Method and apparatus for calibrating a smart antenna array

A smart antenna system includes: an antenna array including a plurality of antenna elements, and at least one calibration element; a plurality of transceiver units each having a port coupled with an associated one of the antenna elements, a receive port, and a transmit port; a transceiver calibration unit including a port coupled with the calibration element via a coaxial cable, a receive port, and a transmit port; and signal processing means communicatively coupled with each of the receive ports and the transmit ports of each of the transceiver units, and coupled with the calibration receive port and the calibration transmit port of the calibration unit. A transmitter calibration path associated with each antenna element extends from the transmit port of the associated transceiver unit to the associated antenna element, from the associated antenna element to the calibration element, and from the calibration element to the receive port of the calibration unit. A receiver calibration path associated with each antenna element extends from the transmit port of the calibration unit to the calibration element, from the calibration element to the associated antenna element, and from the associated antenna element to the receive port of the associated transceiver unit. The signal processing means is responsive to transmit mode resultant signals developed as a result of reference signals propagating through associated transmitter calibration paths. The signal processing means is also responsive to receive mode resultant signals developed as a result of reference signals propagating through associated receiver calibration paths.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates generally to techniques for calibrating an
 adaptive antenna array system, and more particularly to a method and
 apparatus for calibrating a multi-carrier smart antenna array system.
 2. Description of the Prior Art
 Antenna arrays are commonly used in a wide variety of systems that transmit
 and/or receive radio frequency (RF) signals. Examples of such systems
 include wireless communication systems, such as cellular telephone
 systems, and radar systems. An antenna array, which includes a plurality
 of antenna elements, provides improved performance characteristics over a
 single element antenna. The improved characteristics include improved
 signal to noise ratio, improved interference rejection for received
 signals, reduced power requirements for transmitted signals, as well as
 improved directionality.
 For an ideal antenna array, the signal characteristics, including
 attenuation and phase shift, associated with each element of the array are
 identical. An important goal in designing and manufacturing an antenna
 array is to optimize the signal characteristics of the array to be as
 close to ideal as possible. As a result, it is very difficult and
 expensive to manufacture an antenna array system. Antenna array
 calibration provides a means for optimizing the signal characteristics of
 an antenna array using a calibration vector, which is determined based on
 actual signal characteristics of the array, in order to compensate for
 performance variances of the actual signal characteristics of each element
 of the array.
 FIG. 1 shows a schematic block diagram of a prior art beam steering antenna
 array calibration system at 10. The system 10 includes a beam steering
 antenna array transceiver system 12, including: an antenna array 14 having
 a plurality of N antenna elements 16; a plurality of N transceivers 18
 designated TRANSCEIVER.sub.-- 1 TRANSCEIVER.sub.-- 2, . . . ,
 TRANSCEIVER_N, each of the transceivers 18 having a port 20
 communicatively coupled with corresponding one of the antenna elements 16
 via a corresponding coaxial cable 22; and a calibration processing unit 24
 communicatively coupled with each of the transceivers 18 as further
 explained below.
 Each of the transceivers 18 further includes: a duplexer 30 having a port
 32 communicatively coupled with the corresponding one of the antenna
 elements 16 via port 20 of the transceiver and via the corresponding
 coaxial cable 22, a receive port 34, and a transmit port 36; a receive
 processor 38 having an input port 40 communicatively coupled with port 34
 of the duplexer, and an output 42; and a transmit processor 44 having a
 port 46 communicatively coupled with port 36 of the duplexer, and an input
 port 48. The calibration processing unit 24 includes a plurality of
 transceiver ports designated TRANSCEIVER_PORT.sub.-- 1, . . .
 TRANSCEIVER_PORT_N, each of the transceiver ports having an input port 52
 for receiving a signal from port 42 of the receive processor 38 of the
 corresponding one of the transceivers 18, and an output port 54 for
 providing a signal to port 48 of the transmit processor 44 of the
 corresponding one of the transceivers.
 In operation, the beam steering antenna array transceiver system 12 may be
 used in any of a variety of applications including a base station for a
 cellular telephone system. The antenna array 14 receives signals from
 mobile units, and the controlling processor 24 is operative to analyze the
 received signals and determine a position vector associated with the
 corresponding received signal in order to determine the position of the
 mobile unit. The position vector is then used to control a radiation
 pattern generated by the antenna array 14 wherein the beam is controlled
 by varying the phases of signals generated at the output ports 54 of the
 controlling processor 24 in order to focus the beam in the direction of
 the corresponding mobile unit.
 Each of the antenna elements 16 is associated with a corresponding receive
 signal path and a corresponding transmit signal path. The receive path
 associated with each one of the antenna elements 16 extends from the
 corresponding antenna element 16 to the corresponding input port 52 of the
 calibration processing unit 24 traversing the corresponding antenna
 element 16, the corresponding cable 22, the duplexer 30, and the receive
 processor 38 of the corresponding one of the transceivers 18. The transmit
 signal path associated with each one of the antenna elements 16 extends
 from the associated one of the output ports 54 of the calibration
 processing unit 24 to the corresponding antenna element 16 traversing the
 corresponding transmit processor 44, duplexer 30, and coaxial cable 22. In
 an ideal antenna array transceiver system, the signal path characteristics
 associated with each one of the antenna elements 16 are identical to each
 other, and the signal characteristics associated with each one of the
 receiver signal paths are also identical to each other. The signal path
 characteristics include attenuation, or amplitude difference, in a signal
 as a result of propagating through a corresponding path, and the phase
 shift in a signal as a result of propagating through a corresponding path.
 Therefore, each one of the antenna elements 16 has associated sets of
 transmit and receive signal characteristics including the phase shift and
 attenuation associated with the corresponding transmit and receive signal
 paths. Note that each of the antenna elements themselves may have
 different signal characteristics associated therewith as a result of very
 small variances in the dimensions of the antenna elements as well as in
 the material properties of the corresponding antenna elements.
