Studies on the digital communication system for the direct communication links from ground to space shuttle and the links involving the Tracking and Data Relay Satellite (TDRS). Three main tasks were performed:(1) Channel encoding/decoding parameter optimization for forward and reverse TDRS links,(2)integration of command encoding/decoding and channel encoding/decoding; and (3) modulation coding interface study. The general communication environment is presented to provide the necessary background for the tasks and to provide an understanding of the implications of the results of the studies

Huth, G. K.

English

NASA Technical Reports Server

Axiomatix 
(NASA-CE-134183) INTEGBATED S O U B C E  A N D  N74-15845 CHANNEL ENCODED DIGITAL CCBMUIICATIOWE 
SYSTEll D E S I G N  STUDY Final Report 
18 p HC 0 3 . 0 0  CSCL 17B G3/07 28797 
( A x i o s a t i x ,  Earina d e l  b e v ,  Cal i f . )  
Unclas 
Marina del Rey California 
https://ntrs.nasa.gov/search.jsp?R=19740007732 2020-03-23T12:40:59+00:00Z
INTEGRATED SOURCE AND CHANNEL 
ENCODED DIGITAL COMMUNICATIONS 
SYSTEM DESIGN STUDY 
FINAL REPORT 
Contract No. : NAS 9-13467 
Prepared by 
Caylord K .  Huth 
Ax i oma tix 
13900 Panay Way, Suite 1 1  OM 
Marina del Rey,  California 90291 
Prepared for 
Lyndon B .  Johnson Space Center 
Houston, Texas 77058 
Axiomatix Report No. R7.101-2 
25 January 1974 
TABLE OF CONTENTS 
Pace -
I .  INTRODUCTION. . . . . . . . . . . . . . . . . . . .  1 
> 11. SPACE SHUTTLE COMMUNICATION ENVIRONMENT . . .  L 
111. SUMMARY O F  VITERBI DECODER PERFORMANCE 
AND COMPLEXITY . . . . . . . . . . . . . . . . . .  b 
IV. SUMMARY OF SYMBOL SYNCHRONIZER STCDY FOR 
SHUTTLE COMMUNICATIONS. . . . . . . . . . . . . .  9 
V.  SUMMARY OF COMMAND CHANNEL CODING 
PERFORMANCE AND COMPLEXITY. . . . . . . . . . .  1 1  
VI. SUMMARY OF ORBITER TO PAYLOAD COMMAXD 
PERFORMANCE WITH A SPLIT PHASE SYMBOL 
SYNCHRONIZER. . . . . . . . . . . . . . . . . . . .  12 
VII. AREAS FOR FURTHER STUDY . . . . . . . . . . . . .  13 
REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . .  1 5  
i i  
I. INTRODUCTION 
This report summarizes the studies of the digital communication 
system for the direct communication links from ground to Space Shuttle 
and the links involving the Tracking and Data Relay Satellite (TDRS). 
main tasks were performed: 
mization for Forward and Reverse TDRS Links, 2) Integration of Command 
Encoding/Decoding and Channel Encoding/Decoding, and 3) Kodulation-Coding 
Interface Study. Section II presents the general communication environment 
to provide the necessary background for the tasks and to provide an under- 
standing of the implications of the results of the studies. A summary of the 
r e s d t s  of Task 1 is presented in Section I11 with further details in Reference 1. 
Task 2 is summarized in Section V with Reference 3 providing the detailed 
rezults of the study. 
with further details in References 2 and 4. 
Space Shuttle communicatioa system, a number of additional studies that 
need to be performed are identified and presented in Section VII. 
