The NASA/GSFC developed an all digital, real time, programmable Viterbi decoder simulator operating at rates up to 6 Msps. With this simulator, the bit error rate (BER) performance of convolutionally encoded/Viterbi decoded Shuttle-TDRSS return link channels under pulsed radio frequency interference (RFI) conditions has been predicted. The principles of the simulator are described with special emphasis on the channel simulator and the essential interaction between CLASS software and the simulator. The sensitivity of coded BER as function of several illustrative RFI parameters is discussed for two typical Shuttle-TDRSS return link configurations

Herr, D. W.

Schwartz, J. J.

Walvis, D. J. M.

Weinberg, A.

English

NASA Technical Reports Server

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NOVEL APPLICATIONS OF THE NASA/GSFC VITERBI DECODER HARDWARE
SIMULATOR
Dirk J.M. Walvis
Aaron Weinberg
Stanford Telecommunication, Inc.
McLean, Va. 22101
ABSTRACT
Denver W. Herr
John J. Schwartz
NASA/GSFC
Greenbelt, Md. 20771
NASA/GSFC has had developed an al_ digital, real time, programmable Viterbi decoder simulator opera-
ting at rates up to 6 Msps. With this simulator, the BER performance of convolutionally encoded/
Viterbi decoded Shuttle-TDRSS return link channels under pulsed RFI conditions has been predicted.
The principles of the simulator are described with special emphasis on the channel simulator and the
essential interaction between CLASS software and the simulator. The sensitivity of coded BER as
function of several illustrative RFI parameters is discussed for two typical Shuttle-TDRSS return
link configurations.
I. INTRODUCTION
The Shuttle - TDRSS return links, S-band and K-band mode 1, channel 3, employ convolutlonal encoding
/Viterbi decoding. During certain periods the TDRSS g_ound receiver may have to contend with high
level pulsed RFI. These pulses may affect many consecutive information symbols depending on the
symbol rate and RFI pulse duration.
An analytical software package of CLASS (Communicatlon Link Analysis and Simulation System), [1,6],
has the capability to predict the performance of the decoders under RFI conditions when the channel
can be modeled as a memoryless channel. To evaluate the performance when the decoder has to contend
with a series of consecutive pulses affected by RFI (i.e. a non-memoryless channel), NASA GSFC en-
gaged Linkabit Corporation to develop an all digital convolutional encoder/Viterbi decoder simulator
This simulator can be programmed to simulate a large variety of time-varying channels, including
pulsed RFI, and operates at symbol rates up to 6 Msps. It has been used at NASA GSFC to predict the
impact of RFI on the return link performance of TDRSS users such as Space Telescope and Shuttle.
This paper describes the major features of this simulator and its interfaces with CLASS. It pre-
sents the results of tests measuring the impact of illustrative RFI on some Shuttle - TDRSS return
links.
2. OVERVIEW OF THE SIM_JLATOR
The present conflguratlon of the slmulator is shown in Figure I [2]. The slmulator Is configured
and controlled via the desk top calculator, which provides the capability of automated testing. In
addition, the configuration data generated wlth CLASS can be downloaded to the calculator. The
error rate tester is controlled vla the desk top calculator or via front panel controls. A PN se-
quence, which simulates the data source, is generated in the error rate tester and looped back to
the tester for BER measurement after being processed by the encoder, transition probabllity genera-
tor and decoder. Interleaving and deinterleavlng is optlona1. The error rate tester also generates
the symbol rate clock, which allows s3nnbol rates up to 6 Msps. The encoder and decoder are either
rate 1/2 or rate I/3, with constraint length 7. The deinterleaver is integrated with the decoder
and the decoder metric growth is monitored to e_tablish Interleaver/deinterleaver s3mchronization.
I
