Optical OFDM Systems Research Articles

Performance analysis of modified Asymmetrically-Clipped Optical Orthogonal Frequency-Division Multiplexing systems

A modification to the Asymmetrically-Clipped Optical Orthogonal Frequency-Division Multiplexing (ACO-OFDM) technique is proposed through unipolar encoding. A performance analysis of the Bit Error Rate (BER) is developed and Monte Carlo simulations are carried out to verify the analysis. Results are compared to that of the corresponding ACO-OFDM system under the same bit energy and transmission rate; an improvement of 1dB is obtained at a BER of 10−4 . In addition, the performance of the proposed system in the presence of atmospheric turbulence is investigated using single-input multiple-output (SIMO) configuration and its performance under that environment is compared to that of ACO-OFDM. Energy improvements of 4dB and 2.2dB are obtained at a BER of 10−4 for SIMO systems of 1 and 2 photodetectors at the receiver for the case of strong turbulence, respectively.

Next-generation 400 Gb/s Ethernet PMD over SMF at 1310 nm via DD-OFDM with electro-absorption modulator-based transmitters

The technical feasibility of an unamplified 4 x 100 Gb/s wavelength division multiplexed direct-detection optical OFDM system that uses electro-absorption modulators (EAM) as transmitters is numerically demonstrated for up to 10-km reach of single-mode fiber (SMF) in O-band and 800 GHz-channel spacing. Based on the use of currently available optical components, most of which were developed for the current long-reach Ethernet standard, operating around 1300 nm, error-free operation (pre-FEC BER less than 3.8E-5) is achieved. The architecture here put forward represents a backward compatibility alternative that can be used to define next-generation 2- and 10-km 400 Gb/s Ethernet standard.

Low-complexity joint symbol synchronization and sampling frequency offset estimation scheme for optical IMDD OFDM systems.

A low-complexity joint symbol synchronization and SFO estimation scheme for asynchronous optical IMDD OFDM systems based on only one training symbol is proposed. Numerical simulations and experimental demonstrations are also under taken to evaluate the performance of the mentioned scheme. The experimental results show that robust and precise symbol synchronization and the SFO estimation can be achieved simultaneously at received optical power as low as -20dBm in asynchronous OOFDM systems. SFO estimation accuracy in MSE can be lower than 1 × 10-11 under SFO range from -60ppm to 60ppm after 25km SSMF transmission. Optimal System performance can be maintained until cumulate number of employed frames for calculation is less than 50 under above-mentioned conditions. Meanwhile, the proposed joint scheme has a low level of operation complexity comparing with existing methods, when the symbol synchronization and SFO estimation are considered together. Above-mentioned results can give an important reference in practical system designs.

An improved model for estimating the impact of FWM in coherent optical systems

Four Wave Mixing is the main source of nonlinearity in coherent optical OFDM systems that degrades the system performance. In this paper, based on a previously introduced model, we derive a closed form expression that calculates the FWM power. This model can be used for a fast estimation of system performance within a broad range of system parameters and/or can be also used as an integral part of physical layer aware algorithms. The proposed model is in very good agreement with the exact model as well as with numerical results and provides significant lower mismatch than the previously published model. Moreover, this expression is also expanded in Nyquist-WDM systems showing very good accuracy with numerical results while is modified to take into account dispersion compensation, something that was neglected in the previous work.

Improved BER Performance of Real-Time DDO-OFDM Systems Using Interleaved Reed–Solomon Codes

In this letter, we have experimentally demonstrated a Reed–Solomon (RS) coded and multiple symbol interleaved real-time orthogonal frequency-division multiplexing (OFDM) signals transmission over a single mode fiber (SMF)-based direct-detection optical system. To the best of our knowledge, this is the first time to realize the forward error correction (FEC) in real-time optical OFDM systems. The bit error rate (BER) performances of RS-coded OFDM signals are investigated in a low-cost directly modulated laser-based direct-detection optical transmission system. The experimental results show that after 25.26-km SMF-28 transmission of the real-time RS-coded and symbol interleaved optical OFDM signals, a post-FEC BER less than $1\times 10^{-8}$ is successfully achieved by using RS code with 7% overhead for a pre-FEC BER of $1\times 10^{-3}$ .

