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The COFDM
modulation system:
the
heart of digital
audio broadcasting
by P. Shelswell
Digital audio.broadcasting offers the potential to give every radio in
Europe the sound quality of a compact disc.
To
accomplish this, it requires
a
rugged method of transmission. The coded orthogonal frequency division
multiplexing (COFDM) modulation system was developed to meet this need.
This paper describes the reasons why a new modulation process was needed, and
explains how the COFDM system has been optimised t o meet the requirements.
1
Introduction
Th e history of sound broadcasting ha s been one of gradual
improvement. In the early days, most people listened to
signals using amplitude modulation, seated round a large
receiver. It was difficult to conside r portable or mobile
reception because receivers were large, and either were
mains powered or required a sizeable battery.
Since then, frequency modulation and stereo
transmissions have been introduced. These were
originally planned to serve only fixed receivers, but
nowadays there
is
a diversity of receivers providing a rang e
of reception, from the conventional fixed hi-fi system,
through the standard transistor portable and ghetto
blasters to the mob ile car and personal radios.
Thus there is a demand for something that was not
originally part of the broadcast plan: mobile reception.
Originally, the FM broadcast chain was developed
assuming fixed reception with a directional rooftop
antenna. The use of small antennas attached to the
receiver, either indoors or on a car, now me ans that many
of the original planning assump tions are inapprop riate. As
a result the receiver h as to cope with a signal that
is
often
weaker than desirable and contains many echoes. Weak
signal strength combined with multipath propagation
lead s to a characteristic degradation of the received signal.
The programme sound is full of unwanted noises, which
are difficult o describ e in words but are recog nised by the
majority of people who listen to their radio in the car. The
problem
is
one of fading. It
is
common to the majority
of
radio systems, whether used for broadcasting, telephony
or general com munications.
In addition, ther e is an increasing dema nd for more and
more services. This h as led to a congested frequency band
with little room for expansion o r improvement.
One solution to these problems
is
to change
the
transmission standard to a digital system. But this is not
enoug h. Simple digital systems do not work well in the
multipath environment either. A major rethink of the
digital transmission system isneeded.
COFDM (coded orthogonal frequency division
multiplex) is a new digital transmission system which can
provide tugg ed reception, even in the fading channel. Th e
work on this system was initiated by
CCEXTL,2
n France
and developed into a major new broadca sting standard by a
collaborative project, Eureka 147. Th e new DAB (digital
audio broadcasting) system provides good reception in a
range of difficult conditions and is ideally suited to the
application. Reference 3 provides an introductory review of
the
DAB
system.
Th e same concept can also be used for transmission
of
any digital information, and it is interesting to note that
many of the digital television systems that are being
studied also make use of similar techniques.
In this paper, an outline
of
the COFDM modulation
system an d its applications
is
given. Most ofthe description
will be based on the Eureka 147 system, but will be
sufficiently general to allow th e theory to be applied to
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amplitude
t
time or position
frequency
Fig. 1
propagation. The frequency response will vary wit h bot h time and p osition
Frequency selective attenuati on is clearly present
Typical fr equency responseof a channel suffering from multipath
othcr systems.The sound codingand multiplexing aspects
of th e DAB signal will notbe discussed in detail to keep the
paper short.
2 T h e p r o b l e m s of m u l t i p a t h p r o p a g a t i o n
Th e main problem with reception of normal radio signals is
fading caused by multipath propagation. This
is
not an
isolated problem. Delayed signals are the result of
reflections from terrain features such as trees, hills or
mountains, or objects such as people, vehicles or
buildings. Some of these reflections can be avoided by
using a good antenna, but there is a trend towards simple
antenn as on radios nowadays, so this isno longer a realistic
solution.
A characteristic of frequency selective fading is that
some frequencies are enhanced whereas others are
attenuated (Fig. 1).When th e receiver and all the objects
giving rise to the reflections remain stationary, then the
effective frequency response of the channel from the
transmitter to the rec eiver will be substantially fixed.
If the wanted signal is relatively narrowband and falls
into part of the frequ ency band with significant attenuation,
then there will be flat fading and reception will be
degraded.
If
on
the other hand, there is som e movement either of
the vehicle containing the receive r or of any of the
surroundings, then t he relative leng ths and attenuations
of
the various reception paths will change with time.
A
narrowband signal will vary in quality as the peaks and
troughs of the frequency response move around in
frequency. There will also be a noticeable variation in
phase re sponse. which will affect all system s using phase
as a means of signalling. For FM reception, thes e channel
distortionsgive rise to distortion of the sound a s well as the
morc obvious d rop outs a s the signal level falls below the
receiver threshold.
