United States Patent [19J
`Christian et al.
`
`[11] Patent Number:
`[45] Date of Patent:
`
`4,549,293
`Oct. 22, 1985
`
`[75]
`
`[54] TIME DIVISION MULTIPLE ACCESS
`COMMUNICATIONS SYSTEM
`Inventors: J. Robert Christian, Oceanport;
`Louis C. Poli, Hazlet, both of N.J.
`[73] Assignee: The United States of America as
`represented by the Secretary of the
`Army, Washington, D.C.
`
`[21] Appl. No.: 566,767
`(22] Filed:
`Dec. 29, 1983
`[51]
`
`Int. CI.4 ........................... H04J 3/16; H04J 3/06;
`H04B 1/00; H04B 7/00
`[52] U.S. CI ....................................... 370/95; 370/104;
`455/56
`[58] Field of Search ................. 370/104, 100, 103, 95;
`455/56, 53
`
`[56]
`
`References Cited
`U.S. PA TENT DOCUMENTS
`3,908,088 9/1975 Gutleber ........................ 179/15 BC
`4,004,098 1/1977 Shimasaki ........................... 370/104
`4,004,225 1/1977 Glinssonantel ...................... 370/104
`
`4,054,753 10/1977 Kaul et al. .......................... 370/104
`4,301,530 11/1981 Gutleber ............................. 370/103
`
`Primary Examiner-Douglas W. Olms
`Assistant Examiner-Frank M. Scutch, III
`Attorney, Agent, or Firm-Anthony T. Lane; Jeremiah
`G. Murray; Edward Goldberg
`
`[57]
`ABSTRACT
`A covert line of sight time division multiple access
`communications system comprised of a plurality of
`geographically separated transceivers consisting of
`identical hardware and capable of operating either as a
`master station or a slave station, and when desirable as
`a repeater station. The system operates with a non-com(cid:173)
`mitted master station operating within a predetermined
`power circle of operation whereby multiple slave sta(cid:173)
`tions and/or repeaters communicate with each other by
`way of the master station in a one half duplex mode at
`a single frequency in the millimeter wave frequency
`range between 54 and 58 GHz.
`
`13 Claims, 10 Drawing Figures
`
`/
`
`/
`
`/
`
`/
`
`/
`
`Apple Exhibit 1106
`Apple Inc. v. Rembrandt Wireless
`IPR2020-00034
`Page 00001
`
`

`

`U.S. Patent Oct 22, 1985
`
`Sheet 1 of 5
`
`4,549,293
`
`FIG.I
`
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`
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`IPR2020-00034 Page 00002
`
`

`

`U.S. Patent Oct 22, 1985
`
`Sheet2 of5
`
`4,549,293
`
`FIG.2
`
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`IPR2020-00034 Page 00003
`
`

`

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`
`IPR2020-00034 Page 00004
`
`

`

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`
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`
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`
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`
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`
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`
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`
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`
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`
`INDICATOR
`
`OUTPUT
`TO TRANSMIT
`
`IPR2020-00034 Page 00005
`
`

`

`U.S. Patent Oct 22, 1985
`
`Sheet 5 of5
`
`4,549,293
`
`MASTER
`TRANSMISSION
`TM
`
`TOMA TIME FRAME -T
`SLAVES REPLY
`. TRANSMISSION
`Ts-/
`Ts-2
`
`-1 ---
`SYNC MASTER
`SLAVE1'1
`SLAVE#2
`CODES DIGITAL G DIGITAL G DIGITAL G
`BURST
`BURST
`BURST
`
`---
`
`G
`
`Ts-N
`
`SLAVEitN
`DIGITAL G
`BURST
`
`FICJ.6A
`
`SYNC MASTER SLAVE
`CODES
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`
`SLAVE
`#2
`
`G
`SLAVE GUil.RD
`#N
`TIME
`
`FIG.6B
`
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`Tos
`
`SYNC
`CODES
`
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`DATA
`
`I
`
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`GUARD
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`
`FICJ.6C
`
`SYNC
`
`CHI CH2 CH3
`
`SYNC
`
`CHI CH2 CH3
`
`---
`
`---
`
`-- -
`
`---
`
`FIG.6D
`
`CHN icH1 CH2 CH3
`
`I
`
`CHN
`
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`
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`
`I
`
`CHN
`
`FICJ.6E
`
`IPR2020-00034 Page 00006
`
`