Demonstration of 2.97-Gb/s video signal transmissions in DML-based IM-DDO-OFDM systems

Abstract To further investigate the feasibility of the digital signal processing (DSP) algorithms (e.g., symbol timing synchronization, channel estimation and equalization, and sampling clock frequency offset (SCFO) estimation and compensation) for real-time optical orthogonal frequency-division multiplexing (OFDM) system, 2.97-Gb/s real-time high-definition video signal parallel transmission is experimentally demonstrated in OFDM-based short-reach intensity-modulated direct-detection (IM-DD) systems. The experimental results show that, in the presence of ∼12 ppm SCFO between transmitter and receiver, the adaptively modulated OFDM signal transmission over 20 km standard single-mode fiber with an error bit rate less than 1 × 10−9 can be achieved by using only DSP-based small SCFO estimation and compensation method without utilizing forward error correction technique. To the best of our knowledge, for the first time, we successfully demonstrate that the video signal at a bit rate in excess of 1-Gb/s transmission in a simple real-valued inverse fast Fourier transform and fast Fourier transform based IM-DD optical OFDM system employing a directly modulated laser.

Equalization of Dispersion-Induced Crosstalk in Optical Offset-QAM OFDM Systems

In discrete-Fourier-transform-based offset quadrature amplitude modulation (QAM) orthogonal frequency division multiplexing (OFDM), the orthogonality between the inter-carrier interference (ICI) and the signal is broken as dispersion increases, which in turn limits the dispersion tolerance. The ICI on a subcarrier is only from its adjacent two subcarriers, but its influence depends on the frequency of the subcarrier. We find that it is the high-frequency subcarriers that limit the dispersion tolerance and thus propose to equalize these dominant subcarriers to improve the performance. Simulations of 112-Gb/s offset-4QAM and 224-Gb/s offset-16QAM OFDM are performed to verify the analysis. We also experimentally demonstrate the transmission of a 38-Gb/s offset-16QAM OFDM signal over 1200-km single-mode fiber without cyclic prefix (CP). The bit error rate (BER) is reduced from $10^{-3}$ to $10^{-4}$ at $\sim 16$ -dB optical signal to noise ratio by equalizing only the high frequency half of the subcarriers. This is in contrast to conventional OFDM which cannot realize a BER of $10^{-3}$ at 400 km when CP is not applied.

Comparisons of spectrally-enhanced asymmetrically-clipped optical OFDM systems.

Asymmetrically clipped optical orthogonal frequency-division multiplexing (ACO-OFDM) is a technique that sacrifices spectral efficiency in order to transmit an orthogonally frequency-division multiplexed signal over a unipolar channel, such as a directly modulated direct-detection fiber or free-space channel. Several methods have been proposed to regain this spectral efficiency, including: asymmetrically clipped DC-biased optical OFDM (ADO-OFDM), enhanced U-OFDM (EU-OFDM), spectral and energy efficient OFDM (SEE-OFDM), Hybrid-ACO-OFDM and Layered-ACO-OFDM. This paper presents simulations up to high-order constellation sizes to show that Layered-ACO-OFDM offers the highest receiver sensitivity for a given optical power at spectral efficiencies above 3 bit/s/Hz. For comparison purposes, white Gaussian noise is added at the receiver, component nonlinearities are not considered, and the fiber is considered to be linear and dispersion-less. The simulations show that LACO-OFDM has a 7-dB sensitivity advantage over DC-biased OFDM (DCO-OFDM) for 1024-QAM at 87.5% of DCO-OFDM's spectral efficiency, at the same bit rate and optical power. This is approximately equivalent to a 4.4-dB advantage at the same spectral efficiency of 87.7% if 896-QAM were to be used for DCO-OFDM.

Experimental Demonstration of a 16.27 Gb/s 2-D Coherent Optical OFDM System With 3-D Signal Mapper and 2-D IFFT Modulator

We propose a coherent optical OFDM (CO-OFDM) system based on 3-dimensional (3-D) OFDM technique in this paper. By using the 3-D signal mapper and 2-D IFFT module, the minimum Euclidean distance (MED) in constellations is increased by 8.7%, and the peak-to-average power ratio of the generated OFDM signal is reduced by 4 dB. The experimental results show that the 3-D OFDM method can improve nonlinearity tolerance by 2 dB for 100-km standard single mode fiber in a 16.27-Gb/s 2-D coherent optical OFDM system without polarization modulation. Theoretical analysis and simulation results show that the OSNR penalty could be attributed to the increased MED, the extended decision boundaries and the coding gain. To further analyze the performance in long haul transmission system, the simulation setup of 1000-km SSMF CO-OFDM system is built up. The results verify that the proposed method could effectively enhance nonlinearity tolerance and transmission performance of the CO-OFDM system.

