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.

Filter bank multicarrier with offset quadrature amplitude modulation (FBMC/OQAM) can provide higher spectral efficiency and lower out-of-band spectral/power leakages than conventional orthogonal frequency-division multiplexing with QAM (OFDM/QAM). Due to the imperfect frequency response of electrical/optical devices and various interferences, significant subcarrier SNR fluctuation may result in poor bit error rate (BER) performance. In this work, channel-independent precoding techniques are employed for the short-reach FBMC/OQAM-based intensity-modulated and direct-detection (IMDD) system. Four commonly used precoding techniques are experimentally investigated in a directly-modulated laser-based IMDD FBMC/OQAM and OFDM/QAM systems over a 20-km fiber link. The relevant results exhibit that the receiver sensitivity of these two systems can be improved by about 2 dB with precoding techniques at the BER of 3.8e-3. A slight BER enhancement can also be achieved by the precoded FBMC/OQAM compared to the precoded OFDM/QAM.

Digital-to-analog converters (DACs) are widely used in bandwidth-limited visible-light communication (VLC) systems for high spectral-efficiency multi-carrier signal generation. To reduce the implementation complexity of the transmitter, a digital 1-bit low-pass delta-sigma modulation (DSM)-enabled DAC-free filter-bank multi-carrier (FBMC) signal generation scheme is proposed and experimentally investigated in a red light-emitting diode-based VLC system. In this scheme, one field programmable gate array gigabit transceiver in combination with a low-pass filter is employed to realize the 609 MHz baseband FBMC signal generation. Besides, a low-complexity fast Walsh-Hadamard transform (FWHT)-based interleaved block precoding (IBP) technique is also proposed to reduce peak-to-average power ratio and achieve a uniform SNR profile, and then improve the bit error rate (BER) performance. The experimental results exhibit that the net data rate of 2.38-Gbit/s 16-ary quadrature amplitude modulation-encoded FBMC signal, generated by the proposed scheme, can transmit 2.3-m free space with the BER less than 3.8×10 ^-3 . With the help of the FWHT-IBP technique, about 2-dB improvement in receiver sensitivity is achieved. In addition, the BER performance can provide a slight improvement in the BER performance with the proposed scheme, compared to the conventional DAC-based FBMC-VLC.

Orthogonal frequency-division multiplexing-based passive optical networks (OFDM-PON) can provide high spectral efficiency and flexible two-dimensional bandwidth allocation by assigning subcarriers (SCs) and time slots. The direct-modulation and direct-detection (DM-DD) OFDM with Hermitian symmetry constraint has simple hardware implementation, low cost, and low power consumption compared to coherent detection and is more suitable for cost-sensitive PON. In such a Hermitian symmetric optical OFDM (HS-OFDM), only half of the SCs can be used to carry user data, and thus inverse fast Fourier transform (IFFT) and FFT sizes are inefficient in the viewpoint of hardware implementation. In this work, we use a single field programmable gate array and digital-to-analog converter to implement an HS-free OFDM (NHS-OFDM) transmitter without digital/analog up-conversion. We compare the on-chip resource overhead and power consumption between real-time HS-OFDM and NHS-OFDM transmitters under the same bandwidth granularity and data transmission rate. The registers, look-up tables and multipliers are saved up to 45%, 49.5% and 58.7%, respectively. Besides, the on-chip power consumption is also reduced by 15%. Moreover, the two real-time transmitters are experimentally investigated in a short-reach DM-DD transmission system. A similar bit error rate performance can be achieved after 20/50/70 km standard single-mode fiber transmission.