JP5474515B2 - Visible light communication transmitter - Google Patents

Description

  The present invention relates to a transmission device for visible light communication that performs communication using visible light irradiated to a space, and in particular, visible light capable of transmitting a large amount of information at high speed using an OFDM (Orthogonal Frequency Division Multiplexing) system. The present invention relates to an optical communication transmitter.

  Wireless communication using radio waves as a communication medium is used in many fields such as a cellular phone network, a wireless LAN, and short-range wireless communication.

  However, in wireless communication using radio waves as a medium, when transmission / reception is performed near a person, the transmission power cannot be increased in consideration of the influence of electromagnetic waves on the human body. In addition, the frequency band of radio waves used for wireless communication has already been allocated and used in many fields of use, and a wide frequency band cannot be freely used. Furthermore, there are restrictions such as restrictions on the use of radio waves in special environments such as hospitals.

Therefore, in recent years, visible light communication for performing communication using visible light as a communication medium has been developed. In Patent Document 1 and the like, a transmission signal is superimposed on visible light and emitted as spatial light, and the emitted light is received. There has been proposed a visible light communication system in which light is received at the receiver side and communication is performed by demodulating a transmission signal from the received light signal.
JP 2007-266794 A

  This conventional visible light communication system performs visible light communication using illumination light by superimposing a high frequency signal on visible light while using visible light as illumination light, and superimposing a transmission signal on a high frequency carrier wave. The high-frequency signal is input to a light emitting LED (light emitting diode) to cause the LED to emit light, and visible light superimposed with a transmission signal is transmitted to the receiver side. In this visible light communication system, the communication speed is Slowly, it is difficult to transmit a large amount of information such as an audio signal or an image signal at high speed.

  On the other hand, in the field of broadband digital communication such as broadband Internet connection and digital television broadcasting, a communication device that uses the OFDM method and enables transmission of a high-capacity digital signal at high speed has been developed. This OFDM digital communication is a type of multi-carrier transmission communication system that divides a digital signal into a large number of carriers (subcarriers), modulates and transmits the signal, and in particular, a large number of subcarriers are on the frequency axis. Even if they are arranged so densely as to overlap, they are orthogonal to each other and do not interfere with each other. Therefore, the spectrum of subcarriers, that is, subchannels, can be densely arranged, frequency utilization efficiency is increased, and large amounts of parallel data are stored. It can be serialized and transmitted at high speed.

  Therefore, in the visible light communication transmitter using the above-mentioned visible light as a communication medium, in order to realize a large-capacity digital high-speed communication, a large number of subcarriers are modulated by each bit of the digital signal using the OFDM method. Subcarriers are synthesized using an inverse fast Fourier transform algorithm, the synthesized digital signal is converted to analog to form a modulated high frequency signal, and the high frequency signal is input to the LED to cause the LED to emit light. A visible light communication transmitter that radiates visible light as spatial light and performs visible light communication has been studied.

  Visible light communication systems adopting the OFDM system can be used in a favorable state in which, for example, a wide frequency band of 200 kHz to 400 MHz is not easily affected by noise in the natural world or noise of commercial power sources. However, visible light as a communication medium is different from radio waves in that the usable frequency band is not limited by the existing use, and an arbitrary frequency band can be used. For this reason, for example, when a relatively wide frequency band of 4 MHz to 34 MHz is used as a carrier frequency used for visible light communication, for example, an LED that emits visible light on which a high-frequency signal in such a wide frequency region is superimposed is used. It is necessary to emit light with stable frequency characteristics in a wide frequency range.

  However, as shown in FIG. 2, the frequency characteristic of the LED usually has a characteristic that, for example, in the band of 4 MHz to 34 MHz, the light emission level is higher as the frequency is lower and the light emission level is lower as the frequency is higher. The distortion in the frequency characteristics of the LED is considerably large. Also, since the spectrum distribution of the OFDM transmission signal is very finely formed, if the spectrum distribution changes due to non-linear elements associated with the frequency characteristics of the LED and distortion occurs, the signal is normally transmitted on the receiver side. It becomes difficult to demodulate.

  For this reason, in the visible light communication transmission apparatus of the OFDM system that transmits a high-frequency signal distributed in a band of 4 MHz to 34 MHz, which is a composite wave of a large number of subcarriers, the visible light irradiated from the LED for projection is broadband as it is. When the high-frequency signal is superimposed and transmitted, high-speed digital communication becomes difficult due to the nonlinear element (distortion) of the frequency characteristic of the LED. In addition, since the light emission characteristics (current-voltage characteristics) of the LED have nonlinear characteristics, distortion in the amplitude direction occurs in the light emission brightness of the LED, and information superimposed in the amplitude direction of the light emission brightness during modulation is accurately received on the receiving side. It becomes impossible to receive and hinders high-speed communication.

  The present invention has been made in view of the above points, and an object of the present invention is to provide a visible light communication transmitter capable of realizing high-speed communication using visible light using the OFDM method.

In order to achieve the above object, the visible light communication transmitting apparatus of the present invention includes:
Transmits the transmission data signal by dividing the digital signal of the transmission data into a number of subcarriers by the orthogonal frequency division multiplexing method and modulating the modulated high frequency signal to the visible light emitted by the light emitting operation of the LED. In the visible light communication transmitter
A transmission data processing unit for rearranging transmission data so as to assign each bit of the packetized transmission data signal to each subcarrier;
A modulation unit that digitally modulates each bit of the digital signal of transmission data sent from the transmission data processing unit for each subcarrier;
An S / P converter that converts the serial digital transmission signal modulated by the modulator into a data string of the number of subcarriers;
An inverse discrete Fourier transform unit for performing inverse fast Fourier transform on the digital transmission signal converted in parallel by the S / P conversion unit;
The digital transmission signal inversely fast Fourier transformed by the inverse discrete Fourier transform unit corrects in advance the inverse characteristic of the distortion by adding the inverse characteristic of the distortion to the frequency characteristic of the LED, and the LED has A correction calculation unit that performs a correction operation so as to correct in advance the inverse characteristic of the distortion with respect to the nonlinear distortion of the current-voltage characteristic;
A D / A converter that converts the digital transmission signal corrected by the correction calculation unit into an analog signal;
A synthesizing unit that orthogonally modulates and synthesizes the complex data converted into an analog signal by the D / A converter using a cosine wave and a sine wave of the carrier frequency of the subcarrier;
An LED driving unit that drives the LED to amplify and superimpose the analog high-frequency signal synthesized by the synthesizing unit on visible light;
Equipped with a,
The LED is provided in a lamp that emits visible illumination light, the LED is configured to emit visible light as illumination light, and a correction table that corrects the digital transmission signal for dimming the illumination light includes the correction table. It is provided in the correction calculation unit, and the correction table is changed in accordance with the dimming operation to correct frequency characteristic distortion caused by dimming .

