RU2292654C2 - Symbol interlacing method - Google Patents

Abstract

FIELD: technology for exchanging digital data with usage of transfer system based on multiplexing with orthogonal frequency division of channels, including at least one transmitter, and receivers.

SUBSTANCE: method includes selection of operation mode in transmitter from at least one mode, while each operation mode is associated with a number of active carriers for transferring useful data, selection of interlacing symbol in aforementioned selected operation mode, usage of symbol interlacing in transmitter in relation with data element blocks, display of interlaced elements of data on active carriers of aforementioned selected operation mode, receipt of interlaced data elements in receiver, recognition in receiver of symbol interlacing symbol used during data transfer, selection of reversed interlacing symbol in receiver for match with recognized interlacing symbol and reverse interlacing in receiver of received data elements by means of selected reverse interlacing symbol.

EFFECT: increased stability of system due to efficient correction of errors.

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Description

FIELD OF THE INVENTION

The invention relates to interleaving and reverse interleaving. In particular, the invention relates to symbol interleaving and reverse symbol interleaving in a system based on orthogonal frequency division multiplexing (OFDM).

State of the art

Orthogonal frequency division multiplexing (COFDM) type modulation is used for broadcasting (either in multicast or unidirectional transmission) of broadband digital signals from a transmitter to multiple receivers. As one example, the DVB-T (Terrestrial Digital Video Broadcasting) system defined in the ETSI (European Institute of Communication Standards) EN 300 744 (Version 1.4.1) is a system in which an orthogonal frequency division multiplexed, modulated coding modulation method (COFDM) is used for broadcasting (or for multicasting) broadband digital television signals from a DVB-T transmitter to multiple DVB-T receivers.

Typically, digital data exchange utilizes orthogonal frequency division multiplexing (OFDM), OFDM being just one example of this.

As the standard explains, before digital data (for example, encoded according to the MPEG-2 format (developed by the cinematography expert group) video, audio and / or data stream) is ready for transmission, many operations are applied to them by the DVB-T transmitter. These operations include, inter alia, external coding by the Reed-Solomon encoder, external interleaving (i.e., convolutional interleaving), internal coding (using a perforated convolutional code), and internal interleaving.

Internal interleaving is performed in an internal interleaver, which contains a set of bit interleavers for bitwise interleaving, followed by a character interleaver for symbol interleaving. The purpose of internal interleaving is to increase the system's robustness to errors and interference by reordering the digital data to be transmitted in such a way that, basically, the information contained in subsequent / adjacent sequences of digital data is not interrupted due to transmission by adjacent carriers. In this case, interference that occurs only occasionally at a frequency of only one or only a few carrier data can be handled by the effective error correction that the system provides.

In the DVB-T system, the transmitted signal is organized in frames. Each frame has a duration of 68 OFDM symbols. Each OFDM symbol, in turn, is constituted by a set of carriers. The standard describes the use of two operating modes: “2K mode” and “8K mode”. In 2K mode, the number of carriers is 1705, and in 8K, the number of carriers is 6817. However, only 1512 carriers out of a total of 1705 carriers in 2K mode and 6048 carriers out of a total of 6817 carriers in 8K mode are defined as “active” carriers that really carry digital data (e.g. payload, payload). The remaining carriers are mainly used for control purposes. It should be noted that using another counting method that takes into account a special protective frequency band, the total number of carriers 2048 (2K mode) or 8192 (8K mode) can be obtained. These quantities correspond to the FFT size used (FFT) in each case (FFT = Fast Fourier Transform, FFT = Inverse FFT). However, the number of active carriers is still equal to 1512 (2K mode) or 6048 (8K mode).

The mapping of digital data (data words, also called bit words or data elements) to active carriers is performed in the internal interleaver. More precisely, this problem is solved by a symbol interleaver. A separate “2K symbol interleaver” is defined for 2K mode, and “8K symbol interleaver” is defined for 8K mode. In 2K mode, the 2K symbol interleaver displays 1512 data words (i.e. 12 groups of 126 data words, the length v of each data word being v = 2, 4, or 6 bits depending on the modulation method used) coming from a set of bit interleavers 1512 active carriers of one 2K OFDM symbol. Similarly, in 8K mode, 8K symbol interleaver maps 6048 data words (48 groups of 126 data words) to 6048 active carriers of one 8K OFDM symbol.

Recently, the DVB-T system has been evaluated for situations for which it was not originally designed, such as mobile reception. Also, new system usage scenarios, such as IPDC (Broadcast Digital Data over Internet Protocol (IP)), have various new system requirements. Using the existing 8K mode for DVB-T will probably offer a fairly wide coverage for mobile reception. However, the achieved mobility with 8K mode may be too slow, i.e. reception may be unsuccessful, for example, in a fast-traveling vehicle, as at a speed of 120 km / h. 2K mode, on the other hand, will offer sufficient mobility. However, in 2K mode, a high density of base stations will be required, because the so-called guard interval length is not large enough to maintain long distances to the transmitter in this mode. One proposed compromise solution to this problem is to define a new mode: 4K mode.

