This document discusses analog and digital beamforming techniques for antenna arrays. It describes how several analog beamforming network architectures were developed historically, while more recent approaches use digital beamforming to achieve higher flexibility. Some analog topologies can be translated to digital implementations with complexity reductions by exploiting symmetries. The document provides examples of analog beamforming networks and discusses advantages of digital beamforming networks inspired by analog designs.

1. A Digital Revisitation
of
Analog Beam-Forming Techniques
Piero Angeletti¹, Marco Lisi¹¯²
1) European Space Agency, Noordwijk, The Netherlands
2) Special Advisor to the European Commission
19th Ka and 31st AIAA ICSSC Joint Conference
Florence, October 2013
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2. Summary
•
•
•
•
The development of multiple beams antennas and of
reconfigurable active arrays is tightly connected to that of
Beam-Forming Networks (BFN’s);
Several analog BFN architectures were developed in the
past for radar and satellite applications;
More recently, digital beam-forming techniques are being
developed, in order to achieve higher degrees of
reconfigurability and flexibility;
Some analog BFN topologies can be easily translated
into a digital realization. Moreover, by taking advantage
of beams and/or elements symmetries, substantial
reductions in terms of complexity and power consumption
can be achieved.
19th Ka and 31st AIAA ICSSC Joint Conference, Florence, October 2013
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3. Arrays for Space Applications
Key features:
•
•
•
•
•
•
Multi-beam generation: a single
aperture can contemporarily
generate a multitude of beams
Modularity/Scalability: building
blocks approach (i.e. the radiating
elements and its associated T/R
module)
RF Power Pooling: all High Power
Amplifiers (HPAs) contribute to
each beam (the overall RF power
can be dynamically shared among
the beams)
Graceful Degradation: a failure of
some elements will not cause the
loss of the full antenna function
Wide angle scanning
Steering/Pointing Agility
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4. Divide/Combine Beamforming Networks
•
•
The BFN is often the
dominant component, the
“true hearth” of most
multiple beams
antennas.
BFN’s are complex
networks used to
precisely control the
phase and amplitude of
RF energy passing
through them, which is
conveyed to the radiating
elements of an antenna
array.
N
M
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5. Cross-Bar Beamforming Networks
•
Full equivalence
with a cross-bar
topology can be
obtained
identifying the
amplitude/phase
weighting and the
feed accumulation
as a basic cell
element.
M
Single Beam BFN
N
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6. RF versus IF Beam-Forming
NE
1: N E
RF BFN
IF BFN
NE
N B :1
LNAs
NE
RF Power Dividers
NE
IF Power Dividers
RF Phase & Amplitude
Controls
NE xNB
IF Phase & Amplitude
Controls
NB
NB
NE
NE xNB
Feeds
LNAs
RF Power Combiners
NB
IF Power Combiners
NB
Mixers
NE
Mixers
Beams
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7. ESA Multibeam Array Model
(MAM, circa 1980)
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8. S-Band Payload of
Japanese Data Relay Satellite (ETS-VI)
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9. The T. Teshirogi Patent (1982)
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10. Butler Matrix Beam-Forming
Nota Bene: an 8x8 Butler matrix requires 12 hybrid
couplers and 8 phase shifters. A traditional
“divide/combine” BFN would have required 112 hybrid
couplers and 64 phase-shifters!
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11. Butler Matrix as Analog Implementation
of FFT (and viceversa)
•
The complexity reduction of a Butler matrix is equivalent
to that obtained, in digital signal processing, by using the
Fast Fourier Transform (FFT) algorithm to evaluate the
Discrete Fourier Transform (DFT).
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12. Digital Beamforming
•
In digital
beamforming, the
operations of phase
shifting and
amplitude scaling for
each antenna
element, and
summation for
receiving, are done
digitally.
I,Q samples of the
beam signals
Complex Multiplier
and Adder
AI1
AQ1
AIm
AQm
AI M
AQM
BI 1 Q
B 1
BIn Q
B n
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BIN Q
B N
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13. Analog vs. Digital Weight Element
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14. Where does the complexity stand?
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•
•
The total number of
AI
beamforming weights (i.e.
multiplications) plays a key
role in defining the processing
burden
AI EI  AQ EQ
AQ
AI EQ  AQ EI
To carry out the complex
weighting, four real
multiplications and two real
additions are be required
Multipliers cost much more
than Adders
ASIC Gates’ count Example
(10 bits of word length)
Operation
Comp Mul
Comp Add
Gates
6000
300
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15. Digital Beamforming in a Processor
Design Example
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DRA in Tx (2 sets of 475
radiating elements, for 2
polarizations)
Two sets of beamformers, for
the two polarizations, each
one processing 64 user
beams with a 250 MHz
bandwidth
Due to the 2 polarizations on
user side, there are two sets
of output chains (Mx + DAC).
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16. RX Digital Beam-Forming Network
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17. FFT and Butler BFN’s (1/2)
•
Factorised beamforming matrices (e.g.
FFT and Butler Matrices) allow to
reduce the BFN complexity


•
A constituent Radiation Pattern
is chosen as prototype beam.
Out-of-nadir Beams are
generated from the constituent
beam applying the phase
scanning.
The main drawback of the Butler/FFT
BFN is related to the limited array
geometries and beam lattices to
which it can be applied.
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18. FFT and Butler BFN’s (2/2)
•
DBF techniques based on Fast-FourierTransforms (FFTs) on planar lattices
are particularly well suited for periodic
active arrays and have been recently
implemented, tested and validated in a
real-time proof-of-concept demonstrator
(Courtesy of Astrium)
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19. Conclusions
Digital BFN architectures inspired to equivalent analog topologies
and exploiting beams/elements symmetries offer several
advantages, a non exhaustive list of which includes:
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Use of a unique architecture for several array geometries.
Scalable design, able to support from few to
hundreds/thousand beams and elements.
A high degree of modularity, provided by the decomposition
of the BFN in building blocks.
High efficiency in terms of technology and reduced complexity
(i.e. number of devices, mass, area/volume, power
consumption, power dissipation, integration and testing time
and cost).
The solution is applicable both to transmit and receive
beamforming.
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