Method for inter operation of a cellular communication system and a trunking communication system

In a geographic region that contains a trunking communication system and a cellular communication system, where the coverage area of each system substantially overlaps, a method that enables a communication unit to operate in either system is disclosed. Normally in accordance with the present invention, a communication unit will monitor a control channel of the cellular communication system. When the communication unit is needed within the trunking communication system, a communication channel controller of the trunking communication system will place a telephone call to the communication unit via the cellular communication system. The telephone call indicates to the communication unit that it should transfer its affiliation from the cellular communication system to the trunking communication system. Once the communication unit has transferred its affiliation to the trunking communication system, it operates as a trunking communication unit.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
FIG. 1 illustrates a trunking communication system (101) and a cellular 
communication system (100) operably coupled together in accordance with 
the present invention. The cellular communication system (100) comprises a 
mobile telephone switching office (MTSO) (109) and a plurality of cells 
(103) (3 shown), where each cell is operably coupled to the MTSO (109) and 
has a specific coverage area (107). Each cell comprises a limited number 
of communication channels (105), which may be carrier frequencies, 
frequency pairs, and/or time division multiplexing (TDM) slots, wherein 
one of the communication channels is designated a control channel (106). 
The summation of the coverage areas of each cell (107) comprises the 
coverage area of the cellular communication system (108). The cellular 
communication system also comprises a plurality of communication units (2 
shown), where the communication units are equipped with one receiver and 
transmitter that has adequate bandwidth to operate in both the trunking 
communication system and the cellular communication system. 
The trunking communication system (101) comprises a communication channel 
controller (110), a plurality of communication channels (111), wherein one 
of the communication channels is designated a control channel (112), and 
the plurality of communication units (104). The coverage area of the 
trunking communication system (113) substantially overlaps, and may be 
approximately equal to, the coverage area of the cellular communication 
system (108), however, for illustrative purposes the coverage areas are 
shown separately. 
The trunking communication system (101) is operably coupled to the cellular 
communication system by one of three methods. The first and most 
convenient method, especially when the communication systems are 
manufactured by different manufacturers, is to couple the systems together 
through a phone line (114) or a plurality of phone lines, to a phone 
system (102). This coupling method allows the trunking communication 
system to transfer messages to the cellular communication system by 
placing telephone calls to the cellular communication system via the 
telephone system (102). An alternative method, is to directly connect the 
communication channel controller (110) to the MTSO (109) by a direct 
connection (116). Finally, if the communication systems are manufactured 
by the same manufacturer, or an agreement exists between manufacturers, 
the communication channel controller (110) may be coupled directly into 
the communication channel links (117) of the cellular communication system 
by a coupling link (115) that is similar to a telephone link. 
Generally, in accordance with this invention, communication units monitor 
control channels of the cellular communication system and are operating as 
cellular telephones. When a particular communication unit is needed for a 
communication within the trunking communication system, the communication 
channel controller (110) places a telephone call to the particular 
communication unit via the cellular communication system. The telephone 
number that the communication channel controller (110) dials is an 
individual telephone number, or a first access number, for the particular 
communication unit. Once the particular communication unit receives the 
telephone call page, it does not answer the phone as in ordinary phone 
usage, but detects the first access number to indicate that it is to 
transfer its affiliation from the cellular communication system to the 
trunking communication system. (A telephone call page may be equated with 
the ringing of a telephone.) Once the communication unit has transferred 
its affiliation to the trunking communication system it operates as a 
trunking radio. When the communication within the trunking communication 
system is completed, the particular communication unit transfers its 
affiliation back to the cellular communication system. 
FIG. 2 illustrates a logic diagram of the present invention. At step (200), 
the communication channel controller (110), when it determines that a 
particular communication unit is needed for a communication within the 
trunking communication system, transmits a first access number, or places 
a telephone call, to the particular communication unit via the cellular 
communication system. The communication channel controller (110), stores 
in a database, the trunking identification number of each of the 
communication units, and in accordance with this invention, would also 
store a first access number for each communication unit as well. The 
communication channel controller (110) may also store in the database a 
second access number, or second telephone number which would represent a 
group call such that several communication units would transfer their 
affiliation to the trunking communication system relatively simultaneously 
and requiring the communication channel controller (110) to place only one 
telephone call. If the particular communication unit does not detect its 
first access number, either do to the communication unit being off or out 
of range, the communication channel controller (110) retransmits the first 
access number from anywhere from five times to twenty times, if the 
communication unit responds, the process continues, otherwise it ends. The 
process may also end if the communication unit is involved in a 
communication within the cellular communication system (201). 
If the communication unit is not involved in a communication within the 
cellular communication system (201), the communication unit's affiliation 
is transferred from the cellular communication system to the trunking 
communication system (202). Transferring a communication unit's 
affiliation primarily consists of changing the local oscillator frequency 
of the receiver and the transmitter such that the communication unit is 
either operable in the cellular band or the trunking band. Once the 
particular communication unit is affiliated with the trunking 
communication system (202), the communication channel controller (110) 
instructs the communication unit to monitor a particular communication 
channel that contains the communication of interest (203). The 
communication unit will remain affiliated with the trunking communication 
system as long as the communication remains active (204). Once the 
communication has ended, which may be immediately after the communication 
channel is deassigned, or after a predetermined period of time has elapsed 
after deassigning the communication channel, for example 1 to 20 seconds, 
the communication unit transfers its affiliation back to the cellular 
communication system (205). Once the communication unit is affiliated with 
the cellular communication system, the process can repeat again for this 
particular communication unit. Note that while this process is being 
engaged for one particular communication unit, multiple communication 
units could also be involved in a similar process such as when the 
communication channel controller utilizes the second access number. The 
process may also be done for multiple communication units on an individual 
basis. In addition, the present invention does not substantially interfere 
with independent operations of the trunking communication system or of the 
cellular communication system. 
When a user of a communication unit desires to place a telephone call, the 
user simply dials the desired number as he would with any cellular 
telephone. When the user does this, the cellular system processes this 
call as it would any other. When the user desires to place a call in the 
trunking communication system, he activates a push-to-talk button, or 
similar process that generates an inbound signalling word (ISW). When the 
communication unit detects an activation of the push-to-talk button, it 
transfers its affilation to the trunking communication system such that 
the call can be placed.