Multi Beam Concept

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630 IEEEOURNAL ON SELECTEDREASNOMMUNICATIONS, VOL. S A C J , N O . 4, MAY 1987

An Adaptive Multiple Beam SystemConceptAbstract-An adaptiv e multip le beam system, which bas flexibilityto adapt beam power to varying beamraffic, is proposed. The multiplebeam transmitter, which is called hybrid transponder in this paper,

consists of a pair of multiport hybrids and a setf amplifying elements.Power combining, isolation, and intermodulation characteristics of thismultiport network are analyzed. Also, a new interbeam exchange andfeeder link connection concept, which serves as the basis for an adap-tive multiple beam system, is proposed. Finally, characteristics of anexperimental 8-amplifier 8-port hybrid transponder are described.

I I. INTRODUCTIONN satellite communications, demand assigned multipleaccess intended for a large number of customer termi-nals enables an efficient use of a small number of satellitechannels. This ype of service, based on a multicarriersystem using inexpensive ow power terminals, s ex-pected to find extensive use in many areas of communi-cations.

Recently, in the fixed and mobile satellite communi-cations, multiple beam systems have been under consid-eration as advanced satellite communication systems.Multiple beam systems can increaseransmission capacitywith an increase of satellite antenna gain and reuse of al-located frequency band. However, implementation of themultiple beam systems, especially in contiguous domesticcommunications, yields problems for an appropriate sys-tem design. In the case of single beam systems, all thetransponders are connected to a single beam, and everyEarth station has access to all the transponders.. There-fore, traffic variations in a local area do not yield anyproblem, provided they .do not exceed the total transmis-sion capacity. Furthermore, single beam systems inher-ently have high reliability because failure of some tran-sponders does not result in total system breakdown. Onthe other hand, n the case of multiple beam systems, thearea subtended by each beam is a part of the total servicearea, and the transmission capacity of each beam is alsoa part of the total transmission capacity. Thus, the fixedallocation of transponder capacity to each beam degrades.the flexibility of the. system which has varying traffic dis-tribution. Also, it will become difficult to provide a beamto a sparsely populated area with minimal traffic becausesuch a beam requires a transponder with transmittingpower corresponding to the estimated maximum traffic.

Manuscript received August 2, 1985; revised October 21, 1986.S. Egami is with Kamakura Works, Mitsubishi Electric Corporation,M. Kawai is with the Electrical Communication Laboratories, NipponIEEE Log Number 8613620.

Kamimachiya, Kamakura-shi, 247, Japan.Telegraph and Telephone Corporation, Yokosuka-shi, 238, Japan.

Furthermore, in order to assure all beam operations, eachbeam must be highly reliable.In order to cope with these problems, we proposed the

use of an active array multiple beam antenna [l] Also,Kawai proposed a new type of beam switching networkwhich consists of hybrids, l-bit phase shifters, and am-plifiers [2], [3].

In this paper, these concepts are extended yielding anew multiple beam transmitter which consists of multi-port hybrids and a set of amplifiers. A similar conceptusing a Butler matrix was proposed by Sandrin as a Butlermatrix transponder [4]. The system described in this pa-per consists of multiple hybrids and multiple amplifiers,which, inherently realize a wide-band transmission andeasy, mplementation. Multiple fixed phase shifters, whichare necessary in the Butler matrix transponder, are notnecessary in the proposed configuration.

11. THE HYBRIDTRANSPONDERONCEPTA . The Multiport Hybrid Concept

It is well known that a 90 hybrid divides input powerequally into twoports in precise phase conditions.A well-designed hybrid maintains these characteristics under abroad bandwidth. Fig. 1shows the 90 hybrid and hemultiport hybrids derived from it. Input and output sig-nals are represented by complex amplitude variables piand q i respectively. Referring to Fig. l(a ), the input andoutput relation of this hybrid is represented by matrix T I .

As an extension of the 2-port hybrid, a 4-port hybridwhich divides incident power equally into 4 output portscan be derived as shown in Fig. l(b). In order to simplifymatrix representations, the output port numbers are pairedto the input port by the path which does not accompany90 phase shift. Using this notation, the 4-port hybrid isrepresented by matrix T2.(>4 = T2 )

P4

0733-8716/87/0500-0630 01 OO O1987 IEEE

Authorized licensed use limited to: Alcatel Space Industries. Downloaded on January 23, 2009 at 04:43 from IEEE Xplore. Restrictions apply.


