Title of Invention

CHANNEL CALIBRATION FOR A TIME DIVISION DUPLEXED COMMUNICATION SYSTEM

Abstract Techniques are described to calibrate the downlink and uplink channels to account for differences in the frequency responses of the transmit and receive chains at an access point and a user terminal. In one method, pilots are transmitted on the downlink and uplink channels and used to derive estimates of the downlink and uplink channel responses, respectively. Correction factors for the access point and correction factors for the user terminal are determined based on (e.g., by performing matrix-ratio computation or minimum mean square error (MMSE) computation on) the downlink and uplink channel response estimates. The correction factors for the access point and the correction factors for the user terminal are used to obtain a calibrated downlink channel and a calibrated uplink channel, which are transpose of one another. The calibration may be performed in real time based on over-the-air transmission.
Full Text FORM 2
THE PATENTS ACT, 1970
(39 of 1970)
AND
THE PATENTS RULES, 2003
COMPLETE SPECIFICATION
(See section 10; rule 13)
"CHANNEL CALIBRATION FOR A TIME DIVISION DUPLEXED COMMUNICATION SYSTEM"
QUALCOMM INCORPORATED, of 5775 Morehouse Drive, San Diego, California 92121-1714, United States of America;
The following specification particularly describes and ascertains the invention and the manner in which it is to be performed.





BACKGROUND
I. Field
The present invention relates generally to communication, and more specifically
to techniques for calibrating downlink and uplink channel responses in a time division duplexed (TDD) communication system.
II. Background
In a wireless communication system, data transmission between an access point
and a user terminal occurs over a wireless channel. Depending on the system design, the same or different frequency bands may be used for the downlink and uplink. The downlink (or forward link) refers to the communication link from the access point to the user terminal, and the uplink (or reverse link) refers to the communication link from the user terminal to the access point. If two frequency bands are available, then the downlink and uplink may be allocated separate frequency bands using frequency division duplexing (FDD). If only one frequency band is available, then the downlink and uplink may share the same frequency band using time division duplexing (TDD).
To achieve high performance, it is often necessary to know the frequency
response of the wireless channel. For example, the response of the downlink channel may be needed by the access point to perform spatial processing (described below) for downlink data transmission to the user terminal. The downlink channel response may be estimated by the user terminal based on a pilot transmitted by the access point. The user terminal may then send the downlink channel response estimate back to the access point for its use. For this channel estimation scheme, a pilot needs to be transmitted on the downlink and additional delays and resources are incurred to send the channel estimate back to the access point.
For a TDD system with a shared frequency band, the downlink and uplink channel responses may be assumed to be reciprocal of one another. That is, if H represents a channel response matrix from antenna array A to antenna array B, then a reciprocal channel implies that the coupling from array B to array A is given by Hr,




where Hr denotes the transpose of matrix H. Thus, for the TDD system, the channel response for one link may be estimated based on a pilot sent on the other link. For example, the uplink channel response may be estimated based on a pilot received via the uplink, and the transpose of the uplink channel response estimate may be used as an estimate of the downlink channel response.
However, the frequency responses of the transmit and receive chains at the
access point are typically different from the frequency responses of the transmit and receive chains at the user terminal. In particular, the frequency responses of the transmit and receive chains used for uplink transmission may be different from the frequency responses of the transmit and receive chains used for downlink transmission. The "effective" downlink channel response (which includes the responses of the applicable transmit and receive chains) would then be different from the reciprocal of the effective uplink channel response due to differences in the transmit and receive chains (i.e., the effective channel responses are not reciprocal). If the reciprocal of the channel response estimate obtained for one link is used for spatial processing on the other link, then any difference in the frequency responses of the transmit and receive chains would represent error that, if not determined and accounted for, may degrade performance.
There is, therefore, a need in the art for techniques to calibrate the downlink and
uplink channels in a TDD communication system.
SUMMARY
Techniques are provided herein to calibrate the downlink and uplink channels to
account for differences in the frequency responses of the transmit and receive chains at an access point and a user terminal. After calibration, an estimate of the channel response obtained for one link may be used to obtain an estimate of the channel response for the other link. This can simplify channel estimation and spatial processing.
In a specific embodiment, a method is provided for calibrating the downlink and
uplink channels in a wireless TDD multiple-input multiple-output (MMO) communication system. In accordance with the method, a pilot is transmitted on the uplink channel and used to derive an estimate of the uplink channel response. A pilot is also transmitted on the downlink channel and used to derive an estimate of the downlink channel response. Correction factors for the access point and correction factors for the



user terminal are then determined based on the downlink and uplink channel response estimates. The access point may apply its correction factors on its transmit side, or on its receive side, or on both the transmit and receive sides. The user terminal may also apply its correction factors on its transmit side, or on its receive side, or on both the transmit and receive sides. The responses of the calibrated downlink and uplink channels are approximately reciprocal with the access point applying its correction factors and the user terminal also applying its correction factors. The correction factors may be determined using matrix-ratio computation or minimum mean square error (MMSE) computation on the downlink and uplink channel response estimates, as described below.
The calibration may be performed in real time based on over-the-air
transmission. Each user terminal in the system may perform calibration with one or multiple access points to derive its correction factors. Similarly, each access point may perform calibration with one or multiple user terminals to derive its correction factors. For an orthogonal frequency division multiplexing (OFDM) system, the calibration may be performed for a set of frequency subbands to obtain correction factors for each frequency subband in the set. Correction factors for other "uncalibrated" frequency subbands may be interpolated based on the correction factors obtained for the "calibrated" frequency subbands.
Various aspects and embodiments of the invention are described in further detail
below.
BRIEF DESCRIPTION OF THE DRAWINGS
The features, nature, and advantages of the present invention will become more
apparent from the detailed description set forth below when taken in conjunction with the drawings in which like reference characters identify correspondingly throughout.
FIG. 1 shows the transmit and receive chains at an access point and a user
terminal in a MIMO system.
FIG. 2A illustrates the application of correction factors on both the transmit and
receive sides at the access point and the user terminal.
FIG. 2B illustrates the application of correction factors on the transmit side at
both the access point and the user terminal.
FIG. 2C illustrates the application of correction factors on the receive side at
both the access point and the user terminal.




