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TECHNI CAL DESI GN AND SERVI CE ASPECTS
OF
DI GI TAL MOBI LE SATELLI TE COMMUNI CATI ON SYSTEMS
Yutaka YASUDA, Masayoshi OHASHI , Fumaki SUGAYA, Yasuo HI RATA
KDD Megur o R
b
D Laboratori es
1-23, Nakameguro 2I chome,
ABSTRACT
Gl obal mobi l e communi cati on servi ces s uch
as mari t i me, aer onaut i cal and l and mobi l e
communi cati ons are ef f ecti vel y pr ovi ded by
usi ng t he sat el l i t e. Si nce t he power and
bandw dt h avai l abl e f or respect i ve car r i er
are sever el y l i m t ed under t he mobi l e
sat el l i t e communi cati ons envi r onment , t he
appl i cat i on of d i gi t al t ransm ss i on
technol ogi es i s very at t r act i ve to ut i l i ze
the l i m ted sate l l i t e resources ef f i c i ent l y.
The vari ous new i ntegr ated di gi t al communi -
cat i on serv i ces wi l l be al so ef f ect i vel y
provi ded i n the di gi t al t ransm ssi on systems.
I n t hi s paper , t he key techni cal el ement s f or
des i gni ng the di gi ta l mobi l e sate l l i t e
communi cati on syst ems ar e cl ari f i ed together
wi t h the di scussi ons on t he f ut ure t rend of
t he communi cati on servi ces to be of f ered f or
mobi l e user s.
1.
I NTRODUCTI ON
Aut hors had desi gned and devel oped
several di gi t al mari t i me communi cati on
sys t ems
so
f ar, and conduct ed the fi el d
experi ment s by usi ng I NMARSAT (I nter nat i onal
Mar i t i me Sate l l i te Organi zat i on) sate l l i te i n
1984 and 1986 i n order t o i nvest i gat e t hei r
perf ormance characteri st i cs under t he act ual
mari t i me s atel l i t e communi cati on envi r onment
[Ref s.
1-41.
Such a di gi t al voi ce- gr ade
mari t i me sat el l i t e communi cati on system i s
cal l ed Standard- B i n I NMARSAT and i s goi ng
t o be i ntr oduced i n i t s second gener ati on era
[Ref . 51. Based on t he experi ence of
desi gni ng t he di gi t al mari t i me communi cati on
syst ems, we have al so devel oped aeronauti cal
di gi t al voi ce-gr ade communi cati on subsyst em
t o be used f or an experi ment al aer onaut i cal
satel l i t e communi cati on system The f i el d
t r i al of th i s syst emhas been successfu l l y
conduct ed by usi ng passenger ai r pl ane vi a
I NMARSAT Paci f i c ocean r egi on sat el l i t e.
I n thi s paper , bas i c conf i gurat i on o f
di gi t al mobi l e satel l i t e communi cat i on
syst ems i s r evi ewed, and t he key di gi t al
Meguro- ku, Tokyo 153, J apan
t r ansm ssi on t echnol ogi es such as voi ce
codi ng, FEC ( For ward Err or Corr ecti on),
di gi t al modul ati on and synchr oni zati on are
di scussed t aki ng account of our experi ence of
t he har dwar e i mpl ement ati on and t he resul t s
of t he f i el d experi ment s. Then, t he
t rade- of f studi es on t he sat el l i t e e. i . r .p.
r equi r ement s and t he ant enna G/ T of t he mobi l e
t erm nal ar e made based on t he l i nk budget
cal cul ati on. Fur t hermore, new servi ces whi ch
w l l be ef f ect i vel y of f ered to mobi l e users
i n t he di gi t al t ransm ssi on syst ems ar e
st udi ed. These new servi ces cover t he
f acsi m l e and data t r ansm ssi on combi ned wi t h
pers onal computer communi cat i on. The
poss i bi l i ty of o f f er i ng vi deo t ransmss i on
servi ces f or mobi l e users are al so di scussed.
2 KEY TECHNI CAL ELEMENT FOR SYSTEM DESI GN
2.1 System Conf i nur ati on
Fi gure 1 shows the overal l conf i gur ati on
of t he mobi l e sat el l i t e communi cati on syst ems.