 In practice, an antenna array transceiver system provides less than ideal
 performance because the signal characteristics of the transmit paths and
 receive paths associated with each of the antenna elements vary.
 Therefore, it is necessary to determine the signal characteristics of each
 of the receive signal paths and each of the transmit signal paths so that
 calibration compensation values may be determined for each. The
 calibration compensation values are used to determine a calibration vector
 which is used to compensate for variances in the signal characteristics
 associated with each of the transmit signal paths and receive signal paths
 of the transceiver system. Antenna array calibration provides a means for
 implementing an antenna array as closely system which provides acceptable
 performance.
 In accordance with conventional processes for calibrating a beam steering
 directional antenna array transceiver system, either a far-field
 calibration processor 64 or a transponder 60 may be used to determine a
 calibration vector for each of a plurality of beam directions determined
 by positional relationships between the transponder and the array 14. The
 transponder 60 is responsive to signals transmitted thereto from
 corresponding ones of the antenna elements 16, and is operative to
 transmit a return signal back to the antenna array 14. The return signal
 is received by corresponding ones of the antenna elements 16, and provided
 to the input ports 52 of the calibration processing unit 24 via the
 corresponding ones of the coaxial cables 22 and transceivers 18. While
 either of the external calibration processor 64 or transponder 60 may be
 used to calibrate the system 12, use of the external calibration processor
 64 is complicated because the processor 64 must be controlled either via
 remote control or manually by a technician in the field.
 The object of the calibration process is to determine a compensation vector
 for use in operation of the system 12 in order to adjust the transmit
 signals, and receive signals generated and received at ports 52 and 54 of
 the calibration processing unit 24 in order to compensate for differences
 between the signal characteristics of each of the transmit and receive
 signal paths of each of the transceivers 18 and associated elements 16.
 The calibration process generally includes transmitting and receiving
 signals between each one of the antenna elements 16 of the array 14 and
 the transponder 60. The transponder 60 is positioned at a distance far
 enough away from the antenna array 14 so that the distances between each
 of the antenna elements 16 is negligible in comparing the signals
 transmitted and received between the transponder 60 or processor 64 and
 each corresponding one of the antenna elements 16.
 The calibration process includes a receive path calibration process and a
 transmit path calibration process. In the transmit path calibration
 process, the calibration processing unit 24 is operative to provide a
 first reference signal at port 54 of TRANSCEIVER_PORTS.sub.-- 1 to the
 TRANSCEIVER.sub.-- 1 causing a signal to be radiated from the associated
 one of the antenna elements 16 to the transponder 60. Next, the
 calibration processing unit 24 provides a second reference signal at port
 54 of TRANSCEIVER_PORTS.sub.-- 2 to the TRANSCEIVER.sub.-- 2 causing a
 signal to be radiated from the associated one of the antenna elements 16
 to the transponder 60. The transponder 60, which receives the signals, may
 include logic for determining the signal characteristics associated with
 each signal. Alternatively, the transponder 60 may be coupled via a cable
 (not shown) to the calibration processing unit 24 which receives data and
 determines the signal characteristics associated with each of the signals.
 Based on the signal characteristics associated with each of the signals, a
 transmit mode calibration vector is determined for each one of the antenna
 elements.
 In the receive path calibration process, the calibration processing unit 24
 is responsive to resultant signals received at each of its ports 52, each
 of the resultant signals being developed at the ports 52 of the processor
 24 in response to reference signals generated by the transponder 60 and
 received by corresponding ones of the elements 16, and propagating through
 the corresponding one of the cables 22 and transceivers 18. A receive
 calibration vector is determined by determining amplitude differences and
 phase shifts between the resultant signals and associated reference
 signals.
 Note that it is necessary in the beam steering process to move the location
 of the transponder 60, or external calibration processor 64, in order to
 determine signal characteristics associated with each of the transceivers
 and corresponding elements for a plurality of beam directions associated
 with the antenna array 14. The beam must be focused to the position of the
 transponder.
 Another type of antenna array transceiver array system is a smart antenna
 array transceiver system. Such systems include multi-carrier smart antenna
 array systems. Unlike traditional beam steering directional antenna array
 systems which must be calibrated using a far field calibration processor
 or transponder to determine a calibration director vector for each of the
 plurality of directions, a smart antenna array system may be calibrated in
 a different manner. A smart antenna array system is operative to
 adaptively change the beam direction according to the mobile target
 direction. A calibration vector provides compensation for variances of the
 transmit and receive signal paths.
 FIG. 2A shows a schematic circuit block diagram of an internal loop
 calibration system at 80 for calibrating a smart antenna array transceiver
 system 82. The system 82 includes: an antenna array 14 having a plurality
 of antenna array elements 16; a plurality of N internal loop calibration
 transceivers 84 designated TRANSCEIVER.sub.-- 1, TRANSCEIVER.sub.-- 1, . .
 . TRANSCEIVER_N, each of the transceivers 84 including a port 86
 communicatively coupled with a corresponding one of the elements 16 via a
 corresponding one of a plurality of coaxial cables 88, a reference signal
 port 90 communicatively coupled with a reference signal terminal 92, a
 receive signal port 94, and a transmit signal port 96; and a calibration
 processing unit 100 having a plurality of N sets of transceiver ports each
 having a corresponding input port 102 communicatively coupled with port 94
 of a corresponding one of the transceivers 84, and an output port 104
 communicatively coupled with port 96 of the corresponding one of the
 transceivers 84. A reference signal generator 110, having an output 112,
 is used to provide a reference signal to each of the terminals 92 in
 accordance with a prior art "in-loop" calibration process further
 described below.
 FIG. 2B shows a schematic circuit block diagram illustrating further
 details of one of the internal loop calibration transceivers 84 of FIG.
