Optical Optical Orthogonal Frequency-division Multiplexing Transmission Research Articles

Clipping Noise Mitigation in Optical OFDM Using Decision-Directed Signal Reconstruction

In this paper, the performance of a new decision-directed signal reconstruction (DDSR) algorithm with optimal reconstruction thresholds for use with optical orthogonal frequency division multiplexing (OFDM) modulation is investigated. Clipping noise remains one of the main drawbacks in many practical optical OFDM transmission systems. It has been shown in previous studies that a time-domain based DDSR algorithm can be used to effectively reduce the clipping noise. However, in all existing works studying this DDSR algorithm, the decision threshold used to direct the time-domain signal reconstruction is fixed and equal to the level at which the transmitted signal is clipped. In this paper, it is shown that, due to the influences of the unavoidable noise at the receiver, fixing the DDSR threshold at the clipping level is not an optimal solution and can severely limit the advantages of using this algorithm. A detailed theoretical analysis is performed to investigate the key factors that influence the choices of the optimal DDSR thresholds. The presented simulation results are used to verify the accuracy of the theoretical analysis and also show that the performance of the DDSR algorithm is significantly improved by using the obtained optimal thresholds.

Real-time secure optical OFDM transmission with chaotic data encryption

Offline digital signal processing is generally applied in secure optical orthogonal frequency division multiplexing (OFDM) transmission, however the real-time secure optical OFDM transmission is necessary to evaluate the security and transmission performances. Thus a real-time secure optical OFDM transmission with chaotic data encryption is developed to demonstrate the high-speed data transmission with high-level physical-layer confidentiality, where field-programmable gate array (FPGA) boards are employed for the real-time encryption and decryption of optical OFDM signals, along with pipeline analog-to-digital and digital-to-analog converters. The physical-layer security enhancement is attributed to hyper digital chaos, providing a huge key space of 1045. As a proof of concept, the real-time encrypted 16-QAM optical OFDM signal transmission at 435 Mb/s is successfully demonstrated over 22 km standard single-mode fiber (SSMF), without any degradation in transmission performance. The proposed FPGA prototype can be a good candidate for the practical implementation of secure data transmission in OFDM passive optical network.

Laser Phase Noise Tolerance in Direct Detection Optical OFDM Transmission Using Laser Linewidth Emulator

Laser phase noise tolerance in direct detection optical orthogonal frequency division multiplexing (OFDM) transmission system is experimentally demonstrated using laser linewidth emulator. With this linewidth emulator, the linewidth of the laser can be broadened independently without inducing other laser's noise characteristics such as relative intensity noise and chirping. The results show the effect of phase-to-intensity noise conversion due to the laser's phase noise and chromatic dispersion. This induces noise pedestals not only onto the optical carrier, but also to each of the subcarriers, and thus produces intersubcarriers interference. Measurements show 6 dB Q-degradation is suffered by the system when 10 MHz linewidth is used as compared to using narrowest emulator linewidth of 256 kHz. Transmission with the linewidth emulator set to 20 MHz is also shown where it can still be tolerated by 400 km of fiber at the Q of 9.606 dB with respect to $1.6\,\times \,10^{{\rm - 3}}$ bit error rate. Simulation of 16 quadratic-amplitude modulation (QAM) (∼128 Gb/s) and 4 QAM (∼100 Gb/s) using VPItransmissionMaker are also presented to demonstrate the effect of carrier's phase-to-intensity noise conversion when a high data rate and wide OFDM signal bandwidth are used with the linewidth emulator as the system's laser source. The results show that a phase rotation term is insignificant for lower M -size QAM with fiber length less than 500 km. This presents the high effectiveness and reliability of the laser linewidth emulator to discretely determine an independent phase noise tolerance from the other laser's noise characteristics in the optical OFDM transmission system.

Channel Estimation Algorithms in Optical OFDM Transmission

Recent progress and patents on channel estimation algorithms in optical orthogonal frequency division multiplexing (OFDM) systems are reviewed. The channel estimation algorithms can be divided into two categories of pilotaided and blind approaches. For the pilot-aided schemes, the two pilot patterns including the comb-type pilots and blocktype pilots are evaluated by experiments. For the blind schemes, the phase noise caused by the laser linewidth and chromatic dispersion (CD) in the fiber channel can be effectively compensated by the proposed data-aided channel estimation algorithms, which is experimentally demonstrated by the significant improvement in the performances of transmitted OFDM signals.