`

`1
`
`4,549,293
`
`2
`slave stations and which thereafter are linked together
`through the selected master station to provide multi(cid:173)
`plexed digital channel and/or voice communications.
`Covert operation is further provided by operating the
`5 stations in a half duplex mode at a common frequency in
`the millimeter range of the electromagnetic spectrum
`and more particularly in the 54 GHz-58 GHz frequency
`band, a region of high atmospheric signal attenuation.
`In each instance, the respective transceivers include
`10 both an omnidirectional antenna which is utilized when
`operating as a master station, and a directional antenna
`which is utilized when operating as a slave station. By
`coupling two transceivers together at the same location,
`a repeater station can be established whereby one direc-
`15 tional antenna is directed to the master station while the
`other directional antenna is directed to another slave
`station.
`
`TIME DIVISION MULTIPLE ACCESS
`COMMUNICATIONS SYSTEM
`
`The invention described herein may be manufac(cid:173)
`tured, used and licensed by or for the Government for
`governmental purposes without the payment of any
`royalties thereon or therefor.
`
`FIELD OF THE INVENTION
`This invention relates generally to wireless communi(cid:173)
`cations systems and more particularly to a covert, rela(cid:173)
`tively short range time division multiple access commu(cid:173)
`nications system involving a plurality of identical radio
`transceivers selectively operating as a master station
`and slave stations linked together through the selected
`master station.
`
`BACKGROUND OF THE INVENTION
`Time division multiple access communications 20
`(TDMA) systems comprised of a plurality of geograph(cid:173)
`ically separated radio transceivers which communicate
`with one another through a master station are generally
`known. The slave stations do not have direct radio
`contact with each other, but communicate through the 25
`master station in a designated time slot within a channel
`frame period. Also the master station is a piece of appa(cid:173)
`ratus specifically designed to provide control and syn(cid:173)
`chronization and is normally dedicated for that purpose.
`Typical examples of such systems are shown in U.S. 30
`Pat. No. 3,908,088, "Time Division Multiple Access
`Communications System", Frank S. Gutleber, Sep. 23,
`1975, and U.S. Pat. No. 4,301,530, "Orthogonal Spread
`Spectrum Time Division Multiple Accessing Mobile
`Subscriber Access System", Frank S. Gutleber, Nov. 35
`17, 1981. If the master or central station in these systems
`becomes inoperable, the entire network becomes useless
`or at least is impaired to the point where it is substan(cid:173)
`tially useless.
`Accordingly, it is an object of the invention to pro- 40
`vide an improvement in radio communications systems.
`It is another object of the invention to provide an
`improvement in time division multiple access communi(cid:173)
`cations systems.
`A further object of the invention is to provide a time 45
`division multiple access communications system
`wherein covert operation is provided between multiple
`slave stations by way of a master station.
`Still a further object of the invention is to provide an
`improvement in time division multiple access communi- 50
`cations systems which include a plurality of user sta(cid:173)
`tions, any one of which is capable of operating as a
`master station.
`And yet a further object of the invention is to provide
`improvement in time division multiple access communi- 55
`cations systems wherein a plurality of slave stations
`communicate with each other by way of a master sta(cid:173)
`tion while utilizing a single frequency.
`SUMMARY
`The foregoing as well as other objects of the inven(cid:173)
`tion are provided in a time division multiple access
`communications system comprised of a plurality of
`identically implemented transceivers geographically
`separated from one another, with each being capable of 65
`operating as a master or slave station and wherein one
`of the transceivers is selected, on demand, to act as the
`master station whereupon the other stations operate as
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`These and other features of the invention will become
`more readily understood from a consideration of the
`following description taken in connection with the ac(cid:173)
`companying drawings in which:
`FIG. 1 is a simplified diagram of a time division multi(cid:173)
`ple access communications system generally illustrative
`of the subject invention;
`FIG. 2 is a simplified diagram further illustrative of
`the subject invention;
`FIG. 3 is a simplified diagram also further illustrative
`of the subject invention;
`FIG. 4 is an electrical block diagram broadly illustra(cid:173)
`tive of each transceiver making up the system configu(cid:173)
`rations shown in FIGS. 1 through 3;
`FIG. 5 is an electrical block diagram illustrative of
`the details of the RF head assembly shown in FIG. 4;
`and
`FIGS. 6A through 6E are a set of time diagrams
`helpful in understanding the time division multiple ac(cid:173)
`cess operation of the subject invention.
`DETAILED DESCRIPTION OF THE
`INVENTION
`Referring now to the drawings and more particularly
`to FIG. 1, shown therein is a first embodiment ofa time
`division multiple access communications (TDMA) sys(cid:173)
`tem comprised of a plurality of like transceivers lOo ...
`10n in that they are constructed of identical hardware,
`are mobile and can be selectively switched to operate
`either as a master station or as a slave station whereby
`all communications between the slave stations are coor(cid:173)
`dinated by and pass through the master station. As
`shown, reference numeral 100 refers to a transceiver
`designated the system master station through which 1
`through n slave stations 101, 102 ... 10n communicate
`with one another. Each transceiver, moreover, includes
`an omnidirectional antenna 120, 121, 122 ... 12n and at
`least one directional antenna 14o, 141, 142 ... 14n, The
`master station lOocommunicates with slave stations 101,
`102 ... 10n by means of its omnidirectional antenna 120
`60 while the slave stations 121, 122 ... 12n communicate
`with the master station lOo by their respective direc-
`tional antennas 141, 142 ... 14n,
`The transceivers 100 ... 10n operate to transmit and
`receive at the same frequency in the millimeter wave
`frequency band, a region of high atmospheric signal
`attenuation, between 54 GHz and 58 GHz which is near
`the oxygen absorption line of the spectrum and thus
`provides extremely wide bandwidths and covert opera-
`
`IPR2020-00034 Page 00007
`
`

