Patent Publication Number: US-2017353240-A1

Title: Use of very high frequency switching power converters for also modulating light output from solid state emitter to communicate data

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 15/184,004, filed on Jun. 16, 2016, which is a continuation of U.S. patent application Ser. No. 14/508,356, filed on Oct. 7, 2014, now U.S. Pat. No. 9,432,116, issued Aug. 30, 2016, the entire disclosures of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present subject matter relates to techniques and equipment for light based transmission of data, such as Visual Light Communication (VLC), using a solid state emitter type light source, where the source is driven by a very high frequency switching power converter that is also controlled to modulate the light output from the source in a manner to carry data. 
     BACKGROUND 
     Electrical lighting has become commonplace in modern society. Electrical lighting devices are commonly deployed, for example, in homes, buildings of commercial and other enterprise establishments, as well as in various outdoor settings. With the advent of modern electronics has come advancement both in the types of light sources and in the control capabilities within the lighting devices. For example, solid state sources are now becoming a commercially viable alternative to traditional light sources such as incandescent and fluorescent lamps. By nature, solid state light sources such as light emitting diodes (LEDs) are easily controlled by electronic logic circuits or processors. 
     The last decade or so has also seen an explosion of demand for data communication services; and increasingly, this demand include demand for wireless connectivity. Cellular and WiFi services are now quite common. However, there have also been proposals to use light-based communication technologies as an alternative to radio-frequency (RF) based wireless communications. Light wave communications, for example, do not interfere with existing RF are not currently subject to regulation or license requirements. Hence, along with the transition to solid state light sources has come an increased interest in using light as an over-the-air mechanism to transport data. 
     Visual Light Communication (VLC) therefore will likely become a very popular piece of the hybrid network (to increase bandwidth optimization in wireless communication systems). In many cases, the output of a lighting device deployed for a normal lighting purpose, e.g. general white light illumination, is modulated in a manner not readily perceptible by human occupants in the illuminated space but so that the modulated light carries transmitted data. 
     Traditionally, a switching FET is used either in series or parallel with the LEDs to provide dimming (usually pulse width modulation (PWM)) and high speed switching for VLC to implement the modulation. Those methods tend to be lossy because power is wasted in the FET and almost always requires some storage mechanism in parallel with the load. In another method, the power supply (or LED driver) itself is turned ON and OFF to provide dimming and VLC. In any of these methods, the ‘data’ signal is added onto the dimming signal. There are limitations with all of these approach since two types of switching cycles have to be super-imposed. One switching cycle is the cycle of the switcher itself, which provides the requisite current regulation; and the other switching cycle is the cycle of ‘dimming module,’ which requires additional control of LED switching. This can be costly, can be cumbersome, and potentially may create Electromagnetic interference (EMI) issues. These limitations tend to limit the switching speed available for carrying data and result in relative low data transmission rates. 
     There is still room for further improvement in the technologies used for VLC or other similar types of light-based data communications. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawing figures depict one or more implementations in accordance with the present concepts, by way of example only, not by way of limitations. In the figures, like reference numerals refer to the same or similar elements. 
         FIG. 1  is a block diagram of a lighting device including a power and modulation circuit utilizing a very high frequency switching power converter. 
         FIG. 2  is a chart of an example of varying current at varying frequencies, for a VLC application. 
         FIGS. 3A to 3E  are diagrams of examples of variable waveforms for different types of modulations of the light output to carry data. 
         FIG. 4  is a block diagram of a three-channel lighting device, where each channel may include a power and modulation circuit utilizing a very high frequency switching power converter. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, numerous specific details are set forth by way of examples in order to provide a thorough understanding of the relevant teachings. However, it should be apparent to those skilled in the art that the present teachings may be practiced without such details. In other instances, well known methods, procedures, components, and/or circuitry have been described at a relatively high-level, without detail, in order to avoid unnecessarily obscuring aspects of the present teachings. 
     The various examples disclosed herein relate to lighting power and modulation equipment for VLC utilizing very high frequency switching power converters and/or to lighting devices or the like that incorporate such equipment. 
     Reference now is made in detail to the examples illustrated in the accompanying drawings and discussed below.  FIG. 1  is a high-level block diagram that illustrates an example of a lighting device  10  that utilizes a SSL type light source  14  for illumination and operates that source  14  so as to also support light based (optical) communication of data. The lighting device  10  in  FIG. 1  includes a power and modulation circuit  11  utilizing a very high frequency switching power converter  15 . 
     As noted, the lighting device  10  utilizes a SSL type of light source  14 . Although other types of switchable light sources may be used, particularly other types of solid state light emitter(s), in the illustrated example of device  10 , the SSL light emitting source includes some number of (one or more) light emitting diodes (LEDs)  17  that together form the SSL type light source  14 . 
