I r~~AD—AO31 519 ARMY ELECTRONICS COMMAND FORT MONMOUTH N .J F/S 17/2.1V TECHNIQUES FOR ADAPTIVE CONTROL OF DIFFRACTION MODE AND TROPOSC——ETC(tJ)

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c~ Resea rch and Development Technical Report

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kt~ ~TECHNIQUES FORA DA PTIVE CONTROL OFD IFFRACT ION MODEAND TROPOSCATTER ANTENNA S

George E. KrausePaul C. NgCommun icat ions /Automatic Data Processing Labo ratory

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1 Septembe r 1976

~lST RI BiJ T lON STA rF ME NT

Appr oveil for public release:di str ib ut i on un li ~n t ~l (—~rj~çç~ fl

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• ECOMUS A R M Y E L E C T R O N I C S C O M M A N D F O R T M O N M O U T H , N E W J E R S E Y 0 7 7 0 3

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N O T I C E S

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Disclaimers

The rind ings in this report are not to be construed as anofficial Department of the Army position , unless so desig-neted by other authorized documents.

The citation of trade names and names of manufacturer s inthis report is not to be construed as official Governmentindorsement or approval of commercial products or servicesreferenced herein.

• Disposition 4Deetroy this report when it is no longer needed. Do notreturn it to the originator.

/ i . • •U TIl Wh lts SectlisSiC hit kcflis ~~o

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4. TITLE (and SubtitI.) _________ ~f’ ~~LJYPE OF ~ EPilRI_A_VJ~mI

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Techniques for Adaptive Control of K..~ / Interim\~~~~~ Diffraction Mode and Troposcatter Antennas • I

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AREA & WORK UNIT NUMBERSDR L-NL-~~1-3• Comsrunications/ADP Laboratory ~/ lX7637om2t~~17o2Fort Monmouth, N.J. 07703

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II. CONTROLLING OFFICE NAME AND ADDRESS T 12. REPORT DATE

US Army Electronics Commar. d (// ‘ Sept~~~s~ 1976 1 _____

Fort Monmouth, N.J. 07703 ~~~~ ‘3 P IU M B E R O F FA G E S4,

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Approved for public release; distribution unlimited.

17. DISTRIBUTION STATEMENT (of ha abalract .ot.rkd In Block 20. If dlff.rant from R.porf) _________

IS. SUPPLEMENTARY NOTES

Initial Report

19. KEY WORD S (Continua on r,r.raa aid. if nac•aaa.y and Id.ntify by block number)

Adaptive Antenna Control; Beam SteeringAngle Diversity; Antenna Beam Steering

- Troposcatter Communications

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20 ABSTRACT (ConHnu. on ,.ra r.. .1* Sf n.ce..av and Id.nttfr by block nt b.r)

— l’.Be~i-ond the horizon radio communications via troposcatter or diffraction modes•

• are frequently troubled by bending of the RF rays due to ch~iges in theatmospheric index of refraction. This can cause the received angle of

• i arrival to appear outside the antenna ’s main lobe. A possible solution is ± (

~• adapt the antenna’s response to these changes. This report discusses possible

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I state—of-the—art techniques, tradeoffs, potential problems and cost aspects• for developing an adaptive antenna control system for use on e dsting tropo —

scatter/diffraction mode ant enna dishes. ~~DO ~~~~~~~

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TABLE OF CONTENTS • -

• PAGE

t 1. INTRODUCTION 1

• 2 . ADVANTAGES OF AN AUTOMATIC ANTENNA CONTROL SYS TEM 2

A. Technological Standpoint 2

B. Economic Standpoint 2

3. FEASIBILI TY OF AN AUTOMATIC ANTENNA CONTROL SYST~~ 3

t~. DESCRIPTION OF TECHNIQUES 3 ..

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A. Beam Steering Methods 3

• ( i ) Electronic Beam Steeri ng 3

(2) Mechanical Beam Steering

B. Angle Diversity Method

C. System Modes SH

(i) Open—loop Control 5 •

(2) Closed—lo op Control 5

D. Syst~ n Conf igurati ons S(1) Steerable Single Beam S(2) Fixed Multiple Beams S(3) Steerable Multiple Beams 6

5. COMPARISON OF TECHNIQUES AND TRADEOFFS 6

A. Electronic vs. Mechanical 6 1B. Open-loop vs. Closed-loop Control 7

C. Steerable Single Beam vs. Fixed Multiple Beam s Ivs. Steerable Multiple Beams 7 • 1

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COST CONSID~~ AflON S P

• A. Electronic vs. Mechanical 8

3. ~~~~~~— 1co:~ vs . C1os ed— ~ :OP &

C. ~~~~~ O:nfi rati~n Concarisor~ 9

17. TE~:TI~~ ?~ 93L~~I~

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~~ . L~~~~Y .~~~~ CONCLUSIONS 10

~~. 3~cO~~~~ ATIcN S

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1. INT~~DUCT ION

Radio frequency signals propagated beyond the horizon nay be diffractedby obstacles , or scattered by the troposphere. In the case of diff ractiontransniission, variation in the atmospheric index of refraction causes theapparent antenna beam to bend up or down resultin~r in reduced receivedsignals and fadeouts. For tropospheric scatter transmission, variationsin the atmospheric index of refraction give rise to variations in theangle of arrival of the signal at the receiving antenna which~re~ ilts insignal fading. This reduces the average received power and createsoutages, det eriorating system reliability and performance.

