JP7465112B2 - Transmission server, transmission device, receiving device, and program - Google Patents

Description

The present invention relates to the technical fields of satellite broadcasting, terrestrial broadcasting, fixed communication, and mobile communication, and in particular to a transmission server that links broadcasting and communication and uses communication to enable data retransmission in response to a retransmission request from the receiving side, a transmitting device and receiving device related to digital broadcasting, and a program.

In satellite and terrestrial digital broadcasting systems, error-correcting codes are used as a technique for improving transmission performance in the presence of white noise. For example, advanced wideband satellite digital broadcasting uses LDPC (Low Density Parity Check) codes, which are powerful error-correcting codes with performance approaching the Shannon limit, which is the theoretical upper limit of utilization efficiency for the signal-to-noise ratio (see, for example, non-patent documents 1 and 2). However, in digital broadcasting, transmission conditions can deteriorate to the point that the signal cannot be restored by error-correcting codes alone, such as attenuation due to rain in satellite digital broadcasting and fading in terrestrial digital broadcasting.

On the other hand, in digital wireless communication, data retransmission control using ARQ (Automatic Repeat reQuest) is known as a data compensation technique other than error correction code (see, for example, Patent Documents 1 and 2). In digital wireless communication, the use of Hybrid ARQ, which combines error correction code and ARQ, can improve transmission performance, making it possible to transmit data with fewer retransmissions than using ARQ alone, and under conditions that cannot be compensated for by error correction code alone. However, conventional prior art using Hybrid ARQ is limited to two-way communication such as FPU and mobile phone base station communication, and a technique for compensating data with Hybrid ARQ in conjunction with communication in one-way broadcast such as broadcasting has not been established.

Although this does not constitute Hybrid ARQ, a technology has been disclosed in which, when the transmission conditions of digital broadcasting deteriorate, ARQ is performed from the receiving side to the transmitting side via an IP (Internet Protocol) network, which is a communication path that allows two-way communication, and data is resent from the transmitting side (see, for example, Patent Document 3).

A typical IP network is assumed to have an erasure channel (PEC: Packet Erasure Channel) where information is lost due to some kind of failure such as line congestion. Therefore, in Hybrid ARQ in digital wireless communication, error correction codes dedicated to IP communication, which is an erasure channel, such as LDPC-CC (Convolutional Codes) (see, for example, Patent Document 4) and LDGM (Low-Density Generator Matrix) codes (see, for example, Patent Document 5) are used as error correction codes for data compensation. These LDPC-CC and LDGM codes are error correction codes based on LDPC codes, just like advanced broadband satellite digital broadcasting, but they are codes designed with only IP communication, which is an erasure channel, in mind, and it is necessary to prepare an encoder and decoder for error correction codes dedicated to IP communication.

JP 2014-050034 A International Publication No. 2007/069406 International Publication No. 2015/048569 International Publication No. 2010/001610 International Publication No. 2015/022910

R. G. Gallager, “Low-Density Parity-Check Codes,” in Research Monograph series, Cambridge, MIT Press, December 1963 “Advanced Wideband Digital Satellite Broadcasting Transmission Method (ISDB-S3) Standard ARIB STD-B44 Version 2.1”, [online], revised March 25, 2016, ARIB, [Retrieved January 27, 2020], Internet <URL: https://www.arib.or.jp/kikaku/kikaku_hoso/std-b44.html>

As described above, in digital wireless communications, the use of Hybrid ARQ, which combines error correction codes and ARQ, can improve transmission performance, making it possible to transmit data with fewer retransmissions than when using ARQ alone, and under conditions that cannot be compensated for by error correction codes alone.

However, there is no established technique for linking communications with one-way broadcasting and compensating for data using Hybrid ARQ.

Therefore, even in digital broadcasting, a technique is desired that works in conjunction with communications to compensate for data using Hybrid ARQ, which combines error correcting codes and ARQ, by integrating digital broadcasting and communications. For example, when the transmission conditions for digital broadcasting deteriorate, transmission performance can be improved by constructing a transmission system in which ARQ is performed from the receiving side to the transmitting side via an IP network, which is a communication path that allows two-way communication, and the transmitting side resends the data. In other words, since current digital broadcasting only assumes data compensation using error correcting codes, compensating for digital broadcasting data using Hybrid ARQ can also handle deterioration of transmission conditions that cannot be corrected using error correcting codes.

However, if an error correction code for IP communication, as disclosed in Patent Documents 4 and 5, etc., is applied to a system using ARQ as disclosed in Patent Document 3 in order to build a transmission system using Hybrid ARQ that combines broadcasting and communication, it becomes necessary to prepare an encoder and decoder for the error correction code for digital broadcasting and an encoder and decoder for IP communication, and to have a mechanism for cooperating them, which increases the scale of the equipment.

Therefore, there is a demand to efficiently combine "error correction codes used in digital broadcasting" with ARQ, and to build a transmission system that uses Hybrid ARQ by integrating broadcasting and communications.

In other words, there is a demand for improving the error correction capability of digital broadcasting by linking digital broadcasting with communication over an IP network without increasing the size of the encoder in the transmitting device and the decoder in the receiving device, and by configuring a transmission system that realizes Hybrid ARQ, for example, by combining concatenated error correction codes of LDPC codes and BCH codes with retransmission requests using IP communication.

In addition, when retransmission is performed using IP communication, if the number of bits to be retransmitted increases, the load on the communication line increases and the line is expected to become congested. Therefore, a technique is desired that keeps the number of bits during data retransmission as small as possible and efficiently realizes data compensation using Hybrid ARQ.

Therefore, there is a need for a technique to improve the error correction capability of digital broadcasting and realize an efficient Hybrid ARQ by configuring a transmission system that links digital broadcasting and communication via an IP network, for example, by combining a concatenated code for error correction of LDPC codes and BCH codes with a retransmission request using IP communication, without increasing the size of the encoder in the transmitting device for digital broadcasting and the decoder in the receiving device, and by minimizing the number of bits required when retransmitting data.

In view of the above problems, the object of the present invention is to provide a transmission server, a transmission device and a reception device related to digital broadcasting, and a program that link digital broadcasting and communication via an IP network without increasing the size of the decoding device of the reception device, realize a hybrid ARQ that combines error correction using LDPC codes and retransmission requests using IP communication, and also enable selection of an LDPC coding rate according to the correction capability of the error correction code used in digital broadcasting and the packet loss rate of the IP network, thereby minimizing the number of bits when retransmitting data as much as possible, and efficiently use communication to enable data retransmission according to the packet loss rate of the IP network in response to a retransmission request from the reception side based on the coded data of the error correction code used in digital broadcasting.

The transmission server of the present invention is a transmission server that stores a predetermined time portion of the coded data from a transmission device that digitally modulates coded data encoded using an error correction code related to digital broadcasting and transmits the digitally modulated coded data to a reception device via a broadcast transmission path, and enables transmission to a reception device via an IP (Internet Protocol) network, and includes a storage unit that sequentially inputs coded data generated by the transmission device, manages the coded data in a time series manner using a synchronization signal based on frame numbers or time information of error correction code frames that constitute the code length of the error correction code, and stores the predetermined time portion of the coded data managed in a time series while updating it, and receives a retransmission request packet generated when it is determined that bit errors in the coded data cannot be corrected using the error correction code at the reception device via the IP network, and stores packet loss rate information measured based on reception of coded data set by the reception device or retransmitted via the IP network, which are stored in the retransmission request packet, and retransmission request information indicating the coded data of the error correction code frame related to the retransmission request. a coding rate determination unit that refers to a loss correction performance table having a required packet loss rate indicating a maximum packet loss rate that can be lost and corrected for each coding rate based on the packet loss rate information, and determines a coding rate of coded data for retransmission; a coding rate adaptation change unit that determines whether or not to change the coding rate of a block code of coded data generated by the transmitting device and retransmit the coded data based on the coding rate determined by the coding rate determination unit, and when changing the coding rate, changes the coding rate for coded data related to the retransmission request obtained by reading from the storage unit and a predetermined number of coded data consecutive to the coded data based on the retransmission request information, and configures a plurality of error correction code frames; and an IP packet generation unit which generates coded data packets of an IP packet string by adding an identification header for enabling the interleaved frame to be identified, a sequence number for identifying which bit in each error correcting code frame the packet represents, and an IP header which is the header of the coded data packet to the payload obtained by interleaving the error correcting code frames longitudinally, and transmits the coded data packets to the receiving device via an IP network. The coding rate adaptive change unit, when changing the coding rate of the coded data generated by the transmitting device and retransmitting the coded data to the receiving device, adds padding bits known on the receiving device side to information bits of the coded data in accordance with the changed coding rate at insertion positions known on the receiving device side. The error correcting code frame is generated as the coded data for retransmission by removing the padding bit, and the padding bit insertion position is determined in advance, and the padding bit insertion position is determined based on the pre-evaluation of the padding bit insertion position that has excellent performance of erasure correction for packet erasure when changing from a specific coding rate to another coding rate, and is allocated to the front and rear of the information bit in the error correcting code frame for each of a certain number of coding rates that can be used by the coded data for retransmission .

In addition, in the transmission server of the present invention, the predetermined number of coding rates that can be used by the transmitting device include at least 1/2, 3/5, 2/3, 3/4, 4/5, 5/6, 7/8, 9/10, and the insertion position of the padding bit is determined in advance based on the evaluation of the insertion position of the padding bit with excellent erasure correction performance for packet erasure based on the time when changing from coding rate 1/2 to another coding rate, when coding rate is 1/2, no padding bit is inserted, when coding rate is 3/5 , 2/3 , 4/5, 9/10, it is allocated to the rear stage of information bit, when coding rate is 3/4, 5/6, 7/8, it is allocated to the front and rear stages of information bit.

In addition, in the transmission server of the present invention, the coding rate determination unit determines, based on the same coding method as the error correction coding method used by the transmitting device, the highest coding rate among a predetermined number of coding rates available to the transmitting device, which is equal to or higher than the coding rate of the coded data transmitted over the broadcast transmission path and within a range that enables packet losses in the IP network to be corrected.

In addition, in the transmission server of the present invention, the predetermined number of coding rates that can be used by the transmission device include at least 1/2, 3/5, 2/3, 3/4, 4/5, 5/6, 7/8, and 9/10, and the loss correction performance table has a required packet loss rate that decreases according to the increase in the coding rate of the predetermined number of types.

Furthermore, the transmitting device of the present invention is a transmitting device related to digital broadcasting, and is characterized in that it has a means for sequentially outputting coded data that has been coded using error correction codes related to the digital broadcasting to the transmitting server of the present invention.

Furthermore, the transmitting device of the present invention is a transmitting device related to digital broadcasting, and is characterized in that the transmitting server of the present invention is provided inside the device.

Furthermore, a receiving device of the present invention is a receiving device that receives a modulated wave signal that is digitally modulated by a transmitting device using an error correction code related to digital broadcasting and transmitted via a broadcast transmission path, and includes a demodulator that receives and demodulates the modulated wave signal, an error correction decoder that reconstructs an error correction code frame that constitutes the code length of the error correction code from the coded data obtained by demodulation and performs a decoding process corresponding to the error correction code to generate and reproduce received data, a retransmission request packet generator that generates a retransmission request packet in an IP packet format that includes retransmission request information indicating coded data of the error correction code frame related to a retransmission request for requesting retransmission of corresponding coded data when it is determined that a bit error in the coded data cannot be corrected using the error correction code and packet loss rate information measured based on reception of the coded data retransmitted via a setting or an IP network, and transmits the retransmission request packet to a transmission server of the present invention, and a retransmission request packet generator that generates a retransmission request packet in an IP packet format that includes retransmission request information indicating coded data of the error correction code frame related to a retransmission request for requesting retransmission of corresponding coded data when it is determined that a bit error in the coded data cannot be corrected using the error correction code and packet loss rate information measured based on reception of the coded data retransmitted via a setting or an IP network, and transmits the retransmission request packet to the transmission server of the present invention in response to the retransmission request packet, and and an IP packet receiving unit that receives coded data packets of an IP packet sequence storing coded data retransmitted in response to a retransmission request from the transmission server and extracts the coded data retransmitted in response to the retransmission request by performing an inverse process of the interleaving performed by the transmission server, wherein the error correction decoding unit has a means for, for the coded data retransmitted in response to the retransmission request, restoring an error correction code frame without adding padding bits at the coding rate of the coded data obtained by demodulation when the coding rate has not been changed, and restoring the error correction code frame by adding padding bits according to the coding rate at the insertion positions of the padding bits assigned by the transmission server in accordance with the coding rate when the coding rate has been changed, and attempting a decoding process for the retransmitted coded data through a complementation process related to replacement of a likelihood ratio required for decoding the restored error correction code frame , and repeating a request for retransmission of the corresponding coded data until decoding is possible within a predetermined time.

