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combincd analog and digital proccssing technologics
providcd thc most cfficicnt implcmcntation for dcmodulat-
ing thc LDR uplink wavcform. As shown in Fig. 5. thc
frcqucncy dcmcdulation was pcrformcd by a siirfacc
acoustic wave (SAW) dcmodulator followcd by a digital
communications and acquisition proccssor. Now howcvcr.
thc progrcssion of digital tcchnology has advanccti the
staic-of-thc-nrt to thc pint whcrc an all digital approach
is morc cfficicnt. Thc dcvclopmcnt of application spccific
ICs (ASICs) which proccss thc st'mdard EHF waveforms
will contribute significantly to thc rcduction in wcight and
powcr rcquircd by thcsc subsystcms. For furthcr rcduc-
tions in wcight and powcr, thc ASIC dcviccs c h be
intcgatcd into multi-chip modulcs to achicvc ths bcticfits
associated with a wafcr-scalc lcvcl of intcgration as shown
in Fig. 5. In this comparison of zn LDR signal processor
using multichip moclulcs with an I,DR signal procbssor
from thc carly 1960's. an ordcr of magnitudc rcduction iii
wcight is obtaincd while thc powcr is dccrcasrd by inorc
than half for thc samc nlirnbcr or chanricls pmccsscd
The digital Fast Fouricr 'Transform (FFT) dcmodulator is
implcmcntcd wiih two ASIC dcsigns shown in Fig. 6. Thc
samplcd signals arc first prcproccsscd bcforc ihc actud
transform is pcrformcd. In this FIT prcproccssor chip, Ihc
signals arc windowcd. cohcrcntly intcgratcd for iidjusimcnt
of frcqucncy sample spacing, and storcd in mcniory. Thc
hcart of thc dcmodulator is thc in-placc FFT rnip shown
in Fig. 6. Thc data is storcd in mcniory, thc FrT buttcifly
opcrations arc pcrformcd. and aftcr the transform is
coniplctcd. thc 1 and Q sam~!ia arc coltvcrtcd to inagni-
~udc valucs Tor Turthcr prr-cssing by thc uplink processor.
Thc F1.T chip is dcsigncd for usc with (lata or :rccluisitiori
channcls and can pcrfomi a traiisiorrn of up to 256 points.
Thc ASIC for thc in-placc FFT, shown in Fig. 7,' has bccn
dcsigncd, fabricatcd, and tcstcd. Thc FFT ASIC is
dcsigncd to mcci thc priniary rcquircmcnts for a spacc
applicatitm: radiation hardncss, high pcrfnniiancc, low
powcr ccmsumption. mid high rcliability. A significant
dcsign fcaturc in this chip which cnahlcs high pcrfonnancc
in a low powcr configuration is [tic usc of on-chip rnchiory
(RAM and ROM) for thc data bcing proccssctl arid fdr thc
cbcfficicnts uscd in thc FlT. Thc amount of riiciiiory
rcquircd is mininiizcd by employing an innovativc in-place
algorithm using dual port RAM.
Thc Fm chip, thc prcproccssor chip, and a coniniutiications
uplink proccssor (CUP) ASiC can bc confiuurcd into
a comrnunicaiions dcmodulator, which is capablt: of
proccssing up to 16 channcls, as shown in Fig. 8. Thcsc
ASICs, along with thc supprting chips (AA1 cnnvcrtcrs.
CUP RAM, and mission ROM) can be packagcd irito a
multichip modulc which is about 2" x 3" in six.
A
For MDR channcls, similar ASIC tcchnology is cxpcctcd'
to yicld cfficicnt imp:cmcntations for thcsc higher data ratc ,
channcls wcll. A prclirninary dcsign Tor a four channcl ,
MDR subsystcm rcquircs thrcc individual ASIC dcsigns,,
Four dcmodulator chips arc utilizcd in conjunction with a
clock gcncrator chip and an MUR prcrccssor chip to form
thc four channcl MUR suhsystcm. l'hc dcsigris for ihc
clock Rcncrator chip onti thc MI1R pr(tccs?qr, $hip ollow
cascading to support additional MDR dcm,wI,ulatyrs for . .. a*
payloads with mwc than four channcls. , ... . ..' a,.
