International Journal of Engineering Trends and Technology (IJETT) - Volume4Issue4- April 2013
DVB-T (Digital Video Broadcast - Terrestrial) is defined as a Coded Orthogonal Frequency Division Multiplexing
(COFDM) system. The OFDM (Orthogonal Frequency
Division Multiplexing) modulation has many advantages: it is more bandwidth efficient, overcomes the effect of Inter
Symbol Interference (ISI), combats the effect of frequency selective fading and burst errors. It also has some disadvantages: it is more sensitive to carrier frequency offset, suffer in presence of phase noise, have high PAPR
(Peak to Average Power Ratio .
The architecture of the
DVB-T network consists in: program coder, multiplexer,
SFN network adapter.
Asstt. Professor, ECE
Student, M.Tech (ECE)
Department of ECE
Lovely Professional University
Phagwara, Punjab, India.
I.
I
,
NTRODUCTION
Student, M.Tech (ECE)
Department of ECE
Lovely Professional University,
Phagwara, Punjab, India.
Abstract — OFDM is extensively used in modern digital television terrestrial broadcasting (DTTB) systems to support high performance bandwidth-efficient multimedia services, such as the Digital Video Broadcasting for Terrestrial
Television (DVB-T) proposed as the European DTTB standard. The system is based on ofdm technology which allows dvb-t system to exploit the spectrum frequency in a better way, saving bandwidth.This paper is about the analysis of different modes of dvb-t system and to solve the problems of
Bit error rate . The modulation scheme used is QAM. Digital video broadcasting uses this kind of modulation (DVB).The simulation is done using the SIMULINK. The performance of the signal is improved by using the modulation scheme
Keywords-OFDM, dvb-t, SNR
Eventually, the digitally coded signals require less bandwidth than analog signals, with the same quality. For digital video coding, the current standards are referred to as
MPEG-1 to MPEG-4. DVB has decided to make use of the
MPEG standards for source coding and the multiplexing of audio-visual signals and has thus produced guidelines not specifications for implementing MPEG-2 audio, video, and systems in satellite, cable, and terrestrial broadcast applications.
Department of ECE
Lovely Professional University,
Phagwara, Punjab, India.
COFDM (Coded Orthogonal Frequency Division Multiplex) modulator, up converter and transmitter.
The digitization of analog audio and video signals increases the signals’ bandwidth. The application of source coding results in a decreased signal bandwidth, and several bit rates can be selected to support the required quality of service.
Television is the most basic and widely-used multimedia application ever. It has passed many transformations - from the first analog systems to the latest digital ones. New standards and technologies – led by a coalition of broadcasters, leading cellular operators, standardization bodies etc. have set the stage for terrestrial Digital
Television (DTV) services. The DVB (Digital Video
Broadcast) is the leading standard that provide digital video broadcast via different communication technologies. The
Figure 1. Functional description of the MPEG-2 video encoding system
The MPEG-2 standard prescribes a subtraction taking place at the same frequency as that at which a picture on the television screen is updated. At the input of the encoder, successive groups of digital video information are reordered
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International Journal of Engineering Trends and Technology (IJETT) - Volume4Issue4- April 2013 in order to benefit from the equality of successive groups of information. This process is followed by a discrete cosine transform. The DCT not only maps the video signal into the frequency domain, but the division of the amplitudes in the frequency domain shows less correlation as well. Next, the signal is quantized (eight bits), after which redundancy reduction by means of a Huffman code is applied.The feedback circuit includes a video memory and a predictor
II.
BLOCK DIAGRAM
.
An OFDM signal, which is generated at the transmitter end by using DVB-T 2k mode carrier, is transmitted over noisy communication channels i.e. AWGN, Rayleigh and Rician. at the modulation scheme 64 QAM the output will be observed at the value of SNR, and BER is evaluated for the performance of OFDM signal for the above mentioned channels. The diagrammatic approach of the above mentioned scenario is demonstrated in Figure 3.
Figure 2- dvb-t parametres
Interleaver is used to avoid the burst errors, which can produce a lot of consecutive errors causing a high BER in reception ,we must use the interleaver block we can see in the figure. QAM(quadrature amplitude modulation) is the most popular type of modulation combination with
OFDM,espescially rectangular QAM is easy to implement.
