Optical OFDM Systems Research Articles

Accurate and spectral efficient channel estimation using inter-block precoding and superimposed pilots in optical OFDM systems.

A new inter-block precoding-based channel estimation (CE) scheme is proposed and experimentally demonstrated in an optical OFDM system with a superimposed pilot (SP). The proposed inter-block precoding scheme targets on eliminating the statistical mean of the unknown data symbols, and thereby improves the performance of SP-aided CE. We investigate the impact that both the precoding matrix and SP have on the system performance, from which we obtain the optimum value of signal-to-pilot power ratio (SPR) as well as the block length. We show through simulations and experiments that the proposed CE scheme, in comparison with the conventional preamble based scheme, has the advantage of entailing a much smaller overhead size, while offering similar performance in terms of CE accuracy and bit-error ratio (BER) performances. Furthermore, the proposed precoding scheme has no limit to the design of SP, and thus is applicable for any periodic pilots.

Suppression of laser phase noise in direct-detection optical OFDM transmission using phase-conjugated pilots

Due to the unique phase noise (PN) characteristics in direct-detection optical OFDM (DDO-OFDM) systems, the design of PN compensator is considered as a difficult task. In this paper, a laser PN suppression scheme with low complexity for DDO-OFDM based on coherent superposition of data carrying subcarriers and their phase conjugates is proposed. Through theoretical derivation, the obvious PN suppression is observed. The effectiveness of this technique is demonstrated by simulation of a 4-QAM DDO-OFDM system over 1000 km transmission length at different laser line-width and subcarrier frequency spacing. The results show that the proposed scheme can significantly suppress both varied phase rotation term (PTR) and inter-carrier interference (ICI), and the laser line-width can be relaxed with up to 9 dB OSNR saving or even breakthrough of performance floor.

Iterative algorithm for phase noise estimation in coherent optical DFT-S OFDM systems

An iterative algorithm is proposed and investigated for phase noise estimation of discrete Fourier transform spread (DFT-S) orthogonal frequency division multiplexing (OFDM). Compared with common phase estimation (CPE), significant improvement of tolerance to laser bandwidth is obtained and 2.7 dB improvement at BER of 3.8×10−3 is found using the CPE-iter scheme in 103.3 Gb/s coherent polarization-division-multiplexed DFT-S OFDM system over 480-km transmission through simulation.

Total least‐square‐based receiver for asymmetrically clipped optical‐orthogonal frequency divisional multiplexing visible light communication system

This study proposes a receiver for an asymmetrically clipped optical orthogonal frequency-division multiplexing system that utilises the total least-squares (TLS) technique. Commonly used estimation techniques such as least squares, diversity combining, minimum-mean-square-error, and pairwise maximum likelihood ignore the perturbation present while estimating the received signal, which causes the estimated signal to be inaccurate. In this study, a TLS-based receiver is proposed, which takes into consideration the errors present in both the data matrix (channel) and the observation matrix (received signal). While this technique is suboptimal, it is shown that with reasonable computational complexity it has an acceptable performance in a scenario with low processing capability and low battery power. Simulations are carried out for both line-of-sight and non-line-of-sight optical channels and the performance is analysed on the basis of bit error rate versus E b/N 0 where E b is the energy received per information bit and N 0 is the noise power spectral density.

A Fast Hartley Transform based novel optical OFDM system for VLC indoor application with constant envelope PAPR reduction technique using frequency modulation

In this research literature we present a unique optical OFDM system for Visible Light Communication (VLC) intended for indoor application which uses a non conventional transform-Fast Hartley Transform and an effective method to reduce the peak to average power ratio (PAPR) of the OFDM signal based on frequency modulation leading to a constant envelope (CE) signal. The proposed system is analyzed by a complete mathematical model and verified by the concurrent simulations results. The use of the non conventional transform makes the system computationally more desirable as it does not require the Hermitian symmetry constraint to yield real signals. The frequency modulation of the baseband signal converge random peaks into a CE signal. This leads to alleviation of the non linearity effects of the LED used in the link for electrical to optical conversion. The PAPR is reduced to 2 dB by this technique in this work. The impact of the modulation index on the performance of the system is also investigated. An optimum modulation depth of 30% gives better results. The additional phase discontinuity incurring on the demodulated signal at the receiver is also significantly reduced. A comparison of the improvement in phase discontinuity of the proposed technique of combating the PAPR with the previously known phase modulation technique is also presented in this work. Based on the channel metrics we evaluate the system performance and report an improvement of 1.2 dB at the FEC threshold. The proposed system is simple in design and computationally efficient and this can be incorporated into the present VLC system without much alteration thereby making it a cost effective solution.

