U.S. patent application number 12/500977 was filed with the patent office on 2009-11-05 for method and apparatus for compensating for phase noise of symbols spread with a long spreading code.
This patent application is currently assigned to INTERDIGITAL TECHNOLOGY CORPORATION. Invention is credited to Mihaela Beluri, Amith Vikram Chincholi, Kenneth P. Kearney, Philip J. Pietraski, Rui Yang.
Application Number | 20090274197 12/500977 |
Document ID | / |
Family ID | 37010273 |
Filed Date | 2009-11-05 |
United States Patent
Application |
20090274197 |
Kind Code |
A1 |
Pietraski; Philip J. ; et
al. |
November 5, 2009 |
METHOD AND APPARATUS FOR COMPENSATING FOR PHASE NOISE OF SYMBOLS
SPREAD WITH A LONG SPREADING CODE
Abstract
A method and apparatus for compensating for phase noise of
symbols spread with a long spreading code are disclosed. To
compensate for the phase noise, a phase error estimate is generated
from despread symbols with a short spreading code. A phase
correcting phasor is applied to chip rate data before despreading
the data with a long spreading code. A signal-to-interference ratio
(SIR) on a common pilot channel (CPICH) may be calculated by
spreading the data with a parent spreading code in an orthogonal
variable spreading factor (OVSF) code tree and by combining
symbols. Alternatively, a magnitude of the symbols may be used in
estimating the SIR. The SIR of a channel using a short spreading
code and an SIR of a channel using a long spreading code are
measured. The SIR of the channel with the long spreading code may
be compensated in accordance with a difference between degradation
of the SIRs.
Inventors: |
Pietraski; Philip J.;
(Huntington Station, NY) ; Beluri; Mihaela;
(Huntington, NY) ; Yang; Rui; (Greenlawn, NY)
; Chincholi; Amith Vikram; (West Babylon, NY) ;
Kearney; Kenneth P.; (Smithtown, NY) |
Correspondence
Address: |
VOLPE AND KOENIG, P.C.;DEPT. ICC
UNITED PLAZA, SUITE 1600, 30 SOUTH 17TH STREET
PHILADELPHIA
PA
19103
US
|
Assignee: |
INTERDIGITAL TECHNOLOGY
CORPORATION
Wilmington
DE
|
Family ID: |
37010273 |
Appl. No.: |
12/500977 |
Filed: |
July 10, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11301198 |
Dec 12, 2005 |
7561615 |
|
|
12500977 |
|
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|
60662976 |
Mar 18, 2005 |
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60663874 |
Mar 21, 2005 |
|
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|
60665122 |
Mar 25, 2005 |
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Current U.S.
Class: |
375/148 ;
375/E1.016 |
Current CPC
Class: |
H04B 1/707 20130101;
H04B 2201/70701 20130101 |
Class at
Publication: |
375/148 ;
375/E01.016 |
International
Class: |
H04B 1/707 20060101
H04B001/707 |
Claims
1. A receiver that receives data with a short spreading code
transmitted over a first channel and data with a long spreading
code transmitted over a second channel, comprising: a despreader
that receives transmitted data that has been processed as chip rate
data, and that despreads chip rate data generated from received
signals with a short spreading code to generate symbols; a
constellation correction unit that generates a phase error estimate
based on the symbols; a phasor generator that generates a phase
correcting phasor based on the phase error estimate; a multiplier
that multiplies the phase correcting phasor with the chip data rate
to generate phase corrected chip rate data; and a second despreader
that receives the phase corrected chip rate data and despreads the
corrected chip rate data with a long spreading code.
2. The receiver of claim 1 wherein the first channel is a high
speed physical downlink shared channel (HS-PDSCH) and the second
channel is a common pilot channel (CPICH).
3. The receiver of claim 2 further comprising a
signal-to-interference ratio (SIR) estimator for estimating an SIR
from the second channel symbols on the CPICH channel.
