Direct-detection Orthogonal Frequency Division Multiplexing Research Articles

Performance analysis of 4QAM-OFDM-FSO link under rain weather conditions

Abstract Free space optical communication (FSO) is a technique based on transmission of data by propagating the light in free space. FSO is inexpensive, providing high data rates, high capacity, high security, and low power and uses the license free frequency spectrum. Nevertheless, FSO is still facing many problems such as bad weather conditions, atmospheric turbulence, and multipath fading. In this research work, orthogonal frequency division multiplexing (OFDM) FSO system is analyzed for low to heavy rain fall intensities. Different rain rates have been considered for low, moderate, heavy, very heavy, and torrential rains. For all the rain rates, a direct detection OFDM–FSO system and a coherent detection system have been simulated and results are compared. Analysis has been done on the basis of SNR, received power, and constellation diagrams. According to these parameters, link length of both the systems for different rain rates was found. Coherent detection is applied to improve the system sensitivity compared to conventional direct detection. In tropical regions where heavy rainfall is the main reason for the degradation of the system, the OFDM system with coherent detection can perform better with high data rates. Second, over a 11 km FSO, a 4-QAM wireless signal is successfully transmitted under very heavy rainfall conditions. The distance achieved by moderate and heavy rains is 25 km and 15.5 km with an acceptable SNR of 25 dB and a system sensitivity (received power) of −65.1 dBm.

Dynamic polarization-insensitive BOTDA in direct-detection OFDM with CNN-based BFS extraction.

We propose a dynamic polarization-insensitive Brillouin optical time domain analyzer (D/PI-BOTDA) with orthogonal frequency division multiplexing (OFDM) based on intensity-modulated direct-detection (IM-DD). A polarization-division-multiplexed (PDM) pump signal enables polarization diversity of the stimulated Brillouin scattering while a multi-frequency OFDM probe signal realizes dynamic sensing with single-shot transmission. We experimentally demonstrated distributed temperature sensing along a total 940-meter fiber with a temperature sensing coefficient of 1.2°C/MHz. The experimental results indicated a remarkable suppression of Brillouin gain fluctuation up to 4.38 times compared to the case without polarization diversity. To facilitate the Brillouin frequency shift (BFS) extraction process, we also implement a CNN-based BFS extraction method with SE-Res2Net block. The adopted algorithm achieves a higher accuracy than conventional curve fitting method, with a 10-time enhancement in the time efficiency.

Modeling and spectral analysis of high speed optical fiber communication with orthogonal frequency division multiplexing

Abstract Optical communication has emerged as the best solution to high-speed transmission systems due to its higher bandwidth and higher data rates. The higher data rates with simple transmitter and receiver modeling are one of the best solutions to design the spectrally efficient high-speed optical transmission system using different modulation techniques. In the recent past, optical communication networks have been specifically designed and optimized to support different communication standards increasing the capabilities of optic fiber, integrated waveguides, and related optical fiber hardware, and signal conditioning circuits. So, this research paper presents a study, modeling, simulation, and analysis of optical performance under different transmission systems which are considered in a practical high-speed design using orthogonal frequency division multiplexing (OFDM). This can be done by using both coherent detection OFDM and direct detection OFDM to minimize dispersion effects in optical communication because OFDM is basic for modern technology to increase the data rate and increase the requirement of bandwidth in broadband services. Many optical fiber problems, such as chromatic dispersion (CD) and polarization mode dispersion (PMD) are solved using OFDM. The performance analyses made in this work by using different parameters such as bit error rate (BER), optical signal-to-noise ratio (OSNR), quality (Q) factor at different distance transmission by using different quadrature modulation such as 4-QAM, 16-QAM, 32-QAM, and 64-QAM to maintain the OSNR value, spectral efficiency, and the required data rates. The simulated results have shown that the coherent detection OFDM and direct detection OFDM with considered QAM provides the best value of BER and quality of the signal at a specific distance.

