United States Patent (19)
`Raleigh et al.
`
`54 ADAPTIVE BEAM FORMING FOR
`TRANSMITTER OPERATION INA
`WIRELESS COMMUNICATION SYSTEM
`
`75 Inventors: Gregory Gene Raleigh, El Granada;
`Suhas Nagraj Diggavi, Stanford;
`Vincent Knowles Jones, IV, Redwood
`Shores; Arogyaswami Joseph Paulraj,
`Stanford, all of Calif.
`73 Assignees: The Board of Trustees of the Leland
`Stanford Jr. University, Stanford;
`Cisco Technology, Inc., San Jose, both
`of Calif.
`
`*
`
`Notice:
`
`This patent issued on a continued pros
`ecution application filed under 37 CFR
`1.53(d), and is subject to the twenty year
`patent term provisions of 35 U.S.C.
`154(a)(2).
`
`Appl. No.: 08/491,044
`21
`22 Filed:
`Jun. 16, 1995
`Related U.S. Application Data
`63 Continuation-in-part of application No. 08/394,652, Feb. 22,
`1995.
`(51) Int. Cl." ....................................................... H04Q 7/30
`52 U.S. Cl. ............................ 455/561; 455/456; 455/65;
`455/276.1; 342/367; 375/232
`58 Field of Search .................................. 455/33.1, 54.2,
`455/54.1, 56.1, 63, 65, 67.1, 4, 115, 52.3,
`226.1, 276.1, 272, 422, 440, 441, 456,
`457, 517,524,525, 561, 562; 379/58, 59;
`342/350, 367, 368, 372, 373, 378; 395/51;
`375/316, 347, 349, 231, 230, 232
`References Cited
`U.S. PATENT DOCUMENTS
`
`56)
`
`5,132,694 7/1992 Sveenivas ............................... 342/373
`5,260,968 11/1993 Gardner et al. ............................. 375/1
`5,274,844 12/1993 Harrison et al. .......................... 455/25
`5,386,589
`1/1995 Kanai .................................... 455/69 X
`
`USOO61O1399A
`Patent Number:
`11
`(45) Date of Patent:
`
`6,101,399
`*Aug. 8, 2000
`
`5,412,414 5/1995 Ast et al. ............................ 342/372 X
`5,428,712 6/1995 Elad et al. ................................ 395/51
`5,495,256 2/1996 Piper ...........
`342/378 X
`5,515,378 5/1996 Roy, III et al. ......................... 455/525
`5,542,101
`7/1996 Pal ................................... 455/276.1 X
`
`FOREIGN PATENT DOCUMENTS
`0142293 A3 10/1984 European Pat. Off. .......... GO1S 7/28
`0595247 A1 10/1993 European Pat. Off. ......... H01O 3/26
`2266998A 11/1993 United Kingdom ............. H01O 3/26
`WO 94/09568 4/1994 WIPO .............................. HO4B 1/10
`WO9534997 12/1995 WIPO.
`
`OTHER PUBLICATIONS
`
`Per Zetterberg, The Spectrum Efficiency of a Basestation
`Antenna Array System For Spatially
`Selective
`Transmission, Jan. 24, 1994, pp. 1-37.
`Per Zetterberg, Björn Ott4ersten, Experiments using an
`Antenna Array in a Mobile Communications Environment,
`Apr. 21, 1994, 5 pages.
`
`Primary Examiner Fan Tsang
`Assistant Examiner Philip J. Sobutka
`Attorney, Agent, or Firm-Ritter Van Pelt & Yi LLP
`57
`ABSTRACT
`A method for forming an adaptive phased array transmission
`beam pattern at a base Station without any knowledge of
`array geometry or mobile feedback is described. The
`approach is immune to the problems which plague methods
`which attempt to identify received angles of arrival from the
`mobile and map this information to an optimum transmit
`beam pattern. In addition, this approach does not Suffer the
`capacity penalty and mobile handset complexity increase
`associated with mobile feedback. Estimates of the receive
`vector propagation channels are used to estimate transmit
`vector channel covariance matrices which form objectives
`and constraints in quadratic optimization problems leading
`to optimum beam former Solutions for the Single user case,
`and multiple user case. The new invention in capable of
`Substantial frequency re-use capacity improvement in a
`multiple user cellular network.