Requirements in the Future Digital Network (FDN ) call for improvementof the Defense Communications System (DCS) troposcatter and diffractionlinks to provide reliable wideband digital communication capabilities.The requirements on bit error rates (BER) vary fron 1 X l0° to 5 K iü-~depending on path length and desired availability.

The Defense Communications Agency (DCA) in its Systens ImorovementPlan (SIP 1-75) tasked the Army to develop an adaptive antenna controltechnique to combat the fading caus ed by changes in the index of refraction(as discussed above). A letter of agreement (LOA) to imolenent the Army ’saccomplishing the task was signed by the Army Materiel Corinand (A~C)*and the Army Communications Command (ACC ) on 19 Jun e 1975 . The Corunu ni-cations Systems Agency , a joint AMC-ACC organization, then tasked AMC ’ sElectronics Command (ECOM) to perform the engineering effort on thisprogram .

A . Program ~~jectives: The objectives of this program are to:

(1) Investigate the feasibility of an adaptive antenna control (AAC)system;

(2) Determine the optimum technique to accomplish ~~C ; and

(3) Implement and test a prototype of this technique on the RADCtroposcatter test bed in upstat e New York .

B . Program Paraxneters/ConsLraints: Since the AAC is bei ng designedfor operational equipment in the DCS , the following parameters/constraintsare involved :

(i) Structural modifica icns will be limit ed ~~ the feed system;the dish i t self will not be alt ered :

• *Subsequentl y renamed Army Materiel Develo~~rtent and Readiness Command

• (DARC0N )

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(2 ) A ni ri~ nun performance improvement equivalent to one arder cfd±:e~~- i v must be at tained;

(3) ~he -:~~ej~ ~~ll cmero~e on links with the following characteris’~ics:• a. antenna ~‘~~~ ec~ cr diari e±-er to 60 feet , feedhorus at the focal poin~ .

h . t ra r~nission fr equencies fr~~ 600 ~~ z to 5.0 0Hz , J jc’ . ~nooth •~ar~ h n a th lengths from appro~dmately 100 to 320 nautical

• ~1tiles .

I ~~ stenn . an~’id4hs (a~ 3 rIB points) from appro d.mately 0i4° to I

~~ “ or ~~~a~~ 1 signal proces:~ n~

f. b r.d~ i’~~ .: to ~~

• ~~~. ~~~~~~~~~ RF s n r ~~r 1 c 10 KW .

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2. 2~~7A~TAt~ES OF AN AUTOMATTC ANT~~NA CONTROL SYSTEM

A. c n o o ~ ical Standsoint:

I .~ •~~9 ‘~y-tem l~ increase the time availability of the comxnuni c :~~

I i s.k . •1~is ~~~ 11 he aecomnli shed by th~ ability of the antenna to ada~t~ :-:inus~ ‘~nse to ‘he direction of the ma d mum received signal sc~~-r .

•~s -ì rr- -rult , th e rere v~ r sicnai~ te rv~ise ra~~ o will be increased and 4~rr~-r r~

4~~s : ‘ ~ eau~nt iy d~ creased .

~~~~~~cne r , the ~~C c s t ~~ mosse :ses the advantage of having the oPt i on -

•

~c ‘~h~ne ;cith :~ ac~ and/cr frequ ency diversity techni ques to achievech er cua~ 1ty n f serformance.

3. E con - ~~~~~~ 2t~ nd~~ 1nt

The .~~C system would not require any major overhaul of e~~ stingsyst ~a-s . l~ is — ‘ich less complex and ~~~~~~~~~ than a syst~~ ~~plo~~ ng:~~sce v~ rsi v which two or more receiving antennas are used . The

~~~~~~ “reau.~ncv S r~ rian occupancy is much smaller than that for a V~~~~~~~~~~~~ d~ v~ r - ~ ~v

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3. FEASIBILITY OF AN AUTOMATIC. ANTENNA CONTROL SYSTEM

In July 1972, the U.S. Army ran a mechanical beam steering exp erimenton the DCA Schwartzwald-Savona radio link in Europe. During the experi-ment, the transmitting and receiving antenna f eedhorns were steered

• mechanically in both the vertical and horizontal planes through a manualremote control arrangement. It was found that by steering the feedhorn(s),the average received signal power could be increased, thereby reducingpropagation outages due to severe fading. An average signal levelimprovement of approximately 3 dB was obtained .

