GLOBALSTAR is a satellite-based mobile communications system that is interoperable with the current and future Public Land Mobile Network (PLMN) and Public Switched Telephone Network (PSTN). The selection of the transponder type, bent-pipe, or onboard processing (OBP), for GLOBALSTAR is based on many criteria, each of which is essential to the commercial and technological feasibility of GLOBALSTAR. The trade study that was done to determine the pros and cons of a bent-pipe transponder or an onboard processing transponder is described. The design of GLOBALSTAR's telecommunications system is a multi-variable cost optimization between the cost and complexity of individual satellites, the number of satellites required to provide coverage to the service areas, the cost of launching the satellites into their selected orbits, the ground segment cost, user equipment cost, satellite voice channel capacity, and other issues. Emphasis is on the cost and complexity of the individual satellites, specifically the transponder type and the impact of the transponder type on satellite and ground segment cost, satellite power and weight, and satellite voice channel capacity

Louie, Ming

Monte, Paul A.

Wiedeman, R.

English

NASA Technical Reports Server

N92- 14206..
Mobile Telephony Through LEO Satellites: To OBP orNot
Paul A. Monte and Ming Louie
Space Systems/Loral
Palo Alto, CA
R. Wiedeman
Loral Aerospace Corporation
San Jose, CA
GLOBALSTAR is a satellite-based mobile communications system that is interoperable with the
current and future Public Land Mobile Network (PLMN) and Public Switched Telephone Network
(PSTN). The selection of the transponder type, bent-pipe or on-board processing (OBP), for GLOBAL-
STAR is based on many criteria, each of which is essential to the commercial and technological feasibil-
ity of GLOBALSTAR. This paper describes the trade study that was done to determine the pros and
cons of a bent-pipe transponder or an on-board processing transponder.
The design of GLOBALSTAR's telecommunications system is a multi-variable cost optimization
between the cost and complexity of the individual satellites, the number of satellites required to provide
coverage to the service areas, the cost of launching the satellites into their selected orbits, the ground
segment cost, user equipment cost, satellite voice channel capacity and other issues. This paper focuses
on the cost and complexity of the individual satellites, specifically the transponder type and the impact
of the transponder type on satellite and ground segment cost, satellite power and weight, and satellite
voice channel capacity.
Introduction to GLOBALSTAR
GLOBALSTAR is a satellite system which offers global mobile voice and data services and
radio-determination satellite services (RDSS) to and from hand-held and vehicle-mounted transmit and
receive devices. By combining the use of low-earth orbit (LEO) satellites with existing terrestrial com-
munications systems and innovative, highly efficient spread spectrum techniques, the GLOBALSTAR
system provides users throughout the world with low-cost, reliable communications. The system uses a
constellation of 48 operating LEO satellites to provide optimum global coverage.
Because 90 percent of all traffic from a given point will be accommodated by a single gateway,
GLOBALSTAR has been configured to link the mobile unit to a terrestrial gateway through a single
satellite so that the system requires no satellite traffic crosslinks. GLOBALSTAR incorporates existing
terrestrial communications facilities into its overall configuration through gateway earth station inter-
faces. The interoperability of GLOBALSTAR with the PSTN enhances the system's reliability and
decreases costs to the end user by decreasing the complexity of the space segment. By complementing
rather than supplanting existing carriers' networks and by sharing revenues with existing carriers, GLO-
BALSTAR can achieve rapid adoption throughout the United States and the world.
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GLOBALSTAR proposes three alternative spectrum plans. For brevity, this paper will use only
the one employing L-band with C-band feeder links. This system makes bidirectional use of the allo-
cated RDSS spectrum in the L-band (1610-1626.5 MHz).
The GLOBALSTAR system is designed to operate compatibly with other LEO satellite systems
providing RDSS, voice and data services and can operate without causing harmful interference to geos-
tationary, RDSS systems, radio navigation systems and GLONASS. Moreover, the capacity of the
GLOBALSTAR system will be only slightly degraded by operations from those systems.
Criteria & System Definition
The selection of transponder type, bent-pipe or OBP, was based on the simultaneous solution of
many criteria: the complexity and cost of the individual satellites, the weight of the communications
payload, the power requirements of the communications payload, the availability of equipment (or the
amount of development required), satellite voice channel capacity, security (both for privacy and for