 2A. Each of the transceivers 84 further includes: a first RF signal
 coupler 122 having a first port 124 communicatively coupled with the
 corresponding one of the antenna elements 16 via port 86 and via the
 corresponding coaxial cable 88, a coupling port 126 for receiving the
 reference signal, or calibration signal, from the reference signal
 generator 110 (FIG. 2A) via the terminal 92, and a second port 128, a
 second RF signal coupler 130 having a first port 132 communicatively
 coupled with port 128 of the first RF signal coupler 122, a coupling port
 134, and a second port 136; a duplexer 138 having a port 140
 communicatively coupled with port 136 of the second RF signal coupler 130,
 a transmit port 142, and a receive port 144; a transit processor 146
 having an input port 148 communicatively coupled with the corresponding
 one of the ports 104 of the calibration processing unit 100 via port 96 of
 the transceiver, and an output port 150 communicatively coupled with the
 transmit port 142 of the duplexer; an attenuator 152 having an input port
 154 communicatively coupled with port 134 of the second RF signal coupler
 130, and an output port 156; a switch 160 having a port 162
 communicatively coupled with the receive port 144 of the duplexer 138, a
 port 164 communicatively coupled with port 156 of the attenuator 152, and
 a port 166; and a receive processor 170 having an input port 172
 communicatively coupled with port 166 of the switch 160, and an output
 port 174 communicatively coupled with the corresponding one of the receive
 signal ports 102 of the calibration processing unit 100 via port 94 of the
 transceiver 84.
 The switch 160 may be set to connect its port 164 to its port 166, or may
 be set to connect its port 162 to its port 166 for the purpose of
 determining transmit calibration vectors and receive calibration vectors
 as further explained below. The attenuator 152 is also used in the
 calibration process along with the first and second RF signal couplers 122
 and 130 and the reference signal generator 110 (FIG. 2A) which provides
 the reference signal to terminal 92. Typically, a technician in the field
 must connect the reference signal generator 110 (FIG. 2A) to each of the
 reference signal terminals 92 of the transceivers 84 in succession during
 the prior art calibration process which is a laborious task.
 In a receiver calibration mode, switch 160 is set to couple the receive
 port 144 of the duplexer 138 to the input port 172 of the receive
 processor 170 by connecting ports 162 and 166 of the switch Also in the
 receive calibration mode, the corresponding one of the coaxial cables 88
 is disconnected from the corresponding antenna element 16, and the cable
 is terminated in order to isolate the corresponding antenna element from
 the transceiver. Further, in the receive calibration mode, the signal
 generator 110 (FIG. 2A) is connected to the corresponding terminal 92 and
 activated to provide a reference signal to the coupling port 126 of the
 first RF signal coupler 122. The object of the prior art receive
 calibration process is to determine signal characteristics associated with
 a tested receive signal path 180 extending from the coupling port 126 of
 the first RF signal coupler 122 to the input port 102 of the processing
 unit 100 via ports 126 and 128 of the first RF signal coupler 122, ports
 132 and 136 of the second RF signal coupler 130, ports 140 and 144 of the
 duplexer 138, ports 162 and 166 of the switch 160, and the receive
 processor 170.
 By applying the reference signal to the terminal 92 while the switch 160 is
 set in the receive mode and while the cable 88 is terminated as described
 above, a receive calibration mode resultant signal is developed at port
 174 of the receive processor 170 as a result of the reference signal
 propagating through the tested receive signal path 180. The calibration
 processing unit 100 is responsive to the receive mode calibration
 resultant signal received at its port 102 from port 174 of the receive
 processor 170, and operative to determine signal characteristics
 associated with the tested receive signal path 180 based on an amplitude
 difference and phase shift between the reference signal applied to
 terminal 92 and the receive mode calibration resultant signal. In
 accordance with this prior art method, it is assumed that the signal
 characteristics of the tested receive signal path 180 adequately represent
 the signal characteristics of an actual receive path which extends from
 the associated antenna element 16 to the signal path characteristics of
 the associated input port 102 of the processing unit 100 via the
 associated one of the cables 88, ports 124 and 128 of the first RF signal
 coupler 122, ports 132 and 136 of the second RF signal coupler 130, ports
 140 and 144 of the duplexer 138, ports 162 and 166 of the switch 160, and
 the receive processor 170. An important problem associated with the prior
 art internal loop calibration process is that the signal characteristics
 associated with the tested receive signal path 180 do not include the
 signal characteristics associated with the antenna element 16, and the
 associated one of the coaxial cables 88 because these elements are
 bypassed by the injection of the reference signal at terminal 92 which is
 injected at the coupling port 126 of the first RF signal coupler 122.
 Therefore, the described prior art calibration process does not account
 for differences in the signal characteristics associated with each of the
 antenna elements 16, each of the coaxial cables 88, and the path between
 ports 124 and 128 of each of the couplers 122.
 Another problem associated with the prior art internal loop calibration
 process is that the switch 160, attenuator 152, and RF signal couplers 122
 and 130 introduce a significant amount of attenuation in the receive
 signal path of the transceiver 84 which reduces the sensitivity of the
 antenna system. Yet another problem associated with the prior art internal
 loop system is that it is assumed that the attenuator 152 has a precisely
 known attenuation value, while in practice the attenuation value of the
 attenuator 152 may vary.
 In a transmit calibration mode, the switch 160 is set to couple port 156 of
 the attenuator 152 to port 172 of the receive processor 170 by connecting
 ports 164 and 166 of the switch The prior art transmit mode calibration
 process requires measuring signal characteristics of two signals paths in
 accordance with a two step process as further explained below.