As for the internal interleaving of the proposed 4K mode, especially for the symbol interleaver, an obvious solution would be to define a new “4K symbol interleaver” that would map 3024 data words (24 groups of 126 data words) to 3024 active carriers of one 4K mode OFDM symbol ( it is likely that the number of active carriers would be 3024 in 4K mode). However, this solution, of course, requires more space in the DVB-T transmitter and, in particular, in a plurality of DVB-T receivers, since a new 4K symbol interleaver (or reverse interleaver) must be implemented in each device.

To overcome the above disadvantages, there is a need for a new method and system for transmitting digital data, and new types of transmitters and receivers are needed to implement a new method and system.

Disclosure of invention

According to a first aspect of the invention, there is provided a method for exchanging digital data using a transmission system based on orthogonal frequency division multiplexing (OFDM), including at least one transmitter and receivers, the method comprising the following steps:

selecting an operating mode in the transmitter from at least one mode, each operating mode being associated with the number of active carriers for transmitting useful data;

selecting a symbol interleaver in the transmitter from a set of symbol interleavers for symbol interleaving in said selected mode of operation;

the use of symbolic interleaving in the transmitter in relation to blocks of data elements;

mapping interleaved data elements onto active carriers of said selected mode of operation;

receiving interleaved data elements in a receiver;

recognition in the receiver of the symbol interleaver used in data transmission;

selecting a deinterleaver in the receiver to match the recognized symbol interleaver, and

reverse interleaving in the receiver of the received data elements using the selected inverse interleaver.

Since the invention makes it possible to select an operating mode and a symbol interleaver in various ways, it is possible to provide different depths of interleaving depending on various requirements imposed on the system. Previously, this was not possible, since a fixed concrete character interleaver has always been used with a specific mode of operation.

According to a second aspect of the invention, there is provided a transmitter for exchanging digital data using a transmission system based on orthogonal frequency division multiplexing (OFDM), which system has a set of operating modes, and said set contains at least one operating mode, and each mode is associated with a predetermined number of active carriers used for transmitting useful data from the transmitter to the receiver, the transmitter comprising:

a set of character interleavers for character interleaving,

means for selecting an operating mode for transmitting data and

means for selecting a symbol interleaver from said set of symbol interleavers for symbol interleaving in said selected mode of operation.

According to a third aspect of the invention, there is provided a receiver for exchanging digital data using a transmission system based on orthogonal frequency division multiplexing (OFDM), the system having a set of operating modes and said set containing at least one operating mode, and each mode is associated with a predetermined number of active carriers used to transmit useful data from the transmitter to the receiver, and this system further comprises a set of symbolic emezhiteley used for symbol interleaving at the transmitter and the receiver comprises:

means for receiving interleaved data elements;

means for recognizing the symbol interleaver used in data transmission;

a set of symbolic deinterleavers for deinterleaving received data elements that are interleaved in a transmitter in a symbol interleaver, and

means for selecting an inverted symbol interleaver from a set of inverted symbol interleavers corresponding to the recognized symbol interleaver.

According to a fourth aspect of the invention, there is provided a digital data exchange system using a transmission system based on orthogonal frequency division multiplexing (OFDM), comprising at least one transmitter and a plurality of receivers, the system having a set of operating modes and said set containing at least one operating mode, and each mode is associated with a predetermined number of active carriers used to transmit useful data from the transmitter to at least one to the receiver, the transmitter having a set of symbol interleavers used for symbol interleaving with respect to the data element blocks in the transmitter, and at least one receiver has a set of inverted symbol interleavers for reverse interleaving of the interleaved data elements in the receiver, the system further comprising:

means in the transmitter for selecting a mode of operation to be used in transmitting data;

means in the transmitter for selecting a symbol interleaver for symbol interleaving in a selected mode of operation;

means in the transmitter for applying said symbolic interleaving to blocks of data elements;

means in said at least one receiver for receiving transmitted interleaved data elements;

means in said at least one receiver for recognizing a symbol interleaver used in data transmission;

means in said at least one receiver for selecting an inverted symbol interleaver from a set of inverted symbol interleavers corresponding to the recognized symbol interleaver.

According to a fifth aspect of the invention, a symbol interleaver is provided for applying block-based symbol interleaving to data elements, such as data words, to map said data elements to active carriers of a particular operating mode based on an OFDM system, the number of data elements being interleaved in one times, determined by the size of the block, with:

the symbol interleaver is configured to use a block size different from the number of active carriers in said particular mode.

The dependent claims contain preferred embodiments of the invention. The essence disclosed in the dependent claims relating to a specific aspect of the invention is also applicable to other aspects of the invention.

List of drawings

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which:

Figure 1 - internal interleaver according to the standard DVB-T;

Figure 2 - the principle of symbolic interleaving standard DVB-T;

Figure 3 is a symbolic interleaving according to a preferred embodiment of the invention;

4 is a symbolic interleaving according to another preferred embodiment of the invention;

5 is a symbolic interleaving according to another preferred embodiment of the invention;

6 - blocks of a DVB-T transmitter suitable for implementing an embodiment of the invention;

7 - blocks of a DVB-T receiver suitable for implementing an embodiment of the invention;

Fig. 8 illustrates a system in an embodiment of the invention for transmitting digital data; and

9 is a mobile receiver suitable for implementing an embodiment of the invention.

