P25 does not have that capability (TDMA two slot single frequency simplex repeater). That is a very new feature of DMR systems.
Often, the term "simplex repeater" is used in the "old sense" - a "store and forward" "Parrot" repeater. That is NOT what I was writing about. In the old legacy sense, a simplex repeater uses a device that stores voice messages and then retransmits them from its transmitter. So, the initiator would hear his own message transmitted back to him after sending it initially.
No LMR public service repeator that I know of uses this technology - mostly just amateur "Ham" stuff.
The dual time slot "simplex" repeater is a very new concept available for use by LMR systems using newer DMR technology.
As far as I know, no P25 conventional (like the Caltrans repeaters you refer to) can use this type of system. Though it might be technically possible in a TDMA P25 conventional repeater setup (I am not even sure that such a system even exists or can exist since P2 is primarily a trunked system) it, as far as I know, does not yet exist in the wild.
So be VERY careful when talking about a "simplex repeater" as that usually conjurs up the ham radio "parrot" store-and-forward systems that are seldom, if at all, used in professional LMR setups.
I imagine those Caltrans setups you saw were normal conventional two frequency systems or else simplex base stations in the conventional sense.
-Mike
I'm familiar with the parrot repeaters. Locally our sheriff's department sponsored search and rescue team used one on "MRA," which is a simplex frequency of 155.1600 used nationwide for SAR's. The sheriff's department has since obtained another frequency for this use as some of the repeater placements in Mono County can interfere with users on the west side of the Sierra. The repeaters I was referring to were not that of Caltrans, but those of Southern California Edison. They utilized a frequency pair, however, I was incorrect in my thinking that I should watch for single frequency repeaters on Edison's new Phase 2 system. When I read your last post I had one of those "duuuaahh" moments. I assume that these single frequency pair repeaters on this trunked system is capable of transmitting the talkgroup on to the base station and other multi-frequency trunked repeaters and in that limited sense are not truly "conventional repeaters." These are largely low level "fill in" repeaters.
Caltrans is using an interesting system design to fill in stretches of highway not covered by high level, mountaintop type repeaters in areas where conventional systems are in place. These are called "roadside repeaters" that are installed in key roadside locations where they can fill in gaps in coverage. The radio user does not have to keep track of the channel the radio is on, the display shows something like "Caples Lake" (a maintenance station near the Sierra Crest) and the radio itself employs a voting transceiver that automatically switches frequencies to the roadside or high level repeater that is providing the best coverage for the mobile's signal. If voting was not utilized for both the reception and transmission of dispatch and base stations as well as the mobile unit, all radio users would be forced to manually switch from multiple conventional repeaters to roadside repeaters to find the best coverage for that mobile's location on those stretches of highway. The number of roadside repeaters in the final buildout, if there is such a thing, will not be overwhelming numerous. These are not going to be utilized in the urban areas where Caltrans already has trunked systems, these being the Bay Area (with two trunked systems for one district) and the urban portions of L.A., San Bernardino, Riverside and Orange Counties. A conventional repeater system is in place for the rural portions of these counties. In San Diego County Caltrans is using the county's regional trunked system for both San Diego and Imperial Counties, both urban and rural. The only major urban area left is Sacramento, which has terrain that allows coverage for conventional repeaters and sufficient channels for existing radio traffic levels. They also have repeaters placed on the top of a tall building or two in Sacramento if I remember correctly.
The effort Caltrans is making to provide just highway coverage on 700/800 MHz, shows that a statewide trunking system using those frequencies will not work. It shows why the CHP is fine with it's VHF low system. I've been in the field on emergency incidents and have been amazed at how well the CHP system works with so few mountaintop remote bases. I was once the IC on a traffic accident involving a several ton high bed stake side truck that was loaded with 20-30 horses. The USFS responded with an engine, closed traffic off from the entrance station at the end of CA Highway 203 and given that the road was USFS owned and maintained, had to have a USFS investigation to complement the CHP investigation. The driver was driving too fast for the curves and low standard design of the road near Devil's Postpile causing the truck to flip onto its side. This is an area in the uppermost portion of the San Joaquin River, which is accessed by road from Mono County. Mammoth Mountain, the Mammoth Crest the San Joaquin Ridge and the Sherwin Range south of Mammoth Lakes all block this road to the east. When you drop down into this area from the Sierra crest it is like dropping into a 2,000 foot deep bowl. The CHP was there and they were able to communicate with Bishop on either the high level remote base on Silver Peak northeast of Bishop, or the relatively low hilltop remote base located just east of U.S. 395 between Mammoth and June Lake. The officer was using an extender also. They didn't need a remote base on top of Mammoth Mountain, which SCE needs and what I was using on the Inyo National Forest VHF high system to operate in this bowl. When the incident scene was stabilized (4-6 horses had to be shot on scene) I asked him about this amazing coverage and he replied that he really didn't know how it all worked, he just pressed the PTT on the mike in his car or handheld. He indicated it worked in nearly every location he was called to.
We have strayed off topic, but I'm learning a great deal here that is going to be useful in programming and listening to some of the newest radio technology we are now beginning to find in our radio hobby. Your knowledge is very useful in this endeavor and I'm glad to have had the opportunity of this discussion.
