Broadband VLBI Data Downlink of VSOP-2

1
Broadband VLBI Data Downlink of VSOP-2 Yusuke
Kono1, Yasuhiro Murata2, Hisashi Hirabayashi2,
Nanako Mochizuki2, Tomoaki Toda2 , Kiyoaki
Wajima3 1National Astronomical Observatory of
Japan, 2JAXA, 3Korea Astronomy
Observatory Following the success of the VLBI
Space Observatory Programme (VSOP), a next
generation space VLBI mission, currently called
VSOP-2, is being planned. The mission will
provide a downlink data rate of 1 Gbps to improve
sensitivity in observing continuum emission. A
possible band based on frequency allocation
regulations is 37-38 GHz for this data transfer.
The uplink frequency for phase transfer, on the
other hand, will be 40 GHz. We adopt an 8
channel-OFDM (Orthogonal Frequency Division
Multiplex) method for the data transfer to reduce
the out-of-band emission. The link budget is more
severe than that of VSOP, but a reasonable design
is possible.
VSOP-2 Link System
Onboard Radio Astronomy Subsystem
08GHz R/LHCP 22GHz R/LHCP 43GHz R/LHCP
Phase Transfer Link Frequency and phase reference
transferred from the ground station by two way
link. Uplink 40GHz, Downlink 37-38GHz
VLBI Data link Downlink 37-38GHz Bit Rate
1Gbps 8ch-OFDM method QPSK 128Mbps / ch 10W
Transmitting Power
http//vsop.mtk.nao.ac.jp/nagai/submm-ryozanpaku
/presentation/Takeuchi.pdf http//www.fantech.co.
jp/fantech/seihin.html http//web.haystack.mit.ed
u/mark5/Mark5.htm
08GHz R/LHCP 22GHz R/LHCP 43GHz R/LHCP
Phase comparison between up and down link for
Doppler compensation
Demodulator
Optical Fiber
Optical Fiber
lt International VLBI station gt
lt Link station gt
lt Correlator gt
OFDM Software Simulation
OFDM (Orthogonal Frequency Division Multiplex)
technique
QPSK DEM 8ch
TWTA
QPSK MOD 8ch
OFDM is a spread spectrum technique that
distributes the data over a large number of
carriers that are separated by precise
frequencies. This spacing provides the
"orthogonality" in this technique which prevents
the demodulators from seeing frequencies other
than their own.
Benefit of OFDM for VSOP-2 -High spectral
efficiency (Low out-of-band emission)
OFDM signals are sensitive to non-linear
distortions in the amplifier. VSOP-2 will use a
TWTA (Traveling Wave Tube Amplifier). We have
studied the effect on the BER (Bit Error Rate) of
the TWTA non-linearity using MatLab software. The
simulation was carried out considering nonlinear
characteristics of a 20GHz TWTA. The simulation
which indicate that back off of about 2 dB is
needed for the system.
Upper Single carrier QPSK spectrum
(1Gbps) Lower
8-channel OFDM Spectrum
(1Gbps)
OFDM Conceptual Spectrum (8-carriers)
Data Storage transfer
Sampler
OFDM EM Test
One of the problems for VSOP-2 is how to sample
the data on-board. It is difficult to find A/D
sampling LSI chips qualified for use in space. We
tested a 10 Gbps, 1 bit sampler and a 116
de-multiplexer, to demonstrate a possible
solution for the on-board high speed sampler. We
carried out a 1000 krad total dose test and a
single event test. We confirmed we can use these
LSIs on the VSOP-2 orbit.
An Engineering Model which consists of the MOD /
DEM of the OFDM and a TWTA simulator is also
under development.
Mk-V (Haystack Observatory), K5/VSI (NICT),
FANTAS8000 (FANTECH)
A large amount of data from VSOP-2 satellite,
received at the downlink data rate of 1 Gbps,
will be stored at the link stations. An 8 hour
observation produces 4T byte of data. One of the
problems for VSOP-2 is how to store the data and
how to transfer it through optical fibers and/or
transport it to the correlator.
TWTA simulator block diagram
Summary
Feasibility studies of VSOP-2 Link system have
been carried out. The results indicate that a
reasonable design is possible. Hardware EM
tests, and further detailed studies of the data
storage/transfer system and onboard subsystem
will be needed in the near future.
AM/PM characteristic of a 20GHz TWTA to be
simulated.