Detailed description

In the following description of the various embodiments, reference is made to the accompanying drawings, which form a part thereof and in which, by way of illustration, various embodiments are shown according to which the invention can be practiced. It is understood that other embodiments may be used, and structural and functional changes may be made without departing from the scope of the present invention.

First, refer to Fig. 8, where a system 10 for exchanging digital data is shown in the context of transmitting digital data to one or more receivers 600, 601 from one or more transmitters 700, 701 through a plurality of tracks 11, 12, 13, 14 of the air interface. System 10 is, in accordance with an embodiment of the present invention, an orthogonal frequency division multiplexing (OFDM) system.

Figure 1 shows the internal interleaver 100, as defined in EN 300 744 (Version 1.4.1) ETSI for DVB-T. Figure 1 is also applicable to the description of the invention. The internal interleaver can be implemented either in the form of hardware, or in the form of software, or in the form of a combination thereof. Hardware implementation on suitable semiconductor components is preferred. The input stream x ₀ , x ₁ , x ₂ , ... is demultiplexed in the demultiplexer 110 into v substreams, while v is the number of bits per modulation symbol. In the example case of FIG. 1, the modulation method used is 16-position quadrature amplitude modulation (16-QAM), in which the number of bits per modulation symbol is 4 (v = 4).

Bit interleaving (and subsequent character interleaving) is block-based and only performed on payload (payload). Each substream from demultiplexer 110 is processed by a separate bit interleaver using an interleaving sequence, which is defined separately for each bit interleaver. The substream b _0,0 , b _0,1 , ... is transmitted to the first bit interleaver I0. The substream b _1.0 , b _1.1 , ... is transmitted to the second bit interleaver I1. The substream b _2.0 , b _2.1 , ... is transmitted to the third bit interleaver I2, and the substream b _3.0 , b _3.1 , ... is transmitted to the fourth bit interleaver I3. The used block size in bit interleaving is 126 bits. Therefore, each substream b _0.0 , b _0.1 , ..., b _1.0 , b _1.1 , ..., b _2.0 , b _2.1 , ... and b _3.0 , b _3.1 , ... consists of 126 bits.

The first bit interleaver I0 generates an output bitstream of a _0.0 , and _0.1 , .... The second bit interleaver I1 generates an output bit stream of _1.0 , and _1.1 , .... The third bit interleaver I2 produces an output bit stream a _2.0 , a _2.1 , ..., and the fourth bit interleaver I3 produces an output bit stream a _3.0 , and _3.1 , .... Each of the output bit streams a _0.0 , a _0,1 , ..., а _1,0 , а _1,1 , ..., а _2,0 , а _2,1 , ... and а _3,0 , а _3,1 , ... consists of 126 bits.

The output bit streams from v bit interleavers (v = 4 in this example) are transferred to the symbol interleaver 130. The output bit streams are grouped to form data words so that each data word from v bits (v = 4) has one bit from each bit interleaver . In this case, 126 data words are formed, each consisting of v bits. The character interleaver 130 interleaves these data words. It should be noted that in a symbol interleaver, bits of data words are not interleaved, but data words are interleaved. In 2K mode, as defined in the standard, 12 groups of 126 data words (12 · 126 = 1512) are interleaved for the purpose of mapping them onto 1512 active carriers of one OFDM symbol of 2K mode. Accordingly, the block size of the symbol interleaver 2K is 1512 data words. Similarly, in 8K mode, 48 groups of 126 data words (48 · 126 = 6048) are interleaved for the purpose of displaying them on 6048 active carriers of a single 8K OFDM symbol. Accordingly, the block size of the symbol interleaver 8K is 6048 data words. Depending on the implementation, the symbol interleavers of various modes (2K mode, 8K mode) can be implemented as separate components of the symbol interleaver, or they can be integrated into a single "combined" symbol interleaver.

The number of data elements, such as data words, as described above, which are interleaved in a symbol interleaver, and the number of active carriers onto which interleaved data elements are mapped for transmission, are integers in an embodiment of the invention, one of which is a multiple of the other. In another embodiment, the number of data elements that are interleaved in the symbol interleaver and the number of active carriers onto which the interleaved data elements are mapped are even integers, one of which is a multiple of the other. The number of data elements, i.e. the block size, with symbolic interleaving, can be more or less than the number of active carriers in this mode, which are used to transfer data and onto which the interleaved data elements are mapped.

The character interleaver generates an output stream Y ₀ , Y ₁ , ..., where Y ₀ represents a sequence of 1512 (in 2K mode) or 6048 (in 8K mode) data words belonging to the first OFDM symbol 2K or 8K. Accordingly, Y ₁ represents the interleaved data words of the second OFDM symbol.

