U.S. patent application number 10/692671 was filed with the patent office on 2004-07-01 for determining a spatial signature using a robust calibration signal.
Invention is credited to Barratt, Craig H., Boros, Tibor, Trott, Mitchell D., Uhlik, Christopher R..
Application Number | 20040127260 10/692671 |
Document ID | / |
Family ID | 22181228 |
Filed Date | 2004-07-01 |
United States Patent
Application |
20040127260 |
Kind Code |
A1 |
Boros, Tibor ; et
al. |
July 1, 2004 |
Determining a spatial signature using a robust calibration
signal
Abstract
A method and apparatus for estimating the downlink signature for
a remote transceiver which is part of a wireless communication
system that includes a main transceiver for communicating with the
remote transceiver. The main transceiver includes an array of
transmit antenna elements. The method uses the remote transceiver
for receiving signals when the main transceiver transmits downlink
calibration signals. When the main transceiver also has a receive
antenna array, the remote transceiver can transmit uplink
calibration signals to the main transceiver for determining an
uplink signature. The downlink and uplink signatures are used to
determine a calibration function to account for differences in the
apparatus chains that include the antenna elements of the arrays,
and that enable downlink smart antenna processing weights to be
determined from uplink smart antenna processing weights when the
main transceiver includes means for smart antenna processing
according to weights.
Inventors: |
Boros, Tibor; (Sunnyvale,
CA) ; Barratt, Craig H.; (Redwood City, CA) ;
Uhlik, Christopher R.; (Danville, CA) ; Trott,
Mitchell D.; (Mountain View, CA) |
Correspondence
Address: |
BLAKELY SOKOLOFF TAYLOR & ZAFMAN
12400 WILSHIRE BOULEVARD, SEVENTH FLOOR
LOS ANGELES
CA
90025
US
|
Family ID: |
22181228 |
Appl. No.: |
10/692671 |
Filed: |
October 24, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10692671 |
Oct 24, 2003 |
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10135979 |
Apr 29, 2002 |
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6668161 |
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10135979 |
Apr 29, 2002 |
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09295434 |
Apr 20, 1999 |
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6615024 |
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60083875 |
May 1, 1998 |
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Current U.S.
Class: |
455/562.1 |
Current CPC
Class: |
H04L 25/03343 20130101;
H04B 7/0615 20130101; H04L 2025/03426 20130101; H01Q 3/2605
20130101; H01Q 1/246 20130101; H04B 17/12 20150115; H04B 7/005
20130101; H01Q 3/267 20130101; H04B 17/21 20150115; H04B 7/0617
20130101; H04B 7/0848 20130101; H04B 17/10 20150115; H04B 17/11
20150115; H04B 17/14 20150115; H04B 7/0842 20130101 |
Class at
Publication: |
455/562.1 |
International
Class: |
H04M 001/00 |
Claims
What is claimed is:
1. In a wireless communication system comprising a main transceiver
and a remote transceiver capable of receiving signals from and
transmitting signals to the main transceiver, the main transceiver
comprising an array of transmit antenna elements, and at least one
receive antenna element, each transmit antenna element being part
of a transmit electronics chain for transmitting a transmit
apparatus signal using the transmit antenna element, and each
receive antenna element being part of a receiver apparatus chain
for receiving a received antenna signal from the receive antenna
element, the main transceiver and the remote transceiver designed
for mutual communication using waveforms conforming to an air
interface standard, a method for estimating the downlink signature
for the remote transceiver, the method comprising: (a) transmitting
a set of one or more downlink calibration waveforms from the main
transceiver via the transmit antenna array to the remote
transceiver, the set of downlink calibration waveforms
substantially conforming to the air interface standard; (b)
processing the signals received at the remote transceiver
corresponding to the downlink calibration waveforms, the processing
to determine downlink signature related signals related to the
downlink signature for the remote transceiver, (c) transmitting the
downlink signature related signals from the remote transceiver to
the main transceiver using waveforms substantially conforming to
the air interface standard; and (d) determining the downlink
signature of the remote transceiver from the downlink signature
related signals received at the main transceiver.
2. The method of claim 1, wherein the at least one receive antenna
element are a plurality of receive antenna elements forming an
array of receive antenna elements, the number of elements in the
array of receive antenna elements being the same as the number of
antenna elements in the array of transmit antenna elements, the
method further comprising: (e) transmitting a set of one or more
uplink calibration waveforms from the remote transceiver to the
main transceiver, the set of downlink calibration waveforms
substantially conforming to the air interface standard; (f)
processing at the main transceiver the received antenna signals
corresponding to the uplink calibration signals transmitted from
the remote transceiver, the processing determining the uplink
signature for the remote transceiver; and (h) determining a
calibration function for the main transceiver from the uplink and
downlink signatures for the remote transceiver.
3. In a wireless communication system comprising a main transceiver
and a remote transceiver capable of receiving signals from and
transmitting signals to the main transceiver, the main transceiver
comprising an array of transmit antenna elements, and at least one
receive antenna element, each transmit antenna element being part
of a transmit electronics chain for transmitting a transmit
apparatus using the transmit antenna element, and each receive
antenna element being part of a receiver apparatus chain for
receiving a received antenna signal from the receive antenna
element, the main transceiver and the remote transceiver designed
for mutual communication using waveforms conforming to an air
interface standard, a method for estimating the downlink signature
for the remote transceiver, the method comprising: (a) transmitting
a set of one or more downlink calibration waveforms from the main
transceiver via the transmit antenna array to the remote
transceiver, the set of downlink calibration waveforms designed to
be robust to one or more of the set comprising frequency offset,
phase noise, I/Q mismatch, and timing offset; (b) processing the
signals received at the remote transceiver corresponding to the
downlink calibration waveforms, the processing to determine
downlink signature related signal related to the downlink signature
for the remote transceiver; (c) transmitting the downlink signature
related signals from the remote transceiver to the main
transceiver; and (d) determining the downlink signature of the
remote transceiver from the downlink signature related signals
received at the main transceiver.
4. The method of claim 3, wherein the at least one receive antenna
element are a plurality of receive antenna elements forming an
array of receive antenna elements, the number of elements in the
array of receive antenna elements being the same as the number of
antenna elements in the array of transmit antenna elements, the
method further comprising: (e) transmitting a set of one or more
uplink calibration waveforms from the remote transceiver to the
main transceiver, the set of downlink calibration waveforms; (f)
processing at the main transceiver the received antenna signals
corresponding to the uplink calibration signals transmitted from
the remote transceiver, the processing determining the uplink
signature for the remote transceiver; and (g) determining a
calibration function for the main transceiver from the uplink and
downlink signatures for the remote transceiver.
5. The method of claim 3, wherein each of the set of downlink
calibration waveforms conforms to the air interface standard.
6. In a wireless communication system comprising a main transceiver
and a remote transceiver capable of receiving signals from and
transmitting signals to the main transceiver, the main transceiver
comprising an array of transmit antenna elements, and at least one
receive antenna element, each transmit antenna element being part
of a transmit electronics chain for transmitting a transmit
apparatus signal using the transmit antenna element, and each
receive element being part of a receiver apparatus chain for
receiving a received antenna signal from the receive antenna
element, the main communication transceiver designed to transmit
traffic waveforms, the main communication transceiver also designed
to transmit downlink calibration waveforms, a method for estimating
the downlink signature for the remote transceiver, the method
comprising: (a) transmitting downlink calibration waveforms and
traffic waveforms from the main transceiver via the transmit
antenna array to the remote transceiver, the downlink calibration
waveforms interspersed with the traffic waveforms; (b) determining
at the remote transceiver whether the signals received at the
remote transceiver correspond to downlink calibration waveforms or
to traffic waveforms; (c) processing signals received at the remote
transceiver determined in step (b) to correspond to downlink
calibration waveforms, the processing to determine downlink
signature related signals related to the downlink signature for the
remote transceiver, (d) processing signals received at the remote
transceiver determined in step (b) to correspond to traffic
waveforms, the processing to perform normal traffic functions; (e)
transmitting the downlink signature related signals from the remote
transceiver to the main transceiver; and (f) determining the
downlink signature of the remote transceiver from the downlink
signature related signals received at the main transceiver.
7. The method of claim 6, wherein the downlink calibration
waveforms are transmitted during silent periods.
8. The method of claim 6, wherein the down calibration waveforms
are transmitted only after a number of idle waveforms are
transmitted from the main transceiver.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims the benefit of U.S. Provisional
Patent Application Serial No.: 60/083,875 for METHOD AND APPARATUS
FOR DETERMINING SPATIAL SIGNATURES WITH APPLICATION TO CALIBRATING
A BASE STATION HAVING AN ANTENNA ARRAY to inventors Boros, Barratt,
Uhlik, and Trott, Assignee ArrayComm, Inc., filed May 1, 1998.
FIELD OF INVENTION
[0002] This invention relates to the field of wireless
communication systems, and more specifically, to a method and
apparatus for calibrating a communication station that includes an
array of antenna elements.
BACKGROUND
[0003] Smart Antenna Systems
[0004] Antenna arrays may be used in any wireless communication
receiver or transmitter or transceiver (herein under "communication
station") that transmits or receives radio frequency signals using
an antenna or antennas. The use of antenna arrays in such a
communication station provides for antenna performance improvements
over the use of a single element antenna. These antenna performance
improvements include improved directionality, signal to noise
ratio, and interference rejection for received signals, and
improved directionality, security, and reduced transmit power
requirements for transmitted signals. Antenna arrays may be used
for signal reception only, for signal transmission only, or for
both signal reception and transmission.
[0005] A typical application of antenna array communication
stations is in a wireless communication system. Examples include a
cellular communication system consisting of one or more
communication stations, generally called base stations, each
communicating with its subscriber units, also called remote
terminals and handsets. In cellular systems, the remote terminal
may be mobile or in a fixed location, and when fixed such a system
often is called a wireless local loop system. The antenna array
typically is at the base station. Terminology for the direction of
communication comes from conventional satellite communication, with
the satellite replaced by the base station. Thus, communication
from the remote terminal to the base station is called the uplink,
and communication from the base station to the remote terminal is
called the downlink. Thus, the base station antenna array transmits
on the downlink direction and receives on the uplink direction.
Antenna arrays also may be used in wireless communication systems
to add spatial division multiple access (SDMA) capability, which is
the ability to communicate with several users at a time over the
same "conventional" (FDMA, TDMA or CDMA) channel. We have
previously disclosed adaptive smart antenna processing (including
spatial processing) with antenna arrays to increase the spectrum
efficiency of SDMA and non-SDMA systems. See Co-owned U.S. Pat. No.
5,515,378 for SPATIAL DIVISION MULTIPLE ACCESS WIRELESS
COMMUNICATION SYSTEM, U.S. Pat. No. 5,592,490 for SPECTRALLY
EFFICIENT HIGH CAPACITY WIRELESS COMMUNICATION SYSTEMS, U.S. Pat.
No. 5,828,658 for SPECTRALLY EFFICIENT HIGH CAPACITY WIRELESS
COMMUNICATION SYSTEMS WITH SPATIO-TEMPORAL PROCESSING, and U.S.
patent application Ser. No. 08/729,390 for METHOD AND APPARATUS FOR
DECISION DIRECTED DEMODULATION USING ANTENNA ARRAYS AND SPATIAL
PROCESSING. Systems that use antenna arrays to improve the
efficiency of communications and/or to provide SDMA sometimes are
called smart antenna systems.
[0006] With smart antenna communication systems that use linear
spatial processing for the adaptive smart antenna processing,
during uplink communications, one applies amplitude and phase
adjustments in baseband to each of the signals received at the
antenna array elements to select (i.e., preferentially receive) the
signals of interest while minimizing any signals or noise not of
interest--that is, the interference. Such baseband amplitude and
phase adjustment can be described by a complex valued weight, the
receive weight, and the receive weights for all elements of the
array can be described by a complex valued vector, the receive
weight vector. Similarly, the downlink signal is processed by
adjusting the amplitude and phase of the baseband signals that are
transmitted by each of the antennas of the antenna array. Such
amplitude and phase control can be described by a complex valued
weight, the transmit weight, and the weights for all elements of
the array by a complex valued vector, the transmit weight vector.
In some systems, the receive (and/or transmit) weights include
temporal processing, and then are called spatio-temporal parameters
for spatio-temporal processing In such cases, the receive (and/or
transmit) weights may be functions of frequency and applied in the
frequency domain or, equivalently, functions of time applied as
convolution kernels. Alternatively, each convolution kernel, if for
sampled signals, may itself be described by a set of complex
numbers, so that the vector of convolution kernels may be
re-written as a complex values weight vector, which, for the case
of there being M antennas and each kernel having K entries, would
be a vector of KM entries.
[0007] The receive spatial signature characterizes how the base
station array receives signals from a particular subscriber unit in
the absence of any interference or other subscriber units. A
receive weight vector for a particular user may be determined using
different techniques. For example, it may be determined from
spatial signatures. It also may be determined from the uplink
signals received at the antennas of the array from that remote user
using some knowledge about these uplink signals, for example, the
type of modulation used. The transmit spatial signature of a
particular user characterizes how the remote user receives signals
from the base station in the absence of any interference. The
transmit weight vector used to communicate on the downlink with a
particular user is determined either from the receive weight vector
(see below under. "The Need for Calibration") or from the transmit
spatial signature of the particular user and the transmit spatial
signatures of the other users in such a way as to maximize the
energy to the particular user and minimize the energy to the other
users.
[0008] U.S. Pat. No. 5,592,490 for SPECTRALLY EFFICIENT HIGH
CAPACITY WIRELESS COMMUNICATION SYSTEMS describes spatial
signatures and their uses, and U.S. Pat. No. 5,828,658 for
SPECIALLY EFFICIENT HIGH CAPACITY WIRELESS COMMUNICATION SYSTEMS
WITH SPATIO-TEMPORAL PROCESSING, incorporated herein by reference,
describes how to extend this to spatio-temporal processing using
spatio-temporal signatures.
[0009] Thus, while the description herein is provided in terms of
spatial signatures, adding time equalization to provide
spatio-temporal processing is easily accommodated, for example by
adding the concepts of spatio-temporal signatures, which may be
described by MK vectors (both uplink and downlink) when the
temporal processing is using equalizers with K taps (i.e.,
convolution kernels of length K in the weight convolving
functions). Thus, how to modify the invention to accommodate
spatio-temporal processing and spatio-temporal signatures would be
clear to those of ordinary skill in the art, for example in view of
above-referenced and incorporated herein by reference U.S. Pat. No.
5,828,658. Therefore, those in the art would understand that any
time the term spatial signature is used, this might indeed be
referring to a spatio-temporal signature in the context that the
invention is being applied to a communication station equipped with
means for spatio-temporal processing.
[0010] The Need for Calibration
[0011] It is desirable to determine the transmit weight vector from
the receive weight vector for a particular user. More generally, it
is desirable to determine the appropriate transmit signals to use
for transmitting to a particular user from signals received from
that user. Practical problems may make difficult determining the
transmit weight vector from the receive weight vector for a
particular user. Frequency division duplex (FDD) systems are those
in which uplink and downlink communications with a particular
remote user occur at the different frequencies. Time division
duplex (TDD) systems are those in which uplink and downlink
communications with a particular remote user occur at the same
frequency but in different time slots. In a TDD system, because of
the well known principle of reciprocity, it might be expected that
determining the transmit weight vector from the receive weight
vector is straightforward. However, on the uplink, the received
signals that are being processed may be somewhat distorted by the
receive electronics (the receive apparatus chains) associated with
each of the antenna elements of the antenna array. The receive
electronics chain includes the antenna element, cables, filters, RP
receivers and other components, physical connections, and
analog-to-digital converter ("ADC") if processing is digital. In
the case of a multi-element antenna array, there typically is a
separate receive electronics apparatus chain for each antenna array
element, and thus the amplitude and the phase of each of the
received signals at each element may be distorted differently by
each of the receive apparatus chains. In addition, there are RF
propagation effects that take place on the uplink between the
subscriber unit and a particular receiving antenna, such effects
including without limitation the path loss, fading and shading
effects, multipath, and near-field scattering, and these effects
may be different from antenna element to antenna element. Note that
the receive electronics chain and the RF propagation effects
together make up the uplink spatial signature for the remote user.
