Optical OFDM Signals Research Articles

PAPR reduction of asymmetrically clipped optical OFDM signals: Optimizing PTS technique using improved flower pollination algorithm

Orthogonal frequency division multiplexing (OFDM) is a practical approach for indoor optical communications because of its high rate data transmission and high power efficiency. The high peak-to-average power ratio (PAPR) is the main problem of optical OFDM signals which, degrades the system performance due to the LED nonlinearity. Partial transmit sequence (PTS) is the most popular PAPR reduction technique because of its distortionless characteristics and high performance. But, PTS has a high computational complexity, because it requires an exhaustive search to obtain the optimal phase factors. To reduce the search complexity of the PTS, we introduce a new swarm intelligence algorithm, called improved flower pollination (IFP) algorithm. The IFP algorithm is a novel optimization technique derived from the pollination behavior of flowers. The proposed IFP–PTS approach is applied in the asymmetrically clipped optical OFDM (ACO-OFDM) system and evaluated using several PAPR reduction benchmarks and compared with different state-of-the-art approaches in terms of PAPR reduction performance and computational complexity. The results justify the advantages of the IFP–PTS method compared with its counterparts.

Gaussian orthogonal matrix transform approach for PAPR reduction of optical OFDM signals

Optical orthogonal frequency division multiplexing (O-OFDM) has been widely adopted as a high-speed data transmission technique in visible light communication systems. This technique usually suffers from high peak-to-average-power ratio (PAPR). In this paper, a new PAPR reduction technique is proposed for O-OFDM signals. At the transmitter, a matrix transformation with the Gaussian elements is applied to the time-domain O-OFDM signal and at the receiver, the inverse matrix is used to recover the original signal. We show that the Gaussian orthogonal matrices can reconstruct the original signals without degrading the bit error rate (BER) performance. Gram-Schmidt technique is used to orthogonalize the Gaussian matrices. Computer simulations are conducted for 16-QAM baseband modulated symbols and about 3 dB PAPR reduction gain is achieved by the proposed approach compared with conventional O-OFDM.

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.

Spectrum-efficient hybrid PAM-DMT for intensity-modulated optical wireless communication.

In optical wireless communication (OWC), the superimposed optical orthogonal frequency division multiplexing (O-OFDM), such as layered asymmetrically clipped O-OFDM (LACO-OFDM), can improve spectrum efficiency by appropriately combining multiple O-OFDM signals for simultaneous transmission. However, it suffers from increased receiver complexity and latency. Therefore, a novel architecture of hybrid pulse-amplitude-modulated discrete multitone modulation (HPAM-DMT) is proposed in this paper to support a spectrum-efficiency OWC link. In HPAM-DMT, a PAM-DMT signal is carefully designed by using the real parts of subcarriers and is then superimposed on the classic PAM-DMT signal for simultaneous transmission, which fully exploits the spectrum in terms of subcarriers. Moreover, thanks to the well-designed aritecture of the two superimposed PAM-DMT signals, the proposed HPAM-DMT achieves the same spectrum efficiency as the LACO-OFDM, with much lower complexity and latency. Notable improvement of the bit-error rate (BER) performance is observed for the proposed HPAM-DMT compared to LACO-OFDM under the corruption of the transmitter nonlinearity. Moreover, HPAM-DMT achieves a relatively higher power efficiency than conventional O-OFDM schemes, which makes it a competitive O-OFDM scheme for IM/DD-based OWC.

Multilayered optical OFDM for high spectral efficiency in visible light communication system

This paper presents two new schemes for improving the spectral efficiency of an optical orthogonal frequency-division multiplexing-based visible light communication (OFDM-VLC) system. The proposed schemes employed the generalized spatial modulation (GenSM) along with multi-input multi-output (MIMO) techniques as well as modified the coded modulation optical OFDM (CMO-OFDM) approach to attain more bandwidth utilization and higher spectral efficiency. The proposed multilayered/enhanced CMO-OFDM (LeCMO-OFDM) systems ensure the real-valued optical OFDM signals without using Hermitian symmetry and utilize the redundant codes existed in CMO-OFDM for further improvements in system’s spectral efficiency while ensuring acceptable bit error rate (BER) performance. LeCMO-OFDM scheme transmits multilayer optical signals after converting them to unipolar form either by using DC bias (DC-LeCMO-OFDM) or by separating them through employing the GenSM with no DC bias (NDC-LeCMO-OFDM). We present analytical and simulation results to evaluate the effectiveness of the proposed systems and demonstrate the additional capacity gain in terms of spectral efficiency. In addition, the impact of the spacing distance between the light emitting diodes has been addressed. Detailed performance comparison in terms of spectral efficiency and BER between the proposed techniques and other recent works is provided for validation purpose. Finally, results show that our schemes can achieve better spectral and power efficiencies performance with acceptable BER performance when compared with the related methods.

