JP5053302B2 - Digital transmission decoder and receiver - Google Patents

Description

  The present invention relates to a technique for compensating for distortion generated in a satellite transmission path and the like, and more particularly, to a digital transmission decoder and receiver.

  In the digital transmission system, a multi-level modulation system is often used so that more information can be transmitted in the frequency bandwidth available for each service. In order to increase the frequency utilization efficiency, it is necessary to increase the number of bits allocated per modulation signal symbol (the number of modulation levels), but the relationship between the upper limit of the information rate that can be transmitted per frequency 1 Hz and the signal-to-noise ratio is Limited by Shannon limit.

  As an example of a transmission form of information using a satellite transmission path, satellite digital broadcasting can be cited. For example, as illustrated in FIG. 6, the modulated wave signal from the transmission device 100 is transmitted to the reception device 200 via the satellite repeater 300. In such satellite digital broadcasting, the satellite repeater 300 mainly includes an input multiplexer (IMUX) filter, a traveling wave tube amplifier (TWTA), and an output multiplexer (OMUX) filter. Band extraction is performed, gain control is performed by TWTA, and unnecessary frequency components are suppressed by the OMUX filter.

  Thus, TWTA with high power efficiency is often used for the satellite repeater 300 due to hardware limitations. In order to make the most of the limited hardware of the satellite repeater 300, it is desirable to operate the TWTA in the saturation region so that the output of the satellite repeater 300 is maximized. However, since distortion generated in TWTA leads to transmission degradation, phase modulation is often used for transmission signals of transmission apparatus 100 and reception apparatus 200 as a modulation scheme that is resistant to transmission degradation caused by distortion generated in TWTA. .

  Currently, a transmission system called ISDB-S is used as a transmission system for satellite digital broadcasting in Japan, and phase modulation such as BPSK, QPSK, and 8PSK can be used. Moreover, in DVB-S2 which is a European transmission method, a modulation method called amplitude phase modulation (APSK), which further improves the frequency utilization efficiency, has been put into practical use. For example, if 16 APSK, the frequency utilization efficiency is 4 bps / Hz at the maximum, and if 32 APSK, the maximum 5 bps / Hz can be transmitted.

  In satellite digital broadcasting currently used, information correction is performed in a receiving apparatus using an error correction code. By adding a redundant signal called a parity bit to information to be sent, it is possible to control the redundancy (coding rate) of the signal and increase the resistance to noise. Error correction codes and modulation systems are closely related, and the relationship between frequency utilization efficiency and signal-to-noise ratio with redundancy added is defined by the Shannon limit. As one of powerful error correction codes having performance approaching the Shannon limit, an LDPC (Low Density Parity Check) code was proposed by Gallagher in 1962 (see, for example, Non-Patent Document 1).

  The LDPC code is a linear code defined by a very sparse check matrix H (the elements of the check matrix are 0 and 1 and the number of 1 is very small). The LDPC code is a powerful error correction code that can obtain transmission characteristics approaching the Shannon limit by increasing the code length and using an appropriate check matrix. DVB-S2 is a new European satellite broadcasting standard. The LDPC code is also adopted in the broadband wireless access standard IEEE 802.16e. By combining multi-level phase modulation and a powerful error correction code such as an LDPC code, transmission with higher frequency utilization efficiency has become possible.

R.G Gallager, “Low density parity check codes,” in Research Monograph series Cambridge, MIT Press, 1963

  The simulation results of the constellation of the main signal and pilot signal in 32APSK in the conventional transmitter and receiver are shown in FIGS. 13 (a) and 13 (b), respectively. With reference to the constellation of the transmission side main signal in 32APSK (FIG. 13 (a) upper left stage) and the constellation of the transmission side pilot signal (FIG. 13 (b) upper right stage), both constellations having signal points at the same position It is assumed that On the other hand, when the reception side signal point transition of each of the main signal and pilot signal is written in a straight line (the left middle stage and the right middle stage in FIGS. 13 (a) and 13 (b)), the transition between the signal points of the main signal is Although it occurs between all signal points, it can be seen that the pilot signal occurs only between limited signal points. This difference becomes a problem particularly in the APSK modulation method when the influence of distortion under the nonlinear transmission path differs depending on the position of the signal point.

  In the case of APSK, signal points on the circumference having the largest radius are easily affected by distortion caused by the nonlinear transmission path. Assuming the IMUX filter, TWTA, and OMUX filters that make up a general satellite repeater 300, a repeater simulator in which the operating point of TWTA is set to OBO = 3.4 dB is reproduced by computer simulation, and FIG. The constellations when the modulation signals obtained by modulating the main signal shown in the upper left stage and the pilot signal shown in the upper right stage in 13 (b) are respectively input to the lower left stage and the right side in FIGS. 13 (a) and 13 (b). Shown below. From the results shown in the lower left and lower right stages in FIGS. 13 (a) and 13 (b), the constellation of the main signal (the lower left stage in FIG. 13 (a)) has a point on a concentric circle, particularly at the point having the maximum radius. The signal point spreads due to distortion at the center, but in the pilot signal constellation (lower right in FIG. 13B), the signal point spreads due to distortion around a point slightly shifted from the concentric circle. I understand that.

