Zhao Jian, Townsend Paul
Opt Express. 2019 Jan 21;27(2):714-728. doi: 10.1364/OE.27.000714.
We show that the orthogonality between signal and intrinsic imaginary interference (IMI) in offset quadrature amplitude modulation (offset-QAM) orthogonal frequency division multiplexing (OFDM) can still be maintained under a certain condition even when the timing slots of different subcarriers are misaligned. We show that the phase and velocity differences over subcarriers induced by fiber dispersion satisfy this condition. Based on this, we propose a fast channel estimation and equalization scheme without prior channel information including coarse dispersion. We investigate the proposed scheme in a 40-Gbit/s offset-16QAM OFDM experiment and 240-Gbit/s polarization-division-multiplexed offset-16QAM OFDM simulations, both over 1200-km single-mode fiber. It is shown that the proposed scheme gives better performance, reduced overhead for the training sequence, and/or lower complexity than other schemes. We also compare offset-QAM OFDM with the proposed scheme and conventional OFDM, and show that in addition to the elimination of cyclic prefix overhead, offset-QAM OFDM gives better performance and longer transmission reach for a moderate/small number of subcarriers.
我们表明，即使不同子载波的时隙未对齐，在一定条件下，偏移正交幅度调制（offset-QAM）正交频分复用（OFDM）中信号与固有虚部干扰（IMI）之间的正交性仍可保持。我们表明，由光纤色散引起的子载波上的相位和速度差异满足此条件。基于此，我们提出了一种无需包括粗色散在内的先验信道信息的快速信道估计和均衡方案。我们在40 Gbit/s偏移16QAM OFDM实验和240 Gbit/s偏振复用偏移16QAM OFDM仿真中研究了该方案，两者均通过1200公里单模光纤传输。结果表明，与其他方案相比，该方案具有更好的性能、更低的训练序列开销和/或更低的复杂度。我们还将偏移QAM OFDM与该方案和传统OFDM进行了比较，结果表明，除了消除循环前缀开销外，对于中等/少量子载波，偏移QAM OFDM具有更好的性能和更长的传输距离。
