EP2198548A2

EP2198548A2 - Digital transmission of data in white light by leds

Info

Publication number: EP2198548A2
Application number: EP08837453A
Authority: EP
Inventors: Patrick Tortelier
Original assignee: France Telecom SA
Current assignee: Orange SA
Priority date: 2007-09-14
Filing date: 2008-09-12
Publication date: 2010-06-23
Also published as: WO2009047428A2; US20100202780A1; WO2009047428A3; FR2921219A1

Abstract

According to the invention, a device transmits bit sequences (SB1-SB3) from a plurality of light sources (SL1-SL3) having their radiations combined into a substantially white light. Each light source has a different signal-to-noise ratio that depends on the wavelength of the transmitted radiation. The device includes an encoder (COD) for encoding the bit sequence (SB3) to be transmitted by the light source (SL3) having the lowest signal-to-noise ratio, using a code (CB3) having a minimum distance, the product of which by the lowest signal-to-noise ratio is substantially equal to the highest signal-to-noise ratio. The minimum distance of the code is equal to the minimum number of different bits between two words of the code. The binary error ratio of the source transmitting the encoded sequence is reduced and the binary error ratios of the different light sources are homogenised.

Description

Digital transmission of white light data by light-emitting diodes

The present invention relates to a digital data transmission scheme for wireless optical communications.

More particularly, it relates to a correction coding which operates on the digital data to be transmitted by a light source, also used as a light source, for downstream signal transmission.

In the state of the art, there are light sources used both for lighting and for transmitting data. Such a light source is generally a light-emitting diode, which comprises an electronic component able to emit light when it is traversed by an electric current. A light emitting diode then emits monochromatic radiation by means of a PN junction which is forward biased. A light-emitting diode can emit radiation in the visible range or the infrared or the ultraviolet. Subsequently and by way of example, a blue diode will designate a light-emitting diode emitting a blue light, that is to say a radiation whose wavelength in the spectrum of the visible range corresponds to the blue color. A light-emitting diode must emit white light to be used as a light source, for example instead of an incandescent light bulb. There are two ways to emit white light from one or more light-emitting diodes. In a first case, a blue diode emits radiation through a layer of fluorescent material and the latter then emits a yellow light when it is excited by the blue light of the diode. The mixing of the respective radiation colors of the diode and the fluorescent material layer creates a nearly white light. The rate of data transmissions from such a diode is limited by the high response time of the excitation of the fluorescent layer to emit yellow light.

In a second case, light beams of different primary colors emitted respectively by three sufficiently close light emitting diodes are mixed in white light. For example, red, green and blue diodes are combined to emit white light and thus form a diode called "multi-chip white electroluminescent diode". The signals respectively emitted by the three diodes are modulated by the same data to achieve optical digital transmission in white light.

For each different primary color diode, the received signal has a different signal to noise ratio, and therefore a different bit error rate. For example, for a given position of the diodes with respect to a photoreceptor and for an average power received of -30 dBm by the photoreceptor, the bit error rates are 1.8.10, 1.7.10 and 4.2.10 respectively for the diodes red, green and blue.

When one of the three bit error rates, such as the one for the red diode, is much higher than the other two, the error rate of the data signal resulting from the combined emissions of the three diodes depends in a very preponderant way on the highest error rate. This data transmission therefore does not benefit from the low bit error rate, for example relating to the blue diode. Generally, a detector operating only in blue light is used to receive such a signal since the blue light has the best signal-to-noise ratio.

In order to overcome the disadvantages mentioned above, a method according to the invention for transmitting bit sequences respectively by at least two light sources which emit combined radiations into a substantially white light and which have different signal-to-noise ratios depending respectively on the wavelengths of the radiations, is characterized in that it comprises a coding of the sequence of bits to be transmitted by the light source having the lowest signal-to-noise ratio by a code having a minimum distance whose product by the ratio The lowest signal to noise is substantially equal to the highest signal to noise ratio, the minimum code distance being equal to the minimum number of different bits between two words of the code.

