New Dimming Scheme Will Speed Adoption of Visible Light Communications into LEDs

Achievement date: 
2015
Outcome/accomplishment: 

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. 

Impact/benefits: 

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.

Explanation/Background: 

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.