Optical Orthogonal Frequency Division Multiplexing Transmission Research Articles

High security optical OFDM transmission scheme with four-dimensional region joint encryption based on power division multiplexing.

In this paper, a high security chaotic encryption scheme for orthogonal frequency division multiplexing (OFDM) transmission system is proposed by using power division multiplexing (PDM) technology and four-dimensional region joint encryption. The scheme uses PDM to realize simultaneous transmission of multiple user information, which can achieve a good compromise among system capacity, spectral efficiency and user fairness. In addition, bit cycle encryption, constellation rotation disturbance (CRD) and region joint constellation disturbance (RJCD) are used to realize four-dimensional region joint encryption, effectively improving the physical layer security. The masking factor is generated by the mapping of two-level chaotic systems, which can enhance the nonlinear dynamics and improve the sensitivity of encrypted system. A 11.76 Gb/s OFDM signal transmission over 25 km standard single-mode fiber (SSMF) is experimentally demonstrated. At the forward-error correction (FEC) bit error rate (BER) limit -3.8×10-3, the proposed receiver optical power based on quadrature phase shift keying (QPSK) without encryption, QPSK with encryption, variant-8quadrature amplitude modulation (V-8QAM) without encryption and V-8QAM with encryption are about -13.5dBm, -13.6dBm, -12.2dBm, and -12.1dBm. The key space is up to 10128. The results show that this scheme not only improves the security of the system and the ability to resist attackers, but also improves the system capacity and has the potential to serve more users. It has a good application prospect in the future optical network.

Flexible and fast multi‐level modulation format identification using optical orthogonal frequency division multiplexing redundancy subcarriers

AbstractCompared with the orthogonal frequency division multiplexing (OFDM) communication system, the subcarrier‐index modulation OFDM communication system can alleviate the problem of peak‐to‐average power ratio and the frequency offset sensitivity. Therefore, the problem of inter carrier interference (ICI) and low power of the radio frequency amplifier in OFDM communication system can be solved to some extent. A fast and flexible modulation format recognition (MFR) scheme utilizing OFDM redundancy subcarriers is proposed in order to solve the MFR problem in coherent optical OFDM transmission. The scheme uses the silent subcarrier position of OFDM to carry modulation format information (MFI) and the receiving end decodes the MFI index by comparing the modulus values of the redundant symbols. It can recognize the phase shift keying (PSK) and multiple quadrature amplitude modulation (M‐QAM) modulation formats block by block. The number of symbols required by the proposed MFR is equal to the number of OFDM subcarriers, and the average time to get the recognition result in a block is O(1). We numerically investigate the proposed MFR among 20 Gbaud 4/16/32/64 QAM and 8/16 PSK. The results show that the correct recognition rate of the proposed scheme is not affected by the modulation formats. In particular, the proposed MFR is a potential solution for future flexible optical networks.

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.

Time and Carrier Frequency Synchronization for Coherent Optical Communication: Implementation Considerations, Measurements, and Analysis

We develop a measurement scheme for the time and frequency offsets, along with their compensation, for a coherent optical receiver. This relies on a modified constant amplitude zero autocorrelation sequence-based method, which is implemented on a field-programmable gate array. Experiments are performed for an optical orthogonal frequency-division multiplexing transmission system. The performance of the proposed scheme is compared to a MATLAB-based implementation of the original scheme. The contributory factors to the error between these two schemes, as well as the implementation complexity, are investigated. Experimental measurements demonstrate that the proposed scheme closely matches the performance of the original scheme while utilizing fewer hardware resources.

Laser phase noise effect and reduction in self-homodyne optical OFDM transmission system.

We present a laser phase noise (PN) induced effect of a phase-modulation-to-intensity-modulation conversion noise and noise pedestals underneath each of the orthogonal frequency division multiplexing (OFDM) subcarriers in a self-coherent optical OFDM transmission using a self-homodyne technique. We provide a statistical analysis on the received symbols using a histogram to demonstrate the effect of a phase rotation term and inter-subcarrier interference individually and collectively. The PN is then compensated using a simple time delay to realign the phase walk-off of the subcarriers relative to the carrier. Significant quadrature improvements of 6.82 dB using 5 MHz laser linewidth over a 720 km transmission length and 5.38 dB using 20 MHz over 240 km have been obtained with 16 quadrature amplitude modulation (QAM) over 15 GHz OFDM signal bandwidth. The technique also significantly reduced an optical-signal-to-noise ratio requirement at the bit error rate of 1×10-3 by 16.15 dB for 64-QAM over 160 km. With the delay, the system can tolerate three times the chromatic dispersion-length product.

