CA1181134A - Communications system and network - Google Patents

Abstract

COMMUNICATIONS SYSTEM AND NETWORK

INVENTOR: ELLIOT L. GRUENBERG

ABSTRACT OF THE DISCLOSURE

A communications system and network wherein communications links between subscribers are established by retrodirective oscillating loops between each subscriber and a node station. Provision is also made for establishment of links between nodes so that remotely located subscribers can communicate. Information is transmitted between subscribers by using a mixing process in the node whereby information trans-mitted from one subscriber to the node is transferred at the node to a carrier signal transmitted between the node and another subscriber.

Description

3~

D~SCRIPTIOM OF THE INVE~lTION

The invention relates to a communica-tions and control system and network and in particular to a system and network that provides flexible communications and control between stations regardless of their spatial location or relative motion.
Present communications and control sys-tems are used to transmit and receive voice conversation, business and computer data, radio and television programming and graphic material. In such systems, information may be exchanged by a central station and one or more subscribers or individual subscribers may exchange information with each other. Ideally, the communications and control system should be operative without regard to the particular spatial location of a subscriber at any particular time and should ' be operative to provide the means for exchanging information be-tween subscribers who are either at fixed locations or who are moving spatially with respect to one another. Heretofore, fixed communications and control systems have been provided by the switched publlc networ,k and by private or leased-line systems. In present mobile communications and control systems, subscribers utilize omnidirectional broadcasts to make contact with -a base , station that relays all communications between subscribers via omnidirectional ~roadcasting means.
These prior systems suffer from a number of serious drawbac~s For subscribers on a fixed system, the cost of central station switching equipment required to interconnect and direct calls and information is very high. Furthermore, the fixed systems rely on interconnecting subscribers via wire or cable, which means are also presently bandwidth limiting as well as quite expensive in material 3 ~

-and labor cost. Present mobile cvmmunications systems utilize omnidirectional radio broadcasting for base, relay and mobile stations thereby utilizing many frequencies over a general area.
This omnidirectional broadcasting characterist:ic of present mobile systems communications and relay stations drastically re-duces the possible number of simultaneous users because each user requires one operating frequency for the area. One approach to increasing the possible number of simultaneous users in an area'is to segment ~he area into cells with each mobile and relay '-10-: station allowed low po~er limited range omnidirectional broadcas-ts.
This approach, however, increases the cos~ and complexity o~ the mobile system and users must be ~.Titche~ as they move from cell to cell. ~uture-i.ncreases~ in user.demand require that the cells be made smaller and each s~ation's transmi~ter'xeduced in power output.
These, and other problems are overcome by the present invention which provides a communications and control system and ' ne~work that is divided into a fixed grid for the service area. When a user seeks a communication link with another user in the network, the system provides the means -that al.low a co~unicati.ons 20. linkage to develop via radio frequency between the calling and called station or stations through one or more nodes without the necessity ~f intermeaiate switching e~ipment and without regard to their spatial location or relative motion. The highly directional charac-ter of t.he individual links that'form ~his communications and control linkage between communicating stations allows many other users in ~he same area to utilize the same frequency simul-taneously without interference and to operate efficiently wi~h greatly ~educed power over that necessary for present mobile systems.
Future increases in simultaneous user demand will be far better 3.0 accomodated ,by this "space-linking" system and such capacity increases as ar~ required can be met simpl~ by upgrading the cap~city of the network~s relay s-tation~ or noda~.
The liL~kage between s~bscribers is eskablished by the buildup o directional communications and control links -known as retrodi.rective oscillating loops such as are des~
crib~d in U.S~ Patent No. 3,757,335, issued September 4,L973 to Elllot L= Gruenberg. Brieflys U.S. Patent No.3t757,335 describes a retrodirective oscillating loop or link for a communication and control channel between a pair of remotely located antenna terminals. In accordance with the disclos urs, a c~rrier signal for the re~rodirective oscillating loop builds up between two remotely controlled antenna array terminals each of which has retrodirecti.ve properties when sufficient amplification :Ls provided in the loop to overcome losses which occur at the terminals and in the medium be~ween the loops. The retrodirective oscillating loop antenna ~eam.s automatically steer toward each othPr when each terminal is ~ffectiv~ly wi~hin ~he field of view of the other and when the retrodirective antenna develops suffici~nt gain as each of its multiple radiating antenna elements ~ransmits the carri~r wave form in proper phase relationship to one another, whi h will automatically result when the loop gain is greatPr than unity.
According to the present inventlon there is described a communications system having at least one retrodlrectlve node station and at least a paix of subscriber stations for establishing a communlcations link between said subscrlber stations to permlt said subscriber stations to interchange information comprislng means for establlshing first and second i~terdependent retrodirective oscillating loops be tween respectively one of said subscriber statlons and said retrodi.rective node and said other subscrlber station and sald re~rodirec~ive node by contrcllin~ the energy charac~
. terlstics of the link between the first subscrlber station and the retxodirective node for oombina~ion with the energy . charac~eristic of the link between the second subscriber -station and the retrodixective node to bring about the sub-stantially simultaneous establishmen-t of said first and second r~trodirective oscillating loops, and means in said retrodirective node for receiving information directed to said subscriber stations whereby said subscriber stakions can interchange information.
While some present ~ay communication systems do not require switching equipment to establish csmmunications be-tween subscribers, these ~ystems do req~ire a large nu~ber of connections to insure that all subscribers can have access to all other subseribers in the system. One system which does not utilize switchin~ equlpment to establish conn~ctions betwee~ subscribers is described in U~S. Patent 4l00l,691 issued January 4, 1977 to Elli~Lo Gruenberg~
While this cystem would pxovide good communications capabil~
ity~ it requires n~n~ onnections, where n is the number of subscribers in the system.
In accordance with the system of a descrlbed embodi-mant, the complexity and cost of providing multiple paths for subscriber connections is substantially reduced since only a maximum of nf2 connectiQnsare requlred. A connecting path is ~stablished by the calliny party through a transpond-er at a central station, which will be referred to as a node, to the called party~ Only one path is required between the calling party and the node and only one path between ~ the node and the oalled party. Since -the same path is used by both the called party and the calliny party there will be no more than n/~ paths in use at any one ti.me. There are, however~ n channel designations, one for each party of the system who ma~ at one time be a called par-ty and at other times be a calling paxty.
Embodiments of the inver.tion may be made utilizing any microwave or millimeter frequency a~location and may use satellite transpondersO However, other frequencies may be used in some instances. Thus, channels may be of any desir-ed or authorized bandwidth, for example, up -to that which may accomodate high \~

_ _ _ __ _ _ __ _____~

-4a-~quality data or video information or combina-tlons of vid~o, data and voice. ~limination of intermediate switching greatly enhances the convenience and simplici~y of ~he present system and eliminates po-tentia-lly poor transmission paths.

W~lile conventional directional microwave might be used for fixed communica-tions, it cannot be used for mobile communicati~ns. The present system provides the flexibility to be used in fixed, mobile or combired fixed and mobile communi-cations systems with increased transmission quality and greatly lC reduced cos~ due ~o the elimination of complex multiplex and switching equipment. The fle~ible directional aspects of the inventive system i~crease the capacity of communications systems in local areas by perm1tting different streams of data in different directions to utilizelthe same frequencies without interference~
The flexibility of the system thus permits more users to use the same freqùency channel aLlocations with lower power requi~ements.