The l ow gai n antenna i s used f or mobi l e eart h
stat i ons , whi l e the re l at i vel y hi gh gai n
ant enna i s
empl oyed f or t he ground ear t h
Sate i te
CImEKm
C -
b a n d
NCS
Terrestrial)
s
*Or* *
GES
s
L - b a n d
SES
Aeronau t i ca l Ear th Sta t i on
:
Sh ip Ear th Sta t i on
Ground Ear th Sta t i on
:
Network Coordinat ion Stat io n
Fi g. 1 Overal l Syst em Conf i gurat i on f or
Mobi l e Satel l i t e Communi cati ons
1094
34.2.1.
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OO
988 IEEE
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station to enable stable communication with
mobile terminals under the severely
power-limited condition. As for the
access/control of the communication channel,
the demand assignment technique is generally
applied in order to share the limited
satellite channel resources with many mobile
users. In the IlwABSAT system, the network
control station (NCS) is provided in each
ocean region to carry out the centralized
control for the demand assignment operation.
At the mobile earth station, the antenna
and BF unit (up- and down-converters) are
combined with the communication channel
unit.
of the digital voice-grade communication
subsystem. The communication channel bank
composed of the multiple digital communication
subsystems is provided at the ground earth
station. It will be connected at IF stage
with the antenua/RF facilities for ground
earth station which may be commonly used for
different kinds of mobile services.
Figure
2
shows the basic configuration
2.2 Kev Dinital Technolonies
For voice-grade digital communications,
the low bit rate
4.8
kbit/s
-
16 kbit/s)
voice coding scheme which can provide good
voice quality at the given bit rates must be
employed in order to cope with the limited
power/bandwidth constraints of the
communication channel. The APC-MLQ coding
[Ref. 61 is going to be used in the INMARSAT
digital maritime communication system
(Standard-B) and the aeronautical
communication system, because it can provide
high voice quality at reasonable hardware
size and
is
robust to the transmission bit
errors caused in the mobile communications
environment.
advantage that the coding bit rate of the
voice codec can be easily changed according
to the system requirements if it
is
so
designed.
This coding scheme also has an
As for the FEC coding, the effective
coding scheme with high coding gain
1s
required in order to save the power
requirement for the satellite and/or for
mobile earth stations. The soft decision
Viterbi decoding for convolutional code
is
da ta
vo ice
Fig.2
Block Diagram of Digital Communication
Subsystem
widely used for such a purpose. Regarding
the coding rate of the convolutional code, the
punctured codes with the rate higher than 1/2
[Ref. 71 will be suitable in the case that
the bandwidth saving is strongly required for
each channel.
severely degrades the bit error rate
performance of the communication channel, bit
interleaving of the transmission bits (after
FEC coding) will be effective to minimize such
a performance degradation at the FEC decoder.
However, in this case, the increase of the
signal delay due to bit interleaving and
de-interleaving must be taken into account.
When the multipath fading
The frame synchronization is also very
important for the stable operation of the
digital communication equipment. Under the
mobile satellite communication environment,
since the link C/A (carrier-to-noise power
ratio) becomes very low and fluctuates because
of the signal level variation due to various
factors such as multipath fading, the frame
synchronization scheme robust to such a severe
link condition must be employed. In order to
establish a frame synchronization quickly and
to maintain the synchronized status stably,
the framing bits (unique word) of the fixed
length and the fixed bit pattern are
periodically inserted into the transmitted
bit stream. When the coherent demodulation
technique is employed, the influence of the
phase ambiguity caused in such a demodulator
can be also removed by observing the pattern
of the framing bits in the demodulated bit
sequence.
As for the modulation technique, the
power and bandwidth efficient digital
modulation such as
filtered QPSK is generally
desired. However, when a Class-C HPA is used
for the mobile terminal, it will be necessary
to employ a family of constant envelope
modulation schemes such as offset QPSK and
MSK in order to reduce the out-of-band
emission as much as possible at the HPA
output. Furthermore, since the demodulator
must operate at very low C/N condition when
the FEC code with high coding gain is
employed, the stable operation of the carrier
and clock recovery circuit under such a
condition
is
also very important requirement
for the demodulator, When the bit rates of
the transmission channel becomes very low (in
the order of a few kbit/s), the 2-phase
modulation (e.g. BPSK) rather than the 4-phase
modulation such as QPSK
is
applied to ease
the carrier/clock acquisition and tracking.
