GB2540202A

GB2540202A - Enhanced line encoding

Info

Publication number: GB2540202A
Application number: GB1512082.7A
Authority: GB
Inventors: Thoumy François; Achir Mounir
Original assignee: Canon Inc
Current assignee: Canon Inc
Priority date: 2015-07-10
Filing date: 2015-07-10
Publication date: 2017-01-11
Also published as: GB201512082D0

Abstract

A device for encoding a bitstream for transmission over a communication chain comprises a unit 160 for splitting the bitstream into a plurality of substreams, at least two different types of line encoder 170, 180 for respectively encoding the sub-streams, e.g. MB12 and 8b10b encoders, and a transmission unit 190, e.g. a 4-ASK modulator, for transmitting the encoded substreams. The transmission unit provides variable reliability levels for transmission of the encoded substreams through the mapping of bits to symbols during modulation. The line encoders are matched to the reliability level with which the corresponding substreams are transmitted, in particular, an MB12 encoder is chosen for the most reliable bit positions (MSB) since it has a spectrum shape with a smooth increase at low frequencies and so is more robust against filtering of the DC component in the communication chain. The invention takes advantage of the unequal sensitivity to noise of bits during the modulation process thereby improving the tradeoff between the coding rate and the BER/SNR performances.

Description

ENHANCED LINE ENCODING

FIELD OF THE INVENTION

The present invention relates to line encoding.

The invention has applications, for example, in transmission systems wherein the bits of a transmitted bitstream are not equally sensitive to noise such as in ASK modulation.

In particular, the present invention has application in THz communication systems.

BACKGROUND OF THE INVENTION

Line coding is known in wired communications. In wireless communications, it can be implemented in order to shape the spectrum of the signals.

In particular, line coding may be used in addition to the modulation of the signals. This may be the case, for example, in ultra-high speed communications that take advantage of the large bandwidth provided by the terahertz communication technology. In such cases, simple modulation and coding schemes (MCS) may be more appropriate. The complexity must be as low as possible in order to allow an efficient implementation. Some MCS already used in high speed wired communication can be used in terahertz communications like BPSK, OOKor ASK for example.

Line coding technics that can be used in wireless communications are for example 8B10B (A.X. Widmer, P.A. Franaszek, “A DC-Balanced, Partitioned-Block, 8B/10B Transmission Code”, IBM Journal of Research and development, vol. 27, No. 5, Sep. 1983, pp. 440-451), 64B66B (M. Hajduczenia, “64b/66b line code”, proposal for 10 Gigabit Ethernet (IEEE 802.3)), MB810 (D.Y. Young Kim, C. Lee, C. S. Shin, H. W. Jung and Η. H. Lee, “White paper on the MB810 Line Code for 10GbE’, proposal for 10 Gigabit Ethernet (IEEE 802.3), and document US2005/0012646) or the like. These techniques may be implemented in PCI express, Gigabit Ethernet, DVI, HDMI and many other standards.

The purpose of a line code is to shape the spectrum of a signal to make it better suited to the communication chain transporting the signal. In a wireless communication system, the communication chain is typically composed of the transmitting and receiving radio modules (transceivers) and the communication channel. For example, the 8B10B line coding is used to remove the DC (Direct Current) component so that the spectrum of the bitstream has no power at the null frequency. A DC free bitstream is preferable since generally the null frequency is filtered by the decoupling capacitors that are used between the different stages of the transceivers (filters, amplifiers ...etc.). DC free line codes are obtained when the line code generates a bounded value for the Running Digital Sum (RDS). In order to obtain a power of the spectrum at the null frequency as low as possible, the value of the RDS bound must be as low as possible. Then, the best DC free codes have null RDS. A Nyquist free line code (considered as a minimum bandwidth code) is also very interesting for ultra-high speed communications since it has a spectrum which has a null component at the Nyquist frequency. Then, a Nyquist free line code releases the constraints on the design of filters, equalizers etc. and improves the quality of the transmission on limited bandwidth channels.

Nyquist free line codes are obtained when the line code generates a bounded value for the Running Alternate Sum (RAS). In order to obtain a power of the spectrum at the Nyquist frequency as low as possible, the value of the RAS bound must be as low as possible. Then, the best Nyquist free codes have null RAS.

Examples of DC Free/ DC free-Nyquist free line codes are given in A.X. Widmer, P.A. Franaszek, “A DC-Balanced, Partitioned-Block, 8B/10B Transmission Code’’, IBM Journal of Research and development, vol. 27, No. 5, Sep. 1983, pp. 440-451.

This document describes one of the most popular DC free code. The proposed scheme, called 8B10B, encodes each 8 bits and status information obtained from the previous encoding into 10 bits. The idea is to balance continuously between the ones and the zeros so that at any time the encoded bitstream has exactly the same number of ones and the number of zeros and hence the bitstream is DC free.

Another example is given in documents D.Y. Young Kim, C. Lee, C. S. Shin, H. W. Jung and Η. H. Lee, “White paper on the MB810 Line Code for 10GbE”, proposal for 10 Gigabit Ethernet (IEEE 802.3), and US2005/0012646, wherein the authors disclose a new line coding scheme providing a spectrum having null power at the null frequency and a null power at the Nyquist frequency, hence a DC free Nyquist free line code. MB12 line code is particularly described.

Amplitude-shift keying (ASK) is a form of amplitude modulation that represents digital data as variations in the amplitude of a carrier wave. ASK uses a finite number of amplitudes, each assigned to a unique pattern of binary digits (note that a pattern of two consecutive bits is referred to as “dibit”). Usually, each value of the amplitude relates to an equal number of bits. Each pattern of bits forms the symbol that is represented by the amplitude.

One property of ASK modulation is to provide reliability levels, i.e. different levels of sensitivity to noise, for the source bits.

Usually, ASK constellation is generated in order to space regularly the symbols (carrier amplitudes) and the source bits are mapped on the symbols.

Thus, whatever the mapping between the source bits and the symbols, the sensitivity to noise of each source bit is not the same.