`
`13 Claims, 11 Drawing Sheets
`
`-5
`
`COPLEX
`DOWNCONVERTER
`O
`
`a(2)
`
`RECEIVE
`E;
`CHANNE
`
`
`
`
`
`
`
`
`
`
`
`COARANCE
`ESTAFOR
`
`
`
`
`
`CHANNE
`WEIGHTING
`
`
`
`
`
`COPLEX
`UPCON/ERTER
`
`-----------------
`
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`U.S. Patent
`
`Aug. 8, 2000
`
`Sheet 1 of 11
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`6,101,399
`
`Base Station
`
`FIG. f.
`
`CALIBRATION
`PROCEDURE
`
`ESTIMATION OF
`RECEIVE CHANNEL
`STATESTICS
`
`COMPUTE TRANSMIT
`CHANNEL WEIGHT
`VECTOR
`
`
`
`TRANSMIT
`CRITERA
`
`FIG. 4
`
`
`
`
`
`
`
`
`
`
`
`
`
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`U.S. Patent
`
`Aug. 8, 2000
`
`Sheet 2 of 11
`
`6,101,399
`
`
`
`
`
`50
`N-y
`
`FROM
`RECEIVE
`AWIENNA
`ARRAY
`
`TO
`TRANSMIT
`ANTENNA
`ARRAY
`
`
`
`
`
`RE F SES
`
`BEAMFORMER
`
`RECEIVER
`
`
`
`60
`VOICE/DATA
`PROCESSING
`
`54
`
`RECEIVE CHANNEL
`ESITMATION
`INFORMATION
`
`
`
`52
`
`64
`
`RF
`IRANSMITTER
`
`
`
`
`
`E;4
`BEAMFORMER
`
`5%
`VOICEMDA7A
`PROCESSING
`
`FIG. 2A
`
`
`
`RF
`RECEIVER
`
`O
`
`
`
`
`
`RECEIVE
`CHANNEL
`BEAMFORMER
`
`60
`
`TO
`VOICE/DATA
`PROCESSING
`
`54
`
`52
`
`
`
`
`
`
`
`RF
`TRANSMITTER
`
`RECEIVE CHANNEL
`ESITMATION
`INFORMATION
`
`
`
`64
`TRANSMIT ;
`CHANNEL
`VOICEMDA7A
`BEAMFORMER
`PROCESSING
`
`
`
`FIG. 2B
`
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`U.S. Patent
`
`Aug. 8, 2000
`
`Sheet 3 of 11
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`6,101,399
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`
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`
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`
`
`
`
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`U.S. Patent
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`Aug. 8, 2000
`
`Sheet 4 of 11
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`6,101,399
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`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`CALIBRATION
`PROCEDURE
`
`
`
`EST MATION OF
`RECEIVE CHANNEL
`STATISTICS
`
`
`
`COMPUTE TRAWSMIT
`CHANNEL WEIGHT
`VECTOR
`
`NETWORK
`AGREEMENT
`CRITERA
`
`
`
`WOT
`SATISF/ED
`
`TRANSM/T
`CRITERA
`
`NEW
`FREQUENCY
`AVAILABLE
`
`SATISFIED
`
`DETERMINE NEW
`CAWDIDATE
`FREQUENCY
`
`
`
`
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`WEW
`FREQUENCY
`UNAVAILABLE
`TRANSMISSION OUALITY
`SATISFIES
`PREDEFINED MINIMUM
`CRITERA
`
`
`
`
`
`
`
`
`
`NOT
`SATISF/ED
`
`TERMINATE CALL
`OR ISSUE
`HANDOFF REQUEST
`TO MTSO
`
`FIG. 5
`
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`U.S. Patent
`
`Aug. 8, 2000
`
`Sheet 5 of 11
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`6,101,399
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`
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`
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`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`260
`RECEIVER
`CALIBRATION
`SIGNAL
`NECTION
`
`
`
`
`
`
`
`
`
`
`
`
`
`264
`RECEIVER
`CALIBRATION
`COEFFICIENT
`COMPUTATION
`
`
`
`RECEIVER
`CALIBRATION
`EQUALIZER &
`CHANNELIZER
`
`TO
`RECEIVE
`CHANNEL
`WEIGHTING
`
`
`
`
`
`
`
`
`
`TRANSMITTER
`CALIBRATION
`SIGNAL
`INJECTION
`
`- - - - - - - - - - - - -
`
`- -
`
`COMPLEX
