Note: Descriptions are shown in the official language in which they were submitted.
CA 02440064 2007-10-30
Docket No.: 1349.1244
TITLE OF THE INVENTION
CHANNEL EQUALIZER OF SINGLE CARRIER RECEIVER AND EQUALIZING METHOD
THEREOF
BACKGROUND OF THE INVENTION
1. Field of the Invention
[0002] The present invention relates to a channel equalizer of a single
carrier receiver, and
more particularly, to a channel equalizer of a single carrier receiver that
performs equalization of
a received signal with respect to channel environments using a plurality of
decoded symbols
provided by a trellis decoder, and a channel equalizing method thereof.
2. Description of the Related Art
[0003] As for a transmission scheme for digital broadcasting signals, there
are mainly a
vestigial sideband (VSB) modulation scheme and a coded orthogonal frequency
division
multiplexing (COFDM) modulation scheme. The VSB modulation scheme transmits
broadcasting signals with a single carrier. The COFDM modulation scheme
multiplexes and
transmits the broadcasting signal through multiple transmission channels. The
VSB modulation
scheme is a US-oriented digital broadcasting transmission scheme that has been
adopted in
various countries including South Korea and U.S.A., while the CDFDM modulation
scheme is a
European-oriented digital broadcasting transmission scheme.
[0004] A current standard adopted for the VSB modulation of a US-oriented
terrestrial wave
digital television is an ATSC-8VSB that converts the broadcasting signal to be
transmitted into 8
levels. Meanwhile, a VSB receiver receives the broadcasting signal that has
been modulated
by the VSB modulation. The VSB receiver is provided with a channel equalizer
that equalizes
distortions occurring in transmission channels.
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[0005] FIG. 1 is a view showing a channel equalizer adapted for use in a
conventional single
carrier receiver by way of an example. The channel equalizer of the single
carrier receiver
includes a feed-forward filter 10, a feedback filter 30, an adder 40 and a
slicer 50. The channel
equalizer having the feed-forward filter 10 and the feedback filter 30 is
called a decision
feedback equalizer (DFE).
[0006] The feed-forward filter 10 removes a pre-ghost influence from
respective symbols of
the broadcasting signal. The feedback filter 30 removes a post-ghost influence
from the
respective symbols of the broadcasting signal. The adder 40 adds a remnant
value obtained
from the feed-forward filter 10 after the pre-ghost removal, with another
remnant value obtained
from the feedback filter 30 after the post-ghost removal.
[0007] The slicer 50 determines a level of a signal obtained from the adder 40
to be a
nearest one among predetermined levels. The slicer 50 feeds back the
determined signal level
to the feedback filter 30.
[0008] The feed-forward filter 10 includes a buffering unit 12, a multiplier
14, and a second
adder 16. The buffering unit 12 stores and buffers broadcasting signals in
respective buffers Z-'
in an inputting order and in a symbol unit. The multiplier 14 multiplies the
respective symbols
that are stored and buffered in the buffers Z-' of the buffering unit 12 by a
feed-forward filter tap
coefficient of an equalizer (not shown), thereby removing the pre-ghost from
the broadcasting
signals. The second adder 16 adds up values obtained from the multiplier 14
after the removal
of the pre-ghost from the broadcasting signals.
[0009] The feedback filter 30 includes a buffering unit 32, a multiplier 34
and a third adder 36.
The buffering unit 32 stores and buffers level data determined ai: the slicer
50 consecutively in
the inputting order and in the symbol unit. The multiplier 34 removes the post-
ghost from the
respective symbols that are stored and buffered in the respective buffers Z-'
of the buffering unit
32. The adder 36 adds up the values obtained after the removal of the post-
ghost from the
broadcasting signals.
[0010] With the channel equalizer of FIG. 1, the determined level data are
input by the slicer
50 as an input of the feedback filter 30. If an error occurs in the slicer 50
in determining the
level data, the error level data is passed through the feedback filter 30 and
added to an output
value from the feed-forward filter 10 at the adder 40.
