U.S. patent application number 09/782767 was filed with the patent office on 2001-07-26 for bootstrapped, piecewise-asymptotic directivity pattern control mechanism setting weighting coefficients of phased array antenna.
This patent application is currently assigned to Harris Corporation. Invention is credited to Halford, Steven D., Henry, John C. III, Martin, Gayle Patrick.
Application Number | 20010009861 09/782767 |
Document ID | / |
Family ID | 22164309 |
Filed Date | 2001-07-26 |
United States Patent
Application |
20010009861 |
Kind Code |
A1 |
Martin, Gayle Patrick ; et
al. |
July 26, 2001 |
Bootstrapped, piecewise-asymptotic directivity pattern control
mechanism setting weighting coefficients of phased array
antenna
Abstract
Weighting coefficients for a phased array antenna are
iteratively refined to optimal values by a `bootstrapped` process
that starts with a coarse set of weighting coefficients, to which
received signals are subjected, to produce a first set of signal
estimates. These estimates and the received signals are iteratively
processed a prescribed number of times to refine the weighting
coefficients, such that the gain and/or nulls of antenna's
directivity pattern will maximize the signal to noise ratio. Such
improved functionality is particularly useful in association with
the phased array antenna of a base station of a time division
multiple access (TDMA) cellular communication system, where it is
necessary to cancel interference from co-channel users located in
cells adjacent to the cell containing a desired user and the base
station.
Inventors: |
Martin, Gayle Patrick;
(Merritt Island, FL) ; Halford, Steven D.;
(Melbourne, FL) ; Henry, John C. III;
(Indialantic, FL) |
Correspondence
Address: |
Charles E. Wands
Allen, Dyer, Doppelt, Milbrath, Gilchrist P.A.
P.O. Box 3791
Orlando
FL
32802-3791
US
|
Assignee: |
Harris Corporation
|
Family ID: |
22164309 |
Appl. No.: |
09/782767 |
Filed: |
February 13, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09782767 |
Feb 13, 2001 |
|
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|
09081460 |
May 19, 1998 |
|
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6188915 |
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Current U.S.
Class: |
455/562.1 ;
455/446 |
Current CPC
Class: |
H01Q 1/246 20130101;
H04B 7/0848 20130101; H01Q 3/2605 20130101 |
Class at
Publication: |
455/562 ;
455/422; 455/446 |
International
Class: |
H04B 001/38; H04M
001/00 |
Claims
What is claimed:
1. A method of processing signals representative of outputs of
antenna elements of a phased array antenna to derive amplitude and
phase weighting coefficients, through which signal coupling paths
of said antenna elements are controllably weighted to control the
directivity pattern of said phased array antenna, said method
comprising the steps of: (a) providing values of said weighting
coefficients; (b) modifying said signals representative of outputs
of said antenna elements in accordance with said values of said
weighting coefficients provided in step (a); (c) generating
estimates of information signal contents of signals representative
of outputs of said antenna elements as modified in step (b); and
(d) iteratively adjusting said signals representative of outputs of
said antenna elements by (d1) subjecting said signals
representative of outputs of said antenna elements and estimates of
information signal contents of signals representative of outputs of
said antenna elements to a prescribed signal processing operator to
derive improved values of said weighting coefficients, and (d2)
adjusting said signals representative of outputs of said antenna
elements in accordance with said improved values of said weighting
coefficients to produce improved signal outputs of said antenna
elements, and (d3) iteratively repeating steps (d1) and (d2) a
number of times N, where N is greater than or equal to zero.
2. A method according to claim 1, wherein step (d3) comprises
iteratively repeating steps (d1) and (d2), as necessary, to bring
the signal-to-noise ratio of said improved signal outputs of said
antenna elements to within a prescribed improvement value.
3. A method according to claim 1, wherein step (d3) comprises
iteratively repeating steps (d1) and (d2) a predetermined number of
times N, where N is greater than or equal to zero.
4. A method according to claim 1, wherein said prescribed signal
processing operator is one which combines estimates of said
information signal contents of signals representative of outputs of
said antenna elements and signals representative of information
signal contents and noise signal contents of outputs of said
antenna elements to produce respective sets of signal and noise
covariances, and generates said improved values of said weighting
coefficients in accordance with said respective sets of signal and
noise covariances.
5. A method according to claim 4, wherein step (d2) comprises
generating products of said signals representative of outputs of
said antenna elements and said improved values of said weighting
coefficients to produce improved signal outputs of said antenna
elements.
6. A method according to claim 4, wherein step (d2) comprises
generating said improved values of said weighting coefficients in
accordance with products of previous weighting coefficients and
said respective sets of signal and noise covariances.
7. A method according to claim 4, wherein said prescribed signal
processing operator is one which combines estimates of said
information signal contents of signals representative of outputs of
said antenna elements, and signals representative of information
signal contents and noise signal contents of outputs of said
antenna elements, to produce useful signal components and
uncorrelated noise signal components, and is operative to produce
said set of signal covariances in accordance with said information
signal contents of outputs of said antenna elements and said useful
signal components, and to produce said set of noise covariances in
accordance with said noise signal contents of outputs of said
antenna elements and said uncorrelated noise signal components.