     As discussed herein, applicable solid state light emitting elements that may be used alone or in combination to form the SSL source  14  include any of a wide range light emitting or generating devices formed from organic or inorganic semiconductor materials. Examples of solid state light emitting elements include semiconductor laser devices and the like. Many common examples of solid state lighting elements, however, are classified as types of “light emitting diodes” or “LEDs.” This example class of solid state light emitting devices encompasses any and all types of semiconductor diode devices that are capable of receiving an electrical signal and producing a responsive output of electromagnetic energy in the range encompassing visible light and adjacent regions such as infrared (IR) and ultraviolet (UV). Thus, the term “LED” should be understood to include light emitting diodes of all types, light emitting polymers, organic diodes, and the like. LEDs may be individually packaged, as in the illustrated examples. Of course, LED based devices may be used that include a plurality of LEDs within one package, for example, multi-die LEDs that contain separately controllable red (R), green (G) and blue (B) LEDs within one package. Other examples of LEDs may include some light conversion material, such as a phosphor and/or nanophosphor, to convert light of wavelength(s) emitted by the actual diode to one or more other wavelengths, for example, so that the LED device produces a broadband output that appears pastel or appears white. Those skilled in the art will recognize that “solid state” or “LED” terminology does not restrict the source to any particular type of package for the LED or other solid state emitter. Such terms encompass solid state devices that may be packaged or non-packaged, chip on board LEDs, surface mount LEDs, and any other configuration of the semiconductor emitter device that emits light. 
     The color or spectral characteristic of light or other electromagnetic radiant energy relates to the frequency and wavelength of the energy and/or to combinations of frequencies/wavelengths contained within the energy. Many of the examples relate to colors of light within the visible portion of the spectrum, although examples also are discussed that utilize or emit light energy in other spectral ranges. 
     The example of a lighting device  10  shown in  FIG. 1  utilizes a group of LEDs  17  as the SSL light source  14  and utilizes a single power converter and modulator circuit  11  to drive that source  14 . The LEDs  17  forming the SSL source  14  may be connected in parallel, in series or in series-parallel combinations; although in the illustrated example, the LEDs  17  are connected together in a single series string. 
     In a typical general lighting application the LEDs  17  included in the string together produce white light of a desirable color characteristic. For example, the LEDs  17  may all be white (W) LEDs. As an alternate example, configured to produce a somewhat ‘warmer’ white light, some of the LEDs  17  may be white (W) whereas other LEDs  17  in the string may be another color such as red (R) and/ or amber (A). In such white light examples, the LEDs  17  forming the SSL source  14  together provide a broadband visible light output for the general white lighting application, and the circuit  11  supplies driver current and modulates the broadband light output to also carry the data. 
     The light generation and modulation technologies discussed herein are also applicable to more narrowband carriers produced by relatively monochromatic solid state devices. In a simple example, the lighting device  10  of  FIG. 1  might use narrowband LEDs of one color in the SSL source  14 , e.g. all red (R), all green (G) or all blue (B). 
     In most cases, the broadband or narrowband light is in the visible spectrum, however, the present technologies may also be applied in devices that use IR for communication and/or UV (e.g. for communication or to pump phosphor(s) for visible light communication). In a general lighting application where the LEDs do not provide white light (e.g. use R, G, B, UV or IR for data communication), the device  10  could be combined with a separate white light source (not shown) that need not be modulated. Relatively narrowband/&#39;monochromatic&#39; LEDs may utilize the technology better for the higher datarates that broadband LEDs. The phosphor in white LEDs might have persistence which could be longer than the time scale at the high datarate; in which case, colored (or narrow-band) LEDs might work better since they do not have phosphor. A modulated narrowband source, however, may still be part of a broadband lighting device. For example, in a tunable red (R), green (G), blue (B) type lighting device that can provide combined tunable white output, the modulated source may be any one of the three (R, G or B) light sources, although only one such source is shown in  FIG. 1 . As another example, in a tunable white solid state lighting device, the white channel/source (not shown) may be un-modulated, and another tuning channel may have a narrowband source as shown that is then modulated as discussed herein relative to  FIG. 1 . 
     Returning more specifically to the example of  FIG. 1 , the very high frequency switching power converter  15  has an output connected to supply current to drive the LEDs  17  forming the solid state light emitting source  14  to emit light. The very high frequency switching power converter  15  has an input coupled to a source of power. The power source could be a direct current (DC) source, such as a battery. In the example, the device  10  obtains power from alternating current (AC) source at normal line voltage (e.g. around 120V in the US). Although not shown, one or more protective fuses may be provided in the line connection(s). The very high frequency switching power converter  15  connects to the rectified DC output side of a bridge rectifier  19  formed by four bridge connected diodes. Although not shown, some additional smoothing and/or control circuitry may be provided on the power input side, between the bridge rectifier  19  and the very high frequency switching power converter  15 . 