Thus according to the invention, at least one of the bit sequences is advantageously coded in order to reduce the bit error rate relative to the light source emitting the coded bit sequence. Therefore, the average of the bit error rates for the different light sources is also decreased. According to another characteristic of the invention, the method may furthermore comprise encoding by another code of at least one other bit sequence to be transmitted by a light source having another signal-to-noise ratio comprised between the signal-to-noise ratio. the lowest and the highest signal to noise ratio. Said other code then has a minimum distance whose product by said other signal to noise ratio is substantially equal to the highest signal to noise ratio.

The bit error rates relating to the different light sources are thus homogenized by the invention in order to benefit from a low overall bit error rate which is close to the bit error rate provided by the light source having a signal to signal ratio. highest noise.

The invention also relates to a device for transmitting bit sequences respectively by at least two light sources which emit combined radiation into a substantially white light and which have different signal-to-noise ratios respectively depending on the wavelengths of the radiation. The device is characterized in that it comprises means for coding the sequence of bits to be transmitted by the light source having the lowest signal-to-noise ratio by a code having a minimum distance, the product of which by the signal-to-noise ratio lower is substantially equal to the ratio signal to noise is greater, the minimum distance of the code is equal to the minimum number of b _^ ts different between two code words.

According to other features of the invention, the light sources may be diodes emitting. The device may comprise three light sources capable of respectively radiating substantially in blue, green and red, which are primary colors whose combination forms a substantially white light.

The invention also relates to a computer program adapted to be implemented in a device for transmitting bit sequences respectively by at least two light sources which emit combined radiation into a substantially white light and which have signal ratios. with different noise depending respectively on the wavelengths of the radiation. The program includes instructions which, when the program is executed in said device, perform bit sequence coding according to the method of the invention.

Other characteristics and advantages of the present invention will emerge more clearly on reading the following description of several embodiments of the invention given by way of non-limiting examples, with reference to the corresponding appended drawings in which:

FIG. 1 is a schematic block diagram of a white light emitting device according to the invention; and

FIG. 2 is an algorithm of a white light signal emission method according to

The invention.

With reference to FIG. 1, a white light emitting device SE according to the invention comprises at least two light sources arranged close to each other and each emitting a different color radiation, that is to say respectively emitting light waves of different wavelengths in the spectrum of the visible range, so that the combination of emitted radiation by the light sources forms substantially white light.

In general, the transmission device SE comprises K light sources SL 1, with 1 <k <K and K> 2. In order not to overload FIG. 1, only three light sources SL ₁ , SL ₂ and SL ₃ are represented. .

A light source SL 1 serves as a photosensitive transmitter for optical data transmission and also serves as a source of illumination.

For example, the emission device SE covers a relatively closed space, such as a partitioned room inside an office building belonging to a company, or a partitioned room inside an apartment or of a detached house.

By way of example, the transmission device SE is used for broadband data broadcasting, such as the broadcasting of digital video recordings. The transmission device SE can be connected to a computer in which is stored a video file. The video file is then broadcast via the transmission device and received by a photoreceptor adapted to an optical transmission. Another computer connected to the photoreceptor can then read the video file thus diffused. In general, the transmitting device can be used to establish a wireless network between computer entities located in the same room, while illuminating the room. The light waves used in the optical transmission, whose wavelength is of the order of a few hundred nanometers, are partly absorbed and reflected by obstacles located in the room and by the walls of the room. Therefore, two emitting devices used respectively in two different closed rooms can not interfere with each other. In addition, a photoreceptor may receive waves that have been emitted by the transmitting device SE and reflected on walls of the room, which allows the transmitting device to cover a large volume area of the room.

