Some thoughts on the Acceptable RSSI Threshold in Capacity Max

The Acceptable RSSI Threshold is a configurable parameter in a MOTOTRBO radio operating on a Capacity Max DMR Tier 3 system. It plays a role in determining the coverage area where the strength of the signal received by the radio is sufficient to establish and maintain communication with the repeater, whether for a control channel or a trunked channel. This threshold essentially defines the boundary of the "acceptable signal area" within a site's coverage (even though the coverage might be greater)

The Acceptable RSSI Threshold influences roaming, particularly in a poor signal area where the radio actively roams to find an adjacent site with acceptable signal strength.

It can be configured on the radio within a range from -120 dBm to -70 dBm. (default  is -108 dBm) which is more or less typical for the majority of coverage scenarios.

The acceptable signal boundary should also align with the amount of overlap between RF sites. The optimal setting depends heavily on the specific multi-site network coverage type:

• For Dense Overlapping Coverage (Urban) with generous site overlap,  or for Corridor Coverage with slightly overlapping sites, the default value of -108 dBm is recommended.
• For Isolated No Overlapping Coverage (Rural), where sites are isolated with no overlap, the threshold might be configured as low as -120.00 dBm to maximise the base repeater signal range.
• For Multi-Floor Coverage in tall buildings or tunnels with dense, close sites and generous overlap but potential signal barriers between floors, the default of -108.00 dBm is also recommended.

The suggested threshold values are guidelines assuming ideal conditions like a perfect circle boundary for site coverage, which is not always the case in reality. Adjusting the thresholds to align with the actual field environment based on site surveys and testing is crucial.

None the less, adjusting this threshold from the default requires careful consideration because decreasing it makes the acceptable area larger, thereby increasing the distance from the site where the radio deems the signal sufficient. This might lead to an imbalance in coverage, where the radio can still receive the repeater signal, but its lower-power signal cannot make it back. It makes sense therefore, for the radio to roam before this imbalance occurs.

Similarly, increasing the threshold makes the acceptable area smaller, decreasing the distance from the site where the signal is deemed sufficient. This can cause the radio to enter the poor signal area more easily. If this poor area is not covered by an adjacent site, the radio may struggle to establish and maintain communication with any RF site.

When planning, it's important to aim for balanced coverage where the radio and repeater can reach one another within the roaming area. Failure to achieve balanced coverage can result in radios frequently attempting to roam to adjacent sites where they cannot register because their signal is too weak.

Getting the optimal Acceptable RSSI Threshold is, in my opinion, done partially through careful thought and planning but also , to some extent, through coverage testing and subsequent optimisation.

The coverage map module in Consel Plus for example, allows coverage testing to be automated to some extent: the dispatch application can record the location and inbound (radio-to-repeater) RSSI of all the radios, thereby producing a coverage map.

To get the outbound coverage one can use a simple tool like the MOTOTRBO Site Survey Tool to do receive (repeater-to-radio) measurements together with the location.