fraud), and quality of service. There is also a trade-off in cost and complexity between the satellite and
the gateways.
The 48-satellite constellation is the Walker 48/8/1 constellation (ref. 1) at an altitude of 1389 km.
(750 nm.) with an inclination of 52 degrees. This constellation has 48 satellites in eight orbit planes, all
with an inclination of 52 degrees. The phasing of the satellites from one plane to another is shifted by
7.5 degrees. This 48 satellite constellation provides 100% single coverage from the 65 degrees south
latitude to 65 degrees north latitude with a minimum elevation angle (the angle from the horizon to the
line of sight between the user and the satellite) of 10 degrees and provides 100% double coverage of the
continental United States with a minimum elevation angle of 10 degrees with one satellite at a higher
elevation angle than 15 degrees.
With the satellite constellation in a non-polar, inclined orbit, the coverage is constantly changing
for a particular point on the earth (although it is predictable): sometimes the covering satellite is mov-
ing northeast, sometimes southeast; sometimes there are three or four satellites in view. As the cover-
age areas for the satellites move across one another, there will be self-interference (or the system has to
be tightly controlled to prevent it). This will be taken into consideration in determining the transponder.
There might also be interference from other operators.
With the satellite constellation defined, the satellite lifetime defined (7.5 years), input from market
studies and input regarding the usage of cellular phones, a program was run to see the maximum number
of circuits required during the busy hours in the seventh year of operation. In the seventh year, there
will be approximately 1.5 million users which will require a satellite capacity of 1900 channels during
the busy hour.
Code Division Multiple Access (CDMA) is the access of choice. CDMA achieves its high capa-
city by achieving more effective frequency reuse than other methods. This technique allows a statistical
averaging principle known as the "Law of Large Numbers" to come into effect with the result that fre-
quency reuse is more efficient than with Frequency Division Multiple Access (FDMA) or Time Division
Multiple Access (TDMA) techniques. The CDMA technology used in this system exploits the follow-
ing techniques in obtaining high spectral efficiency: voice activity, error detection and correction,
efficient demodulation, antenna directivity (spot beams) and multiple satellites.
Spot beams are required to deliver the capacity required to and from the mobiles. GLOBALSTAR
uses six elliptically shaped spot beams. The major axis of the elliptical beams are aligned with the velo-
city vector of the satellite movement in order to decrease the number of inter-beam handoffs. Figure 1
shows the six spot beams illuminating the United States.
34
The spot beams are designed to compensate for the difference in the satellite-to-user link losses
between the "near" and "far" users, so that the power flux density of the "far" users is about the same as
the "near" users (isoflux design). This antenna design reduces the near-far problem, decreases the range
of power control required for CDMA and increases the capacity of the system. Figure 2 shows a cross
cut of the antenna pattern.
%
,, i
i
I
0 50 I00
__. _,_= =_ Degrees
Figure 1 Spot Beams on the US Figure 2 Isoflux Antenna Pattern
The multiple access technique used for the system is Time Domain Duplex-Frequency Division-
Code Division Multiple Access (TDD-FD-CDMA). The most efficient method of using L-band in both
the user-to-satellite and satellite-to-user directions is TDD. The 16.5 MHz at L-band is divided into 13
sub-bands of 1.25 MHz CDMA channels with each channel being TDD. There are multiple CDMA
users in a 1.25 MHz channel. The frequency plan of how the user uplinks are translated to the gateway
downlinks is shown in Figure 3. Beam hopping is used to minimize the interference from one beam to
another beam and decrease the interference to/from other systems. The system has a 60 msec TDD
frame with six 10 msec time slots. Three time slots are allocated for transmit and three time slots are
allocated for receive. Within an individual time slot, the signals will either transmit or receive two of
the six beams (e.g. beams one and four, or beams two and five, or beams three and six). This is shown
in Figure 4.
The system trade-off left the following limits on the satellites: the cost could not be more than $10
million per satellite; the DC power required by the communications payload could not be more than 750
watts; the satellite could not weight more than 500 kilograms.
A block diagram was designed for the transponders (shown in Figure 5) which shows where the
OBP equipment would be placed.
Link Budgets
Link budgets were produced for the bent-pipe satellite and the OBP satellite to compare the capa-
city and power consumption of the two different transponders. The bent-pipe link budget is shown in
