Joint WHT Precoding and RF-DAC Pre-emphasis Enabled Multi-gigabit LED-based OFDM-VLC System

Due to the limited modulation bandwidth of light-emitting diodes (LED), various advanced modulation formats in combination with pre-equalization/pre-emphasis (PE) or adaptive modulation technologies are usually used to enhance the transmission capacity of visible light communication (VLC) systems. In this paper, the signal PE without additional signal processing is achieved by employing a DAC operating in RF mode. In addition, a low-complexity WHT precoding technique is applied to realize the subcarrier (SC) signal-to-noise ratio (SNR) equalization. The results exhibit that, by using the RF-DAC PE, the power and SNR on high-frequency SCs of 937.5-MHz orthogonal frequency-division multiplexing (OFDM) signal can be effectively enhanced. With the aid of the WHT precoding technique, the SNR fluctuation of SC is reduced from 30 to 3dB, the constellation is more convergent. The bit error rate is reduced by an order of magnitude and less than 3.8e-3 after 1.2-m free-space transmission.