FIG. 3 shows a process for calibrating the downlink and uplink channel
responses in a TDD MMO-OFDM system.
FIG. 4 shows a process for deriving estimates of the correction vectors from the
downlink and uplink channel response estimates.
FIG. 5 is a block diagram of the access point and the user terminal.
FIG. 6 is a block diagram of a transmit (TX) spatial processor.
DETAILED DESCRIPTION
The calibration techniques described herein may be used for various wireless
communication systems. Moreover, these techniques may be used for single-input single-output (SISO) systems, multiple-input single-output (MISO) systems, single-input multiple-output (SIMO) systems, and multiple-input multiple-output (MIMO) systems.
A MIMO system employs multiple (NT) transmit antennas and multiple (NR)
receive antennas for data transmission. A MEMO channel formed by the NT transmit and NR receive antennas may be decomposed into Ns independent channels, with Ns spatial channel of the MIMO channel and corresponds to a dimension. The MIMO system can provide improved performance (e.g., increased transmission capacity) if the additional dimensionalities created by the multiple transmit and receive antennas are utilized. This typically requires an accurate estimate of the channel response between the transmitter and receiver.
FIG. 1 shows a block diagram of the transmit and receive chains at an access
point 102 and a user terminal 104 in a MIMO system. For this system, the downlink and uplink share the same frequency band in a time division duplexed manner.
For the downlink, at access point 102, symbols (denoted by a "transmit" vector
Xdn are processed by a transmit chain 114 and transmitted from Nap antennas 116 over a wireless channel. At user terminal 104, the downlink signals are received by Nut antennas 152 and processed by a receive chain 154 to obtain received symbols (denoted by a "receive" vector r^). The processing by transmit chain 114 typically includes digital-to-analog conversion, amplification, filtering, frequency upconversion, and so on. The processing by receive chain 154 typically includes frequency downconversion, amplification, filtering, analog-to-digital conversion, and so on.


For the uplink, at user terminal 104, symbols (denoted by transmit vector xup )
are processed by a transmit chain 164 and transmitted from Nut antennas 152 over the wireless channel. At access point 102, the uplink signals are received by Nap antennas 116 and processed by a receive chain 124 to obtain received symbols (denoted by receive vector rup).



For a TDD system, since the downlink and uplink share the same frequency
band, a high degree of correlation normally exists between the downlink and uplink channel responses. Thus, the downlink and uplink channel response matrices may be
assumed to be reciprocal (or transposes) of each other and denoted as H and Hr, respectively, as shown in equations (1) and (2). However, the responses of the transmit and receive chains at the access point are typically not equal to the responses of the transmit and receive chains at the user terminal. The differences then result in the


The left-hand side of equation (6) represents one form of the calibrated uplink
channel response, and the right-hand side represents the transpose of one form of the
calibrated downlink channel response. The application of the diagonal matrices, Kut


Rearranging the terms in equation (8), the following is obtained:
Eq(9)
The diagonal matrices have been reshuffled in equation (9) using the property
AB = BA for diagonal matrices A and B.
Equation (9) indicates that the calibrated downlink and uplink channel responses
may be obtained by satisfying the following conditions:
Eq (10a)
Eq(10b)
where a is an arbitrary complex proportionality constant.
In general, correction factors for the access point may be applied on the transmit
side and/or the receive side at the access point. Similarly, correction factors for the user terminal may be applied on the transmit side and/or the receive side at the user terminal. For a given station, which may be the access point or the user terminal, the correction matrix for that station may be partitioned into a correction matrix for the transmit side and a correction matrix for the receive side. The correction matrix for one side (which may be either the transmit or receive side) may be an identity matrix I or an arbitrarily
selected matrix. The correction matrix for the other side would then be uniquely
specified. The correction matrices need not directly address the transmit and/or receive
chain errors, which typically cannot be measured.
Table 1 lists nine possible configurations for applying the correction factors at
the access point and the user terminal. For configuration 1, correction factors are applied on both the transmit and receive sides at the access point, and also on both the transmit and receive sides at the user terminal. For configuration 2, correction factors are applied on only the transmit side at both the access point and the user terminal,








FIG. 2B illustrates the application of correction matrices Kap and Kut on the
transmit sides for configuration 2 to account for differences in the transmit and receive chains at the access point and the user terminal. On the downlink, the transmit vector x^ is first multiplied with the correction matrix Kap by unit 112. The subsequent
processing by transmit chain 114 and receive chain 154 for the downlink is the same as shown in FIG. 1. On the uplink, the transmit vector x„, is first multiplied with the
correction matrix Kut by unit 162. The subsequent processing by transmit chain 164 and receive chain 124 for the uplink is the same as shown in FIG. 1. The calibrated downlink and uplink channel responses observed by the user terminal and access point, respectively, may then be expressed as:

FIG. 2C illustrates the application of correction matrices Kap and K J on the
receive sides for configuration 3 to account for differences in the transmit and receive chains at the access point and the user terminal. On the downlink, the transmit vector Xdn is processed by transmit chain 114 at the access point. The downlink signals are


As shown in Table 1, the correction matrices include values that can account for
differences in the transmit and receive chains at the access point and user terminal. This would then allow the channel response for one link to be expressed by the channel response for the other link. The calibrated downlink and uplink channel responses can have various forms, depending on whether the correction factors are applied at the access point and the user terminal. For example, the calibrated downlink and uplink channel responses may be expressed as shown in equations (7), (11) and (12).