Figure 6 and the OBP link budget is shown in Figure 7.
Each Figure has both the forward path (Gateway to User - columns B and H) and the return path
(User to Gateway - columns C and G) as both paths need to be examined to see the trade-offs in the sys-
tem regarding the link budgets. Lines six through 15 calculate the EIRP. In the ground to space links,
35
125 BEAM L2. OR 3 BEAM 4.5, OR 6
I_ I _ RHCP ' LHCP
1610 M-IZ __1626 5 MHz 16 HZ / 16_5 S _4HZ •
__ i Y........ .... FRE3 . ......
CI , , W.q7 ,'
51_ S M)_Z _2"_ 0 MHr BEAM 3 /t
Figure 3 Frequency Plan / _Y,-_]J ._V ..... ,' r-_,_, : ,/_.......'
ZZ]tJ_J/ _1 .... .... _','_, _,, 6E.M E ,E'.,E',E
,' ' ,' _ , ,, 't BE_k_ E
CHI CH2 CH13
L BAND UP & DOWN UNK$ L BAN0 UP & DOWN LINKS iFOR BEAMS 1.2,3 FOR BEAMS 4,5_5
CDUA
VOICE & DATA
C_AN_ELS
Figure 4 TDD-FD-CDMA
C-BAND L-BAND USER, C-BAND GATEWAY UNKS
RECEIVE
ANTENNA
MULTI-PORT L-BAND
L-BAND AMPLIFIER MULTI-BEAM ANTENNA
1 1 i_l
L-BAND BEAM TIMING DIAGRAMi,.,i,.i- i,. i- i- iC-BAND I"Ce ._: I L___ _ ........
I TIMII_ I: II LEGEND:
LOP IRCP BpF C_BAN_. I ' L_A JL_M -_ ! " --=
NOTE: REDUNDANCY NOT SHOWN
FOR SIMPLICITY
Figure 5 Transponder Block Diagram
the power is ElRP/user whereas in the space to ground links the power is EIRP/I.25 MHz channel. The
number of users denoted is the number of users in a 1.25 MHz channel. Due to the beam hopping and
TDD operation, a transmitter is only on for 1/6 of the time, therefore the transmitter power shown is the
power required when the system is transmitting - not the average power.
As spacecraft antenna are isoflux, the elevation angle does not affect antenna gain minus space
loss. The antenna gain increases as the space loss increases. The antenna gain shown is the gain with
regard to the space loss at nadir (that is why the space loss used is the loss at nadir). There are three
antennas at each gateway and each of the antennas track a specific satellite in view. A 1 dB tracking
loss is taken. The transmitted data rate is six times the actual data rate due to the beam hopping and
TDD operation.
36
All the links are chip synchronous CDMA except for the user to satellite links. Orthogonal codes
are used so there is literally no self-interfence in those links. There are 128 orthogonal codes based on
Walsh functions. There is very little self-interference on the gateway to satellite uplink. The gateways
and satellites (in the OBP case) use convolutional, rate I/2, constraint length 9 encoders with interleav-
ing. The user unit uses a convolutional, rate 1/3, constraint length 9 encoder. There is a 1.3 dB interfer-
ence margin required for the fading, blockage and power control. In the bent-pipe user to gateway link,
a 1.0 dB modem/Doppler loss is taken due to the Doppler estimation required by the gateway. This is
much more accurate in the OBP user to satellite link because the satellite can make faster and more
accurate Doppler estimations.
Figure 70BP Link Budget
For the bent-pipe links, the user to satellite has an additional 10% of the users due to interbeam and
intersatellite interference. Also note that this additional power is used on the satellite to gateway down-
link because it is a bent-pipe (all that is received is transmitted). The satellite to user downlink has 20%
of intra-satellite interference and 25% inter-satellite interference. The satellite to gateway link is the
limiting link due to the fact that the users within a channel are not chip-synchronous. This self-
interference is the limiting factor for the bent-pipe transponder. The bent-pipe satellite can support
1950 simultaneous duplex calls and required 681 watts of DC power (while transmitting).
For the OBP links, the user to satellite path has an additional 10% interference along with the non-
synchronous users in each channel. As in the bent-pipe case, this link is the limiting path. The satellite
to user downlink and the satellite to gateway downlink have 20% if intra-satellite interference while the
satellite to user path has an additional 25% due to inter-satellite interference. The OBP satellite can
support 1950 simultaneous duplex calls while requiring only 565 watts of DC power for the
transmitters. This does not take into account the additional power required for the OBP digital equip-
ment and control.
37
ORIGINAL PAGE IS
OF POOR QUALITY
However, the OBP satellite is not limited to 1950 calls. The OBP satellite can serve up to 2300
duplex calls until the user to satellite path self-interference limits the number of users (the required com-
munications payload power is 720 watts).
Comparison
The link budgets show that the OBP transponder is more RF power efficient than the bent-pipe
transponder (0.29 watts/call versus 0.35 watts/call for the bent-pipe transponder, an 18% savings).
However, the power required for the digital equipment of the OBP transponder is not taken into account.
For 1950 duplex calls, 2000 to 3900 modems are required depending on the design of the satellite-to-
gateway links (only 2000 modems if TDMA is used for the satellite-to-gateways links). For the OBP