4. The receiver of claim 3, further comprising a channel quality
indicator (CQI) mapping unit for generating a CQI from the SIR on
the CPICH.
5. The receiver of claim 2, wherein the despreader is an HS-PDSCH
despreader that uses a spreading factor of 16.
6. The receiver of claim 1, wherein the constellation correction
unit corrects gain and phase errors.
7. The receiver of claim 1, wherein the phase error estimate is an
average of multiple phase error estimates generated by the
constellation correction unit.
8. The receiver of claim 1, further comprising a magnitude
calculator that calculates a magnitude of the symbols.
9. The receiver of claim 1, further comprising an SIR estimator
that estimates a signal to interference ratio (SIR) SIR using the
magnitude of the symbols.
10. A receiver that receives data with a short spreading code
transmitted over a first channel and data with a long spreading
code transmitted over a second channel, comprising: a first
despreader that receives transmitted data that has been processed
as chip rate data, and that despreads chip rate data generated from
received signals with a short spreading code to generate symbols; a
buffer that receives and stores the chip rate data; a constellation
correction unit that generates phase corrected symbols based on the
generated symbols; a phasor generator that generates a unit
magnitude phasor from phase error estimates received from the
constellation correction unit; a multiplier that multiplies the
unit magnitude phasor and chip rate data to generate
phase-corrected chip rate data; a second despreader that receives
the phase-corrected chip rate data and despreads the corrected chip
rate data with a long spreading code.
11. The receiver of claim 10 wherein the first channel is a high
speed physical downlink shared channel (HS-PDSCH) and the second
channel is a common pilot channel (CPICH).
12. The receiver of claim 11 wherein the second despreader
generates dedicated channel (DCH) high speed control channel
(HS-PDSCH) symbols.
13. The receiver of claim 10 wherein the second despreader
generates common pilot channel (CPICH) symbols.
14. The receiver of claim 13 further comprising a signal to
interference ratio (SIR) estimator generates an SIR estimate from
the CPICH symbols.
15. The receiver of claim 14 further comprising a channel quality
indicator (CQI) generator that generates a channel quality
indication from the SIR estimate.
Description
RELATED APPLICATIONS
[0001] This application is a continuation of U.S. Pat. No.
7,561,615 that issued on Jul. 14, 2009, which in turn claims
priority to U.S. Provisional Applications 60/662,976; 60/663,874;
and 60/665,122 filed Mar. 18, 2005; Mar. 21, 2005; and Mar. 25,
2005 respectively. All of these patents and applications are
incorporated by reference as if fully set forth.
FIELD OF INVENTION
[0002] The present invention is related to a code division multiple
access (CDMA) wireless communication system. More particularly, the
present invention is related to a method and apparatus for
compensating for phase noise of symbols having a long spreading
code.
BACKGROUND
[0003] In a CDMA system employing different lengths of spreading
codes, receiver imperfections, such as phase noise, may degrade
transmissions more when using a longer spreading code than using a
shorter spreading code if the nature of the imperfections is
time-varying, such as phase noise on the scale of the spreading
code length. For example, in a universal mobile telecommunication
system (UMTS) frequency division duplex (FDD) system, the spreading
codes may vary from 4 to 512 chips.
[0004] In a third generation (3G) high speed downlink packet access
(HSDPA) system, adaptive coding and modulation (AMC) is based on a
channel quality indication (CQI) estimated by a wireless
transmit/receive unit (WTRU). The CQI is expected to reflect the
channel quality of the high speed physical downlink shared channel
(HS-PDSCH), which uses a spreading factor (SF) of 16. However, the
CQI is generated based on a signal-to-interference ratio (SIR)
measured on a common pilot channel (CPICH), which has an SF of 256.
In an ideal radio environment, this does not present a problem
because different processing gains due to different SFs are easily
factored into the CQI generation. However, phase noise can impact
the SIR measurements made on signals of different SFs by different
amounts. Therefore, the CQI measurement based on the CPICH may not
reflect the channel quality seen by the HS-PDSCH.
