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Receiver synchronization for Digital Audio Broadcasting system based on Phase reference symbol

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The Eureka-147 Digital Audio Broadcasting (DAB) system is a new digital radio technology for broadcasting radio stations that provides high-quality audio and data services to both fixed and mobile receivers and employs coded orthogonal frequency division multiplexing (COFDM) technology. In this paper, we present the efficient synchronization method based on Phase reference symbol (PRS) for the DAB signal at the receiver. Since DAB uses OFDM digital transmission technique, the synchronization block plays an important role in determining bit error rate (BER) performance of DAB system in different channels. The result shows that the synchronization method proposed locates precisely each DAB frame even at low signal to noise ratio (SNR), so that the demodulation can be performed frame by frame, symbol by symbol.
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Receiver Synchronization for Digital Audio
Broadcasting system based on Phase Reference
Symbol
Arun Agarwal, Member IEEE, and S. K. Patra, Senior Member, IEEE
Abstract--The Eureka-147 Digital Audio Broadcasting (DAB)
system is a new digital radio technology for broadcasting radio
stations that provides high-quality audio and data services to
both fixed and mobile receivers and employs coded orthogonal
frequency division multiplexing (COFDM) technology. In this
paper, we present the efficient synchronization method based on
Phase reference symbol (PRS) for the DAB signal at the receiver.
Since DAB uses OFDM digital transmission technique, the
synchronization block plays an important role in determining bit
error rate (BER) performance of DAB system in different
channels. The result shows that the synchronization method
proposed locates precisely each DAB frame even at low signal to
noise ratio (SNR), so that the demodulation can be performed
frame by frame, symbol by symbol.
Index Terms—Eureka-147, DAB, OFDM, phase reference
symbol, synchronization, transmission frame, BER, digital
transmission, multipath fading, SNR,
I. INTRODUCTION
The new digital radio system DAB (Digital Audio
Broadcasting) was developed within the European Eureka-147
project [1], mainly to replace the existing AM and FM audio
broadcast services in many parts of the world. It was developed
in the 1990s by the Eureka 147/DAB project. The new
concepts of digital broadcasting such as perceptual audio
coding (MPEG-1/2), OFDM channel coding and modulation,
the provision of a multiplex of several services and data
transmission protocols makes DAB to provide high-quality
digital audio services (mono, two-channel or multichannel
stereophonic) along with programme-associated data and a
multiplex of other data services (e.g. travel and traffic
information, still and moving pictures, etc.) [2].Use of coded
orthogonal frequency division multiplexing (COFDM)
technology makes DAB system very well suited for mobile
reception providing very high robustness against multipath
reception. It also enables the system to operate in single
frequency networks (SFNs) for high frequency
efficiency.Synchronization is a very important in now-a-days
digital communication systems.
Arun Agarwal is with Siksha ‘O’ Anusandhan University, ITER College,
in Department of Electronics & Communication Engineering, Bhubaneswar-
751030, Odisha, INDIA (e-mail: arun.agarwal23@gmail.com).
S. K. Patra is with the Department Electronics & Communication
Engineering, National Institute of Technology (NIT), Rourkela-769008,
Odisha, INDIA(e-mail: skpatra@nitrkl.ac.in).
978-1-4673-0136-7/11/$26.00 ©2011 IEEE
All digital communication systems require proper
synchronization for decoding of the received signal in order to
produce the original information transmitted. In this paper we
have described in detail the synchronization method based on
phase reference symbol for the DAB signal that improves
performance of DAB system in different transmission
channels. The synchronization block is used to locate precisely
each DAB frame, so that the demodulation can be performed
frame by frame, symbol by symbol. In this paper we developed
a DAB mode-II base-band transmission system based on
Eureka-147 standard [1]. A frame based processing is used in
this work.
Following this introduction the remaining part of the paper
is organized as follows. Section II presents the DAB system
standard. In Section III, the details of the modeling and
simulation of the Synchronization method is presented based
on phase reference symbol (PRS). Then, simulation results
have been discussed in Section IV. Finally, Section V provides
the conclusion.