In general, the communications system connects users of the system by one OE more two - way links or loops, through intermediary transponders or nodes. A control

2~ carrier having a characteristic control carrier frequency is assigned to each system user~ The control carrier is a signal which does not contain intelligence or information (modulation) but which establishes the linkage between subscri~ers, enables modulation to be detected and which is used to direct the intelligence in specific directions and hence along specific paths. To use the system, the calling party selects a control carrier frequency ~hich i5 com~le-mentaxy to the control carrier frequency of the called party.
Selec~ion of this control carrier frequency automatic~lly enables a transmission path between the calling party and the called partv.

3~ For a locaL node call, a retrodirecti~e oscilla-ting loop is established between the call.ing par-ty and a local node (which functions as relaying station) and A second retrodirective .ioop is established ~etween the local node and the called subscribeI~
For long distance communications, the calling party's local node is connected through intermediate nodes to the local node associated with the called party and a retrodirective loo~ between the called party's local node and the called party completes the communications link between -the parties. In another embodiment of the invention! the station coupled to the called party's local la node may ~e the terminus of ano-ther communication system, such as a public switched networl-, which is then used ~o co~plete the connection to the called par~y.

Considering the operaiion of the system in more detail, the calling party's texminal is equipped with a recei~ing antenna array, a transmitting antenna array, each array including the same number o~ antenna elements, an amplifier, and a band pass Eilter coupled between each element of the transmitter and receiver an-tenna array which generate a con-trol caxrier having a preselected frequency. In practice a single array may be used by duplexing the ~ receive and transmit connection to the same antenna element as is well known in the art. An oscillator in the terminal provides signals whi~h ser~e as an offset between the receive and transmit signals and ma~e possible the development of a retrodirective oscillating loop between the cal.ling party's terminal and the calling party's local node, as is more fully described in U. S.
Patent ~o. 3,737,335.

The local node in accordance with t:he desc- ed svstem is equipped with a retrodirective array transceiver including receiving and transmitting antenna arrays. In each of the pa-ths connecting 3(' the receiving antenna elements 3 ~

and the transmitting anLellla eleillentc. of the r~spectivc~ antenna arrays, there is a mixer and a band pass filter which is inserted into the path in advance of the amplifier required to establish the retrodirec-tive oscilla-tion. The filter is chosell to pass only a reference signal centered at frequency 2c, where c is -the fre-quency of -the calling party's con-trol carrier. The mixer provides -tilis reference signal when it receives two complementary signals, the frequency of which are, for example, c-~a and c-a~ Thus~ ir the calling par-ty provides a signal at one of -these frequencies and the called party is set up -to provide automatically a signal at the other frequency to input of the local node, two simultaneous retrodirective oscillating loops will developt one between the calling party and the local node, with the calling party supplying a siynal to the node at frequency c-~a and the other between the called party and the node with the called party supplying a signal to the node at frequency c-a. In the node, the two signals a~e multiplied by the mixer to provide a reference signal at frequency 2c or a suitably translated frequency which is retransmitted bac~ from the node to both the calling and called parties. Each subscriber is equipped to receive the reference signal and the loop is completed. Once the appropriate filter and local oscillator frequencies have been selected, it is only necessary to provide suf~icient elec-tronic and antenna gain to set up the loop gain con-ditions for retrodirective oscillations r Thus, as long as the node, retrodirective antenna and amplifiersat each terminal can support greater than unity loop gain, two retrodircctive loops transmitting control carriers will develop, one between the calling party and the local node and one between the called party and the local node.
The node provides a directional path to and from each party, i.e.
the node retrodirective antenna will receive and transmit the control carrier signals in onl~ the t~o general directions of the ~called and calling party's texminals. Similarly, the an~enna of the called and calling party will receive and transmit in the direction of the local node irrespective of whether the parties are fixed or moving relative to one another. Once the retrodirective loops are esta~lished to in~erconnect the parties, information of any type can be exchanged between the parties.

Info~mation originated at a calling subscriber station is transmitted in the form of radio frequency modulated signals lG to a node along with a carrier signal. From these signals which ~rrive in a fixed spatial phase relationship with respect to each other an information bearing signal is produced at ~he node which does not have any spatial phase relationship. This signal is transferred to the carrier signal or the called subscriber station by a mixing process. In this way the resultant signal contains the in~ormation bearing modulation of the calling station a~d the spatial phases of the called stationO ~'he node processes these phase relationships so that this composite signal will be transmitted by the node in the direc-tion of the called station.

2~
Embodi~ents of the drawings in wnich:

FIG. 1 is a block diagramm2tic representation of the co~munication and control system lllustratina a number of fixed and mobile subscribers interconnected through local and intermediate nodes;

FIG. 2 is 2 simplified block diagrammatic re~resentation of a single node system servicing one pair of subscribers and o~eratincJ :in a full duplex mode;

FIG. 3 is a sim~ ied block diagra~tlc repr~sentation of a single node system servicing two pairs of subscribers stations;

FIG. ~ is a simpliflecl block diagran~at:ic represen-tation of a two node system which illustra-te the operation of the system wi-th remote subscribers;

FIG~ 5 is a block dia~ramma-tic representation of a typical single element of a node in the communication system for establishing ~he connection and for receiving and transmitting information between a calling and called subscriber;

FIG. 6 is a diagrammatic representation showing the control carrier processing unit and the modulation processing unit and the interconnection between the units in the typical single element of the node shown in FIG. 5 Eox processing one pair of subscribers opera-ting in full duplex mode;

FIG. 7 is a block diagrammatic representation of a typical subscriber station;

FIG. 8 is a diagrammatic representation of a carrier signal processing unit at the subscriber station having separa-te ~0 control carrier and modulation channels with one antenna array element pair;

FIG. 9 is a diagrammatic representation of another embodi-ment of a subscriher sta~ion carrier signal processing unit for use when t~le caxrier and modulation occupy the same frequency channel;

FIG. 10 is an alternative embodiment of a carxier selection filter of a subscri~er station carrier signal processing unit; and .~,~,,.

FIG. ll is a diagrammatic representation of an alternate embodimen-t of a single duplex channel of a node sho~Jing modulation and carrier processing units for each arxay elemen-t pair, for use when modulation and carrier occupy the ~ame frequency channel.

Referring ito FIG. l, a typical communications systems is shown. A number of subscri~ers Sl through Sg interconnec-ted by ixed (indicated ~y solid lines) and mobile lindicated by dashed lines) transmission links with local la nodes N~ through ~ or ~hrough sa~ellite node STo Each subscriber is located within the field of view of the antenna cf a node which will be referred to as the local node of ~he particular subscriber.
This relationship between subscrib~r and local node is indicated by the line connecting each subscriber to a node~ 10cal communi-cations are considered to b~ those in which both the called and calling subscribers are in communication via the sam~ local nodeO
For long distance communication/ which i~ defined as communication be~we~n partie~ not communicating via the same local node, inter-mediàte nodes are us~d~ Com~unication b~i~ween an~y two subscribers is accomplished ~hrough the establishment of complementary retro-directive oscillatiGn loops between the calling party and the local node of the calling party, the called party and a local node of the called party and between any intermediate nodes necessary to establish the transmission path ~etween the local nodes of the calling and called parties. If subscriber Sl wishes to ma~e a local call to subscriber S2, subscriber Sl selects a signal at the frequency complement2ry to the frequency of -the sign.~l auto-matically transmitted by su~scriber S2 and transmits that signal to node Nl via, for example, mobile link lO. At t~e same time, node Nl receives a signal a~ the comple~"entary fre~luency from called party S2 via ~lobile communication link 12. Node ~Tl combines these signals and transrnits a reference carrier signal to both parties Sl and S2. Retrodirective loops are thereby established between calling party Sl and node Nl and called party S2 and node Nl to provide a communications lin]c between the parties.
For long distance communications, for e~ample, between calling subscriber Sl and called party S8, calling subscriber Sl trans-mits a signal at a frequency complementary to the frequency auto-matically transmitted by subscriber S8. This signal is received a-t node Nl and -transmitted via lin~s 14~ 16, 18, and 20 to Node N4.
1~ Links 14 through 20 can be radio, microwave, cable or any combir.-ation. Node N4 also receives the complementary signal from called party S8. It generates the reference signal to parties S8 via link 22 and via links 12 through 20 and in nodes N3s~ ST, N2 and Nl back to party Sl to establish communication. Node N3S has the additional capability of e~tracting and inserting intelligence at its location for use of subscribers there, as well as relaying signals to other points in the network.

