Adapting Techniques to Improve Efficiency in Radio Frequency

electronics

Article Adapting Techniques to Improve Eﬃciency in Radio Frequency Power Ampliﬁers for Visible Light Communications

Daniel G. Aller 1,* , Diego G. Lamar 1, Juan Rodriguez 2, Pablo F. Miaja 1 , Valentin Francisco Romero 3, Jose Mendiolagoitia 3 and Javier Sebastian 1 1 Power Supplies Group—Electrical Engineering Department of the University of Oviedo, 33204 Gijon, Spain; [email protected] (D.G.L.); [email protected] (P.F.M.); [email protected] (J.S.) 2 Center for Industrial Electronics—Polytechnic University of Madrid, 28006 Madrid, Spain; [email protected] 3 Thyssenkrupp Elevator Innovation Center S A U—Thyssenkrupp AG, 33203 Gijon, Spain; [email protected] (V.F.R.); [email protected] (J.M.) * Correspondence: [email protected]; Tel.: +34-985-182-578

Received: 30 November 2019; Accepted: 3 January 2020; Published: 10 January 2020

Abstract: It is well known that modern wireless communications systems need linear, wide bandwidth, efficient Radio Frequency Power Amplifiers (RFPAs). However, conventional configurations of RFPAs based on Class A, Class B, and Class AB exhibit extremely low efficiencies when they manage signals with a high Peak-to-Average Power Ratio (PAPR). Traditionally, a number of techniques have been proposed either to achieve linearity in the case of efficient Switching-Mode RFPAs or to improve the efficiency of linear RFPAs. There are two categories in the application of aforementioned techniques. First, techniques based on the use of Switching-Mode DC–DC converters with a very-fast-output response (faster than 1 µs). Second, techniques based on the interaction of several RFPAs. The current expansion of these techniques is mainly due to their application in cellphone networks, but they can also be applied in other promising wireless communications systems such as Visible Light Communication (VLC). The main contribution of this paper is to show how Envelope Tracking (ET), Envelope and Elimination (EER), Outphasing, and Doherty techniques can be helpful in developing more efficient VLC transmitters capable of reaching high bit-rates (higher than 1 Mbps) by using advance modulation schemes. Finally, two examples based on the application of the Outphasing technique and the use of a Linear-Assisted Envelope Amplifier (EA) to VLC are presented and experimentally verified.

Keywords: visible light communication (VLC); light emitting diodes (LED); switching-mode; DC–DC converters and radio frequency power ampliﬁers (RFPA)

1. Introduction Currently, the use of wireless communication systems is essential for the present and the future of our society [1]. Furthermore, the speed demanded by each communication service is continuously growing due to the high bit-rate required (higher than 1 Mbps) by mainstream services. As a result, the Radio Frequency (RF) spectrum is already close to congestion and hence enabling the data traﬃc predicted for upcoming years requires further research into new wireless communication technologies. Visible Light Communication (VLC) is one of the most promising solutions for alleviating the saturation of the RF spectrum [2–4]. VLC uses the wide, unlicensed visible light spectrum (430–750 THz range) to transmit information. The strength and potential of this approach become apparent when the communication task is merged with the lighting functionality of Light-Emitting Diode (LED) lamps.

Electronics 2020, 9, 131; doi:10.3390/electronics9010131 www.mdpi.com/journal/electronics Electronics 2020, 9, 131 2 of 19

Thus, the existing lighting infrastructure can be partially adapted to incorporate communication features. Since its introduction in 2004 [1], VLC has gained attention due to the widespread use of LEDs in lighting installations. Besides their longer lifespan, higher power efficiency, and the fact that they are more environmentally friendly than other lighting technologies, LEDs are able to change their light intensity very quickly, making this technology the most promising one in terms of enabling communication functionalities. The design of the LED driver with communication functionalities (VLC-LED driver) is one of the main challenges of the VLC-transmitter. The VLC-LED driver is responsible for two tasks: guaranteeing the desired lighting level and reproducing, with a high degree of accuracy, the communication signal originating from the other kind circuitry of the VLC-transmitter, which processes the data. Traditionally, the literature focuses on the most important VLC contributions in data processing, advanced modulation schemes, LED modeling and experimental verification using low power prototypes. In other words, the main efforts have been carried out to increase the spectral efficiency of VLC-LED drivers in order to achieve very high bit-rates (higher than 10 Mbps). However, a major issue with regard to VLC-LED drivers is not taken into account: the high efficiency of LED lighting is not only due to the high efficiency of the LEDs in converting electrical power into lighting power, but also to the high efficiency achieved by the VLC-LED driver that delivers the electrical power. Therefore, the VLC-LED driver must also be power efficient [5]. The VLC-LED drivers for VLC-transmitters proposed to date, which can be classified into two subsets, do not achieve both high power efficiency (higher than 90%) and high bit-rates (higher than 1 Mbps):

VLC-LED drivers based on switching the LEDs on and off. Figure1a shows a conventional structure • based on a high efficiency, limited-output-response Switching-Mode DC–DC converter. In this example, a power MOSFET is placed in series to turn the LED on and off. Other structures with the MOSFET placed in parallel are also valid. This VLC-LED driver is simple and high-power efficient (higher than 90%). Only Pulse-Based Modulation (PBM) schemes can be reproduced using this solution [6–9]; however, PBM schemes are not the best option for providing a high bit-rate and for avoiding the multipath issue (an issue arising from the different paths that the light rays can follow from the light focus to the receiver). Although some PBM proposed solutions achieve a very high bit-rate (10 Mbps) at higher power levels associated with the light emitted for communications (up to 10 W) [8,9], they are far from the theoretically achievable bit-rate using advanced modulation schemes. VLC-LED drivers for reproducing advanced modulation schemes. Figure1b shows the conventional • structure, which involves the use of a high efficiency, limited-output-response Switching-Mode converter (DC–DC or AC–DC) to bias the LED, in addition to the use of a low power efficiency, very-fast-output response (i.e., wide bandwidth) RFPA to superpose the communication signal. These VLC-LED drivers focus on reproducing Single-Carrier Modulation (SCM) and Multi-Carrier Modulation (MCM) schemes that allow for providing a higher bit-rate (up to 1 Gbps). These solutions are based on classical configurations of linear Radio Frequency Power Amplifiers (RFPAs) (Class A, Class B, and Class AB) in order to obtain linearity and wide bandwidth. However, they exhibit extremely low efficiency when they manage signals with a wide ratio between average power and peak power [10–15] (i.e., a high Peak-to-Average Power Ratio, PAPR). Although the very low power efficiency only refers to power processing for communication tasks (e.g., efficiencies between 10% and 40%), this fact affects the overall efficiency of the VLC-LED driver (e.g., efficiencies below 70%). Moreover, all the proposed solutions reported to date using linear RFPAs limit the application to very short distances (up to 1 m) with very low output power for both lighting and communications tasks (up to 1 W).

As in any electronics device, a Switching-Mode Converter (either AC–DC or DC–DC [16–19]) will be in charge of generating the DC voltage level needed to supply power to any VLC system. The voltage source represented as VCC in the circuits shown in Figure1 will be the output port of that Electronics 2020, 9, 131 3 of 19

converter.Electronics No2020 speciﬁc, 9, x FOR PEER design REVIEW constrains will distinguish this ﬁrst converter to any other converter.3 of 19 The switching-mode DC–DC converters shown in Figure1 are designed identically as traditional post-regulators, which are being traditionally used in LED drivers.

VCC

Switching- iLED i Mode dc-dc LED Bias control converter LED

Communication control vPBM

vPBM

(a)

Switching- Mode dc-dc VCC converter ibias_LED Bias control

i i v8QAM ac_LED LED Linear iLED v8QAM RFPA +- iac_LED LED

(b)

