Optical Orthogonal Frequency Division Multiplexing Transmission Research Articles

A Novel Energy-Efficient Transmission Scheme in CO-OFDM Elastic Optical Networks

The use of orthogonal frequency division multiplexing (OFDM) technology enables an optical transmission system to break the limitation of wavelength grids due to legacy of wavelength division multiplexing. This constructs a flexible and elastic transmission paradigm so as to achieve high spectrum efficiency and flexibility of fiber resource usage. This paper introduces a novel adaptive transmission strategy in elastic coherent optical OFDM transmission systems, aiming to optimize the system operation in terms of energy and spectrum consumptions for a transmission demand with a required data rate. By jointly considering the nonlinear effects of Mach-Zehnder modulator and amplified spontaneous emission noise from optical amplifiers, as well as the performance impairment due to high peak-to-average-power ratio (PAPR) in the electronic domain, we first provide an analytical model on the bit error rate performance for a single-elastic optical transmission line. To achieve an efficient PAPR reduction, we introduce a new method called simplified null switching, which is considered very suitable in the elastic optical transmission systems due to lower computation complexity and little dependence on the channel side information. Based on the analytical model, an optimization problem is formulated based on the proposed analytical model and solved via mathematical programming. Case studies via extensive numerical experiments are conducted to verify the proposed analytical model and gain better understanding on the solutions of formulated optimization problem.

New low-complexity and robust time synchronization technique for optical IMDD OFDM transmissions

This paper provides an analysis of the performance of conventional preamble-based time synchronization techniques, which have been proposed for Orthogonal Frequency Division Multiplexing (OFDM) radio transmissions, in the context of optical Intensity Modulated and Direct Detected (IMDD) OFDM transmissions over single-mode fiber. A novel preamble structure along with a two-stage synchronization algorithm is proposed. Thanks to the particular structure of the preamble, a first coarse synchronization metric does not present minor peaks. In addition, without the knowledge of the preamble waveform the receiver can perform both coarse and fine synchronization steps. As maximal delay in arrival time between different frequency components of a signal depends on chromatic dispersion and the signal bandwidth, it is also observed that a slowly time-varying preamble is more robust than a fast time-varying one. By means of numerical simulations, the proposed method is shown to provide better performance when compared to other conventional methods in terms of timing offset variance with reduced receiver complexity. In unamplified transmissions the proposed technique guarantees a quasi-optimal transmission capacity even with a short cyclic prefix.

Real-Time Optical OFDM Long-Reach PON System Over 100 km SSMF Using a Directly Modulated DFB Laser

A real-time base-band orthogonal frequency division multiplexing (OFDM) transceiver with a simple symbol synchronization and channel estimation scheme is implemented on only one off-the-shelf commercial field programmable gate array (FPGA). The real-time optical OFDM long-reach passive optical network (PON) system with direct detection over 100 km standard single-mode fiber (SSMF) employing a cost-effective directly modulated distributed feedback (DFB) laser has been successfully demonstrated experimentally. The experimental results show that the simplified symbol synchronization with low complexity and high accuracy is effective for long-reach optical OFDM transmission systems. After 100 km SSMF transmission, the power penalty at a bit-error rate of 1e-3 is only about 1.1 dB.

Nonlinear distortion evaluation of MZM with equivalent mathematical model calculation in IM/DD OOFDM transmission system

We propose an approach by reducing the distortion to signal ratio (DSR) of Mach–Zehnder modulator (MZM) to improve the bit error rate (BER) performance in intensity modulated and direct detected (IM/DD) optical orthogonal frequency division multiplexing (OOFDM) system. The DSR of the MZM, which is a measurement of nonlinear distortions induced by MZM, can be correctly evaluated by building an equivalent polynomial model, and then minimized with partial transmit sequence (PTS) technique without over-sampling. Experimental results demonstrate that compared to the original scheme and peak to average power ratio (PAPR) reduction with PTS scheme, the improvement of the receive sensitivity by the proposed scheme is 2.8dB and 1.2dB, respectively, at the BER of 1×10−3 after transmission over 100km standard single-mode fiber. The theory analysis also shows the proposed scheme can reduce computational complexity significantly compared to the PAPR-PTS scheme.

25.25-Gb/s Real-Time Multi-Band Optical OFDM Transmission Over 300-m MMFs With IQ Modulated Passband

Record-high 25.25-Gb/s real-time end-to-end optical orthogonal frequency-division multiplexing transmissions over an intensity-modulation and direct-detection-based 300-m OM2 multi-mode fiber links are achieved with a tri-sub-band transceiver architecture. A 9.75-Gb/s baseband signal is RF multiplexed with two 7.75-Gb/s sub-bands, which are IQ modulated onto a 6.125-Gdirect-detectionHz RF carrier. The adaptive bit/power-loaded, independently power-optimized sub-bands sampled at 4 GS/s are multiplexed to intensity modulate an EML. Similar BERs and optical penalties are observed for all sub-bands.