A receive weight vector that does not take these receive
electronics chain and RP propagation effects into account will be
in error, causing less than optimal reception at the base station.
However, in practice, communication may still be possible. Also,
when a receive weight vector is determined using some knowledge of
the characteristics of the received signal, for example, the type
of modulation used, such a method already takes into account the
uplink receive electronics chain and RF propagation effects. When
one transmits downlink signals through the antenna array, each of
the signals radiated by an antenna element goes through a different
transmit electronics apparatus chain; thus possibly causing
different amplitude and phase shifts in the transmitted signals. In
addition, there are again RF propagation effects. If the transmit
weight vector was derived from a receive weight vector that did not
take the differences in the receive electronics chains and RP
propagation into account, transmission from the base station may be
hard to achieve. Further difficulty may result if the transmit
weight vector does not take differences in the transmit electronics
chains and transmit RF propagation effects into account, possibly
making communication using such a transmit weight vector
impossible.
[0012] The purpose of calibration is to determine calibration
factors for compensating for the different amplitude and phase
errors that occur in the signals in the receive chain and uplink RF
propagation, and the different amplitude and phase errors that
occur in the transmit chain and downlink RP propagation, the
calibration factors used in a communication station to determine a
transmit weight vector for transmitting to a remote user from the
set of signals received from the remote user. It should be added
that because the phase and amplitude shifts that occur in the
receive and transmit apparatus chains are, in general, frequency
dependent, so in general are the calibration factors frequency
dependent.
[0013] In the case of a TDD system, the uplink and downlink RP
propagation effects cancel so that the calibration factors are
independent of the location of the subscriber unit.
[0014] It is known that compensation can be achieved by convolving
each of the M signals received or transmitted by the antenna
elements by a calibration function (i.e., by a complex valued time
sequence), where each calibration function describes the transfer
function correction required to compensate for the gain and phase
errors a signal undergoes when passing through the transmit and
receive apparatus chains. In some systems, this can be simplified
to multiplicative correction, where each calibration function is a
calibration factor--a complex valued number that describes the
required amplitude and phase correction required for compensation.
In general, the set of calibration functions defines a calibration
vector function with each element a calibration function. In the
case of multiplicative correction, the set of calibration factors
defines a calibration vector with each element a calibration
factor.
[0015] Determining the transmit weight vectors from the receive
weight vectors for a particular user is more difficult in the case
of an FDD system because reciprocity may no longer be assumed. One
needs to additionally take into account the differences in
propagation on the uplink and downlink. Once one does take such
differences into account, there still is a need to determine
calibration factors for compensating for the different amplitude
and phase errors that occur in the signals in the receive chain and
uplink RP propagation and the different amplitude and phase errors
that occur in the transmit chain and downlink RIP propagation. In
general, single calibration factors that are independent of the
location of the remote user may not be possible. In such a case,
one needs to be able to determine the uplink and downlink spatial
signatures.
[0016] In the case of no calibration factors that are independent
of the remote user location being possible, when there is some
functional relationship that enables one to determine the transmit
weight vector to use from the received signals and some parameter,
for example, the angle of arrival, there still is a need to
determine a set of calibration functions for compensating for the
different amplitude and phase errors that occur in the signals in
the receive chain and uplink RP propagation and the different
amplitude and phase errors that-occur in the transmit chain and
downlink RF propagation, these functions being dependent on one or
more parameters of the remote user, for example the angle of
arrival.
[0017] The Need for Signature Estimation
[0018] When no simple calibration (as defined above) is possible,
one still needs to compensate for the different amplitude and phase
errors that occur in the signals in the receive chain and uplink RF
propagation, and the different amplitude and phase errors that
occur in the transmit chain and downlink RF propagation. The
purpose of signature estimation is to determine the uplink and
downlink spatial signatures which characterize these differences.
Thus calibration is a special case of signature estimation when
either 1) the RF propagation effects cancel so that downlink
weights can be determined from uplink signals or weights, or 2)
there is some simple functional relationship of the RP propagation
effects so that uplink weights can be determined from uplink
signals and some parameters of the remote user, for example, the
angle of arrival of the uplink signals.
[0019] Other Methods
[0020] Known methods for determining array calibrations each have
one or more associated drawbacks. Most known methods require
external measuring equipment which may be expensive, unwieldy and
cumbersome to use repeatedly. Secondly, conventional calibration
methods are sensitive to drifts in system parameters, such as
frequency references, over the extended period of time during which
measurements are being taken, and these drifts result in
inaccuracies in the measured array calibrations. In addition, some
known techniques only determine multiplicative rather than
convolution kernel calibrations despite the need to calibrate
frequency dependent components in the antenna array. In order to
eliminate this frequency dependence and still use multiplicative
calibrations, it is necessary to calibrate the antenna array for
each frequency channel of communication. Thirdly, the transfer
characteristics of the RF electronics depend on changing ambient
conditions such as temperature and humidity which make it essential
that antenna arrays be repeatedly calibrated in their ambient
environment.
[0021] Harrison et al. disclose in U.S. Pat. No. 5,274,844 (Dec.
28, 1993) a method for calibrating transmit and, separately,
receive chains (as complex valued vector transfer functions) in two
experiments which involve a data bus connecting a resource
controller to a remote terminal. In the first experiment, the data
bus indicates to the remote terminal to send a known signal to the
base station. This determines the receive apparatus chain
calibration. In a second experiment, the signals received at the
remote terminal are sent back to the resource controller via the
data bus to enable determining the transmit apparatus chain
calibration.
[0022] Co-owned U.S. Pat. No. 5,546,090, issued 13 Aug. 1996, and
assigned to the assignee of the present invention, discloses a
calibration method which can determine both transmit and receive
calibrations using a simple transponder co-located with the to
remote terminal that retransmits to the base station the signals
received at the remote terminal from the base station. Such a
method does not require the wired data-bus of the Harrison et al.
invention. Still, additional transponder equipment is required.
[0023] PCT Patent application publication WO 95/34103 (published.
Dec. 14, 1995) entitled ANTENNA ARRAY CALIBRATION. Johannisson, et
al., inventors, discloses a method and apparatus for calibrating
the transmission (and reception) of an antenna array. For transmit
calibration, an input transmit signal is inputted into each antenna
element one antenna at a time. After the input transmit signal has
passed through a respective power amplifier, the signal transmitted
by each antenna element is sampled by a calibration network. The
resulting signal is fed into a receiver, and a computation means
relates the received signal with the original transmit signal for
each antenna element. Correction factors can then be formed for
each antenna element. The antenna elements may then be adjusted (in
amplitude and phase, or in-phase I and quadrature Q components)
using the correction factors so as to ensure that each element is
properly calibrated during transmission. For receive calibration, a
known input signal is generated and injected using a calibration
network (a passive distribution network) into each antenna element
of the antenna array. The signals pass from the antenna elements
through respective low noise amplifiers, and the signals thus
received by each antenna element are measured by a beam forming
apparatus. The beam forming apparatus can then generate correction
factors by comparing the injected signal with the measured 30
signals so as to individually calibrate each antenna element. The
correction can be described as amplitude and phase corrections, or
as corrections in in-phase I and quadrature Q components.
[0024] U.S. Pat. No. 5,530,449 to Wachs et al. entitled PHASED
ARRAY ANTENNA MANAGEMENT SYSTEM AND CALIBRATION METHOD (herein
under "Wachs") describes a management system and calibration method
for use with a phased array antenna that employs a system level
measurement of amplitude and phase, conducted during nodal
operation, to determine on an element by element basis, the
tracking performance of individual chains for the antennas. The
system and method measure the amplitude and phase of individual
element chains utilizing probe carriers. The required correction
coefficients for each chain are determined from the measured
amplitude and phase data, and each individual element chain is
individually compensated to remedy the amplitude and phase errors.
The system separately calibrates forward and return link phased
array antennas on a phased array antenna communication station
which is on a satellite. In one embodiment, a separate remote
calibration station is used. For calibrating is the transmit paths,
the probe signal is transmitted to an antenna at the calibration
system alternatively from one element (a reference element) and an
element under test. The signals received at the calibration station
are compared to determine the corrections. A separate communication
link also is used to provide communication between the calibration
station and the satellite. In the receive direction, the remote
calibration station is used to transmit to all antenna elements of
the phased array, but only two elements are alternately sampled to
form the calibration carrier. The calibration carrier is then
downlinked at Ka band to a gateway hub station for computation. In
an alternate embodiment, a local sense antenna at the satellite's
communication station is used to sample outputs of the transmit
antenna elements. In both embodiments, separate calibrations are
carried out for receive and transmit paths, and extra equipment is
needed, either a separate remote calibration station, with an
additional link, or a separate sense antenna system. Several
features of Wachs' system are of note. First, additional hardware
is required in the form of a separate calibration station or probe
antenna. Second, special waveforms need to be used for that
calibration, rather than ordinary communication waveforms supported
by standard air interfaces. This means that the communication
station needs additional hardware for forming and transmitting such
waveforms, and the calibration station needs special
receiving/demodulating hardware, and cannot reuse standard
hardware. Thus there is a chance that a Wachs-like system adapted
for use in a wireless communication system may not be allowed to
operate in some countries.
[0025] Thus these known methods provide separate calibrations for
the receive and transmit paths. The methods require special
calibration apparatus. Some known methods and systems use special
waveforms, and thus need additional hardware for processing such
waveforms, and also do not conform to any established air interface
standards, so face the risk of not being allowed to operate in some
countries Those known systems that also calibrate for the different
air paths between the base station antenna elements and the
subscriber unit are more properly classified as spatial signature
estimating techniques under the definition of calibration used
herein.
[0026] Parish et al. in co-owned U.S. patent application Ser. No.
08/948,772 for METHOD AND APPARATUS FOR CALIBRATING A WIRELESS
COMMUNICATION STATION HAVING AN ANTENNA ARRAY, describe a
calibration method for a base station with an array of antenna
elements that does not require any additional calibration
apparatus. One aspect includes transmitting a prescribed signal
from each antenna element using the transmit electronics of that
antenna element while receiving the transmitted signal in at least
one of the receiver electronics chains not associated with the
antenna. This is repeated, transmitting prescribed signals from
other antenna elements using other transmit apparatus chains until
prescribed signals have been transmitted from all antenna elements
for which calibration factors are required. Calibration factors for
each antenna element are determined as a function of the associated
transmit electronics chain and receiver electronics chain transfer
functions. When downlink and uplink communication occurs in the
same frequency channel, a single calibration factor is determined
for any antenna element. In one version of the Parish et al.
invention, the single calibration factor is in phase a function of
the difference between the transmit apparatus chain transfer
function phase and the receiver apparatus chain transfer function
phase associated with a particular antenna element. In another
aspect of the Parish et al. invention, the calibration factors so
determined are used for determining a set of transmit weights from
a set of receive weights.
[0027] While the Parish et al. invention enables determining a
single set of calibration factors for the base station which
enables a downlink set of weights to be determined from an uplink
set of weights without requiring some additional apparatus such as
a transponder, and calibrates for differences in base station
electronics paths, the Parish et al. method cannot be adapted to
estimate spatial signatures to deal with RF propagation path
differences which may occur. In addition, the base station needs to
enter a spatial calibration mode for carrying out the calibration
experiment, and thus cannot be used for any other purpose during
that time.
[0028] Also, there is no mention in the prior art of the capability
of calibrating by combining measurements from a plurality of remote
transceivers.
[0029] Desirable Features
[0030] The main purpose of the calibration process is to acquire
calibration information for the base station. This may involve
measuring the gain and phase differences between the uplink and
downlink channels. Accuracy and high precision are of great
importance during this procedure. If the calibration information is
not accurate, then the beam pattern on the downlink will be highly
distorted. As a consequence, less energy will be radiated toward
the target user, and an excess amount of interference will be
radiated toward co-channel users. This will have a negative effect
on the downlink signal quality and on the downlink range.
Ultimately, a bad calibration strategy may significantly reduce the
capacity of the wireless network.
[0031] One desirable feature of a calibration method is that only a
base station and a subscriber unit are needed for calibration with
no further equipment such as signal generators, transponders,
calibration stations, additional antennas, probes, or other
equipment, being required. Such a system ideally should be able to
calibrate for differences in both the receive and transmit
electronics. Such systems also should use ordinary communication
waveform's substantially conforming to the particular air interface
standard of the wireless communication system in which they
operate. This enables reusing standard hardware, and also ensures
non-violation of standards and maintaining compatibility with any
future modifications with standards. By "conforming to an air
interface standard" we mean conforming to the channel structure and
modulation of an air interface, where "channel structure" is a
frequency slot in the case of FDMA, a time and frequency slot in
the case of TDMA, and a code channel in the case of CDMA, and
"modulation" is the particular modulation scheme specified in the
standard.
[0032] Another desirable feature is that the method can be used for
signature estimation in order to also account for differences in
the RF paths.
[0033] Another desirable feature of a calibration method is ease of
use and the ability to carry out the calibration rapidly and
frequently, even for example, as frequently as several times a
minute. This ultimately increases the downlink processing accuracy
which has a profound effect on signal quality, capacity, coverage,
and possibly other parameters.
[0034] Another desirable feature of a calibration method is that
each and every subscriber unit supports calibration.
[0035] Another desirable feature for a calibration system is the
ability to carry out some or all of the processing of received data
for calibration within the subscriber unit, thus not requiring the
subscriber unit to send the received data back to the base station
and not is requiring the base station to carry out all of the
processing. The computational burden of the base station thus may
be significantly reduced by "distributing" the load across
intelligent subscriber units. This feature is particularly
desirable, for example, for base stations that service many
subscriber units, or that calibrate before each call or even
several times during each call.
[0036] Another desirable feature is the ability to initiate
calibration on any available conventional channel on the base
station, for example, any carrier and any lime slot of a FDMA/TDMA
system. This further enhances flexibility since one can choose any
timeslot and any carrier which is available for use at the
moment.
[0037] Another desirable characteristic for a calibration method is
the ability to calibrate a base station without having to take the
base station off-line for calibration, thus enabling base station
calibration to be performed while the base station services
hundreds of calls, for example, in a FDMA/FDMA/SDMA system on other
carriers (frequency slots)/timeslots/spatial channels. This feature
is especially important for wideband base stations that service
many conventional channels (e.g., carriers for an FDMA/TDMA system)
at the same time.
[0038] Another desirable characteristic for a calibration method is
the ability carry out rapid calibration even several times during
an existing call.
[0039] Another desirable characteristic for a calibration method is
the ability to carry out calibration in a seamless manner during an
ongoing call so that a base station maybe able to continuously
calibrate itself during some calls.
[0040] Another desirable characteristic for a calibration method is
the ability to carry out calibration with several remote
transceivers by combining measurements, each of which may be able
to "see" only a subset of a communication station's antenna array,
or each of which may face a different interference environment.
[0041] Another desirable characteristic for a calibration method is
the ability to determine whether calibration is accurate, for
example by performing statistical measurements, together with the
ability to feed back such information to the communication station
to determine, for example, if the combining from several remote
stations may be necessary.
[0042] Another desirable feature is high accuracy, with immunity to
frequency offset, timing misalignment, I/Q mismatch, and phase
noise that typically might occur in communication with inexpensive
subscriber units.
[0043] Thus there still is a need in the art for a calibration
method and apparatus that include all or most of the above
characteristics. For example, the is a need for a system and method
one that are accurate and simple, both in terms of the equipment
necessary and the time required, so that calibration can be
performed repeatedly and rapidly wherever and whenever desired.
There also is a need in the art for a simple calibration technique
that only uses existing base station electronics and does not
require special calibration hardware. There also is a need in the
art for a method that enables one to determine transmit weight
vectors from receive weight vectors, including calibrating for the
receive electronics and transmit electronics, the calibration
obtained using simple techniques that use existing base station and
subscriber unit electronics and do not require special calibration
hardware.
[0044] Thus there still is a need in the art for efficient methods
that determine uplink spatial signatures for correcting for the
differences in uplink RF paths and receive electronics and downlink
spatial signatures for correcting for the differences in downlink
RF paths and transmit electronics.