Figure 1. Conventional VLC-LED drivers presented to date: (a) VLC-LED driver based on switching the LEDs on and off, reproducing a PBM scheme; (b) VLC-LED driver for reproducing advanced modulation schemes, in this example, 8-Quadrature Amplitude Modulation (8-QAM). Figure 1. Conventional VLC-LED drivers presented to date: (a) VLC-LED driver based on switching Focusingthe LEDs our on attention and off, reproducing on the second a PBM subset scheme; of ( solutionsb) VLC-LED (i.e., driver VLC-LED for reproducing drivers advanced for reproducing advancedmodulation modulation schemes, schemes), in this theexample, low power8-Quadrature efficiency Amplitude of VLC-LED Modulation drivers (8-QAM). is an important limitation that may be overcome so as to obtain high power efficiency and a high bit-rate. The possible solution As in any electronics device, a Switching-Mode Converter (either AC–DC or DC–DC [16–19]) arises from the application of traditional techniques used in the past either to improve the efficiency of will be in charge of generating the DC voltage level needed to supply power to any VLC system. The linear RFPAs or to achieve linear behavior in the case of efficient Switching-Mode RFPAs. In fact, this voltage source represented as VCC in the circuits shown in Figure 1 will be the output port of that is theconverter. main objective No specific of the design present constrains paper: will to describedistinguish the this adaptation first converter of traditional to any other techniques converter. used eitherThe to switching-mode improve the effi DC–DCciency ofconverters or linearize shown RFPAs in Figure to VLC. 1 are In somedesigned cases, identically this adaptation as traditional implies a significantpost-regulators, change inwhich the circuitryare being that traditionally defines these used techniques,in LED drivers. with major associated benefits. For all these reasons,Focusing the our aim attention of this paperon the issecond the revision subset of of solutions these techniques, (i.e., VLC-LED their drivers adaptation for reproducing for VLC, and the experimentaladvanced modulation validation schemes), of two examples:the low power Outphasing efficiency technique of VLC-LED and Linear-Assisteddrivers is an important technique. limitationThe paper that is may organized be overcome as follows. so as to First, obtain a hi summarygh power efficiency of the traditional and a high techniques bit-rate. The used possible in RF to eithersolution improve arises the from efficiency the application of RFPAs of or traditional to linearize techniques the RFPAs used are in given the past in Section either to2. improve The adaptation the of theseefficiency techniques of linear to VLC RFPAs are or given to achieve in Section linear3. The behavior adaptation in the of case the Outphasingof efficient Switching-Mode technique to VLC is givenRFPAs. in Section In fact,4. this The is use the ofmain the objective envelope of amplifiers the present in paper: VLC isto explaineddescribe the in adaptation Section5. of The traditional adaptation techniques used either to improve the efficiency of or linearize RFPAs to VLC. In some cases, this of the Linear-Assisted technique is given in Section6. Section7 presents the circuitry design and the adaptation implies a significant change in the circuitry that defines these techniques, with major experimental results. Finally, Section8 provides the conclusions of the paper. associated benefits. For all these reasons, the aim of this paper is the revision of these techniques, 2. Reviewtheir adaptation of Traditional for VLC, Solutions and the experimental Either to Improve validation the Eofffi twociency examples: of orto Outphasing Linearize technique RFPAs and Linear-Assisted technique. TableThe1 summarizes paper is organized the traditional as follows. techniques First, a summary used either of the to improvetraditional the techniques efficiency used of or in to RF linearize to RFPAs.eitherOn improve the one the hand, efficiency Table of1 showsRFPAs solutionsor to lineari forze increasingthe RFPAs are the given e fficiency in Section of Linear 2. The adaptation RFPAs, which are naturallyof these techniques inefficient to whenVLC are signals given with in Section very 3. high The PAPR adaptation are processed. of the Outphasing On the othertechnique hand, to thereVLC are Electronics 2020, 9, 131 4 of 19 other kinds of techniques for providing linearity to Switching-Mode RFPAs (Class D, Class E, and Class F). Switching-Mode RFPAs are very efficient (theoretical efficiency is 100%) because the active devices (transistors) function as electronic switches instead of linear gain devices. However, they exhibit a lack of linearity because of the nonlinear relationship between the pulses that are used to drive the transistors and the output. In both cases (Linear or Switching-Mode RFPAs), these techniques are based on the use of either very-fast-output response (faster than 1 µs) Switching-Mode DC–DC converters or an arrangement of RFPAs.

Table 1. Traditional techniques either to improve the eﬃciency of or to linearize RFPAs.

Linear RFPAs (Class A & B) Switching-Mode RFPAs (Class D, E & F) Using Switching-Mode DC–DC converters Envelope Tracking (ET) Envelope Elimination and Restoration (EER) Arrangement of RFPAs Doherty Outphasing

In the past, very-fast-output response Switching-Mode DC–DC converters, capable of changing their output voltage at a MHz rate, have been proposed to implement techniques used to increase the efficiency of RFPAs (first row in Table1). The most popular of these techniques are Envelope Tracking (ET) for Linear RFPAs (Figure2a) and Envelope Elimination and Restoration (EER) for Switching-Mode RFPAs (Figure2b) [ 19]. In both cases, a very-fast-output response Switching-Mode DC–DC converter has to generate the time-varying output voltage that is used as the power supply for the RFPA. The variations of this voltage are imposed by the envelope of the communication signal processedElectronics 2020 by the, 9, x RFPA.FOR PEER This REVIEW fast-output response Switching-Mode DC–DC converter is often5 ofknown 19 as an Envelope Modulator or Envelope Amplifier (EA).

venv VCC Envelope Amplifier venv (very-fast-output v Envelope env response Detector v Envelope Switching-Mode oRFPA v control dc-dc converter) env vi

v voRFPA i Linear Delay RFPA

(a)

venv VCC Envelope Amplifier venv (very-fast-output v Envelope env response Detector v Envelope Switching-Mode oRFPA v control dc-dc converter) env

v v voRFPA i PWM Switching Limiter Delay RFPA vPWM

(b)

Figure 2. Conventional block diagram of techniques based on the use of Envelope Ampliﬁers (EA): (a) Envelope Tracking (ET) technique; (b) Envelope Elimination and Restauration (EER) technique. Figure 2. Conventional block diagram of techniques based on the use of Envelope Amplifiers (EA): (a) Envelope Tracking (ET) technique; (b) Envelope Elimination and Restauration (EER) technique.

Moreover, techniques based on an arrangement of several RFPAs can achieve the desired improvements, in this case without using very-fast-output response Switching-Mode DC–DC converters: the Doherty technique for linear RFPAs and the Outphasing technique for Linear RFPAs and Switching-Mode RFPAs. W.H. Doherty originally proposed the Doherty technique in 1936 [20] (Figure 3a). In the case at hand, at least two linear RFPAs must be used. One of the RFPAs operates with the low part of the signal and is called the “carrier” RFPA. The other RFPA only works when the signal level exceeds a specific value and is called the “peak” RFPA. The realization of this technique involves the splitter of the input signal that is to be amplified and the combination of the output signal of each RFPA. The realization of the circuitry that performs these functions (i.e., the splitter and the combiner) is complex and sometimes constitutes the bottleneck of this technique. The outphasing technique was first proposed in the 1930s [21,22] and is based on the following idea: two sinusoidal signals of constant amplitude can be combined to obtain one sinusoidal signal with variable amplitude and phase. Therefore, from this statement, two Switching-Mode RFPAs working at maximum efficiency can be combined to amplify any signal (Figure 3b) by means of appropriate phase shifting between them. In this case, the design and construction of the splitter, the output network of each RFPA, and the combiner are also key points. Electronics 2020, 9, 131 5 of 19

Moreover, techniques based on an arrangement of several RFPAs can achieve the desired improvements, in this case without using very-fast-output response Switching-Mode DC–DC converters: the Doherty technique for linear RFPAs and the Outphasing technique for Linear RFPAs and Switching-Mode RFPAs. W.H. Doherty originally proposed the Doherty technique in 1936 [20] (Figure3a). In the case at hand, at least two linear RFPAs must be used. One of the RFPAs operates with the low part of the signal and is called the “carrier” RFPA. The other RFPA only works when the signal level exceeds a specific value and is called the “peak” RFPA. The realization of this technique involves the splitter of the input signal that is to be amplified and the combination of the output signal of each RFPA. The realization of the circuitry that performs these functions (i.e., the splitter and the combiner) is complex and sometimes constitutes the bottleneck of this technique. The outphasing technique was first proposed in the 1930s [21,22] and is based on the following idea: two sinusoidal signals of constant amplitude can be combined to obtain one sinusoidal signal with variable amplitude and phase. Therefore, from this statement, two Switching-Mode RFPAs working at maximum efficiency

Electronicscan be 2020 combined, 9, x FOR to PEER amplify REVIEW any signal (Figure3b) by means of appropriate phase shifting between6 of 19 them. In this case, the design and construction of the splitter, the output network of each RFPA, and the combiner are also key points.

iRFPA1

vRFPA1 vo

VCC

“Carrier” RFPA iRFPA1 Linear Impedance inverter RFPA1 vRFPA1 Splitter and combiner v vi o iRFPA2 V vi CC iRFPA2 Linear RFPA2 “Peak” RFPA

(a)

vRFPA1

VCC vo vi +α Switching Output network 1 RFPA1 vRFPA1 Splitter Combiner vRFPA2 v vi o

VCC Phase control −α Switching Output network 1 RFPA2 vRFPA2

(b)

Figure 3. Conventional block diagram of techniques based on an arrangement of RFPAs. (a) Doherty Figuretechnique; 3. Conventional (b) Outphasing block technique.diagram of techniques based on an arrangement of RFPAs. (a) Doherty technique; (b) Outphasing technique.

