Modulation and Pre-Equalization Method to Minimize Time Delay in Equalization Digital On-Channel Repeater
This paper presents novel modulation and pre-equalization methods to minimize a signal processing time delay in the equalization digital on-channel repeater (EDOCR) for the ATSC terrestrial digital TV system. The proposed modulation method uses equi-ripple (ER) filter for vestigial side bands (VSB) pulse shaping instead of conventional square root raised cosine (SRRC) filter. And the proposed pre-equalization method calculates pre-equalizer filter coefficients by comparing a baseband signal as a reference signal and a demodulated repeater output signal, and then creates new VSB pulse shaping filter coefficients by the convolution of the ER filter and the pre-equalizer filter coefficients. The new VSB pulse shaping filter minimizes the time delay of EDOCR by adjusting the number of its pre-taps and also compensates the linear distortions due to the use of the ER filter and mask filter.
IEEE TRANSACTIONS ON BROADCASTING, VOL. 54, NO. 2, JUNE 2008 249
Modulation and Pre-Equalization Method to
Minimize Time Delay in Equalization Digital
Heung Mook Kim, Sung Ik Park, Jae Hyun Seo, Homin Eum, Yong-Tae Lee, Soo In Lee, and Hyuckjae Lee
Abstract—This paper presents novel modulation and pre-equal-
ization methods to minimize a signal processing time delay in
the Equalization Digital On-Channel Repeater (EDOCR) for the
ATSC terrestrial digital TV system. The proposed modulation
method uses Equi-Ripple (ER) ﬁlter for Vestigial Side Bands
(VSB) pulse shaping instead of conventional Square Root Raised
Cosine (SRRC) ﬁlter. And the proposed pre-equalization method
calculates pre-equalizer ﬁlter coefﬁcients by comparing a base-
band signal as a reference signal and a demodulated repeater
output signal, and then creates new VSB pulse shaping ﬁlter coef-
ﬁcients by the convolution of the ER ﬁlter and the pre-equalizer
ﬁlter coefﬁcients. The new VSB pulse shaping ﬁlter minimizes the
time delay of EDOCR by adjusting the number of its pre-taps and
also compensates the linear distortions due to the use of the ER
ﬁlter and mask ﬁlter.
Index Terms—ATSC, modulation, on-channel repeater,
ERRESTRIAL television broadcasters in general operate
transmitters and translators according to the geographical
locations of their coverage areas. In both analog and digital tele-
vision broadcasting, Multiple Frequency Networks (MFNs) that
assign different channels to each transmitter and translator have
been used to cover service areas. However, the use of MFNs
is very inefﬁcient in the aspect of using frequencies since it is
unable to share channels among a number of transmitters and
translators unless the distance between two coverage areas is
Therefore, Single Frequency Networks (SFNs) that operate
multiple transmitters and repeaters on the same frequency is de-
sirable for the efﬁcient use of frequencies. Especially, in the re-
cent transition period from analog to digital broadcasting, the
need of SFNs is unavoidable due to the lack of frequencies for
additional transmitters and repeaters. SFNs provide not only
high Signal to Noise Ratios (SNR), but trigger the mobile DTV
Manuscript received February 7, 2007; revised February 22, 2008.
H. M. Kim, S. I. Park, J. H. Seo, H. Eum, and Y.-T. Lee are with the Ter-
restrial Broadcasting Technology Research Team, ETRI, Yuseong-gu, Daejeon,
305-700, Korea (e-mail: email@example.com; firstname.lastname@example.org; email@example.com;
S. I. Lee is with the Broadcasting System Research Department, ETRI,
Yuseong-gu, Daejeon, 305-700, Korea (e-mail: firstname.lastname@example.org).
H. Lee is with the Radio & Communications Laboratory, ICU, Yuseong-gu,
Daejeon, 305-714, Korea (e-mail: email@example.com).
Color versions of one or more of the ﬁgures in this paper are available online
Digital Object Identiﬁer 10.1109/TBC.2008.921371
Fig. 1. Block diagram of EDOCR.
services , . Recently, SFNs are considered for use in ter-
restrial Advanced Television System Committee (ATSC) Dig-
ital Television (DTV) services because of the performance im-
provement of DTV receiver which are able to compensate for
the long-time delay and high level ghost .