SUMMARY
[0045] An feature of the present invention is enabling calibrating
a communication station having an antenna array for differences in
electronics paths, the calibration using only the communication
station and a subscriber unit.
[0046] Another feature of the invention is providing calibration
that enables using a calibrated transmit weight vector, the
transmit weight vector essentially determined from a receive weight
vector, the calibration taking into account differences in
electronics to paths.
[0047] Another feature of the invention is determining spatial
signatures that enable using a calibrated transmit weight vector,
the transmit weight vector essentially determined from a receive
weight vector, the calibrating taking into account differences in
electronics paths and RF propagation paths.
[0048] Another feature of the invention is enabling determining the
uplink spatial signature of a subscriber unit communicating with a
communication station, the determining using only the communication
station and the subscriber unit.
[0049] Another feature of the invention is enabling determining the
downlink spatial signature of a subscriber unit communicating with
a communication station, the determining using only the
communication station and the subscriber unit.
[0050] Still another feature of the invention is calibrating a
communication station having an antenna array that the calibrating
easy and without taking the communication station off the air for
those conventional channels not currently being calibrated.
[0051] Still another feature of the invention is calibrating a
communication station having an antenna array, the calibrating able
to be carried out partially or in total at a subscriber unit.
[0052] Still another feature of the invention is calibrating a
communication station, the calibrating method providing high
accuracy, with immunity to frequency offset, timing misalignment,
I/Q mismatch, and phase noise that typically might occur in
communicating with Inexpensive subscriber units.
[0053] Another feature of the invention is providing a calibration
method and apparatus that can be readily implemented in a radio
frequency system and that make it practical to perform frequent and
routine system calibration, the calibration enabling the use of a
calibrated transmit weight vector, the transmit weight vector
essentially determined from a receive weight vector, the
calibration including correcting for differences in electronic
paths and for differences in RF propagation effects.
[0054] Yet another feature is enabling rapid calibration even
several times during an existing call.
[0055] Yet another feature is enabling carrying out calibration in
a seamless manner during an ongoing call so that a communication
station may be able to continuously calibrate itself during a
particular call.
[0056] Yet another feature is the ability to carry out calibration
with several remote transceivers by combining measurements each of
which may be able to "see" only a subset of a communication
station's antenna array, or each of which may face a different
interference environment.
[0057] Yet another feature is providing the ability to determine
whether calibration is accurate, for example by performing
statistical measurements, together with the ability to feedback
such information to the communication station to determine, for
example, if the combining from several remote stations may be
necessary.
[0058] These and other features will become clear from reading the
detailed description of the preferred embodiments of the invention
provided herein below
BRIEF DESCRIPTION OF THE DRAWINGS
[0059] The present invention will be more fully understood from the
detailed description of the preferred and some alternate
embodiments of the invention, which, however, should not be taken
to limit the invention to any specific embodiments, but are for
explanation and better understanding only. The embodiments in turn
are explained with the aid of the following figures:
[0060] FIG. 1 shows the uplink and downlink signal flow on the base
station;
[0061] FIG. 2 shows the decomposition of the uplink and downlink
channels into "propagation" and "electronics" factors;
[0062] FIG. 3 illustrates the frame structure of a typical TDD
system;
[0063] FIG. 4 shows the receive signal processor and the uplink
weight computation;
[0064] FIG. 5 illustrates the symmetry between the uplink and
downlink signal paths;
[0065] FIG. 6 shows the internal structure of the transmit weight
generator;
[0066] FIG. 7 shows the protocol sequence during calibration;
[0067] FIG. 8 illustrates the decomposition of a 6-element circular
array into 2-element subarrays;
[0068] FIG. 9 illustrates the uplink signature estimation at the
base station;
[0069] FIG. 10 shows the downlink signature estimation at the
subscriber unit;
[0070] FIG. 11 shows a flowchart of one embodiment of a method for
carrying out downlink signature determination with calibration
bursts interspersed with normal TCH bursts;
[0071] FIG. 12 shows the architecture of a typical subscriber unit
in which aspects of the present invention may be implemented;
[0072] FIG. 13 shows the results of testing a two antenna element
implementation of the method for downlink signature estimation;
[0073] FIG. 14 shows the results of testing an implementation of
the method for downlink signature estimation using a single
transmitter and antenna element; and
[0074] FIG. 15 shows the results of testing an implementation of
the method for downlink signature estimation using a single
transmitter and antenna element, but with a different set of
frequencies than used to obtain the results of FIG. 14.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0075] A Note on Reference Numerals
[0076] The first one or two digits in a reference numeral indicate
on which figure that reference numeral is first introduced.
Reference numerals between 100 and 199 are first introduced in FIG.
1, those between 200 and 299 are first introduced in FIG. 2, and so
forth. For example, reference numeral 111 is first introduced in
FIG. 1, 909 is first introduced in FIG. 9, 1009 is first introduced
in FIG. 10, and 1211 is first introduced in FIG. 12.
[0077] General System Description
[0078] The invention preferably is implemented in wireless cellular
communication systems which include a base station (i.e., a
transceiver, a communications station) with a multiple antenna
array that uses smart antenna techniques for uplink or downlink
communication or both. The preferred implementation is in a system
that operates using the Personal Handyphone (PHS) air interface
communication protocol. Two implementations are one in which the
subscriber units are fixed in location, and the other in which
subscriber units may be mobile. Above-mentioned and
incorporated-herein-by-reference co-owned U.S. patent application
Ser. No. 08/729,390 describes the hardware of a base station of a
mobile system in detail, the base station preferably having four
antenna elements. While the invention is useful for mobile and
fixed subscriber unit situations, details are provided herein for
incorporating the invention into a system with fixed location
subscriber units. Wireless systems with fixed locations are
sometimes called wireless local loop (WLL) systems. A WLL base
station into which some aspects of the present invention are
incorporated is described in U.S. patent application Ser. No.
09/020,049 for POWER CONTROL WITH SIGNAL QUALITY ESTIMATION FOR
SMART ANTENNA COMMUNICATION SYSTEMS,
incorporated-herein-by-reference, while the subscriber unit for use
in such a WLL system is described in U.S. patent application Ser.
No. 08/907,594 for METHOD AND SYSTEM FOR RAPID INITIAL CONTROL
SIGNAL DETECTION IN A WIRELESS COMMUNICATI0N SYSTEM. The WLL base
station described in above-referenced U.S. patent application Ser.
No. 09/020,049 includes SDMA and may have any number of antenna
elements, and many of the simulations described herein will assume
a six-antenna array. It will be clear to those of ordinary skill in
the art that the invention may be implemented in any smart antenna
based system using any air interface with one or more than one
spatial channel(s) per conventional channel, and having mobile,
fixed, or a combination of mobile and fixed subscriber units. Such
a system may be analog or digital, and may use frequency division
multiple access (FDMA), code division multiple access (CDMA), or
time division multiple access (CDMA) techniques, the latter usually
in combination with FDMA (TDMA/FDMA).
[0079] Note that while the preferred embodiment is to apply the
invention in a wireless communication system having base stations,
each base station having subscriber units, the invention also is
applicable to peer to peer communication from one radio to another.
There is no inherent need to define the concept of a base station
or subscriber unit, and how to modify this description to
accommodate the peer-to-peer-case would be clear to one of ordinary
skill in the art. Therefore, while the invention is described as
being implemented in a communication station and a subscriber unit,
the communication station in this context may be any radio
transceiver equipped with an antenna array, and the subscriber unit
may be any other radio transceiver remote to the array-equipped
transceiver and able to communicate with the array-equipped
transceiver using some modulation scheme. While the preferred
embodiment describes a base station that has a single array for
both uplink (receive) processing and downlink (transmit)
processing, with means for adaptive smart antenna processing on the
uplink and the downlink, the invention also is applicable to a base
station that has an array only for transmit processing, and for a
base station that uses a separate antenna array for uplink
processing and for downlink processing. When only a single antenna
is used for receiving signals, the calibration factor is the
downlink signature since all received signals pass through the same
receive electronics chain. Also, the "number" of antenna is clearly
the number of "active" antennas, that is, the number of antenna
used for communication.
[0080] While the calibration is intended in the embodiments
described herein for use in adaptive smart antenna processing, the
calibration may be for any other purpose, so that the
antenna-array-equipped transceiver need not even include means for
adaptive smart antenna processing.
[0081] FIG. 1 depicts the uplink and downlink signal flow through a
typical base station (BS) on which the present invention may be
embodied. Base station 101 includes an array of antenna elements
105. The base station communicates with one or more subscriber
units such as subscriber unit 141 and subscriber unit 143. In the
preferred embodiment the base station has a single array of antenna
elements that are used for both receive and transmit, so that a
receive/transmit unit 107 is used. For frequency domain duplexing
unit 107 is a frequency duplexer and for time domain duplexing
(TDD), such as used in the preferred embodiment, unit 107 is a
switch. On the downlink, signals from the subscriber units are
received at the antenna array. Those signals 106 pass through the
switch 107 set to the receive position and these signals pass
through the receive RF electronics 109. In this description, all
the characteristics of the receive RF electronics, including all
the cables and the switch characteristics and the RF receivers, and
other receive paths, are all lumped together. The receive RF
electronic unit 109 converts the RF signals to baseband signals
110. In the preferred embodiment, receive RF electronics unit 109
includes analog RF components, including analog downconversion,
analog to digital converters, and digital downconverter components
to produce digital baseband antenna signals 110, and these baseband
received antenna signals are processed by receive signal processor
111 to generate a received signal from a particular subscriber
unit, for example subscriber unit 141. The receive signal-processor
includes determining a weighted sum of the complex valued (in phase
I and quadrature Q) antenna signals in an optimal manner where the
weighting is in amplitude and phase, and where optimal means that
the desired signal components are enhanced by a maximum amount and
the non-desired components are suppressed by a maximum amount.
[0082] The complex valued receive weights are computed by locking
onto a known training sequence, or by using some decision-directed
technique, or "blindly" by using some other special structure in
the signal. In general, it is not essential to know the phase and
amplitude relations of the receive electronics in order to perform
the computation of the uplink (i.e. receive) weights. See below and
in above-referenced co-owned U.S. patent application Ser. No.
08/729,390 filed Oct. 11, 1996 for more details on how these
weights are computed.
[0083] FIG. 1 shows the output of the receiver part of the base
station as being voice or data 113 with signals which are directed
to the Network Interface Unit (NIU). Thus, as shown in FIG. 1,
receive signal processor 111 also includes all the demodulation
function.
[0084] On the downlink the base station receives voice/data from
the NIU denoted 121 in FIG. 1. The signal is modulated according to
the system specification. A transmit signal processor 123 includes
distributing complex valued weighted copies 124 of the modulated
baseband signal (the weighting according to a set of complex valued
transmit weights), and the weighted transmit antenna signals are
fed to transmit RF electronics unit 125 to generate a set of RF
transmit signals 127, one signal aimed at each antenna element of
antenna array 105. These RF antenna signals are fed to the
corresponding antenna array element through TX/RX switch 107 which
is set in the transmit position. The transmit weights are chosen so
that the antenna array radiates most of the energy towards a
particular subscriber unit ("beam-forming") and it transmits
minimal energy toward co-channel users ("null-placing"). In the
preferred embodiment the set of transmit ts weights 118 is computed
directly from the set of receive weights 115 generated by receive
signal processor 111, and the computation is carried out by
transmit weight generator 117 in real time. However, during this
computation the transmit weight generator 117 must take into
account the gain and phase differences between the uplink and
downlink propagation channels where the channels include both the
air path from and to a subscriber unit and the variation among the
different signal parts within the receive RF electronics and also
within the transmit RF electronics. In the preferred embodiment
this information is stored in calibration storage unit 131 in the
form of a calibration vector 133 as will be described below.
Determining this calibration information is the main goal of the
present invention.
[0085] Uplink and Downlink Signal Path Descriptions
[0086] In this description, the number of elements in the base
station antenna array 105 shall be denoted by M. Thus, on the
uplink there are M signal paths from a subscriber unit, one to each
of the M inputs of the receive signal processor 111. Similarly, on
the downlink, there are M signal paths, one from each of the M
inputs of transmit signal processor 123 to the subscriber unit.
Each of these signal paths is described herein by a complex valued
number that characterizes the phase and amplitude distortion of a
baseband signal. As a compact representation, in this description,
the uplink and downlink channels thus are mathematically described
by M-dimensional complex valued vectors denoted a.sub.rx and
a.sub.tx, respectively, where M is the number of elements in the
base-station antenna array 105, and where each element in the
vector represents the path associated with one of the antenna
elements in array 105. Such a description is particularly accurate
when the differences in propagation times from (or to) a remote
subscriber unit and individual antenna elements (delay spread) are
much smaller than the symbol period for a system that uses a
digital modulation scheme, such as the system of the preferred
embodiment. The vectors a.sub.rx and a.sub.tx may be recognized as
the (un-normalized) uplink spatial signature and downlink spatial
signature, respectively, for the subscriber unit for this base
station.
[0087] Throughout the description, the uplink and downlink
signatures, and the uplink and downlink weights, will all be
described in baseband. It would be clear to those of ordinary skill
in the art that the adaptive smart antenna processing, including
any weighting in amplitude and phase, may alternatively be carried
out in some other band, for example, in intermediate frequency or
in the passband. In such a case, the signature and all its
components similarly would be defined in that frequency band.
[0088] The main goal of the invention is to calibrate the base
station. Assuming identical RF propagation on the uplink and
downlink, a single subscriber unit can be used together with its
base station to carry out the calibration. It also will be apparent
that the method enables the separate determination of the uplink
and downlink signatures for any subscriber unit. The ease with
which such data can be obtained enables one to obtain complete
signature information for any (and even every) active subscriber
unit. Therefore, in addition to calibrating the base station by
running a simple calibration experiment with one of the subscriber
units, the method enables subscriber dependent uplink and downlink
signatures to be determined for any subscriber unit, these
signatures including the effects of the electronic signal paths the
base station hardware and any differences between the uplink and
downlink electronic signal paths for the subscriber unit. One use
of such information is to determine separate calibrations for each
subscriber unit when the RF propagation to and from the subscriber
unit is different. Another use is for calibrating the base station,
but rather than obtaining a single calibration vector using the
base station and a single subscriber unit, using several subscriber
units to determine the single calibration vector. In one
embodiment, the single calibration vector is the average
calibration vector. In another embodiment, it is the weighted
average calibration vector, the weighting given to the estimate
made using a particular subscriber unit dependent on a measure of
the quality of the signal received by that subscriber unit, so that
estimates from subscriber units having better quality signals are
weighed more in the weighted average. A method and apparatus for
determining signal quality is disclosed in above referenced U.S.
patent application Ser. No. 09/020,049. The implementation of the
signal quality estimation method is now described.
[0089] Denote by N the number of samples of a burst to use for the
estimate. The sampled modulus information is first extracted by
forming the sum of the squares of the in phase and quadrature
received signals. The mean power and mean squared power are then
determined using averages over the number of samples for the
expectation operation. 1 R 2 _ = 1 N t = 1 N I 2 ( t ) + Q 2 ( t )
, and R 4 _ = 1 N t = 1 N ( I 2 ( t ) + Q 2 ( t ) ) 2 .
[0090] Note that once the instantaneous power
R.sup.2(t)=I.sup.2(t)+Q.sup.- 2(t) is determined, determining the
squared power R.sup.4(t)=[R.sup.2(t)].- sup.2 requires only a
single additional multiplication per sample, and the estimated
signal-to-interference-plus-noise-ratio (SINR) is determined as the
signal quality estimate, preferably with at most one square root
operation, using 2 SINR = 2 - R 4 _ ( R 2 _ ) 2 1 - 2 - R 4 _ ( R 2
_ ) 2 = A - A 1 - A , where A = 2 - R 4 _ ( R 2 _ ) 2
[0091] Both the ratio 3 R 4 _ ( R 2 _ ) 2
[0092] and the quantity A are sometimes called the kurtosis. This
preferred method of signal quality estimation is insensitive to
frequency offset, and so is a particularly attractive method for
use with the CM method which also is insensitive to frequency
offsets.
