Rainfall attenuation has a severe effect on signal strength and impairs communication links for future mobile and personal satellite communications using Ka-band and millimeter wave frequencies. As rain attenuation compensation techniques, several methods such as uplink power control, site diversity, and adaptive control of data rate or forward error correction have been proposed. In this paper, we propose a TDMA system that can compensate rain attenuation by adaptive control of transmission rates. To evaluate the performance of this TDMA terminal, we carried out three types of experiments: experiments using a Japanese CS-3 satellite with Ka-band transponders, in house IF loop-back experiments, and computer simulations. Experimental results show that this TDMA system has advantages over the conventional constant-rate TDMA systems, as resource sharing technique, in both bit error rate and total TDMA burst lengths required for transmitting given information

Sato, Masaki

Takahashi, Takashi

Takeuchi, Makoto

Wakana, Hiromitsu

Yamamoto, Minoru

English

NASA Technical Reports Server

Adaptive Data Rate Control TDMA Systems
as a Rain Attenuation Compensation Technique
Masaki Sato, Hiromitsu Wakana, Takashi Takahashi,
Makoto Takeuchi, and Minoru Yamamoto
Communications Research Laboratory
Ministry of Posts and Telecommunications
893-1 Hirai, Kashima, Ibaraki 314 Japan
Tel: +81-299-84-4119
Fax: +81-299-84-4149
Ng4"22821
ABSTRACT
Rainfall attenuation has a severe effect on
signal strength and impairs communication links
for future mobile and personal satellite commu-
nications using Ka-band and millimeter wave
frequencies. As rain attenuation compensation
techniques, several methods such as uplink
power control, site diversity, and adaptive con-
trol of data rate or forward error correction have
been proposed. In this paper, we propose a
TDMA system that can compensate rain attenua-
tion by adaptive control of transmission rates.
To evaluate the performance of this TDMA ter-
minal, we carried out three types of experiments:
experiments using a Japanese CS-3 satellite with
Ka-band transponders, in-house IF loop-back
experiments, and computer simulations.
Experimental results show that this TDMA sys-
tem has advantages over the conventional con-
stant-rate TDMA systems, as resource sharing
technique, in both bit error rate and total TDMA
burst lengths required for transmitting given in-
formation.
INTRODUCTION
The Communication Research Laboratory
(CRL) has been carrying out L-band mobile
satellite communications experiments using the
Engineering Test Satellite Five (ETS-V), which
was launched in 1987. This study will be ex-
tended to advanced mobile and personal com-
munications at Ka-band and millimeter wave
frequencies using the Communications and
Broadcasting Engineering Test Satellite
(COMETS), which will be launched in 1997 [1].
Advantages to use such higher frequencies are
much wider available frequency bandwidth and
substantial system size reduction. On the other
hand, we have to overcome such disadvantages
as significant rain attenuation, larger Doppler
shifts, higher RF component losses, larger
phase jitters, etc.
At frequencies above 10 GHz, signal atten-
uation due to rainfall can have significant im-
pairment on space-to-earth communication links.
To compensate rain attenuation, several methods
such as uplink power control[2/[3/, site diver-
sity[4], and adaptive control of data rate and/or
forward error correction techniques[5/have been
proposed. Uplink power control and site di-
versity techniques are very useful for large or
medium size earth stations at large networks
covering different climatic regions. On the other
