US006031877A
`6,031,877
`(114) Patent Number:
`United States Patent 55
`Saunders
`[45] Date of Patent:
`Feb. 29, 2000
`
`
`[54] APPARATUS AND METHOD FOR ADAPTIVE
`BEAMFORMINGIN AN ANTENNA ARRAY
`Inventor: Simon Saunders, Guildford, United
`Kingdom
`
`[75]
`
`5,548,834
`5,646,958
`5,689,528
`5,796,779
`
`8/1996 Suard et al. wee 455/276.1
`7/1997 Tsujimoto .cssecccssseecsssseceseeees 375/233
`
`11/1997 Tsujimoto ............
`we 375/233
`8/1998 Nussbaum etal...eee 375/267
`
`[73] Assignee: Motorola, Inc., Schaumburg,Ill.
`[21] Appl. No.:
`08/913,747
`[22]
`PCT Filed:
`Dec. 16, 1996
`[86]
`PCT No.:
`PCT/EP96/05649
`§ 371 Date:
`Feb. 5, 1998
`§ 102(e) Date: Feb. 5, 1998
`PCT Pub. No; WO97/27643
`PCT Pub. Date: Jul. 31, 1997
`Foreign Application Priority Data
`[30]
`Jan. 27, 1996
`[GB]
`United Kingdom ........c.0 o601657
`[51]
`Tint. C17 oceeeeeecseecees HO04B 7/02; HO4L 1/02
`(8) Fieldof Search9567,290,
`375/259; 455/562, 226.1, 226.2, 226.3
`
`[87]
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`Primary Examiner—Don N. Vo
`Assistant Examiner—Phuong Phu
`Attorney, Agent, or Firm—Heather L. Creps
`[57]
`ABSTRACT
`
`An apparatus and a method for receiving and transmitting
`information from an array of adaptive antenna elements,
`wherein a predictive filter supplies an estimate of received
`signal samples likely to be received in a burst immediately
`preceding a transmission. Combination of this estimate with
`‘received signal samples obtained from actual (historically
`received) signals, received over a predetermined number of
`frames, yield estimates of optimum beamforming coefii-
`ens for application to data for transmission from an
`adaptive array of antenna elements. As such, available
`processing time for obtaining the beamforming coefficients
`is increased.
`
`5,510,796
`
`4/1996 Applebaum ....csssscssssesseeeeee 342/162
`
`10 Claims, 2 Drawing Sheets
`
`40
`
`LO
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`CORRELATION
`MATRIX
`ESTIMATOR
`
`42
`
`MATRIX
`
`ESTIMATOR
`
`1
`
`APPLE ETAL. 1027
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`APPLE ET AL. 1027
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`1
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`

`

`U.S. Patent
`
`Feb. 29, 2000
`
`Sheet 1 of 2
`
`6,031,877
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`LOLA
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`—LYvddldd——
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`2
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`

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`U.S. Patent
`
`Feb. 29, 2000
`
`Sheet 2 of 2
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`6,031,877
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`42
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`40
`
`Lo
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`46
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`50
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`"> A/D CONVERTER
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`49
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`41
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`44
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`}
`
`48
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`82
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`BUFFER
`
`
` WEIGHT
`CALCULATOR
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`
`
`CORRELATION
`MATRIX
`ESTIMATOR
`
`
`
`72g
`
`EQN.1)
`
`WEIGHT
`CALCULATOR
`
`W
`
`opt
`BEAMFORMER
`
`6O—\
`
`pENODULATOR
`
`74
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`62
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`64
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`OUTPUT
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`Tx DATA
`be ee | ee fe eee a
`
`[acauaror
`
`MODULATOR
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`|
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`INPUT
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`78
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`80
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`FICG.8
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`6,031,877
`
`1
`APPARATUS AND METHOD FOR ADAPTIVE
`BEAMFORMINGIN AN ANTENNA ARRAY
`
`BACKGROUND OF THE INVENTION
`
`This invention relates, in general, to communication sys-
`tems and is particularly applicable to communication sys-
`tems using an adaptive beamforming technique.