Figure 2 shows the principle of symbol interleaving standard DVB-T. It should be noted that for clarity, FIG. 2 shows only a simplified case. Figure 2 is divided into two parts: the upper part shows data words S _{n, k} before symbolic interleaving, and the lower part shows data words S _{n, k} after symbolic interleaving. The index n indicates the sequence number of the OFDM symbol in question, and the index k indicates the sequential order of the data words before symbol interleaving. For example, S _0.5 indicates the sixth (index numbering starts at 0) data word belonging to the first OFDM symbol.

As described previously, in the actual case, the number of data words that fall into one OFDM symbol is 1512 (2K mode) or 6048 (8K mode). However, in the simplified case of FIG. 2, the number shown is only 16. This is done for technical reasons.

Words S _0,0 -S _0,15 data belonging to the first OFDM symbol are interleaved with each other. Likewise, data words S _1.0 -S _1.15 belonging to the second OFDM symbol are interleaved with each other, and so on. The lower part of FIG. 2 shows an exemplary interleaving result. It can be seen that the depth of interleaving of symbolic interleaving according to the DVB-T standard is equal to one OFDM symbol, since data words belonging to one OFDM symbol are interleaved only within the region of this one OFDM symbol. Since the used block size is exactly the same as the number of active carriers that correspond to one OFDM symbol, no intersymbol interleaving is done here between data words belonging to different OFDM symbols.

According to a preferred embodiment of the invention, the depth of interleaving varies. Three preferred embodiments are described below. In the first embodiment, 8K symbol interleaving is used in conjunction with the proposed 4K mode. In the second embodiment, 2K symbol interleaving is used in conjunction with the proposed 4K mode. In the third embodiment, 8K symbol interleaving is used in conjunction with 2K mode.

FIG. 3 shows symbol interleaving according to a first preferred embodiment of the invention. It should be noted that again for clarity, FIG. 3 shows only a simplified case. Figure 3 is divided into two parts: the upper part shows data words S _{n, k} before symbolic interleaving, and the lower part shows data words S _{n, k} after symbolic interleaving. Index n indicates the sequence number of the OFDM symbol under consideration, and index k indicates the sequential order of data words before symbol interleaving (index numbering starts at 0). It should also be noted that although the upper part of FIG. 3 (and the subsequent FIGS. 4 and 5) already refer to OFDM symbols and data words belonging to OFDM symbols, OFDM symbols are actually generated only later in the transmitter. However, for clarity, the term “OFDM symbol” has already been used in this context. More specifically, the term "data words belonging to one OFDM symbol" really means only the amount of data, that is, the number of data words that ultimately fall into one OFDM symbol.

Figure 3 illustrates the use of 8K symbol interleaver in 4K mode. If 4K mode is implemented similarly to the existing 2K and 8K modes, then the number of carriers in 4K mode is 3024. Accordingly, the number of data words that fall into one OFDM 4K mode symbol is also 3024. This is exactly half the number of data words that fall into one 8K mode OFDM symbol, and twice the number of data words that fall into one 2K mode OFDM symbol. However, in the simplified case of FIG. 3, the shown number of data words that fall into one OFDM symbol of 4K mode is only 8. This is done for technical reasons.

Now that the 8K symbol interleaver (whose block size 6048 is twice the number of data words that fall into one 4K mode OFDM symbol) is used in 4K mode, the doubled number of data words that fall into one 4K mode OFDM symbol is interleaved at a time according to 8K symbol interleaver rules. In terms of FIG. 3, the first 16 words S _0,0 -S _{1,7 of the} data are interleaved. Of these data words, the first eight data words S _0,0 -S _0,7 belong to the first OFDM 4K symbol, and the next eight characters of data words S _1,0 -S _1,7 belong to the second OFDM 4K symbol. Similarly, data words S _2.0 -S _3.7 belonging to the third and fourth OFDM symbols are interleaved with each other, and so on.

The lower part of FIG. 3 shows an exemplary interleaving result. It can be seen that the symbol interleaving depth according to the first preferred embodiment is equal to two OFDM symbols (4K mode), since data words that would fall into one 4K mode OFDM symbol are interleaved in the region of two OFDM symbols (4K mode). Accordingly, when using the 8K mode symbol interleaver (whose block size 6048 is twice the number of data words that fall into one 4K mode OFDM symbol) in 4K mode, intersymbol interleaving occurs between data words belonging to two adjacent 4K OFDM symbols.

The fact that symbol interleaving covers more than one OFDM symbol improves the system's ability to cope with pulse-like interference and unexpected changes (dynamics) in the channels. Also, using an existing 8K mode symbol interleaver in 4K mode dispenses with the need to define a new symbol interleaver for 4K mode. This will save space both in the DVB-T transmitter, and especially in many DVB-T receivers, since no new circuit is needed for an additional interleaver (or deinterleaver).