In addition, in the receiving device of the present invention, the error correction code is a concatenated code in which an LDPC code is used as an inner code and a BCH code is used as an outer code, and the error correction decoding unit determines that the bit errors in the encoded data could not be corrected when the bit errors could not be completely corrected in the decoding process of the BCH code, or when the errors could be corrected in the decoding process of the BCH code but a predetermined number of BCH errors have been reached.

The receiving device of the present invention is also characterized in that it comprises a packet loss rate measuring unit that counts the number of lost packets based on the sequence numbers of the IP packet string, measures the packet loss rate of the IP network , and notifies the retransmission request packet generating unit of the packet loss rate information.

The receiving device of the present invention is also characterized by having a packet loss rate setting unit that sets the packet loss rate estimated by test communication of a round-trip line with the transmission server or set by a user as the packet loss rate information in the retransmission request packet generating unit.

Furthermore, the program of the present invention is configured as a program for causing a computer to function as the transmission server of the present invention.

According to the present invention, for data loss that cannot be prevented by broadcast reception alone, a retransmission request is made from the receiving side to the transmitting side via an IP network, and the transmitting side can retransmit data, so that the receiving side can supplement the data.In particular, the data that the transmitting side retransmits via an IP network is coded data in the block code of digital broadcasting, and the LDPC coding rate of the coded data that is retransmitted is variably controlled so that the number of bits that are retransmitted is as small as possible according to the packet loss rate of the IP network, so that the packet loss rate of the IP network is shared between the transmitting side and the receiving side in sequence, making it possible to reduce the number of retransmission requests, and furthermore, the transmission efficiency via the IP network can be improved.In addition, by making both the error correction code by broadcast reception and the error correction code used in transmission via the IP network the same as the coding format of the error correction code standardized by the broadcast standard, the receiving side can be realized by simply preparing one error correction decoder, and the equipment scale can be reduced.

In particular, according to the present invention, when the encoding rate is changed from the transmitting server to the receiving device and the encoded data is retransmitted, the insertion position of the padding bits used to change the frame length of the encoded data according to the encoding rate is further variably controlled according to the encoding rate, thereby optimizing the performance for each encoding rate of the error correction code, and high erasure correction performance can be achieved even when burst erasures occur in the IP network.

In addition, according to one aspect of the present invention, the coded data that the receiving side requests the sending side to retransmit is retransmitted from the sending side to the receiving side as coded data packets of an IP packet sequence with sequence numbers, so that the receiving side measures the packet loss rate of the downlink (the line from the sending side to the receiving side) based on the sequence numbers of the received IP packets.Then, the sending side variably controls the coding rate of the coded data to be retransmitted based on the packet loss rate.Therefore, it is possible to select the coding rate of the coded data to be retransmitted more appropriately than when using the packet loss rate estimated by test communication of the round-trip line between sending and receiving or determined by user setting, and the transmission efficiency via IP network can be further improved.

1 is a block diagram showing a schematic configuration of a transmission system including a transmission server, a transmitting device, and a receiving device according to an embodiment of the present invention. 1 is a block diagram showing a schematic configuration of a transmission server and a transmission device according to an embodiment of the present invention; 10 is a flowchart showing a control flow relating to a retransmission request packet between a transmission server and a receiving device in an embodiment of the present invention. 1A to 1D are diagrams conceptually showing a method of generating coded data when the coding rate is changed by a coding rate adaptive change unit in a transmission server according to an embodiment of the present invention. 1A to 1D are diagrams specifically showing a method of generating coded data when the coding rate is changed by a coding rate adaptive change unit in a transmission server in one embodiment of the present invention. 11 is an explanatory diagram of an interleaving process of an IP packet generating unit in a transmission server according to an embodiment of the present invention; FIG. 1 is a block diagram showing a schematic configuration of a receiving device according to an embodiment of the present invention. 10 is a flowchart showing a reception control flow of an example of a receiving device according to an embodiment of the present invention. 10A and 10B are diagrams illustrating measurement and notification of a packet loss rate in an embodiment of a receiving device according to the present invention. A figure showing the position at which padding bits are inserted according to the coding rate, which is changed from the coding rate of 1/2 for coded data transmitted over a broadcast transmission path, by an example coding rate adaptive change unit in a transmission server of one embodiment of the present invention. FIG. 11 is a diagram showing a correspondence table of the insertion positions of padding bits according to the coding rate, which is changed from the coding rate of 1/2 for coded data transmitted over a broadcast transmission path by an embodiment of the coding rate adaptive change unit in a transmission server of an embodiment of the present invention. FIG. 13 is a diagram illustrating a method for evaluating erasure correction performance based on a pattern of padding positions of padding bits according to the present invention. FIG. 13 is a diagram showing the characteristics when changing the coding rate from 1/2 to 3/4 as an evaluation of erasure correction performance according to the padding position pattern of padding bits according to the present invention. FIG. 13 is a block diagram showing a schematic configuration of a receiving device according to a modified example of the present invention.

[Transmission System]
Fig. 1 is a block diagram showing a schematic configuration of a transmission system 1 including a transmission server 6, transmission devices 2 and 3, and a receiving device 5 according to an embodiment of the present invention. The transmission system 1 shown in Fig. 1 includes the transmission devices 2 and 3, the receiving device 5, one or more transmission servers 6, and a load balancing device 7 that is provided when multiple transmission servers 6 are configured.

The transmitting device 2 is a device that generates coded data by applying error correction coding processing to transmission data related to digital broadcasting and adding error correction code parity, digitally modulates the coded data using a predetermined modulation method to generate a modulated wave signal, and emits radio waves related to digital broadcasting via a satellite broadcast transmission path including a broadcasting satellite 4 via a transmitting antenna 2a. For example, the transmitting device 2 can be a transmitting device for advanced wideband satellite digital broadcasting. The transmitting device 2 also has a function of managing the sequentially generated coded data as an error correction code frame, adding frame numbers or time information that function as synchronization signals, and transmitting the data to one or more transmission servers 6, for example, via a local area network.

The transmitting device 3 is a device that generates coded data by applying an error correction coding process to transmission data related to digital broadcasting and adding an error correction code parity, digitally modulates the coded data using a predetermined modulation method to generate a modulated wave signal, and emits radio waves related to digital broadcasting via a terrestrial broadcast transmission path via a transmitting antenna 3a. For example, the transmitting device 3 can be a transmitting device for current or next-generation terrestrial digital broadcasting. The transmitting device 3 also has a function of managing the sequentially generated coded data as an error correction code frame, adding a frame number or time information that functions as a synchronization signal, and transmitting the data to one or more transmission servers 6, for example, via a local area network.

The receiving device 5 has the functions of receiving a modulated wave signal radiated by radio waves from the transmitting device 2 via the receiving antenna 5a, demodulating it, reconstructing an error correction code frame that constitutes the code length of the error correction code, and performing a decoding process corresponding to the error correction coding process in the transmitting device 2 to generate and reproduce received data, and receiving a modulated wave signal radiated by radio waves from the transmitting device 3 via the receiving antenna 5b, demodulating it, reconstructing an error correction code frame that constitutes the code length of the error correction code, and performing a decoding process corresponding to the error correction coding process in the transmitting device 3 to generate and reproduce received data.

In addition, the receiving device 5 has the function of determining whether the coded data obtained from the transmitting device 2 (or transmitting device 3) can be decoded for error correction, and when determining that the coded data can be decoded, it decodes it as it is to generate received data, and when determining that the bit error of the coded data cannot be corrected and the coded data cannot be decoded, it generates a retransmission request packet in the form of an IP packet, which requests the retransmission of the corresponding coded data, and includes the packet loss rate information of the IP network 8 measured on the downlink (the line from the transmitting side to the receiving side) in this retransmission request packet, and transmits it to the transmitting server 6 via the IP network 8. Here, the receiving device 5 has the function of sequentially measuring the packet loss rate of the coded data packet of the IP packet sequence received as a response to the retransmission request to the transmitting server 6. Furthermore, the receiving device 5 has a function of receiving the coded data packet of the IP packet sequence that stores the coded data retransmitted from the transmitting server 6 in response to the retransmission request packet (the coded data reconstructed at the highest coding rate within the range that the coding rate of the coded data transmitted on the broadcast transmission path is equal to or higher than the coding rate of the coded data transmitted on the transmitting server 6 in response to the packet loss rate information and that allows the packet loss of the IP network 8 to be corrected), performing the reverse process of the interleaving process on the transmitting server 6 side, reconstructing and extracting the coded data retransmitted in response to the retransmission request, and performing likelihood conversion so as to use it for decoding process, and repeating the request for retransmission of the corresponding coded data until it can be decoded by the decoding process within a predetermined time.Note that when the receiving device 5 cannot decode the coded data requested for retransmission by the decoding process within a predetermined time, it generates the received data in a state that includes bit errors.

Each of one or more transmission servers 6 is configured as a computer that stores a predetermined amount of coded data from transmission device 2 (or transmission device 3), which digitally modulates coded data encoded using error correction codes related to digital broadcasting and transmits the coded data to reception device 5 via a broadcast transmission path, reconstructs the coded data requested for retransmission from reception device 5 in accordance with packet loss rate information obtained from reception device 5, generates coded data packets of IP packet strings that have been subjected to interleaving processing, and enables transmission to reception device 5 via IP network 8.

That is, the transmission server 6 has a function of sequentially inputting the coded data generated by the transmission device 2 (or the transmission device 3) via, for example, a local area network, managing it in time series by a synchronization signal based on the frame number of the error correction code frame constituting the code length of the error correction code or on time information, and storing it while updating it for a predetermined time period, a function of receiving a retransmission request packet for the coded data together with the packet loss rate information when it is determined that a bit error in the coded data cannot be corrected by the error correction code used by the transmission device 2 (or the transmission device 3) via the IP network 8, and transmitting to the reception device 5, in accordance with the packet loss rate information, a function of transmitting a retransmission request packet for the coded data with a coding rate equal to or higher than the coding rate of the coded data transmitted over the broadcast transmission path and which is also equal to the coding rate of the coded data transmitted over the IP network, The function is to reconstruct the coded data at the highest coding rate within the range that packet loss in the network 8 can be corrected, form m frames of error correction code frames having continuous synchronization signals (frame number or time information) with the error correction code frame constituting the coded data having the synchronization signal (frame number or time information) related to the retransmission request at the head, and generate an interleaved frame with the same coding rate as the coded data having the synchronization signal (frame number or time information) related to the retransmission request, and perform interleaving processing on the interleaved frame in a direction perpendicular to the m frames of error correction code frames to generate coded data packets of IP packet strings, and transmit them to the receiving device 5 via the IP network 8. Note that in this embodiment, the receiving device 5 does not determine the packet loss rate by estimation by test communication of the round-trip line with the transmission server 6 or by user setting, but measures the packet loss rate of the downlink (line from the transmitting side to the receiving side) as described above. Therefore, the packet loss rate information included in the retransmission request packet that the receiving device 5 transmits to the transmission server 6 may indicate that the packet loss rate is unknown when the packet loss rate cannot be measured. Therefore, when the transmitting server 6 receives packet loss rate information indicating that the packet loss rate is unknown from the receiving device 5, it generates encoded data packets of the IP packet sequence at the encoding rate of the encoded data transmitted over the broadcast transmission path.

Note that, although the transmission system 1 illustrated in FIG. 1 has been described as including the transmitting device 2 and the transmitting device 3, it may be configured as including only one of the transmitting device 2 and the transmitting device 3. In the transmission system 1 illustrated in FIG. 1, the transmitting device 2 (or the transmitting device 3) and the transmitting server 6 have been described as being separate entities, but the transmitting device 2 (or the transmitting device 3) may be configured as including the transmitting server 6, and one or more transmitting devices 2 (or the transmitting devices 3) may be configured as being connected by a simple signal cable instead of being connected by a local area network.