  According to the visible light communication transmitting apparatus of the present invention, the digital transmission signal obtained by inverse fast Fourier transform by the inverse discrete Fourier transform unit linearly corrects the non-linear frequency characteristic of the visible light projecting LED to equalize the band. In addition, in order to linearly correct the non-linear current-voltage characteristics of the LED, a correction operation using a predistortion method is performed by adding a reverse characteristic of the distortion in advance to the signal, and therefore, for example, a high frequency band of 4 MHz to 34 MHz is used. When transmitting a signal, the LED tends to cause distortion in the light emission characteristics due to high frequency characteristics and current voltage characteristics, that is, the light emission level is higher as the frequency is lower, and the light emission level is lower as the frequency is higher, Even when an LED having nonlinear distortion in the current-voltage characteristics is used, the transmission signal of the OFDM system is used. The transmission signal can be superimposed on the visible light of the LED without distorting the spectral distribution of the LED, so that a large amount of information can be transmitted at high speed, so that the OFDM signal is demodulated normally on the receiver side. be able to.

  According to the visible light communication transmitting apparatus of the present invention, when the correction calculation unit calculates and corrects the digital transmission signal so as to linearly correct the frequency characteristic of the LED, the correction table is changed according to the dimming operation, The correction value of the transmission signal can be changed according to dimming, and the distortion of the frequency characteristic and current-voltage characteristic of the LED corresponding to dimming, that is, the distortion of the light emission characteristic of the LED can be linearly corrected.

  Further, the LED driving unit is provided with an amplifier that amplifies the analog high-frequency signal synthesized by the synthesizing unit, and adjusts the light emission level of the LED by adjusting the DC offset level of the amplifier according to the dimming operation. The dimming circuit to perform can be connected to an amplifier.

  According to this invention, the direct current offset level of the amplifier can be adjusted according to the dimming operation, and the light emission level of the LED as the illumination light can be dimmed.

  Further, the amplifier of the LED driving unit is connected with a temperature compensation circuit for compensating the temperature characteristic of the LED to make the illuminance constant with respect to the temperature change of the LED, and the detected current / voltage of the LED and the LED or the vicinity thereof. Based on the temperature detection signal of the temperature sensor that detects the temperature of the LED, a signal is output to the amplifier to adjust the DC offset level, while a correction signal is sent to the correction calculation unit so that the correction calculation unit Correction operations can be performed to compensate.

  According to the present invention, for example, when the temperature of the LED rises and the illuminance decreases, the DC offset level of the amplifier is increased to increase the illuminance, while the correction calculation unit performs temperature compensation for the LED temperature change. Thus, it is possible to suppress the decrease in illuminance associated with the temperature rise of the LED and stabilize the illuminance of the LED.

  Further, the LED driving unit is provided with an illuminance stabilization circuit that linearly corrects the non-linear current-voltage characteristics of the LED according to the dimming signal and stabilizes the illuminance of the LED, and according to the output voltage of the LED driver. A signal is output from the illuminance stabilization circuit to the amplifier to adjust its DC offset level, while a correction signal is sent to the correction calculation unit so that the nonlinear distortion of the current-voltage characteristic of the LED is linearly corrected in advance. The inverse characteristic can be corrected and added to the signal.

  According to the present invention, when the characteristic curve portion changes due to dimming with respect to the nonlinear distortion of the current-voltage characteristic of the LED, a signal is output to the amplifier so as to compensate for the nonlinear distortion, and the DC offset level is set. On the other hand, the correction calculation unit can perform correction calculation to linearly correct the distortion of the current-voltage characteristic of the LED, that is, the distortion of the light emission characteristic.

  Thus, according to the visible light communication transmitter, visible light emitted from the LED is used as illumination light, and even when the illuminance of visible light is adjusted by a dimming operation, Large amounts of information can be superimposed on visible light and transmitted at high speed while stabilizing the illuminance of visible light.

  Further, instead of the correction calculation unit, an analog band correction amplifier and a distortion compensation circuit are connected to the output side of the OFDM modulator, high frequency band correction is performed, band equalization is performed, and LED current-voltage characteristics Predistortion correction can be performed.

In this case, the band correction amplifier is configured to amplify the analog high-frequency transmission signal synthesized by OFDM modulation with an inverse characteristic having a slope opposite to the frequency characteristic of the LED. The distortion compensation circuit connects an analog non-linear element to the signal circuit, adjusts the bias voltage applied to the analog non-linear element, and reverses the current-voltage characteristics of the LED with respect to the high-frequency transmission signal flowing through the signal circuit. An inverse characteristic of inclination is added, and the light emission characteristic of the LED is linearly corrected.
As a result, the light emission characteristic of the LED has a frequency characteristic in which the light emission level is higher as the frequency is lower, and the light emission level is lower as the frequency is higher. A large amount of information can be transmitted at high speed by superimposing the transmission signal on the visible light of the LED without causing distortion in the spectrum distribution of the transmission signal, which allows the OFDM signal to be properly transmitted on the receiver side. It can be demodulated.

  According to the visible light communication transmitting apparatus of the present invention, a large-capacity transmission signal modulated by OFDM can be superimposed on the visible light applied to the space and stably transmitted at high speed.

