Radio communication apparatus and method for voice channel busy call back

An apparatus and method for issuing busy call-backs (146) on the voice channel to busied communication units (209,211) on a trunked radio communication system (200) is disclosed. This is accomplished, in part, by issuing a busy signal (104), via a control resource (203), to a communication unit when a requested communication resource is unavailable. Additionally, the radio communication unit (209,211) is capable of receiving the busy signal via the control resource (213) and participating in a second communication while in a busied state. The resource controller (201) is capable of allowing the busied communication unit (209,211) to participate in a second communication on a second communication resource (215) and issuing the call-back signal during the second communication.

FIELD OF THE INVENTION 
The invention relates generally to trunked radio communication systems, and 
includes, but is not limited to trunked radio communication systems which 
allow a busied unit to participate in subsequent communications while 
waiting for their busy call-back signal. 
BACKGROUND OF THE INVENTION 
FIG. 2A shows a simplified block diagram of a typical trunked radio system 
200. In such systems, access to communication resources, for example voice 
channels, is controlled by a central resource controller, for example 201. 
This resource controller is usually coupled, for example via wireline, to 
a plurality of repeaters such as 203, 205, and 207. Resources, or 
channels, are requested through a dedicated repeater acting as a control 
resource, for example 203. When a resource is requested by a communication 
unit, such as portables 209 or mobiles 211, an available channel 215 will 
be chosen and assigned via a channel assignment to that unit. Today, this 
is generally accomplished by transmitting data via a control resource or 
channel. An example of such a transmission is given in FIG. 2A and 
indicated by transmission 213. 
If a communication resource is not available, a "busy" will be issued and 
repeated on the control channel until the time that a suitable voice 
channel becomes available and an assignment can be issued. Once a busy is 
issued to, and received by, a communication unit, that particular unit 
cannot leave the control channel to be involved in any other call as it 
may then miss the assignment for the call originally requested. These 
calls may be a variety of types, for example group dispatch, individual 
dispatch, or telephone interconnect calls. The aforementioned voice 
channel requests, busies, and channel assignment grants are accomplished 
by repetitious signals being transmitted on the control resource, or 
control channel. These transmissions on the control resource are known in 
the art as inbound signalling words (ISW), which originate at the 
communication units, and outbound signalling words (OSW), which originate 
at the resource controller. 
In the arena of radio frequency (RF) communication, trunked systems can be 
found in all different sizes. A smaller system may have as few as two 
repeaters, one of which may be used for voice/data transmission and the 
other remaining channel may be dedicated to the transmission of control 
signals, such as OSWs and ISWs. Large scale trunked communication systems, 
on the other hand, may have as many as 28 channels, 27 of which may be 
used for voice/data transmission and the remaining channel used for 
transmitting control signals. In a larger system, such as the one 
described having 28 channels, there may be as many as two thousand 
communication units being served on that system. It is not difficult to 
imagine the control traffic congestion that occurs when even a fraction of 
these radios are requesting access to a voice channel for communication. 
In today's trunked communication systems, a busied communication unit, that 
is, a subscriber unit whose channel request has been denied until a 
channel of the type requested becomes available, is "locked-in" on the 
control channel until his request is granted, or he cancels his request. 
For a group dispatch call, these busies may typically last anywhere from 
one-half second to eight seconds, depending on the traffic load of the 
system at the time of the request. The actual call, defined as that which 
begins with the pressing of the push-to-talk (PTT) button and ending with 
the release of that button, typically last on the order of twenty seconds. 
For a telephone interconnect call, which may last anywhere from five 
minutes to 30 minutes, the busy duration is typically between one and four 
minutes. It is not difficult to see that the typical duration of a busy 
can be a significant portion of the total transmission time for a given 
call type. In a worst case scenario, the actual busy duration may exceed 
the total call duration. This is often the case on a heavily loaded system 
having only one control resource, or channel. 
Accordingly, there exists a need for a trunked communication system that 
can, through the use of reduced OSW traffic and enhanced subscriber unit 
channel assignment methods, make efficient use of limited resources 
available during times of heavy usage. 
SUMMARY OF THE INVENTION 
The present invention encompasses a radio communication system having at 
least one resource controller which is able to allocate a plurality of 
communication resources. This is accomplished, in part, by issuing a busy 
signal, via a control resource, to a communication unit when a requested 
communication resource is unavailable. The communication unit is capable 
of requesting access to communication resources in order to participate in 
a communication. Additionally, the radio communication unit is capable of 
receiving the busy signal on the control resource and participating in a 
second communication while in a busied state. The resource controller is 
capable of allowing the busied communication unit to participate in a 
second communication on a second communication resource and issuing a 
call-back signal directed to the busied communication unit during, or 
after, the second communication.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
FIGS. 1A-B show two communication control packet streams 100 and 120, 
transmitted on the outbound and inbound control channel, respectively. 
