Modulation and coding research and development at the Jet Propulsion Laboratory (JPL) currently emphasize Deep Space Communications Systems and advanced near earth Commercial Satellite Communications Systems. The Deep Space Communication channel is extremely signal to noise ratio limited and has long transmission delay. The near earth satellite channel is bandwidth limited with fading and multipath. Recent code search efforts at JPL have found a long constraint, low rate convolutional code (15, 1/6) which, when concatenated with a ten bit Reed-Solomon (RS) code, provides a 2.1 dB gain over that of the Voyager spacecraft - the current standard. The new code is only 2 dB from the theoretical Shannon limit. A flight qualified version of the (15, 1/6) convolutional encoder was implemented on the Galileo Spacecraft to be launched later this year. An L-band mobile link, use of the Ka-band for personal communications, and the development of subsystem technology for the interconnection of satellite resources by using high rate optical inter-satellite links are noted

Rafferty, W.

Yuen, J. H.

English

NASA Technical Reports Server

N92-22016
MODULATION AND CODING TECHNOLOGY
FOR DEEP SPACE AND SATELLITE APPLICATIONS
(Invited Paper)
J. H. Yuen and W. Rafferty
Jet Propulsion Laboratory
California Institute of Technology
Pasadena, CA 91109
Abstract
Modulation and coding research and development activities at the
Jet Propulsion Laboratory currently emphasize the following two
areas: Deep Space Communications Systems and advanced near-earth
Commercial Satellite Communications Systems. The Deep Space
Communication channel is extremely signal-to-noise ratio limited
and has long transmission delay. The near-earth (GEO and LEO)
satellite channel is bandwidth limited with fading and multipath.
Recent code-search efforts at JPL have found a long constraint,
low rate convolutional code (15, 1/6) which, when concatenated
with a i0 bit Reed-Solomon (RS) code provides a 2.1 dB gain over
that of the Voyager Spacecraft - the current standard. The new
JPL code is only 2 dB from the theoretical Shannon limit. A
flight qualified version of the (15, 1/6) convolutional encoder
has been implemented on the Galileo Spacecraft - to be launched
later this year. This will result in increased data return from
Jupiter in the 1990s. A decoder for this class of codes is under
development at JPL using parallel processing algorithms and VLSI
technology. An Image Statistics Decoder (ISD) has been developed,
which uses the source statistics of the image (or picture) to
modify the standard Viterbi decoding algorithm in decoding
convolutionally encoded Voyager images. This ISD, which provides
as much as 3 dB coding gain in the region of interest, will be
used as a backup decoder for Voyager's Neptune Encounter in August
1989. Other JPL activities in modulation and coding for deep
space applications will also be discussed.
NASA has played a leading role in the development of satellite
based, fixed and mobile communications for the U.S. with the work
at JPL focused on the L-band, mobile link. This link has
necessitated the development of a new highly bandwidth and power
efficient digital modem. A unique 4.8 kbps, rate 2/3 8 DPSK
Trellis Coded Modulation (TCM) scheme has been derived and
implemented which is robust in the presence of Rician fading, and
doppler shifts up to 10% of the transmitted symbol rate (2.4 ksps)
for a basic 5 kHz channel width. A compatible 4.8 kbps speech
compressor has also been developed which achieves good
intelligibility, and sound "natural" at a low implementation
complexity. New JPL activities in the Satcom area include:
meeting personal communications needs at the turn of the 21st
Century, by exploiting Ka-band; and developing the subsystem
technology for the interconnection of satellite resources by using
high rate optical inter-satellite links.