When multiple transmission servers 6 are configured, the load distribution device 7 is provided between the multiple transmission servers 6 and the IP network 8. When retransmission request packets are received from many receiving devices 5 via the IP network 8, the load distribution device 7 distributes the processing load of the multiple transmission servers 6 and transmits the encoded data packets related to the retransmission request in parts. In the transmission system shown in FIG. 1, the load distribution device 7 is interposed between multiple receiving devices 5 whose users have been registered in advance, so that even if the number of registered users increases, it is only necessary to add a transmission server 6. However, when the transmission servers 6 corresponding to the number of registered users allowed in advance are individually provided, the installation of the load distribution device 7 may be omitted and the transmission servers 6 may be directly connected to the IP network 8.

Below, we will explain in more detail the transmitting device 2 (or transmitting device 3), the transmitting server 6, and the receiving device 5 in that order.

[Transmitting device]
Fig. 2 is a block diagram showing a schematic configuration of the transmission server 6 and the transmission device 2 (or the transmission device 3) according to an embodiment of the present invention. Note that the transmission device 2 (or the transmission device 3) shown in Fig. 2 shows only the main components according to the present invention, and the description of other components such as energy (power) diffusion processing is omitted.

Transmitting device 2 and transmitting device 3 employ error correction coding processing and modulation methods suitable for their respective broadcast transmission paths (satellite broadcast transmission paths or terrestrial broadcast transmission paths), but as shown in FIG. 2, they will be described collectively as including an error correction coding unit 21 and a modulation unit 22.

The error correction coding unit 21 is an encoder that performs error correction coding processing on transmission data related to digital broadcasting, adds error correction code parity, generates coded data, and outputs the coded data to the modulation unit 22. The error correction coding unit 21 also has a function of managing the sequentially generated coded data as an error correction code frame, adding a frame number or time information that functions as a synchronization signal, and transmitting the data to the transmission server 6, for example, via a local area network.

The modulation unit 22 digitally modulates the coded data obtained from the error correction coding unit 21 using a predetermined modulation method to generate a modulated wave signal, and emits radio waves related to digital broadcasting via the broadcast transmission path.

When transmitting data related to digital broadcasting via a terrestrial broadcasting transmission path, the data can be in MPEG2-TS format, which is described in ARIB STD-B31 and used in terrestrial digital broadcasting, and when transmitting data via a satellite broadcasting transmission path, the data can be in TLV (Type Length Value) format, which is described in ARIB STD-B44 and used in advanced wideband satellite digital broadcasting.

In the current terrestrial digital broadcasting, the error correction coding process is performed using a concatenated code in which a convolutional code is used as the inner code and a Reed-Solomon code is used as the outer code. In the next generation of terrestrial digital broadcasting, it is being considered to perform error correction coding using a concatenated code in which an LDPC code is used as the inner code and a BCH (Bose-Chaudhuri-Hocquenghem) code is used as the outer code. In advanced wideband satellite digital broadcasting, the error correction coding process is performed using a concatenated code in which an LDPC code is used as the inner code and a BCH code is used as the outer code. In the following description, a transmission system 1 in which error correction coding is performed using a concatenated code in which an LDPC code is used as the inner code and a BCH code is used as the outer code will be mainly described. In the following description, when an LDPC code is used as the block code in the transmission system 1 of one embodiment, the "LDPC coding rate" is also simply referred to as the "coding rate".

[Sending server]
The transmission server 6 shown in Fig. 2 is configured as a computer, which stores a predetermined time of the coded data from the transmission device 2 (or transmission device 3) that uses the error correcting code related to digital broadcasting to code, digitally modulates the coded data, and transmits it to the receiving device 5 via broadcast transmission line, and reconstructs the coded data according to the packet loss rate information obtained from the receiving device 5 for the coded data that is requested to be retransmitted from the receiving device 5, generates the coded data packet of the IP packet sequence that contains this coded data, and can transmit it to the receiving device 5 via IP network 8, and comprises a storage unit 61, an IP packet generating unit 62, a retransmission request processing unit 63, a coding rate adaptive changing unit 64, a coding rate determining unit 65, and a loss correction performance table storage unit 66.

The storage unit 61 is a functional unit that sequentially inputs the coded data (block-coded signal used in the broadcast transmission path) generated by the transmitting device 2 (or transmitting device 3) based on a synchronization signal (frame number or time information) via, for example, a local area network, and stores the error correction code frames that constitute the coded data while updating them for a predetermined period of time by managing them in chronological order using the synchronization signal (frame number or time information). The coded data obtained from the transmitting device 2 (or transmitting device 3) is assigned a frame number or time information identified by a TMCC (Transmission and Multiplexing Configuration Control) signal, which is a control signal in digital broadcasting used in the broadcast transmission path, as a synchronization signal. For example, in a data transmission method using a broadcast transmission path, the storage unit 61 also receives time information (time information may also be embedded in the main signal) contained in the TMCC signal transmitted together with the coded data in units of error correction code frames, and can manage the coded data in units of error correction code frames sequentially obtained from the transmitting device 2 (or transmitting device 3) using the frame number associated with the time information as a synchronization signal. In addition, the encoded data managed in time series by the storage unit 61 using a synchronization signal (frame number or time information) may be notified from the transmission server 6 via the IP network 8 while being received during digital broadcasting. In other words, in the transmission system 1 shown in FIG. 1, by using the time information or frame number contained in the TMCC signal or main signal as a synchronization signal, it is possible to ensure synchronization between the transmission devices 2 and 3, the reception device 5, one or more transmission servers 6, and the load distribution device 7 that is provided when multiple transmission servers 6 are configured.

The IP packet generator 62 is a functional unit that generates an encoded data packet of an IP packet sequence that stores encoded data related to a retransmission request from the receiver 5 and transmits it to the receiver 5 via the IP network 8. This encoded data packet is generated when the receiver 5 determines that bit errors in the encoded data cannot be corrected using the error correction code used by the transmitter 2 (or the transmitter 3). The IP packet generator 62 has an interleave unit 621 that performs interleave processing to rearrange bits across each error correction code frame based on a predetermined rule that separates the bits of the encoded data related to the retransmission request by a predetermined value or more after adding an identification header for making each interleave frame related to the interleave processing identifiable on the receiver 5 side and a sequence number for the multiple IP packets that make up the encoded data packet, as will be described in detail later with reference to FIG. 6.

When the retransmission request processing unit 63 receives a retransmission request packet from the receiving device 5 via the IP network 8, it extracts the packet loss rate information stored in the retransmission request packet and the retransmission request information indicating the encoded data of the error correction code frame related to the retransmission request. The retransmission request processing unit 63 then notifies the currently received packet loss rate information to the encoding rate determination unit 65, and controls the storage unit 61 to search for the location of the encoded data related to the retransmission request from the storage unit 61 based on the retransmission request information, read out m frames of error correction code frames having consecutive synchronization signals (frame number or time information) starting from the error correction code frame that constitutes the encoded data, and output them to the encoding rate adaptation change unit 64.

Based on the packet loss rate information of IP network 8 obtained from receiving device 5, coding rate determination unit 65 refers to the erasure correction performance table (shown in table 1) stored in erasure correction performance table storage unit 66, determines the highest coding rate (the coding rate with the least parity bit) that is equal to or higher than the coding rate of the coded data transmitted on broadcast transmission line and can correct the packet loss of IP network 8, and controls coding rate adaptive change unit 64.Note that the erasure correction performance table of this example shows the maximum packet loss rate (hereinafter also referred to as "required packet loss rate") that can be achieved error-free for each coding rate of 1/2, 3/5, 2/3, 3/4, 4/5, 5/6, 7/8, 9/10 that is obtained in advance by computer simulation.Therefore, coding rate determination unit 65 can select the coding rate that makes the number of retransmitted bits as small as possible and control coding rate adaptive change unit 64, whether the coding rate of the coded data transmitted on broadcast transmission line is retransmitted without being changed or is retransmitted after being changed. The coding rate adaptive change unit 64 can determine whether to change the coding rate of the block code of the coded data transmitted through the broadcast transmission path and retransmit the data based on the coding rate determined by the coding rate determination unit 64. When the coding rate determination unit 65 receives packet loss rate information indicating that the packet loss rate is unknown from the receiving device 5, it selects the coding rate of the coded data transmitted through the broadcast transmission path.

By the way, in digital broadcasting, usually, multiple types of selectable coding rates are predefined. For example, in ARIB STD-B44, 11 types of coding rates are defined for the LDPC code, which is a block code, namely, 41/120 (≒1/3), 49/120 (≒2/5), 61/120 (≒1/2), 73/120 (≒3/5), 81/120 (≒2/3), 89/120 (≒3/4), 93/120 (≒7/9), 97/120 (≒4/5), 101/120 (≒5/6), 105/120 (≒7/8), and 109/120 (≒9/10). In the transmission system 1 of this embodiment, only 8 types of coding rates in the erasure correction performance table shown in Table 1 are used in the broadcast transmission path. Therefore, when all 11 types of coding rates are used in the broadcast transmission path, an erasure correction performance table that specifies the required packet loss rate corresponding to the corresponding 11 types of coding rates can be used, or in the erasure correction performance table shown in Table 1, for coding rates 1/3, 2/5, and 7/9, the required packet loss rate can be estimated and applied based on the required packet loss rate for other coding rates.

For example, suppose that the coding rate _Fb of the coded data transmitted through broadcast transmission path is 1/2.At this time, when the packet loss rate information of IP network 8 obtained by retransmission request packet from receiving device 5 is 40%, coding rate determination unit 65 refers to erasure correction performance table, and the required packet loss rate that can be error-free transmitted in IP network 8 with 40% packet loss rate is only 1/2, so the coding rate F _i when retransmitting via IP network 8 is determined to be 1/2.On the other hand, when the packet loss rate information of IP network 8 obtained by retransmission request packet from receiving device 5 is 25%, coding rate determination unit 65 refers to erasure correction performance table, and the required packet loss rate that can be error-free transmitted in IP network 8 with 25% packet loss rate is 1/2, 3/5, 2/3, 3/4 from erasure correction performance table, but in order to suppress the line congestion of IP network 8 and improve transmission efficiency, determine the coding rate F _i when retransmitting via IP network 8 to be 3/4, which is the highest coding rate that has the least parity bit.

The coding rate adaptive change unit 64 includes switching units 641 and 642 that switch between changing and not changing the coding rate of the coded data generated by the transmitting device 2 (or transmitting device 3) according to the coding rate determined by the coding rate determination unit 65, and an error correction coding unit 643 that performs an error correction coding process to change the coding rate to the determined coding rate when the coding rate is to be changed.

More specifically, the switching units 641 and 642 are functional units that switch so that when the encoded data related to the retransmission request read from the storage unit 61 (encoded data transmitted over the broadcast transmission path) is retransmitted without changing the encoding rate, the switching units 641 and 642 output the encoded data as is to the IP packet generation unit 62, and when the encoded data is retransmitted after changing the encoding rate, the switching units 641 and 642 output the encoded data reconstructed by changing the encoding rate to the IP packet generation unit 62, when the encoded data is retransmitted after changing the encoding rate.

For example, when the coding rate for transmission via the broadcast transmission path and the coding rate for transmission via the IP network 8 are the same (e.g., F _i = F _b = 1/2), the coding rate adaptive change unit 64 outputs the coded data of the corresponding error correction code frame transmitted by the transmitting device 2 (or transmitting device 3) read from the storage unit 61 directly to the IP packet generation unit 62.

On the other hand, when retransmitting encoded data with a changed encoding rate from transmission via a broadcast transmission path, the encoding rate adaptive change unit 64 changes the encoding rate using the error correction encoding unit 643.