1 is a configuration block diagram of a visible light communication transmission device showing an embodiment of the present invention. It is a graph which shows the frequency characteristic of light emission of LED. It is a graph of the correction coefficient which correct | amends the frequency characteristic of LED. It is a graph which shows the current-voltage characteristic of LED. It is a block diagram of the configuration of a visible light communication transmitter according to another embodiment. FIG. 6 is a circuit diagram of a distortion compensation circuit in the block diagram of FIG. 5.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 shows a configuration block diagram of a visible light communication transmitter. As shown in FIG. 1, the visible light communication transmitting apparatus receives packet data sent through a network such as a LAN, and receives an input data processing unit 1 that recombines the data for OFDM modulation, and is sent from the input data processing unit 1. Each bit of the transmitted data signal is digitally modulated with respect to each subcarrier, the digitally modulated serial data string is converted into parallel, and then the parallel data is converted into time series data by inverse fast Fourier transform. After applying distortion correction to the data, it is converted into an analog signal and synthesized to modulate the transmission data signal for OFDM, and a high-frequency signal of the OFDM-modulated transmission data sent from the OFDM modulator 2 , The high frequency signal is amplified and applied to the LED driver 33, and the LED driver 33 It includes a LED driver 3 for emitting operation (a light emitting diode) 30, a.

  For example, the LED 30 is mounted on an illumination lamp (not shown), and various information signals transmitted through a LAN or the like are superimposed on the visible light for illumination, and the LED 30 of the lamp irradiates the illumination light. An information signal is transmitted to a receiver (not shown) arranged.

  The data processing unit 13 of the input data processing unit 1 includes a CPU, a memory, a register, and the like, and inputs packet data for transmission sent from the LAN through the Ethernet (registered trademark) interface 11. Then, the bucket data is temporarily stored in the data buffer 12, and the data processing unit 13 uses the data memory 14 to reassemble the packet data for OFDM and outputs the packet data from the input / output unit 15 to the OFDM modulator 2. Configured as follows. When a dimming signal for dimming the LED 30 is sent through the input data processing unit 1, the dimming signal is sent to the dimming circuit 36 through the input / output unit 15.

  As described above, the OFDM modulator 2 includes the digital modulation unit 21 that digitally modulates each bit of the transmission data signal for each subcarrier. On the output side of the digital modulation unit 21, a digitally modulated serial data string is In other words, an S / P converter 22 that converts in parallel is connected so that each bit of data is assigned to each subcarrier.

  The digital modulation unit 21 digitally modulates the transmission data signal for OFDM sent from the input data processing unit 1 using a modulation scheme such as 64QAM, 16QAM, or PSK, and the S / P conversion unit 22 performs digital modulation. When N (for example, 1024) subcarriers are used so that each bit of the data string is allocated to each subcarrier, each bit of the data string is converted into N parallel data, and the parallel data is inversely discrete. The result is output to the Fourier transform unit 23.

  The inverse discrete Fourier transform unit 23 performs inverse Fourier transform on the digitized and digitally modulated N symbol data sequences at high speed, and outputs the calculation result to the correction computation unit 24 as complex data in the time domain composed of real numbers and imaginary numbers. To do.

  The correction calculation unit 24 includes a CPU, a memory, a register, and the like, and adds a reverse characteristic of distortion to the signal in advance so as to linearly correct the light emission characteristic based on the nonlinear frequency characteristic and current-voltage characteristic of the LED 30. The equalization is performed and the predistortion type correction operation is performed.

  The correction calculation unit 24 corrects the inverse characteristic of the distortion in advance by adding the signal to the signal so as to linearly correct the nonlinear frequency characteristic of the LED 30, and against the nonlinear distortion of the current-voltage characteristic of the LED. In addition, it is configured to perform a correction operation in advance by adding an inverse characteristic of distortion to the signal. As shown in FIG. 2, the frequency characteristic of the LED 30 in the high frequency region has a characteristic that the light emission level is higher as the frequency is lower, and the light emission level is lower as the frequency is higher. As shown in FIG.

  In this visible light communication transmitter, an information signal is transmitted by superimposing the high frequency signal on visible light emitted from the LED 30 using a high frequency band of 4 MHz to 34 MHz. The frequency characteristics of the LED 30 are shown in FIG. As shown, since there is nonlinear distortion in this frequency region, the correction calculation unit 24 applies band equalizing correction so as to correct the distortion of the frequency characteristics of the LED 30 in advance.

  The correction calculation unit 24 is a graph of the correction coefficient with respect to the frequency having a slope opposite to the frequency characteristic of the LED 30 shown in FIG. 2 with respect to the real and imaginary components of the complex data output from the inverse discrete Fourier transform unit 23. The curve (FIG. 3) is added for correction. Thus, the equalizing correction of the band is performed so that the real and imaginary components of the complex data after correction are smaller as the frequency is lower and are larger as the frequency is higher.

  Further, the correction calculation unit 24 has characteristic curves Ha, having slopes opposite to the current-voltage characteristics of the LED 30 shown in FIG. 4 for the real and imaginary components of the complex data output from the inverse discrete Fourier transform unit 23, respectively. Correct by adding Hb. As a result, the real and imaginary components of the corrected complex data are predistorted so that the current-voltage characteristics of the LED 30 are linearly corrected in advance.

  Further, the dimming signal is input to the correction calculation unit 24 from the dimming circuit 36 via the A / D converter 28, and the temperature compensation circuit 35 corrects the distortion of the temperature characteristic of the LED 30 to the A / D converter. A temperature correction signal is input via the reference numeral 29, and the real and imaginary components of the complex data output from the inverse discrete Fourier transform unit 23 are also predistorted by the dimming signal and the temperature correction signal.

  In order to predistort the real and imaginary components of the complex data in accordance with the dimming signal for dimming the illumination light and in accordance with the temperature correction signal, a correction table is provided in the correction calculation unit 24, and dimming is performed. The correction table is changed according to the signal and the temperature correction signal, and the nonlinear distortion of the LED due to the light control and the temperature change is linearly corrected.