  This constellation shift with respect to the influence of distortion ultimately affects the likelihood calculation performed at the time of LDPC decoding, and causes a deterioration in transmission characteristics. Therefore, as in the energy spreading circuit shown in FIG. 12, which will be described later, when using pilot signal energy spreading by reversing the signs of the I and Q axes, the signal point arrangement obtained by averaging pilot signals is set to the main signal. It is necessary to have some means for correcting so as to converge to a point reflecting the influence of distortion received from the nonlinear transmission path.

  In view of the above problems, an object of the present invention is caused by non-uniformity of signal point transition of a pilot signal when a distortion vector in a transmission path is estimated by a signal obtained by modulating known information called a pilot signal on the receiving side. An object of the present invention is to provide a decoder and a receiving apparatus that improve transmission characteristics by correcting a deviation of a pilot signal point using amplitude and phase information of an ideal signal point.

  The decoder and the receiving apparatus according to the present invention converge the pilot signal point closer to the main signal modulation signal when using the pilot signal subjected to the energy diffusion by the inversion of the sign of the I axis and the Q axis. That is, the decoder and the receiver according to the present invention compare the signal point constellation obtained by averaging the pilot signals (hereinafter referred to as the first signal point constellation) with the ideal signal point defined by the transmission side signal point, The second signal point arrangement corrected by the deviation between the first signal point arrangement and the ideal signal point is obtained, and the second signal point arrangement is used for the likelihood calculation at the time of reception.

  According to an aspect of the present invention, in the decoder and the receiving apparatus, with respect to the amplitude of the second signal point arrangement, the average between signal points of a predetermined amplitude type having the same amplitude at the ideal signal point in the first signal point arrangement. The amplitude is used as the amplitude of the signal point of the same amplitude type. For example, in the case of 32APSK as shown in FIG. 10, signal points having the same amplitude are classified into three types, so that three types of average amplitudes are obtained. By obtaining the average amplitude from the first signal point arrangement, it is possible to suppress the amplitude deviation generated in the first signal point arrangement.

  Furthermore, the decoder and the reception apparatus according to one aspect of the present invention are configured so that the phase at the ideal signal point and the first signal point with respect to the phase of the signal point of the second signal point arrangement are the same as the first signal point arrangement. A phase difference between signal points in the signal point arrangement (hereinafter referred to as a phase deviation) is calculated to obtain a predetermined phase type phase deviation (for example, 32 types of phase deviation in the case of 32APSK). Are grouped into phase / amplitude groups of signal points having the same amplitude (in the case of 32APSK, 32 signal points are classified into three types of phase / amplitude groups having the same amplitude. The types of phase deviations are similarly classified into three types of phase / amplitude groups.) Using the phase deviations included in each phase / amplitude group, the average phase deviation is calculated for each phase / amplitude group. As described above, three types of average amplitude and average phase deviation are obtained.

  Subsequently, the decoder and the reception device according to one aspect of the present invention add the average phase deviation to the phase of the ideal signal point for each signal point belonging to the same phase / amplitude group, and set this value as the second signal point arrangement. Is the phase of the signal point. By generating the likelihood table with the second signal point obtained by adding the average phase deviation to the phase of the ideal signal point in this way, it is possible to suppress the phase deviation caused in the first signal point arrangement. Become.

  That is, the decoder of the present invention is a decoder that corrects a data error in digital transmission using a predetermined likelihood table, and has a period with a predetermined signal point arrangement from a signal demodulated according to a modulation method. Known signal extraction means for extracting a pilot signal to be transmitted in an automatic manner, a first signal point arrangement representing a signal point of the extracted pilot signal, and an ideal signal point arrangement defined by a predetermined transmission side signal point And a deviation calculating means for calculating the deviation between the first signal point arrangement and the ideal signal point arrangement for each symbol, and the first signal point arrangement for generating a likelihood table at the time of decoding. And signal point arrangement correcting means for generating a second signal point arrangement in which the deviation is corrected.

  In the decoder according to the present invention, the deviation includes a deviation in amplitude and phase of the pilot signal.

