CA1288480C - Satellite communications system employing frequency reuse - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A satellite communications system employs separate subsystems for broadcast and point-to-point two-way communications using the same assigned frequency band and employs an antenna system which uses a common reflector (12). The point-to-point subsystem achieves increased communication capacity through the reuse of the assigned frequency band over multiple, contiguous zones (32, 34, 36, 38) covering the area of the earth to be serviced. Small aperture terminals in the zones are serviced by a plurality of high-gain downlink fan beams (29) steered in the east-west direction by frequency address. A special beam-forming network (98) provides in conjunction with an array antenna (20) the multiple zone frequency address function. The satellite (10) employs a filter interconnection matrix (90) for connecting earth terminals in different zones to permit multiple reuse of the entire band of assigned frequencies. A single pool of solid-state transmitters allows rain-disadvantaged users to be assigned higher than normal power at minimum cost and geographically disperses the transmitter intermodulation products. In an alternate embodiment, the satellite (200) employs direct radiating array antennas (202, 204) for reception and transmission. The system 1200) utilizes hybrid-coupled dual amplifiers (251) to reduce amplifier production costs. In another embodiment, both point-to-point and broadcast services are available on a single polarization by allocating one-half of the frequency spectrum to each service and by using separate direct radiating arrays from horizontal and vertical polarization for both reception (235, 236) and transmission (237, 238). The frequency spectrum is reused in each of the contiguous receive zones (220, 222, 224, 226) and the transmit zones (228, 230, 232, 234) because sufficient spatial isolation is achieved by subdividing the receive zones in two halves (220a, 220b, 222a, 222b, 224a, 224b, 226a, 226b), and by using one-half of the frequency spectrum in each subdivided zone.

Description

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-I -SATELLITE COMMUNICATIONS SYSTEM
EMPLOYING FREQUENCY REUSE

TECHN.ICAL FIELO

The present invention broadly relates to satellitc communication systems especially of the type employing a satellite placcd in geosynchronous orbit above the earth so as to form a communication linlt between many small aperture terminals on the earth. More particularly, the invention involves a communication satellite having hyb~id communication 10 capability accommodatin8 both two-way and broa'dcast communication systems. Two-way communications between small aperture earth terminals is achieved through multi-fold reuse of a fixed frequency spectrum in contiguous zones of an area on the earth. ' BACKGROUND ART

In domestic communication satellite systems, which interconnect large numbers of very small aperture earth terminals, the most important parameters affec~ing the system capacity are the Effective Isotropic Radiated Power (EIRP) and the available bandwidth. EIRP refers to a measure of the satellite's transmitter power which takes into consideration the 20 gain of the antenna. EIRP is the power of a transmitter and isotro~ic antennathat would achieve the same result as the transmit2er and antenna which is actually employed.

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In the past, high antenna gain and multiple frequency reuse has been achieved by employing a plurality of up and down link beams covering the regions of a country or other areas of the earth to bc served.
Both frequency division and time division systems have been used or proposcd to interconnect large numbers of signals from many geographically separatcd earth stations. Time division systems permit the satellite transmitters to operate efficiently. This is because only one time division signal at a timc is amplified in a transmitter, so it may be operated at or close to signal channel saturation, the most efficient operating point. However, time division systems require high power ground transmitters and expensivc signal processing and are therefore incompatible with low cost earth stations.
Frequency division systems are better suited to low cost earth stations, but have lower satellite transmitter efficiency because each transmitter handlcs multiple carriers. Since multiple carrier amplifiers ~enerate undesirablc intcrmoduJation products that increase in power as the transmitter efficiency is increased, the optimum compromise ~etween transmitter efficiency and intermodulation product generation results in a relatively low transmitter efficiency.

In Ku band, the satellite communication band most suitable for two-way service between very small terminals, the attenuation of the signals by rain is an important consideration in the design of the system. In the previous systems, this attenuation is overcome on the downlink by using higher satelli~e transmitter power per channel than would be necessary for clear weather service, typically four times as much. This accommodation of rain attenuation ~herefore results in more expensive satellites having fewer available channels.

The available bandwidth of a satellite sys~em is determined by the number of times the allocated frequency spectrurn can be reused.
Polarization and spatial isolation of beams have becn employed to pe~mit reuse of thc frequency spectrum. As the number of isolated beams is increased, however, the problcm of interconnecting all the users becomes very complicated and is one of the factors that limit the number of reuses of the frequency spectrum.

1~8~34B~

The present invention is directed toward overcoming eacb of the deficiencies mentioned above.

SUhIMARY OF THE INYENTION

The present invention provides a satellite communication system for interconnecting large numbers of very small aperture earth terminals and mobile satellite service users which maximizes satellite EIRP as well as thç available bandwidth. The system employs highly directional, contiguous beams on the downlink or transmit signal which substantially increases the EIRP and allows multiple reuse of the assigned frequency spectrum. As a result, the number of communications that can be proYided for point-to-point service is maximized. High multi-carrier ~ransmitter efficiency is achieved as a result of the dispersion of intermodulation productsand the deleterious effects of rain on the downlink channels are easily overcome by the use of pooled transmitter power. The interconnection of the many users is achieved by a combination of a filter interconnection matrix and a highly directional addressable downlink beam.

According to the present invention, a system is provided for interconnecting any of a plurality of earth terminals or mobile terminals within an area on the earth for two-way communication using a communications satellite. A plurality of uplink beams are formed which respectively emanate from contiguous zones coverin~ the area to be serviced by the satellite. The uplink beams carry a plurality of channels over a first preset ran~e of uplink frequencies. Each uplink zone uses the same preset ran8e of frequencies. The uplink frequencies are therefore reused by each zone, thereby effectively multiplying the number of communications channels that can be handled by the satellite. A plurali~y of downlink beams destined for the downlink zones also carry a plurality of channels over a second preset range of frequencies. The beams for each of the downlink zoncs also use tlhe same second preset ran8e of frequencies to provide multiple reuse of these frequencies. The satellite employs a filter interconnection matri~ for interconnecting the channels in the different zones.

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The preset }an8e of frequencies are separated into first and second sets The preset range of frcquencies is spatially distributed over ea~ h zone such that contiguous regions of adjacent zones are not serviced by thc same frequency set. Since contiguous regions of adjacent zoncs communicate 5over different frcquency sets, there is sufficient spatial isolation between contiguous zones to permit frequency reuse. The b~ams operating over the first frequency set are simultaneously transmitted to all of the zones and then the beams operating over the second set are simultaneously transmitted to all the zones. This is achieved by using a two-position switch which insures that 10beams operating over the first and second sets are alternately transmitted A fan beam narrow in one direction, east-west for example, and broad in the orthoonal direc~ion, is generated by a beam-forming network used in conjunction with the transmit array antenna. The transmit array may be a confocal arrangement or a direct radiating array. The east-1~west direction of the beam within the covered area is determined by the downlink frequency, which is related to the uplink frequency by a constant difference. The uplink frequency therefore determines the downlink frequency, which by action of the beam-forming network and the transmit array determine the direction and hence, the destination of the downlink 20beam. Such an arrangement is referred to as a frequency addressable beam.
The side lobes of the beam are designed to be low enough to permit reuse of the frequency spectrum in the adjaccnt zones.

The transmitters, preferably equal power dual solid state amplifiers, are embedded in the transmit array antenna, onc ~mplifier ~eing 25associated with two staves of the array. All of the amplifiers operate at the same power level despite th; unequai powcr inputs and outputs. Since all of the downlink power is provided by this single pool of transmitters, it is easy to provide relatively high power to those relatively few signals direc~ed to rain affected areas with only a very small reduction in power available to she m~lch 30larger number of unimpaired signals.

Because the transmit beam directions are related to the frequencies of their signals, and the frequencies of the intermudulation products generated in the power amplifiers differ from those of the signals which cause them, the intermodulation products go down in different directions than the signals. This process results in the spatial dispersion of the intermodulation products. This dispersion is enhanced by the use of multiple downlink zones. This results in a lower intermodulation product density at all ground terminals in the frequency bands to which they are tuned. This reduced sensitivity to inter-modulation products permits the power amplifiers in the satellite to be operated more efficiently.
Various aspects of the invention are as follows:
A method of communicatively interconnecting any of a plurality of terminals within an area on the earth using an earth-orbiting communications satellite, comprising the steps of: (A) forming a first plurality of groups of radio frequency beams respectively between each o~f a plurality of zones covering said area on the earth and said satellite, the beams in groups thereof carrying communications signals over the same first preselected range of frequencies, such that said first preselected range of frequencies is reused by the beams for all of said zones; (B) separating each group of said radio frequency beams into first and second sets thereof respectively carrying communications signals over first and second sets of frequencies within said first preselected range of frequencies; and (C) alternately transmitting the first and second sets of said beams between said zones and said satellite.