     The power converter  15  is the switcher of a switched-mode power supply (SNIPS) for converting the DC from the bridge rectifier  19  and supplying DC current to the LEDs  17  of the SSL source  14 . Hence, the example implements the converter  15  as a DC-to-DC converter. Depending on the application, e.g. the number of LEDs used to provide the desired maximum light output level and the power requirements of that number of LEDs, the power converter  15  may utilize any appropriate power converter topology (e.g. boost, buck, fly back, etc.), albeit configured to operate at a VHF switching rate, as discussed more fully below. In at least some examples, the VHF converter may be a resonant converter. 
     The modulation and VLC communications techniques discussed herein may use a variety of different types of VHF converters, although some types may provide better performance than others. Hence, it may be helpful to some readers to consider some general information about DC-to-DC converters and then discuss some types that may offer particular advantages in SSL lighting. 
     At sufficiently high converter switching frequencies (&gt;1 MHz), transistor losses become sufficiently large that the loss negates the advantages in reducing inductor sizes. Adding snubbers (e.g. resistor, capacitor diode or “RCD”) does not solve the problem since snubber usage only shifts the losses to RCD component which becomes useless above 200 KHz. To operate at frequencies higher than that, transistor switching losses at Turn ON and Turn OFF must be reduced. This is achieved by resonant converters by associating a resonating LC (inductance, capacitance) circuit with the switching transistor, to render its current sinusoidal rather a square wave in shape. The converter is then arranged to turn the transistor on and off at the zero crossings of the current in the sine wave. There is thus no overlapping of falling current and rising voltage at Turn OFF and rising current and falling voltage at Turn ON and hence no switching losses. This is called Zero Current Switching (ZCS). In addition, even if this loss was eliminated, there is a huge output capacitance associated with the switching MOSFET. Circuits that deal with this are called Zero Voltage Switching (ZVS). ZVS circuits work to ensure that the transistor output capacitance is made so that transistor capacitor is resonant in an LC circuit (the MOSFET&#39;s capacitance being the C). Then the voltage or energy stored in the capacitance when the transistor is OFF is changed to the stored current or energy of the inductor in the resonant circuit. Then later in the cycle, this energy is returned without loss to the power bus. Hence, VHF converters typically have resonant topologies. Class Phi (Φ) inverters use the first several harmonics; and more specifically a class Phi2(Φ 2 ) inverter is tuned for the first 3 harmonics. 
     The VHF converter used in a particular example of the circuits and lighting devices discussed herein is referred to as the Φ 2  converter, which includes or is implanted as a Phi2(Φ 2 ) inverter tuned for the first 3 harmonics. The Phi2 (Φ 2 ) class converter offers different performance from other VHF converters in that the Φ 2  converter can maintain high load efficiency across the entire range of load. This may be particularly advantageous when data is modulated on the load, it forces the load to change between levels. So, using any other type of VHF converter (such as a class E or type Φ converter) would not be as advantageous since the losses would be very high. 
     The DC-to-DC converter includes an internal clock used to drive the actual switch(es) of the convert. All DC-to-DC converters have a ‘controller’ which include this internal clock. In this case, the internal controller is part of the VHF power converter  15 . As an example, by changing the clock speed, it is possible to speed up the switching speed, which would affects the VLC modulation rate. 
     The power converter  15  therefore is configured to convert power from the source of power to direct current to supply via the output to the solid state light emitting source  14 , at least in part by a switching function of the power converter at a very high frequency rate. The VHF switching function, in our examples, switches at a frequency in the range of about 30 MHz (30 MHZ±3 MHz) to about 300 MHz (300 MHz±30 MHz). Some examples are optimized for a switching frequency at or above 200 MHz. 
     A switcher used as the very high frequency (VHF) switching converter, having a switching frequency in the ranges outlined above, can provide load switching, perhaps between 100% to 20%, at a frequency of at least 50 MHz (assuming current mode control and a high bandwidth loop). Such a frequency range used with relatively simple modulation techniques, for example, should be able to provide a data stream of up to at least 10 Mbps for light-based over-the-air data communication. Higher data rates for VLC or other optical transmissions may be possible with higher baud rate modulation encoding techniques. 