`
`SUMMARYOF THE PRIOR ART
`
`The use of adaptive antennas (AA) in communication
`systems(particularly frequency division multiplexed (FDM)
`systems, such as the pan-European digital cellular Global
`System for Mobile (GSM) communication and alternate
`code-division multiple access (CDMA) systems) is becom-
`ing increasingly attractive because such adaptive antennas
`offer general
`improvements in system performance, and
`especially handling(traffic) capacity. As will be appreciated,
`a high degree of beam accuracy is achieved in an adaptive
`antenna system by accurately varying the phase and ampli-
`tude (magnitude) components of a transmitted wave. More
`specifically, phases and magnitudes of a set of transmitted
`waves, emanating from an array of antenna elements of a
`transceiver, are varied by “weighting” individual elements in
`the array such that an antenna radiation pattern (of a base
`site, for example) is adapted (optimised) to match prevailing
`signal and interference environments of a related coverage
`area, such as a cell.
`Adaptive transmit beamforming in duplex communica-
`tion systems requires that beamforming coefficients(i.e. the
`“weighting” factors) are adjusted in response to previously
`received channel information, which received information
`may occurin either an up-link or down-link for the system.
`In fact, when specifically considering a GSM basestation,
`beamforming coefficients for a traffic mode must be calcu-
`lated (estimated) within a period of four time-slot durations
`(namely a time of 4x'56 milliseconds (ms), nominally 2.3
`ms), whereas the period for calculating beamforming coef-
`ficients at a mobile unit may, in fact, be of shorter duration.
`Unfortunately, when one considers the amountof processing
`required to calculate (estimate)
`these beamforming
`coefficients, this limited period of time places severe con-
`straints on an achievable accuracy. Indeed, upon receipt of
`a signal, information contained within the signal (typically)
`must be sampled, stored and then demodulated (by synchro-
`nisation and equalisation processes). Additionally, transmit
`weights must be formed from the received signal and then
`applied to data for transmission prior to loading and modu-
`lation of this data.
`
`Furthermore, the limited time available for processing is
`further eroded by the problems inherently associated with
`such beamforming mechanisms, which problemsprincipally
`result from: (i) the beamformingcoefficients (weights) being
`frequency dependent (bearing in mind that the up-link and
`down-link resources usually operate at different frequencies,
`such that a frequency transposition and a phase-error cor-
`rection is required); and (ii) a time dependent fluctuation in
`channel environment caused by a relative movement
`between a mobile unit and a fixed basestation. In the latter
`respect, the effects of a time variation may be mitigated to
`some extent by averaging several received slots weights, for
`example, but this form of time correction is rather coarse.
`With respect to selection of beamforming coefficients in
`typical communication systems (and as will be understood),
`an optimum selection (corrected, of course, for differences
`between the up-link and down-link frequencies) is provided
`by the Wiener solution:
`
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`-1
`Wopi=Rox yd
`
`(eqn. 1)
`
`where:
`i) X=[X1, Xo, -- - Xg_aye X~n_ay]’ is a received signal vector
`at n branches(i.e. n antenna elements);
`ti) Wo=lWi, Wa -
`-
`- Wont Waal is a vector of
`optimum weights for the n branches;
`ili) r,,=E[x*s] is a correlation of a received signal vector
`with a desired signal vector, s, that is sent during a
`defined training sequence of a burst;
`iv) R,,, is the received signal cross-correlation matrix and
`equals E[x*x"];
`v) R,.”* represents an inverse matrix for the matrix R,,.;
`vi) x* is the complex conjugate of x;
`vii) T is a vector transposition function in which rowsare
`substituted for columns and vice versa; and
`viii) E[.] denotes an expectation value.