A Simple Blind Sampling Frequency Offset Estimation Scheme for Short-Reach DD-OFDM Systems

In this paper, a simple and overhead-free sampling frequency offset (SFO) blind estimation and compensation scheme is proposed and experimentally demonstrated in a short-reach direct detection orthogonal frequency division multiplexing (DD-OFDM) transmission system. After extracting subcarriers of OFDM symbols from multiple OFDM frames, the phase shift is obtained by the fourth-power algorithm, and the obtained results are averaged to perform phase estimation. It can achieve high-precision SFO estimation while ensuring low computational complexity as well as high spectral efficiency. The experimental results show that the SFO estimation accuracy using the proposed scheme can be significantly reduced to as low as ±3 ppm when SFO ranges from -1000 ppm to +1000 ppm after 20-km standard single-mode fiber (SSMF) transmission. Moreover, for the QPSK/16QAM encoded DD-OFDM systems aided by the proposed scheme, up to ±200 ppm SFO can be compensated with slight error vector magnitude (EVM) penalties after 20-km SSMF transmission. Furthermore, the results demonstrate that despite the existence of severe SFO, our proposed SFO compensation scheme can offer nearly negligible power penalties in the DD-OFDM system with optical back-to-back (OBTB) and 20-km SSMF transmission as compared to the ideal case without SFO.

Physical Layer Authentication Based on BER Measurement of Optical Fiber Channel

Physical layer authentication is an important way to ensure the security of optical communication network. We hereby propose a scheme to realize it by measuring the variation of the bit error rate (BER) on both sides of communication. In this scheme, the legitimacy of the receiver is identified by analyzing the BER variation of the optical fiber loop based on the short-term correlation of the channels. We then simulate a 16 PSK optical transmission system with intensity modulation direct detection-orthogonal frequency division multiplexing (IMDD-OFDM). The authentication effect is analyzed in the case of disturbance and beam split, as well as replacement of optical fiber channels caused by eavesdropper (Eve). The results show that this scheme is sensitive to the three kinds of attacks. A high probability of detection (PD) and a low false alarm rate (FAR) can be obtained. The experimental results show that with the increase of the frequency test, PD and FAR tend to be stable, and the authentication effect is better with the accuracy rate 100%.

Vector mode based optical direct detection orthogonal frequency division multiplexing transmission in short-reach optical link

As one solution to implement the large-capacity space division multiplexing (SDM) transmission systems, the mode division multiplexing (MDM) has gained much attention recently. The vector mode (VM), which is the eigenmode of the optical fiber, has also been adopted to realize the optical communications including the transmission over free-space optical (FSO) and optical fiber links. Considering the concerns on the short-reach optical interconnects, the low cost and high integration technologies should be developed. Direct detection (DD) with higher-order modulation formats in combination of MDM technologies could offer an available trade-off in system performance and complexity. We review demonstrations of FSO and fiber high-speed data transmission based on the VM MDM (VMDM) technologies. The special VMs, cylindrical vector beams (CVB), have been generated by the q-plate (QP) and characterized accordingly. And then they were used to implement the VMDM transmission with direct-detection orthogonal frequency division multiplexing (DD-OFDM). These demonstrations show the potential of VMDM-DD-OFDM technology in the large-capacity short-reach transmission links.

Sparse-fast-Fourier-transform-based quick synchronization for optical direct detection orthogonal frequency division multiplexing systems.

Orthogonal frequency division multiplexing (OFDM) has always been a promising candidate for optical access networks. However, it is very sensitive to synchronization errors and requires complex digital signal processing to eliminate this influence, thus increasing the computation complexity, delay, and system cost, which hampers its applications in cost-sensitive and low-latency scenarios of future optical access networks. To deal with this issue, a sparse-fast-Fourier-transform (FFT)-based quick synchronization algorithm for optical direct detection OFDM systems is proposed and demonstrated with greatly reduced computation complexity. Detailed simulations and experimental verifications along 50km standard single-mode-fiber transmission prove the efficiency, accuracy, and feasibility of the sparse-FFT-based synchronization technique in cost- and delay-sensitive applications for next-generation optical access networks.