Each packet in the packet stream represents a distinct message being sent 
to or received by some part of the trunked communication system. Packet 
stream 100 represents an OSW on the control channel. Packet 102 represents 
an interconnect resource grant for a user, for example user-four, which is 
repeated for the duration of that interconnect call. At the same time, 
packet stream 120 representing an ISW, packet 122 shows user-one making an 
interconnect call request for channel assignment. Assuming there are no 
telephone interconnect-capable channels available, the resource controller 
responds to user-one's request by issuing an interconnect busy at packet 
104. In a typical trunked communication system, this interconnect busy 
signal is repeated in a cyclic manner until the requested interconnect 
resource becomes available. Under these conditions, areas 108 of packet 
stream 100 would contain repetitious interconnect busy signals directed to 
user-one, as in packet 104. In the preferred embodiment of the invention, 
this busy is issued a finite number of times, for example four times. In 
this case, areas 108 would have no such repetitious interconnect busy 
signals directed to user-one, thereby effectively reducing the traffic on 
the outbound control channel. Referring back to packet stream 120, packet 
124 shows a dispatch call request initiated by user-two. Because there is 
a dispatch call channel available on the system, the resource controller 
issues a dispatch call channel assignment grant directed to user-two at 
packet 106. Outbound signalling words continue in a cyclic pattern, adding 
new information updates as they become available on the system. During 
this time, there may be a call request initiated by a user on the system 
which invites the participation of user-one. In the preferred embodiment 
of the invention, user-one is allowed to participate in that call and is 
switched to the appropriate voice channel while remaining in a busied, or 
wait, state for the original request. In order for this to occur without 
causing problems on the system, the resource controller must keep track of 
all such busied (i.e., denied) requests currently on the system. This task 
is performed using a standard first-in-first-out (FIFO) queuing process, 
the order of which queue is dynamically altered by the resource controller 
based on predetermined priority criterion. 
Referring to FIG. 1C, stream 140 shows typical voice data 142 transmitted 
via a voice channel. When an interconnect resource becomes available, the 
resource controller issues an interconnect grant 110 directed to user one 
on the outbound control channel, and then issues an interconnect call-back 
directed to user-one, for example on the voice channel at packet 146. 
Referring back to packet stream 120, packet 126 represents the 
acknowledgement of receipt of the interconnect call-back by user-one. This 
acknowledgement may include the pushing of one or more buttons on the 
communication unit 230 of FIG. 2B, for example the `ACK` button 235, 
followed by either the `PRT` button 237 if the user wants to participate 
in the originally requested call, or the `CNC` button 239 if the user 
wishes to cancel the original request. Other buttons 241, 243, and 245 are 
shown which may be used for other functions in accordance with the 
invention. 
The steps executed by the microprocessor 217 at the resource controller 201 
are shown in FIGS. 3A and 3B. Referring to FIG. 3A, flow chart 300 shows 
the relationship of the requisite steps performed by the resource 
controller in servicing requests made by the communication unit. At the 
time when a request is to be serviced at 302, the routine proceeds to a 
decision at 304, which determines whether or not the request is for a new 
call. If it is a new call request, the routine proceeds to another 
decision at 306, where it is then determined whether or not this call has 
already been put on the queue. If this call has been queued already, the 
resource controller re-issues the busy signal at 308 and the routine is 
exited at 319. In the case where the call has not already been queued, the 
routine proceeds to a decision at 314, where it is determined whether or 
not there is a need to queue this call request. When a communication 
resource is available, there is no need to queue the request and the 
resource controller simply processes the call grant at 318, and the 
routine is exited at 319. If the request cannot be granted, as is the case 
when there are no available communication resources of the type requested, 
the resource controller issues a busy signal at 316 and the routine is 
exited at 319. Referring back to decision 304, if the request being 
serviced is not for a new call, the routine proceeds to decision 310, 
where it attempts to determine if the issued busy is being cancelled by 
the communication unit that has been busied. If there is no such cancel 
request, the routine is exited at 319. If there is a request to cancel the 
busy, the resource controller proceeds by removing the busy from the 
system at 312, and the routine is exited at 319. 
FIG. 3B shows a flow chart 320 that describes the sequence of events for 
converting busies on the system 200, in the preferred embodiment of the 
invention. Busy conversion routine 322 begins by getting, from the queue, 
the next busy at 324. The routine then proceeds to decision 326, where it 
is determined whether or not the busied, or target, unit is currently on 
the control channel. If the target is on the control channel, the resource 
controller sends a call-back signal on the control channel at 328. The 
routine then proceeds to decision 336, where it is determined whether or 
not the issued call-back signal has been acknowledged by the communication 
unit. If no such acknowledgement has been received, the resource 
controller re-queues the busy at 330, up to a predetermined maximum number 
of times, and the routine proceeds to the top of the busy conversion 
routine at 322. If the call-back signal has been acknowledged, the 
resource controller assigns the previously busied communication unit to a 
voice channel at 338, and exits the routine at 340. Referring back to 
decision 326, if the target communication unit is not currently on the 
control channel, the routine proceeds to decision 332, where it is 
determined whether or not the target unit is transmitting. If the target 
unit is transmitting, the resource controller re-queues the busy at 330, 
up to a predetermined maximum number of times, and the routine proceeds to 
the top of the busy conversion routine at 322. In the case where the 
target unit is not transmitting, the resource controller sends at 334 the 
call-back signal on the voice channel that the target unit is monitoring. 