When the communication pc~ths between the subscribe~s have been established, transler of information bet~een subscriber Z0 stations Sl and S8 can take place. The modulated signal from S
is designated west for convenience. The ~ west signal is transmitted from subscriber Sl to node N4 where it is transferred to the carrier from subscriber S8. Similarly, signals from sub-scriber S8 modulated with ~ east modulation are transmitted to node Nl and are then transferred to the carrier signal from sub-scriber Sl.

The system operation can be fur-ther understood by considering first the operation of the simplified illustrative system shown in FIG. ~ in which a pair of subscribers communicate 3~ via a single local node. For the purpose of e~planation, assume that the called party, which will also be referred to as -the East party, is assignecl a called frequency of c+a, where c is the reference frequency and a provides an offset from the reference frequency unique to the Eas-t party. The calling party will be referred to as the ~est party. To place a call from the West party to the East party, the West party takes his phone 30 off hook and dials a number code which identifies the East party.
The number code is received in the Wes-t party subscriber sta-tion 32 which translates the code into a local oscillator or a filter adjustment which enables a retrodirective loop to opera-te at a control carrier frequency c~a. The West control carrier signal at frequency c-a and the complementary control carrier signal provided by the East subscriber station 34 at frequency c-~a are received in node 36 via antenna elements 38. At the node 36, the complementary control signals are combined and utilized to generate a reference carrier signal at frequency c, which is transmitted back to both the East and West parties via antenna elements 40. In this manner a communications link including two retrodirective loops, 42, 44 one between each party and node 36 is established between the parties.

It should be understood that this system does not employ independent receivers and transmitters as do conventional systems but instead, a control carrier path is established between receiving and transmitting elements of the party's suhscriber stations and the node by virtue of the operation of retrodirective loops. As shown in FIG. 2, retrodirective loops are established between West party 30 and node 36 and East party 34 and node 36. Node 36 is constructed to permit communication between the parties only when complementary control carrier signals are present.

These control carrier signals are enabled by providing both loops with sufficient electronic amplification and antenna -12~

focusiny power to provide greater th~ln ur-i~y around each loop at the ~esired frequencies to overcome all losses. Thus, t~/o simultaneous retrodirective oscillatiny lOOpa 42 t 44 will be developed read~ for information to be trans~erred bet~leen the parties. Furthermore, all the links will be directional at both ends. Therefore, informa-tion transferred in one direction will not in-texfere with information -transfer in other directions.

When both retrodirective loops 42 ~ 44 are established, the signaling bell in the East party receiving instrument 46 1~ will ~e actuated and the East party completes the connection by -taking his phone off hook. Both parties may no~ use the links which have been established to transfer information between the East and West subscribers via node 36.

Information is modulated on the carrier now existing in West subscriber station 32 and is -transmitted to node 36 where the retrodirective array carrier from the East station 34 is no also present. Node 36 differentiates the two carriers by their carrier frequency and also establis~es the direc~ion of the ca.-riers by the distribution of spatial phase angles on the individual array ,20 antenna elements.

The node removes the modulati.on from the West carrier by a process descri.bed in more detail below and inserts it on the carrier returning to the East station 34. At the same time and by the same process the modulation from the East station is removed from the East carrier and inserted upon the r~est carrier being retransmitted to the l~lest station 32. The simultaneous modulations will not interfere with each other. When the communication is complete, each party goes back on hook. The oscillator in each terminal 32, 3~ is automatically returned to 3~ the assigned called control carrier frequency unique to the party.

~11 stations are, in effect, in a ready st~t~ and emit a low level noise-like signal to the node 36 which includes energy at the party's assigned frequency. The party is again signalled when the node 36 receives a signal at a freq~lency complementary to the party's assigned frequency which causes the es-tablishment of respective retrodirective loops between -the parties and the nodes.

FIG. 3 shows the operation of the system with four parties eommuni~ating through a local node. In FIG. 3 there are two East par-ties 50, 52 having subseriber statisons 54, 56 respeetively, and two West parties 58, 60 having subseriber stations 62, 64 respectively, eo~municating with loeal node 66 via separate communiation ehannels established in the node~
For this deseription, we assume tha-t subscriber 58 wishes to eall subseriber 50 and subseriber 60 wishes to call subseriber 52 and that the ealls are placed simultaneously. Subscribers 58 and 60 go off hook and dial the code number and generate and trans-mit to node 66 control carrier signals at frequeney e-a and e-b which are complementary to the frequeneies assigned to subscribers 50 and 52. Also reeeived at node 66 are noise signals transmitted from subseriber S0, 52 whieh noise signal s~ectral energy inelude eomplementary frequeneies of e~a and e~b respectively Frequeneies c+a and e+b are suffieiently separated to be filtered by separate band pass filters in node 66. The width of -this separation also affeets the build-up time of the eon-trol earrier signal since too narrow a band pass filter would delay the build-up exeessively.
~or this reason, a minimum separation of l,OOOhz is recommended.
This filtering sets up separate pa~hs in node 66 ~or the eontrol carrier signals reeeived from subseribers 58 and 60. In node 66, 3~ the complementary earrier signals are eombined and mixed to provide a eommon signal which passes a band?ass filter cen-tered at frequencv 2c. Tllis signal is used to gencrate a Leference carrier siqnal at freqllency c whic~ is then transmitted bac~ -t~ each subs(riber to establish retrodirective loops 6~, 60, 72 and 7~
between the subscribers and node so long as there is more than unity loop gain between each pair of subscribers. Subscribers 58 and 50 and subscribers 60 and 52 are connected via separate communications channels, since the independent paths provided in node 66 permit the four retrodirec-tive loops ~8, 60, 72 and 74 to operate simultaneously. In fact, a multitude of such retrodirective loops may operate simultaneously and independently, provided sufficient indepen~ent paths are provided in the node. As should now be apparent, these paths or channels can be easily established by providing an independent band pass filter in the node on each chan-nel and by proper selection of the control carrier frequencies for each channel. Modulation may be origina-ted at one s-tation and received at the other station of the pair without interference so long as independent control car-ier frequencies are provided for èach subscriber. No in-terference will be experienced by the subscrihers so long as the corresponding East and West subscribers all do not lie within the same beam width of the nodes even though the pairs use the same modulation channel allocation. This occurs because -the node is made up of an array of individual antenna elements each of which includes processing units which will be discussed in greater detail. The array of processing units are capable of suppressing modula-tion from a direction substantially different from the direction of the carrier developed from the terminal at a given direction as will also be discussed later.