Now consider a signal which is of greater bandwidth.
Some parts of the signal may suffer from constructive
interference and be enhanced in level, whereas o thers may
suffer from destructive interference and be attenuated,
some time s to the point of extinction. In general, frequency
comp onent s close together will suffer variations in signal
streng th which are well correlated. Othe rs which are
further apart will be
less
well
correlated.
The
correlation
bandwidth is often used as a
measure of this phenomenon.
There is no standard definition
of th e correlation bandwidth.
Some workers also use the term
coherence bandwidth, One
typical definition is the
frequency separation of signals
which are correlated by a factor
of
0.9 or better. For a
narrowb and signal, distortion is
usually minimised if the
bandwidth is less than the
correlation bandwid th of the
channel. There
is,
however, a significant chance that the
signal will be subject to severe attenuation on some
occasions. A signal which occupies a w ider bandwidth,
greater than the correlation bandwidth, will be subject to
more distortion, but will suffer less variation in total
received power even
if
it
is
subject to significant levels of
multipath propagation.
If we
look
at the temporal response of the channel, we
see a number
of
echoe s present. There ar e many different
types of echo en vironmen t which ar e typical of different
geographical areas. In cities, echoe s come from reflections
from buildings; there are many separately identifiable
echoes, with a large range of delays observable.
In
the
countryside, the echoes ar e usually less distinct and have a
smaller range
of
delay, especially if there are no nearby
hills, although the presence of large hills and mountains
can increase the range
of
delay observed.
This range
of
delay can be measured statistically.
Different studies use the total range
of
delay,
or
the
average delay. Whichever is chosen, the inverse of this
leads to a good approximation for the correlation
bandwidth.
Measurements by the BBC have shown that the
correlation bandwidth depends very much on the
particular surrou ndings of the rec eiver. Typical results in a
built-up city show correlation bandwidths (using the
90
definition)
of
about
0.25
MHz in the VH F band. Changing
the definition of the correlation bandwidth to reflect a
corr elati on of
0.5
or more gives a bandwidth of over 1MHz.
COFDM is a wideband modulation scheme which is
specifically designed to cope with the problems of
multipath reception. It achieves this by transmitting a large
number of narrowband digital signals over a wide
bandwidth.
3
The
importance ofFDM
In COFDM, the data is divided between a large number
of closely-spaced carriers . This accounts for the frequency
division multiplex part
of
the name COFDM. Only a small
amount of the data
is
camed on each carrier, and this
significantly reduces the influence of intersymbol
interference (Fig 2). In principle, many modulation
A
qua l i ta t ive desc r ip t ion of COFDM
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sche me s could be used to modulate the data at a low bit
rate onto each carrier."
In
DAB, quadrature phase shift
keying
(QPSK*)
is used , with differential encoding of the
data at the transmitter and differential demodulation at the
receiver.
It
is an important part of the COFDM system design that
the bandwidth occupied
is
greater than the correlation
bandw idth of the fading channel. Ag ood under standing of
the propagation statistics
is
needed to ensure that this
condition is met. From the me asurem ents we have made,
the bandwidth occupied is ideally more than a bout
1
MHz.
Then, although some of the carriers are degraded by
multipa th fading, he majority of the carrier s should
still
be
adequately received.
The importance of coding
Th e distribution of the data over many carriers means
that selective fading
will
cause som e bits to be received in
error while others are received correctly. By using an
error-correcting code which adds extra data bits at the
transmitter it is possible to correct ma ny or all of the bits
which were incorrectly received. The information carried
by one of the degraded carri ers is corrected because other
information, which is related
to
it
by the error-correction
code,
is
transmit ted in adifferen t part
of the
multiplex (and,
it is hoped, will not suffer the same deep fade). This
accounts for the 'coded' part of the name COFDM.
Ther e are many types of erro r correcting code that could
be used.',? In the DAB system , the main
channel code
is
a convolutional code,
with a Viterbi receiver. As there is
always a chance
of
residual errors after
a Viterbi decoder, some of the higher
priority data is precoded with a block
code for additional security. These
coding options have been specifically
tailored to the audio and signalling data
that is being broadcast and would
probably not he quite the same if
COFDM were used for other purposes.
The
importance
of
orthogonality
Finally, the 'orthogonal' part
of
the
COFDM nam e indicates that there
is
a
precise mathematical relationship
betwe en the frequencies of the carriers
in thesystem. In anormal FDM system ,
the many carriers are spaced apart in
such a way that the signals can be
received using conventional filters and
demodulators.