hand, adaptive control of data rate and error cor-
rection techniques are more appropriate to small
earth stations, since both small antenna size and
limited RF transmit power impose a restriction
over the range of the uplink power control.
Moreover, these adaptive control techniques can
compensate downlink attenuation as well as up-
link attenuation.
In this paper, we propose a TDMA terminal
with adaptive control capability of transmission
rate to compensate rain attenuation. We carried
out experiments using Japanese Communication
Satellite-Three (CS-3) with Ka-band transpon-
ders to evaluate the performance of this terminal
under rainfall conditions.
ADAPTIVE TRANSMISSION-RATE
TDMA
In this Adaptive Transmission-Rate TDMA
terminal, later referred to as ATR-TDMA, a re-
ceive terminal that is suffering rain attenuation
requests the transmit terminal to reduce trans-
mission rate of the TDMA burst, according to
degradation in bit error rate (BER) or carrier-to-
505
https://ntrs.nasa.gov/search.jsp?R=19940018348 2020-06-16T15:57:25+00:00Z
noisedensityratio (C/No). Sixdifferenttrans-
missionratesareavailablein this terminal.For
example,atransmissionratereductionbyafac-
tor of fourcancompensatethedegradationof 6
dB dueto rain attenuation.Therefore,thissys-
temenhancesthepowermarginby 15dB.inall
theoretically.
Figures1and2 showablockdiagramanda
schemeof modulationanddemodulationof the
ATR-TDMA terminal,respectively.Databit
streamisstoredin abufferanddividedintodata
bursts,the lengthof whichcorrespondsto data
bitswithin oneTDMA frame.DatarateRbis
assignedasfollows:
Rb= Rc/2N-2
for N=I, 2, 3, 4, 5, and6 (1)
whereRc is thereferenceclock rateof 4.144
Mbps in thepresentsystem,N=I for QPSK,
andN=2, 3, 4, 5, 6 for BPSK. Therefore,six
possibledataratesare8288,4144,2072, 1036,
518,and 259kbps.
After thebasebandsignalis scrambledwitha
pseudonoise(PN)codegeneratedat Rcbps,a
carrieris PSKmodulatedin burstmode.Since
PSKmodulatoralwaysworksatthesameclock
rateof Rc,everyTDMA burstsalwaysoccupy
thesamefrequencybandwidth.
Receiveterminalrecoversclockratefromthe
receivedsignalandthenrecoversdataclock
from bursttiming. Thepresenttransmissionrate
canbeknownfrom informationbitsstoredin the
preambleof aTDMA burst.After coherently
demodulatedPSKsignalisdescrambledby the
samePNcode,2N-2 bits areaddedwith syn-
chronoustiming of clock rateandoneinforma-
tion bitcanbereproducedbasedonmajority
rule. Thisprocessof thesynchronizedsumis
equivalento reducethetransmissionratefrom
Rc bpsto Rc/2N-2 bps. Forexample,atrans-
missionratereductionby afactorof twoextends
theburstlengthtwiceandBERperformancecan
be improvedby 3dB in C/No.
TheTDMA frameformatanddataburstfor-
matareshownin Figure3. OneTDMA frame
consistsof a referenceburst,asynchronization
burst,databursts,andinitial acquisitionslot.
Theframeperiodis 15millisecondsandone
controlframeof 1200millisecondperiodcon-
sistsof 80 frames. Eachdataburstis divided
intoapreamblefor receiversynchronizationand
databits. This preambleconsistsof two partsof
P1andP2. In P1,carrierandclock recovery
patterns,andanuniquewordfor identifying
eachterminalandsynchronizingabursttiming
areincluded.In P2,identificationcodesfor
transmitandreceiveterminals,andinformation
bits for controlof transmissionrateareincluded.
Sinceinformationbits in P1areessentialfor all
terminalsin thisTDMA network,thispart issent
atthelowesttransmissionrate. On theother
hand,sincebits in P2isnecessaryonly for
transmitandreceiveterminalsundercommuni-
cation,transmissionrateof P2is thesameas
datarateof eachTDMA burst. Moreover,using
majority-ruledecisionof preambleinformation
thatis receivedoverseveralframes,theTDMA
terminalcanreducepossibilityof incorrectop-
erationsin theTDMA network. Transmission
