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& a time acquisition. Due to thc tactical raquirancnt
that thc MDR EHF systn have a multiple access
capability, it is ~CCUWY for Ihe p~ylod 10 dphcally
assip &U hops and theatres Since a GT in a giv’a
dou no( have knowledge of which hops hive
actual data until Inc rcquisitim has bacn mpletad. bL
synchronization is accompluhcd lhrough che usc of
spccidized synduoruzui~ hops. These hops must ochr
in known locations in Ihc dowrilink datalink structure. DL
aquisition is achkvd by cktating the presence of the
sphrcmizatim bps which vc periodiully transmilted to
each cheatre of opemion. SiKe the detection of Ihew hdps
clrtno~ take place un~ess the antema is pointing at ihe
payropd. he time acquisition procns is perfnmcd
concurrently with the spatial acquisition proccdhc
derribod in scctim 3. As for spatial aquisitinn, the DL
acxpisitia pmcea consists of I coam and fine stage. The
c w ~ Stage e spchmniza Lhc DL timing to within I DL
hop, providing the gmund terminal knowldgc of thc
correct point in thc frqucncy hopping squcnce rclativc to
the GT timc-ofday COD). A sccond fine stagc of DL
time acquisition is mquircd to 8ltain the DL timhg to
within a DPSK chip or spbl period.
It should be notod that the u e of time pcrmulatinn to
combat padd time jamming adds complexity and delay to
he acquisition procxss. The psition of the pcrmutcd
synchronizrtim hops must bc calculated had on thc
estimated TOD and the hopping pattcm. Ihc atklitional
Slay will bc on the orda of the period ovcr which the
permutation takes place. for example a frm.
4.2 Coarsc DL Tlmc Acqulsltlon
Since the DL is in a TDMA format., the SNR in a hop is
rclatcd to the rcquircd symbol SNR by the following
relationship
in which EJN, is the hopencrgy-to-noise-spectraldcnsity
ratio, yN, is thc coded-symbolcnergy-t~noisc-spcclraldenqity
ratio. & is coded symbol rate of a singlc channcl,
q is the number of downlink TDMA slots corrcspnding
to che number of uplink channels, add R, is Ihe hop rate.
For data ccxnmunicatiw the specified valuc of Em, is
fixed by Ihe required bit error rate. and the hop rate is
typically fixed for a given system. Thus as the nunrber of
uplink channels and/or the channel data rate is increased,
the level of E,/No must be incrcascd to maintain thc samc
level of EJN, Since MDR system. mbst support data rate..
up to TI, the rwulting E& it high. A5 M cxarnplc
suppose a MDR syste.~ has 8 channels, rate In coding. a
hop rate of 10 kHz and a TI data rate. For a typical
required system mor rate betwecn 10’ and 1. ‘, the
requhd value of WO is rou~Jy 7 dB. For the values
noted above, Lhe resulhig EJN, will exceed 40 dB.
17-5
Similar values can bc obtained for a junming envirmcnt
Detection of the cxmsc DL synchnmiitim hops can thus
easily bc made bn.& on mgy mcamuanentd. ’he famat
or 111c c:o= ~~hroni;lwtim hops not be cmp~el,
bul should be selected to allow 8 high proba6ility of
detection (PJ, ancl a low Fobability of false alarm (P,).
’I. is is a common radar problem, and numerous
*avefonns can be selectad[4]. A simple example is I
modulated CW tom.
Fine DL lime acquisitich is nuaxwy to obtain the DL
timing to within a symbol period, U well m to rocover the
symbol rate clock. The DL liming ccmacy is dTccted by
the following factors; satellite motion induced dopplcr
effccu. relative clock drift bctwoar the payload rcfcrcnce
clock and the GT clock. group &lay vuiatia introduced
by che channel or the equipment, and the timing jitter
intmduccd by the synchroni7ation process iLsclf. As
discused in suction 2.4.3. the unrcsolvcd dopplcr can be
up to 2 W4z. resulting in a timing variation on the odu of
100 nscdm. Furthermore. the group delay variation can
be on thc ordcr of 40 nut. Given the TDMA nature of the
MDR downlink. and data rates up to TI, the bvst symbol
rate of thr DL will typically be on the order of !O MHz to
100 MH7- dcpeding on the numb 01’ sloc~. In the
cxmplc given above the symbol period would be 40 nw.
Fine timing on the DL can be achieved by .sending fine
synchronizrtim hops consisting of 8 unique word which
can be correlated at thc GT. The numba of finc
synchronizotiin hops and the period bctwan arrival of
fine synchrnni7ation hops is dictatd by magnitude of the
timing mor lhnt a.rumulat~~q betwaen synchroni7adon
hops. The numb of DL synchrcnization hops must be
sufficient to allow for all thcatrcs of operation to be
visitcd. Thc principal sourcc of timing error that will be
rcmovcd by the fine synchronization hops is thc
unpxoived dopplcr error as well as clock drift emr. Once
fine timing has bccn obtained on the DL. he GT must
track thc DL timing. Doppla and clock drift rnw bc
lracked in ordcr Jiat Lhc DL and UL timing of Ihc CT can
be adjusted to compcrwte for these effccts. The timing
jitter induccd by the .gynchroni7ation algorithm is a
function of the complexity of the feedback loop dcsign and
can be separately minimizcd.
4.3 DL Tlme Tracklng
In addition to tracking doppln and clock drift. each DL
data hop must be contpcnsatcd for the elTects of group
dclay */ariation. At the M DR burst symbol ratc, chc group
dclrv variation from one hop to thc next can be air or
niorc symbol pcriods a5:discusscd in soction 4.2. Thus
each data hop must Lc rc.uyrichronized to the DL timing to
ensure that no data symbols arc lost or that Ihe symbol
timing of the nccived symbols is not misrligncd. This can
be achicvd by encoding a known pream!)le at Ihc start of
mch data hop. The GT will correlate ‘the rcccivcd
preamble against thc transmitted pmamble and align the
liming to within half a symbol paid 9 dctccting the
peak of the correlation process. The length of he unique
word selected for the correlation process is a function of