate all the simulcast radios with exact replicas of the one signal, once it has been corrected for transport-time delay.
Motorola has a well-designed transport-time-delay correction scheme with the Digital Simulcast Module, model 2 (DSM-II), which plugs into the digital channel banks (Motorola Tensr by Premisys). The DSM-II uses the one pulse-per-second synchronization from a GPS unit to set and manage time correction from the main site to the remote, high-level site. Harris Constellation microwave radio interconnects the sites.
A typical mountaintop scheme Our typical mountaintop or high-level radio site scheme, as depicted in Figure 3 below, demonstrates how we connect the DSM-II channel signal to the radio. Note that the GPS unit now provides timing information to the channel card at one pulse-per-second and to the radio reference.
The Zyfer Odetics Telecom Accusync GPS unit provides five 10MHz outputs and a couple of one pulse-per-second outputs synchronized to the GPS clocking, thus transferring internal GPS rubidium timing standards from GPS to our public safety radio network. This GPS-disciplined reference for the radio is a vast improvement over traditional HSO units, and because the sites are all locked to the same “transfer reference,” they never drift. No significant routine maintenance is necessary. (Again, the ordinary receiving path has not been illustrated here.)
Technology inventory Let’s review the technology used in this system rebuild. A digital microwave radio interconnect gives us T1 technology for digital cross-connect and total control of our sites and channels. The GPS frequency reference eliminates all long-term, station-to-station carrier drift. (No more wobbling, rolling beats between transmitters and no more zero-beating oscillators during maintenance.) Motorola DMS-II channel cards by Premisys do all the hard work for the system and allow us to send CTCSS throughout the system. Of course, changing our interconnect from a loop to a star connection is cleaner and more manageable, as well as allowing shorter hops on several paths.
The learning curve As this rebuild progresses, we have learned some things. Rain, sleet and the dead of night (along with almost everything else-including project scope-creep), will delay a project. Deliveries will be delayed. Project technicians will have other priorities imposed on them.
Technical details become important, such as running the GPS on battery power. We do. Another agency we reviewed did not. That system lost sync and “wobbled” every time there was a power outage or a standby generator switchover. Then it took a few minutes for it to stabilize. We use a dc power option and the site battery plant for the Accusync GPS, and it has never “burped” with a loss of synchronization.
Have a lot of attenuation pads on hand. First, the radio receivers are designed to feed about 15 miles of wet-paper-insulated No. 22 wire, thus driving at about 110dBm to 120 dBm. Our interconnect microwave wants to see about 216dBm. Without a 26dB pad, the radio output adjustment, whether digital or analog, is at the end of its range and is difficult to set or adjust. With a pad in line, the adjustment is back into its preferred center-of-range and much easier to set up. We use telco-style “66 blocks” with push-on pad units in place of normal bridging clips. Also, the GPS is usually hotter than the radio reference input requirements and is well served with 10dB or 15dB in-line attenuation.
Software runs everything today, but we find that equipment manufacturers don’t always communicate the software information to the poor technician in the field doing a setup. It is hard to find the one person who really knows the software and equipment interactions and settings. For instance, for the Ericsson Mastr-III radios, we use two different software packages with similar-but not identical-features. One program works in batch mode, and thus the technician cannot “tweak” adjustments in real time. The second software works in real time but does not do all the adjustments the first one can do.
You also are never sure you have the latest version, until factory support gives you a solution to a problem along with the qualification “… If you have the latest version and update; otherwise I don’t know how to solve your problem.”
Putting it all together Simulcast solutions is no longer an oxymoron. The end of our project is near because the last of the required hardware just arrived (six months and one week after the order was placed). As soon as it is all installed, we will have a mixture of great technology providing San Luis Obispo County with improved public safety radio communications. N