  Further, in the decoder of the present invention, the signal point arrangement correcting means is an average between signal points of a predetermined amplitude type having the same amplitude according to the modulation method in the ideal signal point arrangement in the first signal point arrangement. Means for determining an amplitude as the amplitude of a signal point of the same amplitude type, and for each signal point of the same amplitude type in the first signal point arrangement, a phase in the ideal signal point arrangement and a signal point in the first signal point arrangement Calculating a phase deviation between the phase difference of the predetermined phase type according to the modulation method, and classifying the phase deviation of the predetermined phase type into a phase / amplitude group of signal points having the same amplitude; Using the phase deviation included in each phase / amplitude group, the average phase deviation is calculated for each phase / amplitude group, and the average phase deviation is ideal for each signal point belonging to the same phase / amplitude group. It was added to the issue point phase, the added value and having a means for correcting the phase of a signal point of the second signal point arrangement in the amplitude.

  The decoder according to the present invention further includes means for generating a likelihood table used for likelihood calculation of decoding according to the second signal point arrangement corrected by the signal point arrangement correcting means.

Furthermore, the decoder of the present invention is characterized in that performs LDPC decoding using the pre Kieu degree table, also characterized as a receiving device including a decoder of the present invention.

  According to the decoder and the receiving apparatus of the present invention, it is possible to transmit information with excellent resistance to a transmission line having distortion.

  Therefore, by generating the likelihood table according to the second signal point arrangement obtained by using the first signal point arrangement, it becomes possible to correct the signal point deviation occurring in the first pilot, It is possible to converge the influence of distortion of the main signal caused by the nonlinear transmission path (that is, mainly the amplifier of the repeater) to the signal point of the signal point arrangement reflecting the distortion on the receiving side.

It is a figure which shows the block diagram of the LDPC decoding part of one Example by this invention. It is a figure which shows the example of an operation | movement flow of the LDPC decoding part of one Example by this invention. It is a figure which shows a response | compatibility with the signal point number in 32APSK of the LDPC decoding part of one Example by this invention, and the same amplitude group number. It is a figure which shows the simulation result of the 1st signal point arrangement | positioning in 32APSK in the LDPC decoding part of one Example by this invention, and a 2nd signal point arrangement | positioning. The computer simulation result of the C / N vs. bit error rate characteristic in 32APSK coding rate 4/5 at the time of using the 1st signal point arrangement | positioning in the LDPC decoding part of one Example by this invention and a 2nd signal point arrangement | positioning is shown. FIG. It is a figure which shows an example of the transmission form of satellite digital broadcasting. It is a figure which shows the structure of the transmission apparatus of an advanced satellite broadcast system. It is a block diagram which shows schematic structure of the conventional receiver of an advanced satellite broadcast system. It is a figure which shows the example of the modulation wave signal format of an advanced satellite broadcast system. It is an example of one transmission form of the pilot signal in 32APSK. It is a figure which shows some LDPC decoding parts of the conventional receiver. It is an example of the energy spreading | diffusion circuit which shows an example of the energy spreading | diffusion part in the conventional transmitter. It is a figure which shows the simulation result of the constellation of the main signal and pilot signal in 32APSK in the conventional transmitter and receiver.

  First, in order to facilitate understanding of the present invention, a schematic configuration of the transmitter 100 and the receiver 200 of the advanced satellite broadcasting system using the LDPC code will be briefly described. For simplification of description, only the portion according to the present invention will be described. However, the advanced satellite broadcasting transmission device 100 and the reception device 200 may replace the LDPC code with another correction code method or use it in combination. Furthermore, interleavers can be used in appropriate combinations.

(Transmitter)
FIG. 7 is a diagram illustrating a configuration of a conventional advanced satellite broadcasting transmission device. The transmission apparatus 100 includes a frame generation unit 110, an LDPC encoding unit 120 (hereinafter also referred to as an encoder), an energy spreading unit 130, a mapping unit 140, and a time division multiplexing / orthogonal modulation unit 150. Prepare. That is, when transmitting a data stream, the transmission apparatus 100 performs a series of processes from generation of a signal of a multiplexed frame of a plurality of slots in FIG. 9 to be described later to generation of a modulated wave signal.

  The frame generation unit 110 functions together with the LDPC encoding unit 120 to generate an LDPC parity. Therefore, the frame generation unit 110 and the LDPC encoding unit 120 generate a frame of a plurality of slots in FIG. The multiplexed frame generated by the frame generation unit 110 is generated so that the number of slots, the amount of dummy, the slot length, the synchronization bit length, the pilot bit length, and the TMCC and parity bit lengths are predetermined numbers. The

  The LDPC encoding unit 120 performs LDPC encoding with a predetermined period on the data and the transmission control signal. The specific method of LDPC encoding is known and is not the subject of this application and will not be described further.

  The energy spreading unit 130 inputs a predetermined number of slots of each multiplex frame, and performs energy spreading (bit randomization) on the entire data.