A light source SL 1 is for example a light emitting diode which emits monochromatic radiation by means of a PN junction at one or more wavelengths given according to the nature of the material making up the PN junction. According to one embodiment of the invention, the emission device SE comprises three light sources SL ₁ , SL ₂ and SL ₃ which are light-emitting diodes each emitting a light radiation of different primary color. For example, the three light sources SL ₁ , SL ₂ and SL ₃ are respectively diodes emitting in blue, green and red, so that the combination of the light emitted by the diodes form a substantially white light.

Each light source SL 1 emits a substantially monochromatic radiation at a given wavelength and is associated with a photoreceptor REC 1, with 1 <k <K, able to receive and analyze the emitted radiation. For example, the REC photoreceptor photodetector as a photodiode which converts the incident light energy into an electrical signal. The photoreceptor then comprises a reverse biased PN junction which absorbs the waves of the radiation emitted by the source SL 1 and generates an electric current whose amplitude is proportional to the incident optical power of the received radiation.

Each photoreceptor REC1 may be equipped with a FIL filter to concentrate and filter the received radiation. This filter selects the wavelength or wavelengths carrying useful data and eliminates parasitic radiation emitted by other light sources, thus reducing the noise induced by the latter. For example, the FIL filter comprises a hemispherical lens centered on the photodiode to increase the angle of incidence of the received radiation with respect to the central axis of the photosensitive face of the photodiode. In particular, the electric current of intensity I generated by the photoreceptor REC ^ is proportional to an optical power PO received by the photoreceptor: I = R x PO, where R a proportionality factor.

The signal-to-noise ratio SNR] ₄ in the photoreceptor REC ^ is then given by the following relation:

SNR _k = (R x PO) ² / σ ² ,

₂ where σ is the variance of the noise.

Each REC ^ photoreceptor is sensitive to a SNR ^ signal to noise ratio which depends on the associated light source SL ^. Each light source SL 1 therefore has a signal-to-noise ratio SNR 1 which is different and depends on the wavelength of the light emitted by the light source. With reference to the example with the three light sources SL] _, SL ₂ and SL ₃ qαi respectively emitting in blue, green and red, the light source SLi emitting a blue radiation has a signal-to-noise ratio SNRi higher than that of the other light sources SL ₂ and SL3.

A digital signal can thus be emitted in the form of light-wave radiation from a light source SL 1 to a photoreceptor REC 1 according to a direct-sensing intensity modulation IM / DD ("Intensity Modulation / Direct Detection"). . Upon reception of the radiation, the photoreceptor REC ^ produces an electric current proportional to the incident optical power and thus performs a direct detection, while the light source SL ^ emits a digital signal in the form of light waves which is modulated in intensity. by the binary data of the signal. For example, the light source SL ^ emits a digital signal X (t) with M bits and modulates in intensity according to the following relation:

M

X (t) = Σ d _m p (t - mt), m = 1 or p (t) = Rect (t) is a rectangular pulse of

1 GÏÏÏΘ width T and d _m is the m bit signal has the state "0" or "1". This intensity modulation is of the "all or nothing" type 0OK ("On-Off Feymg" in English) and consists in directly modulating a substantially monochromatic carrier wave by the binary signal, that is to say to emit αne light pulse only for bits "1" and no pulse for the said "0". This modulation requires a low complexity implementation and consumes energy peα. The photoreceptor then performs a direct detection bxt by bit. In the photoreceptor, the optical power received for a "0" bit is zero and the optical power received for a "1" bit is twice the average received optical power. The transmission device SE processes a series of SER bits which are distributed and transmitted in parallel by means of the K light sources SL 1 ^to SL _K.