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[0011] Meanwhile, the slicer 50 usually has a different performance in
determining the level
data according to the number of levels set for the broadcasting signal. For
example, with
respect to the same electric value of the broadcasting signal, an 8-VSB
modulation having 8
levels has a gap between the signal levels as half as a 4-VSB modulation
having 4 levels.
Accordingly, the possibility that the slicer 50 may have an erroneous level
data determination
increases.
[0012] As the error of the slicer 50 increases, an error propagation occurs at
the feedback
filter 30. Furthermore, a signal to noise ratio (SNR) greatly decreases in
accordance with the
error of the slicer 50, thereby degrading performance of the charinel
equalizer and as a whole,
degrading the receptivity of the single carrier receiver.
[0013] In order to avoid performance deterioration of the channel equalizer
due to the
erroneous level data determination by the slicer 50, a trellis coded
modulation (TCM) may be
adapted to perform trellis coding with respect to the signals being input to
the channel equalizer.
[0014] FIG. 2 is a view showing another channel equalizer adapted for use in
the
conventional single carrier receiver, i.e., the conventional VSB receiver. As
shown in FIG. 2, the
channel equalizer includes the feed-forward filter 10, the feedback filter 30,
the adder 40, the
slicer 50, a computation unit 60 and a trellis coded modulation (TCM) unit 70.
[0015] The feed-forward filter 10 removes an influence by the pre-ghost with
respect to the
respective symbols of the broadcasting signal. The feedback filter 30 removes
the post-ghost
with respect to the respective symbols of the broadcasting signal. The adder
40 adds the
remnant value obtained at the feed-forward filter 10 after pre-ghost removal,
to the another
remnant value obtained at the feedback filter 30 after post-ghost removal. The
slicer 50 decides
the level of the signal obtained at the adder 40 to be the nearest one among
the predetermined
levels.
[0016] The computation unit 60 calculates a difference between output values
from the adder
40 and the slicer 50. The TCM unit 70 performs trellis decodinci with respect
to the output value
of the adder 40. The TCM unit 70 feeds back a resultant value of the trellis
decoding to the
feedback filter 30. Accordingly, the feedback filter 30, based on trellis
decoded data fed back
from the TCM unit 70, removes the post-ghost from the respective symbols of
the broadcasting
signal.
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[0017] The trellis-decoded data is, due to a feedback delay caused by the
trellis decoding,
input to the feedback filter 30 during a predetermined time period in which
the feedback filter 30
removes the post-ghost from the respective symbols based on the output value
from the slicer
50. Accordingly, the feedback filter 30 has to remove the post-ghost from the
respective
symbols based on the level data determined at the slicer 50 until the trellis
decoded data is input
from the TCM unit 70, and this causes the slicer 50 to erroneously determine
the level data.
[0018] Furthermore, under a channel environment where ghost which is robust to
the
broadcasting signal exists prior to an output delay of the TCM unit 70, a high
equalization
performance cannot be guaranteed even from the channel equalizer having the
TCM unit 70
employed therein.
SUMMARY OF THE INVENTION
[0019] Accordingly, it is an aspect of the present invention to provide a
channel equalizer of a
vestigial sideband (VSB) equalizer that uses trellis decoding for ghost
removal, which is capable
of improving an equalization performance even under a channel environment
where a ghost
which is robust to the broadcasting signal exists before a signal output delay
is caused by the
trellis decoding, and a channel equalizing method thereof.
[0020] Additional aspects and advantages of the invention will be set forth in
part in the
description which follows and, in part, will be obvious from the description,
or may be learned by
practice of the invention.