For instance, in case of 4ASK modulation using the constellation 125 of Figure 1a, the MSB (most significant bit), i.e. the left bit of a dibit, is less sensitive to noise than the LSB (less significant bit), i.e. the right bit of a dibit. In what follows, it is named “the most reliable bit”.

This difference in sensitivity to noise is due to the difference between the values of the mean distance when considering all symbols, the distance being the gap between the two symbols.

For the LSB, the mean distance between ‘0’ and T is 3a and for MSB, the mean distance between ‘0’ and T is 4a.

Simple modulation and coding schemes (MCS) are more appropriate for ultra-high speed communications and some DC free line codes have been already proposed, each of them having different values of coding rate and BER performances.

However, there is a limited number of DC free line codes and this limits the granularity of coding rates as well as the related BER performances.

Document US 8,817,910 “Systems and methods for communicating using ASK or QAM with uneven symbol constellation” discloses a system that provides differing BER performances among bits sent from a transmitter without changing the coding rate. This system is based on the creation of a ASK or a QAM constellation with uneven distance between symbols. However, it does not provide any solution for creating various coding rates.

The inventors have found that it may be advantageous to have a new way to use the known line codes to provide a higher granularity in terms of coding rates while keeping the best BER/SNR performance as possible. Preferably, such new line code arrangement should be easy to implement.

The present invention lies within this context.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided a device for encoding a bitstream for transmission over a communication chain comprising: - a split unit for splitting said bitstream into a plurality of substreams, - a plurality of line encoders for encoding said plurality of substreams, said plurality of line encoders comprising at least two types of line encoders, and - a transmission unit for transmitting a plurality of encoded substreams over said communication chain, said transmission unit providing variable reliability levels for transmission of said plurality of encoded substreams, the device being configured to have at least one sub-stream encoded by a line encoder of a type adapted to the reliability level with which said transmission unit transmits said encoded at least one sub-stream.

According to embodiments, at least two line codes, such as DC free line codes, may be combined to create a new coding scheme which takes advantage of the unequal sensitivity to noise of bits during the modulation process to improve the trade-off between the coding rate and the BER/SNR performances.

Embodiments make it possible, in a communication system based on a simple modulation scheme like n-ASK, to take advantage of the unequal sensitivity to noise of bits during the modulation process thereby improving the tradeoff between the coding rate and the BER/SNR performances.

In particular, various Line codes are combined to encode differently the bits of each symbol and take into account the spectrum shape of the Line codes, the channel and the unequal sensitivity to noise of each bit in such a way that the line code which is the best adapted to the channel is applied on the most reliable bit.

For example, the plurality of line encoders comprise a first and a second type of line encoders, the first type line encoder having a spectrum shape that better matches a response of the communication chain than a spectrum shape of the second type line encoder, and wherein the transmission by the transmission unit of the sub-streams encoded by the first type line encoder has a reliability level that is higher than the reliability level for the transmission of the sub-streams encoded by the second type line encoder. In a particular implementation, the first type line encoder has a coding rate lower than a coding rate of the second type line encoder.

According to embodiments, the first type line encoder of the device has a spectrum shape having a smooth increase at low frequencies, causing the sub-stream encoded by the first type line encoder of to be robust against a filtering of the Direct Current (DC) component in the communication chain.

According to embodiments, said transmission unit comprises an amplitude modulation unit, said variable reliability levels being due to modulation characteristics.

According to embodiments, at least one type of line encoder of the device has a spectrum shape that better matches a response of the communication chain than a spectrum shape of another type of line encoder of the device.

For example, said modulation unit carries out a combination of said plurality of encoded sub-streams and said modulation unit gives more weight to encoded sub-streams encoded by a first type of line encoder of the device having a spectrum shape which better matches a response of the communication chain than a spectrum shape of another second type of line encoder of the device.

According to embodiments, said modulation unit performs a linear combination of said plurality of encoded sub-streams and wherein a sum of weights in the linear combination associated with sub-streams encoded with the second type of line encoder is lower than a sum of weights in the linear combination associated with sub-streams encoded with the first type of line encoder.

For example, at least one type of line encoder of the device has a coding rate higher than a coding rate of another type of line encoder of the device.

According to embodiments, said bitstream is split into two substreams and a ratio between the sizes of the blocks of each sub-bitstream is a function of a ratio between the coding rates of two types of line encoders.

For example, said reliability level is an error rate level.

According to embodiments, an error rate is determined for each encoded sub-stream transmitted through said transmission unit.

For example, said transmission unit has a plurality of inputs, each input being associated with a respective reliability level.

According to embodiments, the device further comprises a control unit configured to: - determine a level of reliability of the transmission by the transmission unit over said communication chain, and - based on said reliability level determined: o selecting a line encoder, having a coding rate, for encoding said bitstream, said line encoder being selected based on its coding rate and said reliability level determined, or o using a plurality of line encoders, having respective coding rates for encoding sub-streams resulting from the splitting of said bitstream.

According to a second aspect of the invention there is provided a device for decoding an encoded stream received from a communication chain comprising: - a reception unit for receiving a plurality of encoded sub-streams of said encoded bitstream from said communication chain, said plurality of encoded sub-streams being transmitted and received with variable reliability levels, - a plurality of line decoders for decoding said plurality of encoded sub-streams, said plurality of line decoders comprising at least two types of line encoders, - a combining unit for combining a plurality of decoded sub-streams, the device being configured to have at least one sub-stream decoded by a line encoder of a type adapted to the reliability level with which said substream is received.

For example, said plurality of encoded sub-streams are identified based on a size of blocks in the encoded bitstream.

According to a third aspect of the invention there is provided a method for encoding a bitstream for transmission over a communication chain comprising: - splitting said bitstream into a plurality of sub-streams, - selecting line encoders among a plurality of line encoders for respectively encoding said plurality of sub-streams, said plurality of line encoders comprising at least two types of line encoders, - encoding said sub-streams by respective line encoder selected, and - transmitting a plurality of encoded sub-streams over said communication chain, said transmission providing variable reliability levels for transmission of said plurality of encoded sub-streams, wherein each line encoder is selected as being of a type adapted to the reliability level with which the corresponding encoded sub-stream is transmitted.