245
https://ntrs.nasa.gov/search.jsp?R=19920012773 2020-03-17T13:13:58+00:00Z
JET PROPULSION LABORATORY
DEEP SPACE COMMUNICATIONS
#_.____ 70m ANTENNA & ARRAYLOW NOISE RECEIVE
HIGH POWER TRANSMITTER
smSPACECRAFrANTENNA _ _ !1
LOW POWER TRANSMITTER
• TELEMETRY IS THE PROBLEM
• HIGH DATA RATES FOR IMAGING
• LOW SNRs
• CODING IS CRITICAL TO TELEMETRY
• ENCODERS MUST BE SMALL, LIGHT, LOW POWER
• DECODERS TEND TO BE VERY COMPLEX
246
JET PROPULSION LABORATORY
THE 2 dB CODE SEARCH
OA'rA " i"CONVO'"'SO" 1 'IT 
--ISOLOMON ',,-ILUTIONAt_:_"'_,I CARRIER CARRIER XMITI
BITSIN/ENCOOER | |ENCOOER | I MOO. | " MOD. |
"_o _'- SPACECHANNEL "
I I_ ,_, q, |('sUo._:_, i SYMBOL", I VOTERBI, I REED- ; DECODED
_----! RE I CARR,ER'_,'.. _"I OECO.ER. ISOLO"O" OATACEIVER | I DEMOD., | SYNCH. | | |DECODER BITS
CONCATENATED CODING SYSTEM BLOCK DIAGRAM
• BASELINE PERFORMANCE: BER = 104
• BASELINE CODE: VOYAGER AT URANUS
(7, 1/2) + 8-bitRS Required EJN o = 2.53 dB
• SIMULATED PERFORMANCE OF CONCATENATED CODES WITH 10-bit REED-SOLOMON
OUTER CODE
INNER CONVOLUTIONAL CODES EJN o
(13, 1/4) 0.84 dB
t13, 1/5) 0.68 dB
(14, 1/4) 0.74 dB
t14, 1/5) 0.57 dB
(15, 1/4) 0.80 dB
(15, 1/5) 0.50 dB
(14, 1/6) 0.47 dB
-15;.:1/6)_ i.i ,. .: 0.42 dB I = 2.11 dBGain
We search for codes that will provide significant improvement, say
2 dB, over our Voyager baseline system (7, I/2)convolutlonal
code as the inner code (with Viterbl decoding2 and an 8-bit (255,
223) Reed-Solomon code as the outer code. We use the criterion of
minimizing required bit SNR, Eor a given value of desired BER, for
the goodness of code. The code space is astronomically large for
long constraint length low rate convolutional codes. Using
educated guesses combined with the idea that good codes generate
good codes, we selectlvely search for good codes. These codes
performance are determined by computer simulation. The decoder
complexity is manageable by using concurrent processing technique
and VLSI technology.
247
JET PROPULSION LABORATORY
IMAGE STATISTICS DECODER (ISD)
INFORMATION SYMBOLq (_|, b_ ..... On)
CODED SYMBdDLR {||* $Z ..... Sffi)
_CEZVSD SYNaOZ,S (r I , r z ..... rm)
0j . Ibj-bj.tl
g Z FUNCTION OF BIT SNR
ISD MINIMIZES _ (rl"_'_I)2-_ %n P(bjIDj>
! a
MINIMIZE THIS TERM
ONLY, IF INFORMATION BYTES
ARE ASSUMED INDEPENDENT
I_1 ! . r [ ', ! , i i • _ i •
on
II
U
e'
1.4
ILl
&4
&!
l ,L "
I • O 8 W o| 14 14 IO RO" N! _l
0
01strlbuttoet of Add*cent Pl=ll
Virlitlon 0 for ]Ut fforont
Voyager I==g,s lnd the rin_ case
101
10"z
_ 10 _
W
10_
10"*
104
ISD PERFORMANCE
Image Byte Errors as Compared to Data Compression
' I l i i i [ _ f ' t
I ..... \
i \1\ '°
_,_ _,_
.I _ 3 4 5 6 7 8
E,JNo
30B GAIN
Maximum likelihood (or Viterbi) assumes that all codewords are a
priori equally llkely to be transmitted, this decoding scheme
retrieves the most likely sent codeword. In some cases, though,
codewords are not all equally likely to be transmitted. In
Voyager images, for example, pixel to pixel variations are not
completely random. They are much more likely to be small than
large. In this case, a decoder which makes use of the source
statistics should perform better than a Viterbi decoder. (Image
compression uses these statistics to lower the transmission rate
and thus raise symbol SNR, but some Voyager images are sent
uncompressed because of spacecraft limitations; also, an
alternative would be valuable in the unlikely event of a data
compressor failure before Neptune encounter in 1989.) This is
exactly what our ISD does. It amounts to an additional term to
the usual Viterbi decoder.