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5a A method of communicatively interconnecting a plurality of terminals within an area on the earth using an earth orbiting communications satellite, comprising the steps of: (A) dividing said area into a plurality of sssentially contiguous zones; (B) dividing each zone into at least first and second sections; and (C) alternately transmitting at least first and second radio frequency beams respectively between said first and second sections for each of said zones and said satellite, said ~irst and s~cond beams respectively carrying communications signals over first and second different sets of frequencies defining a first preselected range of frequencies, whersin said first preselected range of frequencies is reused by the first and second beams for all of said zones.
A satellite communications systam for communicatively interconnecting a plurality of earth terminals covering an area on the earth, comprising: a satellite disposed in orbit above the earth; first means for forming a plurality of uplink radio frequency beams between said satellite and each of a plurality of respectively associated zones covering said area, said zones being divided into at least two sections, each plurality of said uplink beams carrying communications signals destined to be received by downlink terminal sites in said zones, the uplink beams for each zone being arranged in first and second sets of said beams respectively originating from said two sections in the zone and carrying communication signals over first and second sets of frequencies, said first and second set of frequencies defining a first range of frequencies, the uplink beams for all of said zones using said first range of frequencies; second means at said satellite for i \ , , 4~3~

5b alternately receiving said first and second sets of beams;
and third means at said satellitP coupled with said second means for forming a plurality of groups of downlink radio frequency beams between said satellite and said zones, each of said groups of downlink beams carrying communication signals destined to be received at downlink terminal sites in one of said zones.
It is therefore an object of an aspect of the invention to provide a communication satellite for inter-connecting large numbers of very small aperture antenna terminals and mobile satellite service users using high satellite transmit antenna gain and allowing multiple reuse of the assigned frequency spectrum, to substantially increase the number of communication channels that can be provided for point-to-point communication service.
An object of an aspect of the invention is to provide the downlink power from a single pool of transmitters, so that signals being attenuated by rain can be easily allocated more satellite transmit power.
An object of an aspect of the invention is to provide a communication satellite which disperses intermodulation products in order to increase transmitter efficiency.
An object of an aspect of the invention is to provide a communication satellite as described above which provides both broadcast, and point-to-point communications service.
An object of an aspect of the invention is to provide a communication satellite which uses direct radiating array antennas in a system which reuses the assigned frequency spectrum.

~, , . i , lX~8~80 An object of an aspect of the in~ention is to provide p~int-to-point and broadcast services on a single polari-zation.
These, and further object and advantages of the inYention will be made clear or will becomc apparent during the coursc sf the following descrip~ion of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

Figure I is a perspectivc view of a communications satellitc, showing the antenna subsys~ems;

Figure 2 is a top plan view of the antenna subsystems shown in Fipure l;

Figure 3 is a sectional view taken along the line 3-3 in Figure 2;

Figure 4 is a sectional view taken along the linc 4-4 in Figure 2;

Figure 5 is a view of the United States and depicts multiple, contiguous receive zones covered by the satellite of the present invention, the primary areas of coverage being indicated in cross-hatching and the areas of contention being indicated by a dimpled pattern;

Fi~ure 6 is a block dia8ram of ~he communication electronics for the communica~ions satellite;

Figure 7 is a schematic diagram of a coupling network which interconnects the point-to-point receive feed horns with the inputs to the communications electronics shown in Figure 6;

Figure 8 is a referenee table of the intcrconnect channels employed ~o connect the receive and transmit zones for the point-to-point system;

Figure 9 is a diagrammatic view of tbc United Statcs depicting multiple contiguous transmit zones covered by the satellite and tlle geographic distribution of the interconnected channels for each zone, across the United States;

Figure 9A is a graph showing the variation in gain of thc transmit antenna beam for each zone in the point-to-point system in relation to 10 the distanse from the center of the beam in the east-west direction;
Figure 9B is a ~raph similar to Figure 9A but showing the variation in gain in the north-south direction;

Fi~urc 10 is a detailed schematic diagram of the filter interconnection matrix employed in the point-to-point system;

Figure 11 is a detailed, plan v;ew of the beam-forming network employed in the point-to-point system;

Figure 12 is an enlargcd, fragmentary view of a portion of the beam-forming network shown in Figure 11;

Figure 13 is a front elevational view of the transmit array 20 for the point-to-point system, the horizontal slots in each transmit element not being shown for sake of simplicity;

Figure 14 is a side view of the transmit element of the array shown in Figure 13 and depicting a corporatc feed network for the element;

Figure 15 is a front, perspective view of one of the transmiS
2J elements ernployed in the transmit array of Figure 13;

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Figure 16 is a front view of the receive feed horns for thc point-to-point system;

Figure 17 is a diagrammatic view showing thc relationsbip betwcen a transmitted wave and a portion of the transmit feed array for the 5 point-to-point system;

Figurc 18 is a perspective view of a deployed mobile satellite which forms an alternate embodiment of the prescnt invenlion;

Figure 19 is similar to Figure 1~ but depicts the mobile satellite in its stowed position;

Figure 20 is an elevational view of a directly-radiating array antenna forming part of the satellite of Figure 18;

Figure 21 is a view similar to Figure 5 but depicting zones covered by the satellite of Figure 18;

Figure 22 is an illustration similar to Figure 9A, but 15 showing the antenna pattern contours for three zones;

Figure 23 is a diagrammatic view showing a ~eam-forming network desi~ned for a system which reuses the frequency spectrum three times;

Figure 24 is similar to Figure 6, but illustrates a block 20 dia8ram for the alternate embodiment;

Figurc 25 is a dia~rammatic view of an equal power dual amplificr and the resulting power distribution;

Figure 26 is a perspectivc view Or another deployed satellite which forms a further embodiment of the present invention, ~8~80 9.
Figure 27 is a view similar to Figurc 5 and Figure 21, but depicting alternate receive zones;

Figure 28 is an alternate illustration, similar to Figurc 9A
and Figure 22, of the reccive beam patterns;

Figure 29 is similar to Figure 9 illustrating alternate transmit zones;

Figure 30 is an illustration of the transmit beam pattern corresponding to the transmit zones in Figure 29;

Figure 31 is a reference table sirnilar to ~hat shown in Fi~ure 8, but for a filter interconnection matrix for the mobile satellite system depicted in Figure 26; and Figure 32 is a block diagram of the repeater employed in the satellite shown in Figure 26.

DESCRIPTION OF THE PREFERRED l~MBODIMENT5 1~ Referrin8 first to Figures 1-4, a communications satellite 10 is depicted which is placed in geosynchronous orbit above the earth's surface.
The satellite's antenna system, which will be described in more detail below, will typically be mounted on an earth-oriented platform so that the antenna system maintains a constant oricntation with respect to the earth.

The satellite 10 is of a hybrid communications-type satellite which provides two different types of communication services in a particular frequency band, for e~ample, the tixed satellite service Ku band. One type of communication servicc, referred to hcreinafter as poin~-to-point service, provides ~wo-way communications between very small aperture antenna 2~ term;nals of relatively narrow band voice and data signals. Through the use of frequency division multiple access (FDMA) and reuse of the assigned frequency spectrum, tens of thousands of such communication channels are 4~C~

accommodated simultaneously on a single linear polarization. The other type of communication service proYided by the satellitc 10 is a broadcast serYicc, and it is carried on the other linear polarization. The broadcast scrvice is primarily used for one-way distribl~tion of video and data throu8hout tbe geo~raphic territory served by the satellite 10. As such,the transmitantcnna beam covers the entire geographic territory. For illustrative purposes throughout this description, it will be assumed that thc geographic area to be serviced by both the point-to-point and broadcast services will be the United States. Accordingly, the broadcast service will bc referrcd to hereinafter as CONUS (Contincntal United ~tates).

The antenna system of the satellite 10 includes a conventional omni antenna 13 and two antenna subsystems for respectively servicing the point-to-point and CONUS systems. The point-to-point antenna subsystem provides a two-way communication link to interconnect earth stations for two- way communications. ~he CONUS antenna system functions as a transponder to broadcast, over a wide pattern covering the entire United States, signals received by one or more particular loca~ions on earth. The point-to-point transmit signal and the CONUS receive signal are vertically polarized. The CONUS transmit and point-to-poin~ receive signals are hori~ontally polarized. The antenna system includes a large reflector assembly 12 comprising two reflectors l~a, 12b. The two reflectors 12a. 12b are rotated relative to each other about a common axis and intersect at their midpoints. The reflector 12a is horizontally polarized and operates with horizontally polarized signals, while the reflector 12b is vertically polarized and therefore operates with vertically polarized signals. Conse~uently, each of the reflectors 12a, 12b reflects signals which the other refiector 12a, 12b transmits.

