Enhanced visible light communication modulators with dual feedback control

This paper presents a modulator for Optical Wireless Communication (OWC) via LEDs, controlled by a dual closed loop. While the potential communication speed and performance of OWC has received a lot of research attention, the design of appropriate modulator circuits has not been studied so widely. This paper addresses control aspects to minimize power losses and to ensure automatic adaptation to component spreads or aging, for ’high power’ LED emitters. We initially address a ’low bandwidth’ system, particularly suited for indoor camera-based detection and then a ’higher bandwidth’ one for LiFi data communication. In contrast to maximizing data throughput as it applies to high speed modulators, low speed modulators aim at high power and high efficiency in general lighting applications. Specifically, for the high-power modulator, we propose a control strategy that allows a wide range of combinations of LED types and LED power supplies. The power consumption of the modulator is minimized by voltage margin setting which is automatically adapt to the modulation and the LED type using the non-linear detector. The common industry practice is to use constant-current LED drivers for powering LEDs. After creating a theoretical framework of some generality to evaluate designs and control loops with rationals and verified models, we realize modulator designs that are supplied by constant current LED drivers and verify their performance by simulation and measurements

Efficient amplitude modulator and ripple canceller for visible light communication

Power loss is an important design factor for Visible Light Communication (VLC). Dissipating only a few percent of the illumination power can still result into a few watts of heat generation which requires additional provisions for cooling the modulator circuit. We show that a transistor in series with LED can be used in linear operation to apply amplitude modulation which only marginally sacrifices efficiency. Yet, this requires a novel active control loop. We report its performance and frequency response for VLC modulation. We show that the modulator also effectively cancels mains ripple. Based on our experimental experience, we also propose a model to quantify the power losses and verify these for pulse amplitude modulation

LED Modelling for Efficient LiFi Modulator Design to Accelerate OOK

This paper studies the response of a Light Emitting Diode (LED) to a stepwise change of the power delivered by a current or voltage supply. This study contributes to the design of efficient electronic circuits to generate On-Off Keying modulation for optical wireless communication using LEDs as an emitter. We combine the Shockley diode equation with the ABC model for electron-hole recombination to propose a new model for rise and fall curves. The fall curve differs for an open or shunted LED, as in the latter case, the carriers (electrons and holes) are pulled out of the LED junction. We quantify these effects and exploit this difference to propose a tristate optimized modulator circuit, which is highly power efficient. In this design, we allow modulating currents to be switched a bit earlier or later than then the bit transition itself

Enhanced visible light communication modulators with dual feedback control

This paper presents a modulator for Optical Wireless Communication (OWC) via LEDs, controlled by a dual closed loop. While the potential communication speed and performance of OWC has received a lot of research attention, the design of appropriate modulator circuits has not been studied so widely. This paper addresses control aspects to minimize power losses and to ensure automatic adaptation to component spreads or aging, for ’high power’ LED emitters. We initially address a ’low bandwidth’ system, particularly suited for indoor camera-based detection and then a ’higher bandwidth’ one for LiFi data communication. In contrast to maximizing data throughput as it applies to high speed modulators, low speed modulators aim at high power and high efficiency in general lighting applications. Specifically, for the high-power modulator, we propose a control strategy that allows a wide range of combinations of LED types and LED power supplies. The power consumption of the modulator is minimized by voltage margin setting which is automatically adapt to the modulation and the LED type using the non-linear detector. The common industry practice is to use constant-current LED drivers for powering LEDs. After creating a theoretical framework of some generality to evaluate designs and control loops with rationals and verified models, we realize modulator designs that are supplied by constant current LED drivers and verify their performance by simulation and measurements

Novel post-distortion to mitigate LED nonlinearity in high-speed visible light communications

This paper addresses dynamic nonlinear effects in the response of typical illumination Light Emitting Diodes (LEDs) to increase the reliability and data rate in Visible Light Communication (VLC) systems. These power LEDs have a limited bandwidth of only several MHz. To conceive a practical receiver, we describe the LED transient response by a nonlinear dynamic differential equation from the physical mechanisms in the Quantum Well (QW) of Double Hetero-structured (DH) LEDs. It includes the transient nonlinear relation between the input current and injection electron concentration, as governed by the dynamic rate equation. Also, we consider the static nonlinear (typically quadratic) relation between the injection electron concentration and output optical power is described by a square operator. Further we consider the static nonlinear relation between the injection current and output optical power, caused by the efficiency droop. We propose a novel post-distorter to overcome LED nonlinearities for high-speed Pulse Amplitude Modulation (PAM)-4 systems. Its performance is validated with measurements of a commercial blue LED

