Abstract:
The invention relates to a wireless optical data transmission system and a method for wireless optical transmission of data. The system comprises a data stream generator for generating at least two parallel data streams ( 18.1, 18.2, 18.3 ). The parallel data streams are transmitted by a number of separate optical transmitting devices ( 8, 9, 10 ) separately by emitting first optical signals. The system further comprises a corresponding number of detectors ( 19, 20, 21 ) for detecting the first optical signals and converting them into respective second signals ( 26.1, 26.2, 26.3 ) and an error correction unit ( 31 ) for correcting the amended second signals. Within a predistortion ( 29 ) unit each second signal ( 26.1, 26.2, 26.3 ) is amended individually with respect to a transmission channel used.

Description:
FIELD OF INVENTION 
   The invention relates to a method for wireless optical transmission of data and to a wireless optical data transmission system. 
   BACKGROUND 
   There are several types of wireless optical data transmission systems. One possibility for realising such a wireless system for transmitting data is using white light emitting devices that are pulsed at a high frequency, which could not be detected by the human eye. Modulation scheme of on/off keying (OOK) for modulation is adopted. It is possible to use different colours of light emitted by a multi-chip-type white LED as parallel transmission channels. 
   The white light of multi-chip-type white LEDs is produced by simultaneously emitting light in the three basic colours, blue, red and green. The superposition thereof results in white light. The problem with using multi-chip-type white LEDs for transmitting data in parallel is that the power ratio of each light emitting device has to be different in order to obtain white light. When a multi-chip-type white LED is used for illumination it is not acceptable to emit bluish or reddish light for example. Thus, the power ratio of the emitted light of the three colour LEDs is fixed and cannot be adjusted. 
   The emitted light of the three LEDs is detected by three separate detectors, but the conversion efficiency of each of the detectors depends on the colour of each of the emitting LEDs. Therefore, the reliability of the output signal of the detectors is different for each colour. 
   For assuring an error rate which is less than 1E-10 e.g. for wireless communications, an error control decoder is required. It is known that the performance of soft decision error control decoders is usually better than the performance of hard decision error control decoders. But even the use of a soft decision error control decoder might not achieve the required error rate, because of the different reliability on each colour. 
   It is therefore an object of the present invention to create a method for wireless optical transmission of data and a wireless optical transmission system to improve the performance of soft decision error control decoding. 
   This problem is solved by a method according to claim  1  and a data transmitting system according to claim  10 . 
   SUMMARY OF THE INVENTION 
   According to the method of claim  1  and the transmission system of claim  10  at least two parallel data streams are produced. Each of these parallel data streams is wirelessly transmitted by use of separate optical transmitting devices. The optical transmitting devices emit first optical signals in order to transmit the data. Each emitted first optical signal is detected by a corresponding detector which converts the received first optical signal. Thus, if three parallel data streams are used e. g. three second signals are output by three separate detectors. Before these signals are fed to an error correction decoder, each of the second signals is amended individually. The amendment of the second signals is performed by a predistortion unit. 
   The error decoding performed on the basis of the amended second signals is much better than performing the error correction on the basis of the output signal of the detectors. By amending the second signals it is possible to consider the poor reliability of the output second signals of a particular detector. On the other hand, a signal with a good reliability obtained from a detector for another light colour is amended with respect to the known good reliability. As a result, the error decoding algorithm of the soft decision error control decoder works in a better way and the overall performance of such a transmission system is improved. Further improvements of the invention are claimed in the subclaims. 
   A preferred embodiment of the present invention is shown in the drawings and will be described in detail below. It is shown: 

   
     BRIEF DESCRIPTION OF THE FIGURES 
       FIG. 1  a diagram of a multi-chip-type white LED 
       FIG. 2  an enlarged view of part II of  FIG. 1   
       FIG. 3  the spectra of emitted light of a multi-chip-type white LED 
       FIG. 4  a block diagram of a optical data transmission system according to the invention and 
       FIG. 5  a flow chart of the method for optical transmission of data according to the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  shows a white LED  1  of the multi-chip-type. The white LED  1  has a moulded cover  2  that is transparent and encloses a support part  3 . Preferably, the support part  3  is formed by an electric connector  4  that is connected to ground. 
   On the surface of the support part  3  three separate LEDs are fixed which is described in greater detail with respect to  FIG. 2 . 