The calibration techniques described herein may also be used for wireless
communication systems that employ OFDM. OFDM effectively partitions the overall system bandwidth into a number of (NF) orthogonal subbands, which are also referred to as tones, subcarriers, frequency bins, or subchannels. With OFDM, each subband is associated with a respective subcarrier that may be modulated with data. For a MIMO system that utilizes OFDM (i.e., a MIMO-OFDM system), each subband of each spatial channel may be viewed as an independent transmission channel.
The calibration may be performed in various manners. For clarity, a specific
calibration scheme is described below for a TDD MIMO-OFDM system.
FIG. 3 shows a flow diagram of an embodiment of a process 300 for calibrating
the downlink and uplink channel responses in the TDD MIMO-OFDM system. Initially, the user terminal acquires the timing and frequency of the access point using acquisition procedures defined for the system (block 310). The user terminal may then send a message to initiate calibration with the access point, or the calibration may be initiated by the access point. The calibration may be performed in parallel with registration/authentication of the user terminal by the access point (e.g., during call setup) and may also be performed whenever warranted.
The calibration may be performed for all subbands that may be used for data
transmission (which are referred to as the "data" subbands). Subbands not used for data transmission (e.g., guard subbands) typically do not need to be calibrated. However, since the frequency responses of the transmit and receive chains at the access point and the user terminal are typically flat over most of the subbands of interest, and since adjacent subbands are likely to be correlated, the calibration may be performed for only a subset of the data subbands. If fewer than all data subbands are calibrated, then the subbands to be calibrated (which are referred to as the "designated" subbands) may be signaled to the access point (e.g., in the message sent to initiate the calibration).
For the calibration, the user terminal transmits a MIMO pilot on the designated
subbands to the access point (block 312). The generation of the MIMO pilot is described in detail below. The duration of the uplink MIMO pilot transmission may be dependent on the number of designated subbands. For example, 8 OFDM symbols may be sufficient if calibration is performed for four subbands, and more (e.g., 20) OFDM symbols may be needed for more subbands. The total transmit power is typically fixed. If the MIMO pilot is transmitted on a small number of subbands, then higher amounts of transmit power may be used for each of these subbands, and the SNR for each subband


is higher. Conversely, if the MIMO pilot is transmitted on a large number of subbands, then smaller amounts of transmit power may be used for each subband, and the SNR for each subband is worse. If the SNR of each subband is not sufficiently high, then more OFDM symbols may be sent for the MMO pilot and integrated at the receiver to obtain a higher overall SNR for the subband.


account for the differences in the transmit and receive chains at the access point and user terminal, as follows:

where K represents a set of all data subbands. Since only estimates of the effective downlink and uplink channel responses are available for the designated subbands during calibration, equation (13) may be rewritten as:


where the ratio is taken element-by-element. Each element of C(k) may thus be computed as:

The mean of the normalized rows is then determined as the sum of the Nut normalized rows divided by Nut (block 424). The correction vector kap (k) is set equal to this mean (block 426), which may be expressed as:



The mean of the inverses of the normalized columns is then determined as the sum of the inverses of the Nap normalized columns divided by Nap (block 434). The correction
vector kut (k) is set equal to this mean (block 436), which may be expressed as:



again subject to the constraint u{ = 1. The minimum mean square error may be obtained by taking the partial derivatives of equation (22) with respect to u and v and setting the partial derivatives to zero. The results of these operations are the following equation sets:





computation are generally better than the correction matrices obtained based on the matrix-ratio computation, especially when some of the channel gains are small and measurement noise can greatly degrade the channel gains.


A pair of correction vectors kap (k) and kut (k) is obtained for each designated
subband. The calibration may be performed for fewer than all data subbands. For example, the calibration may be performed for every n-th subband, where n may be determined by the expected response of the transmit and receive chains (e.g., n may be 2, 4, 8, 16, and so on). The calibration may also be performed for non-uniformly distributed subbands. For example, since there may be more filter roll-off at the edges of the passband, which may create more mismatch in the transmit and receive chains, more subbands near the band edges may be calibrated. In general, any number of subbands and any distribution of subbands may be calibrated, and this is within the scope of the invention.


appropriate correction matrix on the transmit side. However, operating the transmit side at a lower power level results in loss of performance. An adjustment may then be made on the receive side to compensate for the known transmit imbalance. If both the transmit and receive chains have smaller gains for a given antenna, for example, due to a smaller antenna gain, then calibration makes no adjustment for this antenna since the receive and transmit sides are matched.
The calibration scheme described above, whereby a vector of correction factors
is obtained for each of the access point and user terminal, allows "compatible" correction vectors to be derived for the access point when the calibration is performed by different user terminals. If the access point has already been calibrated (e.g., by one or more other user terminals), then the current correction vectors may be updated with the newly derived correction vectors.
For example, if two user terminals simultaneously perform the calibration
procedure, then the calibration results from these user terminals may be averaged to improve performance. However, calibration is typically performed for one user terminal at a time. The second user terminal would then observe the downlink with the correction vector for the first user terminal already applied. In this case, the product of the second correction vector with the old correction vector may be used as the new correction vector, or a "weighted averaging" (described below) may also be used. The access point typically uses a single correction vector for all user terminals, and not different ones for different user terminals (although this may also be implemented). Updates from multiple user terminals or sequential updates from one user terminal may be treated in the same manner. The updated vectors may be directly applied (by a product operation). Alternatively, if some averaging is desired to reduce measurement noise, then weighted averaging may be used as described below.