3. Traditional Solutions Either to Improve the Efficiency of or to Linearize RFPAs and Their Adaptation to VLC The main idea of this section is very simple: to export and adapt any of the techniques indicated in Table 1 to the RFPA of the VLC-LED driver shown in Figure 2b. Figure 4 shows an example of the adaptation of the ET technique to VLC-LED drivers. As already stated, the ET technique requires a very-fast-output response Switching-Mode converter. This converter must include the capability of following the envelope of the signal that is to be amplified (in the example in Figure 4, an 8-QAM signal). Electronics 2020, 9, 131 6 of 19

3. Traditional Solutions Either to Improve the Eﬃciency of or to Linearize RFPAs and Their Adaptation to VLC The main idea of this section is very simple: to export and adapt any of the techniques indicated in Table1 to the RFPA of the VLC-LED driver shown in Figure2b. Figure4 shows an example of the adaptation of the ET technique to VLC-LED drivers. As already stated, the ET technique requires a very-fast-output response Switching-Mode converter. This converter must include the capability of Electronicsfollowing 2020 the, 9, x envelope FOR PEER of REVIEW the signal that is to be amplified (in the example in Figure4, an 8-QAM signal7 ).of 19

Envelope Amplifier Very-fast-output response Switching- VCC Mode dc-dc converter) Envelope control venv Switching- Mode dc-dc Envelope v converter Detector venv dcRFPA ibias_LED v 8QAM Bias control

v vac_LED iLED QAM Linear Delay RFPA - +

iLED LEDs vdcRFPA vac_LED ibias_LED

FigureFigure 4. Appling 4. Appling the the ET ET technique technique to to an an RFPA RFPA of of a VLC-LED driverdriver reproducingreproducing an an MCM MCM scheme scheme (i.e.,(i.e., 8QAM). 8QAM).

However,However, prior prior to to the the adaptation adaptation of of all all the the techniques techniques in Table1 1 to to VLC, VLC, it it is is necessary necessary to to point point outout some some aspects, aspects, which which will will eventually eventually lead lead to to th thee introduction ofof certaincertain constraintsconstraints in in the the real real implementation:implementation: (a) (a)OnOn the the one one hand, hand, thethe maximum maximum bandwidth bandwidth of the of LED the for LED theelectrical-to-optical for the electrical-to-optical power conversion power conversionis a fundamental is a fundamental parameter that parameter must be taken that intomust account be taken for VLC. into Aaccount blue GaN for LED VLC. in combination A blue GaN LEDwith in combination a yellow phosphor with LEDa yellow is the phosphor preferred approachLED is the to preferred obtain white approach light in Solid-Stateto obtain white Lighting light in Solid-State(SSL). However, Lighting this phosphor (SSL). However, layer is optimized this phosphor for color layer conversion, is optimized but not for for color achieving conversion, a very butrapid not for response achieving to changes a very inrapid light response intensity. to In change fact, it limitss in light the LEDintensity. bandwidth In fact, of it white limits light the to LED a bandwidthfew MHz of (i.e., white 3–5 MHz).light to Moreover, a few MHz the bandwidth(i.e., 3–5 MHz). for blue Moreover, light emission the bandwidth is limited to for 10–20 blue MHz light emissiondue to is LED limited technology. to 10–20 On MHz the otherdue to hand, LED the technology. light intensity On the modulation other hand, must the be light fast enoughintensity to be unappreciable to the human eye for safety reasons (i.e., higher than approximately 1 kHz). modulation must be fast enough to be unappreciable to the human eye for safety reasons (i.e., The aforementioned limits define the usable frequency spectrum to modulate the amplitude and higher than approximately 1 kHz). The aforementioned limits define the usable frequency phase of the visible light intensity (VLC frequency spectrum). spectrum to modulate the amplitude and phase of the visible light intensity (VLC frequency (b) The greater part of the previously deﬁned frequency spectrum for the use of visible light (i.e., spectrum). 1 kHz–20 MHz) could be used for many of the possible scenarios (in some cases, the entire VLC (b) The greater part of the previously defined frequency spectrum for the use of visible light (i.e., 1 frequency spectrum). This fact facilitates the use of MCM schemes to achieve high bit-rates kHz–20 MHz) could be used for many of the possible scenarios (in some cases, the entire VLC because there are no spectrum restrictions. This means it is necessary to use wideband RFPAs for frequency spectrum). This fact facilitates the use of MCM schemes to achieve high bit-rates VLC-drivers. Another possible solution is to use several narrowband RFPAs, whose combination because there are no spectrum restrictions. This means it is necessary to use wideband RFPAs for VLC-drivers. Another possible solution is to use several narrowband RFPAs, whose combination could cover the bandwidth required by the MCM scheme. However, the cost of this solution makes it unfeasible. (c) If very wideband RFPAs (when the BW is higher than 10% of the center frequency) operating at relatively low frequencies (due to the bandwidth of the VLC frequency spectrum) are required for VLC-LED drivers, then the adaptation to VLC of some of the techniques mentioned in Table 1 is not possible. For example, Switching-Mode RFPAs require either band-pass resonant circuits (e.g., Class D RFPAs) or transistor-integrated resonant circuits (Class E and F RFPAs) to operate properly because the square waveform generated by transistors must be filtered in order to obtain a sinusoidal carrier. This fact limits the exploitation of the VLC frequency spectrum because it is very difficult to adapt EER and Outphasing techniques using nonlinear RFPAs operating within this frequency range (e.g., from 1 kHz to 20 MHz). Moreover, in the case of the Outphasing technique, the use of selective output networks for the combiner (Figure 3b) is

Electronics 2020, 9, 131 7 of 19

could cover the bandwidth required by the MCM scheme. However, the cost of this solution makes it unfeasible. (c) If very wideband RFPAs (when the BW is higher than 10% of the center frequency) operating at relatively low frequencies (due to the bandwidth of the VLC frequency spectrum) are required for VLC-LED drivers, then the adaptation to VLC of some of the techniques mentioned in Table1 is not possible. For example, Switching-Mode RFPAs require either band-pass resonant circuits (e.g., Class D RFPAs) or transistor-integrated resonant circuits (Class E and F RFPAs) to operate properly because the square waveform generated by transistors must be ﬁltered in order to obtain a sinusoidal carrier. This fact limits the exploitation of the VLC frequency spectrum because it is very diﬃcult to adapt EER and Outphasing techniques using nonlinear RFPAs operating within Electronics 2020, 9, x FOR PEER REVIEW 8 of 19 this frequency range (e.g., from 1 kHz to 20 MHz). Moreover, in the case of the Outphasing technique, the use of selective output networks for the combiner (Figure3b) is mandatory. The real mandatory. The real implementation of these output networks for the VLC frequency spectrum implementation of these output networks for the VLC frequency spectrum is very complex and is very complex and makes the adaptation of this technique unachievable. makes the adaptation of this technique unachievable. (d) In the case of the application of the Doherty technique, impedance inverters, made of discrete (d) In the case of the application of the Doherty technique, impedance inverters, made of discrete components, are needed. Impedance inverters are narrowband, which limit the bandwidth of components, are needed. Impedance inverters are narrowband, which limit the bandwidth of the VLC-driver. the VLC-driver. (e) From all the above, if MCM schemes are to be reproduced for VLC, then only the ET technique (e) From all the above, if MCM schemes are to be reproduced for VLC, then only the ET technique can be applied to single wideband linear RFPAs. In this case, the entire free VLC frequency can be applied to single wideband linear RFPAs. In this case, the entire free VLC frequency spectrumspectrum can can be be exploited. exploited. Another option is either to give up the entire free frequency spectrum of visible light or split it Another option is either to give up the entire free frequency spectrum of visible light or split it up up using several narrowband RFPAs. In this case, all the techniques in Table 1 can be adapted to using several narrowband RFPAs. In this case, all the techniques in Table1 can be adapted to VLC. VLC.Figure Figure5 shows 5 shows the adaptation the adaptation of the Doherty of the technique Doherty totechnique VLC. As can to beVLC. seen, As both can the be “carrier” seen, both RFPA the “carrier”and the RFPA “peak” and RFPA the “peak” are highlighted. RFPA are In highlighted. this case, the In impedance this case, the inverter impedance is designed inverter with isdiscrete designed withcomponents discrete components (two capacitors, (two C,capacitors, and one inductor, C, and one L) insteadinductor, of L) the instead traditional of the quarter-wavelength traditional quarter- wavelengthtransmission transmission line, due to line, the frequenciesdue to the frequencies used in VLC. used in VLC.

Switching- Mode dc-dc vRFPA1 iRFPA1 converter ibias_LED Bias control VCC “Carrier” RFPA iRFPA1 L v iLED Linear ac_LED - + RFPA1 vRFPA1 Splitter C C LEDs vi iRFPA2 V vi CC iRFPA2 Linear RFPA2 v iLED “Peak” RFPA ac_LED

ibias_LED

Figure 5. Appling the Doherty technique to a VLC-LED driver. This scheme is only valid for narrowband Figure 5. Appling the Doherty technique to a VLC-LED driver. This scheme is only valid for approaches. narrowband approaches.