`UPCONVERTER
`
`254
`TRANSMITTER
`CALIBRATION
`EQUALIZER &
`CHANNEL
`COMBINER
`
`
`
`
`
`FROM
`TRANSMIT
`CHANNEL
`WEIGHTING
`
`
`
`CALIBRATION
`DOWNCONVERTER
`
`COWVERSION &
`DIGITAL
`PROCESSING
`
`TRANSMITTER
`CALIBRATION
`COEFFICIENT
`COMPUTATION
`
`280
`
`284
`
`288
`
`FIG. 6
`
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`U.S. Patent
`
`Aug. 8, 2000
`
`Sheet 6 of 11
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`6,101,399
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`260
`RECEIVER
`CEON
`INJECTION
`NETWORK
`
`
`
`
`
`
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`
`
`
`
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`
`
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`
`264
`RECEIVER
`CALIBRATIOW
`COEFFICIENT
`COMPUTATION
`
`
`
`TO
`RECEIVER
`RECEIVE
`CALIBRATION
`CHANNEL
`EQUALIZER &
`CWEEZERWEIGHTING
`
`274
`
`FIG. 7
`
`CALIBRATION
`SIGNAL
`NJECTION
`
`
`
`
`
`
`
`FROM
`TRANSMIT
`
`It is
`
`FROM
`RECEIVE
`AWTENNA
`ARRAY
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`TO
`IRAWSMIT
`ANTENNA
`ARRAY
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`TRANSMITTER
`calleration
`EQUALIZER &
`CHANNEL
`COMBINER
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`
`
`
`
`
`
`CALIBRATION
`DOWNCONVERTER
`
`CONVERSION &
`DIGITAL
`PROCESSING
`
`TRANSMITTER
`CALIBRATION
`COEFFICIENT
`COMPUTATION
`
`IPR2018-01474
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`U.S. Patent
`
`Aug. 8, 2000
`
`Sheet 7 of 11
`
`6,101,399
`
`
`
`
`
`264
`
`RECEIVER
`CALIBRATION
`COEFFICIENT
`COMPUTATION
`
`
`
`
`
`
`
`
`
`
`
`CALIBRATION
`
`EQUALIZER &
`
`TO
`RECEIVE
`CHANNET
`WEIGHTING
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`CALIBRATIOW
`SWITCH
`
`RECEIVE
`CALIBRATION
`SWITCH
`
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`TRANSMITTER
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`COMPUTATION
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`d
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`TRANSMITTER
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`COMBINER
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`TRANSM/T TER
`CALIBRATION
`SIGNAL
`NJECTION
`NETWORK
`
`
`
`
`
`
`
`274
`
`FIG 3
`
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`U.S. Patent
`
`Aug. 8, 2000
`
`Sheet 8 of 11
`
`6,101,399
`
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`US. Patent
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`Aug. 8, 2000
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`Sheet 9 0f 11
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`6,101,399
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`Aug. 8, 2000
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`Apple Inc. EX1025 Page 11
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`U.S. Patent
`
`Aug. 8, 2000
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`Sheet 11 of 11
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`6,101,399
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`

`1
`ADAPTIVE BEAM FORMING FOR
`TRANSMITTER OPERATION INA
`WIRELESS COMMUNICATION SYSTEM
`
`This is a continuation in part of U.S. patent application
`Ser. No. 08/394,652, filed Feb. 22, 1995.
