Note: Descriptions are shown in the official language in which they were submitted.
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ANTENNA ARRAY CALIBRATION FOR MULTI-INPUT MULTI-
OUTPUT WIRELESS COMMUNICATION SYSTEMS
[0001] BACKGROUND
I. Field
100021 The following description relates generally to wireless
communications, and,
amongst other things, to calibrating an antenna array for multi-input multi-
output
wireless communication systems.
II. Background
[0003) Wireless networking systems have become a prevalent means by
which a
majority of people worldwide has come to communicate. Wireless communication
devices have become smaller and more powerful in. order to meet consumer needs
and
to improve portability and convenience. The increase in processing power in.
mobile
devices such as cellular telephones has lead to an increase in demands on
wireless
network transmission systems. Such systems typically are not as easily updated
as the
cellular devices that communicate there over. As mobile device capabilities
expand, it
can bc difficult to maintain an. older wireless network system in a manner
that facilitates
fully exploiting new and improved wireless device capabilities.
[0004] More particularly, frequency division based techniques
typically separate the
spectrtim into distinct channels by splitting it into uniform chunks of
bandwidth, for
=
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example, division of the frequency band allocated for wireless cellular
telephone
communication can be split into charmels, each of which can carry a voice
conversation
or, with digital service, carry digital data. Each channel can be assigned to
only one
user at a time. One commonly utilized variant is an orthogonal frequency
division
technique that effectively partitions the overall system bandwidth into
multiple
orthogonal subbands. These subbands are also referred to as tones, carriers,
subcarriers,
bins, and/or frequency channels. Each subband is associated with a subcarrier
that can
be modulated with data. With time division based techniques, a band is split
time-wise
into sequential time slices or time slots. Each user of a channel is provided
with a time
slice for transmitting and receiving information in a round-robin manner. For
example,
at any given time t, a user is provided access to the channel for a short
burst. Then,
access switches to another user who is provided with a short burst of time for
transmitting and receiving information. The cycle of "taking turns" continues,
and
eventually each user is provided with multiple transmission and reception
bursts.
100051 Code division based techniques typically transmit data over a number
of
frequencies available at any time in a range. In general, data is digitized
and spread
over available bandwidth, wherein multiple users can be overlaid on the
channel and
respective users can be assigned a unique sequence code. Users can transmit in
the
same wide-band chunk of spectrum, wherein each user's signal is spread over
the entire
bandwidth by its respective unique spreading code. This technique can provide
for
sharing, wherein one or more users can concurrently transmit and receive. Such
sharing
can be achieved through spread spectrum digital modulation, wherein a user's
stream of
bits is encoded and spread across a very wide channel in a pseudo-random
fashion. The
receiver is designed to recognize the associated unique sequence code and undo
the
randomization in order to collect the bits for a particular user in a coherent
manner.
[00061 A known type of communication system is a multi-input multi-output
(MIMO) communication system where both the transmitter and the receiver have a
plurality of receive and transmit antennas for communication. A mobile
terminal, with
multiple receive and transmit antennas, within the coverage area of a base
station with
multiple receive and transmit antennas, can be interested in receiving one,
more than
one or all the data streams frorn the base station. Likewise, a mobile
terminal can
transmit data to the base station or another mobile terminal. Such
communication
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between base station and mobile terminal or between mobile terminals can be
degraded
due to channel variations and/or interference power variations. For example,
the
aforementioned variations can affect base station scheduling, power control
and/or rate
prediction for one or more mobile terminals.
[0007] When antenna arrays and/or base stations are employed in conjunction
with a
time domain duplexed (TDD) channel transmission technique, very large gains
can be
realized. A key assumption in realizing these gains is that due to the TDD
nature of the
transmission and reception, both the forward link (FL) and reverse link (RL)
observe
similar physical propagation channels corresponding to a common carrier
frequency.
However, in practice the overall transmit and receive chains, which can
include the
analog front ends and the digital sampling transmitters and receivers, as well
as the
physical cabling and antenna architecture, contribute to the over all channel
response
experienced by the receiver. In other words, the receiver will see an overall
or
equivalent channel between the input of the transmitter digital to analog
converter
(DAC) and the output of the receiver analog to digital converter (ADC), which
can
comprise the analog chain of the transmitter, the physical propagation
channel, the
physical antenna array structure (including cabling), and the analog receiver
chain.
[0008] In view of at least the above, there exists a need in the art for a
system and/or
methodology of calibrating antenna arrays employed in wireless communication
devices.
SUMMARY
[0009] The following presents a simplified summary of one or more
embodiments
in order to provide a basic understanding of such embodiments. This summary is
not an
extensive overview of all contemplated embodiments, and is intended to neither
idcntify
key or critical elements of all embodiments nor delineate the scope of any or
all
embodiments. Its sole purpose is to present some concepts of one or more
embodiments
in a simplified form as a prelude to the more detailed description that is
presented later.
[0010] According to one aspect of the invention, there= is provided a
method of calibrating an antenna
array in a wireless network comprises detennining channel estimates for at
least two antennas of
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at least two access terminals and determining a calibration ratio based upon
each of the
channel estimates for the at least two antennas.
[0010a] According to another aspect of the present invention, there is
provided a
method of calibrating an antenna array in a wireless network employing a time
domain duplex
channel transmission technique, comprising: receiving first channel estimate
information
each corresponding to transmissions to at least two antennas of at least two
access terminals;
determining second channel estimate information each corresponding to
transmissions from
the at least two antennas of the at least two access terminals; and
determining a calibration
ratio based upon each of the first channel estimate information and each of
the second channel
estimate information; wherein at least one of the at least two access
terminals has at least two
antennas; and wherein determining the calibration ratio comprises solving the
equation:
= õ = e 16''rk " = diag(h,,k,õ) = q +
where Zi,k,u is a diagonal matrix whose diagonal
= = Z i,k,u = rl
elements are the elements of reverse link channel estimate information
7, = e'r" , the subscripts i,k,u, are the tone, time, and user indexes,
respectively, g,,k,õ is a forward link channel vector estimate, ii is a
mismatch vector
corresponding to access point antenna array transmit and receive chains,
ni,k,u is a noise
vector, yõ is a gain mismatch corresponding to the access terminal transmit
and receive chains,
and tk,u is a timing error between an access point and an antenna of a u-th
access terminal.