II. SYSTEM SPECIFICATIONS
The working principle of the DAB system is illustrated in
conceptual block diagram shown in Fig. 1. At the input of the
system the analog signals such as audio and data services are
MPEG layer-II encoded and then scrambled. In order to ensure
proper energy dispersal in the transmitted signal, individual
inputs of the energy dispersal scramblers is scrambled by
modulo-2 addition with a pseudo-random binary sequence
(PRBS), prior to convolutional coding [1]. The scrambled bit
stream is then subjected to forward error correction (FEC)
employing punctured convolutional codes with code rates in
the range 0.25-0.88. The coded bit-stream is then time
interleaved and multiplexed with other programs to form Main
Service Channel (MSC) in the main service multiplexer. The
output of the multiplexer is then combined with service
information in the Fast Information Channel (FIC) to form the
DAB frame. Then after QPSK mapping with frequency
interleaving of each subcarriers in the frame, π/4 shifted
differential QPSK modulation is performed. Then the output of
FIC and MSC symbol generator along with the Phase
Reference Symbol (PRS) which is a dedicated pilot symbol
generated by block named synchronization symbol generator is
passed to OFDM signal generator. This block is the heart of the
DAB system. Finally, the addition of Null symbol to the
OFDM signal completes the final DAB Frame structure for
transmission.
Figure 1. Complete DAB transmitter block diagram [1].
A. Channel coding, muliplexing and transmission frame
The channel coding is based on a convolutional code with
constraint length 7. The octal forms of the generator
polynomials are 133, 171, 145 and 133, respectively. The
mother code has the code rate R =1/4, that is for each data bit ai
the encoder produces four coded bits x0,i, x1,i, x2,i, and x3,i. The
individual programme (audio and data) are initially encoded,
error protected by applying FEC and then time interleaved.
These outputs are then combined together to form a single data
stream ready for transmission. This process is called as
Multiplexing. In DAB several programmes are multiplexed
into a so-called ensemble with a bandwidth of 1.536 MHz.
The DAB signal frame has the structure shown in Fig. 2
that helps in efficient receiver synchronization. The period TF
of each DAB transmission frame is of 24 ms or an integer
multiple of it.
Figure 2. DAB transmission signal frame structure.
B. COFDM
DAB uses COFDM technology that makes it resistant to
multipath fading effects and inters symbol interference (ISI).
OFDM is derived from the fact that the high serial bit stream
data is transmitted over large (parallel) number sub-carriers
(obtained by dividing the available bandwidth), each of a
different frequency and these carriers are orthogonal to each
other. OFDM converts frequency selective fading channel into
N flat fading channels, where N is the number of sub-carriers.
Othogonality is maintained by keeping the carrier spacing
multiple of 1/Ts by using Fourier transform methods, where Ts
is the symbol duration. Since channel coding is applied prior to
OFDM symbol generation which accounts for the term ‘coded’
in COFDM.
C. DAB Transmission modes
The Eureka 147 DAB [1] system has four transmission
modes of operation named as mode-I, mode-II, mode-III, and
mode-IV, each having its particular set of parameters as shown
in Table-I.
TABLE I. SYSTEM PARAMETERS FOR THE FOUR DAB MODES
System Parameter Mode -I Mode -II Mode -III Mode -IV
No. of sub-carriers 1536 384 192 768
OFDM symbols/frame 76 76 153 76
Transmission frame
duration
196608
T
49152T
24 ms
49152 T
24 ms
98304 T
48 ms
Null-symbol duration 2656 T
1297 ms
664 T
324
µ
s
345 T
168
µ
s
1328 T
648
µ
s
OFDM symbol
duration
2552 T
1246 ms
638 T
312
µ
s
319 T
156
µ
s
1276 T
623
µ
s
Inverse of carrier
s
p
acin
g
2048 T
1 ms
512 T
250
µ
s
256 T
125
µ
s
1024 T
500
µ
s
Guard interval 504 T
246
µ
s
126 T
62
µ
s
63 T
31
µ
s
252 T
123
µ
s
Max. RF 375
MHz
1.5GHz 3 GHz 750MHz
Sub-carrier spacing 1 kHz 4 kHz 8 kHz 2 kHz
FFT length 2048 512 256 1024
The use of these transmission modes depends on the
network configuration and operating frequencies. This makes
the DAB system operate over a wide range of frequencies from
30 MHz to 3 GHz. Transmission mode –II is designed
principally for Terrestrial DAB for small to medium coverage
areas at frequencies below 1.5 GHz (UHF L-Band).
