Digital video broadcasting terrestrial
The DVB T system stands for Digital video broadcasting terrestrial which is maintained by the DVB t project, an international industry consortium with more than 270 broadcasters, manufacturers, network operators, software developers, regulatory bodies and others in over 35 countries dedicated to designing open technical standards for the worldwide delivery of digital television and data services, and they are published by a Joint Technical Committee (JTC) of European Telecommunications Standards Institute (ETSI), European Committee for Electro technical Standardization (CENELEC) and European Broadcasting Union (EBU).
"Walter Fischer, digital video and audio broadcasting technology, a practical engineering guide, second edition, Springer"
What the DVB T system do is that instead of carrying the data on a single radio frequency carrier, it will work by splinting the digital data stream into large number of slower digital stream each of which digitally modulate a set of closely spaced adjacent carrier frequencies. In the case of DVB T there are three choices of the number of carriers know as 2K, 4K, and 8K, the actual number of carriers are 1705, 3409, and 6817 carriers respectively, which are 4.464 kHz, 2.232 kHz, and 1.116 kHz apart from each other. And there are three nodulation schemes 16 QAM (Quadrature amplitude modulation), 64 QAM, 256 QAM. Table (1) shows the system parameters for the DVB T system that is working in the 8 MHz band.
"Walter Fischer, digital video and audio broadcasting technology, a practical engineering guide, second edition, Springer" and Pluto
There are three carrier numbers from which we can chose as discussed earlier theses are:
- 2K mode: this mode allows extremely high speed reception such as trains and highways, but with small cell radius coverage which is just around 16 km for the ¼ guard interval. The small radius makes the use of on channel repeater to extend the reach of the DVB T transmission an extremely important issue for this mode.
- 4K mode: this mode is mostly used for the DVB H and it allows very high speed reception such streets and highways together with medium cell radius around 33km.
- 8K mode: this mode allows a coverage radius around 67 km with a ¼ guard interval but it is not suited for high speed reception. 8K mode is almost used for all European DVB T networks. Pluto
There are so many ways to deliver digital television and these ways are fully established such as satellite and cable, these two types are available to most people throughout the world, then why the use of another way (DVB T) to deliver digital television which is in addition to be complex and expensive it also require a large amount of maintenance, the additional way to deliver digital television via DVB T is necessary for so many reasons such as:
- Regional requirements (no satellite reception).
- Regional geographic situation.
- Portable TV reception.
- Mobile TV reception.
- Local supplementary community services (regional/urban television).
Many countries throughout the world don have satellite TV coverage, because of the most varied reason of a political, geographic, or nature. And some of these places are unable to be covered by cable because of the permafrost and also often cannot be financed because of sparse population density. This leaves the terrestrial coverage as the only solution. Countries that are close to the equator such as Scandinavia in nature having more problems with satellite reception since, the satellite receiving antenna are almost pointing at the ground. And there are a lot of countries that did not have the analogue satellite as a standard, so, the satellite reception plays a minor role there such as Australia. Also in many countries it is not allowed to have uncontrolled television from the sky because of some political reasons. Some of the regions that have a good quality satellite reception and cable coverage require additional terrestrial TV coverage, mainly for local TV programs. And also portable and mobile reception is not possible only via the terrestrial path. Book J. Cosmas
The functional block diagram of the DVB T system consists of so many blocks, here is a brief description of some of the blocks:
- MPEG 2 source coding and multiplexing (MUX): the compressed data that want to be send compressed data could any kind of data such as video, audio and data stream, are multiplexed together to form program streams (PSs), if more than one program streams are combined together to form a MPEG 2 transport stream (MPEG 2 TS); the MPEG 2 stream is the basic digital stream that is being transmitted and received. Program stream is normally a result of combing one or more stream packets, into a single stream, but all the stream packets must have a common time base, the program stream suitable to be used in environments which are relatively error free and for application that are based on software processing of system information, the program stream packets are of relatively enormous length. The transport stream does combine one or more program stream into a single stream the program streams that are combined together are of different time bases, the transport stream was invented to be used with environments that are likely to have errors such as storage in noisy environments, transport stream packets length are of 188 byte. " ETSI EN 300 744 V1.5.1 (2004 2006), and ISO/IEC 13818 1 second edition 2000 12 01 "
- MUX adaption and energy dispersal: the input stream should be organized in fixed length packets, following the MPEG 2 transport multiplexer the MPEG 2 packet length is 188 bytes. The 188 bytes of the MPEG 2 packet should contain a one synchronization word byte; at the transmitting side the processing order should always start form the most significant bit (MSB) of the synchronization word byte. "ETSI EN 300 744 V1.5.1 (2004 2006)"