Tunable Optical OFDM Demultiplexer Utilizing Slab Star Coupler-Based Optical DFT Circuit

We report on a tunable integrated-optic circuit for demultiplexing optical orthogonal frequency division multiplexing (OFDM) signals with various number of sub-carrier channels. The demultiplexer is composed of tunable optical discrete Fourier transform filter (tunable couplers, delay lines, and a slab star coupler) and optical gates. Various sub-carrier number optical OFDM signals can be demultiplexed by selecting the active delay lines with the tunable couplers. After explaining the demultiplexer and its operating principle, we show some experimental results including transmittance and bit error rate measurement of demultiplexed sub-carriers from various sub-carrier number optical OFDM signals. $2 \times 10$ Gbit/s to $10 \times 10$ Gbit/s OFDM signals could be successfully demultiplexed.

A new post-coding approach for PAPR reduction in DC-biased optical OFDM systems

The high peak-to-average power ratio (PAPR) is one of the main drawbacks in orthogonal frequency division multiplexing (OFDM) communication systems, which also exists in the optical OFDM (O-OFDM) systems. In this letter, a new approach based on the discrete Hartley transform (DHT) post-coding technique is proposed to reduce PAPR of an O-OFDM signal in visible light communication systems. The proposed method is compared with Walsh-Hadamard transform (WHT) and discrete cosine transform (DCT) techniques in terms of PAPR reduction and bit error rate (BER) performance. Experimental results indicate that the proposed DHT post-coding method remarkably reduces the PAPR of an OFDM signal for optical intensity modulated direct detection systems without any corruption in the BER performance.

Integrated demultiplexing and amplification of coherent optical combs.

The explosive growth of the internet during the last few decades has been enabled by two complementary innovations in optical communications: the use of multiple optical channels within a single optical fibre, and the increase in the bandwidth of individual channels to hundreds of Gbps. Further increases in overall bandwidth look to be provided by more spectrally efficient optical superchannels that use coherent sub-carriers generated using optical orthogonal frequency division multiplexing (OFDM). Yet, a cost effective way of generating these signals has not been demonstrated. One crucial, but missing piece is an effective means to separate the closely frequency spaced optical sub-carriers from the coherent optical comb before placing information on each sub-carrier, and thus creating the OFDM signal. Here, we demonstrate a flexible strategy implemented in a compact photonic integrated circuit (PIC) that is used to separate and amplify these sub-carriers using on-chip injection locking.

Impact of adaptive modulated OOFM signals for SD-WDM system using HOA

In this paper, performances of optical orthogonal frequency division multiplexing (OOFM) signals have been analyzed for super-dense wavelength division multiplexing (SD-WDM) system with the support of RAMAN–EDFA–RAMAN–EDFA hybrid optical amplifier. Acceptable rating outcome has been observed in terms of bit error rate (BER) and signal to noise ratio (SNR) for enhancing the high spectral efficiency.

PAPR reduction of optical OFDM signals in visible light communications

Abstract Orthogonal frequency division multiplexing (OFDM) is extensively used in optical communications to achieve high rate transmission. In this paper, Vandermonde like matrix (VLM) pre-coding approach is offered to reduce the high peak-to-average power ratio (PAPR) of DC-biased optical OFDM (DCO-OFDM) and asymmetrically clipped optical OFDM (ACO-OFDM) signals in visible light communications. The proposed method is compared with Walsh–Hadamard transform (WHT), discrete cosine transform (DCT), and discrete Hartley transform (DHT) pre-coding approaches in terms of PAPR reduction performance. Simulation results indicate that the proposed method effectively reduces the PAPR of an optical signal in both DCO-OFDM and ACO-OFDM techniques.

