PROSPECTS
FOR COMMERCIAL
SATELLITE
SERVICES AT Q- AND V-BANDS
J. V, Evans and A. Dissanayake
COMSAT Corporation
6560 Rock Spring Drive
Bethesda, MD 20817
ABSTRACT
Some 16 proposed new satellite systems operating at 36 to
46 GHz (Q-band) and 46 to 56 GHz (V-band) have been
proposed to the U.S. Federal Communications Commission by U.S. companies, Of these, 14 are intended to provide global, or nearly global, service. One is intended for
U.S. domestic service, and one is a package to provide
additional store-and-forward capability on an earlier proposed “Little LEO” system. This paper provides a brief
summary of the 14 global systems, which for the most part
are designed to exploit the wide band offrequencies available for services such as multimedia distribution and Internet access.
Systems are proposed that would use geostationary orbit,
medium earth orbit, low earth orbit, and Molniya orbit
satellites, and in some cases combinations of two of these
orbits. Most of the new systems propose to employ new
technologies such as multiple narrow spot-beam antennas,
onboard demodulation and routing of trafic between beams,
intersatellite links, and in some cases scanning beams to
continuously illuminate the service area as the satellite
flies by.
Some of the difficulties involved infielding systems at these
high frequencies arise from the propagation impairments
that can be expected and the high cost of solid-state power
devices for user terminals, which will drive up costs. It is
concluded that, while the large amount of bandwidth (3
GHz) proposed by the FCC for these systems is attractive,
few if any are likely to be built while spectrum remains
available at Ku-band.
1. PROPOSED
SYSTEMS
In 1997, following applications by Motorola for a satellite
system called M-Star, and Hughes for a system called Expressway, the U.S. Federal Communications Commission
(FCC) opened a Notice of Inquiry offering others the opportunity to propose systems operating at millimeter waves
(Q and V-bands). The FCC-proposed frequencies for the
Q/V-band systems are given in Table 1.
Table 2 lists (alphabetically by company name) the 14
global Q/V-band satellite systems proposed to the FCC [1],
Included in the table are the name of the system, some
indication of its coverage, and the type of orbit(s) to be
empIoyed. It can be seen that geostationary earth orbit
(GEO) remains the orbit of choice for most systems. Geostationary satellites require no tracking by earth station
antennas, which greatly simplifies their cost, installation,
and maintenance. This makes economic sense, given that
most of the cost of a fully deployed system can be in the
ground segment. Low-earth-orbiting (LEO) systems have
been proposed (e.g., by Teledesic at Ka-band) out of concern for the half-second response time encountered over
geostationary satellites, which can limit the speed of access
to the Jnternet and hinder real-time applications such as
teleconferencing.
Two systems (Hughes Communications’ StarLynx system
and TRW’s GESN) are hybrids employing both GEO and
medium earth orbit (MEO) satellites. Hughes took this
approach because its business plan calls for providing service initially over North and South America via GEO satellites, and then launching the MEO system as the amount of
traffic increases worldwide. TRW, on the other hand, has
designed a system that exploits the advantages of both
types of orbits, and one in which intersatellite links will be
used to route traffic along all possible satellite-to-satellite
paths.
Additional information given in Table 2 includes the number of satellites to be deployed; the anticipated total satellite capacity (when fully loaded); whether intersatellite links
will be used; the type of onboard routing contemplated;
and overall projected capital cost. The latter usually includes the cost of the first year of operation.
All of the 14 systems proposing to offer global or nearly
global service employ some form of onboard routing. This
is necessary because designers have had to exploit narrow
spot beams in order to overcome propagation effects (as
discussed below), and are then confronted with the problem of how to route traffic among the beams.