[0093] In alternate embodiments, the single calibration vector
estimate may be obtained using some other function of the several
determinations of calibration vectors, for example, taking from
each calibration vector only the good quality estimates of the
element, and then combining all the subsets to obtain one high
quality calibration vector.
[0094] Note that in the description below, the phase and magnitude
distortions that occur in the various signal paths are described by
the amplitude and phase, respectively of a single complex valued
number, so that a calibration for a one-to-M or M-to-one system is
described by a M-dimensional complex valued vector. For a FDMA or
FDMA/TDMA system, a different complex number may be required to
describe the phase and magnitude distortions for each carrier (each
frequency band).
[0095] Also note that often, while the electronics may be
adequately described by a simple phase and amplitude factor, the RF
propagation part within each frequency band of a carrier is not
adequately described by a complex number, but is adequately
described by a transfer function. Even in such a situation, with
reciprocity in the RF paths between the uplink and downlink, the
transfer functions cancel out when used for calibration, so that a
complex number adequately describes the calibration for one
antenna's uplink-downlink signal path, and a complex valued
M-dimensional calibration vector is adequate.
[0096] Sometimes, even the signal paths through the receiver
electronics or transmit electronics or both are not adequately
describable by complex numbers, but are describable by transfer
functions. In an alternate implementation, this is taken into
account, so each of the uplink and each of the downlink signal
paths is described by a complex valued transfer function for a
baseband signal. How to extend the implementations described herein
to take into account a set of frequencies rather than a
frequency-independent (within a carrier band) phase and amplitude
baseband signal path description would be clear to one of ordinary
skill in the art, and the scope of this invention certainly
includes such extension.
[0097] FIG. 2 shows how the uplink and downlink channel
descriptions are further mathematically decomposed into the product
of "propagation" and "electronic" factors in the following manner.
Between each base station antenna element (an element in 105) and
the antenna 205 of the subscriber unit, there is a complex valued
number that describes the phase and amplitude distortion that
occurs in a baseband signal due to the RF propagation effects on
the uplink and on the downlink. Such propagation effects include
without limitation path loss, fading and shadowing effects,
multipath, and near-field scattering. For each of the uplink and
the downlink, the M such numbers can be combined as M-dimensional
complex valued vectors. Define g.sub.rx and g.sub.tx as these
vectors for the uplink and downlink, respectively. g.sub.rx and
g.sub.tx are called the propagation factors herein. In a typical
low-mobility environment the propagation factors remain constant
over several frames (i.e., tens to hundreds of milliseconds).
[0098] Similarly, there is a complex valued number that describes
the phase and amplitude distortion that occurs in a baseband signal
due to the receive electronics between an element of the antenna
array 105 and the corresponding output terminal of receive signal
processor 111, and another complex valued number that describes the
phase and amplitude distortion that occurs in a baseband signal in
the transmit electronics chain between an input terminal of
transmit signal processor 123 and the corresponding element of the
antenna array 105. These electronics chain phase and amplitude
distortions include those that occur due to cable losses, imperfect
physical connections, variations in the gains of the various active
receive or transmit RF electronics, and group delays in the
particular components that are included in the RF electronics, for
example surface acoustic wave (SAW) filters and other components.
If the base-station hardware is stable, the electronic factors
remain constant over an extended period of time (minutes, hours or
days). There are M electronics based factors for each of the
transmit and receive electronics chains. For each direction, these
factors can be combined as an M-dimensional complex valued vector.
Define receive electronic factor vector e.sub.rx as the vector of
distortions of the M receive electronics chains, and transmit
electronics factor vector e.sub.tx as the set of distortions for
the M transmit electronics chains.
[0099] In FIG. 2 the uplink propagation factors vector g.sub.rx is
shown as 211 and the uplink electronic factors vector e.sub.rx is
shown as 215, while the downlink electronic factors vector e.sub.tx
is shown as 217 and the downlink propagation factors vector
g.sub.tx is shown as 219.
[0100] The multiplicative nature of these factors for each antenna
element for each is direction may be mathematically expressed
as
a.sub.rx=g.sub.rx{circle over (.times.)}e.sub.rx
a.sub.tx=g.sub.tx{circle over (.times.)}e.sub.tx (1)
[0101] where {circle over (.times.)} denotes the elementwise
product (i.e., the Hadamard product).
[0102] The preferred embodiment system is a frequency division
multiple access/time division multiple access (FDMA/TDMA) system in
which each conventional channel is a time slot in a frequency
channel (a frequency channel is referred to as a "carrier" herein
for FDMA/TDMA systems). In particular, time is divided into frames
of timeslots and such a frame is, shown as 301 in FIG. 3. Frame 301
of the preferred embodiment includes eight timeslots. In order,
there are four receive timeslots labeled 0 through 3 (items 305,
307, 309, and 311) followed by four transmit timeslots labeled 0
through 3 (items 315, 317, 319, and 321) in FIG. 3. Thus, in the
preferred embodiment, the uplink and downlink factors are measured
over consecutive receive and transmit slots that are separated by a
relatively short time interval. Therefore, by the principle of
reciprocity, it is reasonable to assume that the uplink and
downlink propagation factors are identical:
g.sub.rx=g.sub.tx (2)
[0103] In an FDD system the relation between the uplink and
downlink propagation factors may be more complicated, and can still
be determined.
[0104] Uplink Weight Computation
[0105] In the preferred embodiment, the uplink weights are computed
at base station 101 by receive signal processor 111. The uplink
weights are summarized by a complex valued M-dimensional complex
valued receive weight vector (also called uplink weight vector) 115
denoted by w.sub.rx herein, each element of which describes the
weighing in amplitude and phase of the baseband received signals.
The result of applying the weighting generates a baseband signal
from the particular subscriber unit. Referring to FIG. 1, the
received signals 106 from the antenna elements are digitized and
converted to baseband by receive RF electronics unit 109. FIG. 4
shows the preferred embodiment (by programming) of receive signal
processing unit 111, including receive (uplink) weights
computation. Receive signal processor 111 first performs pass-band
filtering, and compensates for frequency offset, timing offset, I/Q
mismatch, and other possible distortions. These operations are
commonly labeled as "preprocessing," and are carried out in the
preprocessor shown as 403 in FIG. 4.
[0106] In the next step the transmitted symbol sequence 411 is
estimated from the set of preprocessed received signals 405 by
using a suitable spatial processing and demodulation technique.
Referring to FIG. 4, an estimate of the signal from the particular
desired subscriber unit is determined by spatial processor 407 by
weighting in amplitude and phase by a set of receive weights
described by a receive (uplink) weight vector 115.
[0107] Note that the invention also covers replacing spatial
processor 407 with a spatio-temporal processor which includes time
equalization. With spatio-temporal processing, the weighting is
replaced by a convolution operation in the time domain, or
equivalently, multiplication in the frequency domain. The
convolution usually is finite and on sampled data, and so is
equivalent to combining the spatial processing with time
equalization using a time-domain equalizer with a finite number of
equalizer taps. That is, each of the weights in the weight vector
is replaced by a finite number of values. If the length of each
convolving function is K, then rather than determining a complex
valued M-weight vector w.sub.rx, one determines a complex valued M
by K matrix W.sub.rx.
[0108] Note that a spatial weight determining method can easily be
modified for spatio-temporal processing according to a weight
matrix by re-expressing the problem in terms of matrices and
vectors of different sizes. As throughout this description, let M
be the number of antenna elements, and N the number of samples. Let
K be the number of time equalizer taps per antenna element. A set
of received signal samples can be written as a matrix of row
vectors, each row vector representing the single samples from a
single antenna. All the signal samples can then be represented by
an (M by N) received signal to matrix. To accommodate
spatio-temporal processing, each row vector of N samples of the (M
by N) received signal matrix can be rewritten as K rows of shifted
versions of the first row to produce a received signal matrix of
size (MK by N), which when pre-multiplied by the Hermitian
transpose (i.e., complex conjugate transpose) of a weight vector of
size (MK by 1) produces an estimated received signal row vector of
N samples. The spatio-temporal problem has thus been re-expressed
as a weight vector determining problem. For example, for covariance
based methods, the weight vector is a "long" weight vector of size
(MK by 1). Rearranging terms in the "long" weight vector provides
the required (M by K) weight matrix. Therefore, while the
description herein is in terms of weights and spatial processing,
the scope is intended to include spatio-temporal processing.
[0109] Referring again to FIG. 4 and processor 407, at first, an
estimate of the uplink weight vector 115 is used, for example the
value from the previous frame. The signal estimate 408 is then
demodulated by demodulator and reference signal generator 411 to
generate the estimate of the transmitted symbol sequence 412 which
then is further processed by higher level processing unit 413 to
generate the voice or data signal 113 that is sent to the Network
Interface Unit (not shown). In addition to producing the symbol
sequence 412, demodulation and reference signal generator 411 also
produces a reference signal 410 which is a modulated signal that is
modulated by the estimated symbols and that has a correct signal
structure according to the particular modulation protocol used.
This reference signal, together with the preprocessed receive
signal set 405, is used by weight vector generator 409 to generated
a better estimate of the receive weight vector 115. Weight vector
generator 409 implements an optimization method that determines the
weight vector that minimizes an objective function of weight
vectors; the objective function including a measure of the
deviation of the signal generated through a signal copy spatial
processing operation using the weight vector to the reference
signal 410. In the preferred embodiment, the objective function
also includes a term to limit the magnitude of the weight vector.
The next estimate of the weight vector obtained from weight vector
generator 409 can then be used by signal copy operation 407 and
also may be used by transmit weight generator 117. For more details
on the structure of the base station on which the method of the
present invention is preferably implemented, see above referenced
U.S. patent application Ser. No. 09/020,049. For further details of
the uplink weight vector computation, see above-referenced U.S.
patent application Ser. No. 08/729,390 and U.S. patent application
Ser. No. 09/153,110 for METHOD FOR REFERENCE SIGNAL. GENERATION IN
THE PRESENCE OF FREQUENCY OFFSETS IN A COMMUNICATIONS STATION WITH
SPATIAL PROCESSING.
[0110] Downlink Weight Computation
[0111] The downlink weights 118 may be expressed as an
M-dimensional complex valued vector of weights w.sub.tx (called the
transmit weight vector, also the downlink weight vector). In the
preferred embodiment, the downlink weights are computed directly
from the uplink weights. The symmetry of the uplink and downlink
signal paths is used. This symmetry, illustrated in FIGS. 5A
(uplink) and 5B (downlink), may be expressed as follows:
[0112] 1. The impulse response of the scalar "channel" (in
baseband) between the modulated baseband signal (shown as 503)
transmitted by the subscriber unit and the post-spatial processing
(i.e., demultiplexed) signal (for example, is referring to FIG. 4,
the reference signal 410) is substantially the same as the opposite
direction impulse response from the pre-spatial processing scalar
baseband signal 507 transmitted from the base station to the
received baseband signal 509 at the subscriber unit Mathematically,
this symmetry may be stated as the uplink and downlink weight
vectors substantially satisfying the equation
w.sub.rxa.sub.rx=w.sub.tx*a.sub.tx. (3)
[0113] 2. For receiving from and transmitting to the same
subscriber unit (assuming the subscriber unit uses the same antenna
for receive and transmit), the beam pattern of the antenna array on
the uplink and the downlink should be substantially identical. In
the case that the reciprocity condition (g.sub.rx=g.sub.tx)
substantially holds, this means that the weight vectors should
substantially satisfy
w.sub.rx{circle over (.times.)}e.sub.rx=w.sub.tx{circle over
(.times.)}e.sub.tx, (4)
[0114] where {circle over (.times.)} denotes the elementwise
product (i.e., the Hadamard product). Note that in general the beam
pattern of the antenna array depends on the weight vectors, as well
as on the transfer functions of the RF electronics.
[0115] Eq. (3) has many solutions for w.sub.tx while Eq. (4) has
only one solution:
w.sub.tx=w.sub.rx{circle over (.times.)}e.sub.rx.O
slashed.e.sub.tx, (5)
[0116] where .O slashed. denotes elementwise division.
Consequently, the main equation that governs the transmit weight
generation is given by
w.sub.tx=w.sub.rx{circle over (.times.)}c, (6)
[0117] where the calibration vector 133 (denoted by c) is defined
as
c=e.sub.rx.O slashed.e.sub.tx. (7)
[0118] The internal structure of the transmit weight generator 117
is depicted in FIG. 6. To generate an element of transmit weight
vector 118, the corresponding element of calibration vector 133 is
multiplied by the corresponding element of the receive weight
vector 115 using elementwise multiplication process 603.
[0119] The Calibration Process
[0120] The main purpose of the calibration process is to determine
calibration vector 133 for a base station and one of its subscriber
units which supports the calibration procedure. No additional
calibration equipment such as a transponder, signal generator, or
measuring network is needed. In a typical TDD system the
calibration process consists of the following steps:
[0121] 1. Establish a connection with a suitable subscriber
unit;
[0122] 2. Estimate the uplink channel spatial signature
a.sub.rx;
[0123] 3. Estimate the downlink channel spatial signature
a.sub.tx;
[0124] 4. Assuming reciprocity, compute the calibration vector 113
as
c=a.sub.rx.O slashed.a.sub.tx=e.sub.rx.O slashed.e.sub.tx; (8)
[0125] 5. Terminate the connection with the subscriber unit.
[0126] Clearly in order to determines calibration functions, one
need not explicitly display or store uplink and downlink signatures
(steps 2 and 3 above) and one may instead proceed directly to step
4 of computing the calibration function from intermediate
quantities related to the uplink and downlink signatures. For the
purposes of this invention the computation of the calibration
function from such intermediate quantities is equivalent to
computing the calibration function from uplink and downlink
signatures.
[0127] In the current WLL system in which the preferred embodiment
is implemented, each subscriber unit is able to support the
calibration method. Nevertheless, to maximize the signal to noise
ratio, it is generally recommended to choose a subscriber unit that
is close to the base station. Calibration calls can be initiated on
any carrier and any time slot while the base station is servicing
standard traffic channel (TCH) calls on other carriers and time
slots.
[0128] Note that while the description herein is for the
calibration to occur by the base station communicating with a
subscriber unit, the scope clearly includes the base station
communicating with a special purpose transceiver that performs the
functions described herein, while not necessarily performing any
other functions, for example the typical functions a typical
subscriber unit performs. For example, one can use a subset of the
hardware and software included in a subscriber unit to carry out
the calibration.
[0129] Note that the preferred embodiment uses a system in which
communication occurs burst-by-burst. Hence, the description herein
uses the term "burst" and used terms such as traffic bursts,
calibration bursts, etc. The invention certainly is not limited to
burst-by-burst systems. The general equivalent term to "burst"
applicable to both burst-by-burst and non burst-by-burst systems
used herein is "waveform", and therefore a t "calibration
waveforms" is a calibration burst for a busts-by-burst system, a
"traffic waveforms" is a traffic (or TCH) burst for a
busts-by-burst system, and so forth.
[0130] FIG. 7 shows a typical protocol which includes a calibration
call according to aspects of this invention. Different protocols
may be designed for other implementations. The sequence order is
from top to bottom. The direction of arrows shows the direction of
communication. The protocol starts with a standard call-setup
procedure 703 which includes a paging call 711 from the base
station to the subscriber unit, a link channel request 713 from the
subscriber unit to the base station, resulting in link channel
assignment sent to the subscriber in step 715. Synchronization
("SYNCH") bursts are then sent on the uplink (717) then on the
downlink (719). Finally, in step 721, the page response is sent to
the base station. For the calibration burst phase 705 of the
protocol, the subscriber unit transmits a first uplink calibration
burst or bursts (723) so that the base station can estimate the
uplink channel. Immediately after this, in step 725, the base
station transmits a first downlink calibration burst (or bursts) so
that the subscriber unit can estimate the downlink channel.