For example, the plurality of line encoders comprise a first and a second type of line encoders, the first type line encoder having a spectrum shape that better matches a response of the communication chain than a spectrum shape of the second type line encoder, and the transmission of the sub-streams encoded by the first type line encoder has a reliability level that is higher than the reliability level of the transmission of the sub-stream encoded by the second type line encoder. In a particular implementation, the first type line encoder has a coding rate lower than a coding rate of the second type line encoder.

According to embodiments, the first type line encoder of the device has a spectrum shape having a smooth increase at low frequencies, causing the sub-stream encoded by the first type line encoder to be robust against a filtering of the Direct Current (DC) component in the communication chain.

According to embodiments, said variable reliability levels are due to modulation characteristics of an amplitude modulation performed for said transmitting.

According to embodiments, at least one type of line encoder of the device has a spectrum shape that better matches a response of the communication chain than a spectrum shape of another type of line encoder.

For example, for said modulation, the method comprises carrying out a combination of said plurality of encoded sub-streams and wherein said modulation gives more weight to encoded sub-streams encoded by a first type of line encoder of the device having a spectrum shape which better matches a response of the communication chain than a spectrum shape of another second type of line encoder.

According to embodiments, for said modulation, the method comprises performing a linear combination of said plurality of encoded substreams and wherein a sum of weights in the linear combination associated with sub-streams encoded with the second type of line encoder is lower than a sum of weights in the linear combination associated with sub-streams encoded with the first type of line encoder.

For example, at least one type of line encoder of the device has a coding rate higher than a coding rate of another type of line encoder.

For example, said reliability level is an error rate level.

According to embodiments, the method comprises determining an error rate for each encoded sub-stream transmitted.

For example, said transmission is performed by a transmission unit having a plurality of inputs, each input being associated with a respective reliability level.

According to embodiments, the method further comprises: - determining a level of reliability of the transmission over said communication chain, and - based on said reliability level determined: o selecting a line encoder, having a coding rate, for encoding said bitstream, said line encoder being selected based on its coding rate and said reliability level determined, or o using a plurality of line encoders, having respective coding rates for encoding sub-streams resulting from the splitting of said bitstream.

According to a fourth aspect of the invention there is provided a method for decoding an encoded stream received from a communication chain comprising: - receiving a plurality of encoded sub-streams of said encoded bitstream from said communication chain, said plurality of encoded sub-streams being transmitted and received with variable reliability levels, - decoding, by a plurality of line decoders, said plurality of encoded sub-streams, said plurality of line decoders comprising at least two types of line encoders, - combining a plurality of decoded sub-streams, wherein at least one sub-stream is decoded by a line encoder of a type adapted to the reliability level with which said sub-stream is received.

According to a fifth aspect of the invention there is provided a system comprising: - at least one encoder device, according to the first aspect, for encoding a bitstream for transmission over a communication chain, and - at least one decoder device, according to the second aspect, for decoding an encoded stream received from said communication chain.

According to a sixth aspect of the invention there is provided a computer program product comprising instructions for implementing a method according to the first and/or second aspect(s) when the program is loaded and executed by a programmable apparatus.

There is also provided a non-transitory information storage means readable by a computer or a microprocessor storing instructions of a computer program, for implementing a method according to the first and/or second aspect(s), when the program is loaded and executed by the computer or microprocessor.

The objects according to the second, third, fourth, fifth and sixth aspects of the invention provide at least the same advantages as those provided by the method according to the first aspect.

BRIEF DESCRIPTION OF THE DRAWINGS

Features and advantages of the invention will become apparent from the following description of non-limiting exemplary embodiments, with reference to the appended drawings, in which: - Figure 1a schematically illustrates the architecture of a classical transmitter; - Figure 1b schematically illustrates the architecture of a transmitter according to embodiments; - Figures 2a-2b illustrate the spectra of the modulated signal output by a classical transmitter in case the line code is 8b10b or MB12; - Figures 3a-3b schematically illustrates transmitters according to embodiments; - Figure 4 illustrates the spectra of the modulated signal output by a transmitter according to embodiments; - Figure 5 illustrates the spectra of the modulated signal output by a classical transmitter; - Figure 6 illustrates simulation results related to the transmission of the source bits for the four possible assignments of the 8b10b line codes according to embodiments as illustrated In Figure 1b; - Figure 7 illustrates simulation results related to the transmission of the LSB bits for the four possible assignments of the 8b10b line codes according to embodiment as illustrated in Figure 1b; - Figure 8 is a flowchart of steps of a method according to embodiments; - Figure 9 schematically illustrates a general architecture of a device according to embodiments; - Figure 10 schematically illustrates the architecture of a transmitter according to embodiments.

DETAILED DESCRIPTION OF THE INVENTION

Line coders may present different coding rates (e.g. 0.5 for MB12; 0.8 for 8B10B). Low coding rate line coders (e.g. MB12) are more robust against errors caused by DC cut, i.e. filtering of DC component, because the spectrum shape of these coders presents a smooth increase at low frequencies. This is in contrast with line coders which are more efficient (higher coding rate) and which present a sharper decrease of the spectrum shape around the DC frequency. The spectrum of such coders is thus more distorted when filtering the DC component.

In what follows, embodiments are described wherein a bitstream is transmitted with variable reliability levels, i.e. an uneven quality of transmission for each bit, for example due to modulation, and the line coders compensate for that.

When the modulation is expressed as a linear combination of substreams, the weight or coefficient assigned to each sub-stream represents the quality or reliability level of the corresponding sub-stream. High reliability level is equivalent to high coefficient and low reliability level is equivalent to low coefficient. Thus both representations are considered equivalent herein. Also, the coefficients and weights are considered equivalent terms, and they can be used interchangeably. Same equivalence applies between the terms quality and reliability.