2.3 ExamDle of Channel Parameters
The basic channel parameters
of
the
INMARSAT Standard-B and aeronautical
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communication systems are shown in Table 1
for reference. Figure 3 shows the frame
formats of the digital voice-grade
communication channels for these two systems.
In the aeronautical communication system, the
bit interleaving technique of the block size
of 384
(=
64 x 6) bits is applied to the
coded data sequence to cope with the burst
errors caused by the fast fading under the
aeronautical transmission environment. The
total signal delay due to this interleaving
and de-interleaving is around 36 msec.
Informat on
bit rate
16 kbit/s 9.6 kbit/s
(option: 9.6 kbit/s)
Modulation Offset QPSK
-
~ ~~
Transmission
bit rate
Carrier spacing
T X ~ilters Square root raised-cosine Nyquist
I
filter with 60 roll-off
24 kbit/a 21 kbit/s
20 kHz
17.5 kHz
before FEC coding-
r a t e
1 / 2 )
G T
of mobile -4 dBK -13 dBK
terminal (low
G T:
-10 dBK)
O A T ~ V C ~ C EATA VOICE
M T A VOICE
FIE FIELD FIE FIELD
- - - - - - - - - - - - - -
FIE FIELD
I
2
2
25
25
RF - c h a n n e l
r a m e = 80
ms4
PR EAM BL E
ONLY)
BURST MODE
U W
C O D ED D ATA FB
- - -
448
80
148
+1872 b i ts
3. TRADE-OFF STUDY ON TRANSMISSION PARAMETERS
3.1 Reauired C/Bo Versus Information Bit Rate
Figure 4 shows the theoretical relation
of the required C/Bo (C: carrier power, No:
one-sided noise spectral density) versus
information bit rates to obtain the bit error
rate (BER) of lo-? In this figure, the
employment of ideal coherent PSK modulation
and the F E based o n the soft decision Viterbi
decoding are assumed. As for the coding rate
of convolutional codes for Viterbi decoding,
the rate 1/2 code with constraint length K=7
and the rate 3/4 and 7/8 punctured codes
derived from it are selected. The required
C/No to obtain objective BER for the given
information bit rate (Br) has been calculated
by the following equation, assuming the
theoretical BER versus Eb/No (Eb: energy per
information bit) performance for respective
F E C code [Ref. 71.
C/Bo(dB) = Eb/No(dB) + 10 loglOBr
(1)
s B
VOICE VOICE VOICE VOICE
b e f o r e FEC c o d ing - F I EL D F I EL D F I EL D F I EL D
4
r a t e
3/41 1 3
124 7
320 I
L 3 5 1
E
:
F r a m i n g B i t s
S B Su b - b a n d s i g n a l l i n g f i e l d
(a) Standard-B System
frame
=
500
ms -
F -channel
BLOCK
2
- - - - - - - - - - BLOCK27
k 2 0 4 d
I X for sub-band r l g n a l l l n g and user data )
(b) Aeronautical System
When the rate 1/2 F EC code is applied to
the 9.6 kbit/s information channel for
example, we can see from Figure
4
that the
required C/No at BER
=
is about
44
dBHz. If additional bits are inserted at the
transmission channel after
F EC
coding for the
frame/burst synchronization and for other
purposes such as a signalling transmission,
the transmission channel bit rate must be
increased according to the ratio of such
additional bits. Therefore, in such a case,
it is necessary to take account of an extra
margin in the required C/No.