A frequency selective screen 18 is pro~ided which includes two halves or sections 18a, 18b and is mounted on z support 30 such that the screen halves 18a, 18b ~re disposed on opposite sides of a cen~erline passing diametrically through the satellite 10, as best seen in Figure 2. The frequency selective screen 18 functions as a diplexer for separating diffcren~ bands of frequencies and may comprise an array of discrete, electrically conductive 8 48() elements formed of any suitable material, such as copper. Any of Yariou~
types of known frequency selective screens may be employed in this anter.na system. However, one suitable frequency selective screen, cxhibiting sharp transition characteristics and capable of separating two frequeney bands which are relatively close to each other, is described in U.S. Patent No.
4,785,310, issued November 15, 1988, and assi~ned to Hughes Aircraft Company. The frequency selective screen 18 effecti~ely separates the transmitted and received signals for both the CONUS and point-to-point subsystems. It may be apprGciated that the two halves 18a, 13b of the screen 18 are respectively adapted to separate individual signals which are horizon~ally and vertically polarized.

The CONUS subsystem, which serves the entire country with a single beam, has, in this e~tample, eight conventional transponders each having a high power traveling wave tube amplifier as its transmitter 82 (see Figure 6). The CONUS receive antenna uses vertical polarization, sharing the vertically polarized reflector 12b with the point-to-point transmission system.
CONUS receiYe signals pass through the frequency selective screen half 18b and are focused on the receive feed horns 14 located at the focal plane 28 of reflector 12b. The antenna pattern so formed is shaQed to cover CONUS.
The COI~IUS transmit antenna employs horizontal polarization, and shares reflector 12a with the point-to-point receive system. Signals radiating from the transmit feeds 24 are reflected by the hori~oneally polarized frequency selectivescreen 18a toreflector 12awhose secondarypattern isshaped tocover CONUS.

The point-to-point subsystem broadly includes a transmit array 20, a subreflector 22, and receive feed horns 16. The transmit array 20, which will be described 3ater in more detail, is mounted on the support 30, immediately beneath the screen IB. The subrzflector 22 is mounted forward of thG transmit array 20 and slightly below the screen 18. The signal emanating from the transmit array 20 is refleeted by the subreflector 22 onto one half 18b oî the screen 18. The subreflector 22 in conjunction with ~he main reflector 12 functions to effectively magnify and enlarge the pattern of the signal emanating from the transmit array 20. The signal reflected from ~.

~.28~80 the subreflector 22 is, in turn, reflected by one half 18b of the screen 18 ontothe large rçflector 12b, which in turn reflccts the point-to-point signal to thcearth. Throu~h this arrangement, the performance of a large aperture phase array is achieved. The receive feed horns 16 are positioned in the focal planc 26 of the reflector 12a. It consists of four main horns 50, 54, 58, 62 and threeauxiliary horns 52, 56, 60 as shown in Figure 16.

Referring now also to Figures 13-15, thc transmit array 20 comprises a plurality, for example forty, transmit waveguide elements 106 disposed in side-by-side relationship to form an array, as shown in Figure 13.
Each of the transmit waveguide elements 106 includes a plural;ty, for example twenty-six, of horizontal, vertically spaced slots 108 therein which result in the ~eneration of a vertically polarized signal. As shown in Figure 14, the transmit array 20 is fed with a transm;t signal by means of a corporate fecd network, generally indicated by the numeral 110 which excites the array element in four places 114. The purpose of the corporate feed network 110 is to provide a broadband match to the transmit waveguide element 106. Signals input to the waveguide opening 112 excite the array slots 108 so that the slot excitation is designed to give a flat pattern in the north-south direction.

Attention is now directed to Figure 5 which depicts a generally rectangular beam coverage provided by the horizontally polarized point-to-point receive system. In this particular examplc, the area serviced by the point-to-point system is the continental United States. The point-to-point receive system comprises four beams Rl, R2, R3, R4 respectively emanating from the four uplink zones 32, 34, 36, 38 to the satellite, wherein each of ~he beams Rl-R4 consists of a plural;ty of individual uplink beams originating from individual sites in each zone 32, 34, 36, 38 and carries an individual signal from that site. The upllnk beam si8nals from the individual sites are arranged into a plurality of channels for each zone. For example, zone 32 may include a pluraliiy, e.g. si~steen 27 MHz channels wi~h each of such channels carrying l-undreds of individual beam signals from corresponding uplinlc si~es in zone 32.

~ ~84~30 The signal strength for each of the four beam patt~rn contours, respectively designatcd by numerals 32, 34, 36 and 38, are approximately 3 dB down from peaks of their respectivc beams. The antcnna bcams have becn dcsigncd to achieve sufficient isolat;on bctween thcm to make feasible in the cross-hatched regions 39, 41, 43, 45 rcuse of the frequcncyspectrum four times. In thc dottcd regions 40, 42, and 44, the isolation i5 insufficient to distinguish between signals of the same frequency OriginatiDg in adjacent zones. Each signal originating in thesc regions will gencratc two downlink signals, one intended and onc e~traneous. The gcncration of extraneous signals in these areas will be discussed latcr in more dctail.

It may be readily apprcciated from Figure 5 that thc four zones covered by beams 32, 34, 36, 38 are unequal in width. The East Coast zone covered by beam 32 extends approximately 1.2 degrees; the Central zone covered by beam 34 extends approximately 1.2 degrees; the Midwest zonc covcred by beam pattern 36 extends approximately 2.0 degrees, and; the West Coast zone covered by beam pattern 38 extends approximately 2.0 degrees.
The width of each of the four receive zones 32, 34,36and 38is determined by the number of terminals and thus the population density in the various regions of the country. Thus, beam pattern 32 is relatively narrow to accommodate the relatively high population density in the Eastern part of the United States while beam pattern 36is relatively wide due to the relatively low population density in the Mountain states. Since each zone utilizes the entire frequency spectrum, zone widths are narrower in regions where the population density is high, to accommodate the 8reater demand for channel usage.

2~ As shown in Figure 9, the point-to-point transmit system comprises four beams Tl, T2, T3, T4 respectively covering the four transmit zones 31, 33, 35, 37, wherein each of the beams TI~T4 consists of a plurality ofindividual downlink beams destined for the individual downlink sites in each zone 31, 33, 3S, 37 and carries an individual signal to that site. The downlink beam signals, destined to be received at the individual d~wnlink si~;es, are arranged into a plurality of channels for each zone. Fo~ e~ample, zone 31 rnay include a plurality, e.8. sixteen 27 MHz channels with each of such ~2~ 30 channels carrying hundreds of individual beam signals to corresponding downlink sites in zone 32.

The use of multiple downlink zones and downlink zones of unequal widths assist in causing the intermodulation products, generatcd by the later-discussed solid state power amplificrs, to be geographically dispcrscdin a manner which preYents most of these products from being reccivcd at thc ground tcrminals. The net effect is that the amplificrs may be operatcd morc efficiently because the system can tolerate more intermodulation products.
Although the widths of the transmit zones 317 33, 357 37 are ncarly the same as those of the receive zones Rl7 R2, R3, R4, small differences between the two sets have bcen found to optimizc the capacity of the system.

The half power beam width of the individual transmit beams 29 is substantially narrower ~han that of the transmit zones 31, 33, 35, 37. This results in the desirable high gain, and avoids the zones of contention i~ 40, 42, 44 characteristic of the receive zone arrangement. These individual beams 29 must be steered within the zones in order to maximize the downlink EIRP in the directions of the individual destination terminals. The transmit point-to-point frequency addressable narrow beams 29 are 8enerated by an array 20 whose apparent size is magnified by two confocal parabolas comprising a main reflector 12b and a subreîlector 22. The east-wcst direction oî each beam 29 is determined by the phase progression of its signal along the array 106 of transmit elements 20 (Figures 13 and 15). This phase progression is established by a later-discussed beam-forming network 98 and is a function of the signal frequency. Each of the transmit array elements 20 is ~5 driven by a later-discussed solid state power amplifier. The power delivered to the array elements 106 is not uniform but is instead tapered with the cdge elements being more than 10 dB down. Tapsring of the beams 29 is achieved by adjusting the transmit 8ain according to the position of the transmit array elements 20. Thc e~citation pattern determines the characteristies of the transmit secondary pattern, shown in Fi~ure 9A.
Referring to Figure 9, the closest spacing between transmit zones 31, 33, 3S, 37 occurs between zones 31 and 33 and ;s approximately 1.~ degrees. This means that a signal addressed to zone 33 using a particular frequency would interfere with a signal using the same frequency in zone 31 with its side lobe 1 2 de~reesfrom its beam center. However, the individual transmit gains have been adjusted to provide low side lobe levels, thereby permitting frequency reuse in adjacent zones. Referring to Figure 9A, it is seen that the side lobe level at this angle off beam center is more than 30 dB down, so that such interferenee will be negligibly small. The same frequency uses in zones 35 an~ 37 are further removed in angle, hence the side lobe interferenee in those zones is even smaller.