Three 
1) Channel Encoding/Decoding Parameter Opti - 
Finally, Task 3 i s  summarized in Sections IV and VI, 
As a result of the studies of t h e  
1 
I I. SPACE SHUTTLE COMMUNICATION ENVIRONMENT 
The Tracking and Data Relay Satellite !TDRS) system i s  designed to 
provide tracking, data. teltmentrg, command, and voice relay services to 
several classes of u s e r  satellites, including the space shuttle. 
system will provide increased communication coverage over the existing STDN 
( space flight tracking and data network). In addition, the use of the TDRS 
system will provide NASA with a potential reduction in network operating 
cost by deactivating several of the existing STDN ground stations. 
being proposed consists of two relay satellites located 130 degrees apart in 
longitude and a single ground station located within the continental United 
States. This arrangement will provide about 85 percent coverage for low- 
altitude (160 kilometer) spacecraft as opposed to the average coverage of 
about 27 percent provided by the existing STDN. 
This satellite 
The system 
The advantages of the TDRS system are not without accompanying 
disadvantages in terms of the implementation of the communication equip- 
ment on the spacecraft. 
craft and the STDN ground stations are very strong, even though the spacecraft 
employs low-power transmitters, high-noise receivers, and omnidirectional 
antennas. However, the path length of the link between a low-altitude manned 
spacecraft and the TDRS is significantly longer, and the TDRS h a s  much less 
capability than t h e  STDN ground station, 
links involving t h e  spacecraft and the TDRS are much weaker than the direct 
links between the spacecraft and the ground stations. 
Typically, t h e  links betweeen a low-altitude space- 
Therefore, the communication 
For these links involving 
2 
t- the TDRS, it is extremely important to design i n  efficient communication 
system which minimizes the transmitting and receiving system losses. 
A highly efficient S-band communication system is being studied 
for the communication links involving the spacecraft and t h e  TDRS. An 
all-digital signal design i s  employed because it offers a number of advantages 
for transmission of voice, data, and commands over the more conventional 
analog techniques. First, digital speech with source coding such as variable- 
slope delta modulation is an efficient technique for  voice transmission. Second, 
with digital speech, efricient time-division multiplexing of the speech, data, 
and commands i s  possible. Finally, 'forward error correction coding such 
as convolutional encoding with Viterbi decoding is possible to significantly 
reduce the power requirements and improve the link efficiency. 
The variable-slope delta modulation is chosen for the source coding 
of the voice because of excellent performance at relatively low sampling 
rates, wide dynamic range, relative insensitivity to channel bit e r r o r s  and 
design simplicity. 
to.be 32 kbps. 
The data rate for the digital voice channels is assumed 
The voice quality at this data rate is particularly good for 
a bit e r ro r  probability of le38 than 
Two 32 kbps digital voice channels a r e  time-division multiplexed 
prior to transmission with a 6 . 4  kbps encoded command channel and a 1 .6  
kbps synchronization channel resulting in a data rate of 72 kbps. Discretion 
was used in choosing the data rates of these channel8 to provide significant 
simplifications in both spaceborne and ground station equipment. 
3 
Convolutional encoding with 3 -bit soft decision Viterbi decoding is 
selected for the all-digital system under consideration because of the very 
favorable tradeoff between performance gain and hardware complexity. A 
code rate of 1 /3  and constraint length of K = 7 is  used for transmission of 
voice, data, and commands. 
The S-band transmission system described in the preceding para- 
graphs for the forward link is summarized in figure 2.1. The multiplexed 
data rate i s  72 kbps, with the data rate out of the convolutional encoder equal 
to 216 kbps. 
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111. SUMMARY OF VITERBI DECODER PERFORMANCE 
AND COMPLEXITY 
Viterbi decoding of a convolutional code is employed in the Space 
Shuttle digital communication system to increaee the efficiency of the com- 
munication link. Reference 1 presents the performance, complexity, and 
availability of Viterbi decoder hardware. 
Convolutional encoding with 3 -bit soft-decision Viterbi decoding is 
selected for the all-digital system under consideration because of the very 
favorable tradeoff between performance gain and hardware complexity. A 
code rate of 1 / 3  and constraint length K=7 is selected to be used for trans- 
mission of voice, data, and commands. 
The performance of t h e  Viterbi decoder was predicted using analytical 
techniques. 