   Each of the separate LEDs is fixed on the support part  3  and is electrically connected with conductive paths  5 ,  6  and  7 , respectively. The conductive paths  5 ,  6 ,  7  are connected with a source for supplying a supply voltage to each LED. 
   A multi-chip-type white LED  1  consists of three separate LEDs. Each of these LEDs emits light with a different colour. For emitting white light the colours are green, blue and red. An enlarged view of part II of  FIG. 1  is shown in  FIG. 2 . On top of the support part  3  three light emitting devices  8 ,  9  and  10  are arranged on the ground of a recess  11 . Each of the LEDs is supplied with a voltage individually. Thus, the LEDs  8 ,  9 ,  10  can be switched on an off individually. 
   By controlling the supply voltage of the LEDs  8 ,  9  and  10  individually, the three LEDs  8 - 10  can be used for parallel transmission of data so that a red transmission channel, a green transmission channel and a blue transmission channel are established. The rate at which each supply voltage of the LEDs  8 - 10  is switched on an off is too fast to be detected by the human eye. The human eye therefore only gets the impression of white light being emitted by the white LED  1 . 
   In order to get the impression of white light emitted by the white LED  1  the ratio of the optical emitted power from the red, the green and the blue LEDs  8 ,  9 ,  10  differs. A spectrum of a blue LED, a spectrum of a green LED and a spectrum of a red LED is shown in  FIG. 3  and indicated with reference numerals  12 ,  13  and  14 . It is clear from the shape of the different curves of the blue, the green and the red LED that the emitted power of the three LEDs must not be equal in order to obtain white light. 
   The preferred embodiment of the present invention is now explained referring to the block diagram of  FIG. 4 . Data  15  to be optically transmitted is processed by a data stream generator and is first input into an encoder  16  that encodes the data with respect to a particular protocol chosen for transmission. The encoded data is output from the encoder  16  as serial data and is transferred to a serial to parallel converter  17 . In the preferred embodiment the encoder  16  and the serial to parallel converter  17  form the data stream generator but other sources for parallel data streams are possible. The serial to parallel converter  17  splits the serial encoded data into three parallel data streams  18 . 1 ,  18 . 2  and  18 . 3 . According to the data content of the individual data streams  18 . 1  to  18 . 3  the three light emitting devices  8 ,  9  and  10  are supplied with voltage. 
   According to the voltage supplied the LEDs  8 ,  9  and  10  are switched on and off and emit light. Each LED  8  to  10  forms an optical transmitter for a transmission channel. The red, green and blue transmission channels are used in parallel. As described earlier, the ratio of the emitted power of the different LEDs  8 ,  9  and  10  must not be equal in order to result in white light emitted from the white LED  1 . 
   In order to retrieve the original data  15  to be transmitted three detectors  19 ,  20  and  21  are arranged within a detector unit  22 . Each detector  19 ,  20  and  21  is connected with a filter for filtering the incident white light with respect to the basic colburs. The filtered light thus assigns each detector  19 ,  20 ,  21  to a particular transmission channel. For example, ahead of detector  19 , a bandpass optical filter  23  is arranged that transmits only the red component of the light emitted by the white LED  1 . In the same way a second filter  24  is arranged ahead of detector  20 , which is a bandpass optical filter too and transmits only the green component of the incident light. In the same way a third filter  25  is arranged ahead of the detector  21 , which permits only the blue component of the light to reach the sensitive surface of the detector  21 . LED  8  together with filter  23  and detector  19  build a red transmission channel, whereas LED  9  together with filter  24  and detector  20  build a green transmission channel and LED  10  together with filter  25  and detector  21  build a blue transmission channel. 
   The conversion efficiency of the detectors  19 ,  20  and  21  is depending on the colour of the incident light. The conversion efficiencies of the three detectors  19 ,  20 ,  21  are therefore different in the transmission channels. The detectors  19 ,  20  and  21  convert the light falling on a sensitive surface into a second signal each. Thus, the detectors  19 ,  20  and  21  output three second signals  26 . 1 ,  26 . 2  and  26 . 3 , whereby the reliability of the second signals  26 . 1  to  26 . 3  differ from each other. This means the second signal that corresponds to the data stream  18 . 1  which is transmitted by emitting red light by the LED  8  has a different reliability compared with e.g. the second signal being output by detector  21  which corresponds to the data stream  18 . 3  transmitted by emitting blue light by diode  10 . The three parallel second signals  26 . 1  to  26 . 3  are input into a parallel to serial converter  27 . The parallel to serial converter  27  converts the input parallel second signals  26 . 1  to  26 . 3  into a serial signal, which is a common second signal  28 . 
