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SDR for space systems: overview and perspectives
Télécom Bretagne / Institut Mines Télécom
IEEE International Workshop on Metrology for AeroSpace
!May 29-30, 2014 - Benevento, Italy
L. Franck
Introduction• Software Defined Radio (SDR) is a technological concept
where the processing of RF signals is implemented in re-programmable units rather than application-specific integrated circuits (ASICs)
• Re-programmable units encompasses digital signal processors (DSP), field programmable gate arrays (FPGA) and general purpose processors (GPP or CPU)
• It is made possible thanks to Moore’s law (and a bit of science and entrepreneurship too)
• We’ll discuss here about the applicability of SDR to space systems with a strong focus on satellite communications (satcom)
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L. Franck
Introduction• Devising satcom equipments is challenging:
• Space is a tough environment (temperature variation, vacuum, radiations, scarce energy supply)
• Getting there is techno-demanding (vibration, acceleration) and costly: about 1/4 of the overall satellite cost [≈ 1/4 x $500 millions ]* is dedicated to launch operations
• Once there, always there: a satellite lifetime is about 15 years*
• And “space” can be quite far from Earth (36 000 km for the geostationary orbit)
• At Ku-band (around 12 GHz) that makes a 200 dB free space loss
3* For big satcom geostationary satellites
L. Franck
Introduction
• Selling satcom services is also challenging:
• The markets and business models are different from terrestrial telecommunications: niche and governmental markets (except for TV and radio broadcasting)
• There is strong competition with terrestrial technologies where the satellite and terrestrial market intersect
➡ Could SDR be the swiss army knife of satcoms ?
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L. Franck
SDR for satcom
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User terminals
Telecommunication payloads
Teaching activities
Speciality payloads
Earth stations
“One thing to rule them all ?”
(Ground instrumentation)
L. Franck
User terminals
L. Franck
Categories of user terminals
• Portable terminals
!
• Mobile terminals
!
• Transportable terminals
!
• Fixed terminals
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L. Franck
User terminals• The SDR technology is makes possible the following
capabilities:
• Cognitive radio: adaptivity to various spectrum conditions (as a result of prior sensing) in terms of frequency, bandwidth and emitted power, including the possibility to share spectrum
• For example on the 17.7-19.7 GHz band, the fixed satellite service (FSS) and fixed service (FS) are both primary
• Integration of multiple waveforms (i.e., transmission schemes) into a single hardware platform to (a) save space and cost, (b) ensure upgradability and (c) foster co-operative communications schemes based on ancillary terrestrial components
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User terminals: example• The Inmarsat BGAN service provides data rates up to 492 kbit/s with a
worldwide coverage
• Support for real-time (called streaming, 384 kbit/s) and non-real time IP-based services as well as voice services
• GateHouse has developed a software BGAN implementation compliant with the SCA (Software Communication Architecture) standard and the BGAN specification
• This software can be run on SCA compliantSDR platforms
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[Source: Inmarsat]
The GateHouse BGAN waveform for SDR is a complete waveform for the Inmarsat
BGAN satellite system
BGAN Waveform for SDR
Product Sheet
The waveform is based on the existing GateHouse BGAN Protocol Stack, already incorporated in a large number of BGAN terminals from several suppliers.
The waveform and the SCA compliant hardware will form a complete radio for land, maritime or aeronautical use.
The waveform can be certified as SCA compliant, and is designed for military as well as civilian use. BGAN sup-ports major VPN products and encryption standards.
The waveform is type approved by Inmarsat.
Technical description
The radio will be built on two main components:
�� 2�D42�T`^a]ZR_e�a]ReW`c^��� EYV�8ReV9`fdV�382?�hRgVW`c^�
The SCA platform contains the hardware, including typical SDR items such as RF, FPGA, DSP and GPP and the SCA core framework.
The GateHouse BGAN waveform contains all the neces-sary software.
The platform must support the characteristics of the BGAN system, such as: Operation in the L-Band, the receive band is 1525 MHz to 1559 MHz, the transmit band is 1626.5 MHz to 1660.5 MHz. These bands may be slightly extended in the near future, in order to meet the demands from new users. Channels are aligned to a 1.25 kHz grid in both forward and return directions. The BGAN system is a full duplex system, that is, the platform must be able to receive and
transmit at the same time. The duplex distance, i.e. the RX-TX separation, is not fixed. The BGAN system utilizes a number of channel bandwidths, from 10 kHz to 200 kHz.
