U.S. patent application number 12/490614 was filed with the patent office on 2009-12-24 for method and apparatus for generating equalizer filter tap coefficients.
This patent application is currently assigned to INTERDIGITAL TECHNOLOGY CORPORATION. Invention is credited to Mihaela Beluri, Philip J. Pietraski.
Application Number | 20090316768 12/490614 |
Document ID | / |
Family ID | 36336934 |
Filed Date | 2009-12-24 |
United States Patent
Application |
20090316768 |
Kind Code |
A1 |
Pietraski; Philip J. ; et
al. |
December 24, 2009 |
METHOD AND APPARATUS FOR GENERATING EQUALIZER FILTER TAP
COEFFICIENTS
Abstract
A method and apparatus generating an error signal and an update
vector signal used to generate filter tap coefficients for an
equalizer filter residing in an equalizer. The equalizer filter
outputs an equalized signal in response to receiving a sample data
stream. The error signal is generated by down-sampling the
equalized signal, subtracting the equalized signal from a reference
signal, and filtering and down-sampling the resulting signal.
Simultaneously, the update vector signal is generated by converting
scalar samples of the sample data stream to a data vector signal
and descrambling, filtering, and down-sampling the data vector
signal. A tap coefficients generator is used to generate the filter
tap coefficients for updating the equalizer filter based on the
error signal and the update vector signal.
Inventors: |
Pietraski; Philip J.;
(Huntington Station, NY) ; Beluri; Mihaela;
(Huntington, 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: |
36336934 |
Appl. No.: |
12/490614 |
Filed: |
June 24, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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11216818 |
Aug 31, 2005 |
7555040 |
|
|
12490614 |
|
|
|
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60625627 |
Nov 5, 2004 |
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Current U.S.
Class: |
375/232 |
Current CPC
Class: |
H04L 25/03038 20130101;
H04B 1/7097 20130101; H04L 2025/03477 20130101; H04L 2025/03375
20130101; H04L 2025/03617 20130101 |
Class at
Publication: |
375/232 |
International
Class: |
H03K 5/159 20060101
H03K005/159 |
Claims
1. An equalizer comprising: at least one equalizer filter for
processing at least one input sample data stream with filter
coefficients from at least one antenna to generate at least one
equalized signal; a first multiplier configured to multiply a
scrambling code conjugate signal with the at least one equalized
signal to generate at least one descrambled equalized signal; an
adder configured to generate at least one error signal by
subtracting the at least one descrambled equalized signal from a
reference signal; an error filter configured to filter the at least
one error signal to generate at least one filtered error signal; a
tap coefficients generator configured to generate tap coefficients
based on the at least one filtered error signal for updating the
filter coefficients of the at least one equalizer filter; and a
first down-sampler inserted between the at least one equalizer
filter and the first multiplier, the first down-sampler configured
to down-sampling the at least one equalized signal, wherein the at
least one equalized signal is down-sampled by the first
down-sampler by a factor of over-sampling on a condition that the
at least one sample data stream undergoes over-sampling.
2. The equalizer of claim 1 further comprising: a second
down-sampler inserted between the error filter and the tap
coefficients generator, the second down-sampler configured to
down-sampling the at least one filtered error signal at a desired
down-sampling rate.
3. The equalizer of claim 1 wherein the at least one equalizer
filter is a finite impulse response filter.
4. An equalizer comprising: at least one equalizer filter
configured to process at least one input sample data stream with
filter coefficients from at least one antenna to generate at least
one equalized signal; a first multiplier configured to multiply a
scrambling code conjugate signal with the at least one equalized
signal to generate at least one descrambled equalized signal; an
adder configured to generate at least one error signal by
subtracting the at least one descrambled equalized signal from a
reference signal; an error filter configured to filter the at least
one error signal to generate at least one filtered error signal; a
tap coefficients generator configured to generate tap coefficients
based on the at least one filtered error signal for updating the
filter coefficients of the equalizer filter; a serial-to-parallel
to vector converter configured to convert scalar samples of the at
least one sample data stream to at least one data vector signal; a
second multiplier configured to multiply the scrambling code
conjugate signal with the at least one data vector signal to
generate at least one descrambled data vector signal; and an error
filter vector version unit configured to generate at least one
filtered update vector signal based on the at least one descrambled
data vector signal, wherein the tap coefficients generated by the
tap coefficients generator are also based on the at least one
filtered update vector signal.
5. The equalizer of claim 4 further comprising: a third
down-sampler inserted between the serial-to-parallel to vector
converter and the second multiplier for down-sampling the at least
one data vector signal, wherein if the at least one sample data
stream undergoes over-sampling, the at least one data vector signal
is down-sampled by the third down-sampler by a factor of
over-sampling.