[0131] Note that in the preferred embodiment, the calibration
bursts are calibration waveforms that conform to the particular air
interface standard, in this case, the PHS standard. By "conforming
to an air interface standard" we mean conforming to the channel
structure and modulation of an air interface, where "channel
structure" is a frequency slot in the case of FDMA, a time and
frequency slot in the case of TDMA, or a code channel in the case
of CDMA, and "modulation" is, for example, .pi./4-DQPSK in the case
of PHS, or GMSK in the case of GSM, and so forth. In the two-tone
and multi-tone calibration methods described herein under, the
calibration waveform consists of a sum of two or more waveforms
each conforming to the PHS air interface standard. As such sums
occur naturally in a multiuser communication system with frequency
reuse, a sum of waveforms conforming to an air interface standard
is also considered to conform to an air interface standard for the
purpose of this description.
[0132] While one implementation would be to calibrate the whole
antenna array at once, in the preferred embodiment, one considers
not the whole array of M antenna elements, but subarrays of the
array, each of less than M elements, and calibrates each subarray
independently. In this preferred implementation, one or more
additional uplink calibration bursts and one or more additional
downlink calibration bursts may needed, each for each additional
subarray, and these additional steps are shown as dotted lines 727
and 729, respectively in FIG. 7. Note that while only one downlink
and one uplink additional step is shown dotted, it is to be
understood that this represents as many additional bursts as there
are additional subarrays to be calibrated.
[0133] In the particular implementation, the antennas are
calibrated pairwise with each antenna calibrated with respect to a
fixed reference antenna. Thus, the M-element antenna array is
viewed as a collection of 2-element subarrays and there are M-1
bursts used to calibrate in each direction (steps 727 and 729 each
carried out M-2 times). FIG. 8 shows a circular arrangement of 6
antennas 801, 802, 803, 805, 807, and 809, with antenna 801
arbitrarily chosen as the fixed reference antenna. The subarrays
are shown as the antennas within the dotted line areas. The five
subarrays are: subarray #1 (811) of antennas 801 and 802, subarray
#2 (813) of antennas 801 and 803, subarray #3 (815) of antennas 801
and 805, subarray #4 (817) of antennas 801 and 807, and subarray #5
(819) of antennas 801 and 809.
[0134] In the preferred embodiment, the subscriber unit has some
intelligent signal processing capabilities which allow it to
analyze the downlink calibration burst or bursts. In general, some
of the downlink channel estimation can then be carried out by the
remote subscriber unit, this part of the signature estimation
determining partial results, called "downlink signature related
signals" herein. In the preferred embodiment, the subscriber unit
has sufficient processing power to completely compute the downlink
channel estimate, and in this case, the downlink signature related
signals are the downlink channel estimate components. These results
(whether complete or partial estimates--in general, downlink
signature related signals) are sent back to the base station by
using some standard messaging protocol, including without
limitation SACCH, FACCH, TCH payload as described in the PHS
protocol. The PHS protocols are incorporated herein by reference.
The PHS standard is described, for example, in the Association of
Radio Industries and Businesses (ARIB, Japan) Preliminary Standard,
Version 2, RCR STD-28 and variations are described in Technical
Standards of the PHS Memorandum of Understanding Group (PHS
MoU--see http://www.phsmou.or.jp). This sending is shown as step
731 for the first downlink calibration burst and as dotted line 733
for those implementations that use additional bursts, for example
for the remaining subarrays. Other relevant information (e.g.,
signal quality estimates or the raw I/Q samples) can also be
transmitted back to the base station from the subscriber unit for
use in power control and for other analyses and purposes. See above
referenced U.S. patent application Ser. No. 09/020,049 for a
description of the power control and signal quality to estimation
aspects of a subscriber unit.
[0135] At the end of the calibration process, the base station
computes the calibration vector and terminates the calibration
call. The call termination 709 preferably includes a disconnect
command 735 from the base station followed by a release message 737
from the subscriber unit.
[0136] Uplink Signature Estimation
[0137] In the preferred embodiment, uplink signature estimation
occurs at an active subscriber unit in the vicinity of the base
station. After the service channel is established, the subscriber
unit transmits an uplink calibration burst towards the base
station. In our particular implementation, the uplink calibration
bursts are idle (no-payload) TCH bursts. In alternate embodiments,
other sequences can be used, and how to modify the method to use
other sequences would be clear to one of ordinary skill in the art.
For example, in another embodiment, downlink signature estimation
is carried out first. The downlink signature related signals
computed at the subscriber unit, which preferably are the signature
estimates, are then transmitted to the base station. These signals
are then used to estimate the uplink-signature.
[0138] FIG. 9 describes the elements for determining the uplink
signature a.sub.rx. In the preferred embodiment, subscriber unit
(e.g., unit 141) includes an uplink calibration burst synthesizer
907 implemented as a set of programming instructions on a signal
processor. Synthesizer 907 includes a memory (part of the already
present signal processor memory), and generates the first
calibration burst (in step 723) or the second calibration burst (in
step 727). The burst is transmitted from the subscriber unit
antenna 911 using the subscriber unit's transmit RF electronics
909. The architecture of the preferred embodiment subscriber unit
is described in above referenced U.S. patent application Ser. No.
08/907,594 and in FIG. 12. Referring to FIG. 12, time duplexer 1203
is in the transmit position during transmission and connects the
output of transmit RPF electronics 909 to antenna 911. Normal
traffic burst signals are obtained from telephony interface unit
1213 via a vocoder DSP 1209. The complex valued (I, Q) samples are
formed in a DSP device (TX DSP 1211) which is connected to a memory
1207 shared with another DSP device, the RX DSP 1205 used for
signal reception. For the uplink channel determination
implementation described herein, TX DSP 1211 is programmed to carry
out the function of uplink calibration burst synthesizer 907 in
addition to its normal transmit signal processing functions. The
uplink calibration bursts are received by the base station antenna
array 105 and converted to the baseband signals 110 by the receive
RF electronics 109, as shown in FIG. 9. The signals from the
antenna elements are then processed by the receive signal processor
111 which is made up of one or more digital signal processing
devices (DSPs) programmed to carry out the functions of the
elements 403, 921, and 931. Pre-processor 403 carries out
pre-processing which includes baseband filtering, and removing the
frequency offset, the timing offset, and the I/Q mismatch from the
received signal. In some implementations, baseband equalization may
also be included in the pre-processor 403 if necessary, and how to
so include equalization and would be clear to those skilled in the
art and is not the main concern of the invention. Unit 921 includes
units 407 and 411 and estimates the transmitted symbol sequence (a
reference signal) by carrying out the signal copy operation,
demodulation and reference signal generation. In the preferred
embodiment, the subscriber unit transmits standard TCH bursts, and
therefore the default TCH demodulation method of the base station
can be used for this purpose. In an alternate embodiment, the
subscriber unit transmits a pre-defined calibration sequence that
is explicitly known, and thus may be pre-stored at the base
station. In this case, it is not necessary to demodulate the
received signal. This alternate is shown in dotted lines in FIG. 9,
where the pre-defined burst segments 923 are used instead of the
transmitted signal estimate 410. Channel identification unit 931
uses the transmitted signal estimate 410 and received signals 405,
which are the input and output signals respectively, of the uplink
channel, 933 to estimate the underlying spatial signature 933. Any
standard system identification technique may be used in channel
identification unit 931. The following method is used in the
preferred embodiment. N samples of the received signals 405 and the
transmitted signal estimate 410 are used. In the preferred
embodiment, N=50. That is just 50 samples of the burst are used.
Denote by k the time index of the N samples, where k=0, 1, . . . ,
N-1, by x(k) vector of received signals 405 at time k, and by s(k)
transmitted signal estimate 410 at time k. The estimate of the
uplink channel signature is obtained as
.sub.rx=XS*(SS*).sup.-1 (9)
[0139] where matrix X=[x(0) x(1) . . . x(N-1)] and vector S=[s(0)
s(1) . . . s(N-1)]. Those skilled in the art may recognize this as
the maximum likelihood estimate of the channel signature for
modeling the received signals by
x(k)=a.sub.rxs(k)+v(k), k=0,1, . . . , N-1 (10)
[0140] where v(k) denotes a vector of additive noise at time k, the
noise vector being a vector of statistically independent,
identically distributed Gaussian random processes with a mean
E{v(k)}=0 and covariance matrix E{v(k)v(k)*}=.sigma..sub.v.sup.2I,
where I is the identity matrix. This part of the invention however
does not depend on any modeling assumptions. In alternate
embodiments, more or less sophisticated standard system
identification techniques may be used in place of Eq. (9). The book
by Lyung, L., System Identification: Theory for the User,
Englewood-Cliffs, N.J.: Prentice-Hall, 1987 is a good source for
many alternate system identification methods that may be adapted
for use in the present invention. Note also that the solution of
Eq. (9) and equivalent solutions are sometimes referred to herein
as the maximum likelihood estimates, even when the received signal
model and other conditions for the maximum likelihood are not met,
and it is to be understood that the term "maximum likelihood
estimate" means the solution that would be maximum likelihood when
the appropriate linear signal model and noise conditions hold. For
example, applying Eq. (11) or equivalent would fall under "maximum
likelihood estimate" for any transmitted S and received X using any
or no model with any kind of noise present.
[0141] Downlink Signature Estimation
[0142] In order to estimate the downlink channel, the base station
101 transmits one or more downlink calibration bursts towards
subscriber unit 141. FIG. 10 describes the elements for determining
the downlink signature a.sub.tx. In the preferred embodiment,
transmit signal processor 123 in base station 101 is programmed as
a downlink calibration burst synthesizer 1005 to generate the
downlink calibration burst (the first burst of step 725 or the
second burst of step 727 depending on the number of bursts used in
the embodiment of the method, and the step in that embodiment).
Such a burst preferably is generated by recalling the burst from a
memory in base station 101. The bursts arm transmitted to
subscriber unit 141 by using transmit signal processor 123 for the
required spatial processing (shown in FIG. 10 as part of unit 1005)
and then transmitting through the transmit RF electronics 125 and
antenna array 105.
[0143] The bursts are received in the subscriber unit (e.g., unit
141) on antenna 911 via subscriber unit receive electronics 1009.
Referring again to FIG. 12, the preferred embodiment subscriber
unit includes RX DSP 1205 which for this implementation is
programmed as a pre-processor 1011 to generate a sampled received
signal 1012 denoted y(k) where k is used as a time index, and also
programmed as a downlink channel identification processor 1013
which determines the downlink cannel signature using the received
signal 1012 and a stored version 1019 of the set of transmitted
signals denoted by M-vector z(k). The stored version 1019 is stored
in a buffer formed in memory 1207. The subscriber unit then
transmits the result back to the base station.
[0144] In the particular embodiment, the signals are modulated
using .lambda./4 DQPSK and have a baud rate of 192 kbaud per sec.
The received signal y(k) is four times oversampled. When used for
two-tone calibration (see below), the transmitted calibration
waveforms are appropriately modulated sine waves, and in the
preferred embodiment, to preserve memory, only a single period of
each sine wave is stored in memory 1207, that section of memory
1207 configured as a circular buffer. The data then is repeatedly
read out as a sequence of periods.
[0145] A typical subscriber unit usually has at most a few antennas
(one antenna 911 in the WLL system on which the invention
preferably is implemented), which limits the information that is
available for downlink signature estimation. Also, the hardware for
a typical subscriber unit is simple because f size and cost
constraints and therefore less capable of sophisticated, accurate
processing than a typical base station's hardware. As a result, the
received signal at the subscriber unit may have significant
distortions including, without limitation, frequency and timing
offset effects, and phase noise that may reduce the accuracy of the
downlink channel estimate compared, for example to those of the
uplink estimate. In the future, it is anticipated that more signal
processing (or other computing) power will be available in average
subscriber units to enable these distortions to be corrected in
preprocessor 1011. However, our invention also works so when less
signal processing power is available.
[0146] In an improved embodiment, the base station uses
specifically designed signal sequences that are robust with respect
to effects that include, without limitation, frequency offset,
timing offset, I/Q mismatch, and phase noise. This enables accurate
results to be obtained using even simple inexpensive subscriber
units with some, but limited, signal processing capability. For
example, the downlink calibration burst may consist of pure tones.
This enables RX DSP 1205 programmed as preprocessor 1011 in the
subscriber unit to carry out frequency offset and timing alignment
estimation with little computational effort. Alternatively, the
downlink calibration burst can be synthesized from pseudo-random
signal sequences or chirp (swept frequency) signal sequences which
make it possible to characterize the propagation channel across a
wider range of frequencies.
[0147] Let row vector z(k)=[z.sub.1(k) z.sub.2(k) . . .
z.sub.M(k)], k=0, 1, . . . , N-1 denote the N samples (in baseband)
of M modulated baseband signals z.sub.1(k), z.sub.2(k), . . . ,
z.sub.M(k) that are transmitted from base station 101 from a
calibration burst. Let y(k) k=0, 1, . . . , N-1 denote the N
samples of the received signal (in baseband and after the
preprocessing of 1011) at the subscriber unit. Define vector y and
matrix Z as 4 y = [ y ( 0 ) y ( 1 ) y ( N - 1 ) ] Z = [ z 1 ( 0 ) z
2 ( 0 ) z M ( 0 ) z 1 ( 1 ) z 2 ( 1 ) z M ( 1 ) z 1 ( N - 1 ) z 2 (
N - 1 ) z M ( N - 1 ) ] ,
[0148] respectively. The downlink signature estimate 1017 is
preferably determined In identification processor 1013 according
to
.sub.tx=(Z*Z).sup.-1Z*Y. (11)
[0149] Those skilled in the art may recognize that this is the
maximum likelihood estimate of the downlink signature when the
received signal samples 1012 conform to the model (in baseband)
that
y(k)=z(k)a.sub.tx+n(k), k=0, 1, . . . , N-1 (12)
[0150] where the n(k), K=0, . . . , N-1 denote some additive noise
in the received signal, modeled as N statistically independent,
identically distributed Gaussian random variables. Note to that
this invention does not depend on the received signal samples
conforming to such a model. Note also that the solution of Eq. (11)
and equivalent solutions are sometimes referred to herein as the
maximum likelihood estimates, even when the received signal model
and other conditions for the maximum likelihood are not met, and it
is to be understood that the term "maximum likelihood estimate"
means the solution that would be the maximum likelihood solution
when the appropriate linear signal model and noise conditions hold.
For example, applying Eq. (11) or equivalent would fall under the
term "maximum likelihood estimate" for any transmitted Z and
received Y using any or no model with any kind of noise
present.
[0151] Denoting the noise samples as a vector 5 n = [ n ( 0 ) n ( 1
) n ( N - 1 ) ] ,
[0152] Eq. (12) can then be expressed as
y=Za.sub.tx+n. (13).
[0153] Note that the signature 1017 can be determined according to
Eq. (11) only if Z has linearly independent columns. For this, each
antenna element of the calibrated array (or subarray) transmits M
(or fewer in the case of a subarray) substantially "linearly
independent" signals from the M (or fewer) antenna elements during
downlink calibration. M transmitted signals z.sub.i(k) are linearly
independent if it is impossible to rind constant complex valued
parameters c.sub.1, c.sub.2, . . . , c.sub.M so that 6 i = 1 M c i
z i ( k ) = 0
[0154] for k=0, 2, . . . , N-1. In practice, this requirement can
be fulfilled in various different ways. In one embodiment, the
calibration burst can be divided into segments so that only one
antenna element is active at any given time (orthogonality in the
time domain). Alternatively, the antenna elements can transmit pure
tones with different frequencies (orthogonality in the frequency
domain). Linearly independent signals can also be synthesized from
pseudo-random signal sequences or chirp signal sequences. Other
techniques would be apparent to those of ordinary skill in the
art.
