Note: Descriptions are shown in the official language in which they were submitted.
- ~ . 1 1 6~05 1
45-MR-224
REPEATER TRUNKING SYSTEM
._.
BAC~GROUND OF THE INVENTION
This invention relates generally to radio
repeater systems. More specifically this invention
pertains to systems wherein a plurality of mobile or
portable stations communicate with one another over
several communications channels through repeaters
operating on each of such channels. Even more
specifically, this invention is directed to
`arrangements for establishing communication between
a first radio station and a second radio station
through a repeater and over a radio channel selected
from a plurality of such channels.
In many localities, a plurality of land
mobile radio fre~uency channels are allocated for
communication among vehicles. Generally, there may
be, and usually are, many more vehicles having
mobile radio stations than there are available radio
channels in a given locality. Each of these radio
channels usually includes at least two separate and
distinct frequencies r one for transmitting and the
other for receiving.
Consequently, somQ arrangement is needed
for enabling a mobile vehicle user to obtain access
to a radio channel that is not in use and to
i thereafter establish communication with another
mobile vehicle user over that radio frequency
channel.
One such system is disclosed by J. C.
Berti et al in Can. Patent Application Serial No.
342,007 filed December 14, 1979 and entitled IMPROVED
ARRANG-'SENT AND MET~OD FOR ESTA3LISHING RADIO
- CO~IUNICATION IN A SYSTE~. In the Berti system,
.
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~ 1 ~40S 1
45-MR-224
each mobile or portable radio sta~ion (including a
~' transmitter and receiver) can-be placed in an idle
or call originate mode. When a mobile station is
- placed in the call originate mode, its receiver
scans all of the allocated radio frequency channels
in the system until an unused channel is found.
When an unused channel is found, the station's
transmitter and receiver become operable on that
channel. The mobile station's transmitter sends a
busy signal to the repeater operatin~ on the unused
channel. The repeater receives this busy signal and
transmits a busy signal identical to the busy signal
received. When the mobile station that sent the
busy signal receives it back from the repeater, it
sends out a group signal identifying a particular
subset of mobile or portable radio stations with
which the user desires to communicate. The group
sign~l is re-transmitted by the repeater and is
recognized by each radio station within that subset
and they are activated on the channel selected by
the busy tone exchange.
In the idle mode, the receiver o~ each
mobile or portable station in the system scans all
of the allocate* radio frequency channels for its
predetermined group signal. When the receiver
detects its predetermined group tone or tones on a
channel, it stops scanning, and enables the
transmitter and receiver on that channel for
communication through the repeater with the station
originating the call. There may be a plurality of
such stations associated with a predetermined group
tone or tones.
The Berti system works well under most
circumstances. However, under certain circumstan-
ces, a mobile radio station can erroneously lock
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1 1 6405 1
45-MR-224
onto a channel that is non-vacant. When a mobile
station is placed in the call originate mode, it scans
all of the channels for the presence of a busy signal.
If this scanning takes place during a fade situation or
when for any other reason the busy signal detector of
the call originating station has failed to detect the
busy tone, the call originating station may lock onto a
channel that is non-vacant and it will begin to operate
on that channel.
SUMMARY OF THE INVENTION
Therefore the present invention provides an
arrangement for more reliably establishing
communication among mobile radio stations through a
repeater. Utilizing the arrangement according to the
present invention, a lock onto a non-vacant channel is
much less likely to occur than when using previous
arrangements.
The arrangement according to the present
invention provides for a positive "handshake" between a
mobile station originating a call and the repeater on a
vacant channel.
In the present arrangement, a mobile station
placed in the call originate mode searches the
available channels sequentially until an idle one is
found on which no busy marking signal or tone is
detected. The mobile station stops scanning on the
vacant unmarked channel and transmits a busy signal
which is normally used to mark a busy channel. The
repeater, upon detecting the busy signal transmitted by
the mobile station, transmits an acquisition signal
having different characteristics from the busy signal.
This acquisition signal is transmitted only if the
channel i5, in fact, vacant. The call originating
mobile station only after detecting acquisition tone
1 1 6405 1
45-MR-224
from the repeater will complete the signalling
_ sequence by transmitting a group signal identifyin~
the particular mobile stations with which tne user
would like to communicate, a sub-group signal, if
subgroups are used and a busy signal. Thus, there
is provided a positive indication by the presence of
an acquisition signal rather than merely by the lack
.of a busy signal to define a vacant channel in the
call originate sequence of signalling.
BRIEF DESCRIPTION OF THE DRAWINGS
Many of the attendant advantages of the
present invention will be readily apparent as the
invention becomes better understood by reference to
the followinq detailed desc~iption with the appended
claims, when considered in conjunction with the
accompanying drawings, wherein:
FIGURE 1 is a pictorial diagram of a
typical land mobile communication system for which
the arrangement of the present invention is
intended;
FIGURE 2 is a general block diagram of the
repeater trunking system according to the present
invention;
FIGURE 3 shows a block diagram of a
~5 repeater station;
FIGURE 4 tFIGURES 4A through 4D) sets forth
a schematic diagram of that portion of the present
invention located at a mobile station;
; ` 1164051
45--MR-224
- FIGURE 5 is a graphical representation of
-~ the various operating modes of the repeater trunking
system according to the present invention;
FIGURE 6 is a flow chart showing ~he
operation of the repeater trunking systemin the
n idle" mode;
FIGURE 7 is a flow chart showing the
operation of thè repeater trunking system in the
"wait~ mode;
FIGURE 8 is a flow chart diagram showing
the operation of the repeater trunking system in the
"ready" mode; and
FIGURE 9 is a diagram illustrating the
acquisition signalling sequence between a mobile
station originating a call and a repeater station.
DETAILED DESCRIPTION OF THE PREFER~ED EMB~DIMENT
Referring now to the figures wherein like
reference numerals designate like or corresponding
parts throushout, and specifically referring to
FIGURE 1, there is shown a pictorial diagram of a
typical mobile communication system for which the
present invention is intended. It is assumed that a
particular geographic locality has been allocated
five radio repeater communication channels (for
example ;n bands around 850 Megahertz), each such
channel havin~ a different transmitting and
receiving frequencies. For five channels, five
repeaters Rl through R5 are provided and located at
'' an available site that provides optimum coverage.
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fi ~ O S l
4 5 -MR- 2 2 4
` Each of repeaters Rl through RS includes a
- transmitter, a receiver, a transmitting antenna, and
a receiving antenna.