  The bit randomized signal from the energy spreading unit 130 is switched according to various modulation schemes (BPSK, APSK, etc.) while inserting synchronization and pilot signals as appropriate, and a mapping unit 140 (a plurality of mappings corresponding to the modulation schemes). ).

  The mapping unit 140 performs mapping according to the modulation scheme specified by TMCC synchronization.

  The time division multiplexing / orthogonal modulation unit 150 performs time division multiplexing in units of frames, performs orthogonal modulation, and generates a modulated wave signal.

(Receiver)
FIG. 8 is a block diagram showing a schematic configuration of a conventional receiving apparatus 200 of the advanced satellite broadcasting system. The receiving apparatus 200 includes a channel selection unit 210, an orthogonal detection unit 220, a transmission control signal decoding unit 230, an energy despreading unit 240, and an LDPC decoding unit 250.

  The channel selection unit 210 receives the modulated wave signal from the transmission apparatus 100, selects a channel of a predetermined frequency band, and converts the signal of the channel into a signal of a predetermined frequency handled by the quadrature detection unit 220. For example, if the modulated wave signal is a satellite broadcast wave, a 12 GHz band broadcast wave (modulated wave signal) is received by a BS antenna, and a 1 GHz band BS-IF signal is transmitted using a known frequency converter (not shown). Convert to

  The quadrature detection unit 220 receives a signal (for example, a BS-IF signal) of a predetermined frequency of the channel selected by the channel selection unit 210 and converts it into a synchronous baseband signal.

  The transmission control signal decoding unit 230 receives the synchronous baseband signal converted by the quadrature detection unit 220, first detects a synchronous byte of the TMCC signal, and uses this as a reference to periodically BPSK modulated waves. The position of a certain phase reference burst signal is also detected. Also, TMCC information decoding processing for modulation scheme and error correction information transmitted by the TMCC signal is performed through the energy despreading unit 240. Transmission control information (modulation scheme / error correction TMCC information) decoded by transmission control signal decoding section 230 is input to LDPC decoding section 250.

  The energy despreading unit 240 performs the energy despreading process by adding the same pseudorandom code again by MOD2 in order to restore the process in which the pseudorandom code is added by MOD2 in the energy spreading unit 130 of the transmission apparatus 100.

  The LDPC decoding unit 250 receives the synchronous baseband signal from the quadrature detection unit 220 and also receives the modulation scheme / error correction information detected by the transmission control signal decoding unit 230, and converts the synchronous baseband signal into the LDPC code. The decoding process is performed using the parity check matrix.

  As described above, the advanced satellite broadcasting transmission device 100 and the reception device 200 using the LDPC code can be combined with multi-level phase modulation and a powerful error correction code such as an LDPC code to achieve higher frequency utilization efficiency. Can be transmitted.

  However, although such a strong error correction code such as an LDPC code has an excellent correction capability for white noise, the correction capability for signal deterioration against distortion inherent in a satellite transmission path is not sufficient. In particular, when amplitude phase modulation such as 16APSK or 32APSK is used for a satellite transmission path, signal deterioration due to waveform distortion generated in an amplifier such as TWTA used in the satellite repeater 300 or the earth station is more serious than phase modulation.

  In general, as a method of suppressing waveform distortion generated in an amplifier, a technique of operating the amplifier in a more linear region by lowering the output level from the saturation region is used. However, in this case, transmission degradation due to distortion is reduced, but the output of the satellite repeater 300 is reduced, leading to a reduction in received signals on the ground. Therefore, in order to apply APSK in satellite broadcasting or the like, it is essential to use a transmission method that is resistant to transmission deterioration due to distortion without reducing satellite output as much as possible.

  In the latest satellite digital broadcasting systems such as DVB-S2, a technique (maximum likelihood decoding) for maximizing the posterior probability based on Bayesian theory is used as a decoding method for error correction codes. The posterior probability is obtained by a likelihood function (formula (1)).

  From the definition of the likelihood function, the likelihood function and the Euclidean distance indicating the distance between the received signal and the ideal signal point are closely related. In a transmission path containing only white noise, the received signal point generates a random deviation of a Gaussian distribution according to S / N. In a transmission path including specific distortion other than white noise, Signal point transition with amplitude and phase deviation occurs. In particular, when APSK is input to the non-linear amplifier, the signal points belonging to the concentric circle having the largest amplitude cause a larger signal deviation because of the convenience of transmitting the APSK by combining a plurality of types of concentric circles.