[0155] Two-Tone Downlink Calibration
[0156] In the preferred embodiment, the antenna array is
partitioned into 2-element subarrays with a common reference
element, as shown in FIG. 8, and each subarray is calibrated
independently. In one embodiment, during calibration each antenna
element of a particular subarray transmits a complex valued sine
wave at a different frequency. Denote by .omega..sub.1 and
.omega..sub.2 (in radians per second) (he frequencies of the first
calibration signal through the first antenna element of a
particular subarray and the second calibration signal through the
second antenna element of a particular subarray, respectively. In
this case, the value of M is 2 and the downlink channel estimate
according to Eq. (11) is 7 [ a ^ 1 a ^ 2 ] = [ N j N T - 1 j T - 1
- j N T - 1 - j T - 1 N ] - 1 [ k = 0 N - 1 y ( k ) - j 1 k T k = 0
N - 1 y ( k ) - j 2 k T ] ( 14 )
[0157] where T denotes the sampling period for the signals and
.DELTA..omega.=.omega..sub.2-.omega..sub.1 denotes the frequency
separation between the tones. If N is chosen so that the
observation interval NT is an integer multiple of
2.pi./.DELTA..omega., then e.sup.j.DELTA..omega.NT=1, and we obtain
the simple formulas 8 a ^ 1 = 1 N k = 0 N - 1 y ( k ) - j 1 k T , (
15 a ) a ^ 2 = 1 N k = 0 N - 1 y ( k ) - j 2 k T . ( 15 b )
[0158] One will recognize these as the discrete Fourier transform
(DFT or its rapid implementation, the FFT) of the received signal
at .omega..sub.1 and .omega..sub.2, respectively. One also will
recognize these as proportional to the cross-correlations of the
received subscriber unit signal y with the two calibration bursts,
respectively. Clearly, in implementation, the 1/N factors are not
included in determining the signatures.
[0159] The relative downlink signature for one of the antenna
elements, say the second antenna element, with the first antenna
element as the reference, is computed as the second cross
correlation divided by the first cross correlation.
[0160] In the preferred embodiment implementation, RX DSP 1205 is
programmed as downlink channel identification processor 1013.
Received signal samples y(k) are four times oversampled 192 kbaud
per sec. signals. That is, there are 784 ksamples per second. The
two frequencies used are 24 kHz (divided by 2.pi. for
kradians/sec.) and -72 kHz (recall that the calibration signals are
complex valued). In general, the larger the frequency difference
.DELTA..omega.=.omega..sub.2-.omega..sub.1, the better the
performance. In the preferred implementation, signals are
synthesized by providing particular bit patterns to the .pi./4
DQPSK modulator (the standard for PHS). This enables the tones to
be easily synthesized. However, the .pi./4 DQPSK modulation and the
particular baud rate means that effectively only signals with
frequencies of +72 kHz, +24 kHz, -24 kHz and -72 kHz may be
synthesized. While the greatest separation would be obtained with
the tone pair being at +72 kHz and -72 kHz, the 72 kHz signals
appear less like pure tones than the 24 kHz signals, so the two
tones used in the preferred embodiment are +24 kHz and -72 kHz.
That this performs better than using +24 kHz and -24 kHz tones is
discussed in the "Performance" section herein below. The DSP
program implementing channel identification processor 1013 may be
summarized as follows:
Two-Tone Downlink Procedure
[0161] INPUTS: subscriber received sequence y(0), y(1), . . . ,
y(N-1).
[0162] OUTPUT: The estimated downlink channel in the form 9 [ 1 C ]
.
[0163] 1. Cross-correlate the received sequence with the first
calibration sequence (tone at frequency .omega..sub.1): 10 A = k =
0 N - 1 y ( k ) - j 1 k T .
[0164] 2. Cross-correlate the received sequence with the second
calibration sequence (tone at frequency .omega..sub.2): 11 B = k =
0 N - 1 y ( k ) - j 2 kT .
[0165] 3. Compute the desired quantity C=B/A.
[0166] Note that alternate implementations may use different
methods for synthesizing the tone signals that do not include the
limitations of what tones are available, such methods possibly
requiring more complex implementation, or may use different
orthogonal signals.
[0167] The method using tone calibration bursts is robust with
respect to phase noise and frequency offset for frequency offsets
and phase noises that are small compared to the frequency
difference .DELTA..omega..
[0168] When large timing offsets are present, an improved
embodiment of the two-tone method allows such a timing offset to be
determined and the quantities corrected for the timing offset. Let
.tau. denote the constant time by which the transmitted signal is
delayed. In this improved embodiment, the calibration bursts are
broken up into two time segments, with the break point the same for
the two bursts. During the first time segment, a sum of the first
and second sine waves is transmitted from the same antenna element,
say the first antenna element. Let there be N.sub.1 samples during
the first time segment and denote the received signal at the
subscriber unit by y.sub.1(k), k=0, . . . , N.sub.1-1. Assuming
that the first segment observation interval N.sub.1T is an integer
multiple of 2.pi./.DELTA..omega., an estimate for the timing offset
is determined from the ratio of the cross correlation of the
subscriber unit received signal with the second correlation burst
to the cross correlation of the subscriber unit received signal
with the first correlation burst: 12 j = k = 0 N 1 - 1 y 1 ( k ) -
j 2 k T k = 0 N 1 - 1 y 1 ( k ) - j 1 k T . ( 16 )
[0169] On the second segment of the calibration bursts, the two
sine waves are transmitted via two different antennas as in the
previously described embodiment of the two-tone method. Let there
be N.sub.2 samples during the second time segment and denote the
received signal at the subscriber unit by y.sub.2(k), k=0, . . . ,
N.sub.2-1. If N.sub.2 is chosen so that the observation interval
N.sub.2T is an integer multiple of 2.pi./.DELTA..omega., then 13 k
= 0 N 2 - 1 y 2 ( k ) - j 2 k T k = 0 N 2 - 1 y 2 ( k ) - j 1 k T =
a ^ 2 a ^ 1 - j ( 17 )
[0170] Combining Eqs. (16) and (17) leads to the desired ratio of
the two downlink signature estimates. For simplicity, the two
segments are made of equal length, N.sub.1=N.sub.2. As in the first
two-tone embodiment, the two frequencies used are 24 kHz and -72
kHz (recall that the calibration signals are complex valued). The
DSP program for RX DSP 1205 implementing channel identification
processor 1013 according to the second implementation that includes
correcting for the timing offset may be summarized as follows.
Improved Two-Tone Downlink Procedure
[0171] INPUTS: received sequence y(0), y(1), . . . , y(N), . . . ,
y(2N-1).
[0172] OUTPUT: The estimated downlink channel in the form 14 [ 1 C
] .
[0173] 1. Cross-correlate the first half of the received sequence
with the first half of calibration sequence #1: 15 A1 = k = 0 N - 1
y ( k ) - j 1 k T .
[0174] 2. Cross-correlate the first half of the received sequence
with the first half of calibration sequence #2: 16 B1 = k = 0 N - 1
y ( k ) - j 2 k T .
[0175] 3. Compute C1=B1/A1.
[0176] 4. Cross-correlate the second half of the received sequence
with the second half of calibration sequence #1: 17 A2 = k = N 2 N
- 1 y ( k ) - j 1 k T .
[0177] 5. Cross-correlate the second half of the received sequence
with the second half of calibration sequence #2: 18 B2 = k = N 2 N
- 1 y ( k ) - j 2 k T .
[0178] 6. Compute C2=B2/A2.
[0179] 7. Compute the desired quantity C=C2/C1.
[0180] It would be clear to those of ordinary skill in the art that
various modifications may be made to the methods, including without
limitation; using segments of unequal length, using two sets of two
tone signals (separated by a known amount), and transmitting
different combinations. Different formulas also may be used to
determine the calibration factors.
[0181] It is advantageous to use any two constant modulus signals
whose dot product is a pure tone. Alternatively, one might, for
example, use a tone for the first segment and a chirp signal
sequence for the second.
[0182] One also may generalize the method to deal with more than
two antennas at a time. The following alternative method works for
any number of M antennas. In the first segment (say the first half)
of the segment, the sum of M different single tone signals, each of
the M tones being distinct, is transmitted from the first (say the
reference) antenna element, while no signal is transmitted from the
other antenna elements. In the second segment, a different one of
the M single tone signals is transmitted from the M antenna
elements. The method then proceeds as follows to estimate the
M-antenna element array (or subarray). The notation used is that
the first half correlations are denoted by A.sub.i with the
subscript i denoting which tone the received signal was correlated
with, while the second half correlations are denoted by B.sub.i
with the subscript i denoting which tone the received signal was
correlated with. The M pure tone signals have frequencies denoted
by .omega..sub.1, .omega..sub.2, . . . , .omega..sub.M,
respectively.
Improved M-Tone Downlink Procedure
[0183] INPUTS: received sequence y(0), y(1), . . . , y(N), . . . ,
y(2N-1).
[0184] OUTPUT: The estimated downlink channel in the form 19 [ 1 C
2 C M ] .
[0185] 1. Cross-correlate the first half of the received
sequence.
[0186] With the first half of each calibration sequence to obtain M
correlations A.sub.1, A.sub.2, . . . , A.sub.M, respectively.
[0187] 2. Normalize with respect to the first correlation A.sub.1
corresponding to the reference antenna element to obtain M numbers
1, A.sub.2/A.sub.1, . . . , A.sub.M/A.sub.1, respectively.
[0188] 3. Cross-correlate the second half of the received sequence
with the second half of each of the M calibration sequences
sequence to obtain M correlations B.sub.1, B.sub.2, . . . ,
B.sub.M, respectively.
[0189] 4. Normalize with respect to the first correlation B.sub.1
corresponding to the reference antenna element to obtain M numbers
1, B.sub.2/B.sub.1, . . . B.sub.M/B.sub.1, respectively.
[0190] 5. Compute the M signature elements as 1,
[(B.sub.2/B.sub.1)/(A.sub- .2/A.sub.1)], . . . ,
[(B.sub.M/B.sub.1)/(A.sub.M/A.sub.1)], respectively.
[0191] The above generalization for determining the signature for M
elements simultaneously can be modified to avoid transmitting the
sum of all the M tones on one antenna element in the first segment.
In general, one can assume that the timing offset is the same for
transmissions from all the antenna elements of a base station. In
the system in which the embodiments described herein is
implemented, all the ADCs and all the downconversions and
upconversions are synchronized. In such a case, for example, only
the sum of the tone transmitted from the reference antenna element
and one other antenna tone (e.g., the second) are transmitted from
the first element in the first segment. How to modify the above
generalization in this and many other ways would be clear to one of
ordinary skill in the art.
[0192] Note that while the above discussion mentions canceling out
timing offsets, the dividing of the factors also cancels out any
phase offsets.
[0193] Timing Offset Determination.
[0194] The above discussion also suggests how sending multiple
signals, for example, pure tone signals, can be used to determine
the timing offset in the subscriber unit with very little
computation.
[0195] To determine timing offset, one carries out steps 1, 2 and 3
of the "Improved Two-Tone-Downlink Method" above. In step 3, the
quantity C1 is essentially exp-j(.omega..sub.2-.omega..sub.1).tau..
Thus, taking logarithms and diving by
.DELTA..omega.=(.omega..sub.2-.omega..sub.1) gives an estimate of
the timing offset .tau..
[0196] In an improved timing offset method, one carries steps 1 and
2 of the "Improved M-Tone Downlink Method" above. In step 2, the
quantities 1, A.sub.2/A.sub.1, . . . , A.sub.M/A.sub.1,
respectively, give the M quantities 1,
exp-j(.omega..sub.2-.omega..sub.1).tau., . . . ,
exp-j(.omega..sub.M-.omega..sub.1).tau., respectively. Taking
logarithms of the last M-1 quantities and dividing the first of
these by (.omega..sub.2-.omega..sub.1), the second by
(.omega..sub.3-.omega..sub.1- ), . . . , and the last by
(.omega..sub.M-.omega..sub.1), respectively, gives M-1 estimates of
the timing offset .tau.. These may be averaged to give a final
estimate of the timing offset
[0197] Calibration During Standard Traffic Channel Calls
[0198] In yet another alternate embodiment, instead of using
dedicated calibration calls, it is possible to embed the
calibration procedure into standard telephone calls in both
directions which are used for normal traffic functions. Normal
traffic functions depend on the air interface, and may include
demodulation, timing and frequency tracking, and various control
functions such as power control and handoff. For example, the
uplink channel signature can be estimated from standard uplink
traffic channel (TCH) bursts by using a decision directed technique
as described above. The downlink channel estimation method
described above is modified as follows:
[0199] On the downlink, the base station transmits a mixture of TCH
bursts and calibration bursts towards the subscriber unit in a
random fashion. That is, the calibration bursts are interspersed
with the TCH bursts. Because calibration bursts may cause audible
errors to occur, it is preferable to send such calibration bursts
infrequently and during silent periods. A typical silent period is
longer than a burst, so in an improved embodiment, calibration
bursts are sent (instead of TCH bursts) only after a number of idle
bursts are sent by the base station.
[0200] An illustrative embodiment of processing by the subscriber
unit which includes estimating the downlink channel signature is
shown in FIG. 11. In step 1105 the subscriber unit acquires the raw
burst and first preprocesses the burst in the receive signal
processor programmed as preprocessor 1011. This received
preprocessed signal is stored. The preprocessed signal next is
demodulated in step 1109 as would be a standard TCH burst. In step
1111, it is determined whether or not the demodulated bits are for
a standard TCH burst. As in most standard protocols, the PHS
protocol used in the system of the illustrative embodiment includes
some method to determine when a sequence is correctly received, for
example, the presence of a particular pre-defined bit-sequence. In
the PHS standard, there is such a 32-bit "Unique Word" sequence,
which is prearranged and known to every subscriber unit. Correct
reception is determined in step 1111 by detecting the presence of
the Unique Word. Other protocols use other techniques, and
alternate ways of determining correct reception of a standard TCH
burst in whatever protocol would be clear to those of ordinary
skill in the art using the specification of the protocol. If the
burst is determined to be a standard TCH burst, then the bit
sequence is forwarded in step 1113 to vocoder DSP 1209. If, on the
other hand, the bit sequence is not recognized as a standard TCH
burst, then the subscriber unit in step 1115 determines whether the
received burst is a calibration burst. In the two-tone methods
described herein above, this step 1115 is performed preferably by
carrying out the first correlation step of the calibration method.
If the correlation is high, then there is a high level of
confidence that this is a calibration burst, If the result of step
1115 is that yes, this is a calibration burst, then the downlink
signature estimation method is continued in step 1117 and the
resulting downlink signature is sent to the base station in step
1119.
[0201] Calibration Using SYNCH Bursts
[0202] In yet another alternate embodiment, instead of using
dedicated calibration calls, it is possible to embed the
calibration bursts into SYNCH bursts, the calibration bursts
preferably being the two-segment multi-tone bursts (or two-segment
two-tone busts for pairwise calibration).
[0203] Performance
[0204] The accuracy of the downlink channel estimate for the
two-tone method (improved implementation including timing alignment
correction) was measured by performing experiments using the PHS
base station and a subscriber unit from the WLL system used in the
preferred embodiment. In the first experiment, two antennas of the
PHS base station were used with two different sets of transmit
electronics. Forty sets of calibration bursts were sent to the
subscriber unit, and the subscriber unit was programmed to save the
received signal. The saved received signal was then used to
calculate the relative downlink signature. The calculation was
carried out offline using the MATLAB environment (The Mathworks,
Inc., Natick, Mass.). The results are shown in FIG. 13. As can be
seen, for the carrier frequency of the experiment, the two transmit
electronics/antenna elements had different amplitude gains and
produced the relative phase of about 109 degrees. The two tones
used were +24 kHz and -72 kHz.
[0205] A second experiment was carried out, this time by using the
same transmit electronics and the same antenna. That is, the two
calibration signals (the two tones) were transmitted from the same
electronics and antenna element. FIG. 14 shows the results when the
two tones used were +24 kHz and -72 kHz. As can be seen the phase
angle was close to 0.0, and the magnitude close to 1.0, as would be
expected. This same experiment was repeated with the two tones
being at +24 kHz and -24 kHz. The results are shown in FIG. 15. The
error and variance when using these two tones were larger that when
using the frequencies used for FIG. 14.
[0206] Using Several Subscriber Units
[0207] In another aspect of the invention, the calibration factor
may be obtained using more than one subscriber unit and determined
as a function of signatures obtained from these subscriber units.
These may even be all subscriber units. The function may be, for
example, a principal component, an average, or a centroid. In the
preferred embodiment of the combining step, the principal component
method is used. Signatures a.sub.1, . . . , a.sub.Ns gathered from
subscribers 1, . . . , Ns, respectively, are combined by forming a
matrix A=[a.sub.1 . . . a.sub.Ns] and computing the principal
component (the eigenvector corresponding the eigenvalue of largest
magnitude) of A.sup.HA or, equivalently, by finding the left
singular vector corresponding to the largest singular value of A.