The transition probability generator models the discrete channel formed by the modulator, channel
characteristics and noise, and the quantized output of the demodulator/detector. The TPG maps each
symbol into one of the possible 8 soft decision levels at the input of the deinterleaver/decoder,
based on a probabilistic process for which the data is generated with CLASS.
CHANNEL MODELING
With the TPG the channel is modeled as follows: The time varying channel is divided into as many as
32 different channel conditions or states. Each of these channel conditions is time-invariant and
is modeled as a discrete, binary input to 8-ary output, memoryless channel. The time variation is
then achieved by selecting one of the available channel conditions for each transmitted symbol. The
transitions between channel conditions are modeled as a zero or first order Markov process with up
to 32 states. In addition, it is possible to specify the number of symbols the TPG is to stay (or
hold) in a channel condition, once entered. This is particularly useful when modeling pulsed RFI as
the hold time is equal to the number of channel symbols the RFI pulse overlaps. For the application
described here the transitions between channel conditions are medeled as a zero order Markov process
i.e. the probability of entering a channel condition is independent of the previous channel condi-
tion. For a pulsed RFI environment the channel condition transitions are then determined from the
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duty cycle, Tj, and pulse duration, Mj, of each associated RFI condition. When the phenomenon of
erlapping RFI pulses is neglected, which is a good approximation for low duty cycles, the proba-
bility of entering a channel condition is easily found to _:
1Ti (_1T_Prj - _jj M_i
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with:
Prj - the probability of entering channel condition j.
Tj - the duty cycle of the RFI associated with the j-th channel condition.
Mj - the pulse duration of the RFI associated with the J-th channel condition.
Ne - total num_r of different channel conditions.
Figure 2 shows a functional diagram of the TPG and a typical list of channel conditions. The two
right_st columns show the duty cycle and pulse duration from which C_SS software calculates the
transition probabilities via (I). These probabilities are do, loaded into the desk top calculator.
_st channels are modeled with the pulse duration of the NO-RFI channel condition, approximately an
AWGN channel condition, equal to I as sho_ in the tabl_ of Figure 1. This implies that the TPG is
not forced to stay in the non-RFI condition and that the RFI channel conditions may be selected
after any num_r of channel symbols which are'not affected by RFI.
For each channel condition the discrete binary to 8-ary memoryless channel is fully characterized by
_Iculating the 8-1evel soft decision statistics, assuming that a "I" is transmitted, as the TPG
models each channel condition as a symmetric channel. These soft decision statistics are calculated
by C_SS. For full details on the _thods follo_d in these computations see [I]. In addition to
the RFI para_ters listed in F_gure 2, the following para_ters must _ specified in order for C_SS
to compute the soft decision statistics:
Shuttle: EIRP, Data rate
TDRSS : Retu_/do_ link noise, AM/AM, AM/PM, clipping level
The next section descries so_ results on decoder performance and shows typical soft-decision sta-
tistics.
3. SIMU_TION RESULTS
The simulation results presented in this pa_r illustrate the capability of the simulator to assess
the impact of several RFI para_ters on the perfomance of the Viterbi algorithm decoder. As exa_
ples, the impact of illustrative RFI on the Shuttle - TDRSS K- and S-band has been simulated.
SHUI-_LE-TDRSS K-BAND
For the simulation described here the configur_ion data for the TPG are computed with the following
signal and channel para_ters, [3],:
e Shuttle channel 3, up to SO Mbps, characterized as BPSK with 47.3 dbW EIRP,
rate i/2 c_ing.
• Pulsed RFI