60
BER = 10-5
30
12 2.4 4.8 7296
16
32
64
I n fo r ma t i o n
B i t Rate k b i t / s )
No
Cod ing
7 / 0
314
I
/ 2
(X)
Fig.3 Frame Format of Voice Communication Fig.4 Required C/No vs. Information Bit
Channel (specified in INMARSAT) Rate (Objective BER =
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3.2 Li nk Budget Trade-of f
The channel C/No was cal cul ated as a
f unct i on of t he G/ T or t he e.i .r. p. of t he
mobi l e t er m nal assum ng the t ypi cal
t r ansm ssi on par amet er s of t he I NMARSAT
sate l l i t e l i nk. Si nce the overa l l l i nk C/No
i n t he mobi l e sat el l i t e communi cati on syst ems
i s general l y determ ned by t he l i nk parameters
bet ween t he mobi l e eart h st at i on and t he
sate l l i te , the L -band sate l l i te e. i . r .p. i n
t he f orward (C- to-L) l i nk and t he L- band
sat el l i t e G/ T i n the return (L- to-C) l i nk
were t aken as par ameters i n t hi s cal cul ati on.
Satel l i te I r p
d B W)
20
-15
- I O 5
0
Mobi le Term nal G/ T (dBK)
Fi g. 5 Tot a l L i nk C/No vs. G/ T of Mobi l e
Term nal
Satellite G / T d B K )
-
I O
-
I2
-14
-
f
m
z
U
Fi g. 6 Tot al L i nk C/No vs. EI RP of Mobi l e
Term nal
Fi gur es 5 and 6 show t he per f ormance of
t he t o t al s at e l l i t e l i nk
C/No
ver sus G/ T and
e. i . r .p. of t he mobi l e eart h stat i on. I n
t hese f i gures, t he l i nk paramet er s f or t he
I NMARSAT cur r ent Standar d- A, Standard- B and
aer onaut i cal voi ce- gr ade channel s are marked.
The exampl e of l i nk budget cal cul ati ons f or
t hese syst ems ot her t han t he Standar d-A i s
shown i n Tabl e
2,
whi ch al so i ncl udes the
l i nk budget f or a l ow G/ T -10 dBK) shi p
ear t h st at i on syst emusi ng J apanese
Experi ment al Test Satel l i t e (ETS- V) [Ref . 8 1 .
Si nce t he ETS- V satel l i t e has t wo L- band spot
beams, t he satel l i t e e. i . r.p. and G/ T i n t he
l i nk bet ween t he satel l i t e and mobi l e ear t h
st ati on are f ar hi gher t han those of I NMARSAT
gl obal beam sat el l i t e. Theref ore, t he
hi gh- speed dat a tr ansm ssi on such as
of 64
kbi t / s i nf or mati on rat e wi l l become possi bl e
even f or t he l ow G/ T mobi l e t erm nal . ( See
Sect i on 4.)
Tabl e 2 Exampl e of Li nk Budget Cal cul ati ons
[ Satellite System ] INKARSAT ETS-V
(INTELSAT-V/MCS)
Std B Aero.
low-G/T SES
(a) Ground-to-mobile link
(Frequency) (6.42 GHz) (5.96 GHz)
(47 degs.)5 degs.)
UD
link:
GES elevation
55.5 62.0 66.0
ES EIRP (dBW)
200.9 199.4
ath loss (dB)
0.4 0.2
bsorption
loss
(dB)
Up link C/No (dBHz) 68.8 75.3 87.0
Satellite G/T (dBK) -14 -8
Satellite:
Satellite gain (dB)
161.3 166.5
Satellite C/IMo (dBHz) 61.3 67.8
--
U
Frequency) (1.54 GHz) (1.54 GHz)
32.9
atell ite EIRP (dBW) 15.5 22.0
Path loss (dB) 188.5 187.6
0.4 0.2bsorption loss (dB)
63.7
own link C/No (dBHz) 51.2 48.7
-10
obile G/T (dBK) -4 -13
L i n k
Performance (unfadedl:
63.7
otal C/No (dBHz) 50.7 48.6
(b) Mobile-to-ground
link
UD l i n k : (Frequency) (1.64 GHz) (1.64 GHz)
Mobile terminal elevation (5 degs.) (47 degs.)
