FEEDBACK INTERFERENCE CANCELLER WITH PRE-WHITENING FILTERS
FOR ATSC EQUALIZATION DIGITAL ON-CHANNEL REPEATERS

Young-Jun Lee*, Jongwoo Shin*, Ho min Eum†, Sung Ik Park†, Heung Mook Kim† and Hyoung-Nam Kim*

*School of Electrical Engineering, Pusan National University,
San 30 Jangjeon-dong, Guemjeong-gu, Busan 609-735, Korea
hnkim@pusan.ac.kr
†Electronics and Telecommunications Research Institute,
161 Gajeong-dong, Yuseong-gu, Daejeon, 305-700, Korea

ABSTRACT

The combination of the Equalization Digital On-Channel
Repeater
(EDOCR) and
the Feedback
Interference
Canceller (FIC) may be considered as a good solution for
re-broadcasting recovered DTV signals with high power
after the reception of distorted weak signals. However, the
adoption of the FIC distorts spectrum edge which hinders
the EDOCR’s synchronization. This is because the FIC
cannot eliminate the feedback around the carrier pilot and
the edge of the spectrum clearly. To overcome this obstacle,
an FIC with the pilot-free reference (PFR) was proposed but
an additional cost for the PFR was too prohibitive to be
implemented. Therefore we propose an FIC with pre-
whitening filters (PWFs) which help the FIC estimate the
feedback channel accurately. The proposed FIC with the
PWFs eliminates the spectrum distortion clearly and
suppresses the feedback signal more powerfully with a little
additional cost compared to the FIC with the PFR.

Index Terms—DTV, EDOCR, FIC,
spectrum
distortion, pre-whitening filters

1. INTRODUCTION

Recently single frequency networks (SFNs) have been
considered for the ATSC DTV services [1-3]. Combining
the Equalization Digital On-Channel Repeater (EDOCR)
with the Feedback Interference Canceller (FIC) is an
interesting issue for organizing SFNs. Since the EDOCR
makes a high-quality of re-transmitted signal and the FIC
enhances the re-transmission signal power, the combined
system gives a near optimal solution for re-broadcasting
distorted weak DTV signals [4-9].
However, there is a crucial problem that the spectrum
around the carrier pilot and the edge of the FIC output
signal is significantly distorted when adopting the FIC to the
EDOCR. Spectrum distortion makes
the EDOCR’s
operation difficult by disturbing the carrier and timing
recovery which are preceded before the equalization process.
This disaster comes from the reason that the FIC cannot
estimate the feedback channel properly because of the
strong carrier pilot and band-limited characteristic of the
ATSC DTV signal. The spectrum distortion is an important
research issue to be solved for combining the EDOCR and
the FIC
To solve the spectrum distortion, a FIC with the pilot-
free reference (PFR) which estimates the spectrum around
the carrier pilot properly was proposed [10]. However an
additional implementation cost for the PFR increases
dramatically, developers hesitate to choose the FIC with the
PFR as a partner of the EDOCR. In aspect of minimizing
the implementation cost, a pre-whitening filter (PWF) which
can eliminate the spectrum distortion may be considered as
an alternative to the PFR. The FIC with PWFs has no
spectrum distortion, better feedback cancelling performance
and an affordable implementation cost.
The rest of this paper is organized as follows: In section
2, the conventional FIC and the FIC with the PFR are
introduced briefly and the proposed FIC with PWFs is
discussed in section 3. The simulation results are shown in
section 4 and the paper is concluded in section 5.

2. PREVIOUS WORKS

2.1. Conventional Feedback Interference Canceller

The conventional FIC has been widely used in the field of
acoustic signal processing. The conventional FIC estimates
the feedback channel by cross-correlating the output signal
of the repeater and the feedback signal. Estimated feedback
channel information is updated to the adaptive filter by
Least Mean Square (LMS) algorithm and the replica of the
feedback signal is generated by inner product of adaptive
filter coefficients and reference signal vector. The feedback
signal is simply removed by subtracting this replica signal
from the input signal of the repeater.
As the feedback cancellation process is iterated, the
adaptive filter estimates the feedback channel more
accurately and thus suppresses the residual feedback signal
)
(
ˆ n
s

Fig. 1. The simplified blocks of the combined system: the EDOCR
and the conventional FIC.

