The pan-European L-band terrestrial cellular system (GSM) is expected to provide service to more than 10 million users by the year 2000. Discussed here is the feasibility of a new satellite system at EHF (40/50 GHz) to complement, at the end of the decade, the GSM system or its decendants in order to provide additional services at 64 kbits/s, or so. The main system aspects, channel characteristics, technology issues, and both on-board and earth terminal architectures are highlighted. Based on the performed analyses, a proposal was addressed to the Italian Space Agency (ASI), aimed at the implementation of a national plan

Falciasecca, G.

Paraboni, A.

Ruggieri, M.

Valdoni, F.

Vatalaro, F.

English

NASA Technical Reports Server

N92-24092
Feasibility of an EHF (40/50 GHz) Mobile
Satellite System Using Highly
Inclined Orbits
ABSTRACT
G.Falciasecca(*), A.Paraboni ("),
M.Ruggieri, F.Valdoni, F.Vatalaro ('*')
(*) DEIS, Universita' di Bologna (ITALY)
(**) DIE, Politecnico di Milano (ITALY)
(***) DIE,Universita' di Roma"Tor Vergata"
Via E.Carnevale, 00173 - Roma (ITAI,Y)
Phone: + 39 - 6 - 24990464; Fax: + 39 - 6 -2490519
The pan-European GSM cellular system
is expected to provide service to more than
10 million users by the year 2000. This
paper indicates the feasibility of a new
satellite system at EHF (40/50 GHz) to
complement at the end of the decade the
GSM system or its descendants, in order to
provide additional services at 64 kbit/s, or
so. The main system aspects, channel
characteristics, technology issues, and
both on-board and earth terminal
architectures are highlighted in the paper.
Based on the performed analyses, a
roposal has been addressed to the Italian
pace Agency (ASI) and the process is
advanced aimed to the implementation of a
national research plan.
INTRODUCTION
both high speed data and facsimile, video
services and high quality voice, (2) to widen
coverage to Eastern Europe, to the Middle
East Countries and to Northern Africa, also
filling possible temporary holes of the GSM
system coverage, and (3) to better cope with
the networking needs of private users in
small regional areas.
Being the L-band very congested, it is
mandatory to exploit higher frequency
bands. Our feasibility study identified the
EHF (40/50 GHz) frequency band as the
most suited candidate in a ten year
perspective. Putting into account
technological trend data for RF components
at EHF, some solutions have been proposed
for a new mobile satellite system. This
system utilizes Highly Inclined Orbits
(HIO) with the apogee on the zenith of
central Europe to minimize the probability
of link obstruction.
The pan-European L-band terrestrial
cellular system (GSM) will provide mobile
users with digital voice and low bit rate
data services starting from early 90's.
According to market forecasts, at the end of
the century the GSM system will serve a
population between ten and twenty
millions terminals all over Western
Europe. However, up to now this system is
not intended to transparently provide the
mobile user with 2B+D ISDN channels.
Even a small demand of ISDN services
(few percent of the overall GSM traffic) can
motivate the deployment of a new satellite
system to be integrated with the GSM
system. Main objectives of such a new
system are: (1) to provide higher data rates
(64 kbit/s and possibly n x 64 kbit/s), for
The present paper highlights problems
and possible solutions, both at system and
subsystem level, that characterize the
proposed satellite system. On the basis of
the feasibility study, a proposal [1] was
addressed to the Italian Space Agency
(ASI) and the process is advanced aimed to
the implementation of a national research
plan.
SYSTEM ASSESSMENT
EHF Channel Model
In mobile systems the acceptable link
unavailability generally ranges from 1 to 5
percent. The unavailability time can be
International Mobile Satellite Conference. Ottawa, 1990
131
https://ntrs.nasa.gov/search.jsp?R=19920014849 2020-03-17T12:30:24+00:00Z
estimated through the evaluation of the
blockage probability due to obstacles
(buildings, plants, etc.), the probability of
severe multipath conditions, and the excess
attenuation probability due to atmospheric
effects.
Some experiments on blockage at 1.5
GHz showed that the unavailability time at
43 ° elevation is about 0.2 % on motorways
and more than 50% in an urban
environment [2]. Therefore, on the one
hand the unavailability figure of a HIO
system at EHF can be almost fully allotted
to the atmospheric effects in rural areas.
On the other hand, in metropolitan areas
an adequate grade of service can only be
provided under particular circumstances.
The 40/50 GHz mobile radio channel