 In accordance with a first step of the prior art internal loop transmit
 mode calibration process, the calibration processing unit 100 generates
 reference signals at each of its ports 104, each of the reference signals
 having a known phase and amplitude. The reference signal generated at each
 of the ports 104 propagates through a loop signal path 182 traversing the
 transmit processor 146, ports 142 and 140 of the duplexer 138, ports 136
 and 134 of the second RF signal coupler 130, the attenuator 152, ports 164
 and 166 of the switch 160, and the receive processor 170. The calibration
 processing unit 100 is responsive to a first transmit mode resultant
 signal received at its port 102, the first transfer mode resultant signal
 being developed at the output port 174 of the receive processor as a
 result of the reference signal, generated at the corresponding port 104,
 propagating through the loop signal path 182. The calibration processing
 unit 100 is operative to compare the resultant signal received at port 102
 to the associated reference signal generated at the corresponding one of
 the ports 104 which has a known phase and amplitude. The calibration
 processing unit 100 is further operative to determine the signal
 characteristics associated with the signal path 182. The signal
 characteristics associated with the signal path 182 of each of the
 transceivers 84 (FIG. 2A) are used to determine a vector X which is used
 to determine a transmit mode calibration vector as further explained
 below.
 In accordance with a second step of the prior art internal loop transmit
 mode calibration process, the signal characteristics associated with a
 residual signal path 184 must be measured. The residual signal path 184
 extends from port 126 of the first RF signal coupler 122 to port 102 of
 the calibration processing unit 100 and transfers ports 126 and 128 of the
 first RF signal coupler, ports 132 and 134 of the second RF signal coupler
 130, the attenuator 152, ports 164 and 166 of the switch 160, and the
 receive processor 170. The switch 160 is set to communicatively couple
 port 156 of the attenuator 152 with port 172 of the receive processor 170
 by connecting ports 164 and 166 of the switch. A second reference signal,
 having a known phase and amplitude, is then applied to the reference
 signal terminal 92 using the reference signal generator 110 (FIG. 2A). The
 calibration processing unit 100 is responsive to a second resultant signal
 received at its port 102, and operative to determine the signal
 characteristics associated with the signal path 184 by determining a phase
 shift and amplitude difference between the reference signal provided to
 the reference signal terminal 92 and the second resultant signal which is
 developed as a result of the reference signal propagating through the
 signal path 184. The signal characteristics associated with each path 184
 of the transceivers 84 (FIG. 2A) are used to determine a vector Y.
 A transmit calibration vector associated with a tested transmit signal path
 may be determined in accordance with relationship (1), below.
EQU Transmit calibration vector=X.cndot./Y (1)
 Wherein the vector Y represents the signal characteristics associated with
 each of the residual paths 184 of the transceivers 84 (FIG. 2A), and
 wherein the vector X represents the signal characteristics associated with
 each of the loop signal paths 182 of the transceivers 84 (FIG. 2A).
 Relationship (1), above, yields a transmit calibration vector that is
 determined by considering the signal characteristics associated with a
 tested transmit signal path which extends from port 126 of the first RF
 signal coupler 122 to port 174 of the receive processor 170 via ports 126
 and 128 of the first RF signal coupler, ports 132 and 136 of the second RF
 signal coupler 130, ports 140 and 144 of the duplexer 138, ports 162 and
 166 of the switch 160, and the receive processor 170.
 Another important problem associated with the prior art internal loop
 calibration process is that the signal characteristics associated with the
 tested transmit signal path do not include the signal characteristics
 associated with the antenna element 16, and the associated one of the
 coaxial cables 88 because these elements are bypassed by the injection of
 the reference signal at terminal 92 which is injected at the coupling port
 126 of the first RF signal coupler 122. The signal characteristics
 associated with each of the elements 16 are significant because the
 radiation from each of the elements 16 is different as each of the
 elements 16 has different signal characteristics including slightly
 different dimensions and slightly different material properties.
 SUMMARY OF THE INVENTION
 It is an object of the present invention to provide an apparatus and method
 for calibrating a smart antenna array system where it is unnecessary to
 introduce calibration components, such as switches, couplers, and
 attenuators, into the transmit and receive signal paths which provide
 coupling between the antenna elements of the array and signal processing
 units of the system.
 Another object of the present invention is to provide an apparatus and
 method for calibrating a smart antenna system wherein the sensitivity of
 the system is not reduced by the introduction of calibration components.
 Another object of the present invention is to provide an apparatus and
 method for calibrating a smart antenna system wherein the signal
 characteristics associated with each of the antenna elements, and the
 signal characteristics associated with the cables connected to the antenna
 elements, are taken into account in the calibration process.
 A further object of the present invention is to provide improved accuracy
 in calibrating a smart antenna system, so that the efficiency of the smart
 antenna beam forming will be improved.
 Yet another object of the invention is to provide an apparatus enabling a
 simplified process for calibrating a smart antenna system.
 Briefly, a presently preferred embodiment of the present invention includes
 a smart antenna system including an antenna array including a plurality of
 antenna elements, and at least one antenna calibration element for
 radiating and receiving radiated signals to and from each of the antenna
 elements. The antenna elements are disposed in a generally circular array,
 and the calibration element is disposed proximate a center point of the
 array.
 The smart antenna system further includes: a plurality of transceiver units
 each having an input/output port communicatively coupled with an
 associated one of the antenna elements via am associated antenna coupling
 means, a receive port, and a transmit port; a transceiver calibration unit
 including a calibration input/output port communicatively coupled with the
 calibration element via a coaxial cable, a calibration receive port, and a
 calibration transmit port; and signal processing means communicatively
 coupled with each of the receive ports and the transmit ports of each of
 the transceiver units, and communicatively coupled with the calibration
 receive port and the calibration transmit port of the calibration unit.
 A transmitter calibration path associated with each one of the antenna
 elements extends from the transmit port of the associated transceiver unit
 to the associated antenna element, from the associated antenna element to
 the calibration element, and from the calibration element to the
 calibration receive port of the calibration unit. A receiver calibration
 path associated with each one of the antenna elements extends from the
 calibration transmit port of the calibration unit to the calibration
 element, from the calibration element to the associated antenna element,
 and from the associated antenna element to the receive port of the
 associated transceiver unit.
