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Description  |
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BACKGROUND
1. Field of the Invention
The present invention relates to apparatus and a method for increasing the
utility of a cellular radio communication system. More particularly, this
invention pertains to a microprocessor controlled interface that renders
numerous useful devices compatible with existing cellular systems.
2. Description of the Prior Art
Mobile telephone communication technology has grown from one way radio
transmission, much like that of the Citizens Band (C.B.), to
fully-duplexed audio, both voice and data. In large measure, the progress
of this technology has followed from the development of low power
transmissions that have increased the number of user (and useable)
frequencies.
A further development in the area of mobile telephone communication has
been that of the cell theory of radio transmission. Cellular systems
currently utilize 666 channels assigned by the FCC in an 850 MHz frequency
band. Half of such channels are reserved for wire line carriers (local
operating companies).
In a cellular system, the service area is divided into a plurality of cells
of 8 to 12 miles in radius. The cells may be further subdivided in
accordance with the projected amount of system traffic. As the available
frequencies are apportioned among an increased number of cells, the
required operating power decreases.
Communication by means of a cellular system requires, inter alia radio
contact between the mobile vehicular equipment and cell site transmission
and receiving equipment. Land line facilities are utilized to tie the
vehicular-cell site link into a conventional central switching office
whereby the mobile unit is patched into either a non-mobile,
"conventional" telephone (or modem) or another mobile telephone so that a
full-duplex communication link is established.
The vehicle mounted apparatus of the mobile cellular system commonly
comprises a mobile transceiver and a handset. The handset and the
transceiver communicate through a logic bus whereby the transceiver is
activated to transmit audio entered, as either voice or data, into the
handset and to transfer audio and signalling received from the cell site
transmission and receiving equipment. In existing mobile systems, the user
must become familiar with the particular operational characteristics and
requirements of the handset which are differ from that of the standard
dial or touch telephone. For example, one must become familiar with a
telephone receiver (the handset) that has a peculiar audio "call in"
signal that is quite different from the sound of the electromechanical
telephone ringer. One must also adjust to a receiver that is totally mute
prior to completion of a connection, sounding as if it were a dead line
and providing no indication prior to the completion of dialing that the
vehicle has entered a "no service" area. The user must become fully
conversant with the handset keypad functions, assiduously entering a
manual "send" command when initiating a communication. Of further concern,
the handset and the transceiver must be "matched" in accordance with the
applicable protocol of the manufacturer, further limiting the options of
the user.
SUMMARY OF THE INVENTION
The foregoing limitations imposed upon the potential usefulness of existing
cellular systems and other disadvantages of the prior art are addressed
and overcome by the present invention that provides, in a first aspect,
apparatus for interfacing a standard telephone outlet with a cellular
system of the type wherein a radio transceiver is adapted to communicate
directly with a handset of compatible design so that audio is transmitted
between said handset and a central office. Such apparatus includes means
for observing the state or the logic bus of the transceiver and for
observing the status of the outlet. Means are further provided for
generating at least one signal for controlling the outlet in response to
the state of the bus and the status of the outlet.
Finally, means including a microprocessor in communication with the logic
bus of the transceiver and with the outlet, is provided. Such means is
arranged to identify the protocol of the transceiver and to communicate
the state of the outlet in the identified protocol to the logic bus.
In a further aspect, the invention provides a method for modifying a
cellular system of the type that includes a transceiver characterized by a
predetermined equipment protocol for interfacing with a handset, of
compatible design, so that a telephony device can communicate through such
system. In accordance with such method, the state of the logic bus is
observed. The protocol of the transceiver is then identified. The status
of the device is next observed. Thereafter, at least one signal for
controlling the operation of the device is generated in accordance with
the state of the bus and the status of the device. Further, information is
transmitted onto the bus in response to the status of the device in the
protocol of the transceiver.
The preceeding and other features and advantages of the invention will
become further apparent from the detailed description that follows. This
description is accompanied by a set of drawing figures. Numerals are
employed throughout the written description and the drawings to point out
the various features of this invention, like numerals referring to like
features throughout.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram of mobile vehicular equipment including an interface
unit in accordance with the invention;
FIG. 2 is a schematic diagram of the interface of the invention;
FIGS. 3A and 3B are flow diagrams of the microprocessor control functions
of the invention in response to an external transmission and an
internally-generated transmission, respectively; and
FIGS. 4A and 4B comprise a flow chart of an algorithm for detecting
transceiver protocol in accordance with this invention.