In
such receivers, guard
bands h q e to be introduced between
thc different carriers, and the
introduction of these guard bands in
* It
is
a minor detail. hut the modulation
system is actually n/4 D-QPSK In this
system, the phases of
the reference signals
are
ncreased by
45
each symbol
period. In
the rest
of
this p aper, for simplicity,
normal
QPSK will he assumed.
the frequency dom ain results in a lowering of the spectrum
efficiency.
It is possible, however, to arrange the carriers in a
COFDM signal so that the sidebands of the individual
carriers overlap and the signals can still be received
without adjacent carrier interference. In order to do this
the carriers m ust be m athematically orthogonal.
Th e receiver acts as a bank of demodulators, translating
each carrier down to DC, the resulting signal then being
integrated over a sym bol period to recover t he raw d ata. If
the o ther carriers all beat down to frequencies which, in the
time domain, have a whole number of cycles in the sym bol
period
( T ) ,
then the integration process results in zero
contribution from all these other carriers.
Thus
the
carriers a re linearly independent (i.e. orthogonal)
if
the
carrie r spacin g is a multiple of l r
Mathematically, suppose we have a set of signals Y,
where YD s the pth element in the set. The signals are
orthogonal if
0
1)
Yo t) Yz(t)d t
= K
for p
= q
O
f o r p e q
where the indicates the complex conjugate. It is fairly
simple to show, for example, that the series sin (mw)for m
=
1 , 2 ,
..
isorthogonal over the interval TI ton .
impulsehannelesponse
time
.
1 carrier
V
V
V
\
V v v \
n
n
carriers
V
n
8 c a r r
n
V
I
\rz/ r
Fig.
2
data rate, increasing the number of carriers reduces the data rate that each
indivi dual carrier must convey, and hence (for a given modulation system)
lengthens the symbol period . This means that the in tersymbo l interference
affects a smaller percentage of each symbol as the num ber of carriers and
hence the symbol period increases
The effect of adopting a multicarr ier system. For a given overall
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Much of transform theory makes use of orthogonal
series. Fourier series are a well known example of an
orthogonal series, although they are by no means th e only
example.
Otherfeaturesof
COFIIM
adopted to ensure the maximum robustness:
In the Eureka 147 system, some further refinements are
the data is interleaved, both in frequency and time. Th e
error correction process works hest
if
the errors in the
incoming data are random. To ensure this
is
true, the
transmitted data is interleaved ove r
all
the carriers and
over
a
range of time.
the addition
of
a guard interval allows the system to
cope with echoes
of
moderate d uration, and with small
inaccuracies in the receiver (for example small timing
errors) . It isdiscussed in more detail below.
Tlze
sound coding associated with
COFDM
When transmitting digitally-coded sound signals it is
particularly importan t to ensur e high efficiency in the use
of spectrum. Bit-rate reduction of the sound
is
therefore
used. Th e choice of sou rce coding for DAB
is
essentially
independent of the choice of COFDM for the modulation
scheme.
The Eureka 147 group has developed
a
system called
MUSICAM, which has now been adopted by the
International Standard s Organisation in their
IS0
11172-3
Layrr I1 standard.
This
offers a variety
of
options, in which
bit-rate can he traded for quality. For high-quality stereo
signals, a hit-rate of about 256 kbit/s is needed, hut the
standard offers a range starting at
32
kbit/s and rising
to
384 kbit/s.
In order to occupy sufficient bandwidth to gain the
advantages of the COFIIM system, it isnecessary to go up
a
number of programme s together to form awideb and system.
4 Mathematical description of
COFDM
After the qualitative descrip tion of th e system it isvaluable
to discuss the mathematical definition of the modulation
systcm. This allows
us
to
see
how the signal is generated
and how the receiver must operate, and it gives u s a tool to
understand the effectsof imperfections n th e transmission
channel.
As noted above, COFDM transmits a large num ber of
narrowband carriers, closely spaced in the frequency
domain. In order to avoid a large number of modulators
and filters at the transmitter and c omplem entary filters and
demodulators at the receiver, it is desirable to be able to
use mod ern digital signal processing techniq ues.
Mathematically, each carrier can be described as a
complex wave:
s, t)
=
A,(t )e
l , cf
W ' l
e )
Th e real signal is the real part
ofs,(t).
Both A,(t ) and @
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This is the same condition that was
required for orthogonality (see Section
3). Thus,
one consequence
of
maintaining orthogonality is that the
COFDM signal can be defined by using
Fourier transform procedures.