-4
-3
-2
-1
0
1
2
3
4
-40
-20
0
20
40
Frequency (MHz)
Signal Power (dB)-4
-3
-2
-1
0
1
2
3
4
-40
-20
0
20
40
Frequency (MHz)
Signal Power (dB)
Fig. 2. Spectra of the originally transmitted signal (top) and the
conventional FIC output (bottom)
Feedback Channel
hf
Adaptive Filter
h
f(n)
s(n)
x(n)
fr(n)
)
(
ˆ n
s
High
Power
Amplifier
y(n)
FIC with Pilot-Free Reference
Demodulator
Modulator
Equalizer
`
EDOCR
ypf(n)
Modulator
(w/o DC pilot)
High Power
Amplifier

Fig. 3. The simplified blocks of the combined system: the EDOCR
and the FIC with the PFR.
in the recovered signal more powerfully. Considering the
successful reception of the EDOCR, the FIC should
suppress the feedback signal power at least -4dB below the
main transmitted signal power. This minimum feedback
cancelling requirement is due to the capability of the
equalizer of the existing EDOCR [4, 5].
Fig. 1 shows the simplified blocks of the EDOCR with
the conventional FIC. To investigate
the feedback
cancelling procedure in detail, we will use notations
depicted in Fig. 1. Let s(n) be the main transmitted signal
and f(n) be the feedback signal of the repeater, then the
input signal of the repeater x(n) is as follows;

( )
( )
( )
( )
( )
( )
f
x n
s n
f n
s n
n
n
∗
=
+
=
+
∗
h
y
, (1)

where hf(n) is the real feedback channel vector between the
transmit antenna and the receive antenna and y(n) denotes a
reference signal vector which consists of the output signal
of the repeater. The FIC generates the feedback replica by

( )
( )
( )
r
f n
n
n
∗
=
∗
h
y
, (2)

then subtracts it from the FIC input to remove feedback,

ˆ( )
( )
( )
r
s n
x n
f n
=
−
. (3)

The adaptive filter are updated by the LMS algorithm
simply,

ˆ
(
1)
( )
( ) ( )
n
n
s n
n
μ ∗
+ =
+
h
h
y
. (4)

where μ is a step-size which determines the convergence
speed and the steady-state performance of the FIC.
If the step-size is properly selected, the FIC can
effectively suppress the feedback signal and meets the
minimum feedback cancelling requirement. The crucial
problem, however, occurs around the carrier pilot and the
edge of the spectrum of the recovered signal. Fig. 2 shows
the spectra of the main transmitted signal and the recovered
signal. It can be easily seen that there is severe distortion
around the carrier pilot and edge of the spectrum.
Synchronization circuits of the existing EDOCR do not
work owing to this spectrum distortion hence the EDOCR
cannot process the FIC’s output signal.

2.2. Feedback Interference Canceller with the Pilot-Free
Reference

To overcome the spectrum distortion caused by the carrier
pilot and band-limited characteristic of ATSC DTV signals,
the FIC with the pilot-free reference (PFR) was proposed
[10]. The simplified structure of the FIC with the PFR is
shown in Fig. 3. The feedback cancelling procedure is
identical to that of the conventional FIC except for the
adaptive filter update. The adaptive filter coefficients are
updated by