In the ATSC 8-VSB system, SFNs can be implemented
with DTxT (Distributed Transmitters) that uses the same fre-
quency among a number of transmitters, and/or with Digital
On-Channel Repeaters (DOCRs) that uses the same frequency
between transmitters and repeaters , . The disadvantages
of DTxT are that some devices maintaining the frequency
synchronization between SFN transmitters must be added to
existing transmitters and that the distance between the trans-
mitters can be restricted by the limited equalization range of
receivers. DOCRs do not need to change existing transmitters,
but they produce limited output power and low quality of
signal. As complementary to existing DOCRs, the Equalization
DOCR (EDOCR) has been proposed , .
This paper presents the operational requirements of the
EDOCR modulator and pre-equalizer, and proposes its conﬁg-
uration to meet the requirements. The proposed modulation and
pre-equalization method is analyzed by computer simulations,
and it is also conﬁrmed by laboratory tests.
0018-9316/$25.00 © 2008 IEEE
250 IEEE TRANSACTIONS ON BROADCASTING, VOL. 54, NO. 2, JUNE 2008
Fig. 2. Block diagram of VSB modulator.
II. CHARACTERISTICS OF
DOCRs are used to ﬁll in coverage gaps and to extend
coverage areas which transmitter can not cover. Conventional
DOCRs such as the RF processing DOCR and the IF processing
DOCR offer a short processing time, but they provide limited
transmitting power, low quality of output signal, and inadequate
adjacent channel rejection. The EDOCR has been proposed to
overcome such disadvantages of conventional DOCRs and its
conﬁguration is shown in Fig. 1. The EDOCR system takes the
• Since the EDOCR does not use Forward Error Correction
(FEC) decoding and encoding, it does not have the ambi-
guity problem in which the DOCR output symbol stream
differs from its input symbol stream.
• The EDOCR has good selectivity of the received signal due
to utilizing a matched ﬁlter in demodulation. That is, it is
capable of rejecting adjacent channels.
• The EDOCR uses a blind Decision Feedback Equalizer
(DFE), which includes the trellis decoder as a decision de-
vice with TBD (Trellis Back Depth) of 1 . The DFE is
able to remove noise and multipath signals caused by the
signal paths between the main transmitter and the EDOCR,
so that the quality of output signal is better than that of
the input signal. Also, since the equalizer rejects feedback
signal due to low isolation between transmitting and re-
ceiving antennas, the transmitting power of EDOCR can
be increased more than 10 times higher than that of the
• Because of the re-modulation and pre-equalization, the
EDOCR can transmit good quality of signal.
The EDOCR involves a lot of digital signal processing,
which possibly causes a long time delay between transmitted
and received signals compared with conventional DOCRs. Due
to the non-inclusion of FEC decoding and encoding, however,
its signal processing time can be limited within 6 , . The
time delays of each module are 1 in the demodulator, 1 in the
equalizer, 3 in the modulator, and 1 in the RF systems and
III. EDOCR M
A. EDOCR Modulator
The block diagram of a VSB modulator used in the ATSC
terrestrial DTV transmitter or repeater is shown in Fig. 2, and
its operation has following procedure:
• Step 1: The data consisting of the equalizer output, the
ﬁeld sync and the segment sync is up-sampled after pilot
• Step 2: The up-sampled data is ﬁltered by a VSB I/Q pulse
• Step 3: The VSB ﬁltered I/Q components with the center
frequency of 2.69 MHz is up-converted to the center fre-
, and combined to form the IF signal.
A SRRC ﬁlter is generally used for VSB pulse shaping ﬁlter
in the ATSC system and the VSB I/Q ﬁlters based on the SRRC
where is a time index, is a SRRC ﬁlter coefﬁcient ac-
cording to the time index,
is 2.69 MHz, and is a symbol
time (about 93 ns).
The VSB modulated signal must meet the FCC emissions
mask shown in Fig. 3 and maintain the output SNR greater than
27 dB . Assuming that the up-sampling rate for VSB ﬁltering
is 4, Fig. 4 shows the output SNR and the spectrum shoulder
amplitude according to the number of SRRC ﬁlter taps. The
shoulder amplitude is the power difference between the ampli-
tude of the spectral regrowth spectrum at the channel’s edge and
the total average DTV power.