  In addition, since the OFDM signal is a combined wave of a large number of subcarriers, a plurality of high-amplitude waves that are generated simultaneously are combined depending on the subcarrier combination, and a peak wave with a high peak value appears. Is configured to perform a correction operation for linear compression correction of peak wave data when a threshold wave is set in advance and a peak wave exceeding the threshold appears.

  The real and imaginary complex data corrected by the correction calculation unit 24 are respectively sent to the D / A conversion units 25 and 26 to be converted into analog signals, and then converted into analog signals by the D / A conversion units 25 and 26, respectively. The complex data converted into the signal is sent to the synthesis unit 27. In the synthesizer 27, a pair of complex data is orthogonally modulated and synthesized using a cosine wave and a sine wave of a carrier frequency of 4 MHz to 34 MHz of the subcarrier, and OFDM modulated from the synthesizer 27 for high frequency transmission of 4 MHz to 34 MHz. A signal is output.

  As described above, the OFDM modulator 2 digitally modulates each bit of the transmission data signal transmitted from the input data processing unit 1 for each subcarrier by the digital modulation unit 21, and converts the digitally modulated serial data string into S / P. After conversion into parallel by the conversion unit 22, the parallel data is converted into time-series data by inverse fast Fourier transform at the inverse discrete Fourier transform unit 23.

  The time series data is subjected to distortion correction so as to reversely correct the frequency characteristics, current voltage characteristics, etc. of the LED 30 in the correction calculation unit 24, and then the real and imaginary numbers of the complex data are converted into D / A conversion units. 25 and 26, and converted into analog signals by the D / A converters 25 and 26. The complex data converted into the analog signal is sent to the synthesizer 27, and the pair of complex data is orthogonally modulated and synthesized by the synthesizer 27 using the cosine wave and sine wave of the carrier frequency of the subcarrier, A transmission signal is output from the combining unit 27 to the LED driving unit 3.

  The LED drive unit 3 is configured by a circuit that amplifies a transmission signal having a subcarrier carrier frequency of the OFDM signal, operates the LED driver 33 by the transmission signal, and drives the LED 30 to emit light. The transmission signal output from the combining unit 27 is input to the amplifier 31 of the LED driving unit 3 and amplified, and the output side of the amplifier 31 is connected to the LED driver 33. Since an OFDM signal is a signal having a strict spectral distribution, an amplifier having a wide dynamic range and good linearity is required.

  As the LED driver 33, a circuit for driving the LED 30 with a constant current is used. As shown in FIG. 4, the IV characteristic (current-voltage characteristic) of the LED 30 decreases as the temperature of the LED 30 increases. When the current is reduced by light, the IV characteristic curve is distorted so that the slope of the IV characteristic curve becomes larger than when the current is high. For this reason, the LED drive unit 3 is provided with a temperature compensation circuit 35 that performs LED temperature compensation, and an illuminance stabilization circuit 34 that performs dimming operation stably.

  The temperature compensation circuit 35 detects the temperature of the LED 30 or the vicinity thereof, adjusts the DC offset level of the amplifier 31 based on the temperature, corrects distortion due to the IV characteristic of the LED 30, and performs temperature compensation. When the illuminance of the LED is dimmed by the dimming circuit 36, the illuminance stabilization circuit 34 corrects the distortion of the IV characteristic that occurs according to the dimming operation, and stably performs the dimming operation.

  On the other hand, a dimming circuit 36 is provided to adjust the illuminance of the LED 30 that is also used as illumination light for dimming. The dimming circuit 36 is dimmed by a dimming signal sent from the LAN or a dimming signal sent from a remote controller (not shown). The dimming signal is sent to the amplifier 31, and the direct current offset level of the amplifier 31 is adjusted by the dimming signal, and the illuminance of the LED 30 is changed by adjusting the level of the transmission signal. Further, the DC offset level of the amplifier 31 is adjusted based on the signals from the temperature compensation circuit 35 and the illuminance stabilization circuit 34 so as to correct the distortion of the IV characteristic due to the temperature of the LED 30 and the distortion of the IV characteristic due to dimming. Is done.

  FIG. 4 is a graph showing the current-voltage characteristics of the LED 30. Usually, the temperature coefficient of the LED is about −2 mV / ° C., the LED voltage at high temperature is lower than the LED voltage at low temperature, and the IV characteristic curve is The illuminance decreases as the voltage moves in the lower direction, and the slope of the IV characteristic curve also changes. For this reason, a temperature sensor for detecting the temperature of the LED 30 or the temperature in the vicinity of the LED 30 is provided, and the nonlinear distortion of the IV characteristic curve of the LED 30 is linearized according to the temperature and dimming of the LED 30 based on the detection signal of the temperature sensor. It is to be corrected.

  That is, as shown in the IV characteristic curve of the LED in FIG. 4, when the temperature of the LED 30 rises, the terminal voltage of the LED 30 decreases, and this IV characteristic curve changes depending on the illuminance level of the LED 30. For this reason, the DC offset level of the amplifier 31 that amplifies the transmission signal is adjusted by the temperature signal as well as the dimming signal, so that the temperature compensation of the operation of the LED 30 and the illuminance stabilization are performed.

  The temperature compensation circuit 35 is connected to the amplifier 31 and is based on the temperature detection signal of the temperature sensor that detects the temperature of the LED 30 or the vicinity thereof, and also according to the dimming signal sent from the dimming circuit 36, the DC offset of the amplifier 31. Adjust the level. In addition, a temperature correction signal is sent from the temperature compensation circuit 35 to the correction calculation unit 24 to perform temperature compensation by a predistortion method with respect to a temperature change of the LED 30.

  As shown in the IV characteristic curve of FIG. 4, when the operation IV region of the LED 30 changes due to the dimming signal and the illuminance changes high or low, for example, in the high illuminance region a, the change in the current I with respect to the change in the voltage V Is relatively good, but in the region b where the illuminance is low, the linearity of the change in the current I with respect to the change in the voltage V is poor and the distortion is large. For this reason, when light control is performed while the LED 30 is operating in the region b with low illuminance, it is not possible to perform light control according to the light control operation and the light control width as compared with the region a with high illuminance. Becomes smaller.

