CLASS (Communication Link Analysis and Simulation System) is a software package developed for NASA to predict the communication and tracking performance of the Tracking and Data Relay Satellite System (TDRSS) services. The methods used to verify CLASS are described. The usefulness of a software tool such as CLASS depends strongly on the reliability and accuracy of the results it produces. For this reason, considerable attention was paid to validation throughout the CLASS development

Braun, W. R.

Mckenzie, T. M.

English

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N8 5 .1695¢
VALIDATION OF THE _UNICATIONS LINK ANALYSIS AND SIMULATION SYSTEM (CLASS) 9
Walter R. Braun*
Teresa M. HcKenzie
LtnCom Corporation
P.O. Box 15897
Los Angeles, CA 90015
(213) 381-3701
ABSTRACT
CLASS (Comauntcattons Ltnk Analysts and Simulation System) ts a software package developed for
NASA to predict the communlcatlon and tracking perfomance of the Tracking and Data Relay Satelllte
Syste_ (TDRSS) services. This paper describes the methods used to vertfy CLASS.
1. INTRODUCTION
The Communlcatlons Llnk Analysis end Simulation System (CLASS) presently under development for
NASA loddard Space F11ght Center Is an integrated set of FORTRAN programs capeble of predlctlng the
coa_Datlblllty and performance of the communlcatlo_ and tracking 11nks for all servlces and slgnal
fomats supported by the Tracklng and Data Re1_y Sate111te System (TDRSS).
The capabilities and structure of CLASS are presented tn another paper In these Proceedings,
"Communications Ltnk Analysis and Simulation System" by Robert Godfrey. Hodels of major components of
CLASS are described in a second paper tn the Proceedings, "Modeling Techniques in the Communications
_Link Analysis and Simulation System (CLASS)" by the same authors as the current paper.
The usefulness of a software tool such as CLASS depends strongly on the reliability and accuracy
of the results it produces. For thls reason, considerable attention was pald to validation throughout
the CLASS development. The purpose of thls paper Is to describe those efforts. The models mentioned
in this paper were discussed in the second paper clted in the above paragraph.
Verification has been and continues to be done by making four types of comparisons: conq)artsons : ,',.':,-!_'_:)Le
with analysis (Section 2), with Monte Carlo-typ_ simulations (Section 3), with measurements (Section ,_: ,':
4), and with _RSS test data (Section 5). The prediction of blt error rate (B[R) on links both with iJii:i_)....
and without radio-frequency interference (RFI) has been verified in the first three ways. The _ _ _ :
prediction of pseudo-noise (PN) code acquisition has been checked by comparison wlth analysis. ';L_jC!:
Finally, In the next couple of years the entire CLASS wlll be validated on both subsystem and system _ij._?_:__
levels by TDRSS test data. , .
2. COMPARISON WITH ANALYSIS
One approach taken to verify CLASS Is to coe_pare Its predicted results with those obtained by
analyses, both in-house and publlshed. In this section we describe such efforts for slngle-parameter
sensitivities of BER, the V1terbl decoder perfomance, BER for a link _th RFI, and PN-code acquisition
time.
The BER sensitivity to each user constralnt (member of a set of distortion parameters that
characterlze the TDRSS user's transmitter [i]) was evaluated with CLASS using an otherwise perfect
slgnal and a linear, wldeband chennel. The results were then compared to analytlcal slngle-parameter
sensitivity results. Typlcal results are shown In Figure I. The sllght discrepancy In Figure Ic Is
due to the fact that the sampled signal model does not allow the modellng of Instantaneous phase
transltlons. It can be seen that agrement is excellent.
Co_arlson of CLASS _th purely analytical models of non-ldeal channels is not practical because
of the limitations tt)elatter place on the channel that can be treated. Figure 2 shows that a typical
channel model for analysis does not include filtering effects on the transmitted signal, In distinction
from even a minimal model for simulation.
The R0 approximation method for computing the BER for a convoluttonally encoded Gausstan channel
from Eb/N 0 (ratio of btt energy to noise one-sided power spectral density) was checked by comparing It
+This work was supported byNASA Goddard Space _Ight Center, Greenbelt, HD, under contracts
NAS5-27240, NAS5-25681, and NASG-_3591.
W. Braun is currently wlth Brown, Borer(, & Co., Limited, Baden, Switzerland.
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with two other models: a Linkabit Corporation software model probably based on the union bound and an
in-house Monte Carlo-type _imulatlon. Figure 3 shows that the R0 approximation matches the simulation
for BER's above about 10"_ and is upper-bounded by the Linkablt model for all BER's, as it should be.
The validation of the R0 approxlmatlon for non-Gausslan,RFI links is discussed in Section 3.
The analytical model for BER on an RFI llnk was partially validated by showing that the two sub-
models, one for hlgh bit rates and one for low, glve similar results in the range of bit rates near the
bit rate which is the cross-over point between the sub-models, even though the sub-models themselves
are quite different. Figure 4 shows the good agreement for a typical link. The vertical scale on the
plot is nmttted for national security reasons.
Flnaly, the model for PN code acqulsltlon performance, applicable to both• variable and fixed
dwell-time systems, was verified by showing excellent agreement between its predictions and exact
results for the fixed dwell-time algorithm, as described in [2].
3. COMPARISON WITH MONTE CARLO-TYPE SIMULATIONS
