RESUMEN
Transmitter pre-emphasis is one of the techniques proposed to improve the relatively poor frequency responses of light-emitting diodes (LEDs) used in visible light communications (VLC) as light sources. However, according to the communications theory in linear channels, pre-emphasis may only be as good as the not particularly efficient feed-forward equalizer (FFE). Given the increase in peak-to-average power ratio (PAPR) caused by pre-emphasis, its use in place of receiver equalization can be considered counterproductive. In this study, we show that the performance of the transmitter pre-emphasis is indeed equivalent to the FFE at low input powers, where the LED is a linear system. However, at higher modulation voltages, the transmitter pre-emphasis has a clear advantage over FFE, as it mitigates the nonlinear distortion of the LED. This mechanism is for the first time to our knowledge explained in both theory and experiment.
RESUMEN
In this work, we demonstrated a convenient and reliable method to realize the vertical stack integration of the blue and yellow InGaN micro-LED arrays. The standard white and color-tunable micro-light sources can be achieved by adjusting the current densities injection of the micro-LEDs. The spectra cover violet, standard white, cyan, etc., showing an excellent color-tunable property. And the mixed standard white light can be separated into red-green-blue three primary colors through the color filters to realize full-color micro-LED display with a color gamut of 75% NTSC. Besides, the communication capability of the integrated micro-LED arrays as visible light communication (VLC) transmitters is demonstrated with a maximum total data rate of 2.35 Gbps in the wavelength division multiplexing (WDM) experimental set-up using orthogonal frequency division multiplexing modulation. In addition, a data rate of 250 Mbps is also realized with the standard white light using on-off keying (OOK) modulation. This integrated device shows great potential in full-color micro-LED display, color-tunable micro-light sources, and high-speed WDM VLC multifunctional applications.
RESUMEN
In this Letter, we propose the application of a Volterra predistorter to compensate for the second-order nonlinear distortion generated in light-emitting diode (LED) communication systems. We demonstrate that a predistorter dedicated specifically to the LED has linear (not quadratic) numerical processing complexity. Experimental investigation performed using pulse amplitude modulation (PAM-4) indicates that it achieves similar performance as the classical Volterra equalizer located at the receiver. The highest measured gain in electrical signal to noise and interference power ratio of predistortion over linear equalization was at 300 Mbaud and was approximately 3.5 dB. At the same baud rate, the unpredistorted signal had a received optical power penalty of up to 2.5 dBm compared with the predistorted signal.
RESUMEN
An in-depth study of the second order Volterra kernel of light emitting diodes (LEDs) is provided, with a special focus on compensation of dynamic nonlinear distortion generated by the LED when used as a communications signal transmitter. It is shown that from the factorization of the kernel in the frequency domain follows a simplified time-domain model for LED nonlinearity, consisting of three operations: two linear filtrations and squaring. Next, using the pth inverse theory, the corresponding block structures of nonlinear pre- and postdistorters are derived. As it turns out, they exhibit linear computational complexity. It is shown that the model coefficients defining the kernel may be estimated up to the Nyquist frequency. Numerical results indicate that pre- and postdistortion achieve the same performance with respect to receiver signal to noise power ratio (SNR) and have a considerable advantage over linear equalizers. However, in a practical scenario, the predistortion increases the peak to average power ratio of the transmitted signal (unless it is not high already), possibly leading to performance inferior to postdistortion. Finally, it is shown that the simplified equalizer based on the LED model has roughly identical performance to the regular, quadratic Volterra equalizer.
RESUMEN
In this paper, we propose to calculate the impulse and frequency response of step-index polymer optical fiber in the presence of mode coupling using the matrix exponential method. The computation time in this method is independent of the fiber length and is typically 2 orders of magnitude shortened compared with the standard numerical procedures involving finite difference methods. Results obtained with our method are compared with those of known cases, where an analytical solution for the time-dependent power flow equation exists, and with a finite difference method. We also investigate the impact of mode coupling and differential modal attenuation on impulse response duration and shape.
RESUMEN
In this paper, we present a time domain method for extracting coefficients of nonlinear Volterra-series kernels for white light-emitting diodes (LED) used both for illumination and visible light communications. We show that this method may have several advantages over the thus far more popular frequency domain method. We successfully apply the measured kernel coefficients up to the 3rd order for the modeling of nonlinear distortion impact on advanced modulation formats: pulse amplitude modulation, carrierless amplitude phase and orthogonal frequency division multiplexing. The impact of blue filtering on dynamic nonlinearity is also presented.
RESUMEN
In this Letter, an intensity-modulation direct-detection mode group multiplexing system is investigated. The system does not use highly modally selective optics: multiplexing and demultiplexing is done in commercial multimode couplers and modal diversity is enhanced by modulation rates far beyond the length-bandwidth product of the fiber. Experimental and theoretical results shown for OM1 and OM2 fibers fully confirm this theory.
RESUMEN
By experimental measurement and from a numerical solution to the time-dependent power flow equation, the frequency response, bandwidth, mode coupling, and mode-dependent attenuation are determined for a low-numerical-aperture (NA) plastic optical fiber. Frequency response and bandwidth are specified as a function of fiber length. Numerical results are verified against experimental measurements. Mode coupling and modal attenuation are found to differ substantially between two fiber varieties of the same type (both low-NA, step-index, and plastic), implying their preferential suitability that is application-specific.