access point until they are updated again.
As shown in equations (10a) and (10b), the correction factors for a given station
(which may be an access point or a user terminal) account for the responses of the transmit and receive chains at that station. An access point may perform calibration with a first user terminal to derive its correction factors and thereafter use these correction factors for communication with a second user terminal, without having to perform calibration with the second user terminal. Similarly, a user terminal may perform calibration with a first access point to derive its correction factors and thereafter use these correction factors for communication with a second access point, without having to perform calibration with the second access point. This can reduce overhead for calibration for an access point that communicates with multiple user terminals and for a user terminal that communicates with multiple access points, since calibration is not needed for each communicating pair of stations.



D. Gain Considerations
The calibration may be performed based on "normalized" gains for the downlink
and uplink channels, which are gains given relative to the noise floor at the receiver. The use of the normalized gains allows the characteristics of one link (e.g., the channel gains and SNR per spatial channel) to be obtained based on gain measurements for the other link, after the downlink and uplink have been calibrated.
The access point and user terminal may initially balance their receiver input
levels such that the noise levels on the receive paths for the access point and user terminal are approximately the same. The balancing may be done by estimating the noise floor, e.g., by finding a section of a received TDD frame (which is a unit of downlink/uplink transmission) that has a minimum average power over a particular time duration (e.g., one or two symbol periods). Generally, the time just before the start of each TDD frame is clear of transmissions, since any uplink data must be received by the access point and then a receive/transmit turnaround time is necessary before the access point transmits on the downlink. Depending on the interference environment, the noise floor may be determined based on a number of TDD frames. The downlink and uplink channel responses are then measured relative to this noise floor. More specifically, the channel gain for a given subband of a given transmit and receive antenna pair may first be obtained, for example, as the ratio of the received pilot symbol over the transmitted pilot symbol for that subband of that transmit and receive antenna pair. The normalized gain is then equal to the measured gain divided by the noise floor.
A large difference in the normalized gains for the access point and the
normalized gains for the user terminal can result in the correction factors for the user terminal being greatly different from unity. The correction factors for the access point
are close to unity because the first element of the matrix Kap is set to 1.
If the correction factors for the user terminal differ greatly from unity, then the
user terminal may not be able to apply the computed correction factors. This is because the user terminal has a constraint on its maximum transmit power and may not be capable of increasing its transmit power for large correction factors. Moreover, a reduction in transmit power for small correction factors is generally not desirable, since this may reduce the achievable data rate.
Thus, the user terminal can transmit using a scaled version of the computed
correction factors. The scaled calibration factors may be obtained by scaling the


computed correction factors by a particular scaling value, which may be set equal to a
gain delta (difference or ratio) between the downlink and uplink channel responses.
This gain delta can be computed as an average of the differences (or deltas) between the
normalized gains for the downlink and uplink. The scaling value (or gain delta) used
for the correction factors for the user terminal can be sent to the access point along with
the computed correction factors for the access point.
With the correction factors and the scaling value or gain delta, the downlink
channel characteristics may be determined from the measured uplink channel response, and vice versa. If the noise floor at either the access point or the user terminal changes, then the gain delta can be updated, and the updated gain delta may be sent in a message to the other entity.

2. MIMO Pilot
For the calibration, a MIMO pilot is transmitted on the uplink by the user terminal to allow the access point to estimate the uplink channel response, and a MIMO pilot is transmitted on the downlink by the access point to allow the user terminal to estimate the downlink channel response. A MIMO pilot is a pilot comprised of NT pilot transmissions sent from NT transmit antennas, where the pilot transmission from each transmit antenna is identifiable by the receiving station. The MIMO pilot may be generated and transmitted in various manners. The same or different MIMO pilots may be used for the downlink and uplink. In any case, the MIMO pilots used for the downlink and uplink are known at both the access point and user terminal.
In an embodiment, the MIMO pilot comprises a specific OFDM symbol
(denoted as "P") that is transmitted from each of the NT transmit antennas, where

NT = Nap for the downlink and NT = Nut for the uplink. For each transmit antenna, the
same P OFDM symbol is transmitted in each symbol period designated for MIMO pilot transmission. However, the P OFDM symbols for each antenna are covered with a different N-chip Walsh sequence assigned to that antenna, where N³Nap for the
downlink and N ³ Nut for the uplink. The Walsh covering maintains orthogonality
between the NT transmit antennas and allows the receiver to distinguish the individual transmit antennas.
The P OFDM symbol includes one modulation symbol for each of the NSb
designated subbands. The P OFDM symbol thus comprises a specific "word" of NSb modulation symbols that may be selected to facilitate channel estimation by the receiver. This word may also be defined to minimize the peak-to-average variation in the transmitted MIMO pilot. This may then reduce the amount of distortion and non-linearity generated by the transmit and receive chains, which may then result in improved accuracy for the channel estimation.
For clarity, a specific MIMO pilot is described below for a specific MIMO-
OFDM system. For this system, the access point and user terminal each have four transmit/receive antennas. The system bandwidth is partitioned into 64 orthogonal subbands, or NF = 64, which are assigned indices of+31 to -32. Of these 64 subbands, 48 subbands (e.g., with indices of ±{1, ..., 6, 8,..., 20,22,..., 26}) are used for data, 4 subbands (e.g., with indices of ±{7, 21}) are used for pilot and possibly signaling, the DC subband (with index of 0) is not used, and the remaining subbands are also not used and serve as guard subbands. This OFDM subband structure is described in further detail in a document for IEEE Standard 802.11a and entitled "Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications: High-speed Physical Layer in the 5 GHz Band," September 1999, which is publicly available.
The P OFDM symbol includes a set of 52 QPSK modulation symbols for the 48
data subbands and 4 pilot subbands. This P OFDM symbol may be given as follows:


where g is a gain for the pilot. The values within the {} bracket are given for subband indices -32 through -1 (for the first line) and 0 through +31 (for the second line). Thus, the first line for P(real) and P(imag) indicates that symbol (-1-y) is transmitted in subband -26, symbol (-1 + j) is transmitted in subband -25, and so on. The second line for P(real) and P(imag) indicates that symbol (l-y) is transmitted in subband 1, symbol (-1 - j) is transmitted in subband 2, and so on. Other OFDM symbols may
also be used for the MIMO pilot.
In an embodiment, the four transmit antennas are assigned Walsh sequences of
% = 1111, W2 = 1010, W3 = 1100, and W4 = 1001 for the MIMO pilot. For a given
Walsh sequence, a value of "1" indicates that a P OFDM symbol is transmitted and a
value of "0" indicates that a -P OFDM symbol is transmitted (i.e., each of the 52
modulation symbols in P is inverted).
Table 2 lists the OFDM symbols transmitted from each of the four transmit
antennas for a MIMO pilot transmission that spans four symbol periods, or N = 4.


Table 2

OFDMsymbol Antenna 1 Antenna 2 Antenna 3 Antenna 4
1 +P +P +P +P
2 +P -P +P -P
3 +P +P -P -P
4 +P -P -P +P
For longer MIMO pilot transmission, the Walsh sequence for each transmit antenna is simply repeated. For this set of Walsh sequences, the MIMO pilot transmission occurs in integer multiples of four symbol periods to ensure orthogonality among the four transmit antennas.


Each transmit antenna is assigned one column of F. The elements in the assigned
column are used to multiply the pilot symbols in different time intervals, in similar
manner as the elements of a Walsh sequence. In general, any orthonormal matrix whose
elements have unity magnitude may be used to multiply the pilot symbols for the
MMO pilot.
In yet another embodiment that is applicable for a MIMO-OFDM system, the
subbands available for transmission are divided into Nx non-overlapping or disjoint subsets. For each transmit antenna, pilot symbols are sent on one subset of subbands in each time interval. Each transmit antenna can cycle through the Nj subsets in NT time intervals, which corresponds to the duration of the MIMO pilot. The MIMO pilot may also be transmitted in other manners.



processing and are denoted as such by their subscripts.
The singular value decomposition is described in further detail by Gilbert Strang
in a book entitled "Linear Algebra and Its Applications," Second Edition, Academic
Press, 1980, which is incorporated herein by reference.
The user terminal can estimate the calibrated downlink channel response based
on a MIMO pilot sent by the access point. The user terminal may then perform singular






Again, additional processing (e.g., channel inversion) may also be performed on
the modulation symbols prior to transmission. The spatial processing may then be expressed as:





Table 3
Uplink Downlink
User Terminal Transmit:x,(*)4«(*fa*)I,(*)!,(*) Receive: IdoW = l_1WYltWKrut(%dn(A:)
Access Point Receive:IUpW=l"1wCWKrapWrupW Transmit:



C. Data Transmission on One Link
[0Q113j Data transmission on a given link may also be achieved by applying correction
matrices at a transmitting station and using an MMSE receiver at a receiving station. For example, data transmission on the downlink may be achieved by applying the correction factors on only the transmit side at the access point and using the MMSE receiver at the user terminal. For simplicity, the description is for a single subband and the subband index k is omitted in the equations. The calibrated downlink and uplink channel responses may be given as:





Equations (45) and (46) indicate that the user terminal can obtain the same performance
with the MMSE receiver regardless of whether the correction factors are applied at the
user terminal. The MMSE processing implicitly accounts for any mismatch between
the transmit and receive chains at the user terminal. The MMSE spatial matched filter is
derived with Hedn if the correction factors are not applied on the receive side at the user
terminal and with Hodn if the correction factors are applied.
Similarly, data transmission on the uplink may be achieved by applying
correction matrices on the transmit side and/or the receive side at the user terminal and using the MMSE receiver at the access point.
4. MIMO-OFDM System
FIG. 5 shows a block diagram of an embodiment of an access point 502 and a
user terminal 504 within a TDD MIMO-OFDM system. For simplicity, the following description assumes that the access point and user terminal are each equipped with four antennas that may be used for data transmission and reception.
On the downlink, at access point 502, a transmit (TX) data processor 510
receives traffic data (i.e., information bits) from a data source 508 and signaling and other information from a controller 530. TX data processor 510 formats, encodes, interleaves, and modulates (i.e., symbol maps) the received data and generates a stream of modulation symbols for each spatial channel used for data transmission. A TX spatial processor 520 receives the modulation symbol streams from TX data processor 510 and performs spatial processing to provide four streams of transmit symbols, one stream for each antenna. TX spatial processor 520 also multiplexes in pilot symbols as appropriate (e.g., for calibration).