8. A method according to claim 1, wherein step (c) comprises
demodulating said signals representative of outputs of said antenna
elements as modified in step (b) to generate said estimates of
information signal contents of signals representative of outputs of
said antenna elements.
9. A method according to claim 1, wherein step (a) comprises
providing initial values of said weighting coefficients exclusive
of information available in performing steps (c)-(d).
10. A method according to claim 1, wherein step (c) comprises
subjecting said signals representative of outputs of said antenna
elements as modified in step (b) to a limiter operator to generate
said estimates of information signal contents of signals
representative of outputs of said antenna elements.
11. A method according to claim 1, wherein step (c) comprises
hard-limiting said signals representative of outputs of said
antenna elements as modified in step (b) to generate said estimates
of information signal contents of signals representative of outputs
of said antenna elements.
12. A method according to claim 1, wherein step (d) comprises the
preliminary step (do) of storing said signals representative of
outputs of said antenna elements, step (d1) comprises subjecting
signals stored in step (d0) and said estimates of information
signal contents of signals representative of outputs of said
antenna elements to a prescribed signal processing operator to
derive improved values of said weighting coefficients, and step
(d2) comprises adjusting said signals stored in step (d0) in
accordance with said improved values of said weighting coefficients
to produce improved signal outputs of said antenna elements.
13. A method according to claim 1, wherein said phased array
antenna is installed at a cell base station of a multi-cell, time
division multiple access (TDMA) cellular communication system, and
has its directivity pattern adaptively modified in steps (a)-(d) so
as to form a beam whose gain and/or nulls are defined so as to
maximize the signal to noise ratio in the presence of co-channel
users whose communication time slots overlap a communication time
slot of said desired user.
14. A method according to claim 13, wherein steps (a)-(d) are
performed during an unused time slot, to derive values of said
weighting coefficients that are effective to maximize the signal to
noise ratio in the presence of co-channel users whose communication
time slots overlap a communication time slot of said desired
user.
15. A method according to claim 14, wherein steps (a)-(d) are
carried out by receiving transmissions from said users in said
dispersed cells exclusive of a transmission from said user in said
cell, and further including the step (e) of processing contents of
said transmissions from said users to determine relative offsets
between said users' time slots and said user time slot.
16. A method according to claim 15, wherein step (e) comprises
correlating with synchronization patterns contained in
transmissions from said users to identify times of transitions
between successive ones of said users' time slots relative to a
time of transition of said desired user's time slot, and deriving
said set of weighting coefficients in accordance with said times of
transitions.
17. A method according to claim 16, wherein step (e) further
comprises generating said weighting coefficients, in response to a
transition between successive ones of said users' time slots, and
maintaining said values of said weighting coefficients until a
further transition between successive ones of said users' time
slots.
18. For use with a phased array antenna having a plurality of
antenna elements, signal coupling paths of which are controllably
weighted by amplitude and phase weighting coefficients to control
the beam pattern of said phased array antenna, a method of
processing signals representative of outputs of said antenna
elements, to derive said weighting coefficients, said method
comprising the steps of: (a) generating initial values of said
weighting coefficients; (b) adjusting said signals representative
of outputs of said antenna elements in accordance with said initial
values of said weighting coefficients generated in step (a); (c)
generating initial estimates of information signal contents of
signals representative of outputs of said antenna elements as
adjusted in step (b); (d) subjecting said signals representative of
outputs of said antenna elements and said initial estimates of
information signal contents of signals representative of outputs of
said antenna elements generated in step (c) to a prescribed signal
processing operator to derive improved values of said weighting
coefficients; and (e) adjusting said signals representative of
outputs of said antenna elements in accordance with said improved
values of said weighting coefficients generated in step (d) to
produce improved signal outputs of said antenna elements.
19. A method according to claim 18, further including the steps of:
(f) subjecting said improved signal outputs of said antenna
elements as adjusted in step (e) and said signals representative of
outputs of said antenna elements to said prescribed signal
processing operator to derive further improved values of said
weighting coefficients; (g) adjusting said improved signal outputs
of said antenna elements in accordance with said further improved
values of said weighting coefficients generated in step (f) to
produce further improved signal outputs of said antenna elements;
and (h) iteratively repeating steps (f) and (g) a number of times
N, where N is greater than or equal to zero.
20. A method according to claim 19, wherein step (h) comprises
iteratively repeating steps (g) and (h), as necessary, to bring the
signal-to-noise ratio of said improved signal outputs of said
antenna elements to within a prescribed improvement value.
21. A method according to claim 19, wherein step (h) comprises
iteratively repeating steps (g) and (h) a predetermined number of
times N, where N is greater than or equal to zero.
22. A method according to claim 18, wherein said prescribed signal
processing operator is one which combines estimates of said
information signal contents of signals representative of outputs of
said antenna elements and signals representative of information
signal contents and noise signal contents of outputs of said
antenna elements to produce respective sets of signal and noise
covariances, and generates said improved values of said weighting
coefficients in accordance with said respective sets of signal and
noise covariances.
23. A method according to claim 22, wherein step (e) comprises
generating products of said signals representative of outputs of
said antenna elements and said improved values of said weighting
coefficients to produce improved signal outputs of said antenna
elements.