CFO compensation method using optical feedback path for coherent optical OFDM system

Abstract We investigate feasibility of carrier frequency offset (CFO) compensation method using optical feedback path for coherent optical orthogonal frequency division multiplexing (CO-OFDM) system. Recently proposed CFO compensation algorithms provide wide CFO estimation range in electrical domain. However, their practical compensation range is limited by sampling rate of an analog-to-digital converter (ADC). This limitation has not drawn attention, since the ADC sampling rate was high enough comparing to the data bandwidth and CFO in the wireless OFDM system. For CO-OFDM, the limitation is becoming visible because of increased data bandwidth, laser instability (i.e. large CFO) and insufficient ADC sampling rate owing to high cost. To solve the problem and extend practical CFO compensation range, we propose a CFO compensation method having optical feedback path. By adding simple wavelength control for local oscillator, the practical CFO compensation range can be extended to the sampling frequency range. The feasibility of the proposed method is experimentally investigated.

Secret Key Generation Protocol for Optical OFDM Systems in Indoor VLC Networks

Although the visible light communication (VLC) channel has superior security than radio frequency channels, its broadcast character makes VLC links vulnerable to eavesdropping. Therefore, VLC networks need to support security to protect the user's data against its eavesdroppers. Keyless security approaches have been proposed to provide security for VLC in the literature. Although these approaches support good security against eavesdroppers, they require complicated implementation. Algorithms of keys generation are essential to secure wireless links. Nevertheless, the common keys generation algorithms can be very complex and costly in many setups. They use up limited resources such as bandwidth. In this paper, we enhance the confidentiality of VLC networks by suggesting a new key extraction protocol for optical orthogonal frequency division multiplexing (OFDM) schemes in an indoor environment. We introduce to extract keys from the bipolar OFDM samples produced from optical OFDM schemes. We propose to allocate a portion of cyclic prefix samples placed in the area of low channel impact for keys extraction throughout session duration. The introduced protocol extracts keys for the medium access control (MAC) level and the physical level. It supports high physical-layer security as every OFDM signal frame is encrypted by a various key. Thus, the encryption at the MAC level can be switched off beyond any danger in security. This setup can improve the throughput of the system because it decreases the processing delay in the upper layer process. Also, the presented protocol supports zero bit mismatch rates with key extraction on the fly.

Clipping Noise Mitigation in Optical OFDM Systems

This letter describes a new non-linear algorithm for clipping noise mitigation in intensity modulation/direct detection dc biased optical orthogonal frequency division multiplexing (DCO-OFDM) systems. Clipping noise is often the major limitation in DCO-OFDM. In this letter, we show that extra information about the clipped signal can be extracted using a non-linear process and then used to mitigate the clipping noise. The effectiveness of the new algorithm is demonstrated by simulation and in an optical wireless experiment.

Comparing System Performance of Optical OFDM Systems Using LMSTE, NLMSTE and DFTE

The evaluation of Optical Orthogonal Frequency Division Multiplexing (OOFDM) using Least Mean Square Time-domain Equalizer (LMSTE), its normalized form (NLMSTE) and Decision Feedback Time-domain Equalizer (DFTE) to reduce Cyclic Prefix (CP) length over 1200 Km of Standard Single Mode Fiber (SSMF) is presented. All of these TEQs are used immediately after the fiber channel. They can cancel the residual Inter Symbol Interference (ISI) and Inter Carrier Interference (ICI) caused by both the Group Velocity Dispersion (GVD) and the CP length being shorter than the Channel Impulse Response (CIR). Using these TEQs allow the reduction in size of CP, and consequently leading to system performance improvement. Using DFTE can decrease the noise whereas using NLMSTE can solve the problem of choosing the gain of LMS algorithm to make it stable since LMS algorithm is sensitive to the scaling of its input.

Decision-directed iterative methods for PAPR reduction in optical wireless OFDM systems

In this paper, we propose two iterative decision-directed methods for peak-to-average power ratio (PAPR) reduction in optical-orthogonal frequency division multiplexing (O-OFDM) systems. The proposed methods are applicable to state-of-the-art intensity modulation-direct detection (IM-DD) O-OFDM techniques for optical wireless communication (OWC) systems, including both direct-current (DC) biased O-OFDM (DCO-OFDM), and asymmetrically clipped O-OFDM (ACO-OFDM). Conventional O-OFDM suffers from high power consumption due to high PAPR. The high PAPR of the O-OFDM signal can be counteracted by clipping the signal to a predefined threshold. However, because of clipping an inevitable distortion occurs due to the loss of useful information, thus, clipping mitigation methods are required. The proposed iterative decision-directed methods operate at the receiver, and recover the lost information by mitigating the clipping distortion. Simulation results acknowledge that the high PAPR of O-OFDM can be significantly reduced using clipping, and the proposed methods can successfully circumvent the clipping distortions. Furthermore, the proposed PAPR reduction methods exhibit a much lower computational complexity compared to standard PAPR reduction methods.