|Null-symbol| PRS| FIC (FIBs) | MSC (CIFs) |
Transmission Frame TF
Synchronization
Channel
Fast Information
Channel (FIC)
Main Service
Channel (MSC)
III. THE SIMULATION MODEL
Fig. 3 presents the complete block diagram of the DAB
system which was modeled and simulated by us in MATLAB
environment. The main objective of this simulation study is to
explain the synchronization method based on phase reference
symbol. The simulation parameters are obtained from Table I
for transmission mode-II. A frame based processing is used in
this simulation model. The system model was exposed to
AWGN channel, Rayleigh fading channel and Rice channel to
test the effectiveness of the synchronization block. The
important blocks of the simulation model is discussed in detail
as follows:
A. Phase reference symbol generator
According to DAB standard the first OFDM symbol
(without taking account Null symbol) in the transmission frame
is the phase reference symbol which helps in receiver
synchronization. Since it occurs once in a frame therefore the
detection of this symbol can be used for frame synchronization.
It serves as reference for the differential modulation for the
next OFDM symbols in the transmission frame. The phase
reference symbol is defined [1] by the following expression:
,  
0 0
0 0 (1)
Where

 , (2)
The values of indices i, k’ and the parameter n are given as
functions of the carrier index k for all the DAB transmission
modes. The values of the parameter hi,j is given as a function of
its indices i and j [1].
B. Synchronization
The synchronization block is used to locate precisely each
DAB frame, so that the demodulation can be performed frame
by frame, symbol by symbol. In the DAB frame shown in Fig.
2 the first is the synchronization channel which consists of two
symbols.
One is the Null symbol having duration TNULL. During
null symbol period no information is transmitted. Null symbol
gives coarse time synchronization. It gives the rough frame
timing by envelope detection of the received signal i.e.,
detecting the null symbol by comparing average signal power
during null symbol period TNULL with a set threshold. From
the received signal, a data block of size 664 (equal to TNULL)
samples is taken to measure the average signal power [4].
When this average signal power is less than half of the average
transmitted signal power the null symbol has been detected,
which indicates the start of the new frame. This method of
frame synchronization based on null symbol detection is not
suited for low SNR conditions because high noise power will
provide incorrect frame timing estimate. Therefore phase
reference symbol detection is ideally well suited for correct
symbol timing and frame timing. Fig. 4 illustrates the process
of receiver synchronization.
Figure 3. Block diagram of DAB system simulated.
Information
Source
Energy
dispersal
scrambler
Phase reference
symbol generator
Frequency
interleaving
QPSK
mapping
Differential
modulation
Block
partitioner
Convolutional encoder
(FEC)
Mother code rate R=1/4
Null symbol
generator
IFFT
operatio
n
Guard time
insertio
n
Zero
padding
Channel
(AWGN,
Rayleigh &
Rician)
To DAB Receiver
COFDM
symbols
SYNCHRONIZATION
BLOCK
Figure 4. Block diagram of Symbol and Frame synchronization [13], [4].
Fine time synchronization or symbol timing
synchronization [4] is performed by calculating the Channel
Impulse Response (CIR) based on the actually received time
frequency phase reference symbol (PRS) and the specified PRS
stored in the receiver. To estimate the CIR, training Sequences
(PRS in case of DAB system) are used. This means that a part
(or the whole) of the transmitted signal is known from the
receiver. As the receiver knows which signal is supposed to be
observed, it can evaluate the distortion induced by the
propagation channel and the modulation& demodulation
stages.