The variation of the quality of transmission of bits belonging to the same bitstream may be due to characteristics of the modulation used for transmitting the bitstream. For instance, in case of a pure amplitude modulation wherein the modulated signal is a linear combination of the inputs, the input bits which are multiplied by the highest coefficients are the less sensitive to noise. They are thus allocated to the bits encoded with the line encoder that better matches the spectrum (with less energy around null frequencies in the case described below).

Other types of modulation (other than pure amplitude modulations) may also have uneven quality of transmission for each bit.

The present invention has thus application within this context.

For example, when encoding the bitstream with a 4-ASK modulation (2 bits symbol), 1 bit of the symbol is more robustly transmitted than the other. According to embodiments, a first line encoder that better matches the spectrum (e.g. with low coding rate) is assigned to the sub-stream having a better transmission quality (higher reliability level) and a second line encoder that less matches the spectrum (e.g. with high coding rate) is assigned to the sub-stream having the lower transmission quality (lower reliability level). According to embodiments, a modulation is appropriately selected that assigns more weight to the sub-stream encoded with the lower efficient line coder (but better spectrum matching).

This makes it possible to mitigate the contribution of the less efficient encoder in terms of coding rate while keeping good protection of the bitstream.

The ASK modulation may be modelled as a weighted linear combination of sub-streams.

In the figures and in what follows, identical elements and steps are designated with a same numerical reference.

With reference to Figures 1a and 1b, the architectures of a classical transmitter and a transmitter according to embodiments are described in order to better assess the new aspect according to the present embodiments.

Figure 1a schematically illustrates the architecture of the classical transmitter.

Bits are provided by a data source 105 and are scrambled in a scrambler 110 in order to randomize ‘0’ and T bits before being input to a line coder 115 which carries out an encoding process.

For example, the line coder may carry out an encoding that removes the DC component (i.e. the mean value of the amplitude of the signal) of the incoming stream. The 8b10b or MB12 encoding processes perform such removal for example.

The same encoding is applied on all input bits. Encoded bits are then input to an ASK modulator 120 which generates a modulated wave based on the input bit values. For instance, for a 4 ASK modulation, bits are grouped two by two.

The curves of graphs 130 and 135 respectively show examples of input data and the related generated waveform when using an ASK modulator and the constellation 125.

Figure 1b schematically illustrates the architecture of the transmitter according to embodiments of the invention.

As for the classical transmitter, the bits provided by the data source 105 are scrambled in the scrambler 110 before being input to a split module 160. In split module 160, the bits are split into two streams. A first stream is named “DM” and a second stream named “DL”.

The split is performed based on the coding rates of line encoders 170 (LC1) and 180 (LC2) which encode the two streams.

Source bits are grouped by blocks. Considering a size “D” of the source bits block, “DM” is the size of the blocks of the first bitstream and “DL” the size of the blocks of the second bitstream.

The relationship between these block sizes is as follows: D= DM+DL, DL= DM * (CR2/CR1), where CR1 is the coding rate of the first line encoder and CR2 the coding rate of the second line encoder.

The first bitstream is input to the first line encoder 170 and the second bitstream is input to the second line encoder 180. Outputs of both line encoders are connected to the inputs of a 4-ASK modulator 190. For example, the output of Line encoder 170 (LC1) is connected to the MSB input of modulator 190 and the output of the Line encoder 180 (LC2) is connected to the LSB input.

The acronyms “MSB” and “LSB” do not refer to the significance of the bits issued from the data source. Instead, the MSB input of the 4ASK modulator receives the bits which are the most reliable (less corrupted during the communication) as compared to the bits received by the LSB input.

With reference to Figures 2a and 2b, there are described the spectra of the modulated signal output by the classical transmitter in case the line code used is “8b10b” or “MB12”. Description of these spectra helps to understand how the line codes are selected for each stream in the following embodiments.

Figure 2a illustrates the spectrum of the modulated signal output by the classical transmitter (Figure 1a) in case the line code used is “8b10b”.

Since the 8b10b code is a DC free line code, the energy of the signal at the null frequency is very low. However, the slope of the spectrum magnitude is very steep and the energy of the spectrum becomes quickly significant when the frequency increases.

Figure 2b illustrates the spectrum of the modulated signal output by the classical transmitter (Figure 1a) in case the line code is MB12.

Since MB12 code is a DC free Nyquist free line code, the energy of the signal at the null frequency and at the Nyquist frequency (which is half of the sampling frequency) is very low.

From the spectrum obtained with the MB12 encoder, it can be noticed that near the null frequency, the slope of the spectrum magnitude is less steep than the one obtained with the 8b10b code.

This shows that for a given low frequency, the spectrum of the 8b10b encoded signal has more energy than the spectrum of the MB12 encoded signal.

For instance, for a frequency equal to 20MHz, the amplitudes of the spectra are around -5dB for the 8b10b encoded signal and around -15dB for the 8b10b encoded signal.

With reference to Figures 3a and 3b, a communication system according to embodiments is described.

Figure 3a schematically illustrates a transmitter according to embodiments. MB12 and 8b10b line codes are used in association with a 4-ASK modulation.

The line code which is the best adapted to the communication chain of the communication system is used for the stream which is connected to the input of the ASK modulator which is the most reliable.

In case of a DC cut communication chain, i.e. a chain which does not transmit or attenuates a lot the low frequencies of the signal, the line code which is the best adapted to the communication chain is the one with the lowest energy near the null frequency. Given the spectra of the 8b10b and MB12 line codes (Figure 2), it appears that the MB12 line code is to be connected to the less noise sensitive (most reliable) input of the 4-ASK modulator.

The transmitter illustrated in Figure 3a is then the same as the general embodiment described with reference to Figure 1b. Once the assignment of the line encoders has been done (MB12 line encoder on the most reliable input of the 4-ASK modulator, 8b10b encoder on the other input) the block sizes D, DL, DM can be defined.

The first encoder is an MB12 encoder. Thus, its coding rate CR1 is equal to 0.5. The second encoder is 8b10b encoder. Thus, its coding rate CR2 is equal to 0.8.

Then, we have DL= DM * (CR2/CR1), DL=1.6DM and D= 2.6 DM.

For instance if DM= 5, DL=8 and D=13.