Piecewise Companding Transform Assisted Optical-OFDM Systems for Indoor Visible Light Communications

In visible light communications (VLCs) relying on intensity-modulation and direct detection (IM/DD), the conversion from electrical signals to optical signals and the limited dynamic range of the light-emitting diodes (LEDs) constitute the fundamental impediments in the way of high-integrity communications, especially when orthogonal frequency-division multiplexing (OFDM) is employed. In IM/DD VLCs, only real-valued positive signals are used for signal transmission. However, the Fourier transform of OFDM systems is operated in the complex domain. In order to meet the requirements of the IM/DD VLCs, the complex-to-real conversion is achieved at the cost of reducing the bandwidth efficiency. Moreover, OFDM signals experience a high peak-to-average power ratio; hence, typically clipping is used for confining the positive-valued signals within the LED’s dynamic range. However, hard clipping leads to the loss of orthogonality for optical OFDM (O-OFDM) signals, generating inter-carrier interference. As a result, the performance of the clipping-based O-OFDM systems may be severely degraded. In this paper, the concept of piecewise companding transform (CT) is introduced into the O-OFDM system advocated, forming the CTO-OFDM arrangement. We first investigate the general principles and design criteria of the piecewise CTO-OFDM. Based on our studies, three types of piecewise companders, namely, the constant probability sub-distribution function, linear PsDF (LPsDF), and the non-LPsDF-based CT, are designed. Furthermore, we investigate the nonlinear effect of hard clipping and of our CT on O-OFDM systems in the context of different scenarios by both analytical and simulation techniques. Our investigations show that the CTO-OFDM constitutes a promising signaling scheme conceived for VLCs, which exhibits a high bandwidth efficiency, high flexibility, high reliability, as well as a high data-rate, despite experiencing nonlinear distortions.

基于群智能算法的光OFDM系统PAPR抑制

基于群智能算法的光OFDM系统PAPR抑制

Nonlinear impairment compensation for DFT-S OFDM signal transmission with directly modulated laser and direct detection

We experimentally demonstrate a 16-ary quadrature amplitude modulation (16QAM) DFT-spread optical orthogonal frequency division multiplexing (OFDM) transmission system utilizing a cost-effective directly modulated laser (DML) and direct detection. For 20-Gbaud 16QAM-OFDM signal, with the aid of nonlinear equalization (NLE) algorithm, we respectively provide 6.2-dB and 5.2-dB receiver sensitivity improvement under the hard-decision forward-error-correction (HD-FEC) threshold of 3.8×10−3 for the back-to-back (BTB) case and after transmission over 10-km standard single mode fiber (SSMF) case, related to only adopt post-equalization scheme. To our knowledge, this is the first time to use dynamic nonlinear equalizer (NLE) based on the summation of the square of the difference between samples in one IM/DD OFDM system with DML to mitigate nonlinear distortion.

Study on performance of coherent orthogonal frequency division multiplexing system in exponential atmospheric turbulent channel

We analyze the performance of a coherent orthogonal frequency division multiplexing (OFDM) system and a serial decode and forward relay transmission multihop coherent free-space optical OFDM system using an exponential distribution atmospheric turbulence model under the circumstance of strong atmospheric turbulence. The attenuation of the atmospheric channel fading model mainly considers the light intensity scintillation caused by atmospheric turbulence and interaction between the path consumption, the transmitter and the receiver. The OFDM signal mapping method uses quadrature amplitude modulation. We also derive the formulas of the outage probability and symbol error rate of the coherent OFDM and multihop system, respectively, under the conditions described above. In addition, a simulation is performed, which is essential to evaluate the influence of key factors including coherent detection in a number of relay nodes, the mapping orders, and the number of subcarriers, which have a significant effect on the outage performance and the bit error performance of the OFDM-FSO system under the strong atmospheric turbulence.