248
JET PROPULSION LABORATORY
DE CODER
• Objectives:
v' Develop, Build, test, and demonstrate a prototype Viterbr decoder for
the DSN capable of using (15,1/6) convolutional codes, leading
to flight tests with Galileo's (15,1/4) encoder
• Requirements:
v' Performance:
- Constraint length up to 15, programmable
- Code rate 1/2 to 1/6, programmable
- Data rate 1.1 Mbit/s (Galileo 115, 134 Kb/s)
- Node synch using frame synch pattern, programmable
- Node synch using metric growth rate
SSA._aA
- Include full serf-test capability
v' DSN Compatible
TCT New TPA
l i
_=_,'' -C°mputer!M°nit°r/C°ntr°l .... I
Node Synch !Eat¢rall
_,_,, ., ._,=mb,),I• 41- --
A T
Comparator 1
i Simulator _ )" As_mbly /
J
The major complexity driver of the decoder is constraint length,
since the amount of hardware is roughly proportlonal to the number
of states which is 2 to the (K-l), where K is the constraint
length. Hence, a decoder for K = 15 is approximately 256 times
more complex than a decoder for K = 7. Using concurrent
processing techniques, such a complex decoder can be built, with
current VLSI technology, within reasonable size limitations. The
decoder is under development. It will be ready in 1991, before
Galileo reaches Jupiter in 1995.
249
JET PROPULSION LABORATORY
MODULATION & CODING FOR SATCOM APPLICATIONS
MOBILE PERSONAL
COMMUmCATIONS COMMUmCATIONS
IIII • :,
+: - + _ +_.::::2i!:?:"_
MODULATION & CODING ARE CRITICAL TO
EFFICIENT USE OF SATELLITE RESOURCES
• BANDWIDTH
• POWER
• ORBITAL SLOT
25O
JET PROPULSION LABORATORY
Performance of Galileo Codes
The new encoder ....
26 resistors, 39 capacitors, 20 ICs
_1-:1 I.l_Jl t_-lJ I'l-I I I'1"_,_]
[_lS_Jl.i i _ T] JL I,][ I JLX,I, I t Z - Z_
_..JI_IZI 1 I'lr.zI,ILI.Z.,ILZlr]L -" ._
\
10-=- ,
n-
uJ
m,m10-3.
n-
_ 10"4.
LU
°_
,_ 10.5 .
10" I
-1
%
1 3 5 7 9
Bit SNR (Eb/No), dB
Due to the Space Shuttle Challenger's accident, the launch of the
Galileo spacecraft to orbit Jupiter (and to drop a probe into the
atmosphere of Jupiter) was delayed. The delay wlll require new
trajectory that makes the communication distance from Galileo to
Earth much longer than the original trajectory. We install a
(15, 1/4) convolutlonal code on Galileo -- the RS code remains to
be 8-bit (255, 223). This gives about 1.5 dB over the original
(7, 1/2) code. This provides significant performance improvement
with minimal impact on the existing Galileo design.
251
JilL
JET PROPULSION LABORATORY
4800 BPS 8DPSK TCM MODEM
MODULATOR
-- _
"8OO n_E LL_ ENCCX3_R OlFF[BENTI_t y
8PS ............ S MSCt T
O*rA _frERLF_VER Trl_LS_ rITE R
to o
Pb lo 2
DEMODULATOR
10 3
SIMULATION RESULTS
_°_ER SPR F-_O. 4O Hz
_2a s Yu B_.
dNTE RLE,_ViNG
K ° _0 dB
7 8 9 10 1_ 12
E_
THE MSAT-X 8DPSK TCM MODEM IS A POWER AND BANDWIDTH EFFICIENT,
NARROWBAND MODEM DESIGNED TO COMBAT THE PROPAGATION EFFECTS
OF THE L-BAND LAND-MOBILE SATELLITE CHANNEL. THE TYPICAL CHANNEL
IMPAIRMENTS CONSIST OF MULTIPATH (RICIAN) FADING AND VEGETATIVE
SHADOWING. THE MODULATOR TRELLIS ENCODES, INTERLEAVES, AND
DIFFERENTIALLY ENCODES THE INPUT DATA BEFORE CONVERTING THE
DATA TO PULSE SHAPED I AND Q WAVEFORMS FOR TRANSMISSION THROUGH
THE CHANNEL. THE DEMODULATOR IS IMPLEMENTED WITH A FEEDFORWARD
ARCHITECTURE TO COMBAT THE EFFECTS OF FADES, AND EMPLOYS
DIFFERENTIAL DETECTION USING A NOVEL MATCHED FILTERING AND
DOPPLER ESTIMATION/CORRECTION ALGORITHM TO RECOVER THE
TRANSMITTED DATA. THE DATA IS THEN DEINTERLEAVED AND DECODED
TO PRODUCE AN ESTIMATE OF THE BASEBAND DIGITAL DATA. EXPERIMENTAL
RESULTS HAVE SHOWN THAT THE CURRENT IMPLEMENTATION OF THE
MODEM PERFORMS WITHIN 1 dB OF SIMULATION RESULTS.
