Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L1/00—Arrangements for detecting or preventing errors in the information received
  • H04L1/02—Arrangements for detecting or preventing errors in the information received by diversity reception
  • H04L1/06—Arrangements for detecting or preventing errors in the information received by diversity reception using space diversity
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B7/00—Radio transmission systems, i.e. using radiation field
  • H04B7/02—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas

Definitions

• the field of the invention is that of communication systems, and more specifically that of communication systems for the transmission of multimedia content to portable terminals via digital broadcasting networks.
• DVB-SH (satellite services for mobile devices), a variant of DVB-H, is an example of a digital broadcasting standard for transmitting content to mobile terminals.
• DVB-SH is a hybrid satellite / terrestrial system that allows broadcast broadcast of content from satellites as from terrestrial repeaters to mobile terminals.
• Satellite transmission provides coverage of large areas, while the terrestrial component provides coverage in areas where good direct reception of the satellite signal is not possible, for example in urban areas.
• the DVB-SH standard specifies two modes of operation.
• SH-A mode uses Orthogonal Frequency Division Multiplexing (OFDM) to refer to both orthogonal frequency division multiplexing on both the satellite link and the terrestrial link.
• OFDM Orthogonal Frequency Division Multiplexing
• SH-B mode uses TDM (Time Division Multiplexing) over the satellite link, and OFDM modulation over the terrestrial link.
• the terrestrial transmitters are synchronized to transmit the same bit in the same OFDM carrier at the same time (with an accuracy of 100ns) and use the SFN (Single Frequency Network) transmission mechanisms.
• the satellite transmitter can be synchronized with terrestrial transmitters using the same SFN technique: SHA-SFN mode.
• the satellite transmitter may also not be synchronized with terrestrial transmitters: this is known as SHA -Non SFN mode.
• the DVB-SH compatible mobile TV terminals in SH-B mode or in SHA-Non SFN mode are thus multi-mode terminals having two reception circuits able to receive a compatible data stream.
• DVB-SH via two transmission channels: a terrestrial channel and a satellite channel.
• terminals having a DVB-H reception circuit and a DVB-SH circuit have characteristics equivalent to the terminals mentioned above from the point of view of the applicability of the invention.
• the aim is to improve the reception quality of the multimedia content by the terminal, in a context where the same content arrives by two (or more) different transmission channels.
• the different transmission channels have different rates and error distribution models, so that some information may arrive wrong by one channel while they arrive without error by another channel.
• the state of the art uses combination techniques to improve the quality of reception.
• Combination techniques carried out at the level of the physical layer are also known. These techniques exploit the fact that the architecture of the physical layer allows for different radio layers and relies on common interleaving and protection modules (FECs). This is the case, for example, of DVB-SH whose physical layer allows OFDM or TDM radio levels which feed a common FEC mechanism (TurboCode) as well as a common interleaver.
• FECs common interleaving and protection modules
• the set of services are organized in bursts, the set of MPEG-2 packets making it possible to constitute a burst of each service forms a multiplexing frame whose size in MPEG-2 packets must be a multiple of the length of the packet.
• an SH frame (“SH Frame").
• the services broadcast are grouped into HS services whose length in MPEG-2 packets is fixed and multiples of 8.
• the HS services common to both channels may benefit from the code combination.
• the fact of resorting to HS services whose size is fixed limits the beneficial effects of statistical multiplexing for video streams that by nature have a variable bit rate. It is recalled that the statistical multiplexing uses the variability of the flows to allow the increase of the number of streams emitted simultaneously in a fixed rate transmission channel. This is done because while some streams are transmitting little data, the bandwidth can be used to satisfy the needs of data-intensive streams.
• the gain expressed in the number of additional channels increases with the total number of streams that can participate in the statistical multiplexing operation.
• the invention is part of this approach to improve the quality of reception and aims to propose a technique that allows to benefit from the redundancy of information without presenting the disadvantages of a combination at the radio level or at the physical level .