mainly suffers from rain and snow effects,
clouds and fog, water vapour, molecular
absorption and tropospheric turbulences.
With respect to these channel impairments
the 1 to 5 % unavailability range still lacks
deep investigations. Therefore, to the
purpose of the present preliminary
analysis, some measured data, collected in
the frame of the Sirio and the OTS satellite
programmes, have been elaborated and
extrapolated in frequency (Tab. 1) [3].
Acceptable performance is expected even at
50 GHz with reasonable power margins.
4
,0
tt
z
A'VrEN_UATION _
RAIN 20 4.3
CI,OUDS 0.6 0.4
WATERVAPOUR 0.6 0.6
OXIGEN 0.5 0.5
SCINTILLATION 0.4 0.4
SPINO
FUCINO LARIO
D'ADDA
0.,!, o.,I,
o.4 o.4
0.4 0.4 0.6 0.6
0.4 0.4 0.5 0.5
0.3 0.3 0.4 0.4
TOTAI, 22.1 6.2 17.4 7.8 25.7 6.3
5 RAIN 26.6 5.4 21.1 8.6 31.4 9
0 CLOUDS 1.1 0.8 0.9 0.6 1.1 0.8
G WATER VAPOUR 0.9 0.9 0.6 0.6 0.8 0.8
H OXIGEN 4.5 4.5 3.2 3.2 4.4 4.4
z SCINTILLATION 0.5 0.5 0.4 0.4 0.5 0.5
TOTAL 33.6 12.1 26.2 13.4 38.2 15.5
Tab. 1" Estimated values of excess
attenuation at 40 GHz and 50 GHz.
The use of highly directive earth
terminal antennas that is compatible
with vehicular applications at millimetre
waves strongly reduces the multipath
effects. Therefore, in the limit the
channel can be c onsidered either AWGN
or fully obstructed by ostacles. An
exception to this will presumably be the
case of personal communications where the
channel turns out to be again a Ricean
channel.
The preliminary link design indicates
the following achievable performance [1] :
(1) system availability between 95% and
99 % ; (2) BER <_ 10-6; (3) capacity of
about 1000 channels with less than lkW
class satellites.
HIO Implications
The joint adoption of satellites located on
non-geostationary HIOs and of millimetre
waves presents several implications at
system level to be carefully evaluated.
Among them the following two are here
considered: zooming effect and Doppler
effect.
During the activity period of a satellite
(between two successive satellite hand-
overs) the on-board antenna footprint
dimension continually varies (zooming
effect). In case of Continental coverage
with multibeam antennas, pointing of each
single beam must be independently
controlled by electronic means.
The zooming effect can be partially
compensated by providing the on-board
antenna with a phased array feed
subsystem, so to keep constant beam
boresight directions. Therefore,
individual beam coverage zones have
variable dimensions, being minimum
at hand-over and maximum at apogee.
In order to totally compensate the
zooming effect, the beam radiation
function must be controlled. This can
be achieved either by time varying the
beam gain so that it is maintained
constant at the edge of beam coverage,
or by keeping constant the power
density at edge of coverage.
132
International Mobile Satellite Conference, Ottawa, 1990
In the first case the on-board antenna
main reflector is normally over-
dimensioned, and is used with maximum
illumination efficiency at hand-over. In the
second case the power density is constant at
edge of coverage: this can be achieved by
statically shaping the beam to put into
account the variation of the free space loss
between the hand-over point and the
apogee.
The Doppler effect is primarily due to
the satellite movement and secondarily to
the movement of the vehicle. The
contribution due to the vehicle is about 3
kHz at 50 GHz for an elevation angle of 50 °
and vehicle velocity of 100 km/h.
In case of a transparent satellite, the fixed
earth station can provide a pre-
compensation in the forward link. To avoid
frequency compensation on the mobile, in
the return link the correction of the up-link
and the compensation for the down-link
frequency are performed on-board.
A possible solution is to provide the
Doppler compensation with respect to the
centre of the beam, thus accepting a
residual Doppler that increases towards the
beam edge. The maximum value of the
residual at 50 GHz ranges from about
3.5 kHz to about 6.5 kHz, depending on
orbit selection (case of 500 km beam
diameter).
By assuming the vehicle located in a
given site, the satellite contribution can be
split into a first component in the fixed link
and a second component in the mobile link.
Both these components can be evaluated as
a function of the frequency and of the
orbital parameters. At 50 GHz the
maximum Doppler shift ranges from about
90 kHz to about 400 kHz, depending on the
choice of orbital parameters. Due to the
above extremely high values of Doppler
shifts, some measures have to be taken.
SYSTEM DESIGN
On-board Architecture
A possible payload configuration is
shown in Fig.1. This configuration
assumes frequency division coverage of