`BACKGROUND OF THE INVENTION
`I. Field of the Invention
`The present invention relates to the formation of antenna
`beam patterns (beam forming), and more particularly to a
`technique for adaptive transmit beam forming based on the
`result of adaptive receive beam forming.
`II. Description of the Related Art
`Within wireless mobile communication systems, directive
`antennas may be employed at base Station sites as a means
`of increasing the Signal level received by each mobile user
`relative to the level of received signal interference. This is
`effected by increasing the energy radiated to a desired
`recipient mobile user, while Simultaneously reducing the
`interference energy radiated to other remote mobile users.
`Such reduction in the interference energy radiated to mobile
`users over other wireleSS channels may be achieved through
`generation of Spatially Selective, directive transmission
`beam patterns. Unlike "line-of-Sight' radio links, a number
`of Signal transmission paths typically comprise each wire
`leSS communication channel.
`FIG. 1 shows an illustrative representation of a wireless
`“multipath’ communication channel between a base Station
`2 and a remote mobile user 4. The various signal transmis
`Sion paths within each Such multipath communication chan
`nel arise from reflection of the transmitted Signal by domi
`nant reflection Surfaces 6, and by minor reflection Surfaces
`12, between the base station 2 and remote mobile user 4.
`Accordingly, techniques for improving Signal reception in
`line-of-Sight radio Systems are often not directly applicable
`to multipath Signal environments. For example, in line-of
`Sight System the "gain” of an antenna typically varies
`inversely to antenna beam width. However, if a given
`antenna beam width is made less than the angular region
`encompassing the various signal paths comprising a multi
`path communication channel, further reduction in the beam
`width may reduce the energy radiated along Some of the
`angular paths. In turn, this may actually decrease the effec
`tive time average gain of the antenna.
`Within wireless mobile communication systems, three
`techniques have been developed for improving communi
`cation link performance using directive transmit antennas:
`(i) selection of a particular fixed beam from an available set
`of fixed beams, (ii) adaptive beam forming based on receive
`Signal angle estimates, (iii) adaptive transmission based on
`feedback provided by the remote mobile user, and (iv)
`adaptive transmit beam forming based upon the instanta
`neous receive beam pattern. Each of these techniques is
`described briefly below.
`In the first technique, one of Several fixed base Station
`antenna beam patterns is Selected to provide a fixed beam
`Steered in a particular direction. The fixed antenna beams are
`often of equal beam width, and are often uniformly offset in
`boresight angle So as to encompass all desired transmission
`angles. The antenna beam Selected for transmission typically
`corresponds to the beam pattern through which the largest
`Signal is received. The fixed beam approach offers the
`advantage of Simple implementation, but provides no
`mechanism for reducing the Signal interference power radi
`ated to remote mobile users within the transmission beam of
`
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`2
`the base station. This arises because of the inability of the
`traditional fixed beam approach to Sense the interference
`power delivered to undesired users.
`The Second approach involves “adapting the beam pat
`tern produced by a base Station phase array in response to
`changing multipath conditions. In Such beam forming
`antenna arrays, or “beam formers', the antenna beam pattern
`is generated So as to maximize signal energy transmitted to
`(“transmit beamforming”), and received from (“receive
`beam forming”), an intended recipient mobile user.
`While the process of transmit beam forming to a fixed
`location over a line-of-Sight radio channel may be performed
`with relative ease, the task of transmitting to a mobile user
`over a time-varying multipath communication channel is
`typically considerably more difficult. One adaptive transmit
`beam forming approach contemplates determining each
`angle of departure (AOD) at which energy is to be trans
`mitted from the base Station antenna array to a given remote
`mobile user. Each AOD corresponds to one of the Signal
`paths of the multipath channel, and is determined by esti
`mating each angle of arrival (AOA) at the base station of
`Signal energy from the given user. A transmit beam pattern
`is then adaptively formed So as to maximize the radiation
`projected along each desired AOD (i.e., the AOD spectrum),
`while minimizing the radiation projected at all other angles.