..
6. EXAMPLE EHF PAYLOAD FOR
GEOSYNCHRONOUS ORBITS
An cxample EHF payload, which utilizcs a pair of variable
beamwidth spot bcam antcmias and bah LDR and MDR
signal processing, is shown in Fig. 9. For this example
payload, the uplink spot bcam antennas utilize 7 fceds
cacli and the downlink bcams arc formcd using 1 ked
each. Both LDR and MDR channcls are supportcd in the
spot hams. In addition. the payload providcs LDR c h
covcragc senticc through a pair of carth covcragc horns.
The LDR proczssor supports 16 communications channcls
in each of the bcams using thc EHF common transmission
format. Thc MDR proccssor providcs a total of 4 channels
of scrvicc in thc spat bcams with any mix bctwccn
the two beams.
Thc main coasidcrations in sclccling a spot bcam sizc arc
thc rcquircd gain and llrc covcragc arca prtwidcd by the
ham. Thc 1" :o 3" spot bcam sim in this cxamplc pay-
load, along with thc 6 W solid statc transmittcr. will
support 2.4 kbps scrvicc lo a small tcrminal (2'nw) whilc
in thc widc bcam mode and will support 1 Mbps links to
a mcdium si7x tcrminal (4'/12W) whilc in thc narrow
bcam niodc. Thc payload in Fig. 9 is cstimatcd to wcigh
about 200 Ib and rcquire about 290 W (thcsc cstimatcs in-
cludc 20% margins).
Thc 6 W transmittcr and thc 20 spot bcam antcnnas
providc sufficient EIRP to support both LDR and MDR
links in a varicty of modcs and daia ratcs with the total
throughput for thc payload dcpcnding on thc mix of LDR
and MOR bxnirials in a sccnario. An cxainplc loading
scciiario is shown in Fig. IO. For this cxariiplc. thrcc
typcs or tcrminals wcrc assunicd: a 6', 25 W ground
tcrniinal; a 4', 12 W transpnrtablc tcrminal: and a 2', 2 W
portablc tcrniinal. Thc ground tcrminal is supported by thc
carth covcragc bcani in thc cxarnplc. Thc portable and
transportablc tcrniinals arc supportcd in thc spot hcanis.
Chc of thc spot bcmns is sct to a 3" bcainwidth (ahut
1200 milc diarnctcr covcragc at thc subsatcllitc point)
whilc thc othcr :pot bcam is sct to a 1" bcamwidth (about
400 milc dianctcr covcragc ai thc subs:itcllitc point). in
this cxamplc. 27 LDR networks and 17 MDR links arc
supportcd for a total payload throughput of 3277 kbps.
A range of payload capabilities can bc implcmcntd using
the variablc hamwidth aitcnnas. nulling prtxxssors. and
ttic othcr kcy tcchnologics dcscribd bricfly in Fig. 1 and
2. Thcsc tcchnologics can bc uscd to implcnicnt small
EHF payloads as in thc cxamplc prescntcd hcrc. Howcva.
thc samc tcchnologics can also bc uscd in sccondiiry anti-
jam payloads or multiple function anti-jam payloads on
largc satcllitcs as shown in Fig. 11. In addition. many of
thc smc signal processing and frcqucncy gcncration
tcchnologics arc applicablc for improving El IF tcrniinals
as wcll.
7. SUMMAHY
A kcy fcaturc fur thc flcxiblc usc of Ihc EHF payload is to
providc thc ability to configurc the payload to providc a
varicty of scrviccs. Suppming cithcr LDR. MDK. or both
typcs of chonncls in a varinlk. hciunwitlth utitcririn ticllm
provitlc itiis sort of flcxihility to nwct a troiitl raiigc of
uscr rcquircmcnts. Dcvclopmcnt of the critical technologics
for use in thcsc typcs of payloads hiLC bocn initiatcd.
Thc tcchnology arcas includc variable bmwidth antcnnas.
lightwcighl frcqucncy synthcsi7~~i!.,and high spccd signal
processors for both LDR and MDR channcls.