(i.  e . ,  within 0.1 dB at 
simple to compute. 
single-sided noise spectral density (Eb/No) of 3.3 dB is required to achieve 
These analytical performance estimates are extremely accurate 
probability of e r r o r  per bit) and a r e  also extremelv 
Using these techniques, it was found t h a t  a bit energy per 
a probability of e r ro r  per bit of 
of 1 / 3  and constraint length K=7 convolutional code. 
agrees with computer sirnulatima.* 
the Viterbi decoder since 5 . 2  $8 l e r s  Eb/NO is required with the Viterbi 
decoder than with unc3ded PSK. Alternately, with a Viterbi decoder of 
with a Viterbi decoder for a code rate 
This analytical result 
A performance gain is achieved using 
"B. H. Batson, R, W. Moorehead, and S. 2. H. Taqui, "Simulation Results 
for the Viterbi Decoding Algorithm, '' NASA Technical Report R-396, Wash- 
ington, D. C .  , November 1972 .  
6 
equal complexity for a code rate 1 / 2  and constraint length K=?, i t  is found 
that = 3.6  dB is required to achieve a bit probability of e r r o r  equal 
to There results also have been verified by computer simulations.* 
Hence, the Viterbi decoder for the rate 1/3  code provides an additional 0.4 
dB performance gain over the rate 1 / 2  code. It has been found that this 0.4 
dB is achievable since the additional degradation in the symbol synchronizer 
i s  negligible due to decreasing the rate f rom 112 to 1 / 3  (i.e., increasing the 
signaling rate from 144 kbps to 216 kbps). However, it should be noted that 
the analytical performance estimates indicate that, by increasing the c Bm- 
plexity of the rate 1 / 2  Viterbi decoder by increasing the path memory length 
from 32 to 40, decreases the required Eb/No by 0.1 dB. 
not available for the rate 1 / 3  Viterbi decoder. 
decoder provides only a 0.3 dB performance gain over the more complex rate 
1 / 2  Viterbi decoder. However, th i s  result should be verified wi th  computer 
simulations. 
This decrease is 
Thus, the rate 1 / 3  Viterbi 
Reference 1 presents the results of a survey of the available Viterbi 
decoder hardware. While it was found that there were no space-qualified o r  
flight models of the Viterbi decoder, two companies (Linkabit and Radiation, 
Inc. ) have operational Viterbi decoders for ground equipment. In addition, 
a number of companies have made paper designs and built breadboard versions. 
Predominantly, ,code rate 1 / 2  Viterbi decoders have been built, but all the 
companies surveyed agreed that a rate 1 / 3  Viterbi decoder represents no 
a;c J. A. Heller and 1. M. Jacobs, "Viterbi Decoding for Satellite and Space 
Communication, 
COM-19, No. 5 ,  October 1971, pp. 835-848. 
SEEE Transaction8 on Communication8 Techniques , Vol. 
7 
development difference8 than rate 1/2 and a similar complexity may be 
expected (i. e. 
ment in the near future). Therefore, since a development is required for 
space-quallf . 
80-100 IC# and posttibly 1ee1~. depending on the logic develop- 
I rht equipment, there aeems to be no disadvantage in 
* selecting tale rate 1 ,  Vitarbi decoder for the Space Shuttle, 
8 
IV .  SUMMARY OF SYMBOL SYNCHRONIZER STUDY 
FOR SHUTTLE COMMUNICATIONS 
The Space Shuttle digital communication system was designed to 
have 8 large coding gain which results in a very amall ratio of energy per 
rymb01 to single-sided noise spectral density (E, /NO) available for symbol 
synchronization. 
eynchronizer must introduce negligible degradation. 
Ye t ,  to achieve the expected coding gains, the symbol 
Three fundamental types of symbol synchronizer loops have been 
analyzed in Reference 2. 
t he  early-late gate synchronizer, and the data transition tracking synchronizer 
The synchronizer analysis concluded that the early-late g t te  slightly outper- 
forms the other two synchronizers for the expected doppler of 5 5  kHz and 
doppler rate of 4.1 kHz per seton*: at E,/No = -5  dB. However, the data 
transition tracking synchronizer performs nearly as well and is simpler to 
implement. It H' 
was not significantijl greater for a rate 1 /3  convolutional code than for a 
rate 1/2  code. 
