System level trade-offs carried out during Mobile Satellite (M-SAT) design activities are described. These trade-offs relate to the use of low level beam forming, flexible power and spectrum distribution, and selection of the number of beams to cover the service area. It is shown that antenna performance can be improved by sharing horns between beams using a low level beam forming network (BFN). Additionally, greatly increased power utilization is possible using a hybrid matrix concept to share power between beams

Moody, H. J.

English

NASA Technical Reports Server

N92-2  i 9
Payload System Tradeoffs for Mobile
Communications Satellites
HJ. Moody
Spar Aerospace Limited
21025 TransCanada Highway, Ste-Anne-de-Bellevue, Quebec
H9X 3R2 Canada
Phone: (514) 457-2150
FAX: (514) 457-2224
ABSTRACT
This paper describes system level trade-offs car-
ried out during M-SAT design activities. These
trade-offs relate to the use of low level beam form-
ing, flexible power and spectrum distribution and
selection of the number of beams to cover the ser-
vice area. It is shown that antenna performance can
be improved by sharing horns between beams using
a low level BFN and that greatly increased power
utilization is possible using a hybrid matrix concept
to share power between beams.
INTRODUCTION
Since 1980, Spar Aerospace Limited has been
engaged, under contract to the Department of Com-
munication and to the Department of Industry, Sci-
ence and Technology Canada, in a study and design
of a payload for a mobile communication satellite.
This paper describes some of the trade-offs carried
out during the design of the payuload.
REQUIREMENTS
The design requires the complete coverage of
Canada and the United States including Alaska,
Hawaii and Puerto Rico. The minimum aggregate
EIRP (AEIRP) is to exceed 54 dBW. Frequency
reuse capability is required between the east and the
west areas demanding a minimum of four beams
across the continent. Capability of adapting to
changing traffic patterns is also a requirement. As a
minimum the capability should allow power to be
moved between the two eastern beams and between
the two western beams though not between the east
and the west.
SYSTEM TRADE-OFFS
The system trade-offs described here include the
following:
1. Number of beams covering the continent.
2. Frequency reuse capability.
3. Low level beam forming for improved antenna
performance.
4. Capability of changing the allocation of power
and spectrum between beams to match the
evolving traffic pattern.
NUMBER OF BEAMS
Configurations have been evaluated for 10
beams, 8 beams and 4 beams covering Canada and
the United States not counting Alaska, Hawaii and
Puerto Rico, which require additional beam(s) in all
eases. The 10 beam and 8 beam configurations con-
sisted of 5 and 4 circular beams over Canada and an
equal number of slightly elongated beams over the
United States. The 4 beam configuration has elon-
gated beams each of which covers part of Canada
and part of the United States.
The advantage of fewer beams is the reduction of
spacecraft hardware associated with each beam and
the possibility of using a smaller L-Band reflector.
The advantage of more beams is the increased an-
tenna gain and resulting increased EIRP and in the
case of the 10 beam configuration, the increased
frequency reuse capability.
InternationalMobile SatelliteConference, Ottawa, 1990
423
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FREQUENCY REUSE
Frequency reuse patterns are well known and are
characterized by the number of frequency segments
required (one per beam) so that the beams are
sufficiently far apart and the frequency segments can
be reused. For the triangular beam format, the
frequency segments can number 4, 7, 9 or higher for
a symmetric reuse pattern. For a square beam for-
mat, the number can be 4, 9 or higher. Since the
North American continent favoured a square beam
format, the choice was between 4 and 9 segments of
frequency. The 4 and 9 segment plans require every
other beam and every third beam to use the same
frequency respectively.
To have frequency reuse in every other beam
requires that, not only must the first sidelobe be
