Method and apparatus for communicating an information signal having dynamically varying quality

Subscribers participating in a call may communicate information un-impeded until the occurrence of an event (such as the expiration of a time interval). After detecting this event, the quality or intelligibility of the communicated information is repeatedly reduced to encourage the subscribers to voluntarily conclude the call.

TECHNICAL FIELD 
This invention relates generally to radio frequency (RF) communication 
systems, and more specifically to those RF communication systems that 
control the time period that subscribers are permitted to use 
communication resources, and is particularly directed towards encouraging 
subscribers to quickly conclude a communication so as to effectively 
utilize an RF communication system. 
BACKGROUND ART 
Contemporary radio frequency (RF) communication systems typically allocate 
communication resources (channels or time slots) among several subscriber 
units. Generally, each subscriber desiring access to the communication 
system must transmit a call request code and receive a grant code before 
communication (transmissions) between two or more subscriber units may 
commence. 
Since the communication resources of RF communication systems are limited, 
it is known to limit the duration of any single communication to a 
predetermined time interval. Should the subscribers participating in a 
call attempt to communicate beyond this time interval, a warning tone 
precedes automatic preemption of the communication, and the communication 
resource is reassigned to other subscribers. While designed to improve the 
capacity of a communication system, this practice tends to confuse and 
frustrate subscribers as calls are abruptly terminated. Moreover, such a 
ruthless call preemption scheme impedes effective communication, and may 
actually reduce the efficiency of the communication system, since, in 
practice, subscribers tend to immediately re-request access to the 
communication system to complete their call. This causes an increase in 
signalling traffic that may reduce the efficiency of the communication 
system below that of merely allowing the subscribers to complete the 
original call. Accordingly, a need exists for a method of encouraging 
subscribers to quickly conclude their communications, without hampering 
the capacity and efficiency of the communication system. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide a 
communication system that avoids the detriments of the prior art. 
Briefly, according to the invention, subscribers participating in a call 
may communicate information un-impeded until the occurrence of an event 
(such as the expiration of a time interval) is detected. After detecting 
this event, the quality or intelligibility of the communicated information 
is repeatedly reduced to encourage the subscribers to voluntarily conclude 
the call.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, a block diagram of a time division multiplex (TDM) 
subscriber unit 100 is shown in accordance with the present invention. The 
subscriber unit 100 may comprise a mobile, portable, or control station, 
which communicates information (voice or data) via one or more TDM time 
slots arranged on one or more radio frequency channels. As used herein, a 
portable unit is a communication device designed to be carried on or about 
the person, a mobile subscriber unit is typically designed to be installed 
in vehicles, and a control station comprises a subscriber unit designed to 
be permanently or semi-permanently installed in a building or other fixed 
location. 
Operationally, the subscriber unit 100 processes information signals (voice 
information from the microphone 102, or data information from the data 
source 104) in an information code 106. The information coder 106 
preferably comprises a digital signal processor (DSP), which processes the 
information signal at a coding rate defined by a coding signal 108, which 
is provided by a clock source 110. According to the invention, the clock 
source 110 is programmable (112) by a controller 114 so as to vary the 
coding rate of the information coder 106. Additionally, the coding 
procedure(s) executed by the information coder 106 may be adapted by 
programming line(s) 109 by the controller 114. By adapting the coding rate 
(108) and the coding procedures (109), the information coder 106 may 
affect a plurality of different coding schemes such as subband coding 
(SBC), linear predictive coding (LPC), or other coding procedures known in 
the art. 
The coded information signals are received by the controller 114, and 
stored in a transmit buffer 116 at the coding rate. After accumulating a 
suitable amount of information in the transmit buffer, the controller 114 
extracts and formats a packet of information, which is routed to the 
transmitter 118 for transmission at a suitable channel rate so as to time 
division multiplex the coded information signals of several subscriber 
units. Next, the controller 114 activates (120) an antenna switch 122 so 
as to couple the transmitter 118 to an antenna 124 so that the packet of 
information may be transmitted at an appropriate time so as to occupy one 
or more time slot(s) assigned to subscriber unit 100 to enable 
communication. 