The
Fourier
transform
The Fourier transform allows u s to
relate events in the time domain to
events in the frequency domain. The re
are several versions of the Fourier
transform , and the choice of which one
to use depends on the particular
circumstances of the work.
?he conventional transform relates
continuous signals which are not limited
in either the time or frequency domains.
However, signal processing is made
easier
if
the signals are sampled.
Sampling of signals with an infinite
spectrum causes all sorts
of
problems
because the sampling process leads
to
aliasing, and th e pr ocessing of signals
which are not time limited can lead to
interesting problems with signal storage.
10 void this, the majority of signal
processing uses a version of the
discrete Fourier transform (DET). Th e
DFT is a variant on the normal
transform in which the signals are
sampled in both the time and the
frequency domains. By definition, the
time waveform mus t repeat continually,
and this leads to a frequency spectrum
which repeats continually in the
frequency domain.
7 b e fast Fourier transform (FFT) is
frequency,
0 3
MHzIdivision
merelv a raoid mathematical method for calculatine the
in a domestic system
I
DIT . I t is the availability of this tec hniq ue, and technolog y
that allows it to be im plemented on integrated circuits at a
reasonable price. that has permitted COFDM to be
developed as far as it has. The rapid progress in the
development of FET chip sets has led to many exciting
opportun ities in this field.
The process of transforming from the time domain
representation to th e frequency domain uses the Fourier
transform itself, whereas the reverse process uses the
inverse Fourier transform.
T ~
se
ofthe FFTin COFDM
The main reason that the CO FDM technique has taken
so long to come to prominence has been practical. It has
becn difficult to generate such a signal, and even ha rder to
receive and demodulate the signal. Th e hardware solution
which makes use of multiple modulators and
demo dulators in parallel was somewh at impractical for use
Now, the ability to define the signal in the frequency
domain, in software, and to generate the signal using the
inverse Fourier transform is the key to its current
popularity.The use of the reverse process in the receiver is
essential
if
chea p and reliable receivers are
to
be readily
available. Although the original proposals were made
som e time ago. it has taken som e time for technology to
catch up.
At the transmitter, the signal isdefined in software in the
frequency domain. It is a sampled digital signal, and it is
defined such tha t the discrete Fourier s pectrum exists only
at discrete frequencies. Each COFDM c am er corresponds
to one element of this discrete Fourier spectrum. The
amplitudes and phasesof the cam ers depend on the data to
be transmitted. As each carrier isQPSKin this example, all
the carrier am plitudes are unity, but this
is
not necessary in
the more general case.The phase of each cam er
is
defined
for each transmitted symbol. At the carriers have their data
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Fig.
5
Generation of an
OFDM signal. The incomin g
data is converted from a
single serial bi t stream to a
multip le parallel stream,
which is modulated
o n to
a
large number of carriers.
Generation of the OFDM
signal itself could use a large
bank of oscillators and
multipliers. n reality, that
function (the part of the
system outl ined in th e
coloured box) is replaced by
an inverse
FFT
chip
spectrum
of
compositesignal
_ -
transitions synchronised, and can be processed together,
symbol by symbol. Fig.
5
is a schematic diagram of the
transmitter signal processing.
Using VLSl it is possible to carry out the inverse
FFT.
This provides a series of samples which are the time
domain representation of the signal. Thes e samples can be
applied to a conventio nal digital-to-analogue converter
(DAC) to give the real electrical signal.
To
enable the signal to be generated using an inverse
FFT, it
is
preferable that the number of carriers considered
in
the calculationis a po wer of 2. In practice, it is not always
desirable to have the number of real camers restricted in
this way. H owever, it is convenient to m ake up the actual
number chosen to a power of 2 by setting the amplitudes of
thos e not wanted to zero. This feature also simplifies the
design of the anti-alias filter after th e DAC.
In
the receiver, the reverse process is applied.Assuming
that the receiver has som e means of synchronisatio n, then
the signal is converted from an analogue format to a
sampled digital representation. The samples
corresponding to each symbol are than Fourier
transformed to the frequency domain. This gives the
amplitude and phase of each transmitted carrier.
In the Eureka 147 system, it
is
the change in phase of
each carrier from one symbol to the next which
communicates the information.
Orthogonality
A natural consequence of this method is that it allows us
to generate carriers which are orthogonal. The memb ers
of an orthogon al set are linearly independent.
