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Digital Audio BroadcastingKadri Balli
AbstractSince the publication of the Digital Audio
Broadcasting Terrestrial standard (DAB-T) in 1994,
the broadcasters world has discovered a new and
efficient method to modulate their signals; the
Coded Orthogonal Frequency Division Multiplex
(COFDM).
This full digital modulation technique allows to
supply, to the mobile and the portable receivers,
an interference free digital signal over a
terrestrial radio frequency channel.
Broadcasters to setup such kind of networks, need
not only to implement COFDM modulators but
also to manage their primary distribution networks
in order to synchronize the system both in the
frequency and time domains.
This article presents An Introduction to Digital
Audio Broadcasting with System Overview,
COFDM and the Adaptation of COFDM for DAB.
1. Introduction and System
Overview
1.1 Introduction
When FM was established in the middle of the last
century it was specified for reception with
stationary receivers equipped with directional
antennas mounted in a height of approximately 10
meters. Nowadays this specification is no longer
up-to-date since more than 85% of the radio
receivers are portable or mobile systems
connected to simple non-directional antennas.
Also the expectations in the quality of the received
signal have changed. Certainly the introduction of
Stereo in the end of the 1960's brought a
significant improvement of sound quality, but in
the age of noiseless sound from media like
compact disc, mini disc and digital Audio tape
many radio listeners are no longer satisfied with
the sound quality provided by FM. Especially for
mobile reception, a decreased listening quality is
experienced from noise and hissing which is
caused by multi-path recept ion.
In addition the FM frequency bands get more and
more crowded caused by the growing number of
radio stations all over the world. Especially
because transmitters with overlapping coverage
areas have to be operated on different frequencies
to avoid reception problems caused by phase
shifted and delayed signals.
This and many other reasons made it necessary to
develop new radio systems based on digital technology
with the following requirements:
safe and distortion free reception withstationary, portable and mobile receivers
sound quality comparable with CD increased frequency efficiency due to single
frequency networks (SFN)
ready for Multimedia applications
multi-path reception shall not cause anyproblems
it has to be possible to transmit a signal thatcontains several programs
The broadcasting system DAB, developed by the
EUREKA 147 project and fully standardized by the
European Telecommunications Standards Institute (ETSI)
is a system that totally fulfils the requirements above and
therefore is recommended by the International
Telecommunications Union (ITU). (Standards: ETS 300
401; ITU-R Rec. 774 789 )
1.2 System Overview
1.2.1 Description of the DAB system
The DAB System is a transparent digital transmission
channel that allows to transport any information that can
be expressed in bits and bytes to stationary, portable and
mobile receivers. The capacity of this transmission
channel can be split into a number of sub-channels which
can carry independent Audio or data programs with
different data rates and protection levels. For the
transmission of these digital programs the innovative
modulation technology Coded Orthogonal Frequency
Division Multiplex (COFDM) is used.To maintain an efficient use of the provided transmission
channel it was decided to use a data compression system
according to ISO-MPEG 11172-3 (MPEG 1-Layer II) and
ISO-MPEG 13818-3 (MPEG 2- Layer II).
1.2.2 The basic DAB signal chain:
The Basic DAB Signal Chain is shown in figure: 1.1.
A basic DAB signal chain can be separated into the
following three sides:
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The Service Provider Side, where the basic
audio encoding is done or data signals are
inserted The Ensemble Provider Side, where the
data streams provided by various service
providers are put together in an ensemble
multiplexer to a DAB Ensemble.
The Transmitter Network Provider Side, where the
COFDM Encoding, modulation and terrestrialtransmission is done.
Figure 1.1 The Basic DAB Signal Chain
1.2.2.1 Service Provider Side (Studio)
To insert an audio or data channel into a DABsignal, an Audio Encoder / Data Inserter is needed.
Usually this equipment is located at the studio and
compresses the incoming Audio signal according
to the selected method (ISO-MPEG Layer II) and
selected data rate (between 32 to 384 kBit/s).
