a digital revisitation_of_analog_beamforming_techniques - aiaaicssc2013_lisi

19th Ka and 31st AIAA ICSSC Joint ConferenceFlorence, October 2013

A Digital Revisitation of

Analog Beam-Forming TechniquesPiero Angeletti¹, Marco Lisi¹¯²

1) European Space Agency, Noordwijk, The Netherlands2) Special Advisor to the European Commission

1

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 2

19th Ka and 31st AIAA ICSSC Joint Conference, Florence, October 2013

Arrays for Space ApplicationsKey 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

3


Divide/Combine Beamforming Networks

N

M

• 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.

4


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.

Single Beam BFN

N

M

5

RF versus IF Beam-Forming


EN

BN 1:BN

EN:1

Beams

Feeds

RF BFN IF BFN NE LNAs NE LNAs

NE RF Power Dividers NE IF Power Dividers

NE xNB RF Phase & Amplitude Controls NE xNB

IF Phase & AmplitudeControls

NB RF Power Combiners NB IF Power Combiners

NB Mixers NE Mixers

ESA Multibeam Array Model (MAM, circa 1980)


S-Band Payload of Japanese Data Relay Satellite (ETS-VI)


The T. Teshirogi Patent (1982)


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!

Butler Matrix as Analog Implementationof 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).



AI1

AQ1

AIm

AQm

AIM

AQM

BI1BQ

1BI

nBQ

nBI

NBQ

N

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 Multiplierand Adder

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Analog vs. Digital Weight Element



Where does the complexity stand?• The total number of

beamforming weights (i.e. multiplications) plays a key role in defining the processing burden

• To carry out the complex weighting, four real multiplications and two real additions are be required

• Multipliers cost much more than Adders

Operation GatesComp Mul 6000Comp Add 300

IA

QA

I I Q QA E A E

I Q Q IA E A E

ASIC Gates’ count Example(10 bits of word length)

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Digital Beamforming in a Processor

Design Example• 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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RX Digital Beam-Forming Network


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.

FFT and Butler BFN’s (2/2)


• DBF techniques based on Fast-Fourier-Transforms (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)

Conclusions


Digital BFN architectures inspired to equivalent analog topologiesand exploiting beams/elements symmetries offer severaladvantages, a non exhaustive list of which includes:

• 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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