DETAILED DESCRIPTION
Turning now to the drawings, FIG. 1 is a diagram of a mobile vehicular unit
for a conventional cellular system. The vehicular unit, adapted to be
mounted in an automobile 10, includes an interface 12 in accordance with
the invention. A cellular handset 14 provides for user input (signalling
and audio) that is communicated through a vehicle-mounted radio
transceiver 16. Logic and power (originating with the vehicle battery 18)
is transmitted between the transceiver 16, the interface 12 and the
handset 14 through a transceiver bus 20. A cable 22 delivers power from
the battery 18 to a logic and power control section of the transceiver 16
(discussed below). FIG. 2 is a schematic diagram of the interface 12 of
the invention. The interface as illustrated interacts with a cellular
handset 14 and transceiver 16. The audio interface is typically 600 ohm
balanced. However, it is to be appreciated that the interface is also
compatible with single-ended unbalanced systems.
The transceiver 16 includes three primary functional units. A receiver
section 26 transmits signalling and audio received from the cell site
transmitter. A transmitter section 28 accepts audio and signalling that
originate from either the handset 14, a standard telephone or modem
plugged into the telephone outlet 29, such as a RJ11C (Universal Service
Order Code for standard mini-modular) jack, of the interface 12, or both
and, upon command, transmits to the cell site receiver. A logic and power
control module 30 "talks" to the handset 14 and to the interface 12 over
the transceiver bus 20.
The arrangement of a standard cellular handset 14 includes a speaker 34 or
other conventional audio output device and an input differential receiver
36 in communication therewith. A microphone 38, in combination with a
differential driver 40, is provided for the user's audio input. A standard
cellular keypad 42 is provided for input to a handset microprocessor 43.
Various functions, both incoming and outgoing, are illustrated on a
display panel 44.
The heart of the interface is an eight-bit microprocessor 46 that is
programmed to control various essential interface functions. A system read
only memory (ROM) 48 stores system data. By providing a memory for data
storage, the invention is able to function as illustrated herein
regardless of the manufacturer operating protocol of the
handset-and-transceiver system. The utility of this feature of the
invention resides in the fact that, whereas as single protocol, known by
the acronym of "A.M.P.S." (Advance Mobile Phone Service) once served as
the industry standard, a number of equipment manufacturers have since
adopted different protocols for handset-to-transceiver communication. In
large measure, this charge has reflected a desire to simplify the hardware
requirements. A.M.P.S., a parallel system, requires the dedication of
eight pin connections while serial systems typically require only three
pin connections. Examples of other equipment protocols currently in use
include the serial protocols of the NEC, the OKI, the General Electric and
the Novatel transceivers. In the present invention, known equipment
protocols are detected at the microprocessor through systematic comparison
with the known protocols that are stored in the system ROM 48. As the
transceiver protocol is analyzed and determined by the interface (upon
installation) the microprocessor 46 is prepared to transmit and receive
communications in the proper system protocol. Thus the present invention,
by incorporating an intelligent microprocessor, is rendered compatible
with known transceiver types. A representative algorithm for detecting
transceiver protocol in accordance with this invention is illustrated by
the flow chart that comprises, in combination, FIGS. 4A and 4B of the
drawings.
The audio circuits of the interface 12 are powered directly by 12 volt d.c.
from the automobile battery 18 while a voltage regulator 50 converts the
battery voltage to 5 volts d.c. for supply to the interface logic
circuits. An auxiliary power jack 51 is provided for powering a lap
computer of like apparatus. The audio circuits include a differential
driver 52 and a differential receiver 54 that provide a port for
communicating with the handset 14 while a differential driver 56 and a
differential receiver 58 form a transceiver port. Intercom capability is
afforded by the intercom switches 60 and 62, each under microprocessor
control, that permit handset communication with the telephone device, such
as a home or office telephone that is connected to the outlet 29 of the
interface. The presence of this capability permits conference
communication with the cell site receiver/transmitter. Alternatively, this
feature permits communication between the handset 14 and the device, an
especially desirable feature in the event that a cordless telephone is
plugged into the outlet 29.
The audio path between the handset 14 and the transceiver is further
controlled by microprocessor-controlled audio switches 64 and 66. These
switches are normally (i.e., when no apparatus is plugged into the outlet
29) closed to permit normal usage when desired. Otherwise, when
interrogation of the outlet 29 determines that a device is in
communication therewith, the switches are moved to their open positions. A
jack 67 is provided for attachment of a tape recorder of any of a number
of "hands free" audio input devices.
A variable gain amplifier 68 is located in the interface receiver channel.