Research and experimental works by Waterman1 showed that a narrowbeam could be swung in quick succession through a limited angular sectorby fast control of phasing, and the use of a broadside phased array .Phase and amplitude response patterns were obtained , and used to de erminethe direction in which the received signal level was naximun ’- - ’3 . .Ji thstate—of—the—art technology , it is feasible to autona~ ica11y adapt antennabeam direction to the changes in the received signal ang le of’ arrival.

An alternative to spatial movement of an antenna beam is a syst emwhich employs angle diversity techniques. It has been demons t•rated byMonsen~ as being capable of achieving performance comp~rable with that ofspace or frequency diversity. Experiments by Sureniari0 also ind icc edthat angle diversity is comparable to space diversity in merfo~~ancecapabilities.

~~~. DESCRIPTION OF TECHNIQUES

The centra l problem is to adapt th e antemna response to acc~~nmodat ea changing transmission path for the transmitted signal. Concentually,this can be accomplished by either fixing the direction of the t ran smit t ingantenna beam and adaptively adjusting the receiving antenna beam or byallowing the freedom for both transmitting and receiving antennae todetermine optimum alignment . The adaptive control can be either thespatial movement of an antenna b eam(s), or selecting/combining signalsusing diversity-type techniques. The individual choices are discussedin succeeding paragraphs.

A. Beam Steering Methods:

(i) Electronic Beam Steering : If the antenna beam is to be spatiallysteered, it is feasible to accomplish this via purely electronic means.One possible implementation would be the appropriate phasing of a multi-feedhorn system. The number of horns, their positioning , and the rang e of

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phasing would depend on the range of beam deviation required .• ~~F•

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A orede ~n~ d threshold level for r e c ef ’red 5i~ n a L s oeng •h ~-rou1dthe c - ~~: i :x~ whether or not to enter a beam snang e c”cle . lc~~ --

• --d nathn o~ o~~ f ’cum beam orientat ion could be ‘ìco:v:lis:ed hc- incres: ;it :,L•

• tcial no -v ovevent s , or by utilizio:~ a senosr c echni~ ue. The :es: r~~ “-~-~~m ~o ‘-ca’- an -ngs 1 r sector i d ~

- a n 0 ‘ “ p

~~~~~~~ ~o o~~ aa” ~~~~~~~~ sde ar ~ oh~- e “ r’v~ a~ -’ i- -, ~ a “

n~~-; ho nsed to r1so- :las~ and : E-r :’~ ~~ o disect ion ~~ ~the rec~0--~ --i s d snol ~e -el is s awiri’sv . This is do-ne at a s nld rate ,whic:• is •nden- cm the scan r a t e . f a r r —o~~t be taken to ~n sor~

a r a te is creat er than the marl in fad e ca~ e ~exc1uding fadthtO ~: 1 5 O S O~~~~ In the conron vo1un e~ of th e sommunica~ icm channel. It shc~~ d

• i n the -od or 3 of 20 srac / :ec . doall y , the con~ rr l cycle ~~~~ :t~~~~ -~

y e -:a ne an the scan ra e . o~vmer , the ~o-n~ rc1 cf rou!~~m my:‘~~~~ “ ‘~~- ‘ e sO~ ~

— ‘~~~~~~° 1° ~ a e Ar i”c~~’~ ‘ab le r —

‘ i c a cont r’~l on-o le an :-cr ~ ‘a: ocssihle vYhcut ononins oso~nod to i€v~~- m i - ~~ the d i s o t i :r as n t d c t . the ~1--er we ~~ nnal no er isn-ic - n the ~i~ e I~vLod of the erole .

As with all • :en: described herc- . the re~~~~osn~-ai secye nco- :~~~~~ ~te~ ~ * c~ e~ a e~ ~~~~~~~

=~ ‘-v-s ~ ioahi1it and a su~ t ’a h l e ‘- --a — ine li nnla~- of ~~~~~~-n- ta co ~c’~1d he ~no1ud ed .

( ) k’ -’ anir-al ~~r— ~~~‘crj ac: O:nce ’~allm . ~ohanin-r s n m e st h e o e s ~ nzn i t ~~~~~ e— ° - ~y i s -n : hc- -e’-~ r . there ~o oT -~ ~~

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r e o~~-:io:~ - ‘ - -n - ‘- “ he o~I1h v- n(s ) no conn i th •e~rin ~~

n-wi f led can :+ e ~tnoe::t i~ to~ c c : ;h i c ed by hr - :“ acn - m t tY’A iitkr , - -‘

~~ i r t~ ’ -~ ~~sh o sco the sane feed ho :n ori ts

fo’- ~so’a n - i~~-i- ’ - a d r r o e i : - . n~~: ~e ~v- ahlloaed nba: the cs isOj • -

a s : : t : f e ’-v’d . S 5~~5O ~~~~

‘ ‘ - u ~~ nove’-~~nt ~~~~~~ ~~~~~~~~~~~~~~

Ar ana v i - s vS °n t o an e t e ’vr in ic s y c t~n for ~~cI5’ r 4 as c n f - - z n- nt wc’ild h.~ i-- -~ °n ’ -

~~. ~he in crementa l ria~ -‘

~‘o r -d ~-: n ’~~ ri —c o a t r e :h --:lca ~ones~ n~ -v’ ‘h.p “eed h orn s . ~o -‘ v Oe’ ~en ’ ascan to’- . ou d ”oh~~~i :: h° a- env in: to use nu 1~ i a ] ° ~~ed ‘ sr - sas is ‘th - o :~~. asra;; s:is~~n of ~ I S v - C ~c — s .

