Some thoughts on the Handover and Roaming RSSI Threshold in Capacity Max

In a previous post, I wrote about the Acceptable RSSI Threshold. There are two more settings that will affect radio roaming behavior on a Capacity Max system.

The Handover RSSI Threshold setting defines the signal level at which the radio will preform an in-call handover. This will happen when the signal strength of the current payload channel (a.k.a. traffic channel) is lower than this value; at least one adjacent site is carrying the talkgroup call and has a signal strength of more than 6dB higher than the current payload channel.

The Roaming RSSI Threshold (not to be confused with the setting of the same name used in IPSC and Capacity Plus) determines when a radio should consider roaming to another site when idle. When a radio is registered at a site (a.k.a. its home site), it periodically measures the current control channel signal strength. 

If this drops below this threshold, the radio then samples the adjacent site's control channels. 

If the signal strength of home site's control channel is equal to or higher than this threshold, the radio will not sample adjacent site control channel because roaming is not needed.

To support the measurement of adjacent site signal strength, the system sends a Vote Now Advice PDU that gives radios the opportunity to leave the current control channel and take measurements of an adjacent site's control channel signal strength. During this time, the system will not place calls on that site until the radios return to the home site control channel.

Generally, the Handover RSSI Threshold value should the the same as the Roaming RSSI Threshold value. However, if the Handover RSSI Threshold is set lower than this, the radio will remain on the home site for a little bit longer and might accumulate more adjacent site signal strength measurements before making the move.

While in a talkgroup call, the radio periodically measures the signal strength of its current payload channel (a.k.a. traffic channel). If this signal strength becomes lower than the Roaming RSSI Threshold for the home site, the radio samples the signal strength of adjacent site payload channels carrying the same call.

Therefore, increasing the Roaming RSSI Threshold increases the roaming-in-acceptable area and increases the chance for the radio to roam. It also decreases the distance from the site where the radio starts roaming.

Decreasing the Roaming RSSI Threshold however, decreases the roaming-in-acceptable area, increases the no-roaming area, and increases the distance from the site where the radio starts roaming.

In essence, a lower Roaming RSSI Threshold encourages the radio to stay at the home site longer.

Even if an adjacent site has a higher preference level, a radio within the no-roaming area of the home site will not attempt to automatically roam.

RF sites in a Capacity Max system can be assigned one of five preference levels. If a radio finds more than one control channel with the same signal strength, it will use its configured preference level to determine which site to roam to. At a system level, this has the effect of reducing the load on sites which, because of coverage or location, may otherwise become overloaded.


[The stars are needed because the screenshots came from a customer configuration.]