- Duty cycle: < 1%
- For each test a choice of:
-- In-band or out-of-band CW
-- T_ illustrative RFI po_r levels
-- 1, 10 or 20 consecutive information symbols overlapped
by a single RFI pulse
• TDRSS 6A return link trans_nder
- 2SO MHz noise bandwidth
44 dBW thermal noise level
- ALC with 10 dB clip_r and 1.2 degrees per dB AM/PM
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FIGURE 2: TRANSITION PROBABILITY GENERATOR
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The BPSK signal characterization is an approximation to the actual QDSB signal structure and is rea-
sonable due to the 6 dB dominance of the channel 3 signal relative to the composite of the channel I
and 2 signals.
The TDRS transponder characteristics indicated reflect the actual KSA return link transponder. The
thermal noise level of 44 dBW refers to the fact that the thermal noise power within the 250 MHz
bandwidth may be equivalently obtained by having the TDRS point directly at a 44 dBW thermal noise
source on the surface of the earth that is radiating via an omnidirectional antenna.
The duty cycle of less than 1% is purely illustrative, as is the number of consecutive symbols over-
lapped by RFI. The in-band RFI, modeled in CLASS with a RFI frequency which precisely coincides with
the carrier, leads to worst case performance for a given RFI power level. Six combinations of the
RFI parameters are evaluated. These are listed in Table I. As a companion to.Table I, Table 2 sum-
marizes the soft decision statistics that apply to each RFI presence. The NO RFI channel condition
is the same for each scenario and is also listed in this table.
Table 3 shows the BER obtained through simulation, as well as the channel symbol error rate for each
scenario. As seen, and expected, performance degradation is much more severe under In-band RFI
conditions.
Also to be observed from Table 3 is the expected Incheaslng degradation that occurs as the RFI EIRP
and the number of symbols overlapped increases. Especially interesting is the approach of the de-
coded BER to the channel error rate for the particularly severe scenarios. This phenomenon reflects
that for those scenarios virtual each RFI p_Ise is sufficiently severe and long to cause a burst of
decoding errors. Note that the degradation of the channel during the out-of-band RFI pulses is not
sufficiently severe to consistently cause a burst of decoding errors.
SHUTTLE-TDRSS S-BAND
For the simulation described here the configuration data for the TPG are computed with the following
signal and channel parameters:
Shuttle S-band return link, BPSK with typical EIRP and 96 or 192 Kbps,
rate 1/3 coding.
• Illustrative pulsed RFI
- Severe RFI with a duty cycle < 2%
- I, 2, 3 or 4 consecutive information symbols overlapped by RFI
• TDRSS SSA return link transponder
- 20 MHz noise bandwidth
- 29 dBW thermal noise level
- ALC with 10 dB clipper and 1.2 degrees per dB AM/PM
The design of the rate 1/3 decoder of the Shuttle unique receiver of the TDRSS ground equipment is
slightly different from the decoder used in the simulator. The major difference is the path memory
length which is 64 decoded bits instead of the 3Z decoded bits in the simulator. In addition, the
metric assignment for the different decoder iv+put quantization levels are probably different. The
exact assignments are proprietary. As part of the evaluation of the impact of RFI on the Shuttle
decoder, it was shown that degradation of perfoFmance for both decoders is very comparable. This
was shown by configuring the software decoder simulator "CODES", which is part of CLASS, either as
the Shuttle decoder or as the all digital, real time simulator and comparing the results.
Figure 3 shows the BER degradation due to RFI as function of the.number of consecutive symbols af-
fected by RFI, for a variety of margins of the Shuttle EIRP. We see that for the same margin the
192 kbps is degraded more severely than the 96 kbps. This is due to the higher duty cycle of the