SUMMARY
[0005] The present invention is related to a method and apparatus
for compensating for phase noise of symbols having a long spreading
code. In order to compensate the phase noise, a phase error
estimate is generated from the despread symbols with a short
spreading code. The phase correcting phasor generated from the
phase error estimate is applied to the chip rate data and then the
phase corrected data with long spreading codes is despread. A SIR
on a CPICH may be calculated by despreading the chip rate data with
a spreading code which is a parent code of a spreading code of a
CPICH in an orthogonal variable spreading factor (OVSF) code tree
and by combining symbols. Alternatively, a magnitude of the CPICH
symbols may be used in estimating the SIR. The SIR of a channel
using a short spreading code and an SIR of a channel using a long
spreading code are measured. The SIR of the channel using the long
spreading code may be compensated in accordance with a difference
between degradation of the SIR on the channels.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] FIG. 1 is a block diagram of a receiver for compensating
phase noise for data spread with a long spreading code in
accordance with the present invention.
[0007] FIG. 2 is a flow diagram of a process for compensating phase
noise for data spread with a long spreading code in accordance with
the present invention.
[0008] FIG. 3 is a block diagram of an apparatus for compensating
phase noise in SIR estimation for long SF symbols in accordance
with one embodiment of the present invention.
[0009] FIG. 4 is a block diagram of an apparatus for compensating
phase noise in SIR estimation for long SF symbols in accordance
with another embodiment of the present invention.
[0010] FIG. 5 is a block diagram of an apparatus for compensating
phase noise in SIR estimation for long SF symbols in accordance
with yet another embodiment of the present invention.
[0011] FIG. 6 shows degradation of CPICH SIR and HS-PDSCH SIR in
the presence of radio impairments.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0012] The features of the present invention may be incorporated
into an integrated circuit (IC) or be configured in a circuit
comprising a multitude of interconnecting components.
[0013] The present invention is applicable to any wireless
communication system including, but not limited to, a third
generation partnership project (3GPP) system. The present invention
will be explained with reference to a CPICH and an HS-PDSCH
hereinafter. However, it should be noted that the reference to the
CPICH and the HS-PDSCH is for illustration of the present invention
and the present invention may be applied to any other channel using
any SF.
[0014] FIG. 1 is a block diagram of a receiver 100 for compensating
for phase noise associated with data spread with a long spreading
code in accordance with the present invention. The receiver 100
includes a first despreader 102, a constellation correction unit
104, a buffer 106, a phasor generator 108, a multiplier 110, a
second despreader 112, an SIR estimator 114 and a CQI generator
116. A first channel transmits data spread with a short spreading
code and, simultaneously, a second channel transmits data spread
with a long spreading code. The transmitted data is received and
processed to generate a chip rate data 122. The chip rate data 122
is fed to the buffer 106 and the first despreader 102. The buffer
106 temporarily stores the chip rate data 122. The first despreader
102 despreads the chip rate data 122 with a short spreading code
123 to generate symbols 124. The first despreader 102 may be an
HS-PDSCH despreader for despreading HS-PDSCH transmissions using an
SF of 16.
[0015] Symbols 124 generated by the first despreader 102 are fed to
the constellation correction unit 104. The constellation correction
unit 104 corrects gain and phase errors in the constellation prior
to mapping symbols 124 into phase corrected symbols 125. The
details of the constellation correction unit 104 and the process
for correcting the gain and phase errors are described in a U.S.
patent application Ser. No. 10/980,692 filed Nov. 3, 2004 entitled
"WIRELESS COMMUNICATION METHOD AND APPARATUS FOR PERFORMING
POST-DETECTION CONSTELLATION CORRECTION," which is incorporated by
reference as if fully set forth.