Electronics 2020, 9, 131 8 of 19

4. Adaptation of the Outphasing Technique to VLC The Outphasing technique can be applied using both linear and Switching-Mode RFPAs. The efficiency with linear RFPAs can be increased up to the maximum of each class (50% for Class A and 78% for B), whereas, in the case of Switching-Mode RFPAs, the maximum is theoretically 100%, making this the most suitable adaptation to VLC. The major difficulty in applying the Outphasing technique (Figure3b) is the design of the output combiner and the output networks, which are in charge of connecting the two RFPAs together and summing the sinusoidal signals. On the one hand, the connection of the outputs of the RFPAs is not straightforward, as the output impedance of each RFPA changes over time depending on the output voltage due to the phase shift operation, thereby leading to an undesirable influence between them. On the other hand, the combiner must be designed to prevent the differential mode connection of the load (i.e., the load must be connected to the ground of the circuitry). Traditionally, the combiner is performed by means of two quarter-wavelength transmission lines. Furthermore, the cross effect between the RFPAs due to the output combiner is especially critical when the RFPA is based on a resonant topology (i.e., Class E or Class F), in which the efficiency depends on the correct tuning of the resonant circuit. In this case, the analysis and design of the combiner is complex in order to obtain a trade-off between efficiency and bandwidth. Another key part of the application of the Outphasing technique is the input splitter and phase control. This circuitry is in charge of generating the input signals for each RFPA, whose sum is the desired output communication signal. In the case of Switching-Mode RFPAs, this splitter and phase control can be implemented on a digital platform, as the input signals for these amplifiers are square waveforms. In the case of linear amplifiers, the splitter and phase control have to be implemented over the analog signals, thus increasing the difficulty of the implementation. At this point, we can exploit summing the light output in order to adapt the Outphasing technique to VLC. The idea of light Outphasing is that of splitting the LED load between the two RFPAs and summing the two sinusoidal signals in their light form, as can be seen in Figure6. In the case of Figure6, both LED strings will generate the same DC value of light intensity due to the fact that ibias_LED1 equals ibias_LED2 (generated by a conventional Switching-Mode DC–DC converter). Furthermore, two sinusoidal signals of constant amplitude and variable phase are superposed to each string of LEDs by the actions of both Switching-Mode RFPAs. As the added quantities are two light intensities instead of two electrical signals, the need to use a combiner and connect both Switching-Mode RFPAs electrically is circumvented, thus leading to a huge simplification of the design and avoiding the influence between the two signals (with the resulting increase in power efficiency). In other words, as the outputs of each Switching-Mode RFPA are not connected together and the load is an LED string, the impedance from the point of view of the RFPAs is known and constant. Hence, the possible condition of Zero Voltage Switching (ZVS) of each RFPA does not depend on the other, thereby simplifying the overall design. In a conventional VLC-LED driver, the LED string is connected via a bias T (Figure1b). The bias T is used to connect the amplifier and the Switching-Mode DC–DC converter together, blocking the DC component from passing to the RFPA (using a series capacitor) and the signal components from passing to the Switching-Mode DC–DC (using an inductor). As shown in Figure6, there is no path between the strings at high frequency, meaning that the assumption that there is no influence between the RFPAs is still correct. In the case of Class E RFPAs, due to their intrinsic resonant output filter, DC blocking is already implemented in each amplifier, so there is no need for an additional series capacitor between each RFPA and the corresponding LED string. Electronics 2020, 9, x FOR PEER REVIEW 9 of 19 the resonant circuit. In this case, the analysis and design of the combiner is complex in order to obtain a trade-off between efficiency and bandwidth. Another key part of the application of the Outphasing technique is the input splitter and phase control. This circuitry is in charge of generating the input signals for each RFPA, whose sum is the desired output communication signal. In the case of Switching-Mode RFPAs, this splitter and phase control can be implemented on a digital platform, as the input signals for these amplifiers are square waveforms. In the case of linear amplifiers, the splitter and phase control have to be implemented over the analog signals, thus increasing the difficulty of the implementation. At this point, we can exploit summing the light output in order to adapt the Outphasing technique to VLC. The idea of light Outphasing is that of splitting the LED load between the two RFPAs and summing the two sinusoidal signals in their light form, as can be seen in Figure 6. In the case of Figure 6, both LED strings will generate the same DC value of light intensity due to the fact that ibias_LED1 equals ibias_LED2 (generated by a conventional Switching-Mode DC–DC converter). Furthermore, two sinusoidal signals of constant amplitude and variable phase are superposed to each string of LEDs by the actions of both Switching-Mode RFPAs. As the added quantities are two light intensities instead of two electrical signals, the need to use a combiner and connect both Switching- Mode RFPAs electrically is circumvented, thus leading to a huge simplification of the design and avoiding the influence between the two signals (with the resulting increase in power efficiency). In other words, as the outputs of each Switching-Mode RFPA are not connected together and the load is an LED string, the impedance from the point of view of the RFPAs is known and constant. Hence, the possible condition of Zero Voltage Switching (ZVS) of each RFPA does not depend on the other, therebyElectronics simplifying2020, 9, 131 the overall design. 9 of 19

Conventional Switching-Mode dc-dc converter iLED1 ibias_LED1 Bias control v RFPA1 ibias_LED2 ibias_LED1 VCC vi iLED2 +α Switching i - + LED1 i RFPA1 v bias_LED1 Splitter RFPA1 iLED2

vi vRFPA2 vRX VCC Phase control −α Switching - + RFPA2 vRFPA2 LEDs

Electronics 2020, 9, x FOR PEER REVIEW 10 of 19 Figure 6. Adapting the Outphasing technique to a VLC-LED driver exploiting summing the light. This Figurescheme 6. Adapting is only valid the for Outphasing narrowband technique approaches. to a VLC-LED driver exploiting summing the light. 5. ExploitingThis scheme Envelope is only valid Amplifiers for narrowband (EA) for approaches. Implementing VLC-LED Drivers 5. Exploiting Envelope Amplifiers (EA) for Implementing VLC-LED Drivers As previously stated, ET and EER techniques (Table 1) require EAs (i.e., very-fast-output In a conventional VLC-LED driver, the LED string is connected via a bias T (Figure 1b). The bias responseAs Switching-Mode previously stated, DC–DC ET and EER converters, techniques Figure (Table 2)1) capable require EAsof changing (i.e., very-fast-output their output response voltage at T is used to connect the amplifier and the Switching-Mode DC–DC converter together, blocking the a verySwitching-Mode high frequency DC–DC rate converters, (i.e., MHz) Figure following2) capable the envelope of changing of the their modulation output voltage scheme. at a very The high idea is DC component from passing to the RFPA (using a series capacitor) and the signal components from nowfrequency very simple: rate (i.e., to use MHz) EAs following synthesizing the envelope the overall of thecurrent modulation passing scheme. through The the ideaLED is string. now veryIn this passing to the Switching-Mode DC–DC (using an inductor). As shown in Figure 6, there is no path case,simple: only toone use Switched-Mode EAs synthesizing DC–DC the overall converter current is passing also used through in the the final LED implementation string. In this case, of a only VLC- between the strings at high frequency, meaning that the assumption that there is no influence LEDone driver. Switched-Mode However, DC–DC this converter converter is very is also challenging used in the because final implementation it is in charge ofof a generating VLC-LED driver.both the betweenHowever, the thisRFPAs converter is still correct. is very challengingIn the case of because Class E it RFPAs, is in charge due to of their generating intrinsic both resonant the DC output bias DC bias current for the lighting task and the AC current level corresponding to the information to be filter,current DC forblocking the lighting is already task and implemented the AC current in level each corresponding amplifier, so tothere the information is no need tofor be an transmitted additional transmitted using advanced modulation schemes (Figure 7). Therefore, the cutoff frequency seriesusing capacitor advanced between modulation each schemesRFPA and (Figure the corresponding7). Therefore, the LED cuto string.ff frequency corresponding to the corresponding to the transfer function between the control variable (the converter duty cycle) and transfer function between the control variable (the converter duty cycle) and the output voltage must the output voltage must be as high as the highest frequency component of the ac current that is to be be as high as the highest frequency component of the ac current that is to be used for communication used for communication tasks. In practice, this means that very-fast-output response Switching- tasks. In practice, this means that very-fast-output response Switching-Mode DC–DC converters must Modebe used DC–DC for this converters purpose. must be used for this purpose.