Experimental evaluation of pilot pattern design in direct-detection optical OFDM transmission

Pilot patterns are experimentally investigated to characterize the frequency/time dependence of the optical orthogonal frequency division multiplexing (OFDM) signal transmission. The optical signal performance is evaluated in terms of error vector magnitude (EVM). It is shown that, the quality of the OFDM signals can be improved up to 4dB in EVM, for 10Gb/s, 16-QAM-encoded OFDM signals after 20km single mode fiber (SMF) transmission with intensity-modulation and direct-detection (IMDD). The best performance is obtained by applying pilot tones for all subcarriers, which implies that the optical OFDM transmission is relatively quasi-static with respect to the subcarrier frequencies. The noise of the OFDM signals originated mainly from the amplitude/phase fluctuation of optical signal with time.

Theory and Performance Analyses in Secure CO-OFDM Transmission System Based on Two-Dimensional Permutation

Security in optical orthogonal frequency division multiplexing (OFDM) network has traditionally been considered to be addressed in higher layers of the network, while the physical layer is vulnerable to various attacks. As the dramatic increase of network capacity and secret sensitivity, the issue of physical layer security becomes more and more important. This paper proposes a novel physical-enhanced chaotic secure strategy for optical OFDM system based on two-dimensional (2-D) permutation. A controlled Logistic map is adopted for the chaotic mapping. The 2-D secure algorithm jointly utilizes frequency subcarriers and time-slots permutation to guarantee the secure of physical layer security of optical OFDM transmission system. This method performs dynamic permutations over frequency and time dimensions, resulting in a robust protection on transmitted information. Experiment is performed to confirm the good effectiveness of the proposed method.

Hadamard Transform Combined With Companding Transform Technique for PAPR Reduction in an Optical Direct-Detection OFDM System

In this paper, we propose a novel method to reduce the high peak-to-average power ratio (PAPR) of the optical orthogonal frequency division multiplexing (OFDM) signal. The method is based on the technique of the Hadamard transform combined with a companding transform. We experimentally demonstrate that, in an optical intensity-modulation direct-detection OFDM transmission system, the hybrid method has better performance in reducing the PAPR compared with the case of applying only the Hadamard transform or only a companding transform.

Spectral Efficient Coding Schemes in Optical Communications

Achieving high spectral efficiency in optical transmissions has recently attracted much attention, aiming to satisfy the ever increasing demand for high data rates in optical fiber communications. Therefore, strong Forward Error Correction (FEC) coding in combination with multilevel modulation schemes is mandatory to approach the channel capacity of the transmission link. In this paper we give design rules on the joint optimization of coding and signal constellations under practical considerations. We give trade-offs between spectral efficiency and hardware complexity, by comparing coding schemes using capacity achieving constellations with bit-interleaved coded modulation and iterative decoding (BICM-ID) against applying conventional square quadrature amplitude modulation (QAM) constellations but employing powerful low complexity low-density parity-check (LDPC) codes. Both schemes are suitable for optical single carrier (SC) and optical orthogonal frequency-division multiplexing (OFDM) transmission systems, where we consider the latter one in this paper, due to well-studied equalization techniques in wireless communications. We numerically study the performance of different coded modulation formats in optical OFDM transmission, showing that for a fiber optical transmission link of 960 km reach the net spectral efficiency can be increased by ≈0.4 bit/s/Hz to 8.61 bit/s/Hz at a post FEC BER of <10-15 by using coded optimized constellations instead of coded 64-QAM.In this paper we propose a high spectral efficient coded modulation scheme for implementation in future optical communication systems operating at data rates beyond 400 Gb/s. In detail, we adapt the “Turbo Principle” to BICM-ID[8] and combine it with a high-rate outer algebraic code to obtain a post-FEC BER <10-15, which is a typical demand in optical transponders. Furthermore we give simple design principles for the design of BICM-ID based on the extrinsic information transfer (EXIT) chart analysis[9]. The optical channel is considered to be weakly-nonlinear. Therefore the proposed techniques are also applicable for single-carrier transmission; however we consider OFDM since it appears to be more appropriate for the high order modulation formats and efficient equalization algorithms that are well established in wireless communications.

Performance Analysis for Optical OFDM Transmission in Short-Range IM/DD Systems

This paper investigates the performance of some recently proposed optical orthogonal frequency-division multiplexing (OFDM) transmission techniques in a short-range intensity-modulated, direct-detected (IM/DD) optical channel. Due to the unipolarity of the IM/DD channel, asymmetric clipping and associated nonlinear distortion is inevitable, resulting in an unfavorable dc power and a significant performance penalty. Using a detailed analysis of the clipped signal, our recent work focused on reducing this clipping-resulted dc via both power optimization and frequency planning. In this paper, a set of effective receiver signal-to-noise ratio formulas is proposed, which helps in accurately and efficiently evaluating the system performance across various optical OFDM systems. We conduct a power efficiency analysis to reveal tradeoffs between power and spectral efficiencies among different systems. Our results show that with a cost of additional complexity, a diversity-combining technique offers significant benefits in all cases.