For simplex operation, in the first
repeater, Rl, the receiver and receiving antenna
function on a frequency Fl-R, and the transmitter
and transmitting antenna function on a frequency Fl-
T which is removed from frequency Fl-R by a
sufficient amount to provide good frequency
separation for the transmitter and receiver. Other
frequencies F2-R and F2-T through F5-R and F5-T are
provided for repeaters R2 through ~5 respectively.
In the same locality, different groups of
users may utilize the repeaters with the radio
transmitters and receivers in their mobile vehicles
Vl through V5, for example. Typically, each group
has a common basis, such as the vehicles in a given
business, for example taxi cabs or a fleet of oil
delivery trucks. The number of groups which a
repeater can serve depends, among other things, upon
the number of vehicles in a group, the amount of
time a radio channel is used, and the number of
repeater channels available. For example, five
repeaters (each including a transmitter and
receiver~ operating on a channel different from the
other repeaters can serve as many as two ~housand
vehicles. However, the number of vehicles which can
be efficiently served may change depending upon the
demand for the channels.
If there are a number of repeater channels
àvailable in a given locality, it is desirable that
a group of vehicles be able to quickly ascertain
which channel or frequencies are available, so as to
establish communication with the other vehicles in
that same group. The present invention provides an
1364051
45-MR-224
arrangement and method for providing and
- establishinq this communication in a relatively
simple but highly reliable fashion.
FIGURE 2 is a general block diagram of the
repeater trunking system~ ~ ~ to the present
invention for use in a mobile station, such as in a
vehicle. It is assumed that the vehicle is one of a
qroup of vehicles Vl throuqh V5, shown in FIGURE
l. The mobile radio station in the vehicle includes
a radio transmitter ll capable of operating on any
of ~he frequencies Fl-R through F~-R, and a radio
receiver 12 capable o~ operating on any of the
frequencies Fl-T through F~-T. ~o frequencies are
needed for each channel in order that the mobiIe
station and the repeater can function in a simplex
or push-to-talk arrangement. Thus, the mobile
station would transmit on a repeater receive
f~equency (for example F1-R). The signal on
frequency Fl-R would be received by the repea~er and
trasmitted to the other vehicles on frequency Fl-
T. The other vehicles in tle group would receive
the signal on repeater frequency Fl-T. When
transmitting, t~e other vehicles would transmit on
frequency Fl-R. In such an arrangement, a single
antenna 10 is switched by a relay l9 between the
vehicular or mobile transmitter ll and receiver ~2,
depend`ing upon whether the mobile station is
transmittinq or receiving.
Full duplex operation could be provided by
using these same two frequencies, but with separate
receive and transmit injection, a duplexer, and
separate antennas. Or, full du?lex operation could
be provided by speciying four separate and distinct
frequencies w;thin each channel and for the repeater
associated with that channel. It is assumed in this
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f 164051
45-r~lR-224
presently preferred, non-limitative, exemplary
- embodiment that transmitter 11 and receiver 12 are
frequency modulation devices, although they may be
amplitude moduIation devices.
Transmitter 11 includes an oscillator
circuit ~not shown) which operates at one of five
transmit frequencies dependinq upon the signal
applied to it by a.search generator 13. Receiver 12
is preferably of the superheterodyne type and
includes a local oscillator which also operates at a
frequency coresponding to one of five frequencies
depending upon the signal supplied by the search
aenerator 13. Clock siqnals are Provided by a clock
aenerator 16. Search generator 13 is supplied with
clock signals which cause it to se~uentially pro~uce
fre~uency scanning signals at each o~ five outputs
20. These scanning signals are applied to the
transmitter and receiver oscillators to cause
transmitter 11 to be sequentially operable on radio
- 20 fre~uencies Fl-R throuqh F5-R, and receiver 12 to be
sequentially operable on the radio frequencies Fl-T
throu~h F5-T. Tone encoders, decoders, and gates 18
are provided for decoding or filtering tones
received by receiver 12 and for encoding or
producing ~ones to be gated and transmitted by
transmitter 11.
The mobile station includes operator
controls and indicators 14 to enable the user to
operate his station and to have inaications of the
status of his station. The user controls permit the
operator to place his station in the call originate
mode to establish communication with another vehicle
in the same group, or to place his station in the
idle or scannin~ mode so that he will receive
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45--MR-224
communications from other mobile stations in the
same group.
A control circuit lS supplies clock signals
from clock generator 16 to a sequence generator 17
when the operator controls Place the station in the
call originate mode,or to search qenerator 13 when
the operator controls place the station in the idle
mode.
In the call originate mode, contro.l- circuit
15 supplies clock pulses to the sequence generator
17 to cause the sequence generator to proauce
signals which provide the following sequence of
functions: first, locating an idle channel; second,
transmitting a busy siqnai, suitably an audio tone
of a predetermined frequency to the repeater to
acquire and enable or activate the repeater trans-
mitter and receiver on the idle channel; third,
stopping transmission and causing the receiver to
listen for an acquisition signal from the repeater;
fourth, transmitting a group signal, suitably an
audio tone of a predetermined frequency different
from the frequency of the busy signal to identify
other mobile stations within a predetermined group;
fifth, transmittin~ a subgroup signal, suitably an
audio tone having a frequency different from both
the busy signal and group signal, i~ the stations in
a given group are further subdivided into subgroups;
sixth, stopping transmitting and causing the mobile
station's receiver to listen and ascertain whether
the repeater is sendinq out a trans~ission with a
busy signal on the selec~ed channel, and seventh
repeating the sequence on all available channels if
the repeater transmitter was not enabled or
activated in the second function or if an-
acquisition signal was not received. Tf the
t l 64051
45-MR-224
repeater transmitter was activated, then indications
_~ are provided to the operator that he has acquired
and activated a repeater, and that his station and
the repeater are in a ready mode for operation~
S In the idle mode, control circuit 15
supplies clock pulses to search generator 13 to
cause the search generator to produce signals for
changing the frequency of the transmitter and
receiver oscillators. This causes the receiver 12
to se~uentially tune to each of the five frequencies
on which repeaters can transmit and, at the same
-time, causes transmitter 11 to sequentially tune to
each of the five companion or associated frequencies
on which ~he repeaters can receive. If the group
signal for that particular mobile station is not
detected on a cha~nel, search generator 13 causes
the receiver and ~ ~of that mobile station -
to tune to the next ch-annel frequency. This
sequence of events continues as long as receiver 12
is operable, and stops only when it receives a busy
signal, its predetermined group signal, and its
subgroup signal if utilized. If the group signal
and subgroup 8~ (if used) are received, control
circuit lS provides an indication to the user that
he is being called. The user can then pick up his
handset or microphone and communicate with the
calling station.
