Adaptive phased-array antennas provide cost-effective implementation of large, light weight apertures with high directivity and precise beamshape control. Adaptive self-calibration allows for relaxation of all mechanical tolerances across the aperture and electrical component tolerances, providing high performance with a low-cost, lightweight array, even in the presence of large physical distortions. Beam-shape is programmable and adaptable to changes in technical and operational requirements. Adaptive digital beam-forming eliminates uplink contention by allowing a single electronically steerable antenna to service a large number of receivers with beams which adaptively focus on one source while eliminating interference from others. A large, adaptively calibrated and fully programmable aperture can also provide precise beam shape control for power-efficient direct broadcast from space. Advanced adaptive digital beamforming technologies are described for: (1) electronic compensation of aperture distortion, (2) multiple receiver adaptive space-time processing, and (3) downlink beam-shape control. Cost considerations for space-based array applications are also discussed

Abend, Kenneth

Horton, Charles R.

English

NASA Technical Reports Server

wN94-22 745
Adaptive Array Antenna for Satellite Cellular and Direct Broadcast Communications*
ABSTRACT
Adaptive phased-array antennas provide
cost-effective implementation of large, light
weight apertures with high directivity and
precise beamshape control. Adaptive self-
calibration allows for relaxation of all
mechanical tolerances across the aperture and
electrical component tolerances, providing
high performance with a low-cost, light-
weight array, even in the presence of large
physical distortions. Beam-shape is pro-
grammable and adaptable to changes in tech-
nical and operational requirements. Adaptive
digital beam-forming eliminates uplink con-
tention by allowing a single electronically
steerable antenna to service a large number of
receivers with beams which adaptively focus
on one source while eliminating interference
from others. A large, adaptively calibrated
and fully programmable aperture can also
provide precise beam shape control for power-
efficient direct broadcast from space.
This paper describes advanced adaptive
digital beamforming technologies for: (1)
electronic compensation of aperture distortion,
(2) multiple receiver adaptive space-time
processing, and (3) downlink beam-shape
control. Cost considerations for space-based
array applications are also discussed.
SATELLITE ANTENNA DESIGN
High density RF communications traffic
requires satellite antennas with high gain,
precise beam pointing, and electronic speed
and steering agility. If this is to be supplied
Charles R. Horton and Kenneth Abend
GORCA SYSTEMS INCORPORATED (GSI)
P.O.Box 2325
Cherry Hill, New Jersey
08034-0181 U.S.A.
Phone: 609-272-8200
Fax: 609-273-8288
email: knabend@gorca.com
with a large phased array antenna, problems of
size, weight, cost, and performance arise. As
uplink traffic becomes more intense or
diverse, the need for multiple receivers and
adaptive interference suppression adds to the
cost.
Conventional approaches to maintaining
antenna gain, suppressing interference, and
controlling beam shape are dependent upon
maintaining very stringent tolerances across
the aperture. Even if rigidity and precise
control is provided during manufacture, once
the satellite is deployed it will be subject to (1)
distortion of the structure after repositioning,
(2) thermal distortion across the array, and (3)
potential revised beam requirements due to
changing business plans. The risk is of
reduced operational performance due to gain
degradation, increased sidelobe interference
for uplink signals, and high sidelobe energy
spiliover for downlink traffic and for direct
broadcast.
As an alternative to using sophisticated
structural components and/or auxiliary sub-
systems to measure and correct for deforma-
tions and other sources of error, a class of self-
calibrating adaptive digital beamforming
techniques has been developed by GSI. These
procedures compensate for aggregate errors
without regard to their source: structural
deformations, mutual coupling, solid state
module phase errors, RF delay errors, and
propagation anomalies. These procedures
complement adaptive interference cancellation
approaches. The cost savings due to reduced
reliance on tight electrical and mechanical
tolerances is greater than the cost of the digital
* The conceptual design approaches described herein take advantage
of signal processing algorithms proprietary to GORCA Systems, Inc.
];qNC_'DJ["_G _AGE £_LANK NOT F_t_$E'D
47
https://ntrs.nasa.gov/search.jsp?R=19940018272 2020-06-16T18:30:06+00:00Z
beam forming needed for multiple beam
formation and adaptive interference sup-
pression. The gain control achievable for
direct broadcast saves prime power. The
procedures may permit the antenna structure to
be non-rigid and deformable resulting in a
major weight reduction and consequently a
reduced launch cost with improved perform-
ance. Advantages and benefits of adaptive
processing are listed below.
TABLE 1. ADAPTIV E ARRAY ADVANTAGE_;
Antenna Array
• 30% to 40% reduction in mass