the mobile link, a transparent
transponder on-board and the use of
the 20/30 GHz frequency band in the
fixed link.
M
V •
X •
m
m
F--
I
L "
T •
E •
R
S
DOPPLER [
CORRECTION AND ]
PRE-COMPENSATION]
• i
I ,
o i
BFN
Rx
BFN
Tx
--_u/ct--
• MPA
---4 U/CI-- ---4
/
Fig.l: Payload configuration.
International Mobile Satellite Conference, Ottawa, 1990
133
750 I
70.5
66.0
61.5
57.0
52.5
48.0
43.5
39.0
LAT.
jA
_!
30.0 _L
-10 -6 -2 2' 6 LO 14 1,_ 22 26 30
LOt_G.
Fig.2: Mobile-links multibeam coverage of Europe.
To achieve the required multispot
coverage of the mobile link, a suitable on-
board antenna configuration consists of a
reflector and a phased array feed
subsystem located in the focal region. A
single antenna could be used for both
transmission and reception, however, due
to the antenna limited size (diameter:
about 0.5m), separate reflectors for the
mobile up-link and down-link are
assumed. The European coverage at
40/50 GHz is achieved with about 40
spots (Fig.2), approximately 500 km
diameter each
The 40 GHz high power section is
configured as a Multi-Port Amplifier
(MPA) [4]. This configuration seems
particularly advantageous in the proposed
system thanks to: (a) efficient use of
payload power in the presence of variable
traffic loads; (b) graceful degradation of
performance in case of failures of
amplifying modules. Furthermore, when a
set of low-power amplifiers with adjustable
gain is adopted together with the MPA, the
additional capability of a dynamic spot
power reallocation results.
Most of the challenging aspects of the
proposed payload architecture are related
to the multibeam antenna subsystems and
to the high power section. In particular, a
MPA is envisaged with a number of ports
equal to the number of radiating elements,
adopting 40 GHz SSPA modules.
Link budgets show the need for 150 mW
per carrier i0r an optimized selection of
orbital parameters. This figure seems
compatible with near-term technological
outcomes.
Mobile Terminal
A focal aspect of the proposed system is
the need for a high gain earth terminal
antenna equipped with automatic tracking.
Among possible solutions [5], active direct-
radiating phased arrays with electronic
steering seem to be the most promising in
the envisaged program time scale. Two
separate square array antennas to be
mounted on the same panel have been
considered, one for the reception (900
radiating elements), Fig.3, and one for
transmission (400 elements). Antenna
gains of about 37 dBi and of about 33 dBi
result for reception and for transmission,
respectively.
An overall radiated power of about 6 W
is obtained by no more than 15 mW per
radiating element. This figure is
compatible with near-term power
capabilities of monolithic amplifiers with
FET devices at 50 GHz.
134
I,-flernational Mobile Satellite Conference, Ottawa, t990
A low noise receiver of 500 K noise
temperature is suitable for link budget
needs. This figure is expected to be
achieved at 40 GHz in the near future
from HEMT monolithic amplifiers.
C) C_ C_ C> <25 O C_ ---C)/
RX ARRAY/(_ <::_ <_" (::_ (:_ <:_ /
I I 1
1 ..... • .... __ .... _]_N t
LNA _ _ [_ [_]
,,
TR NG
TO
COMM UNICATION
sis (weights)
I .....
I
I
I
I
I
4,
TO
TX ARRAY
(50 Gliz)
SUM
BEAM
Fig. 3: Schematic of the mobile terminal
receive antenna subsystem.
CONCLUSIONS
The satellite system here illustrated
can be reasonably deployed within the end
of the decade. Early technology
development, experiments on channel
characteristics and system definition can
lead to a system capable to succesfully
complement services provided by existing
and near future terrestrial cellular
systems.
REFERENCES
1. G. Falciasecca, A. Paraboni, M.
Ruggieri, F.Valdoni, F. Vatalaro:
"Proposta di Missione Tecnologica-
Sperimentale in Vista di Futuri Servizi
Radiomobili Via Satellite a Copertura
Europea in Gamma EHF", Universita'
di Roma "Tor Vergata", Int. Report,
Nov. 1989.
2. E.Lutz: "Land Mobile Satellite
Channel - Recording and Modelling",
4th Int. Conf. on Sat. Systems for Mobile
Comms and Nay., IEE-294, 1988, pp.15-
19.
3. A.Paraboni: "Propagation Problems
Regarding Millimeter Waves (40/50
GHz)", Int. Report, Politecnico di
Milano, N.88-023, 1988.
4. S.Egami, M.Kawai: "An Adaptive
Multiple Beam System Concept", IEEE
J. on Sel. Areas in Communications",
vol. SAC-5, N.4, May 1987, pp.630-636.
5. M.Shimada et al.- "A Millimeterwave
Mobile Satellite Communication
System for Personal Use", 4th Int. Conf.
on Sat. Systems for Mobile Comms and
Nay., IEE-294, 1988, pp.52-56.
International Mobile Satellite Conference, Ottawa, 1990
135


Feasibility of an EHF (40/50 GHz) mobile satellite system using highly inclined orbits

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