Each modulator (MOD) 522 receives and processes a respective transmit symbol stream to generate a corresponding stream of OFDM symbols. Each OFDM symbol stream is further processed by a transmit chain within modulator 522 to generate a corresponding downlink modulated signal. The four downlink modulated signals from modulator 522a through 522d are then transmitted from four antennas 524a through 524d, respectively.
At user terminal 504, antennas 552 receive the transmitted downlink modulated
signals, and each antenna provides a received signal to a respective demodulator (DEMOD) 554. Each demodulator 554 (which includes a receive chain) performs processing complementary to that performed at modulator 522 and provides received symbols. A receive (RX) spatial processor 560 performs spatial processing on the received symbols from all demodulators 554 and provides recovered symbols, which are estimates of the modulation symbols sent by the access point. An RX data processor 570 processes (e.g., symbol demaps, deinterleaves, and decodes) the recovered symbols and provides decoded data. The decoded data may include recovered traffic data, signaling, and so on, which are provided to a data sink 572 for storage and/or a controller 580 for further processing.
Controllers 530 and 580 control the operation of various processing units at the
access point and user terminal, respectively. Memory units 532 and 582 store data and program codes used by controllers 530 and 580, respectively.
During calibration, RX spatial processor 560 provides a downlink channel




downlink and is the m-th column of the matrix Vut(k) for the uplink. Each multiplier 652 multiplies the scaled modulation symbols with its eigenvector value v -(/e) and
provides "beam-formed" symbols. Multipliers 652a through 652d provide four beam-
formed symbol substreams (which are to be transmitted from four antennas) to summers
660a through 660d, respectively.
Each summer 660 receives and sums four beam-formed symbols for the four
eigenmodes for each symbol period and provides a preconditioned symbol for an associated transmit antenna. Summers 660a through 660d provides four substreams of preconditioned symbols for four transmit antennas to buffers/multiplexers 670a through 670d, respectively. Each buffer/multiplexer 670 receives pilot symbols and the preconditioned symbols from TX subband spatial processors 640 for the ND data subbands. Each buffer/multiplexer 670 then multiplexes pilot symbols, preconditioned symbols, and zero symbols for the pilot subbands, data subbands, and unused subbands, respectively, to form a sequence of NF symbols for that symbol period. During calibration, pilot symbols are transmitted on the designated subbands. Multipliers 668a through 668d cover the pilot symbols for the four antennas with Walsh sequences W, through W4, respectively, assigned to the four antennas, as described above and shown in Table 2. Each buffer/multiplexer 670 provides a stream of symbols to a respective multiplier 672.



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.
For a software implementation, the calibration techniques may be implemented with modules (e.g., procedures, functions, and so on) that perform the functions described herein. The software codes may be stored in a memory unit (e.g., memory units 532 and 582 in FIG. 5) and executed by a processor (e.g., controllers 530 and 580, as appropriate). The memory unit may be implemented within the processor or external to the processor, in which case it can be communicatively coupled to the processor via various means as is known in the art.
Headings are included herein for reference and to aid in locating certain
sections. These headings are not intended to limit the scope of the concepts described therein under, and these concepts may have applicability in other sections throughout the entire specification.
The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.


WE CLAIM :
1. A method of calibrating communication links in a wireless time division
duplexed (TDD) communication system, comprising:
obtaining a channel response estimate for a downlink channel from an access point to a user terminal;
obtaining a channel response estimate for an uplink channel from the user terminal to the access point; and
determining correction factors for the access point and correction factors for the user terminal based on the channel response estimates for the downlink and uplink channels, the correction factors for the access point and the correction factors for the user terminal being used to obtain a calibrated downlink channel response and a calibrated uplink channel response.
2. The method of claim 1, further comprising:
applying the correction factors for the access point on a transmit side, or a receive side, or both the transmit and receive sides at the access point.
3. The method of claim 1, further comprising:
applying the correction factors for the user terminal on a transmit side, or a receive side, or both the transmit and receive sides at the user terminal.
4. The method of claim 1, wherein the determining the correction factors
for the access point and the correction factors for the user terminal comprises
determining the correction factors for the access point and the correction factors for the user terminal based on the following equation:



5. The method of claim 4, further comprising:
deriving correction factors for a transmit side of the access point and correction factors for a receive side of the access point based on the following equation:

9. The method of claim 4, wherein the determining the correction factors for the access point and the correction factors for the user terminal comprises


14. The method of claim 1, wherein the determining the correction factors for the access point and the correction factors for the user terminal comprises


deriving a first set of matrices of correction factors for the access point for a first set of frequency subbands, and
interpolating the first set of matrices to obtain a second set of matrices of correction factors for the access point for a second set of frequency subbands.
15. The method of claim 1, wherein the determining the correction factors
for the access point and the correction factors for the user terminal comprises
deriving a first set of matrices of correction factors for the user terminal for a first set of frequency subbands, and
interpolating the first set of matrices to obtain a second set of matrices of correction factors for the user terminal for a second set of frequency subbands.
16. The method of claim 1, further comprising:
transmitting a pilot on the uplink channel, wherein the uplink channel response estimate is derived based on the pilot transmitted on the uplink channel; and
receiving a pilot on the downlink channel, wherein the downlink channel response estimate is derived based on the pilot received on the downlink channel.
17. An apparatus in a wireless time division duplexed (TDD) communication
system, comprising:
means for obtaining a channel response estimate for a downlink channel from an access point to a user terminal;
means for obtaining a channel response estimate for an uplink channel from the user terminal to the access point; and
means for determining correction factors for the access point and correction factors for the user terminal based on the channel response estimates for the downlink and uplink channels, the correction factors for the access point and the correction factors for the user terminal being used to obtain a calibrated downlink channel response and a calibrated uplink channel response.
18. The apparatus of claim 17, further comprising:
means for applying the correction factors for the access point on a transmit side, or a receive side, or both the transmit and receive sides at the access point.