Figure 9B is an illustration of the transmit beam pattern in the north-south direction. The twenty six slots 108 in each of the transmit waveguide elements 106 are excited in a manner which creates a nearly flat north-south pattern, extending over the covered ran8e of plus and minus 1.4 degrees from the north-south boresight direction.

Both the point-to-point and CONUS systems may utilize the same uplink and downlink frequency bands, with the point-to-point system using horizontal polarization for its uplink polarization, and the CONUS
system using vertical polarization, as previously mentioned. Fos example.
both services may, simultaneously, utilize the entire 500 MHz uplink frequency band between 14 and 14.5 ~Hz, as well as the entire 500 MHz downlink frequency band between 11.7 and 12.2 GHz. Each of the receive zones 32, 34, 36, 38 and transmit zones 31, 33, 35, 37, employin~ the point-to-point service utilizes the entire frequency spectrum (i.e. 500 MHz).
Furthermore, this total fre~luency spectrum is divided into a plurality of channels, for e~ample, sixteen channels each having a usable bandwidth of 27 MHz and a spacing of 30 MHz. In turn, each of the sixteesl channels may accommodate approximately 800 subchannels. Hence, within each zone, approximate3y 12,500 (16 channels x 800 subchannels) 32 !cilobit per secund channels may be accommoda~ed, at any given moment. As will be discussed belo-v, the communic~tion archi~ecture of the point-~o-point system allows any terminal to communicate directly with any other terminal. Thus, within a single polarization, a total of 50,000 subchannels may be accommodated nationwide.

Referring now particularly to Figures 1, 2, 6, 7 and 16, the point-to-point receive feed array 16 employs seven receive horns 50-62.
Horns 50, 54, 58 and 62 respeetively receive signals from 7OneS 32, 34, 36 and 38. Horns 52, 56 and 60 receive signals from the zones of contention 40, 42 and 44. IJsing a series of hybrid couplers or power dividers Cl-C9, the 3ignals received by horns 50-62 are combined into four outputs 64-70. For exampk, a signal originating from the area of contention 44 and received by horn 60 i5 divided by coupler C2 and portions of the divided signal are respectively delivered to couplers Cl and coupler C4 whereby the split signal i5 combined 1~ with the incomin~ signals received by horns 58, 62 respcctively. Similarly, si~nals originating from the area of contention 42 and reeeived by horn 56 are split by coupler C5. A portion of the split signal is combined, by coupler C3, with the signal output of coupler C4, wllile the remaining portion of the split signal is combined, by coupler C7, with the signal received by horn 54.

lS Attention is now particularly directed to Figure 6 which depicts, in block diagram form, the elec~ronics for receiving and transmitting signals for both the CONUS and point-to-point systems. The point-to-point receive signals 64-70 (see also Figure 7) are derived from the point-to-point receive feed network in Figure 7, whereas the CONUS receive signal 72 2l) derives from the CONUS receive feed horns 14, (Figures 1 and 3). Both the point-to-point and CONUS receive siRnal are input to a switching network 76 which selectively connects input lines 64-72 with five correspondin8 receivers, eight of which receivers are generally indicated at 74. The receivers 74 are of conventional design, three of which are provided for redundancy and are not normally used unless a malfunction in one of the receivess is experienced. In the event of a malfunetion, s~vitching networ3c 76 reeonnects the appropriate incoming line 64-72 with a back-up reeeiver 74. Reeeivers 74 function ts drive the filters in a filter interconnection matrix 90. The outputs of the receivers 74, which are conwcted with lines 64-70, are coupled by a seeond 3D switching network 78 through four receive lines Rl-R4 to a filter in~erconnection matrix 90. As will be diseussed later below, the filter interconnection matrix (FIM) provides interconnections between the receive zones 32, 34, 36, 38, and the transmit zones 31, 33, 35, 37. C)peratin~ in the above-mentioned 500 MHz assigned frequency spectrum, separated into sixteen ~38~8~

27 MH~ channels, four sets of sixtecn filters arc employed. Each set of the sixteen filters utilizes thc entire 500 MHz frequency spectrum and cach fil~er has a 27 MHz bandwidth. As will be discussed later, the filter outputs Tl-T4 are arran8ed in four groups, each group destined for one of the four transmit ~ones 31, 33, 35, 37.

The transmit signals Tl-T4 are respectively connected, via switching network 94, to four of six driving amplifieJs 92, two of such amplifiers 92 being provided for back-up in the event of failure. In thc event of the failure of one of the amplifiers 92, one of the back-up amplificrs 92 will be reconnected to the corresponding transmit signal Tl-T4 by thc switching network 94. A similar switching network 96 couples the amplified output of the amplifiers 92 to a beam-forming nctwork 98. As will be discussed later in more detail, ~he beam-formin8 network 98 consists of a plurality of - transmission delay lines connected at equal intervals along the four delay lines. These intervals and the width of the delay lines are chosen to provide the desired centerband beam squint and the beam scan rate with frequency for the corresponding transmit zones 31, 33, 35, 37 to be serviced. The transmit signals, coupled from the four delay lines, are summed in the beam-forming nctwork 98 as shown in Figures ll and 12, to provide inputs to solid state power amplifiers l00, which may be embedded in the point-to-point system's transmit array 20. In the illustrated embodiment discussed below, forty solid state power amplifiers (SSPAs) l00 are provided. Each of the SSPAs 100 amplifies a corresponding one of the forty signals formed by the beam-fo~ming network 98. The SSPAs l00 possess different power capacities to 2~ provide the tapered array excitation previously mentioned. The output of the SSPA 100 is connected to the input 112 (Fi~ure 14) at one of the elements of thetransmit array 20.

The receive signal for CONUS on line 72 is connected to an appropriate receivcr 74 by switching networks 76, 78. The output of the receiver connccted with the CONUS signal is delivered to an input multiplexer 80 which provides for ei~ht channels, as mentioned above. The purpose of the input multiplexers 80 is to divide the one low leve! CONUS signal into subsi~nals so that the subsignals can be amplified on an individual basis. The lX~

CO~US receive signals are highly amplified so that the CONUS transmit signal may be distributed to very small earth terminals. The outputs of the input multiplexer 80 are connected through a switching nctwork 84 to eight of twelve high power traveling wavc tube amplifiers (TWTAs) 82, four of which TWTAs 82 are employed for back-up in ~he event of failure. The outputs of the eight TWTAs 82 are connected throu~h another switching network 86 ts an output multiplexer 88 which recombines the eight amplified signals to form one CONUS tsansmit si~nal. The output of the multiplexes 88 is dclivered via waveguide to the transmit horns of the CONUS transmitter 24 (Figures 2 aDd 3).

Attention is now directed to Figure 10 which depicts the details of the FIM 90 (Figure 6). As previously discussed, the FIM 90 effectively interconnects any terminal in any of the receive zones 32, 34, 36~ 38 (Figures 5) with any terminal in any of the transmit zones 31, 33, 35, 37. The FIM 90 includes four waveguide inputs 120, 122, 124 and i26 for respectively receiving the receive signals Rl, R2, R3 and R4. As previously mentioned, receive signals Rl-R4, which originate from a corresponding receive zone 32, 34, 36, 38 (Figure 5), each contain the entire assigned frequcncy spectrum, (e.g.
500 MHz), and are separated into a plurality of channels, (e.~. sixteen 27 MHz 20 channels). The channels are fur~her separated into a plurality of subchannels, where each of the subchannels carries a signal from a corresponding uplink site. The FIM 90 inciudes 64 filters, one of which is indicated by the numeral 102. Each of the filters 102 has a passband corresponding to one of the channels (e.g. 1403-1430 MHz). The filters 102 are 25 arran8ed in four groups, one for each receive zone 32, 34, 36, 38, with each group including two banks or sub~roups of eight filters per subgroup. One subgroup of filters 102 contains those filters for the even-numbered channels and the other subgroup in each group contains eight fil~ers for the odd-numbered channels. Thus, for example, the filter group for receive signal Rl 30 comprises sub~roup 104 of filters 102 for odd channels, and subgroup 106 of îilters 102 for even channels. The following tablc relates the receive signals and zones to their filter subgroups:

~28~3~8V

Filter Sub~roups Receive Zone Receive Si~nal Odd Channel~ Even Channels 32 Rl 104 106 The filters are grouped in a unique manner such that when the receiYe signals Rl-R4 are filtered, the filtered outputs are combined to form the transmit signals. The transmit signals Tl-T4 also utilize the entire assigned frequency spectrum, (e.g. 500 MHz). In the illustrated embodiment, each Or the transmit signals Tl-T4 possesses sixteen 27 MHz wide channels, and comprises four channels from each of the four receive zones 32-38 (Figure 5).