Efficient amplitude modulator and ripple canceller for visible light communication

Power loss is an important design factor for Visible Light Communication (VLC). Dissipating only a few percent of the illumination power can still result into a few watts of heat generation which requires additional provisions for cooling the modulator circuit. We show that a transistor in series with LED can be used in linear operation to apply amplitude modulation which only marginally sacrifices efficiency. Yet, this requires a novel active control loop. We report its performance and frequency response for VLC modulation. We show that the modulator also effectively cancels mains ripple. Based on our experimental experience, we also propose a model to quantify the power losses and verify these for pulse amplitude modulation

Characterization of dynamic distortion in LED light output for optical wireless communications

Light-emitting diodes (LEDs) are widely used for data transmission in emerging optical wireless communications (OWC) systems. This paper analyzes the physical processes that limit the bandwidth and cause nonlinearities in the light output of modern, high-efficiency LEDs. The processes of carrier transport, as well as carrier storage, recombination, and leakage in the active region appear to affect the communications performance, but such purely physics-based models are not yet commonly considered in the algorithms to optimize OWC systems. Using a dynamic modeling of these phenomena, we compile a (invertable) signal processing model that describes the signal distortion and a parameter estimation procedure that is feasible in an operational communications link. We combine multiple approaches for steady-state and dynamic characterization to estimate such LED parameters. We verify that, for a high-efficiency blue GaN LED, the models become sufficiently accurate to allow digital compensation. We compare the simulation results using the model against optical measurements of harmonic distortion and against measurements of the LED response to a deep rectangular current modulation. We show how the topology of the model can be simplified, address the self-calibration techniques, and discuss the limits of the presented approach. The model is suitable for the creation of improved nonlinear equalizers to enhance the achievable bit rate in LED-based OWC systems and we believe it is significantly more realistic than LED models commonly used in communications systems

Modeling and analysis of transmitter performance in visible light communications

This work addresses the power penalty in a Visible Light Communication (VLC) emitter that re-uses illumination Light Emitting Diodes (LEDs) for high-speed communications. We quantify the effect of modulation depth for two widely explored modulation schemes in VLC, namely Orthogonal Frequency Division Multiplexing (OFDM) and Pulse Amplitude Modulation (PAM). Extra power is dissipated in the LED and in the modulator due to such modulation. Two popular highspeed VLC transmitter topologies, namely a Bias Tee (Bias-T) and a Serial FET (Serial-F), are compared in terms of efficiency, Extra Energy Per Symbol/Bit (EEPS/B) and complexity. This work presents a theoretical model and compares it to simulation results. We show that PAM outperforms OFDM in terms of better Bit Error Rate (BER), larger dynamic range for modulation depth, less extra power loss, thus higher power efficiency for communication over an Additive White Gaussian Noise (AWGN) channel

Dynamic response-based LEDs health and temperature monitoring

This paper presents a number of novel methods to measure the junction temperature and to estimate the health of gallium nitride light-emitting diodes (LEDs). The methods are based on measurements of the dynamic impedance and optical output. Our experimental analysis reveals temperature sensitive parameters of the electrical and optical responses. Moreover, a correlation between the non-radiative current characterizing the active region defects and the small-signal impedance is shown. The demonstrated methods can be applied to enhance existing temperature-monitoring techniques. The derived dependencies also build a foundation for advanced in-field health monitoring of the LEDs using the infrastructure of visible light communication systems. Such methods and techniques are valuable for predictive maintenance of solid state lighting systems

Novel post-distortion to mitigate LED nonlinearity in high-speed visible light communications

This paper addresses dynamic nonlinear effects in the response of typical illumination Light Emitting Diodes (LEDs) to increase the reliability and data rate in Visible Light Communication (VLC) systems. These power LEDs have a limited bandwidth of only several MHz. To conceive a practical receiver, we describe the LED transient response by a nonlinear dynamic differential equation from the physical mechanisms in the Quantum Well (QW) of Double Hetero-structured (DH) LEDs. It includes the transient nonlinear relation between the input current and injection electron concentration, as governed by the dynamic rate equation. Also, we consider the static nonlinear (typically quadratic) relation between the injection electron concentration and output optical power is described by a square operator. Further we consider the static nonlinear relation between the injection current and output optical power, caused by the efficiency droop. We propose a novel post-distorter to overcome LED nonlinearities for high-speed Pulse Amplitude Modulation (PAM)-4 systems. Its performance is validated with measurements of a commercial blue LED