19. The apparatus of claim 17, further comprising:
means for deriving correction factors for a transmit side of the access point and
correction factors for a receive side of the access point based on the correction factors for the access point.
20. The apparatus of claim 17, further comprising:
means for applying the correction factors for the user terminal on a transmit side, or a receive side, or both the transmit and receive sides at the user terminal.
21. The apparatus of claim 17, further comprising:
means for deriving correction factors for a transmit side of the user terminal and correction factors for a receive side of the user terminal based on the correction factors for the user terminal.
22. The apparatus of claim 17, wherein the means for determining the
correction factors for the access point and the correction factors for the user terminal
comprises
means for performing minimum mean square error (MMSE) computation on the channel response estimates for the downlink and uplink channels to determine the correction factors for the access point and the correction factors for the user terminal.
23. The apparatus of claim 17, wherein the means for determining the
correction factors for the access point and the correction factors for the user terminal
comprises
means for performing matrix ratio computation on the channel response estimates for the downlink and uplink channels to determine the correction factors for the access point and the correction factors for the user terminal.
24. A method of calibrating communication links in a wireless time division
duplexed (TDD) multiple-input multiple-output (MIMO) communication system,
comprising:
transmitting a pilot on a first communication link from a first station to a second station;




obtaining a channel response estimate for the first communication link derived based on the pilot transmitted on the first communication link;
receiving a pilot on a second communication link from the second station;
deriving a channel response estimate for the second communication link based on the pilot received on the second communication link; and
determining correction factors for the first station and correction factors for the second station based on the channel response estimates for the first and second communication links, the correction factors for the first station and the correction factors for the second station being used to obtain a calibrated channel response for the first communication link and a calibrated channel response for the second communication link.
25. The method of claim 24, further comprising:
applying the correction factors for the first station on a transmit side, or a receive side, or both the transmit and receive sides at the first station.
26. The method of claim 24, further comprising:
sending the correction factors for the second station to the second station.
27. The method of claim 24, further comprising:
updating the correction factors for the first station based on calibration with a plurality of second stations.
28. An apparatus in a wireless time division duplexed (TDD) communication
system, comprising:
a transmit spatial processor to transmit a first pilot on a first communication link from a first station to a second station;
a receive spatial processor to receive a second pilot on a second communication link from the second station, to derive a channel response estimate for the second communication link based on the received second pilot, and to receive a channel response estimate for the first communication link derived based on the transmitted first pilot; and
a controller to determine correction factors for the first station and correction factors for the second station based on the channel response estimates for the first and



second communication links, the correction factors for the first station and the correction factors for the second station being used to obtain a calibrated channel response for the first communication link and a calibrated channel response for the second communication link.
29. The apparatus of claim 28, wherein the controller performs minimum mean square error (MMSE) computation on the channel response estimates for the first and second communication links to determine the correction factors for the first station and the correction factors for the second station.
30. The apparatus of claim 28, wherein the controller performs matrix-ratio computation on the channel response estimates for the first and second communication links to determine the correction factors for the first station and the correction factors for the second station.
31. The apparatus of claim 28, wherein the controller derives correction factors for the transmit spatial processor and correction factors for the receive spatial processor based on the correction factors for the first station.
32. The apparatus of claim 28, wherein the controller updates the correction factors for the first station based on calibration with a plurality of second stations.
33. A method of transmitting data in a wireless time division duplexed (TDD) multiple-input multiple-output (MIMO) communication system, comprising:
applying correction factors for a first station on a transmit side, or a receive side, or both the transmit and receive sides at the first station;
transmitting a pilot on a first communication link from the first station to a second station; and
receiving a data transmission sent on a second communication link from the second station to the first station, wherein the data transmission is spatially processed based on a channel response estimate for the first communication link derived from the pilot transmitted on the first communication link.



34. The method of claim 33, further comprising:
spatially processing the received data transmission with a matched filter.
35. The method of claim 33, wherein the receiving the data transmission sent
on the second communication link comprises
receiving the data transmission sent on a plurality of eigenmodes of the second communication link.
36. The method of claim 33, wherein the second station applies correction factors on a transmit side, or a receive side, or both the transmit and receive sides at the second station.
37. An apparatus in a wireless time division duplexed (TDD) multiple-input multiple-output (MIMO) communication system, comprising:
means for applying correction factors for a first station on a transmit side, or a receive side, or both the transmit and receive sides at the first station;
means for transmitting a pilot on a first communication link from the first station to a second station; and
means for receiving a data transmission sent on a second communication link from the second station to the first station, wherein the data transmission is spatially processed based on a channel response estimate for the first communication link derived from the pilot transmitted on the first communication link.
38. An apparatus in a wireless time division duplexed (TDD) multiple-input
multiple-output (MIMO) communication system, comprising:
a transmit processor to transmit a pilot on a first communication link from a first station to a second station; and
a receive processor to receive a data transmission sent on a second communication link from the second station to the first station, wherein the data transmission is spatially processed based on a channel response estimate for the first communication link derived from the pilot transmitted on the first communication link, and wherein the transmit processor applies correction factors to the transmitted pilot, or the receive processor applies correction factors to the received data transmission, or




both the transmit processor applies correction factors to the transmitted pilot and the receive processor applies correction factors to the received data transmission.
39. A method of transmitting data in a wireless time division duplexed
(TDD) multiple-input multiple-output (MIMO) communication system, comprising:
transmitting a pilot on a first communication link from a first station to a second station;
receiving a data transmission sent on a second communication link from the second station to the first station, wherein the second station applies correction factbrs on a transmit side, or a receive side, or both the transmit and receive sides at the second station, and wherein the data transmission is spatially processed based on a channel response estimate for the first communication link derived from the pilot transmitted on the first communication link; and
processing the received data transmission with a minimum mean square error (MMSE) receiver at the first station.
40. The method of claim 39, wherein the receiving the data transmission sent
on the second commumcation link comprises
receiving the data transmission sent on a plurality of eigenmodes of the second communication link.