The incoming receive signals Rl-R4 are divided into the correspondin~ subgroups by respectively associated hybrid couplers 128-134 15 which effectively divert 509~ of the signal power to each subgroup. Hence, for example, one-half of the Rl signal input at waveguide 120 is diverted to transmission line 136 which serYices the subgroup 104 of filters 102, and the remaining half of the Rl signal is diverted to transmission line 138 which services subgroup 106 of filters 102~ In a similar manner, each of the 20 subgroups 104-118 of filters 102 is served by a corresponding distribution line, similar to lines 136 and 138.

The construction of subgroup 104 will now be described in more detail, it being understood that the remaining subgroups 106-118 are identical in architecture to subgroup 104. At intervals along the transmission 25 line 136, there are eight ferrite circulators 140, one associated with each of the odd-numbered channel filters 102. The function of ths circulators 140 is to ConneGt the transmission line 136 to each of the odd channel filters 102 in a lossless manner. Thus, for e~ample, the Rl signal enters the first circulator 140a and circulates it counterclockwise whereby the 27 MHz band of si~nals 30 correspondin~ to channel I passes through it to circulator 142. All other 84~3~

frequencies are reflected. These reflected signals propagate via the circulator toward the ne~t filter where the prscess is repeated. Through this process, the Rl rcceive signal is filtered into sixteen channels by the si~tcco filters 104-108 corresponding to the Rl signals. Hence, the Rl signal with frequencies in the range of channel I will pass through th~ first ferrite circulator 140a and it will bç filtered by filter I of group 104.

The outputs from a filter subgroup 104-118 arc selectively coupled by a second set of ferrite circulators 142 which sums, in a criss-cros~
pattern, the outputs from an adjaccnt eroup of filters 102. For c~amplc, thc 0 outputs of channel filters 1, 5, 9, and 13 of group 104 are summed with thc outputs of channel filters 3, 7, 11 ant 15 of filter group 112. This sum appears at the output terminal for Tl 144. Referring to Figure 8, these signals correspond to the connections between receive zones Rl and R3 and to transmit zone Tl.

1~ Attention is now directed to Figures 8 and 9 which depicthow the transmit and receive signals are interconnected by the FIM 90 to allow two-way communication between any terminals. Specificaliy, Figure 8 provides a table sbowing how the receive and transmit zones are connected together by the interconnect channels while Figure 9 depicts how these 20 interconnect channels are distributed geographically across the transmit zones 31, 33, 35, 37. In Figure 8, the receive signals Rl-R4 are read across by rows of interconnect channels and the transmit signals Tl-T4 are read by columns of interconnect channels. It can be readily appreciated from Figure 8 that each of the transmit signals Tl-T4 is made up of sixteen channels arranged in 25 four groups respectively, where each group is associated with one of the receive signals Rl-R4. The satellite communications systern of the present inYention is intended to be used in conjunction with a $round station referred to as a satellite network control center which coordinates communications between the ground terminals via packet s vitched signa!s. The network 30 control center assigns an uplink user with an uplink frequency based on the location of the dcsircd downlink, assigning the available frequency whose downlink longitude is closest to that of the destina!ion. The frequency addressable downlink transmit beams 29 are thus addressable by the ~884B~

frequencies of the uplink signals. This strategy ma~cimizes the gain of the downlink signal.

As shown in Figure 9, the continental United Statcs i5 divided into four primary zones 31, 33, 35, 37. Zone 31 may be rcferred to as the East Coast zone, zone 33 is the Central zone, zone 35 is thc Mountain zonc, and zone 37 is thc West Coast zone. As preYiously mentioned, each of the zones 31, 33, 3~, 37 utilizes the entire assigned frequcncy spectrum (e g. S00 MHz). Thus, in the case of a 500 MHz assigned frequency band, therc exists sixteen 27 MHz channels plus guard bands in each of the zoncs 31, 33, 35, 37.

The numbers 1-16 rcpeated four times above the beams 29 in Figure 9 indicate the longitude of the beams corresponding to the center frequencies of the channels so numbcred. Because of the frequency sensitivity of the beams, the longitude span between the lowest and highest frequency narrow band signa5 in a channel is approximately ORe channel width. Each beam is 0.6 degrees wide between its half power point, about half the zone width in the East Coast and Central zones and nearly one-third the zone width in the Mountain and West Coast zones. The antenna beams 29 oYerlap each other to ensure a high signal density; the more that the beams overlap, the 8reater channel capacity in a given area. Hence, in the East Coast zone 31, there is a greater overlap than in the Mountain zone 35 because the signal traffic in the East Coast zone 31 is considerably greater than that in the Mountain zone 35.

The interconnect scheme described above will now be explained by way of a typical communication between terminals in different zones. In this e~tample, it will be assumed that a caller in Detroit, Michigan wishes to place a call to a terminal in Los Angeles, Ca;ifornia. Thus, Detroit, Michigan, which is located in thc Central zone 34, is the uplink site, and Los Angeles, California, which is located in the West Coast zone 37, is the downlink destination. As shown in Figurc 9, each geographic location in the continental United States can bc associated with a specific channel in a specific zone. Thus, I,os Angeles is positioned betwecn channels 14 and 1~ in transmit zone 37.

~884~0 Referring now concurrently to Figures 5, 8 and 9 par~icularly, receive and transmit zones Rl and Tl lie within the East Coast zone 32 and 31, R2 and T2 lie within the Central zone 34 and 33, R3 and T3 lie within the Mountain zone 36 and 35, and R4 and T4 lie within the West Coast 5zone 38 and 37. Since Detroit lies in the Central or R2 zone 34, it can be seenthat the only channels over which signals can be transmitted to the West Coast or T4 zone 37 are channels 1, 5, 9 and 13. This is determined in the table of Figure 8 by the intersection of row R2 and column T4. Therefore, from Detroit, the uplink user would uplink on either channel 1, 5, 9 or 13, whiehever10of these channels is closest to the downlink destination. Since Los Angeles is located between channels 14 and 15, the network control eenter would uplink the signal on channel 13 because ehannel 13 is the closest to channel 14. The downlink bearn width is broad enough to provide high gain at Los Angeles.

Conversely, if the uplink site is in Los Angeles and the 15downlink destination is in Detroit, the intersection of row R4 and column T2 in Figure 8 must be consulted. This intersection reveals that the signal can be transmitted on channels 1, 5, 9 or 13 depending upon which channel is elosest to the downlink destination. The netwerk control center would uplink the signal from Los Angeles on channel 9 since channel 9 is closest to channel 11 20which, in turn, is closest to Detroit.

Returning now to Figure 10, the conversion of a receive signal to a transmit signal will be described in connection with the e~ample mentioned above in which the uplink site is in Detroit and the downlink site is in Los Angeles. The uplink signal transmitted from Detroit would be 25transmitted on channel 13 carried by receiYe signal R2. Thus, the R2 receive signal is input to transmission line 122 and a portion of such input signal is diverted by the hybrid eoupler 130 to the input line oî subgroup 108 of filters 102. Subgroup 108 includes a bank of eight filters for the odd ehannels, including channel 13. Thus, the incoming signal is filtered through by filter 3~13 and is output ona line 164 alongwith othersignals fromsubgroups lQ8and 116. The channel 13 signal present on line 164, is combined by the hybrid eoupler 158, with signalsemanating fromsubgroup 106and 114,and formsthe ~1 ~884~3~

T4 signal on output line 150. The transmit signal T4 is then downlinked to Los Angeles.

It is to be understood that the aboYe example is somewhat simplified inasmuch as the network control center would assign a more specific channel than a 27 MHz wide band channel, since the 27 MHz wide channel may actually comprise a multiplicity of smaller channels, for example, 800 subchannels of 32 KHz bandwidth.