A Power and Spectrum Efficient NOMA Scheme for VLC Network Based on Hierarchical Pre-Distorted LACO-OFDM

Recently, a hybrid multiple access (MA) scheme, non-orthogonal MA (NOMA) combined with orthogonal frequency-division MA (OFDMA), has attracted significant attention in the fifth-generation (5G) wireless communication due to its superior spectrum efficiency. However, the advantage of the hybrid MA scheme cannot be fully realized in visible light communication (VLC) networks, since current optical orthogonal frequency-division multiplexing (OFDM) technologies are unable to provide high spectrum efficiency and power efficiency at the same time. In this paper, a hierarchical pre-distorted layered asymmetrically clipped optical OFDM (HPD-LACO-OFDM) scheme is proposed for NOMA, which offers superior spectrum efficiency as well as high optical power efficiency. In HPD-LACO-OFDM, multiple layers of asymmetrically clipped optical OFDM (ACO-OFDM) signals are generated to fill the odd subcarriers successively, and the inter-layer interference is eliminated with successive signal pre-distortion. A comparison of the bit-error-rate (BER) performance between the traditional dc-biased optical OFDM (DCO-OFDM), the layered ACO-OFDM (LACO-OFDM), and the proposed HPD-LACO-OFDM scheme is experimentally performed. The results show that with the same signal power, the HPD-LACO-OFDM-based NOMA-VLC network shows the best BER performance, whereas the demand for direct current voltage is reduced by half of the signal voltage compared to the traditional DCO-OFDM-based NOMA-VLC network.

Analysis and Mitigation of Clipping Noise in Layered ACO-OFDM Based Visible Light Communication Systems

Due to the limited dynamic range of the off-the-shelf electrical and optical components, deliberate digital clipping (DDC) is widely applied to optical orthogonal frequency division multiplexing (OFDM) based visible light communication systems. In this paper, we present a theoretical characterization of the layered asymmetrically clipped optical OFDM (ACO-OFDM) signals subject to peak clipping. We decouple a clipped $L$ -layer ACO-OFDM symbol to $L$ single-layer ACO-OFDM symbols, each corresponding to a layer, and show that these symbols are subject to symmetrical peak clippings at random levels. Using Bussgang’s theorem, the resulting attenuation factors and variances of the additive noise associated with each layer are derived. It is shown that the clipping noise caused by the DDC mainly falls onto the first layer, and its impact is gradually reduced in the subsequent layers. In order to combat the clipping noise, a novel receiver based on decision aided reconstruction is proposed. Simulation results show that the proposed receiver can effectively mitigate the clipping noise, leading to significant improvement of bit error rates over the conventional receiver.

Soliton content in the standard optical OFDM signal.

The nonlinear Schrödinger equation (NLSE) is often used as a master path-average model for fiber-optic transmission lines. In general, the NLSE describes the co-existence of dispersive waves and soliton pulses. The propagation of a signal in such a nonlinear channel is conceptually different from linear systems. We demonstrate here that the conventional orthogonal frequency-division multiplexing (OFDM) input optical signal at powers typical for modern communication systems might have soliton components statistically created by the random process corresponding to the information content. Applying the Zakharov-Shabat spectral problem to a single OFDM symbol with multiple subcarriers, we quantify the effect of the statistical soliton occurrence in such an information-bearing optical signal. Moreover, we observe that at signal powers optimal for transmission, an OFDM symbol incorporates multiple solitons with high probability. The considered optical communication example is relevant to a more general physical problem of the generation of coherent structures from noise.

Balanced Detection of 100-Gb/s Optical OFDM Signals With SSBI Cancellation Based on Fused-Type Fiber Mode-Selective Coupler

We propose a signal-to-signal beating interference (SSBI) cancellation scheme in two-mode fiber (TMF) transmission system based on balanced detection without requirement of a guard band. The SSBI can be eliminated based on the mode selectivity characteristic of mode coupler. The mode selectivity in mode coupler relies on the coupling behavior between the fundamental mode LP01 and the first-order mode LP11, which is highly dependent on the spatial orientation of the fiber modes. We design and fabricate a fused-type fiber mode coupler (FFMC) based on circular-core TMF. Based on the fabricated FFMC, we experimentally demonstrate a multi-band 100-Gb/s optical orthogonal frequency division multiplexing signal transmission over 10-m TMF with SSBI cancellation. Significant performance improvement is observed when compared with conventional direct detection scheme under the condition of insufficient bandwidth of a guard band.

