DESCRIPTION

The present invention relates to a method for generating a VCM or ACM signal having an improved framing compared to the one required by the DVB-S2 protocol.

More in particular, the present invention relates to a method for generating a VCM or ACM signal having a framing that allows for a simpler receiver synchronization procedure, while at the same time reducing the signalling overhead.

The invention is mainly intended for, without being limited to, satellite transmission and reception of digital audio and video signals and interactive applications for domestic and professional use, including Internet access.

It is known that the DVB-S2 system has been designed on the basis of two signal framing layers:

- a first one, at the physical layer (PL), transports a small number of highly protected signalling bits;

- a second one, at the baseband layer (BB), transports a large number of signalling bits to ensure the utmost flexibility of adaptation of the input signal. The first framing layer, i.e. the physical one, has been so conceived as to allow the modulation and coding parameters to be detected prior to demodulation and FEC decoding, and to give the possibility of synchronizing the receiver (carrier and phase retrieval, frame synchronization) in very critical signal-to-noise ratio conditions, as dictated by the high performance of FEC.

The physical layer framing of DVB-S2 is composed of a regular sequence of periodic frames called PLFRAME.

Each PLFRAME is composed of:

- a payload FECFRAME, corresponding to a coded block generated by coding the user bits in accordance with the chosen FEC scheme;

- the header of the PLFRAME, called PLHEADER, containing decoding and synchronization information, i.e.: modulation type and FEC coding rate, FECFRAME length, presence/absence of pilot symbols for facilitating receiver synchronization.

In order to protect the signals from the distortions of the transmission channel, the DVB-S2 system utilizes the LDPC (Low Density Parity Check) coding associated with the QPSK, 8PSK, 16APSK and 32APSK modulations (Figure 1), which are suitable for transmission over a non-linear channel such as the satellite one.

A description of the DVB-S2 standard and LDPC codes can be found, for example, in A. Morello, V. Mignone, "DVB-S2: The Second Generation Standard for Satellite Broad-band Services", Proceedings of the IEEE, Volume 94, Issue 1 , January 2006, pp. 210 - 227.

The LDPC codes of the DVB-S2 standard are characterized by a length of the FECFRAME, or FEC block, equal to 16200 bits (short block) or 64800 bits (normal block).

Depending on the modulation scheme in use, the FECFRAME is transformed into a symbol block XFECFRAME of different length, as shown in Figure 2. A frame PLFRAME is generated by adding at the beginning of each frame a header PLHEADER consisting of 90 modulated symbols and possibly 36 pilot symbols after each group of 1440 symbols of the symbol block XFECFRAME. The PLHEADER contains 26 symbols (SOF) to allow identifying the beginning of the PLFRAME, and 64 further symbols (PLS-code) transporting the following information: the length of the FEC block (normal or short); the type of modulation and coding of the symbol block XFECFRAME; a signalling of presence/absence of pilot symbols to facilitate the synchronization.

The typical structure of a frame PLFRAME is shown in Figure 3.

In the CCM (Constant Coding and Modulation) applications of DVB-S2, the modulation parameters are constant throughout the duration of the transmission; hence the PLFRAME has a constant length and synchronization is simple. However, the DVB-S2 system also provides for using the VCM (Variable Coding and Modulation) and ACM (Adaptive Coding and Modulation) modes, which allow changing the modulation parameters at each PLFRAME.

The result is a sequence of frames PLFRAME having different length and hence different duration, which makes synchronization by a receiver more complex because it is necessary to decode the current PLHEADER in order to identify the position of the next PLHEADER.

This complexity is even more apparent in the evolution of the DVB-S2 system for ultra-broadband transponders (e.g. 250 or 500 MHz), where such decoding must be carried out at a very high speed.

Figure 4 shows the length of the PLFRAME for the QPSK, 8PSK, 16APSK and 32APSK modulations, with or without pilot symbols, whereas Figure 5 shows the efficiency of the PLFRAME in the same configurations.

As can be seen in Figure 4, the length of the PLFRAME may have 16 possible values. The PLHEADER search algorithms must therefore consider 16 possible positions for the next PLHEADER.

It is an object of the present invention to provide a method for generating a VCM or ACM signal with improved framing which is adapted to speed up and make simpler and more robust the synchronization carried out by a receiver using said signal.

It is another object of the present invention to provide a method for generating a VCM or ACM signal with improved framing which is adapted to reduce and make constant the efficiency loss due to the PLHEADER.

It is a further object of the present invention to provide a VCM or ACM signal with improved framing which is adapted to improve the efficiency of a reception system receiving said signal.

Said objects are achieved through a method for generating a VCM or ACM signal with improved framing having the features set out in the appended claims, which are an integral part of the present description.

In brief, the method for generating a signal with improved framing in accordance with the present invention and usable with the DVB-S2 protocol provides for generating a signal with a frame PLFRAME having a constant length in terms of transmitted modulation symbols and in terms of time duration, independently of the modulation scheme adopted to transmit the symbol blocks of the LDPC code.

This is attained by creating a PLFRAME that comprises a sequence of n symbol blocks XFECFRAME while keeping the modulation and the LDPC coding rate constant, where n is the number of bits per symbol of the modulation scheme adopted to code the symbol block XFECFRAME.

