Optical superimposition-based PAM-4 signal generation for visible light communication

A pulse amplitude modulation with 4 levels (PAM-4), generated by optical superimposition, is first proposed and experimentally demonstrated for visible light communication (VLC). It can overcome the shortcomings of the PAM-4 signals generated in the electrical domain, with enhanced tolerance to the modulation nonlinearities of a light emitting diode (LED). A proof-of-concept experiment employing two blue LEDs has been conducted in this work. We achieve a 12.288-Mb/s PAM-4 signal, within a 2.5-MHz system bandwidth, by superimposing two different on-off keying (OOK)-modulated signals in the optical domain and investigate the BER performance over 2-m, 3-m, 4-m, 5-m and 6-m air channels. At this bit rate, the available maximum transmission distance is 4 m in the air and the corresponding BERs of the two OOK signals are 8.39×10^-5 and 1.43 ×10^-3, respectively.