Table 3 provides details of the RF portions of the payloads,
including the number of beams to be used; their size; and
Table 1. FrequenciesProposed for Q/V-bandSystems
Downlink
Uplink
Orbit
(GHz)
(GHz)
Geostationary
37.5-40.5
47.2–50.2
Non-~eostationarv
37.5–38.5
48.2-49.2
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Table2. ProposedU.S. Q/V-bandSatellite Systems
Satellite
Capacity
(Gb/s)
Capital
Investment
($W
Orbit
No. of
Satellites
Pentriad
Molniya
9
25°N–
85°N
<36
No
MSM
1.9
GE Americom
GE*Star Plus
GEO
GS-40
LEO
Global
*70°
-70
-1
2? optical
Glcbalstar L.P.
MSM
MSM
3.4
?
Hughes Comm.,
Inc.
Expressway
GEO
11
80
14
Limited
global
-65
Optical
3 Gb/s
SSTDMA
3.9
Hughes Comm.,
Inc.
SpaceCast
GEO
6
Limited
global
-@
Optical
3 Gb/s
SSTDMA
1.7
Hughes Comm.,
Inc.
StarLynx
GEO &
MEO
4
+80°
2 optical
5 optical
Baseband
2.9
Lockheed
Martin
QJV Band
GEO
9
Global
3 optical
2 radio
ATM
baseband
4.75
Loral Space&
Comm. Ltd.
Cyberpath
GEO
10
Global
17.9
2 radio
ATM
baselmd
1.17
(for 4)
Motorola
M-Star
LEO
72
~60°
-3.6
27 radio
MSM &
SSTDMA
6.4
Orbital Sciences
Corp.
Orblink
MEO
7
*50°
2 radio
MSM
0.9
PanAmSat
VStream
GEO
12
Global
2? radio
MSM
3.5
Spectrum Astro,
Inc.
Aster
GEO
25
Global
2 optical
SSTDMA &
Baseband
2.4
Teledesic
VBS
LEO
72
Global
4 optical
Baseband
1.9
TRW
GESN
GEO &
MEO
14
15
+70°
10 optical
4 optical
Baseband
3.4
Company
D enali Telwom.
System
LLC
Coverqe
<5.9
<6.3
20
545
-75
<3.2
-lo
4
-50
-70
Intersatellite
Link
No
Onboard
Switching
MSM: Microwaveswitchmatrix.
and power of the transponders. In
some instances, the system design is too complex to fit
neatly into this format. For example, a number of designs
employ transponders (the frequency-changing, bandwidthdefining portion of the payload) that are shared by a common power amplifier. This is invariably the case where the
onboard routing is performed via a static switch matrix, for
example. Thus, the number of transponders and the number of power amplifiers are not necessarily the same (as is
usually the case in simpler “bent pipe” satellites). Another
example is the case of active phased arrays, where all the
amplifiers in the array may contribute some power to all
the beams, and the notion of a single power amplifier that
excites a single beam no longer applies.
the number, bandwidth,
2. .Q/V-BAND PROPAGATION
Propagation effects on satellite links have been studied
by a number of groups employing satellite beacons and
other methods. (For review, see Allnutt [2] and Crane [31.)
Table 4 provides a simple checklist of the impairments that
can be encountered in the different wavebands. It is evident that effects become more severe at shorter wavelengths.
2.1 Gaseous Absorption
Gaseous absorption depends on the frequency (Figure 1),
the path length through the atmosphere, and the atmospheric humidity. Figure 2 shows the absorption as a ftmction of humidity for 40 and 50 GHz at an elevation of 20°,
There is little the satellite designer can do to avoid these
effects other than to design a system that allows the earth
station to view the satellites at a high elevation angle. This
is best achieved with MEO satellites in polar or near-polar
orbits, The original plan for the Teledesic Ka-band system
entailed the use of 840 satellites [5], and was driven by a
decision to serve terminals only within 0° to 40° off the
nadir of the satellite. This is obviously an expensive and
extreme solution. Somewhat interesting in tiis regard is
the Pentriad system (Table 2), By exploiting Molniya orbits, the system provides coverage of the more populous
regions (northern hemisphere latitudes between 25”N and
85”N) at elevation angles above 40°.