The error correction coding unit 643 first extracts only information bits excluding a predetermined parity from the coded data related to the retransmission request read from the storage unit 61 (coded data transmitted over the broadcast transmission path). Next, the error correction coding unit 643 adds padding bits consisting of bits known between the transmission server 6 and the receiving device 5 (all bits of "0" or "1", or a regular bit pattern of "0" and "1") to an insertion position known between the sending and receiving devices according to the coding rate to be changed. Then, the error correction coding unit 643 performs error correction coding processing on the predetermined information bits to which the padding bits have been added, based on the same coding method as the error correction coding method used by the error correction coding unit 21 in the sending device 2 (or sending device 3), and at a coding rate different from the coding rate transmitted over the broadcast transmission path that is available to the sending device 2 (or sending device 3) related to the broadcast. Finally, the error correction coding unit 643 replaces the parity added to the coded data generated by the transmitting device 2 (or transmitting device 3) with the parity of the block code with the changed coding rate for the coded data transmitted over the broadcast transmission path, adds it to the information bits, and generates coded data for retransmission with the padding bits removed. The method of generating coded data when the coding rate is changed by the error correction coding unit 643 of the coding rate adaptive change unit 64 will be described later with reference to Figures 4 and 5.

For example, when the coding rate for transmission via the broadcast transmission path is different from that for transmission via the IP network 8 (F _b < F _i ), the coding rate adaptive change unit 64 extracts only the information bits from the coded data of the corresponding error correcting code frame transmitted by the transmitting device 2 (or transmitting device 3) read from the storage unit 61, adds padding bits known between the transmitting and receiving devices to an insertion position known between the transmitting and receiving devices, adds parity of a block code with coding rate F _i , and then generates coded data for retransmission from which the padding bits have been removed, and outputs the data to the IP packet generation unit 62. More specifically, when the code length during transmission via a broadcast transmission path is 44,880 bits, and the coding rate is changed from F _b = 1/2 to F _b = 3/4 and retransmitted, parity bits (10,472 bits) are generated and added by an error correction coding process in which padding bits known between the sender and receiver (for example, padding with all 0s) are added to 22,814 information bits (22,440 bits of the main signal corresponding to the coding rate F _b = 1/2), and an error correction code frame with a frame length of 33,286 bits is reconstructed for retransmission with the padding bits removed.

The coding rate adaptive change unit 64 outputs m error correction code frames having consecutive synchronization signals (frame number or time information) beginning with the error correction code frame constituting the coded data related to the retransmission request to the IP packet generation unit 62 at the same coding rate as the coding rate of the coded data related to the retransmission request (when the coding rate of the coded data related to the retransmission request is changed by the error correction coding unit 643, the other consecutive error correction code frames are also subjected to the same coding rate change processing).

The IP packet generation unit 62 then uses the interleaving unit 621 to construct an interleaved frame using m frames of error correction code frames, and performs interleaving processing on the m frames of error correction code frames in a longitudinal (orthogonal) direction to generate an IP packet sequence (encoded data packets). When the encoding rate has been changed, the IP packet generation unit 62 generates and transmits an encoding rate change notification packet as a notification to that effect, and then transmits the encoded data packet to the receiving device 5.

For example, when the code length during transmission via the broadcast transmission path is 44,880 bits, if the coding rate for transmission via the broadcast transmission path and retransmission via the IP network 8 is the same (for example, F _i = F _b ), the frame length of the error correction code frame obtained from the coding rate adaptive change unit 64 will be 44,880 bits, and when retransmitting via the IP network 8, 44,880 IP packets with sequence numbers from 1 to 44,880 are generated for the same code length of 44,880 bits as in transmission via the broadcast transmission path, and the transmission order is interleaved and output. Furthermore, when the coding rate for transmission via the broadcast transmission path differs from that for transmission via the IP network 8 (F _b < F _i ), for example when the coding rate for retransmission via the IP network 8 is F _i = 3/4, the frame length of the error correcting code frame obtained from the coding rate adaptive change unit 64 is 33,286 bits, and 33,286 IP packets with sequence numbers from 1 to 33,286 are generated and output after interleaving the transmission order.

(Control flow related to retransmission request packet)
FIG. 3 is a flowchart showing a control flow relating to a retransmission request packet between the transmission server 6 and the receiving device 5 in one embodiment of the present invention.

First, the transmission server 6 stores, by the storage unit 61, the coded data with coding rate _Fb transmitted over the broadcast transmission path for a predetermined period of time while updating it (step S50). The coded data transmitted over the broadcast transmission path is data that has been error-correction coded, and is composed of error-correction code frames of block codes such as LDPC codes described in ARIB STD-B44.

The receiving device 5 receives and demodulates the digital broadcast transmitted over the broadcast transmission path, and attempts to determine whether or not bit errors in the encoded data can be corrected. If the transmission environment is good and reception is error-free, or if the effects of white noise, etc. are within the range that can be decoded using error correction coding, the error-corrected and decoded received data is output as is so that it can be reproduced. If the transmission environment is poor, such as when there is rain attenuation, and bit errors in the encoded data received over the broadcast transmission path cannot be corrected, the receiving device 5 generates a retransmission request packet for the corresponding encoded data via the IP network 8 and makes a retransmission request to the transmitting server 6 (step S51). This retransmission request packet contains retransmission request information and packet loss rate information, as described above.

When the transmission server 6 receives a retransmission request packet from the receiving device 5 by the retransmission request processing unit 63 (step S52), the retransmission request processing unit 63 extracts the retransmission request information and packet loss rate information from the retransmission request packet, notifies the currently received packet loss rate information to the coding rate determination unit 65, and controls the storage unit 61 to search for the location of the coded data related to the retransmission request based on the retransmission request information, read out the coded data of m consecutive frames of error correction code frames starting from that coded data, and output it to the coding rate adaptation change unit 64 (step S53).

Next, transmission server 6, by coding rate determining unit 65, refers to erasure correction performance table (shown in table 1 as an example) based on packet erasure rate information, determines the highest coding rate F i that is equal to or higher than the coding rate F _b of the coded data transmitted through broadcast transmission line, and can correct the packet erasure of this _IP network 8 (step S54).

Then, the transmission server 6 compares the coding rate F _i determined under the control of the coding rate determination unit 65 with the coding rate F _b of the coded data generated by the transmission device 2 (or transmission device 3) in the coding rate adaptive change unit 64, and switches between changing and not changing the coding rate F _b (step S55). If the coding rate F _b is to be retransmitted without being changed to a different coding rate (step S55: No; F _b = F _i ), the transmission server 6 proceeds to step S57, and if the coding rate F _b is to be changed to a different coding rate and retransmitted (step S55: Yes; F _b < F _i ), the transmission server 6 proceeds to step S58.

When changing the coding rate _Fb of the coded data transmitted over the broadcast transmission path and retransmitting the coded data (step S55: Yes; _Fb < _Fi ), the coding rate adaptive change unit 64 changes the coding rate Fi to a different coding rate Fi for the coded data (information bits with parity bits) of the error correcting code frame related to the retransmission request searched and read out from the storage unit ₆₁ based on the currently received retransmission request information, using padding bits that are known between the information bits and the transmitter and receiver, and then reconstructs the coded data of the error correcting code frame with the padding bits removed and outputs it to the IP packet generation unit 62 (step S56).

On the other hand, when the coding rate adaptive change unit 64 retransmits the coded data transmitted over the broadcast transmission path without changing the coding rate _Fb (step S55: No; _Fb = F _i ), it outputs the coded data (information bits with parity bits) of the error correction code frame related to the retransmission request searched and read from the storage unit 61 based on the currently received retransmission request information as is to the IP packet generation unit 62 (step S57). That is, when the coding rate adaptive change unit 64 retransmits the coded data transmitted over the broadcast transmission path without changing the coding rate _Fb , it outputs the coded data transmitted over the broadcast transmission path as is to the IP packet generation unit 62.

The coding rate adaptive change unit 64 also outputs to the IP packet generation unit 62 m error correction code frames having consecutive synchronization signals (frame number or time information) beginning with the error correction code frame constituting the coded data related to the retransmission request at the same coding rate as the coded data related to the retransmission request (when the coding rate for the coded data related to the retransmission request has been changed by the error correction coding unit 643, the other error correction code frames are also subjected to the same coding rate change processing).

Finally, the transmission server 6 uses the IP packet generator 62 to construct an interleaved frame using m frames of error correction code frames, interleaves the m frames of error correction code frames in a longitudinal (orthogonal) direction to generate an IP packet sequence (encoded data packets), and when the encoding rate is changed, generates and transmits an encoding rate change notification packet as a notification to that effect, and then transmits the encoded data packet to the receiving device 5 (step S58). Note that the transmission server 6 can also construct an encoded data packet including encoding rate information and transmit it to the receiving device 5. In this example, in order to increase the transmission efficiency of the encoded data packets, when the encoding rate is changed, an encoding rate change notification packet is notified in advance to the receiving device 5 as a notification to that effect.

The transmission server 6 may be configured to transmit the coding rate change notification packet multiple times, or may be configured to obtain a reception confirmation response for the coding rate change notification packet and repeatedly transmit the coding rate change notification packet until a reception confirmation response is received from the receiving device 5. Note that when the coding rate of the coded data transmitted via the IP network 8 is the same as the coding rate of the coded data transmitted over the broadcast transmission path, the receiving device 5 only needs to know the information on the coding rate over the broadcast transmission path, so the transmission server 6 may omit the advance notification of the coding rate change notification packet, but may transmit the coding rate change notification packet each time the coded data packet is retransmitted, regardless of whether the coding rate has been changed.

(Method of generating encoded data when changing encoding rate)
An example of generating coded data when the coding rate is changed will be described with reference to Fig. 4. Fig. 4(a) to Fig. 4(d) are each a conceptual diagram showing a method of generating coded data when the coding rate is changed by the coding rate adaptive change unit 64 in the transmission server 6 according to an embodiment of the present invention.

First, Figure 4(a) conceptually shows the structure of an error-correcting code frame of a block code that represents the encoded data transmitted over a broadcast transmission path.

Incidentally, the information on the coding rate of the coded data transmitted over the broadcast transmission path is usually transmitted by a TMCC signal, which is a transmission control signal in digital broadcasting, but it may be configured to be notified in advance from the transmission server 6 via the IP network 8. As described above, with regard to the information on the coding rate of the coded data to be retransmitted in response to a retransmission request, the transmission server 6 generates and transmits a coding rate change notification packet as a notification indicating that the coding rate has been changed by the IP packet generation unit 62, or transmits a coded data packet including the coding rate information to the receiving device 5. Here, if the coding rate of the coded data transmitted over the IP network 8 is the same as the coding rate of the coded data transmitted over the broadcast transmission path, the receiving device 5 only needs to know the information on the coding rate on the broadcast transmission path, so the transmission server 6 can omit the prior notification of the coding rate change notification packet.

As shown in Fig. 4(a), if the coding rate of the coded data in the block code with a code length of N bits transmitted through the broadcast transmission channel is _Fb , the information bits of the coded data (i.e., the main signal in the block code) are composed of N x _Fb bits, and the parity is composed of N x (1 - _Fb ) bits. Usually, when using a block code in a broadcast transmission channel, the number of bits of the code length of the error correction code frame is often constant regardless of the coding rate so that it is easy to handle in the broadcast transmission channel. For example, in ARIB STD-B44, the code length N of the LDPC code, which is a block code, is constant at N = 44880 bits regardless of the LDPC coding rate. Here, if the coding rate of the coded data to be retransmitted via the IP network 8 is F _i , then even when the parity of coding rate F _i is changed from coding rate F _b , it is sufficient to use N×F _b information bits (i.e., the main signal in the block code) to make a block code of code length N bits, so that the coding rate should be within a range of a predetermined number of coding rates available to the transmitting device 2 (or transmitting device 3) that is equal to or higher than the coding rate of the coded data transmitted over the broadcast transmission path.

Therefore, the transmission server 6 that receives retransmission request packet from receiving device 5 extracts the packet loss rate information contained in retransmission request packet by retransmission request processing unit 63, and notifies coding rate determination unit 65.Based on this packet loss rate information, coding rate determination unit 65 refers to loss correction performance table (shown in table 1 as an example), determines the highest coding rate (the coding rate that makes parity bit the smallest) that is equal to or higher than the coding rate of the coded data transmitted on broadcast transmission line and can correct the packet loss of this IP network 8, and controls coding rate adaptive change unit 64.