4 shows a symbolic interleaving according to a second preferred embodiment of the invention. It should be noted that again for clarity, FIG. 4 shows only a simplified case. 4 is divided into two parts: the upper part shows data words S _{n, k} before symbolic interleaving, and the lower part shows data words S _{n, k} after symbolic interleaving. Index n indicates the sequence number of the OFDM symbol under consideration, and index k indicates the sequential order of data words before symbol interleaving (index numbering starts at 0).

Figure 4 illustrates the use of 2K symbol interleaver in 4K mode. As described previously, the number of data words that fall into one 4K OFDM symbol is equal to 3024. However, in the simplified case of FIG. 4, the number of data words that fall into one 4K mode OFDM symbol is only 8. This is done by drawing -technical reasons.

Now that a 2K character interleaver (whose block size 1512 is half the number of data words that fall into one 4K mode OFDM symbol) is used in 4K mode, half of the number of data words that fall into one 4K mode OFDM symbol is interleaved at a time according to the rules of 2K symbol interleaver. In terms of FIG. 4, the first 4 words S _0,0 -S _{0,3 of the} data are interleaved. Similarly, the next 4 words S _0,4 -S _{0,7 of} data are interleaved with each other, and so on.

The lower part of FIG. 4 shows an exemplary interleaving result. It can be seen that the symbol interleaving depth according to the second preferred embodiment is equal to half the OFDM symbol (4K mode), since the first half and second half of the data words belonging to the same 4K mode OFDM symbol are interleaved independently.

Using a character mode interleaver of 2K mode in 4K mode compared to using a character interleaver of mode 8K in 4K mode has advantages related to inverted character interleaving in a DVB-T receiver. Although the reversed symbol interleaving in the DVB-T receiver in conjunction with the use of the 8K mode symbol interleaver in 4K mode can only begin at the beginning of any second received OFDM 4K symbol (due to the 8K mode symbol interleaver block size being twice the number of data words, which fall into a single 4K mode OFDM symbol), symbol interleaving in conjunction using a 2K mode symbol interleaver in 4K mode may begin at the beginning of any received 4K OFDM symbol. In this case, using 2K mode interleaving in 4K mode provides a shorter delay.

5 shows symbol interleaving according to a third preferred embodiment of the invention. It should be noted that again for clarity, FIG. 5 shows only a simplified case. 5 is divided into two parts: the upper part shows data words S _{n, k} before symbolic interleaving, and the lower part shows data words S _{n, k} after symbolic interleaving. Index n indicates the sequence number of the OFDM symbol under consideration, and index k indicates the sequential order of data words before symbol interleaving (index numbering starts at 0).

5 illustrates an embodiment using the 8K symbol interleaver in 2K mode. As described previously, the number of data words that fall into one OFDM symbol of mode 2K is 1512. However, in the simplified case of Fig. 5, the number of data words that fall into one OFDM symbol of mode 2K is only 4. This is done according to the drawing technical reasons.

Now, when the 8K symbol interleaver (block size 6048 of which is the quadruple number of data words that fall into one 2K OFDM symbol) is used in 2K mode, the quadruple number of data words that fall into one 2K mode OFDM symbol is interleaved at a time according to 8K symbol interleaver rules. In terms of FIG. 5, the first 16 words S _0,0 -S _3,3 data are interleaved. Of these data words, the first four data words S _0,0 -S _0,3 belong to the first OFDM 2K symbol, the next four data words S _1,0 -S _1,3 data belong to the second OFDM 2K symbol, and the next four words S _4,0 -S _7.3 data belongs to the third OFDM symbol of 2K mode, and so on. Similarly, data words S _4.0 -S _4.7 belonging to the fifth, sixth, seventh and eighth OFDM symbols are interleaved with each other, and so on.

The lower part of FIG. 5 shows an exemplary interleaving result. It can be seen that the symbol interleaving depth according to the third preferred embodiment is equal to four 4K mode OFDM symbols, since data words belonging to one 2K mode OFDM symbol are interleaved in the region of four 2K mode OFDM symbols. Accordingly, when using the 8K mode symbol interleaver in 2K mode, intersymbol interleaving is performed between data words belonging to four adjacent OFDM mode 2K symbols.

As described in conjunction with the first preferred embodiment, the fact that symbol interleaving spans more than one OFDM symbol improves the system's ability to cope with pulse-like interference and unexpected changes (e.g., fading) in the channels. The use of the 8K mode symbol interleaver in 2K mode in this way contributes to the resistance to mostly weak impulse noise of 2K mode. This type of use can be useful in mobile systems, and therefore, the high mobility of the 2K mode can now be combined with better resistance to impulse noise of the 8K mode.

6 shows blocks of a DVB-T transmitter 600 suitable for implementing the invention. The DVB-T transmitter as such is known to those skilled in the art. Those skilled in the art also know that the DVB-T transmitter may contain other units than those shown in FIG. 6.