• ~r : s:; s em s w c u 7 d ~ -~n v - T I n the iv- mo :emont of the hr “~~ e~h -

~~~ di r ec ion change:. an~ t n~xo he en ‘abl~ hed th at ~~~~~ ~- -

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wi~~n ’ ’y o~ or ’; t~~ a2~ or ‘ad ’an f n ~ to than -nan lrr .

1- Le • - er:i~ Yoth od : In t h e v-poscat~er node the co~~~ nv - I - n—c - • -~~~~~~~~~~

- - - vf~~~cl~ • :o ’-e’ e scatter : , which gi’: - “ n o-

~I ia!° ry; y th: at ’ are ‘~‘ - v I - --’~ly unc~ r r r ~~~~~~ ~n t heir a~n~~~~~des~~~~~~~~ 4 si~ r - I ’~~v-r~ in cr eas e in “9r ~tv- :thr ’a n t r c a m t h can

-y~~:ar ~~-o 7v iwi’~minaoing n~ ro nan ore area in. n : e

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common volum e, and diversity combining the vari ous ;i rnal :. One way toimplement this is the use of multiple feed horns in a dish , arranged toproduce multiple antenna lobes . This technique is commonly called ang lediversity. To implement angle diversity on d if f r a c t i o n nods; oaths , it

• must be shown that multiple paths , with relatively uncorrelated fadin gcharacteristics, exist.

C. ~ystem Modes:

Cl) Open-loop Control: For the open—loop confimura~ion . with eitherelectronic or mechanical beam steering, ont inal r e rfcrmance is achievedby adaptively controlling the receiving antenna beam(s) based on receivedsignal statistics.

(2) Closed—loop Control: As the name i m pl i e s , the cont ro l inforna-tion loop is completed by giving each terminal access to the re~ iltsobserved by both terminals when an adaptive chanm e is mad e . Th o contro lchannel must be made as reliable as possible , and do s ima ed such that at emporary signal degradation would not cause an interrun*ion due to theneed to re—establish the control loop.

The amount of feedback data that the control channel mnst r-arry willdetermine whether or not the present ordei~ ire chaon’~ls can he used forthe purpose. Some type of data processing will be necessary to- conver 4feedback data to antenna beam control information . To a I~~em~~~r-e the rround trip delay time, between antenna acti on and feedback reaction wiwidetermine the feasibility and effectiveness of closed loom * o~ :~fljq~~~2

D . System Configurations:

(1) Steerable Single Beam~ Operationally , a single ant enna beam issteered in the direction of maximum received signal owes’ . thnscr ecnii n-rnent , or incremental trial and error techniques , can he used to track theoptimal beam pointing angle.

Such a system needs no combining circuitry and is relatively simpleto implement . It can be implemented by either elect.roni c or mechanicalsteering techniques , and can be controlled either open-loop or closed-loop .

(2 ) Fixed Multiple Beams : A fixed multiple beam system employsangle diversity techniques, as discussed in Section B . above. Care mustbe taken in determining the optimal pointing angle of each bean , toensur e that the correlation coefficients among the received si~ nals from

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the diversity channels are less than about 0.6. This is due to t he :1basic requirement of statistical independence arionc diversi th channelsin order to achieve quality performance.

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Aidisional recci--ers and combining circuitry are necessary for m ole-- v -t ing anc Ic diversit-r .* The output of each receiver, corresponding toone diversity channel, is fed to the combiner. Various combining tech—nioue s sai-n be used to enhance the received signal. Among them, maximalratio combining and equal-gain combining techniques are more popular ,:-her v:s sele~ ’ion combirino technique provides slightly lower performance.T r e vo n be exersosed in inolementing the combining to avoid introducinco - :- - :-: i r - ai~~ ~a:a r ’a~ e i thl t raff ic .

sn : le Tvltinle Peans: A steerable multiple be~~ system - -

c:n rsl and an~:le diversity techniques. The- • -~°cas lc -~ach than must he -cn-orocr’iately determined, and be svo~ -

•a ~~~~~

- - a ~ s c~ er~ be * r~~~~ 4 nc signal5 of any two bea”~ ‘e

sensor eovinn:ent s are needed to scan through predeterminedan-nalar :ec:ors c:rre:oon-dinr to the diversity channels. The ootjnaiociat~ uncl e of each bean could be de~ ernined by a data processor.0:-a °~~~~~~~~a~~~-° a-- v--oLshed by either electronic or mechanic~ l

Ic di-:ersi ty, mul*iple receivers are use~~ and the - -

- re fed i n t o a combi ne r for si~ rnai enhancement .

a cteerable multiple bean system can achieve better• - en-~

• nan ~i- her a sm nn le steera~ Ic bean or a multiple fixed bea--

CH ~ IllTS 4SJD T~i~DEC-FFl

sic vs. heohanical: Solid state technology offers a ver-:

~-:eni ea~ hv-lemen - twit-n of electroni c to-othiques. It avoids rn ech an icaJtarts nvich i—cu es an inherent reliability advantage. The . - -

~

-‘es:vve. :irce no mechanical iner ia is inn olved, is many times faster ,llt *le v~o :nion as to i ts abili~ n’ to track the variations in the

an - ’ :f arriva -~~‘ the wi rs-rnrest simrwi . Comp onents likely to need

- - -~~ - - -vovnce on — ‘en -i i r are more conveni ent ly located .

is net ad at r -u ’ ‘advantages, how~~er. It has been

~mnlem ens °~ n :o -v - awion-a l H rcui~ s and has shown an improvønent in:e-

~

ci. There 1-5 no problem in propo~~ ioning high trans-~it ’ er n — ’ : - - ’~’ (tin ‘-0 i~~ I-n-r) between/among feed horns , nor in physicala l o c on- oo t c - ’ -u i ’~~-horr arrays . Disadvantages include b~~iding/flexing of S

‘nI:~- vs a1 ~‘a - ‘~ ech’a rdcr il motion , and the inability to follow angle- - ‘ an ~co in a ~ e fram e shorter than seconds.

-~ - ‘er~~ nf l sy stem i n used . Other circuitry would ber a rod ~~ r~r- ’ I r - f l t echni que.

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- B. ~~en—loop vs. Closed—loop Control: Icon-loop control systems are- -