Architecture
The main architecture of the radio is shown here:
The BGAN waveform contains two integrated compo-
nents:
�� EYV�Ac`e`T`]�DeRT\���� EYV�AYjdZTR]�=RjVc�
The waveform includes all the software necessary to use the BGAN services. The waveform includes code for FPGAs, DSPs and GPPs.
The waveform is able to handle all UE (user equipment) classes; at start up the actual UE class will be configured.
The waveform is developed to be easily portable across SDR platforms and will as far as possible only require standard SCA interfaces.
The waveform has an interface for antenna pointing and control.
The Physical Layer is a complete, high performance physical layer that is able to handle all UE classes.
Protocol Stack
Core Framework
Analog Hardware
Antenna
Physical Layer
FPGA DSP GPP
BGAN Waveform
SCA Platform
[Source: GateHouse]
L. Franck
Conclusions on user terminals• The use of SDR for user terminals is a promising direction
• The signal bandwidth to process is usually limited
• For terminals, the exposure to changing standards, hence the need to adapt is strong
• Using SDR for user terminals adresses the following stakes:
• To decrease the CAPEX of accessing satellite services by favouring convergence between terrestrial and satellite terminal hardware technologies
• To decrease the OPEX of accessing satellite services by favouring seamless hybridisation of terrestrial and satellite access
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Earth stations
L. Franck
Categories of Earth stations
• Gateways / hubs /teleports
!
• TTC stations
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Earth stations• Gateways/hubs and teleports deal with complex processing of
large bandwidth of spectrum
• Possibly not the best case for SDR
• On the other hand, SDR technology is well positioned for the development of specialised receivers that are tailored to situations where a dedicated ASIC development would be too costly
• Example: in the DIGIDSAT ESA project, an antenna tracking system is developed for end-of-line geo satellites that drift to inclined orbit. SDR is used for a building a dedicated beacon receiver that actuates antenna pointing
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Earth stations
• For small satellites (typically low earth orbit) such as cubesats, amateur sats or nanosats, SDR-based Earth stations are popular
• From an SDR standpoint, it is a favourable case since (a) signals are narrowband and (b) transmission schemes are simple (AFSK, BPSK modulations)
• Example: the OSAGS ground station network is based on Ettus Research USRPs
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Earth stations: example
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6.1 THEORETICAL BOX
The information are displayed in three different panels. The first one is dedicated to the features of the system. Both, satellite and Earth station, are characterised. The two others permit to inform about the theoretical values of attenuation according to the conditions of propagation: rain or clear sky. More information about this point are given in the first report.
6.2 EXPERIMENTAL BOX
The overall work performed during this second part is materialised in this section of the program. We would like to create an automatic tool permitting the acquisition, the record and the analysis of the signal. These objectives are reached.
In the first panel the user could control the USRP, see in real time the signal, its power and position and also choose the frequency with which measurements are saved and the emplacement of the record.
Looking at the second panel, the user could read data previously recorded. It is even possible to display on the same graphic two records in order to compare them.
On the last panel, the power of the received signal is no longer displayed. We rather build a graphic with the attenuation suffered from signal during its propagation. According to the values obtained, and comparing them to the theoretical ones, we deduce and indicate to the user the condition of propagation of the record: clear sky or rain.
Figure 3: General view of the interface in LabView.