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63 1G A M I A N D KAWAI: ADAPTIVE MULTIPLE B E AM SYSTEM CONCEPT

91959 39792

998

6

P1p2p 3 9

q19 9

p4 9

4 1p7 9 7P8 15P9plo 92q l op11P12 96

9

a

14

?21516 8q l 6

Fig. 1 . Configurations of the rnultiport hybrid.

As an extension of the 4-port hybrid, an port hybridcan be derived as shown in Fig . l(c). As stated above,the output ports numbers are paired to the input port bythe path without 90 phase shift. Then, the port hybridis represented by matrix T3.

Fig. l(d) shows a l6-port hybrid derived from the 8 -

from the above procedure [ 2 ] :n - 1

t;,k = (I/@) eXp j T / 2 ih @ kh) 5 )h = Owhere @ indicates exclusive OR a @ b = a * b + a *b) . ih and kh =O or 1 are derived from the followingbinary representation of and k :

- -

= io + i121 + i22 + * * + i n 4 2 n - lk =, kO + k12I + k22 + * + k n - 1 2 n - . (6)

B. The Hybrid TransponderConceptUsing a pair of N =2)-port hybrid and N amplifying

elements, an N-port power combining network can be de-rived. ,We call this multiport power combining networkhybrid transponder after Butler matrix transponderproposed by Sandrin [4].

The most simple 2-port transponder is the well-knownbalanced amplifier [5 ]shown in Fig. 2(a). If the complexamplitude gain of the amplifying elements (hereafter ab-breviated as amps) is represented by a , the relationshipbetween the input pi nd output ri is represented usingmatrix T l as follows (suffix of the input p i and output riare numbered from the top. However, in Fig. 2and here-after, he corresponding input and output ports are de-noted by the same port number).

Input p 1 s amplified by two amps and comes out as r2 .As an extension of the balanced amplifier, a 4-port tran-sponder can be derived as shown in Fig. 2(b). In this case,outputs rl - r4 are represented by matrix T2 and inputsP1 - P4 . (::) = aT2T2 7)

P4

port hybrid. In general, an N(=2)-port hybrid consists Similarly, an 8-port transponder can be derived fromof n 2 hybrids, and its matrix T , can be represented by the 4-pOrt transponder as shown in Fig. 2(c). Outputs rlTn- as follows: - r8 are represented by matrix T3 and inputs p 1 - p8.In order to facilitate further analysis, elements of the

matrix T, should be explicitly represented. ForheN =2)-port hybrid, element t i k f the matrix T, can bederived from the following equation which is induced



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632 IEEE JOURNAL ON SELECTEDAREAS NCOMM UNICATIONS, VOL. SAC-5, NO. 4, MAY 1987

42 334 21

(b)

12 873 64 556 4378 21

c )

Fig. 2 . Configurations of the hybrid transponder.

Fig. 2(d) shows a.16-port transponder as an extentiono f the 8-port transponder. For general expression, if ampsgain matrix for the N =2)-port transponder are definedby An,

A,, = ;I 1 O )aN

then output vector r is related to the input vector p by thefollowing equation:

r = T,A,T,p. 1 1 )In practice, when the number of beams is less than the

number of ports, the unused input and output ports areterminated with matching impedances. It is also possibleto decrease the number of ports by deleting unnecessaryhybrjds as shown later in Fig. 5 .111. CHARACTERISTICSF THE HYBRID TRANSPONDERA. Effect of Amplifying Element Nonuniformity

If the phase shift and gain of the N =2 ) amps are thesame; the power combining efficiency and isolation aredetermined by the characteristics of the multiport hybrid.Since the 90 hybrid has excellent coupling, isolation, andphase-shift characteristics over a broad band, amps gainand phase-shift uniformity become the most important.Thus, in this section, effects of the amps gain and phase-shift nonuniformity are analyzed.