Depending on the type of the used audio encoder
Program Associated Data (PAD) and an
independent data stream can be added. If more
than one audio program is produced at a studio
side, then the needed number of Audio Encoders
and Data Inserters can be linked by the WG1/WG2
Bus.
If the Ensemble Multiplexer is at the same location
as the Audio Encoders, the WG1/WG2 Signal can
be connected directly to the Ensemble Multiplexer.
Otherwise the Service Transport Interface (STI)
has to be used to transport the data stream from
the service provider side to the Ensemble
Multiplexer. The STI Interface has been defined in
the ETSI Standard ETS 300 797 and can be
transported on various digital channels (e.g. G704).
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1.2.2.2 Ensemble Provider Side (Ensemble
Multiplexer)
The DAB ensemble contains a number of sub-
channels in various sizes defined by the EnsembleProvider. Each of those sub channels can carry
exactly one Audio or Data program. In the
Ensemble Multiplexer the data streams received
from the different service providers are collected
and the programs for the DAB ensemble are
selected from these data streams.
For the transportation of the DAB Ensemble from
the Ensemble Multiplexer to the various
transmission sites in the network, the ETI - Signal
(ETI ... Ensemble Transport Interface) was defined
in the standard ETS 300 799. Depending on the
setting of the Multiplexer, the ETI signal isprovided according to G-703 or G-704 with a fixed
data rate of 2.048 Mbit/s.
1.2.2.3 Transmission Network
The transmission network consists of all
transmission sites that receive their ETI input
signal from one Ensemble Multiplexer. At each
transmission site the COFDM signal is processed
from the incoming ETI signal and radiated either in
the VHF Band III (176 MHz - 240 MHz) or the L-
Band (1452 MHz -1492 MHz).
1.2.3 Comparison of a DAB TransmissionNetwork with a Standard FM Transmission
Network
In a standard FM transmission network each
Audio program has to be distributed via a separate
transmission network and is shown in figure: 1.2.
Therefore, it is very difficult for local and regional
program providers to reach a high coverage. With
DAB it is now possible to transmit several
programs via the same transmission network. This
innovative concept allows local and regional
program providers for the first time to share the
costs of a transmission network and to combinetheir coverage areas.
On the other hand national or state owned
program providers that operate a DAB Network
can rent a capacity in their DAB Ensembles to any
program provider without the need to build up a
separate transmission network as in FM.
Figure 1.2 Actual FM Networks
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digital data stream. The carriers in the OFDM
signal are called subcarriers. The operation of
several modulators in parallel complies exactly the
algorithm of the Inverse Discrete FourierTransformation. (figure:2.2 )
Each subcarrier is modulated according to the
same digital modulation method. This means that
all subcarriers are either QPSK, 16 QAM or 64
QAM modulated. For DAB D-QPSK (Differential -
Quadrature Phase Shift Keying) was chosen. The
combination of all modulated sub carriers is called
symbol. The number of bits that can be
transported in one OFDM - Symbol depends onthe number of subcarriers and the selected
modulation method.
Figure 2.2 The Principle of OFDM
Examples for QPSK, 64 QAM and 16 QAM
modulated signals:
Example 1: 100 subcarriers per Symbol, each
carrier is QPSK modulated
QPSK Modulation (= 4 QAM)
Logd(4)=2 bits per carrier
100 subcarriers*2 bits 200 bits
per symbol
Example 2: 500 subcarriers per Symbol, each
Carrier is 64 QAM modulated64 -QAM Modulation Logd(64)
6 bits per carrier
500 carriers * 6 bits per carrier
3000 bits per symbol
The time that the symbol is valid, is called symbol
duration. During the symbol duration the bit
combination carried by each subcarrier is not
changed. The symbol duration defines the number
of symbols that can be broadcasted per second
and therefore the net bitrate of the OFDM Signal.