• A simple SDR Ku-band beacon receiver
• The receiver includes frequency tracking to cope with cheap COTS components in the LNB
L. Franck
Conclusions on Earth stations
• SDR best suited to design of dedicated receivers or ground instrumentation for controlling facilities
• Or for LEO small satellites Earth stations
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Payloads
[Source : O3b networks]
L. Franck
Telecommunication payloads• Transparent “Bent pipe” payload architectures are an heritage
from broadcast services (e.g., TV and radio broadcast):
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Receiver IMUX
uplinkHPA
HPA
HPA
HPA
OMUX downlink
Transponder = channels of fixed 36 or 72 MHz bandwidth One “fat” carrier per transponder
Ku or C band
The HPA can be operated close to the saturation point
➡ Current usage is shifting away from this paradigm
L. Franck
Telecommunication payloads• Current usages and satellites display the following characteristics :
• Directed to “telecom” (i.e., bi-directional, point-to-point) services
• The business model changes dramatically and the cost of transmitted Mbit is a strategic issue
• The rate of change of terrestrial standards for networks and services is higher than the typical lifetime of a satellite
• Forward and return link show different constraints and characteristics
• Based on multi-beam architectures
• For example, KA-SAT features 82 user spot beams over Europe at large
• Operating in the Ka-band band and above
• The Ka-band suffers from tougher environment impairments (than the Ku-band). These may vary on a carrier by carrier basis
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Telecommunication payloads• These characteristics are summarised in two challenges:
• Increasing payload capabilities
• In terms of technology figure of merits (e.g., mass, consumption and thermal characteristics)
• In terms of transmission figure of merits (e.g., spectral efficiency)
• Increasing payload flexibility
• In terms of adaptivity to evolving trafic geographic distribution
• In terms of adaptivity to evolving trafic characteristics
➡ Onboard processing contributes to tackling these challenges
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Telecommunication payloads• Categorisation of onboard processing:
• Digital signal processing of the incoming carriers to optimise subsequent channelised HPA operations (e.g., digital transparent processors implementing filtering and carrier routing)
• Digital signal processing of the incoming carriers to accommodate to a flexible definition of channels (in terms of bandwidth and central frequency)
• On-board demodulation for regenerative processing (e.g., different modulations on the uplink and downlink) or higher layer switching (i.e., mesh architectures)
➡ SDR contributes to reconfigurable digital processing for flexible payloads and favours reusability, cutting down costs
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By increasing requirem
ent on reconfigurability
L. Franck
Telecommunication payloads: example• Legacy payloads: the frequency plan determines the (fixed)
switching policy among uplink and downlink channels
22[Source: JSAT int’l]
L. Franck
Telecommunication payloads: example• Digital transparent processing enables programmable switching &
duplicating at carrier granularity among uplink and downlink spots
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2
Fig.1: Digital Transparent Processor functional architecture.
GC division offers great flexibility to customers for the selection of the signal type suitable to the service, the only constraint being the bandwidth.
In Figure 1 a functional DTP architecture is presented; as outlined the main processor elements are:
• Down-conversion, ADC (Analog-to-Digital Converter) and DAC (Digital-to-Analog Converter);
• Frequency compensation and sub_channel demultiplexing by means of suitable digital low-pass filters;
• Switching matrix that performs the channelization function by means of a fine digital filtering of sub-channels; as previously described, the bandwidth is variable and is programmable by ground commands.
The switch function performs a full connectivity routing at
sub-channel level between the input and output ports; this is the core section of the DTP and provides a flexible management of RF spectrum (each up-link traffic channel can be moved to a different position in the down-link frequency plan).
It has to be outlined that all the DTP simulations have been performed using a frequency down-scaling of a third-order factor; scaling has been requested in order to decrease the overall simulations computational load
3. RX SUBSYSTEM In Figure 2 the block diagram of a RX section is shown. Each 36 MHz sub-band is IF processed (filtered and amplified) in order to be suitable for ADC input requirements. The pass-band filtering is such to reject adjacent sub-bands of same or adjacent channels and it is implemented in SAW (Surface Acoustic Wave) technology. Due to sub-band FDMA organization (a maximum of 72 carriers if all the CG are A Type, a minimum of 9 Carriers if
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SDR makes it possible to have reconfigurable processing in the digital realm
L. Franck
Speciality payloads• Speciality payloads implement missions that are different from
the typical “receive, amplify and transmit” telecom payloads
• For example, Telemetry & Telecommand (TM/TC) is present in every satellite and sends health information (TM) about the satellite to Earth and receives orders from the control centre (TC)
• Other examples may be embarked as primary/secondary payloads in geostationary or non-geostationary platforms
• Telemetry data links for observation satellites, scientific payloads, search and rescue, …
• These payloads are also candidates for using SDR technology in order to benefit from its flexibility
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L. Franck
Speciality payloads: examples• The following example is a Telemetry, Tracking and Control
(TT&C) transceiver developed from Com Dev and embarked by the FORMOSAT-7 satellite series (LEO satellites for weather prediction through atmospheric sounding)
• Communication modulation is implemented in an FPGA to offer flexibility depending on the mission and mission phases
• The SCAN NASA testbed aims to test three SDR-based payloads that are compatible with NASA’s TDRSS system
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the logic loaded into the reconfigurable FielGate Arrays (FPGAs). The OE includesboth, as do waveform applications. Thprimarily in C and C++. FPGA configuratwith Verilog and VHDL (Very high speed Hardware Description Language). All tSpaceWire [2] for their data connection Processor. Procurement of the CoNNeCT SDRs
The Harris and GD SDRs were devecompetitively competed cooperative agHarris and GD funded a significant development costs. This approach is ideadevelopment efforts such as CoNNeCT wheshares the risk of the new development, from NASA’s investment. A cooperaprovides industry an opportunity to raise thspace product and gain valuable flight exptest this new development in a space envirbenefits from the opportunity to assess netechnologies of interest to NASA mireconfigurable SDR technology, and Sdevelopment.