Referring now again to Figures 5, 8 and 9, in the event that an uplink signal originates from one o~ the areas of contention, 40, 42, 44 (Figure 5), such signal will not only be transmitted to its desired downlink destination, but a non-~eglible signal will be transmitted to another geographicarea. For example, assume that the uplink signal originates from Chicago, Illinois which is in the area of contention 4~ and that the signal is destined for Los Angeles, California. The area of contention 42 is produced by the overlap of the beams forming zones 34 and 36. Hence, the uplink signal can be transmitted as receive signals R2 or R3. The network control center determines whether the uplink communication is carried by receive signals R2 or R3. In the present example, since Chicago is closer ~o ~one 36, the uplink communication is carried on receive signal R3.

As previously discussed, the downlink destination, Los Angeles, is located in zone 37 and lies between channels 14 and 15. As shown in Fi~ure 8, the intersection of R3 with column T4 yields the possible channels over which the communication can be routed. Thus, the Chicago uplink signal will be transmitted over one of channels 2, 6, lO or 14. Since Los Angeles is closest to channel 14, channel 14 is selected by the network contsol center as the uplink channel. Note, however, that an undesired signal is also transmitted from zone 34 on channel 14. To determine where the undesired signal will be downlinked, the table oî Figure 8 is consulted. The table of FiRure 8 reveals that uplink si~nals carried on channel 14 in the R2 zone 34 aredownlinked to the Tl transmit zone 31. The desired signal is transmitted to Los Angeles and the undesired signal (i.e. an ex~raneous si8nal~ is transmitted to the East Coast (i.e. zone 31). The ne~work control center keeps track of 8~

these extraneous signals when making frequency assignments The effest of these extraneous si~nals is to reduce slightly the capacity of thc system.

Referring now again to Figure 6, the beam-formin~ network 98 receives the transmit signals Tl-T4 and functions to couplc all of the individual communication signals in these transmit signals together so that a transmit antenna beam for each signal is formed. IA thc c~tamplc discussed above in which the assigned frequency spectrum is 500 MHz, a total of approximately 50,000 overlapping antcnna beams are formed by the beam-forming network 98 when the system is fully loaded with narrow band signals.
Each antenna beam is formed in a manner so that it can be poinled in a direction which optimizes the performance of the system. The incremental phase shift between adjacent elements determines the direction of the antenna beam. Since this phase shift is determined by the signal frequency, the system is referred to as frequency addressed.

Attention is now directed to Figures 11 and 12 which depict the details ~f the beam-forming network 98. The beam-forming network, generally indicated by the numeral 98 in Figurc 11, is arranged in the general form of an arc and may be conveniently mounted on the communication shelf (not shown) of the satellitc. The arc shape of the beam-formin~ network 98 facilitates an arrangement which assures that the paths of the signals passing therethrough are of correct length.

The beam-forming network 98 includes a first set of circumferentially e~tending transmission delay lincs 168, 170, a sscond set of transmission delay lines 172, 174 which are radially spaced from delay lines 168 and 170, and a plurality of radially extending waveguide assemblies 176.
In the illustsated embodiment, forty waveguide assemblies 176 are provided, one for each of the elements 106 of the transmit array 21) ~Figure 13~. The waveguide assemblies 176 intersect each of the delay lines 168-1?4 and a}e equally spaced in angle.

~2~848~

Each of the waveguide assemblies 176 defines a radial line summer and intersects each of the delay lines 168-174. As shown in Figure 12, at the points of intersection, between the radial line summers 176 and the transmission delay lines 168-174, a crossguide coupler 180 is provided. The crossguide coupler 180 connects the delay lines 168-174 with the radial linc summers 176. The function of the cross~uide couplers 180 will be discussed later in more detail.

Four delay lines 168-174 are provided rcspectively for the four transmit zones Tl-T4 (Figure 93. Hcnce, transmit signal Tl is pro~ided to the input of delay line 170, T2 is provided to input o~ delay line 168, T3 isprovided to the input of delay line 174, and T4 is provided to the input of delay line 172. As shown in ~igure 12, the distance between the radial line summers is indicated by th_ letter "1~ and the width of each of the radial delay lines is designated by the letter "wn. Although the radial line summers 176 are spaced at equal angular intervals along the delay lines 168-174, the distance between them varies from delay line to delay line due to the fact ~hat the delaylines 168-174 are radially spaced from each other. Thus, the further from the center of the arc, which is formed by the radial line summers 176, the 8reater the distance between the radial line sum ners 176, at the point where they 2U intersect with the delay lines 168-174. In o~her words, the spacing "I" between radial line summers 176 for delay line 168 is less than the spacing between adjacent radial line summers 176 than for delay line 174. Typical values for the dimensions "I" and "w" are as follows:

DelaY Lin~Si~nall. irlchesw, inches 25J 68 T2 1.6~ 0.64 170 Tl 1.72 0.66 172 T4 2.45 0.74 i 74 T3 2.55 0.76 ~ ~8~

The wid~h of the delay lines 168-174, "w", and thc distance "I" between adjacent radial line summers are chosen to providc thc desircd center beam squint and beam scan ratc so that the beam pointing is correct for each channel. This results in the desired start and stop points for each of thc transmit zones Tl-T4.

Refcrring particularly to Figurc 12, the transmit signal T2 propagates down the delay line 168 for a precise distancc, at which point it reaches the first radial line summer 176. A portion of the T2 signal passes through the crossguide coupler 180, which may, for example, bc a 20 dB
iO couplcr, such tha~ one percent of the transmitted power of transmit signal T2is diverted down thc radial linc summer 176. This diverted energy then propagatcs down the waveguide 176 towards a corresponding solid state power amplifier 100 (Figures 6 and 11). This process is repeated for s;gnal Tl which propagates down delay line 170. The portionsof signalsTI andT2 whichare diverted by the crossguide couplers 180 (i.e. 0.01 Tl and 0.01 T23 arc summed together in the radial line summer 176 and the combined signal 0.01 (Tl +T2) propagates radially outwardly toward the next set of delay lines 172, 174.
This same coupling process is repeated for si~nals T3 and T4 in delay lines 174 and 172 rcspectively. That is, 0.01 of si~nals T3 and T4 are coupled via 2G crossguide couplers 180 to the radial line summer 176. The resulting combined signal 0.01 (Tl + T2 ~ T3 + T4) propagates radially outwardly to an associated solid state powcr amplifier 100 where it is amplified in preparation for transmission.

Aftcr encounterin8 the first radial line summer 176, the 2~ remaining 0.99 of signals Tl-T4 propagate to the second radial line summer where an additional one percent of the signals is diverted to the summer 176.
This process of divertin~ one percent of thc signals Tl-T4 is repeated for each of the radial line summcrs 176.

The signals, propagatin~ through the ratial line summcrs 176 towards thc SSPAs 100, are a mi~ture of all four point-to-point transmit signals Tl-T4. Howcver, each of the transmit signals Tl-T4 may comprise 12,500 subsignals. Conscquently, the forty signals p~opagating through the radial line summers 176 may be a mixture of all 50,000 sipnals in the case of the embodiment mentioned above where the assigned frequency spec~rum is 500 MHz wide. Therefore, each of the SSPAs 100 amplifies all 50,000signals which emanate from each of the plurali~y of wave guide assemblies 176.

Since each of the SSPAs 100 amplifics all 50,000 signals which are destined for all regions of the country, it can be appreciatcd that all of the narrow, high gain downlink bearns are formed from a common pool of transmitters, i.e. all of the SSPAs 100. This arrangement may be thought Or as effectively providing a nationwide pool of power since each of the downlink beams coverin~ the entire country is produced using all of thc SSPAs 100.
Consequently, it is possible io divert a portion of this nationwide pool of power to accommodate specific, disadvantaged downlink users on an individual basis without materially rcducing the signal power of the other beams. For example, a downlink user may be ~disadvantaged~ by rain in the 1~ downlink destination which attenuates the signal strength of thc beam. Such a rain disadvanta8ed user may be individually accommodated by increasing the signal strength of the corresponding uplink beam. This is accomplished by diYerting to the disadvantaged downlink beam, a small portion of the power from the pool of nationwide transmitter power ti.e. a fraction of the power supplied by all of the SSPAs 100). The power of an individual uplink beam is proportional to that of the corresponding downlink beam. Consequently, in order to increasc the power of the downlink beam it is merely necessary to increase the power of the uplink beam.