FIGURE 3 is a block diagram of one of the
repeaters shown in FIGURE 1 for use with the
trunking system according to the present
invention. In this repeater, an antenna 3~ is
connected to a repeater receiver 31 for receiving a
particular frequency, for example Fl-R. The output
of repeater receiver 31 is coupled to a busy signal
35 notch filter 32 ~lhich filters out or rejects the
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11
45-MR--224
busy signal, but applies the received audio signals,
~- ~roup signal, and subgroup signal to a repeater
transmitter 33. Repeater transmitter 33 is coupled
to an antenna 37 for transmitting on a compa'nion
channel frequency Fl-T. The companion channel
frequencies Fl-R, Fl-T are separated sufficient for
proper circuit operation. It is possible to operate
repeater rece'iver 31 and repeater transmitter 33 on
tlle same antenna, or to operate alI of the repea~ers
on the same antenna if proper couplinq and matching
networks are used.
The output of repeater receiver 31 is also
applied to a busy signal bandpass filter and
d'etector 34 which detects only t~e busy slgnal
frequency and' applies the busy signal detection
signal to a timer 35. Timer 35 produces an enabling
or keying signal at the beginning of and as long as
a busy signal is detected, and for a selec~ed period
~for example 10 seconds) after termination of the
busy signal. This enabling or keyins siqnal turns
on or enables repeater transmitter 33. When the
enabling signal ends, repeater transm;tter 33 is
turned'off automatically without ~he need Eor an
idle signal. This feature prevents repeater
transmitter 33 from being erroneously keyed or
enabled in the absence of a busy signal. Timer 35
also permits a repeater to become available to
another subsequent group if transmissions by the
acquiring group do not occur within a predetermined
span of time, for example, every 10 seconds. Timer
35 generates a window for the transmission of an
acquisition signal to the call originating mobile
station.
The repeater also includes a bus~ signal
generator 36 and an acquisition signal generztor 38,
1 3 64051
12
45-MR-224
the outputs of which are coupled to the input of
_ repeater transmitter 33 along with the audio and
subgroup signal.
It is preferred that the ~usy signal be
regenerated at the repeater so as to insure that the
mobile stations are kept in operative condition even
though there is momentary absence (for example a few
seconds less than 10) of any mobile station
transmission. ~owever, the group and subgroup
signals are re-transmitted by repeater transmitter
33 as detected by repeater receiver 31 from the
mobile transmitte~. Each of the individual blocks
of FIGURE 3 are known in the art, so that a more
detailed description of them is not believed to be
necessary. Each of the repeaters shown in FIGURE 1
is similar to the exemplary repeater shown in FIGURE
3, but operates at different frequencies.
FIGURES 4A throug'n 4D show a detailed
circuit diagram of our mobile station communication
establishing arrangement of FIGURE 2. FIGURES 4A
through 4D form a single sche~a.ic diagram as shown
in FIGURE 5 which is a composite of FIGU~S 4A-4D.
For clarity, transmitter 11, receiver 12, and
antenna 10 shown in FIGURE 2 have been omitted from
FIGURES 4 and 5. In FIGURE 4, search generator 13;
clock generator 16; tone encoders, decoders, and
gates 18; operator controls and indicators 14; and
sequence generator 17 are shown enclose~ in
respective dashed line rectangles. The remainder of
the circuit outside these rectangles comprises the
control circuit 15. The circuit shown in FIGURE 4
inlcudes four types of logic gates, namely an OR
gate such as Gl, a NOR gate such as G2, and AND gate
such as G3, and an inverter such as Il. As kno~ in
' 35 the art, an OR gate produces a logic 1 at its output
- . t tfi4051
45-MR-224
in response to any o~ its inputs being at a logic 1,
~
and produces a logic 0 at its output in response to
all of its inputs being a logic 0. A NOR gate
produces a logic 0 in rsponse to any of its inputs
being at a logic 1, and produces a logic 1 in
resp~nse to all of its inputs being at a logic 0.
An AND gate produces a logic 0 in response to any of
its inputs being at a logic 0, and produces a logic
1 in response to all of its inputs being at a lo~ic
1. ~n inverter simply produces the logic inversion
B f ~ signal applied to its input. In this circuit,
it is assumed that a logic 1 is a positive volta~e
l~, and that a logic 0 is zero volts or ground.
However, other voltages may be utilized for the
logic 1 and logic 0 levels.
The circuit shown in FIGURES 4 and 5
utilizes a number of bistable flip-flops, such as
the flip-flop FFl. Flip-flop FFl produces a lo~ic 1
at its Q output and a logic G a' its Q output in
response to a logic 1 being momentarily applied to
its set input S; and produces a logic 0 at its Q
input and a logic 1 at its Q input in response to a
logic 1 being momentarily applied to its reset input
R. The circuit utilizes a timer Tl which is
normally reset to produce a logic 0 at is Q output.
when timer Tl is set in response to a logic 1 being
applied to its set input S, it produces a logic 1 at
its Q output for a predetermined time (approximately
.5 se,conds in~ a preferred embodiment). After the
~ time expires, the timer T1 resets so
that its Q output produces a logic 0.
Clock generator 16 includes a pulse
oscillator that produces logic 1 pulses of
approximately 10 milliseconds duration approximately
every 90 milliseconds.
. . 1 164Q51
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45-MR--224
Sequence generator 17 includes a seven
~~ stage shift register SRl having a clock input C and
a reset input R. When shift register SRl is in an
initial or reset condition, a logic 1 is present in
stage 1, and all other stages are at a logic 0.
This logic 1 is seguentially coupled through the
subsequent stages 2 through 7 in response to clock
signals. The stages may include a number of
substages, depending upon ~he timing function
required for a given stage.
In the preferred embodiment for a five
channel system stage 1 has one substage; stage 2 has
B one substage, stage 3 ~as one substage, stage 4 has
~, six substages; stage~has five substages; stage 6 has
two substages and stage 7 has one substage. This is
a total of 17 substages which, for a clock pulse
rate of 90 milliseconds, requires about 1.53 seconds
for one sequence through the shift register SRl.