suppressed but also that the main lobe of the nearest
alternate beam must not extend into the designated
reuse region. This demands very narrow beams
formed using large reflectors and results in a large
gain difference between the peak of the beam and
the edge of the coverage area. This causes a large
difference in carrier power through the reverse tran-
sponder and an increased demand for feeder link
powcr.
These problems are eased by reusing the spec-
trum only in every third beam. This allows use of a
smaller antenna diameter and reduced feeder link
transmitter power. There is a small impact on the
antenna gain with the smaller antenna as the system
is sized on the antenna gain at the cross over point
between beams rather than the peak antenna gain.
FLEXIBLE POWER DISTRIBUTION
In a multibeam environment, the traffic in each
beam may not be known at the time the payload is
designed, and in addition some changes in traffic
distribution can be expected during the life of the
spacecraft. If fixed power is assigned to each beam,
then it should be matched to the expected traffic in
each beam. Preferably, flexibility of power distribu-
tion should be built into the payload so that the
capability can be matched to the traffic as the traffic
evolves. A simplified block diagram of a transpon-
der with fixed power per beam is shown in Figure 1.
The disadvantage of timed power assignment can
be seen from Figure 2. The maximum power utili-
zation factor is shown as a function of the traffic
fraction serviced by a fully powered beam for three
cases; two beams with two equal power pools, four
beams with four equal power pools, and ten beams
with ten equal power pools. The power utilization
factor is defined as the total power radiated by all
beams as a fraction of the total power available in all
beams. This can be determined by looking only at
the performance of one amplifier in one beam. That
is, the power utilization factor is given by the power
radiated by one beam as a fraction of the total power
available in all beams divided by the fraction of the
traffic carried by that beam. The same utilization
factor is obtained by considering beams which are
not fully loaded. Also, power pools may be unequal
as is the case if the beam powers are taylored to the
expected traffic distribution. The utilization faction
will be on the appropriate line in the case of a fully
loaded amplifier and below the line for one not fully
loaded but the same utilization factor will be ob-
tained no matter which amplifier is considered.
As the traffic builds up, there will be a point at
which even the lightly loaded beams reach full power
and the power utilization factor will reach unity.
However, in the meantime, traffic has been rejected
in the heavily loaded beams.
A design approach is to interconnect the amplifi-
ers for a number of beams by means of a hybrid
matrix network I which combines the separate power
pools into a single power pool and allows any beam
to dip into the pool for the number of carriers pres-
ent at each instant of time. The hybrid matrix con-
sists of a network of 3 dB hybrids connected so that
a binary number of input and output ports exist. The
configuration for a 4 X 4 (4 input and 4 output ports)
hybrid matrix is shown in Figure 3. An input hybrid
matrix is placed before the amplifiers which takes
the signal at each input port and divides it equally
between all amplifiers. An output hybrid matrix is
placed after the amplifiers which collects the signal
from all amplifiers and directs it to the correspond-
ing output port. In this way, every signal extracts an
equal amount of power from all amplifiers in the
hybrid matrix power amplifier. A simplified block
diagram of the transponder is given in Figure 4
showing the location of the input and output hybrid
matrix. In this way for example, four beams with
four power pools can be converted to a single power
pool by connecting all beams to the hybrid matrix
network.
FLEXIBLE FREQUENCY DISTRIBUTION
In order to make use of the power flexibility, as
provided by the hybrid matrix, it is necessary to have
a corresponding amount of spectrum flexibility.
424
