Channel assignment decisions in trunked communication systems are carried out by a central controller in response to channel requests from a plurality of communication units.  At times, these trunked systems are installed in geographic proximity with other users of the same channels.  This produces some undesired co-channel interference for both parties.  These co-channel users can be using another trunked system or a conventional system. 
Conventional radio systems add special signalling, such as sub-audible tone modulation on the carrier, to avoid being bothered with other user's traffic.  Turning off this feature before transmitting to avoid interference is quite common.  This enables the receiving speaker to hear any current user on the channel.  If no one is active, the operator can then transmit with little risk of interference.  This method of manual interference avoidance is very common.  Similarly, some conventional radio units provide an option that automatically inhibits transmission if a signal is present at the receiver. 
This method of monitoring the communication channel for co-channel users prior to transmission cannot be readily applied to a trunking system since the communication units never know which channel resource (a voice or data channel) they will be assigned.  Because the system central controller decides which resource to assign, it is logical that interference avoidance in a trunked system could be performed in the central controller.  The centralized interference avoidance method today involves looking for activity on the inbound voice channels at the base station receiver.  Since channels in trunked systems are made up of the same frequency pair (one channel for inbound messages to the base station, one channel for outbound messages from the base station), detecting an inbound signal usually meant that the outbound frequency is active as well.  However, many systems use wireline dispatch centers which do not use the inbound channel to make an outbound call, hence dispatcher co-channel users cannot be detected with this method. 
The present method of signal detection starts a timer as soon as a simple carrier is detected (above a preset signal level threshold) on one of the inbound voice channels.  The timer continues to run as long as the carrier remains present.  A break in the carrier resets and turns off the timer.  When the timer expires, i.e. reaches a predetermined value, say, 50 seconds, the channel is considered to have undesired interference and is taken out of service for a period of time, such as 5 minutes.  This carrier detection method is best suited to detect the class of interference called intermodulation (IM), which is very common at the more congested antenna sites.  It is not suited to instantaneously and reliably detect the presence of a true co-channel user. 
Accordingly, there is a need for a method of co-channel interference detection that can instantaneously and reliably detect co-channel users, especially dispatcher co-channel users, on both inbound and outbound frequencies.