JP4928573B2 - 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. In addition, DVB-S2, which is a European transmission method, uses an amplitude phase modulation called amplitude phase modulation (APSK), and a modulation method that further improves frequency utilization efficiency has been put into practical use. For example, if 16 APSK is used, the frequency utilization efficiency is 4 bps / Hz at the maximum, and if 32 APSK, transmission is possible at a maximum of 5 bps / Hz.

  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

  In the prior art, an IMUX filter, TWT, and OMUX filter that constitute a general satellite repeater 300 are assumed, and a repeater simulator in which the operating point of TWT is set to OBO = 3.4 dB is reproduced by computer simulation. FIG. 13 shows a signal example in which the 32APSK modulated signal is waveform-equalized by the blind equalizer having the FIR filter 10. FIG. 13A shows a constellation before applying blind equalization, and FIG. 13B shows a constellation after applying blind equalization.

  Referring to FIG. 13, it can be seen that the error vector magnitude is improved by performing the blind equalization, and the waveform quality is improved. Next, FIG. 14 shows the positional relationship between the constellation after waveform equalization and the pilot signal points. Referring to FIG. 14, by performing waveform equalization, it can be seen that the pilot signal points separately received in advance and the constellation after waveform equalization are separated from each other particularly in the outermost circle. . This distance affects the likelihood used at the time of demodulation and causes a deterioration in transmission characteristics. Therefore, when more accurate likelihood calculation is performed on a signal that has undergone waveform equalization, an appropriate calculation method that matches the signal after waveform equalization is required.

In particular, referring to FIG. 13, by performing waveform equalization, the constellation of the received signal may not be sufficiently equalized depending on the signal point position, and the influence of transmission path distortion may not be obtained. The remaining constellation becomes a coexisting constellation. Even if it is attempted to evaluate the reception performance of a signal that has not been fully equalized by a conventional decoder and receiving apparatus and has been subjected to waveform equalization, it is computationally expensive to evaluate the performance of each signal point. Will be enormous. For example, in the modulation multi-level number M, there are 2 ^M combinations of selection of pilot signals and transmission signals, and in the case of 32APSK, there are 2 ³² = 4294967296 combinations of selection methods of pilot signals and transmission signal points, It is impractical to examine all reception performance for these.

  Therefore, an object of the present invention is to perform signal ratio calculation of the equalized received signal when performing likelihood ratio calculation for decoding with respect to the received signal whose waveform is equalized for the purpose of channel distortion compensation. A digital transmission system decoder and receiver capable of improving transmission characteristics by performing likelihood ratio calculation using a signal obtained by modulating known information called a pilot signal and a transmission signal point for each arrangement. To provide an apparatus.

  The decoder and the reception apparatus according to the present invention use the pilot signal and the transmission signal point obtained in the reception apparatus as appropriate calculation means for performing more accurate likelihood calculation on the signal subjected to waveform equalization. In combination, the pilot signal and the transmission signal are selected for the equalized signal and calculated so as to be handled as an ideal symbol point at the time of likelihood ratio calculation.

Therefore, for signal points belonging to groups belonging to the same amplitude, a constraint condition is set such that selection of pilot signal points and transmission signal points is not performed. For example, since there are three types of groups having the same amplitude in 32APSK, it is possible to select an ideal symbol point arrangement from two or ^three combinations. Then, by selecting the good combination of the most decoding characteristics from the combinations of the two ^triplicate as ideal symbol point arrangement, with respect to the signal after the waveform equalization, it is possible to obtain a more excellent transmission characteristic .