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Table3. Parametersof U,S. ~-band
No.of
No. of
Beams
System
Pentnad
Beam
Size
80
12
w v -Lmncl
12W-band
40 V-band
9 Ku-baud
400
1° x 3“
16
30 steered
2°
30
Expressway
20 V-band
8 Ku-band
0.15°
1° x 3°
20
8
SpaceCast
40 V-band
16 Ku-band
40
16
StzuLynx
40 GEO
32ME0
0.15°
1° x 3°
0.15°
0.6°
Lockheed
Martin
Q/V Band
9
0.30°
Cyberpath
100
0.42°
M-Star
32
-1.1°
100
VStream
20
8 spot
Aster
8 regional
2 steerable
300
250
18/90
300
250
300
250
270
40
32
48 user
8 gateway
EIRP
(CtB1’v)
73.0
77.7
End-of-Life
Power
(m
Dry Mass’
0%)
Life
(Yr.)
6.000
1,444
10
--5,500
15
100
25
59
52
15,000
48
52
4,500
992
7.5
100
25
55
48
15,000
3,500
15
100
150
72
55
70.5
56
15,000
3,500
15
15,000
3,500
15
12
3,900
4,130
15
19,800
4,315
15
1,530
1,004
8
4,000
1,268
9
5,000
3,500
-’5,000
15
1,120
15
7
100
50
125
Active
phased
array
100
142
40
104
90
800 narrow
20 wide
-0.5°
TWA
Power
(W)
80
78
-0.4°
0.30°
GS-40
Orblink
Transponder
Bandwidth
(MHz)*
Earth-Space
Transponders
Satellite Systems
Active
phased
array
50
1,000
25
64.2 user
62.7 gateway
77.5
21-29 to
MTSO
33-43 to
cell site
62
60
1.5°
80
375
75
60
0.50 spot
1° & 5°
regional
18
470 spot
980 regional
50
66.7
60.7
61.4
63
59
1,250
566
83
78
12,700
15,000
2,769
2,707
VBS
32 Up
40 down
“small”
GESN
32 GEO
48ME0
32
48
?
300/
3,000
32
48
Active
phased
array
15
* Defined here as the passband(s) of the frequency-changing portion(s) of the transponder.
Table4. PropagationFactors AffectingSatelliteLinks
Propagation
Factor
Cband
Kaband
Vband
GaseousAbso@ion
x
x
CloudAttenuation
x
x
x
x
x
RainAttenuation
Kuband
Rain/IceDepolarization
x
x
x
x
TroposphericScintillation
x
x
x
x
One consequence of operating the uplink (V-band) close to
the molecular absorption line (shown in Figure 1) is that
there can be a significant difference in the atmospheric
attenuation between the two edges of the band. This is
illustrated in Figure 3, which shows estimates for this effect, including rain attenuation and other impairments, as a
function of the percentage of time for a station in Singapore
operating at 20° elevation, The curves show that, for
1 percent of the time, attenuation cam exceed 4 dB at the
uplink frequency and 2 dB at the dowrdink frequency.
2.2
Rain Attenuation
By far, the most severe problem facing the designers of
Q/V-band satellite systems is rain attenuation. This problem has been studied by various groups over a wide range
of frequencies and climates. The International Telecommunication Union (ITU) climate model is shown in Figure 4.
Here, the ‘A’ regions are the driest, and the ‘P’ regions
have the heaviest thundershower activity, which is responsible for the severest absorption events.
Figure 5 shows model calculations for rain attenuation for
five of these climatic regions (E, H, K, M, and P) for the
V-band uplink frequency. It can be seen that the design of
0-7803-4902-4/98/$10.00 (c) 1998 IEEE
103
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fi6
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–
&
$4
–
$
K
p2
~
c1
—
o
0.1
I
100
PERCENT TIME ORDINATE EXCEEDED
Figure 3. Cumulative Distribution of Differential Attenuation
Across the 3-GHz Band at Downlink Frequency (40 GHz)
and Uplink Frequency (50 GHz) at Singapore
(elevation angle = 20°) [6]
1IJ-2 I
I
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10
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102
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FREQUENCY (GFk)
Figure 1. Total Zenith Attenuation due to Gases, as a Function
of Frequency (a = range of values. Curve A dry atmosphere.