 The signal processing means is operative in a transmit calibration mode to
 provide a transmit mode reference signal to the transmit port of each of
 the transceiver units, and is responsive to transmit mode resultant
 signals developed as a result of associated ones of the transmit reference
 signals propagating through associated ones of the transmitter calibration
 paths. The signal processing means is also operative to determine a
 transmit mode calibration vector by determining amplitude differences and
 phase shifts between the transmit mode reference signals and the
 associated transmit mode resultant signals.
 The signal processing means is operative in a receive calibration mode to
 provide receive mode reference signals to the calibration transmit port of
 the calibration unit, and is responsive to receive mode resultant signals
 developed as a result of associated ones of the receive mode reference
 signals propagating through associated ones of the receiver calibration
 paths. The signal processing means is further operative to determine a
 receive mode calibration vector by determining amplitude differences and
 phase shifts between the receive mode reference signals and the associated
 receive mode resultant signals.
 An important advantage of the smart antenna system of the present invention
 is that it provides improved calibration accuracy thereby improving the
 beam forming efficiency of the system.
 Another advantage of the smart antenna system of the present invention is
 that the signal characteristics associated with each of the antenna
 elements, and with the coaxial cables connecting the antenna elements to
 the transceivers, are calibrated.
 A further advantage of the present invention is that the sensitivity of the
 smart antenna system is not reduced by the introduction of in-loop
 calibration components which cause attenuation of signals.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 FIG. 3 shows a schematic circuit block diagram of a smart antenna array
 transceiver system at 200 in accordance with the present invention, the
 system 200 having in-system external loop calibration features. The system
 200 includes: an antenna array 202 having a plurality of N antenna
 elements 204 disposed in a generally circular array, and an antenna
 calibration element 206 disposed proximate a center point of the array of
 elements; a plurality of N transceiver units 210 designated
 TRANSCEIVER.sub.-- 1, TRANSCEIVER.sub.-- 2, . . . TRANSCEIVER_N, each of
 the transceiver units 210 having an input/output port 212 connected with
 an associated one of the antenna elements 204 via an associated one of a
 plurality of coaxial cables 214, a receive port 216, and a transmit port
 218; a control processing unit 220 having a plurality of sets of
 transceiver ports each being communicatively coupled with an associated
 one of the transceiver units 210, each of the transceiver ports having an
 input port 222 communicatively coupled with the receive port 216 of the
 associated transceiver, and a port 224 communicatively coupled with the
 transmit port 218 of the associated transceiver, the processing unit 220
 also having a calibration control signal port 226 further explained below;
 and an in-system multi-carrier external loop calibration unit 230 having
 an input/output port 232 communicatively coupled with the antenna
 calibration element 206 via a coaxial cable 234, and a calibration control
 signal port 236 communicatively coupled with port 226 of the control
 processing unit 220. In an alternative embodiment of the present
 invention, the antenna elements 204 and the calibration element 206 may be
 coupled to the associated transceiver units 210 and to the calibration
 unit 230 respectively via wave-guide or any other high frequency
 transmission medium.
 The calibration unit 230 includes: a transceiver calibration unit 238
 having an input/output port 240 communicatively coupled with the antenna
 calibration element 206 via port 232 and via the cable 234, a receive port
 242, and a transmit port 244; and an in-system calibration processing unit
 246 having an input port 248 communicatively coupled with the receive port
 242 of the transceiver calibration unit, an output port 250
 communicatively coupled with the transmit port 244 of the transceiver
 calibration unit, and a calibration control signal port 252
 communicatively coupled with port 226 of the control processing unit 220
 via port 236 of the calibration unit.
 The control processing unit 220 and calibration processing unit 246 provide
 an in-system calibration processing sub-system 219. In one embodiment of
 the present invention, the control processing unit 220 includes a digital
 signal processor 254 and a memory storage unit 255 for storing computer
 executable instructions for execution by the digital signal processor 254
 for implementing an in-system multi-carrier external loop calibration
 process in accordance with the present invention. The calibration
 processing unit 246 also includes a digital signal processor 256 and a
 memory storage unit 257 for storing computer executable instructions for
 execution by the digital signal processor 256 for implementing further
 steps of the in-system multi-carrier external loop calibration process as
 further explained below. In an alternative embodiment of the present
 invention, the calibration processing sub-system 219 is formed as an
 integral unit having a single memory unit and a single digital signal
 processor for storing and executing all required instructions of the
 external loop calibration process of the present invention. Also in an
 embodiment of the present invention, the transceiver calibration unit 238
 and calibration processing unit 246 are enclosed in a shielded enclosure
 for the purpose of isolating the calibration components from potential
 radiation interference from the transceivers 210.
 In accordance with the in-system multi-carrier external loop calibration
 process of the present invention, the control processing unit 220 and
 calibration processing unit 246 are operative to generate reference
 signals, and receive resultant signals via the transceiver units 210 and
 calibration transceiver unit 238 and via corresponding ones of the antenna
 elements 204 and the calibration element 206 as further explained below.
 A transmit mode calibration path associated with each one of the antenna
 elements 204 extends from the transmit port 218 of the associated one of
 the transceiver units 210 to the receive port 242 of the transceiver
 calibration unit 238. The transmit mode calibration path includes: a
 transmit path segment of the associated one of the transceiver units 210
 which couples the input/output port 212 of the associated transceiver to
 the transmit port 218 of the associated transceiver; the associated cable
 214; the associated antenna element 204; the calibration element 206; the
 cable 234; and a receive path segment of the calibration transceiver unit
 238 which couples the input/output port 240 of the calibration transceiver
 to the receive port 242 of the calibration transceiver. Stated
 alternatively, the transmit mode calibration path associated with each of
 the antenna elements extends from the transmit port 218 of the associated
 transceiver unit to the associated antenna element 204, from the
 associated antenna element 204 to the calibration element 206, and from
 the calibration element 206 to the receive port 242 or the calibration
 unit 238.