That is, the decoder of the present invention is a decoder that corrects a data error after digital transmission waveform equalization using a predetermined likelihood table, and is preliminarily specified from a signal demodulated according to a multi-level modulation method. A known signal extracting means for extracting a pilot signal periodically transmitted with the determined signal point arrangement, a pilot signal point arrangement representing a signal point of the extracted pilot signal, and a predetermined transmission side signal point. using a transmit constellation by which the multi-level modulation when the M kinds of types of the same amplitude determined by method likelihood of 2 ^M as a combination of the pilot signal point and the transmission signal points distinguished in each of the same amplitude A transmission characteristic for evaluating data errors after waveform equalization using ideal symbol determination means for determining ideal symbol point arrangement for frequency ratio calculation and the one or more ideal symbol point arrangements for likelihood ratio calculation. And selecting an ideal symbol point arrangement for likelihood ratio calculation that provides the highest transmission characteristic performance in the evaluation while switching from the one or more ideal symbol points for likelihood ratio calculation, and receiving the main signal normally And likelihood table switching means for determining a likelihood table used at the time of decoding.

  In the decoder according to the present invention, the waveform equalization comprises an adaptive equalizer.

  In the decoder of the present invention, LDPC decoding is performed using the predetermined likelihood table.

  Furthermore, the decoder of the present invention is characterized in that LDPC decoding is performed using the predetermined likelihood table, and is also characterized as a receiving apparatus including the decoder of the present invention.

  According to the decoder and the receiving apparatus of the present invention, it is possible to improve the transmission characteristics after waveform equalization for various modulation schemes having a plurality of amplitude levels.

It is a figure which shows the block diagram of the LDPC decoding part of one Example by this invention. It is a flowchart explaining the operation | movement of the LDPC decoding part of one Example by this invention. It is a figure which shows the transmission signal point arrangement | positioning in the case of 32APSK. It is a figure which shows the selection list | wrist of the ideal symbol point arrangement | positioning for likelihood ratio calculation in 32APSK. It is a figure which shows the selection list | wrist of the ideal symbol point arrangement | positioning for likelihood ratio calculation in 32APSK. It is a figure which shows the computer simulation result of the C / N vs. bit error rate characteristic in 32APSK coding rate 4/5 of the blind equalization in the LDPC decoding part of one Example by this invention. 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 the conventional satellite broadcasting system. It is a block diagram which shows schematic structure of the conventional receiver of a satellite broadcast system. It is a figure which shows the example of the modulation wave signal format of a 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 the schematic of the decision feedback type FIR filter with which a blind equalizer is equipped. It is a figure which shows the signal example which waveform-equalized the 32APSK modulation signal by the blind equalizer which has a decision feedback type FIR filter. It is a figure showing the positional relationship between the constellation after waveform equalization, and a pilot signal point.

  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 method (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, In the Advanced Satellite Digital Broadcasting System (hereinafter referred to as Advanced Satellite Broadcasting System) shown in the report “Report of the Information and Communications Council Information and Communication Technology Subcommittee Broadcasting System Committee Report” (July 29, 2008), APSK Consideration is also given to the improvement of transmission characteristics.

  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 a pilot signal, and shows a correspondence diagram of known transmission bits with respect 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.

  Here, use of an adaptive equalizer is mentioned as a method of compensating for transmission path distortion in a receiving apparatus, and a blind or the like that can reduce the influence of transmission path distortion from only a received signal as one form of the adaptive equalizer. (For example, reference: Kil Nam OH, “A Single / Multilevel Modulus Algorithm for Blind Equalization of QAM Signals” IEICE TRANS. FUNDAMENTALS. VOL.E80-A, N.6. June 1997, reference)

  A blind equalizer is a type of adaptive filter that reduces the distortion caused by the transmission path by appropriately selecting the filter tap length, step size, and error vector calculation method, and makes the symbol point close to the transmission signal. This is a waveform equalizer capable of changing the reception signal point. The blind equalizer can be verified using the decision feedback type FIR filter 10 shown in FIG. The step size s is 2E-4, the tap length M of the feedforward filter (FF) 11 is 10, the tap length L of the feedback filter (FB) 14 is 14, and the blind algorithm uses the GCMCA algorithm (see the above-mentioned reference). ).