Modulation originating from and/or transmi-t-ted to directions substantially within -the same beam width of the node will re~uire a different modulation channel to avoid interference.
I{owever, subscriber stations may be assigned different combinations L~

o~ control carrier and modulat-on frequencles so that even if subscribers are mobile and move from beam to beam, inter-ference is minimized. For e~ample, if the number oE stations is S and the number of beam positions (control carriers) is C
and the number of modulation channels is M, then the number of modulation channels (frequency assignments required) is M=S/C;
and if M equals C, ~l equals S. Thus, 10,000 users may be accommo-dated with only 100 channels and at least 100 users may use each beam simultaneously. Only 1% of the channel allocation ~ould be required as opposed to assigning each station a frequency as has been heretofore required. Thus si~nificant savings in band width and/or significant increases in system capacity are readily achieved.
FIG. 4 illustrates how remote subscribers may use the system. Remote subscribers are those which do not share the same field of ~Jiew of a single node but must be reached via two or more nodes. In such a case, intermediate links between nodes must be established. Thus, if subscriber 80 wishes to call subscriber 82, subscriber 80 adjusts the frequency transmitted by sûbscriber unit 84 in the same way as described above to generate the control carrier signal at frequency c-a which is complementary to the tuned fre-quency of subscriber unit 86 associated with subscriber 82. Node 88 receives the control carrier at frequency c-a and is equipped with an antenna containing directional couplers whïch ~ermit the control carrier siqnal received by node ~ to be transmitted to node 90 via communications channel 92 which may be a microwave link or a cable. In essence, node 38 upon receiving a control carrier si~nal automatically attempts to find the subscriber anywhere in the system which is set up to generate thc complementary control carrier signal. In the systems of FIG. 2 and 3, the subscriber was found coupled to a local node. In the system of FIG. 4, the node had to see~ the desire~ subscriber at a distant node. This operation would autornaticallv ta~e place throu~h as many intermediate nodes .IS ~ould be required to establish a connectiorl ~ett~e~ll subscribers L'IG. ~ shows the use of two nodes for illustrative purposes only.
At node 90, a noise signal generated by subscriber 82 continuing frequ~ncy c+a is reccived, applied to the input of node 90 and also transmitted to node 80 via link 92. Receipt of the comple-mentary signal in node 88 enables the signal to pass throush node 88 and provide a reference signal at frequency c, which is trans-mitted ko subscriber sta-tion 84. Similarly, a signal at frequency c-a from node ~8 originating in subscriber station 84 complements a signal at frequency c~a received at node 90 from subscriber sta-tion 86 enabling the signal at frequency c -to be sent to subscriber station 86 to sornplete the loop. Retrodirec-tive loops 94, 96 are enabled by sufficiency of gain around each loop offsetting any losses including those incurred in pa-th 92 and 94. Modulation from subscriber station 84 can now be transferred to the control carrier at frequency c in node 90 and transmitted to subscriber station 86.
Similarly, modulation can be transferred to carrier c from node 8B

directed to subscriber station 84.
The typical node 100 of the sys-tem (examples of nodes are Nl, N2,N3 of FIG. 1, 36 of FIG. 2, 66 of FIC~. 3, 88 and 90 of FIG. 4) is composed of one or more identical node elements. FIG~ 5 shows such a typical node element, referred to as 100-1. Each node will contain n node elements each comprising circuitry. The series of elements will be referred to as -1, -2 The nodes elements (100-1, 100-2 . . . 100-n) are all connected to a common single oscillator unit 118 which is required to invert ~he spatial phases of signals for retransmission back to the terminal units to complete the retrodirective oscillating loops. Each node includes a receiving antenna 10~ ana a irans~ ing antenna lu4; each node element includes, for example antenna elements, 102-1 and 104-1 resPectivel~ hen the physical sPacinq bet~een ~17-clnt~nrla ~lements 102-1 . ..n anQ 10~ 1. ..n are pror~er~ spac~ usu.
Lcss than 0.9 of the wavelenrJth of the frequencv b~ing received or transmitted by the node) the energy received or transmitted will be conccntrated into discrete beams in accordance with -the well l-nown principles of operation of phased arraya. These beams will be directed in accordance with the repetitive phase differences received at the different elements 102-1, 102-2 t 102-3 . . . .
102-n. We will refer to -this pha;e difference from -the West sta-tion as ~ and that from the ~ast station as 3.
ln Each element node 100-1, 100-n may be equipped to handle "m" duplex channels. To do so each node must be equipped with duplicates of modulation signal processor 112 and control signal processor 110. By duplex channel is meant a channel connecting two terminals (stations or subscribers) which can carrier information simultaneously from each terminal -to the other. Each such channel requires a different set of complementary frequencies. The following description describes the functioning of a single duplex channel.
The operation of mul-tiple channels can then be readily inferred from the description of one channel.
2~ The numbers of node elements 100-1, 100-~, 100-3 . . .
100-n which are used in a given node is dependen-t primarily on the number of desired independent beam direc-tions. The particular dependency is determined by the type of phased array used, but in the case of planar arrays "n" node elements are used for "n" beam directions within the field of view of the array. The field of vie~
of the array is the angular volume over which signals may be received from and transmitted to the node from the stations and is principally determined by the element antenna pattern of 102 and 104. (Other ar.ays which can be used include spheric~l and cylindrical arrays.

These arrays have wider field of views than the planar array~. An e~ample ~,ituation ~/ould be a node field of view of 60 and a node beam width of 6 indicatin~ 10 independent beam direc-tions l,ithin the node field of view. Thus 10 terminal stations may use the same frequenci~ band without any interfexence what-soever ~hen they each are located in a different ~eam. Also each user l~ill receive the benefi~ of the an-tenna gain implied by the narro~ 6 beam. This greater gain permits a higher in-Eormation transfer for the same transmitted power and distance between node and user.
l;' The local node 100 is capable of providing multiple co~munication channels for comnunications among subscriber~
within the field of vie~ of the node, the field of view being the angular sector over which node element an-tennas 102-1 . . . n and 104-1 . . . n can physically receive and transmit, and between remote subscribers, which are subscribers not within the field of view of a single local node. Signals from local sub-scribers in direct communica!ion with node 100 are received by receiving antenna 102-1 . . . n and transmitted to these sub-scribers by transmitting antenna, 104-1 . . . . n. These Z0 antennas receive directionally from subscr-~ers at arbitrary directions with respect to the antenna and transmit to the subscribers in the corresponding directions. For communications between remote subscribers, directional antennas 106 and 108 receive and transmit signals from other nodes and are connected to antenna 102 in a manner to be described in more detail below.

FIG. 5 shows a single node element 100-1 includiny a single antenna elements 102-1, 104-1, con-trol carrier siynal processor 110-1 and modulation siynal processor 112-1. These elements work cooperatively to generate the complementary retro 3C directive loops and to tr~nsfer information to the complementarY
stations ~hiCh communic~te ~ith each other. As mentioned before a separate set of processors 110 and 112 is required in each node e1emerlt 100 for each set of m complementary transmission L~aths or cn~nnels which the system is to be c~pable of es-tablishin~. The West originating control signal for the .irst node element will be referred to by the notation "Cw~ he ~est originating modulation signal fox the first node element will be referred to by -the notation ~'MW1" Thé East originating signals will be rèferred to by the subscript "E". The "nth"
element will be referred to by the no-tation n.

