IEEE TRANSACTIONS ON BROADCASTING, VOL. 46, NO. 2, JUNE 2000
101
Comparison of Terrestrial DTV Transmission Systems: The ATSC 8-VSB,
the DVB-T COFDM, and the ISDB-T BST-OFDM
Yiyan Wu, Ewa Pliszka, Bernard Caron, Pierre Bouchard, and Gerald Chouinard
Abstract—this paper compares the performances of the ATSC
8-VSB, the DVB-T COFDM, and the ISDB-T BST-OFDM digital television terrestrial transmission systems under different impairments and operating conditions. First, a general system level
description is presented. It is followed by comparisons based on
laboratory test results and theoretical analyzes. The differences
in the system threshold definitions are discussed. In addition, a
brief performance and implementation analysis is also presented
for the three transmission systems under different network infrastructures. Whenever possible, the impact on the broadcasters or
consumers is discussed. Possible performance improvements are
also identified.
Index Terms—COFDM, data broadcasting, DTV, DTV terrestrial broadcasting, mobile reception, SFN, VSB.
I. INTRODUCTION
A
FTER a decade of intense research and development,
Digital Television Terrestrial Broadcasting (DTTB) has
finally reached the implementation stage. DTTB services have
been available in North America and Europe since November
1998. Many countries have announced their choice for a DTTB
system and their implementation plan. Currently, there are
three DTTB transmission standards:
1) The Trellis-Coded 8-Level Vestigial Side-Band (8-VSB)
modulation system developed by the Advanced Television Systems Committee (ATSC) in the USA [1];
2) The Coded Orthogonal Frequency Division Multiplexing
(COFDM) modulation system adopted as the Digital
Video Broadcasting—Terrestrial (DVB-T) standard in
Europe [2]; and
3) The Band Segmented Transmission (BST)-OFDM
system adopted in Japan for Terrestrial Integrated Service Digital Broadcasting (ISDB-T) [3].
Since there are more than one DTTB approaches, many countries and administrations are now engaged in the process of selecting a DTTB system. Each country has specific characteristics and needs. The selection of a DTTB system must be based
upon how well each modulation system meets specific conditions such as the use of the spectrum resource, coverage requirements and transmission network structure, reception conditions, type of service required, policy, objectives for program
exchange, cost to the consumers and broadcasters, etc.
Manuscript received May 10, 2000; revised July 7, 2000.
Y. Wu, B. Caron, P. Bouchard, and G. Chouinard are with the Communications Research Centre Canada, 3701 Carling Avenue, Ottawa, Ontario, Canada
K2H 8S2 (e-mail: yiyan.wu@crc.ca).
E. Pliszka is with the Telekomunikacja Polska S.A. Research and
Development Centre, Obrzezna 7, 02-691 Warszawa, Poland (e-mail:
epliszka@cbr.tpsa.pl).
Publisher Item Identifier S 0018-9316(00)08224-X.
This paper compares the performances of the ATSC 8-VSB,
the DVB-T COFDM, and the ISDB-T BST-OFDM transmission systems under different impairments and operating
conditions. First, a general system level description is presented. It is followed by comparisons based on laboratory test
results and results from theoretical analyzes. The differences
in the system threshold definitions are discussed. A calculated
performance comparison of the three transmission systems for
a 6 MHz channel is provided. The 7 and 8 MHz systems should
yield the same performance, except for higher bit-rate capacity,
since identical modulation and channel coding schemes are
used. In addition, a brief performance and implementation
analysis is also presented for the three transmission systems
under different transmission network infrastructures. Whenever
possible, the impact on the broadcasters or consumers is discussed. Possible performance improvements are also identified.
It should be pointed out that the performance benchmarks
quoted in this paper are representative of present technologies.
Meanwhile, the tests have been conducted in different laboratories, under different test environments and using receivers
from different manufacturers over more than one generation of
products. This might result in some discrepancies due to actual
equipment implementation. On the other hand, with the technical advances, all systems will improve in performance up to
close to their theoretical limits.
II. SYSTEM DESCRIPTIONS
A. General System Descriptions
The main characteristics of the three DTTB systems are listed
in Table I.
1) The ATSC 8-VSB System: The ATSC Digital Television
Standard was developed by the Advanced Television Systems
Committee in the USA [1].
The ATSC system was designed to transmit high-quality
video and audio (HDTV) and ancillary data over a single 6 MHz
channel. The system was developed for terrestrial broadcasting
and for cable distribution. It can reliably deliver 19.4 Mbit/s
of data throughput in a 6 MHz terrestrial channel and 38.8
Mbit/s in a 6 MHz cable television channel. Two modes of
operation are available: the 8-VSB “simulcast terrestrial mode”
intended to be more immune to the NTSC interference, and
the 16-VSB “high data rate mode” primarily developed for the
cleaner—compared to terrestrial—cable channels.
Although the system was developed and tested with 6 MHz
channels, it can be scaled to any channel bandwidths (6, 7, or
8 MHz) with corresponding scaling in the data capacity.
For terrestrial broadcasting, the system was designed to
allow the allocation of an additional digital transmitter for
0018–9316/00$10.00 © 2000 IEEE
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IEEE TRANSACTIONS ON BROADCASTING, VOL. 46, NO. 2, JUNE 2000
TABLE I
MAIN CHARACTERISTICS OF THREE DTTB SYSTEMS
each existing NTSC transmitter with comparable coverage,
and minimum disturbance to the existing NTSC service in
terms of both area and population coverage. This capability is
met and even exceeded as the RF transmission characteristics
of the system are carefully chosen to cope with an NTSC
environment.
Various picture qualities can be achieved with 18 video formats (SD or HD, progressive or interlaced, as well as different
frame rates). There is a great potential for data-based services
utilizing the opportunistic data transmission capability of the
system. The system can accommodate fixed (and possibly
portable) reception.
The system is quite efficient and capable of operating under
various conditions, i.e. clear channel availability or, as implemented in the US, constrained to fit 1600 additional channel allocations into an already crowded spectrum, and reception with
roof-top or portable antennae.
The system is designed to withstand many types of interference: existing analog NTSC TV services, white noise, impulse noise, phase noise, continuous wave and passive reflections (multipath). The system is also designed to offer spectrum
efficiency and ease of frequency planning.
The system uses a single carrier modulation scheme, eightlevel Vestigial-SideBand (8-VSB) modulation. It is designed
for single transmitter (Multi-Frequency Network, MFN) implementation. However, limited on-channel repeater and gap-filler
operation are possible.
The main characteristics of the ATSC 8-VSB system are
listed in Table I.
2) DVB-T COFDM System: The DVB-T system was developed by an European consortium of public and private sector
organizations—the Digital Video Broadcasting Project [2].
The DVB-T specification is part of a family of specifications
also covering satellite (DVB-S) and cable (DVB-C) operations.
This family allows for digital video and digital audio distribution as well as transport of forthcoming multimedia services.
For terrestrial broadcasting, the system was designed to operate within the existing UHF spectrum allocated to analogue
PAL and SECAM television transmissions. Although the system
was developed for 8 MHz channels, it can be scaled to any
channel bandwidth (8, 7, or 6 MHz) with corresponding scaling
in the data capacity. The net bit rate available in 8 MHz channel
ranges between 4.98–31.67 Mbit/s, depending on the choice
of channel coding parameters, modulation types, and guard interval duration.
The system was essentially designed with built-in flexibility,
in order to be able to adapt to all types of channel. It is capable of coping not only with Gaussian channels, but also with
Ricean and Rayleigh channels. It can withstand high-level (up
to 0 dB) long delay static and dynamic multipath distortion.
