An inter-symbol guard time has been proposed as a means of synchronizing the symbol and slot clocks of an optical pulse-position modulation (PPM) receiver with the symbol and slot periods of an incoming optical PPM signal.The proposal is applicable to the low-flux case in which the receiver photodetector operates in a photon-counting mode and the count can include contributions from incidental light sources and dark current

Far, William

Gin, Jonathan

Quirk, Kevin

Srinivasan, Meera

NASA Technical Reports Server

NASA Tech Briefs, July 2009 15
In a proposed method of determining
the resistances of individual DC electri-
cal devices (e.g., batteries or fuel-cell
stacks containing multiple electrochemi-
cal cells) connected in a series or paral-
lel string, no attempt would be made to
perform direct measurements on indi-
vidual devices. Instead, (1) the devices
would be instrumented by connecting
reactive circuit components in parallel
and/or in series with the devices, as ap-
propriate; (2) a pulse or AC voltage exci-
tation would be applied at a single point
on the string; and (3) the transient or
AC steady-state current response of the
string would be measured at that point
only. Each reactive component(s) associ-
ated with each device would be distinct
in order to associate a unique time-de-
pendent response with that device. 
Using the known time-varying voltage
excitation, the known values of induc-
tance and/or capacitance, and the stan-
dard equation predicting the response
for the known circuit configuration, the
time-varying current response would be
subjected to nonlinear regression analy-
sis. In essence, this analysis would yield in-
dividual device resistances that result in a
best fit between the predicted and actual
time-varying current responses.
This work was done by Dan Hall of Lock-
heed Martin Corp. and Frank Davies of Her-
nandez Engineering, Inc. for Johnson Space
Center. Further information is contained in
a TSP (see page 1).
This invention is owned by NASA, and a
patent application has been filed. Inquiries con-
cerning nonexclusive or exclusive license for its
commercial development should be addressed to
the Patent Counsel, Johnson Space Center,
(281) 483-1003. Refer to MSC-23623-1.
Measuring Multiple Resistances Using Single-Point Excitation
Lyndon B. Johnson Space Center, Houston, Texas
Improved-Band width Trans impedance Amplifier
NASA’s Jet Propulsion Laboratory, Pasadena, California
The widest available operational ampli-
fier, with the best voltage and current
noise characteristics, is considered for
transimpedance amplifier (TIA) applica-
tions where wide bandwidth is required
to handle fast rising input signals (as for
time-of-flight measurement cases). The
added amplifier inside the TIA feedback
loop can be configured to have slightly
lower voltage gain than the bandwidth re-
duction factor (the ratio of the input ca-
pacitance plus the feedback capacitance
to the feed capacitance). This innovation
enables the optimization of design based
on suitable space-approved operational
amplifiers and provides better, stronger
performance under radiation and wide
temperature variations. In many cases,
this approach can eliminate the need to
qualify new amplifiers.
This work was done by Jacob Chapsky of Cal-
tech for NASA’s Jet Propulsion Laboratory. For
more information, contact iaoffice@jpl.nasa.gov.
NPO-45798
VCSEL, designed to operate in the wave-
length range of 795±3 nm, that was gen-
erating about 200 µW of optical power.
(The use of relatively high injection
power levels is a usual practice in injec-
tion locking of VCSELs.)
This work was done by Dmitry Strekalov,
Andrey Matsko, Anatoliy Savchenkov, Nan
Yu, and Lute Maleki of Caltech for NASA’s Jet
Propulsion Laboratory. Further information
is contained in a TSP (see page 1).
NPO-43454
An inter-symbol guard time has been
proposed as a means of synchronizing
the symbol and slot clocks of an optical
pulse-position modulation (PPM) re-
ceiver with the symbol and slot periods of
an incoming optical PPM signal. (Such
synchronization is necessary for correct
identification of received symbols.) The
proposal is applicable to the low-flux case
in which the receiver photodetector op-
erates in a photon-counting mode and
the count can include contributions from