[0016] The phase error estimate 126 of each symbol is calculated
from the constellation correction unit 104. The phase error
estimates 126 are preferably collected over time to generate a
smoothed phase error estimate, which may be generated by filtering
the phase error estimates or by performing a polynomial fit to the
data. The phase error estimate 126 is fed to the phasor generator
108. The phasor generator 108 generates a unit magnitude phasor 130
to correct the phase error in the chip rate data 122. The unit
magnitude phasor 130 is multiplied with buffered chip rate data 132
in the buffer 106 via the multiplier 110 to generate phase
corrected chip rate data 134.
[0017] The phase corrected chip rate data 134 is sent to the second
despreader 112. The second despreader 112 despreads the phase
corrected chip rate data 134 with a long spreading code 135. The
long spreading code 135 may be any length of spreading codes. For
example, the second despreader 112 despreads the phase corrected
chip rate data 134 with spreading codes for a CPICH, a dedicated
channel (DCH), a high speed-shared control channel (HS-SCCH) (or
any other channels) and outputs CPICH symbols 136 and DCH and/or
HS-SCCH symbols 137.
[0018] The CPICH symbols 136 are fed to the SIR estimator 114 for
calculating an SIR estimate 138 on the CPICH. The SIR estimate on
the CPICH is fed to the CQI generator 116 for generating a CQI
140.
[0019] The phase corrected chip rate data 134 may be re-despread
for the short spreading codes iteratively with multiple phase error
corrections. Additional short spreading code despreaders,
constellation correction units and phasor generators may be added
so that the output of the additional despreader may again be used
to do more constellation correction, phase error estimates and
correction.
[0020] FIG. 2 is a flow diagram of a process 200 for compensating
phase noise for data spread with a long spreading code in
accordance with the present invention. A chip rate data is
generated by sampling and descrambling received signals (step 202).
The chip rate data is stored in a buffer temporarily (step 204).
The chip rate data is despread with a short spreading code (step
206). A phase error estimate is generated from symbols obtained by
despreading the chip rate data with the short spreading code (step
208). A phase correcting phasor is then generated from the phase
error estimate (step 210). The phase correcting phasor is applied
to the chip rate data stored in the buffer before despreading the
chip rate data with the long spreading code (step 212).
[0021] FIG. 3 is a block diagram of an apparatus 300 for
compensating for phase noise in SIR estimation for long SF symbols
in accordance with one embodiment of the present invention. The
apparatus 300 includes a despreader 302, a symbol combiner 304 and
an SIR estimator 306. In order to alleviate the impact of the phase
noise on symbols spread with a long spreading code, a shorter
spreading code is used to despread the symbols and soft symbols
output from the despreader 302 is combined to obtain the long SF
symbols. The despreader 302 despreads a post-equalizer chip rate
data 312 with a short spreading code and the symbol combiner 304
combines the symbols 314 output from the despreader 302 in
accordance with timing for CPICH symbol boundary, which will be
explained in detail hereinafter. The combined soft symbols 316 may
be sent to the SIR estimator 306 to calculate a CPICH SIR 318.
[0022] For example, for the case of 3G FDD, a CQI is generated
based on a CPICH SIR estimate. The CPICH SF is 256 and the chip
rate is 3.84 Mchips/s. If the SF of 64 is used for de-spreading,
then four (4) consecutive soft symbols are combined to estimate the
CPICH symbol. A timing signal 320 is provided to the soft symbol
combiner 304 such that the soft symbols 314 to be combined are
aligned to the CPICH symbol boundary.
[0023] For the above example, (a CPICH spreading with an SF=256
code and despreading with an SF=64 code), despreading with a short
spreading code and symbol combining are explained hereinafter.
{right arrow over (s)}=[s.sub.1 s.sub.2 s.sub.3 s.sub.4].sup.T
represents a column vector of soft symbols at the de-spreader
output. {right arrow over (d)}=[d.sub.1 d.sub.2 d.sub.3
d.sub.4].sup.T represents a column vector of symbols transmitted
for each of the 4 codes with SF=256 derived from the common SF=64
parent code in an OVSF code tree. The common SF=64 parent code
corresponds to the OVSF tree branch that the CPICH belongs to.