24. A method according to claim 22, wherein step (e) comprises
generating said improved values of said weighting coefficients in
accordance with products of previous weighting coefficients and
said respective sets of signal and noise covariances.
25. A method according to claim 22, wherein said prescribed signal
processing operator is one which combines estimates of said
information signal contents of signals representative of outputs of
said antenna elements, and signals representative of information
signal contents and noise signal contents of outputs of said
antenna elements, to produce useful signal components and
uncorrelated noise signal components, and is operative to produce
said set of signal covariances in accordance with said information
signal contents of outputs of said antenna elements and said useful
signal components, and to produce said set of noise covariances in
accordance with said noise signal contents of outputs of said
antenna elements and said uncorrelated noise signal components.
26. A method according to claim 18, wherein step (c) comprises
hard-limiting said signals representative of outputs of said
antenna elements as modified in step (b) to generate said estimates
of information signal contents of signals representative of outputs
of said antenna elements.
27. A method according to claim 18, wherein said phased array
antenna is installed at a cell base station of a multi-cell, time
division multiple access (TDMA) cellular communication system, and
has its directivity pattern adaptively modified in steps (a)-(e) so
as to form a beam whose gain and/or nulls are defined so as to
maximize the signal to noise ratio in the presence of co-channel
users whose communication time slots overlap a communication time
slot of said desired user.
28. For use with a time division multiple access (TDMA) cellular
communication system having a plurality of cells, that are
dispersed relative to a cell in which a desired user conducts
communications with a base station in said cell, and wherein said
dispersed cells contain co-channel users which may transmit during
time slots that overlap a desired user's time slot used for
communications between said desired user and said base station, a
base station signal processing arrangement for reducing
interference of communications between said desired user and said
base station, by transmissions from said co-channel users in said
dispersed cells during said time slot, comprising: a phased array
antenna; and a signal processor, coupled to said phased array
antenna and being programmed to process signals received from said
co-channel users, and adaptively controlling values of amplitude
and phase weighting coefficients, through which signal coupling
paths of said antenna elements are controllably weighted to control
the directivity pattern of said phased array antenna in a manner
that maximizes the signal to noise ratio in the presence of
co-channel users whose communication time slots overlap a
communication time slot of said desired user by performing the
steps of: (a) providing values of said weighting coefficients; (b)
modifying said signals representative of outputs of said antenna
elements in accordance with said values of said weighting
coefficients provided in step (a); (c) generating estimates of
information signal contents of signals representative of outputs of
said antenna elements as modified in step (b); and (d) iteratively
adjusting said signals representative of outputs of said antenna
elements by (d1) subjecting said signals representative of outputs
of said antenna elements and estimates of information signal
contents of signals representative of outputs of said antenna
elements to a prescribed signal processing operator to derive
improved values of said weighting coefficients, and (d2) adjusting
said signals representative of outputs of said antenna elements in
accordance with said improved values of said weighting coefficients
to produce improved signal outputs of said antenna elements, and
(d3) iteratively repeating steps (d1) and (d2) a number of times N,
where N is greater than or equal to zero.
29. A base station signal processing arrangement according to claim
28, wherein step (d3) comprises iteratively repeating steps (d1)
and (d2), as necessary, to bring the signal-to-noise ratio of said
improved signal outputs of said antenna elements to within a
prescribed improvement value.
30. A base station signal processing arrangement according to claim
28, wherein step (d3) comprises iteratively repeating steps (d1)
and (d2) a predetermined number of times N, where N is greater than
or equal to zero.
31. A base station signal processing arrangement according to claim
28, wherein said prescribed signal processing operator is one which
combines estimates of said information signal contents of signals
representative of outputs of said antenna elements and signals
representative of information signal contents and noise signal
contents of outputs of said antenna elements to produce respective
sets of signal and noise covariances, and generates said improved
values of said weighting coefficients in accordance with said
respective sets of signal and noise covariances.
32. A base station signal processing arrangement according to claim
31, wherein step (d2) comprises generating products of said signals
representative of outputs of said antenna elements and said
improved values of said weighting coefficients to produce improved
signal outputs of said antenna elements.
33. A base station signal processing arrangement according to claim
32, wherein step (d2) comprises generating said improved values of
said weighting coefficients in accordance with products of previous
weighting coefficients and said respective sets of signal and noise
covariances.
34. A base station signal processing arrangement according to claim
31, wherein said prescribed signal processing operator is one which
combines estimates of said information signal contents of signals
representative of outputs of said antenna elements, and signals
representative of information signal contents and noise signal
contents of outputs of said antenna elements, to produce useful
signal components and uncorrelated noise signal components, and is
operative to produce said set of signal covariances in accordance
with said information signal contents of outputs of said antenna
elements and said useful signal components, and to produce said set
of noise covariances in accordance with said noise signal contents
of outputs of said antenna elements and said uncorrelated noise
signal components.
35. A base station signal processing arrangement according to claim
28, wherein step (a) comprises providing initial values of said
weighting coefficients exclusive of information available in
performing steps (c)-(d).
36. A base station signal processing arrangement according to claim
28, wherein step (c) comprises hard-limiting said signals
representative of outputs of said antenna elements as modified in
step (b) to generate said estimates of information signal contents
of signals representative of outputs of said antenna elements.