incidental light sources and dark current.
The use of the inter-symbol guard time
would be an alternative to a prior syn-
chronization method based on the peri-
odic transmission of a fixed pilot symbol.
The proposal involves a modification
of conventional M-ary optical PPM, in
which each successive symbol period is di-
vided into M time slots (0, 1, 2, ..., M-1),
each slot being of duration Ts. Each time
slot represents a different symbol in an al-
phabet of up to M symbols. At the trans-
mitter, during each time slot, a laser ei-
ther transmits a pulse or no pulse,
depending on which symbol is to be sent.
Synchronization of the receiver symbol
and slot clocks is necessary because the
task of the receiver is to determine which
of the M possible symbols has been re-
ceived by observing the photon counts
accumulated during each of the M time
slots of a symbol period.
In both the prior method and the
method now proposed, the basic idea is
to estimate the symbol and slot timing
boundaries of the received signal by cor-
relating the received-signal counts with a
known component of the transmitted
signal while taking account of the fact
that the received-signal counts are re-
lated to the received-signal intensity
through a Poisson distribution. In the
prior method, the known component of
Inter-Symbol Guard Time for Synchronizing Optical PPM
This method would involve less computation than does the pilot-symbol method.
NASA’s Jet Propulsion Laboratory, Pasadena, California
https://ntrs.nasa.gov/search.jsp?R=20090027783 2019-08-30T07:31:28+00:00Z
16 NASA Tech Briefs, July 2009
the transmitted signal is the pilot sym-
bol, transmitted in place of an informa-
tion symbol at intervals of R symbol peri-
ods. The pilot symbol is embodied as a
pulse in the M/2th time slot of each af-
fected Rth symbol period. In order to ac-
quire the symbol and slot timing of the
pilot signal, the receiver can correlate
the received signal with variously de-
layed replicas of the pilot signal and
choose the delay offset that yields the
maximum correlation. The number of
different correlations at increments of
Ts is limited to MR; a coarse offset can
thus be identified as whichever of these
offsets yields the greatest correlation.
Once the coarse offset has been deter-
mined, a finer estimate can be made by
use of an early-late method with the ad-
jacent correlation statistics.
In the method now proposed, there
would be no pilot symbol. Instead, succes-
sive symbol periods would be separated
by the aforementioned inter-symbol
guard time, equal to one time slot Ts, dur-
ing which no pulse would be transmitted.
In effect, each symbol period would be di-
vided into M + 1 time slots, of which the
first M would be reserved for data pulses
and the M + 1st, containing no pulse,
would be used for synchronization. In
this method, to acquire the symbol and
slot timing of the received signal, one
could seek either (1) the maximum cor-
relation between the received signal and
a synthetic signal consisting of pulses in
all the data time slots or (2) the mini-
mum correlation between the received
signal and a synthetic signal consisting
only of the inter-symbol guard time. In ei-
ther case, the number of different corre-
lations at increments of Ts would be lim-
ited to M + 1. As in the prior method,
once the coarse offset had been deter-
mined, a finer estimate could be made by
use of an early-late method with the adja-
cent correlation statistics.
Because fewer correlations would be
needed to determine the coarse timing in
the proposed method than are needed in
the prior method, the proposed method
could be implemented in a receiver by
means of simpler, lower-power, less-mas-
sive computational circuitry, which is con-
sidered an acceptable trade-off to the
slightly increased estimation error. An-
other advantage would be that unlike in
the prior method, no energy would be ex-
pended in transmitting pilot symbols.
This work was done by William Far,
Jonathan Gin, and Meera Srinivasan of Cal-
tech and Kevin Quirk of Northrop Grumman
Information Technology for NASA’s Jet
Propulsion Laboratory. For further informa-
tion, cintact iaoffice@jpl.nasa.gov.
NPO-43671


Inter-Symbol Guard Time for Synchronizing Optical PPM

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