IIFT this is the most important block for all OFDM system this block generates the orthogonal subcarriers. The orthogonality between subcarriers as well as saving bandwidth. The channel is the block that allows us to transmit the signals from the transmitter to the receiver. The
AWNG channel block adds white Gaussian noise to a real or complex input signal . when the input signal is real, this block adds real gaussian noise and produces a real output signal. When the input signal is complex, this block adds complex Gaussian noise and produces a complex output signal. Typical values of the SNR for an AWNG channel are between 5 and 30. When the values is closer to zero this procedures a high BER , on the other,if the value of SNR increases we will get a better BER. AWNG block does not introduce problems like fading and multipath. FFT the purpose of this block to get QAM symbols at the output, from the subcarriers created in the IFFT block before the channel. The complex QAM symbols at the output of the
FFT block have to be de-mapped .this is the principle task of the QAM de-mapping block. Rectangular QAM demodulates the input signal using the rectangular quadrature amplitude modulation method. The output of this block are integer numbers which belong each one to the gray constellation points .
III METHODOLOGY
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Figure 3Transmission and reception of DVB-T based OFDM signal through various channels.
.In the Source coding and multiplexing compressed video, compressed audio, and data streams are multiplexed into MPEG program streams (MPEG-PSs). One or more
MPEG-PSs are joined together into an MPEG transport stream (MPEG-TS); this is the basic digital stream which is being transmitted and received by TV sets or home set of boxes. the MPEG-TS is identified as a sequence of data packets, of fixed length (188 bytes). With a technique called energy dispersal, the byte sequence is de-correlated. a first level of error correction is applied to the transmitted data, using a non-binary block code, a Reed-Solomon RS
(204, 188) code, allowing the correction of up to a maximum of 8 wrong bytes for each 188-byte packet. convolutional interleaving is used to rearrange the transmitted data sequence, in such a way that it becomes more rugged to long sequences of errors. a second level of error correction is given by a punctured convolutional code, which is often denoted in STBs menus as FEC (Forward error correction). There are five valid coding rates: 1/2, 2/3,
3/4, 5/6, and 7/8. data sequence is rearranged again, aiming to reduce the influence of burst errors. This time, a block interleaving technique is adopted, with a pseudo-random assignment scheme (this is really done by two separate interleaving processes, one operating on bits and another one operating on groups of bits).
IV RESULT
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International Journal of Engineering Trends and Technology (IJETT) - Volume4Issue4- April 2013
Figure 4-spectrum scope of RICIAN channel Figure 7- -Received constellation of 64 QAM via AWGN channel
Figure 5- -Received constellation of 64 QAM via RICIAN channel
Figure 8-spectrum scope of RAYLEIGH channel
Figure 6-spectrum scope of AWGN channel
Figure 9 -Received constellation of 64 QAM via RAYLEIGH channel
The simulations are performed for the 64QAM schemes where the SNR is varied and the signal is analyzed at three
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International Journal of Engineering Trends and Technology (IJETT) - Volume4Issue4- April 2013 channels which are AWGN, Rician and Rayleigh. In each of the figures below, the comparison between the three channels are shown. It is evident from the figure which is representing received constellation of 64QAM signal that the most affected path in terms of noise is Rayleigh channel as the received constellation points are at a large distance as compare to AWGN and Rician received constellation points. From figure it is observed that the transmitted symbol is not received correctly due to large distance between the points representing the symbol. This concludes that the schemes which have higher data rates are more prone to error than the schemes with lower data rates, the performance of different schemes can be made acceptable by increasing the SNR but only to a certain limited extent.
The performance of the Rician channel is remarkable and it is the least effected channel, so there must be a balance between the SNR and the modulation scheme that we are using for DVB-T based transmission so that the originally transmitted data can be recovered as it is quite evident from the graphs of the different schemes and channels that if we increase the data rate the bit error rate also increases even the SNR value failed to effect the reception at very high data rates like 64 QAM or 256 QAM.
V CONCLUSION
The combination of high data capacity, high spectral efficiency, and resilience to interference as a result of multipath effects means OFDM is ideal for the high data applications that are becoming a common factor in today’s communication scene. It has thus gained a significant presence in the wireless market of today. The paper investigated the performance of OFDM and the effect of different multipath channels, namely Rayleigh, Rician and
AWGN. The results which were obtained verify that
Rayleigh is the least suitable and most affected multipath environment due to its non line of sight communication and delay characteristics, whereas Rician shows the most promising result, while the AWGN performance lies in between these two. Different modulation schemes were also applied for the DVB-T based OFDM transmission and reception and it was found that as the modulation scheme increases from 4 QAM to 64 QAM, the chances for error
(for example ISI) also increases because of the reason that high data rates has high probabilities of error due to less symbol period. Furthermore if the number of carrier is increased, the chance for error also goes high and the receiver will not be able to properly recover the signal and it may end up in making wrong decision.
A CKNOWLEDGMENT
I wish to convey warmest thanks to my supervisor Mr. Ravi
Shankar for his support and encouragement. I also thank Mr
Chandika Mohan Babu and Department of Electronic and
Communication Engineering at Lovely Professional
University for their expert help and recommendations. I also would like to thank all staff and my friends of the School of
Engineering at Lovely Professional University for their kind help.
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