Antenna element 102-1 receives West control and modul-ation signals Cwl, Mwl, respectively, at a spatial phase anglewith respect to a reference point, ~1~ whereas East control and modulation signals CEl, ~ are received at spatial angle of ~l.Similarly, antenna ~lement 102-n receives signals C
M~ at an anyle ~n and CEn, MEn at an angle ~n-Each antenna element 102 is equipped with a powerdivider 103 for permitting signals, delivered to directional antenna 108 to be transmitted to a remote node. The signal is also applied to control car~ier signal processor 110 and modulation signal processor 112 via splitter-summer 105. Signals C'~, M'W
from a possible complementary subscriber located near a remote node are received via antenna 106, splitter~summer 105 and are also directed to control carrier signal processor 110 and modulation signal processor 112. Splitter-summer 105 may also be a directional coupler as is well known in the art.

Control carrier signal processor 110-1 operates with one set of antenna elements 102-1 and 104-1. Hence, there are n control carrier signal processors, for each channel~ Similarly, modulation signal processor 112-1 operates with one set of antenna elements 102-1 and 104-1. E~ence, there are also n modulation 3~. signal processors for each chl~nnel.

' Receiving antenna 102 is coupled to control carrier si~nal processor 110, which processes carrier control signals C~ and Cw recei~ed respectively from the East and West subscribers between which communication is to be, or is establishel. As e~plained above, the frequency of signal CEl (equal to C~a)is complementary to -the frequency of signal Cwl (equal to c-a). When the signals are combined in control carrier signal processor 110-1 they yield a signal whose frequency is 2c. If the spatial phase of signal Cwl at a given element oE antenna 1~, 10~ is 01 and the sp2tial phase of the signal CEl at antenna 102 is 91 then the spatial phase of -the resulting signal is 01 ~ ~1 Spatial phase is used herein to mean the phase of -the sine wave radio frequency signal received at one location within the node with respect to the phase of a signal received at another reference location within the node. For convenience, the frequency of the resulting signal is translated to reference frequency c in the control carrier signal processor 110-1. The resul-tant reference carrier signal at reference requency c is transmitted via antenna 104 ~o both the calling and called subscribers.

Wherever carrier or modulation signals are referred to, a simplified notation will be adopted. Instead of referring to a signal as Aei(C-a + ~West)-t+~ ~ for simplicity i-t will be referred to only by the exponential term. Thus, the above signal would be referred to as c -a ~ ~ west -~ ~. The reference to time, t, is dropped because it is not needed for the explanation. d west is equivalent to ~w when the modulation is in the form of phase or frequency modul~tion. While the description shows how the sys-tem works when using exponential modulations (phase or frequency), ~he system will also work with amplitude ~û modulation~
Inverter 113-1 receives input from oscillator 118, 3 ~

inverts the spatial phases of the si~nals recei.ved Erom elementslO2 before transmission via antenna 104 as described more fully below. Two retrodirective oscillating loops are established simultaneously, one between the cal.ling subscriber and the calling node, and the other be-tween the called subscriber and the called node, provided that the loop gain in each path exceeds uni-ty and that at the frequency of operation, the net phase shift around -the loop is zero or a multiple of360 degrees. Part or all of the required gain can be prov-ided by control ~arrier signal 1~ processor 110 as will be described in more detail below Information signals are received and transmitted via separate modulation channels Modulation received via antenna 102 is processed in modulation signal processor 112 which receives modul.ation from both the calling and called sub-scribers and uses control carrier signals Cw and CE received from control carrier signal processor llOIto provide signal products for redirection to the complementary user i.e.
modulation from the west subscriber is retransmitted only to the called subscriber, East, via control carrier CE1 as 2~ will be described in more detail hereinafter.

Modulation signal processor 112 also provides modulation output to the node and receives modulation for retransmission from the node when there are subscribers located at the node or when connection to an external communications system, such as the public telephone network is desired. In the latter case, a signal at frequency c~a is provided to the control carrier signal processor 112 from oscillator reference bank 114. This signal, together with a complementary control signal CWtc a) received from a westc211ing subscriber either within the field of view of the node, or from a distant node, will develop a signal with ~22-fre~uency 2c in control carrier signal proccssor 110 to be used to generate a cont~ol carri~r o-~ frequenc~ c for trans-mission back to the stations and thus enable a retrodirective oscillating loop to exist between the node and the calling or c~lled part~. Demodulator 115 and Modulator 116 are provided to extract the intelligence signals destined for the node location and to inser-t in-telligence for trans-mission to the specific subscriber.
FIG. 6 shows a control carrier signal processor 110-1 and a modulation signal processor 112-1 in greater detail.
lC Referring to FIG. 6, cortrol carrier signal processor 110-1 includes band pass filters 130, 132, which are respectively tuned to the rrequency Cwl = ~c-a) and CEl = (c~a). The output of filters 130 and 132 are applied to mixer 134 which multiplies these signals to provide the output product, 2c ~
Bandpass filter 136 is tuned to frequency 2~ and filters out all other unwanted frequency components. The signal from filter 136 is amplified by amplifier 138 and applied -to inverterjsumming unit 113. A signal from common reference oscillator 118 at frequency c is mixed in mixer 142 with the output of a~lplifier 138 to produce the reference control carrier at frequency c which passes through summer 144 and ~ixer 146 and is transmitted via antenna 104-1 to both the called and calling party, whether or not modulation is present.
The output of filters 130 and 132 are also applied to modulation signal processor 112. Modulation signal processor 112 includes filter 150 which separates the East going and West Wl +01 and ~ 1+ ~1 respectively and delivers this signal to mixers 152 and 154. Mixer 152 mixes the signal received from Eilter 132 with the signal from filter 150 to 3~ produce signal products ~ 1 ~ ~1 CEl ~1 E E.
product represents an in-phase component,that is, all such products in all n of the modulation signal processors 112 wi~ll be in phase with each other. On the o-ther hand, the ~1 + 01 ~ CE~ product has a variable phase angle 01 -~1 in each modulation sic3nal processor and hence will not reinforce as will the first products. Hence, only the in-phase products will he effec-tively coupled through filter 156.

Similarly, mixer 154 mixes the signal Cwl -~ ~1 from Eilter 130 ~ith the modulation signals 1~ ~ .
El + 1 -to obtaln the signal M + e -c - ~ and ~ - C
El 1 Wl 1 1 Wl whlch are input to filter 15~. Only the second of these products is 1~ effectively coupled through filter 158.