Adaptive Three-Dimensional Optimization for Optical Direct-Detection OFDM

We propose an adaptive three-dimensional (3-D) optimization including bit loading, power loading, and trellis-coded modulation for optical direct-detection orthogonal frequency division multiplexing (OFDM) in the fading channel. This approach could improve the bandwidth of the various devices such as electro-optical components, digital to analog convertors (DACs), and analog to digital convertors (ADCs) effectively. We conduct experiments to verify this 3-D optimization and compare the performance with that of 2-D optimization, i.e., bit loading and power loading only. Our experimental results show that the receiver sensitivity of 3-D optimization can be improved by 2 dB at the BER of 10 –3 compared with that of 2-D optimization in the fading channel. With the simple scheme we present of realizing the optimum 3-D optimization adaptively, the data rate is improved by 32% compared with using adaptive 2-D optimization in the fading channel. With the adaptive 3-D optimization, we also transmit a 16-GHz optical OFDM signal using a 10-GHz Mach–Zehnder (MZ) intensity modulator over a 20-km standard single-mode fiber link successfully. The modulation bandwidth of the MZ modulator is improved by 60% and the data rate is improved by 40%.

120 Gbit/s 2 × 2 Vector-Modes-Division- Multiplexing DD-OFDM-32QAM Free-Space Transmission

We demonstrate a 120-Gbit/s mode-division-multiplexing (MDM) system based on two typical vector modes of TE01 and TM01 with direct detection orthogonal frequency division multiplexing (DD-OFDM) and 32-quadrature-amplitude-modulation (32-QAM) signal. The vector mode conversion is achieved by the key q-plate and the mode crosstalk between the converted two vector modes are both less than –20 dB. This crosstalk can be further minimized by high-quality q-plate. In this demonstration, error-free transmission has been realized with the power penalties less than 2 dB. The experimental results show that the scheme proposed in this paper can be a good candidate in large-capacity short-reach optical interconnect.

A Single Pilot Subcarrier-Based Sampling Frequency Offset Estimation and Compensation Algorithm for Optical IMDD OFDM Systems

A sampling frequency offset (SFO) estimation and compensation scheme for asynchronous optical intensity-modulation and direct-detection orthogonal frequency division multiplexing (OFDM) systems by using one single pilot subcarrier is proposed. Experimental demonstrations are also under taken to evaluate the performance of the mentioned scheme, and results show that precise SFO estimation with an accuracy of < ±0.2 ppm can be achieved under the SFO range from −1000 to 1000 ppm after 25 km standard single mode fiber (SSMF) transmission. Meanwhile, an SFO compensation method by extending the channel response with the phase shift correction term is also presented; more than 1000 and 400 ppm SFO effects can be compensated by using the phase shift correction term and its approximation, respectively.

Bandwidth Improvement Using Adaptive Loading Scheme in Optical Direct-Detection OFDM

In current metro and access optical networks, the frequency band beyond the bandwidth of the electro-optical components is always deprecated due to the fading frequency response. On the other hand, an orthogonal frequency division multiplexing (OFDM) system is managed on individual subcarrier and can adapt to the varied channel frequency response by optimally allocating different modulation formats and powers across different subcarriers. Therefore, to reuse the commercial low bandwidth components in high speed networks, we design modified OFDM signals, which are with adaptive power and bit allocation, to improve the bandwidth utilization and maximize the data rate. We successfully transmit 16-GHz optical OFDM signals over a 20-km standard single mode fiber link with a 10-GHz commercial Mach–Zehnder (MZ) intensity modulator and direct detection. Eleven modulation formats are used for bit loading and the maximum data rate can reach 88.8 Gb/s. The modulation bandwidth utilization of the MZ modulator is increased by 60%.

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

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.

On the Filter Narrowing Issues in Elastic Optical Networks

This paper describes the problematic filter narrowing effect in the context of next-generation elastic optical networks. First, three possible scenarios are introduced: the transition from an actual fixed-grid to a flexi-grid network, the generic full flexi-grid network, and a proposal for a filterless optical network. Next, we investigate different transmission techniques and evaluate the penalty introduced by the filtering effect when considering Nyquist wavelength division multiplexing, single side-band direct-detection orthogonal frequency division multiplexing, and symbol-rate variable dual polarization quadrature amplitude modulation. Also, different approaches to compensate for the filter narrowing effect are discussed. Results show that the specific needs per each scenario can be fulfilled by the aforementioned technologies and techniques or a combination of them, when balancing performance, network reach, and cost.

