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8657Suggestion to improve programming of hybrid P25 systems

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  • Milton Engle
    Dec 24, 2013
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      For UPMan, something for the engineers to think about.
      The current layout of programming a P25 site has only a global option to turn attenuation on or off.
      A Motorola provided P25 system I monitor is a hybrid, with one site that is standalone (single group of frequencies) and the other consisting of several locations that are Linear Modulation Simulcast (same frequency set at multiple locations).
      From a fixed location it is easy to simply pick from either the standalone site which gives very good decoding performance, or if the standalone site cannot be heard, the simulcast group which is a crapshoot for decode performance.  If a location is near to the receiver and other locations provide significantly lower signals then the decode performance is fair to good.  If however the scanner receives signals from multiple locations at similar levels the decode performance drops like a rock.  From my research Motorola systems using LSM are set up to have all sites in phase at transmit, which means that the signals at a radio location may be somewhat out of phase when received.  This leads to a limitation of how far apart sites may be located.  P25IP systems from Harris appear to use more conventional simulcast technics which focus on limiting the overlap of multiple signals and attempts to keep the signals in phase at the receiving location.  
      I have found in programming that adding attenuation in LSM systems MAY help, or it may not; each instance is unique. 
      With regard to the above mentioned hybrid system, one option is to program 2 sites, however this usually means that in areas where both the stand alone site and the simulcast group sites are heard the scanner wastes time trying to scan one site, then the other.
      By combining both sets of control channels into a single site the scanner will do a fairly good job of staying on the site that gives the best decode performance and this has been the method I have chosen for portable and mobile scanning. 
      What would be nice would be the ability to select attenuation based on the control channel frequency entry rather than simply by the site.   This would allow me to attenuate the LSM control channel frequencies while keeping the standalone frequencies at maximum sensitivity, thus taking advantage of the best decode on the non-simulcast channels.
      Probably not a firmware option since this would significantly change the menu structure, but a suggestion for future product releases.
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