The system is robust to interference from delayed signals, either
echoes resulting from terrain or building reflections, or signals
from distant transmitters in a single frequency network (SFN)
arrangement.
The system features a number of selectable parameters
that accommodate a large range of carrier-to-noise ratios
and channel behaviors. It allows fixed, portable, or mobile
reception, with a consequential trade-off in the usable bit rate.
This range of parameters allows the broadcasters to select a
mode appropriate to the application foreseen. For instance, a
moderately robust mode (with a correspondingly lower data
rate) is needed to ensure reliable portable reception with a
simple set-top antenna. A less robust mode with a higher
WU et al.: COMPARISON OF TERRESTRIAL DTV TRANSMISSION SYSTEMS
data rate could be used where the service planning uses
frequency-interleaved channels [2], [12]. The less robust modes
with the highest payloads can be used for fixed reception and if
a clear channel is available for digital TV broadcasting.
The system uses a large number of carriers per channel modulated in parallel via an FFT process, a method referred to as Orthogonal Frequency Division Multiplexing (OFDM). It has two
operational modes: a “2k mode” which uses a 2k FFT; and an
“8k mode” which requires an 8k FFT. The system makes provisions for selection between different levels of QAM modulation
and different inner code rates and also allows two-level hierarchical channel coding and modulation. Moreover, a guard interval with selectable width separates the transmitted symbols,
which allows the system to support different network configurations, such as large area SFN’s and single transmitter operation.
The “2k mode” is suitable for single transmitter operation and
for small SFN networks with limited distance between transmitters. The “8k mode” can be used both for single transmitter
operation and for small and large SFN networks.
The main characteristics of the DVB-T COFDM system are
listed in Table I.
3) ISDB-T BST-OFDM: The ISDB-T system was developed by the Association of Radio Industries and Businesses
(ARIB) in Japan [3].
ISDB (Integrated Services Digital Broadcasting) is a new
type of broadcasting intended to provide audio, video and
multimedia services. The system was developed for terrestrial
(ISDB-T) and satellite (ISDB-S) broadcasting. It systematically
integrates various kinds of digital contents, each of which may
include multi-program video from Low Definition TV (LDTV)
to HDTV, multi-program audio, graphics, text, etc.
Since the concept of ISDB covers a variety of services, the
system has to meet a wide range of requirements that may differ
from one service to another. For example, a large transmission
capacity is required for HDTV service, while a high service
availability (or transmission reliability) is required for data services such as the delivery of a “key” for conditional access,
downloading of software, and so on. To integrate different service requirements, the transmission system provides a range of
modulation and error protection schemes which can be selected
and combined flexibly in order to meet each requirement of
these integrated services.
For terrestrial broadcasting, the system has been designed to
have enough flexibility to deliver digital television and sound
programs and offer multimedia services in which various types
of digital information such as video, audio, text and computer
programs will be integrated. It also aims at providing stable
reception through compact, light and inexpensive mobile
receivers in addition to integrated receivers typically used in
homes.
The system uses a modulation method referred to as Band
Segmented Transmission (BST) OFDM, which consists of a set
of common basic frequency blocks called BST-Segments. Each
segment has a bandwidth corresponding to 1/14th of the terrestrial television channel spacing (6, 7, or 8 MHz depending
on the region). For example, in a 6 MHz channel, one segment
103
occupies 6/14 MHz = 428.6 kHz spectrum, seven segments ocMHz = 3 MHz.
cupy
In addition to the properties of OFDM reviewed in the previous section, BST-OFDM provides hierarchical transmission
capabilities by using different carrier modulation schemes and
coding rates of the inner code on different BST-segments. Each
data segment can have its own error protection scheme (coding
rates of inner code, depth of the time interleaving) and type of
modulation (QPSK, DQPSK, 16-QAM or 64 QAM). Each segment can then meet different service requirements. A number
of segments may be combined flexibly to provide a wideband
service (e.g., HDTV). By transmitting OFDM segment groups
with different transmission parameters, hierarchical transmission is achieved. Up to three service layers (three different segment groups) can be provided in one terrestrial channel. Partial
reception of services contained in the transmission channel can
be obtained using a narrow-band receiver that has a bandwidth
as low as one OFDM segment.
Thirteen OFDM spectrum segments are active within one terrestrial television channel. The useful bandwidth is BW
, corresponding to 5.57 MHz for a BW
MHz
channel, 6.50 MHz for a 7 MHz channel, and 7.43 MHz for an
8 MHz channel.
The system was developed and tested with 6 MHz channels
but it can be scaled to any channel bandwidth with corresponding variations in the data capacity. The net bit rate for
one 428.6 kHz segment in a 6 MHz channel ranges between
280.85–1787.28 kbit/s. The data throughput for a 5.57 MHz
DTV channel ranges between 3.65–23.23 Mbit/s.
The main characteristics of ISDB-T BST-OFDM system are
listed in Table I.
B. System Performance Summary
Generally speaking, each system has its own unique advantages and disadvantages. The ATSC 8-VSB [1] system is more
robust in an Additive White Gaussian Noise (AWGN) channel,
has a higher spectrum efficiency, a lower peak-to-average power
ratio, and is more robust to impulse noise. It also has comparable performance to DVB-T and ISDB-T systems on low-level
ghost ensembles and against analog TV interference. Therefore, the ATSC 8-VSB system could be more advantageous for
Multi-Frequency Network (MFN) implementation and for providing HDTV service within a 6 MHz channel to fixed receivers.
The DVB-T COFDM [2] system has performance advantages
with respect to high-level (up to 0 dB), long-delay static and
dynamic multipath distortion. It could be advantageous for services requiring large-scale Single Frequency Network (SFN)
(8k mode) and for mobile reception (2k mode). Hierarchical
channel coding and modulation, which uses multi-resolution
constellation on OFDM carriers (16QAM or 64QAM), is also
available to provide two-tier services within one DTTB channel.
The ISDB-T BST-OFDM [3] system, which uses the same
modulation and channel coding scheme as the DVB-T system,
has similar performance advantages as the DVB-T system. It
was designed to operate under large-scale SFN and, particularly,
in a mobile reception environment. The depth of the time interleaver can optionally be selected to improve the quality of
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IEEE TRANSACTIONS ON BROADCASTING, VOL. 46, NO. 2, JUNE 2000
mobile reception and the immunity against impulse noise. The
band segmentation transmission allows the use of up to three different modulation schemes and coding rates on different channel
segments to meet various service requirements and interference
conditions.
However, it should be pointed out that large scale SFN,
mobile reception, and HDTV service cannot be achieved
concurrently without severe data rate or C/N penalties for all
existing DTTB systems over any channel spacing, whether 6,
7, or 8 MHz. Specific system parameters should be selected for
particular implementations.
III. SYSTEM PERFORMANCE COMPARISON
A. DTTB Signal Peak-to-Average Power Ratios
The COFDM signal can be statistically modeled as a twodimensional Gaussian process [4]. Its Peak-to-Average power
Ratio (PAR) is somewhat independent of the filtering. On the
other hand, the 8-VSB PAR is largely set by the roll-off factor
of the spectrum shaping filter, i.e., 11.5% for the ATSC 8-VSB
signal. Studies show that the PAR of the DVB-T and ISDB-T
signals, for 99.99% of the time, is about 2.2 dB higher than that
for the 8-VSB system [4]–[6]. For 99.9% of the time, the PAR
of COFDM signal is about 2 dB higher. In order to achieve a
35 or 36 dB spectrum attenuation at the channel edges [13], a 6
dB output back-off (OBO) is required for ATSC 8-VSB signals,
whereas for COFDM, the OBO should be 7.5 to 8 dB.