Stage 4 preferably has a period sufficiently long so
that each receiver in a group has sufficient time to
scan all channels for a group signal if it is
present. Stage 6 preferably has a time period
sufficiently long so that activation of the repeater
can be determined. These times will be discussed in
more detail subsequently. When the logic 1 reaches
stage 7, it is fed back to the input for subse~uent
passage through the stages. In search generator 13,
a five state counter is provided for the five to
twenty channel fre~uencies. The counter produces a
5 ~it BCD code which is decoded to produce the
required channel frequency,
In tone encoders, decoders, and gates 18,
~here is provided four tone encoder-decoder circuits
TC-l...TC-4, and one decode only TC-5. Each of
these circuits includes a decoder portion having a
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15 1 164051
45-~lR-224
tone input I coupled to the audio output of receiver
cc 12. The decoder portion includes a filter and
produces a logic 1 at its Q output in response to a
signal of predetermined frequency. While we have
shown the circuits TC-3, TC-4 with their Q outputs
applied to an OR gate Gll, the Q outputs could also
be switched for individual selection by another
section of a switch 46. Each of the circuits ~C-
l..~TC-4 also includes an encoder portion which
produces a particular signal frequency depending
upon the intended function of the signal. For the
busy signal, a relatively high audio frequency of
3000 Hertz is preferred, since the higher frequency
passes through filters and operates more quickly.
The group and subgroup signals may be any suitable
freguency, but are preferab~y in the voice range of
B audio frequencies ~ ~ ~te~, namely between about
400 and 3000 Hertz. Each of the circuits ~C-l...TC-
4 acts as an encoder in response to a logic 1
applied to a control input C, and ac~s as a decoder
in response to a logic 0 applied to its control
input.
The outputs of the encoder portions o~ the
circuits TC-l...TC-4 are selectively applied through
switches S4 through S7 to an amplifier 51. The
output of this amplifier 51 is coupled to trans-
mitter 11 for modulating the transmitted signals in
accordance with its output signal. In tone
encoders, decoders, and gates 18, are provided a
tone burst generator 50 which, along with a ready
lamp 41, provide a short audible indication to the
user that he has received a call, or that he has
enabled the repeater to transmit.
In the circuit, there is utilized a
plurality of switches Sl through S7 indicated by a
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16
4 5--MR--2 ~ 4
switch arm contained in a rectanqle. Because the
c~ circuit operates with ~ogic level signals, switches
Sl through S7 must be fast operating, so that we
preEer to use a switching devic.e such as an FET
transistor. Each of switches Sl through S7 responds
to a logic slgnal applied to a lead indicate,d as a
dashed line in contact with the switch arm. Each
switch opens i`ts switch.path ;n response to a logic
0, and.provides a closed. path in response. to a logic
10 1.
In the operator controls and indicator 14,
we provide a wait lamp 40 to indicate to a user that
his mobi'le station is scanning for a vacant channel
in.call originate mode; a ready lamp 41 to indica~e
to an oper,ato,r that ~is control arrangement has
B enabled''the~r~ ~ and receiver for operation;
a microphone 42 for transmitting voice signals; a
microphone push-to-talk button 43 to key or energize
the transmi'tter for trans~ission; a loudspeaker 44
for hearing received audible signals; a microphone
hook switch 45 having an on hook position when the
microphone 42 is placed on the switch 45, and an off
hook position when it is removed from the switch 45;
and finally a subgroup selector st~itch 46 if
sbugroup signals are utilized. We also provide a
notch filter Fl coupled between the receiver audio
ou`tput and the loudspeaker switch S2 to remove the .
busy signal (preferablY 3000 ~ertz) which would
otherwise be heard by a mobile station use.r. If
desi'red, microphone 42 and loudspeaXer 44 may be
replaced by other devices, such as a telephone
handset which can be placed on the hook s~itch 45.
When a user is in his mobile unit and is
expecting to be called at any time, he places his
. .
station in the idle mode by turning his mobile
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~ 1 6 4 0 5 1
17
45-MR-224
station on. This provides power to our arran~ement,
--`' and the voltage +V is applied as a logic 1 through
the on hook contact of the switch 45, ~apacitor C5
and gate Gl to reset flip-flops FFl, FF3 if they are
not already reset, and through capacitor Cl and OR
gate G16 to reset flip-flop FF2 if it i5 not already
reset. However, the transmitter is not keyed or
- radiating, and the mobile antenna is connected to
the receiver. If conditions re~uire distinction
amon~ mobile stations within the same group,
subgroup signals (one for each desired subgroup) can
be provided. In FIGURE 4, it is assumed that there
are two subgroups available, and that the station
shown is-in subgroup 1. Hence, the switch arm of
subgroup selector switch 46 is shown connected to
subgroup contact 1. This connects tone circuit TC-3
for subgroup 1 into the circuit, and disconnects
tone circuit ~C-4 for subgroup 2 from the circuit.
In the idle mode, the reset flip-flop FFl Q output
is at a logic 1 and its Q output is at a logic ~.
Flip-flop FF3 is also reset so that its Q output is
a logic 0. this causes an AND gate G18 to produce a
logic 0 and prevents clock pulses from being applied
to shift register SRl within sequence generator 17.
If no signal is present, a sensor, such as
busy tone decoder TC-l, coupled to gate Gl~ is at a
logic 0. This sensor may detect a carrier signal,
or busy signal, or group signal as desired. We
prefer that the sensor detect the busy fre~uency.
The logic ~ produced by SRl causes tone circuits TC-
2, TC-3, TC-4 to decode. The output of the OR gate
G22 is a logic 0 which causes tone circuit TC-l to
also decode. But if no tones have been received at
this time, the Q outputs of the decoders TC-l, TC-2,
TC-3, TC-4 are at a logic 0. Hence, the flip-flop
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~ 1`64051 ~
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45-MR--224
FF4 produces a logic 0. Flip-flop FF2 is also reset
so that its Q output is at a logic 0 and its Q
output is at a logic 1. Under this condition, all
inputs to the NOR gate G2 are at a logic 0 so that
gate G2 produces a logic 1. This logic 1 permits
the AND gate G6 to pass the logic 1 clock pulses as
they are produced by generator 16. These clock
pulses are passed by the AND gate G6 to the clock
input C of search generator 13. This causes a BCD
code to be generated at the fi~e outputs of search
generator 13. This BCD code, when decoded, controls
the oscillator frequency in both the trans~itter and
receiver of the mobile station so that the
transmitter and receiver are tuned to the
frequencies FlR and FlT, F2R and F2T, F3R and F3T,
F4R and F4T and F5R and F5T in sequence.
