AC Summer 08 WIN-T Online - United States Army Signal Center of ...

models delivered antennas with 24
inch apertures, which proved too
large for mobile tactical vehicles.
WIN-T has performed multiple
tradeoff studies between throughput
performance and antenna size,
weight, and power to define the
optimal antenna system for each of
the mobile vehicle platforms. WIN-T
Increment 2 will provide 18 inch
aperture antennas for the TCN and
PoP CIs and 16.5 inch antenna for
the SNE CI.
In support of future WIN-T
Increments, PM WIN-T is working
closely with the CERDEC to further
reduce SWAP requirements for
SOTM antennas via sponsorship of
the SOTM Active Quasioptical array
antenna. This hybrid design combines
the benefits from both phase
arrays and real aperture technology
to provide electronic switchable Ku/
Ka-band in a single antenna.
The concept is for the low
profile antenna along with the
positioner motor and electronics to
be enclosed in a 37 x 11 inch pancake
radome. The objective weight of the
antenna is approximately 100
pounds. At least four prototype
antenna units are planned for test in
the first quarter of FY 09. It is
anticipated that this new antenna
design will be ready for production
in the first quarter of FY 10 with a
target AUPC similar to that of the
SNE terminal.
RF link conditions
To address the radio frequency
link condition challenges, PM WIN-
T developed and matured the
Network-Centric Waveform, hosted
on the MPM-1000 modem. NCW
provides four receiver/demodulation
chains and two transmit/
modulation chains supporting
biphase shift keying and offset
quadrature phase key shifting
modulation, and half and threequarter
rate Forward Error Correction.
NCW supports data rates from
32 kbps to 6.144 Mbps. The maximum
user throughput that is
achieved depends on the associated
SATCOM terminal, as well as the
link conditions.
Radio frequency links will
Figure 3. NCW Scheduler Slot Plan
suffer blockages and periodic
outages due to terrain, tunnels,
buildings, vehicular orientation, and
weather effects. The SOTM system
must be able to expediently acquire
the satellite signal, and reacquire it
should it be lost due to line-of-sight
obstruction/blockage. NCW is
designed for fast acquisition and
requisition.
The modem will reacquire
bursts within a second for short
duration or intermittent blockages.
Should the user be in extended
blockage duration, such as behind a
building or in a tunnel, the SOTM
system will automatically reacquire
and reconnect with the network
within 10 seconds when the vehicle
moves out of the obstruction and a
clear line-of-sight is established. In
addition, PM WIN-T is considering
implementation of an automated
persistent slot feature within WIN-T
Inc 3. This feature will use Return
Order Wire statistics to automatically
assign and release satellite
resources during OTM and intermittent
blockage conditions.
The WIN-T SOTM systems will
provide dynamic user throughputs
nominally at 128 kbps for the SNE
and 256kbps for the PoP and TCN
CIs. Higher transmit user rates up to
1024 kbps will be achievable but will
vary and depend on weather conditions,
satellite in use, location in
beam footprint, and other factors.
NCW employs an advanced network
scheduler that maximizes network
data throughput under varying
atmospheric conditions and terminal
population by making optimal use of
satellite power and bandwidth. The
system adjusts itself dynamically
and requires little operator intervention.
The data rate, modulation/
coding and power settings are
dynamically scheduled for each
link/burst in real time based on link
conditions and satellite resource
constraints. Burst scheduling recurs
every 400 milliseconds. This rescheduling
capability is critical,
especially in OTM operations, due to
the dynamic nature of radio frequency
link conditions. A sample
frame structure which depicts the
scheduler’s efficient use of spectrum
while supporting multiple apertures
is provided in (Figure 3.)
Networking protocol adaptations
A major challenge associated
with all satellite communications is
long delay times. When adding the
software-based scheduling delay of
the NCW waveform to the physical
propagation delay of a geosynchronous
satellite, it is not uncommon to
see a one second round-trip time
between two terminals. Standard
TCP implementations will not take
full advantage of available bandwidth
under such long delay conditions
since the TCP protocol implements
a reliable sliding
acknowledgement window with a
limited maximum window size, and
assumes data loss is the result of
network congestion. WIN-T SOTM
systems will make use of a Space
Communications Protocol Standards
Army Communicator 21