A variety of BER predictions made by Monte Carlo-type simulations were compared to those made by
CLASS. The CLASS link was configured as near as possible to the link used in the Monte Carlo
simulations. Three simulation programs of no-RFI links and one of an PJ_Ilink were used.
The first comparison for a no-RFI llnk was _th Champ, a slmulatlon program of Comsat Corp. for
the evaluation of BER and synchronization (tracking) (see, e.g., [3]). BER results for a TDMA link
agreed within 0.5 dB.
The second such comparison was with'Link, a program of TRW, Inc., which is usually run without
uplink noise (see, e.g., [4]). Such published results were duplicated by CLASS.
The third comparison for a no-RFI llnk was wlth an in-house program that modeled the BPSK llnk
shown in Pigure 2a. Here, the pulse shaping filter is modeled as a half-Nyqulst filter with a roll-off
parameter a • .I. The satellite input and output filters are of the Chebyshev type with a bandwidth
equal to three times the data rate and a ripple of .I dB. The receiver low-pass filter is matched to
the pulse-shaping filter. The high-power a_lifler Is linear while the satellite TWTA characteristic
is given by Figure 5. For the Monte Carlo simulation the same 63-blt signal used in CLASS was combined
with 32 different uplink noise waveforms to find the upllnk waveform contribution to the detector
input. _ The downlink noise effect was modeled analytically. No effort was made to smooth the resulting
performance curves in order to de_nstrate the slow convergence of the results. Typical results are
shown in Figure 6 for two different operating points of the l_fTA. Note that the Monte Carlo curves
follow the CLASS results quite well, but wlth some wdld variations, despite the large number of bits
simulated.
The predictions of the analytlcal program for BER on an RFI link have been compared with those
from an in-house Monte Carlo-tybe program for coded links without Interleaver. The two data rates used
in the con,_arison were low enough that the analytical model also included no interleaver. The BPSK
link had uplink noise, a non-linearity, and no downllnk noise or receiver losses. The RFI_ was
severe. The BER's were identical within 10%. _Is finding confirms not only the approach taken in the
analytical program to model the n_ched filter output In RFI but also the applicability of the R0
approach for coded BER computation on a channel wi_ich,is not nearly Gaussian.
4. COMFARISON WITH MEASUREMENTS
Probably the most convincing validation _f results obtatned with software is close agreement wlth
hardware results. Both no-RFI and RFI llnk predictions were checked this way.
One comparison of CLASS with hardware was with Harris, Cor_., breadboard measurements made in 1978
for a TDMA satellite communication system. The link was nonlinear and included uplink and downli_k
thermal noise and adjacent channel interference. Agreement was within i dB.
The RFI BER mOdel was verified with Harris Corp. breadboard measurements using TDRSS hardware and
an RF! test generator. Four RFI scenarios were used. Two distributions of RF! power and pulse arrival
rate were used, a benign environment and a severe environment. On each environment were based two
scenarios, one wtth only noise-type pulses and one wtth the pulses divided between noise-type and
continuous wave (CW)-type. The results in Table 1 demonstrate that the predictions were accurate in
most cases and that they were in all cases pessimistic. The model was designed not to underestimate
the RFI effect.
805
S. COMPARISON WITH TDRSS TEST DATA
Currently an effort is under way to develoo a c_prehensive CLASS valldation plan using the first
TDRS in orbit and the TDRSS ground statton. Baseline hardware losses of TDRSS subsystems wtll be
measured. Then the sensitivity of all the performance measures predicted by CLASS to all the sources
of signal distortion and interference wtll be verified. A great effort has been made to reduce the
number of tests to the minimum necessary for a complete validation.
1.
REFERENCES
"Tracking and Da_ Relay Sate111te System (TDRSS) Users' Guide, Revision 4," STDN No. 101.2, NASA
Goddard Space night Center, Janua_ 1980.
2. Braun, W. R., "Perfo_nce _alysis for the Expanding _arch PN _qulsition Algort_," IEEE Trans.
on C_., Vol. CDM-3O, No. 3, March 1982.
3,
4.
Pa1_r, L. C., and Leb_itz, S., "Including Synchronization in Ti_-D_aln Channel Simulations,"
C_sa_ Technical Revi_, Vol. 7, No. 2, Fall 1977.
Poza, H. B., and _rger, H. L., "Perfomance Charac_rlzation of Advanced Wldeband Da_ Links,"
International Conference on C_unicatlons, 1975. ,
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NOISE, (B) MODULATOR PHASE ERROR, (C) RECOVERED CLOCK PHASE ERROR.
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COMPARISON OF PERFORMANCE MODELS FOR (A) RATE-l/2 CODE, (B) RATE-l/3 CODE.
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Table I. VERIFICATIONOF RFI MODEL: BER DEGRADATION.
RFI PULSES
ENVIRONMENT
DATA RATE
MEASUREMENT
CLASS PREDICTION
NOISE-ONLY NOISE÷_
BENIGNENVIRONMENT BENIGNENVIRONMENT
200 Kbps 6 Mbos 200 Kbos I 6 Mbos
.9 dB 1.5 dB ,B dB I 1,30B (l.g) I
l,q6 3,02 1.19 2,84
RFI PULSES NOISE-ONLY INOISE+CW
ENVIRONMENT SEVERE ENVIRONMENT SEVERE ENVIRONMENT
DATA RATE 200 Kbps
MEASUREMENT 4.0 dB
CLASS PREDICTION 5.32
6 MbOs ' 200 KbOs 6 MbPs
B.O dB .-2.1 dB (3.6) 6.0 dB (9,8)
.1-1,51 5,22 11,67
INtmbers In porentheses come from "RFI Test Stay Second Interim Report "
by Horrls Coro 26 Februory 1980. Others come from "RFI Test Study
Final Rer)ort," by Harris Coro,, 24 April 1980.
809


Validation of the communications link analysis and simulation system (CLASS)

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