If a busy signal is present, the busy
signal decoder TC-1 produces a logic 1 at its Q
output. If a proper group tone is also detected,
the group tone decoder TC-2 produces a logic 1 at
its Q output. This logic 1 is held sufficiently
long by FF4 (timed by timer Tl) for detection of the
subgroup 1 signal. With receipt of the busy signal
and group signal, tone decoder TC-2 produces a logic
1 at its Q output, causing NOR gate G2 to produce a
logic 0. This logic 0 at gate C2 prevents clock
pulses from passing through the AND gate G6 so that
- search generator 13 is held in its present state
which activates the receiver oscillator (and also
the transmitter oscillator) associated with the
channel frequency on which the busy and group
signals were detected. If the proper subgroup 1
signal is detected, then the tone decoder TC-3
produces a logic 1 at its Q output. This logic 1 is
passed by OR gate ~11. t~hen the decoders TC-l,
~ l 6405`1
19
4 5--MR--2 2 4
TC-2, TC-3 simultaneously produce a logic 1, an AND
,-- gate G12 produces a logic 1 which is passed by an OR
gate G13 to set flip-flop FF2. When flip-flop FF2
becomes set, its Q output supplies a logic 1 which
causes NOR gate G2 to continue producing a logic 0
that blocks the clock pulses in AND gate G6. This
logic 1 from the Q output of the flip-flop FF2
closes switches Sl, S2, and S3. Closed switch Sl
connects microphone 42 to the transmitter; closed
switch S2 connects the loudspeaker 44 through the
filter Fl to the receiver audio output; and closed
switch S3 connects the push-to-talk button 43 to an
OR gate G14 for keying the transmitter~ The logic 1
from the Q output of the flip-flop FF2 also turns on
the ready lamp 41 and energizes a tone burst
generator 50 (connected to the loudspeaker 44) to
indicate to the operator that he is receiving a call
and switches a busy tone attenuator 101 into the
busy tone encode ~the busy tone deviation is reduced
in ready mode since voice must be transmitted with
busy tone) line. The transmitter and receiver of
the mobile unit are tuned to the proper transmit and
receive frequencies for communicatins with the
repeater to other mobile units in the group.
The user responds by removing his
- 30 microphone 42 from the hook s~itch 45, or if his
microphone 42 is already removed, the switch 45
supplies a plus voltage or logic 1 to the off hook
contact. A busy signal from a repeater, as detected
by busy tone decoder TC-l or by the encode control
line to busy tone decoder TC-l keeps a busy tone
counter 103 reset via a gate G-40 thereby causing a
logic "0" at its Q output. Hence, flip-flop FF2
r~omains in the set condition. ~Yhen the operator
wishes to talk, he depresses his microphone push-to-
... .
I I fi ~ O 5~
2~
- 45-MR-22ds
talk button 43 which provides a logic 1 through
~: closed switch S3 to OR gate G14 to key the
transmitter and cause it to radiate and to cause
antenna relay 19 (see FIGURE 2) to switch to the
transmitter position. Each time push-to-talk button
43 is depressed, a logic 1 is applied to OR gate G14
to cause a busy signal to be transmitted so as to
keep the repeater timer 35 (see FIG. 3) producing an
enablins signal. Upon release of the push-to-talk
button 43, the transmitter is unkeyed and antenna
lS relay 19 switches to the receive position so that
the operator can hear signals from his loudspeaker
44.
The user remains on the channel as.long as
there is activity. When transmissions cease, the Q
output of the busy tone decoder TC-l becomes a logic
0. This permits busy tone counter 103 to count a
logic 1 that is passed by the OR gate G16 to reset
the flip-flop FF2. This causes the NOR gate G2 to
produce a logic 1 that permits the gate G6 to pass
clock pulses to the search generato 13. Hence that
user's receiver returns to the idle mode and scans.
Or when the user is finished communicating,
he places his microphone 42 back on hook switch
B 45 This C ~ se~ a plus voltage or logic 1 to be
applied to the on hook contact. This logic 1 is
momentarily passed by a capacitor Cl through OR gate
G16 to reset flip-flop FF2. ~hen flip-flop FF2 is
reset, its Q output returns to a logic 0. The logic
- 1 from the on hook contact is also momentarily
passed by the capacitor C5 and the OR gate G3 to
reset the flip-flops FFl, FF3 if they are not
already reset. Removal of the busy signal causes
1ip-flop FF2 to produce a logic 0. Hence, NOR gate
G2 produces a logic 1 that permits clock pulses to
.. . .
1 ~ 6~05 1
45-MR-224
21
pass through gate G6 and cause the search generator 13
to begin searching once again.
In brief summary, when a mobile station is in
the idle mode, search generator 13 scans until a signal
is detected. If a busy signal and the proper group and
subgroup signals are received, scanning stops, and an
indication is provided to alert the user of a call.
The following description relates to the call
originate mode. Once again, it is assumed that the
mobile station is using a group signal and is in
subgroup 1, and that the subgroup 1 signal has been
selected by the subgroup selector switch 46 engaging
contact 1 for connecting the tone circuit TC-3 into the
circuit. Search generator 13 is scanning the
channels. The user may initiate the call originate
mode by either momentarily depressing his push-to-talk
button 43 or by removing his microphone 42 from the
microphone hook switch 45. Either action causes a
call-originating command signal such as a positive
voltage or logic 1 to be momentarily applied by
capacitors C2 or C3 to an OR gate G17. This logic 1 is
applied to the AND gate G3 along with the logic 1 from
the Q output of the reset flip-flop FF2. AND gate G3
produces a logic 1 that sets the flip-flop FFl and
places it in the call originate mode. The Q output of
the flip-flop FFl switches to a logic 1 which turns on
the wait lamp 40 to indicate to the operator that he
must wait until a channel is established. A Q output
- of flip-flop FFl at logic "0" is applied to the reset
input R of a programmable counter 60. Programmable
counter 60 allows sequence generator 17 to run
~ through its entire sequence for each channel. If
; proper communication is not established after one
sequence per channel, programmable counter 60 causes
I
1 1 640~ i -
22
4 5-MR-224
5, its Q output to produce a logic 1 that resets the
_. flip-flop FFl to the idle mode. The logic 1 from
the Q output of the set flip-flop F~l is also
applied,to an AND gate G4 along with whatever logic
is being produced by the inverter Il, If, at the
time the user switched to the call originate mode,
his search generator 13 was on a busy channel, the
receiver signal sensor produced a logic 1 which was
inverted to a logic 0 so that gate G4 produces a - -
`logic 0. This permits the search generator to `~ '
continue until an idle channel is found (indicated
by lack of a carrier or by the lack of a busy
signal)j and the receiver signal sensor produces a
logic 0. This logic 0 is inverted to a logic 1 by
an inverter Il so that gate G4 produces a loqic 1.