[0010b] According to still another aspect of the present invention,
there is provided a
wireless communication apparatus configured to employ a time domain duplex
channel
transmission technique, the apparatus comprising: at least two antennas; and a
processor
coupled with the at least two antennas, the processor configured to determine
a calibration
ratio, based upon first channel estimate information each corresponding to
transmissions to at
least two antennas of at least two access terminals and second channel
estimate information
each corresponding to transmissions from the at least two antennas of the at
least two access
terminals; wherein at least one of the at least two access terminals has at
least two antennas;
and wherein the processor is configured to determine the calibration ratio by
solving the
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equation: g,õ rõ = e = diag(h,,k,õ ) = +
where Zr,k,ti is a diagonal matrix whose
=
diagonal elements are the elements of a reverse link channel vector estimate
, = õ = e'r÷ " , the subscripts i,k,u, are the tone, time, and user
indexes,
respectively, g,,h,õ is a forward link channel vector estimate, is a mismatch
vector
corresponding to access point antenna array transmit and receive chains,
ni,k,u is a noise
vector, yõ is a gain mismatch corresponding to the access terminal transmit
and receive chains,
and Tk,u is a timing error between an access point and an antenna of a u-th
access terminal.
[0010c] According to yet another aspect of the present invention,
there is provided an
apparatus comprising: means for processing first channel estimate information
each
corresponding to transmissions to at least two antennas of at least two access
terminals and
received from the at least two access terminals; means for determining second
channel
estimate information each corresponding to transmissions from the at least two
antennas of the
at least two access terminals; and means for determining a calibration ratio
based upon each of
the first channel estimate information and each of the second channel estimate
information;
wherein the at least two access terminals implement a time domain duplex
channel
transmission technique; wherein at least one of the at least two access
terminals has at least
two antennas; and wherein the means for determining the calibration ratio
comprises means
k õ = 7 õ = e jw'rk " = diag(h,kõ) = 1+
for solving the equation: " , , where Z, k,,, is a
diagonal
¨ = Z 1,k =1+
matrix whose diagonal elements are the elements of reverse link channel
estimate information
= 7õ e = I a''rk " , the subscripts i,k,u, are the tone, time, and user
indexes,
respectively, is a forward link channel vector estimate, ri is a mismatch
vector
corresponding to access point antenna array transmit and receive chains,
ni,k,u is a noise
vector, yõ is a gain mismatch corresponding to the access terminal transmit
and receive chains,
and tk,õ is a timing error between an access point and an antenna of a u-th
access terminal.
[0010d] According to a further aspect of the present invention, there is
provided a
processor-readable medium having stored thereon processor-readable
instructions that, when
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executed by one or more processors, cause the one or more processors to:
process first
channel estimate information each corresponding to transmissions to at least
two antennas of
at least two access terminals and received from the at least two access
terminals; determine
second channel estimate information each corresponding to transmissions from
the at least
two antennas of the at least two access terminals; and determine a calibration
ratio based upon
each of the first channel estimate information and each of the second channel
estimate
information; wherein the at least two access terminals implement a time domain
duplex
channel transmission technique; wherein at least one of the at least two
access terminals has at
least two antennas; and wherein determining the calibration ratio comprises
solving the
= 7õ = e = diag(h,k,õ)=ii+n
1 0 equation: , where Zit,k,u is a diagonal
matrix whose
= ,k ,te Z I,k ,tr .11+ ni,k,u
diagonal elements are the elements of reverse link channel estimate
information
= yu 'e ju''r4 " , the subscripts i,k,u, are the tone, time, and user indexes,
respectively, gi,k,õ is a forward link channel vector estimate, q is a
mismatch vector
corresponding to access point antenna array transmit and receive chains, na,u
is a noise
vector, yõ is a gain mismatch corresponding to the access terminal transmit
and receive chains,
and tk,u is a timing error between an access point and an antenna of a u-th
access terminal.
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4c
[00111 To the accomplishment of the foregoing and related ends, the one or
more
embodiments comprise the features hereinafter fully described and particularly
pointed
out in the claims. The following description and the annexed drawings set
forth in
detail certain illustrative aspects of the one or more embodiments. These
aspects arc
indicative, however, of but a few of the various ways in which the principles
of various
embodiments may be employed and the described embodiments are intended to
include
all such aspects and their equivalents.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 illustrates aspects of a multiple access wireless
communication
system
[0013] FIG. 2 illustrates an antenna arrangement comprising a receiver
chain and a
transmitter chain in accordance with various aspects described herein.
[0014] FIG. 3 illustrates aspects timing for calibration operations.
[0015] FIG. 4 illustrates aspects of logic that facilitates calibrating an
antenna array
to compensate for gain mismatch.
[0016] FIG. 5 illustrates aspects of a system that facilitates calibrating
an antenna
array to compensate for gain mismatch.
[0017] FIG. 6 illustrates aspects of a methodology for calibrating an
array of
antennas.
[0018] FIG. 7 illustrates aspects of a methodology for calibrating an
array of
antennas.
[0019] FIG. 8 illustrates aspects of a receiver and transmitter in a
wireless
communication system
[0020] FIG. 9 illustrates aspects of an access point.
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DETAILED DESCRIPTION
[0021] Various embodiments arc now described with reference to thc
drawings,
wherein like reference numerals are used to refer to like elements throughout.
In the
following description, for purposes of explanation, numerous specific details
are set
forth in order to provide a thorough understanding of one or more embodiments.
It may
bc evident, however, that such embodiment(s) may bc practiced without these
specific
details. In other instances, well-known structures and devices are shown in
block
diagram form in order to facilitate describing one or more embodiments.
[0022] As used in this application, the terms "component," "system," and
the like
are intended to refer to a computer-related entity, either hardware, a
combination of
hardware and software, software, or software in execution. For example, a
component
may be, but is not limited to being, a process running on a processor, a
processor, an
object, an executable, a thread of execution, a program, and/or a computer.
One or
more components may reside within a process and/or thread of execution and a
component may be localized on one computer and/or distributed between two or
more
computers. Also, these components can execute from various computer readable
media
having various data structures stored thereon. The components may communicate
by
way of local and/or remote processes such as in accordance with a signal
having one or
more data packets (e.g., data from one component interacting with another
component
in a local system, distributed system, and/or across a network such as the
Internet with
other systems by way of thc signal).