If the amps gain and phase shift are denoted by ai andB i respectively, they can be represented by their meanvalues ao,Bo and their deviations A i , di as follows:

where NC A i = Oi = l

N6i = 0i = l

If the sum of each amps output power is represented byP o , the combined output by Po,,, and the output at theisolated port by Piso, hen the combining efficiency is ob-tained from (5) and (11) as follows:

P o u , / P o= 1 - N - l ) / N ) A z+ 6) 1 5 )where A and 6 are mean value of gain and phase-shiftdeviation, respectively, which are defined by the follow-ing equations :

\ I/

6 = (+, 6 ? / N ) .Similarly, the isolation at the isolated port is obtained

as follows:piso/p0 I / N ) A + 6. 1 7 )

Fig. 3 shows the combining efficiency and the isolationas functions of mean deviations. Since it is not difficult toprescribe mean gain deviations within 1 dB and meanphase-shift deviations within 10 for 20 dB gain amps,the combining efficiency lower than 0 . 3 dB and isolationover 20 dB can be expected.B. Effects of the Amplifying Elem ent Failu re

Failure of one or several amps decreases the combinedoutput power and degrades isolation. In this section, theeffect of one amp failures analyzed. Failure of more thantwo amps is not included since its effect is not uniform onall output ports. If the combined output is P o , and oneamp failure decreases it to Po,,, then the rate of powerdecrease is obtained by using 5)and 1 1 as follows:

POt/PO = N - l ) / N ) Z . 17)

PisoIPo = 1 / N Y 1 8 )If the output power of the isolated port is denoted byP i s o , hen the degraded isolation is given as follows:

For example, in the case of the 8 amps transponder, thefailure of 1 amp decreases output power by 1.2 dB anddegrades the isolation to 18.1 dB. In case of 16 amps tran-sponder, the failure of 1 amp decreases output power by



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EGAMI AND KAWAI: ADAPTIVE MULTIPLEBEAMSYSTEMCONCEPT 633

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p -0

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Kin- 4 0

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0 2 6 8 1 024 1 6Mean Phase Variation Deq.)0 . 5 1 1 . 5 2Mean Gain Variation dB)

Fig.3 . Decrease of output power and degradation of isolation as functionsof mean phase-shift deviation and mean gain de viation.Low PowerTransponder ~ . . ~ - o . ~ t . . . B ~ o c k . ~ : : : : ~ : : ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ . . . . . . . ~ . ~ . . .2 Port Block Multibeamr .__ .__. . Antenna

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Band H y b r idT ra n sp o n d e rFig. 4. The 8-amps 8-port hybrid transponder implemented to an 8-beam system .

0.8 dB and degrades the isolation to 24 dB. Thus, a largernumber of amps minimizes the effect of 1 amp failure. Ifthe isolation over 20 dB is necessary, more than 16 ampswill be necessary.C. IntermodulationCharacterist ics

If more than one carrier is applied to only one port, theintermodulation characteristics of the hybrid transponderare similar to those of the single port amplifier. However,if one carrier is applied from one port, nd another carrieris applied from another port, the theoretical estimate ofthe intermodulation output ports will be necessary. In thecase of the third-order intermodulation products, it can beverified from (5) and 9) that the following simple ruleexists.

If the carrier frequency applied from port n and port rnis denoted as fn and f respectively, the intermodulationproduct 2f - n appears at port n with the output of fnand 2fn - f appears at port m with the output of f .Therefore, intermodulation product dispersion cannot beexpected as the Butler matrix transponder [4]. However,these characteristics on the intermodulation products mayease the actual system design.

IV. IMPLEMENTATIONN MULTIPLE EAM YSTEMSFig. 4hows an 8-beam system using the 8-amps 8-port

hybrid transponder. The configuration of the 8-port tran-sponder shown in Fig. 2(c) i s rearranged so as to become

symmetrical. By making the configuration symmetrical,the hybrid transponder can be made by the assembly of a2-port and a -port lock as shown by the dotted line inthe Fig. 4. Thiswill facilitate hardware materialization asan onboard transmitter.As shown in Fig. 4, the low power transponder (LPT)provides input to the hybrid transponder. If the total trans-mitting power is 100 W , and the amps gain is 20 dB, thenecessary transmitting power of LPT is 1 W in order totransmit whole power to the corresponding beam. Thislower transmitting power of the LPTs, which are allo-cated for each beam,enables flexible assignment of beampower and adding beams to sparsely populated areas.