[0021] To achieve the above and/or other aspects of the present invention, a
channel
equalizer of a single carrier receiver of an improved equalizatiori efficiency
includes a feed-
forward filter that removes a pre-ghost from respective symbols of a
broadcasting signal, a
feedback filter that removes a post-ghost from respective symbols of the
broadcasting signal, an
adder that adds the pre-ghost removed symbols to the post-ghost removed
symbols, a level
decision unit that determines a level of the symbols added at the adder with
reference to
predetermined level data and then feeds back the determined level to the
feedback filter, a trellis
decoder that performs trellis decoding with respect to the symbols added at
the adder and has a
whole decoding depth as an N (N=natural number), and a whole length of a trace
back delay as
an N x K(K=natural number), an error calculator that calculates an error value
between the
symbols added at the adder and the determined level of the level decision
unit, and a trellis
control unit that controls the trellis decoder so that a plurality of decoded
symbols output from
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Docket No.: 1349.1244
the trellis decoder can be input to the feedback filter in accordance with the
error obtained at the
error calculator.
[0022] The trellis control unit controls the trellis decoder so that the
decoded symbols are
input to the feedback filter when a signal to noise ratio (SNR) corresponding
to the error value is
equal to or more than a predetermined threshold.
[0023] The decoded symbols output from decoding depth states of n
(n S N, N = natural number) are input to a 1+(n x K)th filter tap of the
feedback filter.
Accordingly, the decoded symbols output from the decoding depth states are
input to
respectively corresponding filter taps of the feedback filter.
[0024] With an input of the determined level from the level decision unit, the
feedback filter
removes the post-ghost with respect to the respective symbols based on the
determined level,
and with another input of the decoded symbols from the trellis decoder, the
feedback filter
removes the post-ghost based on the decoded symbols.
[0025] Meanwhile, to achieve the above and/or other aspects of the present
invention, a
channel equalizing method of a single carrier receiver includes removing a pre-
ghost with
respect to respective symbols of an input signal with a feed-forward filter,
removing a post-ghost
with respect to respective symbols of the input signal with a feedlback
filter, combining the pre-
ghost removed symbols with the post-ghost removed symbols, performing a
feedback operation
of deciding a sum of the symbols as a corresponding level with reference to
predetermined level
data to input a determined level to the feedback filter, computing an error
between the sum of
the symbols and the determined level based on a predetermined error updated
algorithm,
performing a trellis decoding operation of performing trellis decoding with a
trellis decoder, and
performing a controlling operation of controlling whether to input a plurality
of decoded symbols
output from the trellis decoder to the feedback filter based on the computed
error.
[0026] The controlling operation inputs the decoded symbols from the trellis
decoder into the
feedback filter when a signal to noise ratio (SNR) corresponding to the error
value is equal to or
more than a predetermined threshold.
[0027] The decoded symbols output from decoding depth states of n
(n <_ N, N = natural number) are input to a 1+(n x K)th filter itap of the
feedback filter.
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Accordingly, the decoded symbols output from the decoding depth states are
input to
respectively corresponding filter taps of the feedback filter.
[0028] The feedback filter removes the post-ghost from the respective symbols
based on the
determined level, and with the input of the decoded symbols from the trellis
decoder, removes
the post-ghost from the respective symbols based on the decoded symbols.