The DVB-T transmitter 600 includes an external encoder 610 that provides Reed-Solomon coding for the digital data to be transmitted. An external interleaver 620 convolutionally interleaves on digital data. The internal encoder 630 encodes the digital data using a perforated convolutional code. The operation of the internal interleaver 100 with respect to the present invention is described above. The internal interleaver 100 corresponds to the internal interleaver of FIG. 1 and comprises a demultiplexer 110, a plurality of bit interleavers, and a set of symbol interleavers 130. The symbol interleaver selector 150 is for selecting one of a set of symbol interleavers. This symbol interleaver selector 150 is connected to the internal interleaver. The display means 640 modulates the carriers according to the selected QAM signal constellation (constellation of signals). In a preferred embodiment of the invention, a mode that relates to the number of active carriers is selected using mode selection means 645. The frame adaptation unit 650 organizes the transmitted signal into frames and adds pilot signals (isolated and continuous) to the frames, as well as TPS carriers (transmission parameter alarms), which it receives from block 655. OFDM modulator 660 performs, among other things, the inverse fast Fourier transform ( OBPF) in order to convert the transmitted signal from the frequency to the time domain. A guard interval introducer 670 introduces a guard interval at the beginning of each OFDM symbol. The digital-to-analog converter 680 converts the transmitted signal from digital to analog. The external interface 690 takes care of transmitting the DVB-T signal through the antenna.

The DVB-T transmitter 600 operates in more than one mode of operation (i.e., 2K mode, 4K mode, or 8K mode) corresponding to the number of active carriers (i.e., 1512, 3024, or 6048 active carriers). The DVB-T transmitter 600 includes means 645 for selecting an operating mode from among the available operating modes (the number of available modes may be one or more) and means 150 for selecting one of the available symbol interleavers 130 (i.e., for example, a 2K, 4K, or 8K symbol interleaver ) The transmitter 600 may also include means for incorporating an indication of the selected symbol interleaver into the transmitted signal of the transmitter 600. The selection of the symbol interleaver 130 from the available symbol interleavers in the internal interleaver 100 may be made so that the selected symbol interleaver is different from the symbol interleaver associated with the selected mode of operation. In this embodiment, the operation mode, which is understood as referring to the number of active carriers, and the block size defining the number of data words to be interleaved at a time, may thus differ from each other.

It should be noted that regardless of the character interleaver used in any mode, the rest of the DVB-T transmitter works as required by the mode in question. Therefore, for example, when the 8K symbol interleaver is used in 4K mode, the inverse fast Fourier transform is nevertheless performed at a time with respect to the number of data words for which it is usually performed in 4K mode. It should also be noted that the used character interleaver does not affect bitwise interleaving, that is, bitwise interleaving is performed in the same way regardless of the character interleaver used.

The digital data transmitted by the DVB-T transmitter may be, in particular, a decoded broadcast digital television signal MPEG-2, an audio signal, a signal from a broadcast system, such as an Internet Protocol broadcast system, or a combination thereof.

FIG. 7 shows an embodiment of blocks of a DVB-T receiver 700 suitable for implementing the invention. The DVB-T receiver as such is known to those skilled in the art. Those skilled in the art will also know that DVB-T receiver 700 may contain units other than those shown in FIG. 7.

DVB-T receiver 700 includes an external interface 710 that receives a transmitted DVB-T signal through an antenna. An analog-to-digital converter 780 converts the received signal from analog to digital. The OFDM demodulator 760 performs, among other things, fast Fourier transform (FFT) in order to convert the received signal from the time to the frequency domain. The frame demultiplexer 750 demultiplexes the received OFDM frames. The inverse display means 740 performs an inverse operation with respect to the display device 640 of the DVB-T transmitter 600. The internal deinterleaver 200 performs an inverse operation with respect to the internal interleaver 100. Accordingly, the internal deinterleaver 200 comprises a set of inverted symbol interleavers, followed by inverted bit interleavers. The control unit 701 is connected to an external interface 790, an analog-to-digital converter 780, an OFDM demodulator 760, a frame demultiplexer 750, an inverse display device 740, and an internal deinterleaver 200 for time and frequency synchronization, general phase error correction, channel estimation, and reliability estimation. In one embodiment, the control unit 701 receives information from other units 790, 780, 760, 750, 740 and 200 connected to it, and generates control signals based on the received information to control data processing in the other units. These operations include recognizing the interleaver used in the transmitted signal and selecting the appropriate inverted interleaver. In this embodiment, the control unit 701 thus acts as a means for recognizing the used interleaver in the transmitter and means for selecting one of the available inverted symbol interleavers for the inverted symbol interleaving, the selection being based on said indication of the selected symbol interleaver at the transmitting end, which A 600 DVB-T transmitter is embedded in the signal that the DVB-T receiver receives. The internal decoder 730, the external deinterleaver 720, and the external decoder 710 perform the operation inverse to the operation of the respective units 630, 620 and 610 of the transmitter 600. Before the transmitted digital data in its original form is received, the received signal is usually further processed in blocks not shown in FIG. 7. These blocks may include descrambling, encoding video, audio and / or data.

The receiver may, for example, be a stationary DVB-T receiver installed in a set-top box, or a mobile DVB-T receiver integrated into a mobile portable device, such as, for example, a portable mobile phone. In addition to broadband reception, the receiver may, in particular, have a return channel through a cellular radio network, such as a GSM, GPRS, WLAN, UMTS or ALL IP network. Alternatively or in addition to this, it can have a fixed return channel using DECT technology or a fixed telephone line.