   The common second signal  28  is transferred to a predistortion unit  29 . The common second signal r(t) (reference numeral  28 ) contains the individual second signals  26 . 1  to  26 . 3  but being arranged in series. Within the predistortion unit  29  the second signals  26 . 1  to  26 . 3  of a common second signal  28  are amended individually. This means that the second signal  26 . 1  is amended in a different way compared to the second signal  26 . 3  e.g. The amendment is performed by considering the different optical power of the corresponding transmission channel and the different conversion efficiency of corresponding detectors  19 ,  20 ,  21 . For amending the second signals individually first a channel factor P i  for each channel i is calculated that considers the emitted optical power of the respective LED  8 ,  9 ,  10  and the corresponding conversion efficiency of the respective detector  19  to  21 . The part of the common second signal  28  corresponding to a particular second signal  26 . 1 ,  26 . 2 ,  26 . 3  is then multiplied by a factor that considers a ratio of the individual channel factor P i  of the respective red, green or blue transmission channel to an overall channel factor. 
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   The channels factors P i  are defined by the ratio of the optical output power M i  multiplied by the square root of the conversion efficiency C i  of the respective detector  19  to  21  (P i =M i *√{square root over (C i )}). Using these functions for amending the second signals results in having more weight on the second signals  26 . 1 ,  26 . 2  or  26 . 3  with a good reliability and therefore having an improved input signal for the error decoder that leads to a better error rate. 
   The predistortion unit  29  outputs an amended common second signal r′(t) (reference numeral  30 ) consisting of parts that represent amended second signals. The amended common second signal  30  is transferred to a soft decision error control decoder  31 . The soft decision error control decoder  31  might be a Viterbi decoder or a Viterbi decoder concatenated with Reed-Solomon decoder or a decoder for Turbo Convolutional Code or a decoder for Turbo Product Code or a decoder for Low Density Parity Check Code for example. The performance of such soft decision decoders is enhanced because of the different weighing of the second signals  26 . 1 ,  26 . 2  and  26 . 3  by amending them, the amendment being performed with respect to the reliability of the signals. On the first hand, the reliability of the signals depends on the optical power emitted by the LED  8  to  10  of the respective transmission channel and on the other hand the reliability depends on the conversion efficiency of the detectors  19  to  21  of the respective transmission channel. 
   The preferred embodiment which is described and shown in  FIG. 4  considers both the different optical power within the transmission channels and the respective conversion efficiency of the detectors  19 - 21  of the transmission channels which differs due to the colour of the emitting LED  8  to  10 . It is also possible to consider only the different optical power or only the different conversion efficiency of the detectors  19  to  21 . 
   In  FIG. 5  a flow chart of the amendment being performed on the common second signal r(t) is shown. In a first step  32  the method is initiated e.g. automatically by inputting a common second signal r(t). In the next step  33  it is questioned whether the actual part of the common second signal r(t) belongs to a second signal  26 . 1  that corresponds to the red transmission channel. If the answer to this question is “yes” then the actual common second signal r(t) is amended by multiplying the common second signal r(t) with a correction term as explained above. If the answer to the question of step  33  is “no” then the next question is, if the actual common signal r(t) is corresponding to a data stream that is transmitted via the blue transmission channel. If the question of step  35  is answered “yes” then the common second signal r(t) is amended by being multiplied with another correction factor considering the channel factor P b  of the blue transmission channel. If the question of step  35  has to be answered “no” then the actual signal must correspond to the data stream  18 . 2 , which is transmitted by the green transmission channel. In this case, the actual common second signal r(t) is amended by a correction factor that considers the channel factor P g  of the green transmission channel. Every time the common second signal  28  is amended, the processing of data ends and the amended signal is transferred to the soft decision error control decoder  31 . 
   The scope of the invention is not limited by the shown preferred embodiments but covers also arbitrary combinations of the shown features.