The bits output by the scrambler are taken by blocks of 13 bits and these blocks are split into blocks of 5 bits for the first bitstream and blocks of 8 bits for the second bitstream. Then, data delivered to both inputs of the 4-ASK modulator have the same data rate.

Blocks 190 to 195 show the way a block of data is processed in the transmitter. An exemplary data block 190 at the output of the scrambler is shown, however, the encoding process is the same whatever the content of the data block 190.

The scrambler 110 receives the data from the data source 105 and scrambles it (using a conventional algorithm). Block 190 is a block of 13 bits output by the scrambler which has the value Ί00011110100Τ in this example.

The module 160 splits the data block 190 Ί00011110100T in two data blocks 191 Ό100Τ and 192 Ί000111Τ. In that the present example, block 191 is composed of the 5 latest bits of the data block 190 and block 192 is composed of the 8 first bits of the data block 190 but any other way to split the data block may be used as long as the reverse function is applied in the receiver.

The MB 12 encoder encodes the data block 191 and outputs the data block 193 Ί101001110’ while the 8b10b encoder encodes the data block 192 and output the data block 194 Ί 000111110’.

The MB12 encoded word (data block 191) is fed to the most reliable input of the 4ASK modulator and the 8b10 encoded word (data block 194) is fed to the less reliable input of the 4ASK modulator.

With regards to the mapping shown in Figure 1, the 4ASK modulator outputs the modulated signal 195 ’3 1 -3 1 -1 -1 3 3 3 -3’

The special effect resulting from the depicted coding scheme is described with reference to Figures 4 and 5 which show the spectra of the modulated signal with regard to the coding schemes and with reference to Figures 6 and 7 which show the performances achieved with the coding schemes.

Figure 3b schematically illustrates a receiver according to embodiments.

The data that are received are demodulated by a 4-ASK demodulator 310 which provides two bitstreams. One bitstream is related to the most reliable signal and one bitstream is related to the less reliable signal.

These bitstreams are processed according to the process in the transmitter, that is to say that the most reliable bitstream is decoded by the MB12 decoder 320 and the less reliable bitstream is decoded by the 8b10b decoder 330.

The decoded bitstreams are then combined in a combination module 340 before being delivered to a descrambler 350 which finally outputs the expected data.

Combination of bits is performed according to the split carried out in the transmitter.

The bits from the first bitstream are taken by blocks of 5 bits. The bits from the second bitstream are taken by blocks of 8 bits. The bits are combined to create blocks of 13 bits. Then, these blocks are serialized to create the data stream delivered to the descrambler.

Blocks 390 to 395 show the way a block of modulated data 390 is processed in the receiver for an exemplary input signal 390.

The 4-ASK demodulator receives the modulated signal 390 ’31-31-1 -1 3 3 3 -3’ and feeds the MB12 encoded word (data block 391) to its most reliable output and the 8b10 encoded word (data block 392) to its less reliable output.

The MB12 decoder decodes the data block 391 Ί101001110’ and outputs the data block 393 Ό100Τ while the 8b10b decoder decodes the data block 392 Ί 000111110’ and outputs the data block 394 Ί 0001111’.

The module 340 combines the data block 393 Ό100Τ and the data block 394 Ί 0001111’ and outputs the data block 395 Ί 000111101001’.

As already discussed hereinabove, the combination of the two streams must be the reverse of what has been done in transmitter. The concatenation of the two streams as detailed here is only given as an example.

The decoded word output from module 340 is then unscrambled in module 350 and data are delivered by the receiver to its destination.

Figure 4 illustrates the spectrum of the modulated signal output by a transmitter according to embodiments.

As shown in the figure, the spectrum is a mix of the spectra of the 8b10b and MB12 encoders. As a consequence, the spectrum of the modulated signal according to embodiments has lower energy near null frequency than the spectrum of the 8b10b line encoder (Figure 2a).

To better identify the advantages of a transmitter according to embodiments, Figure 5 illustrates the spectrum of the modulated signal output by a classical transmitter.

The figure shows the spectrum of the modulated signal which would be obtained if the assignment of the line encoders would be different, i.e. if the 8b10b line encoder output would be connected to the most reliable input of the 4 ASK modulator and the MB 12 line encoder output would be connected to the other input of the modulator.

Comparing that spectrum to the spectrum of the modulated signal output by the transmitter according to embodiments (Figure 4), it is clear that the signal generated according to embodiments of the invention has lower energy near null frequency, which improves the performances of the system when using a DC cut channel.

Figure 6 illustrates exemplary simulation results (the bit error rate BER as a function of the signal to noise ratio SNR) related to the transmission of the source bits for the four possible assignments of the 8b10b and MB12 line codes according to the embodiments illustrated in Figure 1b.

Considering an embodiment using MB12 and 8b10b encoders associated with a 4-ASK modulator, there are four possibilities for assigning the line encoders: - 8b10b line encoders are used for both inputs of the 4-ASK modulator, giving a coding rate equal to 0.8, - MB 12 line encoders are used for both inputs of the 4-ASK modulator, giving a coding rate equal to 0.5, - 8b1 Ob line encoder is connected to the most reliable input of the 4-ASK modulator, and MB12 line encoder is connected to the less reliable input of the 4-ASK modulator giving a coding rate equal to 0.65, and - MB 12 line encoder is connected to the most reliable input of the 4-ASK modulator, and 8b10b line encoder is connected to the less reliable input of the 4-ASK modulator giving also a coding rate equal to 0.65.

The performances obtained for these four configurations are shown in Figure 6.

The use of two MB 12 line encoders gives the best performance (at the cost of a coding rate limited to 0.5) while the use of two 8b10b line encoders give the lowest performance but offer a coding rate equal to 0.8.

Regarding the two ways to get a coding rate equal to 0.65, it can be noticed than the solution according to embodiments of the invention (MB12 line encoder connected to the most reliable input -MSB here- of the 4-ASK modulator, and 8b10b line encoder connected to the other input) gives better performance than the other way.

This result is in line with the analysis of the spectra of Figures 4 and 5.

