Direct-detection OFDM Systems Research Articles

A Simple Nonlinear Companding Transfrom for Nonlinear Compensation of Direct-Detection Optical OFDM Systems

In direct-detection OFDM systems, the nonlinear effects caused by optical modulation and fiber transmission can degrade the system performance severely. In this study, we propose a new nonlinear companding transform to improve the performance of direct detection optical OFDM transmission systems. The demonstration is realized by Monte-Carlo simulation of the intensity modulation and direct-detection DCO-OFDM optical transmission system at 40 Gbps over a 80 km of standard single mode fiber link. The influence of the companding parameters on the performance of system in different nonlinear transmission conditions has been investigated via simulation.

Cost-Effective SFO Compensation Scheme Based on TSs for OFDM-PON

A cost-effective sampling frequency offset (SFO) compensation scheme based on a simple training symbol (TS) is proposed and experimentally demonstrated in an orthogonal frequency-division multiplexing passive optical network (OFDM-PON) downlink transmission system. For SFO estimation and compensation, two TSs are used, while one of them is also used for timing synchronization and channel estimation. In addition, the performance of the SFO compensation scheme is investigated using four optical network units (ONUs) and one optical line terminal in an asynchronous intensity-modulated and direct-detection OFDM system. Moreover, the error vector magnitude (EVM) performance with respect to the SFO error is evaluated. The experimental results indicate that the SFO estimation accuracy of the proposed scheme is within ±2 ppm under an SFO ranging from −200 ppm to +200 ppm. When the SFO error is 2 ppm, the EVM penalties for ONU1-ONU4 are all below 1 dB. In addition, aided by the proposed scheme, up to ±200 ppm SFO can be successfully compensated with an EVM penalty under 1 dB.

OLT-centralized sampling frequency offset compensation scheme for OFDM-PON.

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.

Digital baseband predistortion for SSII cancellation in EAM-based direct-detection OFDM systems

To increase the available bandwidth of electro-absorption modulator (EAM)-based direct-detection orthogonal frequency division multiplexing (DD-OFDM) systems, which suffers from dispersion- and chirp-induced power fading, the multiband subcarrier allocation scheme and adaptive modulation formats are adopted. However, the subcarrier-to-subcarrier intermixing interference (SSII) would degrade signal-to-noise ratio on the increased bandwidth and become a bottleneck of the multiband systems. In this paper, a novel digital baseband predistortion is proposed to minimize the penalty of SSII. The experimental results show that a 30Gbps EAM-based DD-OFDM system with the proposed technique could improve ~3dB receiver sensitivity compared with the system without the technique over 100km long single mode fiber transmission and only decreases ~1.0dB in comparison with the back-to-back situation.

Spectrally efficient polarization multiplexed direct-detection OFDM system without frequency gap.

We experimentally demonstrate a spectrally efficient direct-detection orthogonal frequency-division multiplexing (DD-OFDM) system. In addition to polarization-division multiplexing, removing the frequency gap further improves the spectral efficiency of the OFDM system. The frequency gap between a reference carrier and OFDM subcarriers avoids subcarrier-to-subcarrier beating interference (SSBI) in traditional DD-OFDM systems. Without dynamic polarization control, the resulting interference after square-law direct detection in the proposed gap-less system is polarization-dependent and composed of linear inter-carrier interference (ICI) and nonlinear SSBI. Thus, this work proposes an iterative multiple-input multiple-output detection scheme to remove the mixed polarization-dependent interference. Compared to the previous scheme, which only removes ICI, the proposed scheme can further eliminate SSBI to achieve the improvement of ∼ 7 dB in signal-to-noise ratio. Without the need for polarization control, we successfully utilize 7-GHz bandwidth to transmit a 39.5-Gbps polarization multiplexed OFDM signal over 100 km.

