Experimental implementation of digital equalizer for multilevel signal in visible light communication

An equalization scheme consisting of two square-root-raised-cosine filters and a finite impulse response (FIR) filter is proposed to compensate the channel fading at high frequency induced by light-emitting diodes (LEDs) in visible light communication (VLC). Using the digital equalizer based on this scheme, the performance of the VLC system with different multilevel baseband signals is analyzed experimentally. It is demonstrated that, when the proposed digital equalizer is introduced in the VLC system, the transmission rate is shown to double under forward error correction threshold. As a result, a transmission rate of 120 Mb/s is realized when the LED’s 3-dB bandwidth is only 13 MHz. The number of the FIR tap coefficients was also optimized by considering the transmission performances and computational complexity, which is set to 32 in the adopted digital equalizer. The relation between the system performance improvement and signal Euclidean distance with the same bit per symbol is further investigated. The results reveal that the longer Euclidean distance is beneficial to the improvement of transmission performance for the VLC system with the proposed equalizer.