Phase Noise Effects on Phase-Modulated Coherent Optical OFDM

One of the primary drawbacks of orthogonal frequency-division multiplexing (OFDM) is the large peak-to-average ratio (PAPR). It continues to contribute nonlinear degradations after it was introduced into optical fiber transmissions. However, phase-modulated OFDM can reduce the PAPR to the minimum, since its amplitude can be constant. In this paper, we conduct the theoretical and experimental study on the laser phase noise effect in phase-modulated coherent optical OFDM systems. Due to the laser phase noise, the phase transformation at the receiver is prone to errors due to phase wrapping. Consequently, we propose a mean-phase-compensation (MPC) method, which compensate for the mean phase offset for each OFDM symbol that has a Gaussian-distributed histogram. This method can avoid the problematic unwrapping function in phase transformation. We derive the closed-form expressions for the phase noise spreading and the corresponding signal-to-noise ratios (SNRs) based on the Wiener phase noise model. Unlike conventional coherent optical OFDM, the phase noise spreading to each OFDM subcarrier is decided by the reciprocal of the square of the subcarrier frequency, and the SNR can be improved by using a stronger PM modulation. We also discuss the SNR improvement by discarding a few affected subcarriers closing to dc. In a simulation, we demonstrate that our closed-form expressions are accurate. Finally, we experimentally verify our MPC method and our analysis.

A Multiband DDO-OFDM System With Spectral Efficient Iterative SSBI Reduction DSP

A multiband direct-detected optical orthogonal frequency-division multiplexing (DDO-OFDM) system effectively enhance data capacity without high-end electrical devices. However, signals are optical-to-electrically converted with only one photodetector in a direct detecting scheme, signal–signal beating interference (SSBI) is meanwhile introduced. Traditionally, a large guard band can be applied to evade SSBI, however, it seriously restricts the precious spectral efficiency (SE). In this letter, we propose an iterative digital signal processing (DSP) in the receiving end of a multiband DDO-OFDM system to cope with the SSBI. With this iterative DSP, the corresponding SE theoretically approach to that of single polarization coherent optical OFDM systems. A total 65.4-Gb/s data rate is experimentally demonstrated with a three-band DDO-OFDM signal in 16-QAM format, and the SSBI reduction DSP saturates within two iterations. Various bandgaps and carrier-to-signal power ratios settings are also evaluated to investigate the feasibility of the proposed iterative DSP.

偏振分集相干光OFDM通信系统中的相位噪声消除

偏振分集相干光OFDM通信系统中的相位噪声消除

Theoretical Bit Error Rate in a Circular Polarized Optical OFDM System

Theoretical Bit Error Rate in a Circular Polarized Optical OFDM System

Timing Synchronization Method for Asymmetrically Clipped DC Biased Optical OFDM System

In a typical intensity-modulated direct-detection optical system, the information is carried on the intensity of the optical signal, and therefore can only be positive. Existing synchronization methods for RF-based OFDM system cannot be used directly in asymmetrically clipped DC biased optical of DM (ADO-OFDM) system. In order to solve the above problem, according to the characteristics of ADO-OFDM signal, the modified training symbol is presented. The simulation results show that the modified synchronization method gives good performance.

A combined PAPR-reduction technique for asymmetrically clipped optical OFDM system

In this paper, we propose a combined peak-to-average power ratio (PAPR) reduction technique for asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM) system. The proposed PAPR-reduction technique combines discrete Fourier transform (DFT)-spread technique with peak-clipping technique. For a tradeoff between PAPR and BER performance, the clipping ratio (CR) in peak-clipping technique is set to 2.2. Under this CR, a 6dB of PAPR reduction can be obtained by the combined PAPR-reduction technique at the probability of 10−3 and BER performance is barely affected by the clipping distortion. The transmission experiment over 100-km SSMF has been implemented to verify the feasibility of the proposed scheme. At the forward error correction (FEC) limit, ACO-OFDM with the combined PAPR-reduction technique has 1.8dB improvement of received sensitivity compared to ACO-OFDM with DFT-spread technique and 3.8dB improvement compared to conventional ACO-OFDM.