Joint Influence of the Optical Carrier-to-Sideband Ratio and Guard Band on Direct-Detection SSB-OOFDM System

The signal-to-interference-plus-noise ratio of the noised single-sideband optical orthogonal frequency-division multiplexing (SSB-OOFDM) signal in a direct-detection OFDM system is first deduced theoretically. The relationship of the carrier-to-sideband ratio (CSR) and guard band (GB) to the signal-to-signal beat interference (SSBI) is explored. According to our theoretical analysis, the degradation caused by SSBI on the SSB-OOFDM signal becomes worse as the GB reduces, whereas the increase in the CSR can loosen this degradation. Therefore, a tradeoff between the GB and CSR balances their joint influence on system performance. The simulation for the 20-km optical link with 40-Gb/s 16-quadrature amplitude modulation (16-QAM) noised SSB-OOFDM signal is conducted to confirm our theoretical results. It shows that, without any complex device or algorithm, the GB can be reduced to 40% only by increasing the CSR to $\sim$ 8 dB, and thus, the spectral efficiency of the DDO-OFDM link is improved after optimizing the CSR and GB of the SSB-OOFDM signal. In addition, the influence of the optical signal-to-noise ratio and the electronic filter on the optimum CSR of the system is further analyzed.

A Scaling Scheme for DCT Precoded Optical Intensity-Modulated Direct Detection Systems

A scaling technique is employed to improve the performance of a Discrete Cosine Transform (DCT) precoded optical intensity-modulated direct detection (IM/DD) OFDM system, which fully exploits the dynamic range of a digital-to-analog converter (DAC). The theoretical analysis shows that the proposed scaling scheme can improve the BER performance of DCT precoded and scaled OFDM systems. The experiment results also show that the proposed scheme significantly improves the BER performance without changing the receiver structure. The measured received sensitivity at a BER of 10−3for a 4 G samples/s (2.7 Gbits/s) DCT precoded and scaled OFDM signal and after 100 km standard single-mode fiber (SMF) transmission has been improved by 3 and 1.3 dB when compared with the original OFDM system and conventional DCT precoded OFDM system, respectively.

Performance of 16 QAM-OFDM With New Null Subcarrier Shifting in an Intensity-Modulated Direct Detection System

To reduce the peak-to-average power ratio (PAPR) of the optical orthogonal frequency-division multiplexing (OOFDM) signal, a novel null subcarrier shifting scheme is proposed in an intensity-modulated direct detection OFDM system. At the receiver, a constant subcarrier is used as the location information to implement demodulation accurately. Meanwhile, the computational complexity is degraded apparently by adopting the proposed scheme. The experimental results show that the PAPR reduction of the OFDM signal can reach about 2 dB at a complementary cumulative distribution function of 1×10−4, and the receiver sensitivity after 100 km single-mode fiber transmission at the bit error rate (BER) of 1×10−3 can be improved by about 1.5, 1.8, 2.1, and 2.6 dB when the launch power is 3, 5, 8, and 10 dBm, respectively.

Flipped-exponential Nyquist pulse technique to optimize the PAPR in optical direct detection OFDM system

In this paper, a novel technique based on flipped-exponential (FE) Nyquist pulse method is proposed for peak-to-average power ratio (PAPR) reduction in optical direct detection orthogonal frequency-division multiplexing (DD-OFDM) system. The method is based on a proper selection of the FE Nyquist pulses for shaping the different subcarriers of the OFDM. We experimentally demonstrated an optical DD-OFDM transmission system with this novel technique to achieve significant improvement in PAPR reduction in the optical OFDM system. The received sensitivity of the OFDM signal after suffered from strong nonlinear effects in standard single-mode fiber (SMF) has been experimentally investigated.

Optimization of pilot interval design in direct-detected optical OFDM system

The direct-detected optical OFDM (DDO-OFDM) system in frequency-selective fading fiber channel with three different types of pilot interval design has been experimentally investigated. In these schemes, the 2.28 Gbit/s QPSK-OFDM signal is transmitted over 100km fiber in the system. The experiment results show the scheme with sparse pilot as frequency increasing shows the best performance for DDO-OFDM system.