I'm using a commercial propagation analysis software to analyze a legacy VHF simulcast Land Mobile Radio (LMR) system. I'm trying to analyze the parameter "Delay Spread". The software gives a figure of merit in micro seconds (uS) for analog FM analysis. I understand that delay spread in this context is a measure of how a signal transmitted from multiple sites interferes with itself due to phase delay, but I have no idea what is acceptable for analog FM. The software manual refers to TIA TSB88, but reading that only seems to give me acceptable figures for digital systems. Possibly I'm just not understanding the documents methodology.
I was also thinking maybe I'm the threshold is based on what delay spread in microseconds causes the phase cancellation to push the signal below the acceptable signal of noise floor ratio? Which if that's the case, is there a published minimum acceptable signal of noise floor for Analog FM?

  • \$\begingroup\$ It's not a measure of phase shift, it's a measure of how, if you have more than one path your signal can take, the flight-time difference between the fastest (often direct/line of sight) and the slowest (often longest, reflected off faraway big structures or reflected multiple times in an urban canyon or such) path are. You don't usually evaluate that - it results from the physical scenario you're working in \$\endgroup\$ Jun 19, 2022 at 19:50
  • \$\begingroup\$ It's definitely not a figure of merit! \$\endgroup\$ Jun 19, 2022 at 19:51
  • \$\begingroup\$ @MarcusMüller This is for a simulcast system, where you have multiple transmitters in different geographic locations, transmitting the same signal. So in this case it's not multipath from the same transmitter.. the measurements is used to determine where the overlapping coverage from the multiple transmitters that are transmitting the same signal, is going to be unintelligible. That way you can adjust timing, power, or location of the transmitter. \$\endgroup\$
    – Frank
    Jun 19, 2022 at 23:42
  • \$\begingroup\$ @MarcusMüller: Not quite right. It's really a question of the zones where the signals from two transmitters have approximately the same signal level at the receiver, within the capture window of the receiver. Outside these zones, the receiver simply captures the stronger transmitter and there is no problem. The trick is to "tune" the modulation delays of the individual transmitters so that the delays within the overlap zones are as equal as possible. Or failing that, "push" the poorly-performing zones to areas to places where it doesn't matter as much (open fields vs. buildings and streets). \$\endgroup\$
    – Dave Tweed
    Jun 19, 2022 at 23:43
  • \$\begingroup\$ At one point in my life, I built the telecom equipment to support such systems. We could tune delays with a precision of 1 us, using GPS signals as timing reference at each site. But I really couldn't tell you anything about how the system engineers evaluated the overall system performance. From my conversations with them, I gathered that there was a lot of "black art" to it. It involved studying the topography of the region to be covered and picking both transmitter placement and transmitter power levels optimally, in addition to tuning the delays. \$\endgroup\$
    – Dave Tweed
    Jun 19, 2022 at 23:45


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.