252
JET PROPULSION LABORATORY
JPL 4.8 kbps DIGITAL SPEECH COMPRESSION
VECTOR ADAPTIVE PREDICTIVE CODING
UC-SANTA BARBARA
T TT l
o it_,__ _ %r_(>HT i I LONG
. PNE [)1_ TO,q_ j-- L P_ED_CTOR J
SH_T DECODER
p_f blc tc)_
EDICTON
ENCODER
UCSB VECTOR ADAPTIVE PREDICTIVE CODING
THE VAPC ALGORITHM COMBINES LINEAR PREDICTION WITH VECTOR QUANTIZATION.
ADAPTIVE LINEAR PREDICTORS ARE UTILIZED TO REMOVE REDUNDANCY FROM THE
SPEECH WAVEFORMS. THE REMAINING PREDICTION ERROR IS THEN QUANTIZED BY
EMPLOYING VECTOR QUANTIZATION. THE USE OF VECTOR QUANTIZATION ALLOWS
THE CODING OF THE ERROR SIGNAL AT RATES BELOW ONE BIT PER SAMPLE - AN
ESSENTIAL CHARACTERISTIC FOR LOW BIT RATE COMPRESSION.
THE ENCODING PROCESS BEGINS WITH A LONG DELAY PREDICTOR TO REMOVE THE
REDUNDANCY CORRESPONDING TO THE PITCH STRUCTURE OF SPEECH. A SHORT
DELAY PREDICTOR FOLLOWS TO REMOVE INFORMATION ROUGHLY CORRESPONDING
TO THE FORMANT STRUCTURE OF SPEECH. THE REMAINING ERROR SIGNAL IS
QUANTIZED AS TIME SEQUENCES OR VECTORS BY EXHAUSTIVELY COMPARING
THE ERROR VECTORS TO STORED VECTORS AND CHOOSING THE STORED VALUES
THAT MINIMIZE THE MEAN SQUARED ERROR.
253
JET PROPULSION LABORATORY
MULTIPLE SYMBOL DIFFERENTIAL
DETECTION OF MPSK
MULTIPLE BIT DETECTOR
N=3
\\
N
BIT SNR
8 DPSK PERFORMANCE
N = 2, 3, 5, _ (COHERENT)
CONVENTIONAL DIFFERENTIAL DETECTION OF MPSK (MDPSK) USES THE PREVIOUS SYMBOL AS
A DEMODULATION REFERENCE FOR THE CURRENT SYMBOL. THE ERROR PROBABILITY
PERFORMANCE OF BINARY DPSK VARIES AS EXP(-Es/No).
MULTIPLE SYMBOL DIFFERENTIAL DETECTION OF MPSK OBSERVES THE RECEIVED SIGNAL
PLUS NOISE OVER N (MORE THAN TWO) SYMBOL INTERVALS. A MAXIMUM-LIKELIHOOD
SEOUENCE ESTIMATION ALGORITHM IS USED TO DETECT THE CURRENT SYMBOL USING ALL
OF THE PREVIOUS SYMBOLS. IT REQUIRES IDENTICAL DIFFERENTIAL ENCODING OF THE
INPUT DATA PHASES AS FOR CONVENTIONAL (N=2) DIFFERENTIAL DETECTION. THE ERROR
PROBABILITY VARIES BETWEEN THAT FOR CONVENTIONAL DIFFERENTIAL DETECTION AND
COHERENT DETECTION. IN THE LIMIT OF INFINITE SYMBOL OBSERVATION, THE ERROR
PROBABILITY BECOMES IDENTICAL TO THAT OF COHERENT DETECTION OF MPSK WITH
DIFFERENTIALLY ENCODED INPUT PHASES. IN PRACTICE, WITH ONLY A FEW ADDITIONAL
OBSERVATION INTERVALS, ONE CAN APPROACH COHERENT DETECTION PERFORMANCE.
