ITTO20110310A1 - METHOD OF PROCESSING DIGITAL SIGNALS AND RELATIVE TRANSMISSION AND RECEPTION SYSTEMS - Google Patents

Description

DESCRIPTION of the Industrial Invention entitled:

"METHOD OF PROCESSING DIGITAL SIGNALS AND RELATED TRANSMISSION AND RECEPTION SYSTEMS"

DESCRIPTION

The present invention relates to a method of processing digital signals and related transmission and reception systems.

The invention is intended to be applied above all, though not exclusively, to the reception and transmission of digital audio and video signals, and in particular to those relating to the broadcasting of second generation digital television signals on mobile and / or portable terminals.

The second generation system for broadband satellite transmissions (DVB-S2) uses the LDPC (Low Density Parity Check) encoding associated with the QPSK, 8PSK, 16APSK and 32APSK modulations, illustrated in Figure 1, which are suitable for transmission on a non-linear channel, such as the satellite one. An exposition 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.

For a better exploitation of the potential of the codes, the DVB-S2 standard provides that an interlacer is interposed between the LDPC encoder and the mapper on the constellations 8PSK, 16APSK and 32APSK, which allows a better association between the bits of the coded word and the bits carried by the points. of the constellation.

The interlacer defined in the DVB-S2 standard requires that the packet encoded at the output of the LDPC encoder (formed by a number of bits equal to 16200 or 64800, a number generally indicated with the "NFRAME" symbol) is written by columns in a matrix with N columns, where N is the number of bits carried by the constellation (N is 3 for 8PSK, 4 for 16APSK, 5 for 32APSK), and NFRAME / N rows, as shown in Figure 2, and is read for lines; the reading takes place from left to right for all the coding rates foreseen by the standard except for the 3/5 rate, where the reading takes place from right to left. The association to the points or coordinates of the constellations occurs as illustrated in Figure 1.

The second generation system for the transmission of digital terrestrial audio / video signals (DVB-T2) uses the same LDPC coding system used in DVB-S2, associated with an error rate between 1/2 and 5/6 and to the QPSK, 16QAM, 64QAM and 256QAM modulations.

Currently it is planned to use the same coding scheme of the DVB-T2 standard, i.e. the same LDPC codes, for the reception and transmission of digital audio and video signals relating to the broadcasting on mobile and / or portable terminals of second generation digital television signals. , associated with QAM modulations ("Quadrature Amplitude Modulation"); in particular, the QPSK, 16QAM and 64QAM modulations are provided, illustrated in Figure 3. In the case of error rates lower than 1/2, the coding scheme of the DVB-T2 standard is integrated by the LDPC codes of the DVB-S2 system for error rates below 1/2.

The Applicant has realized that, with the QAM modulations, the LDPC codes provide good performances, but not totally satisfactory in terms of the signal to noise ratio (SNR) necessary to reach the QEF ("Quasi Error Free") condition; as known, this condition corresponds to the case in which less than one error is received for each hour of program received.

The present invention has the general purpose of overcoming the problem mentioned above and, in particular, of associating in a better way the bits at the output of the LDPC encoder to the coordinates of the constellations of the QAM modulations; more particularly, the present invention deals with LDPC coding with coding rate 1/3 or 2/5, and with 16QAM modulation.

The aforesaid objects are achieved through the digital signal processing method and the relative transmission and reception systems having the characteristics set out in the appended claims which form an integral part of the present description.

The invention will now be described in detail in some of its preferred embodiments, given by way of non-restrictive example, with reference to the attached drawings, in which:

- Figure 1 is a schematic representation of the constellations QPSK, 8PSK, 16APSK and 32APSK provided for example by the DVB-S2 standard;

- Figure 2 represents an explanatory diagram of an interlacer envisaged by the DVB-S2 standard, specifically the 8PSK modulation;

Figure 3 is a schematic representation of the QPSK, 16QAM and 64QAM constellations applicable to the reception and transmission of audio and video signals relating to the broadcasting of second generation digital television signals; Figure 4 represents a very simplified block diagram of a modulating digital signal processing system according to the present invention; - Figure 5 represents a general explanatory diagram of the interlacer, or interleaver, of Figure 4;

Figure 6 schematically represents the function performed by a "Demux" block of Figure 5 according to a first example of embodiment of the present invention relating to the 16QAM modulation and LDPC coding with coding rate 1/3;

Figure 7 schematically represents the function performed by a "Demux" block of Figure 5 according to a second embodiment of the present invention relating to 16QAM modulation and LDPC coding with coding rate 2/5.