6. The equalizer of claim 5 further comprising: a fourth
down-sampler inserted between the error filter vector version unit
and the tap coefficients generator for down-sampling the at least
one filtered update vector signal at a desired down-sampling
rate.
7. The equalizer of claim 1 wherein the tap coefficients generator
generates the tap coefficients based on a normalized least mean
square algorithm.
8. An equalizer comprising: an equalizer filter for processing at
least one input sample data stream with filter coefficients from at
least one antenna to generate at least one equalized signal; a
serial-to-parallel to vector converter which converts scalar
samples of the at least one sample data stream to a data vector
signal; a first multiplier for multiplying the scrambling code
conjugate signal with the at least one data vector signal to
generate at least one descrambled data vector signal; an error
filter vector version unit for generating at least one filtered
update vector signal based on the at least one descrambled data
vector signal; and a tap coefficients generator for generating tap
coefficients based on the at least one filtered update vector
signal for updating the filter coefficients of the at least one
equalizer filter.
9. The equalizer of claim 8 further comprising: a first
down-sampler inserted between the serial-to-parallel to vector
converter and the first multiplier for down-sampling the at least
one data vector signal, wherein if the at least one sample data
stream undergoes over-sampling, the at least one data vector signal
is down-sampled by the first down-sampler by a factor of
over-sampling.
10. The equalizer of claim 9 further comprising: a second
down-sampler inserted between the error filter vector version unit
and the tap coefficients generator for down-sampling the at least
one filtered update vector signal at a desired down-sampling
rate.
11. The equalizer of claim 8 wherein the equalizer filter is a
finite impulse response filter.
12. The equalizer of claim 8 further comprising: a second
multiplier for multiplying a scrambling code conjugate signal with
the at least one equalized signal to generate at least one
descrambled equalized signal; an adder for generating at least one
equalizer error signal by subtracting the at least one descrambled
equalized signal from a reference signal; and an equalized signal
error filter for filtering the at least one equalizer error signal
to generate at least one filtered equalizer error signal, wherein
the tap coefficients generated by the tap coefficients generator
are also based on the at least one filtered equalizer error
signal.
13. The equalizer of claim 10 further comprising: a second
multiplier for multiplying a scrambling code conjugate signal with
the at least one equalized signal to generate a descrambled
equalized signal; an adder for generating at least one equalizer
error signal by subtracting the at least one descrambled equalized
signal from a reference signal; and an equalized signal error
filter for filtering the at least one equalizer error signal to
generate at least one filtered equalizer error signal, wherein the
tap coefficients generated by the tap coefficients generator are
also based on the at least one filtered equalizer error signal.
14. The equalizer of claim 10 further comprising: a third
down-sampler inserted between the at least one equalizer filter and
the second multiplier for down-sampling the equalized signal,
wherein if the at least one sample data stream undergoes
over-sampling, the at least one equalized signal is down-sampled by
the third down-sampler by a factor of over-sampling.
15. The equalizer of claim 14 further comprising: a fourth
down-sampler inserted between the second error filter and the tap
coefficients generator for down-sampling the at least one filtered
equalizer error signal at the desired down-sampling rate.
16. The equalizer of claim 8 wherein the tap coefficients generator
generates the tap coefficients based on a normalized least mean
square algorithm.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation of U.S. application Ser.
No. 11/216,818, filed Aug. 31, 2005, which claims the benefit of
U.S. Provisional Application No. 60/625,627, filed Nov. 5, 2004,
which is incorporated by reference as if fully set forth.
FIELD OF THE INVENTION
[0002] The present invention relates to an equalizer used in a
receiver. More particularly, the present invention relates to a
method and apparatus for generating tap coefficients for an
equalizer filter residing in the equalizer.
BACKGROUND
[0003] Adaptive equalizers, such as normalized least mean square
(NLMS) equalizers which are used in wireless transmit/receive units
(WTRUs) and base stations, optimize their associated filter tap
weights through an iterative procedure to reach a convergence. In
the case of a pilot-directed equalizer, an error signal used to
generate an update of the equalizer tap weights is derived by
measuring the difference between the locally generated reference
signal and the output of the equalizer. For a frequency division
duplex (FDD) system, this amounts to supplying a reference signal
that corresponds to a scrambled, spread and/or scaled pilot signal
such that data symbols have the desired amplitude.
[0004] When operated at a chip rate, the output of a pilot-directed
equalizer includes a plurality of signals superimposed on one
another whereby only one of which is the pilot signal. Since the
pilot signal is small in comparison to the total output signal, the
error signal generated for filter coefficient adaptation includes
mostly undesired signals.