For the same level of adjacent channel spill-over, which is the
major source of adjacent channel interference, the DVB-T and
ISDB-T systems require a transmitter that can handle a higher
peak power to accommodate the 2 dB additional output power
back-off, or a better channel filter with additional side-lobe attenuation. However, the high PAR has no impact on system performance. It just increases the start-up investment cost and operational power consumption cost for the broadcasters. In some
cases, however, the PAR may have an impact on the transmitting antenna and transmission line, depending on their respective peak power-handling capabilities.
B. Thermal Noise
1) C/N Thresholds: Theoretically, from a modulation point
of view, the OFDM and single carrier modulation schemes, such
as VSB and QAM, should have the same C/N threshold over Additive White Gaussian Noise (AWGN). It is the channel coding,
channel estimation, and equalization schemes, as well as other
implementation margins (phase noise, quantization noise, intermodulation products), that result in different C/N thresholds.
All three DTTB systems use concatenated forward error correction and interleaving. The DVB-T and the ISDB-T outer code
) code with 12 R–S
is a Reed–Solomon (R–S) (204, 188,
block interleaving. The R–S (204, 188) code, which is shortened
from the R–S (255, 239) code, can correct 8-byte transmission
errors and is consistent with the DVB-S (satellite) and DVB-C
(cable) standards for commonality and easy interconnectivity.
The ATSC system implements a more powerful R–S
) code, which can correct 10-byte errors,
(207,187,
and uses a much larger (52 R–S block) interleaver to mitigate
impulse and co-channel NTSC interference. The different
R–S code implementations may result in a small difference in
C/N performance. Computer simulations show that the ATSC
system has a small (0.3–0.5 dB) advantage over the DVB-T
and ISDB-T systems [7].
The ATSC system implements a
trellis-coded modulation (TCM) as the inner code, while the DVB-T/ISDB-T
system used a punctured convolutional code (the same as the one
used in the DVB-S standard for commonality). Again, this gives
a slight coding advantage in favor of the ATSC system, which is
estimated to be around 0.5–1.0 dB. Therefore, the implementation difference in forward error correction gives the ATSC
system an estimated total C/N advantage of between 0.8–1.5
dB. This difference could, in the long term, be reduced with
technical advances or system improvements, for example using
iterative de-coding schemes in the DVB-T/ISDB-T systems.
Although it is not mandatory, all of the ATSC receivers on
the market implemented a Decision Feedback Equalizer (DFE).
The DFE causes a very small noise enhancement, but it also results in a very sharp Bit Error Rate (BER) threshold, because
of the error feedback. In a DVB-T/ISDB-T receiver which is
properly implemented to allow fast multipath channel tracking
and interference rejection, various degradations arise which are
well understood and quantified [33]. These degradations add up
to some 1.5–2 dB [8], [9]. Therefore the aggregate C/N performance difference, based on today’s technology, is estimated to
be around 2–3 dB in favor of the ATSC system over an AWGN
channel [6], [10], [11].
However, the AWGN channel C/N performance is only one
benchmark for a transmission system. It is an important performance indicator, but it might not represent a real-world channel
model. Meanwhile, the channel estimation, equalization and
Automatic Gain Control (AGC) systems designed to perform
well in an AWGN channel might be slow to respond to signal
variations and/or moving echoes.
In Europe and Japan, the Ricean channel model was used in
the DTTB spectrum planning process [9], [12]. The computer
simulation results show that the C/N threshold difference between Gaussian channel and Ricean channel (direct path to muldB) is about 0.5–1 dB, depending on
tipath power ratio
the modulation and channel coding used [2]. The actual C/N
threshold values recommended for the planning process factored in a 2 dB noise degradation caused by the channel estimation and equalization over the receiver noise floor [9].
The frequency planning for the ATSC system has been
done using different approaches. In the USA, the FCC used a
Gaussian channel performance [6]. In Canada, a generous 1.3
dB C/N margin was set aside for multipath distortion (direct
dB), which is similar
path to multipath power ratio
to the European approach [13].
Table II presents the C/N thresholds (AWGN channel) for the
three DTTB systems based on computer simulations [1]–[3] and
the most recent laboratory RF back-to-back test results available [6], [9], [11], [14]–[17]. It should be pointed out that the
C/N threshold measurement is somewhat dependent on the measurement conditions. For example, changing the RF channel frequency and signal levels might result in minor (a few tenths of
a dB) C/N threshold differences.
WU et al.: COMPARISON OF TERRESTRIAL DTV TRANSMISSION SYSTEMS
105
TABLE II
C/N THRESHOLDS BASED ON TEST RESULTS
2) Fair Comparison of the System C/N Performances: It
should be pointed out that the threshold values presented in
Table II are not a fair comparison, because the systems have
different data rates, and their thresholds are also defined differently (TOV vs. QEF). For the DVB-T and ISDB-T systems,
selection of different guard intervals, while maintaining the
same channel coding, will result in different data throughputs,
despite the same C/N threshold.
, or carrier-to-noise ratio
One alternative is to use the
per unit of data capacity (bit) and unit of bandwidth (Hz), to
evaluate the system performance, as it takes account of the
is a digital transmission
system data rate and bandwidth.
system’s spectrum and power efficiency measurement, which is
widely used in digital communications’ literature. The smaller
value, the more efficient the transmission system.
the
It is defined as:
dB
(1)
is the system data throughput and
is the system
where
bandwidth. For the 6 MHz ATSC system, the data rate is
Mbit/s [1]. The comparable DVB-T and ISDB-T systems
coding and 1/32 guard interval)
(6 MHz system, with
Mbit/s [2] and 17.7 Mbit/s [3], redata rates are
spectively. Thus, for the DVB-T and ISDB-T systems, using the
same coding but a different guard interval length, the system
will be different, due to
C/N should be the same, while
the different data throughput.
The DVB-T and ISDB-T systems’ threshold was defined at
before the R–S decoding
a Bit Error Rate (BER) of
[2], [3]. After R–S decoding, this corresponds to a BER of less
, or Quasi Error Free (QEF) reception, which
than
is equivalent to one error hit every few hours. This threshold
definition is typical of performance required for high-speed data
transmission.
The ATSC threshold was actually derived subjectively from
the video picture “Threshold Of Visibility” (TOV), assuming
that some video error concealment or resilient techniques are
implemented in the receiver. The corresponding objective mea, or Segment Error
surement was defined at BER
Rate (SER) =
, after the R–S decoding. This SER translates into an 8-VSB symbol error rate after the equalizer (before trellis decoding) of 0.2. It also indicates a byte error rate of
after the trellis decoding [18]. It can be seen
about
that the ATSC threshold is more relaxed than that of the DVB-T
and ISDB-T systems. A correction factor should be added for
a fair comparison. For an AWGN channel, the ATSC system
correction factor, between TOV and QEF, should be around
0.8 dB [19]. For the DVB-T and ISDB-T systems, the correction factor was reported as 1.3 dB [15]. It should be pointed out
that measurement on different receivers may result in different
values depending on their implementation.
Based upon the above discussion, factoring in the data
rate and the threshold definition differences, the calculated
system
thresholds for AWGN channels are presented
and
in Table III. Two convolutional coding rates,
are selected for DVB-T and ISDB-T systems, the latter
one providing comparable data rates as with the ATSC system.