• complex RF components replaced by solid state
electronics with firmware algorithms
• 1130:1 easing of mechanical tolerances
• adaptive response provides immediate recovery from
mechanical, electrical and RF disturbances
• design approach takes advantages of technology
advancements in MIMIC, VLSI, etc.
• low cost assembly and test due to relaxed mechanical
requirements, ability to correct for errors, etc.
Satellite
• 10:1 reduction in system pointing requirement and
"fine" attitude control, due to adaptive calibration of
antenna and beam steering capability
• up to 50% reduction in satellite mass, some of whlch
can be applied to increased on-orblt life
• significant reduction in solar array and battery power,
due to more effective use of RF power
• conventional attitude control components could be
replaced by attitude and position sensing using the
array electronics
• self-calibrating features provide immunity to thermal or
other mechanical distortions
• immunity to contentious uplinks due to antenna hulling
• low cost integralion and test, and accelerated schedules
• significant reduction in cost of launch services
Communications Applications
• lower uplink RF power, or higher G/T margin, due to more
precise beamshaping
• higher effective downlink EIRP due to precise spot beams
• software beamforming allows operational flexibility to
meet changing requirements, traffic diversity, business
plans, etc.
• very large number of users accommodated
• digital beam steering and software beam shaping
provides instantaneous response to changing
requirements
• very large aperture systems can be developed which
could otherwise not be built
ADAPTIVE DIGITAL BEAMFORMING
We consider a phased array antenna
consisting of transmit/receive modules with a
receiver and A/D converter behind each. By
sampling the aperture we obtain complete
software control over transmit and receive
beam formation. This includes the ability to
(a) adaptively determine sets of weights that
compensate for mechanical, electrical, and
environmental errors, (b) adaptively home in
on a transmitted signal while simultaneously
suppressing contending signals, (c) digitally
form multiple simultaneous receive beams,
and (d) design and revise the transmitter foot-
print for broadcast operation.
Digital beamforming weights an array so
as to (1) sense a desired signal and (2) attenu-
ate interference. The received signal at each
array element is assumed to have been het-
erodyned, filtered, sampled, and digitized so
that en(t ) is a complex number representing the
in-phase and quadrature components of the
wavefront at the n th array element at time t.
The beamforming concept is illustrated in
Figure 1.
A beam is formed in the digital processor
as a linear combination,
N
y-_]w_en = whe, (1)
n-I
where w h = (wi* , w2*, ...,w_*) is a vector of
complex weights to be applied to the compo-
nents, el, e2,...,% , of the received data vector,
e. The weights are obtained as the conjugate
transpose (h) of the optimum weight vector,
w = ctR-ls , (2)
where s is a Iow-sidelobe "steering vector" that
would be used if interference were spatially
homogeneous (R=o2I ") such as thermal noise.
The directionality of the interference is
accounted for in the NxN correlation matrix,
R, whose ij th element is the correlation
between the interference wave front at
elements i and j,
R =E{ e eh}.
Note that many beams may be formed simul-
taneously by choosing a different steering
vector for each.
48
y(O) is the complex signal output
I :E
! )4
_ w_
eN
e3
e2
e_
r
y
Digital Beamformer
Adaptive
Weight
Estimation
w(e) = _Rls(e)
Earth
- 5- - - -
e_ e2 e3 eN
steering _//6_direction "_
of "*',,.i
---../. orro 
-'/,,\ i\ nor o,
- communications terminal
Antenna Modules
-- -- "3 y=_W_en
where
en is the complex
signal received
by the nth antenna
element
Wn is a complex
weight developed
by the signal
processor
R is the spatial
covariance
matrix
s = s(e]= low sidelobearray
steeringvector
FIGURE 1. BEAMFORMING
Beamforming is said to be adaptive if the for radar applications. They involve the use of
weight vector is computed on the basis of
received signals (as opposed to being specified
a priori). We note that there are two parts to
adaptive beamforming. One relates to using
received signal data to estimate the steering
vector, s. The other relates to using received
interference data to estimate R -1.
Estimating s is called self calibration, self
cohering, or adaptive focusing. Estimating R -_
is called adaptive nulling or interference
cancellation. GSI is in the forefront of both
aspects of adaptive beamforming. We will
discuss the self-calibration aspect first.
ELECTRONIC COMPENSATION FOR
APERTURE DISTORTION
Table 2 summarizes a number of adaptive
self calibration algorithms that were developed
a variety of phase synchronizing sources to
furnish the calibration pilot signal and they
have been successfully applied to the
calibration of real apertures, synthetic
apertures, and inverse synthetic apertures.
When used to establish a phase reference
across a synthetic aperture they are equivalent
TABLE 2. SELF-COIIERING TF_IINIQI_ES
)ominant Scatterer
• Minimum Variance Algorithm - Steinberg (U of P)
• Multiple Scatterer Algorithm (MSA) - Attia (GSr)
• MSA with Subarray Processing - Attia/Kang (GSIAIofP)