To receive information, the antenna switch 122 is activated (120) to couple 
the antenna 124 to a receiver 126. The receiver 126 provides baseband 
(recovered) coded information signals to a clock recovery circuit 128 and 
the controller 114. The clock recovery circuit provides a recovered clock 
signal 130 that is used by the controller 114 to synchronously receive the 
recovered coded information signals. The controller 114 stores the 
received information in a receive buffer 132 at the channel rate, and 
retrieves this information from the receive buffer 132 at the rate the 
information was coded. To recover (decode) the information signals, the 
controller 114 routes the stored information to an information synthesizer 
134 that restores the encoded information to an intelligible form. To do 
this, the information synthesizer 134 must be programmed (136) by the 
controller 114 so as to synthesize the information using a complementary 
coding function that was used by the information coded at the transmitting 
subscriber unit. That is, both the coding procedures (type) (i.e., LPC or 
SBC) and the encoding rate (for example, 9.6 kbs, 4.8 kbs, or 2.4 kbs) 
must be identical at the receiving and transmitting subscriber units. 
After being programmed by the controller 114, the information synthesizer 
134 operates to produce an information signal that is suitable for 
presentation to the subscriber. For voice information, the synthesized 
signals are routed to an audio amplifier 140 and a speaker 142. 
Conversely, recovered data information is provided to a data sink (such as 
a printer or a display) for presentation to the subscriber. 
According to the invention, subscriber units may communicate unimpeded 
until an event is detected (for example, the expiration of a predetermined 
time interval). After detecting this event, the controller 114 operates to 
reduce the coding signal (108) so as to intentionally degrade the quality 
and/or intelligibility of the information signal. Additionally, the coding 
procedures (type) may be altered such as by adapting from 9.6 kbs SBC to 
2.4 kbs LPC, which would reduce the intelligibility (or quality) of the 
message. In this way, the subscribers are encouraged to terminate the 
communication as the transmitted information signals gradually become more 
difficult to understand. Thus, abrupt automatic termination of calls is 
avoided in favor of a gradual quality degradation, which encourages the 
subscribers to voluntarily wind up the communication. Accordingly, 
communication resources become available for use by other subscribers, 
while the existing practice of re-requesting access to the communication 
resources is reduced since subscribers will tend to complete their 
message, albeit at reduced intelligibility (or quality), rather than 
re-dial to access the system. Optionally, certain subscribers (executives 
or other high-level officials) may operate within the communication system 
without having their communications impededed in any way (i.e., without 
intelligibility or quality reduction). Such subscribers are contemplated 
as being permitted to communicate indefinitely, and therefore, are 
encouraged to terminate their communications. 
In the preferred embodiment of the present invention, the event detected to 
trigger intelligibility (or quality) reduction comprises the expiration of 
a predetermined time interval. According to the invention, the subscribers 
may communicate unimpeded (i.e., a full quality information signal) for 
this time interval, which is determined by the controller 114 using any 
suitable software or algorithm that determines when a time period has 
lapsed. Of course, other events may be used singularly or in conjunction 
with a time interval to trigger intelligibility (or quality) reduction. 
For example, the controller 114 may detect the beginning of a busy period 
(determined either by real-time or by measuring the communication 
traffic), or the accumulated time that each subscriber requests access to 
the communication resources may be detected. Thus, the preferred 
communication system can trigger both the reduction of the unimpeded 
communication interval and the rate of which intelligibility (or quality) 
is reduced so as to more quickly discourage prolonged communication by the 
subscribers. 
Referring still to FIG. 1, it was previously mentioned that the preferred 
intelligibility (or quality) degradation comprises a reduction in the 
coding rate and/or a variation in the coding procedures (type). 
Alternately, the intelligibility (or quality) of the information signal 
can be degraded in a number of other ways. For example, a limiter 146 
could be disposed between the microphone 102 and the information coder 106 
so as to increase distortion components in a voice signal. This may be 
accomplished by amplifying or hard limiting the information signal so as 
to intentionally clip the signal. As is known, clipped (or squared off) 
signals contain a high degree of distortion components. Therefore, one 
method of reducing the intelligibility (or quality) of the signal is to 
increase the distortion components of the information signal to be 
transmitted. Also, the audio bandwidth of the information signal could be 
reduced (such as by filtering or discarding bits in the information coder) 
to reduce the intelligibility (or quality) of the information signal. 
