Dual Capacity Direct Mode is a feature available in all second generation MOTOTRBO radios which allows two slot operation on simplex. Not only does it offer two slot operation but is also currently the only product which complies with ETSI TS102-361 §1-4 v2.1.1 for 6.25e operation on simplex.
DCDM - also known as 6.25 kHz Equivalence Direct Mode (6.25e DM) - provides additional spectral efficiency by supporting up to two simultaneous Direct Radio-to-Radio Mode subscriber calls within a 12,5kHz allocation of spectrum.
|The CPS DCDM settings in a simplex channel|
In DCDM radios automatically and cooperatively select a Channel Timing Leader for the RF Channel (regardless of slot). So that all radios receive the Direct Mode Channel Timing signalling communication, this signalling is communicated using the “All Site” Color Code (15).
In the CPS, there is one major setting relating to DCDM: the Timing Leader Preference. There are three choices: Preferred; Eligible and Ineligible. A likely candidate for Preferred would be a base station because it doesn’t leave the channel; is connected to the mains (or a large battery) and has better coverage on simplex.
One of the subscribers with the higher leader preference setting (usually Preferred) is elected as the Channel Timing Leader and if that leader leaves the channel, the system selects a new leader. The Channel Timing Leader periodically announces the Channel Timeslot Structure using special Beacons (600ms transmission every 4,5 minutes). If the timing is accepted, it is then re-transmitted using “Forced Propagations” to the next lot of radios further away from the leader.