The output of fllter 158 is mixed, in mi~er 160 with the signal CEl~ el from filter 132 and the resultant product is ~ 1 ~ Cwl ~ CEl + el. This resultant is appro~imately ~71 + ~1 because CEl Cwl is a small negligible constant offset term, and the resultant is the modulation from the calling or West subscribers station redirected to -the complementary East or called station by virtue of the angle 91. Similarly, the output of filter 156 is mixed in mixer 162 -,~ith the signal Cwl + 01 from filter 130 to form the signal ME -C ~ C ~ 0 1 El Wl 1 or approxlmately 20 ME + ~1 ~

These signals are added in summer 164 and the summed signal supplied to the summer 144 of summer-inverter 140 where they are addecl linearly to C in summer laa~ Signals rom com~on reference oscillator 118 of frequency 2C are used to provide spatially phase inverted output signals C ~1 ~ e~

~nd ~ 1 _ el from mixer 146. These signals will be directed by phased array operation back to the respective subscribers ex-cept that modulation signal ~ 1 ~ ~1 will be direc~ed to the East and MEl ~ ~1 to the West station because they are inverted in s?atial ?hase and because the Mw si~Jnal no~ is associated ~ith the spa-tial angle 2C of the east terminal arld i~E with 0 of the west terminal as described more fully below. In order for c~rrier control signal at frequency 2C .rom filter 136 to be transmitted at frequencies similar to the carrier frequencies of the modulation signals ~1 and MEl it is desirable -to translate its frequency. Reference oscillator 118 su~plies the sigr.al of frequency C which is applied to mixer 142 which translates the frequency to the modulation frequency C without 10 affecting the spatial phase 01or ~1~ Reference oscilla-tor 118 also supplies a si~nal at frequency 2C which mixes with the summed signals in summer 142 and provide the output products C - ~n ~ ~n' 0111 ~ 01 and ~ 1 el. The phases oE the signals at frequency 2c from reference oscillator 118 are the same Eor all inverters 113.-1 ... n.
FIG. 7 shows a block diagramatic representation of a typical subscriber station 200. For -the purposes of explanation this subscriber unit will be considered the ~ast or called sub-scriber. Antenna unit 202 may be a combined rec_ive/-transmit unit or may be composed of separate receive and transmit units. The antenna unit 202 may be either a directional antenna or a retro-directive array. A directional antenna could be used when the sub-scriber station is fixed and knows the node direction. Both the modulation signals and control carrier signals are transmitted and received through antenna Unit 202. Signal processing uni-t 204, which will be described later converts and amplifies the~
control carrier signals in a manner similar to the control carrier signal processor 110. Snould the subscriber unit receive the ref-erence control carrier at frequency c it will respor.d with the control carrier signal at frequency CE completing the retrodirective os-cillatina loop bet~een the su~scriber station and its local node station~

~ lodulation unit 20~ pro~ides modulation iVlE for trans-mission Erom subscriber unit 200. Demodulation unit 208 demodulates -25- , ~odulation ~ received from the other party. Selector ~lO
selects the complementary control carrier frequency C~ which will set up the communications path between this subscriber sta~ion and the complementary West subscriber station Multiplexer 212 and demultiplexer 2l4 may be used to permit several users, indicated by lines 215 and 218,to share the same tr~nsmission path and modulation channel. It will be understood t~at e~cept as described below, the ele~en-ts of the typical subscriber station 200 are of conventional design.

FIG. 8 shows .he subsc-iber station and in pzrticular tne signal proces,or ~Q~ ln more detail. It is unde~stooc that the sinal processor is for one array element pair. In cases where the subscriber station uses a con~entional directional antenna, only one element pair is used. However, when retro-directive ~rray terminals are used, n pairs are requixed in a similar manner as described for node lO0. The reference carrier signal at frequency C and modulation signal ~ received at anten~a 23~ are separated ~y filters 232 ~n1 234 respectiJely.

These signals, which have a spatial phase angle ~ependent u~on direction signals are received, are apDlied to mixer 236. The re~

sultant signal has 7ero phasean~le and may be su~med wit~ other signals from other subscriDer station array ~airs. The su~ed signals from ~ixers 236-l . . . n are de~odula-ted i~ discrir,linator ~08.
The control carrier signal is amplified in am?lirier 238 to provide loop gain for carrier buildup. T~e selecticn unit 210 selects the frequency of oscillator 240. If the subscriber ~;ishes ~o call ano~he.r party, the frequency CE, (c~a) complementary to th2t par~y's frequency is selected~ If "on hook" or "idle", the subsc-iber's own frequencv i5 set to per~it t.~e s~at_on ~o ,o ~e called ~y an~ other party sendins t~e co~.ple~.entarv -~eq~enc~..

-2~

The output of oscillator 2~0 is mixcd ,~ith the output of amplifier 238 in mixer 242 to provicle transmi-tted output to antenna clement 2~ mplifier 238 is preferably of ~he limiting type.
'I'he signal from cscillator 2~0 is modulated by mo~ulator 246 with modulation signal Mw and oscillator 240 supplies modulation and carrier signals to all the elements in the su~scriber's antenna array.
The frequency of oscillator 240 is selec-ted so tnat mixer 242 provides a spatially phased inverted outpu-t to antenna element 24~ wi-th respect to signals received by element 230 so -that transmitted signals may be -transmitted in the direction from which the control signal was received.