In the transmission device SE, a se-parallelepair converter CSP converts the series of bits SER into K sequences of bits SBi ^to SB _K, which are distributed respectively over K parallel transmission channels terminated by the light sources SLi ^a. SE transmission comprises a coder COD which optionally transforms a sequence of bits SB ^ ^{into a} respective coded bit sequence SCj ₄ which is then transmitted by the respective light source SLj ₄ . Each coded bit sequence SC] ₄ may be coded differently. Moreover, at least one bit sequence is not coded. The sequence αe bit which is not coded is that which is emitted by the light source which has the highest signal to noise ratio among the different light sources. According to the example with the three light sources SL ₁ , SL ₂ and SL ₃ which are respectively blue, green and red diodes, the bit sequence SB i is not coded.

Some terms and concepts useful for understanding the invention are defined below. A sequence of bits SB] ₄ can be coded into a coded bit sequence SCj ₄ by a binary code CB of length n, containing 1 bits of useful information, with 1 <n, and of minimum distance d. The minimum distance is equal to the minimum number of bits that differs between two words of the code CB. A code has a high correction capacity when the minimum distance is large.

The Hamming weight is defined as the number of non-zero bits in a code word. An enumerator polynomial of the weights P (X) is the polynomial of which each coefficient P ₁ of the monomial respectively X is equal to the number of weight code words of Hamming equal to 1:

where n, the length of the code, is also the maximum Hamming weight of the code.

A first enumerator polynomial of weight A (X, z), denoted by IOWEF ("Input Output Weight Enumerator Function" in English), is defined as follows: A (X, z) = Σ Σ A ₁ , -, 7? X ¹ ,

I = O] = O or A _{1 ^} -, is the number of words with a Hamming weight equal to ai and having j non-zero useful information bits, X is a monomer associated with the Hamming weight, z is a monomial associates with the number j of non-zero bits. The coefficients A ₁ ^ are zero for

0 <i <d, by definition of the minimum distance of the code. On the other hand the index - \ takes values between

1 and 1 since the index j indicates on how many bits of information different two words of the code of length n and dimension 1, the dimension being the number of information bits of the codewords.

A second enumerator polynomial of weight B (X) is defined in the following way: n B (X) - XB ₁ X ^X ,

or B ₁ is defined according to the following relation: U = = Σ j A 1 ']

J = I

In a first example, the binary code CB is a BCH (Bose-Chaudhuri-Hocquenghem) code of length n = 31, containing 1 = 21 bits of useful information and of minimum distance d = 5. First terms of the first and second enumerating polynomials of the BCH code are: A (X, z) = 1 + X ⁵ (5z + 29z ² + 63z ³ + 67z ⁴ + 22z ⁵ ) + X ⁶ (3z + 57z ² + 173z ³ + 279z ⁴ + 24Oz ⁵ + 54z ⁶ ) + ..., and B (X) = 630 X ⁵ + 3276 X ⁶ + ....

The term 29 z X for the first polynomial A (X, z) means that the BCH code has Α ^, ₂ ⁼ 29 code words with the Hamming weight equal to i = 5 and having j = 2 bits of useful information not invalid.

In a second example, the binary code CB is a Golay code of length n = 24, containing 1 - 12 bits of useful information and of minimum distance d = 8. First terms of the first enumerator polynomial of the Golay code are:

A (X, z) = 1 + X ⁸ (12z + 6Oz ² + 18Oz ³ + 255z ⁴ + 18Oz ⁵ + 6Oz ⁶ + 12z ⁷ ) + X ¹² (6z ² + 4Oz ³ + 24Oz ⁴ + 60Oz ⁵ + 804z ⁶ + 60Oz ⁷ + 24Oz ⁸ + 4Oz ⁹ + 6z ¹⁰ ) + X ¹⁶ (12z ⁵ + 6Oz ⁶ + 180z ⁷ +, 2o5c5cz ⁸ +, -1io8nOz ⁹ +, r6rO, z ¹⁰ +, + X ^ ²⁴ z ¹² In a third example, the binary code CB is a code length n parity containing 1 = n-1 bits of useful information and minimum distance d = 2. In such a code, parity bit is equal to "1" only when the weight of the n-1 bits of useful information is odd. The first and second enumerative polynomials of the parity code are:

A (X, Z) v X ^1] i = A ', A, 6 _t ... i = 2,4,6, ... where C is the combination operator.

