Title of Invention

A METHOD FOR CODED MODULATION OF DIGITAL DATA AND A TRANSMITTER FOR SAID METHOD

Abstract

A METHOD FOR CODED MODULATION OF DIGITAL DATA AND A TRANSMITTER FOR SAID METHOD" Method for coded modulation Abstract (IN/PCT/2002/01282/CHE) A method is proposed for coded modulation of digital data, with the user bits being split into parallel signal streams, and the user bits then being channel-coded at different code rates depending on their error sensitivity. The user bits, which are thus differently channel-coded, are then scrambled separately from one another in time. In this case, the different code rates that are used and the respective number per transmission frame of the user bits, which are channel-coded at the different code rates, are signalled to the receivers, so that the receiver is able to carry out the descrambling and channel decoding. (Figure 2)

Full Text

Method for coded modulation Prior art The invention is based on a method for coded modulation according to the generic part of the independent patent claim. It is already known for coded modulation to be used, in which the channel coding and modulation are optimized jointly. The expression multilevel coding is also known, and is equivalent to the term coded modulation. Source coding is carried out for data reduction prior to the coded modulation. Advantages of the invention The method according to the invention for coded modulation and having the features of the independent patent claim in contrast has the advantage that nonuniform error protection is used for coded modulation, with different code rates being used in this case for user bits with different error sensitivity. The code rate and the number of user bits to be coded can in this case be chosen independently of one another. The method according to the invention allows simpler implementation, overall. The measures and developments described in the dependent claims allow advantageous improvements of the method, specified in the independent patent claim, for coded modulation. It is particularly advantageous for the user bits, which are in each case channel-coded at different code rates, to be distributed between parallel scramblers for time scrambling, in order to then once again be joined together, by means of a multiplexer, to form a signal stream. This advantageously also results in only those user bits which have been channel-coded at the same code rate being scrambled with one another. The channel decoding and the descrambling, which precedes the channel decoding, in the receiver are thus correspondingly simple. Alternatively, it is advantageous to use a scrambler per stage after the coder, which scrambler scrambles the user bits, which are channel-coded at different code rates, successively and separately in time. A scrambler is thus used which has the necessary intelligence in order to subdivide the user bits, which are channel-coded at the different code rates, into corresponding groups, and then to scramble each of these groups in time with one another. It is furthermore advantageous for the at least two differently used code rates for channel coding and the respective number of user bits which are channel-coded at the different code rates to be signalled to the receiver, to a receiver [sic] . It is thus clear to the receiver the code rates with which the differently error-sensitive user bits have been channel-coded and what the number of differently coded user bits is, in order to carry out appropriate descrambling and channel coding correctly. It is furthermore advantageous for both a transmitter and a receiver to have means for carrying out the method according to the invention. Drawing Exemplary embodiments of the invention are illustrated in the drawing, and will be explained in more detail in the following description. Figure 1 shows partitioning of a 4-ASK, Figure 2 shows a block diagram of the transmitter according to the invention. Figure 3 shows a block diagram of the receiver according to the invention, and Figure 4 shows a flowchart of the method according to the invention. Description Digital Radio Mondiale (DRM) digital transmission is currently being developed for transmission bands below 30 MHz. It has been decided that multilevel coding (MLC) will be used for the channel coding. In this case, the channel coding and the modulation are optimized jointly, so that this is also referred to as coded modulation. Channel coding adds redundancy to the data, on the basis of which transmission errors can be identified and, possibly, corrected. For a higher-level modulation method with a q-nary signal constellation, the signal alphabet has q, and only q, values. The basis for MLC is formed by the partitioning of the signal alphabet into subsets. Each division step is assigned a component of the address vector in the signal space representation. In this case, each component is protected by a dedicated code. If one considers a 2'"-stage signal constellation, this results in splitting into n stages, if m = n, corresponding to the address vector c (= CQ, Ci, ..., Cn - 1). The stepping m of the modulation is, for example, not necessarily equal to the number of steps, if 64-QAM (quadrature amplitude modulation) is used. Figure 1 shows the partitioning of 4-ASK (amplitude shift keying). Four states are thus coded in 4-ASK. The data stream is coded using n parallel coders, with the code Co having the lowest