  In addition, http: // www. soumu. go. jp / joho_tsusin / policyreports / joho_tsusin / bunkakai / 080729_1. In response to Advisory No. 2023 “Technical Conditions for Broadcasting Systems” from the Minister of Internal Affairs and Communications, published as “Document 60-1-2 Broadcasting System Committee Report” to html, Advanced Satellite Digital Broadcasting System (referred to as the Advanced Satellite Broadcasting System in this specification) shown in the report “Report of the Information and Communication Council Information and Communication Technology Subcommittee Broadcasting System Committee Report” (July 29, 2008) However, improvement of transmission characteristics for APSK is also considered.

  FIG. 9 shows an example of a modulated wave signal format of the advanced satellite broadcasting system. The modulation wave signal can be changed in transmission parameters including a modulation method and an error correction code coding rate in units of frames. One frame is composed of 120 modulation slots, and can be transmitted by specifying a modulation method and a coding rate for every five modulation slots. In each modulation slot, a 24-symbol synchronization signal, a 32-symbol pilot signal, and a pair of 136-symbol data and a 4-symbol TMCC signal are time-division multiplexed 66 times.

  FIG. 10 is an example of a transmission form of pilot signals in 32APSK, and is a diagram illustrating the correspondence of known transmission bits to transmission symbols whose transmission order is determined in advance. As shown in FIG. 10, in the case of 32APSK, pilot signals are transmitted in order of signal points corresponding to symbols “00000” to “11111”. In FIG. 10 (a), FIG. 10 (b), and FIG. 10 (c), signal points corresponding to the first symbol, the second symbol, and the 32nd symbol are represented by “black circles”, respectively.

  FIG. 11 shows a part of an LDPC decoding unit 250 of a conventionally known receiving apparatus 200 using this pilot signal. In the case of a system that does not include a pilot signal, LDPC decoding is performed on the I and Q signals subjected to quadrature detection while referring to a likelihood table. The LDPC decoding unit 250 averages the pilot signal for each symbol. To obtain the signal point constellation after being affected by nonlinear distortion in the transmission path, and to generate or update the likelihood table based on the obtained signal point constellation, the performance can be improved (same as above). (Refer to Reference Material 1-8 “Improvement of reception characteristics by pilot signal”).

  Specifically, the LDPC decoding unit 250 includes a pilot signal extraction unit 252, a pilot signal averaging processing unit 253, a likelihood table generation unit 254, and a likelihood stored in a predetermined memory (not shown). A table 255 and an LDPC decoder 251 are provided.

  The pilot signal extraction unit 252 inputs the synchronous baseband signal converted by the quadrature detection unit 220, and uses the signal of the synchronization information (pilot timing signal) indicating the pilot timing in the TMCC signal detected in advance, to generate the pilot signal. The position of the signal point is extracted and sequentially transmitted to the pilot signal averaging processing unit 253.

  Pilot signal averaging processing section 253 averages pilot signal signal points for each symbol, and sends this information to likelihood table generation section 254.

  The likelihood table generation unit 254 represents a decoder output log likelihood ratio (LLR) for the signal points of the pilot signal averaged for each symbol, and is used for likelihood calculation in LDPC decoding. 255 is generated and stored in a predetermined memory or updated. The likelihood table 255 is preferably generated individually according to the modulation scheme and coding rate with a predetermined slot length, and information on the modulation scheme and coding rate used to generate the likelihood table 255. Is obtained from a signal (referred to as a modulation scheme / coding rate selection signal) according to TMCC information in the TMCC signal detected in advance.

  Based on the LLR of the likelihood table 255 and the modulation scheme / coding rate selection signal, the LDPC decoder 251 performs LDPC decoding on the information of the synchronous baseband signal converted by the quadrature detection unit 220 and transmits a decoded signal. .

  Note that if such a fixed pattern (that is, a repetition pattern of pilot signal points) is periodically multiplexed, a line spectrum is generated in the frequency component of the modulation signal, so that a pilot signal is also transmitted. Energy diffusion on the side. An energy diffusion circuit showing an example of the energy diffusion unit 130 shown in FIG. 12 includes an addition circuit 261 for repeatedly adding output data sequentially to input data 260 (D1 to D15) and re-inputting to the head, A level conversion circuit 262 that generates a pseudo-random sequence (PN sequence) composed of 0 or 1 and inverts the sign of the I-axis and Q-axis of the signal point of the pilot signal, and the signal point (I, Q) to be spread On the other hand, there are provided multiplication circuits 263 and 264 for transmitting spread output signal points (I ′, Q ′) obtained by multiplying the values subjected to level conversion by the level conversion circuit 262 and performing energy diffusion of pilot signals. For details of such an energy diffusion circuit, refer to section 2.2.8 “Energy diffusion method” in the aforementioned report.