The amplifier 68 is controlled by the microprocessor 46 to adjust the
volume of the incoming transmission, as desired. The input from the
telephony device for commanding a volume adjustment comprises a
recognizable coded transmission during the audio phase of a transmission
such as the application of a code using the buttons of a touch tone
telephone or the dialing of a predetermined code of a pulse (i.e. rotary)
dial telephone.
Referring now to the signalling section of the interface face 12, a loop
current generator 70 comprises electrical current sinking circuit that
includes the telephone outlet 29. When the telephony device that is
plugged into the outlet 29 is taken off the hook, the outlet changes from
an open to a closed circuit, thereby causing current to flow from the 12
volt power supply through the outlet and device to ground.
A off hook detect electrical circuit 72 is in electrical connection with
the loop current generator 70. The electrical potential of a preselected
node of the circuit 72 changes from 12 volts to ground as a consequence of
a current flow in the loop current generator 70. This grounding of the
node effects a change in the state of the output of the hook detect from
"low" to "high"The design of circuitry for effecting a detectable change
in the state of the output of the circuit 72 in response to a change in
the status ("on" or "off" hook) of the device plugged into the outlet 29
is understood to be well known in the circuit design art.
The output of the circuit 72 is directed to an input of an OR gate 74, the
other input of which comes from a ring off hook detector circuit 76. The
ring off hook detector circuit 76 is discussed below. Essentially, this
circuit serves to inform the interface, in the midst of a ring burst of a
ring generator 78, that the telephony device has been picked up. The
interface, of course, moves to a new mode of operation when the device
plugged into the outlet 29 is picked up either to initiate a call or to
answer a call. By ORing together these outputs, the system provides
similar effects regardless of the phase of operation of the device at the
time the receiver is picked up.
The output of the OR gate 74 is applied to (1) an amplifier 80 and a light
emitting diode (LED) 75 that provides a visual confirmation to the user
that the telephony device is off hook, (2) the microprocessor 46 for
processing as discussed in conjunction with the following figure and (3)
an interface audio switch 85 that closes in response to the state of the
gate 74 to permit transmission of audio from the device to the
differential driver 56 of the transceiver output section of the interface.
It should be noted that the differential receiver 58 of the transceiver
output section of the interface is always on line to the device plugged
into the outlet 29.
The ring generator 78 is activated by the microprocessor 46 that is, in
turn, responsive to the presence of incoming call signalling on the
transceiver bus 20. In response to such signalling, the microprocessor 46
transmits a 5 volt 20 Hz (ringing frequency) square wave to the generator
78. Circuitry of the ring generator 78 first amplifies this signal to a 12
volt amplitude then increases this signal to a 60-to-80 volts r.m.s. a.c.
waveform (by means of a step-up transformer) for actuating the ringing
mechanism of the standard dial or touch tone telephone that may be plugged
into the outlet 29.
At the same time that it generated the above-referenced square wave, the
microprocessor 46 transmits a signal on the conductor 79 that causes a
ring relay switch 80 to open. This normally-closed switch connects hybrid
audio duplexer circuits, including a receiver circuit 82 and a transmitter
circuit 84 to both logic and audio circuits of the interface 12. The
opening of this switch during the ringing function protects such circuitry
from the 168 peak-to-peak voltage of the ring generator 78.
The receiver audio hybrid circuit 82 accepts audio from the handset 14
through the intercom switch 60 (when the switch 60 is enabled by
transmission of the appropriate code from the telephony device) and passes
dialing tones generated by the telephony device that is plugged into the
outlet 29 to a decoder 86. The decoder is preferably an integrated circuit
of the DTMF (for dual tone multi-frequency) type that converts incoming
frequencies into hexidecimal words. The decoded tones are fed to the
microprocessor 46 as set forth in greater detail with reference to the
following figure.
The transmitter audio hybrid circuit 84 accepts a sinusoidal dial tone from
a low pass filter and mixer 88 that is provided when the off hook
condition, without incoming signalling, is detected by the microprocessor
46. This sinusoid is directed through the circuit 86 to the outlet 29 so
that the user of the device attached thereto receives exactly the same
response when taking the telephone off the hook as he would in a
non-mobile system. The sinusoid is generated from a pair of square waves
applied to the filter and mixer 88 over the conductors 90 and 92. These
square waves are generated in the microprocessor 46 at the frequencies of
350 Hz and 440 Hz, well known and recognized in the telephone
communication arts as "precision frequencies" for dial tone generation.