Low-complexity recombined SLM scheme for PAPR reduction in IM/DD optical OFDM systems.

High peak-to-average power ratio (PAPR) causes nonlinear impairments in intensity modulation direct detection (IM/DD) optical orthogonal frequency division multiplexing (O-OFDM) systems. Selective mapping (SLM) is a well-known effective PAPR reduction technique, but it suffers from high computational complexity due to the bank of inverse fast Fourier transforms (IFFTs) required to generate the set of candidate signals. In this paper, we propose a recombined SLM scheme that can generate up to 2U2 symbol candidates with U IFFTs. The candidate sequences are first partitioned and then recombined to generate new candidate signals, where the addition operation replaces the IFFT block and reduces the computational complexity significantly. Simulations and a real-time end-to-end IM/DD O-OFDM transmission system with line rate 10.5 Gb/s are set up to verify the performance of the proposed scheme. It is demonstrated that compared with conventional SLM, the proposed scheme achieves similar PAPR reduction performance with considerably lower computational complexity and no bit error rate degradation.

Analytical Solution of Stage-Dependent Bit Resolution of Full Parallel Variable Point FFTs for Real-Time DSP Implementation

Digital signal processing (DSP) is a major driving force for cost-effectively realizing “software-defined anything” required by future converged networks. The fast Fourier transform (FFT) is a fundamental building block of an overwhelming majority of those DSP algorithms. For practical real-time implementation, the logic resource usage reduction of FFT operations is critical for considerably decreasing the hardware cost and power consumption. In this paper, a simple and effective solution of stage-dependent minimum bit resolution of full parallel variable-point FFTs is analytically derived, for the first time, whose validity and robustness are rigorously verified, both numerically and experimentally, over intensity modulation and direct detection optical orthogonal frequency-division multiplexing transmission systems. The developed solution has unique advantages including great simplicity, excellent accuracy and robustness, and significant saving in logic resource usage. The solution can ease the practical real-time FFT DSP design, decrease the DSP complexity, and maximize the overall system performance by making full use of available transceiver/system design parameters.

On Adiabatic Chirp and Compensation for Nonlinear Distortion in DML-Based OFDM Transmission

This paper provides detailed descriptions of the interplay between dispersion and chirp (specifically adiabatic chirp), in an optical orthogonal frequency-division multiplexing (OFDM) transmission system based on a directly modulated DFB laser (DML). We experimentally investigated various amounts of dispersion and adiabatic chirp by, respectively, varying the length of the fiber (0–150 km) and the bias current of the laser. By implementing DSP-based compensation for nonlinear distortion while taking adiabatic chirp into account, we determined that adiabatic chirp dominates nonlinear distortion after dispersive transmission. Adiabatic chirp was also shown to mediate dispersion-induced power fading and even provide a power gain. This is an indication that a specific amount of adiabatic chirp may be beneficial to transmission performance, particularly when the nonlinear distortion is mitigated by DSP-based compensation. The proposed adiabatic-chirp-related nonlinear compensation scheme enables an improvement of up to 11 Gbps in maximum capacity, and the presence of adiabatic-chirp-related gain makes it possible to achieve higher capacities following dispersive transmission.

Secure OFDM Transmission Precoded by Chaotic Discrete Hartley Transform

We propose a chaotic discrete Hartley transform (DHT) to enhance the physical-layer security and simultaneously improve the transmission performance of optical orthogonal frequency division multiplexing (OFDM) in passive optical network. The theoretical analysis and numerical simulation show that the chaotic DHT precoding matrix after independent row/column permutations can also effectively reduce the peak-to-average power ratio (PAPR) of OFDM signals, which can be generated for OFDM data encryption using digital chaos. The multifold data encryption provides a huge key space of ∼10178 to ensure the physical-layer confidentiality. An encrypted data transmission of 8.9 Gb/s 16-QAM optical OFDM signals is successfully demonstrated over 20 km standard single-mode fiber. Due to the effective reduction in PAPR by the chaotic DHT precoding matrix, the receiver sensitivity is improved by ∼1.4 dB ${(\text{BER}@ 10}^{- 3})$ . Moreover, the proposed scheme has low computational complexity, which also provides high spectral efficiency since no additional sideband information is required.