To meet the emissions mask requirement, the shoulder ampli-
tude must be greater than 47 dB. Suppose that the number of taps
of the matched ﬁlter is 121 to measure the SNR while observing
200,000 symbols through an ideal channel. According to the
Fig. 4, the VSB pulse shaping ﬁlter based on the SRRC ﬁlter
should theoretically have more than 420 taps to meet the output
SNR and emissions mask requirements simultaneously. How-
ever, when the symbols are over-sampled at 4 times the ATSC
KIM et al.: MODULATION AND PRE-EQUALIZATION METHOD TO MINIMIZE TIME DELAY IN EQUALIZATION DIGITAL ON-CHANNEL REPEATER 251
Fig. 3. FCC emissions mask.
Fig. 4. SNR and shoulder amplitude according to the number of the SRRC
system symbol rate, it causes a time delay of about 4.9 which
results in a relatively long delay in the EDOCR system. Since
the time delay is critical, a new pulse shaping ﬁlter is required
for the EDOCR modulator. Since the number of ﬁlter taps to
satisfy the requirements is determined by the shoulder ampli-
tude rather than the SNR according to the Fig. 4, the new pulse
shaping ﬁlter must be designed to have large shoulder amplitude
while maintaining the number of taps as small as possible.
An ER ﬁlter that has good capability of out-of-band suppres-
sion while allows relatively lots of in-band ripples can be used
as a pulse shaping ﬁlter in the EDOCR system for a short time
delay. The VSB I/Q ﬁlters based on the ER ﬁlter are
Fig. 5. SNR and shoulder amplitude according to the number of the ER ﬁlter
where is a time index, is an ER ﬁlter coefﬁcient according
to the time index,
is 2.69MHz, and is a symbol time.
Fig. 5 shows the output SNR and the shoulder amplitude ac-
cording to the number of ER ﬁlter taps, and ER ﬁlter coefﬁ-
cients are calculated by Parks-McClellan algorithm , .
The ER ﬁlter with greater than about 140 taps can meet the
output SNR and emissions mask requirements simultaneously
according to the Fig. 5. When the symbols are over-sampled at
4 times the symbol rate, it causes a time delay of about 1.6 which
is adequate as a pulse shaping ﬁlter in the EDOCR system. The
ER ﬁlter has good capability of out-of-band suppression, but it
causes lots of in-band ripples which are not ideal characteristic
of Nyquist pulse shaping ﬁlter. Therefore, the output SNR of
252 IEEE TRANSACTIONS ON BROADCASTING, VOL. 54, NO. 2, JUNE 2008
Fig. 6. The modulator and conventional pre-equalizer.
the ER ﬁlter is lower than that of the SRRC ﬁlter when the same
number of taps is used.
B. Pre-Equalization Method
To meet the FCC emissions mask requirement, an EDOCR
uses a mask ﬁlter which is capable of out-of-band suppression
after a high power ampliﬁer. The mask ﬁlter with good out-of-
band suppression capability causes a lot of in-band group delay
which degrades the output signal quality. Also, there is a pos-
sibility of additional SNR degradation due to the use of an ER
ﬁlter as a pulse shaping ﬁlter. To compensate these SNR degra-
dations, a pre-equalizer is used. Fig. 6 shows the conﬁgura-
tion of the modulator and the conventional pre-equalizer. In
the pre-equalizer, its ﬁlter coefﬁcients are calculated by com-
paring the baseband signal to be transmitted and the demodu-
lated channel ﬁlter output signal of the EDOCR.
A pre-equalizer ﬁlter in general is a linear ﬁlter and its coefﬁ-
cients can be calculated using Least Mean Square (LMS) algo-
rithm. To update the coefﬁcients, the following variables must
baseband signal to be transmitted at time
demodulated signal after channel ﬁltering at time
pre-equalizer output signal at time
-th ﬁlter tap coefﬁcient of pre-equalizer at time .