Figure 7 illustrates exemplary simulation results (the bit error rate BER as a function of the signal to noise ratio SNR) related to the transmission of the LSB bits for the four possible assignments of the 8b10b line codes according to the embodiments illustrated in Figure 1a.

The simulations as the simulations of Figure 6 have been run but the measurements concern the bit error rate of the bit connected to the less reliable input (named LSB in Figure 3) of the ASK modulator instead of the bit error rate of the source bits.

Again, the use of two MB12 line encoders gives the best performance while the use of two 8b10b line encoders gives the lowest performance.

However, regarding the two ways to get a coding rate equal to 0.65 and the bit error rate of only that LSB bit, it can be noticed that the BER is lower when the MB12 line encoder is connected to the MSB input and the 8b10 line encoder is connected to the LSB input.

Another way to analyze the curves consists in comparing the BER of the LSB bit for the two cases: - MB 12 line encoder connected on the MSB input of the 4-ASK modulator and 8b10b line encoder connected on the LSB input of that modulator, and - 8b10b line encoder connected on the MSB input of the 4-ASK modulator and 8b10b line encoder connected on the LSB input of that modulator.

While keeping the same line coding scheme for the LSB bit (8b10b then a coding rate equal to 0.8), the BER for that bit is improved (divided by 300 for the SNR value 21 dB) when a MB12 line encoder is used for the MSB bit.

This result is another advantage of implementing the coding scheme based on two line encoders according to embodiments of the invention.

With reference to the flowchart of Figure 8, a process according to embodiments is described.

In the present example, the change of the line coding scheme is based on the signal to noise ratio (SNR) value and guarantees that the expected bit error rate (BER) is obtained.

According to embodiments, the SNR parameter can be measured in the receiver and sent back to the transmitter. Thus, transmitter and receiver know the SNR value and use the most appropriate coding rate scheme using the process of the flowchart in Figure 8.

According to other embodiments, the SNR parameter can be measured in the receiver and sent back to the transmitter also. Then, the transmitter selects the most appropriate coding rate scheme using the process of the flowchart in Figure 8 or another process. The transmitter then informs the receiver of the coding rate scheme used by inserting one appropriate data in the header inserted at the beginning of the frame.

The main principle is that when the signal to noise ratio is high (higher than value a SNR_HCR), the line code with a high coding rate is used. When the SNR is low (lower than a value SNR_LCR), the performance (number of error) is obtained by using the line code which has a low coding rate.

When the SNR is between the values SNR_FICR and SNR_LCR, a new line code scheme as described hereinabove is carried out which makes it possible to get the expected performances as well as an intermediate coding rate.

In a first step 810, the spectrum of the channel is acquired. For example, the spectrum of the channel may be acquired by using a database which gives the spectrum with regard to the communication chain (use of a model of the chain) or by transmitting random data on the channel without coding, computing the spectrum in the receiver and getting back the spectrum.

Next, the SNR of the channel is measured in a step 820 (using a conventional measurement method).

The measured SNR is then compared with a predefined value SNR_FICR. For example, the value SNR_HCR is defined by computation (or experimentation) as the lower value which guarantees that the expected BER is obtained for the line code which has the higher coding rate.

If the value of the measured SNR is greater than the value SNR_HCR (‘Yes’ branch of test 830), the Line code with the higher coding rate (for instance 8b10b) is selected in step 840 and the transmission of the data starts.

If the value of the measured SNR is lower than the value SNR_HCR (‘No’ branch of test 830), in step 850, the value of the measured SNR is compared with a predefined value SNR_LCR. The value SNR_LCR has been defined by computation (or experimentation) as the lower value which guarantees to get the expected BER for the line code scheme using two line codes as proposed in the embodiment of the invention described previously.

If the value of the measured SNR is lower than the value SNR_LCR HCR (‘Yes’ branch of test 850), the line code with the lower coding rate (for instance MB12) is selected in step 860 and the transmission of the data starts.

If the value of the measured SNR is higher than the value SNR_LCR HCR (‘No’ branch of test 850), the line code scheme described hereinabove is used.

In step 870, the spectra of each line code are compared to the spectrum of the channel and the best matching line code is selected.

In next step 880, the more reliable input of the modulator is defined (for instance by calculating the distance between codewords).

In next step 885, the line code selected during step 870 is assigned to the more reliable input of the modulator defined in step 880.

In next step 890, the remaining line code is assigned to the remaining input of the modulator.

In next step 895, data are encoded using the new line code scheme proposed and the transmission of the data starts.

In order to dynamically adapt the coding scheme to the SNR, if tests 830 and 850 are positive (‘Yes’ branches) and step 890 is terminated, a timer is started in step 865. Once the timer started, its value is continuously evaluated in test 868 in order to check if the predefined max value of the timer is reached or not. If not, the timer continues and is incremented and the transmission of data continues. When the maximal value of the timer is reached, the algorithm goes back to step 820 to evaluate again the SNR value and to change eventually the line code scheme.

An alternative way (not represented in the flowchart) to control the line code scheme may consist in coming back to the SNR evaluation with regard to the number of errors detected at the reception.

With reference to Figure 10, another architecture for transmitters 1050 according to embodiments is described. A data source 105 provides data that are scrambled in a scrambler 110 before being input to a split module 1060. In split module 1060, the bits are split into several streams T to ‘n’.

The split is performed based on the coding rates of line encoders 1070 (LC1), 1071 (LC2) ... 1073 (LCn-1), 1080 (LCn) which encode the streams and output respective signals ii(t), i2(t) ... j„-i(t), in(t).

Source bits are grouped by blocks. The size of the blocks are defined with regard to the coding rates of the Line codes in order to provide simultaneously data to each input of the modulator

For instance, in case of a modulator with 3 inputs (like 8-ASK modulator), it may be decided to use 3 different line encoders.

The line encoder 1 which process data stream D1 has a coding rate CR1, the line encoder 2 which process data stream D2 has a coding rate CR2, and the line encoder 3 which process data stream D3 has a coding rate CR3.