code rate RQ, that is to say-adding the most redundancy and protecting that position in the address vector which is most susceptible to errors. Four states are marked by solid circles on the uppermost state line in Figure 1. The individual codeable states for 4-ASK are then given by the two central state lines. The first stage is either CQ = 0 or 1. In a corresponding way, the four solid circles are distributed between two numerical lines, which have mutually complementary solid and empty circles. The individual states for 4-ASK, namely 00, 01, 10 and 11, are now coded for the four lower state lines. In this case, the state 00 has a solid circle on the extreme left, which is then followed by three empty circles. Starting from the left, the state 01 has the solid circle in third place. Starting from the left, the state 10 has the solid circle in second place, and the state 11 has the solid circle on the extreme right. The other positions are symbolized by empty circles for a 0. Figure 2 shows a block diagram of the transmitter according to the invention. The data memory 1 contains data which is to be transmitted by the transmitter according to the invention. However, other data sources can also be used here. This data is transmitted from the data memory 1 to a source coder 2, which carries out source coding in order to reduce the amount of data to be transmitted. The data source-coded in this way is then transmitted with the user bits to a bit multiplexer 3, which distributes the data stream between n parallel lines. Each of these n lines, which are numbered successively from 0 to n-1, is connected to a respective coder, which channel-codes one of the data streams (qo...qn-i)- By way of example, a coder 5 is shown here in the line 0, and a coder 4 in the line n-1. The signals CQ and Cn-i are produced at the output of the respective coders. The coders 4 and 5 use convolution coding to carry out the channel coding, and thus add further redundancy to the user bits. In this case, the coders 4 and 5 will use two different code rates. The source coder 2 has identified those user bits which require first error protection, and the remaining user bits which require second error protection. The channel coders 4 and 5 then each use a different code rate for this purpose. The user bits are thus distributed among the stages in accordance with predetermined rules in this case, without the user bits being evaluated for this purpose. The user bits are then first of all distributed between the stages and the coders 4 and 5 which are channel-coded at the first code rate, and this is followed by the user bits which are channel-coded at the second code rate. The code rate for all the coders that are present is then switched for this purpose. Since differently channel-coded user bits are now produced at the output of the coders 4 and 5, these user bits are scrambled separately in time in the downstream scrambler. A multiplexer 61 following the coder 5, and a multiplexer 51 following the coder 4, distribute the differently coded user bits between respective scramblers 62 and 63, as well as 52 and 53. The scramblers are also referred to as interleavers. Scrambling means that bits which are close to one another in time and have information with a high correlation level with adjacent bits are separated from one another in time so that it is still possible to correct a sufficient number of errors by means of the channel decoding at the receiving end, in the event of a burst error, so that the number of audible errors is minimized. The scrambled user bits originating from the scramblers 52 and 53 are then joined together once again in the demultiplexer 54 to form a signal stream. The scrambled user bits from the scramblers 62 and 63 are joined together once again in an analogous manner in the demultiplexer 64 to form a signal stream. The data which has been channel-coded in this way is then allocated, in block 6, to signal space points, in order then to produce the respective modulation symbols. Convolution codes with puncturing are used as the component codes in the individual coders 4 and 5. In the case of MLC, at least two different code rates are used for each stage here. After the modulation in the function block 6, signalling data is added to the signal stream, signalling to a receiver which different code rates have been used for channel coding, and how many bits per frame have been channel-coded at the first code rate, and how many bits have been channel-coded at the second code rate. It is then possible for a receiver to carry out correct demodulation and descrambling, as well as channel decoding. The modulation symbols coded in this way in Figure 2 are then transmitted from the function block 6 to an OFDM (Orthogonal Frequency Division Multiplex) modulator 7, which distributes the individual modulation symbols between frequency carriers which are close to one another but are orthogonal to one another. The OFDM signals produced in this way are then mixed in an analogue radio-frequency section 8, are amplified, and are then transmitted using an antenna 9. Figure 3 shows a block diagram of the receiver according to the invention. An antenna 10 for receiving the OFDM signals is connected to one input of a radio-frequency receiving section 11. The radio-frequency receiving section 11 converts the received "signals to an intermediate frequency, amplifies them, and filters