  Specifically, the decision feedback type FIR filter 10 includes a feedforward filter (FF) 11, an equalizer output unit 12, a decision unit 13, a feedback filter (FB) 14, an adder 15, and a filter coefficient. An update unit 16 is provided.

The feedforward filter (FF) 11 has filter coefficients W _FF : [W ₀ , W ₁ ,..., W _M ] corresponding to the tap length M, and inputs an input signal vector x (n). Vector sequence x _FF : [x (n), x (n−1),..., X (n−M)] ^T is held, and the product sum of the feedforward filter coefficient W _FF and the input vector sequence X _FF Perform the operation. Here, T represents transposition of the matrix.

The feedback filter (FB) 14 has filter coefficients W _FB : [W _{M + 1} , W _{M + 2} ,..., W _{M + L} ] corresponding to the tap length L, and a determiner output d (n) of the determination unit 13 described later. respect, the determination output sequence _{d FB: [d (n-} 1), d (n-2), ···, d (n-L)] holding the ^T, the feedback filter coefficients W _FB and judgment output Perform product-sum operation with column d _FB .

  The adder 15 outputs a value obtained by subtracting the output 14 a of the feedback filter (FB) 14 from the output of the feedforward filter (FF) 11 to the equalizer output unit 12.

  The determination unit 13 performs minimum Euclidean distance determination with an ideal symbol point determined by a known modulation method for the equalizer output z (n) obtained by the equalizer output unit 12 to obtain the minimum Euclidean distance. The ideal symbol point is output to the feedback filter (FB) as the determiner output d (n). At the same time, the filter output d (n) is output to the filter coefficient update unit 16 for updating the filter coefficient.

  The equalizer output unit 12 sends out the equalizer output z (n) obtained from the adding unit 15.

z (n) = x ′ (n) ^T · W (n) (2)
Where x ′ (n) = [x (n), x (n−1),..., X (n−M),
d (n-1), d (n-2), ..., d (n-L)]

  At the same time, the equalizer output z (n) is output to the filter coefficient updating unit 16 for updating the filter coefficient.

The filter coefficient updating unit 16 obtains an error vector e (n) from the equalizer output z (n) and the determiner output d (n), and feedforwards it using the LMS algorithm using e (n) and the step size s. The filter (FF) coefficient W _FF and the feedback filter (FB) coefficient W _FB are sequentially updated. (See references above).

  Therefore, according to the decoder and the receiving apparatus of the embodiment of the present invention, the pilot signal and the transmission signal point obtained in the receiving apparatus 200 are used together, and the pilot signal and the transmission signal are selected with respect to the equalized signal. Thus, it is calculated so that it is handled as an ideal symbol point at the time of likelihood ratio calculation, and more accurate likelihood calculation can be performed.

  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 list generation unit 54, and one stored in a predetermined memory (not shown). Likelihood table switching unit 55 for switching the above likelihood table, LDPC decoder 51, blind equalizer 56, BER evaluation unit 57, and transmission signal stored in advance in a predetermined memory (not shown) A point table 58. The pilot signal extraction unit 52, the pilot signal averaging processing unit 53, and the LDPC decoder 51 in the LDPC decoding unit 50 of the present embodiment are respectively the pilot signal extraction unit 252 and the pilot signal in the LDPC decoding unit 250 shown in FIG. It has the same functions as the averaging processing unit 253 and the LDPC decoder 251 and operates in the same manner. Therefore, the LDPC decoding unit 50 according to the present embodiment includes the blind equalizer 56, the BER evaluation unit 57, and the transmission signal point table 58, and includes a likelihood table switching unit 55 instead of the likelihood table 255. This is different from the LDPC decoding unit 250 shown in FIG.

  The blind equalizer 56 is an equalizer that functions as the decision feedback type FIR filter 10 shown in FIG. 12. For the I signal and the Q signal obtained from the quadrature detection unit 220 through the energy despreading unit 240, a waveform or the like is obtained. The converted output is sent to the LDPC decoder 51.