`

`4,549,293
`
`3
`tion for either multiplexed, multichannel communica(cid:173)
`tion or single channel voice communication in the form
`of digital word bursts. While line of sight (LOS) com(cid:173)
`munication is required between the master station 100
`and any slave station 101 ... 1011, line of sight is not 5
`required between the slave stations themselves due to
`the fact that all communication between slave stations is
`through the master station. Accordingly, at millimeter
`wave frequencies, a covert power circle 16 having, for
`example, a one kilometer radius is generated around the 10
`master station 100 which provides covert communica(cid:173)
`tions between all members 101 ... 1011 within the power
`circle 16.
`Whereas TDMA systems of the known prior art
`include a singularly dedicated master station specifi- 15
`cally designed for the purpose of acting as a master
`station and whereas the slave stations are specifically
`designed to act as slave stations, the present invention
`departs from such a conventional concept in that, upon
`demand, any of the slave stations 101, 102 ... 1011 can be 20
`subsequently designated as a new master station in the
`event that the originally designated master station 100
`becomes inoperative for any number of reasons. This
`becomes particularly advantageous when used by the
`military under battlefield conditions.
`This now leads to a consideration of FIG. 2 wherein
`transceiver 10s is now designated the new master station
`due to the fact that the original master station 100 be(cid:173)
`comes inoperable or has been put out of action, e.g.
`destroyed, as shown by the phantom lines. Now the 30
`slave stations 101 ... 104, 106 ... 1011 communicate to the
`omnidirectional antenna 12s of the new master station
`10s via their respective directional antennas 141 ... 144,
`146 ... 1411. Additionally, the covert power circle has
`shifted from that shown by reference numeral 16 as 35
`shown in FIG. 1 to a new power circle 16' whose radius
`originates at the new master station 10s.
`Since the system of the subject invention normally
`provides communication between the slave stations 101
`... 1011 within the power circle 16, any transceiver, for 40
`example, station 1011 can further be converted into a
`repeater station 10' 11 as shown in FIG. 3, whereby a like
`transceiver 1011+ 1 as the slave stations 101, 102, 103 can
`access the system through the master station 100 and its
`omnidirectional antenna 120. In order to implement this 45
`mode of operation, the repeater station 10' 11 is config(cid:173)
`ured from two colocated transceivers 1011 which are
`connected together so as to include a second directional
`antenna 14' 11 which is directed at the directional antenna
`1411+ 1 of a slave transceiver 1011 + 1 which lies outside of 50
`the power circle 16.
`As noted above, the system operates at a single fre(cid:173)
`quency in a half duplex mode with digital word bursts
`being provided between participating stations through
`the master station as shown in FIGS. 6A-6E and which 55
`will be considered subsequently. Since a single fre(cid:173)
`quency is used among all participating stations, a con(cid:173)
`servation in bandwidth is achieved. Also, it permits the
`system operating frequency to dynamically move with(cid:173)
`out a great deal of difficulty and thus permits the utiliza- 60
`tion of relatively unsophisticated hardware while pro(cid:173)
`viding protection against undesired interception and
`jamming. Moreover, due to the likeness of all of the
`apparatus, an extremely mobile TDMA line of sight
`communications system comprised of multiple geo- 65
`graphically separated transceivers having a non-com(cid:173)
`mitted master station operating within a predetermined
`power circle is achieved.
`
`4
`Having thus considered the system details of the
`subject invention, direction is now turned to the details
`of the transceivers too ... 1011 • Referring now to FIG. 4,
`reference numeral 10 designates radio apparatus for
`implementing the transceivers lOo ... 1011 and is com(cid:173)
`prised of a control unit 18 which is connected to an RF
`head assembly 20 by means of a multi-conductor cable
`22. The RF head assembly 20 is shown in further detail
`in FIG. 5 and will be considered subsequently. FIG. 4,
`however, is intended to show that the RF head assem(cid:173)
`bly 20 is separated from the control unit 18 and is
`adapted to be mounted on top of a mast or tripod lo(cid:173)
`cated, for example, on a mobile vehicle, not shown, for
`selectively coupling energy to and from an omnidirec(cid:173)