Very-fast-output iLED response Switching- VCC Mode dc-dc converter vd_ac v + + d iLED + vd Switching-Mode dc-dc control vd_dc vd_dc LEDs

Figure 7. Very-fast-output Switching-Mode DC–DC converter for synthesizing the overall current Figure 7. Very-fast-output Switching-Mode DC–DC converter for synthesizing the overall current passing through the LED string (VLC-LED driver). This scheme is valid for reproducing PBM, SCM passing through the LED string (VLC-LED driver). This scheme is valid for reproducing PBM, SCM and MCM schemes. and MCM schemes. Over the years, several designs of very-fast-output response Switching-Mode DC–DC converters conceivedOver the to work years, as EAsseveral have beendesigns proposed of [very-fa23]. Mostst-output of these designsresponse derive Switching-Mode from the buck DC–DC DC–DC convertersconverter, conceived as this converter to work exhibits as EAs have linearity been between proposed the [23]. control Most and of outputthese designs voltage derive operating from in the buck DC–DC converter, as this converter exhibits linearity between the control and output voltage operating in Continuous Conduction Mode (CCM). The basic structure of the buck DC–DC converter (Figure 8a) presents a second-order filter at its output. Using Pulse-Width Modulation (PWM), the buck DC–DC converter can reproduce signals with frequencies up to 10–20 times its switching frequency. The challenge is now to propose circuit derivations aimed at reducing the gap between the switching frequency and the frequency of the signal to be reproduced without distortion: (a) Increase the order of the output filter [24] (Figure 8b). For a certain switching frequency value, the higher the filter order, the higher the rejection of the switching-related harmonics of the PWM signal at the input of the buck DC–DC converter filter. In the case of VLC-LED drivers, as the dynamic resistance of LEDs is quite constant, the conventional filter theory can be used to design a 4th- or 6th-order output filter, thus reducing the gap between the switching frequency and the frequency of the signal to be reproduced up to 4–5 times. (b) Use multi-input structures [25] (Figure 8c). The idea is to reduce the power of the switching- related harmonics of the PWM signal. Given that the voltage at the input of the buck DC–DC converter filter is a PWM voltage waveform, the switching-related harmonics are determined by the amplitude of the pulses. The lower the amplitude of the pulses, the lower the power of the switching-related harmonics at the input of the buck DC–DC converter filter and hence the higher the cutoff frequency of the filter for a specified switching frequency. Moreover, multi- input structures of the buck DC–DC converter mean lower switching losses due to the fact that the transistor switches between lower voltages (in the circuit in Figure 7, S3 switches when S2 is on and therefore withstands the voltage VCC3-VCC2).

Electronics 2020, 9, 131 10 of 19

Continuous Conduction Mode (CCM). The basic structure of the buck DC–DC converter (Figure8a) presents a second-order filter at its output. Using Pulse-Width Modulation (PWM), the buck DC–DC converter can reproduce signals with frequencies up to 10–20 times its switching frequency. The challenge is now to propose circuit derivations aimed at reducing the gap between the switching frequency and the frequency of the signal to be reproduced without distortion: (a) Increase the order of the output filter [24] (Figure8b). For a certain switching frequency value, the higher the filter order, the higher the rejection of the switching-related harmonics of the PWM signal at the input of the buck DC–DC converter filter. In the case of VLC-LED drivers, as the dynamic resistance of LEDs is quite constant, the conventional filter theory can be used to design a 4th- or 6th-order output filter, thus reducing the gap between the switching frequency and the frequency of the signal to be reproduced up to 4–5 times. (b) Use multi-input structures [25] (Figure8c). The idea is to reduce the power of the switching-related harmonics of the PWM signal. Given that the voltage at the input of the buck DC–DC converter filter is a PWM voltage waveform, the switching-related harmonics are determined by the amplitude of the pulses. The lower the amplitude of the pulses, the lower the power of the switching-related harmonics at the input of the buck DC–DC converter filter and hence the higher the cutoff frequency of the filter for a specified switching frequency. Moreover, multi-input structures of the buck DC–DC converter mean lower switching losses due to the fact that the Electronics 2020, 9, x FOR PEER REVIEW 11 of 19 transistor switches between lower voltages (in the circuit in Figure7,S 3 switches when S2 is on (c) Useand thereforemultiphase withstands structures the (Figure voltage V8d).CC3 -VTheCC2 ).multiphase operation allows the complete (c) eliminationUse multiphase of the structures switching (Figure frequency8d). The multiphasecomponent operation of PWM allows modulation. the complete Although elimination the sidebandsof the switching components, frequency which component are also ofgenerated PWM modulation. when working Although with a the non- sidebandsconstant components, duty cycle, canwhich only are be also attenuated. generated Moreover, when working this techniqu with a non-constante and the use duty of cycle,a higher can order only beoutput attenuated. filter becomeMoreover, a very this useful technique solution and the for useEAs of [26]. a higher order output filter become a very useful solution (d) Thefor EAscombination [26]. of multiphase and multi-input structures is also possible [27]. (d) The combination of multiphase and multi-input structures is also possible [27].

iLED iLED

L L1 L2 S S V C C VCC C CC D 1 2 D LEDs LEDs

(a) (b)

iLED iLED L LA1 L2 S3 VCC3 D C SA C C2 D LEDs VCC 1 2 DA LEDs LB1 S2 VCC2

D1 S1 SB VCC1 DB

FigureFigure 8. DerivationsDerivations of of the the buck buck DC–DC DC–DC converter. converter. ( (aa)) conventional; conventional; ( (bb)) higher higher order order output output filter; ﬁlter; ((cc)) multi-input; multi-input; ( d) multiphase.

ToTo increase the the transient transient response response of of the the Swit Switching-Modeching-Mode DC–DC DC–DC converter, converter, the the combination combination of switchingof switching and and linear linear stages stages can can provide provide certain certain benefits. beneﬁts. This This solution solution has hasalso also been been explored explored for ET for applications [28,29]. The proposal is based on the following idea: the greater part of the power is processed by a very-fast-output Switching-Mode DC–DC converter, while only extreme variations at the output are tracked by the linear RFPA. A good trade-off between extremely-fast-output response and high power efficiency is achieved using this solution. The two most common linear switching combinations are depicted in Figure 9. Series combination (Figure 9b) is often regarded as less efficient, as all the current through the LEDs has to pass through a dissipative linear stage. However, if the input voltage to this stage is carefully selected, it can outperform the parallel combination [29]. One of the main difficulties is to drive the transistor, S, whose source terminal is not referred to ground.

6. Adaptation of the Linear-Assisted Technique to VLC Taking the block diagram in Figure 9a as a reference, Figure 10 shows the proposed adaptation of the linear-assisted technique for VLC. The proposal follows the same principle: the high efficiency power stage comprises a Class E RFPA instead of a Switching-Mode DC–DC converter, which will be linear-assisted by a linear RFPA.

Electronics 2020, 9, 131 11 of 19

ET applications [28,29]. The proposal is based on the following idea: the greater part of the power is processed by a very-fast-output Switching-Mode DC–DC converter, while only extreme variations at the output are tracked by the linear RFPA. A good trade-off between extremely-fast-output response Electronicsand high 2020 power, 9, x FOR effi PEERciency REVIEW is achieved using this solution. The two most common linear switching12 of 19 combinations are depicted in Figure9. Series combination (Figure9b) is often regarded as less e fficient, as all the current through the LEDs has to pass through a dissipative linear stage. However, if the input i Electronicsvoltage to2020 this, 9, stagex FOR PEER is carefully REVIEW selected, it canLED outperform the parallel combination [29]. One12 of iofLED the 19 main difficulties is to drive the transistor, S, whose source terminal is not referred to ground. V CC Very-fast-output VCC Very-fast-output i response Switching- i LED LED response Switching- iLED Mode dc-dc Splitter Splitter Mode dc-dc V converter converter S v CC VCC i + Very-fast-output vi Very-fast-output + iLED response Switching- iLED response Switching- Mode dc-dc Splitter Splitter Mode dc-dc v Linearconverter v converterLinear S vi v i i + RFPA i + RFPA LEDs LEDs Linear vi vi Linear (a)RFPA RFPA(b) LEDs LEDs Figure 9. Linear-assisted technique applied to VLC-LED drivers. (a) parallel connection scheme; (b) series connection scheme.(a) (b)