     Although other connection arrangements may be used, in the example, the output of the very high frequency switching power converter  15  provides the DC voltage and current to the anode side of LEDs  17  in the lighting device  10 , to drive the LEDs  17  to emit light at a desired illumination level and modulated to carry data. In the device  10 , the power and modulation circuit  11  also includes a control circuit  21  coupled to respond to input data and coupled to the power converter  15 , to control at least one parameter of operation of the very high frequency power converter  15  and thus the current output from the power converter  15  to drive the solid state light emitting source  14 , such that light emitted from the solid state light emitting source is modulated to carry the input data. 
     As used herein direct current or DC refers to current that does not swing between positive and negative (does not pass through 0 or neutral). The output of the bridge rectifier  19 , for example, is a half-wave rectified current, and we will consider that DC although there is still extensive variation between peaks and 0. The output of a switcher-based DC-to-DC converter will be relatively constant DC but will still include some ripple. In the device  10  of  FIG. 1 , the output of the VHF converter  15  can be considered DC for all intents and purposes (even though it is not the same output as from a battery). The converter output will have a ripple at VHF frequencies. For the data transmission application, the DC output of the VHF converter  15  will also be modulated in one of several ways to carry the data that is being transmitted via the LED light output. 
     A variety of strategies/technologies may be used to provide the illumination level control as well as the current and light modulation to transmit the data over the output light. The example includes a sense resistor (Rs) in series with the SSL source  14 , and the control circuit  21  includes a comparator (not separately shown) to compare an input signal from a baseband signal processor  23  to the voltage across the sense resistor (Rs) to produce and appropriate control voltage as a feedback signal input to the very high frequency switching power converter  15 . The baseband signal processor  23  is responsive to input data to provide a baseband signal representing the data as an input to the control circuit. Although separate illumination level control may be provided, in the example the baseband signal processor  23  also adjusts the signal provided to the control circuit  21  so that the very high frequency switching power converter  15  achieves and maintains an output current level to drive the SSL source  14  to provide the desired overall illumination level as well as to modulate the light output to carry the data. 
     The lighting device  10  may implement a variety of overall host control/operation technologies that provide the high level logic to control operation of the device including data transmission. As lighting devices become increasingly intelligent and ‘connected,’ such devices utilize ever more sophisticated controllers. The trend in intelligent lighting devices is to utilize programmable processors to implement the host control and communication logic. Although processor based lighting devices may use microprocessors similar to those in computers or mobile devices, the illustrated example of device  10  uses a micro-control unit (MCU)  25 , sometimes referred to as a microcontroller. 
     The MCU  25  implements the control logic for the device  10 , that is to say, controls operations of the device  10  based on execution of its embedded ‘firmware’ instructions. The MCU  25  may be a microchip device that incorporates a processor serving as the programmable central processing unit (CPU)  27  of the MCU and thus of the lighting device  10  as well as one or more memories  29  accessible to the CPU  27 . The memory or memories  29  store the executable programming for the CPU  27  as well as data for processing by or resulting from processing of the CPU  27 . The memory or memories  29 , for example, may store illumination level settings and/or may temporarily store data the device is sending or receiving via VLC. The MCU  25  may be thought of as a small computer or computer like device formed on a single chip. Such devices are often used as the configurable control elements embedded in special purpose devices rather than in a computer or other general purpose device. A variety of PIC16 and PIC32 type MCU chips, for example, may be used as the MCU  25  in the lighting device  10 . 
     The lighting device  10  also includes a communication interface  31  coupled to a communication port of the MCU  25 . The interface  31  provides a communication link to a telecommunications network that enables the MCU  25  to send and receive digital data communications through the particular network. The network may be wired (e.g. metallic or optical fiber), wireless (e.g. radio frequency or free space optical) or a combination of such network technologies; and the interface  31  in a particular installation of the device  10  will correspond to the most advantageous network available (based on considerations such as cost and bandwidth) at the location of the installation. For example, if the network is a particular type of local area network (LAN), the communication interface is of a type for linking to and communication through the available type of LAN. The communication interface  31  is therefore accessible by the processor/CPU  27  of the MCU  23 , and the communication interface  31  is configured to enable the processor to communicate information about its operations as wells as data sent or received as VLC communication through the LAN or other communications network. 
     Data sent and received via the interface  31  may relate to lighting operations, e.g. to report device status to some other equipment and/or to receive commands and/or setting data to control light output, for example, to turn the light output ON to a particular overall intensity level. In the example, however, the network and the interface  31  also enable the device  10  to receive data for VLC transmission to a device in the space illuminated by light output from the SSL source  14 . 