`The beamforming coefficients necessarily calculated for a
`succeeding frame of information must be estimated from
`historic received signals because correlation matrices R,,,
`and r,, are not available directly (inasmuch as one cannot
`know whatthese correlation matrices are until such time as
`a signal relating to these matrices has been received). In this
`respect, an estimation R,,. (denoted bythe bar) suitable for
`use in calculating approximate weights for a succeeding
`frame (n+1) is given by the equation:
`n
`Ram+D=5 » x(x)
`k=n-B+1
`
`(eqn. 2)
`
`where B is the number of sample portions (such as bursts)
`that are taken into consideration per estimation (which may,
`in certain circumstances involve more than one burst per
`frame), as expressed in the article “Signal Acquisition and
`Tracking with Adaptive Arrays in the Digital Mobile Radio
`System IS-54 with Flat-Fading” by J. H. Winters, published
`in IEEE Transactions on Vehicular Technology in November
`1993, 42(4), pages 377-384. As such, an estimation of the
`correlation matrices is based on actual received signals.
`As such,it is desirable, generally, to provide a reliable but
`improved mechanism (particularly in terms of increased
`efficiency) by which beamforming coefficients are calcu-
`lated.
`
`SUMMARYOF THE INVENTION
`
`Apparatus for receiving and transmitting information
`from an array of adaptive antenna elements, the apparatus
`comprising storage meansfor storing received information
`and characterised by: a predictive filter for estimating, in
`response to the received information, predicted information
`likely to be received by the apparatus in at least one future
`transmission to the apparatus; and means for combining the
`previously received information and the predicted informa-
`tion to generate beamforming coefficients for weighting
`information to be transmitted subsequently from the array of
`adaptive antenna elements, thereby allowing beamforming
`coefficients to be calculated prior to receipt of information to
`be received by the apparatus in at least one future transmis-
`sion to the apparatus.
`invention there is
`An a second aspect of the present
`provided a methodof receiving and transmitting information
`in an apparatus having an array of adaptive antenna
`elements, the method comprising the step of storing received
`information and characterised by the steps of: estimating, in
`
`4
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`

`

`6,031,877
`
`3
`response to the received information, predicted information
`likely to be received by the apparatus in at least one future
`transmission to the apparatus; and combining the previously
`received information and the predicted information to gen-
`erate beamforming coefficients for weighting information to
`be transmitted subsequently from the array of adaptive
`antenna elements,
`thereby allowing beamforming coeffi-
`cients to be calculated prior to receipt of information to be
`received by the apparatusin at least one future transmission
`to the apparatus.
`Exemplary embodiments of the present invention will
`now be described with reference to the accompanying
`drawings.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is a representation of a prior art duplex commu-
`nication channel.
`
`FIG. 2 illustrates a relative timing advantage obtained
`through the implementation of the present
`invention in
`relation to processing of the duplex communication channel
`of FIG. 1.
`
`FIG. 3 is a functional diagram illustrating a mechanism
`and apparatus (in accordance with a preferred embodiment
`of the present invention) for adaptive beamforming.
`
`DETAILED DESCRIPTION OF A PREFERRED
`EMBODIMENT
`
`Referring to FIG. 1 there is showna representation of a
`prior art duplex communication channel 10, which com-
`prises a plurality of frames 12-18 (in this specific instance
`only four framesare illustrated for the sake of brevity). Each
`frameis dividedinto eightdiscrete time-slots t,-t, (although
`it will be appreciated that the numberof time-slots may vary
`according to the system and that each time slot may be of
`differing duration). As will be understood, the duplex com-
`munication channel 10 maybea traffic channel (TCH)or a
`broadcast control channel
`(BCCH), with a distinction
`between these differing forms of channel being realised by
`the assignmentofat least one dedicated time-slot (usually t,)
`in the BCCHfor system control purposes. If we consider the
`duplex communication channel 10 to be a TCH,then time-
`slot ty would typically be assigned as a down-link, whereas
`time-slot t, would be assigned to a corresponding up-link.
`The remaining time-slots would be assigned/paired in a
`similar fashion. Therefore, in this example, a buffering of
`two time-slot occurs between down-link transmission and
`
`up-link reception in each frame 12-18, and a buffering 20 of
`four time-slots (t,-t,) occurs between up-link reception and
`down-link transmission in contiguous frames, as explained
`above. Clearly, in the case of a mobile unit, the buffering is
`correspondingly reversed.