A cooperative agreement poses a challengflight hardware since the consequences of acost impact are shared among both theNASA. This shifts a portion of the deveNASA in exchange for the cost sharing froIn the end, the cooperative agreement wasfor the Harris and GD SDRs. The delivunits, and documentation deliverables through close technical oversight by Ncommitment by Harris and GD to deliver onclose and open working relationship to sharproblems, discuss solutions, and take actionthroughout development. The JPL SDR was procured using an existtask contract between NASA and JPL.
Common Development Approach Discussio
The focus of this paper is the uniquedeveloping and testing reconfigurable platMany aspects of the development and test oSDRs follow typical radio hardware developAll SDRs underwent subsystem testing at thplatform and waveform requirements weenvironmental tests such as thermal vacuumEMI were performed across the range values. Once the unit was delivered to Center, and at each subsequent integrationwas tested for basic functionally, exeplatform requirements and waveform requirnothing changed or degraded as it procesintegration steps. Once integrated into
3
ld Programmable s components in he GPP code is tions are defined integrated circuit three SDRs use to the Avionics
eloped under a greement, where
portion of the al for technology ere the developer while benefiting
ative agreement he TRL of a new perience to fully ronment. NASA ew concepts and issions such as STRS standards
ge when used for a late delivery or e developer and elopment risk to m the developer.
s very successful very of the SDR
were achieved NASA, a strong n schedule, and a re status, disclose ns to mitigate risk
ting development
n
e challenges of tforms for space. of the CoNNeCT pment processes. he vendor, where ere verified and m, vibration, and of the specified Glenn Research n point, the unit ercising various rements to ensure ssed through the the system, and
along with the RF subsystem (e.isolator, diplexer) the integrated syssystem level requirements. characterization tests were conducteof the radio pre-flight as a basis for to advance the SDR and assessVerification and characterization signal characteristics including performance, frequency scharacterization, and on the receitracking and acquisition thresholdsothers. Many of the end-to-end throughout the system thermal vaunderstand performance over tepresence of other signals. Documentation delivered with earequired by the STRS ArchitectuInterface Description (HID) documDocument, and an FPGA waveformGuide to aid future waveform devwaveforms and facilitate the port document provides a description ofavailable to a waveform developewrapper provides template files foand provides sample interfaces. Tintended to abstract hide the Spaprotocols. Also included are standfor connecting to the FPGA andsimulation of the FPGAs Harris Corporation Software Define
The Harris radio utilizes the TDRSreturn service. The Harris SDR conIV FPGAs, a 1 GFLOP floatinProcessor (DSP), an AITech 950utilizing the VxWorks Operating SKa-band RF converter. The Harrithe three in that it uses a second Spand interface to the flight avionics. with the Harris radio uses the Vx W
Figure 3. Harris SD
g. TWTA, coax cables, stem was tested to verify
In addition, several ed to assess performance the experiments planned
s performance on-orbit. tests included output power, and spectral
stability, bandwidth ive side tests measured
s, BER performance, and link tests were repeated acuum test and EMI to emperature and in the
ach radio platform (and ure) include a Hardware ment, an Interface Control
m application Wrapper’s velopers to develop new
to the SDR. The HID f the hardware resources er. The FPGA Module or future FPGA designs
The sample interfaces are acewire and internal bus dard naming conventions d prototype files to aid
ed Radio
SS Ka-band forward and ntains four Xilinx Virtex-ng point Digital Signal single board computer
System, and an S-band to s radio is unique among
paceWire link for control The STRS OE supplied
Works RTOS.
