Some things to think about if you have a MSCP system with an unequal number of repeaters at each site

A Multisite Capacity Plus system with an unequal number of channels on each site is a valid configuration. This would be the choice when there is an uneven distribution of radio users at each location or in a situation where a proportion of users will seldom go to certain locations.

The above example (fictitious) system consists of three sites. Site 1 has three repeaters (channels); site 2 two repeaters and site 3 one repeater. 

There are 150 radio users on the system. Around two-thirds of these users only work in the area covered by Site 1 (Roosendaal) with the rest of the users mostly located at the facility covered by Site 2 (Breda).

Multisite Capacity Plus is different from Capacity Max, in that talkgroup calls are streamed statically to preconfigured sites, even to sites where no talkgroup parties may be present. This means that some planning is needed, since you do not want to occupy timeslots on sites where there are no radio users listening. Multisite Capacity Plus also uses all start: this means that a multi-site talkgroup call will only go ahead if a timeslot is free on all sites.

Remember that there is no Trunking Site Controller or Switching and Management Infrastructure to process mobility information (i.e. which radio is on which site and which talkgroup does that radio want to listen to). With some careful planning, the system can be optimised to ensure a good user experience.

This example system has 16 talkgroups. Most talkgroups will only be active on site 1. Some only on site 2 while a few talkgroups could be on any (all) site(s). To ensure talkgroup call voice traffic is limited to relevant sites, there is a table in the Master Repeater, which can be used to assign specific sites to any talkgroup.

This table only needs to be provisioned in the Master Repeater. The Peer Repeaters will get this information when they register upon power up. Also, should the Master Repeater go down, the Peers will continue to operate normally with minimal interruption to service.

While the above table will prevent talkgroup calls from streaming to unwanted sites, it doesn’t prevent users on those talkgroups from roaming to sites where the currently selected talkgroup is not streamed. This might not be good, as the user will wonder why they get signal but can’t hear anyone or make a call.

To prevent this, several Capacity Plus Site Lists can be created in the radio codeplug. These site lists would then be associated with the Talkgroup(s) via the Personalities in the Zone. 

In the below example, there are three Capacity Plus Site Lists. Site List “All Sites” contains the site number and Voice List of all three  sites in the system. Similarly, Site List “site 1 and 2” contains only site 1 and 2’s details and Site List “Site 1 only” only has the details of site 1 in it.

The Personalities in the Zone then refer back to the above three Site Lists. So, when the user changes the channel, a different site list is used. This means, that if the user is on Talkgroup 10, the radio will never roam to site 2 or 3 since the Site List used only refers to Site 1.

A possible problem with any multisite radio system, is that one or more RF sites may have better coverage than others. This could result in radios roaming to sites that aren’t intended to serve that location. This may not be desirable as talkgroup voice transmissions originating from any site will consume more upstream bandwidth and if a site has very good coverage; the statistical probability that more radio will roam there, is higher.

Also, private calls made to/from radios at a site will consume one timeslot. Since the number of slots on Site 2 and 3 are less than Site 1, it makes sense to keep as many radios on Site 1 while they are within its coverage area.

The coverage from Site 2 is significantly greater than the coverage from the other two sites. This needs to be compensated for in the radio.

In Multisite Capacity Plus; with private call, only the sites where the call parties are roamed to, will be used (lit). So if many radios have roamed to a specific site and those users make a larger number of private (individual) calls, the likelihood of that site being busied is a lot higher.

The solution for this is to use a Per-Site (Roaming) RSSI Threshold. In this case, the RSSI Threshold for Site 2 (Breda) is -105dBm whereas the other two are -108dBm. This is in the radio's codeplug.

IP Site Connect and Multisite Capacity Plus use a process called roaming to automatically select the next available RF site, as the radio user moves through the towns and cities covered by the system. This process involves measuring the signal strength of each RF site and selecting the one with a signal that is above this threshold.

Setting the RSSI Threshold higher means that the signal from this site; must be comparatively stronger than the signal from the other two sites, in order for the radio to select it. 

In Multisite Capacity Plus, an idle radio uses the rest channel beacon to measure signal strength (RSSI). Every 5th beacon transmission from a site contains information about the neighbouring site(s) that saves the radio from having to scan all system channels thereby speeding up the roaming process.

The RSSI threshold is in dBmV and is therefore logarithmic. A signal of -105dBm is twice as strong as a signal of -108dBm. A MOTOTRBO radio will be able to receive signals when the RSSI is as low as -123dBm but -108dBm is a safe default value to start with.