Consider that our set of transmitted carriers, Y, is an
orthogonal set , such t hat
If
this
is truely orthogonal, then the orthogonality
relationship in eqn. 1 hould hold, that is
h
h
=
(b - a fo rp
=
q
= O f o r P t q a n d ( b - a ) = s ( 1 2 )
(remember thatp and
q
are integers)
Thus
the carrie rs, which are separated in frequency by
l/z,
meet the requirements
of
orthogonality provided that they
are correlated over a period z This is the formal derivation
of the result quoted earlier.
If the ana lysis is extended to include the phase of each
carrier, then this
is
recovered by the process defined in
eqn. 12. This is exactly the computation that is needed in
the receiver.
The method can be extended to show the effect
of
frequency and timing erro rs in the sys tem, but there is
insufficient space to d o so here.
Synchronisation and the guar d interval
In the receiver it is necessary to sample the incoming
signal and perform a transform to recover the carriers.
If
the system is all locked up, and the sampling frequency is
correct and in the right phase, there is no problem.
However this ideal case will be difficult to achieve,
especially when the receiv er is first switched on. 'Ihere
is
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therefore a need for acquiring timing lock.
In the DAB system, coarse
synchronisation is provided by a simple
technique: all the carriers are switched off
on a regular basis. By using a simple
amplitud e detection circuit, it is possible to
generate an approximation to th e timing.
However, the timing will not be perfect,
and all of the samples could be displaced
by a k e d time offset, giving rise to
intersymbol interference (see Fig. 6).
In DFT transform theory, the assum ed
waveform is a continuously repetitive
sequence, rather like wallpaper patterns
repeating across a wall. The minimum
information required
is
one cycle
of
this
pattern. To avoid the timing problems,
more than one complete symbol
is
transm itted the part of the symbol that is
repeated has been named the guard
intcrval (see Fig.
7).
With the guard interv al, he initial timing
accuracy only needs
to
ensure that the
samples are taken from one symbol (see
Fig. 8).Th e longer the guard interval, the
more rugged the syste m, but at a penalty of
the power needed to transmit the guard
interval. Ther e is obviously a compromise
to
be reached, but more of that later. In
practice, it is convenient to think
of
the
transm itted sym bol in two parts: the guard
interval precedes th eactive symbol period,
which is so called because in a correctly
aligned receiver the F l T window is in that
time
slot.
This gives ruggedness in the
uresence of echoes.
symbol
M
symbol
M
1
ymbol
M i
w w w
w
w w w
a Correctly timed samples
w w
w
w w
b
Incorrectly timed samplesgiving ntersymbol
interference
Fig. 6
must be decoded by sampling withi n the symbol period and then
perf orm ing a fast Fourier transform t o calcu late the phases of the
individual carriers. With a timi ng error, th ere can be considerable
intersymbol interference
The effect of timi ng errors o n reception of a signal. The signal
symbol
M 1
symbol symbol
M 1
uard active
period
interval
Fig.
7
parts. The whole signal is contained in the acti ve symbol (shown
highlig hted or the symbol
M),
the last part of which (shown in a lighter
colour ) is also repeated at the st art of the symbol and is called the
guard interval
Example of the guard in terval. Each symbol is made up of t wo
Once coarse synchronisation has beenobtained, there
is
then the question of how to improve it. After the null
symbol, a reference signal
is
transmitted. Correlation of
Another use of the guard interval
is
to minimise the
effectof echoes. If the echo isshort compared with the total
symbol period, then any energy conveyed from one
the received version of this symbol with the known
transmitted signal provides the impulse response of the
channel. From this, a much more accurate timing can be
obtained.
Similarly, the same signal also provides a
measure of the frequency error of both the sampling
frequency and the actual frequency
of
the signal (and
hence allows accurate automatic frequency control to be
implemented).
symbol
to
the next by the echo only degrades the gua rd
interval. Th e active symbol period contains direct ene rgy
and reflecti ons whose delay is less than the guard interval,
and istherefore derived from the sa me symbol period. This
is not intersymbol interference, but a form of linear
distortion. In the receiver the calculated phase of the
symbol for each spectral component is distorted by th e
multipath signal. If the channel is not changing rapidly
I
I I
I
I
symbol
M - i i:zA j
symbol
M ~
symbol
M +1
w w
w w w w w
aCorrect lyt im ed samples
w w
b
Incorrectly timed but decodable samples
w w w w w w
c Incorrectly timed samples giving intersymbol interference
Fig.
