08-01-2013, 02:36 PM
Direct-To-Home (DTH) Report
[attachment=46687]
Introduction
The Direct-To-Home (DTH) service is a digital satellite service that provides television services direct to subscribers anywhere in the country. Since it makes use of wireless technology, programs are sent to the subscriber's television direct from the satellite, eliminating the need for cables and any cable infrastructure. This is particularly valuable in remote and difficult to reach areas where cable and in many cases, terrestrial television services are poor or nonexistent. DTH services also provide the finest of picture and sound quality which is considered to be second to none worldwide. Now surround sound, home theaters, live concerts and daily television programming are all delivered to your home with the same quality as any modern movie theater.
DTH System Architecture
DTH system encompasses the uplink system for digitizing, compressing, encoding and transmitting multiple television programs using DVB-S standards and downlink system for decoding, switching of video signals, program storage. Other elements are also used for the management of DTH system like billing, customer turn on-off etc.
Mainly DTH system architecture consists of 3 major parts:
1). Programming Source.
2). Broadcasting Center.
3). Receiving terminal.
Programming Source:
The programming source is that we have seen the channels in television like SET MAX, SONY, COLORS, etc. DTH service provider doesn’t generate the program. The job of service provider is that to collect all the international programs and various local programs to one station that called Broadcasting Center. The information can be gathered at DBS (digital broadcasting system) center via any appropriate medium. The provider pays to programming source for broadcasting the program.
Broadcasting center:
After receiving all the programs by programming source, the job of the broadcasting center is to do multiplexing, encoding, compressing, modulating and transmission. Broadcasting center has a digital video broadcasting system to do these jobs. Generally, a digital video broadcasting (DVB) system valid for all media carries a flexible combination of MPEG-2 video, audio, and data using the common MPEG-2 transport stream multiplex. The common service information system gives details of the programmes. Modulation and additional channel coding system, if any, are chosen to meet the requirements of different transmission media. A common scrambling system and a common conditional access interface are available.
DVB-S is the DVB standard for satellite delivery. It is an extension of MPEG-2 standard with specific instructions for implementing the satellite links. This standard is widely adopted. The details are explained in to chapter 3.
Receiving terminal:
Receiving terminal is used for getting signals which is coming from satellite to earth by using small aperture dish antenna.LNB( Low noise block converter) is used for the converting RF wave signal to the electrical signal and more the signal is fed to the device is called as set top box. That is doing decoding, de multiplexing, error correction etc. The details of this terminal are discussed in Chapter No. 5.
DVB-S Transmission:
Many of the formats and transmission aspects of satellite DVB services are standardized by international bodies such as the International Organization for Standardization (ISO) and the International Telecommunications Union (ITU).The standard developed by the European Telecommunication Standard Organization (part of ITU) applies to Ku-band satellites operating at 11/12 GHz. It is designed to provide quasi error-free (QEF) service at bit error rates (BERs) of 10-10 to 10-11. By using a fairly robust error-prevention scheme, which can be varied depending on the channel environment, it can provide this QEF rate to channels with non-corrected error rates of 10-1 to 10-2.
Processing of AV signals and DATA signals:
The functional block diagram of a DVB-S channel is shown in Fig3.1. The MPEG encoder unit can take in several compressed video channels (including the programmed audio and other digital data). All the data is compressed to produce a single MPEG data block of 188 bytes. Latest MPEG encoders can compress together up to ten regular video channels. The whole DVB-S system operates in the time-division multiplexing (TDM) mode. The input data must be in 188-byte blocks with 1-byte sync word at the beginning. Figure 3.1 shows the functional block diagram of the DVB-S transmission and each block is explained below.
Data coding:
A Reed Solomon code (204/188) is applied to the data. This coding can correct up to eight errors. In it, 16 bytes of overhead are added to the 188 bytes from the MPEG encoder. On the receive side, the Reed Solomon decoder can take in data coming at a BER of approximately 10-4 and convert it into a BER of 10-10 or lower.
Interleaving:
The data is then optionally Fornay interleaved (convolutional interleaving with depth 12). It is delimited by occasional sync packets. On the receiver side, the interleaver provides a gain of approximately 3 dB. This enhances the ability to correct burst errors that have been missed by the inner convolutional decoder.
Inner code:
The data is then convolutionally coded depending on the transponder size and channel quality desired. (Increasing the code rate reduces the redundancy from the base rate. Increasing the code rate increases the information rate and hence the error rate but reduces Eb/N0 requirements.) The basic code rate is ½ with K=7. But this rate can be increased by puncturing the code at code rates of 2/3, 3/4, 5/6, 7/8, and others. Each code rate is tried and then locked using the sync data. On the receive side, the convolutional decoder can take in a service quality of 10-2 and improve it to an error rate of 10-4.