With reference to Figure 4, the procedure for associating the bits of the modulating information flow to the points or coordinates of the constellation of the QAM modulation is schematically represented.

An "Encoder" block receives the modulating information flow at its input and generates at its output a coded information flow organized in packets consisting of NFRAME bits, equal to 64800 or 16200; the code used is the LDPC code, in particular that of the DVB-S2 standard, with coding rate 1/3 or 2/5. Within an "Interleaver" block, these packets are written in an interlacing matrix, of total size NFRAME; this matrix is made up of mxN columns and NFRAME / (mxN) rows.

A "Demux" block performs a permutation of the bits received from the "Interleaver" block; such bits are received by the interlacing matrix in groups of mxN at a time, where N is the number of bits carried by the constellation (N = 2 for the QPSK, N = 4 for the 16QAM, N = 6 for the 64QAM), and " m "is an integer greater than or equal to 1. The" Demux "block associates them in m groups of N bits, exchanges them according to pre-established schemes taking into account the type of modulation (ie the QAM level), the code and the type of transmission channel and provides them at the output.

A "Mapper" block associates the N-ples of bits at the output of the "Demux" block to the points or coordinates of the constellation, for example as indicated in Figure 3 for the QAM modulations.

It is worth highlighting that the blocks represented in Figure 4 are only those essential to the understanding of the present invention; therefore the presence of intermediate blocks must not be excluded, for example between the "Demux" block and the "Mapper" block, suitable for carrying out specific signal processing functions.

The present invention proposes particular permutation schemes that can be adopted for QAM modulations and LDPC codes with different coding rates as provided, for example, by the DVB-S2 / T2 standard in association with different types of interlacing.

The preferred embodiments of the present invention refer to the 16QAM modulation and to the LDPC code of the DVB-S2 standard with 1/3 or 2/5 coding rate.

The preferred embodiment of the present invention provides that an interlacer equal or similar to that of the DVB-S2 standard illustrated in Figure 2, or equal or similar to that of the DVB-T2 standard with a number of bits / columns dependent on the type of levels, is used. of QAM modulation. Preferably, then, this preferred embodiment provides that the N-ples of bits outgoing from the "Demux" block are associated with the points of the QAM constellations through the "Mapper" block according to the labeling used in the DVB-S2 standard (see Figure 3 ), or DVB-T2.

In the case of 16QAM modulations, the present invention provides that a matrix "Interleaver" interleaver is used, realized as a matrix with 2xN columns and NFRAME / (2xN) rows, written by columns from top to bottom, and read by rows, from left to right. In this case, the "Demux" block of Figure 4 can operate for example with m equal to 2.

In the case of 16QAM modulation and 1/3 coding rate, the 2xN bits in input to the "Demux" block can be permuted for example as specified in Figure 6 and associated with two consecutive symbols of the 16QAM modulation.

That is, given the 2xN bits from bO to b7, the 2xN bits carried by the constellation 16QAM from yO to y 7 are thus determined, in which the bits yO and bO are respectively the most significant bits of the words Y and B and the bits y7 and b7 are respectively the least significant bits of words Y and B:

y0 = b2, yl = bl, y2 = b0, y3 = b3, y4 = b7, y5 = b5, y6 = b6, y7 = b4.

In the case of 16QAM modulation and 2/5 coding rate, the 2xN bits in input to the "Demux" block can be permuted for example as specified in Figure 7 and associated with two consecutive symbols of the 16QAM modulation.

That is, given the 2xN bits from bO to b7, the 2xN bits carried by the constellation 16QAM from yO to y 7 are thus determined, in which the bits yO and bO are respectively the most significant bits of the words Y and B and the bits y7 and b7 are respectively the least significant bits of words Y and B:

y0 = b3, yl = b2, y2 = b0 / y3 = bl, y4 = b7, y5 = b5 / y6 = b6 / y7 = b4.

As is evident to the person skilled in the art, if the methods described above are applied in the transmission phase, inverse methods must be applied in the reception phase.