TheFigure idea 9. ofLinear-assistedLinear-assisted adapting the technique technique linear-assisted applied applied to toVLC-LEDtechni VLC-LEDque drivers. drivers.for VLC (a) (parallela )resides parallel connection connectionin exploiting scheme; scheme; the(b) light, series connection scheme. allowing(b )summing series connection both scheme.signals in terms of light, instead of electrically. This modification greatly simplifies6. AdaptationThe the idea design. ofof theadapting On Linear-Assisted the theone linear-assisted hand, Technique the design techni to of VLCque the forcircuitry VLC thatresides combines in exploiting both signals the light, is no longer necessary, while, on the other, eliminating the electrical connection avoids the influence allowingTaking summing the block both diagram signals in Figurein terms9a asof alight, reference, instead Figure of electrically. 10 shows the This proposed modification adaptation greatly of between the different circuitry involved. This influence is especially critical in resonant circuits such simplifiesthe linear-assisted the design. technique On the forone VLC. hand, The the proposaldesign of follows the circuitry the same that principle: combines theboth high signals eﬃciency is no as Class E RFPAs, in which efficiency and correct behavior greatly depend on the correct tuning of longerpower stagenecessary, comprises while, a Classon the E RFPAother, instead eliminat ofing a Switching-Mode the electrical connection DC–DC converter, avoids the which influence will be the resonant circuit. betweenlinear-assisted the different by a linear circuitry RFPA. involved. This influence is especially critical in resonant circuits such as Class E RFPAs, in which efficiency and correct behavior greatly depend on the correct tuning of the resonant circuit. Conventional Switching-Mode dc-dc converter iLED1 Conventional i Switching-Mode bias_LED1 Bias control v i dc-dc converterlinear ibias_LED2 ibias_LED1 LED1 V i v CC bias_LED1 PSK Bias control v linear ibias_LED2 ibias_LED1 iLED2 Linear VCC i vPSK - + LED1 i RFPA1 v i bias_LED1 Splitter linear iLED2 LED2 Linear iLED1 - + ibias_LED1 vi RFPA1 v Splitter vClassElinear iLED2 vRX vi VCC vClassE v V RX Class E CC - + RFPA2 vClassE Class E - + RFPA2 vClassE LEDs LEDs Figure 10. Adapting the Linear-Assisted technique to a Class E RFPA as a VLC-LED driver, exploiting Figuresumming 10. Adapting the light. the This Linear-Assisted scheme is only technique valid for PSK to a modulation Class E RFPA schemes. as a VLC-LED driver, exploiting summingFigure 10. the Adapting light. This the scheme Linear-Assisted is only valid technique for PSK to a modulation Class E RFPA schemes. as a VLC-LED driver, exploiting summing the light. This scheme is only valid for PSK modulation schemes. In this case (Figure 10), instead of having one string of LEDs, the load is split into two strings, each oneIn beingthis case connected (Figure 10),to the instead Class of E havingRFPA or one th est linearring of RFPA, LEDs, respectively.the load is split Both into strings two strings, of LEDs areeach biased one beingby a conventionalconnected to the Switching-Mode Class E RFPA or DC the–DC linear converter, RFPA, respectively. which controls Both stringsthe bias of LEDscurrent (ibias_LED1are biased = ibias_LED2 by a). conventional By controlling Switching-Mode the bias current, DC both–DC LEDconverter, strings which are always controls working the bias within current the linear(ibias_LED1 region, = ibias_LED2 regardless). By controlling of the temperature the bias current, effect. Asboth can LED be stringsseen in are Figure always 10, working each string within of LEDsthe haslinear the same region, bias regardless current, thusof the generating temperature the effect. same AsDC can light be intensityseen in Figure in order 10, toeach perform string theof LEDs task of illumination.has the same Moreover, bias current, due thus to thegenerating connection the same of each DC stringlight intensity of LEDs in to order either to theperform Class the E RFPAtask of or linearillumination. RFPA, its Moreover, combination due generates to the connection the AC lightof each intensity string ofrelated LEDs toto theeither communication the Class E RFPA task. or linear RFPA, its combination generates the AC light intensity related to the communication task.

Electronics 2020, 9, 131 12 of 19

The idea of adapting the linear-assisted technique for VLC resides in exploiting the light, allowing summing both signals in terms of light, instead of electrically. This modification greatly simplifies the design. On the one hand, the design of the circuitry that combines both signals is no longer necessary, while, on the other, eliminating the electrical connection avoids the influence between the different circuitry involved. This influence is especially critical in resonant circuits such as Class E RFPAs, in which efficiency and correct behavior greatly depend on the correct tuning of the resonant circuit. In this case (Figure 10), instead of having one string of LEDs, the load is split into two strings, each one being connected to the Class E RFPA or the linear RFPA, respectively. Both strings of LEDs are biased by a conventional Switching-Mode DC–DC converter, which controls the bias current (ibias_LED1 = ibias_LED2). By controlling the bias current, both LED strings are always working within the linear region, regardless of the temperature eﬀect. As can be seen in Figure 10, each string of LEDs has the same bias current, thus generating the same DC light intensity in order to perform the task of illumination. Moreover, due to the connection of each string of LEDs to either the Class E RFPA or linear RFPA, its combination generates the AC light intensity related to the communication task. With this proposed adaptation, only SCM schemes can be reproduced—in this particular case, only a Phase Shift-Keying (PSK) modulation scheme. The Class E RFPA is used to provide the great amount of power associated with the communication signal at high eﬃciency. The linear RFPA only supplies high slew-rate parts (phase changes) and corrects the distortion at the output.

7. Circuitry Design and Experimental Section This section reports the design, building, and experimental validation of two prototypes as a proof of concept of the adaptation of the two aforementioned techniques for VLC.

7.1. Outphasing Technique Using Two Class E RFPAs (Exploiting Summing the Light) The ﬁrst approach is performed to adapt the Outphasing technique to VLC by exploiting summing the light. Figure 11 shows the complete circuit of the transmitter made up of: Two Class E RFPAs, two LED strings, a DC–DC converter, and an FPGA.

7.1.1. Circuitry Design The Class E RFPAs were built and each one was connected to an LED string of 8 XLamp MX-3 HB-LEDs from Cree (Durham, North Carolina, USA). An external DC–DC converter is necessary to bias and control the average current through each LED string. In order to make the LED string work in the middle of its linear region, the average current is kept at 250 mA for each string. The entire process is controlled by an Artix-7 FPGA from Digilent (Pullman, WA, USA). Control of the average current is also necessary in order to avoid the temperature drift suffered by LEDs. This means that the DC–DC converter ensures that the LED always works in its linear region, regardless of the temperature. It is worth mentioning that the DC–DC converter does not required any adaptation and can be implemented using the same topologies used in normal LED drivers. The only difference is that the DC–DC converter is connected through a Lbias = 5.6 µH biasing inductor acting as a low-pass filter. The value of the inductor is obtained as follows: RL Lbias = , (1) 2π fsw/10 in order to have a cutoff frequency 10 times lower than the communication signal frequency. RL = 17.6 Ω is the resistive equivalent of the LED string and fsw is the communication frequency. This enables the DC biasing current to pass to the LEDs and also provides a high impedance path for the communication signal. Electronics 2020, 9, x FOR PEER REVIEW 13 of 19

With this proposed adaptation, only SCM schemes can be reproduced—in this particular case, only a Phase Shift-Keying (PSK) modulation scheme. The Class E RFPA is used to provide the great amount of power associated with the communication signal at high efficiency. The linear RFPA only supplies high slew-rate parts (phase changes) and corrects the distortion at the output.

7.1. Outphasing Technique Using Two Class E RFPAs (Exploiting Summing the Light) The first approach is performed to adapt the Outphasing technique to VLC by exploiting summing the light. Figure 11 shows the complete circuit of the transmitter made up of: Two Class E RFPAs, two LED strings, a DC–DC converter, and an FPGA.

7.1.1. Circuitry Design The Class E RFPAs were built and each one was connected to an LED string of 8 XLamp MX-3 HB-LEDs from Cree (Durham, North Carolina, USA). An external DC–DC converter is necessary to bias and control the average current through each LED string. In order to make the LED string work in the middle of its linear region, the average current is kept at 250 mA for each string. The entire process is controlled by an Artix-7 FPGA from Digilent (Pullman, WA, USA). Control of the average current is also necessary in order to avoid the temperature drift suffered by LEDs. This means that the DC–DC converter ensures that the LED always works in its linear region, regardless of the temperature. It is worth mentioning that the DC–DC converter does not required any adaptation and can be implemented using the same topologies used in normal LED drivers. The only difference is that the DC–DC converter is connected through a 𝐿 = 5.6 µH biasing inductor acting as a low- pass filter. The value of the inductor is obtained as follows: 𝐿 = , (1) in order to have a cutoff frequency 10 times lower than the communication signal frequency. 𝑅 = 17.6 Ω is the resistive equivalent of the LED string and 𝑓 is the communication frequency. This enablesElectronics the2020 DC, 9, 131biasing current to pass to the LEDs and also provides a high impedance path13 for of 19the communication signal.

Figure 11. Experimental setup of the adaptation of the Outphasing technique using two Class E RFPAs. Figure 11. Experimental setup of the adaptation of the Outphasing technique using two Class E 7.1.2.RFPAs. Communication Scheme

7.1.2. CommunicationThe first part of Scheme the communication design is the selection of the communication scheme to be used. Amplitude and phase modulation are chosen to test the capability of the adaptation of theThe Outphasing first part technique. of the communication The communication design schemeis the selection used is 16-QAMof the communication digital modulation scheme with to abe used.carrier Amplitude frequency and of 5 phase MHz. Themodulation symbol periodare chosen is five to signal test the periods, capability reaching of the a bit-rate adaptation of 4 Mbps. of the OutphasingThe communication technique. scheme The communication defines the minimum scheme bandwidth used is 16-QAM necessary digital for the modulation amplifiers. with According a carrier frequencyto [30], an of approximation 5 MHz. The ofsymbol the minimum period is bandwidth, five signalBW periods,min, of the reaching modulation a bit-rate is of 4 Mbps. The communication scheme defines the minimum bandwidth necessary for the amplifiers. According to 2 [30], an approximation of the minimumBW bandwidth,min = = BW2 MHzmin, of, the modulation is (2) Ts

where Ts = 1 µs is the symbol period.