FIG. 9 shows another embodiment of the signal processor 204' of the subscriber station in which the frequency of the control carrier is located in the same frequency band as the frequency o the modulation signal. The subscriber substation operates with the node shown in FIG. 11. Signals received from an an-tenna element 230 are applied to ~oth mixers 260 and 262. If the correct carrier signal is present it will be delivered to narrow band filter 264 which passes the carrier signal to mixer 262, via limiting amplifier 274. FIG. 9 shows a West station. The total signal, modulation and carrier, C + a - ~ +
~East, received, mixes with the carrier signal in mixer 262 to provide a modulation frequency product, ~Easr, which passes through modu-la-tion band pass filter 266 and via limiting amplifier 273 to mixer 260. This signal mixes with the input signal in mixer 260 to provide an input to filter 264 which retains the spatial phasing of this signal as received while at the same time producing a modulation sisnal of frequency ~East, which has a spatial phase of zero.
This signal is summed with all such similarl~J in-phase signals from other element pairs and applied to discriminator 208 for demodulation At thc same time, the carrler siynal c -~ a - ~ from filt^r 264 i5 ~mpLified by amplifier 268 and is transmitted around a retrodirective loop via mixer 270 and antenna 244.
Amplifier 268 is preferably a limiting amplifier. Oscillator 272 provides a signal at frequency 2c to the mixer 270. The frequency of this signal is approximately twice the received refexence carrier frequency, the difference being the prescribed reference offset frequency. The resultant transmitted signal c - a ~
is inverted in spatial phase with respect to the input sisnals.
Oscillator 272 is also modulated by modulator 246 and supplies the modulatea signals to all array elemencs, so that -the resulting signal is c - a ~ ~ f~ ~est-Selector unit 210 controls the band pass frequency of carrier selector filter 264. When idle, the subscriber unit is set to receive its prescribed frequency so that other parties may call it. When calling, the selective unit sets the frequency to that of the desired party as in the present case.
For convenience it may be preferable to adjust one oscillator frequency rather than tuning several filters. In that case, filter 264 may be replaced by the arrangement of FIG.
10. Offset oscillator 280 provides a signal at a frequency con-trolled by selection unit 210. When a given offset frequency, a, is selected it mixes in mixer 282 with the incoming signal c + a + ~'from mixer 260 (FIG. 9) has a frequency c with spatial phase 0'and can pass through fixed narrow band pass filter 284 to produce an output sicJnal c + ~'. The output of filter 284 mixes in mixer 286 with the offset signal from offset oscillator 280 to produce the signal c + a + ~'which is the desired output.
FIG. 11 shows another embodiment of a processing unit 300 used at the node element 100 with a modulation separation system in which the control carrier frequency lies within the ~28-mo~1ulatLon ~)arld if deslrcd. ~lhcn car~ier frcclu~nc~ i-. in thc~
modu].acion band this node processor must be used wi.th the subscriber station of the type shown in EIG. 9. The processing unit 300 sho~,ln in FIG. 11 would replace both the control carrier signal processor 110 and modulation signal processor 112.
Signals from the antenna element 102, c - a -~ bWest and c + a + ~ East, are applied to mi.xers 301 and 302 which receives the East modulation channel from a pair of .stations, and mixers 304 and 306 which receive the West channel. Mixer 301, filter 308, and filter 310, and limiting amplifiers 332 and 334 operate to generate carrier CE of frequency c + a when the retrodirecti~e oscillating loop is properly completed with a subscri.ber station to pro-vide the SIgnal c ~ a + ~ ~ ~East to the antenna 102 input. At the same time, modulation ~East, with no spatial phase, is developed at the output of filter 310. Carrier CE at the output of filter 308 retains the spatial phase ~ of the input signal and is delivered to linear amplifier 311. This is the path of the retrodirective oscillating loop and the amplifier 311 provides gain for sustaining the chosen carrier.
In a similar fashion, control carrier C~ of frequency c -is extracted from the incoming signal of frequency c - a ~ ~ + ~ West by the operation of mixer 304, limiting amps 334 and 335, filter 312 and mixer 306. Filter 312 passes only frequencies close to c - a and preserves the spatial phase angle 0. When this signal mi~es with the incoming signal c - a + ~ + ~ West in mixer 306 the difference frequency resultant is ~West, having no spatial phase angle, which is the output of filter 314. The output of filter 312 feeds amplifier 316 which is used to develop the control carrier Cw by providing gain to the retrodirective oscillatin~ loop between the node and the West station. So that the West station may activate the East station, the two loops between the stations and the node are simultaneously activated. For this purpose the sicJnals rom arnplifiers 317 and 316 are supplied to mixer 318.
~he resultant sum frequency signal is selected by filter 320 and is c -t a + ~ -~c - a -~ ~)= 2c + ~ ~ 0. This signal is mi~ed with the output of amplifier 311 in mixer 322 and the desired resultant is the difference signal 2c -~ ~ + ~ -(c + a + ~)=c-a+~
This signal is mi~ed wi-th the output of Eilter 310, ~ East, in mixer 324 to produce c ~ a -~ 0 + ~! East. At the same time the output of amplifier 316 is mixed ~Jith the output of filter 320 in mixer 326 to produce 2c 1 ~ + 0 -(c - a -~ ~ = c + a -~ ~ and this signal is mixed in mi~er 328 with -the modulation output of filter 314 to produce the signal c + a -~ ~ + ~ West.
At this point modulation from the West has been transferred to the East and vice versa since, for example, West was associated at the input with the spatial angle 0 and it is now at this point associated with spatial angle ~, the spatial angle received from the East. In order for this modulation to be transmitted to the East, it is necessary for the sense of the spatial angle to be inverted i.e. changed to -~, as it is well known in the art that for a retrodirective arxay to transmit to a direction from which it receives the phase sense of signals on the array elements must be reversed (inverted).
Inversion is accomplished by mixing these signals in mixer 330 with signals from reference oscillator 118 whose frequency is 2c.
The resulting outputs to antenna element 104 are 2c - (c - a +
East) = c t a - 0 - ~ East and similarly c - a - e - ~ West.
It is now apparent tha-t siynal c + a ~ East will be sent to the West terminal because 0 represents the spatial angle of the direction of the West terminal. c + a is tne frequency o the carrier C~7 receivable at the West terminal but it will bear ~ East, the modulation originated at the East terminal. Similarly the signal c - a ~ West will be sent to the East termina~ station because it co~tains ~ which \

represents the spatial an~le received from the East direction The transmitted carrier CE ~ill be of frequency c - a and will bear modulation ~ ~est. I~he node will -thus properl~l redirec-t modulations between pairs of terminal stations on the basis of proper frequency designator a which may take on many values Al, A2 A3, ...An. Each value will set up a specific pair. The node element should contain a sinyle unit equivalen-t to FIG. 11 for each value of A (and duplex channel to be used), as previously pointed out, to avoid cross modula-tion be~ween channels.
Note that in this embodimen-t of FIG. 11 only two modulated signals are transmitted per duplex channel; i.e. c + a ~ West and c - a - ~ West instead of three signals;i.e., modulations Me, M and carrier c transmitted in the embodiment of FIG. 6~
Conferencing between stations is effected by a station initiating calls to several other stations in the same manner as previously described. The conference initiating party then bridges his instrument across the inputs and outputs oE
the several channels.
If it is desired that each party may independently talk to all other of the conferring parties, then each terminal will call up the other remaining conferees after the conference is initiated.
Another conferencing method is to assian the same channel designator Al to several stations, say ten. Then when any of the stations is called all these stations will answer and all will be able to receive and transmi-t informa-tion from and to the o-ther parties.
It is usual practice to provide an offset in frequency between transmission and reception in order to prevent the trans-mitter power from interfering with the weak received signals. Thisrequirement has not been emphasi~ed in the above explanation and o~

dcscription of the invention in the interes-t of simpli-ficatiorl. Such offset frequency is readily provided, by, for e~anlple adjusting the reference oscillation 118 in the nodes to be offset from 2c by an amount z. Similarly the subscriber st~tion oscilla-tors 240 may be adjusted to provide for the correc-t matchin~ offse-t to allow -the retro-directive loops to operate with separa-ted up and down frequency bands.
The offset z may be m2de different for different co~munication paths. For e~ample, a subscriber station may be within the field of view of two nodes. Each node is capable of transmitting to the subscriber on the same frequency channel.
But Node A requires an offset oE Za to complete the loop whereas Node B use a different offset Zb The subscriber may choose which path by choosing the offset frequency even though his receiving frequency remains the same.
This operation has practical importance in situations where two subscribers lie in the same direction with respect to a ~ode A but not with respect -to a Node B. If the two ~0 encounter interference using -the same received frequency channel one may switch the offset of his station so as to receive from the other node. In effect, this makes possible much greaLer frequency reuse.

Similarly, a subscriber station may not only be assigned a specific receive frequency f but also a specific offset z. Then the number of subscriber channels would be nfnz, where nf is the nun~ber of different frequency assignments and n is the number of offset assignments.
In mobile cases if n is made equal to the number of independent beam positions no interference will occur if the nun~er of subscribers equals nfn and they are uniformly distributed. This leads to a potential reduction in frequency assignments to ~ to serve n subscribers~
~32-XnterferomPtrie methods well kno~n in the state - of the art may be used at th~ nodes and retrodirective terminals to observe spa~ial phas~ angles between array 1 elements and ~o de~ermine from these obsexvat~ons the dir-i ectlon from which signals are being received. Irhe present invention facilitates such measurements because the carrier signals are readily identified. Fur~hermore~ because the system requires a complete loop to operate, modulation signals may be sent around the loop to determine distance from tim~ delay observation between subseribers and inter-mediate nodes. These data can be used to compute the -location of c~mmunicating subscribers.
As described, a series of links are set up between ; re~rodirecti~e relay sta~ions or nodes such that several directional links can be established. An important feature illustrated is that the system does no~ require computation or knowledge of the location of the desired parties in order for a co~mu~ications linkage to be established between subOcribers. Ano~her important feature is that switching equipme~t is not required on the nodes to establish or maintain the communications linkage between subscribers.
Wh~ot has been described are presently preferred, hence, illustrative embodiments of ~he invention. Those skilled in the art will immedia~ely recognize that modifications can be made while still coming within the scope Oc the in~ention which has been described and which is set forth in the claims.