in-band RFI, which is a result of the wider receive filter. When we compare the _-margin curves
with the B-margin curves, we see that the degradation due to consecutive symbols affected by RFI, is
more severe when the margin is higher.
It is interesting, as well, to compare the simulation speed of the software package CODES, which
allows detailed specification of different convolutional codes and hardware constraints, wlth the
simulation speed of the hardware simulator. A ten million bit simulation takes 5 to 10 hours with
CODES on the CLASS computer. With the hardare simulator such a simulation takes less than 20 min-
utes, which includes inputting the data into CLASS and operating the hardware simulator.
825
TABLEI
RFI SCENARIOS CONSIDERED IN SECTION 3
Desired slgnal £1RP = 47.3 dBW
SCENARIO RFI
NO, [X_rYCYCL_
< 1%
2 < 1%
s < 1%
6 < I%
t
rZ , [ l
Rrl RFI
CLA.% EI_
IN-BAND [1"
IN-BANO E2"
i !
IN-BAND [2
OUT-Or-BAND E1
OUT-Or-BAND r Z
OUT-OF-BAND E2
TABLE 2
NO. OF C_($ECm'IYE
SYI_ OVE_
BY BFI PULSE
2O
10
20
2O
10
ZO
CONDITIONAL SOFT DECISION PROBABILITY DISTRIBUTIONS FOR RFI SCENARIOS OF TABLE 1
OLMLNT ] ZATIOq NO I N-BAND , IN-BAND . OUT-OF-BAND, OUT-OF-BAND ,
LEVEL" Rf ] E 1 E2 E1 EZ
1 1.6 _ 10 -$ .0_2 .ZlO .015 .006
2 10 *4 .044 .034 .0_3 .029
3 6.S • 10 .4 .057 J .031 .021 .03S
4 3.1 x 10 .3 .064 .029 .033 .042
i
5 .011 .063 .OZ7 .047 .048
6 .033 .058 .OE6 .063 .0_,4
7 .074 .052 .OZ6 .079 .(_9
8 .BTB .610 .559 .729 .64S
Assumes transmission of a + symbol polarity.
te be symmetric.
The channel is assumed
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MEASURED ERROR RATES FOR RFI SCENARIOS OF TABLE I
DECODED , CHANNEL SYMBOL
SCENARIO BER ERROR RATE
1 1.4 x 10 "4 4.7 x 10 "3
1.1 x 10 "3""
1.3 x 10 "3
8.4 x 10 -7
6.0 x lO"5
1.6 x I0"4
5.3 x 10 -3
5.3 x lO"3
4.2 x 10 "3
I 4,6 x 10 "3
I
I
4.6 x 10 "3
i
HAD INTERLEAVING/DEINTERLEAVING _EEN EMPLOYED DECODED BER WOULD BE
<<I0 "6 IN ALL CASES.
827
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10-3
10-4
10-5
10- 6
FIGURE 3:
MARGIN
D dB
/
/
/
/
/
" ] _96 Kbps
.....o-- ...... t 192" Kbps
A dB
I
2
o MEASUREDDATA
3 4
RFI PULSE DURATION
SYMBOL DURATION
ILLUSTRATIVE IMPACT OF RFI PULSE DURATION ON SHUTTLE-TDRSS S-BAND RETUR_I LINK.
(A > D > C > B)
82B
4. SUMHARY
The NASA/GSFC Ylterbl decoder hardware simulator extends the capability of CLASS to include non-
interleaved channels when the impact of pulsed RFI on the coded BER of the TDRSS return link is
assessed. The discrete, bursty channel Is simulated by computing a set of soft decision probabil-
ity distributions with CLASS. The simulator chooses one soft decision probability distribution for
each channel symbol.
The slmuhtlon results show the sensitivity of coded BER to a variety of illustrative RFI parameters
The sensitivity of coded BER to the RFI pulse duration can be easily assessed as is shown with the
Shuttle-TORSS S-band simulations. As an example of the impact of the RFI power levels, pulse dura-
tion and of the RFI being on-band/out-of-band, the coded BER of the Shuttle-TDRSS K-band Is evaluated
for some illustrative RFI parameters.
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3.
4,
REFERENCES
Weinberg, A., "Coded BER Performance for the Nonlinear TDRSS Communication Channel Corrupted
by RFI, "STI/E-TR-8115, Stanford Telecommunications, Inc., McLean, VA (March 1980).
Final Report for a Simulation System for Validating the Analytical Prediction of Performance
of the Convolutional Encoder and Symbol Interleaved TDRSS S-Band Return Link Service in a
Pulsed RFI environment, Linkabit Corporation, San Diego, CA (1981).
Walvls, D.J.M., Zakrzewskl, e., Wllkinson, W., Swerdlin, S., Weinberg, A.: Hardware and
Software Simulation of Viterbi Decoder Performance in Pulsed RFI. STI/E-TR-25003A, Stanford
Telecommunications, Inc., McLean, VA (1982).
COSAM User's Guide, Stanford Telecommunications, Inc., McLean, VA (1980).
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Novel applications of the NASA/GSFC Viterbi decoder hardware simulator

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