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in which P- is tk prabrbility of bit erro~ at the ohput
of Lhe dunaJul~a.
ud P- U Ihe probability of bit km
of a DPSK modulated signal. U he danodulator is
followed by divasity cumbining. he value of P, c8n be
poor, and yet still allow dust performance. For exatnplc
wuming a basic maprity logic combining approach. in
which hops arc divcnifiad he RER at the output of the
diversity canbindon circuitry will be givm by
in which Pav is the probability of bit error at he output of
hc diversity combination cui:hy. Plots of P, versus
EJNo are given in Figure 3. It can be s en that even uith
a modcst diversity of L = 11, thhpt a BER of less than 0.02
CUI be obtained at low E& c:en for P, values a5 poor as
0.7. This PER into a good FIX will give an overall RER
of less than 10'. In an operational system. other more
robust diversity combining techniques may be cmploycd.
4A Unique Word &lectlon
Qe probability of false alarm in the detection of UI
cmmcous preamble is dicuted by the si7z of the sidelobes
of Ihe unique word Unique word. with ideal mlation
propaties such as Barka type codes can only be fourid for
a few shorter length sequences. where ideal comclatib of
8 uniqie word of length M shall is defined as hl for
perfect alignment and +/- 1 elsewhat. By shortenink the
decision window of the unique word and considcring,only
sidelobes within the defined window, it is possible to
attain a higha processing gain for L!C unique word. The
widih of hc window must be long enough howevdr. to
accOmmDdllte nny potential group delay variation and
accumulated clbck &iR.
45 UL T h e Acqulslllon and TracklnR
When the GT initially beginq acquiring. the total :time
uncertsinty is dominated by the e m in the estimate of the
range 10 the satellite ud the error in the position of the
GT. Acquiring the DL timing resolves one of lhese
pu~metac. In order to resolve the second parameter, thz
time at which the UL trrnmwsl * '~ocaumustbedjustai
to uuun lblt the drt. hop urive at the payload wihin
the pooaahg window far Ibs UL bop. Thir is but
achieved by a clmd Imp pnrrea~ m which spaidid synchnmizalibn probts lllt tnnsminbd in baa wrvefam
locations to he paylord Tbc payload detactr the positioa
of tbc hopl relative to the rmnind loutions Md
tnnsmirs spocirlir#l responswto IbsCT tb.1 contain the
requid adj*mt of the GT UL hing in arda to
ensure that the UL hopr trmmniaal by che GT urive at
he payload with th m t limine. As is thecut fa the
DL Liming, Ihe UL probes will coatah Unique words that
xhould be judiciously dmun to alloar iar m qtinial value
of P, and P,. relative to he size of the -eh window for
UL Liming.
in which T- is the UL timing hop mismatch, T, is the
e m in the DL timing estimate. TI is Ihe wcust case pup delay variation. and T, is the residual enor in he UL
timing from the previous UL timc cstimatc The residual
UL arclr T, is no worn t h the ~ a c c v pvidcd by
the come 01 h e DL synchroniution faponsu. For MDR
sysiuns. T, will excad the paid of a DL symbol.
rquiring that the 'JL dah hops have'prembla with
unique words to pcnnit hop-to-hop comlation of the
timing. The size of T- will dictate the size of the
processing window in the payload and the size of guard
bands in the UL hops.
An additional concidaation in the design of an UL timing
and tracking pro~ocol is the placement of the probes in the
UL frune structurc. Robes CUI be assigned on eitha a
channel basis over the entire frame, or on a TDMA time
sla his, spanning some or all of he available channels.
Both of lhesc approaches have advantages and
disadvantagca UL probes assigned on a chand basis will
provide m m Uuoughput to an assigned user, however. the
payload must posscss separate frontcnd processing to
detect pmbcs h m one theam of opcration whk it
processes data channels from a separate heatre of
operaticin. In contrast, if probes arc assigned on a TDMA
time slot basis. the payload can detect probes from one
heatre while rweiving data communications from another
theatre 111 within Ihe same frunc. with no additional
hardware. The disadvantage to this approach is that the
amcunt of probe capacity available to the GT is more
restricted than in the FDMA approach.
5 FREQUENCY ACQUISlTfON
The principal degradation in system performance due to
frequency m will be due to ke accuracy of he hit
timing in the DPSK demodulator of the M DR receiver. ln