Generation of faster-than-Nyquist coherent optical DFT-spread OFDM signals with high-baud and high-order modulations

Abstract 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).