[0029] As a result, even when the SNR corresponding to the error value
obtained at the error
calculator is equal to or more than the predetermined threshold, degradation
of an equalization
efficiency due to an erroneous level decision of the level decision unit is
prevented by the
feedback filter which performs filtering based on the decoded data input from
the trellis decoder.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] These and/or other aspects and advantages of the invention will become
apparent
and more readily appreciated from the following description of the preferred
embodiments, taken
in conjunction with the accompanying drawings of which:
FIG. 1 is a view showing a channel equalizer adapted for use in a conventional
single
carrier receiver;
FIG. 2 is a view showing another channel equalizer adapted for use in the
conventional
single carrier receiver ;
FIG. 3 is a schematic block diagram of a single carrier receiver according to
an
embodiment of the present invention;
FIG. 4 is a block diagram of a channel equalizer adapted for an equalization
performance improvement of the single carrier receiver shown iri FIG. 3;
FIG. 5 is a view illustrating an output of trellis decoded data of a trellis
decoder of FIG. 4;
FIG. 6 is a table listing a signal to noise ratio according tc- an output
delay of trellis
decoding of the trellis decoder of FIG. 4; and
FIG. 7 is a flowchart illustrating a channel equalizing method the single
carrier receiver
shown in FIGS. 3 and 4.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMEINTS
[0031] Reference will now be made in detail to the present preferred
embodiment of the
present invention, examples of which are illustrated in the accompanying
drawings, wherein like
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reference numerals refer to the like elements throughout. The embodiment is
described below
in order to explain the present invention by referring to the figures.
[0032] Hereinafter, the present invention will be described in detail with
reference to the
accompanying drawings.
[0033] A structure of a VSB receiver, one of single carrier receivers, will be
explained with
reference to FIG. 3 showing a schematic block diagram of the VSB receiver
according to an
embodiment of the invention. The VSB receiver is described hereinafter as an
example of the
single carrier receivers. However, the invention is not limited thereto.
[0034] As shown in FIG. 3, the VSB receiver includes a demodulator 81, a
distortion
compensator 82, a comb filter 83, a channel equalizer 84, a phase recovery
unit 85, a trellis
decoder 87, a de-interleaver 88 and a reed-solomon demodulatcir 89.
[0035] The demodulator 81 converts a received broadcasting signal from an RF
bandwidth
into a base bandwidth. The distortion compensator 82 recovers a segment
synchronization
signal, a field synchronization signal and a symbol timing signal with respect
to the broadcasting
signal output from the demodulator 81. The comb filter 83 removes an NTSC
(National
television system committee) interference signal from the broadcasting signal
output from the
demodulator 81.
[0036] The channel equalizer 84 compensates for channel distortion that occurs
during
transmission of the broadcasting signal through a transmission channel. The
phase recovery
unit 85 recovers a phase of the channel distortion-compensated broadcasting
signal. The trellis
decoder 87 performs trellis decoding with respect to the broadcasting signal
with phase
distortion which is recovered at the phase recovery unit 85. The de-
interleaver 88 performs de-
interleaving with respect to the trellis decoded broadcasting signal in
correspondence to
interleaving performed at a transmitter. The reed-solomon demodulator 89
performs decoding
with respect to the de-interleaved broadcasting signal in correspondence to
reed-solomon
encoding performed at the transmitter.
[0037] FIG. 5 shows one example of the trellis decoder 87 shown in FIG. 3. A
whole
decoding depth of the trellis decoder 87 has a state of 0,1,2,3,...,N, and in
a case of performing
de-interleaving in a unit of K (K=natural number) symbol(s), a whole trace
back delay has a
symbol length of K x N. In other words, when the whole decoding depth of the
trellis decoder
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87 is in the state of 0, 1, 2, 3, ..., N, a plurality of certain decodecl data
do , d, ,......, dn (n <_ natuYal number) that are randomly chosen from whole
decoded data
dold, ,......., dN , are fed back to the channel equalizer 84. Accorclingly,
based on the certain
decoded data fed back from the trellis decoder 87, the channel equalizer 84
performs channel
equalization with respect to respective symbols of the broadcasting signal.
[0038] FIG. 4 is a block diagram of a channel equalizer adapted for use in the
single carrier
receivers, for example, the VSB receiver shown in FIG. 3, to impirove an
equalization
performance . The channel equalizer of FIG. 4 is used as the channel equalizer
84, the phase
recovery unit 85, and the trellis decoder 87 shown in FIG. 3.
[0039] As shown in FIG. 4, the channel equalizer includes a feed-forward
filter 110, a
feedback filter 120, an adder 130, a trellis decoder 140 (the trellis decoder
87 of FIG. 3), a level
decision unit 150, an error calculator 160 and a trellis control unit 170.
