New Dimming Scheme Will Speed Adoption of Visible Light Communications into LEDs
Researchers at the NSF-funded Lighting Enabled Systems & Applications ERC (LESA), headquartered at Rensselaer Polytechnic University, have demonstrated for the first time a type of dimming-compatible light transmission (using optical orthogonal frequency division multiplexing, or OFDM) based on the recently proposed reverse polarity optical OFDM (RPO-OFDM) scheme. The scheme they devised combines the industry-standard dimming technique—pulse width modulation (PWM)—with OFDM, the highly efficient modulation widely implemented in various communication standards.
By supporting the PWM dimming format, the new scheme has promise for speeding the adoption of visible light communications (VLC) technology into light-emitting diode (LED) luminaires.
VLC technology allows LED luminaries to be used simultaneously for both illumination and broadband wireless communication. Optical OFDM is a promising modulation technique for VLC systems, in which the OFDM signal is used to modulate the power of the optical carrier to achieve gigabit data rates. However, a major design challenge that limits the commercialization of VLC is how to incorporate the industry-preferred PWM light-dimming technique while maintaining a broadband and reliable communication link. A novel signal format, RPO-OFDM, is intended to combine the fast optical OFDM communication signal with the relatively slow PWM dimming signal, where both signals contribute to the effective LED brightness.
The RPO-OFDM maps the OFDM signals onto the on and off periods of the PWM signal through an inversion process. The figure shows how dimming at different levels (70% and 20% duty cycles) is possible while still preserving the OFDM signal shape. In this case, the data rate and the bit-error performance are maintained within a wide dimming range of operation. The advantages of using RPO-OFDM include: (1) the data rate is not limited by the frequency of the PWM signal; (2) the LED dynamic-range of operation is fully utilized to minimize the nonlinear distortion of the OFDM signal; and (3) the bit-error performance is sustained over a large fraction of the luminaire dimming range. In addition, RPO-OFDM offers a practical approach to utilize off-the-shelf LED drivers.