View Single Post
  #9 (permalink)  
Old 03-18-2011, 11:57 PM
bldavis bldavis is offline
Member
   
Join Date: Jun 2009
Posts: 20
Default

To the original poster - You didn't specify whether this was a vanilla commercial system, or a Public Safety system, and that makes a large difference in terms of budget, and needs. Again, seek expert help. Although there may be a few pros on this forum (and a ton of scanner enthusiasts who really know very little about what goes into the planning and requirements of some of these systems), you need to work with someone right there, with a track record of building out successful systems.

Regardless, there's simply not enough information presented to give you an answer to your question.

Quote:
Originally Posted by flecom View Post
if you are doing simulcast you also need to delay the audio so all three xmitters are sending out the exact same audio at the same time, and your xmitters will need to gps disciplined to minimize intermod in the overlap areas
Yeah, that's not really the story. The time source is more about ensuring consistent frequency and phasing at the base stations, and timestamping packets on digital, or timebasing the comparators on old analog gear. There are a number of schemes for doing simulcast, and they vary by manufacturer and system configuration. The idea isn't to make the transmitters fire simultaneously, it's to adjust phase, carrier, etc to deal with phase distortion or carriers far enough off to destructively interfere. Good system designs also balance transmitter output looking for that magic 3dB receive signal differential.


Quote:
Originally Posted by davidgcet View Post
yep, try setting the simulcast on 10 sites where some locations get 3 or more sites in on overlap. it gets REAL fun then. and if you go telco based expect to redo the simulcast at least once a year because they will move your circuits and change the delay in the lines. i'm glad all my remaining simulcast sites are RF linked and GPS synced now, cause i absolutely hated the old telco links we used to maintain.
It's a good deal easier with modern digital systems, because you get some new metrics to use to help balance things out. Some of them do a form of auto-balancing, most of them have new and far easier methods. The best of them have done away with prime sites altogether, and comparators as well.

And as with all things, it depends on the kind of telco links. In urban areas, you can usually buy fractional fiber optic links at prices comparable to the old DS-x V.35 stuff. And Fiber systems are very stable, and can have performance metrics that make microwave backhauls look like two tin cans on a string. In my case, I constructed my own fiber system between my sites, and in the long term, I saved a fair sum of money over RF backhauls.

Last edited by bldavis; 03-18-2011 at 11:59 PM..
Reply With Quote