Mob ile EIRP (dBW) 33 25.5 27.2
Pat h loss (dB) 189.0 188.2
Absorption loss (dB)
0.4 0.2
Satellite G/T (dBK)
-13
-4.9
Up link C/No (dBHz)
59.2 51.7 62.5
Satellite:
Satellite gai n (dB) 150.9 165.1
Satellite C/IMo (dBHz) 62.5 55.0 --
Down link: (Frequency) (4.2 GHz) (5.2 GHz)
Satellite EIRP (dBW) -5.5 -13.0 3.9
Absor ption loss (dB) 0.4 0.2
GES G/T (dBK) 32 33.7
Pat h loss (dB) 197.2 198.2
Down l i nk
C/No (dBHz) 57.5 50.0 67.8
Link Derformance (unfaded):
Tot al C/No (dBHz) 54.5
47 O
61.4
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It should be also noted that the multipath
fading margi n is not included in the link
budget calculations shown
in
Table 2.
Therefore, wh en the elevation angle of the
mobile earth station is low, the appropriate
fading margin must b e taken into account.
The required C/No under the unfaded condition
is determined from the BER objective shown in
Figure
4
by adding hardware implementation
margin, etc. The amount of the fading margin
depends on the antenna size of the mobile
earth station and the elevation angle of it
toward the satellite. Figure
7
shows the
measured results of the fading depth versus
elevation angle of the ship earth station
antenna under the actual INMARSAT maritime
satellite communication link. These data
were obtained by the field experiment on the
digital ship earth stati on systems using a
high-G/T
-4
dBK) parabolic antenna and a
low-G/T (-10 dBK) short-backfire antenna with
fading reduction capability [Refs.
4,
91.
4.
SERVICE CONSIDERATION
In the digital mobile satellite
communication systems, the various integrated
digital communication services will be
provided
in
addition to the conventional
voice services. These new services include
high-speed facsimile transmission, personal
computer (PC) communications and compressed
video/picture transmission, etc. In this
section, the future evolution of such
communication services is considered, taking
account o f the possible technical progress.
6
z
M 4
2 2
a
c
a
.-
A
0
OHigh G/T
o L o w - G / T F R
A
FR
0
0 0
0
0
o p
0 0
O N )
:
OFF)
0
0
0 1 1 1 1 1 1 1 1 1 1 1
4 8
10
12
14
Elevation Angle deg.)
High-G/T: 85c m parabolic antenna (G/T = 4 dBK)
Low-G/T : 40cm short backfire (G/T = -10 dBK)
with fading reduction (FR) capability
Fig.7
Measure Results of Fading Depth vs.
Elevation Angle (maritime)
Voice
In the INMARSAT Standard-B system, the
APC-MLQ voice coding of 16 kbit/s is going to
be used
in
order to provide toll quality voice
communication services. In the aeronautical
communication systems, 9.6 kbit/s APC-MLQ
voice coding is also going to be used. In
the future land mobile communication systems,
further reduction of voice coding rate (e.g.
4.8 kbit/s) may be needed i n order to
minimize the power and bandwidth requirement
per channel. Suc h a low rate voice coding
will be also effectively employed when two or
more voice channels must be integrated into
single digital communication channel. For
instance, two independent voice channels of
4.8
kbit/s can be integrated
in
a single
9.6
kbit/s digital voice-grade channel in the
aeronautical communication systems in which
the multiple voice communication capability
for passengers and cabin crews is strongly
desired.
Facsimile
In the current INMARSAT Standard-A system
using analog companded FM modulation, 63
facsimile transmission of up to 2.4 kbit/s i s
guaranteed. Thi s capability is also
maintained wh en 16 kbit/s APC voice codec is
applied in the INMARSAT Standard-B system.
Furthermore, in the mobile satellite
communication systems wit h a digital
transmission channel at the bit rate of 9.6
kbit/s or more, the
63
facsimile transmission
at a speed of 9.6 kbit/s becomes possible by
providing appropriate analog-digital interface
at the ground earth station. In this case,
the facsimile signal is transmitted in the
digital form via digital satellite
communication channel, whereas in the
terrestrial analog PSTN (public switched
telephone network), it is transmitted through
the conventional V.29 voice-band modem
incorporated i n the 63 facsimile equipment or
through the V.32 modem which enables both-way
9.6 kbit/s data transmission via 2-wire local
line. Whe n the digital communication
channels become widely available in the
public terrestrial networks, the satellite
data channel of the arbitral bit rate is
directly connected to suc h a terrestrial data
channel without using voice-band modem.
such a case, higher speed 64 facsimile
transmission suc h as 16 kbit/s will also
become available for the relatively high G/T
mobile earth station systems.