[0023] Furthermore, various embodiments are described herein in connection
with a
subscriber station. A subscriber station can also be called a system, a
subscriber unit,
mobile station, mobile, remote station, access point, base station, remote
terminal,
access terminal, user terminal, user agent, user equipment, etc. A subscriber
station
may be a cellular telephone, a cordless telephone, a Session Initiation
Protocol (SIP)
phone, a wireless local loop (WLL) station, a personal digital assistant
(PDA), a
handheld device having wireless connection capability, or other processing
device
connected to a wireless modem.
[0024] Moreover, various aspects or features described herein may be
implemented
as a method, apparatus, or article of manufacture using standard programming
and/or
engineering techniques. The term "article of manufacture" as used herein is
intended to
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encompass a computer program accessible from any computer-readable device,
carrier,
or media. For example, computer readable media can include but are not limited
to
magnetic storage devices (e.g., hard disk, floppy disk, magnetic strips...),
optical disks
(e.g., compact disk (CD), digital versatile disk (DVD)...), smart cards, flash
memory
devices (e.g., card, stick, key drive...), and integrated circuits such as
read only
memories, programmable read only memories, and electrically erasable
programmable
read only memories.
[0025] Referring to FIG. 1, a multiple access wireless communication
system
according to one embodiment is illustrated. A multiple access wireless
communication
system 1 includes multiple cells, e.g. cells 2, 4, and 6. In FIG. 1, each cell
2, 4, and
6 may include an access point that includes multiple sectors. The multiple
sectors are
formed by groups of antennas each responsible for communication. with access
terminals in a portion of the cell. In cell 2, antenna groups 12, 14, and 16
each
correspond to a different sector. In cell 4, antenna groups 18, 20, and 22
each
correspond to a different sector. In cell 6, antenna groups 24, 26, and 28
each
correspond. to a different sector.
[0026] Each cell includes several access terminals which are in
communication with
one or more sectors of each access point. For example, access terminals 20 and
22 are
in communication with access point base 42, access terminals 24 and 26 are in
communication with access point 44, and access terminals 28 and 40 are in
communication with access point 46.
[0027] Controller 50 is coupled to each of the cells 2, 4, and 6.
Controller 50 may
contain one or more connections to multiple networks, e.g. the Internet, other
packet
based networks, or circuit switched voice networks that provide information
to, and
from, the access terminals in communication with the cells of the multiple
access
wireless communication system 1. The controller 50 includes, or is coupled
with, a
scheduler that schedules transmission from and to access terminals. In other
embodiments, the scheduler may reside in each individual cell, each sector of
a cell, or a
combination thereof.
[00281 In order to facilitate calibration of transmissions to the
access terminals, it is
helpful to calibrate the access point gain calibration loop to deal with
mismatches due to
the transmit and receive chains of the access point. However, due to the noise
in the
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channel, any calibration estimates based on the signals received at the access
terminals,
forward link, and transmitted from the access terminals, reverse link, may
contain noise
and other channel variations that may call into question the estimates
provided. In. order
to overcome the channel noise effects, multiple calibrations on both the
forward link
and reverse link are utilized for multiple access terminals. In certain
aspects, multiple
transmissions to and from each antenna of each access terminal are taken into
account to
perform calibration of a given sector. In certain aspects, the multiple
antennas may be
used to calibrate communication for a single access terminal. In other
aspects, one or
less than all of the antennas for a group of access terminals may be utilized
for
communication with all of the antennas for the group of access terminals.
[0029] hi certain aspects, either the transmit chain of the access
point or receive
chain of the access point may be calibrated. This may be done, for example, by
utilizing
a calibration ratio to calibrate the receive chain of the access point to its
transmit chain
or calibrate its transmit chain to its receive chain.
[0030] In the case of a MIMO system, each antenna of each access
terminal may be
treated as a separate access terminal for the purposes of determining a
calibration ration.
Then when the calibration ratios are combined, each separate calibration ratio
or
calibration information for each antenna of each access terminal may be
utilized as a
separate component.
[0031] As used herein, an access point may be a fixed station used for
communicating with the terminals and may also be referred to as, and include
some or
all the functionality of, a base station, a Node B, or some other terminology.
An access
terminal may also bc referred to as, and include some or all the functionality
of, a user
equipment (UE), a wireless communication device, terminal, a mobile station or
some
other terminology.
[0032] It should be noted that while FIG. 1, depicts physical sectors,
i.e. having
different antenna groups for different sectors, other approaches may be
utilized. For
example, utilizing multiple fixed "beams" that each cover different areas of
the cell in
frequency space may be utilized in lieu of, or in combination with physical
sectors.
Such an approach is depicted and disclosed in copending US Patent Application
Serial
No. 11/260,895, entitled "Adaptive Sectorization In Cellular System."
=
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[0033] Referring to FIG. 2, an antenna arrangement 100 comprising a
receiver chain
102 and a transmitter chain 104 in accordance with various aspects described
herein.
Receiver chain 102 comprises a down converter component 106 that down converts
a
signal to a baseband upon receipt. Down converter component 106 is operatively
con-nected to an automatic gain control (AGC) functionality 108 that assesses
received
signal strength and automatically adjusts a gain applied to the received
signal to
maintain receiver chain 102 within its associated linear operation range and
to provide a
constant signal strength for outputting through transmitter chain 104. It will
be
appreciated that AGC 108 can be optional to some embodiments described herein
(e.g.,
automatic gain control need not be performed in conjunction with every
embodiment).
AGC 108 is operatively coupled to an analog-to-digital (A/D) converter 110
that
converts the received signal to digital format before the signal is smoothed
by a digital
low-pass-filter (LPF) 112 that can mitigate short-term oscillations in the
received signal.
Finally, receiver chain 102 can comprise a receiver processor 114 that
processes the
received signal and can communicate the signal to one or more components of
transmitter chain 104.
[0034] Transmitter chain 104 can comprise a transmitter processor 116 that
receives
a signal from receiver chain 102 (e.g., transmitter receives a signal that was
originally
received by receiver chain 102 and subjected to various processes associated
with the
components thereof, ...). Transmitter processor 116 is operatively coupled to
a pulse
shaper 118 that can facilitate manipulating a signal to be transmitted such
that the signal
can be shaped to be within. bandwidth constraints while mitigating and/or
eliminating
inter-symbol interference. Once shapcd, thc signal can undergo digital-to-
analog (D/A)
conversion by a D/A converter 120 before being subjected to an operatively
associated
low-pass filter (LPF) 122 in transmitter chain 104 for smoothing. A pulse
amplifier
(PA) component 124 can amplify the pulse/signal before up-conversion to the
baseband
by an up-converter 126.