- The outer coder and outer interleaver: the outer coding and interleaving is applied to the input packet, the coding type that is used is the Reed Solomon RS (204, 188, t=8) shortened code this code has a length of 204 bytes dimension of 188 bytes and can allow correction of up to 8 random erroneous bytes in a received word of 204 bytes, this RS code was derived from the original systematic RS (255, 239, t=8) code, this coding will be applied to each transport packet to generate an error protected packet, the Reed Solomon coding will be applied to the synchronization byte, either inverted or non inverted. The outer interleaver is a convolutional byte wise interelaving with depth I = 12 will be applied to the error protected packets, the results is an interleaved data structure. The interleaver will rearrange the transmitted data in a way such that become more robust to extensive sequence of errors. "ETSI EN 300 744 V1.5.1 (2004 2006)"
- Inner encoder and interleaver: the inner encoder is based on the punctured convolutional codes; there could be different coding rates such as 1/2, 2/3, 3/4, 5/6, and 7/8. This coding will give a second level of error correcting. The inner interleaver is again rearranging the data sequence in order to make more robust to the effect of the burst of errors; this part determines the inner interleaving to be used with the 2K, 4K and 8K transmission mode. "ETSI EN 300 744 V1.5.1 (2004 2006)"
- Mapper: the DVB T uses Orthogonal Frequency Division Multiplexing (OFDM) transmission all data carriers are modulated using either 16 QAM (Quadrature Amplitude Modulation), 64 QAM or 256 QAM modulation schemes. Usually the DVB T system contain two independent MPEG transport streams, one is called the high priority stream and the other is called low priority stream these two streams are mapped onto the signal constellation by the mapper and the modulator which is by that way will have the corresponding number of inputs. "ETSI EN 300 744 V1.5.1 (2004 2006)"
- Frame adaptation: the frame adaptation does mange the process of the pilot insertion this process totally depends on which transmission mode being used such as 2K, 4K or 8K, the symbols are grouped in block of constant length, the block length can be either 1512, 3024 or 6048 symbols per block, the result generated by this process is a frame the frame length 68 blocks long, and the super frame is built by 4 frames. "http://www.xilinx.com/publications/3rd_party/products/MW_DVB TH.pdf" "ETSI EN 300 744 V1.5.1 (2004 2006)"
- Pilots and TPS insertion: the pilots and TPS (transmission parameters signalling) is used to make the reception process on the terrestrial radio channel easier, during the synchronization (frame synchronization, frequency synchronization and time synchronization), and equalization phase the pilot signals are used the pilots are also used for the channel estimation, while the TPS is used send the parameters of the transmitted signal and to determine the transmission cell. In order for the receiver to gain access to the information it must be able to decode, synchronize and equalize. "ETSI EN 300 744 V1.5.1 (2004 2006)"
- OFDM modulation: The modulation type that is used to modulate theses blocks is OFDM (Orthogonal Frequency Division Multiplexing) modulation using either 2048, 4096, or 8192 carriers for (2K, 4K, or 8K mode respectively) the advantage of the Orthogonality is that it will allow orthogonal signal to be transmitted via an overlapping spectrum, thus, significant amount of bandwidth can be saved. Figure (2) shows the overlapping spectrum of the OFDM. Book J. Cosmas " Runhe Qui, Zhongli Wu, Shan Zhu, Donghua University, Shanghai, China, a novel PAPR reduction method in OFDM systems by using CAZAC matrix transform 2008"
- Guard interval insertion: in order to reduce the complexity of the receiving process, an extra bit is added to each block by coping the cyclic prefix in front each block, the width of this type of guard interval can be either one of the values shown between the brackets (1/4, 1/8, 1/16 or 1/32 length of the original block length), the cyclic prefix is essential for the operation of single frequency networks, where there is an existence of an in eliminable interference coming from a number of sites that may be transmitting the similar program on the same carrier frequency. "ETSI EN 300 744 V1.5.1 (2004 2006)"
- Digital to analogue converter: the digital to analogue convertor transforms the digital signal to an analogue signal.
- Front end: the analogue signal form the DAC is modulated by the front end to radio frequency (VHF, UHF), the bandwidth occupied by the single DVB T signal is either 6, 7 or 8 MHz wide. "ETSI EN 300 744 V1.5.1 (2004 2006)"
DVB T carriers:
In the DVB T system, an IFFT (Inverse Discrete Fourier Transform) with number of points equal to either 2048, 4096 or 8192 are used, in theory these carriers would be available for the data transmission, however, not all the carriers are being used for the data transmission. For the 2K mode there are 1512 carriers out of 2048carrier that are used for the data transmission, for the 4K mode there are 3024 carriers that are used for the data transmission out of 4096 carriers, and for the 8K mode there are 6048 carriers that are used for the data transmission out of 8192 carriers, thus all of the modes (2K, 4K, 8k modes) has the same data rates because if for example we take the 2K mode and the 8K modes then the 8K mode has exactly four times as many payloads carriers as the 2K mode but since the 2K mode has a symbol rate that is 4 times higher than that of the 8K mode, both modes will have the same data rates, given that the condition of transmission is the same for both modes. DVB T contains different types of carriers: Book J. Cosmas
- Inactive carriers with fixed position (set to zero amplitude).