Fine time synchronization is based on the phase reference
symbol which is the dedicated pilot symbol in each DAB
transmission frame. Since the modulation each carrier is known
[12], multiplication of received PRS with complex conjugate of
PRS at the receiver results in cancellation of the phase
modulation of each carrier. The phase reference symbol can be
converted to impulse signal or CIR can be obtained by an IFFT
operation of the resultant product as illustrated in (3) below
CIR = IFFT {Received PRS• PRS*} (3)
Where PRS* is the complex conjugate of the phase
reference symbol. The peak of the impulse signal obtained
from (3) will give position of the start of the PRS compared to
a set threshold (T) providing symbol timing as well as frame
timing. According to Fig. 4 from the received signal a data
sample block of FFT length is taken. Then FFT operation is
performed on the block to convert the samples into frequency
domain. Since FFT window length is 512 and size of PRS at
the receiver is 384 (mode-II) therefore zero padding removal
and data rearrangement has to be done [4]. The resulting
sample block is of size 384 same as PRS. Now sample block is
multiplied by the complex conjugate of the PRS known at the
receiver which is then transformed into impulse signal in time
by performing IFFT operation on the product.
The highest peak detection will indicate the start position of
the PRS. To get a precise synchronization decision the peak
obtained from every sample block taken from the received
signal is compared to set threshold level (T). When the
detected peak is less than the threshold level, then the peak
found is not the desired peak and does not indicate the accurate
start of the PRS. So the loop process has to be continued by
taking the next sample block till the desired peak is obtained.
The peak will be greater than the threshold only for the sample
block which has phase reference symbol in it, since PRS have a
high correlation with itself.
IV. SIMULATION RESULTS AND DISCUSSION
In this section we have presented the simulation results
along with analysis under worst signal to noise ratio (SNR)
conditions for AWGN channel, Rayleigh fading channel and
Rice channel. The results are shown for transmission mode-II
and the simulation parameters are taken as per the DAB
standard [1].
First of all the Threshold level will be determined by
observing the magnitude of the highest peak obtained by
multiplication of the PRS with its complex conjugate and IFFT
applied to the product, both in presence and absence of noise.
Fig. 5 presents the phase reference impulse symbol in presence
and absence of noise. It is observed that the highest peak is
obtained in the absence of noise therefore the threshold level
was set to be greater than half the magnitude of this peak. This
ensures that noise peak will not be mistaken as desired peak
during peak detection. Threshold level was set to be T= 140.
Fig. 6 shows the successful detection of the desired peak in
AWGN channel with SNR of 20dB. It may be easily evaluated
form Fig. 6 that the highest peak is located at sample index
791. According to DAB standard the first symbol in the DAB
frame is a Null symbol of size 664 zeros followed by a guard
interval of 126 samples of PRS. Thus sum of null symbol and
guard interval samples equals 790, therefore the peak is located
exactly at the starting point of useful phase reference symbol.
After successful verification of fine time synchronization, the
DAB system will be tested for peak detection under worst SNR
of -11dB. Fig. 7 presents the peak detection with SNR of -11
dB in AWGN channel.
Envelope
detector
FFT
Complex conjugate of Phase
reference symbol
IFFT Peak
detection
Sync.
Decision and
position
compare
Null symbol detection
Synchronized
data
Received
DAB
si
g
nal
Figure 5. Threshold level determination.
Figure 6. Desired peak detection in AWGN channel with 20 dB SNR.
Figure 7. Desired peak detection in AWGN channel with -11 dB SNR.
From the Fig. 7 it can be evaluated that the highest peak is
located at sample index 791 even at worst SNR at -11 dB
providing correct fine time synchronization. The peak is
located exactly at the starting point of useful phase reference
symbol.
Fig. 8 and Fig. 9 presents the peak detection in Rayleigh
fading channel with doppler shift of 40 Hz with SNR of 20 dB
and -11 dB, respectively. It is observed that in both figures we
have successful peak detection with correct position of PRS.
From this point, the DAB frame and hence the ODFM symbols
can now be demodulated to extract the original information.