[0035] Antenna array 100 may exist for each antenna of both an access point
and
access terminal. As such, there may be a noticeable difference observed
between
transfer characteristics of transmitter chain 104 and receiver chain 102
and/or samples
thereof, reciprocity of the equivalent channel and/or transmitter/receiver
variations may
not be assumed. When calibrating an array of antennas 100, an understanding of
the
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magnitude of variations, in terms of -the effects on the phase and/or
amplitude, of signals
propagated. along the transmitter and receiver chains and their influence on
the accuracy
of a reciprocity assumption may be utilized in order to facilitate the
calibration process.
Furthermore, in the case of an antenna array, generally each antenna 100 has a
different
transmit-ter chain 104 and a receiver chain 102 than each other antenna.
Therefore, each
different transmitter chain 104 may have different effects, in terms of phase
and/or
amplitude, as any other transmitter chain 104, respectively. The same can be
true for
receiver chains 102 of each antenna 100.
[0036] The mismatches in the effects can be due to the physical structure
of the
antenna 100, component differences, or a number of other factors. Such
mismatches
can include, for example, mutual coupling effects, tower effects, imperfect
knowledge
of element locations, amplitude and/or phase mismatches due to antenna
cabling, and
the like. Additionally, mismatches can be due to hardware elements in
transmitter chain
104 and/or receiver chain 102 of each antenna 100. For example, such
mismatches can
be associated with analog filters, .1 and Q imbalance, phase and/or gain
mismatch of a
low-noise amplifier or a pulse amplifier in the chains, various non-linearity
effects, etc.
[0037] For an access point, to calibrate each transmit chain to its
corresponding
receive chain (i.e. the receive chain corresponding to the same antenna)
independently
would require a complex and potentially unwieldy process. Further, any
specific
feedback, for forward link transmission, or pilots, used for reverse link
transmission, for
any given access terminal is subject to the noisc for that user. Therefore,
for any givell
calibration ratio estimated based on both the forward and reverse links, there
is some
error introduced by the channel variation and noise. Therefore, in several
aspects, one
or more calibration ratios estimated for a number of different antennas of
different
access terminals are combined in order to obtain a single calibration ratio to
be used by
the access point for transmission to one or all of the access terminals. In
certain aspects,
the combination may constitute an average of all of the calibration ratios for
each
antenna of each access terminal communicating with the access point, or some
predetermined subset. In another aspect, the combination may be done in a
joint
optimization fashion where the channel measurements from and for each antenna
of
each access terminal are combined to estimate a single calibration ratio that
is a
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combination of the gain mismatches for each antenna of each access terminal,
without
calculating an individual calibration ratio for each antenna of each access
terminal.
[0038] For any given antenna of each access terminal, the access point uses
its
reverse link channel estimates for that antenna as well as the forward link
channel
estimates, which are performed at the access terminal and fed back to the
access point,
in order to estimate or calculate the calibration ratio, based on that antenna
of that
access terminal.
[0039] A forward link channel estimate, , may be
estimated at the access
terminal for transmissions from the access point's i-th transmit antenna to
the antenna of
the access terminal. However, any channel estimate will have components
related to the
noise of the channel, along with any gain or distortion caused by the access
points
transmit chain and the access terminals receive chain. The forward link
channel
estimate may then be written as:
= cx(i) = h + n.
AT ¨ AT AP
AT receive chain AP transmit chain Physical Measurement
gain mismatch gain mismatch channel noise
(1)
In Equation. 1, channel estimate is a function of the gain mismatch PAT of the
access
terminal receiver chain for the particular antenna, the gain mismatch
a(,;(1):, of the
transmitter chain of the access point, hiwhich is the physical channel between
the two
antennas being measured, and the noise 721 of the channel that is part of the
channel
estimate.
[0040] In the case of reverse link transmissions, the channel estimate at
the access
point's i-th receive antenna due to transmission from the antenna of the
access terminal
4, is essentially an inverse of Equation 1. This can be seen in Equation 2
below:
=h +
AP = a AT=AP = i
AT transmit AP receive chain Physical Measurement
chain gain mismatch channel noise (2)
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In Equation 2, this channel estimate is a function of the gain mismatch aAT of
the access
terminal transmit chain for that antenna, the gain mismatch n of the access
point
receiver chain, hi which is the physical channel between the two antennas
being
measured, and the noise v, of the channel that is part of the channel
estimate.
[0041] In order to calibrate the antenna array the mismatch errors between
receiver
chains 102 and transmitter chains 104 of the antennas 100 therein is shown
below in
Equation 3. It should be noted that other methodologies and mathematical
relationships
may be employed to achieve array calibration in conjunction with, in lieu of,
the
methodologies and mathematical relationships described herein.
4(i) R(1) R(i) (3)
AP a AT AP AP
(i)
Cz= y = (1) = 7 == 77.
a
AT 13 AT a(i)
AP AP
In Equation 3, ei is the overall mismatch ration between reverse link
transmissions and
forward link transmission, y is the mismatch ratio of the gains between
transmit and
receive chains of the access terminal for the particular antenna, and is the
mismatch
ratio of the receive and transmit chains for the ith antenna at the access
point. It should
be noted that y is substantially constant for each antenna pair at the access
point. Also,
in some regards Equation 3 is idealized, as the noise estimate is not included
therein.
[0042] The calibration ratios e, , i =1,...,M , where M is the number of
antennas in
the access point antenna array can be grouped into one vector ë, for each
antenna at the
access terminal, which may be termed a "calibration vector."
771
1
62 772 z2
e . + . -==y-i+n (4)
_OM ijM zm
[00431 In Equation 4, the entries of vector e correspond to the estimates
for each
antenna of the access point with respect to a single antenna of an access
terminal. It
should be noted that the elements of vector e may be complex numbers including
both
the amplitude and phase mismatch for each transmit and receive chains of the
access
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12
point antenna array as well as common mismatch corresponding to the transmit
and
receive mismatch of the access terminal transmit and receive chains for the
particular
antenna. It should be noted that while Equation 4, describes a vector that has
entries for
one access terminal antennas, it may include entries for multiple access
terminals or
multiple antennas of an access terminal.
[0044] Thc noise vector n includes effects of channel measurement
errors (NISE)
and also the effects of channel measurement de-correlation, since the
measurements of
the gains are performed at different times thus allowing channel variation
over time as
well as temperature and other variations to effect the measurement.
