Efficiency of power loading strategies for visible light communication

Optical Wireless Communication via LEDs faces the problem that LEDs are band-limited but show a gentle roll off above the 3 dB bandwidth. This requires an appropriate power loading of the different frequency components of the signal spectrum. OFDM is an attractive modulation method to appropriately or even optimally distribute the available power among its sub-carriers. It is now commonly accepted and experimentally verified that the LED transfer function declines exponentially with frequency. This motivated us to derive the theoretical capacity of this channel and evaluate to what extent waterfilling and uniform power loading strategies perform w.r.t. this capacity bound. For waterfilling, we managed to derive a novel closed-formula for the relation between the power budget and the achievable data rate and for the optimum bandwidth to be used. Yet, in practice these adaptive loading schemes need to achieve good performance with only discrete choices for QAM constellations. To approach the effect of waterfilling, Hughes-Hartogs proposed an iterative scheme. We generalized this to also find optimal signal bandwidth, power and constellation per sub-carrier for uniform power loading. We found that, for LEDs, waterfilling does not yield any significantly larger rate than the simpler uniform loading

Capacity of the first-order low-pass channel with power constraint

This paper bounds the performance of a uniformpower\u3cbr/\u3eand a waterfilling algorithm for low-pass channels. It is\u3cbr/\u3eshown that a visible light communication channel with double\u3cbr/\u3ehetero-structure LEDs in their linear regime can be approximated\u3cbr/\u3eby a first order low-pass channel. Closed form expressions\u3cbr/\u3eare derived to relate the maximum achievable data rate (capacity)\u3cbr/\u3eand the corresponding optimum bandwidth usage to the input\u3cbr/\u3etotal (modulation) power

Modeling and compensating dynamic nonlinearities in LED photon-emission rates to enhance OWC

\u3cp\u3eLEDs can be modulated at relatively high speeds to support wireless optical data communication (OWC). Yet, particularly LEDs optimized for illumination act as a non-linear low-pass communication channel. It has become clear in recent literature that their non-linearity and low-pass behavior cannot be seen as two separable, cascaded mechanisms. Although standard nonlinear equalizer schemes, e.g. based on Volterra Series, have been proposed and tested before, our recent research results show that a more dedicated approach in which we specifically analyze the hole-electron recombination mechanisms, yield a very effective and computationally-efficient compensation approach. In this manuscript, we will review the non-linear differential equations for photon emissions, its electrical equivalent circuit and a discrete-time variant with delays and non-linearities. This can be inverted, in the sense that we can actively eliminate or mitigate the non-linear dynamic LED distortion by adequate signal processing. We propose an aggressive simplification of the compensation circuit that allows us to use a relatively simple structure with only a couple of parameters.\u3c/p\u3

Sub-carrier loading strategies for DCO-OFDM LED communication

LEDs, particularly those used for Visible Light Communications (VLC), have a limited bandwidth, while above their 3 dB bandwidth, the roll-off is relatively gentle. If the modulation bandwidth would be limited to the 3 dB LED bandwidth, the achievable rate would be unacceptably constrained. Hence, effective communication systems need to optimize the use of bandwidth significantly above this 3 dB point. Orthogonal Frequency Division Multiplexing (OFDM) is a popular method to fine-tune the amount of power and constellation as a function of the channel response over different frequencies. Various power and bit loading strategies have been proposed and simulated in literature, but their performance was not captured in expressions. This manuscript derives these for optimal waterfilling, uniform and pre-emphasized power loading for the LED channel, that severely attenuates high frequencies. We also investigate the influence of practical discrete constellations and verify our new results experimentally. Interestingly, simple uniform loading only falls less than 1∼2% short of the throughput achieved by waterfilling, but when we restrict OFDM to discrete QAM constellation sizes, the penalty for uniform loading is 1.5 dB. Inspired by the good performance of uniform power loading, we propose an algorithm to find the best discrete bit loading for uniform power within an optimized band. As pre-emphasis is nonetheless attractive because a flattened channel does not need adaptive sub-carrier loading, we quantify its penalty. This can be modest provided that the system can adapt its transmit bandwidth, thereby adaptively switching upper sub-carriers to zero power

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

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

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

Effect of blue filter on the SNR and data rate for indoor visible light communication system

\u3cp\u3eFor an indoor Visible Light Communication system with phosphorescent LEDs, we show a blue receive filter is particularly suitable for cool-white LEDs but less attractive for warm-white LEDs. Distance and sunlight also influence whether a blue filter should be used.\u3c/p\u3

DC-bias for Optical OFDM in Visible Light Communications

The requirement of a DC-bias is known to make DC-biased Optical Orthogonal Frequency Division Multiplexing (DCO-OFDM) less energy efficient. This can be improved by Asymmetrically Clipped Optical OFDM (ACO-OFDM), Pulse Amplitude Modulated OFDM (PAM-OFDM) or Flip-OFDM, but these variants use the bandwidth inefficiently. Our trade-off between energy and spectrum efficiency considers a given limited channel bandwidth of the Light Emitting Diode (LED) and then attempts to get the highest throughput per unit of energy. We investigate previous findings that clipped OFDM can be more attractive in a low-SNR regime. More specifically, we consider Visible Light Communication (VLC) in which the average light level, i.e., the bias, is prescribed by illumination requirements, thus comes for free. ACO/PAM/Flip-OFDM can convert the DC-bias into power for communication, but all variants of OFDM, including DCO-OFDM consume extra electrical power. We conclude that in this scenario, advantages attributed to ACO/PAM/Flip-OFDM vanish, as DCO-OFDM outperforms its variants in all SNR conditions, in terms of achieved throughput over a bandlimited channel as a function of extra electrical power required. For hybrid solutions, such as Asymmetrically clipped DC biased Optical OFDM (ADO-OFDM) and Hybrid ACO-OFDM (HACO-OFDM), we optimize a new adaptive power and rate splitting between odd (clipped) and even (biased/clipped) subcarriers to balance power and bandwidth efficiency

Mitigating LED nonlinearity to enhance visible light communications

\u3cp\u3eThis paper addresses the nonlinear memory effects in the response of typical illumination light emitting diodes (LEDs), in order to enhance the performance of visible light communication (VLC) systems. These LEDs have a limited bandwidth of only several MHz. To reflect the physical mechanisms in the quantum well, we describe the LED transient response by a nonlinear dynamic differential equation. Three different mechanisms of the nonlinearity are relevant in the double hetero-structure LEDs, which result in dynamic nonlinearities, that is, a mixture of nonlinearities and memory effects. Hitherto, generic pre-distorter and non-linear equalizers have been studied for the LEDs. Yet this paper shows that recombination rates of photon generation can be translated into an equivalent discrete-time circuit that can be inverted. This allows us to develop a new pre-distorter with a simpler and more efficient structure than previously studied and overly generic approaches. The novel pre-distorter along with a parameter estimation can effectively overcome LED nonlinearity for high-speed VLC with amplitude-based single carrier modulations, including ON-OFF keying and pulse amplitude modulation-4 systems, and with the multi-carrier orthogonal frequency-division multiplexing. We report experimentally obtained eye-diagrams, first to justify our choice for the LED model on which our nonlinear pre-distorter have been based, and second to verify the effectiveness in enhancing the VLC link performance to the extent predicted by our model.\u3c/p\u3