- simpler , and more economical to implement than closed-loop systems. Amajor advantage for closed loop control is that the results of transmitter

-- ant enna beam movements are available back -at the tran smitting terminal

~

via the closed loop contro l chann el . This is particularly inoort-ant for- a mechanical beam steering system since it is probable that the trans-

mitting and receiving antenna horn (s) may physically move together . Inan open loop system , it must- not be assumed that the omt,Lmnon ori entation

- for both transmit and . receive beams is coincident . Transmi t and receivefrequencies are normally well separated to insure that the high power of

• the transmitter does not di sable its co-located receiver . It must beestablished that this frequency separation does not disnim~- an assumptionof coincident ray paths before coincident movement of t ransmit and receivebeams is accepted .

It is possible that a mechanical or electronic beam steering open loopcontrol system could experience excessive hunting . If one terminalrealigns its feed horn structure to change its angle of arrival , it alsochanges its transmit angle (ang le of launch) . The distant terminal wouldnotice a change in its angle of arrival due to the changed transmit angleat the original terminal. A realignment by the distant terminal couldset up a hunting sequence. This t:roe of hunting could be prevented in aproperly instn~mented closed-loop cysten wherein actions of each t enwi nalare known by its partner .

Round-trip delay time , which can be in the order -of 3 milliseconds- (depending on path length , path geometry , and oroc~ssing speed . etc.)

is a factor to be considered in closed-loop control syst~ n s . It is to be- -

determined whether delays of this order are significant . This in turn• depends on how fast a fade rate the system will be designed to follow.

An obvious advantage of open-loop control is that it does not require- - a channel to send feedback information. This requirement is rir-iplified

for a closed-loop system if existing orderwire channels can he used , whilestill satisfying the reliability criteria.

The potential gain in circuit reliability for closed—loop control-

‘ must be weighed against the advantages of a simplified syst em for the- open-loop control .

- C. Steerable Single Beam vs. Fixed Multiple Beams vs. Steerable- Multiple Beams: A steerable single beam receiver offers the advantage of

adaptive control, which maximizes the average received signal strength.Closed-loop control can be employed to further improve system performance.The need for a sensor equipment is o f f s e t by the fact that no combining

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- - circuitry is necessary . Choices for critical system param eters such as-

- -

- the scan rate for the sensor equipment and the decision criterion for

-

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- beam movement, determine the maxi m um i~th l ity of the system .

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A Sived n u i w i n l e P r - -in r r - c -~~ - vs -~mr Tovs -nv’ l~= ~i--er : i”.- t echr i ’a s e : .yh-o s~ rerr~~r--;. - - - has : -~- -rn n - - v - n to - nnr - - -v’h thos e o f scace and frec - - i c -nn --J ivers i’ y t~~~~ci :ues a . To sen v~r eaai n •ern n is ne eded , but - extra rccei- :or~and/ or - - b4.nin- ~inciñ~~ ir : ~ he - n ~~~~~~~~~ P r - n o r chcico of bea

:‘ n , ’ annles is e5 -e n w i - J s ce : r f- 1 o - ~~~’i~ r - .

A -‘ - --- - - ~‘-d~~e -~v~ ’ i - i e boar- --r -~m no~’n~ - bc~ P sensor and cm~binic:’-‘nns , -i - 1 Is e- --c- --” i - - o and - - : • -n- --d~~~~~

- - : T - ~-- sn~ . To at ~ sin a-

~

‘s I . i t * h’s-:e~~- n : n d ’ ~ce substan t iaJer-n : n - - er : - - -- r n:: ’- : . Tt s complexity and cost

~~ fO - -- ~h •e o~~lv on roub ---none - ~ - ‘fr ac t ion mode inkl:; that have an ‘-“ s h e :c-r-~ ”r c- m : c c - r n~~. a

• - - - :e~ I n n : . ~he ce-n rniI :— rchles- and :v:t~~. phjlosomhv

a - a -~ s ~~c- -- ~‘- - ‘ ~‘— x a ~ea’~v~~Lty study an 1ai-a T - -wi~ to solve the - n : l en. I- should be ‘arr ived at via a contractual

-a -d°s~ nr. t l a c a -’ ~ino r’ am -i id - s o t echnique , and a t -es 4the - - ‘- acn e 1m r- - ° - 0m m an~ icimat -ed . The techniaue

be im~- :-- °n n - i -~rnd t a n n e d or: a -- rocoscatter t e sn link . The ±~enrhnsvld ~ -~ Jccn- oiv ~n a fi rn: l tyne resort . 3’udy , an ’aTh’n i: ,

-. ‘~~~~ c ab Ut Si -~~n~ -c Imo~ °— e~-

• - ‘v n - . and d c- s-v-nt a i-:n -no u ld r---:uire ~j n -add it ional twelve m o n t h s .

— — a - ° speci c

-

_ s-Ce: - -~ - a T n a ~ n the va r iou s t -xrn_siques, it is in-o c-rt ant to ‘-onsi -j er- - o - j~~~c -~~ - - -- e~~~ -s-f ~~~~~

- ‘:. The 5- l: icn :0fe rs some i n s igh t s .

A . ~ -r ~ ~~

- -~ - - . - -~ 1n - s-~ n: The state—of—the—art is relativela-

- - ° - - -~~~~~~~~~~~~

- :Y~~m n , : - r on~ rast to the d~rnamics of eleetr:niri’ - - - ç- : -- -- ’ . Th e r’-os t s of icnlencn -~~s--’ ~tn - cl e x nr r ceswing continues a:

-‘ .-o -‘ — t o ‘~~~~

‘~~t elec ecn a ~e less ex o en ’-\ o

— r 1 — - ‘ - act 4 —~~ mon o S e’— s using rd d -~~a e

- ‘~~

- b - - : - r r -~ved imhoren y mor e re l iab le, and less costly tor- e co e electroni c ir’o1enier~- -- io-’

- ‘:005’ - r no oc~~~~~li l l irss - --- r -~~ -r: :-

P . - ~lc- c n. Tth - - - 1- s-v IT~~- -ed loop ~on~ rol systems require ala na n~ o-:- - sn - -on d - r - d ot - a in the feedback loop. If the-95 i 5 - ~L a ~- s ‘n° - ‘ “n n cns~-nt or - r -~ rr circ udts , there will

cc - I- :- - c or . cr~~ rn a n n sir n canal I ~it n - . Ti- adv antages discussed tn - -- ’ t - ssln .- f -- s --‘ Ic - ed —l oon- o -n r’- l nrc ho ’s-nh a~ a vri cc of increased

r’ - - : : ‘ - n u: -‘a :-c :nib le need for more _

-

-~

