Polarization-independent preamble design in Alamouti code-based simplified coherent system for PON downstream

Benefiting from the time slot orthogonality of Alamouti coding, the simplified coherent system for PON downstream using a single-polarization coherent receiver can achieve polarization-insensitive reception via a MIMO equalizer without PBS. However, frame synchronization (FS) and frequency offset estimation (FOE) in the digital signal processing (DSP) are still affected by link noise and polarization rotation, and it is difficult to make full use of this orthogonal property to achieve polarization insensitivity. To solve these problems, we propose and demonstrate a reliable and efficient preamble design for the Alamouti code-based simplified coherent system to realize the three DSP functions consisting of polarization-independent FS, polarization-independent FOE, and fast channel estimation (CE). The feasibility of the proposed preamble scheme is simulated and experimentally verified in a 30-GBaud simplified coherent system using 16 quadrature amplitude modulation (QAM). The simulation and experiment results demonstrate that the proposed preamble, jointly with the data-aided DSP, achieves polarization-independent FS and FOE, and fast convergence through CE even with only a single polarization detection. The optical power budget of about 34.8 dB is also experimentally achieved at the 20% soft-decision forward error correction (FEC) limit.