[00451 An estimated calibration vector i". corresponding to access
terminal u, may
be determined as shown below in Equation 5.
(5)
where n is the gain mismatch corresponding to the access terminal antenna's
transmit
and receive chains and q is the mismatch vector corresponding to the access
point
antenna array transmit and receive chains. The vector "oõ is determine for all
of thc
antennas of the access point antenna array with respect to each antenna of
each access
terminal.
[0046] In the above it should be noted that there are several methods
to combine
different calibration vector estimates (corresponding to measurements from
different
antenna's of different access terminals) to generate an overall or combined
calibration
vector. One way to do this combination is to average all the calibration
vector estimates
to obtain a single estimate.
[0047] In this approach, each calibration vector estimate includes a
multiplicative
factor, n, which is different for different access terminals. In a case where
one or more
access terminals have a very large gain mismatch Yu, simple averaging may lead
to
results that bias the average toward the antenna's having the largest gain
mismatch yõ.
[0048] In another aspect, each calibration vector estimate,
corresponding to a
specific access terminal, is normalized according to an element of the vector.
This may
provide minimization in those cases where one or more access terminals have
high gain
mismatch n. This process is depicted below in. Equation 6.
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13
u
= =--Tr DI (6)
u=i
It should be noted that, in certain aspects, the normalizing element may be
any element
of the calibration vector, as long as it is the same element for each
calibration vector
estimate, e.g. the first element. The sum of the normalized elements is then
divided by
the total number of elements Uof the vector é.
[0049] Another approach that may be utilized to combine different
calibration
vector estimates may be based upon combining the estimated vectors in a
matrix. For
instance, in certain aspects, it may that that each calibration vector
estimate is a rotated
and scaled version of the same vector II and the rotation and scaling are due
to the
different mismatches ru for the different access terminals. One way to get rid
of this
scaling and rotation is to first normalize each calibration vector to have a
unit norm.
Then, a matrix Q whose columns are the normalized calibration vector estimates
may
be formed from the calibration vectors. A single estimate for the calibration
vector is
obtained by performing a decomposition of the matrix, e.g. a singular value
decomposition on the matrix Q. The eigenvector corresponding to the maximum
singular value may be used as the overall calibration vector estimate, e.g. as
shown in
Equation (7) below.
.
Q= [c1 Z2 '" e. =- i=1,...,u
jlcjll (7)
SVD(Q)=U-S=V
[0050] As exemplified in the three approaches above, a calibration ratio is
generally
estimated in two steps. First, values corresponding to the elements of
calibration
vectors are calculated for the antenna array, or those antennas of interest.
The
calibration vectors are then combined according to one or more different
mathematical
processes.
[0051] An alternative to calculating multiple calibration vectors is to
utilize a joint
optimization procedure using multiple access point and access measurement as
follows.
CA 02628374 2012-12-18
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14
In some cases, the access terminal antennas and access point may generate
their channel
estimates for different frequency tones and at different time instants.
Further, there may
be a timing error of rõ,õ between the access point and the u-th access
terminal at time k.
In such a case, the forward link channel vector estimate g,,k measured at the
antenna of
the access terminal may be related to the reverse link channel vector estimate
measured at the access point. One approach, utilizing the calibration vector
n, and the
mismatch yzi of the access terminal antenna is depicted in Equation 8 below.
2/. = = diag(hi,k,.)-11+ni,k,u
ri,k,u Z 1,k,u =114.11r,k,u (8)
In Equation 8, Z,),,u is a diagonal matrix whose diagonal elements are the
elements of
the reverse link channel vector estimate hi,k,u and Y1,u= a/. e-iwirkA The
subscripts i,k,u,
are the tone, time, and user indexes, respectively. In the above equation, the
unknowns
are the calibration vector and the access terminal specific mismatch Yi,h,u. A
feature
of Equation 8 is that access terminal mismatch includes the effect of the
timing
mismatch between the access point and the antenna of the access terminal in
addition to
the gain mismatch due to the access terminal transmit and receive chains for
that
antenna. One way to obtain a solution for n and yi,k is to utilize a minimum
mean
squared error (MMSE) approach as shown in Equation 9.
p E11,k,u= = 1(
1,k ,u
(9)
{1,7404} arg min pi,(11,71,k,u) s=t= 111111=1
Solutions for and may be given by Equation 10 below.
m inimal eigenvector of F = K =
,k ,u
(10)
g,,,,, = I,k .11
71,k ,u
141,k,ugi,k,u
where, for a vector x, the orthogonal projection operator rt may be defined as
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nix I --7¨xx (11)
x x
[00521 To compensate for the mismatches, the calibration ratios may be used
to alter
the gain, in terms of both, or either, the phase and amplitude of the
transmitter chain of
the access point to match it to its receiver chain or equivalently to alter
the gain of the
receive chain of the access point to match it to its transmit chain.
[00531 More specifically, the access point may use maximal ratio combining
(MRC)
beamforming, equal gain combining (EGC) beamforming, or any other spatial pre-
processing techniques for transmission to any access terminal. That is, if the
reverse
link channel vector is h, the access point uses the following prc-processing
weights for
transmission:
WuRc (h) =h*/1h1 , 1111= 4-77i; for MRC
1 (12)
WEGC (h) = exPe-fin) , (ph = h
for EGC
With a calibration vector estimate 11, the access point may uses the following
pre-
processing weights to compensate for its transmit and receive chain
mismatches:
WmRc =""-= d iag(n) = h* , =h for MRC
1(13)
WEGC d iag(o ) = exp(¨ j(ph ) , (ph =ll h for EGC
VA/
where diag(%) = diag(0
[0054] While FIG. 2, depicts and describes one embodiment of receiver chain
102
and transmitter chain 104 other layouts and structures may be utilized. For
example, a
different number of components may be used in both receiver chain 102 and
transmitter
chain 104. Additionally, different devices and structures may also be
substituted.
[00551 It should be noted that the combined or joint calibration vectors
may be
formed by treating each antenna, or group of antennas, of a given access
teiminal as a
separate access tcrminal. In that way, the calibration process may be
simplified and
each access terminal need not be calibrated independently.