     Although the device  10  could provide one-way data transmission, for purposes of the present example, the device  10  is also capable of receiving data via VLC transmission from a device in the illuminated space. Hence, the device  10  also includes an optical receiver  35 , for example, including one or more photodiodes to receive light; and a signal conditioning circuit  37  processes the receiver output signal to recapture (e.g. demodulate the signal to obtain) data carried on the received light. Such received data is provided to the MCU  25  for processing and forwarding via the interface  31  for communication over the network. 
     The lighting device may also include an additional input/output (I/O) interface  39 , for communication between the MCU  25  and another local device, such as a local user interface device. 
     Returning now more specifically to the discussion of VLC over-the-air data communication, the device  10  typically receives data for VLC transmission via the network and the communication interface  31  and supplies the data to the MCU  25 . The MCU  25  may also generate some signaling data or the like locally for communication via VLC to the other device. The MCU  25  provides level setting commands to the baseband signal processor  23 . The MCU  25  may also instruct the baseband signal processor  23  as to the appropriate modulation format, and the MCU  25  supplies the received data to the baseband signal processor  23 . The baseband signal processor  23  responds to these inputs from the MCU  25  to generate a signal for input to the control circuit  21 . Using the voltage across the sense resistor (Rs), the circuit generates a signal to set the output current from the very high frequency switching power converter  15  to the intended level for the desired illumination intensity, albeit with the drive current and thus the light output modulated as instructed so as to carry the data over the air. 
     With the VHF switcher approach to DC-to-DC conversion and data modulation, the problems outlined earlier may be mitigated since the switcher itself is switching so fast. The dimming profile (which corresponds to the switching between different load current at a fixed or variable frequency) can be inherently controlled by the switcher. One can imagine doing all sorts of things with the load profile. For example, in the graph of  FIG. 2 , there are varying current levels and varying frequencies, which creates a spread spectrum effect. By way of example,  FIG. 2  depicts three possible variations in the output of the VHF converter. The output current can change in magnitude. The output current can also vary in frequency and/or duty cycle. So, the graphs represents the profile of the output current. Corresponding instructions would be provided by the baseband processor. By using a combination of such modulation schemes, the VLC designer now has the freedom to use any type of communication scheme. So, this type of an output profile can then manage dimming and also provide modulation for high bandwidth optical communication. Somewhat traditional multiplexing (such as orthogonal frequency-division multiplexing (OFDM)) or non-traditional keying (such as reverse polarity orthogonal frequency-division multiplexing (RP-OFDM) can be used. 
     Flicker is of great concern in design and manufacture of SSL lighting. Since the DC-to-DC converters used in the examples operate at such high switching frequencies, a designer using a lighting device incorporating such a converter would not have to worry about flicker switching and modulation will occur at such high frequencies. Variations in the light output below about 10 kHz cause a stroboscopic effect on humans that in some cases may produce adverse medical effects. It is desirable for any variation in the light output to take a form, e.g. in magnitude and/or frequency that unobjectionable/harmless to humans. The VHF switcher approach discussed herein produces unobjectionable/harmless modulation of the light output of the device. 
     The very high frequency switching power converter  15  offers high efficiency at the entire load range; and as result, these drivers are compact and have ultra-high power densities. One can envision such a design as being particularly suited for on-board drivers, which means integrating the driver electronics with the LEDs on the circuit board in many types of LED lighting products. Such an on-board integration approach will reduce overall cost for electronics, for example, in RGB and tunable white type drivers, e.g. by reducing board count and hence increases mean time between failures (MTBF). 
     The input data signal provided to the VHF converter  15  will affect the DC output of the converter  15 , and thus the light output from the SSL source  14 . Data can be embedded in a variety of different ways.  FIGS. 3A to 3E  are diagrams of examples of variable waveforms for different schemes for modulation of the light output to carry data. 
       FIG. 3A , for example, shows a modulation scheme in which one or more current levels represent data. As shown, the load is switched from a variable low current level x 1  to a variable high current level x 2 . Each value x 1 , x 2  may vary between  0  and the maximum current level (I max ) of the converter output, for the particular configuration on the converter  15  in the device  10 . For each cycle of the waveform, the value of one or both of x 1 , x 2  may represent a data value. If x 1 =x 2 , the load presented by the SSL source  14  is ON at the instantaneous value of x 1  (or x 2 ) amps. If x 1 &lt;x 2 , the load presented by the SSL source  14  will be switching between current levels x 1  and x 2 . 
     In other scenarios, x 1  can vary but x 2 , is fixed, or x 2  can vary but x 1  is fixed. In either of these modulation schemes, the load presented by the SSL source  14  will be switching between the variable and fixed current levels, and the level of the variable at a particular time or point on the waveform represents the data value. 