[0040] The feed-forward filter 110 removes an influence of a pre-ghost with
respect to the
respective symbols of the broadcasting signal. The feedback filter 120 removes
a post-ghost
with respect to the respective symbols of the broadcasting signal. Here, the
feed-forward filter
110 and the feedback filter 120 are constructed so as to sequentially buffer
the input of the
broadcasting signal in a unit of the symbol and then remove the pre- and post-
ghosts from the
symbols buffered at respective buffers (not shown). The feed-foivvard filter
110 and the
feedback filter 120 of FIG. 4 have structures which are identical to the feed-
forward filter 10 and
the feedback filter 30 of FIG. 1, respectively.
[0041] The adder 130 adds a pre-ghost removed value M output from the feed-
forward filter
110 with a post-ghost removed value L output from the feedback filter 120. The
trellis decoder
140 performs the trellis decoding with a resultant value Y obtained at the
adder 130.
[0042] The level decision unit 150 determines a level of the resultant value Y
from the adder
130 to be a nearest one among predetermined levels. Accordingly, the level
decision unit 150
provides the feedback filter 120 and the error calculator 160 with determined
level data D. The
level decision unit 150 may determine the levels in corresponderice with the
predetermined
broadcasting signal, such as 4-level, 8-level, or 16-level.
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[0043] The error calculator 160 calculates an error value E, which is the
difference between
the resultant value Y from the adder 130 and the determined level data D from
the level decision
unit 150.
[0044] The error calculator 160 provides the calculated error value E to the
feed-forward filter
110, the feedback filter 120 and the trellis control unit 170. The feed-
forward filter 110 and the
feedback filter 120 update a tap coefficient based on the error value E
provided by the error
calculator 160.
[0045] Based on the error value E provided by the error calculator 160, the
trellis control unit
170 controls an operation of the trellis decoder 140. More specifically, when
the signal to noise
ratio (SNR) corresponding to the error value E obtained at the error
calculator 160 is equal to or
more than a predetermined threshold, the trellis control unit 170 controls the
trellis decoder 140
so that the certain decoded data d.,d,,....,dn corresponding to certain
decoding depth states
which are randomly selected from the whole decoding depth states 0, 1, 2, ...,
N are input to a
plurality of corresponding filter taps of the feedback filter 120.
[0046] For example, FIG. 5 shows a situation where the decoded data
do,d,...... dn output
from the whole decoding depth states of 0, 1, 2, ..., n(n <_ N, natural
nurnber).
Corresponding to a trace back delay length, the decoded data d.,d....... dn
are input to
corresponding filter taps of the feedback filter 120, respectively.
Accordingly, the decoded data
do output from the decoding depth state '0' is input to a first filter tap of
the feedback filter 120,
and the decoded data d, output from the decoding depth state '1' is output in
the unit of the K
symbol, thus to the (1 +K)th filter tap. The decoded data dn output from the
decoding depth
state 'n' is input to the l+ (n x K) th filter tap of the feedback filter 120.
[0047] With performing the trellis decoding on the resultant value Y, the
trellis decoder 140
under a control of the trellis control unit 170 inputs the decoded data
do,d,...... dn that are
output from the certain decoding depth states, into respectively corresponding
filter taps.
Accordingly, the feedback filter 120 removes the post-ghost with respect to
respective the
symbols based on the decoded data do,d,...... dn input from the trellis
decoder 140.
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[0048] Accordingly, the feedback filter 120 performs the post-ghost removal
filtering based on
the determined level data fed back from the level decision unit 150, and
performs the post-ghost
removal of the respective symbols based on the decoded data do,d,,....,dn when
the decoded
data do ,d,,....,dn are input from the trellis decoder 140.
[0049] As a result, error propagation of the feedback filter 120 due to
erroneous level data
determination of the level decision unit 150 can be avoided.