 A receive mode calibration path associated with each one of the antenna
 elements 204 extends from the transmit port 244 of the transceiver
 calibration unit 238 to the receive port 216 of the associated one of the
 transceiver units 210. The receive mode calibration path traverses: a
 transmit path segment of the calibration transceiver unit 238 which
 couples the input/output port 240 of the calibration transceiver to the
 transmit port 244 of the calibration transceiver; the cable 234; the
 calibration element 206; the associated antenna element 204; the
 associated one of the cables 214; and a receive path segment of the
 associated one of the transceiver units 210 which couples the input/output
 port 212 of the associated transceiver to the receive port 216 of the
 associated transceiver. Stated alternatively, the receive mode calibration
 path associated with each one of the antenna elements 204 extends from the
 transmit port 244 of the calibration unit 238 to the calibration element
 206, from the calibration element 206 to the associated antenna element
 204, and from the associated antenna element 204 to the receive port 216
 of the associated transceiver unit
 In a receive calibration mode, the calibration processing unit 246
 generates a receive mode reference signal at its port 250. The transceiver
 calibration unit 238 is responsive to the receive mode calibration
 reference signal received at its port 244 and operative to generate a
 signal at its port 240, the signal then being propagated via the coaxial
 cable 234 and radiated from the calibration antenna element 206. Each of
 the antenna elements 204 is responsive to the radiated signal radiated
 from the calibration antenna element 206. The control processing unit 220
 is responsive to calibration control signals received at its input 226
 from port 252 of the calibration processing unit 246, the calibration
 control signals indicating the magnitude and phase of the receive mode
 reference signals generated at port 250 of the calibration processing unit
 246. The control processing unit 220 is also responsive to receive mode
 calibration resultant signals received at corresponding ones of the input
 ports 222 associated with corresponding ones of the antenna elements 204.
 The receive mode calibration resultant signals are developed at ports 216
 of associated ones of the transceiver units 210 as a result of the receive
 mode reference signals propagating through the associated receive mode
 calibration paths. The control processing unit 220 is operative to analyze
 the phase and amplitude of the receive mode resultant signals, and
 operative to compare the receive mode resultant signals to the associated
 receive mode reference signals generated at output port 250 of the
 calibration processing unit 246. The control processing unit 220 is
 operative to determine phase shifts and amplitude differences between the
 receive mode reference signals and the associated receive mode resultant
 signals to yield a receive mode calibration vector as further explained
 below.
 In a transmit calibration mode, the control processing unit 220 is
 operative to generate a transmit mode reference signal at each of its
 output ports 224. Each of the transceiver units 210 is responsive to the
 receive mode reference signal provided to its transmit port 218, and
 operative to generate a signal at its port 212 in order to cause a
 radiated transmit mode calibration reference signal to be radiated from
 the associated one of the antenna elements 204. The calibration element
 206 is responsive to the radiated transmit mode calibration reference
 signals, and the transceiver calibration unit 238 is responsive to signals
 developed by the calibration antenna element 206 in response to the
 radiated transmit mode reference signals. The calibration transceiver unit
 238 is operative to provide a transmit mode calibration resultant signal
 at its receive port 242. The trait mode resultant signals are developed at
 the receive port 242 of the transmit calibration unit 238 as a result of
 the associated transmit mode reference signals propagating through the
 associated transmit mode calibration paths. The calibration processing
 unit 246 is responsive to the transmit mode resultant signals associated
 with each one of the antenna elements 204, and operative to analyze the
 transmit mode resultant signals. The in-system calibration processing unit
 246 is operative to compare the transmit mode resultant signals to the
 transmit mode reference signals, and is operative to determine a phase
 shift and amplitude difference between corresponding ones of the transmit
 mode reference signals and the transmit mode resultant signals to yield a
 transmit mode calibration vector V.sub.tc as further explained below.
 FIG. 4 shows a schematic circuit diagram illustrating further details of
 one of the transceiver units 210, and the transceiver calibration unit 238
 of the system 200 (FIG. 3) of the present invention. Each of the
 transceiver units 210 includes: a duplexer 264 having a port 266
 communicatively coupled with the associated one of the antenna elements
 204 via port 212 of the transceiver unit and via the associated one of the
 coaxial cables 214, an output port 268, and an input port 270; a transmit
 processor 272 having an input port 274 communicatively coupled with port
 224 of the calibration processing unit 220 via the transmit port 218 of
 the transceiver unit, and an output port 276 communicatively coupled with
 port 270 of the duplexer; and a receive processor 278 having an input port
 280 communicatively coupled with the output port 268 of the duplexer, and
 an output port 282 communicatively coupled with port 222 of the
 calibration processing unit 220 via port 216 of the transceiver unit.
 The transceiver calibration unit 238 is very similar to the transceiver
 units 210 and includes: a duplexer 290 having a port 292 communicatively
 coupled with the calibration antenna element 206 via port 240 of the
 transceiver calibration unit and via the coaxial cable 234, an output port
 294, and an input port 296; a calibration transmit processor 298 having an
 input port 300 communicatively coupled with port 250 of the calibration
 processing unit 246 via port 244, and an output port 302 communicatively
 coupled with port 296 of the duplexer; and a calibration receive processor
 304 having an input port 306 communicatively coupled with port 294 of the
 duplexer, and an output port 308 communicatively coupled with port 248 of
 the calibration processing unit 246 via port 242 of the calibration
 transceiver unit.
