Continuous transmission mode radio with control information monitoring capability

In a communication system where outbound communications are comprised of a TDM format, and inbound communications are not, and wherein the outbound communications occasionally include system control information, a method for allowing a transmitting unit to occasionally interrupt its transmissions to allow reception of system control information.

TECHNICAL FIELD 
This invention relates generally to RF communication systems, and 
particularly to time division multiplex (TDM) systems that provide system 
control information. 
BACKGROUND ART 
Trunked RF communication systems are known in the art. Such systems operate 
by allocating a limited number of frequency resources amongst a number of 
communication units (as used herein, "communication units" refers to 
two-way radios, including mobile radios, portable radios, and fixed 
location radios). To facilitate this allocation, such systems typically 
provide system control information, either in a dedicated control channel 
or control slot, or in a distributed manner, by including the control 
information subaudibly with message information such as voice signals. 
In some applications, system resources are best utilized when a 
communication unit has the capability of monitoring system control 
information at all times. Such capability allows a communication unit to 
respond quickly to changing resource needs or other situations. In a 
communication system where messages are transmitted from a central to the 
communication units via a TDM format, and where communications from the 
communication units to the cental are in the form of continuous 
transmission signals, such capability is difficult to provide in a cost 
effective and satisfactory manner. 
SUMMARY OF THE INVENTION 
The invention functions, in this embodiment, in an RF communication system 
that includes at least a first station, such as a fixed base, and a second 
station, such as a communication unit, wherein the first station transmits 
information, including both message information and system control 
information, to the second station in as TDM format, and wherein the 
second station transmits information to the first station in a non TDM 
format, such as a continuous transmission signal (for example, standard 
FM). When operating in a transmit mode, the second station can monitor 
system control information transmitted by the first station by 
interrupting its own transmission activity from time to time and receiving 
system control information transmitted by the first station during these 
interruptions. 
In one embodiment, the second station delays final processing of its 
message transmissions, while simultaneously monitoring its message 
information to detect the occasional occurrence of predetermined message 
information events, such as natural nulls that occur during typical voice 
communications (i.e., natural pauses between words, syllables, sentences, 
and the like). Some of these detected nulls are then removed from the 
message signal as finally transmitted to provide time for the radio to 
occasionally interrupt its transmission mode without losing any of the 
message information. Such interruptions of the transmission mode allow the 
radio to receive system control information then being transmitted to the 
radio.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, the system (100) includes fixed end equipment (101) 
such as a trunking central and associated repeaters, as understood in the 
art. The system (100) also includes a plurality of communication units 
(102) (only one is depicted here). Outbound communications (103) from the 
fixed end (101) to the communication units (102) are configured in a TDM 
format. In addition, for purposes of this explanation, it will also be 
presumed that the fixed end (101) has a plurality of frequency resources, 
and therefore outbound messages are also frequency division multiplexed. 
Inbound transmissions (104) from the communication units (102) to the 
fixed end (101) are frequency division multiplex only, and may be, for 
example, a continuous transmission type of signal such as standard FM. 
FIG. 2 depicts the frame and slot format for outbound messages (103). It 
may be presumed that each frequency resource is divided into time frames 
(201), and that each frame (201) is subdivided into four slots (202). It 
will also be presumed that two of the slots (202) in each frame (201) of a 
single frequency resource are control slots (A and B) that carry system 
control information, such as frequency and slot assignments for 
communication units (102). (Additional information regarding such a 
multiple control slot FDM/TDM configuration can be found in copending U.S. 
Ser. No. 071,300,392 now U.S. Pat. No. 4,492,570, entitled Multiple 
Control Slot TDM/FDM Communication System, filed on even date herewith and 
being owned by the same assignee hereof.) Voice information can be 
supported in the remaining time slots depicted. In addition, of course, 
other frequencies available for use in the systems (100) could support 
other slots, or these two control slots could bse divided between two 
frequency resources, so long as there are two control slots per frame 
window. (It should be understood that two control slots are depicted in 
this embodiment for purposes of explanation only; the benefits of the 
invention could be realized in a single control slot system as well.) 
With reference to FIG. 3, a communication unit (102) includes generally a 
microphone (301) for receiving audible voice signals from a user. The 
microphone (301) transduces these audible voice signals in a known manner 
to create electric signals representative of the voice signals, which 
signals are then processed in a transmit audio processing unit (306) as 
appropriate (for example, these signals may be pre-emphasized, limited, or 
filtered as appropriate and in a mannner well understood in the art). 
These signals are provided to both a variable delay (302) and to a null 
detector (303). The variable delay (302) could be comprised, for example, 
of a series of tapped delay units, with the output being determined by 
appropriate selection of one of the delay taps. The null detector (303) 
could be comprised of any of a number of known and well understood null 
detectors. 
When the null detector (303) detects nulls, signals indicative of this 
detection are provided to a delay control (304). Provided with both 
information regarding the existence and duration of nulls in the voice 
signal, and of the impending occurrence of control slot transmissions from 
the fix end equipment (102), the delay control provides signals to the 
variable delay unit (302) to control the actual delay of the voice signal. 
The audio output of the variable delay (302) is of course coupled to an 
appropriate transmitter. 
Referring to FIG. 4, a more detailed description of the operation of the 
invention will be provided. 
The envelope signal representing the original voice input is set forth in 
FIG. 4B. Such a signal will have occasional nulls. In this case, for the 
purpose of explanation, it is presumed that the voice envelope signal 
(403) has a first null (404) having a duration of 40 ms. and a second null 
(405) having a duration of 80 ms. For purposes of comparison, it may be 
noted that the control slots (401 and 402) (FIG. 4A) have a duration, in 
this example, of 100 ms. 
The variable delay (302) functions, in part, to delay the voice envelope 
signal by a fixed predetermined amount. This fixed amount provides the 
communication unit (102) with sufficient time to peform the other 
functions described herein. For purposes of this explanation, it may be 
presumed that the fixed delay is 100 ms., and this delayed signal (408) 
can be seen in FIG. 4D. 
The null detector (303) detects both the occurrence and duration of the 
first and second nulls (404 and 405). The null detector (303) provides 
this information to the delay control (304), which also receives 
information about contol slot transmission timing. The delay control (304) 
uses this information to control the variable delay (302) by deleting at 
least some of the nulls from the voice signal as finally transmitted 
(409). For example, in this particular embodiment, the first null (404) 
has been substantially eradicated from the transmitted signal, as denoted 
by the reference character 410. All but 20 ms of the second null (405) 
have been similarly deleted as denoted by the reference character 411. 
So configured, the communication unit (102) functions to interrupt its own 
transmissions (409) from time to time by interrupting its own transmission 
control line (406) (FIG. 4C) in order to allow receipt of control slot 
information (412). These interruptions are 100 ms in length (since this 
embodiment presumes that the contol slots are each 100 ms in length). The 
communication unit (102) then uses detected nulls to regain the lost 100 
ms. In the example depicted, the first null provided 40 ms of regained 
time, and the second null provided 60 ms (from the original 80 ms of null 
time). 
Therefore, the communication unit (102) is able to occasionally receive TDM 
control slot information while simultaneously supporting an effective 
continuous transmission operating mode. In particular, none of the 
original voice information is lost, because interruptions to the 
transmission mode are made at the expense of nulls, which do not include 
any voice information.