In practice, the previously mentioned nctwork control centcr keeps track of all of shose regions of the country in which it is rainingand determines which of the uplink users arc placing communications to downlink destinations in rain affectcd areas. The network control center thcn instructs each of these uplink uscrs, using packet switched signals, to increase its uplink power for those signals destined for a rain affected area.
Thc increase in power of the uplink user's signals results ir; greater collective amplification of these signals by the SSPAs 100, to produce corresponding downlink bcams to the rain affected areas, which have power levels increased sufficiently to compensatc for rain attenuation. Typically, the number of ~.288~

signals destined for rain affected areas is small relative to the total number of signals being handled by the total pool of SSPAs 100. Accordingly, other downlink users not in the rain affected zones do not suffer substantial signal loss since the small loss that may occur in their signals is spread out over themany thousand users.

The SSPAs 100 (Figures 8 and 11) may be mounted, for example, on the rim of the communication shelf (not shown) of the satellite.
The signals amplified by the SSPAs 100 are fed into the corresponding elements 106 of the transmit array 20 (Figure 13 and 14).

As previously discussed, an incremental phase shift is achieved between the signals that are coupled in the forty radial line summers 176. Hence, the beam-forming network 98 permits the antenna beams emanating from the transmit array 20 (Fi~ures 1, 2, and 13) to be steered by frequency assignment. The incremental phase shift ;s related to the time delay between the waveguides 176 as we!l as frequency. At~ention is now directed to Figure J7 which is ~ diagrammatic view of four of the forty transmit array elements 106 (Figure 133, showing the wavefront emanating therefrom, wherein "d" is equal to the spacing between transmi~ array elements 106. The resulting antenna beam has an angular tilt of ~ , where ~ is defined as the beam scan angle. This means that ~ is the angle from normal of the transmit beam center. The incremental phase shift produced by the delay line arrangement is ~ . The relationship between ~ and is given by 2 ~t 2~ where:
A - signal wavelength ~3 ~ beam scan angle - d e spacing between array elements 4~3~

Hence, the east-west direction of the antenna beam is determined by thc incremental phase shift which is different for the four delay lincs 168-174 of the beam-forming network 98, resulting in the four transmi~ zoncs Tl-T4 previously noted.

Reference is now made to Figures 18 and 19 whercin still another alternate form of the invention illustrated in Figures 1~19 is depicted. In the alternate embodiment, the satellite 200 employs a direct radiating array antenna 202 for both the reception and transmission and is designed for L
band and Ku band service. In Figure 18, the satellite 200 is illustrated in its deployed position. The satellite 200 is body-stabilized, and has solar panels 206 and antenna arrays 202, 204 arran~ed so that they may be stowed, as illustrated in Figure 19, in a compact arrangement during launch.
Specifically, the outer portions of the L band array 202 are hinged so that theymay be swung inwardly to a stowed position for launch. Similarly, the solar panels 20~ which occupy the north and south faces of the body-stabilized bus are hinged so that they may be stowed compactly for launch and the Ku band array 204 is positioned so that it facilitates stowing of the satellite during launch. Referring now also to Figure 20, the L band directly radiating array antenna 202 is a close-packed, two-dimensional array of circularly-polarized cupped dipoles 205. In the illustrated embodiment, the array 202 is 2.4 m by 8.0 m, and has thirty-two columns, wherc each column of dipoles is driven in pairs 207 so that only sixteen drive points are needed. The array 20~ forms both transmit and recci~e ~eams.

Referring to Figure 21 and 22 concurrently, a typical geographic area (greater North America3 serviced by the satellite 202 is illustrated. The satcllitc system is designed to reuse the L band frequency spectrum three timcs, thc frequency band being reused i~a each zone 206, 208, 210. In each zone 206, 208, 210, the low end of the frequency band is directet to the wcst cdge of the zone, and ~he high end is directed to the east edgs of the zone. As illustrated in Figure 22, where the zones 209, 211 meet, the high-frequency end of one zonc coincides with the low frcquency end of the other zone. By using a tapered illumination of the array, suitably low side lobe ~ ~884~30 -3~-levels are generated so that the interference caused by the simultaneous usc of each frequency channel three times is reduced to acceptable le-rels.

In order to achieve reuse of the frequency spectrum three times, a three-zone beam-forming network 212, as shown in Figure 23, is employed. The beam-forming network 212 functions similar to the previously discussed beam-forming network illustrated in Figure 11. Thc beam-forming network 212 also includes transmission delay lines 214 wherein there is one delay line associated with each zone 206, 208, 210, and there arc three transmission lines 216 for each pair of the sixteen drive points in the lU array antenna 202. The outputs of the three ~onal si3nals are added by hybrid couplers 218 to form single outputs corresponding to the ~ntenna drive points.

Reference is now madc to Figure 24, wherein a block dia~ram of the satellite 200 is shown which JS particularly suited for use with mobile ground stations. With mobile servicc, the mobile users and the stationary parties are connccted by base stations.

The satellite 200 and the base stations for the fixed terminal service communicates over assigned frequencies in the Ku-band, whereas the satellite 200 and the mobile service users communicate over assigned frequencies in the L band on both the uplink and downlink. Ku-band signals from base stations are received by the Ku array antenna 204 and receivers 257.
The signal frequency is converted from Ku-band to L-band by down converters 255. The converted L-band signals from the multiple zones, are combined in a three zone beam-forming network 212, amplified by dual power amplifiers 251, divided by a diplexer 259, and transmitted to mobile service users via thc L-band array 202. This is referred to as the "forward communication link~. The "return communication link", frnm the mobile users is transmitted to thc satellite 200 over L-band and reeeived by the L-bandarray 202. Thc L-band signals from different receiYe zones, are amplified by amplificr 263, combined by a beam fosming network 212, and upconverted 253 to the Ku-band frcquency. The signals are transmitted 261 via the Ku-band array 204 to the base stations in various zones.

~ ~8848~) Reference is now made to Figure 25, wherein the power distribution 252 for the array 202 is shown. In order to avoid thc design and production costs associated with systems employing several multiple Icvel power amplifiers, the unequ~l power Icvels, required by the array elemcnts 205, are generated by hybrid-coupled dual amplifiers 251, In this arrangement, thcre are eight hybrid-coupled dual amplificrs wherein all of the amplifiers operate at the samc power level despite thc uncqual power levels at their inputs and outputs. By the use of hybrid-coupled dual amplifiers 251, the intermodulation products are dispersed geographically, thus reducing their level at the user's locations. In addition, by using power amplifiers 251, in which each amplifier contributes to thc amplification of all of the signals, maximum flexibility and powcr assi~nment is achicvcd.

Referring to Figure 26, another alternate embodiment of the invention is illustrated wherein the satellite system provides for a private ~Cu-16 band network system 250 utilizing four dircct-radiating arrays: two receive arrays 235, 236 and two transmit arrays 237, 238. ~here are two arrays for both the transmit and receive because both the broadcast and point-to-point services are accommodated on a single polarization. One receive array 235 is dedicated for horizontal polarization while the second receive array 23~ is dedicated to vertical polarization. Similarly, one transmit array 237 is dedicated for horizontal polarization and the second transmit array 238 is dedicated for the vertical polarization. Both services are made available on one polarization by allocating one half of the frequency spectrum to broadcast service and one half of the spectrum to point-to-point servicc. By providing both services on one polarization, the aperture of the ground terminals may be small, thus r~ducing the cost of the ~round stations. Two parabolic reflectofs 244 are used to provide shaped beam coverage over the geographic region scrviced by the satellite. The Ku-band domestic system, like the L band mobile system, is able ~o overcome rain attenuation on the downlink with only 3~ slight penalty tg the si~nals in rain-free aréas. Since only a small portion of the country experiences rain at one time, large power increases to those disadvantaged areas causes only a slight decreasc in power a~ailablc elsewhere.
As with thc L band mobile system, the solar panels 24B are extendable and are stowed during launch.

~ ~8~30 The illustrated Ku-band satellite system permits frequcncy reuse in the priva~e network system by utilizing very small aperturc earth terminals on user premises. The private network may bc either a ~star~
network, which connects the earth terminals to a central data basc and provides broadcast video capacity, or a "mesh" network, which connects the earth terminals by circuit-switched intcrconnections to providc services such as teleconferencing, high-speed graphics, and voice communication.

Attention is now directcd to Figure 27 which illustrates contiguous receive zones 220, 222, 224, 226 for a typical dornestic Ku-band private network satellite system. In this system, pairs of users are directly connected without thc use of intermediary base stations, thus precluding the use of a frequency addressable beam on both thc uplink and downlink.
- Consequently, a set of frequency independent beams is used on the uplink, so that frequency may be used to address the downlink destination.