As explained above, each photoreceptor REC _k receives a signal-to-noise ratio SNR _k which depends on the associated light source SL _k . Each light source SL _k emits a sequence of bits SC ^ possibly encoded by a binary code CB _k of minimum distance d _k . When a photoreceptor REC _k receives a signal which has been modulated according to a modulation of the "all or nothing" type 0OK and which contains a sequence of bits SC _k coded by a code CB _k of minimum distance d _k , an error rate binary BER _k can be estimated after a decoding of the received signal as follows:

1 ^

BER _k ≈ Σ ^B i ^x Q ^ ix SNR _k ), l ^k i = d _k where 1 _< is the dimension of the considered code and Q (x) is the following error function: For example, for a BCH code of minimum distance d = 5, the dimension of the code is 1 = 21.

Since the error function Q (x) decreases exponentially, the bit error rate BER _k can be approximated as a function of the first term of the sum, that is, by:

BER _k - B _d , x Q (jd _< x SNR _k ).

1

According to the invention, coded bit sequences

SE _k can be encoded respectively by CB _k codes of minimum distances d _k so that the different bit error BER _k corresponding rates to the signals respectively comprising the coded bit sequences SC 1 and transmitted respectively by the light sources SL ₁ are close to each other and possibly close to the bit error rate corresponding to the signal comprising a sequence of uncoded bits and transmitted by the source light with the lowest signal-to-noise ratio.

For example, a coded bit sequence SCj ₄ is to be transmitted by the light source SLj ₁ having the lowest signal-to-noise ratio SNR ^ among the different light sources and the bit sequence SC ^ is coded by a minimum distance code d ^ so that the product of the lowest signal-to-noise ratio SNR i by the minimum distance d i of the code is substantially equal to the highest signal-to-noise ratio among the different light sources, or at least equal to the second signal ratio the lowest noise among the different light sources. In all cases, the light source having the highest signal-to-noise ratio transmits a bit sequence SBj, which is not coded.

With reference to FIG. 2, the white light signal emission method according to the invention comprises steps E1 to E3 automatically executed in the transmission device SE.

In step El, the transmission device SE receives as input a series of SER bits to be transmitted by means of the light sources SL 1. The serial-parallel converter CSP distributes the series of SER bits in K bit sequences SB] _ to SBK, at the rate of one bit among K successive bits of the SER series by light source. The K bit sequences SB 1 to SB 2 are supplied to the coder COD so as to transmit them respectively by the K light sources SL] _ a SL _K.

In step E2, the coder COD encodes one or more sequences of bits SB ^ into coded bit sequences SC ^ which are to be transmitted respectively by light sources having the lowest signal-to-noise ratios among the different light sources. In a complementary manner, the coder COD does not code the other bit sequence (s) that are to be transmitted respectively by the other light sources having the highest signal to noise ratios among the different light sources. Therefore, in addition to the coded bit sequence to be transmitted by the light source having the lowest signal-to-noise ratio, other bit sequences may be encoded if they are transmitted by other light sources having signal-to-noise ratios. noise strictly between the lowest signal to noise ratio and the highest signal to noise ratio.

According to one embodiment of the invention with the three light sources SLi, SL ₂ and SL ₃ which emit respectively in blue, green and red, only the light source SL] _ which has a signal to noise ratio much higher than those of the other light sources SL ₂ and SL ₃ emit a sequence of bits SB] _ which is not coded, while the other light sources SL ₂ and SL ₃ respectively transmit coded bit sequences SC ₂ and SC ₃ .