From the RF back-to-back test data, the ATSC system presently
has a few dB advantage for an AWGN channel. Again, it should
be mentioned that improvements are possible for all systems,
and the AWGN channel might not be the best channel model
for DTTB, especially for indoor reception.
Since all three systems can be scaled for different channel
spacings, i.e., 6, 7, and 8 MHz, without changing the channel
values presented in Table III
coding scheme, the system
are generally valid for 6, 7, and 8 MHz systems.
C. Multipath Distortion
The COFDM modulation system used by DVB-T and
ISDB-T has a strong immunity against multipath distortion.
It can withstand echoes of up to 0 dB relative to the most
powerful received signal. High level echoes are usually found
in urban areas, where direct line of sight to the transmitter
is blocked, and when indoor or set-top antennae are used.
The implementation of a guard interval can totally eliminate
the inter-symbol interference, except for echoes with excess
delays larger than the extent of this guard interval. However,
the in-band fading will still have an impact on the required
C/N, especially when high order modulations are used on the
COFDM carriers. A strong inner error correction code and a
good channel estimation system are required for the DVB-T
and ISDB-T systems to withstand 0 dB echoes and a higher
C/N will be needed to deal with such strong echoes. With the
convolutional coding, it needs about 6 dB more signal
power to deal with the 0 dB echoes [5], [10]. However, some
of this increased C/N requirement will be compensated by the
signal power arriving from the echoes [20]. The balance of
these requirements will depend on the code rate selected. Soft
decision decoding using an eraser technique can significantly
improve the performance [21]. For static echoes with levels
less than 4 to 6 dB, the 8-VSB system, using a Decision
Feedback Equalizer (DFE), yields a smaller noise enhancement
[11], [18].
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IEEE TRANSACTIONS ON BROADCASTING, VOL. 46, NO. 2, JUNE 2000
TABLE III
SYSTEM E =N THRESHOLDS
For very close-in echoes, produced by nearby structures,
wide-band selective fading will be experienced. These close-in
echoes will affect more predominately narrow-band systems,
such as the segments used in the ISDB-T system, which might
cause reception failure on some segments or the entire system.
Wide-band systems, such as the ATSC and the DVB-T will be
more immune to these types of channel impairment.
The DVB-T and ISDB-T system guard interval can be used to
deal with both advanced or delayed multipath distortions. This is
important for SFN operation. The ATSC system cannot handle
long pre-echoes, as it was designed for an MFN environment
where they usually do not happen in outdoor fixed reception
conditions. SFN operation can provide significant savings in
spectrum requirements as all the transmitters in one area can
operate on the same frequency (see Section IV-F). It can also
provide for significant savings in total transmission power because of the increased probability of receiving the signal from a
number of transmitters, the so-called “network gain.”
is necessary to consider interference from more than one analog
television system and, in such cases, a fixed set of notch filter
frequencies may be less appropriate. Considerable attention was
paid to this aspect during the design and development of the
DVB-T system. COFDM systems using 8k FFT should outperform systems using smaller sizes of FFT.
D. Co-Channel Analog TV Interference
F. Impulse Noise
Co-channel analogue television interference (with its energy
concentrated around the visual carrier and, to a lesser extent,
the color sub-carrier and the aural carriers) will interfere with a
limited number of COFDM carriers in specific portions of the
DTTB band. A good channel estimation system combined with
soft decision decoding using an eraser technique should result
in good performance against the analog TV interference.
The ATSC system uses a much different approach. A carefully designed comb-filter or notch filter is implemented to
notch out the analog TV’s visual, aural and color sub-carriers to
improve the system performance. Actually, the OFDM system
can also implement a notch filter to improve the performance
against co-channel analog TV interference.
The ATSC approach is relatively simple to adopt in the case
where there is only one interfering analog television system to
be considered. In Europe and in some other parts of the world, it
The impulse noise interference usually occurs in the VHF
and low UHF bands, and is caused by industrial equipment and
home appliances, such as microwave ovens, fluorescent lights,
hair-dryers, and vacuum cleaners. High-voltage power transmission lines, which often generate arcing and corona, are also
a source of impulse noise.
Theoretically, OFDM modulation should be more robust to
time-domain impulse interference, because the FFT process in
the receiver can average out the short duration impulses. Therefore, an OFDM system with larger FFT size, e.g., 8k FFT, will
perform better against impulse than the system with smaller
FFT size, e.g., 2k FFT [41]. However, as mentioned previously,
the channel coding and interleaver implementation also play an
important role. The stronger R–S(207, 187, 10) code with the
52-segment interleaver makes the ATSC system more immune
to impulse interference than the DVB-T and ISDB-T systems
E. Co-Channel DTV Interference
Good co-channel DTV C/I performance will result in less interference into the existing analog TV services. It will also mean
better spectrum efficiency once the analog services are phased
out. All three DTV signals behave like additive white Gaussian
noise. Therefore, the co-channel DTV interference performance
should be highly correlated with the C/N performance, which
is largely dependent upon the channel coding and modulation
used. There is about a 3–4 dB advantage for the ATSC system,
see Tables III and IV, as it benefits from its better forward error
correction system.
WU et al.: COMPARISON OF TERRESTRIAL DTV TRANSMISSION SYSTEMS
107
TABLE IV
DTV PROTECTION RATIOS FOR FREQUENCY PLANNING.
which use R–S(204, 188, 8) code with a 12-segment interleaver
[11]. With respect to the inner code, the shorter constraint length
of 2 for the ATSC system (7 for DVB-T and ISDB-T) also results
in shorter error bursts, which are easier to correct by the outer
code. However, the ISDB-T system with the option of using a
long time interleaver also proved to be robust to impulse noise
[41].
The robustness of the carrier recovery and synchronization
circuits against impulse noise can also limit the system performance. This is however primarily a receiver design issue and
not a system issue.
G. Continuous Wave (CW) Interference
Since a COFDM system is a frequency-domain technique,
which implements a large number of closely spaced carriers,
a single CW or narrow band interference will destroy only a
few of these carriers, but the lost data can be easily recovered
by the error correction code. On the other hand, CW interference will cause eye closing for the 8-VSB modulation. The
adaptive equalizer could reduce the impact of the CW interference, but, in general, DVB-T and ISDB-T systems should
outperform the ATSC system on CW interference by a large
amount ( 10 dB) [5], [11]. However, tone interference is just
another performance benchmark. In the real world, a DTTB
system should never experience a tone interference dominated
environment, as a well-engineered spectrum allocation plan is
designed to avoid such a problem. A badly designed receiver
front-end might generate inter-modulation products falling in
the signal band, which would result in “CW-like” interference.
But this is a receiver design issue, not a system issue.
H. Phase Noise Performance
For a single carrier modulation system, such as 8-VSB [40],
the phase noise generally causes constellation rotation and jitter
that can mostly be tracked by a phase-locked loop.
Theoretically, the OFDM modulation is more sensitive to the
tuner phase noise. The phase noise impact can be modeled into
two components [28], [29]: 1) a common rotation component
that causes a phase rotation of all OFDM carriers; 2) a dispersive
component, or inter-carrier interference component, that results
in noise-like defocusing of the carriers’ constellation points. The
first component can easily be tracked by using in-band pilots
as references. However, the second component is difficult to
compensate. It will slightly degrade the DVB-T and ISDB-T
system noise threshold.
A tuner with a more optimized phase noise performance will
be needed for the DVB-T and ISDB-T systems [30]. Using a
single conversion tuner or a double conversion tuner will also
cause differences in performance. Single conversion tuners have
less phase noise and better dynamic range, but are less tolerant to
adjacent channel interference, especially on the image channel.
