CA1164051A - Repeater trunking system - Google Patents

Abstract

REPEATER TRUNKING SYSTEM

ABSTRACT OF THE DISCLOSURE

Arrangement for establishing communication between a first mobile radio station and a second mobile radio station through a repeater over a radio frequency channel selected from a plurality of such channels. In an idle mode, a mobile station searches the channels for a predetermined group signal iden-tifying it as belonging to a particular group of mobile stations. When that predetermined group signal is received on a channel, the mobile station is enabled on that channel so that it can communicate with a call originating mobile station belonging to the same group. In a call originate mode, a mobile station searches for an idle or unused channel identified by the absence of a busy signal. When an idle channel is found, the call originating station transmits a busy signal. The repeater, hearing the busy signal, transmits an acquisition signal. This acquisition signal is detected by the call originating mobile station which thereafter transmits a predetermined group signal for enabling other mobile stations in the common group on that same channel.

Description

- ~ . 1 1 6~05 1 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, .
...... .

.

~ 1 ~40S 1 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 ...

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 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 - 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.
....

fi ~ O S l ` 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 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 .: ,. 1 ' , ' 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 , . .

~ J 6405 1 g 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 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 . ` . I `
t l 6405 1 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, 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 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 ` 14 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 .....

.. -,. ~ ~ ! ' , 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 : ; jt t l 6405 1 ` 1.

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 ......

~ 1 6 4 0 5 1 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 .

~ 1`64051 ~

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 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 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 -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.
... .

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1 ~ 6405 1 . . 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 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 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 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 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 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.
~, ....

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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 ., 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 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- ' `

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 -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.

Claims (9)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a system including a plurality of radio stations capable of communicating with one another via an enabled one of a plurality of repeaters, each repeater operating on a different channel wherein the channels being used are indicated by the presence of a marker signal appearing thereon and the channels being unused are indicated by the absence of such marker signal, an improved method for automatically establishing a dedicated communication channel between a call-originating first radio station and a call-receiving second radio station on a presently unused channel in response to a call-originating command signal generated at the first radio station, said method comprising the steps of:
automatically scanning by the first radio station of the plurality of channels to select an unused unmarked channel in response to said call-originating command signal; and automatically exchanging signals on the thus selected channel between the first radio station and the repeater associated with the selected channel to enable the repeater on the selected channel, the exchanging of signals including (1) transmitting a busy signal from the first radio station, (2) receiving the busy signal at the thus selected repeater, (3) transmitting by the thus selected repeater of an acquisition signal different from the busy signal, and (4) receiving the acquisition signal at the first radio station, and only thereafter dedicating the thus selected repeater for communication between the first and second radio stations in response to successful completion of the entire exchanging step.

2. A method for establishing communication according to claim 1 wherein the step of scanning includes scanning the plurality of channels in a predetermined sequence beginning with a randomly selected channel.

3. A method for establishing communication according to claim 1 wherein the step of scanning includes scanning the plurality of channels in a predetermined sequence beginning with a predetermined one of said channels.

4. A method for establishing communication according to claim 1 wherein the step of scanning includes scanning the plurality of channels in a random sequence beginning with a randomly selected channel.

5. A method for establishing communication according to claim 1 wherein the step of scanning includes scanning the plurality of channels in a random sequence beginning with a predetermined one of said channels.

6. A method for dedicating a channel for communication between a first radio station through a radio repeater and a second radio station over a radio channel selected from a plurality of such channels comprising the steps of:
placing the first radio station in a call originate mode and scanning the plurality of channels until an idle channel without busy signal is located, and enabling the first station on the idle channel;
transmitting by the first radio station a busy signal to the radio repeater to enable the radio repeater on the idle channel;
receiving by the radio repeater the busy signal transmitted by the first radio station;
transmitting by the repeater of an acquisition signal, different from the busy signal;
receiving at the first radio station the acquisition signal transmitted by the radio repeater;
transmitting by the first radio station in response to the acquisition signal at least one predeter-mined group signal to the radio repeater on the idle channel for retransmission by the radio repeater to enable the second radio station.

7. A method according to claim 6 further including the steps of:
scanning the plurality of channels by the second radio station in the idle mode until a predetermined group signal is located and enabling the second station on the channel having the predetermined group signal; and communicating by the first radio station with the second radio station through the radio repeater in response to the repeater transmitting the busy signal on the idle channel.

8. A method according to claim 7 further including in response to the failure of said radio repeater to transmit said acquisition signal on said idle channel, the steps of:
again transmitting by the first station the busy signal to the radio repeater to enable the radio repeater on another idle channel;
again listening for the acquisition signal transmitted by the repeater;
again transmitting by the first radio station in response to the acquisition signal the group signal to the radio repeater on the idle channel for retrans-mission by the radio repeater to enable the second station;
again receiving by the first radio station on the idle channel to determine whether the radio repeater is transmitting the busy signal on the idle channel; and communicating by the first radio station with the second radio station through the radio repeater in response to the radio repeater transmitting the busy signal, or resuming scanning said channels in response to the failure of the radio repeater to transmit the acquisition signal the idle channel.

9. A method for establishing radio communication from a first radio station through a radio repeater to a second radio station over a radio channel selected from a plurality of such channels comprising the steps of:
scanning by the first radio station the plurality of channels until an idle channel without busy signal is located, and enabling the first station on the idle channel;
transmitting by the first radio station a predetermined busy signal to the radio repeater to enable the radio repeater on the idle channel;
listening for an acquisition signal transmitted by the repeater in response to the busy signal transmitted by the first station;
transmitting by the first radio station at least one group signal to the radio repeater on the idle channel;
receiving by the first radio station on the idle channel to determine whether the radio repeater is trans-mitting the acquisition signal on the idle channel; and communicating by the first radio station with the second radio station through the radio repeater in response to the radio repeater transmitting the acquisition signal on theidle channel.