37. A base station signal processing arrangement according to claim
28, wherein steps (a)-(d) are performed during an unused time slot,
to derive values of said weighting coefficients that are effective
to form a beam whose gain and/or nulls maximize the signal to noise
ratio in the presence of co-channel users whose communication time
slots overlap a communication time slot of said desired user.
38. A base station signal processing arrangement according to claim
37, wherein steps (a)-(d) are carried out by receiving
transmissions from said users in said dispersed cells exclusive of
a transmission from said user in said cell, and further including
the step (e) of processing contents of said transmissions from said
users to determine relative offsets between said users' time slots
and said user time slot.
39. A base station signal processing arrangement according to claim
38, wherein step (e) comprises correlating with synchronization
patterns contained in transmissions from said users to identify
times of transitions between successive ones of said users' time
slots relative to a time of transition of said desired user's time
slot, and deriving said set of weighting coefficients in accordance
with said times of transitions.
40. A base station signal processing arrangement according to claim
28, wherein said antenna comprises a non-linear array of antenna
elements having a variable spacing between adjacent antenna
elements.
41. A base station signal processing arrangement according to claim
40, wherein said phased array antenna comprises a generally
circular array of antenna elements, and wherein spacings between
successive antenna elements of said generally circular array vary
such that, for any point on a radial line in the plane of said
circular array and passing through an element of said circular
array, the vector distance to any two antenna elements is unequal
and uniformly distributed modulo 2.pi..
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present invention relates to subject matter disclosed in
the following co-pending patent applications, filed coincidentally
herewith: Ser. No. ______ (hereinafter referred to as the ______
application), by K. Halford et al, entitled: "Selective
Modification of Antenna Directivity Pattern to Adaptively Cancel
Co-channel Interference in TDMA Cellular Communication System," and
Ser. No. ______ (hereinafter referred to as the ______
application), by R. Hildebrand et al, entitled "Circular Phased
Array Antenna Having Non-Uniform Angular Separations Between
Successively Adjacent Elements," each of which is assigned to the
assignee of the present application and the disclosures of which
are incorporated herein.
FIELD OF THE INVENTION
[0002] The present invention relates in general to communication
systems, and is particularly directed to a new and improved signal
processing mechanism for rapidly and optimally setting weighting
coefficient values of respective elements of a multi-element
antenna, such as a phased array antenna employed to controllably
form a beam whose gain and/or nulls are defined so as to maximize
the signal to noise ratio. Such improved functionality makes the
invention particularly useful in association with the phased array
antenna of a base station of a time division multiple access (TDMA)
cellular communication system, where it is necessary to cancel
interference from co-channel users located in cells adjacent to the
cell containing a desired user and the base station.
BACKGROUND OF THE INVENTION
[0003] As described in the above-referenced ______ Halford et al
application, in a TDMA cellular communication system, a simplified
illustration of which is diagrammatically shown in FIG. 1,
communications between a base station BS and a desired user 11-1 in
a centroid cell 11 are subject to potential interference by
co-channel transmissions from users in cells dispersed relative to
the cell of interest (cell 11), particularly immediately adjacent
cells shown at 21-71. This potential for co-channel interference is
due to the fact that the same frequency is assigned to multiple
system users, who transmit during respectively different time
slots.
[0004] In the non-limiting simplified example of FIG. 1, where each
cell has a time division reuse allocation of three (a given channel
is subdivided into three user time slots), preventing interference
with communications between user 11-1 and its base station BS from
each co-channel user in the surrounding cells 21-71 would appear to
be an ominous task--ostensibly requiring the placement of eighteen
nulls in the directivity pattern of the antenna employed by the
centroid cell's base station BS.
[0005] In accordance with the invention disclosed in the ______
application, this problem is successfully addressed by determining
the times of occurrence of synchronization patterns of monitored
co-channel transmissions from users in the adjacent cells, and
using this timing information to periodically update a set of
amplitude and phase weights (weighting coefficients) for
controlling the directivity pattern of a phased array antenna.
Namely, the weighting coefficients are updated as participants in
the pool of interferers change (in a time division multiplexed
manner), so as to maintain the desired user effectively free from
co-channel interference sourced from any of the adjacent cells.
[0006] In addition to being applied to the weighting elements, the
updated weighting coefficients are stored in memory until the next
cyclically repeating occurrence of the time slot of the last (in
time) entry in the current pool of co-channel participants. In
response to this next occurrence, the set of weight control values
for the current pool is updated and used to adjust the phased
array's directivity pattern, so that the nulls in the directivity
pattern effectively follow co-channel users of adjacent cells. The
newly updated weight set is then stored until the next
(periodically repeated) update interval for the current co-channel
user pool, and so on.
[0007] Since the maximum number of nulls than can be placed in the
directivity pattern of a phased array antenna is only one less than
the number of elements of the array, the fact that the number of
TDMA co-channel interferers who may be transmitting at any given
instant is a small fraction of the total number of potential
co-channel interferers (e.g., six versus eighteen in the above
example) allows the hardware complexity and cost of the base
station's phased array antenna to be considerably reduced. However,
because the locations of co-channel interferers and therefore the
placement of nulls is dynamic and spatially variable, the antenna
directivity pattern must be controlled very accurately; in
particular, excessive sidelobes that are created by grating effects
customarily inherent in a phased array having a spatially periodic
geometry must be avoided.