Curve B: exponential water-vapor atmosphere of 7.5 g/m3 at
ground level. Scale height = 2 km) [4]
165°
135°
105°
75°
45”
15°
o“
15”
45”
75”
105”
Figure 4. ITU-R Rain Zones [7]
- —.
o
20
40
60
REIATIVE HUMIDITY
80
100
I 00 -
(Y.)
80 -
Figure 2. Gaseous Absorption as a Function of Relative
Humidity bath elevation angle = 20°, surface
temperature = 20°C) [4]
global systems operating at these frequencies becomes especially challenging. To achieve an availability of 99.5
percent in all regions requires an excess margin of approximately 55 dB on the uplink. Since such large amounts of
excess link margin are economically impractical, the actual
availability will vary with climate. In addition, provision
of some services may be unacceptable once availability
60 401
20<
n
-0.1
10
1
PERCENT TIME ORDINATE EXCEEDED
Figure 5. Rain Attenuation Cumulative Distributions in
Different ITU Rain Zones at the Uplink Frequency (50 GHz)
Using the DAH Model (elevation angle= 20°) [61
0-7803-4902-4/98/$10.00 (c) 1998 IEEE
100
drops below a certain level, so that marketing of the system’s
capabilities may have to be carried out at a regional level.
2.3 Total Attenuation
Figure 6 provides model calculations for the combined
effects of all of the impairments cited in Table 4 at Ka-,
Q-, and V-bands for two locations, according to a methodology proposed by Dissanayake et al. [6]. Clarksburg, Maryland, is a mid-latitude station with a moderate rainfall climate (’K’ in Figure 4), while Singapore has a tropical
climate (’P’ in Figure 4) and represents a “worst case.”
Also, the elevation angle of 20° used in these calculations
represents the worst situation likely to be encountered in
any of the designs.
3. SYSTEM DESIGN CONSIDERATIONS
Section 1 described a wide range of approaches taken in
designing the Q/V-band systems that were filed with the
FCC. For the 14 global systems, there are seven system
designs that employ GEO satellites, three LEO, one MEO,
one Molniya, and two combinations of GEO and MEO.
Three designs employ fixed antenna beams, but two allow
beams to be selected (from a larger number of possible
choices) after launch. Five systems employ scanning or
steered beams, and six have beams of two different types.
The choice among routing schemes is equally varied. Six
systems employ frequency-selective routing (i.e., using a
static switch matrix), while five intend to switch at baseband. TWOof the systems will employ time-division multiple access (TDMA) and use satellite-switched TDMA
(SSTDMA) for routing (i.e., briefly connect each receiving
beam to some, or all, of the transmit beams in a cyclical
fashion). l%o systems employ two of these schemes for
routing.
This spread of design approaches reflects the fact that all
of the systems represent fairly radical departures from current GEO bent-pipe satellite practice, and a new universally recognized optimum approach to serving the data/
multimedia market has yet to emerge. All of the systems
share a few common design themes. Achieving adequate
link margins is of paramount importance in view of the
propagation impairments discussed above. The subsections
that follow discuss some of the pros and cons of the design
choices that were made.
3.1 Choice of Rain Fade Mitigation Scheme
‘al
1
10
103
PERCENT TIME ORDINATE EXCEEDED
(a)
150
1
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Clarksburg,
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90 –
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60 <
30 -
n
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10
1
100
PERCENT TIME ORDINATE EXCEEDED
(b) Singapore
Figure 6. Predicted Cumulative Distribution of Totat
Attenuation (elevation angle = 20°)
Most of the designs incorporate features to mitigate rain
fade. To reduce the link margin required for good bit error
ratio (BER) performance, satellite links are currently designed with forward error correction protection. The best
practice involves the use of concatenated coding [8]. Figure 7 shows the performance improvement achieved by
using rate 3/4 convolutional coding/Viterbi decoding, and
when such coding is concatenated with an outer (ReedSolomon 208,188) block code. It can be seen that the
signal-to-noise ratio required to achieve a BER of 10-10has
been decreased from about 13 dB to 5 dB. This approach
will be a necessity for Q/V-band.
