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ADVANCED TECMNOILOGIIES FOW LIGHTWEIGHT EHF TACTICAL
@BMMUNICATP[ONS SATELLITES*
David R. McEIroy. Dcan P. Kolha. William L. Cimnbcrg. ad Marilyn D. Scmprucci
Lincoln Laboratory. Massachusettq htiiutc of Technology
P.O. Box 73
Lexington, MA 02173-9108 - USA
1. ABSTRACT
'The communicatiom capabilitics prdvidcd by-!iHF satcl-
litcs can range from low data ratc scrviccs (75 to 2W' bps
pcr channcl) to mcdiuni data ratc links (4.8 kbps to 1:>44
Mbps pcr link) dcpcnding on thc payload configuratlon.
Through thc usc of EHF wavcfonn standards. thc EHF
payloads kill be compatible with cxisting and planncd
EHF terminals. Atlvanccd tcchnologics pcrmit thc dcbcl-
opmcnt 0; '.ighly capable, lightwcipht payloads which can
bc utilizcd in a varicty of rolcs. 'Ric kcy payload IcAdol-
ogics includc adaptivc uplink antennas; high spccd, ]ow
powcr digital signal proccssing subsystcms: liphtwc\ght
frcqucncy hopping synthcsizcrs; and cfficicnt solid-statc
transmittcrs. Thc focus in this papcr is on thc sipal
proccssing and frcqucncy gcncration tcchnologics and tlicir
application in a lightweight El4 F p;tj.load for tactical
applications.
2. INrHODUC'I'ION
A motivating factor for thc tramsition to EHF corrimunica-
lions (i.c., 44 Gklz uplinks and 20 Gtf?. downliiiks) is thc
rcquircrncnt for iinprovcd intcrfcrcncc protcctiw with
small mohilc prwl rnancportahlc tcnninals for tiictical and
stratcgic ucr~s. Enicrginp tcchnologics nllow El IF ctm-
municatiom systcrns. which can s uppi both low data ratc
(LDR) and medium data ratc (MDR) scrviccs, to tx implc-
mcntcd in lightweight. low powcr configuration% Sbn-
dad EHF payloads, utilizc both atlvanccd antcnna systcrns
and on-board signal gcncration antl prnccssing tcchniqucs
to improvc pcrformancc and protcction. A payload which
utilizcs thcsc fcaturcs for a thcatcr covcxigc application is
described in this papcr. The application of advanccd
signal gcnnation and proccssing tcchnologics to this light-
weight payload rcsult in a payload which can bc incorpo-
rated into sstcllitcs of many sizcs, ranging from large,
multiplc function satcllitcs to small, augrncntation satcllitcs
1141.
3. ANTENNAOFTIONS
Advanccd antcnna fcaturcs for tactical applications can
includc thc ahility to providc a variablc covcragc uplink
spot hain pattcm, an autonomous nulling capability within
the uplink spot bcam pattcrn, or both. Variable bcamwidth
EIIF antcnnas can bc utilized in a varicty of applications
as shown in Fig. I. For payloads in clliptical orbits, a
variablc barn width fcaturc can bc utilizcd to maintain an
essentially fixcd covcragc arca indcpcndcnt of satcllitc
altitudc 141. In a gcosynchronous orbit application, thc
bcam variability can be uscd to satisfy diffcrcnt covcragc
and gain rcquircmcnis such as in supporting tactical
thcatcn of varying sizc and capacity rcquircmnts. Onc
approach to obtaining the bcam variability is by employing
multiplc fwds in thc antcnna. Thcsc frcds arc combined
wirh a vanablc pown conibincr nctwork hcforc going to
thc rcccivcr. With 7 uplink fccds in thc anicnna. a $to- 1
*This work was sponsorcd by thc Dcpmmcnt of thc
Amy. thc Dcpartmcnt of thc Air Forcc. antl the Ihfcnsc
Advanccd Rcscarch Projccts Agcncy undcr Air Forcc
Contract FIWI~R-~O-C-OOZ.
variation in bctunwidlh can bc achieved. whilc 19 uplink
fccds givc a 5-10-1 variation. Ry incorporating both phasc
and amplitude co~trol in thc hamforming nctwork and
including a proccssor, thc variablc bcamwidth anicnna can
also includc autonomous nulling [5].