Thus, the pre-equalizer output is
where is the number of the pre-equalizer ﬁlter taps. The
number of taps is determined by the degree of linear distortion
such as group delay. To obtain the update formula for ﬁlter tap
coefﬁcients, the error signal
The ﬁlter tap coefﬁcients are updated as
where is a step size which determines convergence speed and
steady state Mean Square Error (MSE). For a large step size
value the convergence speed is fast, but the steady state MSE
is large. Otherwise, for a small step size value the steady state
MSE is small, but the convergence speed is slow. To update the
tap coefﬁcients, the EDOCR uses known symbols as a training
sequence, instead of the decision symbols of the VSB Demod-
ulator output in Fig. 6. Therefore, it is recommended to use
a small step size for a small steady state MSE although the
convergence speed is slow . The modulator including the
pre-equalizer can compensate the linear distortions and reduce
the ripples caused by the use of a mask ﬁlter and an ER ﬁlter, so
that the output SNR of EDOCR can be improved.
C. Combination of Pre-Equalizer Filter and Pulse Shaping
The symbol level pre-equalizer ﬁlter shown in Fig. 6 is one of
the factors causing a time delay in the EDOCR. To minimize the
time delay, the method of combining the pre-equalizer ﬁlter and
the pulse shaping ﬁlter, and adjusting the number of the com-
bined ﬁlter’s pre-taps is proposed in this section. Precisely, the
time delay can be minimized by truncating the number of pre-
taps after convolution of the pre-equalizer ﬁlter and the pulse
shaping ﬁlter. The conﬁguration of the EDOCR modulator in-
cluding the proposed pre-equalizer is shown in Fig. 7, and the
process of combining two ﬁlters and adjusting the number of
pre-taps of combined ﬁlter is shown in Fig. 8.
Assume that there are a pre-equalizer ﬁlter in which the total
number of taps is
and the main tap is positioned at
, and a VSB I/Q ﬁlter in which the total number of taps
and the main tap is positioned at . After
convolution of the pre-equalizer ﬁlter and the VSB I/Q ﬁlter, a
combined VSB I/Q ﬁlter functioning pre-equalization in which
the total number of taps is
main tap is positioned at
is created. And some of
the left most ﬁlter coefﬁcients of the combined VSB I/Q ﬁlter
are truncated to reduce the processing time delay of EDOCR.
So the truncated VSB I/Q ﬁlters have the total number of taps
and its main tap is positioned at where
KIM et al.: MODULATION AND PRE-EQUALIZATION METHOD TO MINIMIZE TIME DELAY IN EQUALIZATION DIGITAL ON-CHANNEL REPEATER 253
Fig. 7. The conﬁguration of modulator and proposed pre-equalizer.
Fig. 8. Process of combining pre-equalizer ﬁlters and pulse shaping ﬁlter and adjusting the number of pre-taps.
and . The post-taps can
also be truncated to be accommodated in limited hardware re-
sources. By such adjustment of the pre-taps, the truncated VSB
pulse shaping ﬁlter can minimize the time delay in the EDOCR.
Due to the pre-equalization, it can also compensate the linear
distortions and reduce the in-band ripples so that the output SNR
of the EDOCR can be signiﬁcantly improved.
IMULATION AND LABORATORY TEST
A. Simulation Results
The computer simulations have been performed based on the
conﬁguration of the EDOCR modulator shown in Fig. 7. The
up-sampling rate for VSB ﬁltering was assumed as 4, and the
ER ﬁlter with 191 taps was used as a pulse shaping ﬁlter. The
linear distortions which can be caused by a high power ampliﬁer
were not considered, and the mask ﬁlter was modeled as the 8th
order Chebyshev ﬁlter. Fig. 9 shows the magnitude and group
delay characteristic of the designed mask ﬁlter.