SUMMARY
[0005] The present invention is related to a method and apparatus
generating an error signal and an update vector signal used to
generate filter tap coefficients for an equalizer filter residing
in an equalizer. The equalizer filter outputs an equalized signal
in response to receiving a sample data stream. The error signal is
generated by down-sampling the equalized signal, subtracting the
equalized signal from a reference signal, and filtering and
down-sampling the resulting signal. Simultaneously, the update
vector signal is generated by converting scalar samples of the
sample data stream to a data vector signal and descrambling,
filtering, and down-sampling the data vector signal. A tap
coefficients generator is used to generate the filter tap
coefficients for updating the equalizer filter based on the error
signal and the update vector signal.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] A more detailed understanding of the invention may be had
from the following description, given by way of example and to be
understood in conjunction with the accompanying drawings
wherein:
[0007] FIG. 1 is a block diagram of an exemplary adaptive equalizer
including an equalizer filter in accordance with the present
invention; and
[0008] FIG. 2 is a flow diagram of a process for generating tap
coefficients for the equalizer filter of the adaptive equalizer of
FIG. 1.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0009] The preferred embodiments will be described with reference
to the drawing figures where like numerals represent like elements
throughout.
[0010] Hereafter, the terminology "WTRU" includes but is not
limited to a user equipment (UE), a mobile station, a laptop, a
personal data assistant (PDA), a fixed or mobile subscriber unit, a
pager, or any other type of device capable of operating in a
wireless environment. When referred to hereafter, the terminology
"base station" includes but is not limited to an access point (AP),
a Node-B, a site controller or any other type of interfacing device
in a wireless environment.
[0011] 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.
[0012] The present invention is applicable to both a pilot-directed
equalizer and a data-directed equalizer. For simplicity, the
present invention will be explained with reference to only the
pilot-directed equalizer.
[0013] Hereafter, the present invention will be explained with
reference to an NLMS algorithm. However, it should be noted that
any type of adaptive equalization or filtering, such as least mean
square (LMS), Griffith's algorithm, recursive least square (RLS),
channel estimation based NLMS (CE-NLMS), and other iterative or
recursive algorithms using error signal feedback in filter
coefficient adaptation may be used.
[0014] FIG. 1 is a block diagram of an exemplary adaptive equalizer
100 in accordance with the present invention. The adaptive
equalizer includes a serial-to-parallel (S.fwdarw.P) to vector
converter 104, down-samplers 108, 122, 134, 154, descrambling
multipliers 112 and 140, an error filter vector version unit 118, a
tap coefficients generator 126, an equalizer filter 130, an adder
144 and an error filter 150.
[0015] An input sample data stream 102 is input to the equalizer
filter 130 and the S.fwdarw.P to vector converter 104. The
equalizer filter 130 is preferably a finite impulse response (FIR)
filter. The equalizer filter 130 processes the sample data stream
102 with filter coefficients which are updated by the tap
coefficients generator 126. The sequence of the sample data stream
102 may be generated at any sampling rate, but preferably two times
(2.times.) the chip rate. The equalizer filter 130 outputs an
equalized signal 132 which is down-sampled by the down-sampler
134.
[0016] If the sample data stream 102 undergoes over-sampling (OS),
the equalized signal 132 is down-sampled by a factor of OS, by the
down-sampler 134. The down-sampler 134 generates a down-sampled
signal 136 which is then multiplied with a scrambling code
conjugate signal 114, P(n), by the descrambling multiplier 140 to
generate a descrambled equalized signal 142 which is always
maintained at the chip rate. The descrambled equalized signal 142
is then subtracted from a reference signal 146 by the adder 144 to
generate an error signal 148 which is input to the error filter
150. The reference signal 146 may be a scaled pilot signal, (e.g.,
a pilot in a common pilot channel (CPICH)). The error signal 148 is
filtered by the error filter 150. For example, an N-moving average
filter may be used as the error filter 150, whereby N is a
despreading factor that is applied to the reference signal 146.
[0017] The equalized signal 132 includes a plurality of signals
superimposed on one another, whereby only one is the pilot signal.
Since the pilot signal is small in comparison to the total
equalized signal 132, the resulting error signal 148 is
substantially an undesired signal. The error filter 150 (e.g., a
low pass filter (LPF)) filters the undesired signal components from
the error signal 148 to generate a filtered error signal 152 which
is optionally down-sampled by the down-sampler 154 at a desired
down-sampling rate M to generate a down-sampled error signal 156.
The down-sampled error signal 156 is input to the tap coefficients
generator 126.