 The receive mode calibration reference signals, generated at port 250 of
 the calibration processing unit, propagate from port 250 of the
 calibration processing unit to the output of the calibration element 206
 via a calibration unit transmit signal path 312 which extends from port
 250 of the calibration processing unit to the output of the calibration
 antenna element 206 via the calibration transmit processor 298, ports 296
 and 292 of the duplexer 290, the coaxial cable 234, and the calibration
 antenna element 206. In the receive calibration mode, a receive mode
 resultant signal, received at port 222 of the control processing unit 220
 is developed as a result of the associated receive mode reference signal
 propagating through the receive mode calibration path. As the receive mode
 reference signal propagates through the receive mode calibration path the
 reference signal traverses the calibration unit transmit signal path 312,
 is radiated from the calibration element 206 to the associated antenna
 element 204, and ultimately propagates through a transceiver receive path
 314 which extends from the input of the corresponding antenna element 204
 to port 222 of the control processing unit 220 via the associated antenna
 element 204, the associated coaxial cable 214, ports 266 and 268 of the
 duplexer 264, and the receive processor 278.
 In the transmit calibration, a transmit mode reference signal generated at
 port 224 of the control processing unit 220, propagates via a transceiver
 transmit path 316 which extends from port 224 of the control processing
 unit 220 to the output of the associated antenna element 204 via ports 274
 and 276 of the transmit processor 272, ports 270 and 266 of the duplexer
 264, the associated coaxial cable 214, and the associated antenna element
 204. A transmit mode resultant signal, received at port 248 of the
 calibration processing unit 246, is developed as a result of the
 associated transmit mode reference signal propagating through the
 associated transceiver transmit path 316, radiating from the associated
 antenna element 204 to the calibration element 206, and ultimately
 propagating through a calibration unit receive signal path 318 which
 extends from the input to the calibration antenna element 206 to port 248
 of the calibration processing unit 246 via the calibration antenna element
 206, the coaxial cable 234, ports 292 and 294 of the duplexer 290, and via
 the calibration receive processor 304. Each of the transmit mode
 calibration paths includes an associated one of the transceiver transmit
 paths 316, an associated radiation path extending from the associated
 element 204 to the calibration element 206, and the calibration unit
 receive signal path 318.
 Note that each of the transceiver units 210 does not include any
 supplemental calibration components. This is in contrast with the
 "internal loop" calibration transceiver units 18 (FIG. 2B) of the prior
 art. The calibration components of the smart antenna system of the present
 invention are referred to as "in-system" because no external reference
 generator signal is required as in the prior art system of FIGS. 2A and
 2B. Also, the components of the transceiver calibration unit 238 are
 referred to as "external loop" components because no calibration
 components are inserted in the signal paths 314 and 316 of the transceiver
 units 210. Because there are no calibration components, such as switches,
 couplers, and attenuator in the transceiver units 210, the smart antenna
 transceiver system 200 (FIG. 3) provides optimal sensitivity to signals
 received by the elements 204.
 In the receive calibration mode, the calibration processing unit 246
 generates a receive mode reference signal at its port 250, and the receive
 mode reference signal propagates via the calibration unit transmit signal
 path 312. A receive mode resultant signal is developed at port 222 of the
 control processing unit 220 as a result of the receive mode calibration
 reference signal propagating through a receive mode calibration signal
 path traversing the calibration unit transmit signal path 312, a radiation
 path between the calibration element 206 and the associated antenna
 element 204, and the associated transceiver receive path 314. The control
 processing unit 220 is operative to determine phase shifts and amplitude
 differences between the receive mode reference signals generated at port
 250 of the calibration processing unit 246 and the receive mode resultant
 signals developed at port 222 of the control processing unit 220 to yield
 a receive calibration vector V.sub.rc that is indicative of the signal
 path characteristics associated with each one of the receive mode
 calibration paths. The receive mode calibration vector may be expressed in
 accordance with relationship (2) below.
EQU V.sub.rc =V.sub.r.cndot.V.sub.ct '=[v.sub.rc1, v.sub.rc2, . . . , v.sub.rcN
 ]=[v.sub.r1, v.sub.r2, . . . , v.sub.rN ].cndot.[v.sub.ct1, v.sub.ct2, . .
 . v.sub.ctN ]' (2)
 Wherein the vector V.sub.r is a receive vector indicative of signal path
 characteristics associated with the transceiver path 314 of each of the
 transceiver units 210 (FIG. 3). The vector V.sub.ct is a calibration unit
 transmit vector indicative of the signal path characteristics associated
 with the calibration unit transmit signal path 312. Note that the
 calibration unit transmit signal path 312 and the transceiver receive path
 314 cascade. Therefore the signal characteristics associated with the
 total receive mode calibration path, including paths 312 and 314, may be
 determined by determining the dot product of the vectors including the
 complex numbers representing the signal characteristics of each of the
 paths. Linear algebra is employed in order to determine the receive mode
 calibration vector V.sub.rc because there are a plurality of N of the
 transceiver units 210 (FIG. 3) and N of the antenna elements 204. Note
 that all elements of the vector V.sub.ct are the same because there is
 only one calibration unit 238 and only one.
 In the transmit calibration mode, each of the transceiver units 210
 transmits a signal via the corresponding antenna element 204. The
 calibration processing unit 220 generates a transmit mode calibration
 reference signal at the transmit port 224 which is connected to the
 associated one of the transceiver unit 210. The signal propagates via the
 corresponding transceiver transmit path 316 to the associated antenna
 element. The transmit mode resultant signal developed at port 248 of the
 calibration processing unit is developed as a result of the associated
 transmit reference signal propagating via the transceiver transmit path
 316, radiating from the associated antenna element 204 to the calibration
 element 206, and propagating via the calibration unit receive signal path
 318. The calibration processing unit 246 is operative to determine phase
 shifts and amplitude differences between the transmit mode reference
 signals generated at the ports 224 of the processing unit 220, and the
 associated transmit mode resultant signals received at port 248 of the
 calibration processing unit 246 to yield a transmit mode calibration
 vector V.sub.tc which is indicative of the signal path characteristics
 associated with each one of the transmit mode calibration paths.