Estimating the Performance of DD OFDM Optical Systems Impaired by In-Band Crosstalk Using the Moment Generating Function

A theoretical method to assess the impact of in-band crosstalk on the performance of orthogonal frequency division multiplexing (OFDM) communication systems with direct-detection (DD) is presented. The proposed method relies on the moment generating function (MGF) to estimate the bit error rate (BER) of the DD OFDM communication system and takes into account in-band crosstalk, detected noise, and arbitrary electrical filtering at the receiver. By comparison with Monte Carlo simulation, we show that the MGF-based method predicts accurately the BER for different crosstalk levels, subcarriers numbers, 4-ary, 16-ary, and 64-ary quadrature amplitude modulation mappings, carrier-to-signal power ratios, ideal and nonideal electrical filtering, and different types of linearly mapped OFDM signals used in DD. The accuracy of the proposed method may be severely diminished in presence of significant signal distortion due to the nonlinearity of the optical modulator as, in this case, the assumptions of the proposed method do not hold. We also show that the DD OFDM optical systems tolerance to in-band crosstalk is lower than in single-carrier coherent detection optical systems, and it is significantly lessened when the modulation order in the OFDM subcarriers increases.

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.

Unified direct and coherent orthogonal frequency division multiplexing optical transmission scheme

We experimentally demonstrate a novel unified direct and coherent orthogonal frequency division multiplexing (OFDM) scheme. This self-coherent OFDM scheme simplifies receiver architecture and provides interchangeability between direct and coherent receivers using the same unified transmitter. We have experimentally verified the resilience of this scheme to fiber nonlinearities and achieved receiver sensitivity improvement of up to 1.73dB as compared to the conventional intensity modulation and direct detection OFDM scheme. We have also verified the effectiveness of a dual-analyzer-based balanced detection scheme.

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.

New hybrid peak-to-average power ratio reduction technique based on carrier interferometry codes and companding technique for optical direct-detection orthogonal frequency division multiplexing system

In direct-detection optical orthogonal frequency division multiplexing (OFDM) systems, the high peak-to-average power ratio (PAPR) will cause nonlinear effects in both electrical and optical devices and optical fiber transmission when the nonlinear amplifiers are employed. A new hybrid technique based on carrier interferometry codes and companding transform has been proposed and experimentally demonstrated to reduce the high PAPR in an optical direct-detection optical OFDM system. The proposed technique is then experimentally demonstrated and the results show the effectiveness of the new method. The PAPR of the hybrid signal has been reduced by about 5.7 dB when compared to the regular system at a complementary cumulative distribution function of 10 −4 . At a bit error rate of 10 −4 , after transmission over 100-km single-mode fiber with a μ of 2, the receiver sensitivity is improved by 3.7, 4.2, and 5 dB with launch powers of 3, 6, and 9 dBm, respectively.

Analytical characterization of four wave mixing effect in direct-detection double-sideband OFDM optical transmission systems

A closed-form expression for the variance of the four-wave mixing (FWM) induced in each subcarrier of a double sideband orthogonal frequency division multiplexing (OFDM) system employing direct detection is proposed and validated. Particularly, using a small signal analysis, equivalent transfer functions that characterize the frequency response of the FWM effect are derived taking into account the walkoff effect between the modulated pump waves and the FWM wave. The accuracy of the variance estimates provided by the closed-form expression is assessed for different sets of system parameters. The closed-form expression provides good variance estimates of the FWM-induced degradation caused by degenerate and nonsymmetric nondegenerate FWM components. For symmetric non-degenerate FWM components, the proposed expression provides reliable but pessimistic variance estimates, not exceeding the actual FWM variance in 1.5 dB for modulation indexes of interest.