Adaptability-Enabled Record-High and Robust Capacity-Versus-Reach Performance of Real-Time Dual-Band Optical OFDM Signals Over Various OM1/OM2 MMF Systems [Invited

Real-time dual-band optical orthogonal frequency-division multiplexing (OOFDM) transceivers incorporating digital-to-analog and analog-to-digital converters operating at sampling speeds as low as 4 GS/s and field-programmable gate-array-based digital signal processing clocked at only 100 MHz are employed to explore the maximum achievable transmission performance of electro-absorption modulated laser-based multimode fiber (MMF) systems subject to conventional optical launching. Making use of adaptive subcarrier bit and subcarrier/subband power loading, record-high 20.125 Gb/s transmission over 800 m OM2 MMF of real-time dual-band OOFDM signals is experimentally demonstrated, for the first time, with an optical power penalty as low as 1 dB. In addition, extensive experimental explorations of dual-band OOFDM capacity versus reach performance are also undertaken over the aforementioned systems consisting of different MMF types and lengths. It is shown that 20.125 Gb/s over 100 m and 19.625 Gb/s over 1000 m OOFDM transmissions are obtainable in OM2-only MMF systems and that 20 Gb/s over 100 m and 19.375 Gb/s over 1000 m OOFDM transmissions are achievable in OM1-only MMF systems. Furthermore, in various OM1 and OM2 MMF systems, minimum optical power penalties of 19 Gb/s over 1000 m transmission of dual-band OOFDM signals are practically feasible in any legacy MMF systems.

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.

A Survey on OFDM-Based Elastic Core Optical Networking

Orthogonal frequency-division multiplexing (OFDM) is a modulation technology that has been widely adopted in many new and emerging broadband wireless and wireline communication systems. Due to its capability to transmit a high-speed data stream using multiple spectral-overlapped lower-speed subcarriers, OFDM technology offers superior advantages of high spectrum efficiency, robustness against inter-carrier and inter-symbol interference, adaptability to server channel conditions, etc. In recent years, there have been intensive studies on optical OFDM (O-OFDM) transmission technologies, and it is considered a promising technology for future ultra-high-speed optical transmission. Based on O-OFDM technology, a novel elastic optical network architecture with immense flexibility and scalability in spectrum allocation and data rate accommodation could be built to support diverse services and the rapid growth of Internet traffic in the future. In this paper, we present a comprehensive survey on OFDM-based elastic optical network technologies, including basic principles of OFDM, O-OFDM technologies, the architectures of OFDM-based elastic core optical networks, and related key enabling technologies. The main advantages and issues of OFDM-based elastic core optical networks that are under research are also discussed.

HERMITIAN SYMMETRY BASED FIBER NON-LINEARITY COMPENSATION IN OPTICAL OFDM NETWORKS

Orthogonal Frequency Division Multiplexing (OFDM) is a modulation technique which is now used in most new and emerging broadband wired and wireless communication systems such as standard 802.11a/b/g/n, Digital Video Broad casting Television (DVB-TV), and Long Term Evolution (LTE) in the next mobile generation, due to its capacity in solving the problems of Inter-Symbol Interference (ISI) caused by the effects of the dispersive channel. Very recently researches focus on applying OFDM technology in optical fiber communication systems. Optical OFDM is well suited for high speed transmission systems with high spectral efficiency and attracted significant attention from the optical communication community. One of the major issues that degrade the performance of optical OFDM networks is its fiber non-linearity. Fiber non-linearities represent the fundamental limiting mechanisms to the amount of data that can be transmitted on a single optical fiber. Non-linear effects arise as optical fiber data rates, transmission lengths, number of wavelengths, and optical power level increases. Therefore, the effect of non-linearity in high data rate optical networks needs to be controlled to enhance link performances. In this paper, a nonlinearity compensation technique (Hermitian Symmetry) is implemented to improve the performance of OFDM based optical networks. This would provide high spectral efficiency, low ISI and very good Bit Error Rate (BER) performances without increasing the complexity of the network. The optical OFDM transmission system with fiber non-linearity compensation is simulated using Virtual Photonics Integrated (VPI) software.

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.

Wavelet packet transform–based optical orthogonal frequency-division multiplexing transmission using direct detection

As an alternate to fast Fourier transform-based orthogonal frequency-division multiplexing (OFDM), wavelet packet transform (WPT)-based OFDM (WPT-OFDM) does not require cyclic prefix to avoid inter-symbol-interference. The wavelet has many varieties and therefore, it can provide more freedom for system design to suit different applications. We propose a real-valued WPT-OFDM that uses intensity modulation/direct detection. We also conduct an experiment to verify its performance through a 75-km standard single-mode fiber.

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.