[0008] In accordance with the invention described in the
above-referenced ______ Hildebrand et al application, and
diagrammatically illustrated in FIGS. 2 and 3, this unwanted
sidelobe/grating effect is minimized by using a spatially aperiodic
phased array geometry, in which a plurality of N antenna elements
(such as dipole elements) 31, 32, 33, . . . , 3N are unequally
distributed or spaced apart from one another in a two-dimensional,
generally planar array 30, shown as lying along a circle 40 having
a center 41. This unequal distribution is effective to decorrelate
angular and linear separations among elements of the array.
[0009] Each dipole 3i of the circular array is oriented orthogonal
to the plane of the array, so as to produce a directivity pattern
that is generally parallel to the plane of the array. Via control
of amplitude and phase weighting elements coupled in the feed for
each dipole element, the composite directivity pattern of the array
is controllably definable to place a main lobe on a desired user,
and one or more nulls along (N-1) radial lines `r` emanating from
the center 41 of the array toward adjacent cells containing
potential interfering co-channel users. Namely, for any angle of
incidence of a received signal, the vector distance from any point
along that radial direction to any two elements of the array is
unequal and uniformly distributed in phase (modulo 2.pi.).
[0010] What results is a spatially decorrelated antenna element
separation scheme, in which no two pairs of successively adjacent
antenna elements have the same angular or chord separation. Without
spacial correlation among any of the elements of the array,
sidelobes of individual elements, rather than constructively
reinforcing one another into unwanted composite sidelobes of
substantial magnitude, are diminished, thereby allowing nulls of
substantial depth to be placed upon co-channel interferers.
[0011] As further described in the ______ Halford et al
application, non-limiting examples of weighting coefficient
algorithms that may be employed for determining the values of the
weighting coefficients and thereby the directivity pattern of the
base station's phased array antenna include the "Maximum SNR
Method," described in the text "Introduction to Adaptive Arrays,"
by R. Monzingo et al, published 1980, by Wiley and Sons, N.Y., and
the PSF algorithm described in U.S. Pat. No. 4,255,791 (the '791
patent) to P. Martin, entitled: "Signal Processing System," issued
Mar. 10, 1981, assigned to the assignee of the present application
and the disclosure of which is herein incorporated.
SUMMARY OF THE INVENTION
[0012] The present invention is directed to an alternative approach
to the PSF algorithm described in the above-referenced '791 patent,
that is particularly useful in a dynamic environment, such as a TDM
cellular communication system environment, in which the number of
and spatial location of participants may undergo changes, mandating
the need for a weighting coefficient control mechanism that is able
to make rapid real time adjustments with effectively little or no
knowledge of the environment being addressed.
[0013] Pursuant to the invention, this objective is successfully
achieved by an iterative or `bootstrapped`, piecewise-asymptotic
directivity pattern control mechanism, that is operative to
continuously monitor signals as received by a plurality of antenna
elements and to process these signals in accordance with an
iterative weighting coefficient processing mechanism, so as to
produce a set of (amplitude and phase) weighting coefficients
through which the directivity pattern is controlled so as to
maximize the signal to noise ratio. The received signals for the
monitored user channel of interest, as modified by the adaptively
updated weighting coefficients, are then output to a downstream
demodulator.
[0014] In order for the adaptive weighting coefficient control
mechanism of the present invention to `bootstrap` itself, it starts
off with a relatively coarse, but reasonably well defined set of
weighting coefficients, that have a positive signal-to-noise ratio,
such as a bit error rate on the order of one in ten, as a
non-limiting example. The actual signals received by the antenna
elements are modified by this initial set of weights to produce a
first set of estimates of the information signal contents of the
received signals. Using this initial set of signal estimates and
the actual signals received by the antenna elements (and buffered
as necessary for iterative signal processing, as will be
described), the initial set of weighting coefficients are refined
by means of a prescribed signal processing operator.
[0015] The signal processing operator includes a data decision
unit, to which the modified received signal estimates are supplied,
and a signal transform operator, to which both the unmodified or
`raw` data representative of the received signals from the antenna
elements and the output of the data decision unit are applied. If a
priori knowledge of the signal is available, the data decision unit
may comprise a data demodulator or other similar component, that
uses such knowledge to derive an initial data estimate output
signal. Alternatively, the data decision unit may comprise a
relatively simple signal processing component, such as a
hard-limiter or bit-slice unit, that does not require a priori
knowledge of the signal, as long as the received signal has some
degree of coherence.
[0016] Using the signal processing scheme described in the
above-identified '791 patent, the signal transform operator
produces an output containing two components--one containing the
desired information signal component S(t) and a noise component
n(t) of the form Ad(t)cos(.omega.t+.phi.)+n(t), where d(t) is data
and A is amplitude, and the other of which is a transformed noise
signal component .eta.(t) that is uncorrelated with any other
signal, including the noise component n(t). Since the transformed
noise signal component .eta.(t) is uncorrelated with any other
signal, then the correlated energy E is such that E((n(t)*S(t))=0,
E((.eta.(t)*n(t))=0, and E((.eta.(t)*S(t))=0, leaving only
E((S(t)*S(t)) proportional to S.sup.2(t).