DR Picture
General Dynamics (GD) Software Defined R
The GD radio, shown in Figure 4, utilforward and return links to TDRSS or dirfrom a ground station. The GD SDR coRTAX, one 3 million gate Xilinx QPROProgrammable Gate Array (FPGA), a processor, and RF conversion module, and The radio transmit output power is approxThe operating system is the VxWorks RealSystem (RTOS). The GD radio is controlcomputer avionics via a MIL-STD-1553Btheir heritage design.
Figure 4. GD SDR Picture
Jet Propulsion Laboratory (JPL) Software D
The JPL radio, shown in Figure 5, utilforward and return links to TDRSS or dirfrom a ground station. The JPL SDR alsfrequencies of L1 (1575.42 MHz), L2 (122L5 (1176.45 MHz). The JPL SDR contai2000 and two 3 million gate Virtex II XActel AT697 with SPARC V8 processosection, and a power amplifier. The raminimum of 7.5 Watts at 2287 MHz. The for the JPL radio is via MIL-STD-1553B. The STRS OE supplied with the JPL radisource RTEMS [3] RTOS. POSIX styleprovide the abstracted interface to GPP hardware devices, including those instantXilinx FPGAs. The OE implements a coand basic engineering telemetry and mainterface.
4
Radio
lizes S-band for rect links to and ontains an Actel
O Virtex II Field ColdFire micro power amplifier.
ximately 8 Watts. l Time Operating lled by the flight B link based on
e
Defined Radio
lizes S-band for rect links to and so receives GPS 27.60 MHz), and ins Actel RTAX ilinx FPGAs, an
or, RF converter adio transmits a control interface
io uses the open e device drivers software of the
tiated within the ommand handler anages the 1553
Figure 5. JPL SDR
3. HARRIS S
Requirement’s Approach
The Ka-band SDR transceiver was twell defined and tested requiremexist. The requirement developmeKa-band SDR involved scalingGeneration IV transponder requireKa-band missions (although missiocompliance to operate within thTDRSS), and in-house analysis. Lrequirements were divided amongand waveform requirements. Theplatform separate from the wavefocapabilities could be described waveform for future waveform deplatform and waveform requireStandard provided to the developedocument was created and tracked f There were a number of key requSDR. For the platform, key operation at Ka-band frequency, coarchitecture by providing an opabstract the application and controlsingle board computer, have fullprocessing, provide an FPGA absaccepting new waveforms, providestandard interface (Spacewire), anupsets and immune to single event the space environment. Other included on the RF section of the rabandwidth for operation with TDRpower to drive the TWTA. A key goal of the project (and substransmitting 100 Mbps from theNetwork. While the RF implementfell to the RF subsystem, the radioThe available modulations for tbandwidth service were QPSK anRequirements were then specified application using SQPSK to minfrom the TWTA at 100 Mbps uncod
R Picture
DR
the first for NASA, and a ments document did not
nt process for the Harris g the S-band TDRSS ements, looking at other ons were transmit only), he Space Network (i.e. Like the other SDRs, the g platform requirements e goal was to define the orm, so that the platform
independently of the evelopers. Based on the ements and the STRS er, a single specification for compliance.
uirements for the Harris requirements included
ompliance with the STRS perating environment to l software running on the ly reconfigurable signal straction layer to enable e data transfer through a nd mitigate single event latchups for operation in
platform requirements adio included a 225 MHz SS, and sufficient output
sequent requirement) was e radio over the Space tation of this requirement provided the waveform. the planned 225 MHz
nd Staggered QPSK [5]. to operate the waveform
nimize spectral regrowth ded and 100 Mbps coded
General Dynamics (GD) Software Defined R
The GD radio, shown in Figure 4, utilforward and return links to TDRSS or dirfrom a ground station. The GD SDR coRTAX, one 3 million gate Xilinx QPROProgrammable Gate Array (FPGA), a processor, and RF conversion module, and The radio transmit output power is approxThe operating system is the VxWorks RealSystem (RTOS). The GD radio is controlcomputer avionics via a MIL-STD-1553Btheir heritage design.