In the present example, the relationship between the sizes of the data streams and the coding rate of the affected line code is the following: D1/CR1 =D2/CR2=D3/CR3, and the size of the block input in the splitter module is D=D1 +D2+D3.

For instance with CR1=0.5, CR2=0.65, CR3=0.8, the following values may be selected: D1 = 100, D2=130, D3=160 and D=390.

More generally, if n Line encoders are used, the relationship between the sizes of the data streams and the coding rate of the affected line code is the following: D1/CR1=D2/CR2=... Dn/CRn and the size of the block input in the splitter module is D=D1 +D2+D3+ ... +Dn.

Of course, it is possible to use a same line encoder for several input of the modulator. A modulator 1090 is used which has the property to generate a modulated signal with an amplitude which is a linear combination of the amplitudes of its inputs (example of such modulation: ASK, QPSK, n-QAM).

Such modulator delivers the modulated signal s(t) from the n inputs signals

In such a case, the Line code which generates an output signal having a spectrum which matches the best the communication chain are allocated to the inputs of the modulator which have the higher coefficients.

In case of two types of Line codes, it may be considered to allocate the Line code of the type which matches the best the channel to k input signals among the n input signals and the second type of line code to the (n-k) remaining input signals.

As long as the sum of the k coefficients

is higher than the sum of the other coefficients

performance of the system is improved with regards to other embodiments giving the same coding rate but with another allocation of the Line encoders.

For instance for a 4-ASK, we may have

for a 8-ASK we may have

anc* so on-

In others words, with the present architecture, it is made possible to create a system which has an intermediate coding rate between the coding rates of each of the two Line codes with improved performance with regards to the performance which would be obtained with a random allocation of the two Line codes on the modulator inputs.

Figure 9 is a schematic block diagram of a general architecture of a device 900 for implementing of one or more embodiments of the invention. The device 900 comprises a communication bus connected to: -a central processing unit 901, such as a microprocessor, denoted CPU; -a random access memory 902, denoted RAM, for storing the executable code of the method of embodiments of the invention as well as the registers adapted to record variables and parameters necessary for implementing a method according to embodiments, the memory capacity thereof can be expanded by an optional RAM connected to an expansion port for example; -a read only memory 903, denoted ROM, for storing computer programs for implementing embodiments of the invention; -a network interface 904 is typically connected to a communication network over which digital data to be processed are transmitted or received. The network interface 904 can be a single network interface, or composed of a set of different network interfaces (for instance wired and wireless interfaces, or different kinds of wired or wireless interfaces). Data are written to the network interface for transmission or are read from the network interface for reception under the control of the software application running in the CPU 901; - a user interface 905 for receiving inputs from a user or to display information to a user;

- a hard disk 906 denoted HD - an I/O module 907 for receiving/sending data from/to external devices such as a video source or display

The executable code may be stored either in read only memory 903, on the hard disk 906 or on a removable digital medium such as for example a disk. According to a variant, the executable code of the programs can be received by means of a communication network, via the network interface 904, in order to be stored in one of the storage means of the device 900, such as the hard disk 906, before being executed.

The central processing unit 901 is adapted to control and direct the execution of the instructions or portions of software code of the program or programs according to embodiments of the invention, which instructions are stored in one of the aforementioned storage means. After powering on, the CPU 901 is capable of executing instructions from main RAM memory 902 relating to a software application after those instructions have been loaded from the program ROM 903 or the hard-disc (HD) 906 for example. Such a software application, when executed by the CPU 901, causes the steps of a method according to embodiments to be performed.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive, the invention being not restricted to the disclosed embodiment. Other variations to the disclosed embodiment can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure and the appended claims.

In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. The mere fact that different features are recited in mutually different dependent claims does not indicate that a combination of these features cannot be advantageously used. Any reference signs in the claims should not be construed as limiting the scope of the invention.

Claims

1. A device for encoding a bitstream for transmission over a communication chain comprising: - a split unit for splitting said bitstream into a plurality of substreams, - a plurality of line encoders for encoding said plurality of substreams, said plurality of line encoders comprising at least two types of line encoders, - a transmission unit for transmitting a plurality of encoded substreams over said communication chain, said transmission unit providing variable reliability levels for transmission of said plurality of encoded substreams, the device being configured to have at least one sub-stream encoded by a line encoder of a type adapted to the reliability level with which said transmission unit transmits said encoded at least one sub-stream.

2. The device according to claim 1, wherein the plurality of line encoders comprise a first and a second type of line encoders, the first type line encoder having a spectrum shape that better matches a response of the communication chain than a spectrum shape of the second type line encoder, and wherein the transmission by the transmission unit of the sub-streams encoded by the first type line encoder has a reliability level that is higher than the reliability level for the transmission of the sub-streams encoded by the second type line encoder.

3. The device according to claim 2, wherein the first type line encoder has a spectrum shape having a smooth increase at low frequencies, causing the sub-stream encoded by the first type line encoder to be robust against a filtering of the Direct Current (DC) component in the communication chain.

4. The device according to claim 1, wherein said transmission unit comprises an amplitude modulation unit, said variable reliability levels being due to modulation characteristics.

5. The device according to claim 4, wherein at least one type of line encoder of the device has a spectrum shape that better matches a response of the communication chain than a spectrum shape of another type of line encoder of the device.

6. The device according to claim 5, wherein said modulation unit carries out a combination of said plurality of encoded sub-streams and wherein said modulation unit gives more weight to encoded sub-streams encoded by a first type of line encoder of the device having a spectrum shape which better matches a response of the communication chain than a spectrum shape of another second type of line encoder of the device.

7. The device according to claim 6, wherein said modulation unit performs a linear combination of said plurality of encoded sub-streams and wherein a sum of weights in the linear combination associated with sub-streams encoded with the second type of line encoder is lower than a sum of weights in the linear combination associated with sub-streams encoded with the first type of line encoder.

8. The device according to any one of the preceding claims, wherein at least one type of line encoder of the device has a coding rate higher than a coding rate of another type of line encoder of the device.