A Tone Reservation-Based Optical OFDM System for Short-Range IM/DD Transmission

This paper investigates the use of tone reservation for optical orthogonal frequency-division multiplexing (OFDM) transmission in a short-range, intensity-modulated, direct-detected (IM/DD) optical channel. Due to the unipolarity of the IM/DD channel, any asymmetric clipping of the bipolar OFDM signal and associated nonlinear distortion will result in an unfavorable dc power and significant performance penalty. Using a detailed analysis of the clipping distortion, we propose a new framework to enable IM/DD optical OFDM transmission, where certain subchannels are reserved to produce a negative peak cancellation (NPC) signal via a frequency-domain digital filter. This NPC signal guarantees a unipolar OFDM signal, hence eliminating any additional biasing and/or clipping. It also produces no in-band or out-of-and distortions and requires no transmission of any side information. Beneficial tradeoffs are identified that enable an efficient use of both the available power and spectrum.

On the Phase Noise Impact in Direct-Detection Optical OFDM Transmission

In this letter, we characterize the impact of laser phase noise (PN) in direct-detection optical orthogonal frequency-division multiplexing (OFDM) and emphasize its several differences from those in coherent optical OFDM. We also analyze the system performance in the presence of PN for various quadrature-amplitude-modulation formats and provide the bit-error-rate estimation method which can yield reliable results when the PN-induced optical signal-to-noise ratio penalty is lower than ~2 dB.

A hybrid IQ imbalance compensation method for optical OFDM transmission

Transmitter and receiver IQ imbalance causes image interference that degrades performance in high capacity and high spectral efficiency optical orthogonal frequency division multiplexing (OFDM) schemes. Digital compensation is an attractive method to relax component specifications. In this paper we report the details of a hybrid compensation method for IQ imbalance compensation, comprising of orthogonal training symbol-based method for transmitter-side compensation and an iterative image reduction-based method for receiver-side imbalance compensation. We demonstrate performance improvement using the hybrid method in presence of frequency dependent imbalance by both simulation and back-to-back direct detection optical OFDM experiment. We report on the tolerable limit of transmitter IQ imbalance under presence of carrier frequency offset.

Achievement of Subchannel Frequency Spacing Less Than Symbol Rate and Improvement of Dispersion Tolerance in Optical OFDM Transmission

We propose a novel optical orthogonal frequency-division multiplexing (OFDM) transmission, where the frequency spacing of subchannels is less than the symbol rate. We show experimentally that the novel OFDM transmission achieves an optical SNR (OSNR) sensitivity level and dispersion tolerance roughly equivalent to those of the conventional OFDM. We also show the influence of chromatic dispersion from the transmission channel on the OFDM signal. The chromatic dispersion causes a desynchronization of the symbol timing between subchannels and breaks the frequency orthogonality in OFDM. We confirm experimentally that it is possible to mitigate the signal degradation due to the chromatic dispersion by optimizing the symbol timing and orthogonally polarizing the subchannels to the neighboring subchannels.

Sensitivity to signal quantization of 43 Gb/s and 107 Gb/s optical 16-QAM OFDM transmission with coherent detection

The sensitivity of optical orthogonal-frequency division multiplexing (OFDM) transmission to the finite resolutions of the digital-to-analog (DAC) and analog-to-digital converters (ADC) has been analyzed using numerical simulations. We show that for back-to-back configuration the requirements regarding the number of quantization bits for the DACs are similar to the ones of the ADCs. After transmission over metro and long-haul distances a higher resolution of the DAC/ADC is required compared to back-to-back configuration. We show that after transmission it is sufficient to enhance either the ADC resolution or the DAC resolution by 1 bit.

Guard-band influence on orthogonal-band-multiplexed coherent optical OFDM

Coherent optical orthogonal frequency-division multiplexing (OFDM) has recently been demonstrated beyond 100 Gb/s using orthogonal band multiplexing (OBM). OBM-OFDM gives a multitude of benefits in implementing high-speed optical OFDM transmission. We investigate the system sensitivity to the guard band of OBM-OFDM. The result suggests that when the guard band equals a multiple of the subcarrier spacing, the Q penalty practically vanishes. We also show that two OFDM bands can be demodulated simultaneously with one fast Fourier transform, owing to interband orthogonality, and its performance is almost the same as one-band detection.