Abstract
Techniques are described to calibrate the downlink and uplink channels to account for differences in the frequency responses of the transmit and receive chains at an access point and a user terminal. In one method, pilots are transmitted on the downlink and uplink channels and used to derive estimates of the downlink and uplink channel responses, respectively. Correction factors for the access point and correction factors for the user terminal are determined based on (e.g., by performing matrix-ratio computation or minimum mean square error (MMSE) computation on) the downlink and uplink channel response estimates. The correction factors for the access point and the correction factors for the user terminal are used to obtain a calibrated downlink channel and a calibrated uplink channel, which are transpose of one another. The calibration may be performed in real time based on over-the-air transmission.


Documents:

1275-MUMNP-2007-ABSTRACT(31-1-2011).pdf

1275-MUMNP-2007-ABSTRACT(31-3-2011).pdf

1275-mumnp-2007-abstract.doc

1275-mumnp-2007-abstract.pdf

1275-MUMNP-2007-CLAIMS(AMENDED)-(31-1-2011).pdf

1275-MUMNP-2007-CLAIMS(AMENDED)-(31-3-2011).pdf

1275-MUMNP-2007-CLAIMS(MARKED COPY)-(31-3-2011).pdf

1275-mumnp-2007-claims.doc

1275-mumnp-2007-claims.pdf

1275-MUMNP-2007-CORRESPONDENCE(11-8-2010).pdf

1275-mumnp-2007-correspondence(27-12-2007).pdf

1275-mumnp-2007-correspondence(ipo)-(291-2010).pdf

1275-mumnp-2007-correspondence-others.pdf

1275-mumnp-2007-correspondence-received.pdf

1275-mumnp-2007-description (complete).pdf

1275-MUMNP-2007-DRAWING(31-1-2011).pdf

1275-mumnp-2007-drawings.pdf

1275-MUMNP-2007-FORM 1(31-3-2011).pdf

1275-mumnp-2007-form 13(31-3-2011).pdf

1275-MUMNP-2007-FORM 2(TITLE PAGE)-(31-3-2011).pdf

1275-MUMNP-2007-FORM 26(31-3-2011).pdf

1275-mumnp-2007-form 3(11-8-2010).pdf

1275-MUMNP-2007-FORM 3(11-8-2010).tif

1275-mumnp-2007-form 3(22-8-2007).pdf

1275-mumnp-2007-form 3(27-12-2007).pdf

1275-MUMNP-2007-FORM 3(31-1-2011).pdf

1275-MUMNP-2007-FORM 3(31-3-2011).pdf

1275-mumnp-2007-form-1.pdf

1275-mumnp-2007-form-18.pdf

1275-mumnp-2007-form-2-1.doc

1275-mumnp-2007-form-2.doc

1275-mumnp-2007-form-2.pdf

1275-mumnp-2007-form-26.pdf

1275-mumnp-2007-form-3.pdf

1275-mumnp-2007-form-5.pdf

1275-mumnp-2007-form-pct-ib-304.pdf

1275-mumnp-2007-form-pct-ib-373.pdf

1275-mumnp-2007-form-pct-isa-237.pdf

1275-mumnp-2007-form-pct-separate sheet-237.pdf

1275-mumnp-2007-general power of attorney(22-8-2007).pdf

1275-MUMNP-2007-OTHER DOCUMENT(31-1-2011).pdf

1275-mumnp-2007-pct-search report.pdf

1275-MUMNP-2007-PETITION UNDER RULE 137(31-1-2011).pdf

1275-MUMNP-2007-PETITION UNDER RULE 137(31-3-2011).pdf

1275-MUMNP-2007-REPLY TO EXAMINATION REPORT(31-1-2011).pdf

1275-MUMNP-2007-REPLY TO HEARING(31-3-2011).pdf

1275-mumnp-2007-wo international publication report(22-8-2007).pdf

abstract1.jpg


Patent Number 247623
Indian Patent Application Number 1275/MUMNP/2007
PG Journal Number 17/2011
Publication Date 29-Apr-2011
Grant Date 27-Apr-2011
Date of Filing 22-Aug-2007
Name of Patentee QUALCOMM INCORPORATED
Applicant Address 5775 MOREHOUSE DRIVE, SAN DIEGO, CALIFORNIA 92121-1714,
Inventors:
# Inventor's Name Inventor's Address
1 WALLACE MARK S. 4 MADEL LANE, BEDFORD, MASSACHUSTTS 01730
2 KETCHUM JOHN W. 37 CANDLEBERRY LANE, HARVARD, MASSACHUSTTS 01451
3 WALTON J. RODNEY 85 HIGHWOODS LANE, CARLISLE, MASSACHUSETTS 01741
4 HOWARD STEVEN J. 75 HERITAGE AVENUE, ASHLAND, MASSACHUSTTS 01721
PCT International Classification Number H04B7/005 H04B7/08
PCT International Application Number PCT/US2006/003203
PCT International Filing date 2006-01-27
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 11/045,781 2005-01-27 U.S.A.