[0017] The actually received signal input (S+N) and the output
(S+.eta.) of the signal transform operator are applied to a
correlation--multiplier operator to produce a noise signal
set/matrix (.eta.-N). The individual signal components of the
signal input (S+N) are multiplied by signal components of the
output (S+.eta.), while the components of the noise signal
set/matrix (.eta.-N) are multiplied to produce a desired signal
covariance matrix Rs and a noise covariance matrix Rn. In order to
derive the actual values of the updated weighting coefficients,
these desired signal and noise covariance matrices Rs and Rn are
applied to a coefficient multiplier, which generates the matrix
product of the inverse of the noise covariance matrix Rn.sup.-1,
the useful signal matrix Rs and the previous values of the
weighting coefficients W. This matrix product is a set of refined
or updated set of weighting coefficients Wu, that replace the
previous set of weights, such as an initial set of weights used at
the start of the iterative process. The temporarily buffered
signals are then modified by the updated weights Wu via a matrix
multiplier, to produce an `improved` signal estimate.
[0018] For each subsequent iteration of the weighting coefficient
update sequence, the values of the signal estimates are applied to
the data decision unit in place of the previous estimates. Since
the updated weighting coefficients produce better estimates of the
received signals, the improved signal estimates will result in more
accurate weighting coefficients at the next iteration. Analysis has
shown that the degree of improvement of each iteration follows a
non-linear track, that is asymptotic to some final `ideal` value,
and that the improvement differential between sequential iterations
along this asymptotic variation typically becomes very small after
only a small number of iterations, e.g., only two in the case of a
TDM cellular system. This rapid iterative asymptotic refinement is
significant in real time or quasi real time signal processing
applications, where throughput delay must be minimized. The number
of iterations is preferably determined by simulating the signal
processing application of interest.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 is a simplified diagrammatic illustration of the cell
distribution of a time division multiple access (TDMA) cellular
communication system;
[0020] FIGS. 2 and 3 are respective diagrammatic plan and side
views of an embodiment of the spatially decorrelated antenna array
according to the present invention;
[0021] FIG. 4 diagrammatically shows the overall signal processing
arrangement of a cellular communication system base station of the
type described in the above-referenced ______ Halford et al
application;
[0022] FIG. 5 diagrammatically illustrates the general signal
processing architecture employed by the time slot processing unit
100 of FIG. 4 to implement the iterative weighting coefficient
control operator of the present invention for a respective user
channel of a cellular application;
[0023] FIG. 6 diagrammatically illustrates the functional signal
processing modules carried out by the adaptive weighting
coefficient control mechanism within the control processor of the
weighting coefficient control operator of FIG. 5;
[0024] FIG. 7 is a flow chart of the respective steps associated
with the functional modules of FIG. 6;
[0025] FIG. 8 shows the composition of the signal processing
operator 64 of FIG. 6; and
[0026] FIG. 9 diagrammatically illustrates the iterative processing
scheme of the present invention for an arbitrary number N of
iterations.
DETAILED DESCRIPTION
[0027] Before describing in detail the new and improved iterative
weighting coefficient generation scheme in accordance with the
present invention, it should be observed that the invention resides
primarily in what is effectively a prescribed arrangement of
conventional communication circuits and associated signal
processing components and attendant supervisory control circuitry
therefor, that controls the operations of such circuits and
components. Consequently, the configuration of such circuits and
components, and the manner in which they are interfaced with other
communication system equipment have, for the most part, been
illustrated in the drawings by readily understandable block
diagrams. These diagrams show only those details that are pertinent
to the present invention, so as not to obscure the disclosure with
details which will be readily apparent to those skilled in the art
having the benefit of the present description. Thus, the block
diagram illustrations are primarily intended to show the major
components of the system in a convenient functional grouping,
whereby the present invention may be more readily understood.
[0028] Referring now to FIG. 4 the overall signal processing
arrangement of a cellular communication system base station of the
type described in the above-referenced ______ Halford et al
application is diagrammatically shown as comprising a phased array
antenna 30 having a plurality of antenna elements 31, 32, . . . ,
3N, coupled to respective weighting circuits 41-1, 41-2, . . . ,
41-N. Each respective weighting circuit 41-i is coupled to receive
a set of amplitude and phase weighting coefficients
(W.sub.a,W.sub..PHI.).sub.i, shown as weights W.sub.1, . . . ,
W.sub.N supplied by a weighting coefficient control operator
employed by a time slot processing unit 100. This weighting
coefficient control operator (to be described in detail below with
reference to FIGS. 5-9) adjusts a set of values of the amplitude
and phase weighting coefficients (W.sub.A, W.sub..PHI.) for each
respective weighting circuit 41-i of the antenna array 30, as
necessary, to form a desired beam.