As previously indicated, the blue-emitting light source SL 1 has the highest signal-to-noise ratio SNR 1. The SB ₂ and SB ₃ bit sequences are coded respectively into SC ₂ and SC ₃ bit sequences, coded respectively by a code CB2 of minimum distance d ₂ and a code CB ₃ of minimum distance d ₃ . Each code CB ₂ , CB3 is selected so that the product of the signal-to-noise ratio SNR ₂ , SNR ₃ by the minimum distance d ₂ , d ₃ of the code is substantially equal to the signal-to-noise ratio SNR ₁ :

SNR ₁ ≈ d ₂ x SNR ₂ ≈ d ₃ x SNR ₃ .

By way of example, for a given position of the light sources SL ₁ , SL ₂ and SL ₃ with respect to photoreceptors and for an average power received of -30 dBm by the photoreceptors, the linear signal-to-noise ratios are SNR ₁ = 19 , 86, SNR ₂ = 8.6 and SNR ₃ = 4.28 respectively for blue, green and red diodes. The uncoded bit sequence SB ₁ contains U ₁ = 31 bits and therefore I ₁ ^~ 31 bits of useful information. The coded bit sequence SC ₂ may be encoded by a parity code CB2 of length n ₂ , containing I ₂ = n ₂ -l bits of useful information and of minimum distance d ₂ = 2. The coded bit sequence SC ₃ may be encoded by a BCH code length CB3 n3 = 31, containing I ₃ = 21 bits of useful information and minimum distance d ₃ = 5.

Moreover, if each bit sequence SB ₁ , SC ₂ , SC ₃ to be transmitted contains n] _ = n ₂ = n ₃ = 31 bits, the overall efficiency p of the transmission of the bit sequences is: p = (I ₁ + I ₂ + 1 ₃ ) / (3 xn _x ), ie p = (31 + 30 + 21) / (3 x 31) = 88%.

In another example, the uncoded bit sequence SB ₁ contains rt] = 24 bits and thus I ₁ = 24 bits of useful information. The encoded bit sequence SC ₂ may be encoded by a parity code CB ₂ of length n] _, containing I ₂ = n] _- l = 23 bits of useful information and of minimum distance d ₂ = 2. The sequence coded bit rate SC ₃ can be encoded by a Golay code CB ₃ of length ni, containing I3 = 12 bits of useful information and minimum distance d ₃ = 8. In this case, the overall efficiency p of the transmission of the bit sequences is: p = (24 + 23 + 12) / ( 3 x 24) = 82%.

The use of a Golay code with respect to a BCH code offers a high correction capacity but generates a lower overall transmission efficiency. Alternatively, if the light sources SL SLi ^e t ₂ respectively have signal to noise SNRi and SNR2 close relationships, the sequences of SB ₂ and SB corresponding bits are not encoded and only the bit sequence SB ₃ is coded. In step E3, the coder COD provides the sequences of coded or uncoded bits respectively to the light sources. SLi the light source emits the bit sequence SBi ^uncoded and the light sources SL ₂ and SL ₃ respectively transmit the encoded bit sequences SB ₂ and SB _3.

For example, each light source emits the bit sequence as a monochromatic radiation by means of a forward-biased PN junction, after applying 0OK "all-or-nothing" intensity modulation to the bit sequence .

The transmission of the bit sequences via several light sources results in a decrease in the relative amplitude of each light source, which contributes to better remeαier intersymbol interference during the reception of the bit sequences.

The invention described herein relates to a method and a device for transmitting bit sequences. According to one embodiment, the steps of the method of the invention are determined by the instructions of a computer program incorporated in the device such as the transmission device SE. The program comprises program instructions which, when said program is executed in a processor of the device whose operation is then controlled by the execution of the program, perform the steps of the method according to the invention. Accordingly, the invention also applies to a computer program, including a computer program recorded on or in a computer readable recording medium and any data processing device, adapted to implement the computer program. 'invention. This program can use any programming language, and be in the form of source code, object code, or intermediate code between source code and object code such as in a partially compiled form, or in any other form desirable to implement the method according to the invention.