ˆ
(
1)
( )
( )
( )
pf
n
n
s n
n
μ ∗
+ =
+
h
h
y
. (5)
Feedback Channel
hf
f(n)
s(n)
x(n)
fr(n)
)
(
ˆ n
s
High
Power
Amplifier
y(n)
Proposed FIC
Demodulator
Modulator
Equalizer
EDOCR
Transversal Filter
h
PWF
Adaptive Filter
hw
copy
frw(n)
xw(n)
yw(n)
)
(
ˆ n
sw
PWF
Fig. 4. The simplified blocks of the combined system: the EDOCR
and the proposed FIC with PWFs.
-4
-3
-2
-1
0
1
2
3
4
-40
-20
0
20
40
Signal Power (dB)Frequency (MHz)
-4
-3
-2
-1
0
1
2
3
4
-40
-20
0
20
40
Frequency (MHz)
Signal Power (dB)
Fig. 5. Spectra of the FIC with the PFR output (top) and the FIC
with PWFs output (bottom)
Table I. Simulation Parameters
Parameters
Specifications
Feedback signal power
+ 20 dB to main signal power
Feedback signal delay
0.45 ㎲ to repeater output
Gain increasing speed
0.0116 sec to +20dB
Adaptive filter length
40 baseband taps
Pre-whitening filter length
40 baseband taps


ypf(n) denotes the pilot-free reference vector which is
generated by subtracting the DC pilot at the modulation
block of the EDOCR and passing an additional analog
devices including D/A converter, HPA and so on.
The FIC with the PFR performs more exact feedback
channel estimation and compensates the spectrum distortion
around the pilot. However an extra line for the PFR needs
the same digital/analog devices used in main signal-path
line and also modification of the existing EDOCR. These
prohibitive additional implementation costs are obstacles for
adopting the FIC with the PFR to the EDOCR.

3. PROPOSED METHOD

Since the above-mentioned FIC compensates the spectrum
distortion around the carrier pilot by using the PFR, we can
reach the conclusion that the same result may be obtained if
we remove the carrier pilot in reference signals. A pre-
whitening filter (PWF) also suppresses the carrier pilot in
reference signals by de-correlating the input signal hence
the PWF may be considered as an alternative to the PFR [9,
11-12]. Contrary to the FIC with the PFR, the PWF is
realized in FIC circuits internally and does not need a large
hardware resource.
Fig. 4 shows the simplified blocks of the EDOR with the
proposed FIC. The feedback cancelling process is also
identical to that of the conventional FIC except for the
feedback channel estimation in the whitened-signal domain.
To perform feedback channel estimation in whitened-signal
domain, both the input signal and the reference signal
convolve with the PWF on each signal’s path. The whitened
replica is generated by

( )
( )
( )
rw
w
f
n
n
n
∗
=
∗
h
y
, (6)

where yw(n) denotes a whitened reference signal vector
which consists of the output signal of the PWF. Then the
whitened recovered signal is calculated by

ˆ ( )
( )
( )
w
w
rw
s n
x n
f
n
=
−
. (7)

where xw(n) is the whitened input signal. The adaptive filter
coefficients are updated by the LMS algorithm with the
whitened recovered signal and the whitened reference
vector as follows,

ˆ
(
1)
( )
( )
( )
w
w
n
n
s n
n
μ ∗
+ =
+
h
h
y
. (8)

Estimated adaptive filter coefficients are copied to the
transversal filter and the FIC removes the feedback signal
using eq. (2), (3) in real signal domain. Because the PWF
eliminates all negative causes of ATSC DTV signals for
feedback channel estimation, the proposed FIC suppresses
the feedback successfully without any spectrum distortion.

4. SIMULATION RESULTS

We analyzed the performance of the proposed FIC with
PWFs by comparing to that of the conventional FIC in two
aspects: spectrum distortion and feedback cancelling
performance. All of parameters associated with simulations,
such as power of the feedback signal, adaptive filter length,
pre-whitening filter length and so forth, are summarized in
Table I. Fig. 5 shows output signal spectrum of each FIC.
0
3
6
9
12
15
x 105
-16
-12
-8
-4
0
4
# of samples
RFP (dB)

Minimum requirement
Conventional FIC
FIC with PFR
FIC with PWFs

Fig. 6. RFP curves of all FICs

The FIC with the PFR preserves the carrier pilot perfectly
but there is still the distortion in the spectrum edge. On the
contrary, the proposed FIC maintains the carrier pilot
satisfactorily and removes the spectrum edge distortion
clearly.
The measuring index of the feedback cancelling
performance is a residual feedback power (RFP), which
means the remaining feedback signal power in the
recovered signal. The RFP is defined by