ratescanbechangedonlyoncewithin onecon-
trol frame.
RAIN ATTENUATION
COMPENSATION EXPERIMENTS
Satellite Experiments
We carried out experiments using the CS-3
satellite to evaluate the performance of the ATR-
TDMA terminal at Ka-band satellite link.
Communication channels were monitored by
measuring both BER and C/No with measuring
equipments and a personal computer.
Since allowable BER without FEC is de-
signed to be 10 -3 in this system, we assigned
BER of 8.0* 10 -4 or C/No that corresponds to
BER of 8.0"10 -4 as the threshold to change
transmission rates. When BER or C/No is
worse than the threshold for Td seconds,
transmission rate is reduced half. When BER or
C/No is better than the threshold for Tu seconds,
transmission rate is increased twice.
Figure 4 shows the experimental result that
was controlled from degradation in C/No with
Td = 5 seconds and Tu = 5 seconds. Top of this
figure shows rain attenuation of eighteen-hour
measurement at satellite loop-back link ( includ-
ing both 20 GHz and 30 GHz attenuation ) and
20GHz-beacon. Controlled transmission rates
in kbps and measured BER are shown in middle
and bottom of Figure 4, respectively. According
to rain attenuation, the ATR-TDMA terminal
controlled its transmission rate to keep BER
better than the allowable BER with as high
transmission rates as possible. This terminal can
compensate rain attenuation actually up to about
15 dB ( although theoretically 16dB ). When
506
rainattenuationis largerthan15dB or rainrate
becomesfasterthanresponsetimeof thissys-
tem,BERbecomeslargerthantheallowable
value. However,suchunavailabletimeduration
is improvedandbecomesconsiderablyshort.
With thelowesttransmissionrateatcon-
stant-rateTDMA terminals,wecouldkeepBER
betterthanthethresholdlike theATR-TDMA
terminal. However,totalTDMA burstlengths
requiredfor transmittingthesameinformation
will becomemuchlargerthanATR-TDMA.
Next, westudyBER andtotalburstlengthsof
ATR-TDMA asafunctionof differentTd andTu
values.
In-house Experiments
Values of parameters Td and Tu are very im-
portant for effective control of transmission rate.
In the TDMA network, time duration in one
TDMA frame is a common resource for every
terminals. When many user terminals suffer rain
attenuation, several terminals will wait to access
until a vacant time slot appear and some termi-
nals will break down due to the degradation of
communication links.
In in-house experiments, the receive terminal
connects with the transmit terminal through IF
port and monitors BER to detect degradation in
communication link. C/No is degraded by addi-
tive white Gaussian noise according to rain at-
tenuation data that were measured and stored
using satellite links. Using several values of Tu
and Td, BER and total burst lengths are mea-
sured and shown in Figure 5. As Tu becomes
larger, BER becomes smaller, but TDMA burst
lengths, with respect to the burst length of the
fastest rate, become considerably large. Both
improvement in BER and increase of burst
lengths is more remarkable as Td becomes
smaller.
Figure 6 shows comparison of this experi-
mental result with the result obtained by the
constant-rate TDMA terminal. The lower
transmission rates of N=4 and 5 can provide
very good BER performance but the TDMA
burst of this terminal will occupy a large part in
one TDMA frame. The number of user termi-
nals than can access this TDMA network is very
restricted. The ATR-TDMA terminal has better
performance in both BER and burst lengths than
the conventional constant-rate TDMA.
Computer Simulation
To study the effect in BER and burst lengths
under different rain conditions and different Tu,
and Td, we have carried out computer simulation