`tional antenna 12 or a directional antenna 14. The con(cid:173)
`trol unit 18 is also adapted to be located on or in the
`vehicle and includes, among other things, a manually
`operable control switch 24 which is operable to switch
`the operational mode of the apparatus so as to operate
`either as a master station or a slave station and, when
`desirable, as a repeater station. In the master and slave
`operational modes, only a single piece of apparatus is
`shown in FIG. 4 as utilized; however, in the repeater
`mode, the control unit 18 as well as the RF head assem-
`25 bly 20 including the omnidirectional antenna 12 and
`directional antenna 14 is duplicated and electronically
`connected together by means of a multi-conductor
`cable assembly 26 so that, for example, energy transmit-
`ted to one directional antenna 14 can be coupled to and
`from the other directional antenna, not shown, on the
`other RF head assembly, also not shown, through the
`cable assemblies 22, 26 and respective control units 18
`to provide a double directional antenna configuration as
`shown in FIG. 3 with respect to the repeater station
`10' 11. The omnidirectional antennas 12 in both head
`assemblies are not used in the repeater mode.
`Further as shown in FIG. 4, the control unit 18 has
`two power supply potentials, 110 volts AC and 28 volts
`DC, applied thereto. These power supply potentials are
`fed to well known supply circuits, not shown, to gener(cid:173)
`ate the necessary power voltages for operating, for
`example, signal amplifiers and switching devices within
`the transceiver apparatus. The control unit 18, more(cid:173)
`over, is adapted to be coupled to an operator hand set 28
`and/or a multi-channel multiplexer-demultiplexer unit
`30 via respective cable assemblies 32 and 34. Thus the
`control unit 18 provides the required interconnection
`between the RF head assembly 20 and the operator
`hand set 28 as well as the multiplexer-demultiplexer 30
`or both. In order to communicate these signals, the
`control unit 18 additionally includes a synthesizer 36
`shown in FIG. 5 which is operable to convert the
`TDMA and voice input signals to digital burst signals
`having a carrier frequency in the GHz range from 1.0 to
`1.14 GHz, for example, where they are further multi(cid:173)
`plied up to the millimeter wave (55 to 57 GHz) RF
`frequency in the head assembly 20. However, it should
`be noted that, when desired, other known means of
`achieving the necessary millimeter wave frequencies
`can be utilized. The head assembly 20 is further adapted
`to convert received RF signals from the master station
`to an IF frequency where they are then fed to a demod(cid:173)
`ulator 38 in the control unit 18 to provide output
`TDMA or voice communications signals back to the
`multiplexer-demultiplexer 30 or operator hand set 28.
`Turning attention now to FIG. 5 which discloses a
`typical example of achieving the required millimeter
`wave transceiver and considering the details of the RF
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`5
`head assembly shown therein, all of the input and out(cid:173)
`put signals connected thereto as well as the power sup(cid:173)
`ply potentials are included in the multiconductor cable
`22 shown in FIG. 4. The control unit 18, moreover,
`includes power supply circuitry, not shown, which is 5
`adapted to generate in this case + 15 VDC, +28 VDC,
`and +s VDC. These DC power supply potentials are
`coupled into the RF head assembly 20 by means of
`respective line filters 40, 42 and 44. The + 15 VDC and
`+28 VDC supply potentials are coupled to a voltage 10
`regulator 46 through a pulse modulator which is used
`only in the slave mode to provide regulated + 15 VDC,
`+ 10 VDC and +20 VDC power supply potentials.
`The voltage regulator 46, moreover, is adapted to be
`operated in a keyed mode by a modulation drive input 15
`signal applied via signal lead 50 during TDMA opera(cid:173)
`tion in order to deactivate the transmitter portion of the
`transceiver during the receive mode. The signal output
`from the synthesizer 36 is fed to a pair of bi-polar tran(cid:173)
`sistor amplifiers 52 and 54 which operate, for example, 20
`in the 1.0 to 1.14 GHz range for providing amplification
`of the TDMA and voice signal digital inputs which are
`then fed to a frequency multiplier and filter 56 which is
`operable to multiply the frequency by a factor of 10
`(XlO) to provide an output of signals in a range of 11.0 25
`to 11.4 GHz. These signals are next fed to three power
`field effect transistor amplifiers 58, 60 and 62. The out(cid:173)
`puts of the amplifiers 60 and 62 are commonly fed to a