When the coding rate adaptive change unit 64 leaves the coding rate of the coded data transmitted over the broadcast transmission path unchanged (F _b = F _i ), it outputs the coded data of code length N bits relating to the retransmission request shown in Figure 4(a) read from the storage unit 61 as is to the IP packet generation unit 62.

On the other hand, when the coding rate adaptive change unit 64 changes the coding rate _Fb of the coded data transmitted over the broadcast transmission path to a different coding rate _Fi ( _Fb < _Fi ), it first extracts only N x _Fb information bits from the coded data of code length N bits related to the retransmission request shown in Figure 4(a) read from the storage unit 61 (see Figure 4(b)).

Next, the coding rate adaptive change unit 64 adds N×(F i -F b ) bits of padding bits with a value known between the sender and the receiver (for example, all 0 _bits ) to the N×F _b bits of information bits read from the storage unit ₆₁ by the error correction coding unit 643 at an insertion position known between the sender and the receiver, and performs error correction coding processing to obtain a coding rate F _i on the information bits and padding bits to generate a parity of N×(1 - F _i ) bits (see FIG. 4(c)). For example, the error correction coding unit 643 changes the coding rate F _b to a different coding rate F _i that can be used by the sending device 2 (or the sending device 3) while using the N×F _b bits of information bits as the coding rate F _i .

Then, the error correction coding unit 643 removes the padding bits, reconstructs the coded data for retransmission by adding N×(1−F _i )-bit parity to the N×F _b- bit information bits, and outputs the reconstructed coded data to the IP packet generating unit 62 (see FIG. 4(d)). Since the frame length of the reconstructed coded data for retransmission is shorter than the original code length of N bits, the transmission efficiency is higher than when coded data for the code length of N bits is retransmitted to the receiving device 5.

(Method of generating encoded data when changing encoding rate in one embodiment)
As described above, normally, in digital broadcasting, multiple types of selectable coding rates are predefined, but here, as those used in this transmission system 1, the predefined eight types of LDPC coding rates shown in Table 1 are used, and for these eight types of LDPC coding rates, for example, the check matrix and the check matrix initial value table for forming the check matrix, which are predefined in ARIB STD-B44, can be used as they are. Therefore, as a more specific embodiment, an example of generating coded data when the coding rate is changed within the range of the number of types of LDPC coding rates shown in Table 1 conforming to ARIB STD-B44, will be described with reference to FIG. 5.

The example shown in Figures 5(a) to 5(d) is an example in which the transmission server 6 receives coded data of advanced wideband digital satellite broadcasting, which began broadcasting in December 2018 as advanced wideband digital satellite broadcasting, with a modulation method of 16APSK and an LDPC coding rate of 61/120 (≒ 1/2), and stores it in the storage unit 61. When the radio wave reception conditions deteriorate from the receiving device 5, the coding rate of the coded data requested for retransmission via the IP network 8 is changed to 109/120 (≒ 9/10) and retransmits it.

As shown in FIG. 5(a), here, the coding rate of the coded data in the LDPC code with code length N=44880 bits transmitted through the broadcast transmission path is F _b =61/120 (≈1/2). In ARIB STD-B44, one error correcting code frame has a constant code length of 44880 bits regardless of the LDPC coding rate, and is configured in slot units based on the set partitioning method, so that the information bits to be coded by the LDPC code are the "slot header", "main signal (data to be transmitted)", "BCH code parity", and "stuff bit" that have been power-dispersed, and one error correcting code frame is configured by adding the parity of the LDPC code. Then, in the storage unit 61 in the transmission server 6, the coded data in units of one error correcting code frame shown in FIG. 5(a) is managed in chronological order with a synchronization signal (frame number or time information) added, and is stored while updating for a predetermined time. The LDPC code parity at an LDPC coding rate of 61/120 (≈1/2) is composed of 44880×(1−61/120)=22066 bits.

Then, the transmission server 6 that receives the retransmission request packet from the receiving device 5 extracts the packet loss rate information contained in the retransmission request packet by the retransmission request processing unit 63 and notifies the coding rate determination unit 65. Based on the packet loss rate information, the coding rate determination unit 65 refers to the loss correction performance table (illustrated in Table 1) to determine the optimal coding rate for retransmission via the IP network 8 in terms of transmission efficiency, and controls the coding rate adaptation change unit 64.

When the coding rate adaptive change unit 64 leaves the coding rate of the coded data transmitted over the broadcast transmission path unchanged (F _b = F _i ), it outputs the coded data of code length N bits relating to the retransmission request shown in Figure 5 (a) read from the storage unit 61 as is to the IP packet generation unit 62.

On the other hand, when the coding rate adaptive change unit 64 changes the coding rate _Fb = 61/120 (≈ 1/2) of the coded data transmitted over the broadcast transmission path to a different coding rate _Fi = 109/120 (≈ 9/10) ( _Fb < _Fi ), first, it extracts only the information bits to be coded with the N x _Fb = 22814-bit LDPC code (i.e., the BCH coded bits that have been power-dispersed) from the coded data of code length N = 44,880 bits related to the retransmission request shown in Figure 5(a) read from the storage unit 61 (see Figure 5(b)).

Next, the coding rate adaptive change unit 64 adds, via the error correction coding unit 643, N×(F _i -F _b ) = 17,952 bits of padding bits with a value known between the sender and the receiver (for example, all 0 bits) to the 22,814 information bits (i.e., the BCH coded bits after power dispersion) read out from the storage unit 61 at an insertion position known between the sender and the receiver, and performs error correction coding processing on these information bits and padding bits with a coding rate of F _i = 109/120 (≈ 9/10) to generate N×(1 - F _i ) = 4,114 bits of parity (see Figure 5 (c)).

Then, the error correction coding unit 643 removes the padding bits, reconstructs the coded data for retransmission (26,928 bits) by adding 4,114 bits of parity to the 22,814 bits of information bits, and outputs it to the IP packet generating unit 62 (see FIG. 5(d)). Since the frame length of this reconstructed coded data for retransmission, 26,928 bits, is shorter than the original code length of 44,880 bits, the transmission efficiency is higher than when coded data of the code length of 44,880 bits is retransmitted to the receiving device 5.

In this way, when the transmission server 6 receives a retransmission request packet, it adaptively changes the coding rate of the coded data related to the retransmission request based on the packet loss rate information contained in the retransmission request packet, and generates a coded data packet through interleaving processing of m error correction code frames, and transmits it to the receiving device 5.

(Method of generating encoded data packets)
When transmitting encoded data for which a retransmission request has been made to the receiving device 5 via the IP network 8, in order to achieve high correction performance even in the case of burst packet losses occurring in the IP network 8, it is preferable for the transmitting server 6 to construct an n x m interleaved frame, which will be described later with reference to Figure 6, by means of the IP packet generating unit 62, generate an IP packet sequence with an identification header, sequence number, and IP header, interleave the sending order of the IP packets according to the sequence number, and transmit the encoded data packets to the receiving device 5.

More specifically, the method of generating IP packets by the IP packet generator 62 in the transmission server 6 will be described with reference to FIG. 6. FIGS. 6(a) to 6(d) are explanatory diagrams of the method of generating IP packets by the IP packet generator 62 in the transmission server 6 in one embodiment of the present invention.

First, the IP packet generating unit 62 inputs m frames of error correcting code frames having consecutive synchronization signals (frame number or time information) starting from an error correcting code frame constituting coded data having a synchronization signal (frame number or time information) related to a retransmission request, each of which is composed of a code length of n bits with the same coding rate _{F i} , from the coding rate adaptive changing unit 64, and forms an n×m interleaved frame. Note that m is a fixed value, but can be variably set from the outside, and is a value shared between the sender and the receiver. Also, n=N when the code length n bits of the coding rate F _i has not been changed from the code length N bits of the coding rate F _b used in the broadcast transmission path, and n<N when it has been changed from the code length N bits of the coding rate F _b .

Therefore, the IP packet generation unit 62 temporarily stores m error correction code frames, including the error correction code frame as the encoded data related to the retransmission request, in a vertically arranged memory unit (not shown) (see FIG. 6(a)).

Then, the IP packet generation unit 62 reads each bit from the beginning of the m-frame error correction code frame using the interleave unit 621, and generates n packets of m-bit IP payloads as the packet length excluding the header of the IP packet to be generated (see FIG. 6(b)). That is, the IP payload is m bits obtained by collecting the same bits from bits 1 to n of each error correction code frame by one bit each.

Then, the IP packet generation unit 62 adds an identification header to the IP payload of n packets generated by the interleaving unit 621, which enables the receiving device 5 to identify each interleaved frame involved in the interleaving process, a sequence number that specifies which bit in each error correction code frame it represents, and an IP header as the header of the encoded data packet, thereby generating n packets of IP packets (see FIG. 6(c)). Note that in this example, the sequence numbers are expressed as 1 to n for ease of understanding, but any expression form can be used as long as it allows the receiving device 5 to identify which bit in the error correction code frame each IP packet represents.

As shown in FIG. 6(b), one interleaved frame is composed of IP payloads for n packets generated from multiple error correction code frames. If the IP packet generator 62 assigns, for example, ID="1" to an identification header to be added to a certain interleaved frame, it assigns ID="2" to the identification header to be added to the next interleaved frame, and ID="3" to the identification header to be added to the next interleaved frame while incrementing, so that the interleaved frame to which the identification header belongs can be identified by its value. Therefore, as shown in FIG. 6(c), the identification header is added so that it has the same value within one interleaved frame. Therefore, the receiving device 5 can identify which interleaved frame the IP payload of the received encoded data packet belongs to by referring to the identification header.

Then, the IP packet generating unit 62 uses the interleaving unit 621 to interleave the generated n packets of IP packets based on the sequence numbers and a predetermined rule for the order of transmission to the IP network 8 (for example, the packets may be sent in ascending order of sequence numbers, or a different order of sequence numbers determined between the sender and the receiver), and transmits them as encoded data packets to the receiving device 5.

In this embodiment, the IP packet generation unit 62 requires m (m is an integer of 1 or more) error correction code frames including an error correction code frame as the encoded data related to the retransmission request. In other words, the interleaving unit 621 cannot perform interleaving until m error correction code frames are available. Therefore, if time loss is a problem, only a predetermined number of bits n1 (<n) as sequence numbers of the n-bit error correction code frames related to the retransmission request may be extracted from m (m is an integer of 1 or more) error correction code frames to reduce the total number of IP packets holding the corresponding sequence numbers. Furthermore, m and n are fixed values but can be variably set from the outside, so that the packet length of each IP packet can be adjusted.

In addition, while the IP packets are generated by collecting one bit from each error correction code frame, it is also possible to collect multiple bits from each error correction code frame. Conversely, it is also possible to treat two error correction code frames as one frame, or to generate 2n packets of m bits. The IP payloads of n packets thus generated are treated as one interleaved frame, and an identification header for making each interleaved frame identifiable on the receiving device 5 side and a sequence number for identifying each IP payload are assigned. In other words, other interleaving techniques can be applied as long as the order of sending each bit of the error correction code frame is rearranged so that packet loss in the IP network 8 can be predicted in advance and mitigated. In general, the longer the period (signal length) to be subjected to interleaving processing, the stronger the resistance to burst packet loss, so the interleaving unit 621 is configured to perform optimal interleaving processing within an allowable period for the entire transmission system 1.

[Receiving device]
7 is a block diagram showing a schematic configuration of a receiving device 5 according to an embodiment of the present invention. The receiving device 5 includes a demodulator 51, an error correction decoder 52, a retransmission request packet generator 53, and an IP packet receiver .

The demodulation unit 51 receives and demodulates the modulated wave signal emitted by radio waves from the transmission device 2 (or the transmission device 3) via the broadcast transmission path (satellite broadcast transmission path or terrestrial broadcast transmission path), and outputs the encoded data obtained by this demodulation process to the error correction decoding unit 52.

The error correction decoding unit 52 is a decoder that calculates the log-likelihood ratio (LLR) of each bit of the encoded data obtained from the demodulation unit 51 prior to the decoding process of the error correction code, reconstructs the error correction code frame that constitutes the code length of the error correction code using this prior log-likelihood ratio (prior LLR), and performs a decoding process corresponding to the error correction encoding process in the transmitting device 2 (or transmitting device 3) to generate received data and output it externally in a reproducible manner.