~~~~~~~~~~~~~

r -•

‘ ‘ -~~

- -i~~— -~

- ~~~~~~~~~~~~~~~~~~ ——L~~~~~~~~~~~ —

C. System Configuration Comparison:

(i) Steerable single b ean system: The complexity and cost of asteerable single beam system will vary according to its intrlerrentation .Some t-ype of information processing capability will be moc0s:ary, butthere is a choice , for example, between a scanning t~’s-e sensor systemand a ‘vial—and-error approach . There is sufficient flexibility suchthat. a well considered design could make the steerable :intle be-an systemattractive from both a cost and performance standpoint .

(2 ) Fixed multiple beam system : The fixed multiple beam r:~stera willnrobah Iv be the closest rival to the steerable single bean system . Thesensor rvs ’em or trial—and—error technique would he rernlacr- r I by anai-an4a4-ior, of well proven angle diversity techniques. Due consideration--‘s-t be taken in the initial setup since the receiving lobes are fixed .Considering the fact that this technique has been shown exc erimentallyto approach the performance of space or frequency diversi ty , the system

• ir romises to he competitive in p~~fonnance with steer-able single beansvr ton S.

(3) Steerable multiple beam system: A steerable multiple beam syst~~is core expensive to implement than either a st eerab le sinole hear- orfixed multiple beam system , and would include all ~he necessarm- equip-ments for the other two configurations . The procassor must have analgorithm able to follow fast (less than 1 second) fades. The addedimprovement for most links would probably not justify its cost and

- - ‘ complexity.

- i7. POTENTIAL PROBL~~S

Many technical problems are expected in implementing an AAC system.The following are significant and probable examples:

A. Suboptimal threshold level for a decision to ir~plement a beammovement cycle: Wh en the threshold level is too high , oscillation ofant enna beam position may occur~ if it is too low, little or rio adaptivecontrol is accomplished .

3. Slow respons e time: System performance would be deteriorated ifthe antenna beam is not adapted to the desired or optimal pointing anglewithin a reasonably short instance of time , after contro l action is

- - initiated . This may be due to hardware limitations and characterized bylarge overshoots .

C. Phase variations: Due to the differences in propagation path- - ~~~

- , length , phase variations among signals from the diversity channels maydeteriorate a system employing multiple beans .

:~~~~.

________________________________________

- _ _~~~~

_—-J — - — — ______

I

~- . ;,- -

- - - - _

- - — — -c - ’ ; ~~~~~~~~~~ steering only): Sb- -:

S - - - — - ‘

~~- - n n - - nvsl by adaptive control, if

-s: ’ai-li -t f’ f:~~~~- -