The recording medium may be any entity or device capable of storing the program. For example, the medium may comprise storage means on which is recorded the computer program according to the invention, such as a ROM, for example a CD ROM or a microelectronic circuit ROM, or a USB key, or a magnetic recording medium, for example a floppy disk or a hard disk.

On the other hand, the recording medium may be a transmissible medium such as an electrical or optical signal, which may be conveyed via an electrical or optical cable, by radio or by other means. The program according to the invention can in particular, be downloaded on an internet-type network.

Alternatively, the recording medium may be an integrated circuit in which the program is incorporated, the circuit being adapted to execute or to be used in the execution of the method according to the invention.

Claims

1 - Process for transmitting bit sequences (SB] _-SBκ) respectively by at least two light sources (SLi ~ SL _κ ) which emit combined radiations into a substantially white light and which have different signal-to-noise ratios respectively depending wavelengths of the radiation, characterized in that it comprises an encoding (E2) of the bit sequence (SB ^) to be emitted by the light source (SL] ^) having the lowest signal-to-noise ratio by a code (CB ^) having a minimum distance whose product by the lowest signal-to-noise ratio is substantially equal to the highest signal-to-noise ratio, the minimum code distance being equal to the minimum number of different bits between two words code (CB ^).

2 - Method according to claim 1, further comprising a coding by another code of at least one other sequence of bits to be transmitted by a light source having another signal-to-noise ratio comprised between the lowest signal-to-noise ratio and the highest signal-to-noise ratio, said other code having a minimum distance whose product by said other signal to noise ratio is substantially equal to the highest signal-to-noise ratio.

3 - Apparatus 1.SE) for emitting sequences of bits fSB] _- SBκ) respectively by at least two light sources (SL ^ -SL ^) which emit combined radiations in a substantially white light and which have Siαnal reports a noise different depending respectively on the wavelengths of the radiation, characterized in that it comprises means (COD) for coding the sequence of bits (SB) J to be emitted by the light source (SLjJ having the lowest signal-to-noise ratio by a code (CBjJ having a minimum distance whose product by the lowest signal-to-noise ratio is substantially equal to the highest signal-to-noise ratio, the minimum distance of the code being equal to the minimum number of different bits between two words of the code (CBj ₁ ).

4 - Device according to claim 3, wherein the light sources (SL] _ a SL ^) are light emitting diodes.

5 - Device according to claim 3 or 4, comprising three light sources capable of respectively radiating substantially in blue, green and red.

6- A computer program adapted to be implemented in a device (SE) for transmitting bit sequences (SBχ-SB _κ ) respectively by at least two light sources (SLi-SL _κ ) which emit combined radiation into one substantially white light and which have different signal-to-noise ratios respectively depending on the wavelengths of the radiation, said program being characterized in that it comprises instructions which, when the program is executed in said photoreceptor, perform a coding (E3 ) of the bit sequence (SBjJ to be emitted by the light source (SL) J having the lowest signal-to-noise ratio by a code (CBjJ having a minimum distance whose product by the lowest signal-to-noise ratio is substantially equal to the highest signal-to-noise ratio, the minimum distance of the code being equal to the minimum number of different bits between two words of the code (CBj ₁ ).

7 - Device-readable recording medium for transmitting bit sequences (SB-SB _K ) respectively by at least two light sources (SLχ-SL _κ ) which emit combined radiations into substantially white light and which have reports signal with different noise depending respectively on the wavelengths of the radiation, characterized in that it has recorded a computer program comprising instructions for the execution of an encoding (E3) of the bit sequence (SBj ₄ ) to be emitted by the light source (SL] ₄ ) having the lowest signal-to-noise ratio by a code (CBj ₄ ) having a minimum distance whose product by the lowest signal-to-noise ratio is substantially equal to the signal-to-noise ratio highest noise, the minimum distance of the code being equal to the minimum number of different bits between two words of the code (CB _k ).