`According to eqn. 2, a received signal vector, x(k), of a
`frame k can be derived (from a cross-correlation of bits of
`a training sequence, such as a known mid-amble sequencein
`the specific case of GSM) onceperburst transmission, while
`the numberof bursts required per estimation, B, is adjusted
`according to an anticipated rate-of-change of R,,,. However,
`eqn. 2 requires the use of x(n) and is therefore subjectto the
`limited available time between reception and transmission of
`information by a communication device, e.g. the base station
`or the mobile unit.
`
`The preferred embodiment of the present invention uti-
`lises linear predictive filtering to supply an estimate of
`received signal samples, x(n), likely to be received in the
`burst immediately preceding a transmission, and combines
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`4
`this estimate with received signal samples obtained from
`actual (historically received) signals received over an arbi-
`trary (predetermined) numberof bursts or frames,e.g. three
`frames. As will be understood,linear predictive filtering may
`be modelled on the equation:
`n-1
`X(n) = » alx(m)
`m=n-M
`
`(eqn. 3)
`
`where:
`
`i) a,, are the vectors offilter coefficients obtained using
`techniques known in those of ordinary skill in the art
`(see the reference book “Adaptive Filter Theory” by
`Simon Haykin, 2nd Edition, New Jersey, U.S.A;
`Prentice-Hall, 1986. ISBN: 0-13-01326-5 for a method
`of optimising the choice of a,,);
`ii) M is a length of the linear predictivefilter;
`iii) m is an index integer; and
`iv) n is the current frame.
`Therefore, according to a preferred embodiment of the
`present invention, an estimation of the correlation matrix is
`provided by:
`
`Rent l= if,
`
`nol
`» x(a!(ke) 4 (x(n)
`=n-B+1
`
`(eqn.
`
`4
`
`4)
`
`The mechanism of the present invention therefore allows
`beamforming coefficients to be calculated in advance of the
`receipt of a burst (because previously received signals
`influence subsequent beamforming coefficients), such as
`before time-slot t, in the case of the base station of FIG. 1.
`Consequently, additional time-slots are made available for
`processing between reception and transmission of data,
`thereby providing increased buffering 30. This increased
`buffering is shown in FIG. 2 in which a relative timing
`advantage obtained through the implementation of the
`present invention can be seen relative to a corresponding
`processing time for the duplex communication channel of
`FIG. 1. It will be understood that the increased buffering 30
`may be an entire frame or greater, but it is at least the
`additional period provided between the last actual received
`burst and the burst estimated by the linear predictive filter
`(which may occur in the same frame).
`Although predictive filtering in itself requires processing
`within a microprocessor (or the like) of a communication
`device, the additional time provided to the communication
`device allows either the use of more sophisticated decoding
`and beamforming algorithms (the latter of which will
`improve the resolution and accuracy for beamforming
`within the communication system, generally) or the use of a
`slower (and hence less expensive) processor. However, the
`additional processing required in the communication device
`may be optimised by an appropriate limitation of the number
`of bursts, B, used during estimation.
`For the sake of brevity the mechanism for the calculation
`of R,, has been described in detail, although it will be
`understood that an identical mathematical approachis pref-
`erably adopted for the estimation of r,,; albeit that appro-
`priate substitutions are required, namely that x’ or x
`becomes’.
`The basic concept of the present invention may be devel-
`oped further by weighting each term in eqn. 4 by a factor
`appropriate to an anticipated rate-of-change of R,,,, thereby
`making the correlation matrix estimation itself predictive.
`
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`

`6,031,877
`
`5
`This can be expressed mathematically as:
`
`Rent l= i, » clk—n+Bxkx"(k)+c(BOX"(nx?(n)
`
`nl
`=n-B+1
`
`(eqn.
`
`5
`
`9)
`
`. c(B)]’ is estimated
`.
`where a set of values c=[c(1), c(2), .
`in advance to minimise estimation error through empirical
`measurements of point received data over a coverage area
`(as measured between a mobile unit and a fixed base
`station). Therefore, this predictive weighting takes account
`of an actual rate-of-change of the correlation matrix R,... As
`such, the inclusion of the coefficients c provides a relative
`weighting of terms within the series of eqn. 5 to minimise an
`error in estimation for R,.,,.