  For this reason, the LED drive unit 3 is provided with an illuminance stabilization circuit 34 that linearly corrects the nonlinear current-voltage characteristics of the LED 30 to stabilize the illuminance of the LED. When the illuminance stabilization circuit 34 adjusts the direct current offset level of the amplifier 31 by the dimming signal from the dimming circuit 36 and performs dimming, as shown in FIG. The DC offset level of the amplifier 31 is corrected by a correction circuit having a correction curve Ha having a characteristic opposite to that of a curve. For example, in a region b where the illuminance is low, the correction circuit having a correction curve Hb having a characteristic opposite to that of the characteristic curve is used. The DC offset level of the amplifier 31 is corrected.

  Further, a correction signal corresponding to the temperature is sent from the temperature compensation circuit 35 to the correction calculation unit 24 through the A / D converter 29 to perform temperature compensation by a predistortion method with respect to the temperature change of the LED 30, while dimming. A correction signal corresponding to the dimming is sent from the circuit 36 to the correction calculation unit 24 through the A / D converter 28, and the correction table of the correction calculation unit 24 is corrected along with the correction curve of FIG. It is selected according to Ha and Hb, and the current-voltage characteristic of the LED 30 is dynamically predistorted in accordance with the temperature and dimming of the LED 30 to greatly widen the dimming range.

  Next, the operation of the visible light communication transmitter configured as described above will be described. The visible light communication transmitter uses the LED 30 for illumination, and is used by attaching the LED 30 to a lamp attached to, for example, an indoor ceiling portion in order to use it as a projector for visible light communication.

  On the other hand, a receiving device (not shown) is installed at a position capable of receiving visible light emitted from the LED 30. The LED 30 of the lamp illuminates the room or the like, and transmits a digital signal such as an image or sound sent from a LAN or the like to visible light by OFDM modulation and transmits it to the receiving device as described later.

  When this visible light communication transmission device is activated, the data processing unit 13 of the input data processing unit 1 inputs the packet data for transmission sent from the LAN through the Ethernet interface 11 and uses the data memory 14 to transmit the packet data. The data is reassembled for OFDM, and the data is output from the input / output unit 15 to the OFDM modulator 2.

  When the digital modulation unit 21 of the OFDM modulator 2 receives the transmission data signal for OFDM sent from the input data processing unit 1, the digital modulation unit 21 digitally converts each bit of the transmission data signal for each subcarrier using a modulation scheme such as 64QAM. The modulated serial data signal is sent to the S / P converter 22.

  When the digitally modulated serial data is input, the S / P converter 22 uses N (for example, 1024) subcarriers so that each bit of the data sequence is assigned to each subcarrier. Each bit is converted into N pieces of parallel data, and the parallel data is output to the inverse discrete Fourier transform unit 23.

  Next, the inverse discrete Fourier transform unit 23 performs high-speed inverse Fourier transform on the parallel symbol data sequence that is digitally modulated and configured to assign each bit of the data sequence to each subcarrier, and the operation result is converted to a real number. Are output to the correction calculation unit 24 as time domain complex data consisting of imaginary numbers.

  The correction calculation unit 24 is a graph curve of the correction coefficient with respect to the frequency, with the real and imaginary components of the complex data output from the inverse discrete Fourier transform unit 23 having slopes opposite to the frequency characteristics of the LED 30 shown in FIG. (FIG. 3) Or it corrects using a computing equation. As a result, the equalizing correction of the band is performed so that the real and imaginary components of the complex data after correction are smaller as the frequency is lower and larger as the frequency is higher. In addition, the dimming signal from the dimming circuit 36 and the temperature correction signal from the temperature compensation circuit 35 that corrects the distortion of the temperature characteristics of the LED 30 are input to the correction calculation unit 24, and the dimming signal and the temperature correction signal are input to the correction calculation unit 24. Accordingly, a pre-stored correction table is selected, and the real and imaginary components of the complex data output from the inverse discrete Fourier transform unit 23 are predistorted.

  The real and imaginary complex data corrected by the correction calculation unit 24 are respectively sent to the D / A conversion units 25 and 26 to be converted into analog signals, and then converted into analog signals by the D / A conversion units 25 and 26, respectively. The complex data converted into the signal is sent to the synthesis unit 27. In the synthesizer 27, the pair of complex data is orthogonally modulated using a cosine wave and a sine wave of a carrier frequency of, for example, 4 MHz to 34 MHz of the subcarrier, and OFDM-modulated from the synthesizer 27, and the high frequency of 4 MHz to 34 MHz. The transmission signal is output to the amplifier 31 of the LED driving unit 3.

  Then, the amplifier 31 amplifies the transmission signal having the subcarrier carrier frequency of the OFDM signal, the amplified transmission signal is input to the LED driver 33, and the LED driver 33 drives the LED 30 to emit light. As a result, the LED 30 emits light, and visible light on which an OFDM-modulated high-frequency transmission signal is superimposed is emitted from the LED 30 for visible light communication and illumination. A light receiver of a receiving device (not shown) receives the visible light emitted from the LED 30, and takes in and demodulates the OFDM signal superimposed on the visible light.

  As described above, the visible light communication transmission device emits the LED 30 to emit visible light, and transmits the visible light by superimposing the OFDM signal on the visible light. However, as shown in FIG. Even if an LED with distortion is used, a digital transmission signal obtained by performing inverse fast Fourier transform in the inverse discrete Fourier transform unit 23 in the correction calculation unit 24 is converted into a signal having an inverse characteristic of the distortion in advance with respect to the nonlinear distortion of the frequency characteristic of the LED 30. In addition, band equalization correction is performed. For this reason, for example, when transmitting a signal using a high frequency band such as 4 MHz to 34 MHz, an LED having a frequency characteristic in which the light emission level is higher as the frequency is lower and the light emission level is lower as the frequency is higher is visible light projection. Even when used for light, a large amount of information can be transmitted at high speed by superimposing the transmission signal on the visible light of the LED without causing distortion in the spectral distribution of the transmission signal of the OFDM system. On the receiver side, the OFDM signal can be normally demodulated by a receiver having a relatively simple circuit configuration.