Baseband pulse shaping:
Pulse shaping is the process of changing the waveform of transmitted pulses. Its purpose is to make the transmitted signal better suited to its purpose or the communication channel, typically by limiting the effective bandwidth of the transmission. Here Baseband pulses are then gray-coded and root-raised cosine filtered. The roll-off rate is 0.35.
QPSK modulation:
This single carrier is now quaternary phase shift keying (QPSK) modulated. The table shown below provides data rates and transponder sizes. This is an example, and the parameters for individual systems may vary. The code rate is dynamically variable. So when the link is clean, the transmitter may be transmitting at a high code rate (less overhead). But if the link deteriorates, say, due to rainfall, the transmitter switches to a higher coder rate to provide the same BER. This means that the system must be designed to oper-ate in the worst condition. To guarantee a certain link quality, the system must pro-vide the highest Eb/N0 listed in the table3.1.
Channel hopping considerations:
The response time during channel hopping can be acceptably short with MCPC access, so long as the service information is delivered at an adequate rate. The maximum delay is likely to occur when switching from one multiplex to another multiplex, which requires retuning of the receiver to the new carrier frequency. With SCPC access, the response time can be as high as 5 seconds. This is partly because the data transmission rate is much lower than for MCPC transmissions, leading to a lower rate of service information transfer for the same degree of overhead (percentage of the capacity allocated to the service information). Switching between different multiplexes, and hence receiver retuning, will also occur more frequently, as SCPC transmissions will generally carry only one or very few digital TV programmes.
Indoor Data Terminal:
As seen in the figure, indoor terminal consists of 70 MHz modulator and power supply. We know that video signal has a bandwidth of 5 MHz and DTH service provider has to transmit all the television programs. So after doing multiplexing, encoding, compression, the data signal should be converted in to intermediate frequency that is 70 MHz or 140 MHz. Conversion to an intermediate frequency is useful for several reasons. When several stages of filters are used, they can all be set to a fixed frequency, which makes them easier to build and to tune. Lower frequency transistors generally have higher gains so fewer stages are required. It's easier to make sharply selective filters at lower fixed frequencies.
[attachment=46687]
Introduction
The Direct-To-Home (DTH) service is a digital satellite service that provides television services direct to subscribers anywhere in the country. Since it makes use of wireless technology, programs are sent to the subscriber's television direct from the satellite, eliminating the need for cables and any cable infrastructure. This is particularly valuable in remote and difficult to reach areas where cable and in many cases, terrestrial television services are poor or nonexistent. DTH services also provide the finest of picture and sound quality which is considered to be second to none worldwide. Now surround sound, home theaters, live concerts and daily television programming are all delivered to your home with the same quality as any modern movie theater.
DTH System Architecture
DTH system encompasses the uplink system for digitizing, compressing, encoding and transmitting multiple television programs using DVB-S standards and downlink system for decoding, switching of video signals, program storage. Other elements are also used for the management of DTH system like billing, customer turn on-off etc.
Mainly DTH system architecture consists of 3 major parts:
1). Programming Source.
2). Broadcasting Center.
3). Receiving terminal.
Programming Source:
The programming source is that we have seen the channels in television like SET MAX, SONY, COLORS, etc. DTH service provider doesn’t generate the program. The job of service provider is that to collect all the international programs and various local programs to one station that called Broadcasting Center. The information can be gathered at DBS (digital broadcasting system) center via any appropriate medium. The provider pays to programming source for broadcasting the program.
Broadcasting center:
After receiving all the programs by programming source, the job of the broadcasting center is to do multiplexing, encoding, compressing, modulating and transmission. Broadcasting center has a digital video broadcasting system to do these jobs. Generally, a digital video broadcasting (DVB) system valid for all media carries a flexible combination of MPEG-2 video, audio, and data using the common MPEG-2 transport stream multiplex. The common service information system gives details of the programmes. Modulation and additional channel coding system, if any, are chosen to meet the requirements of different transmission media. A common scrambling system and a common conditional access interface are available.
DVB-S is the DVB standard for satellite delivery. It is an extension of MPEG-2 standard with specific instructions for implementing the satellite links. This standard is widely adopted. The details are explained in to chapter 3.