A tuner that covers both VHF and UHF bands will be slightly
worse than a single-band tuner.
IV. DISCUSSION
ON THE MEANING OF THE
PERFORMANCE
SYSTEM
A. Indoor Reception
Indoor reception of DTTB systems needs more investigation.
There is no published large-scale field trial data to support a
meaningful system comparison. In general, indoor signals suffer
from strong multipath distortion, due to reflections from indoor
walls, as well as from outdoor structures. The movement of
108
human bodies and even pets can significantly alter the distribution of indoor signals, causing time varying echoes and field
strength variations.
The indoor signal strength and its distribution are related to
many factors, such as building structure (concrete, brick, wood),
siding material (aluminum, plastic, wood), insulation material
(with or without metal coating), and window material (tinted
and metal coated glasses, multi-layer glass). Typical building
attenuation for VHF/UHF signals is around 10 and 25 dB.
Measurements on indoor set-top antennae showed that gain
and directivity depend very much on frequency and location
[12]. For “rabbit ear” antennae, the measured gain varied from
about 10 to 4 dBi. For five-element logarithmic antennae,
the gains are between 15 to 3 dBi [12]. Meanwhile, indoor
environments often experience high levels of impulse noise interference from power lines and home appliances.
B. Mobile Reception
Distribution of multimedia services (digital TV, audio, data,
etc.) to portable and vehicular receivers may become an important application for terrestrial broadcasters. DVB-T and ISDB-T
systems can be used to provide mobile services, but a lowerorder modulation on the OFDM carriers and a lower rate of conor
) are recommended to
volutional coding (e.g.,
provide reliable services [22], [23]. There is a penalty in data
throughput for mobile reception in comparison to fixed recepor
, or 16 QAM with
tion. Usually, QPSK with
are the preferred modes for reliable mobile reception with data rates in the order of 5 to 12 Mbit/s [22], [23].
With higher order modulation, the system will be sensitive to
fading/shadowing and Doppler effects, which, in turn, would require more transmission power.
In these conditions, it is not possible to achieve a 19 Mbit/s
data capacity required for a single good quality HDTV program and associated multi-channel audio and data services in
the envisaged channel bandwidths (6–8 MHz) in a mobile environment [24]. However, providing mobile multimedia services
(multi-program SDTV, audio and data services) seems to be a
possible option [22], [23], [25], [34], [42].
The DVB-T system was originally designed for fixed and
portable reception, but some of its rugged modes, e.g., 2k FFT
with QPSK or 16-QAM modulation and strong coding, can provide for mobile reception of DTV services [2], [23]. Theoretical
analysis, computer simulations, laboratory tests and field trials
show that the channel Doppler spread is not the fundamental
limitation of the DVB-T system, although such Doppler spread
needs to be compensated for by a somewhat higher C/N. It is
the lack of proper time interleaving which restricts the performance of the system in a mobile environment [23]. Field tests
of mobile reception with the DVB-T system have shown that
increasing the field strength is the prime means to counter this
lack of time interleaving and provide a satisfactory mobile service. In MFN situations, an additional 6–8 dB is needed in the
case of a Ricean channel to provide for mobile reception of the
[23].
2k DVB-T mode with
Since the ISDB-T system was designed from the outset for
mobile reception, it can optionally use a large time interleaver—
IEEE TRANSACTIONS ON BROADCASTING, VOL. 46, NO. 2, JUNE 2000
up to approximately 0.5 second—to improve the mobile reception service quality [3], [25], [42]. The robust DQPSK modulation can be selected for the OFDM carriers and a 2k or 4k FFT
mode can be used for mobile service. In the DQPSK modulation
mode, channel estimation is not necessary. This can reduce the
receiver complexity and power consumption. It could be advantageous for small hand-held battery-operated receivers.
In the case of mobile reception in an SFN environment,
where the mobile receiving terminal is travelling at a different
speed relative to each SFN transmitter, there will likely be
strong Doppler spread effects on strong multipath signals
which have to be dealt with by channel estimation and error
correction systems. The “nonpunctured” convolutional inner
code,
, is recommended for mobile implementation
[23].
One potential problem with mobile services is the spectrum
availability. Since mobile reception requires different modulation and channel coding from the fixed services, it might be advantageous to provide mobile services in other dedicated channels rather than share the same channel with the fixed reception DTV/HDTV services, which usually opt for the maximum
data throughput. Since many countries have difficulties allocating one fixed service DTV channel to every existing analog
TV broadcaster, finding additional spectrum for mobile service
might be difficult.
One alternative to provide fixed and mobile services within
one channel is, however, to use the hierarchical channel coding
and modulation (DVB-T and ISDB-T systems), which will be
discussed in Section IV-H.
Another alternative is to time-share the transmission facility
for fixed and mobile services. For example, the HDTV service
may be provided during prime time, while the mobile service is
offered during traffic rush hour. However, in the case of several
broadcasters sharing a multiplex of SDTV programs on a terrestrial channel, this approach might be difficult to implement.
It should be mentioned that digital audio and video services
are more robust to transmission errors than data services. With
error concealment and muting capabilities, a BER in the order
can probably meet the DTV service requirements [35].
of
On the other hand, a data service might need error-free transor less). A mobile reception environment
mission (i.e.,
might have an irreducible transmission error floor, which can
not achieve error free transmission. Mobile data services, may
use either a return link (e.g., via cellular phone) to acknowledge the transmission error and initiate data re-transmission, or
blind multiple data re-transmission, via so called data carousels,
which will significantly improve the transmission performance
over error prone channels.
It should be pointed out that since mobile services are mostly
intended to deliver audio, data, and low-resolution video services to car drivers or passengers on public transportation systems (buses and trains) [25], [42], they are in direct competition
with Digital Audio Broadcasting (DAB) and the third generation Personal Communications System services (IMT-2000).
They might also need special approval from the proper regulatory authorities.
WU et al.: COMPARISON OF TERRESTRIAL DTV TRANSMISSION SYSTEMS
C. Spectrum Efficiency
OFDM, as a modulation scheme, is slightly more spectrum-efficient than single carrier modulation systems, since its
spectrum can have a very fast initial roll-off even without an
output spectrum-shaping filter. For a 6 MHz channel, the useful
%)
(3-dB) bandwidth is as high as 5.7 MHz (or
%)
for the DVB-T system [2], and 5.6 MHz (or
for the ISDB-T system [3], in comparison with the 5.38 MHz
%) useful bandwidth of the ATSC system [1].
(or
OFDM modulation has, therefore, an advantage of up to 5% in
spectrum efficiency.
However, the guard interval that is implemented to provide
enhanced freedom from multipath distortion and the in-band pilots inserted for fast channel estimation reduce the data capacity
for the DVB-T and ISDB-T systems. For example, the DVB-T
offers a selection of system guard intervals, i.e., 1/4, 1/8, 1/16
and 1/32 of the active symbol duration. These are equivalent
to data capacity reductions of 20%, 11%, 6%, and 3%, respectively. The 1/12 in-band pilot insertion results in an 8% loss
of data rate. Overall, the data throughput reductions are up to
28%, 19%, 14%, and 11% for the different guard intervals. Subtracting the previous 5% bandwidth efficiency advantage for the
OFDM system, the total data capacity reductions for the DVB-T
system, in comparison with the ATSC system, are 23%, 14%,
9%, and 6%, respectively. This means that, for a 6 MHz system,
assuming equivalent channel coding and modulation schemes
), the DVB-T system will offer data rates
(64 QAM,
of 14.9, 16.6, 17.6, and 18.1 Mbit/s for the guard interval ratios identified above. The ISDB-T system will provide data rates
of 14.6, 16.2, 17.2, and 17.7 Mbit/s. The corresponding ATSC
system data rate is a fixed 19.4 Mbit/s.