`second frequency multiplier and filter 64 comprised of,
`for example, an avalanche diode which operates to 30
`increase the frequency by a factor of five (XS). Accord(cid:173)
`ingly, a 55 to 57 GHz RF signal is coupled to a transmit(cid:173)
`receive {T/R) switch 66 implemented, for example, by
`means of a PIN diode. The T /R switch 66 is coupled by
`means of a waveguide transmission line 68 to a wave- 35
`guide switch 70 which is mechanically switchable be(cid:173)
`tween the omnidirectional antenna 12 and the direc(cid:173)
`tional antenna 14, depending upon whether the equip(cid:173)
`ment is being utilized as a master station or a slave sta(cid:173)
`tion. The T /R switch 66 is driven by means of two pulse 40
`type signals appearing on circuit leads 72 and 74 from a
`switch driver circuit 76 having + 15 VDC and +5
`VDC supply voltages applied thereto in addition to a
`T /R input signal from the control unit 18.
`In the transmit mode, a portion of the RF signal cou- 45
`pied to the waveguide 68 is fed to an RF detector 80
`through an RF coupler 82 inserted in the waveguide
`transmission line intermediate the waveguide switch 70
`and a waveguide connection 84 out of the RF head
`assembly 20. The output of the detector 80 is fed to a 50
`buffer amplifier 81 which provides a signal to a transmit
`output indicator, not shown, located, for example, on
`the control unit 18. In the receive mode, the output of
`the T /R switch is fed to a mixer 86 along with a local
`oscillator input signal (LO) which provides an IF out- 55
`put signal having a frequency in the order of, for exam(cid:173)
`ple, 100 MHz±lS MHz. The IF output signal is next
`fed to a three stage low noise IF preamplifier comprised
`of amplifiers 88, 90 and 92 coupled together in series
`with the output of the third preamplifier 92 being cou- 60
`pied to the demodulator 38 in the control unit 18.
`As noted above, a single frequency is used in a half
`duplex mode for transmitting digital burst signals be(cid:173)
`tween communicating stations and time division multi(cid:173)
`ple access is used to enable the member stations to com- 65
`municate with each other through the system master
`station. A typical TDMA time frame and subdivisions
`thereof is shown in FIGS. 6A through 6E.
`
`6
`As shown in FIG. 6A, the TDMA time frame is
`divided into a master transmission time Tm and a plural(cid:173)
`ity of slave reply transmission times T 5.J, Ts-2 ... Ts-n• It
`is to be noted that a dead or guard time G is spaced
`between the individual master and slave transmission
`and has for its purpose the allowing for different propa(cid:173)
`gation delays to the member slaves and to avoid multi(cid:173)
`path interference. The master transmission time Tm is
`further shown in FIG. 6B and includes a time slot
`(SYNC) containing synchronizing codes to assure syn(cid:173)
`chronization with the listening slave stations. Following
`this, a digital data burst in the time Tc is transmitted
`containing data relating to system control and channel
`routing. Following this, sequential data bursts Tdm are
`transmitted containing the actual multiplexed system
`channels 1 through n followed by the guard time G.
`Digital voice communications signal burst would ap(cid:173)
`pear within any of the channel bursts, e.g. the channel n
`burst. The time Dm is further shown in FIG. 6D.
`With respect to the slave reply transmission time,
`FIG. 6C discloses one such time. As shown, the period
`Ts-n includes a time (SYNC) wherein sync codes are
`transmitted to the master station followed by a data
`burst time T ds which includes the various channel trans(cid:173)
`mission times as shown in FIG. 6E followed by the
`guard time G. This technique and format is well known
`to those skilled in the art and is simply disclosed herein
`to provide one with the manner in which the system is
`operated while utilizing a millimeter wave RF medium
`and a non-committed replaceable master station in a
`TDMA communications system employing a plurality
`of identical transceivers which are selectively adapted
`to operate at either a slave station or a master station
`within a covert power circle which establishes the max(cid:173)
`imum range of master-slave communications.
`It is to be noted that the foregoing detailed descrip(cid:173)
`tion has been made by way of illustration and not limita(cid:173)
`tion, and accordingly all modifications, alterations and
`changes coming within the spirit and scope of the inven(cid:173)
`tion as set forth in the appended claims are meant to be
`included.
`We claim:
`1. A time division multiple access communication
`system including a non-permanently committed master