 Wherein the transmit mode calibration vector V.sub.tc may be expressed in
 accordance with relationship (3) below.
 V.sub.tc =V.sub.t.cndot.V.sub.cr '=V.sub.tc =[v.sub.tc1, v.sub.tc2, . . .
 v.sub.tcN ]=[v.sub.t1, v.sub.t2, . . . v.sub.tN ].cndot.[v.sub.cr1,
 v.sub.cr2, . . . v.sub.crN ] (3)
 The vector V.sub.t is a transmit vector indicative of signal path
 characteristics associated with the transceiver transmit path 316 of each
 of the transceiver units 210 (FIG. 3). The vector V.sub.cr is a
 calibration unit receive vector indicative of the signal path
 characteristics associated with the calibration unit receive signal path
 318. Note that the transmit mode calibration vector V.sub.tc is determined
 by the product of the transmit vector V.sub.t and the calibration unit
 receive vector V.sub.cr because the paths 316 and 318 are cascade. Note
 that each of the elements of the calibration unit receive vector V.sub.cr1
 are equal and equal to a complex constant because the calibration unit
 receive path 318 is common to the receive mode signal path associated with
 each of the antenna elements 204.
 One advantage of the in-system external loop calibration of the present
 invention is that there is no need for a technician in the field to apply
 a reference signal via a reference signal generator, and there is no need
 to terminate the coaxial cables during the calibration process. The
 calibration process described above is automatic and may be remotely
 initiated.
 Each of the elements 204 is associated with a corresponding plurality of
 channels, or carriers. In one embodiment, each of the elements 204 has 8
 channels associated with it, and the transceiver units 210, transceiver
 calibration unit 238, and calibration processing sub-system 219 are
 configured to process the data carried by each of the eight channels.
 FIG. 5 shows a schematic block diagram illustrating further details of the
 calibration transceiver unit 238 (FIG. 3) at 320. The calibration transmit
 processor 298 includes: a first digital up-converter 322 having an input
 port 324 communicatively coupled with port 250 of the calibration
 processing unit 246 via port 300 of the calibration transmit processor,
 and an output port 328; a second digital up-converter 330 having a first
 input port 334 communicatively coupled with port 328 of the first digital
 up-converter 322, a second input port 332 communicatively coupled with
 port 250 of the processing unit 246 via port 300 of the calibration
 transmit processor, and an output port 336; a digital-to-analog converter
 (D/A converter) 340 having an input port 342 communicatively coupled with
 port 336 of the second digital up-converter 330, and an output port 344; a
 band pass filter 346 having an input port 348 communicatively coupled with
 port 344 of the D/A converter 340, and an output port 350, a transmit path
 mixer 354 having an input port 356 communicatively coupled to receive a
 band pass filtered signal from port 350 of the filter 346, an input port
 358 for receiving a transmit path reference signal, and an output port
 360; a reference signal generator 362 having an output port 364 for
 providing the transmit path reference signal to input port 358 of the
 mixer 354; and a preamplifier 366 having an input port 368 communicatively
 coupled with port 360 of the mixer 354, and an output port 370
 communicatively coupled with the input port 296 of the duplexer 290 via
 port 302 of the calibration transmit processor.
 The calibration receive processor 304 includes: a preamplifier 380 having
 an input port 382 communicatively coupled with port 294 of the duplexer
 290 via port 306 of the calibration receive processor, and an output port
 384; a receiver path mixer 386 having an input port 388 communicatively
 coupled with port 384 of the amplifier 380, an input port 390 for
 receiving a receive path reference signal, and an output port 392; a
 receive path reference signal generator 394 having an output port 396 for
 providing the receive path reference signal to port 390 of the mixer 386;
 a band pass filter 400 having an input port 402 for receiving a mixed
 receive signal from the output 392 of the mixer 386, and an output port
 404 for providing a band pass filtered receive signal; an
 analog-to-digital converter (A/D converter) 408 having an input port 410
 communicatively coupled with port 404 of the band pass filter for
 receiving the filtered receive signal, and an output port 412 for
 providing a digital receive signal to a node 414; a first digital
 down-converter 416 having an input port 418 for receiving four channels of
 the digital receive signal from port 412 of the A/D converter via node
 414, and an output port 420 communicatively coupled with port 248 of the
 in-system calibration processing unit 246 via port 308 of the calibration
 receive processor; and a second digital down-converter 424 having an input
 port 426 for receiving four channels of the digital receive signal
 provided at port 412 of the A/ID converter 408 via node 414, and an output
 port 428 communicatively coupled with port 248 of the calibration
 processing unit 246 via port 308 of the calibration receive processor.
 The digital up- and down-converters 322, 330, 416, and 424 provide digital
 control for tuning to the corresponding channels, or carriers, of each of
 the transmit and receive signals transmitted to and received from the
 antenna calibration element 206.
 The in-system external loop calibration smart antenna system of the present
 invention may be used in a wide variety of smart antenna transceiver
 system applications. In one embodiment, the system is used in a base
 station of a cellular telephone system which may be operated in accordance
 with any of a variety of well-known protocols including TDMA and CDMA. For
 different systems, the channels are allocated and used differently. Note
 that in the calibration process, the calibration vectors may be determined
 for each channel one at a time, successively. However in the preferred
 embodiment, the calibration vectors are determined simultaneously for each
 of the eight channels, or carriers, associated with each of the elements
 204 of the antenna array 202.
 Although the present invention has been particularly shown and described
 above with reference to a specific embodiment, it is anticipated that
 alterations and modifications thereof will no doubt become apparent to
 those skilled in the art. It is therefore intended that the following
 claims be interpreted as covering all such alterations and modifications
 as fall within the true spirit and scope of the invention.