[0018] The S.fwdarw.P to vector converter 104 converts the scalar
samples of the sample data stream 102, x(n), to a data vector
signal 106, X(n), such that X(n)={x(n), x(n-1), . . . , x(n-L)},
where L is the length of the equalizer filter 130. The S.fwdarw.P
to vector converter 104 is similar to a tapped delay line (TDL) of
the equalizer filter 130, whereby the data vector signal 106
indicates the state of the TDL used to generate the equalized
signal 132. The data vector signal 106 undergoes the same vector
version of the processing (i.e., down-sampling, descrambling,
filtering, followed by down-sampling) that the equalized signal 132
has undergone, such that the down-sampled error signal 156 and the
data vector signal 124 are kept aligned.
[0019] The data vector signal 106 is down-sampled by the
down-sampler 108 to generate a down-sampled vector signal 110. If
the sample data stream 102 undergoes over sampling (OS), the
down-sampled signal 110 is down-sampled by the down-sampler 108 by
a factor of OS and is then multiplied with the scrambling code
conjugate signal 114, P(n), by the descrambling multiplier 112 to
generate a descrambled vector signal 116. The error filter vector
version unit 118 is essentially a bank of filters, where each
filter in the bank is the substantially identical to the error
filter 150. The number of filters in the bank is equal to the
length of the descrambled vector signal 116. Each element of the
vector is effectively filtered in the same way as the error filter
150. The error filter vector version unit 118 generates a filtered
update vector signal 120 which is optionally down-sampled by the
down-sampler 122 at a desired down-sampling rate M to generate a
down-sampled update vector signal 124. The down-sampled update
vector signal 124 is input to the tap coefficients generator
126.
[0020] The tap coefficients generator 126 generates tap
coefficients 128 for use by the equalizer filter 130 based on the
down-sampled update vector signal 124 and the down-sampled error
signal 156. The tap update may be performed using any type of
adaptive equalization or filtering, such as LMS, Griffith's
algorithm, RLS, CE-NLMS, or any other iterative or recursive
algorithms using error signal feedback in filter coefficient
adaptation. For example, the equation for the NLMS would be
w .fwdarw. n = .alpha. w .fwdarw. n - 1 + .mu. x .fwdarw. H x
.fwdarw. 2 + e ##EQU00001##
where the down-sampled update vector signal 124 is x, e is
down-sampled error signal 156, parameters .alpha., .mu. are
optional leakage and step size parameters, respectively, and w is
the updated tap coefficients 128. The subscripts n and n-1 indicate
that the previous value of w is used to compute the current value
of w. The parameter .epsilon. is used to optionally prevent
division by zero.
[0021] In accordance with the present invention, the tap
coefficients generator 126 operates with a cleaner error signal and
provides better performance in terms of convergence speed and
miss-adjustment. Down-sampling also permits lower complexity
operation in very slow moving channels.
[0022] The present invention may be applied to diversity
structures. For example, the outputs of two equalizer filters 130
operating on two receive diversity antennas may be combined. The
combined signal may then be descrambled and subtracted from a
reference signal. The resulting error signal may be used to drive a
tap coefficients generator 126 associated with each antenna.
[0023] FIG. 2 is a flow diagram of a process 200 including method
steps for generating tap coefficients 128 for the equalizer filter
130 of FIG. 1. In step 205, the equalizer filter 130 is used to
process an input sample data stream 102 with filter coefficients to
generate an equalized signal 132. In step 210, the equalized signal
132 is down-sampled (by a factor of OS if the sample data stream
undergoes over-sampling). In step 215, a scrambling code conjugate
signal 114 is multiplied with the down-sampled equalized signal 136
to generate a descrambled equalized signal 142. In step 220, an
error signal 148 is generated by subtracting the descrambled
equalized signal 142 from a reference signal 146. In step 225, the
error signal 148 is filtered to generate a filtered error signal
152. In step 230, the filtered error signal 152 is down-sampled at
a desired down-sampling rate to generate a down-sampled filtered
error signal 156. In step 235, scalar samples of the sample data
stream 102, x(n), are converted to a data vector signal 106, X(n),
such that X(n)={(x(n), x(n-1), . . . , x(n-L)}, wherein L is the
length of the equalizer filter 130. In step 240, the data vector
signal 106 is down-sampled (by a factor of OS if the sample data
stream undergoes over-sampling). In step 245, the scrambling code
conjugate signal 114 is multiplied with the down-sampled data
vector signal to generate a descrambled data vector signal 116. In
step 250, a filtered update vector signal 120 is generated based on
the descrambled data vector signal 116. In step 255, the filtered
update vector signal 120 is down-sampled at the desired
down-sampling rate. In step 260, tap coefficients are generated for
the equalizer filter 130 based on the down-sampled filtered error
signal 156 of step 230 and the down-sampled filtered update vector
signal 124 of step 255.
[0024] While the present invention has been described in terms of
the preferred embodiment, other variations which are within the
scope of the invention as outlined in the claims below will be
apparent to those skilled in the art.
* * * * *