An approach for Tomlinson-Harashima Precoding in Visible-Light-Communications Systems

We present an approach for channel equalization based on a variation of the Tomlinson-Harashima Precoder (THP) suited for practical Visible Light Communication (VLC) channels. This approach combines a traditional THP with an adaptive receiver. The precoder compensates for the long impulse response of the luminaire (e.g. white-phosphor Light-Emitting Diodes (LEDs)), and the adaptive receiver compensates for timing and impulse response variations of the physical channel between the transmitter and the receiver. Using this approach the precoder information (typically estimated at the receiver) needs to be communicated from the receiver to the transmitter very infrequently making this approach practically feasible. We experimentally tested this approach using two different luminaires and found that the overhead information that needs to be communicated from the receiver to the transmitter can remain valid for long periods of time, and that the adaptive receiver can compensate for receiver motion, without requiring the precoder to be retrained, avoiding significant overhead. Additionally, the bounded amplitude of the transmit signal in this approach results in a significant reduction in Peak-to-Average Power Ratio (PAPR), compared to Orthogonal Frequency-Division Multiplexing (OFDM) which is commonly used in VLC systems making this an attractive approach for channel equalization in practical VLC systems.