Also, as shown in the figure, it is assumed that all ele-ments and LPTs share a common broad bandwidth. RFchannel assignment will be carried out considering inter-beam interference. The allocation of a common broadband to each beam enables variations of the number ofcarriers from 0 to the maximum value limited by the totaltransmitting power. The flexibility or adaptability willliquidate the stated problems in the ordinary multibeamsystems.

Fig. 5shows a 16-amps 10-port transponder. Unnec-essary ports and hybrids are eliminated from the original16-port configuration. Although 1 amp failure among 16amps decreases 12 percent of the total transmission ca-pacity, it does not bring about any specific beam failure.At the s.ame ime, isolation degrades to 24 dB, which stillallows cochannel frequency reuse. If the number of amps



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634 IEEE JOURNAL ON SELECTEDAREAS IN COMMUNICATIONS, VOL. SAC-5, NO. 4 MAY 1987Low PowerTransponder 4,..~.o.rt...Bl~ck...~..2........._... ::::Er: ?:k

H y b r i d Transponder

Fig. 5 . The 16-amps 10-port hybrid transponder mp lemented to a 10-beam systemMultibeamAntenna Common Channel Unit MultibeamAntenna

Band, fu

Receiving 1st IFransmittingstF

M I X ; MixerSYN; Frequency SynthesizerRx ; Receiving Down ConverterT K ; Transmitting Up Converter

f l f 2 f 3 . f 4 f 5 f 6 f 78

n 8n 7FdFd

nE x

FdFd

Band, Fdlommon D/L

Feeder Link, To Terrestrial NetworkFig. 6 . Adaptive multibeam system concept using the hybrid transponder.

is small and high isolation is required, providing sparesto each elements will become necessaly.

V . INTERBEAMXCHANGE USING COMMON CHANNELUNITSBy employing the hybrid transponder, the number of

carriers transmitted from each beam can be changed from0 to the maximum value limited by the total transmittingpower. In order to facilitate an adaptive carrier assign-ment, a new onboard system configuration is proposed.Fig. 6 shows the proposed adaptive multibeam systemconfiguration. All beams share the same frequency bandwhich corresponds to the allocated bandwidth. The samefrequency band f transmitted from each beam is con-verted by receivers RXI - RX o different contiguous 1stIF bands5- s which are then combined by the resistivecombining network. If all the traffic is connected to the

existing terrestrial network by a feeder link, these 1st IFsignals are directly sent to the feeder link Earth station.If interbeam traffic exists, channel units are connected tothe combined 1st IF signals. A channel unit selects a car-rier from the 1st IF bands - 8, and changes its re-quency to the appropriate frequency slot of the transmit-ting 1st IF bands F1- F8. f it must be connected to beam1, the channel unit changes its frequency to any frequencyslot of F , The transmitting 1st IF bands F1- F8 are thenconverted to the same transmitting frequency band Fd bythe transmitting converter 7 X 1 - 7 X 8 . If all the traffic i soriginated from the feeder link, the transmitting 1st IFsignals are directly supplied from the feeder link. Sincethe channel units are used in common, he number ofchannel units can be conspicuously decreased comparedto the former SS-FDMA system concept which provideschannel units to each beam [ 6 ] ,



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EGAMI ANDKAWAI: ADAPTIVE MULTIPLE BEAM SYSTEM CONCEPT 635

Fig. 7 . Outside view of the 2.5 GHz 8-amps 8-port experimental hybridtransponder.36 i I I / 4 5

I I I I 1 I I L O S SCombiningEm 3 0Uaaa

4J 2 0-0 10 20

Unit amp input (dBm)Fig. 8. A typical amp output and the combined output as functionsf ampsinput power. In the figure, gain and phase-shift deviation o f 8-amps arealso shown.