Additionally, a masking signal could be injected at the input of the 
information coder 106 from a masking signal source 148. The masking signal 
may merely be a noise signal, which would increase noise floor of the 
received signal, or it could be an audio tone (or other suitable signal) 
that combines with the information signal so as to reduce the quality 
and/or intelligibility of the information signal at the receiving end. 
Moreover, the masking signal could be supplied at the receiving end by 
combining the noise or audio signals at the output of the information 
synthesizer 134. Any of these techniques would reduce the intelligibility 
(or quality) of the information signals. Accordingly, as a general 
statement, any process that reduces the quality or the intelligibility of 
the information signal is effective for use with the present invention to 
encourage subscribers to relinquish the communication resource for use by 
other subscribers. 
The intelligibility (or quality) reduction used by the present invention to 
encourage voluntary communication resource relinquishment, is preferably 
performed in steps so as to effect declining intelligibility (or quality) 
over time or in response to detection of a variety of events. Depending 
upon the implementation, the intelligibility (or quality) could be reduced 
to zero (at which point the communication resources would be reclaimed), 
or alternately, the communication resource may be reduced to some minimum 
level of intelligibility (or quality), at which the point the subscriber's 
communication may continue indefinitely. For example, the present 
invention may permit unimpeded communication for a three minute time 
interval, after which the information coding rate is adapted from 9.6 kbs 
SBC to 4.8 kbs SBC. After another time interval (perhaps one minute) the 
information coding scheme by change to 4.8 kbs LPC, thus degrading 
intelligibility (or quality) one step further. After another time interval 
(for example, one minute which may be reduced to thirty seconds during 
busy hour) the coding scheme is again adapted to 2.4 kbs LPC, which 
establishes a floor intelligibility (or quality) rate. At this point, the 
information signal would be intelligible, but may sound hollow and 
mechanical. Nevertheless, if the subscribers desire to continue to 
communicate at this level of intelligibility (or quality), they are 
permitted to do so. 
Referring to FIG. 2, the present invention prefers to reclaim portions of 
time slots that are not used (within frames) during the time when the 
intelligibility (or quality) is reduced. Those skilled in the art will 
appreciate that a reduced quality information signal (i.e., coded at a 
lower rate) will take less time to transmit. Accordingly, the invention 
preferably operates to reallocate the time slot for use by other 
subscribers, or for use by the communication system to transmit signalling 
or control information to maintain the efficient operation of the 
communication system. 
FIG. 2a illustrates a time slot 200 suitable for carrying toll grade voice 
during the time interval that communication is permitted at full 
intelligibility (or quality). After a time interval, the coding scheme is 
adapted to reduce the intelligibility (or quality) of the information 
signal, and the controller 114 reformats the reduced information packet 
into a time slot 200'. According to the invention, the remainder or the 
original time slot 200 may be reformatted into a sub-slot 202, that may be 
used by another subscriber unit operating in a reduced intelligibility (or 
quality) mode. Thus, capacity is increased, and the grade of service of 
the communication system is improved. Thereafter, the coding scheme may be 
again adapted to reduce the information to reside in a time slot 200", 
which allows sub-slots 204 to accommodate two other subscribers operating 
in a minimum intelligibility (or quality) mode. 
Alternately, FIG. 2b illustrates the time slot 200 being reformatted into 
the sub-slot 200' thereby defining a portion 206 that may be used for 
signalling or control information. As the information signal is further 
reduced in intelligibility (or quality), it may reside in the time slot 
200". Thus, more signalling and control information may be placed in the 
portion 206'. In this way, the communication system may improve its 
efficiency by effective placement of signalling and control information. 
This practice will expand capacity by apportioning and subdividing time 
slots into sub-slots to accommodate additional users at reduced 
intelligibility (or quality) rates (for example, during busy hour or as 
otherwise needed). 
Referring to FIG. 3, a block diagram of a frequency division multiplex 
(FDM) communication system 300 is shown in accordance with the present 
invention. In the communication 300, a subscriber 302 communicates with 
another subscriber unit 302' via a repeater 304. To accomplish this, an 
information signal provided by the microphone 102 is routed to an 
attenuator 306, which is programmed (308) by a controller 310. The 
attenuated information signal is routed to a transmitter 312, which passes 
the information signal through a duplexer 314 to an antenna 318. Received 
information comes from the antenna 318 (via the duplexer 314) to a 
receiver 316, which routes the recovered information signals to an 
amplifier 140 for presentation to the operator via the speaker 142. 