International Mobile Satellite Conference, Ottawa, 1990
Ku BAND
SECTION
(_J-LIK.BANDL
__J IRECEIVERI
I IF I L-BAND
I _SECTION SECTION
I
I
I
1
I
I
I
I
I
BANK
9_O135
Fig. 1. Forward repeater with f'Lxed power per
beam
2.7_
z
0.6.
_.5_
"_,4.
_.3.
re :
\ \,wo
POWER
FOUR _OLS
POWER
\ ,oocs
POOLS
IlllllUll
.1 .2 .3 .4 .$ .$ .7 ,8 .9
FRA_N _ TOTAL_AFFIC IN FULLY POWERED BE_
Fig. 2. Maximum power utilization curves for
2, 4 and 10 pools of power
/X
\
ii==,,_m_ I
QO0_ 38
Fig. 3. Block diagram of a 4X4 hybrid matrix
Ku BAND
SECTION
/_ KuBAND __RECEIVER
IF I L-BANDSECTION SECTION
INPUT
HYBRID
MATRIX
FILTER
BANK
OUTPUT
HYBRID
MATRIX
Fig. 4. Forward repeater with flexible power and spectrum distribution
International Mobile Satellite Conference, Ottawa, t990
425
Table 1. Comparison between filter bank and
filter switch matrix approach to
spectrum flexibility
Filter
switch
matrix
Filter
bank
Total bandwidth (MHz) 29 29
Filter bandwidth (MHz) 0.5 - 2.0 3.5
Switches per filter 3 to 5 one
Several filters connectable to yes yes
one beam
Several beams connectable yes yes
to one filter
Number of filters 40 per 8 per
use beam
This is provided in a stepwise fashion by switching
filters rather than in a continuous fashion as pro-
vided for power with the hybrid matrix. There are
two ways of implementing the filter switching as
shown in Figures 5and 6, namely a filter switch ma-
trix approach and a filter bank approach.
The filter switch matrix approach connects the
filters permanently to the feeder link and connects
them to the desired beam by means of switches. A
full 29 MHz of filters is provided for each frequency
use and connected to the feeder link for transmis-
sion to the ground.
The filter bank approach provides a full 29 MHz
of filters permanently connected to each beam and
these are connected or disconnected as desired.
The filter bank approach tends to have more
filtering and less switching whereas the filter switch
matrix approach tends to have less filtering and
more switching. However, to minimize the number
of filters in the filter bank approach, the filter
bandwidth is increased. A summary comparison be-
tween the two approachs is given in Table 1.
Table 2. Comparison of shared and un-shared
horn approaches
Reflector size (m)
Edge of coverage gain
Peak to edge gain delta
Reuse isolation
Un-
Shared
shared
hornshorns
5X6 5X5
Ref. + 1 dB
5-6 dB 2.5-3 dB
Ref. + 2-3 dB
LOW LEVEL BEAM FORMING
To provide good overlap between beams, it is
necesary to share horns between beams, that is to
use the same horn as part of the horn cluster for
adjacent beams. Because of the loss in the beam
forming network, the network is placed before the
final power amplifiers (or after the initial LNA's on
the receive side) with one amplifier used for each
radiating element. This is shown in simplified form
in Figure 7. The advantage of sharing horns in this
way _s a higher antenna gain at the cross-over point
between beams and a lower gain variation across the
beam compared to un-shared horns. An additional
advantage is the reduction of sidelobes in the reuse
region. The disadvantage with shared horns is that
the antenna and the transponder are intermingled
and the performance of each can not be separately
evaluated. Table 2 summarizes the performance
obtained for M-SAT with both shared and un-
shared horn approachs.
REFERENCES
1. Egami and Kawai. May 1987. An Adaptive
Multiple Beam System Concept. IEEE Jour-
nal on Selected Areas in Communications,
SAC5, pp. 630-636.
426
International Mobile Satellite Conference. Ottawa, 1990
BEAM PORTS
1 2 N
FIRST USE
-- 29 MHz
_ SECOND USE
29 MHz
900132
Fig. 5. Filter switch matrix configuration
BEAM PORTS1 7
I
I
I
I
I
I
I
I
I
I
I
I
I
I
N'--
FILTER BANK 1
29 MHz
i
FILTERBANK
29 MHz
-- FIRST USE
--SECOND USE
900133
Fig. 6. Filter bank configuration
FILTER
BANK
LOW
LEVEL
BFN
L-BAND
SECTION
INPUT I OU )
HYBRIDI I HY
MATRIX % MA
-i
J i
9OO134
Fig. 7. Forward repeater with low level beam forming and flexible power and spectrum distribution
International Mobile Satellite Conference, Ottawa, 1990
427



Payload system tradeoffs for mobile communications satellites

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