47[1] KADRI BALLI MAKALESI

Embed Size (px)

Citation preview

  • 8/3/2019 47[1] KADRI BALLI MAKALESI

    1/12

    33

    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:

  • 8/3/2019 47[1] KADRI BALLI MAKALESI

    2/12

    34

    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).

  • 8/3/2019 47[1] KADRI BALLI MAKALESI

    3/12

    4

    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

  • 8/3/2019 47[1] KADRI BALLI MAKALESI

    4/12

  • 8/3/2019 47[1] KADRI BALLI MAKALESI

    5/12

    4

    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

  • 8/3/2019 47[1] KADRI BALLI MAKALESI

    6/12

    4

    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

  • 8/3/2019 47[1] KADRI BALLI MAKALESI

    7/12

    4

    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

  • 8/3/2019 47[1] KADRI BALLI MAKALESI

    8/12

    4

    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

  • 8/3/2019 47[1] KADRI BALLI MAKALESI

    9/12

    4

    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:

  • 8/3/2019 47[1] KADRI BALLI MAKALESI

    10/12

    4

    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

  • 8/3/2019 47[1] KADRI BALLI MAKALESI

    11/12

    4

    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).

  • 8/3/2019 47[1] KADRI BALLI MAKALESI

    12/12

    4

    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)