Example 3: 1000 subcarriers per symbol, each sub
carrier 16 QAM Modulated symbol duration 1 ms
16 QAM Modulation
Logd(16) 4 bits per carrier
1000 carriers* 4 bits per carrier
4000 bits per carrier
Symbol duration 1 ms 1000
Symbols per second
1000 Symbols per second
4000 bits*1000 symbols/s = 4 Mbit/s
The frequency bandwidth of the complete OFDM
signal is defined by the sum of the frequency
bandwidth of each subcarrier. The selection of the
frequencies for the subcarriers can not be freely
defined. To maintain a distortion free signal
retrieval in the receiver, the frequencies of the
subcarriers have to be orthogonal. This means that
the frequency spacing between the carriers is the
inverse of the symbol duration. The frequency
spectrum of the subcarriers overlap but due to the
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digital modulation of the subcarriers the
demodulation is possible. (See figure 2.3).
Since the reflected signal can arrive delayed to the
directly received signal, it is necessary to insert adelay time between the symbols. This delay time is
called guard interval and is a very important
parameter for single frequency operation. The
complete symbol duration is therefore the sum of
the useful symbol duration plus the duration of the
guard interval. Since no information can be
broadcasted during the guard interval the total
transmission capacity is reduced, but the
protection against distortions caused by multi
path reception is increased.
2.3 The difference between OFDM and
COFDMCOFDM is a variation of the OFDM very suitable
for mobile applications since an additional
protection against frequency selective fading is
introduced. In OFDM usually the bits carried by
the subcarriers are assigned to the subcarriers in
the way they are received from the signals source.
Therefore adjacent bits in the incoming serial bit
stream are assigned to adjacent carriers in theOFDM. If a number of adjacent subcarriers are
distorted by frequency selective fading, then the
adjacent bits in the bit stream are distorted and the
receiver is not able to compensate the errors by
Forward Error Correction (FEC).
ln COFDM the bits of the incoming bit stream are
assigned to the carriers of the OFDM signal by a
defined code. Therefore, Coded Orthogonal
Frequency Division Multiplex. In the receiver the
bit stream is brought to the correct order again by
decoding with the same code. A distortion of
adjacent carriers in the COFDM signal causes
distortions of non adjacent bits in the bit streamwhich can be usually corrected in the receiver by
Forward Error Correction. (See figure2.4).
Figure 2.3 Orthogonal Carrier Frequency Spacing Figure 2.4 OFDM and COFDM
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3. The Adaptation of COFDM for
DAB
This third section is mainly focused on the
modulation technology, "Coded Orthogonal
Frequency Division Multiplex" - COFDM which
was adapted for DAB.
3.1 COFDM Subcarrier modulation used for
DAB
As explained in section two a COFDM signal
consists of a defined number of orthogonal
subcarriers that can be either QPSK, 16-QAM or
64-QAM modulated. Since it was the target of theEureka 147 project to establish a system that works
even with poor signal to noise ratios, it was
decided to use D-QPSK (Differential QPSK). On
one hand, this decision limited the number of bits
that can be transported on each subcarrier to 2 and
on the other hand, the D-QPSK signal is the most
robust signal possible with COFDM. (See figure
3.1).
In the transmission channel from the DABtransmitter to the receiver, noise is added to the
transmitted signal. This means that the phase and
the amplitude of each subcarrier is influenced. The
more noise is added the more the received
constellation diagram differs from the constellation
diagram of the transmitted signal. The dashed
circle in figure 3.1 indicates the maximum deviation
caused by noise so that the receiver is still able to
reproduce the correct data stream.
In QPSK (and QAM) each possible bit
combination of the data stream transported via a
subcarrier is assigned to a point in theconstellation diagram. In QPSK the amplitude of
the carrier is always the same but for each bit
combination the carrier has a different phase.
Figure 3.1 Comparison Between QPSK and QAM
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Due to this fact the DAB receiver has to recover
the signal stream from the phase of the received
subcarriers. Since the phase shift of thetransmission channel is different for each
reception point in the coverage area, normally it
would be necessary to transmit a phase reference
signal together with the subcarrier. To avoid this,
D-QPSK was chosen for DAB.