This logic 1 sets flip-flop FF3 and its Q output
produces a logic 1 for the remainder of the call
, originate sequence. This logic 1 from flip-flop FF3
cause's NOR gate G2 to produce a logic 0 so that gate
G6 prevents he clock pulses from reaching search
generator 13 for the remainder of the call originate
se~uence. The logic 1 from tbe flip-flop FF3 is
applied to AND gate G18 to permit the clock pulses
from clock generator 16 to be applied to the cloc~
input of shift register SRl. Shift register SRl was
previously reset so that the circulating logic 1 was
in stage 1. Logic 1 signals produced by the SRl
'- stages 2,4 and 5 places tone circuits TCl, TC2, TC3,
or TC4 in the encodin~ mode.
In the call originate sequence, clock
pulses' are passed by the gate G18 to cause the logic
1 to shift from stage 1 to stage 2 of shift register -
SRl. This logic 1 is applied by an OR gate G22 to
the control input of switch S4 and also to tone
~~ circuit TC-l to switch it to the encoding mode.
... .
. . ~ ` J ` . ` ~
.; .
1 ~ 6405 1
23
. . 4 5~ 2 2 4
S Tone circuit TC-l produces a busy tone of 3000 ~ertz
-~ which is passed by switch S4, ampiified by tone
amplifier 51, and applied to the mobile transmitter
11. The logic 1 in stage 2 is also passed by OR
gate G14 to key the mobile transmitter 11 so that
the busy signal is transmitted to the repeater. As
mentioned before, the repeater transmitter 33 o~
FIGURE 3 is enabled by receipt of this busy signal,
and transmits a new busy signal on the selected
channel to mobile stations. After the appropriate
number of clock pulses, the logic 1 in stage 2 is
then transferred to stage 3 of shift register S~l.
This transfer opens the switch S4 and removes the
busy signal. This activates an acquisition tone
decoder TC-5. If an acquisition tone is not
detected, then its Q output is logic "ln. This "ln
at G44 resets SRl to State 1 thru G20 and flip-flop
FF-3 through gate G42. The search generator
advances to the next idle channel and SRl starts
over. This continues until an acquisition signal is
found or until counter 60 causes the station to go
into idle mode. If an acquisition tone is located,
then shift register SR-l shifts to its fourth
- step. The transferred logic 1 in stage 4 again
passes through the gate G14 to key mobile
transmitter 11. The logic 1 is also applied to the
switch S5 associated with the tone circuit TC-2.
Tone circuit TC-2 is in the encoding mode. When the
switch SS is closed, the qroup signal (for example a
tone of 1050 ~ertz) produced by the encodinq tone
circuit TC-2 is applied to the tone amplifier ~1 and
keyed transmitter 11 which transmits this group
si~nal to the repeater which retransmits it to the
mobile stations. The logic 1 is held in stage 4 for
a sufficient length of time to permit all of the
g ~ 6405 1
24
45-MR--22'
search generators in the other mobile stations to
~- n pass through all five channels of their ~eY#K~r
~L~, search ~enerator since it is possible that a search
generator could have just left the acquired channel
at the time communication was being established.
Those stations in the designated group be~in to
respond.
After a suitable number o~ clock pul~es,
the logic 1 in the shift register SRl is shifted
from state 4 to stage 5. This trans~er terminates
the group signal provided by the encoder in tone
circuit TC-2. The logic 1 in stage 5 is passed by
gate G14 to key the mobile transmitter 11. This
logic 1 is also coupled through subgroup selector
switch 46 and its contact to close the switch S6.
(If selector switch 46 had been in position 2, the
logic 1 ~rom stage 4 would,have been applied to the
switch S7.) The tone from the subgroup signal
circuit TC-~ (for example a 400 Hertz tone) is
passed by the switch S6 to tone amplifier 51 and
applied to transmitter 11 which is keyed and trans-
mits the tone to the repeater which re-transmits it
to the other mobile stations.
After a suitable number of clock pulses,
the logic 1 is shifted from stage 5 to stage 6 of
shift Eegister SRl. This trans~er terminates the
- subgroup signal provided by the encoder of tone
circuit TC-3. Under this condition, no logic 1 is
applied to the gate G14, so that the mobile
transmitter 11 becomes unkeyed. At this time, both
inputs to the OR gate G22 are at a logic 0, and this
logic 0 causes the busy tone circuit TC-l to switch
to the decode mode. Since the transmitte~ is
unkeyed, the receiver is enabled, and is listening
''~ on the selected channel. The reoeater should have
: . ~ 1 6405 1
9 5-MR- 2 2 4
been enabled and keyed by this time, and busy signal
_ - should be present on the channel~ If busv tone is
present, and received by the call originating
mobile, the tone causes the decoder of tone
circuit TC-l to produce a logic 1 at its Q ouptut.
This logic 1, along with the logic 1 from staqe 6 of
shift register SRl, is applied to an AND ga~e G23 to
cause gate G23 to produce a logic 1. This logic 1
is applied through O~ gate Gl to the reset input R
of the flip-flop FFl to reset that flip-flop, and
--15 sto~ the calling seguence. Resettin~ of the flip-
flop FFl extinguishes wait lamp 40. This logi~ 1
from the gate Gl also resets the flip-flop FF3, and
its Q output switches to a logic 0 to prevent any
further clock puises from passing through AND gate
G18. This logic 1 from the ~ate Gl passes through
an OR gate G20 to reset the shift register SRl. The
logic 1 from the gate G23 is also applied through
the OR gate G13 to set flip-flop F~2. When flip-
flop FF2 becomes set, its Q output switches to a
logic 1 which turns on the ready lamp 41, closesswitches Sl,.S2, S3, and energizes tone burst
generator 50 so that the user is alerted that he has
obtained a channel. The closed switches Sl, S2, 53
. 30 connect the speaker 44 to the receiver 12, and
connect microphone 42, and push-to-talk button 43
through the gate G14 to the transmitter 11. With
- flip-flop FF2 held in the set condition, its Q
output of a logic 1 insures that the gate G2
produces a logic 0 so that clock pulses cannot be
applied to search genertaor 13, and cause the
established channel to be switched. Each time the
operator depresses his push-to-talk button 43, the
. logic 1 produced passes through the O~ gate G22 to
_. 40 cause the tone circuit TC-l to supply busy signal to
., .
