Transmitter IQ Skew Estimation for Alamouti Coding-Based Simplified Coherent Systems

The simplified coherent system combining Alamouti coding and single balanced photodetector reception has emerged as a promising solution for short reach scenarios. However, Alamouti coding is one kind of space-time coding that has a higher requirement on orthogonality, which results in greater sensitivity to IQ skew. In this paper, we propose a polarization time interleaving twin subcarriers estimation (PTI-TSE) method to estimate the transmitter IQ skew for Alamouti coding-based simplified coherent systems. The training sequences of PTI-TSE are designed to be conjugate or negative-conjugate on symmetric subcarriers and time-interleaved on X- and Y-polarization to mitigate the frequency aliasing and polarization interference. Then, the Tx IQ skew is extracted from the phase offset of the difference between twin subcarriers. We verify the effectiveness of the PTI-TSE method by simulation and experiment of a 40 GBaud PM-QPSK DSCM transmission. Simulation and experimental results show that the PTI-TSE is tolerant to 500 MHz residual frequency offset and its estimation accuracy is within ±0.2 ps. Besides, it is also robust to imperfections such as Tx IQ imbalance, linewidth variations, and state-of-polarization (SOP).