Scintillation Robust Adaptive Optical Signal Detection in Free Space Optical Communications using CSI Prediction

Adaptive multi-level symbol detection technique in free-space optical communication (FSOC) is presented in this paper. Generally, the on-off keying (OOK) format is used in intensity modulation/direct detection (IM/DD) based free-space optical (FSO) system. For OOK symbol detection in FSOC, the maximum likelihood (ML) based optimal detection scheme is adopted. The ML detection techniques need the accurate marginal distribution of the turbulence-induced fading. However, it is non-attainable in practical system, and the distribution model is also not fixed in the FSO link. The response of intensity fluctuation in turbulence is relatively slow compared with that of the data rate with a cutoff frequency of 1 kHz. For this reason, there is a variation in time about the optimal threshold, causing significant burst errors when a fixed threshold is used. Applying a DC block or AC coupling of PD can stabilize symbol detection in OOK symbol. However, when it comes to multi-level pulse amplitude modulation (PAM) format in FSO system, the symbol detection problem cannot be solved with previous detection techniques. In this paper, the channel state information (CSI) prediction technique for adaptive multi-level optical symbol detection is proposed. Considering the slow varying characteristic of scintillation channel, we used an adaptive linear prediction (ALP) filter to predict the CSI of the upcoming frame. The proof-of-concept experiments were conducted with a Mach-Zehnder modulator (MZM) based channel emulator. The validity of the proposed symbol detection technique was experimentally demonstrated by comparing with the fixed threshold decision (FTD) and adaptive threshold decision (ATD) scheme.