The operation of the interface will become further apparent from the
control operations of the microprocessor 46 that are set forth in the flow
diagrams of FIGS. 3A and 3B. FIG. 3A sets forth the operation of the
microprocessor 46 in response to a transmission from a telephone unit
outside the mobile vehicular unit comprising the transceiver 16, the
device that is plugged into the outlet 29 fo the interface 12 and the
handset 14. Figure 3B depicts the operation of the microprocessor 46 as
initiated by the operator of the device plugged into the outlet 29 of the
interface 12.
Referring now to the flow chart of FIG. 3A, the microprocessor 46, having
identified the operating protocol of the transceiver 16, continuously
interrogates the transceiver bus 20 for the appropriate code indicating an
incoming call (step S-1). When this code is detected, the microprocessor
generates a 20 Hz square wave (step S-2) and transmits this signal to the
ring generator 78 and an "open circuit" signal to the ring relay 80. As
mentioned above, the ring generator 78 includes an amplifier and a step-up
transformer for increasing the voltage of this signal from 5 volts to 60
volts r.m.s. As explained, the ring relay 80 protects the remaining
circuitry of the interface 12 during the ringing of the device by opening
the connection of the audio circuits to the outlet 29 during ringing.
After the ringing of the device has been actuated, the microprocessor 46
interrogates the appropriate port for the status of the output of the OR
gate 74 (step S-3). The output of the gate 74 is responsive to the
detection of an "off hook" condition. When such a condition is detected,
the microprocessor ceases to transmit the 20 Hz square wave and the
ringing of the device ends in the same manner as does that of a telephone
connected to a conventional wired system.
Once the device has gone off hook, the microprocessor 46 opens the audio
switches 64 and 66. It should be noted at this point that the switches are
generally closed and will remain closed if an off hook condition is not
detected. This permits handset-to-transceiver communication in the event a
device is not connected to the outlet 29 or the user chooses to use the
handset rather than the device. The subsequent removal of the device from
the hook will disconnect the handset unless an intercom mode is chosen.
At the same time the microprocessor 46 detects an off hook condition, the
signal at the output of the OR gate 74 causes the interface audio switch
85 to close, connecting the receiver audio hybrid circuit 82 to the
transmitter section 28 of the transceiver. As mentioned, the receiver
section 26 of the transceiver is continually connected to the transmitter
audio hybrid circuit 84. Thus, full duplex communication is established
with the device plugged into the outlet 29.
The microprocessor outputs a "send" signal, properly coded in the
transceiver protocol, over the transceiver bus 20 whereby the transceiver
is energized for for full duplex audio transmission at the same time the
audio circuits are opened (step S-4). A number of user options, though not
all, are indicated in steps S-5 through S-8. Numerous features are made
available via the capacity of the microprocessor 46 to decode tones from
the DTMF decoder or pulses from the off hook detector. The microprocessor
46 is able to accept data at a very high rate from the decoder chip 86 and
from the off hook detector 72, acting as a buffer so that signalling data
is sent to the transceiver 16 at an acceptable rate. In this manner, the
interface 12 is able to accommodate the various "fast dialing/redialing"
options currently offered for telecommunications systems.
As indicated at step S-5, the user may choose to increase or decrease the
volume of the incoming audio by either rotary or touch tone dialing or by
entering a code on the touch tone telephone. In accordance with the system
mode in operation (independent of the protocol of the equipment with which
the device is being interfaced). the microprocessor will decode the volume
command (step S-6) and adjust the variable gain amplifier 68. A second
indicated option, that of intercon with the handset 14, is similarly
activated by signalling to the microprocessor 46 (step S-7) whereby the
microprocessor is caused to closed the intercom switches 60 and 62 (step
S-8). Other options that may be similarly actuated by the transmission of
a coded input from the device connected to the outlet 29 (a representative
user code for this purpose would be a five digit transmission beginning
"000" followed by a two-digit command) might include a "self test" in
which the dialed signal is observed on the handset display 44 prior to the
transmission of a coded send signal by the microprocessor 46, a "ring
check" to determine whether the ringer of the device is operational and
the like. All of such options are made feasible by the incorporation of a
microprocessor into the interface 12.
The period of full duplex audio transmission is indicated by the vertical
dashed line of FIG. 3A. The microprocessor continues to interrogate the
pin connected to the output of the OR gate 74 during audio transmission
for the "hang up" mode. Once this occurs (step S-9), the microprocessor 46
generates and outputs the appropriate "end of call" protocol over the
transceiver bus 20 (step S-10) whereby the transceiver perform the
appropriate functions such as power down, etc.
