FPGA Implementation of Distance-Independent Symbol Timing Synchronization in IM/DD OFDM-PON

Intensity modulation/direct detection (IM/DD) orthogonal frequency division multiplexing (OFDM) is very suitable for high-speed cost-sensitive passive optical network (PON) applications. However, real-time OFDM systems are sensitive to synchronization bias, and thus symbol timing synchronization (STS) becomes a critical issue. We proposed a proportional-sign cross-correlation (PS-CC) STS algorithm with the advantages of distance-independence and low resource utilization. In the proposed PS-CC STS algorithm, the sign operation is applied instead of the multiplication operation with high computational complexity to decrease the resource utilization and the characteristic of low resource utilization will drive its application in real-time systems. Meanwhile, we use field programmable gate array (FPGA) to design a baseband IM/DD OFDM receiver with pipeline structure and built a universal loop test system, which is applied to implement the proposed PS-CC STS algorithm and evaluate its performance. In the experiment of a 16-QAM OFDM-PON system, a 30-km fiber transmission only results in 1.98% ripple on the peak amplitude of correlation, which verifies the distance-independence of the proposed method.