From the above description the characteristics of the present invention are therefore clear, as well as its advantages.

The methods described above can advantageously be used in a digital signal transmission system based on a 16QAM modulator, in particular in a transmitter of digital audio and video signals for broadcasting digital television signals on mobile and / or portable terminals.

As known, the transmission of television signals is carried out by radio frequency transmitters, while the reception of television signals is carried out by television receivers typically installed in the homes of users of the television service or by receivers installed on mobile or portable devices. There are numerous possible variants to the method of processing digital signals and related transmission and reception systems described as an example, without thereby departing from the principles of novelty inherent in the inventive idea, just as it is clear that in its practical implementation the forms of the details illustrated may be different, and the same may be replaced with technically equivalent elements.

It is therefore easy to understand that the present invention is not limited to a method of processing digital signals and related transmission and reception systems, but is subject to various modifications, improvements, replacements of equivalent parts and elements without however departing from the idea of invention, as better specified in the following claims.

Claims (8)

CLAIMS 1. Method of processing digital signals, in particular audio and video, to be sent to a modulator of the 16QAM type, said digital signals being encoded according to an LDPC code, in which a bit permutation is performed before the constellation mapping function, characterized in that said bit permutation is performed on words having a length of 8 bits or further multiples of 4 bits. 2. Method according to claim 1, wherein said LDPC code has an encoding rate equal to 1/3 and said permutation is carried out on 8-bit words and consists in generating a word Y comprising in the order the bits yO yl y2 y3 y4 y5 y6 y 7, starting from a word B comprising in the order the bits bO bl b2 b3 b4 b5 b6 b7, the bits yO and bO being respectively the most significant bits of the words Y and B and the bits y7 and b7 being the least significant bits of words Y and B respectively, and in which: y0 = b2, yl = bl, y2 = b0, y3 = b3, y4 = b7, y5 = b5, y6 = b6, y7 = b4. 3. Method according to claim 1, wherein said LDPC code has an encoding rate equal to 2/5 and said permutation is carried out on 8-bit words and consists in generating a word Y comprising in the order the bits yO yl y2 y3 y4 y5 y6 y 7, starting from a word B comprising in the order the bits bO bl b2 b3 b4 b5 b6 b7, the bits yO and bO being respectively the most significant bits of the words Y and B and the bits y7 and b7 being the least significant bits of words Y and B respectively, and in which: y0 = b3, yl = b2, y2 = b0, y3 = bl, y4 = b7, y5 = b5, y6 = b6, y7 = b4. 4. Digital signal transmission system comprising a modulator of the QAM type, characterized in that it comprises means for implementing the method according to any one of claims 1 to 3. 5. Method of processing digital signals, in particular audio and video, received by a demodulator of the 16QAM type, said signals being encoded according to an LDPC code, in which after the constellation demapping function a bit permutation is performed, characterized by the fact that said bit permutation is carried out on words having a length of 8 bits or further multiples of 4 bits. 6. Method according to claim 5, wherein said LDPC code has an encoding rate equal to 1/3 and said permutation is carried out on 8-bit words and consists in generating a word B comprising in the order the bits bO bl b2 b3 b4 b5 b6 b 7, starting from a word Y comprising in the order the bits yO yl y2 y3 y4 y5 y6 y 7, the bits yO and bO being respectively the most significant bits of the words Y and B and the bits y7 and b7 being the least significant bits of words Y and B respectively, where: y0 = b2, yl = bl, y2 = b0, y3 = b3, y4 = b7 / y5 = b5, y6 = b6 / y7 = b4. 7. Method according to claim 5, wherein said LDPC code has a coding rate equal to 2/5 and said permutation is carried out on 8-bit words and consists in generating a word B comprising in the order the bits bO bl b2 b3 b4 b5 b6 b7, starting from a word Y comprising in the order the bits yO yl y2 y3 y4 y5 y6 y 7, the bits yO and bO being respectively the most significant bits of the words Y and B and the bits y7 and b7 being the least significant bits of words Y and B respectively, in which: y0 = b3, yl = b2, y2 = b0, y3 = bl, y4 = b7, y5 = b5, y6 = b6, y7 = b4. 8. System for receiving digital signals comprising a demodulator of the QAM type, characterized in that it provides the method according to any one of claims 5 to 7.