In optical communication systems, orthogonal frequency division multiplexing (OFDM) is widely used to combat inter-symbol interference (ISI) caused by multipath propagation. Optical systems which use intensity modulation and direct detection (IM/DD) can only transmit real valued symbols, but the inverse discrete Fourier transform (IDFT) or its computationally efficient form inverse-fast Fourier transform (IFFT) required for the OFDM waveform construction produces complex values. Hermitian symmetry is often used to obtain real valued symbols. For this purpose, some trigonometric transformations such as discrete cosine transform (DCT) are also used, however these transformations can eliminate the ISI only under certain conditions. In this paper, we propose a completely different method for the construction of OFDM waveform with IFFT to obtain real valued symbols by combining the real and imaginary parts (CRIP) of IFFT output electrically (E-CRIP) or optically (O-CRIP). Analytical analysis and simulation works are presented to show that compared to the Hermitian symmetric system, the proposed method slightly increases the spectral efficiency, eliminates ISI, significantly reduces the amount of needed calculation and does not effect the error performance. In addition, the O-CRIP method is less affected by clipping noise that may occur due to the imperfections of the transmitter front-ends.

中文翻译：

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结合IDFT的虚部和虚部构造OFDM光学波形

在光通信系统中，正交频分复用（OFDM）被广泛用于对抗由多径传播引起的符号间干扰（ISI）。使用强度调制和直接检测（IM / DD）的光学系统只能传输实值符号，但是OFDM波形构造所需的离散傅立叶逆变换（IDFT）或计算效率高的快速傅立叶逆变换（IFFT）会产生复杂的价值。埃尔米特式对称性通常用于获得实值符号。为此，还使用了一些三角变换，例如离散余弦变换（DCT），但是这些变换只能在特定条件下消除ISI。在本文中，我们提出了一种完全不同的方法，通过将IFFT电输出（E-CRIP）或光学输出（O-CRIP）的IFFT的实部和虚部（CRIP）组合起来，使用IFFT构造OFDM波形以获得实值符号。分析和仿真工作表明，与Hermitian对称系统相比，该方法可以稍微提高频谱效率，消除ISI，显着减少所需的计算量，并且不会影响误差性能。另外，O-CRIP方法受限幅噪声的影响较小，限幅噪声可能是由于发射机前端的缺陷而发生的。分析和仿真工作表明，与Hermitian对称系统相比，该方法可以稍微提高频谱效率，消除ISI，显着减少所需的计算量，并且不会影响误差性能。另外，O-CRIP方法受限幅噪声的影响较小，限幅噪声可能是由于发射机前端的缺陷而发生的。分析和仿真工作表明，与Hermitian对称系统相比，该方法可以稍微提高频谱效率，消除ISI，显着减少所需的计算量，并且不会影响误差性能。另外，O-CRIP方法受限幅噪声的影响较小，限幅噪声可能是由于发射机前端的缺陷而发生的。