A second approach that is contemplated for many of the
designs is uplink power control. This entails raising the
power of the uplink earth station during rain. The decision
to go to high power can be made by command from the
satellite, or autonomously by observing the strength of the
received (40-GHz) signals. Because of the onboard processing employed in some systems, it may not be necessary to maintain the signal arriving at the satellite at a
prescribed level, since no one carrier can “steal” all o-fthe
transponder power, and a simple switch to “high power”
may suftice. However, high power may require an increase
of 10 dB or more—perhaps requiring switching from a
solid-state amplifier to a traveling wave tube-and is likely
to be very expensive.
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10-2
high power, Given that for most of these systems the earth
station equipment cost is likely to exceed the space segment cost, this seems a particularly poor compromise.
10-4
Approach 3 appears to be the only credible approach where
the number of beams, n, is large (creating n2possible routes);
however, it exacts severe cost and weight penalties. It also
forces users to operate at one of the few standard rates and
modulation methods the onboard processor is designed to
support.
K
~ 10-6
NO CODING
1o-8 :
~:
R = y4 CODING
CONCATENATED
CODING
.
0
3.3 Earth Terminal Technology Considerations
5
10
15
Eb/No (dB)
Figure 7. Theoretical Performance of QPSK Modulation
(a) With No Coding, (b) For Coding With Rate 3/4
Convolutional Coding and Viterbi Decoding, and
(c) For Reed-Solomon (RS208,188) Coding
Concatenated With Convolutional Coding
3.2 Choice of Onboard Routing Approach
All 14 of the global multimedia/data distribution systems
discussed herein employ some form of onboard routing.
This is necessary in order to route traffic from beam to
beam, and in many cases from satellite to satellite, via the
intersatellite links. Onboard routing can be implemented in
one of three ways:
1. Subdivide the traffic in each beam into several (e.g.,
10) channels, using filters, and employ a static switch
matrix in the IF stages of the transponders to crossstrap traffic streams in various beams.
2. Employ SSTDMA, in which each uplink beam is con-
nected for a brief period, cyclically, to a large number
of the dowrdink beams.
Satellite systems intended to serve a consumer market must
be designed to operate with inexpensive terminals if they
are to be commercially successful. Unfortunately, at present
there appears to be no such thing as an “inexpensive Q/Vband terminal,” and this could be the most significant drawback in all of the systems proposed.
While the indoor portion of the terminal can largely be
constructed of application-specific integrated circuits
(ASICS) at low cost, this is not true of the outdoor unit.
Currently, the only practical solid-state devices operating at 40 and 50 GHz are pseudomorphic high-electronmobility transistors (P-HEMTs). Single devices are capable
of generating up to approximately 200 mW of power and,
by combining devices, an amplifier of approximately 1 W
can be constructed. Above this level, the losses in the combining networks make further paralleling of amplifiers unattractive and it becomes more sensible to combine in space,
using a phased array. For receive applications, P-HEMTs
must again be used in the low-noise amplifiers and are
capable of providing a noise figure of approximately 3 dB.
Absent a very large demand (in the millions), the cost of
these devices will remain high.
and results in the saturation of some pathways through the
satellite, often while others are greatly underutilized.
4. CONCLUDING REMARKS
It is tempting to dismiss all of these systems as “paper
satellites” that will never be built, but that would be a
mistake. The telecommunications satellite industry has been
transformed by the decisions of four major U. S. manufacturers to enter the service business. First to take this step
was Hughes, and Loral has since followed suit. Lockheed
Martin and Motorola intend to join them. The intentions of
TRW and Boeing remain to be determined. These large
companies have the financial wherewithal and the technical ability to propose, develop, and launch systems qpite
beyond the scale of the more cautious intergovernmental organizations (INTELSAT and Irunarsat) or regional
investors.