The invention will now be described in detail in some of its preferred embodiments, which are provided herein by way of non-limiting example, with reference to the annexed drawings, wherein:

- Figure 1 is a schematic representation of the QPSK, 8PSK, 16APSK and 32APSK constellations included, among others, in the DVB-S2 standard;

- Figure 2 is a table that shows the length of a FEC block in a frame of a DVB- S2 signal as a function of the type of modulation in use;

- Figure 3 is a schematic representation of a PLFRAME according to the DVB- S2 standard;

- Figure 4 is a table that shows the length of a PLFRAME according to the DVB-S2 standard;

- Figure 5 is a table that shows the efficiency of the PLFRAME of Figure 4;

- Figure 6 is a table that shows the length of a PLFRAME generated in accordance with the method of the present invention;

- Figure 7 is a table that shows the efficiency of the PLFRAME of Figure 6;

- Figure 8 shows a PLFRAME which is generated in accordance with the method of the present invention as a function of the type of modulation in use. The method for generating a signal according to the invention requires that a PLFRAME be built as a sequence of n XFECFRAME's, where n is the number of bits per symbol of the modulation, i.e. n = 1 for BPSK modulation, n = 2 for QPSK modulation, n = 3 for 8PSK modulation, n = 4 for 16APSK modulation, n = 5 for 32APSK modulation, and n = 6 for 64APSK modulation (the latter has not yet been defined in DVB-S2), preceded by a PLHEADER.

By way of example, the PLHEADER as defined by the DVB-S2 standard will be taken into account herein; of course, the scheme proposed by the invention is also applicable to any type of PLHEADER, which is to be understood as a sequence of data that precedes the transmission of a frame of modulated data, containing information useful for synchronization, demodulation and decoding purposes. In particular, the invention is applied herein to the case of a new PLHEADER which has been improved over the one of the DVB-S2 standard, and which comprises a specific bit (or a sequence of symbols, e.g. SOF multiple sequences) to indicate to the receiver that the signal being transmitted is based either on the improved framing of the present invention or on other protocols (e.g. the classic DVB-S2 protocol).

Sequences of pilot symbols may possibly be entered into the PLFRAME in order to improve the synchronization of the system.

Figure 6 shows the duration of the PLFRAME generated in accordance with the method of the present invention, both in the case wherein there are no pilot symbols and in the case wherein pilot symbols having characteristics similar to those defined by the DVB-S2 standard have been entered.

As can be observed, the method of the invention advantageously allows to reduce the length of the PLFRAME's, and hence the duration thereof, to 4 possible cases (as opposed to the 16 prior-art cases), thereby allowing to simplify the reception synchronization algorithms to be used for searching for the PLHEADER.

Figure 7 shows the transmission efficiency of the PLFRAME generated in accordance with the method of the present invention. As can be inferred through a comparison with Figure 4, the method according to the invention allows to reduce the overhead of the PLHEADER, thereby significantly improving the system's transmission efficiency.

Figure 8 highlights that, regardless of the type of modulation, the method of the invention allows to generate a constant transmission framing, thus simplifying the synchronization procedures and reducing the signalling overhead.

The method according to the invention can be easily extended to different n- point constellations, which may possibly be used to form the PLFRAME by joining n FEC blocks.

The above-described method may advantageously be used in a digital signal transmission system based on the LDPC coding schemes of DVB-S2.

As is apparent to those skilled in the art, if the above-described method is applied in transmission, a reverse method will have to be applied in reception. The most important advantage, in terms of receiver synchronization procedures, is attained by keeping constant the length of the FRAME of the LDPC code (normal or short) and the configuration of the pilot symbols (present/absent) during all transmission.

It must be pointed out that the configuration flexibility of DVB-S2 is mainly due to the possibility of changing the modulation and coding parameters; therefore, keeping constant during all the transmission duration, the length of the frame of the LDPC code and the configuration of the pilot symbols will have no impact upon the system's flexibility.

In this case, unlike the DVB-S2 receiver, which must decode the current PLHEADER in order to known where the next one can be found or make multiple search attempts among 16 possible configurations, or alternatively demodulate the PLHEADER, a receiver receiving a signal according to the invention will be able to find the next PLHEADER, given the position of the current PLHEADER, because the length of the PLFRAME is fixed.

This technique is even more advantageous in the evolution of the DVB-S2 standard still under definition, in order to allow the receiver to operate at very high transmission speeds on ultra-broadband transponders. In this case, in order to know the signal framing, the receiver will simply have to identify one PLHEADER: by decoding it, it will obtain the complete framing.

The features of the present invention, as well as the advantages thereof, are apparent from the above description.

The method for generating a VCM or ACM signal with improved framing described herein by way of example may be subject to many possible variations without departing from the novelty spirit of the inventive idea; it is also clear that in the practical implementation of the invention the illustrated details may have different shapes or be replaced with other technically equivalent elements.

It can therefore be easily understood that the present invention is not limited to a method for generating a VCM or ACM signal with improved framing, but may be subject to many modifications, improvements or replacements of equivalent parts and elements without departing from the inventive idea, as clearly specified in the following claims.