       FIG. 3B  illustrates an example of a modulation scheme in which the input data signal may request the VHF converter  15  to output a variable current, having a fixed frequency but a variable duty cycle. Variations in the duty cycle of the otherwise fixed frequency output of the converter  15  and thus the SSL light source  14  carries the data. For simplicity, the drawing shows only a few cycles of the modulation. The first cycle/waveform exhibits a 60% duty cycle (ON for 60% of the overall cycle/waveform, OFF for the other 40% of the overall cycle/waveform). The second cycle/waveform exhibits a 50% duty cycle, whereas the third cycle/waveform exhibits a 20% duty cycle. The fourth cycle/waveform exhibits an 80% duty cycle, whereas the fifth cycle/waveform exhibits a 20% duty cycle, and the sixth cycle/waveform exhibits a 50% duty cycle. The different duty cycles represent different data values. The load current goes in incremental steps between 0% and 100%, albeit in different cycles. This approach embeds data (or information) in varying lengths of the ON/OFF portions forming the duty cycle of one or more waveforms of the fixed frequency current/light outputs. 
     In the further example of  FIG. 3C , the input data signal may request the VHF converter  15  to output a variable current, having a fixed duty cycle but variable frequency, where in the frequency variations represent different data values, thus embedding the data in the frequency modulation. Hence, the modulation waveform of  FIG. 3  shows some number of cycles at each of three different frequencies f 1 , f 2 , f 3 . For all of the different frequencies, the duty cycles are the same. Although 50% duty cycle is shown for convenience, a VLC modulation technique of this type may use a higher or lower percentage duty cycle. 
     It should be apparent that modulation schemes may be combined to increase the data transmission rate. It may be helpful to consider a couple of examples of combining two or more of the modulation techniques discussed above. 
       FIG. 3D , for example, represents a modulation scheme that combines amplitude modulation (varying the load current supplied by the VHF converter  15 ) like that of  FIG. 3A  with duty cycle modulation like that of  FIG. 3B  and frequency modulation like that of  FIG. 3C . As shown, the length of time t and thus the frequency varies from cycle to cycle of the waveform, based on some of the data to be transported. Also, the percentage duty cycle varies from cycle to cycle of the waveform, based on some of the data to be transported. Differences in the high and low levels of each cycle of the waveform also represent other information from the data that the device is transmitting via VLC. 
       FIG. 3E  represents a simplified example of a waveform that might result from combining the duty cycle and frequency modulation schemes of  FIG. 3B and 3C . 
     Increasing the number of waveform parameters that change based on the input data signal, increases the amount of data that can be carried on each cycle of the waveform and thus the number of bits of data represented by each symbol transmitted by the modulated current signal and thus the VLC output light, which in turn increases the baud rate and overall data transmission rate. 
     Although the examples above mostly focused on modulation of visible light to carry the data, those skilled in the art will appreciate that the same technique can be used to transmit data over light wavelengths in other spectral regions, for example, on infrared (IR) or ultraviolet (UV) light wavelengths. For example, if the device  10  where not itself intended to emit visible light itself, the  10  would use LEDs  11  of a type configured to emit IR or UV light. 
     The examples discussed so far have assumed a single light communication channel, i.e. transmission of data by modulation of the light output from a single switchable SSL source  14  (via a single converter  15 ). The present technologies, however, are also applicable to multi-channel data transmission. 
       FIG. 4 , for example, is a high-level block diagram that illustrates an example of a lighting device  100  that utilizes light sources of different color characteristics. In this example, the device includes three separately controllable sources, specifically a red (R) source  14   R , a green (G) source  14   G  and a blue (B) source  14   B . Adjustment of the outputs of the sources  14   R ,  14   G ,  14   B  can provide tunable illumination. Individual modulation of the outputs of any or of the outputs of the sources source  14   R ,  14   G ,  14   B  can also support light based transmission of data on up to three different light wavelength bands. 
     The lighting device  100  utilizes SSL type of light sources. Although other types of switchable light sources may be used, particularly other types of solid state light emitter(s), in the illustrated example of device  100  each of the SSL light emitting sources includes some number of (one or more) light emitting diodes (LEDs)  17  that together form the respective SSL type light source  14 . Hence, each source  14   R ,  14   G ,  14   B  includes a group of LEDs of a corresponding color, in this example, red (R) LEDs  17   R , green (G) LEDs  17   G , and blue (B) LEDs  17   B . Similar to the LEDs  17  of  FIG. 1 , each group of colored LEDs  17   R ,  17   G , or  17   B  may be connected in parallel, in series or in any viable series-parallel combination; although in the illustrated example, each respective group of colored LEDs  17   R ,  17   G , or  17   B  is connected together in a single series string. 