• Phase Gradien! Auto focus Algorithm - Jakowitz (Sandla)
;patial Correlation
• Spatial Correlation Algorithm (SCA) - Atlia (GSI)
• Multiple Lag SCA - Subbaram (GSI)
• Shear Averaging - Feinup (ERIM)
• Iterative SCA - Atria (GSI)
Phase llistory Reconstruction
• Image Plane Differential Phase Smoothing - Kupiec (MITLL)
• Aperture Plane Differential Phase Smoothing - Stockburger
Energy Constraint Method - Tsao?Suhbaram (GSI)
49
to motion compensation or auto-focus
techniques for SAR or ISAR imaging.
The key concept is that signals from the
earth are received by individual array elements
and combined in order to establish a phase
reference that compensates for all errors.
Correction for mechanical shape distortion is
illustrated in Figure 2.
The simplest algorithms use the wave-
front at the aperture from a strong signal as a
phase synchronizing source for all array ele-
ments. Other algorithms use multiple sources,
estimate parameters of an aperture distortion
model, or adapt in an iterative manner. The
spatial correlation algorithm correlates the
signals received at adjacent array elements and
introduces a time shift determined by the time
of the correlation peak as well as a phase
correction. This allows for correction of large
distortions without ambiguity.
SPACE-TIME PROCESSING
Since the aperture samples the wavefront
in both space and time, the signals can be
simultaneously processed in both domains to
separate signals according to angle of arrival
and spectral content. In Figure 1, Equation (1)
is applied in the spatial domain to implement a
process called beamforming. In general,
Equations (1) and (2) refer to to linear space-
time processing. The temporal counterpart of
beamforming is filtering and the temporal
counterpart of self calibration is channel
equalization. In the special case where
w=s=E{e]signal}, the spatial weights represent
a matched field steering vector and the
temporal weights represent a white-noise
matched filter for the signal.
With reference to Equation (1) and Figure
1, the formation of multiple beams to
service multiple ground stations while elimi-
nating uplink contention is relatively straight
forward. The N dimensional data vector, e, is
processed in an on-board digital processor to
produce a set of K linear combinations of N
complex numbers: Yl, Y2, ..., YK" Each output
\
\
\
\
\
\ ' \
\ '
\ \ e
1
\, \ \ e2
COMMUNICATtOI/S \ \ e3 ,
UPLINK(S) , e(fROMONEORMORE \, \ \
\
UPLINKLOCATION) \ \ '
\ \ \ \ ' ' \ /./k
' \ \ /k'"
\ \ \, \, \ ,
, k \ \ \/- , \
\ , , ., \/ \ \
• \ \ \/ \
\ , ./_ , / \ \
' \ /X 0 /_ , , \
• \ .._- , xN," - , \ \ , \\ , . \ \ -- \ ,
.._ \ , \ \
k_ k k ' k '
\ , \ k \
\ \ \ ' \
\ \ \ \
IT/RECEIVEMODULES(QIY N)
\
ARRAYNORMALFOR
PERFECTSYSTEM
-- PLANEOFARRAY
FORPERFECTSYSTEM
\ -- SHAPEOFACTUALARRAY
• WITHDISTORTIONA D
\ POINTINGERROR
FIGURE 2. ELECTRONIC CORRECTION FOR MECHANICAL DISTORTION & POINTING ERROR
50
representsthesignalreceivedfrom oneground
station. If 0 a represents the location of a
particular ground station, then Yd is formed
using Equation (1) with w=w(0_). In applying
Equation (2) to obtain this weight vector, each
component of the steering vector is given the
same phase as the phase of that same
component of a data vector, e(0d), that would
is underdetermined, the minimum norm
solution for f is the desirable one because the
norm of f represents the required transmitter
power. Thus, if the array is well formed or
properly calibrated, open-loop beamshaping
by computer is relatively straight forward.
As described earlier, the problem of
calibrating a large receiver array, using signals
be received from that station. The NxN transmitted from the ground, has a reasonably
correlation matrix could even be pre-computed precise solution. Data is collected at each
as a diagonally loaded version of the rank K-1 element and processed so that the signals add
matrix whose mn th component is
K
_e,,(Ok)e_(Ok)"
k,_d
The resulting Yd supplies a receiver with one
signal while nulling all other signals.
DOWNLINK BEAMSHAPE CONTROL
For a properly calibrated array, the aper-
ture illumination function, f(x,y), and the
azimuth-elevation far-field pattern, F(u,v), are
related by a two dimensional Fourier
transform. The larger the aperture, the finer
the control on where power is directed. At an
altitude of h, the diffraction limited resolution
(or pixel diameter) on the ground is at least
h_./D, where _. is the rf wavelength and D is
the aperture diameter.
The desired intensity distribution
_F(u,v)[ 2, is specified as 1 for values of u=sin 0
and v=cos d_ corresponding to the location of
cities that must be reached in a direct
broadcast application. A criterion can be
devised such as minimizing the average power
outside of a given region. We specify F as 0
for values of (u,v) outside of the coverage
region and as 1 for the location of target cities,
and we find the least squares solution of the
set of equations for f.1 If the set of equations
IAcually, if only the power density I/I" is speofted, then only the the
spatial autocorrelation function of the aperture illumination is
constrained.
<[(x+a,y+b)f'(x,y)> = R(a,b)
where R and I/_ 2 are a Fourier transform pair.
coherently. The crucial question is whether it