`Turning nowto FIG. 3, a functional diagram of a mecha-
`nism and apparatus 40 for adaptive beamforming (in accor-
`dance with a preferred embodiment ofthe present invention)
`is illustrated. The apparatus 40 is a communication device,
`such as a base station or a mobile unit (as appropriate), that
`comprises an array of antenna elements 41 for receiving and
`transmitting encoded signals 42. The array of antenna ele-
`ments 41 is coupled to an array of antenna switches 44
`arranged to selectively couple an array of receivers 46 or an
`array of transmitters 48 to the array of antenna elements 41.
`In a receive path,
`information bearing signals (i.e. x)
`received by the array of antenna elements 41 and processed
`by the array of receivers 46 are coupled to a buffer 49
`through an analog-to-digital converter 50. The buffer 49 is
`arrangedto store at least B bursts. Data x stored in the buffer
`49 is input into a correlation matrix estimator 52 that is also
`responsive to a register 54 containing a stored replica of the
`training sequence, s. The correlation matrix estimator 52
`provides values for R,,, and r,, (in accordance with eqn. 2)
`in response to x and s. A weight calculator 56 receives R,,,
`and r,,; to implement eqn. 1 to produce values of w,,,(ie.
`the beamforming coefficients for the receive path) that are
`applied to respective samples from buffer 49 in a beam-
`former 58. An output from the beamformer58 is coupled to
`a demodulator 60 that in turn provides a decoded output
`signal 62 to output device 64, such as a speech decoder or
`a visual display unit (VDU).
`In a transmit path, the data stored in the buffer 49, relating
`to the previous frames, is input into a signal predictor 68
`arranged to calculate x, according to eqn. 3. The data x
`stored in the buffer 49 is also input into a correlation matrix
`estimator 70 (further responsive to x andalsothereplica of
`the training sequence, s, stored in the register 54) which
`implements one of eqn. 4 or eqn. 5 to produce R,,, and £,,.
`A second weight calculator 72 (which may be weight
`calculator 56) receives R,,. and r,, to implement eqn. 1 to
`produce values of w,,, (for the transmit path) that are
`applied, in a beamformer 74 (which may be beamformer
`58), to data 76 from an input device, such as a modem or
`keyboard. An output from the beamformer 74 is coupled to
`an array of modulators 80 that in turn provide encoded
`output signals 82 to the array of transmitters 48 and,
`ultimately, to the array of antenna elements 41 through the
`array of antenna switches 44.
`As will be appreciated, correlation matrix estimators 52
`and 70, weight calculators 56 and 72, beamformers 58 and
`74 and signal predictor 68 are typically implemented within
`a microprocessor 90, while register 54 can be located
`internally (as shown) or externally to the microprocessor 90.
`The information received by the communication device
`during the burst may be data or encoded voice, for example.
`Furthermore, in the specific case of data, several frames may
`
`6
`be buffered at the beginning of a communication so as to
`allow accurate transmit beamforming. However,
`in the
`instance of voice communication, it may be necessary to
`commence the communication with an omni-directional
`pattern of estimated beamforming coefficients and coverage
`to optimise initial weighting factors, and then to introduce
`the mechanism of the present invention to the communica-
`tion at the earliest possible time, i.e. after receipt of at least
`one burst transmission.
`
`Although the present invention has been described in
`relation to the GSM pan-European digital cellular commu-
`nication system,
`it will be appreciated that
`the present
`invention is applicable to any two-way system, including
`those using time division multiplexed (TDM) protocols,
`acoustic waves and duplex systems. Furthermore, imple-
`mentation of the present invention may be at a mobile unit
`or at a base station responsible for control of many mobile
`units.
`It will, of course, be understood that the present invention
`has been given by way of example only and that modifica-
`tions in detail may be made within the scope of the
`invention, e.g. the predictive filtering technique(that is used
`in collaboration with actual received data, which predictive
`filtering technique need notberestricted to linear predictive
`filtering as specifically described in relation to the exem-
`plary embodimentof the present invention) may be extended
`to more than one frame in advance of the immediate burst
`transmission. Therefore, although processing time will be
`increased, accuracy will be corresponding diminished.