These loop6 are the sample function synchronizer, 
,o found that the degradation of a symbol synchronizer 
The loop bandwidth required for  the early-late gate sy..chronizer to 
schieve negligible ryr tem performance degradation is 64 Hz while the other 
loops'invcstigated need a bandwidth of 15 Hz for the same degradation. 
the early-late gate would acquire symbol synchronization faster than the 
other loopr. 
with their optimized tracking loop bandwidths can acquire in a reasonable 
time for the conditions of E,/No = - 5  dB and the expected doppler and doppler 
Thus, 
But the acquirition analysis show8 that none of the synchronizers 
9 
rate. It is shown, however, that acquirition ir porsible in 0.05 seconds 
for these conditions if a separate asquirition device i s  employed. Thus, 
either the acquisition mumt be performed by a reparate device and handed 
over to the tracking loop or a wider I w n  bandwidth must be used during 
acquisition which i s  narrowed for tracking. 
10 
V. SUMMARY OF COMMAND CHANNEL CODING PERFORMANCE 
AND COMPLEXITY 
A simply implemented comma;d encoding/decoding scheme for space 
shuttle application is presented in Reference 3. 
scheme was designed to  be used on weak (data relay) links employing Viterbi 
decoding for e r ror  correction and possiLiy COMSEC equipment for security. 
Also, the command coding can be used alone on strong (direct) links with 
possibly COMSEC equipment for security. 
equipment, while providing improved bit e r ro r  rate and security, respectively, 
result in a deleterious burst e r ro r  environment for the command coding 
scheme. Reference 3 describes in detail the environment, a simple command 
decoder implementatian, and the performance of the command coding scheme. 
Specifically, the coding 
The Viterbi decoding and COMSEC 
The burst e r ror  statistics of the (K = 7, R = 1/3) Viterbi decoder 
and the e r ro r  extension phenomenon of COMSEC equipment were investigated. 
A command channel encoder and decc.der were designed to be compatible 
with the Shuttle multiplexing format rnd the presence of the Viterbi decoder 
and the COMSEC equipment. 
and decoder was very simple, consisting of a 50 stage feedback shift register. 
The resulting decoder more than met the specification for the probability oi 
acceptance of an incorrect command to be less than 10'l8. Also, the speci- 
fication of a probability 01 command rejection of 
dB at the Viterbi decoder when no COMSEC unit i s  uaed and with Eb/No = 4 . 2  
dB when the COMSEC unit is used. When the COMSEC unit is used alone, a 
probability of error  equal to  10'' or  less i s  required at  the input to the COMSEC 
unit in order to meet the loo2  probability of command rejection requirement. 
The implementation of the command encoder 
is  met with Eb/No = 3.4  
i l  
VI. SUMMARY OF ORBITER TO PAYLOAD COMMAND PERFORMANCE 
WITH A SPLIT PHASE SYMBOL SYNCHRONIZER 
-4 command link is being designed to operate between the Space Shuttle 
Orbiter .rnd a payload. 
chronizer slip has on this link. In Reference 4, the command link is discussed 
along with the format of the command word. 
link was analyzed and shown to have an extremely small probability of slipping 
a cycle. However, to maintain the required probability of acceptance of an 
erroneous command, a strategy for detecting a cycle slip and disregarding 
the previous command was presented. The results of the analysis demon- 
strated that, with the expected symbol e r ro r  rate of this link and a typical 
loop bandwidth, the loop SNR would be very large ( ss 48.5 dB) so that the 
probability of a cycle slip is incalculably small. 
cycle slip using the PN sync word was given. 
of falsely declaring a slip (thus falsely rejecting a command) could be made 
to be negligible (e 
Also, the probability of missing a slip if it occurs is shown to be approxi- 
mately 8 ~ 1 0 ~ ~ ~ .  
to occur when the number of acceptable e r ro r s  is K = 10. 
of using the sync word to detect a slipped cycle was shown to yield excellent 
performance. 