• the invention proposes a method of protection against losses in a terminal able to receive a stream of data transmitted in the form of bursts via at least one first channel and one second channel. characterized in that it implements forward error correction based on the union of a burst received from the first channel with the same burst received from the second channel.
• bursts carry IP datagrams and the bursts of the first channel at least are formed of data sections in which are encapsulated IP datagrams and forward error correction sections, and wherein the union of bursts consists in mapping the IP datagrams of both bursts into an Application Data Table.
• the second burst includes forward error correction sections, to join the forward error correction sections of the one and the other bursts in an RS Data Table, failing this, forward error correction sections of the salvo received from the first channel in an RS Data Table;
• the forward error correction sections comprise intra-burst error correction sections, and forward error correction is implemented from the Application Data Table and the RS Data Table to recompose lost data inside a salvo;
• the forward error correction sections also comprise inter-burst error correction sections, and in which following the implementation of the intra-burst correction:
• the united IP datagrams are interleaved in a plurality of Application Data Tables and the united inter-burst correction sections are interleaved in a corresponding plurality of RS Data Tables,
• a direct error correction is implemented for each corresponding Application Data Table and RS Data Table among the plurality of Application Data Tables and RS Data Tables, in case of total or partial loss a salvo, recompose the IP datagrams of said lost salvo;
• the forward error correction sections comprise inter-burst error correction sections bearing the number of the burst, and wherein it is recognized that a salvo received from the second channel is the same salvo as that received since the first channel by exploiting the burst numbers entered in the inter-burst error correction sections; - in the process:
• the united IP datagrams are interleaved in a plurality of Application Data Tables and the united inter-burst correction sections are interleaved in a corresponding plurality of RS Data Tables,
• a direct error correction is implemented for each corresponding Application Data Table and RS Data Table among the plurality of Application Data Tables and RS Data Tables, in case of total or partial loss a salvo, recompose the IP datagrams of said lost salvo;
• the reception of the same burst from the second channel is waited for a predetermined duration, and the union and the direct error correction are implemented only when the burst has been received from the second transmission channel during said predetermined time;
• a direct error correction is implemented on the basis of only the sections of the burst received from the first channel;
• a direct error correction of the burst received from the first channel is implemented, and a direct error correction is implemented on the basis of the union the salvo received from the first channel previously autonomously corrected with the same salvo received from the second channel;
• the transmission channels are a terrestrial channel and a satellite channel
• the stream of data carried on the first channel is DVB-H or DVB-SH compatible
• the invention is based on a hybrid transmission system in which a data stream comprising a plurality of elementary streams (ES) cut out temporally to be transmitted in the form of bursts, is issued to a terminal via at least two different transmission channels.
• ES elementary streams
• a preferred, but not limiting, example of such a hybrid transmission system is a system compatible with the DVB-SH standard (and with either of its modes SH-A and SH-B), the channels in this example a satellite channel and a terrestrial channel.
• the invention relates to the reception of the data stream by a multi-mode terminal able to receive said data stream via at least two different transmission channels, for example via a channel satellite and a terrestrial channel in the case of a DVB-SH compatible system, and aims to improve the quality of the reception by proposing in a first aspect a method of protection against losses.
• the invention proposes to benefit at the terminal correction power which is attached to an elementary stream, and to achieve a combination of correction power attached to the stream elementary as received from one of the transmission channels with the correction power attached to the same elementary stream as received from the other of the transmission channels.
• the data stream comprises a plurality of elementary streams, each temporally divided to be transmitted in the form of bursts.
• Each burst carries IP datagrams encapsulated in MPE (Multi-Protocol Encapsulation) sections, which are themselves transported in MPEG-2 packets.
• MPE Multi-Protocol Encapsulation
• a forward error correction (FEC) mechanism can be implemented to provide protection against data loss.