To calculate the pre-equalizer ﬁlter coefﬁcients, the LMS
algorithm was used. The total number of the pre-equalizer
ﬁlter taps was set to 101 and its main tap was positioned at
51 to maintain the output SNR greater than 35 dB in symbol
rate data. The time delay of the pre-equalizer ﬁlter itself is
4.74. that is relatively long. The number of taps of the matched
ﬁlter was set to 121 to measure the SNR and an ideal channel
with no multi-path and no additive noise was assumed while
observing 200,000 symbols. Fig. 10 shows the simulation
results of the pre-equalization when the pre-equalizer ﬁlter
and the pulse shaping ﬁlter were used separately as shown in
Fig. 6. Fig. 10(a) shows the EDOCR output constellation before
pre-equalizing, in which the output SNR is about 14.1 dB, and
Fig. 10(b) shows that after pre-equalizing, in which the output
SNR is about 35.3 dB. The output SNR after pre-equalizing is
greater than that of the ER ﬁlter with 191 taps (32.88 dB) in
the Fig. 5 since the pre-equalizer can reduce in-band ripples
due to the use of the ER ﬁlter. Fig. 11 shows the SNR and
the shoulder amplitude in case of adjusting the number of the
pre-taps after convolution of the pre-equalizer ﬁlter and the
pulse shaping ﬁlter. In order to maintain the shoulder amplitude
254 IEEE TRANSACTIONS ON BROADCASTING, VOL. 54, NO. 2, JUNE 2008
Fig. 9. Magnitude and group delay characteristic of 8th order Chebyshev ﬁlter.
(a) Magnitude characteristic. (b) Group delay characteristic.
greater than 47 dB and the SNR greater than 27 dB, the number
of the combined ﬁlter’s pre-taps should be greater than 95 and
then the time delay becomes about 2.21. Therefore, the newly
created ﬁlter by adjusting the number of the pre-taps after
convolution of the pre-equalizer ﬁlter and the pulse shaping
ﬁlter minimizes the time delay while maintaining the required
SNR and shoulder amplitude.
B. Laboratory Test Results
To verify the performance of the proposed modulator and
pre-equalizer in the EDOCR, a hardware was implemented and
the EDOCR output was measured by RFA300A, the VSB test
and measurement equipment. The implemented EDOCR system
used the ER ﬁlter with 191 taps and the pre-equalizer ﬁlter in
which its main tap was positioned at 51 in symbol rate was cal-
culated by LMS algorithm. To reduce the time delay as possible
without violation of the EDOCR requirements, the number of
the pre-taps was adjusted as 95 which is the same number of
the pre-taps of the ER ﬁlter after convolution of the ER ﬁlter
and the pre-equalizer ﬁlter. Thus, the time delay in the modu-
lator including the pre-equalization is 2.21 that are the same as
in the ER ﬁlter only. The EDOCR output signal was veriﬁed by
Fig. 10. Constellation of EDOCR output signal before and after pre-equal-
ization. (a) Constellation of EDOCR output signal without pre-equalization
. (b) Constellation of EDOCR output signal with pre-equal-
Fig. 11. SNR and shoulder amplitude according to the number of pre-taps after
convolution of pre-equalizer ﬁlter and pulse shaping ﬁlter.
RFA300A, and its spectrum, frequency response, group delay,
and constellation before and after pre-equalization are shown
in Fig. 12. Fig. 12 proves that the EDOCR output meets the
KIM et al.: MODULATION AND PRE-EQUALIZATION METHOD TO MINIMIZE TIME DELAY IN EQUALIZATION DIGITAL ON-CHANNEL REPEATER 255
Fig. 12. Spectrum, frequency response, group delay, and constellation before and after pre-equalization. (a) Spectrum of EDOCR output signal (Left: before
pre-equalization, Right: after pre-equalization). (b) Frequency response and group delay of EDOCR output signal (Left: before pre-equalization, Right: after pre-
equalization). (c) Constellation of EDOCR output signal (Left: before pre-equalization, Right: after pre-equalization).
spectrum mask and SNR requirements. Usually the linear dis-
tortions caused by mask ﬁlter and other RF components in re-
peater system are not so severe that they can be compensated
by a linear ﬁlter with relatively small number of taps compared
to that of the VSB pulse shaping ﬁlter. According to the labora-
tory test results, it can be predicted that if the effective number
of pre-taps of the combined ﬁlter is greater than that of the orig-
inal pulse shaping ﬁlter, the proposed system would not perform
as well as when the two ﬁlters are not combined but it still meets
the FCC requirements.