[0029] The outputs of the respective weighting circuits 41-1-41-N
are summed in a summing unit 42, and coupled to an RF-IF
downconverter 44, the output of which is coupled to a first port 51
of a mode switch 50. Mode switch 50 has a second port 52 coupled to
time slot processing unit 100, and a third port 53 coupled to a
transceiver 200. Under the control of the base station's
supervisory processor 300, the mode switch 50 selectively couples
the elements of the antenna array 30 to one of the time slot
processing unit 100 and the base station transceiver 200.
[0030] In timing acquisition mode, the phased array 30 is coupled
to time slot processing unit 100 during one of the time slots
available to users in the cell 11 for traffic signalling, but
currently unassigned to any of those users, so that the timing
relationship between the time slots assigned to users within the
base station's cell and those of the adjacent cells containing
potential co-channel interferers may be determined, as described in
the above-referenced ______ Halford et al application. This timing
relationship information is then used by a weighting coefficient
control operator to dynamically update the antenna's weighting
coefficients. (In traffic signalling mode the array is coupled to
base station transceiver 200).
[0031] As described briefly above, the characteristics of a
cellular system may provide one or more a priori known parameters
(such as aspects of the control channel to be handed off to traffic
channel, the order of switching of the traffic channels' TDM time
slots, which traffic channels are currently unassigned and
therefore may be monitored for noise content, etc.,) that enable
the initial weighting coefficients of the iterative weighting
coefficient operator of the present invention to be set at a
reasonably high degree of accuracy (e.g., on the order of
ninety-percent). As a result, in a cellular system application, the
iterative weighting coefficient operator of the present invention
is able to rapidly converge (e.g., usually within one or two
iterations) to a final set of weighting coefficients, using only
reduced length data segments (which are subject to change at the
time division multiplex switching rate of the time slots of the
users of the cellular system) as inputs.
[0032] The general signal processing architecture employed by the
time slot processing unit 100 which may be used to implement the
iterative weighting coefficient control operator of the present
invention for a respective user channel of such a cellular
application is diagrammatically illustrated in FIG. 5. As shown
therein a respective channel signal received through each antenna
element's weighting circuit 41, after initial downconversion in a
downconverter 49, is digitized in a respective analog-to-digital
converter 54-i and then further digitally downconverted via a
digital downconverter 55-i to fall within the digital signal
processing baseband parameters of an associated digital signal
processor (DSP) 56-i. Each digital signal processor 56 is coupled
via a communications industry standard VME bus 57, having an
associated bus controller 59, to a supervisory control processor
60.
[0033] As will be described, the supervisory control processor 60
is operative to continuously monitor signals as received by each
antenna element of the phased array and to process these signals in
accordance with the iterative weighting coefficient processing
mechanism of the invention, so as to produce a set of weighting
coefficients through which the directivity pattern is controlled so
as to maximize the signal to noise ratio. The received signals for
the monitored user channel of interest, as modified by the stored
weighting coefficients adaptively updated by the weighting
coefficient algorithm executed by the control processor 60, are
then output to a downstream demodulator (not shown).
[0034] FIG. 6 diagrammatically illustrates the functional signal
processing modules carried out by the adaptive weighting
coefficient control mechanism within the control processor 60 of
the weighting coefficient control operator of FIG. 5, while FIG. 7
is a flow chart of the respective steps associated with the
functional modules of FIG. 6.
[0035] As described above, in order for the present invention to
`bootstrap` itself, it starts off with a relatively coarse, but
still, reasonably well defined set of weighting coefficients, shown
as an initial set of weights 61 in FIG. 6, and as step 701 in FIG.
7. By reasonably well defined is meant weighting coefficients that
have a positive signal-to-noise ratio, such as a bit error rate on
the order of one in ten, as a non-limiting example. While such a
coarse performance parameter may be unacceptable for a finally
processed signal, its ninety percent accuracy value will enable the
invention to rapidly converge the antenna's weighting coefficients
to a final set of values.
[0036] Empirical examination has shown that only two iterations are
required for the cellular TDM system application of the present
example. (As pointed out above, the initial values of the weights
may be those associated with the control channel, or derived from a
precursor observation of the background noise for an unassigned
traffic channel of interest, to provide a reasonably `good` first
set of weighting coefficients, upon which the refinement algorithm
of the invention may operate.)
[0037] As shown at 62 in FIG. 6 and step 702 in FIG. 7, the actual
signals 63 received by the antenna elements of the phased array are
then modified by this initial set of weights 61 to produce a first
set of estimates of the information signal contents of the received
signals. Using this initial set of signal estimates 62 and the
actual signals 63 received by the antenna elements, the initial set
of weighting coefficients 61 are then refined at a step 703 by
means of a prescribed signal processing operator 64. As shown by a
set of sub-steps 731-735 embodied within step 703, and as will be
described below with reference to FIG. 8, operator 64 generates
improved values of the weighting coefficients 65 using respective
sets of signal and noise covariances that it has derived by
correlating the estimates and the raw data. At step 704, the
actually received signals are temporarily stored in buffer 66, to
accommodate the processing throughput of the signal processing
operator 64. These buffered data values are then modified via a
matrix multiplier 67 to produce an `improved` signal estimate 68,
at step 705.