8
tolerance of adding a guard
interval. With a guard interval
included in the signal, the
tolerance on timi ng th e samples
i s considerably more relaxed
The effect on the timing
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then successive symbols of any one carrier will be
perturbed in a similar manner. Ifwe use differential coding,
the receiver looks at the difference in phase from one
symbol o the next, and the e rrors cancel out (Fig. 9), Thus,
provided that the echo ha s a delay less than the additional
guard interval, there is no degradation to reception of the
active symbol period.
[t does not matter whether th e echo
is
passive, such as
one from a hill o r a building, or active, such a s one from
another transmitter. This is a major feature which allows
DAB to use the s am e frequency from all of its transmitt ers
in the network. Provided that th e transmitters radiate the
sam e signal at the sam e time, then reception will he good,
even in the overlap zones between the transmitters. This
reuse of the spectrum is a major saving over conventional
systems which would need different frequencies in
adjacent service areas. CO FDM, is, for a single channel,
only as efficient as the underlying modulation system.
However when the planning of a complete service is
considered. The single-frequency network (SFN)
operation is a major advantage." Th e spectral reus e leads
to major spectrum efficiencies.SeeSection 5for details.
5
Let us use the
DAB
system as an example of a COFDM
system and consider the choice
of
parameters.
First there
is
the question of what bandwidth to use for
The
appl icat ion
of
COFDM
t o
DAB
symbol
echo 1
resultant
1
symbol
2
77
esultant
1
echo 2
symbol 2
a b
Fig. 9
signal in th e presence of a single echo. Here, two
adjacent symbols are depicted, bot h affected by a single
echo. Arbitrarily, the
QPSK
signal has been coded such
that there i s a
90
phase shift fro m one symbol to th e
next. When there are echoes present, the received phase
i s distorted. In
a)
he symbol and t he echo have a fixed
relationship. This occurs when the echo contains energy
which comes exclusively from t he same symbol period .
An example would be if t he samples for t he signal are
timed as in Fig. 83 and
b .
The phase relationship
between the two resultant symbols is therefore
maintain ed. As differen tial decodi ng is used, the
constant phase shift is of no consequence. b )When
echoes are delayed by more than the guard inter val, as is
th e case in Fig.8c. matters are different. The phase of
the signal in the previous symbol does not have a fixed
relationship wi th th e current symbol, and so the
relationship between the phases of t he two resultant
symbols changes, and this distortion could degrade the
signal
Vector diagram of the di stortion caused to a
the COFDM system. The wider the bandwidth, the more
likely that he system exceeds the correlation bandwidth of
the channel. Short delay ec hoes are the main problem to
overcome, and as hese are always present, there is no hard
bound. The narrower the bandwidth, the m ore likely it is
that th e whole signal will he affected. Ther e is
a
trade-off
between bandwidth and transmitter power.
The original experiments were carried out with a
bandwidth of about 7 MHz, and sho wed few problems."
Then t he bandwidth was successively reduced to below
2
MHz, at which point, there is a degradation equivalent
to about 1dB in performance. This is not much, but the
degradation starts to increase quite rapidly when the
bandwidth
is
reduced below 1.5MHz. If the bandwid th is
reduced to th e
200
kHz used for
FM
sound, then the
margin required would b e an additional
6
dB or so.
T h u s
a figure of about
1.5
MHz for the bandwidth of the system
is
a good compromise for the type
of
propagation
conditions that apply to mobile and portable radio
reception.
One of the parameters that is directly affected by the
bandwidth is the available bit-rate. Th e modulation sy stem
on each carrier is QPSK. The carriers are separated in
frequency by about the inverse
of
their symbol period.
' h u s
the maximum bit-rate available is
2
bit/s/Hz of
bandwidth.This figure
is
reduce d by th e inefficiency of the
guard interval, the null symbol and t he er ror coding. For
DAB.
this brings the useful bit-rate down to about
1bit/s/Hz of bandwidth.
Thus a DAB system will provide just unde r
1.5
Mbit /s of
useful data. Thi s is considerably more than t he
256
kbit/s
that is needed for a high-quality stereophonic progr amm e,
so the implication is that several broadcast program mes
will sh are th e same multiplex.