The error correction decoding unit 52 has a function of judging whether the encoded data constituting the error correction code frame can be decoded for error correction, and when it judges that the encoded data can be decoded, it decodes it as it is to generate received data, and when it judges that the bit error of the encoded data cannot be corrected and the encoded data cannot be decoded, it outputs retransmission request information for the encoded data constituting the error correction code frame to the retransmission request packet generating unit 53. Note that in a data transmission method using a broadcast transmission path, the encoded data in units of error correction code frames obtained by the receiving device 5 receiving it sequentially via the demodulation unit 51 can be identified based on the time information included in the TMCC signal or main signal transmitted together, and can also be managed by a synchronization signal (frame number or time information) like the transmission server 6. Therefore, as a synchronization signal, this retransmission request information may include time information that makes it possible to identify the error correction code frame to be retransmitted, or may also include the frame number.

The error correction decoding unit 52 has a function of extracting coded data related to the retransmission request from the coded data packet received from the transmission server 6 via the IP packet receiving unit 54 in response to the transmission of the retransmission request packet by the retransmission request packet generating unit 53, and performing a conversion related to the replacement of the likelihood ratio required for decoding the error correction code frame of the coded data, thereby complementing the coded data received on the broadcast transmission path.

The error correction decoding unit 52 is configured to repeatedly request retransmission of the encoded data via the IP network 8 for the encoded data received via the broadcast transmission path that it has determined cannot correct bit errors in and cannot be decoded, until the encoded data can be decoded by error correction code decoding processing within a predetermined time using the encoded data acquired via the IP network 8. Note that when the error correction decoding unit 52 cannot decode the encoded data by the decoding processing within the predetermined time, it generates the received data with the bit errors still included.

In addition, in this example, when the transmission server 6 changes the coding rate of the coded data to be retransmitted, it generates and transmits a coding rate change notification packet as a notification to that effect, and then transmits the coded data packet to the receiving device 5. Therefore, the error correction decoding unit 52 determines whether the coding rate of the coded data obtained via the IP network 8 is different from that at the time of broadcast reception based on the presence or absence of a coding rate change notification, adds padding bits as appropriate according to the coding rate, and performs conversion related to the replacement of the likelihood ratio required for decoding, thereby complementing the coded data received on the broadcast transmission path.

The retransmission request packet generating unit 53 is a functional unit that generates a retransmission request packet in an IP packet format that includes retransmission request information indicating a request to retransmit the encoded data and packet loss rate information obtained from the packet loss rate measuring unit 542 in the IP packet receiving unit 54, in order to request the transmission server 6 to retransmit the encoded data when the error correction decoding unit 52 determines that bit errors in the encoded data cannot be corrected and the encoded data cannot be decoded, and transmits the packet to the transmission server 6 via the IP network 8.

The IP packet receiving unit 54 is a functional unit that receives from the transmission server 6 an encoded data packet of an IP packet sequence that stores the encoded data retransmitted in response to the retransmission request packet, acquires the encoded data related to the retransmission request, and outputs it to the error correction decoding unit 52. The IP packet receiving unit 54 has a deinterleaving unit 541 that uses an identification header and a sequence number to perform the reverse process of the interleaving unit 621 on the transmission server 6 side to reconstruct the encoded data retransmitted in response to the retransmission request, and a packet loss rate measuring unit 542 that measures the packet loss rate of the downlink (the line from the transmitting side to the receiving side) based on the sequence number attached to each IP packet of the received encoded data packet.

The packet loss rate measurement unit 542 is a functional unit that sequentially measures and retains the packet loss rate of the downlink (the line from the sender to the receiver) based on the sequence number assigned to each IP packet of the received encoded data packet, and notifies the retransmission request packet generation unit 53 each time it generates a retransmission request packet.

(Example: Reception control flow in a receiver for advanced wideband digital satellite broadcasting)
8 is a flowchart showing a reception control flow of one example of the receiver 5 according to one embodiment of the present invention. Since the receiver 5 operates in the same way whether it receives a signal from the transmitter 2 or the transmitter 3, the reception control flow of the receiver 5 that receives a modulated wave signal radiated from the transmitter 2 and demodulates and decodes the signal will be described below taking as an example a satellite digital broadcasting reception conforming to the advanced wideband satellite digital broadcasting.

First, the receiving device 5 receives and demodulates the modulated wave signal emitted from the transmitting device 2 via the broadcast transmission path of the satellite digital broadcast using the demodulation unit 51, and outputs the encoded data obtained by this demodulation process to the error correction decoding unit 52 (step S1).

Then, in order to execute a decoding process corresponding to the error correction encoding process in the transmitting device 2 by the error correction decoding unit 52, the receiving device 5 first calculates the a priori LLR of each bit of the encoded data from the likelihood table and reconstructs the error correction code frame (step S2). Since the number of bits per frame of encoded data in advanced wideband digital satellite broadcasting is 44,880 bits, and the error correction encoding process uses an LDPC code as the inner code and a BCH code as the outer code, the likelihood table is used to calculate a priori log-likelihood ratios indicating the likelihood that a bit is "0" and the likelihood that a bit is "1".

Then, the receiving device 5 performs LDPC decoding using a sum-product algorithm based on a log-likelihood ratio by the error correction decoding unit 52 (step S3), and then performs power despreading processing and decoding processing of the BCH code (step S4). ARIB STD-B44 specifies that if bit errors cannot be corrected completely during the decoding processing of the BCH code of the outer code and the data does not become error-free, the data is replaced with a null packet, a flag is set indicating that an error exists, etc.

The error correction decoding unit 52 then determines whether or not the coded data obtained from the transmitting device 2 can be decoded for error correction (specifically, determines whether or not the bit errors could not be completely corrected in the decoding process of the BCH code, or whether or not the errors were corrected in the decoding process but the number of BCH errors was a predetermined number) (step S5). That is, in this embodiment, focusing on the fact that the correction capability of the LDPC code is uncertain and the correction capability of the BCH code is deterministic, an efficient retransmission request in Hybrid ARQ is realized by utilizing the presence or absence of an error-free BCH code connected to the LDPC code.

Here, the error correction decoding unit 52 determines whether the encoded data obtained from the transmitting device 2 can be decoded for error correction. If it determines that the encoded data can be decoded, it decodes the encoded data as is to generate reproducible received data (step S5: No). In this case, it continues to receive the modulated wave signal emitted by the transmitting device 2.

On the other hand, if the error correction decoding unit 52 determines that the bit errors in the encoded data obtained from the transmitting device 2 via the broadcast transmission path cannot be corrected and the data cannot be decoded (if it determines that the bit errors could not be completely corrected in the BCH code decoding process, or that the errors were corrected in the decoding process but a predetermined number of BCH errors occurred) (step S5: Yes), it generates retransmission request information for the encoded data to be decoded, and outputs this to the retransmission request packet generating unit 53 to instruct it to generate a retransmission request packet indicating a retransmission request for the encoded data.

In response to an instruction from the error correction decoding unit 52, the retransmission request packet generating unit 53 generates a retransmission request packet in the form of an IP packet that includes retransmission request information indicating a request for retransmission of the encoded data to be decoded and packet loss rate information obtained from the packet loss rate measuring unit 542 in the IP packet receiving unit 54, and transmits the packet to the transmission server 6 via the IP network 8 (step S6).

Therefore, the transmission server 6 refers to the erasure correction performance table (shown in Table 1) based on the packet erasure rate information obtained from the receiving device 5 for the coded data requested to be retransmitted from the receiving device 5, and reconstructs the coded data with the highest coding rate that is equal to or higher than the coding rate of the coded data transmitted on the broadcast transmission path and that can correct the packet erasure of the IP network 8.In addition, the transmission server 6 forms an interleaved frame by forming m frames of error correction code frames with continuous synchronization signals (frame numbers or time information) with the same coding rate as the coded data with the synchronization signal (frame number or time information) related to the retransmission request, starting with the error correction code frame that constitutes the coded data with the synchronization signal (frame number or time information) related to the retransmission request, and performs interleaving processing on the interleaved frame in the direction perpendicular to the error correction code frame of m frames to generate coded data packets of IP packet strings, and transmits them to the receiving device 5 via the IP network 8.In addition, when the coding rate is changed, the transmission server 6 generates and transmits a coding rate change notification packet as a notification indicating that, and then transmits the coded data packet.

The receiving device 5 receives, via the IP packet receiving unit 54, from the transmission server 6, encoded data packets (IP packet sequence) of m error correction code frames including the encoded data to be decoded that was retransmitted in response to the retransmission request packet (step S7).

When the transmission server 6 changes the encoding rate, it generates and transmits an encoding rate change notification packet as a notification to that effect, and then transmits the encoded data packet. Therefore, the receiving device 5 can determine, by the IP packet receiving unit 54 and the error correction decoding unit 52, whether the encoding rate of the encoded data obtained via the IP network 8 is different from that at the time of broadcast reception.

Then, the IP packet receiving unit 54 uses the identification header and sequence number added to the IP packet by the deinterleaving unit 541 to perform the reverse process of the interleaving unit 621 on the transmitting server 6 side, reconstructing the coded data retransmitted in response to the retransmission request, and outputs it to the error correction decoding unit 52 (step S8).

If the coding rate of the coded data obtained via the IP network 8 is different from that at the time of broadcast reception, the error correction decoding unit 52 adds padding bits that are known between the sender and the receiver according to the coding rate to an insertion position that is known between the sender and the receiver in order to restore the error correction code frame with the changed coding rate (step S9). On the other hand, if the coding rate of the coded data obtained via the IP network 8 is the same as that at the time of broadcast reception, there is no need to add padding bits.

Then, the error correction decoding unit 52 extracts the coded data related to the retransmission request from the coded data packet received from the transmission server 6 via the IP packet receiving unit 54 in response to the transmission of the retransmission request packet by the retransmission request packet generating unit 53, performs conversion related to the replacement of the likelihood ratio required for decoding the error correction code frame of the coded data, and complements the coded data received on the broadcast transmission path (step S10). Assuming that the IP network 8 is a loss communication path, the error correction decoding unit 52 determines the correct bit value for the coded data that was received via the IP network 8, and regards the correct bit value as unknown for packets that were lost due to packet loss or the like on the communication path on the way to the IP network 8. More specifically, the error correction decoding unit 52 is configured as an error correction decoder for block codes and performs decoding using the log-likelihood ratio, so that for coded data acquired via the IP network 8, the log-likelihood ratio when the bit value is 0 is set to +∞ (the maximum value as to the likelihood that it is "0"), the log-likelihood ratio when the bit value is 1 is set to -∞ (the maximum value as to the likelihood that it is "1"), and if a packet loss occurs and a non-delivery bit occurs, the log-likelihood ratio is replaced with 0.

The error correction decoding unit 52 then performs LDPC decoding with a check matrix corresponding to the coding rate transmitted via the IP network 8, and then performs power despreading and BCH decoding, and repeats a request for retransmission of the coded data obtained from the transmitting device 2 until the bit errors in the coded data can be decoded by the error correction code decoding process within a predetermined time (steps S3 to S10). After generating received data based on the decoded coded data, if the current receiving path is via the IP network 8, the receiving path is switched to reception via the broadcast transmission path, and the demodulation and decoding process for the next coded data in the time series is started. Here, the error correction decoding unit 52 can be configured to completely restore the received data by repeatedly requesting retransmission until there are no more bit errors in the BCH code decoding process, but it is preferable to avoid infinite loop processing by setting a limit within a predetermined time. Note that when the error correction decoding unit 52 cannot decode the data by the decoding process within the predetermined time, it generates received data that still contains bit errors, and switches the receiving path to reception via the broadcast transmission path.

The retransmission request packet can be a non-delivery notification packet that is commonly used in IP packet format, and the encoded data packet is configured to be retransmitted in response to the non-delivery notification packet. Therefore, the error correction decoding unit 52 can replace the encoded data obtained from the demodulation unit 51 with the encoded data obtained by retransmission and perform decoding again. By repeatedly requesting retransmission until the received data can be restored within a specified time, it is possible to output the data in a reproducible manner.