Actually, the best indication of spectrum efficiency and
values listed in
transmission power requirement is the
Table III. The apparent loss of spectrum efficiency for DVB-T
and ISDB-T systems must be considered in the light of the
performance improvement in: a) severe multipath environment,
b) fast moving multipath environment, c) Single Frequency
Networks (SFN), d) mobile reception and e) nondirectional
receiving antennae situations.
The above analysis of spectrum efficiency is based on an
MFN approach. In an SFN environment, it is possible to have a
number of transmitters re-use the same frequency (channel) to
cover a large geographical area, which results in overall saving
of spectrum and transmission power for DVB-T and ISDB-T
systems.
D. HDTV Capability
Research on digital video compression showed that, based on
current technology, a data rate of at least 18 Mbit/s is required to
provide a satisfactory HDTV picture for sports and fast action
programming (1920 by 1080 format) [24]. Additional data capacity is required to accommodate multi-channel audio and ancillary data services. The ATSC system data rate is 19.4 Mbit/s.
Based on the DVB-T and ISDB-T standards, with modulation
and channel coding schemes equivalent to the ATSC 8-VSB
), the 6 MHz DVB-T and ISDB-T
system (64 QAM,
systems’ data throughput ranges from 14.9–18.1 Mbit/s and
109
14.6–17.7 Mbit/s, respectively, depending on the guard interval
selected. To achieve a higher data rate, a weaker error correction
coding would need to be selected. For example, by increasing
the convolutional coding rate to
, the range for the
data rates becomes 16.8–20.4 Mbit/s for the DVB-T system,
and 16.4–20.0 Mbit/s for the ISDB-T system. However, this
approach would require about 1.5 dB of additional signal power
[2], [16], [17]. The estimated system performance is listed in
Table III. Increasing the coding rate will also compromise the
performance against multipath distortion, especially for indoor
reception and SFN environments.
E. Interference into Existing Analog TV Services
Since both VSB and COFDM signals behave more or less
like white noise. They have the same impact to the analog TV
systems.
In many countries, the government policy requires analog
television and DTTB to co-exist for an extended period of time,
and no additional spectrum resources are available for DTTB
implementation. DTTB services can only be implemented in
channels that cause limited interference into existing analog
television reception. It is expected that one of the limiting factors will be the DTTB interference into the existing analog television services during the analog television-to-DTV transition
period.
In an MFN environment, the current 4 dB C/N difference
in planning parameters (see Table IV) requires that DVB-T or
ISDB-T systems transmit 4 dB more power than the ATSC
system to achieve the same noise limited coverage area. This
might make the frequency planning more difficult and cause
additional interference into analog television systems. Extra
measures would need to be taken to increase the co-channel
spacing, or reduce the DTV transmission power (or coverage).
F. Single Frequency Network (SFN) and On-Channel Repeater
Capability
The 8k mode in the DVB-T and ISDB-T systems was included for large scale (nation-wide or region-wide) synchronous
SFN operation, where a cluster of transmitters, all fed from the
same source, is used to cover a designated service area [38]. It
uses a small carrier spacing, which can support very long guard
intervals. It can also sustain 0 dB multipath distortion, if a strong
). However, about 6 dB
convolutional code is selected (
more signal power is required to deal with the 0 dB multipath
distortion [5], [10], [23]. One alternative to reduce the excess
transmission power is to use a directional receiving antenna,
which would likely eliminate most 0 dB multipath distortion
conditions. Such an antenna would also improve the reception
of the ATSC 8-VSB signals in such circumstance. It should be
noted that the probability of finding a location with two equal
power signals in a coverage area is relatively small.
The SFN approach can provide stronger field strength
throughout the core coverage area and can significantly improve the service availability. The receivers have more than one
transmitter from which they can receive the signal (diversity
gain). They have better chances to have a strong signal path to
a transmitter to achieve reliable service.
110
By optimizing the transmitter network (transmitter density,
tower height and location, as well as the transmission power at
each transmitter) SFN can yield better coverage with lower total
transmit power and provide better spectrum efficiency. Tighter
control of the rate of decay of the field strength outside the service area is also possible through the design of the SFN in order
to maintain a satisfactory level of interference to and from close
neighboring networks [26].
Special measures must be taken to minimize the frequency
offset among the repeaters and flexibly address each transmitter
with respect to its exact site, power, antenna height and the insertion of specific local signal delays. For DVB-T systems, this
can be achieved by using the DVB MIP (Mega-frame Initialization Packet) system. All transmitters in an SFN network can,
then, be synchronized in time and frequency.
One problem that might impact a large-scale SFN implementation is co- channel and adjacent channel interference. In
many countries, it might be difficult to allocate a DTV channel
for large-scale SFN operation that will not generate substantial
interference into existing analog TV services during the analog
TV to DTV transition period. Finding the additional tower
sites at desired locations and the associated expenses (such
as property, equipment, legal, construction, operation, and
environmental studies) might not be practical or economically
viable.
The DVB-T and ISDB-T systems also allow the use of
on-channel repeaters to improve the coverage at the edge of
the coverage area (coverage extenders) and fill holes within
the coverage area (gap-fillers). In this case, these on-channel
repeaters are designed to capture the signal off-air as emitted
by the main transmitter, amplify it and re-transmit it. The
maximum transmission power is limited by the amount of
isolation that can be achieved between the directional receiving
antenna and the transmit antenna. Such limitation could be
alleviated by regenerating the DTTB signal at the repeater,
i.e., the received off-air signal is demodulated, decoded, and
re-modulated. In such a case, the transmission errors generated
in the first hop could be corrected. The signal transmitted
from such an on-channel repeater would normally result, at the
receiver, in the creation of an advanced active echo (from the
main transmitter) with an excessive time difference. In such a
case, sufficient isolation from the main transmitter would need
to be provided by the home receiving antenna.
On the other hand, the ATSC system was not designed for
synchronous SFN implementation. Nevertheless, implementation of on-channel repeaters and gap-fillers for coverage extension is possible, if enough isolation between the reception of
the original signal and the re-transmitted signal can be achieved
[27]. The limitations in designing the on-channel gap fillers to
avoid signal feedback between their transmit and receive antennae are the same for all systems. Local signal re-generation
would also be beneficial for all systems but results in a need for
increased home antenna discrimination toward interfering transmitters resulting from the additional excess delay generation.
The key difference between a DTV and an analog TV system
is that the DTV can withstand at least 20 dB of co-channel interference, while the analog TV co-channel threshold of visibility
is around 50 dB (30 to 35 dB for CCIR Grade 3). In other words,
IEEE TRANSACTIONS ON BROADCASTING, VOL. 46, NO. 2, JUNE 2000
DTTB is 10 to 30 dB more robust than analog TV, which provides more flexibility for the repeater design and siting.
In the case of an ATSC system repeater implementation [27],
using a directional receiving antenna will increase the location
availability as well as reduce the impact of fast-moving or longdelay multipath distortions. The operational parameters will depend on the population distribution, terrain environment, and intended coverage area.
It should be pointed out that under any circumstances, an
RF transmission system (ATSC, DVB-T or ISDB-T; SFN or
MFN), can not realistically provide a service with a 100% location availability.
G. Noise Figure
Generally speaking, the noise figure of a receiver is an implementation issue. It is system-independent. A low noise figure
receiver front end can be used in any DTTB system to reduce
the minimum signal level required. The critical parameter for
planning purposes is sensitivity, which accounts not only for the
noise figure, but also the susceptibility of the system to effects
such as self-interference and inter-modulation products.