In one embodiment of the invention, as illustrated in FIG. 9, the portable mobile phone 800 has a DVB-T receiver 802 for receiving OFDM and a second transceiver 801 for conventional mobile communications. The portable mobile telephone may also comprise a display 810. The DVB-T receiver in the portable mobile telephone comprises means for recognizing an interleaver used in transmitting the OFDM signal, a set of deinterleaved interleavers, and means for selecting a deinterleaver based on the recognized interleaver.

As described above in connection with embodiments of the invention, an internal interleaver (especially a character interleaver) different from the interleaver originally designed for the operating mode (2K, 4K or 8K) can be used. An embodiment of the invention provides for selecting one operating mode from a plurality of available modes and selecting one symbol interleaver (or an internal interleaver containing a symbol interleaver) from a plurality of available symbol interleavers for use in the selected mode. In other words, this embodiment provides symbolic interleavers with any operating modes, thereby providing different interleaving depths. The following table shows alternatives. The table can be expanded if any other mode is defined, such as 1K or 16K.

Mode of operation Character interleaver Interleaving Depth 8K 8K One OFDM Character 8K 4K The character is interleaved twice 8K 2K The character is interleaved four times 4K 8K Two OFDM Symbols 4K 4K One OFDM Character 4K 2K The character is interleaved twice 2K 8K Four OFDM Symbols 2K 4K Two OFDM Symbols 2K 2K One OFDM Character

In the DVB-T receiver, symbolic and bitwise deinterleaving is performed in the internal deinterleaver 200 (FIG. 7).

If a character interleaver is used that is different from that specifically designed for a particular mode, information about the used interleaver can be transmitted from the transmitter to the receiver so that the receiver can correctly deinterlace the transmitted OFDM symbols.

One possibility is to indicate the used character interleaver with one or more TPS bits. In this alternative embodiment, the symbol interleaver selection is encoded with one or more bits, and these bits form part of the TPS bits. In the transmitter (FIG. 6), blocks 650 and 655 set these bits according to the used symbol interleaver. At the receiver (FIG. 7), the control unit 701 and the internal deinterleaver respectively interpret the received bits.

Another possibility is to indicate the used character interleaver at the application level (or at least at a higher protocol level in the protocol stack than the physical layer).

In a system in which the signaling of the used interleaving mode (symbol interleaver) is not implemented at all, the receiver can, according to one embodiment of the invention, determine the used interleaving mode by trying some mode of reverse interleaving (for example, 8K mode interleaving) and measuring the bit error rate. If the received bit error rate is too high, the receiver tries another reverse interleaving mode until the correct mode is found.

Next, the receiver should obtain the location information of the interleaved symbols. However, due to the fact that the well-known TPS synchronization is in any case performed at the beginning of the connection, this will also provide synchronization for the internal deinterleaver.

It should be noted that although only the use of symbolic interleavers of the 2K, 4K, and 8K modes is described, the application is not limited to only these modes. The invention can also be used in the context of other possible operating modes, such as, for example, 1K mode, 0.5K mode or 16K mode.

Specific implementations and embodiments of the invention are described. Although the DVB-T system is used as an example of an OFDM-based system in this description, the invention can also be applied to other OFDM-based systems, such as Japanese Integrated Digital Services Terrestrial Broadcasting System (ISDB-T). It should also be noted that in one embodiment of the invention, a transmitter that has only one operating mode but more than one symbol interleaver can be used. A suitable character interleaver may be selected for this mode depending on the desired interleaving depth.

One skilled in the art will appreciate that the invention is not limited to the details of the above embodiments, but that it can be implemented for other embodiments using equivalent means without deviating from the characteristics of the invention. The scope of the invention is limited only by the attached claims.

Claims (27)