. . li l 6405 1
26
45-MR-224
the mobile transmitter 11. Th~ attenuator
_ 101 in the busy tone output reduces the busy tone
deviation to allow voice to also modulate the
transmitter.
In the sequence above where a }ogic 1 was
in stage 6 of the shift register SRl, if no busy
signal had been detecte~ by the decoder in the tone
circuit TC-l, the flip-flop FFl would have remained
set, flip-flop ~2 would have remained reset, and
the flip-flop FF3 would have remained set. This
-- 15 would permit gate G18 to pass another clock pulse
and cause the logic 1 to be shifted from stage 6 to
stage 7 which resets SRl to state 1.
If a channel is successfully acquired, the
operator can communicate. As mentioned earlier,
because of the ~imer 35 provided at the repeater, it
is necessary for a busy tone to be received by the
repéater during the timing interval in order to
retain acquisition of the channel. After the
communication between two mobile stations through a
repeater is ended, the control is restored to the
idle by replacing the microphone on the hook switch
45. This causes a positive voltage or a logic 1 to
be passed by capacitor C5 through the OR gate Gl to
reset flip-flops FFl, FF3 (already reset) and passed
by capacitor Cl through OR qate G16 to reset flip-
flop FF2. Their Q outputs return to a logic 0. The
group tone decoder in circuit TC-2 is producing a
logic 0. Hence, all inputs to the gate G2 are at a
logic O so that gate G2 produces a logic 1 to permit
clock pulses from the generator 16 to be passed by
the gate G6 to the search generator 13. In the
B - event the mobile operator neglects to hang up ~ h~
icr~ nh~ ~
c~6*e 42 after a communication has ended and
return his radio to the idle mode, a logic "1" from
~ .
1 1 6~05 1
27
45-MR-22ds
. busy tone circuit TC-l keeps busy tone counter 103
_ reset. Once the repeater has timed out, the busy
signal drops off of ~he channel and a logic "0"
appears at the Q output of TC-l. BUsy tone counter
103 counts for approximately one second and provides
a logic "1" at its Q output to reset flip-flop FF2,
returning the station to the id~e mode.
A logic "1" is fed back from stage 7 of
shift register SRl to gate G2~ and to a gate G42 to
reset shift register SRl and flip-flop FF-3. The
channel selection advances for each clock pulse
coupled to search generator 13. Upon receipt of
further clock pulses, the sequence described above
and starting at stage 1 is repeated. The sequence
will continue until the channel counter reaches 5
(this casé) or if, when the logic 1 reaches stage 6,
a busy tone is heard from the repeater, then flip-
flop FF2 becomes set and flip-flop FFl becomes reset
to provide the user an indication that he has
acquired a channel. He should take his microphone
42 off hook before. the timer in the repeater times
out (approximately 10 seconds),and resets.
The operation of the repeater,trunking
system will be further explained in terms of its
various operating modes.
30 Referring now to FIGURE 5, there is shown a
pictorial diagram indicating the various modes of
operation of the repeater trunking system according
to the present invention. The "call originate" and
n idle" modes are further broken down into the
35 . functional modes "idlen, ~wai~" and "ready". When
power is initially applied, a mobile station is
placed in the idle mode. In the id~e mode, a mobiIe
station scans all channels for a call identifying a
particular group with ~hich it is associated. All
~ 1 6405 1
45-MR-224
28
audio circuits are inhibited in this mode so that the
user is undisturbed. If a call is decoded indicating
that a particular mobile station is within a group
being called, that mobile station is placed in the
ready mode.
In the ready mode, the mobile station is
locked onto a channel and ready to communicate. The
audio circuits are enabled and the ready lamp 41 is
on. Alert beeps (four) are sounded via speaker 44. If
the repeater goes off the air or communication is
otherwise interrupted, the mobile station will return
to the idle mode from the ready mode (hook switch 45
hung-up at other mobile station).
From the idle mode, if push-to-talk button 43
is depressed, the mobile station will operate in the
wait mode. Similarly, taking the microphone off of
hook switch 45 will cause the station to enter the wait
mode. In the wait mode, the station stops on the first
idle channel and attempts to acquire the repeater. The
station sends a busy signal to the repeater and waits
to hear an acquisition signal. During this procedure,
wait lamp 40 is on and the audio circuits are
disabled. After all channels have been tried without
acquiring a repeater, a single alert beep is heard via
speaker 44.
If all channels are busy or if the mobile
station has failed to establish communication with a
repeater, the station returns to the idle mode.
However, if a repeater is acquired, as indicated by
the confirming hand shake of the acquisition signal
transmitted by the repeater and received by the
mobile station, the station proceeds to transmit the
desired group signal. Flow charts indicating the
function of the trunking system according to the
'
` ~6~051
2~
~ 45-MR-224
present invention with respect to its various modes
~ are shown in FIGURES 6, 7 and 8.
Referring now to FIGURE 6, there is shown a
flow chart of the operation of the repeater trucking
system according to the present invention operating
in the idle mode. Operating in the idle mode, each
of the channels is scanned so that a mobile station
can determine whether its particular group signal is
among any group signals being transmitte~ over any
of the channels. If the particuIar group signal is
not detected, this system determines whether
microphone push to talk switch 43 or the o~f hook
switch 45 has been activated. If either of these
switches has been activated, this system goes into
the wait mode. If neither of these switches has
been activated, then the receiver listens to the
next channel to determine whether the particular
group signal is present.
Once a group signal for that particular
mobile station has been detected, a group latch is
set and the qroup signal is examined. When the
group signal has ended, the receiver looks for a
subgroup signal if subgroups and subgroup signals
are being utilized. If the appropriate subgroup is
detected a subgroup latch is set and the group latch
is cleared. The receiver then listens for a busy
signal transmitted by the repeater. If no busy
signal is detected the receiver then tunes to the
next channel and a~ain listens for its particular
group si~nal. ~owever, if a busy signal is
detected, then a call latch is set and the mobile
station enters the ready mode to establish
communication with the mo~ile station c~lling the
group with which it is associated.