In an improved embodiment, each subscriber unit also obtains a
signal quality estimate, and these estimates are sent to the base
station. Any subscriber unit implemented signal quality determining
method may be used, and the method (and apparatus) for determining
signal quality used in the preferred embodiment is the kurtosis
based method disclosed in above referenced U.S. patent application
Ser. No. 09/020,049 and also described herein above. Note also that
signal quality related measures may already be available at the
base station for power control purposes. When signal quality
estimates are available, a weighted average calibration factor is
obtained, the weighting for a calibration factor using a subscriber
unit according to the received signal quality for that subscriber
unit. For example, using the principal component method, the
signature estimate is the principal component of the weighted
signature matrix A=[.beta..sub.1a.sub.1 . . .
.beta..sub.Nsa.sub.Ns], where .beta..sub.1, . . . , .beta..sub.Ns
are the weighting factors for respective subscriber units 1, . . .
, Ns.
[0208] In yet another aspect, the calibration factor may again be
obtained as a function of calibration factors obtained from several
(even all) subscriber units. However, the function takes into
account the relative "quality" of each element of the signature
estimate from each of these subscriber units. This is applicable to
the case when for a subscriber unit, one or more of the base
station antenna elements are "weak" compared to the other elements.
In such a case, some of the signature estimate elements and the
corresponding calibration factor elements are discarded. For
example, one might discard signature elements that have a smaller
(normalized) magnitude than some magnitude threshold.
Alternatively, one might use the signature estimates to compare
predicted received signals to actual received signals, and thus
determine residual error (for example, error squared averaged over
a burst) for each element and discard signature elements that
produce a large residual error. One then can combine several such
"incomplete" calibration factor estimates that include at least one
estimate of very one of the calibration factor elements. As an
example, suppose there are four antenna elements in an array (or
subarray), and at three subscriber units denoted SU1, SU2, and SU3,
respectively, the first and second elements, second and third
elements, and third and fourth elements, respectively, are deemed
sufficiently accurate. Denoting the jth calibration factor element
using the ith subscriber unit by C.sub.ij, the four elements of the
complete calibration factor estimate are determined as C.sub.11,
C.sub.12, C.sub.23(C.sub.12/C.sub.22) and C.sub.34
(C.sub.12/C.sub.22) (C.sub.23/C.sub.33), respectively. This can be
generalized to any set of complete or incomplete SU determinations
as follows: Let C.sub.ij, be the jth calibration factor element
determined from the ith subscriber unit and let Q.sub.ij be the
estimate quality associated with the measurement of C.sub.ij where
i=1, . . . , Ns and j=1, . . . , M. With the above-mentioned method
of determining signature reliability, Q.sub.ij has value 0 if the
component is deemed unreliable or value 1 if it is deemed reliable.
Other methods of mathematically indicating reliability also are
possible, as will be clear to those of ordinary skill in the art.
The complete calibration vector D=[D.sub.1 D.sub.2 . . . D.sub.M]
is determined: by performing a joint minimization over D and the
complex-valued parameters B.sub.1, . . . , B.sub.Ns. That is,
defining B=[B.sub.1 . . . B.sub.Ns], is determined by carrying out
the operation 20 min D min B ij Q ij D j - C ij B i 2 .
[0209] This minimization can be carried out using standard methods,
for example by performing a grid search over D to approximately
locate the global minimum, and then performing a gradient descent
to refine the estimate. Alternative methods would be clear to those
of ordinary skill in the art.
[0210] Other Aspects
[0211] As will be understood by those of ordinary skill in the art,
many changes in the methods and apparatuses as described above may
be made without departing from the spirit and scope of the
invention. Variations include, without limitation:
[0212] The method can be modified for estimating uplink signatures
or downlink signatures rather than only for determining a
calibration factor to use for estimating a downlink weight vector
from an uplink weight vector.
[0213] Each uplink signature or downlink signature may be
determined as a vector of transfer functions. The methods described
herein would be modified to include standard transfer function
system identification techniques.
[0214] The uplink or downlink channel signatures may be obtained
using formulas other than derived from Eq. (9) or Eq. (11), based
on different models for the channels and different estimation
techniques.
[0215] The uplink or downlink channel signatures may be described
in other than baseband, as would be applicable to the case of the
uplink and downlink weights being applied at a base station to
signals in other than baseband.
[0216] The methods can be adapted for different types of
communication systems, including, without limitation, systems with
mobile subscriber units, or systems using different protocols, or
both. The methods also can be adapted to non-digital modulated
systems, such as the common AMPS FDMA system. The method also can
be adapted to non TDMA digital systems. In such cases, the uplink
and downlink frequencies are in general different, so that separate
uplink and downlink signatures need to be obtained for each
subscriber unit. Note that we can then determine downlink weight
vectors knowing all the downlink signatures for the subscriber
units.
[0217] Different pre-defined calibration signals may be used.
[0218] Different subarray configurations (of more than two antenna
elements) may be used, or all the antenna elements in the array
calibrated simultaneously.
[0219] More or less of the downlink processing can occur in the
subscriber units, depending on how much computation and storage
power is available in the subscriber unit and the base station.
[0220] Several aspects of the invention described herein were
described implemented as programs run on one or more DSP devices.
Given sufficient economic incentive, DSP functionality, including
DSP programs, may be incorporated into special purpose hardware,
for example as part of an application specific integrated circuit
(ASIC) or as part of a very large scale integrated circuit (VLSI).
DSP functionality may also be met by other processors, for example
a general purpose microprocessor. In addition, a DSP device running
a program may be converted into a special purpose piece of
hardware. Thus, the terms digital signal processor, DSP, and DSP
device as used herein include these equivalent alternatives.
[0221] As will be understood by those skilled in the art, the
skilled practitioner may make many changes in the methods and
apparatuses as described above without departing from the spirit
and scope of the invention. For example, the communication station
in which the method is implemented may use one of many protocols.
In addition, several architectures of these stations and subscriber
units are possible. The invention may be applied in a system
comprising any antenna-array-equipped transceiver and another
transceiver communicating with the array-equipped transceiver. Many
more variations are possible. The true spirit and scope of the
invention should be limited only as set forth in the claims that
follow.
[0222] Embodiments of the present invention may be described
as:
[0223] 1. In a wireless communication system comprising a main
transceiver and a remote transceiver capable of receiving signals
from and transmitting signals to the main transceiver, the main
transceiver comprising an array of transmit antenna elements, and
at least one receive antenna element, each transmit antenna element
being part of a transmit electronics chain for transmitting a
transmit apparatus signal using the transmit antenna element, and
each receive antenna element being part of a receiver apparatus
chain for receiving a received antenna signal from the receive
antenna element, the main transceiver and the remotre transceiver
designed for mutual communication using waveforms conforming to an
air interface standard, a method for estimating the downlink
signature for the remote transceiver, the method comprising:
[0224] (a) transmitting a set of one or more downlink calibration
waveforms from the main transceiver via the transmit antenna array
to the remote transceiver, the set of downlink calibration
waveforms substantially conforming to the air interface
standard;
[0225] (b) processing the signals received at the remote
transceiver corresponding to the downlink calibration waveforms,
the processing to determine downlink signature related signals to
the downlink signature for the remote transceiver;
[0226] (c) transmitting the downlink signature related signals from
the remote transceiver to the main transceiver using waveforms
substantially conforming to the air interface standard; and
[0227] (d) determining the downlink signature of the remote
transceiver from the downlink signature related signals received at
the main transceiver.
[0228] 2. The method of 1, wherein the at least one receive antenna
element are a plurality of receive antenna elements forming an
array of receive antenna elements, the number of elements in the
array of receive antenna elements being the same as the number of
antenna elements in the array of transmit antenna elements, the
method further comprising:
[0229] (e) transmitting a set of one or more uplink calibration
waveforms from the remote transceiver to the main transceiver, the
set of downlink calibration waveforms substantially conforming to
the air interface standard;
[0230] (f) processing at the main transceiver the received antenna
signals corresponding to the uplink calibration signals transmitted
from the remote transceiver, the processing determining the uplink
signature for the remote transceiver; and
[0231] (h) determining a calibration function for the main
transceiver from the uplink and downlink signatures for the remote
transceiver.
[0232] 3. In a wireless communication system comprising a main
transceiver and a remote transceiver capable of receiving signals
from and transmitting signals to the main transceiver capable of
receiving signals from and transmitting signals to the main
transceiver, the main transceiver comprising an array of transmit
antenna elements, and at least one receive antenna element, each
transmit antenna element being part of a transmit electronics chain
for transmitting a transmit apparatus signal using the transmit
antenna element, and each receive antenna element from part of a
receiver apparatus chain for receiving a received antenna signal
from the receive antenna element, the main transceiver and the
remote transceiver designed for mutual communication using
waveforms conforming to an air interface standard, a method for
estimating the downlink signature for the remote transceiver, the
method comprising:
[0233] transmitting a set of one or more downlink calibration
waveforms from the main transceiver via the transmit antenna array
to the remote transceiver, the set of downlink calibration
waveforms designed to be robust to one or more of the set
comprising frequency offset, phase noise, I/Q mismatch, and timing
offset;
[0234] (b) processing the signals received at the remote
transceiver corresponding to the downlink calibration waveforms,
the processing to determine downlink signature related signals
related to the downlink signature for the remote transceiver;
[0235] (c) transmitting the downlink signature related signals from
the remote transceiver to the main transceiver; and
[0236] (d) determining the downlink signature of the remote
transceiver from the downlink signature related signals received at
the main transceiver.
[0237] 4. The method of claim 3, wherein the at least one receive
antenna element are a plurality of receive antenna elements forming
an array of receive antenna elements, the number of elements in the
array of receive antenna elements being the same as the number of
antenna elements in the array of transmit antenna elements, the
method further comprising:
[0238] transmitting a set of one or more uplink calibration
waveforms from the remote transceiver to the main transceiver, the
set of downlink calibration waveforms;
[0239] (f) processing at the main transceiver the received antenna
signals corresponding to the uplink calibration signals transmitted
from the remote transceiver, the processing determining the uplink
signature for the remote transceiver; and
[0240] (g) determining a calibration function for the main
transceiver from the uplink and downlink signatures for the remote
transceiver.
[0241] 5. The method of claim 3, wherein each of the set of
downlink calibration waveforms conforms to the air interface
standard.
[0242] 6. In a wireless communication system comprising a main
transceiver and a remote transceiver capable of receiving signals
from and transmitting signals to the main transceiver, the main
transceiver comprising an array of transmit antenna elements, and
at least one receive antenna element, each transmit antenna element
being part of a transmit electronics chain for transmitting a
transmit apparatus signal using the transmit antenna element, and
each receive element being part of a receiver apparatus chain for
receiving a received antenna signal from the receive antenna
element, the main communication transceiver designed to transmit
traffic waveforms, the main communication transceiver also designed
to transmit downlink calibration waveforms, a method for estimating
the downlink signature for the remote transceiver, the method
comprising:
[0243] (a) transmitting downlink calibration waveforms and traffic
waveforms from the main transceiver via the transmit antenna array
to the remote transceiver, the downlink calibration waveforms
interspersed with the traffic waveforms;
[0244] (b) determining at the remote transceiver whether the
signals received at the remote transceiver correspond to downlink
calibration waveforms or to traffic waveforms;
[0245] (c) processing signals received at the remote transceiver
determined in step (b) to correspond to downlink calibration
waveforms, the processing to determined downlink signature related
signals related to the downlink signature for the remote
transceiver;
[0246] (d) processing signals received at the remote transceiver
determined in step (b) to correspond to traffic waveforms, the
processing to perform normal traffic functions;
[0247] (e) transmitting the downlink signature related signals from
the remote transceiver to the main transceiver; and
[0248] (f) determining the downlink signature of the remote
transceiver from the downlink signature related signals received at
the main transceiver.
[0249] 7. The method of 6, wherein the downlink calibration
waveforms are transmitted during silent periods.
[0250] 8. The method of 6, wherein the downlink calibration
waveforms are transmitted only after a number of idle waveforms are
transmitted from the main transceiver.
[0251] 9. The method of 6, wherein the at least one receive antenna
element are a plurality of receiver antenna elements forming an
array of receive antenna elements, the number of elements in the
array of receive antenna elements being the same as the number of
antenna elements in the array of transmit antenna elements, the
method further comprising:
[0252] (h) transmitting a set of one or more uplink calibration
waveforms from the remote transceiver to the main transceiver, the
set of downlink calibration waveforms;
[0253] (h) processing at the main transceiver the received antenna
signals corresponding to the uplink calibration signals transmitted
from the remote transceiver, processing determining the uplink
signature for the remote transceiver; and
[0254] (j) determining a calibration function for the main
transceiver from the uplink and downlink signatures for the remote
transceiver.
[0255] 10. The method of 6, wherein the downlink calibration
waveforms transmitted from the main transceiver in step (a) are
designed to be robust to one or more of the set comprising
frequency offset, phase noise, I/Q mismatch, and timing offset.
[0256] 11. In a wireless communication system comprising a main
transceiver and a remote transceiver capable of receiving signals
from and transmitting signals to the main transceiver, the main
transceiver coprising an array of antenna elements and an array of
receive antenna elements, each transmit antenna element being part
of a transmit electronics chain for transmitting a transmit
apparatus signal using the transmit antenna element, and each
receive antenna element being part of a receiver apparatus chain
for receiving a received antenna signal from the receive antenna
element, the number of elements in the array of receive antenna
elements being the same as the number of antenna elements in the
array of transmit antenna elements, the main transceiver comprises
means for uplink adaptive smart antenna processing including linear
uplink adaptive smart antenna processing according to an uplink
weight vector, and downlink adaptive smart antenna processing
including linear downlink adaptive smart antenna processing
according to a downlink weight vector, the main transceiver and the
remote transceiver designed for mutual communication using
waveforms conforming to an air interface standard, the main
transceiver further comprising means to determine the uplink weight
vector for the remote transceiver, a method for determining the
downlink weight vector for the remote transceiver, the method
comprising:
[0257] (a) transmitting a set of one or more downlink calibration
waveforms from the main transceiver via the transmit antenna array
to the remote conforming to the air interface standard;
[0258] (b) processing the signals received at the remote
transceiver corresponding to the downlink calibration waveforms,
the processing to determine downlink signature related signals
related to the downlink signature for the remote transceiver;
[0259] (c) transmitting the downlink signature related signals from
the remote transceiver to the main transceiver;
[0260] (d) transmitting the downlink signature related signals from
the remote transceiver to the main transceiver;
[0261] (e) processing at the main transceiver the received antenna
signals corresponding to the uplink calibration signals transmitted
from the remote transceiver, the processing determining the uplink
signature for the remote transceiver;
[0262] (f) determining the uplink weight vector for the remote
transceiver from any signals received at the main transceiver from
the remote transceiver; and
[0263] (g) determining the downlink weight vector for the remote
transceiver from:
[0264] the determined uplink weight vector, the determined uplink
signature, and
[0265] the received antenna signals corresponding to the downlink
signature related signals received at the main transceiver.
[0266] 12. The method of 11, wherein the downlink weight
determining step comprises:
[0267] (i) determining a calibration function for the remote
transceiver from the determined uplink signature and the received
antenna signals corresponding to the downlink signature related
signals received at the main transceiver, and from the downlink
signature related signals received at the main transceiver, and
[0268] (ii) determining the downlink weight vector from the
determined uplink weight vector and the calibration function.
[0269] 13. In a wireless communication system comprising a main
transceiver and plurality of remote transceivers each capable of
receiving signals from and transmitting signals to the main
transceiver, the main transceiver comprising an array of transmit
antenna elements, and at least one receive antenna element, each
transmit antenna element being part of transmit electronics chain
for transmitting a transmit apparatus signal using the transmit
antenna element, and each receive antenna element being part of a
receiver apparatus chain for receiving a received antenna signal
from the receive antenna element, a method for estimating the
downlink signature for the remote transceiver, the method
comprising:
[0270] (a) transmitting a set of one or more downlink calibration
waveforms from the main transceiver via the transmit antenna array
to the remote transceivers;
[0271] (b) processing the signals received at each remote
transceiver corresponding to the downlink calibration waveforms,
the processing to determine downlink signature related signals
related to the downlink signature for the remote transceiver;
[0272] (c) transmitting the downlink signature related signals from
each remote transceiver to the main transceiver using waveforms
substantially conforming to the air interface standard;
[0273] (d) determining a downlink signature for each remote
transceiver from the downlink signature related signals received at
the main transceiver from the remote transceiver; and
[0274] (e) combining the downlink signatures for the remote
transceivers to determine a combined downlink signature.