(Measurement and Notification of Packet Loss Rate)
Fig. 9 is a diagram for explaining the measurement and notification of the packet loss rate of one embodiment in the receiving device 5 of one embodiment of the present invention. First, as shown in Fig. 9, when the receiving device 5 receives the coded data packets of the IP packet sequence transmitted from the transmission server 6 by the IP packet receiving unit 54, in order to deinterleave the coded data packets of the IP packet sequence by the function of the deinterleaving unit 541, rearrange the packets based on the sequence number attached to each IP packet, and try to reconstruct an n x m interleaved frame. At this time, if there is an IP packet with a packet number of L that has been lost, an interleaved frame of (n-L) x m is constructed (step S21). For example, if the IP packets with sequence numbers 2 and n-1 have been lost, the number of lost packets is L (in this case L = 2).

Then, the IP packet receiving unit 54 counts the number of lost packets based on the sequence number using the function of the packet loss rate measuring unit 542, and calculates the packet loss rate ε of the downlink (step S22). The packet loss rate ε is expressed as ε = L/n × 100 [%].

Then, the packet loss rate measurement unit 542 automatically notifies the retransmission request packet generation unit 53 of the packet loss rate ε every time the IP packet reception unit 54 receives an encoded data packet (step S23). Note that, in the control flow shown in FIG. 8, an example has been described in which the error correction decoding unit 52 repeats a retransmission request for the encoded data to be decoded based on a determination of whether decoding is possible for a predetermined period of time. However, in this embodiment, the retransmission request packet generation unit 53 may repeat a retransmission request for the encoded data to be decoded until the packet loss rate notified by the packet loss rate measurement unit 542 within the predetermined period of time indicates that no IP packets have been lost.

When the packet loss rate measurement unit 542 cannot measure the packet loss rate, for example, when a retransmission request is made immediately after the start of communication in the receiving device 5 to the transmission server 6, it notifies the retransmission request packet generation unit 53 that the packet loss rate is unknown. Upon receiving this notification, the retransmission request packet generation unit 53 transmits packet loss rate information indicating that the packet loss rate is unknown to the transmission server 6 along with the retransmission request information. The transmission server 6, which has received the packet loss rate information indicating that the packet loss rate is unknown from the receiving device 5, generates an encoded data packet of an IP packet sequence at the encoding rate of the encoded data transmitted over the broadcast transmission path, and transmits it to the receiving device 5 via the IP network 8.

For example, if an IP packet sequence related to coded data with code length n=44880 has L=11220 packet losses, the packet loss rate at this time is 11220/44880×100=25%. In this case, the retransmission request packet generator 53 transmits packet loss rate information indicating a packet loss rate of 25% to the transmission server 6 together with the retransmission request information. The transmission server 6 that has received the packet loss rate information indicating a packet loss rate of 25% determines the coding rate of the coded data to be retransmitted by referring to the loss correction performance table shown in Table 1 based on the packet loss rate of 25%, as described above. Then, when the coding rate is changed, the transmission server 6 generates a coded data packet of an IP packet sequence related to coded data with a changed coding rate of the coded data transmitted over the broadcast transmission path, and transmits it to the receiving device 5 via the IP network 8.

According to the transmission system 1 of this embodiment, for data loss that cannot be prevented by broadcast reception alone, a retransmission request is made from the receiving device 5 to the transmission server 6 via the IP network 8, and the transmission server 6 can retransmit the data, so that the receiving device 5 can supplement the data. In particular, the data retransmitted by the transmission server 6 via the IP network 8 is encoded data in the block code of digital broadcasting, and the encoded data is variably controlled to a coding rate that minimizes the number of bits to be retransmitted as much as possible according to the packet loss rate of the IP network 8, thereby making it possible to reduce the number of retransmission requests and further improve the transmission efficiency via the IP network 8. In addition, by making both the error correction code used for broadcast reception and the error correction code used in transmission via the IP network 8 the same as the coding format of the error correction code specified by the broadcast standard, the receiving device 5 can be realized by simply preparing one error correction decoder, and the equipment scale can be reduced.

In particular, in the transmission system 1 of this embodiment, the coded data requested to be retransmitted from the receiving device 5 to the transmitting server 6 is retransmitted from the transmitting server 6 to the receiving device 5 as a coded data packet of an IP packet sequence with a sequence number assigned thereto, so that the receiving device 5 measures the packet loss rate of the downlink (the line from the transmitting side to the receiving side) based on the sequence number of the received IP packet.Then, the transmitting server 6 variably controls the coding rate of the coded data to be retransmitted based on the packet loss rate.Therefore, according to the transmission system 1 of this embodiment, it is possible to select the coding rate of the coded data to be retransmitted more appropriately than when using the packet loss rate estimated by test communication of the round-trip line between the transmitting server 6 and the receiving device 5 or determined by user setting, and the transmission efficiency via the IP network 8 can be further improved.

(Variable control of padding bit insertion position according to coding rate)
In the examples shown in FIGS. 4 and 5 described above, the error correction coding unit 643 of the coding rate adaptive change unit 64 in the transmission server 6 inserts padding bits in the error correction code frame when the coding rate is changed between information bits (power disperse BCH coded bits in this embodiment) and parity bits. However, it has been found that, depending on the coding rate, the correction performance of the error correction code can be further improved by allocating and arranging the padding bits before and after the information bits due to the characteristics of the check matrix of the LDPC code.

In this embodiment, the error correction coding unit 643 of the coding rate adaptive change unit 64 determines the insertion position of padding bits in the error correction code frame for each coding rate of the LDPC code and shares this position between the sender and the receiver (between the transmitting server 6 and the receiving device 5), thereby changing the coding rate without degrading the correction performance of the LDPC code.

More specific examples are shown in Figures 10 and 11. Figure 10 is a diagram showing the positions at which padding bits are inserted according to the coding rate, which is changed from the coding rate F _b = 1/2 of the coded data transmitted over a broadcast transmission path, and Figure 11 is a corresponding table.

10 and 11 show the insertion positions of padding bits when the coding rate F _b =1/2 of the coded data transmitted over the broadcast transmission path is changed to coding rates F _i =3/5, 2/3, 3/4, 4/5, 5/6, 7/8, and 9/10 and the coded data is retransmitted via the IP network 8. In the example shown in Fig. 10, all padding bits to be inserted are set to 0. As the insertion positions of the padding bits, front-stage padding bits Pd_f allocated to the front of the information bit i and rear-stage padding bits Pd_r allocated to the rear of the information bit i are set, and how to allocate them for each coding rate F _i is determined.

Note that Figures 10 and 11 show the number of bits when changing from coding rate F _b = 1/2 to another coding rate _Fi ; however, when changing from another coding rate F _b (for example, 3/5) to another coding rate _Fi , the method of allocating the front-stage padding bits Pd_f and the rear-stage padding bits Pd_r is the same, and further, the number of bits of the front-stage padding bits Pd_f is also set to a fixed value for each coding rate, and the number of bits of the rear-stage padding bits Pd_r is adjusted according to the coding rate.

However, the example shown in Figure 10 and Figure 11 is the example of obtaining the insertion position of the padding bit with excellent erasure correction performance based on the time when coding rate F _b = 1/2 is changed to another coding rate F _i , therefore, in all combinations when coding rate F _b is changed to another coding rate F _i , the distribution method of front padding bit Pd_f and rear padding bit Pd_r, and the bit number of the fixed value of each coding rate of front padding bit Pd_f can be further optimized and determined.

(Method of evaluating erasure correction performance based on padding position pattern)
Here, referring to Fig. 12 and Fig. 13, the simulation of obtaining the insertion position of the padding bit with excellent erasure correction performance is described based on the time when coding rate F _b =1/2 is changed to another coding rate F _i . Fig. 12 is the diagram of the example of the evaluation method of erasure correction performance by the padding position pattern of the padding bit according to the present invention.In addition, Fig. 13 is the diagram of the characteristic when coding rate is changed from 1/2 to 3/4 as the evaluation of erasure correction performance by the padding position pattern of the padding bit according to the present invention.

As shown in FIG. 12, first, a predetermined number of padding position patterns are prepared (step S31). Here, as shown in FIG. 12, 29 padding position patterns P0-P28 are shown when changing the coding rate from 1/2 to 3/4.

In other words, the simulation conditions were information bits i = 22,814 bits, total padding bits Pd (Pd_f + Pd_r) = 10,472 bits, parity bits p = 11,594 bits, the padding bits Pd were divided into 374-bit units, and the positions of the front-stage padding bits Pd_f located at the beginning of the error correcting code frame (LDPC frame) and the rear-stage padding bits Pd_r located after the information bits were adjusted.

Then, the case where the information bits i = 22814 bits, the front padding bits Pd_f = 0 bits, the rear padding bits Pd_r = 10472 bits, and the parity bits p = 11594 bits are arranged in this order is defined as P0, the case where the front padding bits Pd_f = 374 bits, the information bits i = 22814 bits, the rear padding bits Pd_r = 10098 bits, and the parity bits p = 11594 bits are arranged in this order is defined as P1, and similarly up to P28 are defined.

Next, an interleaved frame is constructed while changing the padding bits (front padding bits Pd_f and rear padding bits Pd_r) for each pattern of padding positions P0-P28, and a simulation is performed to calculate the bit error rate (BER) of the transmission of the interleaved coded data packets by simulating a burst loss transmission path, and to find a position where the BER can be reduced (step S32). Note that the simulated burst loss transmission path has a variable packet loss rate, and loses a number of packets according to the set packet loss rate in units of 8 packets. In addition, by simulating a transmission in which interleaving/deinterleaving is performed between the transmitter and receiver, continuous packet losses are randomized.

In the simulation, for each pattern of padding positions P0-P28, padding bits were inserted after deinterleaving, and the same pattern was repeatedly retransmitted until the erasures could be corrected by error correction decoding, and the bit error rate (BER) after the erasure correction was measured.

Figure 13 shows the BER calculation results for each padding position P = P0-P28 pattern when the packet loss rate is set to 30%. In Figure 13, since the BER was smallest at P2, pattern P2 was determined to be the padding position that can bring out the loss correction performance when changing the coding rate from 1/2 to 3/4. Similar calculations were performed for other coding rates, and the padding bit insertion position was determined for each coding rate, as shown in Figures 10 and 11.

In this way, when coding rate F _b =1/2 is changed to another coding rate F _i , can obtain the insertion position of the padding bit with excellent erasure correction performance; furthermore, when coding rate F _b is changed to another coding rate F _i , in all combinations, can also optimize and determine the distribution method of front padding bit Pd_f and rear padding bit Pd_r, and the bit number of the fixed value of front padding bit Pd_f.

As can be seen from the evaluation results illustrated in FIG. 13, the error correction coding unit 643 of the coding rate adaptive change unit 64 in the transmission server 6 allocates padding bits to be inserted before and after the information bits according to the coding rate in the error correction code frame when the coding rate is changed, thereby making it possible to further enhance the correction performance of the error correction code.

[Modification of Receiving Device]
The receiving device 5 of one embodiment shown in FIG. 7 described above is a preferred example for measuring the packet loss rate. However, even in a configuration in which the packet loss rate is estimated by test communication of the round-trip line between the transmitting server 6 and the receiving device 5 or set by a user setting, as in the receiving device 5 of the modified example shown in FIG. 14, it is possible to "variably control the insertion position of the padding bits according to the coding rate" as described with reference to FIGS. 10 to 13.

Figure 14 is a block diagram showing a schematic configuration of a modified receiving device 5 according to the present invention. The receiving device 5 includes a demodulator 51, an error correction decoder 52, a retransmission request packet generator 53, an IP packet receiver 54, and a packet loss rate setting unit 55. Note that the same components as those in the receiving device 5 shown in Figure 7 are given the same reference numbers.

The modified receiving device 5 shown in FIG. 14 has a demodulation unit 51 and an error correction decoding unit 52 that operate in the same manner as those shown in FIG. 7, so further explanation will be omitted, but it differs in that the IP packet receiving unit 54 does not have the packet loss rate measurement unit 542 shown in FIG. 7, but instead has a packet loss rate setting unit 55 that sets packet loss rate information.