Signal Shaping and Modulation for Optical Wireless Communication

In this paper, a signal shaping framework for optical wireless communication (OWC) is proposed. The framework is tailored to the single-carrier pulse modulation techniques, such as multi-level pulse position modulation (M-PPM) and multi-level pulse amplitude modulation (M-PAM), and to multi-carrier transmission realized through multi-level quadrature amplitude modulation (M-QAM) with orthogonal frequency division multiplexing (OFDM). Optical OFDM (O-OFDM) transmission is generally accomplished via direct-current-biased optical OFDM (DCO-OFDM) or asymmetrically clipped optical OFDM (ACO-OFDM). Through scaling and DC-biasing the transmitted signal is optimally conditioned in accord with the optical power constraints of the transmitter front-end, i.e., minimum, average and maximum radiated optical power. The OWC systems are compared in a novel fashion in terms of electrical signal-to-noise ratio (SNR) requirement and spectral efficiency as the signal band- width exceeds the coherence bandwidth of the optical wireless channel. In order to counter the channel effect at high data rates, computationally feasible equalization techniques such as linear feed-forward equalization (FFE) and nonlinear decision-feedback equalization (DFE) are employed for single-carrier transmission, while multi-carrier transmission combines bit and power loading with single-tap equalization. It is shown that DCO-OFDM has the highest spectral efficiency for a given electrical SNR at high data rates when the additional direct current (DC) bias power is neglected. When the DC bias power is counted towards the signal power, DCO-OFDM outperforms PAM with FFE, and it approaches the performance of the more computationally intensive PAM with DFE.

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.

Clipping Noise in OFDM-Based Optical Wireless Communication Systems

In this paper, the impact of clipping noise on optical wireless communication (OWC) systems employing orthogonal frequency division multiplexing (OFDM) is investigated. The two existing optical OFDM (O-OFDM) transmission schemes, asymmetrically clipped optical OFDM (ACO-OFDM) and direct-current-biased optical OFDM (DCO-OFDM), are studied. Time domain signal clipping generally results from direct current (DC) biasing and/or from physical limitations of the transmitter front-end. These include insufficient forward biasing and the maximum power driving limit of the emitter. The clipping noise can be modeled according to the Bussgang theorem and the central limit theorem (CLT) as attenuation of the data-carrying subcarriers at the receiver and addition of zero-mean complex-valued Gaussian noise. Analytical expressions for the attenuation factor and the clipping noise variance are determined in closed-form and employed in the derivation of the electrical signal-to-noise ratio (SNR). The validity of the model is verified through a Monte Carlo bit-error ratio (BER) simulation. Finally, the BER performance of ACO-OFDM with DCO-OFDM is compared for different clipping levels and multi-level quadrature amplitude modulation (M-QAM) schemes.

Directly Modulated VCSEL-Based Real-Time 11.25-Gb/s Optical OFDM Transmission Over 2000-m Legacy MMFs

In directly modulated vertical cavity surface-emitting laser (DM-VCSEL)-based, end-to-end real-time, 11.25-Gb/s optical orthogonal frequency division multiplexing (OOFDM) systems utilizing legacy OM1/OM2 multimode fibers (MMFs), as well as simple intensity-modulation and direct-detection (IMDD) detailed experimental explorations, are undertaken, for the first time, for the effectiveness of various optical launching conditions, including center launching and conventional offset launching in maximizing the transmission distances of the legacy MMF systems. It is shown that, in the aforementioned legacy MMF systems, center launching enables an adaptive power-loaded OOFDM transmission at a raw signal bit rate of 11.25 Gb/s over an unprecedented distance of 2000 m with an optical power penalty as low as 0.8 dB. When center launching is replaced by conventional offset launching, the transmission distance of the legacy MMF system is reduced to 300 m, and the corresponding optical power penalty is increased to 2.3 dB. Comparisons of measured maximum achievable transmission distances of various MMF IMDD system configurations subject to different launching conditions show that DM-VCSEL-based, adaptive power-loaded OOFDM at center launching is a viable and cost-effective solution for use in legacy MMF systems. This work may have great potential for upgrading installed MMF local area networks to 10 Gb/s and beyond.

Numerical analysis of SOA performance over a wide optical bandwidth in a CO-OFDM transmission system

We present a numerical analysis of the impact of the optical amplification by semiconductor optical amplifiers (SOAs) in a Coherent Optical-Orthogonal Frequency Division Multiplexing transmission link at 100 Gb/s. The numerical modeling of SOA is developed to be able to simulate all of nonlinear effects of the SOA, particularly four-wave mixing effect. This model is integrated into a co-simulation platform to perform a simulation at a system level. Error Vector Magnitude (EVM) measurement is given with respect to the number of subcarriers and phase-amplitude coupling. We show also the dependence of the EVM at the signal wavelength by performing our simulations on a wide optical bandwidth, taking into account the main parameters of the SOA—such as the phase-amplitude coupling factor, the saturation power and the noise figure—that influence the non-linear effects.

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.