A single conversion tuner has a better dynamic range, and low
phase noise, but its noise figure, which tends to be lower, is not
consistent over the full VHF and UHF TV bands. Single conversion tuners provide less suppression on adjacent channel interference. This suppression is also not consistent over all the channels. On the other hand, a double conversion tuner has a higher
noise figure, less dynamic range, and higher phase noise. It can
achieve better adjacent channel suppression, especially on the
image channel. Its noise figure and adjacent channel suppression are also much more consistent across the frequency bands.
Tuner performance is very much linked to the cost (materials,
components, frequency range, etc.). With today’s technology,
for low-cost consumer-grade tuners, the single conversion tuner
has a noise figure of about 7 dB. A double conversion tuner typically achieves 9 dB. However, tuner noise figure only impacts
the system performance at the fringe of the coverage, where
signal strength is very low and there is no co-channel interference present. This situation might only represent a very small
percentage of the intended coverage areas, since most of the
coverage is interference-limited. However, some countries do
regulate the noise figure of the receivers.
Different countries might opt to use different noise figures
in their frequency planning process. For example, DVB recommended the use of a 7 dB noise figure in their UHF band frequency planning [9], [12]. In the USA, the FCC used a 10 dB
noise figure as requested by the receiver manufacturers [31]. In
Canada, a 5 dB noise figure was used taking into account future
technology developments [13].
H. Hierarchical Modulation and Services
Hierarchical modulation can, among other things, help overcome the problem of a total signal disruption in areas with low
field strength by providing a lower rate, more reliable bit stream
as part of the transmitted signal. The DVB-T system can use
multi-resolution constellations on the OFDM carriers (16 QAM
or 64 QAM) to provide two tiers of service within one DTTB
WU et al.: COMPARISON OF TERRESTRIAL DTV TRANSMISSION SYSTEMS
channel [2]. It can offer a robust channel with a low data rate and
), and
a high error protection (e.g., 4.5 up to 6.3 Mbit/s,
simultaneously a less robust channel with a higher data rate and
), deweaker error protection (e.g. 15 up to 20 Mbit/s,
pending on the channel bandwidth (6, 7, or 8 MHz) [32]. These
two channels, the high priority (HP) stream and the low priority (LP) stream can be used, either for independent services
or for simulcasting of the same services. But they are not intended to provide scalable or hierarchical video coding, since,
presently, all three DTTB standards only implement MPEG-2
Main Profile (see Table I) which does not support hierarchical
or multi-resolution video source coding.
For example, the HP stream can provide the basic DTV service and some audio programs, while the LP stream is used for
additional DTV or data services or for one service of HDTV-like
quality. The difference in the required C/N between the HP and
LP for fixed reception is in the order of 10 dB [32], [34].
From the coverage point of view, the LP data stream can be
used to cover the core service area, but a fixed roof-top directional antenna would be required in most locations. Generally,
within the LP stream coverage, the HP stream should always
be available. The HP data stream can be used to provide three
classes of services/coverage:
• Class I: The HP stream, due to its low C/N requirement,
can be used to provide extended coverage using a fixed
roof-top antenna.
• Class II: The HP stream can be used to provide service
to mobile terminals, e.g., with a car-mounted omni-directional antenna.
• Class III: The HP stream can be used to provide service to
a portable indoor set-top antenna with limited directivity.
However, coverage depends on many factors, such as terrain,
transmitting tower height and power, receiving antenna height
and gain/directivity. For example, for terrain-limited coverage
area (due to mountains, valleys, horizon or man-made structures), the core-coverage (LP data) and Class I (HP data) coverage areas might be quite close. They can achieve different service areas only if the coverage is limited by signal power or interference. For relatively flat terrain, a 10-dB difference in C/N
requirement will result in a coverage difference of about 10 to
15 km in radius.
For the mobile reception case, gain/directivity of the receiving antennae are usually much lower than in the case of
the roof-top antennae (a difference of about 10 dB in the UHF
band). The C/N requirement for mobile reception is usually
at least 6 dB higher than for fixed reception (Rayleigh vs.
Gaussian channels) [23]. The reduced receiving antenna height,
1.5 m car-mounted antennae vs. 10 m roof-top antennae, will
also reduce the received signal strength due to the increased
presence of blockage and shadowing. For indoor reception,
the main problem is building penetration loss, which ranges
between 10–25 dB. The coverage of mobile and indoor portable
reception of the HP stream (Class II and III coverage) is likely
to be less than the core coverage area, i.e., where the LP data
stream can be received with fixed roof-top antennae.
It should also be pointed out that, due to terrain blockage
and shadowing, the real coverage areas for different classes of
111
service are usually not as simple as a series of concentric circles, but rather are akin to a Swiss cheese—with many “holes”
within the coverage—and terrain-limited end-of-coverage contours with irregular shapes.
In the case of the ISDB-T system, the band segmentation
transmission concept allows the use of up to three different modulation schemes and coding rates on different channel segments
to meet various service requirements and interference conditions. It can also independently decode the signals in the different band-segments.
The penalty for providing hierarchical services, using hierarchical modulation and channel coding (DVB-T) or band segmented modulation and channel coding (ISDB-T), is that it either increases the C/N requirement (DVB-T case) or reduces the
total data rate (ISDB-T case), in comparison with the nonhierarchical approach. More studies and tests are required to demonstrate the viability of providing tiered services [34] and to establish the required minimum C/N difference between two layers
of services to support two distinctive or meaningful services.
I. System Flexibility
The DVB-T and ISDB-T standards offer broadcasters a large
selection of operational modes. The reasons for providing this
wide range of choices are the different applications foreseen and
the different introduction scenarios expected in various countries. The parameters that can be chosen for a given application
are:
a) The size of FFT, which specifies the number of OFDM
carriers (for DVB-T: 2K and 8K FFT; for ISDB-T: 2k, 4k
and 8k FFT);
b) Carrier modulation (for DVB-T: QPSK, 16 QAM and 64
QAM; for ISDB-T: DQPSK, QPSK, 16 QAM, and 64
QAM);
c) Coding rate for the inner error correction code (1/2, 2/3,
3/4, 5/6, and 7/8);
d) Guard interval width (1/4, 1/8, 1/16, and 1/32 of the duration of an OFDM symbol);
e) Non-hierarchical or hierarchical modulation and channel
coding.
These selections result in a large number of nonhierarchical
and hierarchical mode combinations. The standard receiver
should be able to automatically detect which mode is being
used. In addition, these two standards are fully specified for all
three existing channel spacings: 6 MHz, 7 MHz, and 8 MHz.
The choice of transmission mode will set the system data capacity and will affect the C/N performance, and, therefore, the
coverage corresponding to different types of receiving installations—fixed roof-top antennae, indoor portable receivers, and
mobile receivers at high speeds.
Depending on the chosen combination, the DVB-T and
ISDB-T systems will be able to accommodate SFN’s of various transmitter densities resulting in improved coverage and
spectrum efficiency.
One special feature for the ISDB-T system is that it can independently decode signals in part of the DTTB band, i.e., in
band-segments [3]. This can be used, for example, to facilitate
narrow-band audio/data broadcasting.
112
IEEE TRANSACTIONS ON BROADCASTING, VOL. 46, NO. 2, JUNE 2000
The ATSC 8-VSB system was designed to maximize the data
capacity. The data rate for a 6 MHz channel is 19.4 Mbit/s.