The IP packet receiving unit 54 is a functional unit that receives from the transmission server 6 an encoded data packet of an IP packet sequence that stores the encoded data resent in response to the resend request packet, acquires the encoded data related to the resend request, and outputs it to the error correction decoding unit 52. The IP packet receiving unit 54 also has a deinterleaving unit 541 that uses the identification header and sequence number to perform the reverse process of the interleaving unit 621 on the transmission server 6 side and reconstructs the encoded data resent in response to the resend request.

The packet loss rate setting unit 55 is a functional unit that attempts test communication with the transmission server 6, measures in advance in a predetermined period of time either the delay related to the communication with the transmission server 6 or the amount of packet loss related to the communication with the transmission server 6, or both, and sequentially estimates the packet loss rate based on either the delay in the communication line or the amount of packet loss, or determines the packet loss rate in advance as specified by the user and sets it in the retransmission request packet generating unit 53 as packet loss rate information.

The delay in the communication line indicates a transmission delay estimated from the time difference between the transmission time of the retransmission request packet sent from the receiving device 5 to the transmission server 6 and the reception time of the encoded data packet retransmitted from the transmission server 6 in response to the retransmission. By utilizing the fact that the packet loss rate tends to increase as the transmission delay increases, a conversion table (not shown) that previously associates the transmission delay with the packet loss rate according to the present invention can be maintained to estimate the packet loss rate according to the present invention.

The amount of packet loss is measured based on communication performance over a specified period of time, assuming that a packet loss has occurred when a corresponding encoded data packet has not been received from the transmission server 6 even after a specified time has elapsed since the transmission time of the retransmission request packet. The packet loss rate can be calculated directly from the amount of packet loss, or the packet loss rate according to the present invention can be estimated by holding a conversion table (not shown) that previously associates the amount of packet loss and transmission delay with the packet loss rate according to the present invention, taking into account the transmission delay related to the communication with the transmission server 6.

That is, even in the modified receiving device 5 shown in FIG. 14, packet loss rate information can be transmitted together with retransmission request information in a retransmission request packet to the transmitting server 6. And, in the modified receiving device 5 shown in FIG. 14, the error correction decoding unit 52 also operates in the same manner as the receiving device 5 shown in FIG. 7, so that if the coding rate of the coded data obtained by retransmission from the transmitting server 6 is different from that at the time of broadcast reception, padding bits that are known between the transmitting and receiving parties according to the coding rate can be added to an insertion position that is known between the transmitting and receiving parties.

Therefore, the modified receiving device 5 shown in FIG. 14 can also perform, together with the above-mentioned transmitting server 6, "variable control of the insertion position of padding bits according to the coding rate" as described with reference to FIGS. 10 to 13.

In the above embodiment, a program for making each means of a computer functioning as the transmission server 6 function can be preferably used. Also, a program for making each means of a computer functioning as the receiving device 5 function can be preferably used. Specifically, a control unit for controlling each means can be configured with a central processing unit (CPU) in a computer, and a storage unit for appropriately storing a program required to operate each means can be configured with at least one memory. That is, the function of each of the above-mentioned means can be realized by having the CPU execute the program in such a computer. Furthermore, a program for making each of the means function can be stored in a predetermined area of the above-mentioned storage unit (memory). Such a storage unit can be configured with a RAM or ROM inside the device, or can be configured with an external storage device (for example, a hard disk). Also, such a program can be configured as a part of the software on the OS used by the computer (stored in a ROM or an external storage device). Furthermore, a program for making such a computer function as each means can be recorded on a computer-readable recording medium. Also, each of the above-mentioned means can be configured as part of hardware or software, and can be realized by combining each of them.

The above-mentioned examples, applications, and modifications of one embodiment have been described as representative examples, but it will be apparent to those skilled in the art that many modifications and substitutions can be made within the spirit and scope of the present invention. For example, an example has been described in which the transmission server 6 inputs encoded data directly from the transmission device 2 (or transmission device 3), but in relay broadcasting, etc., the receiver may first receive modulated waves from the transmission device 2 (or transmission device 3) and demodulate them, and then input the encoded data obtained by demodulation. Therefore, the present invention should not be construed as being limited by the above-mentioned examples, but is limited only by the scope of the claims.

The present invention makes it possible to efficiently combine error correction codes used in digital broadcasting with retransmission requests, making it useful for data compensation related to digital broadcasting.

LIST OF SYMBOLS 1 Transmission system 2 Transmitter for satellite broadcast transmission path 2a Transmitting antenna for satellite broadcast transmission path 3 Transmitter for terrestrial broadcast transmission path 3a Transmitting antenna for terrestrial broadcast transmission path 4 Broadcasting satellite 5 Receiving device 5a Receiving antenna for satellite broadcast transmission path 5b Receiving antenna for terrestrial broadcast transmission path 6 Transmission server 7 Load balancing device 8 IP network 21 Error correction coding unit 22 Modulation unit 51 Demodulation unit 52 Error correction decoding unit 53 Retransmission request packet generation unit 54 IP packet receiving unit 55 Packet loss rate setting unit 61 Storage unit 62 IP packet generation unit 63 Retransmission request processing unit 64 Coding rate adaptive change unit 65 Coding rate determination unit 66 Erasure correction performance table storage unit 541 Deinterleaving unit 542 Packet loss rate measurement unit 621 Interleaving unit 641, 642 Switching unit 643 Error correction coding unit

Claims (11)

A transmission server that digitally modulates coded data that has been coded using an error correction code related to digital broadcasting, and transmits the digitally modulated coded data to a receiving device via a broadcast transmission path, stores the coded data for a predetermined period of time, and enables the data to be transmitted to the receiving device via an IP (Internet Protocol) network,
a storage unit which sequentially receives the coded data generated by the transmitting device, manages the coded data in a time series manner by a synchronization signal based on frame numbers of error correction code frames constituting a code length of the error correction code or on time information, and stores the coded data managed in a time series manner for a predetermined time while updating the coded data;
a retransmission request processing unit that receives a retransmission request packet via an IP network, the retransmission request packet being generated when it is determined in the receiving device that a bit error in the encoded data cannot be corrected using the error correction code, and extracts packet loss rate information stored in the retransmission request packet, the packet loss rate information being measured based on reception of the encoded data set in the receiving device or retransmitted via the IP network, and retransmission request information indicating the encoded data of the error correction code frame related to the retransmission request;
A coding rate determination unit that refers to a loss correction performance table having a required packet loss rate indicating a maximum packet loss rate that can be lost and corrected for each coding rate based on the packet loss rate information, and determines a coding rate of the coding data for retransmission;
a coding rate adaptive change unit that determines whether or not to change the coding rate of the block code of the coded data generated by the transmitting device and retransmit the coded data based on the coding rate determined by the coding rate determination unit, and when changing the coding rate, changes the coding rate of the coded data related to the retransmission request obtained by reading from the storage unit and a predetermined number of coded data consecutive to the coded data based on the retransmission request information, thereby forming a plurality of error correction code frames;
an IP packet generating unit that generates an interleaved frame using the plurality of error correction code frames, adds an identification header for enabling the interleaved frame to be identified, a sequence number for identifying which bit in each error correction code frame the interleaving is performed on, and an IP header that is a header of the encoded data packet, to a payload obtained by performing interleaving across the error correction code frames, thereby generating an encoded data packet of an IP packet string, and transmits the encoded data packet to the receiving device via an IP network;
the coding rate adaptive change unit has a means for, when changing the coding rate of the coded data generated by the transmitting device and retransmitting the coded data to the receiving device, adding padding bits known at the receiving device to information bits of the coded data in accordance with the coding rate to be changed, to an insertion position known at the receiving device, performing an error correction coding process, replacing the error correction code parity added to the coded data generated by the transmitting device with an error correction code parity obtained by performing the error correction coding process, and removing the padding bits to generate an error correction code frame as coded data for retransmission;
The inserting position of the padding bit is determined in advance, and the inserting position of the padding bit with excellent performance of erasure correction is obtained based on the pre-evaluation when changing from a specific coding rate to another coding rate. The inserting position of the padding bit is determined in advance, and the ... with excellent performance of erasure correction is obtained based on the pre -evaluation when changing from a specific coding rate to another coding rate. the predetermined number of coding rates available to the transmitting device include at least 1/2, 3/5, 2/3, 3/4, 4/5, 5/6, 7/8, and 9/10;
The insertion position of the padding bit is determined based on a prior evaluation for determining an insertion position of a padding bit having excellent loss correction performance for packet loss based on a change from a coding rate of 1/2 to another coding rate.
When the coding rate is 1/2, no padding bits are inserted.
When the coding rate is 3/5 , 2/3 , 4/5, or 9/10, the information bits are followed by
2. The transmission server according to claim 1, wherein it is previously determined that the information bits are allocated to the front and rear stages when the coding rates are 3/4, 5/6, and 7/8. The transmission server according to claim 1 or 2, characterized in that the coding rate determination unit determines, based on the same coding method as the error correction coding method used in the transmission device, the highest coding rate among a predetermined number of coding rates available to the transmission device, which is equal to or higher than the coding rate of the coded data transmitted through the broadcast transmission path and within a range in which packet losses in the IP network can be corrected. the predetermined number of coding rates available to the transmitting device include at least 1/2, 3/5, 2/3, 3/4, 4/5, 5/6, 7/8, and 9/10;
4. The transmission server according to claim 3, wherein the loss correction performance table has, as the required packet loss rate, a rate that decreases according to the level of the coding rate of the predetermined number of types. A transmitting device for digital broadcasting,
5. A transmitting device comprising: a unit for sequentially outputting, to the transmitting server according to claim 1, coded data that has been coded using an error correction code related to said digital broadcasting. A transmitting device for digital broadcasting,
A transmission device comprising the transmission server according to any one of claims 1 to 4 therein. A receiving device receives a modulated wave signal transmitted via a broadcast transmission path by digitally modulating coded data that has been coded by a transmitting device using an error correction code related to digital broadcasting, the receiving device comprising:
A demodulation unit that receives and demodulates the modulated wave signal;
an error correction decoding unit that reconstructs an error correction code frame constituting a code length of the error correction code from the encoded data obtained by demodulation and performs a decoding process corresponding to the error correction code to generate received data so that the data can be reproduced;
a retransmission request packet generating unit that generates a retransmission request packet in an IP packet format, the retransmission request packet including retransmission request information indicating coded data of an error correcting code frame related to a retransmission request for requesting retransmission of the corresponding coded data when it is determined that bit errors in the coded data cannot be corrected using the error correcting code, and packet loss rate information measured based on reception of the coded data retransmitted via a setting or an IP network, and transmits the retransmission request packet to a transmission server according to any one of claims 1 to 4;
an IP packet receiving unit that receives an encoded data packet of an IP packet sequence storing encoded data retransmitted in response to the retransmission request from the transmission server in response to the retransmission request packet, and performs a reverse process of the interleaving performed by the transmission server to extract the encoded data retransmitted in response to the retransmission request,
the error correction decoding unit, for the coded data retransmitted in response to the retransmission request, restores an error correction code frame without adding padding bits at the coding rate of the coded data obtained by demodulation when the coding rate has not been changed, and restores the error correction code frame by adding padding bits according to the coding rate at the insertion position of the padding bits assigned by the transmission server according to the coding rate when the coding rate has been changed , attempts a decoding process for the retransmitted coded data through a complementation process related to replacement of a likelihood ratio required for decoding the restored error correction code frame , and repeatedly requests retransmission of the corresponding coded data until decoding is possible within a predetermined time. The error correction code is a concatenated code in which an LDPC code is concatenated as an inner code and a BCH code is concatenated as an outer code,
The receiving device according to claim 7, characterized in that the error correction decoding unit determines that bit errors in the encoded data could not be corrected when bit errors could not be completely corrected in the decoding process of the BCH code, or when errors could be corrected in the decoding process of the BCH code but a predetermined number of BCH errors have been corrected. 9. The receiving device according to claim 7, further comprising a packet loss rate measuring unit that counts the number of lost packets based on sequence numbers of the IP packet string, measures a packet loss rate of the IP network , and notifies the retransmission request packet generating unit of the packet loss rate information. The receiving device according to claim 7 or 8, further comprising a packet loss rate setting unit that sets a packet loss rate estimated by test communication of a round-trip line with the transmission server or determined by a user setting as the packet loss rate information in the retransmission request packet generating unit. A program for causing a computer to function as a transmission server according to any one of claims 1 to 4.