  Further, when the light of the LED 30 used as the illumination light is dimmed by the dimming circuit 36, as shown in the current-voltage characteristic curve in FIG. In the region a, the linearity of the change in the current I is relatively good with respect to the change in the voltage V, whereas in the region b where the illuminance is low, the linearity of the change in the current I is poor with respect to the change in the voltage V. Is big. For this reason, when dimming is performed while the LED 30 is operating in the area b with low illuminance, high-speed transmission according to the dimming operation cannot be performed and the dimming width is larger than in the area a with high illuminance. Becomes smaller.

  However, when the illuminance stabilization circuit 34 of the LED drive unit 3 performs dimming by adjusting the DC offset level of the amplifier 31 using the dimming signal from the dimming circuit 36, for example, as shown in FIG. In the area a, the DC offset level of the amplifier 31 is corrected by a correction circuit having a correction curve Ha having a characteristic opposite to that of the characteristic curve. For example, in the area b where the illuminance is low, the correction curve Hb having a characteristic opposite to that of the characteristic curve is corrected. The DC offset level of the amplifier 31 is corrected by a correction circuit having. Thereby, it is possible to stably perform the light control of the LED 30 according to the light control signal and dynamically correct the superimposed signal.

  Further, the temperature compensation circuit 35 of the LED driving unit 3 linearly corrects the nonlinear distortion of the IV characteristic curve of the LED 30 according to the temperature and dimming of the LED 30 based on the detection signal of the temperature sensor that detects the temperature of the LED 30. .

  As described above, the temperature coefficient of the LED is about −2 mV / ° C., the LED voltage at high temperature driven by constant current is lower than the LED voltage at low temperature, and the IV characteristic curve moves in parallel to the lower voltage. As a result, the illuminance decreases and the slope of the IV characteristic curve also changes. However, the temperature compensation circuit 35 is based on the detection signal of the temperature sensor that detects the temperature of the LED 30 or the temperature in the vicinity of the LED 30, and nonlinear distortion of the IV characteristic curve (FIG. 4) of the LED 30 according to the temperature and dimming of the LED 30. As shown in the IV characteristic curve of FIG. 4, when the temperature of the LED 30 increases, the terminal voltage of the LED 30 decreases. Therefore, the DC offset level of the amplifier 31 is increased in accordance with the increase in temperature. It operates so as to lower the DC offset level of the amplifier 31 in accordance with the temperature drop. Thereby, the instability of illuminance accompanying the change in the temperature of the LED 30 can be eliminated, and the temperature compensation of the LED and its drive unit can be performed.

  Further, a correction signal corresponding to the temperature is sent from the temperature compensation circuit 35 to the correction calculation unit 24 through the A / D converter 29 to perform temperature compensation by a predistortion method with respect to the temperature change of the LED 30, while dimming. A correction signal corresponding to the light control is sent from the circuit 36 to the correction calculation unit 24 through the A / D converter 28. Thus, along with the correction of the DC offset level in the amplifier 31, the correction calculation unit 24 selects the correction table so as to match the correction curves Ha and Hb of FIG. 4, and the current voltage of the LED 30 according to the temperature and dimming of the LED 30. The characteristics can be dynamically predistorted and the light control range can be greatly widened.

  FIG. 5 shows a configuration block diagram of a visible light communication transmitter according to another embodiment. The visible light communication transmitter of FIG. 5 is configured to perform equalization correction of the band in the light emission characteristic of the LED and to correct correction of the IV characteristic of the LED by an analog circuit.

  That is, in this visible light communication transmitting apparatus, as shown in FIG. 5, a distortion compensation circuit 44 and a band correction amplifier 32 as shown in FIG. 6 are used instead of the digital correction calculation unit 24. The distortion compensation circuit 44 corrects the predistortion of the current-voltage characteristics of the LED 30, and the band correction amplifier 32 corrects the equalization of the distortion of the frequency characteristics. In this visible light communication transmitter, the same configuration and operation blocks as those of the visible light communication transmitter in FIG. 1 are given the same reference numerals as those in FIG. 1 and description thereof is omitted.

  As shown in FIG. 5, in this visible light communication transmitter, a distortion compensation circuit 44 that performs distortion predistortion correction by an analog circuit is connected to the output side of the combining unit 27 of the OFDM modulator 2A. The current-voltage characteristic of the LED 30 exhibits non-linearity as shown in FIG. 4, whereby the light emission luminance of the LED 30 changes non-linearly according to the dimming signal. This distortion compensation circuit 44 uses a diode D (FIG. 6) as an analog nonlinear element having a nonlinear current-voltage characteristic similar to that of the LED 30, and uses the current voltage of the LED 30 as an input signal output from the synthesis unit 27. It is configured to perform a predistortion correction by giving a reverse characteristic having an inclination opposite to the characteristic.

  As shown in FIG. 6, in the distortion compensation circuit 44, the anode of the diode D is connected to the signal circuit, the bias voltage of the diode D is connected to be applied through the resistor R, and the capacitor C1 is connected to the input side of the signal circuit. The output side is connected to the input side of the amplifier 31 via the capacitor C2. As a result, the bias voltage of the diode D is adjusted so as to add reverse characteristics (graph curves Ha and Hb in FIG. 4) having a slope opposite to the current-voltage characteristics of the LED 30 to the input signal.

  Thus, when the high frequency transmission signal output from the combining unit 27 and subjected to OFDM modulation is input to the distortion compensation circuit 44 through the capacitor C1, the input high frequency signal is generated by the LED 30 shown in FIG. The predistortion of the current / voltage characteristics of the high frequency signal is preliminarily corrected so as to produce a characteristic having a slope opposite to that of the current / voltage characteristics, so that the distortion of the current / voltage characteristics of the LED 30 generated when the LED 30 emits light is compensated. It has become. Although the diode D is used as the analog nonlinear element in the distortion compensation circuit 44 of FIG. 6, analog nonlinear elements such as other transistors can also be used.