7.1.3. Class E RFPA Design The circuit of the Class E RFPAs that comprises the Outphasing circuit is shown in Figure 11. Each Class E RFPA comprises a PD84010S-E RF MOSFET, a high speed EL7155 driver, a bias inductor, and an output resonant circuit. The high switching frequency needed for the communication signal requires the use of a high frequency RF MOSFET and a high speed driver. The bias inductor works as a low-pass filter to bias the MOSFET and creates a high impedance path for the communication signal. This means that no communication components flow through the power supply. The switching frequency of fsw = 5 MHz is chosen in accordance with the communication signal. The bandwidth of the Class E RFPA depends on the quality factor, Q, of the output resonant filter. The higher the Q, the smaller the bandwidth. According to the definition of Q

fsw 5 MHz Q = = = 2.5, (3) BW 2 MHz the value of Q is 2.5 for the selected bandwidth and switching frequency. Following the design guidelines [31], the values of L, C and C1 that make up the output ﬁlter of the Class E RFPA are obtained as follows: 2.85RL 0.7124 0.219 L = , C = , C1 = , (4) 2π fsw 2πRL fsw 2πRL fsw and the values are given in Figure 11.

7.1.4. Experimental Results

𝐵𝑊 = =2 𝑀𝐻𝑧, (2)

where Ts = 1 µs is the symbol period.

7.1.3. Class E RFPA Design The circuit of the Class E RFPAs that comprises the Outphasing circuit is shown in Figure 11. Each Class E RFPA comprises a PD84010S-E RF MOSFET, a high speed EL7155 driver, a bias inductor, and an output resonant circuit. The high switching frequency needed for the communication signal requires the use of a high frequency RF MOSFET and a high speed driver. The bias inductor works as a low-pass filter to bias the MOSFET and creates a high impedance path for the communication signal. This means that no communication components flow through the power supply. The switching frequency of fsw = 5 MHz is chosen in accordance with the communication signal. The bandwidth of the Class E RFPA depends on the quality factor, Q, of the output resonant filter. The higher the Q, the smaller the bandwidth. According to the definition of Q 𝑄= = =2.5, (3) the value of Q is 2.5 for the selected bandwidth and switching frequency. Following the design

guidelines [31], the values of 𝐿, 𝐶 and 𝐶 that make up the output filter of the Class E RFPA are obtained as follows:

. . . 𝐿= ,𝐶 = ,𝐶 = , (4)

and the values are given in Figure 11.

7.1.4. Experimental Results

The main waveforms of the light-outphasing process are shown in Figure 12a. Iph1 and Iph2 are the currents through each LED string. The light emitted by each string is proportional to the current Electronicsflowing2020 through, 9, 131 it. As the amplitude of the sine waves is kept constant, the amplitude of the14 light of 19 emitted by them is also constant. Vrx is the light received by an optical receiver, PDA10A-EC, which receives the contribution of both strings. It can be seen in the figure that the received light is able to reproducereproduce amplitude amplitude changes changes even even though though the the individual individual lights lights of both of both strings strings are constant. are constant. This eThisffect iseffect achieved is achieved due to thedue phaseto the shiftphas betweene shift between the two the waveforms. two waveforms. TheThe waveforms waveforms of of the the Class Class E E RFPA RFPA when when a a phase phase step step occurs occurs are are shown shown in in Figure Figure 12 12b.b. A A Class Class E RFPAE RFPA is able is able to reachto reach high high effi efficiencyciency due due to to the the zero zero voltage voltage switching switching (ZVS)(ZVS) achieved in the the main main switch.switch. The The condition condition ofof ZVSZVS isis metmet beforebefore andand afterafter a phase shift in the the Class Class E E RFPA, RFPA, as as can can be be seen seen inin the the drain drain to to source source voltage, voltage, VVdsds,, butbut thenthen aa phasephase shift occurs, ZVS ZVS is is lost, lost, and and hence hence the the power power effiefficiencyciency decreases. decreases. Moreover, Moreover, the the higher higher thethe bit-rate,bit-rate, thethe moremore the phase shifts per per second second and and hence hence thethe lower lower the the e efficiency.fficiency.Accordingly, Accordingly, aa trade-otrade-offff betweenbetween eefficiencyfficiency and bit-rate has has to to be be defined defined in in orderorder to to keep keep the the e efficiencyfficiency and and the the bit-rate bit-rate withinwithin admissibleadmissible values.values.

(a) (b)

Figure 12. (a) communication waveforms of the light-Outphasing system; (b) main waveforms of the Class E RFPA during a phase shift.

The proposed transmitter reproduces 16-QAM digital modulation, achieving a bit-rate of up to 4 Mbps at a distance of up to 1 m. The Class E reaches an efficiency of 78% (which is the signal generation efficiency). This efficiency is measured by comparing the input DC power delivered by the 9 V biasing voltages, shown in Figure 11, and the RMS value of the communication signal in the output. The communication signal is being recorded by an oscilloscope and post processed by computer. The efficiency of the Class E is higher than the maximum efficiency reached by the Class A and B alternatives for the 16-QAM modulation. The overall efficiency reached by the whole prototype is 92%, when the signal and the lighting tasks are considered. The efficiency of the lighting task is calculated by comparing the input power of the DC–DC converter and its output power, which is the biasing power on the LED strings.

7.2. Linear-Assisted Technique for a Class E RFPA (Exploiting Summing the Light) The second experimental approach is made to adapt the linear-assisted technique for a Class E RFPA. A linear-assisted Class E amplifier was suitably designed and built to provide experimental results (Figure 13). The circuitry is divided into four parts: Class E amplifier, error amplifier, linear amplifier, and external DC–DC converter. The design resides in the idea of summing the signals in terms of light, instead of electrically.

7.2.1. Circuitry Design The Class E amplifier and the linear amplifier are each connected to an individual LED string comprising 8 XLamp MX-3 HB-LEDs. Both strings are biased by an external DC–DC converter which controls the average current through them. The DC–DC converter can be implemented by using a normal LED driving stage. The only modification required are the two biasing inductors Lbias = 28 µH Electronics 2020, 9, 131 15 of 19

that are used to create a biasing path for the biasing current and to avoid any communication current passing through the converter. The inductor is obtained as follows:

RL Lbias = , (5) 2π fsw/10

in order to have a cutoﬀ frequency 10 times lower than the communication signal frequency. RL = 17.6 Ω is the resistive equivalent of the LED string and fsw is the communication frequency. The Class E RFPA reproduces the communication signal, while the linear ampliﬁer (LT1206) reproduces the error signal between the ideal communication signal and the one delivered by the Class ElectronicsE RFPA. 2020 The, 9, x entire FOR PEER process REVIEW is controlled by an Artix-7 FPGA. 16 of 19

FigureFigure 13. 13. ExperimentalExperimental setup setup of ofthe the adaptation adaptation of of the the linear-assisted linear-assisted technique technique for for a a Class Class E E RFPA. RFPA. 7.2.2. Communication Scheme 7.2.2. Communication Scheme The first part of the design consists in the selection of the communication scheme to be used. In this case,The digital first part phase of modulation the design (16-PSK)consists in was the chosen. selection The of Class the communication E RFPA reaches itsscheme maximum to be efficiencyused. In thiswhen case, the digital amplitude phaseof modulation the communication (16-PSK) signalwas chosen. does not The change Class over E RFPA time. reaches In phase its modulation, maximum efficiencythe amplitude when isthe kept amplitude constant whileof the the communication phase varies over signal time. does The chosennot change carrier over frequency time. In is 1phase MHz, modulation,the modulation the amplitude uses 16 different is kept symbols, constant whichwhile encodethe phase four varies bits each, over giving time. aThe maximum chosen bit-ratecarrier frequencyof 0.5 Mbps. is 1 MHz, In line the with modulation what was uses previously 16 different done symbols, and according which toencode [30], an four approximation bits each, giving of the a maximum bit-rate of 0.5 Mbps. In line with what was previously done and according to [30], an minimum bandwidth, BWmin, of the modulation is approximation of the minimum bandwidth, BWmin, of the modulation is 2 𝐵𝑊 = == 250 𝐾𝐻𝑧, BWmin 250 KHz,(6) (6) Ts where Ts = 8 µs is the symbol period. where Ts = 8 µs is the symbol period.