[0050] FIG. 6 is a table listing the signal to noise ratio (SNR) of an output
signal of the trellis
decoder 140 according to an output delay of the trellis decoding. The
broadcasting signal used
in the experiment was a Brazilian D channel.
[0051] As shown in FIG. 6, the Brazilian D channel has a greatest SNR when the
decoding
depth of the trellis decoder 140 is '0'. Accordingly, it is possible that the
trellis decoded data
do be input from the trellis decoder 140 to the feedback filter 120.
Accordingly, an equalization
efficiency is greatly improved because the trellis decoded data do is input
from the trellis
decoder 140 to the feedback filter 120 even when there is a robust ghost
included in a front
portion of the broadcasting signal in the Brazilian D channel.
[0052] FIG. 7 is a flowchart illustrating a channel equalizing method that
uses the channel
equalizer shown in FIG. 4 to improve a channel equalizing efficiency. Some
parts of FIG. 7 that
have already described above with reference to FIGS. 4 and 5 will be omitted
below.
[0053] First, the feed-forward filter 110 removes the pre-ghost from the
respective symbols of
the broadcasting signal, thereby outputting a signal M in operation S110. The
feedback filter
120 removes the post-ghost from the respective symbols of the broadcasting
signal, thereby
outputting a signal L in operation S120.
[0054] The adder 130 adds the signal M from which the pre-ghost is removed,
and which is
output from the feed-forward filter 110, with the signal L from which the post-
ghost is moved,
and which is output from the feedback filter 120, thereby outputting a sum
signal (the resultant
value) Y in operation S130. The trellis decoder 140 trellis-decodes the sum
signal Y of the
adder 130 in operation S140.
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[0055] The level decision unit 150 determines the level of the sum signal Y of
the adder 130
to be the nearest one among the predetermined levels in operation S150.
Accordingly, the level
decision unit 150 provides the signal D of the determined level to the
feedback filter 120 and the
error calculator 160. The error calculator 160 calculates the error value E,
i.e., a difference
between the sum signal Y of the adder 130 and the determined le:vel data D of
the level decision
unit 150 according to a predetermined error update algorithm in operation
S160. The error
calculator 160 provides the calculated error value E to the feed-fiorward
filter 110, the feedback
filter 120 and the trellis control unit 170. The feed-forward filter 110 and
the feedback filter 120
update coefficients based on the error value E provided by the error
calculator 160.
[0056] Meanwhile, based on the error value E provide by the error calculator
160, the trellis
control unit 170 in the operation S170 controls a time point when the trellis-
decoded data of the
trellis decoder 140 is input into the feedback filter 120. When the SNR
corresponding to the
error value E obtained by the error calculator 160 is equal to or niore than
the predetermined
threshold, the trellis control unit 170 in the operation S170 controls the
trellis decoder 140 to
input the decoded data to the feedback filter 120.
[0057] Accordingly, the feedback filter 120, performing the post-ghost removal
filtering based
on the determined level data feed back from the level decision uriit 150,
performs the post-ghost
removal of the respective symbols based on the decoded data received from the
trellis decoder
140.
[0058] Accordingly, by preventing the error propagation at the feedback filter
occurring due to
the error of the level decision unit, the equalization performance of the
channel equalizer is
enhanced.
[0059] According to the present invention, even when the SNR corresponding to
the error
value obtained at the error calculator is equal to or more than the
predetermined threshold,
degradation of the equalization efficiency due to the erroneous level decision
of the level
decision unit is prevented by the feedback filter which performs filtering
based on the decoded
data input from the trellis decoder.
[0060] Although an embodiment of the present invention has been described, it
will be
understood by those skilled in the art that the present invention should not
be limited to the
described preferred embodiment, but various changes and modifications can be
made within
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the spirit and scope of the present invention as defined by the appended
claims and their
equivalents.
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