[0056] FIG. 3 illustrates a timing cycle for a calibration from a single
access
terminal, where a TDD system having a single forward link frame or burst
adjacent to a
single reverse link frame or burst is utilized. As can be seen, one or more
pilots
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16
transmitted, from each of the antennas, on the reverse link is(are) measured
at the access
point. The time period of the measurement is a function of the decoding time
of the
access point. During this decoding period one or more pilots are transmitted
on the
forward link to the access terminal. The access terminal then measures the
pilots to
estimate the forward link channel for each receive antenna. As with the
reverse link
estimates, some decoding lag exists. The decoded forward link estimates need
to be
transmitted back to the access point in order to generate the calibration
ratio. Therefore,
it can be seen that there is some minimum amount of time, and therefore
maximum
access terminal velocity, for which calibration can be maintained without
drift being a
strong or substantially interfering factor.
[0057] As can be seen from FIG. 3, if multiple channel estimates from
multiple
access terminals are utilized the noise and drift associated may be reduced or
at least
sampled over a range of times and receive chains. Further, if multiple
antennas for
each access terminal is utilized and treated independently the drift and noise
may be
better estimated since the noise and drift may be more uniform for those
antennas for a
single access terminal thus mitigating any anomalies for a given antenna.
[0058] FIG. 4 illustrates aspects of logic that facilitates calibrating an
antenna array
to compensate for gain mismatch. The system 300 comprises a calibration
component
302 that includes a mismatch estimation component 304 that analyzes models
receiver
chain output signals and/or comparisons between receiver chain output signals
and a
ratio aggregation calculator 306 that calculates ratios that are used to
generate vector II
and aggregates them for use using one of the methods described above to
combine
different measurements from different antennas of different access terminals.
[0059] FIG. 5 illustrates aspects of a system that facilitates calibrating
an antenna
array to compensate for gain mismatch. The system 400 comprises a processor
402 that
is operatively coupled to an antenna array 404. Processor 402 can determine
gain
mismatches for individual antenna combinations at the access terminal and
access point
utilizing calibration component 406. Processor 402 further comprises a
calibration
component 406 that determines the calibration ratios and then generates and
utilizes the
vector n.
pnoi System 400 can additionally comprise memory 408 that is operatively
coupled to processor 402 and that stores information related to array
calibration, ratio
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generation and utilization, and generating calibration data, etc., and any
other suitable
information related to calibrating antenna array 404. It is to be appreciated
that
processor 402 can be a processor dedicated to analyzing and/or generating
information
received by processor 402, a processor that controls one or more components of
system
400, and/or a processor that both analyzes and generates information received
by
processor 402 and controls one or more components of system 400.
[0061] Memory 408 can additionally store protocols associated with
generating
signal copies and models/representations, mismatch estimations, etc., such
that system
400 can employ stored protocols and/or algorithms to achieve antenna
calibration and/or
mismatch compensation as described herein. It will be appreciated that the
data store
(e.g., memories) components described herein can be either volatile memory or
nonvolatile memory, or can include both volatile and nonvolatile memory. By
way of
illustration, and not limitation, nonvolatile memory can include read only
memory
(ROM), programmable ROM (PROM), electrically programmable ROM (EPROM),
electrically erasable ROM (EEPROM), or flash memory. Volatile memory can
include
random access memory (RAM), which acts as external cache memory. By way of
illustration and not limitation, RAM is available in many forms such as
synchronous
RAM (SRAM), dynamic RAM (DRAM), synchronous DRAM (SDRAM), double data
rate SDRAM (DDR SDRAM), enhanced SDRAM (ESDRAM), Synchlink DRAM
(SLDRAM), and direct Rambus RAM (DRRA_M). The memory 408 of the subject
systems and methods is intended to comprise, without being limited to, these
and any
other suitable types of memory.
[0062] In certain aspects, memory 408 can store the calibration vectors e-
z, for each
state, i.e. level of amplification, of the AGC. In such aspects, for each
transmission, the
processor 402 may access the calibration vector for the AGC state without
performing a calibration. The decision as to whether to perform an additional
calibration or access a prior calibration vector Za for a give transmission
may be based
upon a time period or number of transmissions since the calibration vector Z.
for the
AGC state was obtained. This may be system parameter or may vary based upon
channel conditions, e.g. loading of the channel.
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[0063] Referring to FIG. 6, a methodology relating to generating
supplemental
system resource assignments are illustrated. For example, methodologies can
relate to
antenna array calibration in a TDMA environment, an OFDM environment, an OFDMA
environment, a CDMA environment, or any other suitable wireless environment.
While, for purposes of simplicity of explanation, the methodologies are shown
and
described as a series of acts, it is to be understood and appreciated that the
methodologies are not limited by the order of acts, as some acts may, in
accordance with
one or more embodiments, occur in different orders and/or concurrently with
other acts
from that shown and described herein. For example, those skilled in the art
will
understand and appreciate that a methodology could alternatively be
represented as a
series of interrelated states or events, such as in a state diagram. Moreover,
not all
illustrated acts may be required to implement a methodology in accordance with
one or
more embodiments.
[0064] FIG. 6 illustrates a methodology for calibrating an array of
antennas for
transmission. Channel estimates for the forward link are received from access
terminals
for each of the receive antennas of the access terminal, block 500. As
discussed above,
these channel estimates may be generated from forward link pilots transmitted
by the
access point. Additionally, channel estimates for the reverse link
information, e.g.
reverse link channel pilots, are generated by the access point for each
transmit antenna
of the access terminal, block 502.
[0065] After both forward link and reverse link channel estimates arc
collected,
calibration ratios for each access terminal antenna and access point antenna
may be
determined, block 504. In certain aspects, the most recent forward link and
reverse link
channel estimate with respect to each other in time is utilized to form a
calibration ratio.
In such cases, multiple estimates for a given access terminal may be performed
based
upon consecutive channel estimate pairs of forward link and reverse link
estimates.
[0066] As discussed with respect to FIG. 3, there may be some time lag
between the
different calculations and transmissions. Further, the functionality for
blocks 500 and
502 may occur substantially simultaneously or at different times for the same
or
different access terminals, although they are likely to be the same for
different antennas
of a single access terminal. Therefore, a calibration ratio may be determined
for a given
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antenna of a given access terminal based upon channel estimates of the forward
link and
reverse link transmissions that may or may not be consecutive in time.
[0067] The calibration ratios are then combined to form a calibration
estimate over
multiple access terminals, block 506. This combined calibration ratio may
include
calibration ratios to some or all of the antennas of the different access
terminals in a
given sector or cell, and have an unequal or equal number of calibration
ratios for each
access terminal antenna for which one or more calibration ratios are being
obtained.