  However, when the energy spread by the PN sequence is used, the generation of the line spectrum can be prevented, but in the pilot signal, each signal point is repeatedly transmitted in a fixed order, so that the positions where the signal points can be shifted are the I axis and the Q axis. There are only two types, the case where the above sign is inverted and the case where it is not inverted, and only a limited pattern can be taken between the signal points. The transition between the limited signal points is shown in FIG.

  In this case, even if the signal points of the pilot signal are averaged for each symbol, only the signal points having a limited transition are averaged, and the distortion received in the nonlinear transmission path is also biased. For this reason, the signal point arrangement obtained as a result is slightly different from the signal point arrangement obtained by averaging the signal points having undergone all transitions. This means a lack of certainty when receiving the actual data signal (main signal).

  That is, since the modulation signal of the main signal is subjected to modulation mapping for each symbol with respect to sufficiently energy-spread data, the transition positions of the modulation signal points in the main signal are all available mapping positions. Transition is possible at random. Therefore, although the pilot signal and the main signal have the same signal point arrangement, the signal point arrangement after the averaging of both is slightly different. This deviation of the signal point arrangement is shown in FIG.

  The decoder and the receiving apparatus according to the present invention provide a configuration for converging to a pilot signal point closer to the main signal modulation signal when using the pilot signal subjected to energy spreading by inversion of the sign of the I axis and the Q axis. To do.

  Hereinafter, a decoder and receiver according to an embodiment of the present invention will be described.

  FIG. 1 shows a block diagram of a decoder (LDPC decoder 50) according to an embodiment of the present invention. Note that the LDPC decoding unit 50 of the present embodiment is replaced with the conventional LDPC decoding unit 250 in the receiving apparatus 200 shown in FIG. 8, thereby constituting the receiving apparatus of one embodiment according to the present invention. Therefore, the operation of the receiving apparatus excluding the LDPC decoding unit 50 is the same as that shown in FIG. 8, and further detailed description is omitted.

  The LDPC decoding unit 50 according to this embodiment includes a pilot signal extraction unit 52, a pilot signal averaging processing unit 53, a likelihood table generation unit 54, and a likelihood table stored in a predetermined memory (not shown). 55, an LDPC decoder 51, and a second signal point generator 56. Note that the pilot signal extraction unit 52, the pilot signal averaging processing unit 53, the likelihood table generation unit 54, the likelihood table 55, and the LDPC decoder 51 in the LDPC decoding unit 50 of the present embodiment are respectively shown in FIG. The decoding unit 250 has the same functions as the pilot signal extraction unit 252, the pilot signal averaging processing unit 253, the likelihood table generation unit 254, the likelihood table 255, and the LDPC decoder 251, and operates in the same manner. Therefore, the LDPC decoding unit 50 of the present embodiment is different from the LDPC decoding unit 250 shown in FIG. 11 in that the second signal point generation unit 56 is provided.

  The second signal point generation unit 56 includes a first signal point arrangement obtained by averaging pilot signals extracted at the time of reception supplied from the pilot signal averaging processing unit 53, and a predetermined transmission side signal point. The ideal signal point to be defined is compared, a deviation between the first signal point arrangement and the ideal signal point is calculated, a second signal point arrangement in which the deviation is corrected with respect to the first signal point arrangement is generated, and the likelihood is calculated. It has a function of sending to the table generation unit 54.

  Therefore, the LDPC decoding unit 50 of the present embodiment extracts a pilot signal periodically transmitted in a signal point arrangement defined in advance from a signal demodulated according to the modulation method, and the signal point of the extracted pilot signal Is calculated for each symbol by comparing the first signal point constellation representing the ideal signal point constellation defined by a predetermined transmission-side signal point with respect to each of the first signal point constellation and the ideal signal point constellation. Then, in order to generate a likelihood table at the time of decoding, it functions to generate a second signal point arrangement in which the deviation is corrected with respect to the first signal point arrangement. In particular, the LDPC decoding unit 50 has a predetermined amplitude type signal point having the same amplitude according to the modulation scheme in the ideal signal point arrangement in the first signal point arrangement (in the case of 32APSK, there are three types of signal points having the same amplitude). The average amplitude between them is determined as the amplitude of signal points of the same amplitude type, and for each signal point of the same amplitude type in the first signal point arrangement, between the phase at the ideal signal point and the signal point of the first signal point arrangement The phase deviation of a predetermined phase type according to the modulation scheme (for example, 32 types of phase deviation in the case of 32APSK) is obtained, and the phase deviation of the predetermined phase type is calculated for the signal point group having the same amplitude. Classify into phase / amplitude groups (in the case of 32APSK, 32 signal points are classified into 3 types of phase / amplitude groups with the same amplitude, so there are 3 types of 32 phase deviations as well. The average phase deviation is calculated for each phase / amplitude group using the phase deviation included in each phase / amplitude group, and each signal point belonging to the same phase / amplitude group The average phase deviation is added to the phase of the ideal signal point, and the added value is corrected as the phase of the signal point of the second signal point arrangement at the amplitude. Further, the LDPC decoding unit 50 according to the present embodiment has a function of generating a likelihood table used for calculating the likelihood of decoding according to the second signal point arrangement, and performing LDPC decoding of the main signal using the likelihood table. .