After analyzing the peak detection in AWGN channel and
Rayleigh channel, the fine time synchronization using PRS in
Rician channel will be investigated next. Fig. 10 and Fig. 11
presents the peak detection in Rician fading channel with
Doppler shift of 40 Hz with SNR of 20 dB and -11 dB,
respectively. It is observed that in both figures we have
successful peak detection.
Figure 8. Desired peak detection in Rayleigh fading channel with 20 dB
SNR.
Figure 9. Desired peak detection in Rayleigh fading channel with -11 dB
SNR.
050 100 150 200 250 300 350 400
0
200
400
600
SAMPLE INDEX -->
AMPLITUDE
Phase reference s ymbol Impuls e signal
050 100 150 200 250 300 350 400
0
200
400
600
SAMPLE INDEX -->
AMPLITUDE
Phase reference s ymbol Impul se si gnal with AW GN Noise
0200 400 600 800 1000 1200 1400 1600
0
50
100
150
SAMPLE INDE X -->
AMPL ITUDE
Peak loc ation = 791
PRS Start = 791
0200 400 600 800 1000 1200 1400 1600
0
20
40
60
80
100
120
140
160
SAMPLE INDEX -->
AMPLITUDE
PEAK DETECTION
Peak location = 791
PRS S tart = 791
0200 400 600 800 1000 1200 1400 1600
0
20
40
60
80
100
120
140
160
SAMPLE INDEX -->
AMPLITUDE
PEA K DETECTION
Peak loc ation = 791
PRS St art = 791
0200 400 600 800 1000 1200 1400 1600
0
20
40
60
80
100
120
140
160
180
200
SAM PLE INDE X -->
AMPLITUDE
PEAK DETECTION
Figure 10. Desired peak detection in Rician channel with 20 dB SNR.
Figure 11. Desired peak detection in Rician channel with -11 dB SNR.
V. CONCLUSION
The proposed synchronization method of fine time
synchronization using phase reference symbol provided correct
frame synchronization even in the low SNR condition in all the
three channels. It was observed that in all channel condition
successful peak detection was obtained indicating exact
position of PRS. From this position, the DAB frame and hence
the ODFM symbols can be demodulated to extract the original
information.
VI. REFERENCES
Standards:
[1] ETSI, "Radio Broadcasting Systems; Digital Audio Broadcasting (DAB)
to mobile, portable and fixed receivers," EN 300 401, V1.3.3, (2001-05),
April 2001.
[2] ETSI TR 101 496-3, "Digital Audio Broadcasting (DAB); Guidelines
and rules for implementation and operation; Part 3: Broadcast network,"
V1.1.2 (2001-05), 2001.
[3] F. Kozamernik, "Digital Audio Broadcasting – radio now and for the
future," EBU Technical Review, no. 265 Autumn 1995.
Dissertations:
[4] Petro Pesha Ernest, "DAB implementation in SDR," University of
Stellenbosch, Master’s thesis December 2005.
[5] Hector Uhalte Bilbao, "Dab Transmission System Simulation,"
Linkoping Institute of Technology, Master’s thesis August 2004.
Technical Reports:
[6] A. J Bower, "DIGITAL RADIO--The Eureka 147 DAB System,"
Electronic Engineering BBC, April 1998.
Books:
[7] Wolfgang Hoeg & Thomas Lauterbach, Digital Audio Broadcasting-
Principles and Applications.: John Wiley & Sons, Ltd., 2001.
[8] H. Harada & Ramjee Prasad, Simulation and Software Radio for mobile
communications.: Artech House, 2003.
[9] R P Singh and S D Sapre, Communication Systems, 2nd ed.: Tata
McGraw-Hill Education Pvt. Ltd., 2007.
[10] John. G. Proakis, “Digital Communications”, 3rd edition, McGraw-Hill,
1995.
Papers from Conference Proceedings (Published):
[11] K. Taura, "A digital audio broadcasting (DAB) receiver," in IEEE
Transactions on Consumer Electronics, vol. Vol. 42, August 1996, pp.
322-327.