Now consider the number
of
carriers. The more there
are,
he greater is the resolution
of
the diversity offered by
the system. Ther e is, however, a relationship between th e
symbol period and the carrier spacing. Th e carrier spacing
is I/s. For the differential demodulatio n to work properly,
the multipath environment must change slowly from
symbol to symbol. Thus there is a limit to the symbol
period and hence the number of camers. For static
reception, this is not a major problem. For mobile
reception, however, the motion
of
the vehicle leads to
changes in the multipath environment. Over a symbol
period, a vehicle moving at a velocity v m/s will travel
UT
x f / c wavelengths. This ishrw avelength s, where f,
is
the maximum Doppler shift. If this is to introduce
negligible phase distortion, then the function
f ~
ust he
small. A figu re of f ,7
8/10/2019 Sistema de modulacin COFDM en audio digital
9/10
significant echoes would all be relatively short. Surveys
indicate that agu ard interval of the ord er of 10
ks
would be
satisfactory for the majority of locations, n th e UK at least.
The use of several transmitters puts a limit on the
minimum guard interval that should be used. The
transmissions from areas so me distance away can reach
quite high levels on occasions of abnorm al propagation. O n
the days that th e weatherman announce s that television
reception may be subject to interference, ther e is also a
strong possibility that interference from remote DAB
transmitters may cause a problem. It
is
possible to plan a
service without worrying too mu ch about this if there is
suftlcient signal from the local transmitters. Then it
becomes a simple matter of deciding the spacing of the
main transmitters. This is a compromise between a small
number of high-power transmitters spaced by about 50 km,
or a much larger number of lower-power transmitters.
Because the first option is likely to be the che apest, the
guard interval is set to about 250 ms quivalent to a
maximum difference of about
80
km in transmission
distance.
The symbol period need not be directly related to the
guard interval. It
is
just a qu estion of how much of the
symbol period is repeated in the gua rd interval. This
is
a
straigh t question of efficiency, as the power transm itted in
the guard interval does not form a useful part of the
information in the receiver unless there are substantial
echoes. To minimise the power loss by this me chanism, it
is
desirable to keep the guard interval to as low a
perce ntage a s possible of the sym bol period. In practice a
guard interval of the order of 25%of the symbol period h as
been found to be agoo d compromise.
This leads to a symbol period of 1ms and hence , using
eqn. 10, to a carrier spacing
of
about 1 kHz. Thus about
1500 carriers are accomm odated in the minimum
bandwidth desirable for on e COFD M transmission.
In Section 4, it was suggested that the DAB system is
spectrum efficient. We now have the data to expand on this
staiement. Within the multiplex we are achieving about
one programme p er 250 kHz of bandwidth. It
is
possible to
transmit the sa me signal through out the full exten t of any
area requiring the sa me se t of prog rammes, thu s offering
four programmes per megahertz in that area. To permit
separate regional or national requirements, by the m a p
colouring theorem, four frequency blocks should be
adequate to provide one single-frequency network (SFN)
in ta ch country. This leads to a minimum overall spectrum
requirement of 1 MHz per stereo prog ramme to provide
flexibility of progra mm es w orldwide.
Hy comparison. at least 2.2 MHz
is
needed to provide
VHF FM programmes in the UK, and this extends to
3.3
MHz in Europe, where the countries are not
surrounded by sea. Optimistic estimates to provide
separa te service s of conventional digital radio, for examp le
using the NICAM system used for television sound,
indicate that the minimum required spectrum would be 3.4
MHz per programme.
A
figure closer to 10 MHz per
programme
is
thought m ore appropriate.
T h u s he SFN a pproach usin g COFD M offers significant
economies in spectrum.
When the signal
is
generated, it is defined in the
frequency domain, and then transformed into its time
domain representation. Sampled digital signals of course
suffer from aliasing. In a real system , the spec tral repeats
are not wanted, and
so
must b e filtered
off.
To make the
filtering easier, the spec trum is not defined such th at allthe
avai lable carr iers a re used. Th e outer c am ers
are
defined
to be zero and this leaves a gap in the spectrum which can
be used to minimise the complexity of the output filter.
When deciding exactly how many carriers are used, and
how many are processed, the decision is guided by th e fact
that the
FFT
works most effectively if the number of
samples in either domain isapo wer of 2. Thu s for DAB, the
number of carriers defined
is
1536, and th e processing is
based on 2048 carriers in the system. When DAB was first
being developed, the need to include a 2048 point FFT in
the receiver was seen as a disadvantage. Now, however,
technology has advanced to the point where higher orde r
FFTs
are bein g pro posed for digital television applications.
6 Other systems
Th e DAB system discussed here, as defined by the Eu reka
147 partners,
is
not the only system that could be defined.
The Eureka group itself has other options which are
tailored for use with systems operating in the top e nd o fthe
UHF range, and which have satellite transmission as a
major objective.