254
JPL JET PROPULSION LABORATORY
ADVANCED OPTICAL MODULATION
D
o,(
FREQUENCY STABILIZED LASER WITH
TEMPERATURE AND PIEZO-ELECTRIC
TUNING CAPABILITIES
SPAC_ SI,&CL _i
L L4NP(
/ :
_ATA
TO DATA
DEMODULATOR
)
BLOCK DIAGRAM OF A COHERENT
OPTICAL LINK
THIS WORK IS FOCUSED ON THE KEY TECHNOLOGIES REQUIRED FOR THE
DEVELOPMENT OF COHERENT FREE SPACE OPTICAL COMMUNICATIONS. RECENT
DEVELOPMENTS OF FREQUENCY-STABILIZED SOLID STATE LASERS WILL PERMIT THE
REALIZATION OF PHASE COHERENT FREE-SPACE OPTICAL COMMUNICATION SYSTEMS
PREVIOUSLY NOT ACHIEVABLE WITH SEMICONDUCTOR LASERS. POTENTIAL DATA
MODULATION SCHEMES FOR COHERENT OPTICAL LINKS INCLUDE PULSE POSITION
MODULATION, FREQUENCY SHIFT KEYING AND PHASE SHIFT KEYING. CURRENTLY,
WORK IS UNDERWAY FOR A LOW DATA RATE PHASE COHERENT SYSTEM
DEMONSTRATION USING BINARY PPM. THE EVENTUAL GOAL IS TO DEVELOP THE
TECHNOLOGY AND SYSTEM ARCHITECTURE SUITABLE FOR HIGH DATA RATE PSK
SYSTEMS.
255
00
0
JET PROPULSION LABORATORY
OTHER ACTIVITIES
CPM/TRELLIS ENCODING
0 MOBILE SATELLITE APPLICATIONS
0 OFFERS GOOD POWER PERFORMANCE AND SPECTRAL EFFICIENCY
CODING DIVERSITY FOR SOUND BROADCASTING SATELLITE (SBSAT)
APPLICATIONS
0 TRELLIS CODING COMBINED WITH MULTIPLE SYMBOL DIFFERENTIAL
DETECTION OF MPSK HAS ADVANTAGES OVER CONVENTIONAL
DIFFERENTIAL DETECTION OF CONVOLUTIONALLY ENCODED HPSK
0 ALLOWS MORE CHANNELS TO BE ACCOMMODATED WITHIN A GIVEN
BANDWIDTH AND AVAILABLE SATELLITE POWER
0 ENABLES DIVERSITY RECEPTION
VARIABLE RATE MODEM FOR HIGH FREQUENCY BAND PERSONAL ACCESS
SATELLITE SYSTEM (PASS)
0 A PRACTICAL WAY TO COMBAT RAIN ATTENUATION FOR HIGH
FREOUENCY COMMUNICATIONS CHANNEL
CONTINUOUS PHASE MODULATION (CPM) HAS A CONSTANT ENVELOPE AND GOOD SPECTRAL
PROPERTIES, I.E., LOW OUT-OF-BAND POWER. CPM COUPLED WITH TRELLIS ENCODING CAN
EFFICIENTLY UTILIZE THE AVAILABLE SATELLITE POWER AND BANDWIDTH, BOTH OF WHICH
ARE PRECIOUS FOR A MOBILE SATELLITE SYSTEM.
SOUND BROADCASTING SATELLITE (SBSAT) CHANNELS SUFFER FREOUENCY AND TIME
SELECTIVE FADING, WHICH CAN BE OVERCOME THROUGH CHANNEL DIVERSITY, I.E., BY
PROVIDING A LARGE NUMBER OF CHANNELS THAT USERS CAN SELECTIVELY TUNE IN.
TRELLIS CODING COMBINED WITH MULTIPLE SYMBOL DIFFERENTIAL DETECTION OF MPSK
HAS BETTER SPECTRAL EFFICIENCY AND POWER PERFORMANCE THAN CONVENTIONAL
DIFFERENTIAL DETECTION OF CONVOLUTIONALLY ENCODED MPSK, HENCE MORE
CHANNELS OR SELECTION FOR A GIVEN BANDWIDTH AND SATELLITE POWER.
A PERSONAL ACCESS SATELLITE SYSTEM (PASS) OPERATING AT KA-BAND WOULD PROVIDE A
DIVERSITY OF SERVICES TO USERS IN CONUS USING SMALL HAND-HELD TERMINALS. TO
COMBAT THE SEVERE RAIN ATTENUATION AT KA BAND, A VARIABLE RATE MODEM (.I- 4.8
KBPS) IS BEING INVESTIGATED. THIS MODEM CAN AUTOMATICALLY DETECT AND/OR
INITIATE THE CHANGE OF DATA RATE WITHOUT NETWORK COORDINATION.
256


Modulation and coding technology for deep space and satellite applications

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server