`Several well known algorithms (e.g., MUSIC, ESPRIT, and
`WSF) may be used to estimate an AOA spectrum corre
`sponding to a desired AOD spectrum.
`Unfortunately, obtaining accurate estimates of the AOA
`Spectrum for communications channels comprised of numer
`ous multipath constituents has proven problematic. ReSolv
`ing the AOA spectrum for multiple co-channel mobile units
`is further complicated if the average Signal energy received
`at the base Station from any of the mobile units is signifi
`cantly less than the energy received from other mobile units.
`This is due to the fact that the components of the base station
`array response vector contributed by the lower-energy inci
`dent Signals are comparatively Small, thus making it difficult
`to ascertain the AOA spectrum corresponding to those
`mobile units. Moreover, near field obstructions proximate
`base Station antenna arrays tend to corrupt the array cali
`bration process, thereby decreasing the accuracy of the
`estimated AOA spectrum.
`In the third technique mentioned above, feedback infor
`mation is received at the base station from both the desired
`mobile user, and from mobile users to which it is desired to
`minimize transmission power. This feedback permits the
`base station to “learn' the “optimum” transmit beam pattern,
`i.e., the beam pattern which maximizes transmission to the
`desired mobile user and minimizes transmission to all other
`users. One disadvantage of the feedback approach is that the
`mobile radio needs to be significantly more complex than
`would otherwise be required. Moreover, the information
`carrying capacity of each radio channel is reduced as a
`consequence of the bandwidth allocated for transmission of
`antenna training Signals and mobile user feedback informa
`tion. The resultant capacity reduction may be significant
`when the remote mobile users move at a high average
`Velocity, as is the case in most cellular telephone Systems.
`The fourth conventional technique for improving com
`munication link performance involves use of an optimum
`receive beam pattern as the preferred transmission beam
`pattern. After calibrating for differences between the antenna
`array and electronics used in the transmitter and receiver, it
`is assumed that the instantaneous estimate of the nature of
`the receive channel is equivalent to that of the transmit
`
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`3
`channel. Unfortunately, multipath propagation and other
`transient channel phenomenon can Substantially eliminate
`any Significant equivalence between frequency-duplexed
`transmit and receive channels, or between time-division
`duplexed transmit and receive channels Separated by a
`Significant time interval. As a consequence, communication
`link performance fails to be improved.
`
`OBJECTS OF THE INVENTION
`Accordingly, it is an object of the invention to provide an
`adaptive transmit beam forming technique which enhances
`remote user received signal quality by utilizing the uplink
`Signal energy received from remote users without the need
`for feedback from the mobile user.
`It is another object of the invention to provide an adaptive
`transmit beam forming technique which accounts for the
`presence of multipath fading inherent in the communication
`channel.
`It is yet another object of the invention that the beam
`forming technique be independent of antenna array
`geometry, array calibration, or of explicit feedback control
`Signals from remote users.
`It is another object of the invention to provide adaptive
`transmit beam forming which improves signal quality
`received by a desired user and while simultaneously reduc
`ing interference energy received by other undesired users So
`as to, within a cellular communication network, improve
`communication traffic capacity, and/or to increase base
`Station coverage area, and/or to improve call quality.
`
`SUMMARY OF THE INVENTION
`The adaptive transmission approach of the invention
`offers the advantages of adaptive transmission using feed
`back without the associated mobile radio complexity
`increase and information capacity penalty. The technique
`has been developed to exploit Structured variation which
`occurs in the multipath fading present in the wireleSS
`antenna array channel. Thus, multipath propagation effects
`are explicitly accounted for in the problem approach. The
`technique is blind in that the antenna beam is formed in the
`absence of explicit knowledge of the array geometry, and
`without the necessity of array calibration or mobile feed
`back. The basic approach is to estimate the optimum trans
`mit antenna beam pattern based on certain Statistical prop
`erties of the received antenna array Signals. Recently
`developed results in blind Signal copy of multiple
`co-channel Signals using antenna arrays are thus exploited to
`make possible the estimation of the receive signal Statistics.