is possible to use the receive calibration
corrections to correct errors in the transmitted
wavefront.
This problem turns out to be more diffi-
cult. The signals transmitted from all array
elements must be synchronized so that they
add coherently in the target region. If errors
exist in the transmitter chain, a closed loop
procedure is required. Transmitter time or
phase synchronization requires the return of a
transponding signal from the target on the
earth so that the two way propagation can be
measured. While phased array receiver cali-
bration is relatively well established [1,2],
retrodirective transmitter calibration is a rela-
tively new technology. However, GSI per-
sonnel have established feasibility in a ground
based demonstration [3]. While further devel-
opment is still needed, the potential payoff for
adaptive transmitter self calibration is
enormous.
COST CONSIDERATIONS
Many advanced array systems have been
proposed and studied for space applications
and less advanced array systems have been
developed and built. Space-based applications
include both remote sensing and communica-
tions, but the degree to which array technology
has been actually deployed has been limited
due to cost considerations.
The cost drivers include cost of electron-
ics, due to the large number of array element
modules; structural co_t of arrays and space-
._I 51
craft, to meet launch loads and minimize
thermally-induceddistortion or pointing error
whenon-orbit; the high cost of integration and
test, for both payload and satellite; and launch
cost, driven by the increased mass of the array
structure and the spacecraft which are needed
to meet system performance objectives.
The cost of electronics is being addressed
via MMIC technology advancements, efficient
signal processing electronics, design
innovations and manufacturing efficiencies
achievable with a large number of array
modules. However, the other cost drivers have
not been sufficiently addressed to make many
advanced array concepts economically viable.
As an example, a typical application of array
technology to achieve power efficient satellite
communications [to achieve very small spot
beams which can be dynamically moved to
match communications traffic requirements]
results in a very large, complex satellite which
also results in high launch costs.
The design concepts presented herein
address these other cost drivers. The array
structure can be built using very low cost, low
mass designs since stiffness and distortion
requirements are minimal. The allowable
satellite and array pointing errors can be
greatly relaxed, further simplifying other
satellite subsystems and satellite operations,
and significantly reducing over-all on-orbit
mass. The costs of integration and test are
minimized since alignment and component
precision issues largely disappear. Finally, the
launch costs are greatly reduced due to the
large reduction in lift-off mass. A typical
communications satellite constellation could
be deployed with a net savings of tens of
millions of dollars.
SUMMARY AND CONCLUSIONS
The performance obtainable by space-
based array systems for communications and
other applications is well known; however, the
implementation of array technologies into
operational systems has been limited due to
the high cost of arrays, satellites and launch
services. The adaptive array design approach
described herein applies advanced signal
processing technology originally developed
for radar systems to next generation commu-
nications satellites. All of the advantages of
array systems, for communications or remote
sensing applications, are retained while mak-
ing it possible to develop and deploy opera-
tional systems at affordable cost.
REFERENCES
[1 ] Steinberg, Microwave Imaging with Large
Antenna Arrays. New York: Wiley, 1983.
[2] B.D. Steinberg and H. Subbaram,
Microwave Imaging Techniques. New York:
Wiley, 1991.
[3] E.H. Attia and K. Abend, "An
Experimental Demonstration of a Distributed
Array Radar," IEEE AP-S International
Symposium Digest, pp. 1720-1723, London,
Ontario, June 1991.
GORCA Systems, Inc. [GSl], an aerospace
engineering company based in Cherry Hill, NJ, has
made innovative contributions to the aerospace
industry in the fields of commercial space,
government, civil, and DoD space systems, advanced
signal processing and remote sensing systems
technologies and applications. GSI has also developed
several spin-off technologies for commercial products.
Phone:609-273-8200.
Dr. Kenneth Abend is Director of GSrs Advanced
Signal Processing Laboratory. He is an expert on
adaptive space-time processing and coherent imaging
wilh distorted apertures. He has thirty five years
experience in research, development, and analysis of
advanced digital systems and techniques as applied to
radar, sonar, communications, and pattern recognition.
Charles R. tlorton, President of GSI, has a diverse
background which includes technical and management
contributions to NASA, DoD and commercial space
systems and specialized components [both space and
ground segments]. His background also includes
development of advanced consumer electronics
products.
52


Adaptive array antenna for satellite cellular and direct broadcast communications

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