In 2004 appeared standard of broadcasting SAT called DVB-S2 which was modification of the previous standard DVB-DSNG, providing maximum available compatibility with the system of digital satellite broadcasting DVB-S. For giving larger universality of application and efficiency increase at work on channels with a sufficient power stock, in technical norms on initial distribution systems are added options of transfers regimes based on modulation of 8 PSK and 16 QAM types.
New DVB-S2 standard is urged to cover lacks as of DVB-S standard (low speeds of streams at the expense of format QPSK modulation), and DVB-DSN Standard (work of SAT transmitters at the lowered output power at the requirement of providing lower distortions). Necessity for revision of present standards has been caused by several reasons.
The main factor of creation of new DVB-S2 standard became plans of mass HDTV start for today is observed deficiency in a frequency resource even while SDTV translation. If all SAT programs are broadcasted in HDTV, then available frequency resource will be not enough even at transition to more perfect systems of TV signal compression. In addition, the prospect of HDTV appearance has required development of formats of the channel coding, more effectively using available frequency resources (i.e. DVB-S2).
The second reason of DVB-S2 standard appearance is obliged to unsatisfactory work of available reception systems of Ka-range. Quality of reception in this range strongly depends on weather conditions, first of all, from rain. That is why for broadcasting in this diapason is often required higher noise immunity, than in C- and KU-diapasons.
The third reason of DVB-S2 standard appearance – is appearance of interactive SAT networks with address services. Such networks demand large transport resource and to optimize its usage is possible, only by adapting parameters of each address flow for conditions of reception of specific addressee. Old standards of such possibilities are not offered.
So, from new DVB-S2 standard the following was required:
* to raise efficiency of transport channel usage , i.e. to give possibility in a strip of the standard channel to transfer more bit of helpful information to a noise stability;
* to allow differentiated approach as a choice of transport parameters for different services transferred in one channel.
Besides, DVB-S2 standard should provide compatibility with previous standards and ways of smooth migration from the old equipment to new.
First two requirements were not executed at the expense of introduction in the standard more various schemes of modulation, usage of more effective systems of protective coding and introduction of additional coefficients of rounding, providing more abrupt fronts of the modulated signal.
Flexibility of channel formation has been reached by the same methods, as the efficiency of spectrum methods usage, and also at the expense of introduction of regimes VCM (Variable Coding and Modulation) and АСМ (Adaptive Coding and Modulation). The first mode allows various level of noise immunity of the services, transferred in one channel, and the second - additional possibility of adaptation of transport parameters to current conditions of service reception.АСМ regime is intended for networks with the return channel where reception systems have possibility to forward on head station the information about reception conditions.
As a result was executed universal standard (DVB-S2) on its base can be built networks for distribution of TV of programs of standard or a high definition, networks for suggesting interactive services, for example, access to the Internet, networks for professional appendages, such as transfer of digital TV from studio to studio, gathering of news and signal distribution on radio extenders. New DVB-S2 standard is also convenient for formation of networks of data transmission and creation of IP-highways.
The majority of the effective mechanisms put in DVB-S2, have appeared incompatible with old standards. Therefore, for execution of compatibility downwards requirements, developers entered two regimes into the standard. One – compatible downwards, but less effective, and another, using all new possibilities, but not allowing to use receiving sets of DVB-S standard.
The first DVB-S2 standard is recommended for traditional services, for migration to the new standard, and the second – for usage in professional networks and for transfer of new services which can't be accepted by old receiving sets.
New DVB-S2 standard provides four possibilities of the modulation scheme (pic.1). First two, QPSK and 8 PSK, are intended for use in broadcasting networks. Transmitters of transponders work there in regime, close to saturation that doesn't allow to modulate carring on amplitude. More high-speed schemes of modulation, 16 APSK and 32 APSK, are focused on professional networks where weaker land transmitters are used, which are not entering onboard retransmitters in a non linear operating regime, and on the reception side professional converters (LNВ) are established, allowing with high accuracy to value phase of the accepted signal. These schemes of modulation can be used and in broadcasting systems, but in this case channel forming equipment should support complicated variants of emphasis, and on the reception aide must be provided higher level of the relation a signal/noise. Symbols inside constellation APSK field of the modulated signal are placed on circles. Such variant is the most noiseproof in terms of transfer amplitude symbol and allows to use transmitters in regimes close to the point of saturation.
Let's pay attention that, in comparison with QPSK, the top scheme of modulation, 32 APSK, allows to raise the general speed of flow in 2.5 times.
Simultaneously with introduction of higher modulation levels DVB-S2 standard provides possibility of usage of two additional coefficients of rounding off alpho (α). To used in DVB-S α = 0.35, in the new standard factors α = 0.20 and α = 0.25 are added. New, lower meanings of coefficient provide a large spectrum of impulses that allows to use a spectrum more effectively. On the other hand, decrease promotes increase of nonlinear distortions that especially affects while transfer to the one carring on transponder. Therefore specific coefficient meaning gets out taking into account all parameters of transfer.
