Adaptive BPSK/QPSK Switching for FSO Links by Real-Time Scintillation Index Monitoring

An adaptive BPSK/QPSK switching scheme is proposed and experimentally demonstrated based on scintillation index (SI) monitoring using a field-programmable gate array (FPGA) to mitigate random and time-varying atmospheric channel conditions in free-space optical (FSO) communication links. The SI is real-time detected at the receiver and fed back to the transmitter. The modulation format is switched between BPSK and QPSK according to the feedback signal with a dual-threshold hysteresis mechanism. A rotating phase plate (RPP) and a variable optical attenuator (VOA) are used to simulate the dynamic atmospheric channel, which is equivalent to a 12.5-km near-ground horizontal weak-to-moderate atmospheric environment with relatively good visibility. The effective throughput is defined to evaluate the achievable link capacity. This capacity is significantly enhanced across varying weather conditions by employing format switching, yielding an improvement of up to 132% compared to fixed modulation formats on sunny days.