transponder to be more power efficient than the bent-pipe transponder, the D.C. power of modem needs
to be less than 0.06 watts each. It is not believed that this can be achieved until after the mid 1990's.
Currently, this type of CDMA modem requires a power consumption of approximately 0.5 watts. This
raises the call power from 0.29 watts/call to over 1.2 watts/call. This higher call power is 368% more
than the bent-pipe transponder. Therefore, the bent-pipe transponder is more power efficient than the
OBP transponder when the power for the OBP equipment is added.
There is more volume required for the OBP payload than the bent-pipe payload and the OBP pay-
load dissipates more power. This extra dissipation could present a problem for small spacecraft. The
OBP satellite will be more complex than the bent-pipe satellite, requires more design effort, and the risk
is higher.
The OBP transponder does have advantages. The OBP transponder makes better use of the
satellite's EIRP by only transmitting the signals of the calls going through that satellite (whereas a
bent-pipe satellite transmits everything it receives). This gives an increase in call capacity for a given
region due to decreased interference and the OBP satellite is able to make better use of satellite double
coverage. For CDMA operation, the OBP transponder will have better power control. Also with an
OBP transponder, the call set-up procedure can be moved to the satellite.
The cost and complexity of the OBP satellite gateways versus the bent-pipe satellite gateways
depends on the design of the OBP gateway-to-satellite links. By using TDMA links, the OBP gateway
costs might be higher than the bent-pipe solution. By using CDMA links, the OBP gateway costs might
be lower than the bent-pipe gateway costs, but doubles the number of modems in the satellite from 1950
to 3900. The price of a space qualified CDMA modem is not available today since no qualified unit
exists. However, it is obvious a cost increase will be incurred with an OBP transponder. For example,
3900 CDMA modems reduced to a single VLSI chip when space qualified would be on the order of
$500 each. At this cost, the price of a satellite chip set is nearly two million dollars not including other
OBP equipment, integration, and test. A 48 satellite set would incur additional costs of $100 million not
including the additional satellite cost of the increased power requirements.
Another decisive factor is the non-standardization of the cellular systems in the world today. The
GLOBALSTAR system must be compatible with many types of mobile operations. Europe, the United
States and Japan all have different standards. A bent-pipe transponder allows for these different stan-
dards to be used with the satellite. There is not enough flexibility in an OBP transponder to handle the
different standards and protocols.
38
Conclusions
The first generation GLOBALSTAR satellite will be bent-pipe due to the flexibility of the tran-
sponder to handle different signal formats. A bent-pipe transponder offers inexpensive capacity to users
without the prohibitive research and development, power and cost requirements that would be incurred
with an OBP solution. The bent-pipe communications subsystem is a classic repeater which uses exist-
ing satellite communications techniques and many off-the-shelf parts. This keeps the nonrecurring
research and development costs low as well as keeping the satellite equipment and testing costs low.
Table 1 give an overview of the comparison between OBP and bent-pipe.
Table 1
Transponder Comparison
OBP Bent-Pipe
Cost/Channel X
Signal flexibility X
Payload Complexity x
Capacity x
Use of Multi-coverage X
Payload Power X
Payload Weight x
Payload Volume x
Thermal x
Risk x
R&D Required x
Gateway Cost - -
Signal Qualityt x
t at maximum capacity
The return link self-interference due to the non-synchronous CDMA operation limits the capacity
of both the bent-pipe and OBP transponders. The OBP transponder is limited to 2300 simultaneous
duplex calls while the bent-pipe transponder is limited to 1950 simultaneous duplex calls. However, the
OBP transponder will use more power than the bent-pipe transponder, which is critical for this LEO
satellite system.
Acknowledgements
The authors wish to express their gratitude to the team of people who contributed to the GLOBAL-
STAR FCC filing. From Loral Aerospace: Alex Tsao, Randy Tyner, Andy Turner, Steve Ames, Vic
Mosley and Dr. Bob Kwan; from Alcatel Espace: Denis Rouffet, Patricia Jung, Jean-Francois Migeon,
Frederic Berthault and Yannick Tanguy; from Qualcomm Inc.: Allen Salmasi and Klein Gilhoussen -
each person gave technical assistance in developing the GLOBALSTAR system and contributed a
significant set of ideas to this paper.
References
1) J.G. Walker, Continuous Whole-Earth Coverage by Circular-Orbit Satellite Patterns, Royal
Aircraft Establishment Technical Report 77044, 24 March, 1977.
39



Mobile telephony through LEO satellites: To OBP or not

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