~~~ :~~~ t} - - fast variations in

F . ‘ -- ~~~~~~ :-lo ~~~

- - : n - m n a non area: For a system wit -h0 1 - ‘-‘~

- rsically place the —

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ . *

L ~-:~r t : : -; n of a: - er~ a hor~ ~:r’ - one lOS antennas, the ::s-

~~~~~~ both t- -v ni on and receptions in.~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ra::- s er. which employs an ’af’a:-~ --o

s h - ~~°. ab s --v n’em-un - :5 ie -- -~:ri-’-in ’ hear’ may offset the ccni-- ’s--v inY - .’ n n I - ’ ‘ ‘ cc -‘-s- ::- i~n i r n n -nn.. orohlem may be exeer-~ i-- r-.-

:- -~~-

- E : 2’~~~~:- . - -~~~~°,“~ ‘ :--d ica ~ i:n exists between -~~rnn :n - i sY -’ n : re ’- - - i ~~cn m oran -i s a - - n- dea- -w-’n--ining the optimum

Ira -=: °-n -icn oor’binir~’: b r a system witi. ’—’sI~ ir --en- ~:° - - -‘

~~~~~~-v ~~~~~~~~ :1:v - 5~~’hing speed among ‘he- ‘ - -e i-nr-aiat i :r. This problem

-~ inf:ao-oninn han. i ‘ n - ’: Tn: a sbo re d—loop system, soc e

- vm s- i -m ‘ - - n c o l a~~ :-”a - -- -~ °-n- -h e feedback ioop . Oneis m~ t n - f mo~ -ni- : .osing the feedback ir~ :m - ’i : .

t , ~~ 1r . n r ~ h lem s ruch as j nsiiJficient-- - u I n s i l i - : -~cd a - n - : I v ° - I - - -- - f ” I n n - cn- ’ P ”~~- cce ujn ’ered .

a . -~~ :~ v :‘~~ - -

~~

-~ ~~~~~~~~~~~~~~~~~~~~ on ’ ~ n sin-’ sn’ signal erhan cement h::.nrn r : - ; 0 -ino: n_

~r a c h a r ~ ing angle of a~-‘~-; :-nP _i :r e-o - -.- n~ ~~“- r~s-.• T :en. “ m e -- ‘s-~-:’artzwald (Germany ) —

- -

-

s ~- °n t ’aa k e d i:- - JCA tc develop an adan ’ l - - ° •- r n :- - - - o-- - cli: - — - ra -r-n - °r -:rd diffraction mode ant on.a ~au ed by changes -

r~~.

1? —

— — ___________________________

— ~~~~~~~~~~~~~~~ ~~~~~~~~~~ ~~~~~~_~~~~Tl_ _____

C. There are several possible approaches , apoli cahl.e to DCA an~ enna s(dishes up to 60 f4- . in diameter, with feedhorns at- the focal point):

• (1) mechanical techniques wherein feedhorms at/near the focal pointare physically displaced ,

(2) electronic techniques. wherein ohysical feedhorn mo’yemaent isreplaced by beam steering via phase array techniques,

(3) angle diversity wherein an antenna has two or more fixed mainbeams whose responses are diversity combined .

I. In designing an adaptive antenna control system , tradeoffs arepossihie, for example , closed loop vs. open loop cont rol.

(i) Open loop control systems are less complex , and more economicalto imolement and maintain , however they are effec~ ive only in correctingreceiver antenna response, whereas,

~2) Closed loop control can effectively a dj u s t a t r ansmi t t ing antennar beam since it is aware of the effect on the receining antenna of the

adjustment. It is, however , more expensive and comd ex, and requires a-

-

highi reliable data channel.