  Here, the second signal point generator 56 can be configured by a logic circuit. Alternatively, the second signal point generation unit 56 can be configured as a computer including a predetermined memory and a central processing unit (CPU), and a program for realizing the function of the LDPC decoding unit 50 and each modulation method. The ideal signal point arrangement data can be stored in advance in the predetermined memory. Further, the second signal point generator 56 can be configured to receive and determine a modulation scheme / coding rate selection signal in order to select ideal signal point arrangement data for each modulation scheme.

  Hereinafter, the operation of the decoder (LDPC decoding unit 50) of this embodiment will be described in detail with reference to FIGS.

  FIG. 2 shows an example of an operation flow according to the LDPC decoding unit 50 of the present embodiment. Here, although not limited, in order to facilitate the understanding of the invention, the signal point sequences shown in FIG. 2 are all assumed to be complex signals. Further, as described above, it is assumed that a signal is transmitted to the satellite repeater 300 (or satellite transmission path). Further, it is assumed that the transmission path distortion occurs only in the process from the transmission apparatus 100 to the reception apparatus 200 (or the decoder) via the transmission path. Further, description will be made assuming 32APSK.

  Referring to FIG. 2, in step S1 (Step 1), the pilot signal extraction unit 52 receives a modulated signal including a pilot signal sequence having a known pattern as an information source, and sets (I_1st_pilot_m) as a first signal point arrangement. .n, Q_1st_pilot_m.n). Since the first pilot signal point is a signal obtained by modulating a known pattern, it can be obtained by simply averaging the corresponding pilot signal. Here, n is defined as the same amplitude group number, and m is defined as a signal point number within the same amplitude group. Also, m_max.n is defined as the maximum signal point number belonging to the same amplitude group n. Taking 32APSK as an example, there are 3 types of the same amplitude group having the same amplitude, and 32 types of signal points for each of the 3 types of the same amplitude group are composed of 4, 12 and 16 signal points in order of decreasing amplitude. are categorized. Therefore, the maximum signal point number m_max.1 = 4, m_nmax_2 = 12, m_nmax.3 = 16. FIG. 3 shows the correspondence between the signal point number and the same amplitude group number in 32APSK.

Subsequently, at step S2 (Step 2), the pilot signal averaging processing unit 53 uses the first signal point arrangement (I_1 ^st _pilot_m.n, Q_1 ^st _pilot_m.n) obtained to determine each same amplitude group number n. The average amplitude Ave_Amp_n at is obtained from equation (2).

  In the case of 32APSK, since there are three types of the same amplitude group, three types of average amplitudes are obtained as the average amplitude Ave_Amp_n.

Subsequently, at step S3 (Step3), the second signal point generation unit 56, the first signal constellation ^{(I_1 st _pilot_m.n, Q_1 st _pilot_m.n} ) and the ideal signal constellation (I_ideal_m.n, Q_ideal_m. Using n), the phase error amount Δφm.n at each mn is obtained by equation (3).

  In the case of 32APSK, since there are 32 types of m.n, 32 types of phase error amounts Δφm.n are obtained.

  Subsequently, in step S4 (Step 4), using the phase error amount Δφm.n at each m.n, the average phase error amount Ave_Δφn at each n is obtained by Equation (4).

  In the case of 32APSK, since there are three types of the same amplitude group, three types of average phase error amount Ave_Δφn are obtained.

Subsequently, in step S5 (Step 5), the second signal point generator 56 uses the average amplitude Ave_Amp_n and the average phase error amount Ave_Δφn for each n to generate the second signal point arrangement (I_2 ^nd _pilot_m.n) for each n. , Q_2 ^nd _pilot_m.n) is obtained by equation (5).

  The second signal point arrangement can be obtained by using the amplitude and the phase in which the influence of the nonlinear distortion included in the first signal point arrangement is reduced by the averaging process according to the equation (5).

  Finally, in step S6 (Step 6), the second signal point generation unit 56 uses the second signal point arrangement as an ideal signal point in the likelihood ratio calculation formula shown in Formula (1), thereby decoding characteristics. Can be improved.

  By generating or updating the likelihood table based on the second signal point arrangement thus obtained, it is possible to improve the decoding characteristics.

  FIG. 4 shows a simulation result of the first signal point arrangement and the second signal point arrangement in 32APSK in the LDPC decoding unit 50 of the present embodiment. In particular, in the outermost circle, it can be seen that the second signal point arrangement converges to a point closer to the center of the received signal.