The routine then proceeds to decision 336, where it is determined whether 
or not the call-back signal has been acknowledged by the communications 
unit. If the call-back has not been acknowledged, the resource controller 
re-queues the busy at 330, up to a predetermined maximum number of times, 
and the routine returns to the top of the busy conversion routine at 322. 
If the acknowledge has been received from the communication unit, the 
resource controller assigns a voice channel to the communication unit at 
338 and the routine is exited at 340. 
FIG. 4 shows a flow chart 400 which describes the operation of the 
microprocessor 219 at the communication unit, in the preferred embodiment 
of the invention. Operation 401 begins when the communication unit, for 
example 230, first receives the busy signal from the resource controller 
at 403. The routine then proceeds to decision 405, where it is determined 
whether or not the busied request is to be cancelled. If the request is to 
be cancelled, the communication unit sends a cancel busy signal at 407, 
which may be initiated by the user of the communication unit, and exits 
the routine at 433. If the busied request is not to be cancelled, the 
routine proceeds to decision 409, where it is determined whether or not a 
call-back signal for that busied request has been received. If the 
call-back signal has not been received, the routine then proceeds to 
decision 413, where it determines whether or not the channel assignment 
has been received. If the channel assignment has not been received, the 
routine is then exited at 433. If a channel assignment has been received, 
the communication unit monitors the call on the assigned voice channel at 
419, after which the routine proceeds to decision 421, where it is 
determined whether or not the call being monitored has ended. If the call 
has ended, the routine returns to the decision point at 405, near the 
beginning of the described operation at 401. If the call has not ended, 
the routine proceeds to decision 423, where it is determined whether or 
not the communication unit 230, or radio, has been keyed, for example by 
depressing the units push-to-talk button 233. If the radio has been keyed, 
the communication unit sends a code, for example its PTT-ID, and transmits 
on the voice channel at 425. In those systems not having PTT-ID 
capability, decision 423 is bypassed. In this case, or in the case where 
the radio is not keyed, the routine proceeds to decision 427, where it is 
determined whether or not the busied request is to be cancelled. If the 
busied request is to be cancelled, the communication unit sends a cancel 
busy signal at 431, and exits the routine at 433. If the busied request is 
not to be cancelled, the routine proceeds to decision 429, where it is 
determined whether or not a call-back signal has been received by the 
resource controller. Receipt of call-back may be supplemented, for 
example, by an audible tone generated by the called-back unit, or through 
the use of a display 231 showing the user which of his requests are being 
serviced. If no such call-back has been received, the communication unit 
continues to monitor the call at 419. If the call-back signal has been 
received, the communication unit acknowledges the call-back at 417, for 
example when the user presses the `ACK` button 235. The user then either 
cancels the original request, for example by pressing the `CNC` button 
239, or participates in the originally requested communication by pressing 
the `PRT` button 237. Then the communication unit gathers the voice 
channel assignment at 411, transmits a message on the voice channel at 
415, and the routine is exited at 433. Referring back to decision 409, if 
a call-back signal has been received from the resource controller, the 
communication unit acknowledges the call-back signal at 417, gathers the 
voice channel assignment at 411, transmits a message on the voice channel 
at 415, and the routine is exited at 433. 
In general, the present invention allows for a multi-threaded environment, 
where the communication unit may initiate, or participate in, a second 
communication on a voice channel while in a busied state resulting from an 
earlier request. This is made possible by adding new functional elements 
to both the communication, or subscriber, unit and the central resource 
controller, per the invention. In addition, the invention seeks to 
minimize control channel traffic by reducing redundancy in information 
generally transmitted via OSWs on the control channel. By significantly 
reducing the amount of traffic on the control channel, the invention 
allows for further expansion of a given trunked communication system and 
allows for a much more efficient use of the available resources on today's 
systems. 
The benefits obtained through the use of the invention can be summarized 
into two categories. The first involves direct gains seen from the 
communication unit user's perspective, and the second involves the 
benefits seen by the resource controller. From the users perspective, 
since the communication unit no longer must sit idle waiting i.e. in a 
wait state, for a channel assignment, the user is free to participate in 
other communications, thereby increasing his own productivity through 
efficient use of the system. Furthermore, by reducing inbound control 
channel traffic on systems experiencing long interconnect busy queues, use 
of the invention precludes the possibility of frustrated users 
continuously requesting interconnect channels only to reject the busy and 
try again later due to the lengthy idle waits they must endure. From the 
resource controller's perspective, the significant reduction of outbound 
control channel traffic is achieved by limiting the number of busy OSW 
repeats. As such, more control channel time slots are then open for other 
operations, for example status messages, additional voice channel 
assignments, or other new features. Furthermore, multi-system scans with 
large scan lists can be achieved much faster since communication units 
spend less time on a control channel to receive a complete OSW cycle.