  In the LED driving unit 3A, the band correction amplifier 32 is connected to the amplifier 31, and the high frequency transmission signal is amplified by the band correction amplifier 32 so as to apply the equalizing correction of the band. A high frequency signal with a wide frequency band is amplified with good linearity. An LED driver 33 that drives the LED 30 to emit light is connected to the output side of the band correction amplifier 32.

  As shown in FIG. 2, the frequency characteristics of the LED 30 are nonlinear characteristics such that the lower the frequency, the higher the light emission level, and the higher the frequency, the lower the light emission level. For this purpose, the band correction amplifier 32 is configured to amplify the high-frequency transmission signal while performing band correction by adding the inverse characteristic of the frequency characteristic of the LED 30 to the signal.

  Thus, for example, when transmitting a signal using a high frequency band such as 4 MHz to 34 MHz, the LED 30 having a frequency characteristic in which the light emission level is higher as the frequency is lower and the light emission level is lower as the frequency is higher. Even when used for light, the transmission signal is superimposed on the visible light of the LED without distortion in the spectral distribution of the OFDM transmission signal so that a large amount of information can be transmitted at high speed. It has become.

  In the visible light communication transmitter configured as described above, in the same manner as described above, the inverse discrete Fourier transform unit 23 converts a parallel symbol data sequence that is digitally modulated and each bit of the data sequence is assigned to each subcarrier at a high speed. And inverse Fourier transform, and the calculation result is output as complex data in the time domain consisting of real and imaginary numbers. The output complex data is sent to the D / A converters 25 and 26, converted into analog signals, and then converted into analog signals by the D / A converters 25 and 26, respectively. Is sent to the synthesis unit 27.

  In the synthesizer 27, a pair of complex data is orthogonally modulated and synthesized using a cosine wave and a sine wave having a carrier frequency of 4 MHz to 34 MHz, for example, of subcarriers, and the high frequency of 4 MHz to 34 MHz modulated by OFDM from the synthesizer 27. The transmission signal is output to the distortion compensation circuit 44.

  When the high-frequency transmission signal is input to the distortion compensation circuit 44, as described above, the analog non-linear element diode D gives the high-frequency transmission signal an inverse characteristic with a slope opposite to the frequency characteristic of the LED 30, and a predistortion system. Compensate for distortion.

  Thereafter, the high-frequency transmission signal subjected to distortion compensation is output to the amplifier 31 of the LED drive unit 3A, and the amplifier 31 amplifies the input signal, that is, the transmission signal having the subcarrier carrier frequency of the OFDM signal. Further, the transmission signal amplified by the amplifier 31 is amplified by the band correction amplifier 32. At the same time, the band equalizing correction is performed. For example, the frequency characteristic of the LED 30 with respect to a high frequency signal having a very wide frequency band such as 4 MHz to 34 MHz decreases in luminance at a high frequency as shown in FIG. The band correction amplifier 32 amplifies the high-frequency transmission signal so that the band is emphasized and the LED 30 emits light with good linearity.

  The amplified transmission signal is sent to the LED driver 33, and the LED driver 33 drives the LED 30 to emit light. As a result, the LED 30 emits light, and visible light on which a high-frequency transmission signal subjected to OFDM modulation is superimposed is emitted from the LED 30 for visible light communication and illumination.

  As described above, the distortion compensation circuit 44 gives a reverse current characteristic having a reverse polarity to the nonlinear high-frequency current characteristic as shown in FIG. Since compensation is performed, even if the current-voltage characteristic of the LED 30 is non-linear, the non-linear distortion can be corrected and the luminance level of visible light can be kept constant, and the OFDM signal superimposed on the visible light can be transmitted stably. can do.

  Further, the band correction amplifier 32 applies a reverse characteristic of distortion to the high-frequency transmission signal in advance so as to compensate for the nonlinear distortion of the frequency characteristic of the LED 30, and performs band equalization correction. Accordingly, when transmitting an OFDM signal using a high frequency band such as 4 MHz to 34 MHz, for example, an LED having a frequency characteristic in which the emission level is higher as the frequency is lower and the emission level is lower as the frequency is higher is visible light. Even when used for light projection, a large amount of information can be transmitted at high speed by superimposing the transmission signal on the visible light of the LED without causing distortion in the spectrum distribution of the OFDM transmission signal.

  Further, when the light of the LED 30 used as the illumination light is dimmed, the dimming signal from the dimming circuit 36 adjusts the direct current offset level of the amplifier 31. At this time, the LED 30 is dimmed. As shown in FIG. 4, the current and voltage at the operating point change on the graph, and it is difficult to perform dimming linearly. However, the operation of the illuminance stabilization circuit 34 prevents the illuminance (luminance) of the LED 30 from becoming unstable. That is, as shown in FIG. 4, the LED 30 corrects the DC offset level of the amplifier 31 by a correction circuit having a correction curve Ha having a characteristic opposite to the characteristic curve, for example, in a high illuminance area a. In the low region b, the DC offset level of the amplifier 31 is corrected by a correction circuit having a correction curve Hb having a characteristic opposite to that of the characteristic curve. Thereby, light control of LED30 can be performed stably according to a light control signal.

  Further, the temperature compensation circuit 35 of the LED drive unit 3A increases the DC offset level of the amplifier 31 according to the temperature rise based on the detection signal of the temperature sensor that detects the temperature of the LED 30, and according to the temperature fall. Since the amplifier 31 operates so as to lower the DC offset level and linearly corrects the non-linear distortion of the IV characteristic curve of the LED 30, the instability of illuminance associated with the change in the temperature of the LED 30 can be eliminated.

