Help, my Capacity Plus system is (almost) blocked!

Although we're not really involved with technical support (anymore), my teammates and I still get to help fix MOTOTRBO systems on occasion. This one came in to one of my colleagues from a Reseller in Europe.

This one was interesting in that one of the lesser-known repeater features was probably at play here.

Problem Description

The Capacity Plus (single site) system we just deployed at a large event; seems to be overloaded. We get a lot of call failures and only the 3rd to 4th call attempt seems to be successful.
The system consists of eight channels (repeaters); 250 radios and a location tracking application using control stations.

Solution

On closer examination, it was found that 50 of the radios had a location cadence (update rate) of 30 seconds.  Also, one of the repeaters had an incorrect frequency pair programmed into it.

Based on a call rate of 10 calls per user per hour and a location update rate of 30 location updates per user per hour, you can see that the traffic far exceeds what Capacity Plus can provide.

Reducing the location cadence to 120 seconds brought the demand back within what Capacity Plus (without data revert etc.) can cope with.

The above screenshots come from the MOTOTRBO System Design Tool.

What about the repeater with wrong frequencies?

From R2.4 onwards, Capacity Plus systems have a feature called Adaptive Rest Channel Rotation. This feature ensures that a rest channel is available, even in the presence of hardware failure or interference. 

The conditions under which Adaptive Rest Channel Rotation; would be useful in a Capacity Plus system, are:

1. There is interference on the receive frequency of one or more repeaters.
2. There is desense occuring on the receive frequency of one or more repeaters.
3. There is a fault with the RF filtering hardware resulting in one or more of the repeaters becoming deaf.
4. A repeater has a faulty receiver.

Essentially, it forces the rest channel to automatically move to another repeater if there is no incoming call within a certain amount of time. The amount of time is variable but the maximum is equal to the Rest Channel TOT value in the CPS/RM.

So in the above system, when the rest channel moves to the repeater with incorrect frequencies, the radios would give an out of range indication. At the same time, the repeater would sense that there are no calls being set up and in less than (in this case) eight minutes, the rest channel would be moved to another repeater.

The minimum Adaptive Rest Channel Rotation time is equal to one beacon interval (see above) plus the subscriber inactivity time (SIT). So ,in this case, it could be that the rest channel moves from the incorrectly configured repeater in as little as 7,92 seconds.

Putting the correct frequencies into the offending repeater and reducing the location update rate made a significant improvement to the user experience.

Had it not been for Adaptive Rest Channel Rotation, the rest channel would move to the incorrectly configured repeater and stay there until someone cycled power on the site or something like that.

The MOTOTRBO System Design Tool is a handy application to have on your PC as it saves you from guesswork and long-winded calculations. It can also confirm whether a radio system is working as expected - as seen above.