`The optimum transmit beam pattern is then found by Solving
`a quadratic optimization Subject to quadratic constraints.
`The adaptive transmission System is Suitable for use in
`conjunction with either a diplexed transmit/receive antenna
`array, or with Separate transmit and receive arrayS.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`Additional objects and features of the invention will be
`more readily apparent from the following detailed descrip
`tion and appended claims when taken in conjunction with
`the drawings, in which:
`FIG. 1 is an illustrative representation of a multipath
`propagation channel between a base Station and a remote
`user Station.
`FIG. 2A is a block diagram of the physical implementa
`tion of a beam forming network configured to perform adap
`tive beam forming in accordance with the present invention.
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`FIG. 2B is a block diagram of the physical implementa
`tion of a beam forming network in which a diplexor is
`employed to allow the antenna array to be used for both
`transmit and receive operation.
`FIG. 3 is a functional block diagrammatic representation
`of a beam forming network of the present invention.
`FIG. 4 provides a flowchart Summarizing an exemplary
`procedure for using an optimization function to determine an
`optimum transmit weight vector.
`FIG. 5 is a flowchart outlining a general procedure for
`determining an optimum transmit weight vector by using
`transmit criteria in conjunction with network agreement
`criteria.
`FIG. 6 is a block diagram of an exemplary calibration
`System incorporated within the adaptive beam forming net
`work of FIG. 3.
`FIG. 7 shows an implementation of a calibration system
`within an adaptive beam forming network having different
`transmit and receive arrayS.
`FIG. 8 provides a block diagram of a self-calibrated
`adaptive beam forming network in which the receive Section
`of the beam forming network is advantageously utilized in
`lieu of a transmit Section calibration receiver.
`FIG. 9 is a block diagram of an implementation of a
`calibration equalizer and channelizer in which a single
`wideband calibration equalizer is coupled to each of the
`outputs of a complex downconverter within an RF receiver.
`FIG. 10 depicts a block diagram of an implementation of
`a transmitter calibration equalizer & channel combiner in
`which a Single transmit wideband calibration equalizer pre
`cedes each of the inputs to a complex upconverter.
`FIG. 11 is a block diagram of an implementation of a
`calibration equalizer and channelizer which utilizes a plu
`rality of Sets of narrowband calibration equalizers.
`FIG. 12 depicts a transmitter calibration equalizer &
`channel combiner within which the equalization function is
`performed Separately for each channel.
`DETAILED DESCRIPTION OF THE
`INVENTION
`I. Overview of Beamforming Network
`Turning now to FIG. 2A, a block diagram is shown of the
`physical organization of a beam forming network 50 config
`ured to perform adaptive beam forming in accordance with
`the present invention. In an exemplary embodiment the
`beam forming network 50 is disposed within a base station of
`a cellular communications network, in which is included a
`transceiver comprised of a radio frequency (RF) transmitter
`52 and an RF receiver 54.
`In the embodiment of FIG. 2A, a base station antenna
`array 56 Serves to produce independent transmit and receive
`antenna beams for facilitating communication with one or
`more mobile units (not shown). The term “receive channel
`vector” is employed to indicate that each antenna element
`within the base station antenna array 56 will form a propa
`gation channel to a given remote user. The composite array
`channel may be repressed as a vector having elements
`corresponding to each individual antenna channel. AS is
`described herein, Statistical characterization of the receive
`channel vector provides information which may be used by
`the base Station to determine an “optimal’ transmit beam
`pattern, i.e., a transmit beam pattern which maximizes the
`average Signal-to-interference power delivered to a given
`mobile user. This obviates the need for the mobile unit to
`provide feedback information to the base Station relating to
`propagation characteristics of the transmit channel. This in
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`turn simplifies implementation of the mobile unit, and
`preserves channel resources for data transmission by elimi
`nating the need for “mobile unit feedback' relating channel
`characteristics to the base Station.
