Simplex radio interference concern

[Pradio13]

Hello everyone,

I am working on an LMR radio project near the NYC area. The goal is to extent the two way radio coverage from an existing site via a hybrid DAS system to a large warehouse environment using GTR8000 base stations to generate the many simplex VHF frequencies for the client's LMR operations. On this antenna system we are also looking to pick up off-air first responded frequencies in the UHF/800MHz range. This means that the system will work with simplex and duplex frequencies. For the system to cater to all frequencies we have decided to combine all the TX frequencies (off-air TX and base stations TX) and all the RX frequencies. The TX frequencies will be transmitted throughout the warehouse via dedicated transmit antennas and the RX frequencies will be picked up via dedicated receive antennas. Since this site has many metal and concrete obstructions we have to place many antenna pairs(TX/RX) throughout the site. For the UHF/800MHz frequencies I am not so concerned as they are duplex but for the VHF I am worried that the transmit antennas will interfere with the receive antennas as they are receiving and transmitting on the same frequency. What are some ways to avoid this interference? Am I supposed to assume that because the scheme is TDM that I should not have a problem as there is time separation between send and receive? Does this P25 base station offer some DSP where it filters out signals that are received directly from the transmit antenna? Or do I have to rely on low power antennas with sufficient spatial separation?

I am new to the radio design field and not that well versed so any insight is greatly appreciated. Please let me know if my request is unclear and I will do my best to answer any system configuration questions.

Many Thanks!

 

[mmckenna]

Engineering,and installing public safety in-building radio coverage systems requires specific training, skills and in most places, licenses.

Setting up any system that is going to connect to someone else's radio system needs to be done with the approval and cooperation of the system licensee.

Designing/installing this system incorrectly will cause interference to the radio system, which could easily put first responders lives at risk, not to mention the FCC headaches, fines, etc.

I can appreciate what you are trying to do, however if you have these sorts of questions, you are in way over your head. Asking for advice on a scanner hobbyist website sends up a LOT of red flags.

Contact a competent, trained, and experienced engineer to design this system. Most of all, you need to talk to whoever's system you are going to connect this to.

 

[zz0468]

Wow.

The potential to cause interference to other systems is enormous. If your client is an LMR licensee, then your concern starts and stops there. If first responder agencies have coverage requirements in this project, then their system engineers need to address it, not scanner hobbyists on a web forum.

 

[TampaTyron]

I second the above comments......................
As someone who works with large LMR systems on a daily basis, you have many engineering challenges ahead of you. A multiband DAS covering many different frequencies without significant filtering is a steep order. Add in that you are trying to boost simplex frequencies and you now have some serious issues. Can you explain a little more detail about what exactly you are trying to accomplish in relation to actual radio services and systems? TT

 

[12dbsinad]

Well, I can answer at least one your questions. No, P25 will not "filter out" signals. It is a type of modulation and that is all. It does not change the fundamentals of RF, unless you can figure out a way to install a square tire on your vehicle and have it ride really well.

Other than that, questions like these do raise major red flags. Exp with pubic safety systems.

 

[KK4JUG]

While there are participants in this forum that have tremendous knowledge on a variety of technical issues, your situation is fraught with so many different potential problems, your best bet is to go to an expert where you can talk face-to-face.

 

[Pradio13]

Sorry I misspoke, I was not referring to P25 being able to filter out frequencies but rather the capabilities of the actual base station. This system is already operational in an adjacent site with all the parameters that I described. I am trying to tap into the existing head end with fiber to the client's other site that has the same first responder and operational frequency requirements.

I am part of a team of other professional radio engineers that are also designing the system but since I am new I am doing most of the legwork and research to get a better understanding of the implications. Searched the web countless times for similar designs and cannot seem to get a compelling answer other than it is very difficult to achieve a unified solution. Reaching out to the designer of the original system is not an option.

The way the previous contractor seems to have done it, is with a DAS that uses low power radiating cables with dedicated UL and DL paths. Even though this approach works, the client has expressed that installation of the radiating cable was not preferred as it was very labor intensive. I have been looking at using directional antennas but as it was described below the potential for interference is great.

I should have simplified my questions to the following:
What methods can you use to mitigate interference between the UL and DL simplex frequencies?
why would there be interference between the other frequencies if they are on other bands whose channels are duplex?

I realize that my knowledge is limited which is why I am trying to reach out to as many knowledgeable people as I can.

 

[mmckenna]

What methods can you use to mitigate interference between the UL and DL simplex frequencies?

Antenna separation, and lots of it.

why would there be interference between the other frequencies if they are on other bands whose channels are duplex?

Unless you are using a channelized system, it's probably the lack of filtering. If this is a bi-directional amplifier system, you might be expecting too much from it. A purpose built channelized system is really the way to go. Improperly designed and installed BDA systems can cause a lot of issues.

I realize that my knowledge is limited which is why I am trying to reach out to as many knowledgeable people as I can.

I understand, but this is really not the right place to be asking these sorts of questions. While there are some of us here that are knowledgeable about this sort of stuff, the site is generally aimed at scanner listeners and amateur radio operators. You'd be better off asking this sort of stuff on an appropriate forum.

If you are trying to interface to fiber, I'd have to ask if the system is designed for that? If it is, contact the manufacturer and talk to them. This needs to be a carefully designed system.

 

[Pradio13]

mmckenna and everyone else responding to this thread. Thank you for your advice, I am trying to find a dedicated DAS forum. In the meantime, on the topic of simplex frequencies, since I am dealing with simplex channels which rely on TDM, doesn't that mean that the nature of this scheme can limit interference since the RX/TX does not occur at the same time?