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.

Experimental demonstration of 60 Gb/s optical OFDM transmissions at 1550 nm over 100 m OM1 MMF IMDD system with central launching

Record-high 60 Gb/s optical orthogonal frequency division multiplexing (OFDM) transmissions over intensity modulation and direct-detection (IMDD)-based 100 m optical mode (OM1) multi-mode fiber (MMF) links are experimentally demonstrated, utilizing 10 GHz electro-absorption modulated laser intensity modulators at a single 1550 nm wavelength. Adaptive bit loading and a simple central launching scheme of the proposed scheme show an effective way for combating the channel fading and simplifying the system structure. It shows good potential in short reach data center interconnections.

General optical discrete z transform: design and application.

This paper presents a generalization of the discrete z transform algorithm. It is shown that the GOD-ZT algorithm is a generalization of several important conventional discrete transforms. Based on the GOD-ZT algorithm, a tunable general optical discrete z transform (GOD-ZT) processor is synthesized using the silica-based finite impulse response transversal filter. To demonstrate the effectiveness of the method, the design and simulation of a tunable optical discrete Fourier transform (ODFT) processor as a special case of the synthesized GOD-ZT processor is presented. It is also shown that the ODFT processor can function as a real-time optical spectrum analyzer. The tunable ODFT has an important potential application as a tunable optical demultiplexer at the receiver end of an optical orthogonal frequency-division multiplexing transmission system.

Chaotic Encryption Algorithm Against Chosen-Plaintext Attacks in Optical OFDM Transmission

We propose and demonstrate a physical-layer encryption algorithm against chosen-plaintext attacks (CPAs) in optical orthogonal frequency-division multiplexing (OOFDM) transmission, where the cyphertexts are dynamically generated for the first time by incorporating the random feature of the input data. The multiple-fold encryption algorithm consists of chaotic subcarrier allocation scrambling in time/frequency domains, chaotic phase rotation of subcarriers, as well as chaotic training sequence, which creates a total huge key space of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">194</sup> by using a 4-D hyperdigital chaos. The transmission of 8.9-Gb/s 16-QAM encrypted OOFDM data is demonstrated over 20-km standard single-mode fiber. The proposed encryption scheme provides excellent confidentiality of OOFDM transmission against the brute-force attacks and CPAs.

Asymmetric peak to average power ratio impacts on direct‐detected optical OFDM transmission link

ABSTRACTIn this article, the impact of asymmetric peak to average power ratio (PAPR) of orthogonal frequency division multiplexing (OFDM) signal is investigated in direct detection OFDM transmission for the first time. The asymmetric PAPR is produced due to the positive definiteness of direct optical OFDM modulation. It gets bigger and bigger due to the generation of different time delay of each OFDM subcarrier, which is caused by chromatic dispersion. The performance of direct detection optical OFDM link in the presence of asymmetric PAPR is evaluated measuring error vector magnitude (EVM) of quadrature phase shift keying (QPSK) and 16‐qudrature amplitude modulation (QAM) symbols according to the optical transmission length and the transmission rate; 14% EVM (bit error rate of 10−3) of QPSK symbol is observed at the optical transmission length of 400 km despite the usage of channel equalization as well as the insertion of cyclic prefix when the asymmetric PAPR of input optical OFDM signal is above 25 dB. © 2016 Wiley Periodicals, Inc. Microwave Opt Technol Lett 58:2662–2665, 2016

Blind Phase Noise Estimation for CO-OFDM Transmissions

In this paper, we discuss in detail the performance of different blind phase noise estimation schemes for coherent optical orthogonal frequency-division multiplexing transmissions. We first derive a general model of such systems with phase noise. Based on this model, the phase cycle slip probability in blind phase noise estimation is calculated. For blind phase tracking, we present and discuss the implementation of feedback loop and digital phase tracking. We then analyze in detail the performance of a decision-direct-free blind scheme, in which only three test phases are required for phase noise compensation. We show that the decision-direct-free blind scheme is transparent to QAM formats, and can provide a similar performance to the conventional blind phase search employing 16 test phases. We also propose two novel cost functions to further reduce the complexity of this scheme.