Discrete sliding norm transform‐based 50% PAPR reduction in asymmetrically clipped optical OFDM systems for optical wireless communications

Orthogonal frequency division multiplexing (OFDM) systems have inferior nonlinear performance due to their high peak to average power ratio (PAPR) characteristics, which degrades the performance in optical wireless communication systems. A modified asymmetrically clipped optical (ACO)-OFDM system with low PAPR by introducing a discrete sliding norm transform (DSNT) as low-complexity post-inverse discrete Fourier transform processing is proposed. Numerical results show that the proposed DSNT-ACO-OFDM achieves about 50% PAPR reduction, while maintaining a competitive bit error rate performance compared with the conventional schemes.

Experimental Demonstration of Real-Time High-Level QAM-Encoded Direct-Detection Optical OFDM Systems

In this paper, high-level quadrature amplitude modulation (QAM)-encoded real-time orthogonal frequency division multiplexing (OFDM) transceivers are implemented with two field programmable gate arrays and high-resolution digital-to-analog converter (DAC) and analog-to-digital converter (ADC). Some key digital signal processing (DSP) algorithms for real-time direct-detection optical OFDM (DDO-OFDM) system are presented and described in detail. To improve the effective number of bits of ADC and reduce quantization noise, the DAC operates at 5 GS/s with an oversampling factor of 2. Meanwhile, the optimal digital clipping ratio at the transmitter is also investigated by numerical simulation to optimize the performance of the real-time transmitter. The results show that the real-time measured BERs after 10-km SSMF are below the hard-decision forward error correction threshold of $3.8 \times 10^{-3}$ . For comparison, the off-line BER performance is also analyzed using off-line DSP approaches. It shows that there is a negligible power penalty between the offline and real-time processing results. To the best of our knowledge, we have achieved the highest modulation format (1024-QAM) for real-time optical OFDM systems.

Simple sampling clock synchronisation scheme for reduced‐guard‐interval coherent optical OFDM systems

A simple data-aided scheme for sampling clock synchronisation in reduced-guard-interval coherent optical orthogonal frequency division multiplexing (RGI-CO-OFDM) systems is proposed. In the proposed scheme, the sampling clock offset (SCO) is estimated by using the training symbols reserved for channel estimation, thus avoiding extra training overhead. The SCO is then compensated by resampling, using a time-domain interpolation filter. The feasibility of the proposed scheme is demonstrated by means of numerical simulations in a 32-Gbaud 16-QAM dual-polarisation RGI-CO-OFDM system.

New nonlinear companding technique to decrease peak to average power ratio of the optical orthogonal frequency division multiplexing system based on Hartley transform

The problem of optics orthogonal frequency division multiplexing (optical-OFDM) communication lies in the peak to average power ratio (PAPR), which seriously affects the quality of communication systems. A composite technique, which combines the Hartley transform and KC companding technique to reduce the PAPR of an optical-OFDM system, is proposed. The proposed technique can obtain the same quality of OFDM signals and offer an improved bit error rate performance by using Hartley transform instead of the tradi- tional Fourier transform, while the computational complexity is almost halved. Theoretical analysis and simu- lation results show that the proposed technique has a superior performance for reducing the PAPR when

A PAPR reduction scheme based on a new spreading code in optical direct detection OFDM system

In this paper, we propose and experimentally demonstrate a peak-to-average power ratio (PAPR) reduction scheme based on a new spreading code in direct detection optical orthogonal frequency division multiplexing (OFDM) system. The new spreading code with low cross correlation and high auto-correlation can support $$2N+1$$2N+1 users. Thus, $$2N+1$$2N+1 users or data symbols can be transmitted over only N subcarriers. The experimental results show that, after transmission over 70 km single-mode fiber, at the bit error rate of $$10^{-3}$$10-3, with fiber launch power of 2.75 dBm, the receiver sensitivity can be improved 2.1 dB by using the proposed scheme based on new spreading code. The PAPR can be reduced about 4.6 dB, compared with the original OFDM signal at a complementary cumulative distribution function of $$10^{-4}$$10-4.