[0068] The combined calibration ratio may be obtained by simply averaging
the
calibration ratios or utilizing the other approaches discussed with respect to
FIG. 2, e.g.
the approaches discussed with respect to Equations 5 or 7.
[0069] Each transmission from each transmission chain of the access point
is then
weighted with weights based upon the combined calibration ratio for that
transmit
chain. Also, a combined or joint set of calibrations weights may be utilized
for one or
more transmit chains of the access point. Alternatively, it is possible to
transmit this
combined calibration ratio or a calibration instruction based upon the
combined
calibration ratio to one or more access terminal antennas. The access
terminals would
then apply the weights based upon the combined calibration ratio to decoding
of the
transmissions received at the antenna of the access terminal.
[00701 Also, in some aspects, the calibration weights are utilized for a
particular
AGC state and not for other AGC states. As such, block 508, would then only
apply to
thc AGC state during block 500.
[0071] FIG. 7 illustrates another methodology for calibrating an array of
antennas
for transmission. Channel estimates for the forward link are received from
access
teadinals for each of the receive antennas of the access terminal, block 600.
As
discussed above, these channel estimates may be generated from forward link
pilots
transmitted by the access point. Additionally, channel estimates for the
reverse link
information, e.g. reverse link channel pilots, arc generated by the access
point for each
transmit antenna of the access terminal, block 602.
[0072] After both forward link and reverse link channel estimates are
collected, a
calibration ratio that utilizes multiple channel estimates for multiple
antennas of
multiple access terminals, block 604. In certain aspects, the most recent
forward link
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and reverse link channel estimate with respect to each other in time is
utilized. In such
cases, multiple estimates for a given access terminal may be performed based
upon
consecutive channel estimate pairs of forward link and reverse link estimates.
[0073] As discussed with respect to FIG. 3, there may be some time lag
between the
different calculations and transmissions. Further, the functionality for
blocks 600 and
602 may occur substantially simultaneously or at different times for the same
or
different access terminals, although they are likely to be the same for
different antennas
of a single access terminal. Therefore, the channel estimates may be
determined for a
given antenna of a given access terminal based upon channel estimates of the
forward
link and reverse link transmissions that may or may not be consecutive in
time.
[0074] The joint calibration ratio may be obtained by utilizing a joint
optimization
process as discussed with respect to FIG. 2, e.g. to Equation 8.
[00751 Each transmission from each transmission chain of the access point
is then
weighted with weights based upon the joint calibration ratio for that transmit
chain.
Also, a combined or joint set of calibrations weights may be utilized for one
or more
transmit chains of the access point. Alternatively, it is possible to transmit
this joint
calibration ratio or a calibration instruction based upon the joint
calibration ratio to one
or more antennas of one or more access terminals. The access terminals would
then
apply the weights based upon the joint calibration ratio to decoding of the
transmissions
received at the antenna of the access terminal.
[0076] Also, in some aspects, the calibration weights arc utilized for a
particular
AGC state and not for other AGC states. As such, block 608, would then only
apply to
the AGC state during block 600.
[0077] Fig. 8 illustrates an exemplary wireless communication system 1300.
The
wireless communication system 1300 depicts one base station and one terminal
for sake
of brevity. However, it is to be appreciated that the system can include more
than one
base station and/or more than one terminal, wherein additional base stations
and/or
terminals can be substantially similar or different for the exemplary base
station and
terminal described below. In addition, it is to be appreciated that the base
station and/or
the terminal can employ the systems (Figs. 1-5) and/or methods (Figs. 6-7)
described
herein to facilitate wireless communication there between.
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[00781 Referring to FIG. 8, a transmitter and receiver in a tnultiple
access wireless
communication system is illustrated. At transmitter system 1310, traffic data
for a
number of data streams is provided from a data source 1342 to a transmit (TX)
data
processor 1344. In an embodiment, each data stream is transmitted over a
respective
transmit antenna. TX data processor 1344 formats, codes, and interleaves the
traffic
data for each data stream based on a particular coding scheme selected for
that data
stream to provide coded data. In some embodiments, TX data processor 1344
applies
beamforming weights to the symbols of the data streams based upon the user to
which
the symbols are being transmitted and the antenna from which the symbol is
being
transmitted. In some embodiments, the bearaforming weights may be generated
based
upon channel response information that is indicative of the condition of the
transrnission
paths between the access point and the access terminal. The channel response
information may be generated utilizing CQI information or channel estimates
provided
by the user. Further, in those cases of scheduled transmissions, the TX data
processor
1344 can select the packet format based upon rank information that is
transmitted from
the user.
[00791 The coded data for each data stream may be multiplexed with pilot
data
using OFDM techniques. The pilot data is typically a known data pattern that
is
processed in a known manner and may be used at the receiver system to estimate
the
channel response. The multiplexed pilot and coded data for each data stream is
then
modulated (i.e., symbol mapped) based on a particular modulation scheme (e.g.,
BPSK,
QSPK, M-PSK, or M-QAM) selected for that data stream to provide modulation
symbols. Thc data rate, coding, and modulation for each data stream may be
determined by instructions executed by a processor 1330, which is coupled to a
memory 1332. In some
embodiments, the number of parallel spatial streams may be varied according to
the
rank information that is transmitted from the user.
[00801 The modulation symbols for all data streams are then provided to a
TX
MIMO processor 1346, which may further process the modulation symbols (e.g.,
for
OFDM). TX MIMO processor 1346 then provides NT symbol streanas to
&transmitters
(TMTR) 1322a through 1 322t. In certain embodiments, TX MIMO processor 1346
applies beamfortning weights to the symbols of the data streams based upon the
user to
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which the symbols are being transmitted and the antenna from which the symbol
is
being transmitted from that users channel response information.
10081] Each transmitter 1322 receives and processes a respective symbol
stream to
provide one or more analog signals, and further conditions (e.g., amplifies,
filters, and
upconverts) the analog signals to provide a modulated signal suitable for
transmission
over the MIMO channel. NT modulated signals from transmitters 1322a through
1322t
are then transtnitted from NT antennas 1324a through 1324t, respectively.
[0082] At receiver system 1320, the transmitted modulated signals are
received by
NR antennas 1352a through 1352r and the received signal from each antenna 1352
is
provided to a respective receiver (RCVR) 1354a through 1354r. Each receiver
1354
conditions (e.g., filters, amplifies, and downconverts) a respective received
signal,
digitizes the conditioned signal to provide samples, and further processes the
samples to
provide a corresponding "received" symbol stream.