The Spatial Dimming Scheme for the MU-MIMO-OFDM VLC System

Multiuser visible light communication (MU-VLC) systems utilizing multiple-input multiple-output (MIMO) and orthogonal frequency-division multiplexing (OFDM) are gaining increased attentions recently. Visible light communication (VLC) links are expected to work under different illumination conditions and, thus, the need for dimming control mechanisms. However, the traditional analog- and digital-based dimming schemes have adverse effects on the data communications performance, such as clipping distortion and the variation of the duty cycle. In this paper, spatial dimming schemes based on the zero-forcing and the minimum mean-squared error precoding schemes are proposed for direct-current biased optical OFDM based indoor MU-MIMO VLC system, and the bipolar optical OFDM signal is biased by a fixed dc level. Transmit antenna selection algorithms are designed for the optimum working light emitting diodes (LEDs) subset at each dimming level. Owing to the simultaneously exploration of the selection diversity of LEDs-based lights and the channel state information, the proposed spatial dimming schemes outperform the traditional dimming schemes, which is also verified by simulation results. Thus, the proposed schemes are shown to have a great potential to be applied in practical MU-MIMO-OFDM VLC systems.

Multi-Gbit/s real-time modems for chaotic optical OFDM data encryption and decryption

Real-time high-speed modems are proposed and demonstrated for physical-layer secure encryption and decryption of optical orthogonal frequency division multiplexing (OFDM) signals using field programmable gate array (FPGA). In the modems, the multi-fold data encryption is applied, including chaotic XOR, chaotic permutation and chaotic dynamic mapping, to enhance the physical-layer security for optical OFDM data during transmission. Real-time modems for encrypted 16-QAM OFDM signals at 4.8 Gbit/s are successfully demonstrated, using parallel multiple channels design on FPGA. The security performances of the real-time encrypted OFDM signals are evaluated, while a huge key space of ∼ 10 168 is created to resist the exhaustive attacks.

Estimation and Compensation for Signal-Signal Beating Interference in Direct Detection Fiber-Optical Transmission Systems of OFDM Signals

A mathematical model is developed for calculating the beating noise that occurs during photodetection of a single-band optical OFDM signal. The model allows one to estimate the spectral distribution of the beating noise and the distortion of the OFDM signal subcarrier levels due to fiber dispersion. The influence of the beating noise on orthogonal subcarrier channels, possible ways of increasing the system transmission capacity, and the method of noise compensation on the receiving side are investigated.

Raman-enhanced optical phase conjugator in WDM transmission systems.

Optical phase conjugation (OPC) can be applied to boost the performance of long-haul transmission by mitigating the impairments from fiber nonlinearity. Unfortunately, noticeable nonlinear noise in the conjugator for optical orthogonal frequency division multiplexing (OFDM) systems often degrades the signal quality. In this paper, we demonstrate nonlinear distortion mitigation in OPC by introducing backward Raman amplification to the conjugator. Raman amplification allows a lower input signal power, thus suppressing the OPC distortion while maintaining the conjugated output power. We investigate the performances of Raman-enhanced OPC in both back-to-back (BTB) and transmission systems with 3 × 25 Gbaud optical OFDM signals. In the BTB OPC system, Raman amplification boosts the tolerance to system nonlinearity, achieving a 3-dB improvement in the output power, a 2.4-dB improvement in the Q factor, and a 6-dB improvement in the input dynamic range. In the transmission system with Raman-enhanced OPC, the optimum launched power is increased by 2 dB and the maximum Q factor is increased by 0.4 dB compared to direct transmission. Similar performances are observed in all the wavelengths, indicating that our scheme works well with WDM transmission systems.

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.

Chaotic Constellation Mapping for Physical-Layer Data Encryption in OFDM-PON

A physical-layer encryption scheme is proposed and experimentally demonstrated in orthogonal frequency division multiplexing passive optical network (OFDM-PON). In the proposed multifold encryption scheme, quadratic-amplitude modulation (QAM) symbols are scrambled and distributed onto the complex plane independently. The dynamic parameters of constellation shifting are generated by a 3-D hyper digital chaos, in which a key space of ~10162 is created to enhance the security level of OFDM data encryption during transmission. An encrypted data transmission of 9.4-Gb/s, 16-QAM optical OFDM signals is successfully demonstrated over 20-km standard single-mode fiber.