FIG. 3B illustrates the microprocessor routine that occurs when a call is
initiated at the device that is plugged into the outlet 29. Steps S-11
through S-19 of FIG. 3B replace steps S-1 through S-3 of FIG. 3A.
As in step S-1 of the prior flow chart, the microprocessor continually
interrogates the state of the output of the Or gate 74 to determine
whether the device is off hook (step S-11). When this occurs, the
microprocessor 46 generates a pair of square waves, one at 350 Hz and the
other at 440 Hz, the precision frequencies for generating a standard
telephone dial tone. As mentioned earlier, these signals are applied to
the low pass filter and mixer 88 by means of the conductors 90 and 92
(step S-12).
Once the microprocessor has generated the signals for providing the caller
a dial tone, it prepares to accept the dialing. While tone dialing will be
seen to be processed through the DTMF decoder 86, pulsed dialing signals,
such as those generated by a standard rotary telephone, are transmitted
through the off hook detector circuit 72.
The microprocessor, after transmitting the dial tone-generating square
waves, examines the output state of the OR gate 74 (step S-13). In the
event that the user is presently in the process of generating tone dialing
signals, the device will remain off hook and the microprocessor processor
proceeds to step S1-14. In the event that the user has either decided to
terminate or is in the process of dialing with a pulse dialing apparatus,
such as a rotary telephone, the microprocessor proceeds to steps S-17
through S-19, a routine that is discussed below.
Assuming that the receiver has remained off hook, the microprocessor 46
receives the dialing tones as hexadecimal-encoded words, in parallel form
(4 bit words), from the decoder 86. Each such word is stored (step S-14).
The microprocessor continues to store such information provided a fixed
increment of time is not exceeded between receipt of the hexadecimal
words. A preferred time increment for ascertaining the completion of a
dialing sequence is four (4) seconds. When this time increment is exceeded
(step S-15), the microprocessor encodes the dialing sequence in accordance
with the transceiver protocol whereby this signalling is transmitted over
the transmitter section 28 of the transceiver 16. Once the completion of a
circuit is indicated on the transceiver bus 20, the system proceeds to the
steps indicated in steps S-4 through S-10 of FIG. 3A.
When the state OR gate 74 indicates to the microprocessor that the
telephony device has returned to the on hook position (step S-13), the
microprocessor then examines the period of time that it remains on hook. A
preselected time is programmed into the routine which corresponds to the
period of a dialing pulse. Such pulses are generally of about 100
millisecond duration. In the event that the device should return to the
off hook position within this time frame, the input from the OR gate 74 is
pulsed dialing. If this time period is exceeded, the microprocessor
determines that the caller has decided to abort the call prior to dialing.
Upon examination of duration of the on hook period at step S-17, the
microprocessor returns to step S-11 to look for the user to begin another
call when the dialing pulse period is found to have been exceeded. In the
event that the dialing pulse period was not exceeded, the microprocessor
proceeds to a standard routine for counting and storing dialing pulses
(step S-18). As in the case of tone dialing, the microprocessor waits
after the input of each pulse dialing input to ascertain the end of
dialing (step S-19). When a set period of time is exceeded, the routine
proceeds to the previously-discussed series of steps that begin with step
S-16 of this figure.
In the event that the device plugged into the outlet 29 goes off hook when
the vehicle is in a "no service" area, the interface will generate a
signal identical to the "fast busy" generated by a central office to
indicate that all trucks are occupied. The low or no signal condition is
transmitted over the bus 20. When the off hook is detected thereafter, the
microprocessor 46 generates and transmits square waves of 480 and 620 Hz
(the appropriate frequencies for generating a precision fast busy signal)
over the conductors 90 and 92. The low pass filter and mixer 88 will
process these signals so that this well recognized audio signal is
generated by processing at the hybrid audio circuits 84 and 82 and
transmitted to the device plugged into the outlet 29.
Thus it is seen that the present invention provides new and useful
apparatus for enhancing the utility of existing cellular communication
systems. By applying the teachings of this invention, one is able to
incorporate familiar telephony devices and to operate such devices in
thoroughly familiar modes while accessing cellular radio communication
systems. In this manner, the usefulness of such systems is enhanced. By
providing a microprocessor-controlled system, the user is able to obtain
an interface that is universally compatible with existing cellular
transceivers and handsets.
While this invention has been described with reference to a presently
preferred embodiment, its scope is not so limited. Rather, this scope of
this invention is limited only insofar as defined by the following set of
claims and all equivalents thereof.
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