Approach 2 forces users to employ TDMA and drives up
the cost of the earth station equipment, which now requires
Despite these advantages, the absence of components for
Q/V-band systems—combined with the severity of the rain
3. Demultiplex (i.e., separate in frequency) each of the
arriving carriers, demodulate them, remove any coding,
and then route the packets (according to a header containing an address) via a baseband digital switch (in
some filings called an “ATM switch”) to the appropriate port, where the packets can be re-encoded and modulated onto a single carrier that serves the desired downlink beam.
Each approach has both advantages and disadvantages.
Approach 1 is easiest to implement but not very flexible,
0-7803-4902-4/98/$10.00 (c) 1998 IEEE
fade and atmospheric attenuation problems—are expected
to delay the introduction of nearly all of the systems described here until the Ka-band spectrum is saturated. That
is, the orderly progression of C- to Ku- and Ku-to Ka-band
is likely to continue, with few companies venturing into
Q~-bmd
[5]
[6] A. W. Dissanayake, J. E. Allnutt, and F. Haidara, “A Prediction Method That Combines Rain Attenuation and Ckher
Propagation Impairments Along Earth-Satellite Paths,”
IEEE TransactionsonAntennas andPropagation,Vol. AP45, No. 10, October 1997, pp. 1546-1558.
any time soon.
REFERENCES
[1]
M. A. Sturza, “Architectureof the TeledesicSatellite System,” InternatiomdMobile Satellite Conference, Ottawa,
1995,Proc., pp. 212-218.
FCC Filings, U.S. Federal Communications Commission,
1997.
[7]
ITU Recommendation ITU-R P.837, International ‘lrelecommunication Union - Radio Sector, Geneva, 1997.
-
Denali Telecom, LLC - Pentriad System (NGSO)
File No. 160-SAT-P/LA-97(13)
[8]
G. C. Clark, Jr., and J. B. Cain, Error-CorrectionCoding
for Digital CommunicationsSystems, New York: Plenum,
–
GE American Communications, Inc. – GE*StarPlus System (GSO)
File Nos. 139 through 147-SAT-P/LA-97
1981.
— Globalstar, L.P. – GS-40 System (80 NGSO satellites)
File Nos. 157/158-SAT-P/LA-97
— Hughes Communications, Inc. – Expressway Systems
(GSO)
File Nos. 90-SAT-P/LA-97 (A) and 119 through 127SAT-PLA-97
— Hughes Communications,
Inc. – SpaceCast System
(GSO)
File Nos. 148 through 15l-SAT-P/LA-97
— Hughes Communications, Inc. - StarLynx System (GSO/
NGSO)
File Nos. 157/158-SAT-P/LA-97 and 159-SAT-P/LA97(20)
Motorola Global Communications, Inc. – M-Star Systems (NGSO)
File Nos. 157-SAT-P/LA-96(72), 29-SAT-AMEND-97,
and 128-SAT-AMEND-97
–
ORBLINKLLC – NGSO System
File No. 138-SAT-P/LA-97(7)
— PanAmSat Corporation - VSTREAM System (GSO)
File Nos. 162 through 172-SAT-P/LA-97
Spectrum Astro, Inc. - Aster Satellite System (GSO)
File Nos. 173 through 177-SAT-P/LA-97
–
Teledesic
File No. 178-SAT-P/LA-97
— TRW-EHF Satellite Network
File Nos. 112 through 116-SAT-P/LA-97
[2]
J. E. Allnutt, Satellite-to-GroundRadiowavePropagation,
London: Peregrinus, 1989.
[31 R. K. Crane,ElectromagneticWavePropagationThrough
Rain, New York: John Wiley, 1996.
[4]
ITU Recommendation ITU-R P.676, International Telecommunication Union - Radio Sector, Geneva, 1997.
0-7803-4902-4/98/$10.00 (c) 1998 IEEE