Assuming that the subscriber units 302 and 302' are operating at a distance 
that prohibits direct communication, it is known to use repeater unit 304 
to increase the effective communitcation range of each individual 
subscriber unit. Accordingly, a repeater 304 receives the signal 
transmitted by the subscriber 302 via its antenna 319. This received 
information is routed via a duplexer 320 to a receiver 322 that provides a 
recovered signal 324 to an attenuator 326. The attenuator 326 is 
controlled by the controller 332 to provide an attenuated signal 328 to a 
transmitter 330. The transmitter 330 transmits (repeats) the signal (via 
the duplexer 320 and the antenna 319) to the subscriber unit 302. 
According to the invention, the information signal may have its quality and 
intelligibility (or quality) reduced by the controller 310 and the 
attenuator 306 by progressively increasing the attenuation of the 
information signal. Therefore, the subscribers are permitted to 
communicate at full intelligibility (or quality) (i.e., zero attenuation) 
for a certain time interval, after which the attenuation is gradually 
increased to reduce the intelligibility (or quality) of the information 
signal. This encourages the subscribers to terminate the call and 
relinquish the communication resource so that other subscriber units may 
have access to it. 
As can be seen with reference to FIG. 3, the intelligibility (or quality) 
reduction may be accomplished either at the subscriber unit 302 (via the 
attenuator 306) or at the repeater 304 (via the attenuator 326). That is, 
the intelligibility (or quality) reduction can be either (or both) 
centralized or decentralized in accordance with the desires of the 
particular embodiment of a communication system. In this way, the 
intelligibility (or quality) of the information signals can be reduced (by 
increasing the attenuation) to zero or to some threshold level of 
intelligibility (or quality) to encourage the subscribers to terminate the 
call. 
Referring to FIG. 4, a flow diagram illustrating the steps executed by the 
present invention are shown. The routine begins in step 400, where the 
information signal is transmitted. During the transmission, the controller 
is continually checking to determine whether the call has been completed. 
If so, decision 402 returns control to other routines or processes used to 
control the subscriber unit. However, if the determination of decision 402 
is that the call is not yet complete, the routine proceeds to decision 
404, which determines whether the event to trigger intelligibility (or 
quality) reduction has been detected. As previously mentioned, the 
detection of this event may be the expiration of a time interval, the 
detection of a busy period, or the detection of an information capacity 
level above a predetermined threshold. In any case, when this event is 
detected, the routine proceeds to decision 406, which determines whether 
the information signal is being coded at a minimum level of 
intelligibility (or quality). If so, the routine returns to step 400, 
where the information signal continues to be transmitted. However, if the 
determination of decision 406 is that the information signal has not yet 
reached its minimum level of intelligibility (or quality), the routing 
proceeds to step 408, where the intelligibility (or quality) is reduced by 
variation of the coding rate or coding procedures (type), or by other 
methods, such as increasing the distortion components of the information 
signal, adding a masking signal to the information signal, or attenuating 
the information signal to reduce its intelligibility (or quality). 
Following this, the routine returns to step 400, where the transmission 
continues, albeit, with the reduced intelligibility (or quality) signal. 
The entire process of FIG. 4, continues in a loop until the call is 
completed and decision 402 returns control to other processes. At each 
pass through the loop, the event detection performed by decision 404 may 
change so as to quicken the rate that information intelligibility (or 
quality) is reduced. That is, the time intervals at which intelligibility 
(or quality) is reduced may be shortened, or the rate at which 
intelligibility (or quality) is reduced may be increased by factors such 
as system loading, emergency traffic, or the amount of time that this 
particular subscriber unit has occupied the system. 
In summary, the present invention operates to gradually reduce the 
intelligibility (or quality) of an information signal so as to encourage 
subscribers to voluntarily terminate their communications and relinquish 
communication resources for the benefit of other subscribers. By reducing 
intelligibility (or quality) to a minimum intelligibility (or quality) 
threshold, the present invention avoids abrupt preemption of communication 
to ease the frustration and confusion experienced by subscribers operating 
in other communication systems.