     The lighting device  100  in  FIG. 4  includes a power and modulation circuit  11  as the drive and control channel for each light output channel provided by the different color sources  14   R ,  14   G ,  14   B . Although not separately sown in  FIG. 4  for convenience, each power and modulation circuit  11  includes a very high frequency switching power converter  15 , such as a Phi2 (Φ 2 ) class converter. 
     Although there may be differences, in the example, all three circuits  11  are the same, and each circuit  11  may be the same or similar to the circuit  11  of  FIG. 1 . For example, for each group of colored LEDs  17   R ,  17   G , or  17   B  forming a colored SSL source  14   R ,  14   G , or  14   B , the corresponding circuit  11  includes a very high frequency switching power converter  15 . For example, in the device  100 , the very high frequency switching power converter  15  ( FIG. 1 ) in each power and modulation circuit  11  ( FIG. 4 ) has an output connected to supply current to drive the LEDs  17   R ,  17   G , or  17   B  forming the respective solid state light emitting source  14   R ,  14   G , or  14   B , to emit modulated light of the respective color. The very high frequency switching power converter  15  in each power and modulation circuit  11  has an input coupled to a source of power. 
     Again, the power source could be a direct current (DC) source, such as a battery; but in the example, the device  100  obtains power from alternating current (AC) source at normal line voltage (e.g. around 120V in the US). Although not shown, one or more protective fuses may be provided in the line connection(s). The very high frequency switching power converter  15  in each power and modulation circuit  11  connects to the rectified DC output side of a bridge rectifier  19  formed by four bridge connected diodes. Although not shown, some additional smoothing and/or control circuitry may be provided on the power input side, between the bridge rectifier  19  and the very high frequency switching power converters in the power and modulation circuits  11 . 
     Each VHF power converter is the switcher of a switched-mode power supply (SNIPS) for converting the DC from the bridge rectifier  19  and supplying DC current to the respective set of LEDs  17   R ,  17   G , or  17   B , both for a portion of the illumination and for modulation to carry one channel of the data transmission. In this example, the device  100  implements each VHF converter as a DC-to-DC converter. Depending on the application, e.g. the number of LEDs used to provide the desired maximum light output level in the respective channel and the power requirements of that number of LEDs, each VHF power converter may utilize any appropriate power converter topology (e.g. boost, buck, fly back, etc.), albeit configured to operate at a VHF switching rate, as discussed herein. 
     In this example, each power converter in a circuit  11  is configured to convert power from the source of power to direct current to supply via the output to the respective solid state light emitting source, at least in part by a switching function of the power converter at a very high frequency rate. Again, the VHF switching function, in our examples, switches at a frequency in the range of about 30 MHz (30 MHZ+3 MHz) to about 300 MHz (300 MHz+30 MHz). Some examples are optimized for a switching frequency at or above 200 MHz. 
     Although not separately shown, each power and modulation circuit  11  includes a control circuit and possibly a baseband processor, similar to the control circuit  21  and the baseband processor  23 , to control operation of the VHF power converter  15  to set the overall output level (dimming control) and modulate the current and thus the light output of the respective colored LEDs  17   R ,  17   G , or  17   B  forming the solid state light emitting source  14   R , M G , or  14   B . Although any of the three channels/sources may be used to carry data, or any two of the channels/sources; in the example, all three of the channels/sources may carry modulated data transmissions similar to those provided by the single channel/source in the example of  FIG. 1 . Hence, in our example, each power and modulation circuit  11  receives a separate independently controllable input signal from a MCU. 
     As in the earlier example, the lighting device  100  may implement a variety of overall host control/operation technologies that provide the high level logic to control operation of the device including data transmission; although the illustrated example uses a MCU  125 . The MCU  125  implements the control logic for the device  100 , that is to say, controls operations of the device  100  based on execution of its embedded ‘firmware’ instructions. The MCU  125  may be a microchip device that incorporates a processor serving as the programmable central processing unit (CPU)  127  of the MCU and thus of the lighting device  100  as well as one or more memories  129  accessible to the CPU  127 . The memory or memories  129  store the executable programming for the CPU  127  as well as data for processing by or resulting from processing of the CPU  127 . The MCU  125  is similar to the MCU  25  of  FIG. 1 , except that the MCU  125  has three outputs to independently provide the control and data signals to the three power and modulation circuits  11 . The three outputs may be individual output ports or a single port with signals addressed to the different baseband processors. 
     Similar to the earlier example, the lighting device  100  also includes a communication interface  31  coupled to a communication port of the MCU  125 . The interface  31  provides a communication link to a telecommunications network that enables the MCU  125  to send and receive digital data communications through the particular network. The communication interface  31  is therefore accessible by the processor/CPU  127  of the MCU  123 , and the communication interface  31  is configured to enable the processor to communicate information about its operations as wells as data sent or received as VLC communication on any of the three light channels through the LAN or other communications network. 