H.sub.4 represents a 4.sup.th order Hadamard matrix.
[0024] In the absence of noise, the soft symbols output from the
despreader 302 are written as follows:
{right arrow over (s)}=H.sub.4{right arrow over (d)}. Equation
(1)
[0025] The transmitted symbols are estimated from the despread soft
symbols as follows:
{right arrow over ({circumflex over (d)}=H.sub.4.sup.-1{right arrow
over (s)}. Equation (2)
[0026] Using the well known property of the Hadamard matrix:
H.sub.NH.sub.N.sup.T=NI.sub.N, Equation (2) can be rewritten as
follows:
d .fwdarw. ^ = 1 4 h 4 T s .fwdarw. . Equation ( 3 )
##EQU00001##
[0027] For applications where only the CPICH symbols are of
interest, there is no need to perform matrix multiplication. The
matrix multiplication can be replaced with a vector dot-product
operation.
[0028] FIG. 4 is a block diagram of an apparatus 400 for
compensating for phase noise in SIR estimation for long SF symbols
in accordance with another embodiment of the present invention. The
apparatus 400 includes a deapreader 402, a magnitude calculator 404
and an SIR estimator 406. Post-equalizer chip rate data is despread
with a long spreading code and an SIR estimate is calculated by
using the symbol magnitude instead of the complex symbol. The
post-equalizer chip rate data 412 is despread by the despreader 402
using the same spreading code used in transmission, (i.e., a long
spreading code). The symbols 414 are then fed to the magnitude
calculator 404 for calculating magnitude of the symbols. The SIR
estimator 406 uses the magnitude values 416 for calculating a CPICH
SIR 418.
[0029] FIG. 5 is a block diagram of an apparatus 500 for
compensating phase noise in SIR estimation on a channel using a
long spreading code in accordance with yet another embodiment of
the present invention. The apparatus 500 includes an SIR estimator
502, a mapping unit 504 and a CQI generator 506. The SIR estimator
502 estimates an SIR on both a channel using a short spreading code
and a channel using a long spreading code. The SIR measured on a
channel using the long spreading code is then mapped to a
compensated SIR by the mapping unit 504. For example, a measured
SIR on a CPICH (which uses a long spreading code) is compensated by
the channel quality seen by a HS-PDSCH (which uses a short
spreading code). The compensated SIR is then mapped to a CQI by the
CQI generator 506.
[0030] The CPICH SIR mapping is performed in accordance with
performance difference anticipated for different SFs in the
presence of phase noise or other radio impairments. Different types
of radio impairments can be isolated and simulated to quantify the
difference in performance of different spreading codes with
different SFs. For example, for any given set of radio impairments,
simulations can be run for a range of SIR values to measure the
degradation of both the CPICH SIR and the HS-PDSCH SIR with respect
to the ideal values. The difference between the degradation of the
CPICH SIR and the degradation of the HS-PDSCH SIR can then be used
to bias the CPICH SIR or the CQI. In this way, the CQI correctly
reflects the channel quality experienced by the HS-PDSCH.
[0031] Once the difference in performance is quantified, a
framework for the compensation can be constructed. An example of
how the various radio impairments have different impact on the long
SF symbols (e.g., CPICH symbols) versus the short SF symbols (e.g.,
HS-PDSCH symbols) for a 3G FDD system, is shown in FIG. 6.
[0032] The mapping by the mapping unit 504 may be implemented as an
equation evaluation or a as a look-up table (LUT). The compensation
may be implemented prior to mapping to a CQI, or alternatively, may
be applied directly to the CQI generated by an uncompensated CPICH
SIR.
[0033] Although the features and elements of the present invention
are described in the preferred embodiments in particular
combinations, each feature or element can be used alone without the
other features and elements of the preferred embodiments or in
various combinations with or without other features and elements of
the present invention.
* * * * *