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IEEE TRANSACTIONS ON BROADCASTING, VOL. 54, NO. 2, JUNE 2008
249
Modulation and Pre-Equalization Method to
Minimize Time Delay in Equalization Digital
On-Channel Repeater
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,
pre-equalization, SFN.
I. INTRODUCTION
TERRESTRIAL 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
far enough.
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: hmkim@etri.re.kr; psi76@etri.re.kr; jhseo@etri.re.kr;
hmeum@etri.re.kr; ytlee@etri.re.kr).
S. I. Lee is with the Broadcasting System Research Department, ETRI,
Yuseong-gu, Daejeon, 305-700, Korea (e-mail: silee@etri.re.kr).
H. Lee is with the Radio & Communications Laboratory, ICU, Yuseong-gu,
Daejeon, 305-714, Korea (e-mail: hjlee@icu.ac.kr).
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TBC.2008.921371
Fig. 1. Block diagram of EDOCR.
services [1], [2]. 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 [3].
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 [4], [5]. 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 [6], [7].
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 EDOCR
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
following advantages:
• 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 [8]. 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
conventional DOCRs.
• 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 [5], [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
cables.
III. EDOCR MODULATOR AND PRE-EQUALIZER
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
insertion.
• Step 2: The up-sampled data is filtered by a VSB I/Q pulse
shaping filter.
• Step 3: The VSB filtered I/Q components with the center
frequency of 2.69 MHz is up-converted to the center fre-
quency of
, 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
filters are
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 [9]. 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
filter taps.
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
taps.
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 [10], [11].
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
be defined.
: 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
is defined
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 [12]. 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
Filter
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
is
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
and the
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
of
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.
IV. SIMULATION AND LABORATORY TEST RESULTS
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
(SNR = 14:1 dB). (b) Constellation of EDOCR output signal with pre-equal-
ization (SNR = 35:2 dB).
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. CONCLUSIONS
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.
REFERENCES
[1] A. Mattsson, “Single frequency networks in DTV,” IEEE Trans. Broad-
casting, vol. 51, no. 4, pp. 413–422, Dec. 2005.
[2] Y. T. Lee, S. I. Park, S. W. Kim, C. T. Ahn, and J. S. Seo, “ATSC terres-
trial digital television broadcasting using single frequency networks,”
ETRI Journal, vol. 26, no. 2, pp. 92–100, April 2004.
[3] O. Bendov, “Areas of cochannel interference and multi-path created by
8-VSB modulated distributed transmitters in flat terrain,” IEEE Trans.
Broadcasting, vol. 52, no. 1, March 2006.
[4] ATSC, “Standard A/110: Synchronization Standard for Distributed
Transmission,” Advanced Television Systems Committee, Wash-
ington, D.C., July 14, 2004.
[5] ATSC, Recommended Practice A/111: Design of Synchronized Mul-
tiple Transmitter Networks Advanced Television Systems Committee,
Washington, D.C., Sep. 3, 2004.
[6] S. W. Kim, Y.-T. Lee, S. I. Park, H. M. Eum, J. H. Seo, and H. M.
Kim, “Equalization digital on-channel repeater in single frequency net-
works,” IEEE Trans. on Broadcasting, vol. 52, no. 2, June 2006.
[7] Y.-T. Lee, S. I. Park, H. M. Eum, J. H. Seo, H. M. Kim, S. W. Kim,
and J. S. Seo, “A design of equalization digital on-channel repeater
for single frequency network ATSC system,” IEEE Trans. on Broad-
casting, accepted for publication.
[8] H.-N. Kim, S. I. Park, and S. W. Kim, “Performance analysis of error
propagation effects in the DFE for ATSC DTV receivers,” IEEE Trans.
on Broadcasting, vol. 49, Sept. 2003.
[9] ATSC, “Standard A/64-Rev.A: Transmission Measurement and Com-
pliance for Digital Television,” Washington, D.C., May 30, 2000.
[10] T. W. Parks and J. H. McClellan, “Chebyshev approximation for nonre-
cursive digital filters with linear phase,” IEEE Trans. on Circuit Theory,
vol. CT-19, pp. 189–194, 1972.
[11] T. W. Parks and J. H. McClellan, “A program for the design of linear
phase finite impulse response filters,” IEEE Trans. on Audio Electroa-
coustics, vol. AU-20, pp. 195–199, 1972.
[12] G. A. Clark, S. K. Mitra, and S. R. Parker, “Block implementation of
adaptive digital filters,” IEEE Trans. on Circuits and Systems, vol. 28,
no. 6, June 1981.
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
television.
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.
249
Modulation and Pre-Equalization Method to
Minimize Time Delay in Equalization Digital
On-Channel Repeater
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,
pre-equalization, SFN.
I. INTRODUCTION
TERRESTRIAL 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
far enough.
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: hmkim@etri.re.kr; psi76@etri.re.kr; jhseo@etri.re.kr;
hmeum@etri.re.kr; ytlee@etri.re.kr).
S. I. Lee is with the Broadcasting System Research Department, ETRI,
Yuseong-gu, Daejeon, 305-700, Korea (e-mail: silee@etri.re.kr).
H. Lee is with the Radio & Communications Laboratory, ICU, Yuseong-gu,
Daejeon, 305-714, Korea (e-mail: hjlee@icu.ac.kr).
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TBC.2008.921371
Fig. 1. Block diagram of EDOCR.
services [1], [2]. 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 [3].
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 [4], [5]. 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 [6], [7].
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
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Fig. 2. Block diagram of VSB modulator.
II. CHARACTERISTICS OF EDOCR
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
following advantages:
• 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 [8]. 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
conventional DOCRs.
• 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 [5], [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
cables.
III. EDOCR MODULATOR AND PRE-EQUALIZER
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
insertion.
• Step 2: The up-sampled data is filtered by a VSB I/Q pulse
shaping filter.
• Step 3: The VSB filtered I/Q components with the center
frequency of 2.69 MHz is up-converted to the center fre-
quency of
, 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
filters are
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 [9]. 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
filter taps.
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
taps.
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 [10], [11].
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
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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
be defined.
: 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
is defined
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 [12]. 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
Filter
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
is
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
and the
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
of
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.
IV. SIMULATION AND LABORATORY TEST RESULTS
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
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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
(SNR = 14:1 dB). (b) Constellation of EDOCR output signal with pre-equal-
ization (SNR = 35:2 dB).
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.
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IEEE TRANSACTIONS ON BROADCASTING, VOL. 54, NO. 2, JUNE 2008
V. CONCLUSIONS
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
television.
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.