Personal Computer Communication
In the mobile satellite communication
systems such as maritime and aeronautical
ones, the data transmission by using the
personal computer is an effective mean to
deal with the various communication services
in an integrated digital form. For the
In
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aer onaut i cal appl i cat i ons, t he use of a handy
l apt op PC comput er wi l l be pract i cal and ver y
att r act i ve i n order to communi cat e f r omt he
cabi n wi t h t he user on the gr ound.
per sonal comput er communi cat i on i ncl udes t he
f i l e t r ansf er and database access i n addi t i on
t o t he convent i onal message exchange. I f t he
appropr i ate i mage scanner and/ or a f acsi m l e
equi pment i s combi ned wi t h t he PC t erm nal ,
t he f i gur e/ i mage t r ansm ssi on bet ween mobi l e
user and t he gr ound user wi l l become
an
att r act i ve servi ce. The hi gh speed and
err or- f r ee both- way dat a tr ansm ssi on bet ween
PC t erm nal s i s r eal i zed by usi ng t he HDLC
speci f i ed i n t he OS1 (Open Syst ems
I nt erconnect i on) protocol .
i t i s i mport ant t o sel ect t he appropri at e
packet si ze as wel l as t he wi ndow si ze f or
t he ARQ cont r ol t o get t he maxi mum t hroughput
at t he mobi l e satel l i t e communi cati on channel
wi t h f ai r l y l arge s i gnal t r ansmss i on del ay.
Compr essed Vi deo/ St i l l Pi ct ur e
speed dat a t r ansm ssi on channel such as 64
kbi t / s can be al so pr ovi ded.
can accommodat e t he i ntegrat ed audi ohi de0
t ransm ssi on f or v i sual t el ephone servi ces by
usi ng t he hi ghl y compressed voi ce/ pi ct ure
codi ng equi pment . I n
t hi s year, we are
pl anni ng to conduct such an exper i ment on the
64
kbi t / s audi ohi de0 t r ansm ss i on over
mari t i me communi cat i on channel vi a J apanese
ETS- V sat el l i t e by usi ng the I NVI TE
64
system
devel oped by KDD [ Ref . l o]. I n addi t i on t o
t hi s , t he st i l l pi ct ur e
of
qual i f i ed col or
i mage wi l l be al so t ransm t t ed vi a l ower rat e
channel such as 9.6 kbi t / s .
The
I n such a system
I f t her e i s enough power margi n, t he hi gh
Such a channel
5. CONCLUSI ON
The key techni cal el ement s i n desi gni ng
t he di gi t al mobi l e sat el l i t e communi cat i on
syst ems were i dent i f i ed and t he tr ade-of f
st udi es on the tr ansm ssi on parameters of t he
sat el l i t e l i nk wer e made based on our
exper i ence of t he exper i ment al syst em
devel opment and the r esul t s of t he f i el d
tr i al s. Then, t he f ut ure evol ut i on of the
vari ous new communi cat i on ser vi ces t o be
of f ered i n the di gi t al mobi l e communi cat i on
sys t ems wer e di scussed.
I n the di gi t a l mobi l e sate l l i t e
communi cat i on sys t ems whi ch w l l be oper ated
under t he condi t i on of l ow C/ N and wi t h a
si gnal l evel f l uctuat i on due to mul t i pat h
f adi ng etc., i t i s the most i mpor t ant
r equi r ement t o est abl i sh t he f r ame
synchr oni zati on qui ckl y and mai nt ai n i t
s tabl e i n addi t i on to t he s tabl e
carr i er/ cl ock tr acki ng under such a sever e
l i nk condi t i on.
Thi s was one of t he maj or
r esul t s whi ch we obt ai ned t hrough t he var i ous
f i el d experi ment s on t he di gi t al mari t i me and
aer onaut i cal satel l i t e communi cat i on systems.
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