9. The device according to any one of the preceding claims, wherein said bitstream is split into two sub-streams and wherein a ratio between the sizes of the blocks of each sub-bitstream is a function of a ratio between the coding rates of two types of line encoders.

10. The device according to any one of the preceding claims, wherein said reliability level is an error rate level.

11. The device according to claim 10, configured to determine an error rate for each encoded sub-stream transmitted through said transmission unit.

12. The device according to any one of the preceding claims, wherein said transmission unit has a plurality of inputs, each input being associated with a respective reliability level.

13. The device according to any one of the preceding claims comprising a control unit configured to: - determine a level of reliability of the transmission by the transmission unit over said communication chain, and - based on said reliability level determined: o selecting a line encoder, having a coding rate, for encoding said bitstream, said line encoder being selected based on its coding rate and said reliability level determined, or o using a plurality of line encoders, having respective coding rates for encoding sub-streams resulting from the splitting of said bitstream.

14. A device for decoding an encoded stream received from a communication chain comprising: - a reception unit for receiving a plurality of encoded sub-streams of said encoded bitstream from said communication chain, said plurality of encoded sub-streams being transmitted and received with variable reliability levels, - a plurality of line decoders for decoding said plurality of encoded sub-streams, said plurality of line decoders comprising at least two types of line encoders, - a combining unit for combining a plurality of decoded sub-streams, the device being configured to have at least one sub-stream decoded by a line encoder of a type adapted to the reliability level with which said sub-stream is received.

15. The device according to claim 14, wherein said plurality of encoded sub-streams are identified based on a size of blocks in the encoded bitstream.

16. A method for encoding a bitstream for transmission over a communication chain comprising: - splitting said bitstream into a plurality of sub-streams, - selecting line encoders among a plurality of line encoders for respectively encoding said plurality of sub-streams, said plurality of line encoders comprising at least two types of line encoders, - encoding said sub-streams by respective line encoder selected, and - transmitting a plurality of encoded sub-streams over said communication chain, said transmission providing variable reliability levels for transmission of said plurality of encoded sub-streams, wherein each line encoder is selected as being of a type adapted to the reliability level with which the corresponding encoded sub-stream is transmitted.

17. The method according to claim 16, wherein the plurality of line encoders comprise a first and a second type of line encoders, the first type line encoder having a spectrum shape that better matches a response of the communication chain than a spectrum shape of the second type line encoder, and wherein the transmission of the sub-streams encoded by the first type line encoder has a reliability level that is higher than the reliability level of the transmission of the sub-stream encoded by the second type line encoder.

18. The method according to claim 17, wherein the first type line encoder has a spectrum shape having a smooth increase at low frequencies, causing the sub-stream encoded by the first type line encoder to be robust against a filtering of the Direct Current (DC) component in the communication chain.

19. The method according to claim 16, wherein said variable reliability levels are due to modulation characteristics of an amplitude modulation performed for said transmitting.

20. The method according to claim 19, wherein at least one type of line encoder of the device has a spectrum shape that better matches a response of the communication chain than a spectrum shape of another type of line encoder.

21. The method according to claim 20, comprising, for said modulation, carrying out a combination of said plurality of encoded sub-streams and wherein said modulation gives more weight to encoded sub-streams encoded by a first type of line encoder of the device having a spectrum shape which better matches a response of the communication chain than a spectrum shape of another second type of line encoder.

22. The method according to claim 21, comprising, for said modulation, performing a linear combination of said plurality of encoded substreams and wherein a sum of weights in the linear combination associated with sub-streams encoded with the second type of line encoder is lower than a sum of weights in the linear combination associated with sub-streams encoded with the first type of line encoder.

23. The method according to any one of claims 16 to 22, wherein at least one type of line encoder of the device has a coding rate higher than a coding rate of another type of line encoder.

24. The method according to any one of claims 16 to 23, wherein said bitstream is split into two sub-streams and wherein a ratio between the sizes of the blocks of each sub-bitstream is a function of a ratio between the coding rates of two types of line encoders.

25. The method according to any one of claims 16 to 24, wherein said reliability level is an error rate level.

26. The method according to claim 25, comprising determining an error rate for each encoded sub-stream transmitted.

27. The method according to any one of claims 16 to 26, wherein said transmission is performed by a transmission unit having a plurality of inputs, each input being associated with a respective reliability level.

28. The method according to any one of claims 16 to 27, further comprising: - determining a level of reliability of the transmission over said communication chain, and - based on said reliability level determined: o selecting a line encoder, having a coding rate, for encoding said bitstream, said line encoder being selected based on its coding rate and said reliability level determined, or o using a plurality of line encoders, having respective coding rates for encoding sub-streams resulting from the splitting of said bitstream.

29. A method for decoding an encoded stream received from a communication chain comprising: - receiving a plurality of encoded sub-streams of said encoded bitstream from said communication chain, said plurality of encoded sub-streams being transmitted and received with variable reliability levels, - decoding, by a plurality of line decoders, said plurality of encoded sub-streams, said plurality of line decoders comprising at least two types of line encoders, - combining a plurality of decoded sub-streams, wherein at least one sub-stream is decoded by a line encoder of a type adapted to the reliability level with which said sub-stream is received.

30. The method according to claim 29, wherein said plurality of encoded sub-streams are identified based on a size of blocks in the encoded bitstream.

31. A system comprising: - at least one encoder device, according to any one of claims 1 to 13, for encoding a bitstream for transmission over a communication chain, and - at least one decoder device, according to any one of claims 14 to 15, for decoding an encoded stream received from said communication chain.

32. A computer program product comprising instructions for implementing a method according to any one of claims 16 to 30 when the program is loaded and executed by a programmable apparatus.

33. A non-transitory information storage means readable by a computer or a microprocessor storing instructions of a computer program, for implementing a method according to any one of claims 16 to 30, when the program is loaded and executed by the computer or microprocessor.

34. A device substantially as hereinbefore described with reference to, and as shown in, Figures 1b, 3a-3b, 9 and 10 of the accompanying drawings.

35. A method substantially as hereinbefore described with reference to, and as shown in, Figure 8 of the accompanying drawings.