`station and at least two slave stations communicating
`with one another through the master station, compris(cid:173)
`ing:
`a plurality of indentical transceivers utilizing the same
`operational frequency near a region of high atmo(cid:173)
`spheric signal attenuation, one of said transceivers
`operating a master station and the other transceivers
`operating as slave stations within line of sight com(cid:173)
`munication to said master station,
`each of said transceivers including an omnidirectional
`antenna providing operation as a master station and a
`directional antenna providing operation as a slave
`station, and means for selectively connecting one of
`said antennas for operation of each transceiver as a
`master station capable of communication with each
`slave station or as a slave station capable of communi(cid:173)
`cation only with said master station, and
`wherein said one transceiver operating as the master
`station establishes a covert power circle of operation
`with the remainder of said transceivers acting as slave
`stations geographically located within said circle to
`establish a line of sight communications link with the
`master station but being selectively operable upon
`demand to act as a master station in the event the
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`4,549,293
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`7
`previously selected master station becomes inopera(cid:173)
`ble.
`2. The system as defined by claim 1 wherein said
`operational frequency is in the region of the oxygen
`absorption line of the spectrum.
`3. The system as defined by claim 1 wherein said
`operational frequency is in the millimeter wave portion
`·
`of the electromagnetic spectrum.
`4. The system as defined by claim 3 wherein said
`slave stations communicate with said master station and
`said master station communicates with said slave sta(cid:173)
`tions in a half duplex mode of operation of alternating
`master-slave transmissions in the frequency band be(cid:173)
`tween 54 GHz and 58 GHz.
`5. The system as defined by claim 4 wherein said
`alternate transmissions comprise alternating master(cid:173)
`slave bursts in designated time slots of a time division
`multiple access time frame.
`6. The system as defined by claim 3 wherein each
`transceiver is additionally comprised of an intercon(cid:173)
`nected control unit and RF head assembly switchably
`coupled to said omnidirectional antenna and said direc(cid:173)
`tional antenna, said control unit having switch means 25
`for selectively rendering said transceiver a master sta(cid:173)
`tion or a slave station and having means for receiving
`externally applied input communications signals which
`are coupled to said head assembly and thereafter radi- 30
`ated selectively from said omnidirectional antenna or
`said directional antenna,
`each said transceiver further having means for provid(cid:173)
`ing output communications signals received by either
`
`8
`said omnidirectional antenna or said directional an(cid:173)
`tenna.
`7. The system as defined by claim 6 wherein said
`communications signals comprise a plurality of commu-
`5 nications signals carried by a plurality of multiplexed
`signal channels within a time division multiple access
`time frame.
`8. The system as defined by claim 7 wherein said
`communications signals additionally include voice com-
`10 munications signals.
`9. The system as defined by claim 8 wherein said
`communications signals are in the form of digital burst
`signals.
`10. The system as defined by claim 6 wherein said
`15 control unit additionally includes means for providing
`synchronization, channel routing, and system control
`for all slave stations when operating as a master station.
`11. The system as defined by claim 6 wherein two
`colocated transceivers are directly coupled together by
`20 way of their respective control units for operating as a
`repeater station whereby a transceiver outside of said
`covert power circle of operation is linked to a slave
`station inside of said power circle through said repeater
`station and said master station.
`12. The system as defined by claim 11 wherein said
`repeater station communicates with said station outside
`of said covert circle and said master station by respec(cid:173)
`tive directional antennas of said directly coupled trans(cid:173)
`ceivers.
`13. The system as defined by claim 12 and wherein
`the control unit of said directly coupled transceivers
`includes switch means for interconnecting signals be(cid:173)
`tween said transceivers.
`* * * * *
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