Reconfigurable Communication Equipment on SmartSat-1
Nozomu NISHINAGA Makoto TAKEUCHI Ryutaro SUZUKI
Wireless Communications Department
National Institute of Information and Communications Technology
4-2-1 Nukuikita, Koganei, Tokyo, 184-8795 Japan
E-mail: {nisinaga,takeuchi,ryutaro}@nict.go.jp
Abstract Expanding the capacity of satellite communication systems requires the use of onboard switching
and regenerative relay techniques. However, there are obvious difficulties in applying new communication
technology that has appeared since a launch if a new modem and baseband switch are required to be loaded on a
satellite. Currently, the lifetime of a communications satellite in orbit is from 10 to 20 years, and it is impossible
to exchange or upgrade mission equipment loaded on a satellite after its deployment in space. This paper describes reconfigurable communication equipment to be loaded on a small experimental satellite, “SmartSat-1”.
The reconfigurable communication equipment includes an onboard software radio system that consists of reconfigurable devices. New functions can be added by installing new configurations sent from a ground terminal
after launching.
1. Introduction
As a result of the development of terrestrial digital-subscriber lines, the so-called
broadband (1.5 Mbps according to the ITU
definition) network has been deployed to
numerous residences. However, the service
area is limited for economic reasons. The
issue then arises of constructing a complementary network system in areas where
broadband service is not available. It is
common knowledge that a satellite communication system has the potential to solve
this issue of the so-called ‘digital divide’.
To establish a wide-area digital-subscriber
network, the satellite communication system
must be capable of servicing many users,
and thus it is essential to be able to enlarge
the system’s capacity. Investigations of the
technology required to expand the capacity
of geostationary communication satellites
have focused on multi-beam transponder,
regenerative-relay, and onboard switching
systems
[1-4].
Regenerative-relay
or
onboard switching systems are generally
used in combination with multi-beam transponder systems. The regenerative-relay
system, which differs from a conventional
bent–pipe transponder system, needs an
onboard modem to regenerate baseband
signals from signals (radio waves) transmitted from earth stations. The baseband sig-
nals are re-modulated and sent to their destination (earth stations). The regenerative-relay system separates uplinks and
downlinks completely, enabling flexible link
design as well as a regenerative-relay gain
(3 dB maximum). Demodulating signals into
baseband signals enables links to be aggregated, and the system’s capacity and number
of users connecting to the system simultaneously to be increased by statistical multiplexing effects. To establish broadband satellite links, Ka (20-30 GHz) and higher
bands must be used. However, a large rain
margin (to allow for signal attenuation
caused by rain) is required to use these
bands. An onboard multi-rate modem that
accepts various rates of transmitting signals
is required to keep the rain margin low.
Therefore, constructing a broadband satellite
communication system requires onboard
communication payloads with high functionality.
The dilemma in loading high-function
payloads is that these payloads cannot, of
course, keep up with paradigm shifts in terrestrial systems. In recent years, there has
been a trend towards designing commercial
satellites with a 10- to 20-year lifetime to
reduce launch risks and costs. In general,
payloads cannot be repaired and replaced
during orbit. Onboard equipment thus often
becomes an obsolete dead weight, or out-
began in 2004. Some experimental items,
[SmartSat-1b ]
[ SmartSat-1a ]
Figure 1: SmartSat-1
dated in terms of functionality and
throughput before reaching the end of its
expected lifetime (i.e. it drops from high to
low functionality). However, if onboard
payloads were reconfigurable, the value of
their payloads could be continuously redefined and maintained at a high level.
In this paper, we report the development
of reconfigurable communication equipment
(RCE) for a small experimental satellite,
“SmartSat-1”. The RCE has five components: a transmitting antenna, receiving antenna, X-band transponder, X-band solid-state amplifier, and onboard software-defined radio (OSDR). The OSDR
consists of reconfigurable gate arrays, and
the configuration of these gate arrays is rewritable during orbit.
2. SmartSat project
The SmartSat project is a space application-proofing program using small satellites;
the National Institute of Information and
Communications Technology (NICT) plays a
central role in the project. As a first step in
this project, the Smartsat-1 program is now
in progress. Twin 150-kg (80-kg payload)
small satellites will be launched in 2008; the
bus system and payloads for these satellites
are under development at present. Basic design (bread-board model) of the bus system
and payloads was completed in 2003 and
development of the engineering flight model
including space-weather observation, orbital-maintenance, optical inter-satellite
communication, and reconfigurable communication experiments, are planned.
The latest configurations of SmartSat-1a
and -1b are shown in Fig. 1. These satellites
are designed on the assumption of a piggy-back launch and will be thrown into a
geostationary transfer orbit.
3. Reconfigurable
Communication
Equipment
3.1. Objectives
There are two objectives for the RCE.