them. Furthermore, the radio-frequency receiving section 11 passes these signals to a digital section 12, which digitizes the received signals and carries out OFDM demodulation. The modulation symbols obtained in this way are then demodulated in the processor 13, and are subjected to descrambling, channel decoding and source decoding, in order to convert them to analogue signals. The analogue signals are then amplified by an audio amplifier 14 in order, finally, to reproduce them by means of the loudspeaker 15. In this case, in particular, the signalling data is evaluated, used [sic], which indicates the two code rates used and the number of user bits per frame for one code rate. It is then possible for a receiver to carry out correct descrambling and channel decoding. Instead of two code rates, it is also possible to use a aumber of code rates. Instead of the source coder 2, the bit multiplexer 3 can also decide which bits are -hannel-coded at which code rate. Furthermore, this can also be allocated, as a task, to a higher-level processor, which is arranged in the transmitter according to the invention. Alternatively, it is also possible in this case to receive multimedia data, which is then reproduced visually. F'igure 4 shows the method according to the invention for coded modulation in the form of a flowchart. In nethod step 16, the data is provided from the data aemory 1, and is subjected to source coding by the source coder 2. The source coder 2 then determines \7hich user bits are channel-coded at which code rate. vionuniform error protection is thus defined. In method 3tep 17, the data stream produced in this way is split into parallel-running data streams by means of the bit multiplexer 3. As an alternative in this case, it is possible for the bit multiplexer 3 to split the user bits for the different channel coding. In method step 18, the individual coders 4 and 5 carry out the channel coding. In this case, the user bits are channel-coded at two different code rates, as has been defined by the source coder 2. In method step 19, the user bits, which are thus channel-coded differently, are scrambled in time. In this case, the only user bits which are scrambled with one another in time are those which have also been channel-coded at the same code rate. This may either be done by means of parallel-connected scramblers or by means of an intelligent scrambler, which sequentially scrambles the user bits, which have been channel-coded at the different code rates, separately from one another. In the function block 6 in method step 20, the channel-coded user bits produced in this way are allocated signal space points, in order to produce modulation symbols. In method step 21, the modulation symbols are subjected to OFDM modulation and, in method step 22, the OFDM signals are amplified and transmitted. In addition in this case, the two different code rates for the user bits and the number per transmission frame of the user bits per code rate used [sic] are transmitted by the transmitter as signalling to the receivers, so that the receivers are able to descramble the differently channel-coded user bits, and to channel-decode them. WE CLAIM: 1. Method for coded modulation of digital data with the digital data having user bits: - carrying out the coded modulation in a number of stages - subdividing the user bits on their basis of the error sensitivity - splitting the user bits into parallel signal streams - subjecting each signal stream by a coder to channel coding with at least two different code rates per stage, setting the different code rates as a function of the error sensitivity of the user bits, channel-coding the user bits at different code rates and scrambling those user bits separately in time - allocating the charmel-coded and scrambled user bits to signal space points in order to produce modulation symbols. 2. The method as claimed in claim 1 comprising the step of: - distributing the user bits which are channel-coded at different code rates between parallel scramblers (52, 53, 62, 63) for time scrambling, joining together the time scrambled user bits by means of a demultiplexer (54, 64) to form a signal stream. 3. The method as claimed in claim 1 comprising the step of scrambling the user bits per stage after the coder by a scrambler, the user bits being chaimel-coded at different code rates, successively and separately in time. 4. The method as claimed in any one of the preceding claims comprising the step of signalling the at least two different code rates and a respective number of the user bits, which are charmel-coded at different code rates to a receiver. 5. The transmitter for carrying out the method as claimed in any one of claims 1 to 4, wherein the transmitter has a first multiplexer (3), a coder (4, 5) for channel coding per stage, a scrambling unit (51, 52, 53, 54, 61, 62, 63, 64) per stage and means for allocating the channel-coded user bits to the signal space points (6). 6. The transmitter as claimed in claim 5, wherein the transmitter has a scrambling unit with a scrambler. 7. The transmitter as claimed in claim 5, wherein, per scrambling unit, the transmitter has one second multiplexer (51, 61), one demultiplexer (54, 64) and one scrambler per code rate (52, 53, 62, 63) that is used.

Documents:

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in-pct-2002-1282-che abstract.pdf

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