10
[ ( )
( )]
10log
[ ( )
( )]
E e n e n
RFP
E s n s n
∗
∗
⎛
⎞
=
⎜
⎟
⎜
⎟
⎝
⎠
(9)

where e(n) is a difference between the main signal s(n) and
the recovered signal
)
(
ˆ n
s
. Fig. 6 shows the RFP curves of
each FIC. All of FICs have a sufficient performance beyond
the minimum feedback cancelling requirement of the
existing EDOCR but the proposed FIC has the most
powerful feedback cancelling performance.

5. CONCLUSIONS

We proposed a feedback interference canceller (FIC) with
pre-whitening filters (PWFs) to compensate for the
spectrum distortion which causes a critical problem in the
synchronization of the EDOCR. The proposed FIC can
remove the spectrum distortion and suppress the feedback
signal more powerfully than previous FICs. It is expected
that the proposed FIC with PWFs can contribute to the
commercialization of the advanced system and the coverage
extension of ATSC services.





ACKNOWLEDGEMENT

This work was supported by the IT R&D program of
MKE/KCC/KEIT.
[KI002067,
Development
of
transmission efficiency enhancement
technology
for
terrestrial DTV system]

REFERENCES

[1] A. Mattsson, “Single Frequency Networks in DTV,” IEEE
Transactions on Broadcasting, vol. 51, no. 4, pp. 413-422, Dec.
2005.
[2] ATSC, “Standard A/110: Synchronization Standard for
Distributed Transmission,” Advanced Television Systems
Committee, Washington, D.C., July 2004.
[3] ATSC,
“Recommended Practice A/111: Design of
Synchronized Multiple Transmitter Networks,” Advanced
Television Systems Committee, Washington, D.C., Sept. 2004.
[4] 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 networks,” IEEE Trans. on Broadcasting, vol. 52, no. 2,
June, 2006.
[5] Y. T. Lee, S. I. Park, S. W. Kim, C. Ahn, and J. S. Seo, “ATSC
terrestrial digital television broadcasting using single frequency
networks,” ETRI Journal, vol. 26, no. 2, pp. 92–100, April 2004.
[6] S. I. Park, H. M. Eum, S. R. Park, G. Kim, Y.-T. Lee, H. M.
Kim, and W. Oh, "Novel Equalization On-Channel Repeater with
Feedback Interference Canceller in Terrestrial Digital Multimedia
Broadcasting System," ETRI Journal, vol. 31, no. 4, Aug, 2009.
[7] BBCA. Wiewiorka and P.N. Moss, “BBC R&D White Paper
WHP120,” Sep. 2005.
[8] Nasr, Karim M.; Cosmas, John; Bard, Maurice; Gledhill, Jeff,
“Performance of an Echo Canceller and Channel Estimator for On-
Channel Repeaters in DVB-T/H Networks,” IEEE Trans. on
Broadcasting, Vol. 53, no.3, pp. 609 – 618, Sept., 2007.
[9] Y.-J. Lee, H. M. Eum, Y.-T. Lee, K. S. Son and H.-N. Kim,
“Performance of Feedback Cancellers for T-DMB On-Channel
Repeaters,” IEEE Trans. Broadcasting, vol. 55, no. 4, Dec, 2009.
[10] K.-H. Suh, Y.-J. Lee, S. I. Park, H. M. Eum, H. M. Kim, and
H.-N. Kim, "Feedback Cancellation with a Pilot-Free Reference
for the ATSC terrestrial DTV Repeaters," 59th Annual IEEE
Broadcasting Symposium, Alexandria, VA, USA, Oct. 2009.
[11] Sophocles J., Orfanidis, Optimum signal processing: An
Introduction, 2nd ed. McGraw Hill, 1988.
[12] S. Haykin, Adaptive Filter Theory, 4th ed. Prentice Hall, 2002.