256 IEEE TRANSACTIONS ON BROADCASTING, VOL. 54, NO. 2, JUNE 2008
V. C ONCLUSIONS
This paper presents the modulation and pre-equalization
methods to minimize the time delay of the EDOCR. The
proposed modulation method uses an ER ﬁlter as a VSB pulse
shaping ﬁlter instead of a conventional SRRC ﬁlter. And the
proposed pre-equalization method calculates the pre-equalizer
ﬁlter coefﬁcients by comparing a baseband signal as a reference
signal and a repeater output signal, and then creates new VSB
pulse shaping ﬁlter coefﬁcients by the convolution of the ER
ﬁlter and the calculated pre-equalizer ﬁlter coefﬁcients. Ac-
cording to the computer simulation and laboratory test results,
the proposed methods have met the FCC requirements without
causing signiﬁcant system delay.
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Heung Mook Kim received the B.S. and M.S.
degrees in electronics and electrical engineering
from POSTECH, Pohang, Korea, in 1993 and 1995
respectively. From February 1995 to January 2002,
he was with POSCO Technology Research Labo-
ratory in the ﬁeld of Measurement and Monitoring
as research engineer. Since February 2004, he has
been with the Broadcasting System Research Group,
Electronics and Telecommunication Research Insti-
tute (ETRI), where he is a senior member of research
staff. Also, he is currently at Information and Com-
munications University (ICU) pursuing Ph.D. degree. His research interests are
in the areas of digital and RF signal processing and RF transmission for digital
communications and digital television.
Sung Ik Park received the BSEE from Hanyang
University, Seoul, Korea, in 2000 and MSEE from
POSTECH, Pohang, Korea, in 2002. Since 2002,
he has been with the Broadcasting System Re-
search Group, Electronics and Telecommunication
Research Institute (ETRI), where he is a member
of research staff. His research interests are in the
areas of error correction codes and digital commu-
nications, in particular, signal processing for digital
Jae Hyun Seo received the BSEE and MSEE from
Kyungpook National University, Daegu, Korea, in
1999 and 2001 respectively. Since January 2001,
he has been with the Broadcasting System Research
Group, Electronics and Telecommunication Re-
search Institute (ETRI), Daejeon, Korea, developing
advanced transmission and reception technology for
terrestrial digital television. His research interests
include digital signal processing, spatiotemporal
signal processing, in particular, signal processing for
digital television and digital communications.
Homin Eum received the BSEE and MSEE from
Korea University, Seoul, Korea, in 1998 and 2000
respectively. Since May 2000, he has been with
Electronics and Telecommunication Research Insti-
tute (ETRI), where he is a senior member of research
staff. His main research interests are in the areas
of digital communication systems, digital signal
processing and DTV transmission systems.
Yong-Tae Lee received the BSEE and MSEE from
Hankuk Aviation University in 1993 and 1995 re-
spectively and Ph.D. degree from Yonsei University,
Seoul, Korea in 2007. Since 1995, he has been with
the Radio Signal Processing Department and Broad-
casting System Research Department, Electronics
and Telecommunication Research Institute (ETRI),
where he is a senior member of research staff. His
research interests are in the area of digital signal
processing and RF signal processing, in particular,
signal processing for digital television, digital
communication and analog narrow band communication.
Soo In Lee received the M.S. and Ph. D degrees,
all in electronics engineering from Kyungpook Na-
tional University, Daegu, Korea, in 1989 and 1996. In
1990, he joined Electronics and Telecommunication
Research Institute (ETRI), Daejeon, Korea, where he
has been working on broadcasting system technolo-
gies. Currently he serves as the Director for Broad-
casting System Research Group. His research inter-
ests include terrestrial DTV and DMB systems, dig-
ital CATV systems, and 3DTV systems.
Hyuckjae Lee was born in Inchon, Korea. He
received B.S. degree in electronic engineering from
Seoul National University, Korea, in 1970, and the
Ph.D. degree in electrical engineering from Oregon
State University, Corvallis, in 1982, where he spe-
cialized in electromagnetic ﬁelds and microwave
engineering. Since 1983, he has been with the Radio
Technology Department, Electronics and Telecom-
munications Research Institute (ETRI), and has been
working in the ﬁelds of radio technology, IMT2000,
broadcasting technology, and satellite communica-
tions system. He is currently a professor of Information and Communications
University, Daejeon, Korea.