• This mechanism is applied at the level of the data link layer according to the OSI model, more precisely at the level of the MAC ("Medium Access Control") layer of the link layer of data.
• This mechanism can, for example, make it possible to correct against the loss of data within a burst.
• the implementation of this correction (which will subsequently be referred to as 'intra-burst FEC') is described in ETSI EN 301 192: “Digital Video Broadcasting (DVB); DVB specification for data broadcasting ", in particular paragraph 9:” Time slicing and MPE-FEC ".
• IP datagrams of a burst are stored vertically in the columns of an Application Data Table (ADT according to the English terminology). "Application Data Table" with a maximum of 191 columns and a maximum of 1024 rows.
• a Reed Solomon encoder processes each line of the ADT, which leads to compute for each line of 191 bytes of data, a parity of 64 bytes. These parity bytes are stored in the rows of a RS Data Table (RSDT) comprising 64 columns.
• RSDT RS Data Table
• the ADT and RSDT arrays together form an MPE-FEC frame. In this way, the data sections formed from the IP datagrams of the ADT are associated and by using the MPE (“MultiProtocol Encapsulation”) encapsulation protocol, intra-burst error correction sections constituted by the different columns. of the RSDT.
• each burst of a plurality of B + S bursts of an elementary stream is interleaved into a plurality B of Application Data Tables (ADTs). Then a RS Data Table (RSDT) is calculated for each of the B Application Data Tables (ADTs). The calculated correction is then dispersed over a plurality of bursts of the elementary stream, by transmitting intersalves FEC columns in a burst, said columns being selected from the Fo columns of a plurality B + S of said RS Data Tables (RSDT). .
• the data sections formed from the IP datagrams of the ADT are associated with inter-burst error correction sections constituted by different columns of the plurality of RSDTs.
• one or both of these correction mechanisms are implemented on the data stream received from one of the transmission channels.
• the data stream received from the terrestrial channel is thus processed independently of the data stream received from the satellite channel to correct the losses.
• the sections of a burst of the stream received from the first channel are arranged in a first ADT S Application Data Table and in FIG. a first data table RS RSDTs, and the one or more error correction mechanisms (intra-burst FEC, inter-burst FEC) are applied on the basis of these tables ADT S and RSDT S.
• the sections of a burst of the stream received from the second channel are arranged in a second ADT T Application Data Table and in a first RS RSDT T Data Table, and the error correction mechanism or mechanisms (intra-burst FEC, inter-burst FEC) are applied on the basis of these tables ADT T and RSDT T.
• a multi-mode terminal 1 able to receive a data stream from a first and a second transmission channel, an error correction mechanism based on the union, at the data link layer, of the data stream received from the first transmission channel with the same data stream received from the second channel. More specifically, it is proposed to perform the union of a burst from the first transmission channel with the same burst received from the second transmission channel, and to implement an error correction based on this union.
• This correction is represented by block 10 in FIG. 1; it outputs corrected loss-corrected burst data in an ADT 0 Application Data Table.
• Such a union is particularly advantageous insofar as, as we have seen previously, the transmission channels (terrestrial, satellite) have different types of error.
• the transmission on the satellite channel is subject to long losses (typically several consecutive sections, even whole bursts) while transmission on the terrestrial channel is subject to shorter losses (typically a section).
• the bursts of the data stream carry IP datagrams and the bursts of the at least one first channel are formed of data sections (MPE sections) in which the IP datagrams and error correction sections are encapsulated.
• MPE sections data sections
• error correction sections are encapsulated.
• the union then consists of mapping the IP datagrams of both bursts into an ADTu Application Data Table and arranging the forward error correction sections (MPE-FEC sections and / or MPE-OFEC sections) of the burst received from the first channel in a RS RSTu Data Table (these sections being optionally joined to the direct error correction sections of the burst received from the second channel, when the second channel also implement an MPE-FEC and / or MPE-OFEC protection mechanism).
• MPE-FEC sections and / or MPE-OFEC sections forward error correction sections
• the data stream is substantially isochronically transmitted over the channels so that a burst arrives substantially at the same time to the terminal from both (with temporal accuracy dt) of the channels.
• the terminal having received a salvo from one of the channels, can determine that the salvo it subsequently receives, in a time interval corresponding to said precision dt, on the other channel is the same salvo as that received on the first channel.