The principle of D-QPSK is quite simple. Instead of
assigning the absolute phase defined for a certain
bit combinations to the subcarrier the sum of the
phases defined by two adjacent symbols is
assigned to the subcarrier. In the receiver simply
the phase difference has to be built for adjacent
symbols to generate the correct bit stream. It isvery important that all phase calculations are made
in the mathematical positive direction (anti-
clockwise) (figure 3.2).
Since the phase shift in the transmission channel
is the same for adjacent symbols the phase
difference between two adjacent symbols is
constant. Therefore there is no phase reference
needed for the receiver.
3.2 The Application of COFDM for DAB:
In the Standard ETS 300 401 the DAB signal is
defined as a COFDM signal with a bandwidth of
1,536 MHz and a net bit rate of up to 2,4Mbit/s. A
part of this bitrate is needed to organise the
COFDM signal and provide redundancy for FEC
(Forward Error Correction). Therefore the useful
bitrate of the DAB signal is approximately
1,5Mbit/s.
Each DAB symbol consists of a certain number ofD-QPSK modulated carriers. The number of
carriers is depending on the chosen DAB Mode.
In the ETSI Standard ETS 300 401 the following
four Modes have been defined for the DAB signal:
Figure 3.2 Principle of Differential QPSK
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DAB Mode 1:
Defined for national and regional SFN (Single
Frequency Networks) in Band III. Used nearly inall countries working on DAB.
DAB Mode 2:
Defined for regional and local Single Frequency
Networks (SFN) in L-Band. Used for example in
France, Canada and Germany.
DAB Mode 3:
Defined for Satellite DAB (S-DAB). At the moment
this mode is not used. The idea is to use a satellite
for the main coverage and terrestrial transmitters
synchronised with the satellite to cover areas
where the satellite signal cannot be received.
DAB Mode 4:
Defined for national and regional Single Frequency
Networks (SFN) in L-Band. At the moment this
mode is mainly used in Canada.
Important Parameters of the DAB Signals:
Number of subcarriers
The main difference between the different DAB
modes is the number of subcarriers in the COFDM
Signal.
DAB Mode 1 DAB Mode 2 DAB Mode 3 DAB Mode 4
1536 subcarriers 384 subcarriers 192 subcarriers 768 subcarriers
Subcarrier spacing:
Since the frequency bandwidth is 1.536 MHz for all four DAB Modes the spacing between the subcarriers can
be calculated as follows:
DAB Mode 1 DAB Mode 2 DAB Mode 3 DAB Mode 4
f Bandwidth: 1.536 MHz
1536 subcarriers 384 subcarriers 192 subcarriers 768 subcarriers
Subcarrier spacing=
1.536 MHz / 1536=
Subcarrier spacing=
1.536 MHz / 384=
Subcarrier spacing=
1.536 / 192=
Subcarrier spacing=
1.536 / 768=
1 kHz 4 kHz 8 kHz 2 kHz
Useful symbol duration:
As mentioned before the subcarriers have to be
orthogonal. The subcarriers in a COFDM signal
are orthogonal if the symbol duration is the
inverse of the carrier spacing:
Ts = 1 / ? f Ts symbol duration
?f sub-carrier spacing
Therefore the useful symbol duration of a DAB
symbol can be calculated as follows:
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DAB Mode 1 DAB Mode 2 DAB Mode 3 DAB Mode 4
Useful symbol
duration=1 / 1 kHz
Useful symbol
duration=1 / 4 kHz
Useful symbol
duration=1 / 8 kHz
Useful symbol
duration=1 / 2 kHz
1000 s 250 s 125 s 500 s
size of the guard interval, total symbol
duration
To avoid an ISI (Inter-Symbol-Interference)
between each symbol a guard interval has to be
inserted. In DA B it was defined to add a guardinterval with the time of 24,6 % of the useful
symbol duration. The guard interval can be
understood as a delay time between two adjacent
symbols. Therefore, the total symbol duration is
the sum of the guard interval and the useful
symbol duration.
