Experimental demonstration of 4-PAM for high-speed indoor free-space OW communication based on cascade FIR-LMS adaptive equalizer

Inter-symbol interference (ISI) is one of the key problems that seriously limit the transmission data rate in optical communication system. To eliminate ISI and further improve the system performance, we design a specific cascade finite impulse response-least mean square (FIR-LMS) equalizer. To the best of our knowledge, it is the first time this kind of equalizer is employed for high-speed indoor free-space optical wireless (OW) communication system. After LD driver printed circuit board (PCB) validated the design, a special miniaturized and inexpensive LD, and a spatial photodetector are employed at the transmitter and receiver, respectively. Based on these, we experimentally investigate the scheme of 4-pulse amplitude modulation (4-PAM) optical signal combined with cascade FIR-LMS adaptive equalizer in an indoor free-space OW communication system. The convergence performance between LMS and FIR-LMS is compared, the bit error rate (BER) performances of FIR, LMS and FIR-LMS equalizer at different propagation distances are also demonstrated. With the utility of the proposed equalizer, the OW communication system can successfully achieve the bit rate 10 Gbit/s transmission over 500 cm indoor free-space link distance. The BER significantly below the 7% forward error correction (FEC) limit of 3.8×10−3, which is lower than LMS by more than an order of magnitude.