of the ATR-TDMA system. C/No is controlled
with measured data of rain attenuation and BER
is calculated from the measured BER perfor-
mance of the TDMA terminal. Table 1 shows
typical values of Tu and Td, which correspond
to different traffic conditions in a satellite
transponder. In situation of mode #3, since only
a few vacant time slots remain in one TDMA
fame due to high traffic condition, transmission
rate cannot be reduced quickly and has to be
brought back to its rate as soon as possible after
the weather improves. On the other hand, mode
#4 prefers to maintain the communication quality
rather than to shorten used burst lengths.
Figure 7 and 8 show the probability that
BER is worse than the threshold as a function of
used total burst lengths using rain attenuation
measured for twenty rainy days. In Figure 8,
data, which are picked up frorn severe rain
events, are normalized with the result of mode
#4.
Slow reduction of transmission rates in
modes #2 and #3 increases the probability that
BER is worse than the threshold up to about
20% in severe rain events as shown in Figure 7.
Simulation results, however, shows that ATR-
TDMA is effective to compensate rain attenua-
tion under many different rain conditions.
Figure 8 shows that values of Tu is insensi-
tive to the probability that BER is worse than the
threshold and total burst lengths decrease as Td
becomes smaller. Parameter Td is more impor-
tant than Tu to improve BER performance under
rain attenuation.
Since the response time should be less than
rain attenuation rate for successful control of
transmission rate, the optimum value of Td must
be as small value as possible. However, one-
way propagation delay for a geostationary satel-
lite link is on the order of 0.25 seconds, and a
similar amount of time is required for receive
terminals to request reduction of transmission
rates to their transmit terminals. Moreover, It
will take a couple of seconds for receive termi-
nals to measure BER or C/No for measuring rain
attenuation, and there are also additional system
processing delays including delay due to control
algorithm. Therefore, control delay of trans-
mission rates from instantaneous rain attenuation
is inevitable. In spite of such delay factors, ex-
507
perimental results showed that Td of 3 to 5 sec-
onds are effective to compensate rain attenua-
tion.
CONCLUSION
Adaptive transmission-rate TDMA system to
compensate rain attenuation is presented. At the
beginning, this system was developed for busi-
ness networks consisting of different size earth
stations at Ka-band frequency Therefore,
transmission rates available in this system may
be too fast for applications of mobile and per-
sonal satellite communications. However, the
same adaptive control scheme can be applied to
these systems at lower transmission rates.
To evaluate the performance, we carried out
three kinds of experiments: satellite experiments,
in-house experiments, and computer simula-
tions. The ATR-TDMA system showed advan-
tages over the the conventional constant-rate
TDMA system to maintain both communication
quality and capacity under rainy weather.
When we apply adaptive control of data rate
to SCPC systems, the reduction of data rate will
result also in a reduction of service quality.
However, this TDMA scheme reduce transmis-
sion rate in RF, but does not reduce data rate at
baseband. Thus service quality is not reduced
except very severe rain and heavy traffic condi-
tions. This adaptive control method is suitable
to future mobile or personal satellite communica-
tion systems at higher frequencies such as Ka-
band or millimeter wave for compensating rain
attenuation.
REFERENCE
[1] S. Ohmori, et al. "Advanced Mobile Satellite