DAB Mode 1 DAB Mode 2 DAB Mode 3 DAB Mode 4
Guard interval =
1000 s*0.246 =
Guard interval =
250 s*0.246 =
Guard interval =
125 s*0.246 =
Guard interval =
500 s*0.246 =
246 s 62 s 31 s 123 s
DAB Mode 1 DAB Mode 2 DAB Mode 3 DAB Mode 4
Total symbol duration
= 1000s + 246 s
Total symbol duration
= 250s + 62 s
Total symbol duration
= 125s + 31 s
Total symbol duration
= 500s + 123 s
1246 s 312 s 156 s 623 s
In the table below a summary of the most important parameters for all four DAB modes are given:
DAB Mode 1 DAB Mode 2 DAB Mode 3 DAB Mode 4
Frequency
Bandwidth1536 kHz
Number of
Subcarriers1536 384 192 768
Subcarrier
Spacing1 kHz 4 kHz 8 kHz 2 kHz
Total symbol
Duration1246 s 312 s 156 s 623 s
Useful symbol
Duration1000 s 250 s 125 s 500 s
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Guard
Interval246 s 62 s 31 s 123 s
The decision about the chosen mode is mainlydepending on the frequency band in which the
SFN will operate. The carrier spacing and the time
period of the Guard interval are important
parameters for the planning of a Single FrequencyNetwork.
Figure 3.3 Important Parameters of DAB Signals
4. ConclusionSince the beginning of the radio broadcast era,
frequency planning aims to avoid the interference
caused by the overlapping of the transmitters
service areas. Unfortunately, transmitters overlap
is not the unique source of interference; the
terrestrial channel has a complex propagation
model which produces echoes (multi-path
propagation) and when addressing mobile
receivers, Doppler frequency shift. As a
consequence, in each point of a service area, the
signal captured by the receivers results as the sum
of several elementary signals including the original
signal, some delayed replicas and channel noise.
To bypass this physical degradation, the
traditional method was to increase the power of
the original signal (e.g.: the transmitting power).As a direct consequence, this method enlarges the
limit of the channel reusability and accordingly
contributes to the artificial increase of the radio
frequency spectrum occupancy.
A modulation system have been studied which is
sufficiently robust and efficient to carry digital
signals and to save radio frequency spectrum; the
Coded Orthogonal Frequency Division Multiplex
(COFDM).
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List of Abbreviations:
Band III 174- 240 MHz
CD Compact Disk
COFDM Coded Orthogonal Frequency
Division Multiplex
DAB Digital Audio
Broadcasting.
DAB - Ensemble Name for the
combination of a
number of service data
streams
Data Rate Describes how many
bits are transported via a transmission
channel per second the
units are usually kbit/s or Mbit/s
ETSI European
Telecommunications Standards Institute
ETS 300401 DAB Standard: DAB to
mobile, portable; and fixed receivers
ETI Ensemble Transport
Interface
FM Frequency Modulation
FM Band 87.5 MHz -108 MHz
G.703/ G.704 Standards for digital
signals
ITU International
Telecommunications Union
L-Band 1452 MHz -1492 MHzMPEG Motion Picture Experts
Group
Multi Path Reception Usually the signal
received by an antenna is a combination of
signals directly
received from the transmitter and signals reflected
by mountains,
buildings etc. This reflected signals are time
delayed and have a
different phase than the signal received
directly from the
transmitter. Depending on the signal strengths of
these reflected signalsdistortions occur.
Protection Level Describes the grade of
error protection in
numbers from 1 to 5
(1very high error
protection, 5 ... very
low error protection)
SFN Single Frequency
Network
STI Service Transport
Interface (defined in ETS 300 797)
Sub Channel Part of the DAB
Ensemble, each sub channel carries.
WG1 / WG2 Bus Work Group 1 / Work Group 2
Bus.
References:
[1] A Seminar on Digital Audio Broadcasting
(DAB), by Hirschmann, Austria GmbH
(Istanbul, 1998)
[2] A Guide to Digital Radio for Engineers ( NTL
Broadcast Radio-BR/(04)/98)
[3] A Seminar on Digital Audio Broadcasting, by
Hirschmann, Austria GmbH and ITIS,
France (Ankara, 1999)