~33O

Claims (16)

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

1. A communications system having at least one retrodirective node station and at least a pair of subscri-ber stations for establishing a communications link between said subscriber stations to permit said subscriber stations to interchange information comprising means for establishing first and second interdependent retrodirective oscillating loops between respectively one of said subscriber stations and said retrodirective node and said other subscriber station and said retrodirective node by controlling the energy characteristics of the link between the first subscri-ber station and the retrodirective node for combination with the energy characteristic of the link between the second subscriber station and the retrodirective node to bring about the substantially simultaneous establishment of said first and second retrodirective oscillating loops, and means in said retrodirective node for receiving information directed to said subscriber stations whereby said subscriber stations can interchange information.

2. The communications system of claim 1, wherein said means for establishing said retrodirective oscillating loops includes means for establishing a transmit and receive signal having frequencies which are selectively offset to enable communication between selected subscribers.

3. A communications system including at least one retrodirective node station and at least a pair of subscriber stations each identified by a particular signal frequency for establishing a communications link between said sub-scriber stations to permit said subscriber stations to interchange information, comprising, means at first subscri-ber station for providing a signal having a frequency related to the particular signal frequency associated with the second subcriber station, and means in said retrodirective node responsive to the signal of said first subscriber station and a signal provided by said second subscriber station for combining and processing these signals and for transmitting signals at particular frequencies to each of said subscribers to establish substantially simultaneously interdependent retrodirective oscillating loops between the subscriber stations and the retrodirective node whereby information can be transferred between subscriber stations by way of the retrodirective node.

4. A retrodirective node for use in a communication system including at least two subscriber stations, said retrodirective node comprising antenna means for receiving a first control carrier signal of the characteristic frequency transmitted by one of said subscriber stations and a second control carrier signal from said second subscriber station having a frequency related to the characteristic frequency of said first signal, a control carrier signal processor coupled to said antenna means for receiving said first and second control carrier signals and for combining said first and second control carrier signals to produce control carrier signals at particular frequencies, and antenna means coupled to said control carrier signal processor for transmitting a control carrier signal at one of the particular frequencies to each subscriber station whereby interdependent retrodirec-tive oscillating loops are established substantially simul-taneously between the subscriber stations and the retrodirec-tive node to permit communications between subscriber stations.

5. A retrodirective node for use in a communica-tion system including at least two subscriber stations, said retrodirective node comprising first means for receiving from the respective subscriber stations first and second control carrier signals having related frequencies, second means for combining said first and second control carrier signals to produce reference carrier signals at particular frequencies and correct phase for transmission to each sub-scriber station thereby establishing substantially simultan-eously interdependent retrodirective oscillating loops be-tween each subscriber station and the retrodirective node, and third means coupled to said first means for receiving information containing signals from the subscriber stations, said third means combining an information signal from one of said stations with a control carrier signal from the other said station to direct the information containing signal from one subscriber station to the other subscriber station to enable communication of information between subscriber stations.

6. The retrodirective node according to claim 5, wherein said third means includes first means for mixing said control carrier signal from said first subscriber station with information signals from said first and second subscriber stations, and second means for mixing said control carrier signal from said second subscriber station with information signals from said first and second subscri-ber stations so that properly spatially phased information signal components are created for transmission to the correct subscriber station so that information can be transmitted between said first and second subscriber stations.

7. A retrodirective node for use in a communica-tions system according to claim 6, wherein said third means further includes a first filter means coupled to an output of said first mixing means, said first filter means arranged to produce an output signal consisting of only said information signal and said control carrier signal from said first subscriber station and a second filter means coupled to an output of said second mixing means, said second filter means arranged to produce an output signal consisting of only said information signal and said control carrier signal from said second subscriber station.

8. A retrodirective node for use in a communications system according to claim 7, wherein there is further included means for mixing an output signal from said first filter means with said control carrier signal from said second subscriber station and means for mixing an output signal from said second filter means with said control carrier signal from said first subscriber station.

9. A communication system having at least two retrodirective node stations, at least one subscriber station associated with each retrodirective node station and a communications link between said at least two retrodirective node stations, said communication systems comprising means for establishing a first retrodirective oscillating loop between one of said subscriber stations and its associated retrodirective node station and for establishing a second retrodirective oscillating loop between the other of said subscriber stations and its associated node stations, said first and second retrodirective oscillating loops being interdependent and being established substantially simultan-eously via said communications link which conveys energy characteristic of a first retrodirective oscillating loop to said second retrodirective node and which conveys energy characteristic of said second retrodirective oscillating loop to said first retrodirective node so that the energy characteristic of each of said retrodirective oscillating loops combine in each of said retrodirective nodes and means for transferring information between said subscriber stations via said first and second retrodirective oscillating loops and said communications link.

10. A communications system according to claim 9, wherein said communications link includes a satellite.

11. A retrodirective node station for use in a comm-unications system having at least two subscriber stations, said retrodirective node station comprising a retrodirective array transceiver including receiving and transmitting antenna arrays, each antenna array having individual antenna elements, means coupling said receiver antenna elements to said transmitting antenna elements including filter means having a pass band selected to pass a reference carrier signal provided by each subscriber station, said reference carrier signals being of complementary frequencies, and means for receiving signals passed by the filter means to produce therefrom characteristic signals for transmission to each subscriber station thereby establishing substantially simultaneously interdependent retrodirective oscillating loops between subscriber stations and said retrodirective node station to permit communications between the subscriber stations.

12. The retrodirective node station of claim 11, wherein the characteristics signals transmitted to each subscriber station have the same frequency and spatial phases dependent upon the receiving subscriber station.

13. The retrodirective node station of claim 11, wherein the characteristic signals transmitted to each subscriber station have unequal and related frequencies and spatial phases dependent upon the receiving station.

14. a communication system having at least two retrodirective node stations and a communications link between said node stations and at least one subscriber station associated with each retrodirective node station, each of said retrodirective node stations comprising a retrodirective array transceiver including receiving and transmitting antenna arrays, each antenna array having individual antenna elements, means coupling said receiver antenna elements to said transmitting antenna elements incl-uding filter means having a pass band selected to pass reference carrier signals of complementary frequency provid-ed by each subscriber station, mixer means for receiving signals passed by the filter means and producing therefrom signals of a characteristic frequency, means for combining the signals of characteristic frequency with the reference carrier signals to produce control carrier signals for transmission to each subscriber station for establishing substantially simultaneously first and second interdependent retrodirective oscillating loops between the subscriber stations and the respective retrodirective node stations, said first and second interdependent retrodirective oscilla-ting loops being established via said communications link which conveys energy characteristic of said second retrodir-ective oscillating loop to said first retrodirective node and which conveys energy characteristics of said first retrodirective oscillating loop to said second retrodirec-tive oscillating node so that energy characteristics of each of said retrodirective oscillating loops combine in each of said retrodirective nodes and means for transferring information between said subscriber stations via said first and second retrodirective oscillating loops and said communications link.

15. A communications system according to claim 14, wherein said mixer multiplies the signals to produce a signal having a characteristic frequency equal to the sum of the complementary frequencies.

16. A communications system according to claim 15, wherein said means for producing control carrier signals includes a filter coupled to an output of said mixer, said filter having a pass band capable of passing the output signal from the mixer, a reference oscillator for producing a reference oscillator signal having a frequency related to the frequency of the reference carrier signals and means for mixing an output signal of said filter with said reference oscillator signal.