Communication using COMETS Satellite in
MM-wave and Ka-Band," Proc. IMSC'93,
Pasadena, California, June 16-18, 1993.
[2] I. Nishiyama, R. Miura, and H. Wakana,
"Closed-loop Uplink Power Control
Experiments in K-band using CS-2," Proc.
ISTS, Tokyo, Japan, 1986.
Transmit
Portion Variable data-rate modulator
I------ -'1 data r "1
IData port.l_ I
L___I ICompression ! Ii Rd >_I i
Preamble _ buffer ill/ PN !__ I .... III
Information I t t I _]enerator Rc
..... I
T)( timing LU Carrier I
controller Iq sw 1
Receive
Portion Variable data-rate demodulator
Modulated
burst
signal
Rd : Date clock rate
Rc : Reference clock rate
CWR : Carrier recovery
CCR : Clock recoveryI
I 3bit D,-, DCR " Data clock recovery
I _ n,., A ,
t l .... I .1',
I _ _i PN I-_ _\--- I'_ I -,--I- t_ iP,°..dI'data II-Ea'_"
! _ _ Igenerat°r| Sync Sum I Rd I I IExpansion _ ,..._ --I , . __._. . / . I I bufferI_____Preamb_e
I........... --l"-/ ----Ii /T I I 1 "- Inf°rmati°n
IBurst SYNCI_1 RXtiming _ DCRI !
IController _ controller I I - i l I
! I ! I t -------J
f
Figure 1. Block diagram of ATR-TDMA system
508
data I I®
PN code ....
112 ......... m I
data _) P N
(scramble) j_
PSK MOD
PSK DEM
J3
outputfrom QPSK demod.
®
PN code .....
de scramble _[--l--['_
112 ......... mI
Sync. sum
hard bit decision I I
Figure 2. Scheme of ATR-TDMA
_1¢ 1 Frame (15 msec}
CBTR UW TX ID_X I_ TC
I I I L)_ t ._C_ | i _i
. _ Preamble _,
DATA
R : reference burst S : synchronization burst
A : initial acquisition slot Dn ( n=l, 2, ... ) : data burst
UW : unique word CBTR : carri_ and burst timing recovery
TC : transmission-rate control bits TX/RX ID : transmit/reseive station ID
P1 : Preamble ( all stations need to receive )
P2 : Preamble ( transmit and receive stations receive }
[3] V. O. Hentinen, "Error Performance for
Adaptive Transmission on Fading Control,"
IEEE Trans. Commun., vol. COM-22, pp.
1331-1337, 1974.
[4] CCIR Rep. 564-4, "Propagation Data and
Prediction Methods required for Earth-Space
Telecommunication Systems,"
Recommendations and Reports of the CCIR,
vol. 5, ITU, Geneva, 1991.
[5] J. Hagenauer, "Rate Compatible Punctured
Convolutional Code," Proc. ICC'87, pp.29.1.1-
29.1.5, Seatle, USA, 1987.
t-
O
.g
m
,,,i
t-
10-
Figure 3 TDMA frame format
(dB
0
5- W loop-back
signal
20GHz-beacon
' ' I ' ' I ' ' I" ' I '
0 3 6 9 12
time
(kbps)
(a)
15 18
(hour)
(b)
2072 .......
518 - -- --
209 --
i i i i I I I i I I i i d
(c)
0 3 6 9 12 15 18
time (hour)
10 .2
re" 10-3 _
UJ 10"4
m 10.6
10 4
1 0 .7
<-7
clear sky
(dB)
%
clear sky
5
Figure 4. Experimental results using the CS-3
satellite (a) rain attenuation (b) transmission
rate (c) BER
509
II:
tlJ
ii1
10"2 I
10-3
10-4
10-5
10-6
0
, . , . ._ (xlO6)"i-a
--0-- 5sec .-7."- _
15sec .-;.T:"" - 1.8 _,
+ cosec _-':--_: .1.6 --
-''.,a--" "'_- burst length
J_ ss
_._-'"" _,/B E R -1.4
1.0
0.8 =
i I
1O0 200 300
T U (sec)
Table 1. Four Control modes using
Control
modes
different
mode #4
Td in
SCC.
Td and Tu, _.......
Tu in
sec.
mode #1 3 3
mode #2 83 103
mode #3 83 3 A
3 103
i
Sym-
bols
er
(3
Figure 5 BER and used burst lengths as
a function of Tu and Td
IIC
tlJ
I11
10"3
10"4
10"5
10"6
10-7
10"8
"N-'I " ' " ' " ' " ' "I'd' "
_o_,.c
N=5
1 2 3 5 6
used total burst lengths ( x 10 6 )
Figure 6. Comparison with constant-rate TDMA
"o 30
o
r.
#)
o
_- 20
I--
A
ri-
m 10
_ o
5
I
A
O
O_
15
used total burst lengths ( x 10 5 )
25
Figure 7. BER > threshold versus burst lengths
"o
O
/-
1-
I-
^
UJ
I11
10
" A
N
A
Q • •
.1 , I n I n
0.4 0.6 0.8
normalized burst lengths
Figure 8. BER > threshold versus burst lengths
normalized with the result of mode #4
510


Adaptive data rate control TDMA systems as a rain attenuation compensation technique

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