  The transmission signal point table 58 is a table indicating signal points on the transmission side transmitted by TMCC information, and is supplied to the likelihood table list generation unit 54.

The likelihood table list generation unit 54 uses the transmission signal point table 58 to roughly divide into M types according to groups belonging to the same amplitude. For example, P is a pilot signal point to be received, and a transmission signal is based on the transmission signal point table 58. A point is defined as T, and ^2M kinds of ideal symbol points for likelihood ratio calculation are calculated from combinations of pilot signal points and transmission signal points distinguished for each same amplitude, and these ideal symbols for likelihood ratio calculation For each point, generate a likelihood table used for likelihood calculation in LDPC decoding, which represents a decoder output log likelihood ratio (LLR) for a signal point of a pilot signal averaged for each symbol, Store in a predetermined memory or update.

  The BER evaluation unit 57 outputs a bit error in the output of the LDPC decoder 51 using each likelihood table for the waveform equalized pilot signal output from the blind equalizer 56 before normal reception of the main signal (data). The bit error rate determined by the rate or the predetermined number of iterations of the LDPC decoder 51 is evaluated as a transmission characteristic result, and this evaluation result is output to the likelihood table switching unit 55.

The likelihood table switching unit 55 acquires transmission characteristic results of pilot signals sequentially input from the BER evaluation unit 57, compares the evaluation results using the likelihood table of ^2M ideal symbol point arrangements, and The likelihood table of the ideal symbol point with the best performance (low bit error rate) is selected from the likelihood tables corresponding to the ideal symbol points, and the likelihood used for receiving the main signal to the LDPC decoder 51 Determine the table. The likelihood table switching unit 55 holds the ideal signal point arrangement with the best decoding performance when the main signal is received, and continues to use it for subsequent reception.

  Next, the operation of the decoder of this embodiment will be described with reference to FIGS.

  FIG. 2 is a flowchart for explaining the operation of the decoder of this embodiment. Here, as described above, all the symbol sequences are assumed to be complex signals. Further, it is assumed that the receiving apparatus 200 including the decoder 50 according to the present embodiment receives a signal via the satellite repeater 300 illustrated in FIG. 6, and transmission path distortion is received from the transmitting unit via the transmission path. It is assumed that it occurs only in the process of going through. In the following description, 32APSK will be described as an example.

  FIG. 3 shows a transmission signal point arrangement in the case of 32APSK. In the case of 32APSK, the reception point candidates are 32 points. The reception point candidates are roughly classified into three types according to groups belonging to the same amplitude by the likelihood table list generation unit 54. For example, the pilot signal point is defined as P, the transmission signal point is defined as T, and the description of the combination of P and T is defined as (T or P, T or P, T or P,...) In ascending order of amplitude. To do. As an example, in 32APSK, since it can be classified by three same amplitudes, when the inner circle and the innermost circle are selected as pilot signal points and the outermost circle as a transmission signal, they can be expressed as (P, P, T). it can.

  Referring to FIG. 2, in step S1, receiving apparatus 200 receives a pilot signal obtained via pilot signal extraction unit 52 and pilot signal averaging processing unit 53 before normal reception of the main signal, and the likelihood. The degree table list generation unit 54 determines the signal point arrangement of the pilot signal.

Next, in step S2, the reception apparatus 200, the likelihood table list generation unit 54, ^{2 triplicate} consisting of a combination of transmit constellation obtained from TMCC information (in the case of 32APSK, a M = 3) Determine the ideal symbol point placement. 4A and 4B show selection lists of ideal symbol point arrangements for likelihood ratio calculation in 32APSK.

Next, in step S3, the reception apparatus 200, the BER evaluation unit 57, using the ideal symbol points likelihood ratio for the calculation of two ways ^3, on the signal after the waveform equalization, bit error rate or bit The transmission characteristic for each ideal symbol point arrangement is evaluated by using the error rate or the number of times of iterative decoding at the time of LDPC decoding.