`I claim:
`1. Apparatus (40) for receiving and transmitting informa-
`tion (42) from an array (41) of adaptive antenna elements,
`the apparatus comprising storage means (49) for storing
`received information (x) and characterised by:
`a predictive filter (68) for estimating, in response to the
`received information, predicted information (x) likely
`to be received by the apparatus in at least one future
`transmission to the apparatus; and
`means (70) for combining the previously received infor-
`mation (x) and the predicted information (x) to generate
`beamforming coefficients (w,,,,) for weighting infor-
`mation (76) to be transmitted subsequently from the
`array (41) of adaptive antenna elements,thereby allow-
`ing beamforming coefficients to be calculated prior to
`receipt of information to be received by the apparatus
`(40) in at least one future transmission to the apparatus.
`2. Apparatus according to claim 1, wherein the predictive
`filter (68) is a linear predictive filter of the form:
`n-1
`X(n) = » alx(m)
`m=n-M
`
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`where:
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`55
`
`i) x(n) is the predicted information
`ii) a,, are vectorsof filter coefficients;
`ili) x(m) is the received information;
`iv) T is a vector transposition function in which rowsare
`substituted for columns and vice versa;
`v) M is a length of the linear predictive filter;
`vi) m is an index integer; and
`iv) n is a current frame.
`3. Apparatus according to claim 2, wherein the means(70)
`for combining includes a correlation matrix estimator for
`estimating a correlation matrix between the predicted infor-
`
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`6,031,877
`
`7
`mation (x) andthe received information (x), according to the
`form:
`
`nl
`; detox"+R")
`k=n-B+1
`
`where:
`i) X=[Xq, Xo, -- - Xqeay Xia)’ is a received signal vector
`at the array of adaptive antenna elements;
`ii) x* is a complex conjugate of x; and
`vii) B is a number of sample portions taken into consid-
`eration per estimation.
`4. Apparatus according to claim 2, wherein the means (70)
`for combining includes a correlation matrix estimator for
`estimating a correlation matrix between the predicted infor-
`mation(x) and the received information (x), accordingto the
`form:
`
`nol
`1
`B =n-B+1
`
`if, » clk —n+B)x*(k)x?(k)+c(BX"(yx?(n)
`
`where:
`X=[X1, Xo,-- + Xqay Xgl’ is a received signal vector at
`the array of adaptive antenna elements;
`ii) x* is a complex conjugate of x;
`iti) B is a numberof sample portions taken into consid-
`eration per estimation; and
`. c(B)] appropriate to an
`.
`iv) cis a set of constants [c(1) .
`anticipated rate-of-change of the correlation matrix.
`
`8
`5. Apparatus according to claim 1, wherein the receiving
`and transmitting of information is in bursts.
`6. Apparatus according to claim 5, wherein the bursts are
`in a time division multiplexed (TDM) communication sys-
`tem.
`
`7. Apparatus according to claim 5, wherein the received
`information is obtained from a predetermined number of
`bursts.
`
`10
`
`8. Apparatus according to claim 1, wherein the apparatus
`is a base station.
`
`9. Apparatus according to claim 1, wherein the apparatus
`is a mobile unit.
`
`15
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`10. Amethodof receiving and transmitting information in
`an apparatus(40) having an array (41) of adaptive antenna
`elements, comprising the step of storing (49) received infor-
`mation (x) and characterised by the steps of:
`estimating (68), in response to the received information
`(x), predicted information (x) likely to be received by
`the apparatus (40)in at least one future transmission to
`the apparatus; and
`combining (70) the previously received information and
`the predicted information to generate beamforming
`coefficients for weighting information to be transmitted
`subsequently from the array of adaptive antenna
`elements, thereby allowing beamforming coefficients
`to be calculated prior to receipt of information to be
`received by the apparatus in at least one future trans-
`mission to the apparatus.
`*
`*
`*
`
`*
`
`*
`
`7
`
`