1. Space-proofing onboard software defined radio technology
2. Space-proofing
versatile
mission
equipment based on concepts of functional redundancy and graceful degradation
The first objective is to launch an onboard
modem with a flexible link design and large
bandwidth capable of being adapted to the
latest communication technology. By loading a versatile modem that can be reconfigured via software or hardware configurations
on a satellite, optimum modulation and demodulation methods and type of error-correcting code can be selected according to link conditions. In addition, the latest
communication technologies and protocols
can be added to the onboard modem by uploading new software or hardware configurations after the satellite has been deployed
in space. With these features, the problem of
rain attenuation can be overcome by establishing a broadband link with a higher carrier frequency, and high interoperability
with terrestrial communication systems can
be maintained by uploading new technology.
The second objective is to take advantage of the flexibility of software-defined radio to ensure the onboard
modem system has the required reliability.
The current approach to ensuring the relia-
Figure 3: First-generation OSDR
ciency can be reduced by reconfiguration
during orbit and by constructing mission
equipment that carries out its required function on time. Total system redundancy can
be decreased by using reconfigurable, versatile mission equipment that still provides
the same level of reliability.
The concept of degrading gracefully
means that equipment problems are not regarded as losses of function but rather as
Figure 4: Second generation OSDR
Figure 2: Configuration of RCE
bility of conventional satellite mission
equipment is to use stand-by or triple-module redundancy, which requires two
or three times the level of system resources
for one piece of equipment or function. Essentially, specific mission equipment is required only when that function of the mission is being operated. This means that with
conventional methods, the weight of the
equipment continues to consume system resources whether the equipment is required or
not. However, it is the function rather than
the equipment that is essential. This ineffi-
reduced capability. The reuse of faulty
equipment becomes a soft-fault rather than a
hard-fault decision.
Different levels of
computational complexity are required to
carry out various functions. Thus, by assigning a lighter load to a degraded piece of
versatile mission equipment, satellite resources can be used more effectively. For
example, demodulation processing and decoding forward error correction codes imposes higher computational costs than modulation processing and encoding codes. Thus,
the lifetime of the whole system can be ex-
tended by assigning modulation processing
Figure 5: Block diagram of OSDR
and encoding to degraded equipment and
demodulation processing and decoding to
high-functioning equipment.
3.2. The RCE Configuration
The configuration of the whole RCE is
shown in Fig. 2. As mentioned above it consists of five components: a transmitting antenna
(TX-ANT),
receiving
antenna
(RX-ANT), X-band transponder, X-band
solid-state amplifier, and onboard software-defined radio (OSDR). To conserve
system resources, two patch-type antennas
are used as the TX-ANT and RX-ANT. The
total weight of the RCE is about 16 kg (not
including the support structure) and the
electrical power consumption is about 80 W.
At present, we are attempting to reduce
these values.
3.3. Configuration of the OSDR
An investigation of space proofing the
OSDR began in 2001. As part of the experimental production of an OSDR, a pre-BBM
model was manufactured (Fig. 3). In this
model, three 100,000-gate-class FPGAs
(field-programmable gate arrays) were used.
One of the three was used to control the
other two. Using an onboard analog-to-digital
converter
and
digital-to-analog converter, inputted analogue
signals were processed by two FPGAs, producing analogue processed signals. The
main feature of this model was that two operating modes, a normal operating mode and
a ‘degenerate’ operating mode, were imple-
mented. In the normal mode, two FPGAs
acted as a full-spec digital-signal processor
(FIR filter). In the degenerate mode, we assumed that one of the two FPGAs had collapsed and that the collapsed FPGA had detached logically. In this mode, the signal-processing performance was half that of
the normal mode (i.e. the cut-off rate was
halved).
Based on the results of designing and manufacturing the pre-BBM model, a second-generation OSDR model was manufactured in 2003, as shown in Fig. 4. In this
model, a multi-rate QPSK modulation and
demodulation function from 2 kbps to 2
Mbps is implemented using seven FPGAs.
Each FPGA has about 500,000 gates (Xilinx
XC2VP4). A block diagram of the model is
shown in Fig. 5. It has two FPGA banks,
each containing three FPGAs. The remaining
FPGA is used to control the other FPGAs. In
the flight model of the OSDR, the control
FPGA will be changed to a non-volatile type
to ensure space-radiation tolerance.
The six FPGAs are symmetrically connected by a switch fabric configured in the
control FPGA. The model has two service
classes determined by the reliability and
computational complexity required. The details are described in the next subsection.
3.4. OSDR operating modes
The second-generation OSDR model has
three operating modes.