- Payload carriers with fixed position.
- Continual pilots with fixed position.
- Scattered pilots with changing position in the spectrum.
- TPS carriers with fixed position.
The "payload carriers" are these carriers that are used for the actual data transmission, the edge carriers at the upper and lower channel edge are set to zero, i.e. they are inactive and do not carry any modulation at all, thus their amplitude is equal to zero. The pilots that are located on the real axis are the continual pilots; those pilots have defined amplitude. The continual pilots are 3 dB higher as compared with the average signal power, and the main use for the continual pilots are in the receiver as they are used as a phase reference and also for the automatic frequency control (AFC), thus, it will be used to look the frequency of the receiver with the frequency of the transmitter. The scattered pilots are scattered along the whole spectrum of the DVB T channel from symbol to symbol and virtually comprise a sweep signal for the channel estimation. In every symbol there is a scattered pilots every 12 carrier, each scattered pilots jumps forward 3 positions in the next symbol so there will be two intermediate carriers in each case figure (3) shows the process of changing position for scattered pilots. Book J. Cosmas
COFDM:
Since the beginning of the electrical transmission of messages almost 100 years ago, the method of transmitting information that have been used is single carrier method, however, the characteristics of most the transmission paths are such that the single carrier methods prove to be sensitive to interference and inadequate. In now days the demand for higher data rates cannot be satisfied by the single carrier method and it is reaching its limits. The terrestrial radio link that always had high bandwidth such as television with normally 8 MHz bandwidth, which is crying out for reliable digital transmission method, one of the ways to provide reliable digital data transmission is the using of multi carrier method, by using the multi carrier method the information is transmitted digitally via not only one carrier but via many carriers, sometimes via thousands of carriers with multiple error protection and data interleaving this method is known as the COFDM. This method is widely used for applications such as:
- Digital Audio Broadcasting (DAB).
- Digital Video Broadcasting (DVB T).
- Asymmetrical Digital Subscriber line (ADSL).
- Transmission of data signals via power lines.
- ISDB T. Japanese standard for digital terrestrial television.
- DMB T. Chinese standard for digital terrestrial.
The terrestrial transmission mediums are extremely difficult to deal with because it has the following characteristics features:
- Multipath reception via various echo paths caused by reflections from buildings, mountains, trees, etc.
- Additive white Gaussian noise (AWGN).
- Narrow band or wide band interference sources caused by internal combustion engines, streetcars or other radio sources.
- Doppler shift, i.e. frequency shift in mobile reception.
What is COFDM:
Orthogonal frequency division multiplexing is a multicarrier method with thousands of carriers that are orthogonal to each other which mean they don't interfere. The data to be transmitted should be interleaved to many subcarriers, and must be added with suitable error protection, resulting in code orthogonal frequency division multiplexing (COFDM). COFDM produces longer symbols than the single carrier method, the longer symbols with a suitable guard interval means that it is possible to eliminate inter symbol interference due to the echoes, because of the fact that the data is interleaved among many sub carriers and the presence of the error protection, it is possible to recover the original data without any errors even if there is any echoes or fading. Book J. Cosmas
CAZAC sequence:
Constant Amplitude Zero Auto Correlation sequences are known for their optimal transmission efficiency and tight time location properties because of that the CAZAC sequences are important in waveform design, CAZAC sequence have the property of ideal autocorrelation, before days these sequence were used in the radar system for precise target tracking and synchronization in communication systems, the CAZAC sequence have zero autocorrelation except when the delay is equal to zero. "Pluto, and Andrew Kebo, Ioannis Konstantinidis, John J. Benedetto, Michael R. Dellomo and Jeffrey M. Sieracki, Norbert Wiener Centre, Department of Mathematics, University of Maryland, 2007, ambiguity and sidelobe behavior of CAZAC coded waveform"
In order to generate a CAZAC sequence the following procedure must be followed:
For example for a sequence of length z2:
- Select a number z>1.
- The z different roots of unity (1, R1, R2, Rz 1) are formed.
- The z2 sequence is formed through the matrix M:
CAZAC sequence of lengths 256 and 900 are generated, there are 16 and 30 roots of unity for CAZAC sequences of length 256 and 900