[0038] Referring now to FIG. 8, the signal processing operator 64
is shown as including a data decision unit 81, to which the
modified received signal estimates 62 are supplied, and a signal
transform operator 83, to which both the unmodified or `raw` data
representative of the received signals from the antenna elements
and the output of the data decision unit 81 are applied, as shown
at sub-step 731 of step 703. As a non-limiting example, if a priori
knowledge of the signal is available, the data decision unit 81 may
comprise a data demodulator or other similar component, that uses
such knowledge to derive an initial data estimate output signal.
Alternatively, data decision unit 81 may comprise a relatively
simple signal processing component, such as a hard-limiter or
bit-slice unit, that does not require a priori knowledge of the
signal, as long as the received signal has some degree of coherence
(namely, other than a white noise characteristic).
[0039] The signal transform operator 83 may correspond to the
signal recognizer described in the above-referenced '791 patent,
except that the reference signal for the signal transform operator
is derived from the data decision unit 81, rather than being an a
priori known signal. At sub-step 732, the signal transform operator
83 produces an output containing two components--one of which
contains the desired information signal component S(t) in the
received signal (which includes both a desired signal component
S(t) and a noise component n(t) of the form
Ad(t)cos(.omega.t+.phi.)+n(t), where d(t) is data and A is
amplitude), and the other of which is a transformed noise signal
component .eta.(t) that is uncorrelated with any other signal,
including the noise component n(t). Since the transformed noise
signal component .eta.(t) is uncorrelated with any other signal,
then not only is the correlated energy E((n(t)*S(t))=0, but
E((.eta.(t)*n(t))=0, and E((.eta.(t)*S(t))=0, leaving only
E((S(t)*S(t)) proportional to S.sup.2(t).
[0040] At sub-step 733, the raw signal input (S+N) to, and the
output (S+.eta.) of, the signal transform operator 83 are processed
via a correlation--multiplier function to produce a noise signal
set/matrix (.eta.-N). At sub-step 734, the individual signal
components of the raw signal input (S+N) are then multiplied in a
multiplication operator 85 by signal components of the output
(S+n), while the noise differential components of the noise signal
set/matrix (.eta.-N) are multiplied in a multiplication operator
86, to produce a useful or desired signal covariance matrix Rs and
a noise covariance matrix Rn. In order to derive the actual values
of the updated weighting coefficients, these useful signal and
noise covariance matrices Rs and Rn produced by multiplication
operators 84 and 85 are applied at sub-step 735 to a coefficient
multiplier 86, which generates the matrix product of the inverse of
the noise covariance matrix Rn.sup.-1, the useful signal matrix Rs
and the previous values of the weighting coefficients Ws. This
matrix product is a set of refined or updated set of weighting
coefficients Wu, that are to replace the previous set of weights,
such as an initial set of weights used at the start of the
iterative process. Next, at step 705, the temporarily stored or
buffered signals are then modified by the updated weights Wu via
matrix multiplier 67, to produce an `improved` signal estimate
68.
[0041] For each subsequent iteration of the weighting coefficient
update sequence described above, the values of the signal estimates
produced at 68 are applied to the data decision unit 81 at step 703
in place of the previous estimates (which used the initial coarse
weight values during the first iteration, as described). Because
the updated weighting coefficients produce better estimates of the
received signals, the improved signal estimates, in turn, will
result in more accurate weighting coefficients at the next
iteration. Analysis has shown that not only does the degree of
improvement of each iteration follow a non-linear track, that is
asymptotic to some final `ideal` value, but that the improvement
differential between sequential iterations along this asymptotic
variation typically becomes very small after only a small number of
iterations, e.g., only two in the case of a TDM cellular system, as
described above.
[0042] This is of particular significance in real time or quasi
real time signal processing applications, where throughput delay
must be minimized. To this end, the number of iterations is
preferably determined by simulating the signal processing
application of interest, rather than using a signal-to-noise ratio
comparator between iterations. The iterative processing scheme of
the present invention for producing an asymptotically optimized
signal estimate from N iterations is diagrammatically illustrated
in the flow sequence of FIG. 9, wherein the respective steps are
identified by the reference numerals of the flow chart of FIG. 7
described above.
[0043] As will be appreciated from the foregoing description, the
adaptive weighting coefficient control mechanism of the present
invention is able to `bootstrap` itself, starting with a relatively
coarse, but reasonably well defined set of weighting coefficients,
that have a positive signal-to-noise ratio. Received signals are
subjected to this initial set of weights to produce a first set of
signal estimates. These estimates and the received signals are
iteratively processed a prescribed number of times to refine the
weighting coefficients to optimal values, such that the gain and/or
nulls of antenna's directivity pattern will maximize the signal to
noise ratio. Such improved functionality makes the invention
particularly useful in association with the phased array antenna of
a base station of a time division multiple access (TDMA) cellular
communication system, where it is necessary to cancel interference
from co-channel users located in cells adjacent to the cell
containing a desired user and the base station.
[0044] While we have shown and described an embodiment in
accordance with the present invention, it is to be understood that
the same is not limited thereto but is susceptible to numerous
changes and modifications as known to a person skilled in the art,
and we therefore do not wish to be limited to the details shown and
described herein, but intend to cover all such changes and
modifications as are obvious to one of ordinary skill in the
art.
* * * * *