4. LIGHTWEIGHT SIGNAL GENERATORS
In ordcr to providc cffwtivc intcropcrability. it is impor-
tant for all thc EHF payloads to work with thc samc typc
of uscr tcrminals. S:andard ELIF transmission formats and
dynamic acccss/configuration control arc important fca-
turcs in providing Lhis intcropcrability. Thc standard EHF
wavcform rcquircs thc dchopping and clcmtdulation of
communications signals on-board thc satcllite. As shown
in Fig. 2, lightwcight frcqucncy hoppinp synthcsizcrs can
bc irnplcmcnted using dircct digital synthcsis tcchniqucs
along with high-spccd. hybridized bandwidth expansion
circuitry. Thcsc advanccd frcqucncy gcncrators yicld
alrnost an ordcr of magnitudc rcduction in wcight ovcr
frcqucncy synthcsizcr subsystcms of thc carly 1980's whilc
also rcquiring significantly lcss than hall thc powcr.
Thc kcy clcsign critcria for a payload frcqiicacy synthcsizcr
arc a low powcr cmifigurati(m which hiis thc aliility to
gcncratc signals with low spurious frcqucncy cantcnt whilc
mccting thc frqucncy switching spccdi cquircmcnt. Thcsc
factors arc kcy in sclccting thc bancbidth expansion
approach as shown in Fig. 3. A switchcd filtcr bank
approach is a straightforward implcmcntatiori. flowcvcr.
film sizc limits thc niimhcr of frcqucncics (N) which can
bc sclcctcd for mixing with the dircct digital symthcsizcr
(DDS) output. thus impacting thc amount of bandwidth
which must bc gcncralcd by thc DDS. This advcrscly
affccb both thc powcr and spur constraints by rcquirinp a
highcr powcr DDS and by gcncrating largcr spurs. Tlic
altcmativc afpoach shown in Fig. 3 was sclcctcd for tltc
payload frqucncy synthcsizcr. This approach utilizcs a
high spccd phasc-lockcd loop to gcncratc thc sct of N
frqucncics which arc mix4 with thc. DDS output to
expand thc bandwidth. 'Thc widc bandwidth of thr loop
allows N to bc Inrgc. thus rcquiring a smallcr Dl>S
bandwidth. Thc rcduccd DDS bandwidth allows thc usc
of a low powcr, CMOS based DDS and rcsults in lowcr
spur Icvcls. Thc kcy icchnology chdlcngcs in iniplcmcnt-
ing thc widc bandwidth loop arc thc high-spcctl countcr
and thc custim voltagc controllxi oscillator (VCO).
Somc of thc tcst rcsults from a brcadboard synthcsizcr
which utilizcs :'IC wick bandwidth loop for bandwidh
cxpansion arc shown in Fig. 4. Thc brcadboard synthcsiz-
cr mwts thc switching spccd rcquircmcnt by scttling to
withit? 7.5" of thc final pha.sc in a littlc nvcr 0.8 pscc.
Thc goal for spur lcvelr is also mct hy thc hrcntltward
synthcsizrr. A typic;ll.ob'tpt spcctrum is shown iri Fig. 4.
5. JII(;lI-SP1.:ED SIGNAL PROCESSORS
High-spccd digital signal prtxcssing advanxs can bc uti-
lizcd to providc lightwcight. low-power dcmtxlulaiors and
signal proccssing subsystcrns capahlc of supporting many
LDR and MDR channcls. In ihc carly 19HO's. ihc use of