Achievable information rates calculation for optical OFDM few-mode fiber long-haul transmission systems.

We propose a method to estimate the lower bound of achievable information rates (AIRs) of high speed orthogonal frequency-division multiplexing (OFDM) in spatial division multiplexing (SDM) optical long-haul transmission systems. The estimation of AIR is based on the forward recursion of multidimensional super-symbol efficient sliding-window Bahl-Cocke-Jelinek-Raviv (BCJR) algorithm. We consider most of the degradations of fiber links including nonlinear effects in few-mode fiber (FMF). This method does not consider the SDM as a simple multiplexer of independent data streams, but provides a super-symbol version for AIR calculation over spatial channels. This super-symbol version of AIR calculation algorithm, in principle, can be used for arbitrary multiple-input-multiple-output (MIMO)-SDM system with channel memory consideration. We illustrate this method by performing Monte Carlo simulations in a complete FMF model. Both channel model and algorithm for calculation of the AIRs are described in details. We also compare the AIRs results for QPSK/16QAM in both single mode fiber (SMF)- and FMF-based optical OFDM transmission.

Intra-Cavity Chromatic Dispersion Impacts on 10-Gb/s Optical OFDM Transmissions Over 25-km Dual-RSOA-Based Self-Seeded PON Systems

Owing to the lack of strong physical mechanisms capable of elaborately balancing a large number of longitudinal modes created in a self-seeded passive optical network (PON), the resulting coherent-like optical signals are vulnerable to frequency-dependent optical phenomena such as chromatic dispersion (CD). In this paper, based on our recently reported real-time dual-reflective semiconductor optical amplifier (RSOA)based self-seeded adaptive optical orthogonal frequency-division multiplexing (OOFDM) transmitters, detailed experimental explorations are undertaken, for the first time, of the impacts of intra-cavity CD on 10-Gb/s OOFDM transmissions over 25-km self-seeded PON systems. The cavity length-dependent intra-cavity CD effect is identified to be one of the most prominent physical mechanisms determining the practically achievable performance of such PON systems. In comparison with a 10-m-long dual-RSOA self-seeded fiber cavity, the accumulated intra-cavity CD effect associated with a 1-km-long dual-RSOA self-seeded fiber cavity not only considerably broadens the optical spectral width of the self-seeding-created coherent-like optical signal but increases the signal relative intensity noise (RIN) by 4.8 dB (at a 2-GHz signal spectral region) as well. In addition, a further 1.1-dB signal RIN growth is also measured after the 25-km single-mode fiber (SMF) PON system transmissions. As a direct result, the dual-RSOA self-seeded PON with a 1-km-long fiber cavity suffers from a 2.2-dB power penalty degradation for 10 Gb/s over 25-km SMF intensity modulation and direct detection OOFDM transmissions.

Datacenter-Oriented Elastic Optical Networks: Architecture, Operation, and Solutions

With the exponentially increasing Internet traffic and emergence of more versatile and heterogeneous applications, the design of datacenter networks (DCNs) is subject to unprecedented requirements for larger capacity and more flexible switching granularities. Envisioning Optical-Orthogonal Frequency Division Multiplexing (O-OFDM) as a promising candidate for such a scenario, we motivate the use of O-OFDM as the underlying switching technology in order to provide sufficient switching capacity and elastic bandwidth allocation. For this purpose, this article reviews the recent progresses of DCN deployment and assesses the scenario where the O-OFDM transmission and switching technology is employed in the underlying transport plane. We discuss the key issues of the datacenter-oriented O-OFDM optical networks, and in particular, elaborate on a number of open issues and solutions including system interconnection architecture, routing and resource assignment, survivability, and energy-efficiency.

Combining discrete cosine transform with clipping for PAPR reduction in intensity-modulated OFDM systems

In this paper, the peak-to-average power ratio (PAPR) of orthogonal frequency division multiplexing (OFDM) signal is reduced by combining the discrete cosine transform (DCT) with clipping in optical intensity-modulated direct-detection (IM/DD) OFDM systems. First, the data are transformed into new modified data by DCT. Second, the proposed scheme utilizes the clipping technique to further reduce the PAPR of OFDM signal. We experimentally demonstrate that the optical OFDM transmission system with this proposed scheme can achieve significant performance improvement in terms of PAPR and bit error rate (BER) compared with the original optical OFDM systems.