Receiving terminal:
Receiving terminal is used for getting signals which is coming from satellite to earth by using small aperture dish antenna.LNB( Low noise block converter) is used for the converting RF wave signal to the electrical signal and more the signal is fed to the device is called as set top box. That is doing decoding, de multiplexing, error correction etc. The details of this terminal are discussed in Chapter No. 5.
DVB-S Transmission:
Many of the formats and transmission aspects of satellite DVB services are standardized by international bodies such as the International Organization for Standardization (ISO) and the International Telecommunications Union (ITU).The standard developed by the European Telecommunication Standard Organization (part of ITU) applies to Ku-band satellites operating at 11/12 GHz. It is designed to provide quasi error-free (QEF) service at bit error rates (BERs) of 10-10 to 10-11. By using a fairly robust error-prevention scheme, which can be varied depending on the channel environment, it can provide this QEF rate to channels with non-corrected error rates of 10-1 to 10-2.
Processing of AV signals and DATA signals:
The functional block diagram of a DVB-S channel is shown in Fig3.1. The MPEG encoder unit can take in several compressed video channels (including the programmed audio and other digital data). All the data is compressed to produce a single MPEG data block of 188 bytes. Latest MPEG encoders can compress together up to ten regular video channels. The whole DVB-S system operates in the time-division multiplexing (TDM) mode. The input data must be in 188-byte blocks with 1-byte sync word at the beginning. Figure 3.1 shows the functional block diagram of the DVB-S transmission and each block is explained below.
Data coding:
A Reed Solomon code (204/188) is applied to the data. This coding can correct up to eight errors. In it, 16 bytes of overhead are added to the 188 bytes from the MPEG encoder. On the receive side, the Reed Solomon decoder can take in data coming at a BER of approximately 10-4 and convert it into a BER of 10-10 or lower.
Interleaving:
The data is then optionally Fornay interleaved (convolutional interleaving with depth 12). It is delimited by occasional sync packets. On the receiver side, the interleaver provides a gain of approximately 3 dB. This enhances the ability to correct burst errors that have been missed by the inner convolutional decoder.
Inner code:
The data is then convolutionally coded depending on the transponder size and channel quality desired. (Increasing the code rate reduces the redundancy from the base rate. Increasing the code rate increases the information rate and hence the error rate but reduces Eb/N0 requirements.) The basic code rate is ½ with K=7. But this rate can be increased by puncturing the code at code rates of 2/3, 3/4, 5/6, 7/8, and others. Each code rate is tried and then locked using the sync data. On the receive side, the convolutional decoder can take in a service quality of 10-2 and improve it to an error rate of 10-4.
Baseband pulse shaping:
Pulse shaping is the process of changing the waveform of transmitted pulses. Its purpose is to make the transmitted signal better suited to its purpose or the communication channel, typically by limiting the effective bandwidth of the transmission. Here Baseband pulses are then gray-coded and root-raised cosine filtered. The roll-off rate is 0.35.
QPSK modulation:
This single carrier is now quaternary phase shift keying (QPSK) modulated. The table shown below provides data rates and transponder sizes. This is an example, and the parameters for individual systems may vary. The code rate is dynamically variable. So when the link is clean, the transmitter may be transmitting at a high code rate (less overhead). But if the link deteriorates, say, due to rainfall, the transmitter switches to a higher coder rate to provide the same BER. This means that the system must be designed to oper-ate in the worst condition. To guarantee a certain link quality, the system must pro-vide the highest Eb/N0 listed in the table3.1.
Channel hopping considerations:
The response time during channel hopping can be acceptably short with MCPC access, so long as the service information is delivered at an adequate rate. The maximum delay is likely to occur when switching from one multiplex to another multiplex, which requires retuning of the receiver to the new carrier frequency. With SCPC access, the response time can be as high as 5 seconds. This is partly because the data transmission rate is much lower than for MCPC transmissions, leading to a lower rate of service information transfer for the same degree of overhead (percentage of the capacity allocated to the service information). Switching between different multiplexes, and hence receiver retuning, will also occur more frequently, as SCPC transmissions will generally carry only one or very few digital TV programmes.
Indoor Data Terminal:
As seen in the figure, indoor terminal consists of 70 MHz modulator and power supply. We know that video signal has a bandwidth of 5 MHz and DTH service provider has to transmit all the television programs. So after doing multiplexing, encoding, compression, the data signal should be converted in to intermediate frequency that is 70 MHz or 140 MHz. Conversion to an intermediate frequency is useful for several reasons. When several stages of filters are used, they can all be set to a fixed frequency, which makes them easier to build and to tune. Lower frequency transistors generally have higher gains so fewer stages are required. It's easier to make sharply selective filters at lower fixed frequencies.