The primary concern is the effect that a symbol syn- 
The symbol synchronizer in this 
A strategy of detecting a 
It was shown that the probability 
4 for the expected symbol e r ro r  rate of p = 10- . 
These values show acceptable performance and were found 
Thus, the strategy 
12 
VU. AREAS FOR FURTHER STUDY 
During the course of the contract, a number of problem areas  were 
identified that require further study. 
7.1 SYSTEM MODELING AND PARAMETER OPTIMIZATION 
It is very important to accurately predict the performance of the 
communication system since the communication link margins are extremely 
small. 
the system are required to develop a mathematical model capable of accurately 
Therefore, further investigations of the individual components of 
predicting nominal and worst-case system behavior. Considering the trade- 
offs between performance, complexity, and availability of hardware, the 
critical system parameters need to be further identified and optimized. 
7.2 SYSTEM ACQUIS1"XON CHARACTERISTICS 
The acquisition characteristics of the digital transmission system for  
communications either directly from ground to the Space Shuttle or  via TDRS 
need to be determined. Carr ier ,  symbol synchronization, Viterbi decoder 
branch synchronization, and multiplexer f rame synchronization acquisition 
need to be studied, and a total system acquisition tiint should be predicted 
as a function of the received signal-to-noise ratio. Effects such a s  antenna 
switching on system acquisition should be investigated. A basic set  of 
acquisition techniques and procedures need8 to be developed for both di rect 
and TDRS links. 
7.3 MODULATION-CODING INTERFACE CHARACTERISTICS 
Sources of performance degradation on the Viterbi decoder due to 
the PSK demodulation and bit synchronization need to be identified. An 
13 
analysis is  needed to determine the design parameters of the Cortas (or 
squaring) loop for PSK demodulation including the bandwidths of t he  low pass 
filters when split-phase (Manchester coding) is employed and the character- 
istic of the limiter (if needed) preceding the Coatas hop.  Also, the types of 
AGC need to be compared a d  the effects that thc ACC has on the system per- 
formance need to be investigated. The interface between the receiver PSK 
demodulator and the digital signal pruceasor needr to be carefully analyzed 
to minimiee the signal processor performance degri: iation. 
7.4 SECUFUTY INTERFACING FOR PAYLOAD k . 4 D  GROUND 
COMMUNICAXTON LINKS 
An analysis and evaluation is needed of the system impact of inter- 
facing communieationr eecurity equipment with the remaining orbiter com- 
munication subsystem equipment for periodic use on payload and ground 
communications links. Attention should be given to the impacts, including 
implementation considerations, of this processing on command, control, data, 
.id voice channels. 
14 
REFERENCES 
1. C. K. Huth, "Viterbi Decoder Performance and Complexity, " 
Axiomatix Report No. R7401-1 (Under NASA Contract NAS 9-13467). 
January 23, 1974. 
2. R. D. Rey, %ymbol synchronizer Study for Shuele Communications, I' 
Axiomatix Report No. R7310-1 (Under NASA COntr8Ct NAS 9-13467), 
October 10, 1973. 
3. G. K. Huth, llCommand Channel Coding for Shuttle Communicrtionr, 'I 
Axiomatix Report No. 7307-1 (Under NASA Contract NAS 9-13467). 
July 20, 1973. Also, G. K. Huth and B. H. Batron, "A Command 
Encoding Scheme for a Multiplexed Space Communication Link, l 1  
NTC '73 Conference Record, Atlanta, Georgia, November 26-28, 
1973. 
4. R. D. ,Rey, "Orbiter to Payload Command Performance with a Split 
Phase Symbol Synchronizer, '' Axiomatix Report No. R7312-1 (Under 
NASA Contract NAS 9-13467). December 7, 1973. 
15 


Integrated source and channel encoded digital communications system design study

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