Robust Key Generation From Optical OFDM Signal in Indoor VLC Networks

This letter improves the confidentiality of visible light communication links by proposing a robust key-generation mechanism for optical orthogonal frequency division multiplexing (OFDM) systems in an indoor environment. In this letter, we propose to generate keys from the bipolar OFDM samples at the output of optical OFDM systems. We suggest to assign a part of cyclic prefix samples located in the region of small channel effect for keys generation during session period. The proposed scheme generates keys for medium access control (MAC) sublayer and physical layer. It provides high computational security against passive eavesdroppers in the physical level, because each signal frame is encrypted by a different key. So, the encryption in MAC level can be turned off without any risk in security. This setting can enhance the system throughput. Also, the implemented method achieves zero bit disagreement rates with key generation on the fly.

Novel Training Symbol Structure for Transmitter IQ Mismatch Estimation for PDM Coherent Optical OFDM System

In this paper, two novel training symbol structures are proposed to mitigate transmitter IQ mismatch and channel distortion for polarization division multiplexed (PDM) coherent optical orthogonal frequency division multiplexing (CO-OFDM) systems. Accordingly, two corresponding estimation methods are proposed. Compared with the conventional estimation approach, the advantage of the first method is that IQ mismatch and channel distortion are estimated independently, and the benefit of the second method is that the overhead of training symbols is reduced. The theoretical analysis is validated by numerical Monte Carlo simulations of PDM CO-OFDM system. Under the simulation conditions: the phase mismatch 15°, the amplitude mismatch 3 dB, the transmission bit rate 100 Gb/s, the standard signal-mode fiber link 480 km, and bit error rate 1e-3, 24 dB optical signal-to-noise ratio (OSNR) is necessary for the first method, and 28 dB OSNR for the second one.

Common phase error estimation in coherent optical OFDM systems using best-fit bounding box.

In this paper, we investigate and characterize a new approach of adopting best-fit bounding box method for common phase error estimation in coherent optical OFDM systems. The method is based on the calculation of the 2-D convex hull of the received signal constellation, which is generally adopted in image processing area to correct the skew of images. We further perform detailed characterizations including root mean square error analysis, laser linewidth tolerance, noise tolerance, and computation complexity analysis, via numerical simulations and experiments. The results show the proposed method achieves much improved spectral efficiency and comparable system performance than the pilot-aided method, while it exhibits good estimation accuracy and reduced complexity than the blind phase searching method.

Efficiency Improvement for Optical OFDM Systems

The evaluation of Optical Orthogonal Frequency Division Multiplexing (OOFDM) using Least Mean Square Time-domain Equalizer (LMSTE), Recursive Least Squares-Time-domain Equalizer (RLSTE) and Decision Feedback Time-domain Equalizer (DFTE) to reduce Cyclic Prefix (CP) length over 1200 Km of Standard Single Mode Fiber (SSMF) is presented. All of these TEQs are used immediately after the fiber channel. They can cancel the residual Inter Symbol Interference (ISI) and Inter Carrier Interference (ICI) caused by both the Group Velocity Dispersion (GVD) and the CP length being shorter than the Channel Impulse Response (CIR). Using these TEQs allow the reduction in size of CP, and consequently leading to system performance improvement. DFTE can decrease the noise whereas RLSTE can achieve a higher convergence and better performance compared to LMSTE.

Channel Estimation on Individual Subcarrier Using Image Processing in Optical OFDM System

In an optical orthogonal frequency division multiplexing (OFDM) system, channel impairments, such as residual chromatic dispersion, residual synchronization error, and clock frequency offset, induce phase shifts over subcarriers. We propose a method of using image processing for phase channel estimation with negligible overhead, which is applicable to various subcarrier modulation formats. We conduct the experiment of 32-, 40-, and 48-Gbit/s direct detection optical OFDM with 16QAM, 32QAM, and 64QAM subcarrier modulation and simulation of 112-Gb/s coherent optical OFDM to verify this method and compare the results with those of the conventional method. Then, we investigate the effects of the numbers of required test phases and OFDM symbols with experimental data. We also present an approach of removing the inner part of constellation points to reduce the complexity.

Efficient joint timing and frequency synchronization algorithm for coherent optical OFDM systems.

A joint timing and frequency synchronization algorithm is proposed for coherent optical orthogonal frequency-division multiplexing (CO-OFDM) systems. The timing and frequency synchronization is realized by using only one training symbol which is composed of conjugated symmetric sequence. The timing estimation of the proposed algorithm has the advantage of being robust to poor optical signal-to-noise ratio (OSNR) and chromatic dispersion (CD), and the frequency estimation range of fractional subcarrier spacing can be achieved. The feasibility and effectiveness of the proposed joint synchronization algorithm is verified in both simulations and a 47.3 Gbit/s 16-ary quadrature amplitude modulation (16-QAM) CO-OFDM system.