[12] Lukas M. Gaetzi and Malcolm O. J. Hawksford, "Performance
prediction of DAB modulation and transmission using Matlab
modeling," in IEEE International Symposium on Consumer Electronics
– Proceedings, pp. 272-277, 2004.
[13] C.C. Lu, and C.C.Huang Y.L. Huang, "Synchronization system of
digital audio broadcasting (DAB) receiver," in in Proceedings of IEEE
International Conference on Consumer Electronics, June 1997, p. 370–
371.
[14] Kai-Sheng Yang, Chao-Tang Yu Yu-Pin Chang, "Improved Channel
Codec Implementation and Performa nce Analysis of OFDM based DAB
Systems," in IWCMC 2006 - Proceedings of the 2006 International
Wireless Communications and Mobile Computing Conference, pp. 997-
100, 2006.
[15] A. Agarwal, S.K. Patra, “Performance analysis of OFDM based DAB
systems Using Concatenated coding technique”, in ICMENS 2010 -
Proceedings of the 2010 6th International Conference on MEMS,
NANO & Smart Systems (ICMENS-2010), Vol.1, pp 6-12, Deccember
2010.
[16] A. Agarwal, S.K. Patra, “Performance prediction of OFDM based DAB
system using Block coding techniques”, in ICETECT 2011 -
Proceedings of the 2011 International Conference on Emerging Trends
in Electrical and Computer Technology (ICETECT-2011), Vol.2, pp
792-796, March 2011.
[17] A. Agarwal, S.K. Patra, “Performance prediction of OFDM based
Digital Audio Broadcasting systems using Channel protection
mechanisms”, in ICECT 2011 - Proceedings of the 2011 3rd
International Conference on Electronics Computer Technology
(ICECT 2011), Vol.2, pp 257-261, April 2011.
0200 400 600 800 1000 1200 1400 1600
0
20
40
60
80
100
120
140
160
SAMP LE INDEX -->
AMPLITUDE
PEAK DETECTION
Peak location = 791
PRS Start = 791
0200 400 600 800 1000 1200 1400 1600
0
20
40
60
80
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160
SA MPLE INDEX -->
AMPLITUDE
PEAK DETECTION
Peak l ocati on = 791
PRS Start = 791
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The European Eureka project has developed a digital audio broadcasting (DAB) system to address the problem of multipath interference that distorts radio signals during broadcasts. The DAB system uses ISO/MPEG1 (International Standardization Organization/Motion Pictures Experts Group) audio coding for source coding and orthogonal frequency division multiplexing (OFDM) for modulation. MPEG1 allows bit rate reduction by using a psychoacoustic model of the human ear which preserves the subjective quality of the audio signals. OFDM is a multicarrier scheme which transfers high speed data using a number of orthogonal carriers with tight frequency spacings. DAB has four transmission modes suitable for satellite and terrestrial communications.
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The world's first DAB services were launched in the United Kingdom and in Sweden at the end of September. With several other broadcasters also preparing for the 'big day', the author reports on the current status of DAB technology, and summarizes the progress being made to implement DAB services worldwide.
Book
Digital Audio Broadcastingrevised with the latest standards and updates of all new developments The new digital broadcast system family is very different from existing conventional broadcast systems. It is standardised in a large number of documents (from ITU-R, ISO/IEC, ETSI, EBU, and others) which are often difficult to read. This book offers a comprehensive and fully updated overview of Digital Audio Broadcasting (DAB, DAB+) and Digital Multimedia Broadcasting (DMB), and related services and applications. Furthermore, the authors continue to build upon the topics of the previous editions, including audio coding, data services, receiver techniques, frequencies, and many others. There are several new sections in the book, which would be otherwise difficult to locate from various sources. Key Features: The contents have been significantly updated from the second edition, including up-to-date coverage of the latest standards Contains a new chapter on Digital Multimedia Broadcasting "Must-have" handbook for engineers, developers and other professionals in the field This book will be of interest to planning and system engineers, developers for professional and domestic equipment manufacturers, service providers, postgraduate students and lecturers in communications technology. Broadcasting engineers in related fields will also find this book insightful.