  FIG. 5 shows a computer simulation result of C / N vs. bit error rate characteristics at 32 APSK coding rate 4/5 when the first signal point arrangement and the second signal point arrangement in the LDPC decoding unit 50 of the present embodiment are used. Indicates. Here, a satellite transmission path was assumed as the transmission path system used in the computer simulation. As a satellite transmission path, an IMUX filter, a TWTA, and an OMUX filter constituting a general satellite repeater 300 are assumed, and the system is reproduced by computer simulation. The TWTA operating point was calculated for two types of OBO = 3.0 dB and 3.4 dB.

  From FIG. 5, when the second signal point arrangement is used, C / N improvement of 0.2 dB at BER = 1E-5 point at OBO = 3.4 dB, and BER = 1E-5 point at OBO = 3.0 dB. It can be seen that C / N improvement of 0.3 dB can be obtained.

  Therefore, in the implementation of distortion compensation of a nonlinear transmission path by a pilot signal using a known pattern, it is possible to improve transmission performance by using the present invention. In addition, since the present invention can select the signal point number and the same amplitude group when calculating the average amplitude and the average phase error according to the modulation method, it can be applied to any modulation method, and various modulation methods. It is possible to improve the distortion with respect to.

  In the above-described embodiment, the case where a specific satellite repeater, a modulation scheme, and an LDPC code are applied has been described. However, the present invention is not limited to a terrestrial repeater, another modulation scheme, and an arbitrary likelihood calculation. It is clear from the description of the embodiments that the present invention can be applied to error correction codes. Accordingly, the invention should not be construed as limited by the embodiments described above, but only by the claims.

  Since the present invention can suitably reduce channel distortion on the receiving side, it is useful for decoders and receiving apparatuses of any digital transmission system.

DESCRIPTION OF SYMBOLS 50 LDPC decoding part 51 LDPC decoder 52 Pilot signal extraction part 53 Pilot signal averaging process part 54 Likelihood table production | generation part 55 Likelihood table 56 2nd signal point production | generation part 100 Transmission apparatus 110 Frame generation part 120 LDPC encoding part 130 Energy spreading section 140 Mapping section 150 Time division multiplexing / orthogonal modulation section 200 Receiver 210 Channel selection section 220 Orthogonal detection section 230 Transmission control signal decoding section 240 Energy despreading section 250 LDPC decoding section 251 LDPC decoder 252 Pilot signal extraction section 253 Pilot signal averaging processing unit 254 Likelihood table generating unit 255 Likelihood table 260 Data 261 Adder circuit 262 Level converter circuit 263, 264 Multiplier circuit

Claims (6)

A decoder for correcting a data error in digital transmission using a predetermined likelihood table,
A known signal extracting means for extracting a pilot signal periodically transmitted at a predetermined signal point arrangement from a signal demodulated according to a modulation method;
The first signal point arrangement and the ideal signal point arrangement are compared by comparing the first signal point arrangement representing the signal points of the extracted pilot signal and the ideal signal point arrangement defined by the predetermined transmission side signal points. Deviation calculating means for calculating the deviation for each symbol;
Signal point arrangement correction means for generating a second signal point arrangement in which the deviation is corrected with respect to the first signal point arrangement for generating a likelihood table at the time of decoding;
A decoder comprising:   The decoder according to claim 1, wherein the deviation includes a deviation in amplitude and phase of the pilot signal. The signal point arrangement correcting means is
Means for determining an average amplitude between signal points of a predetermined amplitude type having the same amplitude in accordance with a modulation method in the ideal signal point arrangement in the first signal point arrangement as the amplitude of the signal point of the same amplitude type;
For each signal point of the same amplitude type in the first signal point arrangement, a phase deviation between the phase in the ideal signal point arrangement and the signal point in the first signal point arrangement is calculated, and a predetermined phase type according to the modulation method Means for classifying the phase deviation of the predetermined phase type into a phase / amplitude group of signal points having the same amplitude;
Using the phase deviation included in each phase / amplitude group, the average phase deviation is calculated for each phase / amplitude group, and for each signal point belonging to the same phase / amplitude group, the average phase deviation becomes the phase of the ideal signal point. The decoder according to claim 2, further comprising means for adding and correcting the added value as the phase of the signal point of the second signal point arrangement at the amplitude.   4. The method according to claim 1, further comprising: means for generating a likelihood table used for calculating likelihood of decoding according to the second signal point arrangement corrected by the signal point arrangement correcting unit. 5. Decoder described. And performing LDPC decoding using prior Kieu degree table, the decoder according to any one of claims 1-4.   A receiver comprising the decoder according to claim 5.