  In the above-described embodiment, the LED 30 is attached to a lamp as illumination light, and an information signal is transmitted by superimposing an OFDM signal on visible light as illumination light. However, the information signal is not used as illumination light but is used only for visible light communication. It is also possible to use LEDs without dimming.

DESCRIPTION OF SYMBOLS 1 Input data processing part 2 OFDM modulator 3 LED drive part 11 Ethernet interface 12 Data buffer 13 Data processing part 14 Data memory 15 Input / output part 21 Digital modulation part 22 S / P conversion part 23 Inverse discrete Fourier transform part 24 Correction calculation part 25 D / A converter 27 Composite unit 30 LED
31 Amplifier 32 Band Correction Amplifier 33 LED Driver 34 Illuminance Stabilization Circuit 35 Temperature Compensation Circuit 36 Dimming Circuit

Claims (5)

Transmits the transmission data signal by dividing the digital signal of the transmission data into a number of subcarriers by the orthogonal frequency division multiplexing method and modulating the modulated high frequency signal to the visible light emitted by the light emitting operation of the LED. In the visible light communication transmitter
A transmission data processing unit for rearranging transmission data so as to assign each bit of the packetized transmission data signal to each subcarrier;
A modulation unit that digitally modulates each bit of the digital signal of transmission data sent from the transmission data processing unit for each subcarrier;
An S / P converter that converts the serial digital transmission signal modulated by the modulator into a data string of the number of subcarriers;
An inverse discrete Fourier transform unit for performing inverse fast Fourier transform on the digital transmission signal converted in parallel by the S / P conversion unit;
The digital transmission signal inversely fast Fourier transformed by the inverse discrete Fourier transform unit corrects in advance the inverse characteristic of the distortion by adding the inverse characteristic of the distortion to the frequency characteristic of the LED, and the LED has A correction calculation unit that performs a correction operation so as to correct in advance the inverse characteristic of the distortion with respect to the nonlinear distortion of the current-voltage characteristic;
A D / A converter that converts the digital transmission signal corrected by the correction calculation unit into an analog signal;
A synthesizing unit that orthogonally modulates and synthesizes the complex data converted into an analog signal by the D / A converter using a cosine wave and a sine wave of the carrier frequency of the subcarrier;
An LED driving unit that drives the LED to amplify and superimpose the analog high-frequency signal synthesized by the synthesizing unit on visible light;
Equipped with a,
The LED is provided in a lamp that emits visible illumination light, the LED is configured to emit visible light as illumination light, and a correction table that corrects the digital transmission signal for dimming the illumination light includes the correction table. A visible light communication transmitting apparatus provided in a correction calculation unit, which changes the correction table according to a dimming operation and corrects distortion of frequency characteristics accompanying dimming .   The LED driving unit is provided with an amplifier that amplifies the high frequency signal synthesized and outputted by the synthesizing unit, and adjusts the light emission level of the LED by adjusting the DC offset level of the amplifier according to the dimming operation. 2. The visible light communication transmitting apparatus according to claim 1, wherein a dimming circuit is connected to the amplifier. A temperature compensation circuit that compensates for the temperature characteristics of the LED and makes the illuminance constant with respect to a temperature change of the LED is connected to the amplifier of the LED driving unit, and a temperature sensor that detects the temperature of the LED or the vicinity thereof is connected. Based on the temperature detection signal, a signal is output to the amplifier to adjust the DC offset level, while a correction signal is sent to the correction calculation unit so that the correction calculation unit performs temperature compensation for a temperature change of the LED. visible light communication transmission apparatus according to claim 1 or 2, wherein the correcting calculation to.   The LED driving unit is provided with an illuminance stabilization circuit that linearly corrects the nonlinear current-voltage characteristics of the LED to stabilize the illuminance of the LED, and the LED driving unit corresponds to the output voltage of the LED driver of the LED driving unit. A signal is output from the illuminance stabilization circuit to the amplifier to adjust the DC offset level, while a correction signal is sent to the correction calculation unit so that nonlinear distortion of the current-voltage characteristic of the LED is linearly corrected. 4. The visible light communication transmitting apparatus according to claim 3, wherein the correction calculation unit performs correction calculation in advance by adding an inverse characteristic of distortion to the signal. Transmits the transmission data signal by dividing the digital signal of the transmission data into a number of subcarriers by the orthogonal frequency division multiplexing method and modulating the modulated high frequency signal to the visible light emitted by the light emitting operation of the LED. In the visible light communication transmitter
A transmission data processing unit for rearranging transmission data so as to assign each bit of the packetized transmission data signal to each subcarrier;
A modulation unit that digitally modulates each bit of the digital signal of transmission data sent from the transmission data processing unit for each subcarrier;
An S / P converter that converts the serial digital transmission signal modulated by the modulator into a data string of the number of subcarriers;
An inverse discrete Fourier transform unit for performing inverse fast Fourier transform on the digital transmission signal converted in parallel by the S / P conversion unit;
A D / A converter for converting a digital transmission signal obtained by inverse fast Fourier transform in the inverse discrete Fourier transform unit into an analog signal;
A synthesizing unit that orthogonally modulates and synthesizes the complex data converted into an analog signal by the D / A converter using a cosine wave and a sine wave of the carrier frequency of the subcarrier;
The high-frequency signal output from the synthesis unit is input, and the non-linear distortion of the current-voltage characteristic of the LED is added to the high-frequency signal in advance by using an analog non-linear element to compensate for the distortion. A strain compensation circuit;
A band correction amplifier that amplifies the frequency characteristics of the LED by linearly correcting the frequency characteristics of the LED by adding an inverse characteristic of a slope opposite to the nonlinear frequency characteristic of the LED to the high-frequency signal;
An LED driving unit that drives the LED by superimposing a high-frequency signal output from the band correction amplifier on visible light;
Equipped with a,
A diode is connected as an analog nonlinear element to the distortion compensation circuit, and a bias voltage applied to the diode is adjusted, and a reverse characteristic with a slope opposite to the frequency characteristic of the LED with respect to a high-frequency signal flowing in the circuit. To compensate for distortion of the frequency characteristics of the LED .

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