1.
triple-redundancy mode
2.
daisy-chain mode
3.
degenerate mode
The configuration of the triple-redundancy
mode is shown in Fig. 6. This operating
mode is for missions that require high reliability. These missions include receiving a
configuration stream uploaded from an earth
station and writing the stream to the
non-volatile memory installed onboard. The
same configuration data is written to all the
FPGAs belonging to the same bank; the triple-module-redundancy voter configured in
the control FPGA detects differences in the
output of FPGAs belonging to the same
Figure 7: Daisy-chain mode
Figure 8: Degenerate mode
Figure 6: Triple-module redundancy mode
banks. It is well known that the data stored
in an FPGA can be changed by single-event
upsets caused by space radiation and that not
only stored data but also configuration data
(i.e. the circuit itself) may be altered. In the
worst case, a pin assigned as an input by the
original configuration data will be assigned
as an output by the single event, thus damaging the circuit. If the mutual comparison
of all FPGA outputs shows a discrepancy,
the
configuration
data
of
the
non-conforming FPGA must be corrected
immediately.
The configuration of the daisy-chain mode
is shown in Fig. 7. Although this operating
mode provides high throughput, it is less
reliable than the triple-redundancy mode. In
this mode, all the FPGAs can be used independently, enabling about three times the
number of gates to be used in comparison
with the triple-redundancy mode. Since a
single-event upset cannot be detected in this
mode, all the configuration data must be
inspected via a readback operation. Unfortunately, there is little information available
on readback data and mask-data formats for
the Xilinx Virtex II Pro series. The total
amount of configuration data required for
six FPGAs to carry out a mission is about 18
Mbits, and another 36 Mbits is needed for
the readback inspection (mask data and expected readback data). Because these 54
Mbits are too large to store onboard, we
considered using an on-the-fly decompression technique to reduce memory costs. To
date, we have found that the mask-data
stream can be divided into two parts, that is,
configuration-dependent
and configuration-independent parts (i.e. a base part) by
hacking these binary streams. Each
mask-data stream can be constructed using
the configuration-dependent part and base
part via a binary operation. In general, the
density
of
“1”
in
the
configuration-dependent part is relatively light and
the base part consists of several types of
fixed data chunks. It is therefore easy to
achieve a high-compression ratio using a
conventional compression algorithm. The
expected readback stream can be constructed
from the original configuration data by a
result of hacking. Thus, these techniques
reduce the total amount of memory required.
The degenerate mode is shown in Fig. 8 In
this mode, a collapsed FPGA is detached
logically from use. A bank that includes a
collapsed FPGA is assigned a modulation/encoding function that requires lighter
computational complexity than a demodula-
Cross Section (cm2/bit)
1.E-06
1.E-07
1.E-08
1.E-09
V2-Pro
1.E-10
V2
1.E-11
0
10
20
30
40
LET(MeV cm2/mg)
50
60
70
Figure 9: Results of block RAM tests
1.0E-08
2
Cross Section[cm /bit]
1.0E-07
1.0E-09
1.0E-10
V2-Pro
V2 (iMPACT)
N
Kr
Ne
V2 (FIVIT)
Kr( 35°)
1.0E-11
0
10
20
30
40
50
60
70
2
LET [Mev cm /mg]
Figure 10: Results of testing memory configuration
tion /decoding function. To decide whether
an FPGA is collapsed or not, a readback inspection and triple-module redundancy
check are used simultaneously. Needless to
say, it is not possible to rescue the OSDR
from every fault. Depending on the fault,
other devices may also collapse as a result of
the failure of a device and methods of preventing this sort of chain reaction are under
consideration.
4. Radiation
testing
of
commercial
S-RAM-type FPGA
To evaluate the effect of space radiation, a
radiation test was carried out in November
2003 at TIARA in Takasaki, Japan. The device selected for testing was a Virtex II Pro
(XC2VP7-5FG456). In the test, three heavy
ions, nitrogen, neon, and krypton, were injected into the device and the number of
single-event upsets was counted. The results
of this test were compared with the results of
Virtex II tests described elsewhere [5]. Figure 9 shows the results of testing the block
RAM region. Fixed data were written to the
block RAM via an external interface and the
outputs of the RAM were compared with the
original data outside the FPGA. The results
of tests of the configuration memory region
are shown in Fig. 10. A single-event upset
can be detected through a JTAG interface. In
these figures, the saturated cross section and
threshold LET show no differences between
the results of the Virtex II.
5. Summary
In this paper, we reported the ongoing
development of reconfigurable communication equipment (RCE) for a small experimental satellite, “SmartSat-1”. The RCE has
five components: a transmitting antenna,
receiving antenna, X-band transponder,
X-band solid-state amplifier, and onboard
software-defined radio (OSDR). The OSDR
consists of reconfigurable gate arrays; the
configuration of these arrays is rewritable in
orbit. Engineering design of a flight model
of the OSDR began this year.
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