Channel independent precoding technique has been widely used in optical discrete multi-tone (DMT) transmission systems to compensate unbalanced impairments induced by bandwidth limitations and imperfect frequency responses of electrical/optical devices and various interferences. However, the comparison of different precoding techniques in terms of peak-to-average power ratio (PAPR) reduction, nonlinear distortion tolerance, implementation complexity, and bit error rate (BER) improvements has not been fully studied. In this paper, we comparatively investigate seven most commonly used precoding techniques, i.e., discrete Fourier transform (DFT), orthogonal circulant matrix transform (OCT), constant amplitude zero autocorrelation sequence-based matrix transform (CAZACT), Zadoff-Chu matrix transform (ZCT), discrete cosine transform (DCT), discrete Hartley transform (DHT) and Walsh-Hadamard transform (WHT), through both numerical simulations and offline experiments. Simulations show that the ZCT can achieve the best PAPR reduction, and the OCT cannot reduce the PAPR. Besides, DFT, CAZACT, ZCT, DCT, and DHT precoded DMT signals have superior error vector magnitude performance after passing through nonlinear models. And the corresponding precoded QPSK-DMT signals have better BER performance than both OCT/WHT precoded and conventional ones in the distortion-limited scenarios. However, the precoded 16/64QAM-DMT signals, excluding OCT precoded one, are more sensitive to nonlinear distortions and provide minor BER improvement or even may degrade the BER performance. Complexity analysis exhibits the WHT precoding does not require multiplications and therefore has the lowest implementation complexity. In the inter-symbol interference-limited case, OCT precoding can still achieve a good signal-to-noise ratio (SNR) balance and provide the best BER performance. A simple timing synchronization point adjustment (TSPA) method is employed to enhance SNR balance. Simulated and experimental results exhibit that similar BER performance can be achieved by using these precoding techniques together with TSPA in noise-limited scenarios. Considering the implementation complexity WHT precoding may be a good option to compensate unbalanced impairments in the short-reach DMT transmission system.

Probabilistically shaped orthogonal frequency-division multiplexing (PS-OFDM) signal is proposed and experimentally demonstrated in a low-cost intensity-modulation and direct-detection (IM-DD) system for optical access networks. The concatenation of constant composition distribution matcher (CCDM) and low-density parity-check (LDPC) code sequentially implements probabilistic amplitude shaping (PAS) and channel coding. Thanks to the precoding scheme, all data subcarriers can be treated equally by one optimized probabilistic distribution. Experimental results demonstrate that constant amplitude zero autocorrelation sequence (CAZAC) precoding can offer optimal performance compared to orthogonal circulant transform (OCT) precoding and discrete Fourier transform spread (DFT-spread) in terms of increasing generalized mutual information (GMI) and reducing PAPR. Moreover, probabilistically-shaped 64-QAM OFDM signal can provide shaping gains of 1.04/0.94/0.64 dB at 4.0/3.6/3.0 bits/QAM symbol compared to traditional 64-QAM OFDM signal. Meanwhile, compared to traditional 16-QAM OFDM, it can offer shaping gains of 1.64/0.64 dB at 3.6/3.0 bits/QAM symbol. In addition, net data rate ranging from 32.66 to 43.55-Gb/s over 20-km single mode fiber (SMF) can be achieved by only adjusting the probabilistic distribution.

We propose an optical line terminal (OLT)-centralized sampling frequency offset (SFO) compensation scheme for adaptively-modulated OFDM-PON systems. By using the proposed SFO scheme, the phase rotation and inter-symbol interference (ISI) caused by SFOs between OLT and multiple optical network units (ONUs) can be centrally compensated in the OLT, which reduces the complexity of ONUs. Firstly, the optimal fast Fourier transform (FFT) size is identified in the intensity-modulated and direct-detection (IMDD) OFDM system in the presence of SFO. Then, the proposed SFO compensation scheme including phase rotation modulation (PRM) and length-adaptive OFDM frame has been experimentally demonstrated in the downlink transmission of an adaptively modulated optical OFDM with the optimal FFT size. The experimental results show that up to ± 300 ppm SFO can be successfully compensated without introducing any receiver performance penalties.