`As is indicated by FIG. 2A, a diplexer 58 can be employed
`to allow the antenna array 56 to be used for both transmit and
`receive operation by isolating the RF receiver 54 from the
`RF transmitter 52. A receive channel beam former 60 coop
`erates with the RF receiver 54 to adaptively optimize the
`receive antenna beam in order to improve received signal
`quality. Similarly, a transmit channel beam former 64 coop
`erates with the RF transmitter 52 to adapt the transmit
`antenna beam to optimize Some characteristic of transmis
`Sion quality. In an exemplary embodiment the transit chan
`nel beam former 64 and receive channel beam former 60 are
`each implemented as a Special purpose digital signal pro
`cessor (DSP).
`In another embodiment of the invention, distinct antenna
`arrays are used for Signal reception and transmission as
`illustrated in FIG. 2B. In the embodiment of FIG. 2B, a
`diplexer is not required since a dedicated receive antenna
`array (not shown) is connected to the RF receiver 54, and a
`dedicated transmit antenna array (not shown) is connected to
`the RF transmitter 52. The receive and transmit antenna
`arrays are designed to provide identical radiation character
`istics when operated at the receive and transmit frequencies,
`respectively. Accordingly, in many instances the physical
`geometries of the transmit and receive antenna arrays are
`Simply physically Scaled to account for the fractional dif
`ference in the receive and transmit RF wavelengths. The
`embodiment of FIG. 2B Substantially eliminates the poten
`tial introduction of error arising from use of a single antenna
`array and diplexer.
`Turning now to FIG. 3, a functional block diagrammatic
`representation is provided of a beam forming network of the
`present invention. In FIG. 3, Solid lines are used to represent
`functional elements and dashed lines are employed to iden
`tify the physical components of FIG. 2. The RF receiver 54
`is functionally comprised of a low-noise amplifier (LNA)
`network 110, and a complex downconverter 112. The com
`40
`pleX downconverter 112 is disposed to frequency downcon
`Vert the received RF signal energy after processing by the
`LNA network 110. The downconverted signal energy is then
`digitally Sampled and provided to a receive channel weight
`ing module 116 of the receive channel beam former 60. The
`weights applied by the receive channel beam former 60 to
`each of the M downconverted antenna element outputs
`X(k), m=1 to M, of the complex frequency downcon
`verter 112 are determined by a receiver weight vector
`adaptation module 118. In the exemplary embodiment the
`receiver weight vector adaptation module 118 determines a
`receive channel weight vector, W, which maximizes the
`Signal quality received over the desired inbound frequency
`channel.
`In the embodiment of FIG. 3, a vector channel covariance
`estimator 140 within the transmit beam former 64 operates to
`produce a Statistical characterization of a receive channel
`vector using: (i) the outputs X(k), m=1 to M, of the
`complex frequency downconverter 112, and (ii) an estimate
`of the desired signal S,(k) generated in the receive channel
`beam former 60. For present purposes the receive channel
`vector may be viewed as being representative of the multi
`path communications channel from a mobile user (not
`shown in FIG. 3) to the antenna array 56. In an exemplary
`embodiment the Statistical characterization carried out
`within the covariance estimator 140 yields an estimated
`receive channel covariance matrix used during the transmit
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`beam forming proce SS. Throughout the following
`description, Scalar quantities are represented using lower
`case characters of Standard type, vector quantities are rep
`resented using lower case characters of bold type, and matrix
`quantities are represented using upper case characters of
`bold type.
`Within the transmit channel beam former 64, an optimal
`transmit beam pattern weight vector is generated by a
`transmit channel weight vector adaptation module 150 based
`on the results of the Statistical characterization of the receive
`channel vector. This weight vector, W(t), optimizes a
`particular aspect (e.g., average desired signal to undesired
`interference ratio) of the Signal energy within the transmis
`Sion range of the base Station array 56. In the exemplary
`embodiment, the optimal transmit beam pattern weight
`vector is generated using the estimated desired receive
`channel covariance matrix, R(k), and undesired interfer
`ence covariance matrix, R(k), both of which are compiled
`within the covariance estimator 140.