7.2.3.7.2.3. Class Class E RFPA E RFPA Design Design FigureFigure 13 13 shows shows the the Class Class E ERFPA RFPA circuit. circuit. It It comprises comprises the the same same RF RF PD84010S-E PD84010S-E MOSFET MOSFET and and EL7155EL7155 high high speed speed driver driver used used previously, previously, a abias bias inductor, inductor, and and a a resonant resonant filter. filter. The The switching switching frequencyfrequency is ischosen chosen according according to to the the modulation modulation sc scheme;heme; in in this this case, case, 1 1 MHz MHz is is used. used. The The resonant resonant filterfilter is isdesigned designed according according to to the the previously previously calculated calculated modulation modulation BWBW. .According According to to the the definition definition of ofQ,Q , fsw 1 MHz Q𝑄== = = = 4,=4, (7) (7) BW 250 kHz thethe necessary necessary Q Qvaluevalue toto provide provide enough enough BWBW forfor the the communications communications signal signal is is 4. 4. Following Following the the design design guidelinesguidelines in in[31], [31 the], the values values of of𝐿,L, 𝐶,C, 𝑎𝑛𝑑and C 𝐶 1 thatthat make make up up the the output output filter filter of of the the Class Class E E are are obtained obtained as asfollows: follows: 3.75RL 0.4166 0.2150 L =. , C =. , C.1 = , (8) 𝐿= 2π,𝐶fsw = 2πR,𝐶Lfsw= 2,π RL fsw (8) and given in Figure 13. In order to be able to measure the error on the communication signal delivered by the Class E RFPA, a shunt resistance, Rsense, is placed in series with the LED string of the Class E RFPA. The voltage across the shunt is used in the error amplifier, generating verr.

7.2.4. Linear-Assisted Circuit Design

The linear RFPA and the error amplifier are shown in Figure 13. The signal, vclass-E, passes through a 2nd-order Butterworth high-pass filter (HPF) in order to obtain only the communication signal, without the biasing contribution. The cutoff frequency is 10 kHz. Moreover, the FPGA generates the input signal modulated in PWM. In order to obtain the ideal communication signal, vin, from the PWM, a 2nd-order Butterworth HPF and low-pass filter (LPF) are respectively needed to eliminate the DC component and the PWM harmonics. The cutout frequencies are 10 kHz and 3 MHz,

Electronics 2020, 9, 131 16 of 19 and given in Figure 13. In order to be able to measure the error on the communication signal delivered by the Class E RFPA, a shunt resistance, Rsense, is placed in series with the LED string of the Class E RFPA. The voltage across the shunt is used in the error ampliﬁer, generating verr.

7.2.4. Linear-Assisted Circuit Design

The linear RFPA and the error amplifier are shown in Figure 13. The signal, vclass-E, passes through a 2nd-order Butterworth high-pass filter (HPF) in order to obtain only the communication signal, without the biasing contribution. The cutoff frequency is 10 kHz. Moreover, the FPGA generates the input signal modulated in PWM. In order to obtain the ideal communication signal, vin, from the PWM, a 2nd-order Butterworth HPF and low-pass filter (LPF) are respectively needed to eliminate the Electronics 2020, 9, x FOR PEER REVIEW 17 of 19 DC component and the PWM harmonics. The cutout frequencies are 10 kHz and 3 MHz, respectively.

Then,respectively. vclass-E Then,is subtracted vclass-E is fromsubtracted vin, thereby from v obtainingin, thereby the obtaining error signal, the error verr. Thesignal, error verr. signal, The error verr, signal,is ampliﬁed verr, is by amplified the linear by RFPA, the linear LT1206, RFPA, whose LT1206, output whose signal output is delivered signal tois adelivered string of to eight a string XLamp of eightMX-3 XLamp HB-LEDs. MX-3 HB-LEDs.

7.2.5. Experimental Results Figure 14 shows the waveforms of the communicationcommunication system reproducing 16-PSK digital modulation. The carrier frequency is 1 MHz and the achieved bit-rate is 0.5 Mbps. vin shows the modulation. The carrier frequency is 1 MHz and the achieved bit-rate is 0.5 Mbps. vin shows the desired and ideal communication signal that is used as a reference. vclass-E is the signal delivered by the desired and ideal communication signal that is used as a reference. vclass-E is the signal delivered by Class E RFPA and measured across the shunt resistance, Rsense. By subtracting vclass-E from vin, the error the Class E RFPA and measured across the shunt resistance, Rsense. By subtracting vclass-E from vin, the verr is obtained, ampliﬁed by the linear RFPA and delivered to the LED strings. An optical receiver, error verr is obtained, amplified by the linear RFPA and delivered to the LED strings. An optical PDA10A-EC, receives the vout signal, the result of the sum in light of vclass-E and verr. The addition of receiver, PDA10A-EC, receives the vout signal, the result of the sum in light of vclass-E and verr. The verr makes vout closer to the ideal signal, vin, by reducing the distortion and error introduced by the addition of verr makes vout closer to the ideal signal, vin, by reducing the distortion and error introducedClass E RFPA, by especiallythe Class E during RFPA, phase especially shifts. during phase shifts.

Figure 14. CommunicationCommunication waveforms waveforms of the linear-assisted transmitter.

The proposal is is able able to to reproduce reproduce 16-PSK 16-PSK phase phase digital digital modulation modulation with with a acarrier carrier frequency frequency of of 1 1MHz, MHz, achieving achieving a abit-rate bit-rate of of 0.5 0.5 Mbps. Mbps. Since Since the the Class Class E E amplifier amplifier delivers delivers most most of of the the power at a high efficiency efficiency and the linear amplifier amplifier stage only de deliverslivers the error signal, the electrical efficiency efficiency of the signal generation generation is is 75%. 75%. This This efficiency efficiency is is being being calculated calculated by by comparing comparing the the bias bias input input power power of bothof both amplifiers amplifiers (Class (Class E Eand and linear linear amplifier) amplifier) and and comparing comparing it it with with the the RMS RMS value value of of the output communication signal. The The communica communicationtion signal signal is is being being recorded recorded by by an oscilloscope and post processed byby computer.computer. TheThe signal signal generation generation e ffiefficienciencycy is is higher higher than than the the alternatives alternatives based based solely solely on ona linear a linear amplifier. amplifier. When When the the signal signal and and biasing biasing power power are are taken taken into into account,account, thethe overall efficiency efficiency reached is up to 85%.

8. Conclusions VLC technology is a promising alternative for alleviating the congested RF spectrum, although one of the main drawbacks of this technology is the low power efficiency of the driver when the communication capability is added. Most of the research work related to VLC focuses on increasing the bit-rate, more complex modulations, or communication schemes, among other issues, whereas little has been published on the improvement of power efficiency. This paper presents the adaptation of some RF techniques aimed at improving overall efficiency. Two different techniques are developed and adapted to VLC by exploiting summing the light: Outphasing and linear assistance. Both techniques are based on summing two signals, which is done electrically. By using the sum of the light in both techniques, the design is greatly simplified. On the one hand, an adaptation of the Outphasing technique is presented based on two Class E RFPAs. The proposed transmitter reproduces 16-QAM digital modulation, achieving a bit-rate of up to 4 Mbps at a distance of up to 1 m. The prototype reaches an electrical efficiency of 78% in terms of

Electronics 2020, 9, 131 17 of 19

8. Conclusions VLC technology is a promising alternative for alleviating the congested RF spectrum, although one of the main drawbacks of this technology is the low power efficiency of the driver when the communication capability is added. Most of the research work related to VLC focuses on increasing the bit-rate, more complex modulations, or communication schemes, among other issues, whereas little has been published on the improvement of power efficiency. This paper presents the adaptation of some RF techniques aimed at improving overall efficiency. Two different techniques are developed and adapted to VLC by exploiting summing the light: Outphasing and linear assistance. Both techniques are based on summing two signals, which is done electrically. By using the sum of the light in both techniques, the design is greatly simplified. On the one hand, an adaptation of the Outphasing technique is presented based on two Class E RFPAs. The proposed transmitter reproduces 16-QAM digital modulation, achieving a bit-rate of up to 4 Mbps at a distance of up to 1 m. The prototype reaches an electrical efficiency of 78% in terms of signal generation (higher than the Class A and B maximum efficiency for 16-QAM modulation) and an overall efficiency of 92% when the communication and lighting tasks are considered. On the other hand, a linear-assisted VLC transmitter comprising a Class E amplifier and a linear amplifier is presented. The proposal is able to reproduce 16-PSK phase digital modulation with a carrier frequency of 1 MHz, achieving a bit-rate of 0.5 Mbps. Due to the fact that the Class E amplifier delivers most of the power and the linear amplifier stage only delivers the error signal, the proposal achieves an electrical efficiency of 75% in terms of signal generation (higher than the alternatives based solely on a linear amplifier) and an overall efficiency of 85% considering the signal and the biasing contribution of the DC–DC converter.

Author Contributions: J.R. and P.F.M. conceived and performed the experiments. D.G.A. and J.S. developed the adaptation of solutions to VLC and the design procedure. D.G.A. and D.G.L. analyzed the data and wrote the paper. V.F.R. and J.M. contributed with the definition, the design, and the evaluation of the experiments and the revision of the paper. All authors have read and agreed to the published version of the manuscript. Funding: This work was funded by the Spanish Government under Project MINECO-17-DPI2016-75760-R, by the Principado de Asturias under Projects SV-PA-17-RIS3-4 and FC-GRUPIN-IDI/2018/000179, and the scholarship BP17-91, as well as by European Regional Development Fund (ERDF) grants. Conflicts of Interest: The authors declare no conflict of interest.

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