[0275] 14. The method of 13, wherein the main transceiver and the
remote transceivers are designed for mutual communication using
waveforms conforming to an air interface standard, and wherein each
waveform in the set of downlink calibration waveforms substantially
conforms to the air interface standard.
[0276] 15. The method of 13, wherein the at least one receive
antenna element are a plurality of receive antenna elements forming
an array of receive antenna elements, the number of elements in the
array of receive antenna elements being the same as the number of
antenna elements in the array of transmit antenna elements, the
method further comprising:
[0277] (f) transmitting a set of one or more uplink calibration
waveforms from each remote transceiver to the main transceiver;
[0278] (g) processing at the main transceiver the received antenna
signals corresponding to the uplink calibration signals transmitted
from each remote transceiver, the processing determing an uplink
signature for each remote transceiver;
[0279] (h) combining the uplink signatures for the remote
transceivers to determine a combined uplink signature; and
[0280] (j) determining a calibration function for the main
transceiver from the uplink and downlink combined signatures.
[0281] 16. The method of 15, wherein the main transceiver comprises
means for uplink adaptive smart antenna processing including linear
uplink adaptive smart antenna processing according to an uplink
weight vector, and downlink adaptive smart antenna processing
including linear downlink adaptive smart antenna processing
according to a downlink weight vector, the method further
comprising:
[0282] (k) determining at the main transceiver the uplink weight
vector for receiving from the subscriber unit by processing
received antenna signals received while the remote transceiver is
transmitting to the main transceiver; and
[0283] (l) determining at the main transceiver the downlink weight
for transmitting to the remote transceiver from the determined
uplink weights and the calibration factor.
[0284] 17. The method of 15, wherein the signature combining is
carried out by the principal component method.
[0285] 18. The method of 17, wherein each remote transmitter also
transmits a remote transceiver received signal quality estimate to
the main transceiver and wherein the signature combining is a
weighted combining, the weighting of the signature for each remote
transceiver being the remote transceiver received signal quality
estimate or the remote transceiver.
[0286] 19. The method of 15, wherein any component in a signature
estimate is discarded if it corresponds to a weak receive or
transmit antenna element relative to the other antenna
elements.
[0287] 20. The method of 2, wherein the uplink calibration signals
are idle traffic waveforms.
[0288] 21. The method of 2, wherein the uplink calibration signals
are the downlink signature related signals.
[0289] 22. The method of 2, wherein the main transceiver comprises
means for uplink adaptive smart antenna processing including linear
uplink adpative smart antenna processing according to an uplink
weight vector, and downlink adaptive smart antenna processing
including linear downlink adaptive smart antenna processing
according to a downlink weight vector, the method further
comprising:
[0290] (h) determining at the main transceiver the uplink weight
vector for receiving from the subscriber unit by processing
received antenna signals received while the remote transceiver is
transmitting to the main transceiver; and
[0291] (j) determining at the main transceiver the downlink weight
for transmitting to the remote transceiver from the determined
uplink weights and the calibration factor.
[0292] 23. The method of 1, wherein the downlink signature is
determined in relation to a reference antenna element of the
transmit antenna array, and wherein the downlink calibration
waveforms are selected so that the signals transmitted from each
transmit antenna elements are substantially orthogonal.
[0293] 24. The method of 23, wherein the downlink calibration
waveforms are modulated constant modulus calibration signals
selected so that the dot product of any two calibration signals
transmitted from any two distinct antenna elements of the transmit
array is a pure tone.
[0294] 25. The method of 23, wherein the downlink calibration
waveforms comprise combinations of M distinct modulated constant
modulus calibration signals, M being the number of antenna elements
of the antenna array for which a downlink signature is being
determined, each calibration signal comprising two segments,
denoted a first segment and a second segment, respectively, the two
segments being identically timed for each calibration signal,
wherein during the first segment time interval, a first set of
linear combinations of the calibration signals is transmitted from
each of the antenna elements of the transmit array, and during the
second segment time interval, a second set of linear combinations
of the calibration signals is transmitted from each of the antenna
elements of the transmit array.
[0295] 26. The method of 23, wherein the signals transmitted from
each antenna element of the transmit array are modulated tone
signals, the frequencies of the tone signals from distinct arrays
being distinct, the downlink signature related signal determining
processing step and downlink signature determining step together
comprising:
[0296] cross correlating the signals received at the remote
transceiver with each of the tone signals, and
[0297] normalizing the correlations with the signals transmitted
from the reference element.
[0298] 27. The method of claim 23, wherein there are M antenna
elements, the first set of linear combinations being a sum of M
distinct tone signals being transmitted from the reference antenna
element, and none of the tone signals being transmitted from the
other transmit antenna elements, the frequencies of the tone of the
distinct tone signals being distinct, the second set of linear
combinations being a different one of the tone signals being
transmitted from each of the antenna elements, the frequencies of
the tones from distinct arrays being distinct, processing to
determine the downlink signature related signals and the downlink
signature determining together comprising:
[0299] cross correlating the signals received during the first
segment at the remote transceiver with each of the first segment
signals transmitted by each antenna element to obtain first segment
correlations;
[0300] normalizing the first segment correlations with the first
segment correlation with the signal transmitted from the reference
element, the normalizing forming first segment normalized
correlations;
[0301] cross correlating the signals received during the second
segment at the remote transceiver with each of the second segment
signals transmitted by each antenna element to obtain second
segment correlations;
[0302] normalizing the second segment correlations with the first
segment correlation with the signal transmitted from the reference
element, the normalizing forming second segment normalized
correlations; and
[0303] dividing each the second segment normalized correlation with
the corresponding first segment normalized correlations to form the
downlink signature estimate components.
[0304] 28. The method of 1, where the downlink signature related
signals comprise the downlink signature for the remote
transceiver.
[0305] 29. The method of 1, wherein the array of transmit antenna
elements and the one or more receive antenna elements comprise
common antenna elements.
[0306] 30. The method of 2, wherein the downlink signature estimate
is determined as the maximum likelihood estimate.
[0307] 31. The method of 1, wherein the communication system is a
cellular system comprising one or more base stations, each having
one or more subscriber units, and wherein the main transceiver is
one of the base stations.
[0308] 32. The method of 31, wherein the remote transceiver is a
subscriber unit of the main transceiver.
[0309] 33. The method of 1, wherein the air interface standard is
PHS.
[0310] 34. A wireless communication system comprising
[0311] (a) a main transceiver comprising:
[0312] (i) an array of transmit antenna elements, each transmit
antenna element being part of a transmit electronics chain for
transmitting a transmit apparatus signal from the transmit antenna
element,
[0313] (ii) one or more receive antenna elements, each receive
antenna element being part of a receive apparatus chain for
receiving a received antenna signal from the receive antenna,
and
[0314] (iii) one or more main transceiver signal processors for
processing received antenna signals and for forming transmit
apparatus signals; and
[0315] (b) a remote transceiver capable of receiving signals from
and transmitting signals to the main transceiver using a waveforms
conforming to an air interface standard, the remote transceiver
comprising:
[0316] (i) a remote transceiver receiver including a remote
transceiver receive antenna for receiving remote transceiver
received signals,
[0317] (i) a remote transceiver transmitter including a remote
transceiver transmit antenna for transmitting remote transceiver
transmit signals to the main transceiver, and
[0318] (iii) one or more remote transceiver signal processors for
processing remote transceiver received signals; and for forming
remote transceiver transmit signals,
[0319] wherein at least one of the main transceiver signal
processors is programmed to:
[0320] transmit a set of downlink calibration waveforms from the
main transceiver via the transmit antenna array to the remote
transceiver, the set of downlink calibration waveforms
substantially conforming to the air interface standard,
[0321] wherein at least one of the remote transceiver signal
processors is programmed to:
[0322] process the signals received corresponding to the
transmitted downlink calibration waveforms at the remote
transceiver to determine downlink signature related signals related
to the downlink signature for the remote transceiver;
[0323] transmit the downlink signature related signals from the
remote transceiver to the main transceiver using waveforms
substantially conforming to the air interface standard, and
[0324] wherein at least one of the main transceiver signal
processors is programmed to:
[0325] process the downlink signature related signals received at
the main transceiver from the remote transceiver to determine the
downlink signature for the remote transceiver.
[0326] 35. The system of 34, wherein the at least one receive
antenna element are a plurality of receive antenna elements forming
an array of receive antenna elements, the number of elements in the
array of receive antenna elements being the same as the number of
antenna elements in the array of transmit antenna elements, wherein
at least one of the remote transceiver signal processors is
programmed to:
[0327] transmit a set of one or more uplink calibration waveforms
to the main transceiver, and
[0328] wherein at least one of themain transceiver signal
processors is programmed to:
[0329] process the received antenna signals corresponding to the
uplink calibration waveforms transmitted from the remote
transceiver, the processing determining the uplink signature for
the remote transceiver; and
[0330] determine a calibration function for the main transceiver
from the uplink and downlink signatures for the remote
transceiver.
[0331] 36. The system of 35, wherein the main transceiver further
comprises means for uplink adaptive smart antenna processing
including linear uplink adaptive smart antenna processing according
to an uplink weight vector, and downlink adaptive smart antenna
processing including linear downlink adaptive smart antenna
processing according to a downlink weight vector, wherein at least
one of the transceiver signal processors is programmed to:
[0332] determine the uplink weight vector for receiving from the
subscriber unit by processing received antenna signals received
while the remote transceiver is transmitting to the main
transceiver; and
[0333] determine the downlink weight for transmitting to the remote
transceiver from the uplink weights determined for the remote
transceiver and the calibration factor.
[0334] 37, A Wireless communication system comprising
[0335] (a) a main transceiver comprising
[0336] (i) an array of transmit antenna elements, each transmit
antenna element being part of a transmit electronics chain for
transmitting a transmit apparatus signal from the transmit antenna
element,
[0337] (ii) an array of receive antenna elements, each receive
antenna element being part of a receive apparatus chain for
receiving a received antenna signal from the receive antenna, the
number of active elements in the receive array being the same as
the number of active elements in the transmit array, and
[0338] (iii) one or more main transceiver signal processors for
processing received antenna signals and for forming transmit
apparatus signals; and
[0339] (b) a plurality of remote transceivers each capable of
receiving signals from and transmitting signals to the main
transceiver, each remote transceiver comprising:
[0340] (i) a remote transceiver receiver including a remote
transceiver receive antenna for receiving remote transceiver
received signals,
[0341] (i) a remote transceiver including a remote transceiver
transmit antenna for transmitting remote transceiver transmit
signals to the main transceiver, and
[0342] (iii) one or more remote transceiver signal processors for
processing remote transceiver received signals; and for forming
remote transceiver transmit signals,
[0343] wherein at least one of the main transceiver signal
processors is programmed to:
[0344] transmit a set of downlink calibration waveforms from the
main transceiver via the transmit antenna array to the plurality of
remote transceivers,
[0345] wherein at least one of each remote transceiver's remote
transceiver signal processors is programmed to:
[0346] process the signals received at the remote transceiver
corresponding to the transmitted downlink calibration waveforms to
determine downlink signature related signals related to the
downlink signature for the remote transceiver,
[0347] transmit the downlink signature related signals from the
remote transceiver to the main transceiver using waveforms
substantially conforming to the air interface standard, and
[0348] transmit a set of one or more uplink calibration signals
from the remote transceiver to the main transceiver, and
[0349] wherein at least one of the main transceiver signal
processors is programmed to:
[0350] process the downlink signature related signals received at
the main transceiver from each remote transceiver to determine a
downlink signature for the remote transceiver,
[0351] process the received antenna signals corresponding to the
uplink calibration signals from each remote transceiver to
determine an uplink combined signature for the remote
transceiver,
[0352] combine the downlink signatures for the transceivers to
determine a downlink combined signature,
[0353] combine the upline signatures for the transceivers to
determine an uplink combined signature, and
[0354] determine a calibration fractor for the main transceiver
from the downlink combined signature and from the uplink combined
signature.
[0355] 38. The system 35, wherein the uplink calibration signals
are idle traffic waveforms.
[0356] 39. The system of 35, wherein the uplink calibration signals
are the downlink signature related signals.
[0357] 40. The system 34, wherein the downlink signature is
determined in relation to a reference antenna element of the
transmit antenna array, and wherein the downlink calibration
waveforms are selected so that the signals transmitted from each
transmit antenna elements are substantially orthogonal.
[0358] 41. The system of 40, wherein the downlink calibration
waveforms are designed to be robust to one or more of the set
comprising frequency offset, phase noise, I/Q mismatch, and timing
offset.
[0359] 42. The system of 41, wherein the downlink calibration
waveforms are modulated constant modulus calibration signals
selected so that the dot product of any two calibration signals
transmitted from any two distinct antenna elements of the transmit
array is a pure tone.
[0360] 43. The system of 41, wherein the downlink calibration
waveforms comprise combinations of M distinct modulated constant
modulus calibration signals, M being the number of antenna elements
of the antenna array for which a downlink signature is being
determined, each calibration signal comprising two segments,
denoted a first segment and a second segment, respectively, the two
segments being identically timed for each calibration signal,
wherein during the first segment time interval, a first set of
linear combinations of the calibration signal is transmitted from
each of the antenna elements of the transmit array, and during the
second segment time interval, a second set of linear combination of
the calibration signals is transmitted from each of the antenna
elements of the transmit array.
[0361] 44. The system of 42, wherein the signals transmitted from
each antenna element of the transmit array are modulated tone
signals, the frequencies of the tone signals from distinct arrays
being distinct, the downlink signature signals determining and the
downlink signature determining together comprising:
[0362] cross correlating the signals received at the remote
transceiver with each of the tone signals, and
[0363] normalizing the correlations with the signal transmitted
from the reference element.
[0364] 45. The system of 41, wherein there are M antenna elements,
the first set of linear combinations being a sum of M distinct tone
signals being transmitted from the reference antenna element, and
none of the tone signals being transmitted from the other transmit
antenna elements, the frequencies of the tone of the distinct tone
signals being distinct, the second set of linear combinations being
a different one of the tone signals being transmitted from each of
the antenna elements, the frequencies of the tones from distinct
arrays being distinct, the downlink signature signals determining
and the downlink signature determining together comprising:
[0365] cross correlating the signals received during the first
segment of the remote transceiver with each of the first segment
signals transmitted by each antenna element to obtain first segment
correlations;
[0366] normalizing the first segment correlations with the first
segment correlation with the signal transmitted from the reference
element, the normalizing forming first segment normalized
correlations;
[0367] cross correlating the signals received during the second
segment at the remote transceiver with each of the second segment
signals transmitted by each antenna element to obtain second
segment corrections;
[0368] normalizing the second segment correlations with the first
segment correlation with the signal transmitted from the reference
element, the normalizing forming second segment normalized
correlations; and
[0369] dividing each the second segment normalized correlation with
the corresponding first segment normalized correlations to form the
downlink signature estimate components.
[0370] 46. The system of 34, wherein the communication system is a
cellular system comprising one or more base stations, each haveing
one or more subscriber units, and wherein the main transceiver is
one of the base stations.
[0371] 47. The system of 34, wherein the remote transceiver is a
subscriber unit of the main transceiver.
[0372] 48. The system of 34, wherein the air interface standard is
PHS.
[0373] 49. The system of 34, where the downlink signature related
signals comprise the downlink signature for the remote
transceiver.
[0374] 50. The system of 34, wherein the array of transmit antenna
elements and the one or more receive antenna elements comprise
common antenna elements.
[0375] 51. The system of 34, wherein the downlink signature
estimate is determined as the maximum likelihood estimate.
* * * * *
References