     Although the device  100  could provide one-way data transmission, for purposes of the present example, the device  100  is also capable of receiving data via VLC transmission from a device in the illuminated space. Hence, the device  100  also includes an optical receiver  35 , for example, including one or more photodiodes to receive light; and a signal conditioning circuit  37  processes the receiver output signal to recapture (e.g. demodulate the signal to obtain) data carried on the received light. Such received data is provided to the MCU  125  for processing and forwarding via the interface  31  for communication over the network. As represented by the dotted lines in the drawing, the device  100  may include one or more additional sets of receivers  35  and signal conditioning circuits  37  (e.g. sensitive to wavelengths in different bands) for additional upstream channels of VLC light reception. 
     The lighting device  100  may also include an additional input/output (I/O) interface  39 , for communication between the MCU  125  and another local device, such as a local user interface device. 
     Returning now more specifically to the discussion of VLC over-the-air data communication, the device  100  receives data for VLC transmission via the network and the communication interface  31  and supplies the data to the MCU  125 . The MCU  25  may also generate some signaling data or the like locally for communication via VLC to the other device(s). The MCU  125  identifies data for transmission over each of the three output light channels R, G or B. For each such channel, the MCU  125  provides level setting commands to the baseband signal processor in the respective power and modulation circuit  11 . The MCU  125  may also instruct the baseband signal processor as to the appropriate modulation format, and the MCU  125  supplies the received data to the baseband signal processor. 
     Each baseband signal processor in a respective power and modulation circuit  11  responds to these inputs from the MCU  125  to generate a signal for input to the associated control circuit; and the control circuit generates a signal to set the output current from the very high frequency switching power converter in the power and modulation circuit  11  to the intended level for the desired illumination intensity for the respective channel R. G or B, albeit with the drive current and thus the light output modulated as instructed so as to carry the data routed to that channel via the over the air light transmission. 
     Each channel may implement any of the light modulation schemes discussed by way of examples earlier to transmit the data via the light output of the respective colored LEDs  17   R ,  17   G , or  17   B  forming the solid state light emitting source  14   R ,  14   G , or  14   B . All three channels may use the same modulation scheme, or one or more of the channels may use different modulation schemes. Modulation schemes, for example, may be dynamically adjusted for one or more of the channels based on performance, e.g. in response to feedback from devise receiving the VLC data transmissions. Different downstream transmission channels on different wavelength channels may be assigned for transmissions to different devices in the illuminated space; or two or more such transmission channels may be used to provide a combined higher rate transmission for one device or intended for common reception by a number of devices in the illuminated space (e.g. broadband broadcast). 
     The use of VHF converters to provide drive current modulation for VLC communication, as described herein, will be amenable to a variety of modifications and adaptations to particular lighting and/or communication applications. The technology also offers a number of advantages over prior approaches to VLC, some of which have been outlined above. In particular, use of Φ2 inverters which are a type of VHF converters, is of interest because such converters lend themselves to VLC application, compared to other VHF converters (for example Class E of Class Φ converters), since the Φ2 inverters are designed to provide high efficiency across the entire load range (5% -100% load can be 80% efficient through the range). This is particularly advantageous when a lighting device modulates the optical output because it does not make the lighting application suffer in efficiency due to changing load current. The load current might have to travel from 0% to 100% depending on the type of modulation used for data transmission. 
     It will be understood that the terms and expressions used herein have the ordinary meaning as is accorded to such terms and expressions with respect to their corresponding respective areas of inquiry and study except where specific meanings have otherwise been set forth herein. Relational terms such as first and second and the like may be used solely to distinguish one entity or action from another without necessarily requiring or implying any actual such relationship or order between such entities or actions. The terms “comprises,” “comprising,” “includes,” “including,” or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. An element proceeded by “a” or “an” does not, without further constraints, preclude the existence of additional identical elements in the process, method, article, or apparatus that comprises the element. 
     Unless otherwise stated, any and all measurements, values, ratings, positions, magnitudes, sizes, and other specifications that are set forth in this specification, including in the claims that follow, are approximate, not exact. They are intended to have a reasonable range that is consistent with the functions to which they relate and with what is customary in the art to which they pertain. 
     While the foregoing has described what are considered to be the best mode and/or other examples, it is understood that various modifications may be made therein and that the subject matter disclosed herein may be implemented in various forms and examples, and that they may be applied in numerous applications, only some of which have been described herein. It is intended by the following claims to claim any and all modifications and variations that fall within the true scope of the present concepts.