However, if there is demand from the broadcasters, 2-VSB and
4-VSB versions of the modulation are possible, which, at the
expense of reduced data rate, would provide more robust reception [40].
J. Systems Scaled for Different Channel Bandwidths
The DVB-T system was originally designed for 7 and 8
MHz channels. By changing the system clock rate, the signal
bandwidth can be adjusted to fit 6–8 MHz channels. The corresponding hardware differences are the channel filter, IF unit,
and the system clock. On the other hand, the ATSC and ISDB-T
systems were designed for a 6 MHz channel. These systems
can operate over 7 and 8 MHz bandwidths also by changing
the system clock, as in the DVB-T case. However, the ATSC
system implemented a comb-filter or a notch filter to reduce
the impact of co-channel NTSC interference. This would need
to be changed to deal with different analog TV systems. The
use of a comb-filter or a notch filter is not mandatory and might
not be needed, if co-channel analog TV interference is not a
major concern. For instance, some countries might implement
DTV on dedicated DTV channels where there is no analog
co-channel interference.
Generally speaking, a narrower channel results in a lower data
rate for all three modulation systems, due to slower symbol rate.
However, it also means a longer guard interval for DVB-T and
ISDB-T systems and longer echo correction capability for the
ATSC system.
One minor weak point for the 6 MHz COFDM 8k systems is
that their narrow carrier spacing of less than 1 kHz (75% compared to 8 MHz system) might cause the systems to be more
sensitive to phase noise and Doppler spread in the case of mobile reception. The 5.7 MHz useful bandwidth of the 6 MHz
DVB-T system might also need steep RF filtering [2] to reduce
any adjacent channel interference into analog television services
produced by nonlinearity in the high power amplification stages
at the transmitters (ISDB-T 6 MHz system useful bandwidth is
5.6 MHz).
V. DTV IMPLEMENTATION PARAMETERS
Planning parameters for digital terrestrial TV are given
in ITU Recommendation 1368 [39]. Countries choosing the
same DTTB system could still use different implementation
approaches, emission masks, and technical parameters in their
spectrum allotment process, depending on their spectrum
resources and policy, population distribution, required service
quality, etc.
For example, Canada adopted the ATSC DTTB system, but
implemented different DTV technical parameters and emission
masks [13] than the USA. Table IV lists the Canadian [13], the
American [6], the European [9], [12] and the Japanese [36], [37]
DTV technical parameters, more specifically the protection ratios, used in DTV planning.
In the Canadian plan, a generous 1.3 dB C/N margin
has been allocated for multipath distortion, which is similar to the EBU approach that uses a Ricean channel
performance threshold as a planning parameter [9]. Since
noise and co-channel DTV interference are additive, a total
dB was allocated in Canada as the system
dB,
threshold [in Table IV,
dB]. Also in
Table IV, a Canadian co-channel NTSC to DTV interference
threshold of 7.2 dB was used. This allows the system to
withstand a co-channel NTSC interference of 7.2 dB, and, at
the same time, either a C/N of 19.5 dB or a co-channel DTV
interference of 19.5 dB. The adjacent channel DTV interference
parameters are generally the same as the American ones, as
shown in Table IV.
It should be pointed out that the protection ratios for DTV interference into analog TV systems depend on many factors, such
as the analog TV standards (NTSC, PAL and SECAM) and the
system bandwidths (6–8 MHz), as well as the subjective evaluation methods (CCIR Grade 3, Threshold of Visibility, continuous or tropospheric interference). Other nontransmission or
spectrum-related factors, such as audio coding (stereo, or surround sound), video format (SDTV or HDTV, progressive or interleaved), error concealment, and reception conditions (fixed,
portable or mobile) should also be considered.
VI. CONCLUSIONS
There are three DTTB standards available. The final choice
of a DTV modulation system is based on how well the system
can meet the particular requirements or priorities of each
country, as well as other nontechnical (but critical) factors,
such as geographical, economical, and political relations with
the surrounding countries and regions. Each country needs
to clearly establish its needs, then investigate the available
information on the performances of different systems to make
the best choice. It is hoped that the information provided in this
paper will be helpful in reaching that goal.
ACKNOWLEDGMENT
The authors would like to acknowledge the help and guidance
from W. Luplow, W. Bretl and R. Citta of Zenith Electronics
Corp.; E. Williams of PBS, T. Gurley of MSTV, Dr. C. Weck
of IRT, E. Wilson of DigiTAG/EBU, P. MacAvock of DVB,
E. Stare of Teracom, B. Sueur of CCETT, Dr. P. Pogrzeba of
Deutsche Telecom, C. Nokes and A. Oliphant of BBC R&D;
Dr. S. Moriyama, Dr. O. Yamada and Dr. T. Kuroda of NHK
Science and Technical Research Laboratories.
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Dr. Yiyan Wu is a Senior Research Scientist with the Communications Research Centre, Ottawa, Canada. His research interests include digital video compression and transmission, signal and image processing, LMDS/MMDS, satellite and mobile communications. He is actively involved in the ATSC technical and standard activities and ITU-R digital television and data broadcasting
studies. He is an Adjunct Professor of Carleton University, Ottawa, Canada, a
member of the IEEE Broadcast Technology Society Administrative Committee
and an Associate Editor of the IEEE TRANSACTIONS ON BROADCASTING.
Ewa Pliszka received her M.Sc. from the Electronics Faculty, Technical University of Warsaw in 1973. She also completed post-graduate studies in 1977
at the Electronics Faculty, Technical University of Warsaw and, in 1993, at the
National Institute of Telecommunications, Evry, France. In 1973, she joined
Research and Development Centre, Telekomunikacja Polska S.A. as a Research
Engineer. Her first activities covered the design and construction of measurement equipment for transmission parameters of audio and video analog signals.
Later on she became interested in the transmission of digital audio and video signals. Her work now covers many aspects of the transmission of digital TV and
sound signals. Her current work also involves—among other things—collaboration with the Radiocommunication Sector of International Telecommunication
Union (ITU), participation in Study Group 11 (Television Broadcasting) and
Study Group 10 (Sound Broadcasting), and also participation in the activities of
the Polish Standardization Committee, especially in the area of digital TV.
Bernard Caron received a B.Sc. in electrical engineering from Laval University, Quebec in 1978 and a M.Sc. from University of Ottawa in 1984. Since 1979,
he has been with the Communications Research Centre Canada and has worked
on Teletext, mobile data transmission, video channel characterization and simulation. He is now the Manager of the Television Systems and Transmission
program in the Broadcast Technologies branch. He is currently involved in the
introduction of digital television terrestrial broadcasting in Canada.
Pierre Bouchard is a Research Engineer in the Television Systems and Transmission group at the Communications Research Centre Canada, in Ottawa. He
is currently involved in research on DTV transmission, DTV coverage, and on
broadband broadcasting technologies (MMDS and LMCS/LMDS).
Gerald Chouinard received a B.Sc. in electrical engineering in 1975 from University of Sherbrooke. He worked five years for the Canadian Broadcasting Corporation (CBC) in the field of International Technical Relations. He joined the
Communication Research Centre (CRC) in 1981, where he became involved in
technical and spectrum-orbit utilization studies related to satellite DBS. In 1992,
he became Director, Radio Broadcast Technologies Research. Since April 1998,
he reports to the CRC President as a Senior Advisor on Broadcast Technologies. In his work, Dr. Chouinard has been closely involved in standards setting
activities for both Digital Television and Radio Broadcasting in Canada, internationally through the ITU-R, as well as in North-America by his participation
in the work of the ATSC and the NRSC.