VI:8-AMPS8-PORT HYBRID TRANSPONDEREXPERIMENT

The 8-amps 8-port hybrid transponder is experimentedat 2.5 GHz band to confirm the stated electrical charac-teristics. The amps have 20 dB small signal gain and 1.5W output power. Branch line-type hybrids made by an airsuspended triplate were used.Fig. 7 shows an outsideview of the experimental transponder.A . CombiningEficiency

Fig. 8 shows a typical unit amp output and the com-bined output as functions of amps input power. Since itsscale is shifted 9 dB for combined output, the differencebetween the two curves represents combining loss. In thesame figure, the gain and phase-shift deviations of 8 ampsare shown. For an input level of 14 dBm, the output levelis distributed from 31.9 to 32.5 dBm (1.5-1.8 W ) , andthe phase shift within * . This level of uniformity canbe attained by a slight adjustment on the devices. In thisoperating condition, combined output power was 40.6dBm 11.5 W). Since the sum of each amps output powerwas 13.7 W , the combining .loss was 0.8 dB. Since thisvalue includes the 8-port hybrid loss of 0.5 dB, the netcombining loss can be estimated to be around 0.3 dB.B. Isolation

The isolation at the isolated port at the stated operatingcondition is shown in Fig. 9.Excluding port 3, the iso-

8- por t - H ybr i d? .....tandem : -.

m

1 2 4 5 6 7 6P o r tFig. 9. Isolation characteristics of the experimental 8-amps 8-port hybridtransponder.

40- 20Ea

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- 2 0m3 40uu 20

0- 20-4172.39 10 MHz/div. 2.49

H zH z,g. 10. Output power spectrumof output ports 1 and 4, when equal powtwo carriers are applied from input ports 1 and 4 simultaneously.Fi

lation was well over 30 dB. As a reference, the isolationwithout amps (tandem connection of port hybrid) isshown by the dotted line. Also, in ordero simulate singleelement failure, one of the elements is turned off. Thisdecreases isolation to 17 dB,which is estimated to be 18.1dB theoretically.

C . IntermodulationTwo equal power carriers with slightly different fre-

quencies f l and f are applied to ports 1 and 4, respec-tively. The input level of each carrier is about 6 dBm atthe input of the. amps. Fig. 10 shows the output powerspectrum at the corresponding output port. At output port1 , carrierf, and intermodulation product 2f4 - l appear.At output port 4, carrier f and 2f, - appear. In otherports, 3rd-order intermodulation products do not appear.These experimental results verify the estimation stated inSection 111-C.



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636 IEEEJOURNAL ON SELECTEDAREAS NCOMMUNICATIONS.VOL.SAC-5, NO. 4, MAY 1987VII. CONCLUSION

An adaptive multiple beam system using a new hybridtransponder concept is described. By the experiments,carried out at 2.5 GHz by 8-amps 8-port configuration,theoretical characteristics are confirmed. These conceptswill facilitate implementation of multibeam multicarriersatellite communications in future mobile or customerpremises services.

REFERENCES[ l ] S.Egami and M. Kawai, Adaptive multibeam satellite communica-tions using active array, in Proc.14thInt. Symp. Space Technol.S c i . , Tokyo, Japan, 1984, pp. 793-798.[2] M. Kawai, Multiport coupling beam switching network for satelliteuse, Trans. IECEJapan, vol. J66-D, no. 3 , pp. 329-336, Mar. 1983in Japanese).[3] M. Kawai, K. Suzuki, and S. Egami, Concept of multiport couplingbeam switching network and its basic performance in K-band, Elec-tron. Lett . , vol. 19, no. 23, pp. .990-991, Nov. 10, 1983.[4] W . A. Sandrin, The Butler matrix transponder, COMSAT Tech.[5] R. S.Engelbrecht and K. Kurokawa, A wide-band low noise L-bandbalanced transister amplifier, Proc. IEEE, vol. 53, pp. 237-247, Mar.1965.[6] J . D. Kiesling, Direct access satellite communications using SS-FDMA, in AIAA 9th Commun. Satellite Syst. Conf., 1982, pp. 627-63 3.

R e v . , vol. 4 , no. 2 , pp. 319-345, Fall 1974.

Makoto Kawai 77) received the B.S . andM.S. degrees from Kyoto University, Kyoto, Ja-pan, in 1972 and 1974, respectively.He joined the Electrical Communications Lab-oratories, Nippon Telegraph and Telephone Pub-lic Corporation, Kanagawa, Japan, in 1974, andi s presently a Senior Research Engineer of the Sat-ellite Communications Department, Radio Com-munication Networks Laboratories. He is cur-rently involved in systems engineering studiesrelated to future satellite communications sys-tems.

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Multi Beam Concept