There are four receive zones 220, 222, 224,226 servicing the area because the frequency spcctrum is reused on the uplink four times, the entire frequency spectrum bein8 used in each of the zones 220, 222, 224, 226.
In order for each uplink zone to be contiguous and non-interfering, each zone is further subdivided into two half zones, illustrated in Figure 27, 220a and 2U 220b, 222a and 222b, 224a and 224b, and 226a and 226b. The receive beam patterns corresponding to zones 220a, 220b, 222a, 222b, 224a, 224b, 226a, and 226b are illustrated in Figure 28. Figure 28 shows the cen~er frequency for each receive beam. The beam patterns 221, 223, 225, 227,229, 231,233 may be generated, for example by an array 80 wavelengths wide driven at 16 drive points tkrough a Butler matrix having 16 antenna terminals and providing 8 bearn outputs. The beam patterns 221, 223, 225, 227, 229, 231, 233, 235 have suffi ient spatial isolation eo permit frequency reuse among the beams separated by at least one beam but not among adjacent beams. The satellite u es four two-position time switches so that the "a" and "b" beams arc used alternately; this is effectively a type of time division arrangement. The switches, operating at the low speed of, for e~ample, 5û Hz, provide four receive zone outputs to the four receiYers that amplify and convert thc signals ~Z~84~3V

frorn 14 to 12 GHz. Usin~ a 50 Hz switch and creating a 20 msec period means that during one time period while the "a" frequencies are simultaneously used in the four zones, the ~b" set of frequencics are not "on" and thus are not u5cd.
Hence, ~he "a" and ~b" frequencies will not interfere with one anothcr, Sincc the duty cycle is one half, the powcr requiremcnt is doublcd at each tcrmina1.

Referring also to Fi~ures 29 and 30, the transmit zones and the transmit beam patterns are respectively shown. Thc transmit zones 228, 230, 232, 234 illustrated in Figure 29 are esscntially thc same as thc rcccivc zones 220, 222, 224, 226 illustrated in Fi~ure 27. The transmit zonc 228 corresponds to the receiYe zone 220, transmit zonc 230 corresponds to receivc zone 222, transmit zone 232 corresponds to the receive zonc 224, and transmit zone 234 corresponds to receive zone 226. As with the uplink, the downlink spectrum is also reused four times, the entire spectrum being reused in each of the transmit zones 228, 230, 232, 234. As shown in Figure 30, each of thc transmit beams 237,239, 241,243 respectiveJy correspond to the transmit zones 228, 230, 232, 234.

Referring to Figure 31, a filter interconnect mat}i~ 252 is shown which includes an assembly of thirty-six filters that provides the connections between the receive zones 220a, 220b,222a, 222b, 224a, 224b, 226a, 226b (Rl A, Rl B, R2 A, R2 B, R3 A, R3 B, R4 A, R4 B) and transmit ~ones 228, 230, 232, 234 (Tl, T2, T3, T4). The filter interconnection matri~ 252 is similar to the previously discussed filter matrix 90. The rows of the matrix are driven by the receiver outputs, and the preselected groups of output from nlne filters are directed to the four transmit zones Tl, T2, T3, T4.

2;~ Referring to Figure 32, a block diagram of the alternate embodiment of the present invention is illustrated. Signals received by the receive array antenna 236 cnter a beam forming network 265, similar to the previously discussed BFN 98 and 212 to combine the received signals. The combined signals are routed to receivers 269 by time division switches 267 where the signals are preselectively filtcrcd by a filter interconncction matri~252 which performs in substantially the same manner as the previously discussed FIM 90. The filtered signals to bc transmitted are also combined in a beam forming network 26~ after which the signals are amplified at 273 and transmitted by the directly radiating transmit array antenna 238.

Having thus described the invention, it is recognized that those skilled in the art may make various modifications or additions to the S preferred embodiment chosen to illustrate the invention without departing from the spirit and scope of the present contribution to the art. Accordingly, it is to be understood that the protection sought and to be afforded hcreby should be deemed to extend to the subject matter claimed and all equivalents thereof fairly within the scope of the invention.

Claims (16)

1. A method of communicatively interconnecting any of a plurality of terminals within an area on the earth using an earth-orbiting communications satellite, comprising the steps of:
(A) forming a first plurality of groups of radio frequency beams respectively between each of a plurality of zones covering said area on the earth and said satellite, the beams in groups thereof carrying communications signals over the same first preselected range of frequencies, such that said first preselected range of frequencies is reused by the beams forall of said zones;
(B) separating each group of said radio frequency beams into first and second sets thereof respectively carrying communications signals over first and second sets of frequencies within said first preselected range of frequencies;
(C) alternately transmitting the first and second sets of said beams between said zones and said satellite.

2. The method of claim 1, wherein step (B) is performed by spatially distributing the first preselected, range of said frequencies over each zone such that contiguous sections of the area serviced by the beams in adjacent zones are respectively covered by different sets of said beams to avoid communication interference in the areas serviced by the beams in said adjacent zones.

3. The method of claim 2, wherein step (C) is performed by simultaneously transmitting the beams in the first set thereof for all the zonesand then simultaneously transmitting the beams in the second set thereof for all the zones.

4. The method of claim n 1, wherein step (C) is performed by transmitting the first and second sets of beams from each of said zones to said satellite.

5. The method of claim 4, including the steps of:
(D) receiving said first and second sets of beams at said satellite; and (E) transmitting a second plurality of groups of radio frequency beams respectively from said satellite to said zones, the beams in each group of said second plurality thereof carrying communications signals over the same second preselected range of frequencies, whereby said second preselected range of frequencies is reused by all of the groups of beams in saidsecond plurality thereof.

6. The method of claim 5, wherein step (E) is performed by spatially distributing the beams in each of said second plurality of groups thereof across the corresponding zone as a function of said second preselected range of frequencies.

7. A method of communicatively interconnecting a plurality of terminals within an area on the earth using an earth orbiting communications satellite, comprising the steps of:
(A) dividing said area into a plurality of essentially contiguous zones;
(B) dividing each zone into at least first and second sections;
(C) alternately transmitting at least first and second radio frequency beams respectively between said first and second sections for each of said zones and said satellite, said first and second beams respectively carrying communications signals over first and second different sets of frequencies defining a first preselected range of frequencies, wherein said first preselected range of frequencies is reused by the first and second beams for all of said zones.

8. The method of claim 7, including the step of forming said first and second beams for each of said zones such that said first and second beam for each of said zones collectively define a rectangularly shaped area of communications service on the earth which corresponds to the associated zone.

9. The method of claim 8, wherein the rectangularly shaped areas of communications service are essentially contiguous.

10. The method of claim 7, wherein at least two of the sections of respective contiguous zones are contiguous to each other, and the sets of frequencies of said contiguous two sections are different from each other, whereby to avoid communications interference between said contiguous two sections.

11. The method of claim 7, including the steps of:
(D)) receiving said first and second beams at said satellite; and (E) transmitting to each of said zones a plurality of downlink radio beams, each plurality of said downlink beams carrying communications signals over the same, second preselected range of frequencies, whereby said second preselected range of frequencies is reused by the downlink beams for all of said zones.

12. The method of claim 11, wherein step (E) includes the step of spatially disbursing the downlink beams in each plurality thereof as a function of frequency.

13. A satellite communications system for communicatively interconnecting a plurality of earth terminals covering an area on the earth, comprising:
a satellite disposed in orbit above the earth;
first means for forming a plurality of uplink radio frequency beams between said satellite and each of a plurality of respectively associated zones covering said area, said zones being divided into at least two sections, each plurality of said uplink beams carrying communications signals destined to be received by downlink terminal sites in said zones, the uplink beams for each zone being arranged in first and second sets of said beams respectively originating from said two sections in the zone and carrying communication signals over first and second sets of frequencies,said first and second set of frequencies defining a first range of frequencies, the uplink beams for all of said zones using said first range of frequencies;
second means at said satellite for alternately receiving said first and second sets of beams; and third means at said satellite coupled with said second means for forming a plurality of groups of downlink radio frequency beams between said satellite and said zones, each of said groups of downlink beams carrying communication signals destined to be received at downlink terminal sites in one of said zones.

14. The system of claim 13, wherein each of said groups of downlink beams covers one of said zones with each group carrying a plurality of signals over a second range of frequencies such that the same range of frequencies are used by all the groups of downlink beams.

15. The system of claim 13, wherein said second means includes a receiving antenna array, a receiver and a two position switch for successively switching said receiver to alternately receive said first and second sets of frequencies respectively.

16. The system of claim 15, wherein said third means includes a transmitting antenna array for transmitting said downlink beams, said receiving antenna array and said transmitting antenna array each being defined by a two dimensional array of direct radiating elements.