Figure 4. GD SDR Picture
Jet Propulsion Laboratory (JPL) Software D
The JPL radio, shown in Figure 5, utilforward and return links to TDRSS or dirfrom a ground station. The JPL SDR alsfrequencies of L1 (1575.42 MHz), L2 (122L5 (1176.45 MHz). The JPL SDR contai2000 and two 3 million gate Virtex II XActel AT697 with SPARC V8 processosection, and a power amplifier. The raminimum of 7.5 Watts at 2287 MHz. The for the JPL radio is via MIL-STD-1553B. The STRS OE supplied with the JPL radisource RTEMS [3] RTOS. POSIX styleprovide the abstracted interface to GPP hardware devices, including those instantXilinx FPGAs. The OE implements a coand basic engineering telemetry and mainterface.
4
Radio
lizes S-band for rect links to and ontains an Actel
O Virtex II Field ColdFire micro power amplifier.
ximately 8 Watts. l Time Operating lled by the flight B link based on
e
Defined Radio
lizes S-band for rect links to and so receives GPS 27.60 MHz), and ins Actel RTAX ilinx FPGAs, an
or, RF converter adio transmits a control interface
io uses the open e device drivers software of the
tiated within the ommand handler anages the 1553
Figure 5. JPL SDR
3. HARRIS S
Requirement’s Approach
The Ka-band SDR transceiver was twell defined and tested requiremexist. The requirement developmeKa-band SDR involved scalingGeneration IV transponder requireKa-band missions (although missiocompliance to operate within thTDRSS), and in-house analysis. Lrequirements were divided amongand waveform requirements. Theplatform separate from the wavefocapabilities could be described waveform for future waveform deplatform and waveform requireStandard provided to the developedocument was created and tracked f There were a number of key requSDR. For the platform, key operation at Ka-band frequency, coarchitecture by providing an opabstract the application and controlsingle board computer, have fullprocessing, provide an FPGA absaccepting new waveforms, providestandard interface (Spacewire), anupsets and immune to single event the space environment. Other included on the RF section of the rabandwidth for operation with TDRpower to drive the TWTA. A key goal of the project (and substransmitting 100 Mbps from theNetwork. While the RF implementfell to the RF subsystem, the radioThe available modulations for tbandwidth service were QPSK anRequirements were then specified application using SQPSK to minfrom the TWTA at 100 Mbps uncod
R Picture
DR
the first for NASA, and a ments document did not
nt process for the Harris g the S-band TDRSS ements, looking at other ons were transmit only), he Space Network (i.e. Like the other SDRs, the g platform requirements e goal was to define the orm, so that the platform
independently of the evelopers. Based on the ements and the STRS er, a single specification for compliance.
uirements for the Harris requirements included
ompliance with the STRS perating environment to l software running on the ly reconfigurable signal straction layer to enable e data transfer through a nd mitigate single event latchups for operation in
platform requirements adio included a 225 MHz SS, and sufficient output
sequent requirement) was e radio over the Space tation of this requirement provided the waveform. the planned 225 MHz
nd Staggered QPSK [5]. to operate the waveform
nimize spectral regrowth ded and 100 Mbps coded
[Sou
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L. Franck
Speciality payloads: example• AIS (Automatic Identification System) is beaconing system for tracking
ships
• Beacons are broadcasted (≈ 160 MHz at 9.6 kbit/s) by ships and collected by land stations located
• While it was not initially designed for, it turned out that beacons could be collected by LEO satellites in order to provide a more global coverage
• Collisions among beacons and weak signals are the two main challenges
• The Aalborg University has devised an SDR AIS receiver which is onboard the AAUSAT3 cubesat
26Fig. 3. The SDR AIS receiver. The colored part is the SDR based receiver.The remaining part is a traditional AIS receiver, which has been included forperformance comparason.
144
8.6 Demodulator
The received symbol Y j = (Yj,1, Yj,2)T is determined in the timeinterval
t 2 [jT, (j + 1)T ).
This time interval can also be written as
t = jT + τ, τ 2 [0, T ).
The demodulation for τ = t � jT 2 [0, T ), j = 0, 1, 2, . . . , canthen be implemented as follows:
PSfrag replacements
Y (t)
Yj,1
Yj,2
T!
2T
cos(2πf1τ )
cos(2πf2τ )
!
2T
" T
0 (.)dτ
!
2T
" T
0 (.)dτ
Land, Fleury: Digital Modulation 1 NavCom
Fig. 4. Textbook matched filter implementation of demodulator [2, p. 144]
however is insignificant compared to running a demodulatorbank.