  Next, in step S4, the receiving apparatus 200 selects and holds the signal point arrangement with the best decoding performance from the evaluation result of the transmission characteristic for each ideal symbol point arrangement by the likelihood table switching unit 55, and thereafter It is used continuously for reception.

  FIG. 5 shows a computer simulation result of C / N versus bit error rate characteristics at the blind equalization 32APSK coding rate 4/5 in the above-described eight types of ideal symbol point arrangements. Here, a satellite transmission line (see FIG. 6) was assumed as the transmission line system used in the computer simulation. As a satellite transmission path, an IMUX filter, a TWT, and an OMUX filter constituting a general satellite repeater were assumed, and the system was reproduced by computer simulation. The operating point of TWT was calculated for 3.4 dB. As the blind equalizer, a decision feedback type FIR filter shown in FIG. 12 was used. The step size is 2E-4, the tap length of the feedforward filter is 10, and the feedback filter tap length is 14. As the blind algorithm, the GCMCA algorithm was used.

  According to the decoder of the present embodiment, among the eight types of combinations, the outermost circle is a transmission signal, and the inner and outermost circles are combinations of pilot signals (P, P, T) (FIG. 4B (b) It can be seen that the best decoding characteristics can be obtained in the above pattern). Therefore, it is possible to improve the transmission performance by using this method for the signal after waveform equalization. Although the blind equalizer is used as the adaptive equalizer of the present embodiment, this method can be applied even when an adaptive equalizer other than the blind equalizer is used. Further, the present system is not limited to 32APSK, and it is possible to improve the transmission characteristics after waveform equalization for various modulation systems having a plurality of amplitude levels.

  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.

10 Judgment feedback type FIR filter 11 Feed forward filter (FF)
12 Equalization Control Unit 13 Judgment Unit 14 Feedback Filter (FB)
DESCRIPTION OF SYMBOLS 15 Adder 50 LDPC decoding part 51 LDPC decoder 52 Pilot signal extraction part 53 Pilot signal averaging process part 54 Likelihood table list generation part 55 Likelihood table switching part 56 Blind equalizer 57 BER evaluation part 58 Transmission signal point table DESCRIPTION OF SYMBOLS 100 Transmission apparatus 110 Frame generation part 120 LDPC encoding part 130 Energy spreading part 140 Mapping part 150 Time division multiplexing / orthogonal modulation part 200 Reception apparatus 210 Channel selection part 220 Orthogonal detection part 230 Transmission control signal decoding part 240 Energy despreading part 250 LDPC decoding unit 251 LDPC decoder 252 Pilot signal extraction unit 253 Pilot signal averaging processing unit 254 Likelihood table generation unit 255 Likelihood table

Claims (4)

A decoder that corrects a data error after waveform equalization of 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 the multi-level modulation method;
Using the pilot signal point constellation representing the signal points of the extracted pilot signal and the transmission signal point constellation defined by the predetermined transmission side signal point, the type of the same amplitude determined by the multi-level modulation method is determined. When M types are used, ideal symbol determination means for determining 2 ^M kinds of likelihood symbol calculation ideal symbol point arrangements from combinations of pilot signal points and transmission signal points distinguished for each same amplitude ;
Transmission characteristic evaluation means for evaluating data errors after waveform equalization using the one or more ideal ratio calculation for likelihood ratio calculation;
The ideal symbol point arrangement for likelihood ratio calculation that provides the highest transmission characteristic performance in the evaluation is selected while switching from the one or more ideal symbol points for likelihood ratio calculation, and is used for decoding at the time of normal reception of the main signal. Likelihood table switching means for determining a likelihood table;
A decoder comprising: The decoder according to claim 1 , wherein the waveform equalization comprises an adaptive equalizer. The decoder according to claim 1 or 2 , wherein LDPC decoding is performed using the predetermined likelihood table. A receiving apparatus comprising the decoder according to claim 1.

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