If the syste m is for futed reception, as is usually the cas e
with television, then many of the pro blem s with multipath
propagation are less severe. Th ere is less variation in the
channel, both because the antenna is capable of being
selective and because the channel is naturally not as
variable as it would be in a moving vehicle. As a
consequence it is possible to use shorte r guard intervals,
or even
no
guard interval at all.
The system used to modulate the individual carriers
does not need to be QPSK. It could be a higher order
system such as 16,6 4or even 256 QAM. Indeed a mixture
of modulation systems
is
possible,
so
long as they are
orthogonal. This restriction is usually met
if
the symbol
periods are the same . Th e use of coherent demodulation
will give better performan ce if the channel
is
not varying
too much. For mobile reception of DAB, the channel
response may vary rapidly in phase, and
so
the potential
benefits of coherent demodulation are lost in the
implementation. For the more static channel, these
benefits can be realised, and
so
it
is
useful to ad opt the
technique. Th e main challenge of using the se higher o rder
systems
is
to be able to cope with the changing
environment,
so
synchronisation and equalisation
strategie s pay a key part in their design.
7 Conclusions
In this short paper, it has only been possible to give an
outline of COFDM. This modulation system
is
being
increasingly used fo r digital transmission in environments
where multipath propagation can cause significant signal
distortions.
ELECTRONICS COMMU NICATION ENGINEERING JOURNAL J U N E 1995 135
8/10/2019 Sistema de modulacin COFDM en audio digital
10/10
Now that the Eureka 147
DAB
system has been fully
defined and accepted
as a
European Standard,13 many
broadcasters
are
preparing to start services.
In
the UK, the
BBC is planning
to
start broadcasting using DAB in
September 1995. Although, at the time of writing this
paper, we have yet to
see
the first dom estic receivers,
the
rapid pace of development m eans that the complexities of
COFDM are likely to
be
implemented
in
domestic
equipment before the paper
is
published.
The
future
is
exciting.
Acknowledgments
Th e author would like to thank the many eng ineers in the
Eureka 147 project who have contributed to his
understanding
of
the system, and to the BBC for
permission t o publish this paper.
References
1 POMMIER. D.. and WU. Y.: 'Interleaving or spectrum
spreading
in
digital radio intended for vehicles', EBUReview,
June
1 9 8 6 , 2 1 7 ,pp.12%142
2 ALARD M., and LASSALLE,R: Principles ofmodulation and
channel coding for digital broadcasting for mobile receivers'.
EBLJ Collected Papers on concepts for sound broadcasting
into the 2lst century, August
1988, pp.47-69
3 PRICE. H.M.: 'CD by radio: digital audio broadcasting', IEE
K w i e w . lGAp ril199 2,38, 4), pp.131-135
4
FAILLI, M.: 'COST 207 Digital land mobile radio
communications', Commission of the European
Communities.
1988. p.137
etseq.
5 PROAKIS. J.G.: 'Digital communications' (McGraw Hill, 1989,
2nd edn.), p.708
6
AGHVAMI. AH .: 'Digital m odulation techniques for mobile
and personal communication system s',Electron. Commun.
Eng.J., 1993.5 , (3) , pp .125132
7 BURR,
A.G.:
'Block versus trellis: an introduction to coded
modulation', Electron. Cummun. Eng. J., 1993,
5,
(4),
pp.24&248
8
KRIGHAM,
E.O.:
The
fast Fourier transfomi' (Prentice Hall,
New Jersey, 1974)
9 WEIN jTEIN.S.B., and EBERT, P.M.: 'Data transmission by
frequency-division multiplexing using the discrete Fourier
transform'. IEEE Trans., October 1971. COM-19, (5).
pp.62&534
10 BELL, C.P. , and WILLIAMS, W.F.: 'Coverage aspects of a
single frequency network designed for digital audio
broadcasting'. BBC Research Department Report No.
RD 1993/3
11
SHELSWELL, P., et al.: 'Digital audio broadcasting, the first
UK
field trial'. BBC Research Department Report No.
RD 1991/2
12 LE FLOCH, B., HALBERT-LASALLE,R.. CASTEWN,
R.:
'Digital sound broadcasting to mobile receivers', IEEE TYQW. ,
13 European Telecommunication Standard
EXS
300401, 'Radio
broadcast systems; Digital Audio Broadcasting (DAB) to
mohile, portable and fixed receivers'
August 1989, CE-3 5, (3), pp.493-503
EE:
1995
First received 24th February
1994
and in revised
form
3rd March
1995
The author is with the BBC Research and Development
Department, Kingswood Warren, Tadworth. Surrey, KTZO 6NP,
U K.
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International
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1995
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