E. Several potential problems suggest th ems elves ~nd ocm’ aredependant on the particular approach. Poten~iai n-noble-c areas include :

I(1) a proper choice of a decision threshold ,

-1(2~ an adequate system response time,

(3) the phase variations among multipath signals (for high data rate -

‘

transmissions), and

(ti ) the handling of feedback informatI on.

9. RECO~’U€NDATI0NS

It is recom~ ended that a study be ini ia 4 ed to de~ r~~ ine the feasi—bility of adaptively controlling the r esnon se of ~repossa t t e r anddiffraction mode antennas , and to de~ °r ’— ine the ~ptjr-i~r- adoc ’ive c’~n~roltechnique.

Method: An effort should he initiated to :

-~~~~‘- (1) determine the optimum apnro -arh and crecare a desI gn r~ an f-n m’

- - p.~ a prototype of it -s hardware Imnlementa t ion , and

-~

11

— --~~~~~~~~~~~ ~~~--- ~~~~~ -~~~ - -

—

~~~~~~~~~ ~~ T1 ITT TT--~7T~ ~~~~

(2 ) v-ro ve i’s ~-xas ihi l it - . and de~ e:m : - - I - s- od’r-intage by fab rl — - -. -

and l abo r : t -OTT ‘e:- 4 irt~~ :b e cro ’ flI~~-~ o~ lii. :- ou -~ be ‘o J I - j i-:pd h-. in-

t on eauincena in a ‘co t bad , to cie -o- rrnir .e i~ -~ advant-a~’e over avaraT~ e-l nmcom ’ro iod li’-’-ii~~~.

• ii ‘

•

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10. BIBLIOG RAPHY

1. A. Wat erman , “A Ran-id Beam-Cwing~ nn-’ F:-:s-eriment in Transhorizon

‘ Propagation”, IRE Trans. A?:? , Vol An-tI , PP 8_i !:2, Oct l~ 5P- .

2. 1. Cox , “Phase as:-] ~mn1itnde Mear crem~ n f s of ~Ii rn:-:a. r-~ PronagatedBeyond the Horizon ”. Ph.D. ciisse-rt-aYrn . f t a m c f o r d RIo - ’ . Labs., Tech . Rep.277t~l, Dec i°6’~.

~~. D. Ccx & A. Waterman , ‘Phase and k m c l l 4 ude o surcmen s of Trans—horizon Microwaves: Angular Response P-a ’~~e--rnc 1’: F eva~ ic r ” , iEEE TransA&P , Vol. AP—19, No. 2, March 1071 .

s. P. Monsen , “Performance of an E-,-re r’icor.~ il iIn~ Ip-DIn’ernnit vTroposcatter System ”, IEEE Trans. Comm., Vol fO ~~- 1 , No. 2 . PP 2~~2 — 2 L ~7 ,Aori l 1a 72 .

~~. D . Surenian , “Experimental Results of Angle Di- - rnrs i ty Sys t emTest” , IEEE Trans. Ccmm. , Vol COM-l3 , June 1 .

~~ . D . Rrer~nan, “Linear Diversity Combining Techniques” , Proc . IRE , ‘

,P 1 n e I ~~~~~

7 . J . Strchh ehn I- A . Waterman, “hIn-:nliated Arann l -ir Ressorare- Patternsfor Tr-a i i shor izon Pronagat ion ” , R a d io Sci . -J . of Re s . , Vol ~

‘‘D , Feb ~~~~ ‘ -

-

~~~. J. f-trohhehn ~ A. Waterman, “Reflection 0e- Radi o Waves form Tn—

f u l a t~ n~ ‘ron nnn n-herl c Layers ”, J. c-f Res. , N . d . 3 ., Vol (n -I , Nov —D ec li6 -~ .

0 . J~ ftrnhbehn & A . Wa terman , “ Transhorizon Pr~ n - -~g~i~ io n leasurem ent sfr n m a ~icu 1 -rineou s Froouency and Ang le Scan Exm erir -nen~ “, Rad I o Science ,1 ( N r ~ : er ie s ), July 10(1

10. R. Kiehurtz & I. Fantera , “Angle-of-Arrival Measurements PerformedOver a 2~ ’- —Kn T roposca~ ter Path” , Radio Science, 1 (New C e r i e s) , Nov 1 ~ ( .

11. C. Grzenda, “Investigation of Angle Diversity Techniques forTactical Troro sca t te r Applications ” , USAECOM — CADPL Report No. 1~ls,July V~70.

12. J. Ve~ e tman , “Tropospheric Scat ter Sy st em Using An gle Diver sI ty ” ,Proc. IRE , Vol. 17, May 1°~°.

13. JTA C , “Radio Transmission by Ionospheri c -and Tronomoherl c -lra~ t-er ” ,Proc . IRE , Jan 1~ (fl .

Th . W . Richard & M. BickeThaupt , “Multiple Angle Diversity DesignConsiderations ” , Tech . Rep . RADC-i~ — (n- , (In ffiss AFB , N.Y.. Ass’i I 1°(~O.

_ 13

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