~, ....
l l B40~ ~ `
4 5-MR--224
S Referring now to FIGURE`7, there is shown a
_ flow chart illustrating the operation of the
repeater trunking system in the ~ait mode. In the
wait mode, wait lamp 40 is li~hted. If a busy
signal is detected, it is determined whether or not
all channels have been tried, if not, the next
channel in the s~quence is lis~ened to. ~owever, if
- all channels have been tried a beep alert sounds
throu~h speaker 44 and wait light 40 is extinguished
and the trunking system enters the idle mode.
lS Whenever a busy signal is not detected, a busy
signal is encoded and an acquisition signal is
listened for. If an acquisition signal is present,
then the group signal is encoded. After the group
signal is encoded, the subgroup signal is encoded.
After group and subgroup signals have been encoded,
a busy signal is listened for. If the busy signal
is present, wait lamp 40 is extinguished ;the call
latch is set and the station enters the ready mode.
Referring now to FIGURE 8, there is sho~m a
flow chart illustrating the operation of the
repeater trunking system in the ready mode.
Operating in the ready mode, ready lamp 41 is lit
and four alert beeps are sounded through speaker
44. All audio circuits are turned on. If the
microphone push-to-talk switch 43 is keyed then
at~enuated busy signal is encoded. If the carrier
control timer has timed out then an audio alert is
sounded. If the transmitter is keyed and there are
20 seconds of audio alert, the transmitter is shut
down.
After the audio circuits have been turned
on, if a busy signal is detected, a fade margin
counter is reset and it is again determined whether
the microphone has been keyed. If the microphone
.,
6405 1
31
4 5-MR--~ 2 4
.
has ~ot been keyed and a busy signal has not been
~ detected, it is determined whether the fade margin
(no busy tone) timer has timed out. If it has timed
out, the call latch is cleared, ready lamp 41 is
extinguished and the station goes into the idle -` - ~
mode.
Referring now to FIG~RE 9, there is
pictorially illustrated the repeater acquisition
sequence. This chart details the hand shake between
a mobile station attempting to access other mobile
stations within its group through a repeater. A
mobile station first searches for a free channel and
then transmits a busy signal. After it has sent the
busy signal, the mobile listens on the channels on
which the busy signal is sent for an acquisition
signal. Upon receipt of the acquisition signal, the
hand shake has been effected and the mobile proceeds
to send group, subgroup, and busy signals.
The repeater, when not in use, is listening
for a busy signal. When a busy signal is received
from the mobile, an acquisition signal is sent.
Assuming the acquisition signal is followed by
receipt of group, subgroup and busy signals being
sent from the mobile, the repeater receives all of
these signals and in turn retransmits the group and
subgroup signals and regenerates and transmits a
busy Signal. This busy signal serves as notice to
all other mobiles not in the particular group and
subgroup defined that the repeater is in use so that
they will not attempt to gain control of the
repeater.
In brief summary, when a mobile or control
station of the repeater trunking system is in the
call originate mode, the shift register SR2 causes
` 35 scanning until an idle channel is found. This
. ' `?
4 0 5 f
32
45-MR-224
causes the shift register SRl to go through its
~ sequence of causing the transmitter to send a busy
signal, listen for an acquisition signal, and then
send a group signal, and subgroup signal to activate
the repeater and appropriate mobile stations. If ~
the repeater is activated, an indication is provided
to alert the user that he has acquired a channel,
and should transmit within a predetermined time (10
seconds). If the repeater is not activated, the
shift register SRl goes through its sequence on the
next channel, and so on through each channel until a
channel is acquired, or until all channels have been
sequenced. If no channel is acquired, the user can
start the entire sequence over again and again.
It will thus be seen that there has been
provided a new and improved control circuit and
method for enabling the user of a mobile station to
readily and quickly acquire a single unused radio
frequency channel from a plurality of such channels
for transmitting, or to be called on an available
channel. The arrangement is relatively simple, and
does not require extensive or complex circuits.
While there has been shown only one embodiment of
our invention, persons skilled in the art will
appreciate the many modifications that may be
made. For exa~ple, there has been shown a 5 channel
system, but there may be almost any number of
channels and repeaters. However, good trunking
practice limits the channels to about 20. Different
logic circuit arrangements and logic levels may be
used, such as a microprocessor circuit. The various
circuits may have almost any desired timing periods,
particularly those in the shift register SRl and in
the timing circuit Tl. Tne timers and counters may
be in analog or digital form. These are matters of
:' -` i 4 1- ' `
45-MR-224
preference and expediency depending upon the
~~particular communication conditions involved. If a-
system is free from radia.ion interference, the busy
signal may be omitted and operation made dependent
S upon the proper carrier. Otherwise, it is preferred `
that the receiver signal sensor produce a logic 1 in
response to a busy tone, and a logic 0 in response
to the absence of busy signal. While we have shown
one group circuit and two sub~roup circuits, one of
- 10 which is selectable, additional subgroups may be
provided to provide further selection of the
particular mobile units being called. Or, less
subgroup circuits may be provided so that a more
general or less select group of mobiles may be
called. Or, group and subgroup signals may be
transmitted simultaneously rather than sequentially,
or may be frequency stepped functions of one common
encoder and decoder. The arrangement may also be
used directly between mobile stations without the
use o the repeater. Persons skilled in the art
will also appreciate that with a repeater
transmitter frequency and a repeater receive
frequency, only push-to-talk or simpleY. operation
can be provided. However, additional frequencies or
auxiliary equipment may be provided so that full
duplex operation can be provided. Again, this is a
matter of preference and frequency availability.
While it is preferred that the busy signal and
acquis;tion signal frequencies be above the audio
frequency, for fast opertaion, the busy signal and
acquisition signal may be almost any desired
frequency or format. Like~ise, the group and
subgroup siqnals may be almost any desired
frequency. However, we believe it preferable that
`~ 35 the busy signal and acquisition signal be at a
. . . ` 1 1 640~ 1 -
34
4 5--MR- 2 2 4
frequency just above the transmitted audio
- frequencies, and that the group and subgroup signals
be in the band transmitted voice frequencies. This
present no problem, since the group and subgroup
tones are transmitted only when communication is
being established. Therefore, while the invention
has been described with reference to a particular
preferred embodiment, it is to be understood that
modifications may be made without depa~ting from the
spirit of the invention or from the scope of the
claims.