100831 An RX data processor 1360 then receives and processes the NR
received
symbol streams from NR receivers 1354a through 1354r based on a particular
receiver
processing technique to provide the rank number of "detected" symbol streams.
The
processing by RX data processor 1360 is described in further detail below.
Each
detected symbol stream includes symbols that are estimates of the modulation
symbols
transmitted for the corresponding data stream. RX data processor 1360 then
demodulates, deinterleaves, and decodes each detected symbol stream to recover
the
traffic data for the data stream whicb is provided to data sink 1364 for
storage and/or
further processing. The processing by RX data processor 1360 is complementary
to that
performed by TX MIMO processor 1346 and TX data processor 1344 at transmitter
system 1310.
[0084] The channel response estimate generated by RX processor 1360 may
be used
to perform space, space/time processing at the receiver, adjust power levels,
change
=-modulation rates or schemes, or other actions. RX processor 1360 may further
estimate
the signal-to-noise-and-interference ratios (Was) of the detected symbol
streams, and
possibly other channel characteristics, and provides these quantities to a
processor 1370,
which is coupled to a memory 1372. RX data processor 1360 or processor 1370
may further derive an
estimate of the "effective" SNR for the system. Processor 1370 then provides
estimated channel
information (CSI), which may comprise various types of information regarding
the
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communication link and/or the received data stream. For example, the CSI may
comprise only the operating SNIt. In some embodiments, the channel information
may
comprises signal interference noise ratio (S1NR). The CSI is then processed by
a TX
data processor 1378, which also receives traffic data for a number of data
streams from
a data source 1376, modulated by a modulator 1380, conditioned by transmitters
1354a
through 1354r, and transmitted back to transmitter system 1310.
[0085] At transmitter system 1310, the modulated signals from receiver
system
1350 are received by antennas 1324, conditioned by receivers 1322, demodulated
by a
demodulator 1390, and processed. by a RX data processor 1392 to recover the
CSI
reported by the receiver system and to provide data to data sink 1394 for
storage and/or
further processing. The reported CST is then provided to processor 1330 and
used to (1)
determine the data rates and coding and modulation schemes to be used for the
data
streams and (2) generate various controls for TX data processor 1344 and TX
MIMO
processor 1346.
[0086] The processor 1390 may also be configured to perform generation
of the
calibration ratios and combined calibration ratio, or the joint calibration
ratio as
discussed with respect to FIGS. 2, 6 and 7 respectively. Further, each antenna
1352a-to
1352r may treated as a separate terminal for the purposes of a combined or
joint
calibration estimate.
[0087] Referring to FIG. 9, an access point 1400 can comprise a main
unit (MU) 1450
and a radio unit (RU) 1475. MU 1450 includes the digital baseband components
of an
access point. For example, MU 1450 can include a baseband component 1405 and a
digital intermediate frequency OF) processing wait 1410. Digital IF processing
unit
1410 digitally processes radio channel data at an intemiediate frequency by
performing
such functions as filtering, channelizing, modulation, and so forth. RU 1475
includes
the analog radio parts of the access point. As used herein, a radio unit is
the analog
radio parts of an access point or other type of transceiver station with
direct or indirect
connection to a mobile switching center or corresponding device. A radio unit
typically
serves a particular sector in a communication system. For example, RU 1475 can
include one or more receivers 1430 ccmnected to one more antennas 1435a-t for
receiving radio communications from mobile subscriber units. In an aspect, one
or
more power amplifiers 1482 a-t are coupled to one or more antennas 1435 a-t.
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Connected to receiver 1430 is an analog-to-digital (A/D) converter 1425. AID
converter
1425 converts the analog radio communications received by receiver 1430 into
digital
input for transmission to baseband component 1405 via digital IF processing
unit 1410.
RU 1475 can also include one or more transmitters 120 connected to either the
same or
different antenna 1435 for transmitting radio communications to access
terminals.
Connected to transmitter 1420 is a digital-to-analog (D/A) converter 1415. D/A
converter 1415 converts the digital communications received from baseband
component
1405 via digital IF processing unit 1410 into analog output for transmission
to the
mobile subscriber units. In some aspects, a multiplexer 1484 for multiplexing
of
multiple-channel signals and multiplexing of a variety of signals including a
voice
signal and a data signal. A central processor 1480 is coupled to main unit
1450 and
Radio Unit for controlling various processing which includes the processing of
voice or
data signal.
[0088] For a multiple-access system (e.g., a frequency division multiple-
access
(FDMA) system, an orthogonal frequency division multiple-access (OFDMA)
system, a
code division multiple-access (CDMA) system, a time division multiple-access
(TDMA) system, etc.), multiple terminals may transmit concurrently on the
reverse link.
For such a system, the pilot subcarriers may be shared among different
terminals. The
channel estimation techniques may be used in eases where the pilot subcarriers
for each
terminal span the entire operating band (possibly except for the band edges).
Such a
pilot subcarrier structure would be desirable to obtain frequency diversity
for each
terminal. The techniques described herein may be implemented by various means.
For
example, these techniques may be implemented in hardware, software, or a
combination
thereof. For a hardware implementation, the processing units used for channel
estimation may be implemented within one or more application specific
integrated
circuits (ASICs), digital signal processors (DSPs), digital signal processing
devices
(DSPDs), programmable logic devices (PLDs), field programmable gate arrays
(FPGAs), processors, controllers, micro-controllers, microprocessors, other
electronic
units designed to perform the functions described herein, or a combination
thereof.
With software, implementation can be through modules (e.g., procedu'res,
functions, and
so on) that perform the functions described herein. The software codes may be
stored in
memory unit and executed by the processors 1390 and 1350.
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[0089] What has been described above includes examples of one or more
embodiments. It is, of course, not possible to describe every conceivable
combination
of components or methodologies for purposes of describing the aforementioned
embodiments, but one of ordinary skill in the art may recognize that many
further
combinations and permutations of various embodiments are possible.
Accordingly, the
described embodiments are intended to embrace all such alterations,
modifications and
variations that fall within the scope of the appended claims. Furthermore, to
the extent that the term. "includes" is used in either the detailed
description or the
claims, such term is intended to be inclusive in a manner similar to the term
"comprising" as "comprising" is interpreted when employed as a transitional
word in a
claim.
