In this paper, we successfully demonstrate the generation and reception of faster-than-Nyquist (FTN) orthogonal frequency-division multiplexing (OFDM) signals with coherent detection. We can improve the spectrum efficiency by using higher order QAMs. However, we believe that the realization of super-Nyquist channel has the following advantages to improve spectral efficiency: Firstly, when we use higher order modulation, the requirements for equipment will be increased, such as the linearity of the modulator. Secondly, flexibility. The signal compression ratio can be arbitrarily set within the maximum compression ratio in our experiment. However, if we increase the modulation order, it can only be an integral multiple of 2, such as 4QAM to 8QAM, which can’t fully and effectively use the channel resources. In the FTN OFDM transmission system, frequency shaping is realized by a delay-and-add filter (DAF) at the transmitter to ensure the energy of signal is concentrated at low frequencies, which will make the OFDM much more robust to strong filtering effect to some extent. We generate a quadrature phase-shift keying OFDM (QPSK-OFDM) signal with equivalent baud rate (In this paper, the generation of FTN signal is in the frequency domain since we used M-point DFT and N-point IDFT while M is greater than N. In other words, the high-frequency parts are discarded to increase spectrum efficiency. Hence, we use “equivalent baud rate” instead of a simple “baud rate”) of 100–120 Gbaud, which is much higher than the sampling rate (80 GSa/s) of digital-to-analog converter (DAC). With this FTN Discrete-Fourier transform spread (DFT-spread) OFDM signal generation technique, we achieve the highest equivalent baud rate (120 GBaud) QPSK-OFDM signal generation and transmission over 80 km single mode fiber (SMF) with bit error rate (BER) under soft-decision forward-error-correction (SD-FEC) limitation of 2.4 × 10⁻². We also investigate the generation and reception of 32 Gbaud FTN 16-ary quadrature-amplitude modulation OFDM (16QAM-OFDM) with coherent detection to achieve higher spectrum efficiency in this paper. The experimental results show that there is negligible optical signal to noise ratio (OSNR) penalty when 32 Gbaud FTN DFT-spread 16QAM-OFDM is compressed and transmitted within 28 GHz bandwidth with the BER at 2.4 × 10⁻² in optical back to back (OBTB).

在本文中，我们成功地演示了具有相干检测功能的比奈奎斯特（FTN）快的正交频分复用（OFDM）信号的生成和接收。我们可以通过使用更高阶的QAM来提高频谱效率。但是，我们认为实现超奈奎斯特信道具有以下优点，可以提高频谱效率：首先，当我们使用高阶调制时，对设备的要求将会提高，例如调制器的线性度。其次，灵活性。在我们的实验中，信号压缩率可以在最大压缩率范围内任意设置。但是，如果增加调制阶数，它只能是2的整数倍，例如4QAM到8QAM，不能充分有效地使用信道资源。在FTN OFDM传输系统中，频率整形是通过发射机处的延迟和加法滤波器（DAF）实现的，以确保信号的能量集中在低频上，这将使OFDM在某种程度上对强大的滤波效果更加鲁棒。我们生成具有等效波特率的正交相移键控OFDM（QPSK-OFDM）信号（本文中，由于我们使用M点DFT和N点IDFT，而M更大时，FTN信号的生成在频域中）换句话说，高频部分被丢弃以提高频谱效率，因此，我们使用“等效波特率”代替简单的100–120 Gbaud的“波特率”，这比采样要高得多。数模转换器（DAC）的速率（80 GSa / s）。借助这种FTN离散傅里叶变换扩展（DFT扩展）OFDM信号生成技术，⁻²。本文还研究了具有相干检测功能的32 Gbaud FTN 16进制正交幅度调制OFDM（16QAM-OFDM）的生成和接收，以实现更高的频谱效率。实验结果表明，将32 Gbaud FTN DFT扩展的16QAM-OFDM压缩并在28 GHz带宽内以BER在2.4×10 ^-2的光背对背传输时，光信噪比（OSNR）损失可忽略不计（ OBTB）。