`As is indicated by FIG. 3, the signal information (S) to
`be transmitted to the desired mobile radio unit is used to
`modulate a digital baseband carrier within a modulator 154.
`The modulated Signal is then applied to a transmit channel
`weighting module 158 disposed to weight, on the basis of
`the optimized transmit pattern weight vector, the input
`Signals corresponding to each element of the antenna array
`56. The weighted set of input signals produced by the
`weighting module 158 are then upconverted in frequency
`within a complex frequency upconverter 160 of the RF
`transmitter 52. The resultant frequency-upconverted Signals
`are then amplified by a bank of power amplifiers 164, and
`provided to the antenna array 56 for transmission via
`diplexer 58.
`In the exemplary embodiment an improved estimate of
`the received signal is obtained through utilization of a
`demodulator 170 and remodulator 172. The received signal
`is demodulated within demodulator 170 in order to recover
`the essential characteristics of the modulating Signal. In the
`case of an analog FM signal, this involves recovery of the
`FM waveform. In the case of a digitally modulated signal
`(BPSK, FSK, QPSK, etc.), the demodulator 170 forms hard
`decisions as to the value of each digital Symbol. The
`demodulated Signal is then processed based upon Some
`predefined characteristic of the Signal and modulation pat
`tern. For example, a demodulated analog FM signal could be
`lowpass filtered based upon a known Signal information
`bandwidth as a means of obtaining an improved post
`demodulation Signal estimate. In the case of digital
`modulation, error correction could be implemented in order
`to remove bit errors, thus improving estimated Signal qual
`ity. In addition, training Signals (i.e., pilot tones, SAT tones,
`etc.) may optionally be employed in lieu of, or in conjunc
`tion with, the aforementioned “blind” techniques.
`Again referring to FIG. 3, the processed demodulated
`signal is then used by demodulator 172 to remodulate an RF
`carrier, thereby producing an improved modulated Signal
`estimate. The improved Signal estimate is then used by the
`receiver weight vector adaptation block 118 and the cova
`riance estimate 140. Other techniques, which do not rely
`upon Such a demodulation/remodulation procedure, can be
`devised for obtaining a Sufficiently accurate received signal
`estimate. FIG. 3 Simply illustrates a particular exemplary
`embodiment incorporating a demodulator and remodulator.
`In the present embodiment, the demodulator 170 and
`remodulator 172 or the receive channel beam former 60 are
`operative to produce a received signal estimate S. The
`quantity S, is then employed by the covariance estimator
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`140 to estimate the covariance matrix of the receive channel.
`The receive channel beam former 60 could of course be
`replaced by numerous alternative Structures including, for
`example, multi-antenna Sequence estimators, maximum
`likelihood receiver Structures, or multiple element decision
`feedback equalizers. Any of these alternative Structures
`could also be used to provide the quantity S, for use in
`estimating the received channel covariance Statistics.
`A detailed description of the adaptive transmit beam form
`ing contemplated by the invention is provided in the fol
`lowing Sections. Specifically, Section II Sets forth a descrip
`tion of transmit and receive multipath antenna channel
`models, as well as an introduction to the approach of
`utilizing receive channel Statistics during transmission chan
`nel beam forming. Section III discusses the extension of the
`Single-user beam forming approach of Section II to an envi
`ronment which includes multiple mobile users.
`In section IV, an exemplary method is described for
`estimating receive channel Statistics useful for effecting the
`transmit channel beam forming of the present invention.
`Finally, Section V provides a description of various tech
`niques for calibrating the transmit and receive channel
`beam forming apparatus.
`II. Blind Beamforming Using Multipath Signals
`AS mentioned above, an initial Step within the present
`invention involves Statistically characterizing a receive
`channel vector representative of the multipath communica
`tions channel from a mobile user to a base Station antenna
`array 56. In order to facilitate understanding of this process,
`a mathematical description will first be