The (other) RSSI Threshold

To facilitate the use of a shared physical channel, a repeater in an IP Site Connect; Capacity Plus  or Linked Capacity Plus system, will monitor its RX frequency and will not transmit if the RSSI from signals from another radio system is greater than a configurable threshold. This ensures that the system will not use a channel if another nearby system is currently using that channel.

This RSSI threshold is CPS configurable anywhere between -40dBm and -130dBm. The threshold should be chosen wisely as:
  • Setting it too low may result in interference from background noise inhibiting a repeater from transmitting. 
  • Setting it too high may result in a channel being used while interference is present. In Capacity Plus and Linked Capacity Plus systems, this would result in data messages and call setup requests being missed.

RSSI Threshold in a Capacity Plus or Linked Capacity Plus Channel.

While setting up the system, or performing a FAT, the RSSI Threshold can be set to -40dBm. Once deployed on site, the RSSI Threshold should be set to a reasonable value, based on the average noise level on site. Setting the the RSSI Threshold to this high value, while setting up and testing in the laboratory, will eliminate RSSI Threshold  as a root cause for the system failing to operate as expected.

RSSI Threshold in a Conventional Channel.


Using RDAC, one can measure the average signal level, under no traffic conditions, on the repeater channel. It is important to only consider the RSSI Threshold while there is no traffic on the channel. It is also important to observe the RSSI Threshold over several minutes to ensure a reliable "noise floor" value is obtained. Once this is done, the RSSI Threshold can be set  to 10dB above this "noise floor". It can also be tweaked at a later stage if anything changes.

4 comments:

  1. Hi

    What about Connect Plus and the other RSSI Threshold?

    ReplyDelete
    Replies
    1. I don't think this is used.
      Then again I must also confess to not being a Connect Plus expert.

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  2. I want to add that this threshold is a real PITA and in one-repeater-per-site systems useless anyway. We operate a world wide network with thousands of such sites, and setting this value too low is a common issue when Motorola repeaters are used. Also people usually do have no clue what to look for when their repeater refuses to repeat, and even experienced people regularly fail in identifying this value as the reason for problems.

    For such installations I only can recommend setting the value to -40 dB. A one-repeater-site anyway has no reason to disable a repeater, because a repeater with noise is better than no repeater at all :)

    ReplyDelete
    Replies
    1. Actually setting the RSSI threshold to -40dBm will create other problems: some users will not always get a channel grant; data and GPS will not always work; Transmit Interrupt will often fail and possibly some others.

      My recommendation is to always set this to somewhere between 3 and 10dB above the average noise floor. So if the RSSI measured during no traffic is around -125 then set the RSSI threshold to something in the range of -122 to -115dBm.

      Setting the RSSI threshold to -40 only makes sense if the repeaters are all next to each other in a laboratory and you want to test the system functionality. Once this is over, the RSSI threshold must be set to a reasonable value (e.g. -110) and then adjusted accordingly once on site.

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