For protection against hindrances in new standard DVB-S2, as well as in previous, is used alternation of data and imposing of two-level code for direct correction (Forward Error Correction - FEC). But systems of external and internal code protection – are another, than in DVB-S standard. As external code protection instead of code of Read-Solomon the code of Bouza-Choudhuri-Hokvingema (BCH) is used, and as internal, instead of convolution code, - a code with low density of checks on parity (Low Density Parity Check Codes – LDPC).
For additional decrease frequency errors, an external level of code protection ВСН is used, working at small density of errors. In most regimes code allows to correct till 12 errors, but in some – till 8 or till 10 errors.
Standards DVB-S and DVB-DSNG are rigidly focused on transfer of transport MPEG-2 TS flow. The structure of transport shot of the new standard isn't linked to a certain format. It allows to transfer both transport packages MPEG-2, and any flows with continuous or package structure.
DVB-S2 provides two-level packing of the flow, entered for a solution of synchronization problem with of reception system in working conditions with low level of signal/noise relation.
Regimes with compatibility downwards basically are intended for broadcasting networks and moreover – for operators giving grants for a purchase of user's receiving set. They can use these regimes for the period of change park of receiving sets, and then be switched tomorrow effective, incompatible regimes.
Standard DVB-S2 allows such two regimes. In the first one executed simultaneous transfer of signals of DVB-S and DVB-S2 standards, asynchronously combined in one frequency channel. In the second signal DVB-S2 is imposed on signal DVB-S by means of hierarchical modulation. That is DVB-S flow represents itself as a signal of the top priority, and DVB-S2 flow – as a signal of the bottom priority (fig. 2). Signal DVB-S2 is transferred by means of modulation 8 PSK with non-uniform structure of constellations. Two points of constellation placed in each quadrant, display one symbol of a signal with top priority. Imposing of DVB-S2 signal is carried out by shift of symbols in constellation field by circle on corner ± θ. Such signal can be transferred by transmitter working in a regime, close to saturation.
Regimes compatible downwards don't allow to use completely potential of the new DVB-S2 standard and are difficult enough in realization. Therefore, most likely, they won't receive a wide circulation.
Irrespectively from the chosen mode of noiseproof coding and the modulation scheme, signal/noise level, allowing to accept a signal on the reception side, fluctuates from-2.4 dB (at modulation QPSK and FEC with relative speed 1/4) to +16 dB (32 APSK and FEC 9/10). These meanings are fair for the Gaussian channel and the ideal demodulator. They have been received by a method of computer modeling. Under condition of admissibility BER at level 10Е-7 the signal power exceeds a limit of Shennona of only on 0.7 – 1.2 dB.
In comparison with DVB-S, new DVB-S2 standard provides increase of transfer speed of useful information on 20-35 % or at the same efficiency of spectrum usage gives a store on signal level in 2-2.5 dB.
On pic.2 are shown variants of useful speed, reached at different configurations of the system, and also useful speeds of signals of DVB-S and DVB-DSNG standards.
The prize in efficiency of transfer appears even more considerable at the usage of АСМ regime, intended for interactive address appendages, such as transfer IP unicast. This regime allows to exclude store of power in 4-8 dB, put in satellite signals for adverse conditions of reception that gives the chance to double or treble throughput transponder.АСМ regime is the most effective with reference to translations of Кα-diapason, and also for tropical zones of reception.
On pic.3 there is a scheme satellite system work in this regime. The system includes АСМ sluice, DVB-S2 modulator with АСМ support, transferring land station, satellite and system of reception of the satellite signal, connected to АСМ sluice through reverse channel.
In АСМ regime format of noiseproof coding and the modulation scheme can change from shot to shot. In conditions of the raised attenuation of signal, service can be supported at the expense of decrease in speed of transfer useful information with simultaneous increase of redundancy of a noise-suppressing code and/or transition to more noiseproof scheme of modulation. Quality of an accepted signal is estimated by parameter C/N + I.
Each receipting system measures size of this parameter and by reverse channel sends result to АСМ sluice.
It is necessary to notice that consortium DVB Project doesn't assume that the new standard will replace old in the nearest future. Today in the world a lot of commercially successful satellite networks of DVB-S standard works, and their translations are accepted by millions of decoders, capable to serve not one year. Therefore the most probable scheme of new DVB-S2 standard introduction and its usage for translation of services which can't be accepted traditional receiving sets. For example, TV signals, compressed in new formats and/or transferred with the high permission.
Quite probably that new DVB-S2 standard will quickly find application also in networks of satellite news gathering. At least in sight of considerable advantages which gives АСМ regime. But speed of its mass introduction, possibly, will depend on appearance of new services incompatible with available reception equipment.