A short description of the elements in method 2:• 2 x Channel Pre-filter. Each wide enough to include clock
drift and Doppler Shift.• 2 x Message and Frequency Detection, used to tune
channel filter and matched filter per message.• 2 x Channel filter, tuned with the estimated Fc per
detected message.• 2 x Matched filter demodulator, tuned with the estimated
Fc per detected message.• 2 x Bit-sync. Adaptive based on zero-crossing NCO.• 2 x Decision rule. Soft decision by adjusting threshold
from zero when needed.
III. RESULTS
In the following the results obtained from the SDR basedAIS receiver since launch on February 25th 2013 will beanalysed.
Fig. 5. Overview of the SDR algorithms, which has been improved forspace-base monitoring
A. Reception
The default AIS decoder on the satellite, as described aboveis currently implemented as a store and process algorithm. Thismeans, that the satellite first samples 1 second of data, and thenprocess it. Once finished it requests a new second of data andprocesses it. This means, that the receivers online time is onlyaround 5%-10%. However, even with this limited online time,the receiver is still capable of receiving and correctly decodingaround 10.000 AIS messages per day.
A plot of most of the ships received and successfullydownloaded through the Aalborg ground station can be seenin fig. 6 at the end.
Fig. 6. Plot of ship positions received globally by the SDR based AISreceiver. Each triangle represents one received ship position.
As seen with other satellite based AIS receivers, the problemwith the extended FOV, as described earlier, is clearly seen invery crouded waters These includes e.g. the MediterraneanSea, near the east coast of China and the east coast of USA.
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Conclusions on payloads• As far as payloads are concerned, the role of SDR is a two-fold
question
• Where to put the boundary between analog and digitalprocessing ?
• What is the added value of SDR for onboard (digital) processing ?
• The answers depend on the available technology and the mission requirements
• The present opportunity for SDR-enabled payloads is where the requirements show a combination of limited throughput and complex processing
!
• Note: antenna processing is not covered here
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Teaching activities
L. Franck
Teaching activities• Context: “Space Communication Systems” programme for the
3rd (last) year of engineering degree & advanced master
• SDR devices used for small projects (teams of two students working during 70 h) and workshops
• Our SDR platforms
• 4 x National Instruments USRP 2920 (50-2200 MHz, 20 MHz of bandwidth, Gigabit Ethernet transport)
• 2 x National Instruments VST 5644R (65-6000 MHz, 80 MHz of bandwidth, instrument grade, PXIe transport)
• Programmable with LabVIEW
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Teaching activities: example• Receiving weather images from space
• Based on NOAA polar orbiting satellites (APT mode)
• Weather images (line by line scans) are downlinked to Earth at 137 MHz with an FM + AM modulation
• One line = 2 x 909 pixels, 1 pixel = 16 km2
• Extending our LEO/amateur satellite station, the students developed an SDR-based receiver and decoder for weather images
• Pointing/tracking of the satellite
• Signal RX and demodulation
• Image processing
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Teaching activities: example
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Teaching activities: example• Flipped teaching for DVB/S2 lectures
• Prior to attending lectures on DVB/S2, students go through an introductory workshop
• The workshop is extensively based on our lab DVB/S2 platform:
• Co-located DVB/S2 modems for remote and hub sites
• An SDR-based channel emulator for geostationary link [covered in a separate presentation]
• The topics covered are: spectral efficiency, C/N0 calculation and measurement, protocol overhead, TCP performance over geostationary links
• Conditions similar to “real transmission” are reproduced thanks to SDR channel emulation
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Teaching activities: example
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Conclusions on teaching activities
• It creates “creates opportunities for projects not possible before” and it “contributes to a better understanding of the lectures” (students say)
• Having to choose between MATLAB or USRP + LavVIEW, 40 % of our students would go for USRPs
• It also calls for multi-disciplinary teaching teams
• SDR = RF instrumentation + (real-time) programming
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Overall conclusions• Software defined radio offer many
opportunities for satcom applications:
• Cognitive or advanced RF processing
• Reconfiguration during operations
• Reusability of existing hardware/software (from similar products or by favouring convergence with terrestrial technologies)
• Specific developments not affordable by means of ASIC technology
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• The associated challenges are
• To bridge the gap between terrestrial and space qualified SDR technology
• Including for ADCs & DACs
• To devise powerful development frameworks and methodologies
• To be adventurous
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Thank you
