Swarm Communications Using a Sparse Array of...
Transcript of Swarm Communications Using a Sparse Array of...
Swarm Communications Using a Sparse Array of
Small Antennas
February 2018
Jason Weaver, Greg Shreve
GSAW 2018
Northrop Grumman Corporation
Approved For Public Release #18-0133; Unlimited Distribution
© 2018 by Northrop Grumman Corporation.Published by The Aerospace Corporation with permission.
Overview and Motivation for MIIMIO
• Microsats have a small power budget, which intrinsically limits downlink transmitter power and data capacity
– Microsats may collect more data than they can downlink– Large downlink bandwidth requires a large receiving aperture on the ground– Microsat swarms compound the downlink problem
• The Multiple Independent Inputs, Multiple Independent Outputs (MIIMIO) sparse ground receive array concept provides microsatswarm downlink advantages
– Provides large receive aperture, to enable wideband downlinks with low transmitter power
– Enables simultaneous downlinks from all microsats in the swarm, using same frequency
– Microsats transmit independently, without coordination– Low cost ground antennas point open loop, providing the field of regard and total
aperture gain
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MIIMIO Downlink Overview System Drawing
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Satellite swarmSwarm orbit
trajectory
Effective beamforming aperture –antennas may be distributed over
hundreds of meters
Sum of aperture area equivalent
to one large dish
Relatively wide beamwidthof each MIIMIO antenna
(the Field of Regard)
Narrow beamwidth of one MIIMIO effective receive aperture
Traditional approach – one large antenna with equivalent aperture gain• Can only point at one or a couple of adjacent
satellites in the swarm• Needs frequency division multiplexing
Downlink footprint of each satellite
MIIMIO Sparse Ground Receiver Array
• Digital beamforming creates a narrow high gain receive beam for each satellite in the swarm
The swarm may subtend angle of 1 degree or more, as seen from ground array
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MIIMIO Features
• Transmitters on the microsats are independent – no coordination required among them
• The initial MIIMIO application is for reception of separate, re-used frequency downlinks, from a swarm of closely-spaced microsats
• Sparse array of receiving antennas on the ground– Similar to high resolution sparse array interferometers in radio astronomy– Ground antenna elements are typically 100 to 1,000 wavelengths apart– Compute very narrow beams to isolate each downlink– Total receiving area (antenna aperture x number of array elements) sets link
budget– Each microsat can downlink a large bandwidth
• This is -not- MIMO; no need for channel modeling
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Sparse Aperture Digital Receive Beamforming Details
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DigitizeBufferD/C
Network
Digital Beamforming(& Nulling)
Demodulators
N Beams
Ethernet Network from Antennas
MIIMIO arrayM Antennas
N Outputs
Digital true time delay signal combining
Downconverters and digitizer sampling clocks can & should be freq/phase locked for all antennas
Software demods
N satellites
Each satellite downlink
Electronics at each antenna:Downconverter, digitizer, memory buffer
MInputs
Frequency & Timing for Iterative Trajectory Reconstruction
Swar
m
Proc
essi
ng
Linear beamforming and nulling matrix
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Numerical Example
• X-band downlink, 9 GHz
• 36 antennas, 1.8 meter diameter, open loop pointing– Half-power beamwidth (HPBW) provides 1.2° field of regard
• Equivalent single dish collection area = 10 meter diameter– Total RX gain = 56 dB
• Sparse array distributed over 150 meters diameter– Each formed beam HPBW = 0.015°– On average, only one satellite in the swarm will be in the full-gain boresight
• Nulls can be computed to further reduce co-channel interference in sidelobes and grating lobes
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Notional Nulling in Swarm
• Can use traditional receive beamforming to accomplish reduction of interferers by nulling
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X
XXX
X
X X
O
Zenith
Horizon
Swarm
Array gain peak on
Satellite n
Other satellites nulled
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Notional Simulation - Beamforming Plus Multiple Nulls
• Sharp nulls can be computed for each beam, to null some or all of the other satellite downlinks in the swarm to further reduce the cross-channel interference
• A number of well-formed nulls can be multiplied with little reduction of main beam gain, while providing significant reduction of interferers
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‐20
‐15
‐10
‐5
0
‐2 ‐1.5 ‐1 ‐0.5 0 0.5 1 1.5 2
Gain in
dB
Boresight angle in degrees
Synthesized Beams with Nulling
Beam 1 Beam 2 Beam 3
Clutter Rcvr Noise Field of Regard
NULL 1 NULL 2
Nulls not adaptively centered; have little effect on beam gain
Nulls centered on beams not of interest; suppression of >15 dB
‐20
‐15
‐10
‐5
0
‐2 ‐1.5 ‐1 ‐0.5 0 0.5 1 1.5 2
Gain in
dB
Boresight angle in degrees
Synthesized Beams with Nulling
Beam 1 Beam 2 Beam 3
Clutter Rcvr Noise Field of Regard
NULL 1 NULL 2
Synthesized Beams without Nulling
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Simulation - Comparison of Transmitted Signal vs. Recovered Signal
• In simulation, use time domain (burst transmissions) to show received signals relative to interference and noise
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Mag
nitu
de (V
)M
agni
tude
(V)
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Simulation - Signals Received at Antenna Array
• Interference Signals at [30, 45.5 ] & [31, 45.5] ([az,el])
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Mag
nitu
de (V
)
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Normalized Power (dB), Broadside at 0.00 degrees
Elevation Cut (azimuth angle = 45.0°)
-40
-30
-20
-10
-150
0
30
60
-120
90
-90
120
-60
150
-30
180 0
Simulation - Polar Radiation Plot of 36 Element Array and Parabolic Pattern Superimposed
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This pattern includes a superimposed antenna element gain with < 3° HPBW
Swarm
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Conclusions
• Mature sparse array beamforming provides swarm support
• Microsats are independent and can be as simple as possible– No crosslinks needed for on-orbit coordination
• Beams can be computed after the LEO flyover– Iteratively optimize LEO tracking, formed beam gain, nulling and cross-channel
cancellation across the flyover arc
• Low cost ground– Small antennas with open loop pointing– Commodity digitizers– Offline beam computing
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