1. A method for exchanging digital data using a transmission system (10) based on orthogonal frequency division multiplexing (OFDM), including at least one transmitter (600, 601) and receivers (700, 701), comprising stages at which choose a mode of operation in the transmitter from at least one mode, and each mode of operation is associated with the number of active carriers for transmitting useful data; selecting a symbol interleaver in a transmitter from a set of symbol interleavers for symbol interleaving in said selected mode of operation; apply symbolic interleaving in the transmitter in relation to blocks of data elements; displaying interleaved data elements on active carriers of said selected mode of operation; receiving interleaved data elements at the receiver; recognizing at the receiver a symbol interleaver used in data transmission; select a deinterleaver in the receiver to match the recognized symbol interleaver and perform in the receiver the reverse interleaving of the received data elements using the selected inverse interleaver. 2. The method according to claim 1, in which the number of data elements in the block in respect of which symbol interleaving is applied differs from the number of active carriers in said selected mode. 3. The method according to claim 2, in which the number of data elements in the block and the number of active carriers in said selected mode are integers, one of which is a multiple of the other. 4. The method according to claim 3, in which the number of data elements in the block and the number of active carriers in said selected mode are even integers, one of which is a multiple of the other. 5. The method according to any one of claims 2 to 4, in which the number of data elements in the block is greater than the number of active carriers. 6. The method according to claim 5, in which the number of data elements in the block is equal to twice or a multiple of twice the number of active carriers. 7. The method according to any one of claims 2 to 4, in which the number of data elements in the block is less than the number of active carriers. 8. The method according to claim 7, in which the number of active carriers is equal to twice or a multiple of twice the number of data elements in the block. 9. The method according to claim 1 or 2, in which the choice of character interleaver for character interleaving in the selected mode of operation is based on the desired depth of interleaving. 10. The method according to claim 1 or 2, wherein said set of character interleavers comprises at least a character interleaver 8K and a character interleaver 2K and at least a 4K mode of operation can be selected for a DVB-T (terrestrial digital video broadcasting) system. 11. The method according to claim 1 or 2, wherein said set of symbol interleavers comprises at least a symbol interleaver 8K and at least a 2K mode of operation can be selected for a DVB-T system. 12. The method of claim 1, wherein the data elements are data elements of one or more OFDM symbols. 13. The method according to claim 1, in which the digital data transmission system is one of the following systems: DVB-T (terrestrial digital video broadcasting) system, ISDB-T (terrestrial broadcasting system with integrated digital services). 14. The method according to claim 2, in which the data elements form part of either a broadband digital television transmission or broadcast digital data. 15. A transmitter (600, 601) for exchanging digital data using a transmission system based on orthogonal frequency division multiplexing (OFDM), the system having a set of operating modes and said set containing at least one operating mode, and each mode is associated with a predetermined number of active carriers used to transmit useful data from the transmitter to the receiver (700, 701), the transmitter comprising a set of character interleavers (130) for character interleaving, means for selecting an operating mode for transmitting data and means for selecting a symbol interleaver from said set of symbol interleavers for symbol interleaving in said selected mode of operation. 16. The transmitter of claim 15, wherein said set of symbolic interleavers (130) forms part of the internal interleaver (100) of the transmitter. 17. The transmitter according to any one of paragraphs.15 and 16, in which the transmitter (600) is configured to transmit information indicating said selected symbol interleaver for the OFDM receiver (700). 18. The transmitter of claim 17, wherein one or more TPS (Transmission Parameter Signaling) bits are used to transmit said information indicating said selected symbol interleaver. 19. A receiver (700) for exchanging digital data using a transmission system based on orthogonal frequency division multiplexing (OFDM), the system having a set of operating modes and said set containing at least one operating mode, and each mode associated with a predetermined number of active carriers used for transmitting useful data from the transmitter (600) to the receiver, and this system further comprises a set of symbol interleavers (130) used for symbol interleaving in the transmitter, and the receiver contains means for receiving interleaved data elements; means for recognizing the symbol interleaver used in data transmission; a set of symbolic deinterleavers for deinterleaving received data elements that are interleaved in a transmitter in a symbol interleaver, and means for selecting an inverted symbol interleaver from said set of inverted symbol interleavers corresponding to the recognized symbol interleaver. 20. The receiver according to claim 19, in which the receiver (700) is configured to receive information indicating the used character interleaver. 21. The receiver of claim 19, wherein the output of the means for recognizing the symbol interleaver used in data transmission is information indicative of the recognized symbol interleaver. 22. The receiver according to any one of paragraphs.19-21, in which the receiver (700) is either a stationary receiver or a mobile receiver. 23. The receiver according to any one of paragraphs.19-21, in which the receiver (700) contains means for the return channel through a cellular radio network and / or through a fixed network. 24. A digital data exchange system using a transmission system based on orthogonal frequency division multiplexing (OFDM), comprising at least one transmitter (600, 601) and a plurality of receivers (700, 701), and this system has a set operating modes and said set contains at least one operating mode, and each mode is associated with a predetermined number of active carriers used to transmit useful data from the transmitter to at least one receiver, the transmitter having a set of symbol interleavers (130) used for symbol interleaving with respect to the data element blocks in the transmitter, and said at least one receiver has a set of inverted symbol interleavers for reverse interleaving of the interleaved data elements in the receiver, the system further comprising means in the transmitter for selecting a mode of operation to be used in transmitting data; means in the transmitter for selecting a symbol interleaver for symbol interleaving in a selected mode of operation; means in the transmitter for applying symbolic interleaving to blocks of data elements; means in said at least one receiver for receiving transmitted interleaved data elements; means in said at least one receiver for recognizing a symbol interleaver used in data transmission; means in said at least one receiver for selecting an inverted symbol interleaver from a set of inverted symbol interleavers corresponding to the recognized symbol interleaver. 25. The system according to paragraph 24, in which the ratio between the number of active carriers in different modes of operation is an integer. 26. The system according to paragraph 24, in which the ratio between the number of active carriers in different modes of operation is two or a multiple of two. 27. The system according to any one of paragraphs.24 and 25, in which the number of symbolic interleavers in said set of symbolic interleavers is less than the number of operating modes of the system.

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