We experimentally demonstrate a heterodyne W-band fiber-wireless system using twin single-side-band orthogonal frequency division multiplexing (Twin-SSB-OFDM) transmission scheme with low-cost electrical filters. The transmission scheme is proposed to halve the required sampling rate of the baseband receiver. Moreover, since the W-band signal generation is based on optical heterodyne with non-ideal laser sources, there is a time-varying carrier frequency offset (CFO) in the system. To mitigate the CFO effect and simultaneously realize down-conversion, a blind carrier recovery method is proposed and validated in the system. The experimental results show that after 22-km SSMF transmission and 1-m wireless transmission, the system can achieve a 40.07 Gb/s data rate with a bit-error-ratio (BER) below 3.8 × 10-3 when using a two-channel 12-GSa/s arbitrary waveform generator (AWG) as transmitter and two emulated 12-GSa/s baseband receivers. In addition, by setting the AWG at 5-GSa/s and using a 5-GSa/s real-time field programmable gate array (FPGA)-based receiver, an additional experiment is carried out by implementing the proposed method in the real-time receiver. Although the frequency swing of the intermediate frequency (IF) carrier of the received signal is up to ~300 MHz, the system can also achieve a relatively stable transmission performance. The additional experiment further verifies the effectiveness and practical feasibility of the proposed carrier recovery method and the Twin-SSB-OFDM transmission scheme.

A field-programmable gate arrays-based real-time orthogonal frequency-division multiplexing (OFDM) transceiver is implemented for the uplink of an X-band radio-overfiber (RoF) system with heterodyne detection for the first time. Digital intermediate-frequency I/ Q modulation and demodulation, and forward error correction are applied. Minn's timing synchronization method is slightly modified with reduced implementation complexity for the proposed OFDM-RoF system. By using Reed-Solomon code with symbol interleaving technique, the real-time transmission of the X-band 16-ary quadrature amplitude modulation OFDM-RoF signal over 10-m wireless and 2.26-km single mode fiber link is successfully achieved with a post-FEC bit error rate below 1×10 ^-9 .

Filter bank multi-carrier (FBMC) can provide higher spectral efficiency and lower out-of-band spectrum leakages than orthogonal frequency division multiplexing (OFDM) or discrete multi-tone (DMT). Due to a crystal or oscillator usually having a frequency tolerance and stability of a few tens of parts per million (ppm), the sampling frequency offset (SFO) between digital-to-analog converter and analog-to-digital converter in the transceiver will seriously degrade the performance of these asynchronous optical multi-carrier communication systems. The SFO-induced impairments and their estimation and compensation methods have been widely studied in optical OFDM/DMT systems. However, the SFO problem is rarely investigated in the optical fiber FBMC system. In this work, the SFO-induced impairments in the asynchronous optical FBMC system are first theoretically analyzed, and then we propose an inter-symbol guard (ISG) and training symbol (TS)-aided SFO estimation method and compare two commonly used SFO compensation methods, i.e., phase compensation (PC) and digital interpolation (DI) regarding estimation accuracy and bit error rate (BER) or error vector magnitude performance by both numerical simulation and offline experiment. The results exhibit that an SFO estimation accuracy of less than ±1 ppm is achieved in the intensity modulation and direct detection FMBC system with a 20-km single-mode fiber link under initial SFOs up to ±200 ppm. Besides, FBMC can afford a similar BER performance with DMT with PC or DI methods in the presence of SFO up to ±100 ppm, and achieve a negligible power penalty at the BER of 3.8e-3, compared to the SFO-free cases. Moreover, FBMC is more robust to SFO estimation errors than DMT.