• the time difference dt between the emission of the same burst on the first channel considered as master and the time of emission of the same burst on the various other channels must, in the general case, be less than the burst repetition interval without being equal to zero (perfect isochronia) or close to zero. It should be noted that large dt values create temporal diversity that increases the efficiency of the code combination. Since the forward error correction sections include MPE-FEC intra-burst error correction sections, the intra-burst FEC forward error correction is implemented from the Application Data Table. ADTu and RS RSTu Data Table in which the union of the same burst received from either channel was performed to recompose the lost data inside the burst.
• the inter-burst forward error correction is also implementation from the ADTu Application Data Tables and RS RSTu Data Tables in which the union of the same burst received from the one and the other of the channels has been achieved, in case of total loss or part of a salvo, recompose the data sections of said lost salvo.
• This correction is for example carried out in the manner recommended by the Applicant in the patent application filed in France on April 4, 2007 under the number FR0754285, by performing the following steps:
• a FEC forward error correction is implemented for each corresponding Application Data Table and RS Data Table from among the plurality of Application Data Tables and RS Data Tables, to recompose the united IP datagrams in case of total or partial loss of the burst.
• the terminal having received a burst from one of the channels, uses the burst_number field of an MPE-OFEC section to determine the number of the burst. The terminal can thus verify that the burst it subsequently receives on the other channel has the same number and is therefore the same burst as that received on the first channel.
• the inter-burst error correction is implemented by using, as indicated above, Tables ADTu and RSDTu in which the bursts of the same number and received from the one and the other channels have been united.
• the recognition of two identical bursts is based on the analysis of their contents.
• the bursts carry IP datagrams which are uniquely identified on the Internet by the concatenation of the source and destination addresses network (IP) and transport (TCP / UDP) as well as by the identifier of the datagram.
• IP IP
• TCP / UDP transport
• the criterion for deciding the identity of two bursts may for example consist of checking that they have in common one or more IP datagrams.
• This identification mode also allows the combination of a channel based on the DVB-H or DVB-SH standard with a Wimax channel on which the transmission is also operated by bursts, but without necessarily using the MPE encapsulation mechanisms, neither the MPE-FEC / MPE-OFEC protection mechanisms, nor complementary signaling mechanisms indicating the position of the datagram in a virtual ADT Table.
• a conventional error correction using only the sections of the burst received on this channel is first implemented on the burst received from one of the channels. Then the union of this autonomously corrected burst with the same burst as received from the other channel (possibly also already independently corrected), is then performed, to then implement an error correction applied to the united sections of to combine the power of correction.
• the invention is of course not limited to a method according to its first aspect, but also extends to a multi-mode terminal adapted to receive a stream of data transmitted in the form of bursts through at least a first channel and a second channel, the terminal being characterized in that it comprises means configured to implement the method according to the first aspect of the invention.
• the invention also extends to an electronic component intended to be integrated in a terminal adapted to receive a stream of data transmitted in the form of bursts formed of a plurality of sections through at least one first channel and a second channel, characterized in that it comprises means adapted to perform the union of a burst received from the first channel with the same burst received from the second channel and to implement an error correction directly on the bursts thus united.

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• Engineering & Computer Science (AREA)
• Computer Networks & Wireless Communication (AREA)
• Signal Processing (AREA)
• Detection And Prevention Of Errors In Transmission (AREA)
• Error Detection And Correction (AREA)

Priority Applications (1)

Applications Claiming Priority (2)

Publications (2)

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• 2008
  • 2008-04-16 FR FR0852568A patent/FR2930388A1/fr active Pending
• 2009
  • 2009-04-10 EP EP09753735A patent/EP2274856A2/de not_active Withdrawn
  • 2009-04-10 WO PCT/EP2009/054351 patent/WO2009144080A2/fr not_active Ceased

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