Optical Frequency Division Multiplexing Research Articles

Optical Beam Steerable Orthogonal Frequency Division Multiplexing (OFDM) Non-Orthogonal Multiple Access (NOMA) Visible Light Communication Using Spatial-Light Modulator Based Reconfigurable Intelligent Surface

Optical Beam Steerable Orthogonal Frequency Division Multiplexing (OFDM) Non-Orthogonal Multiple Access (NOMA) Visible Light Communication Using Spatial-Light Modulator Based Reconfigurable Intelligent Surface

Performance of a free space optical link employing DCO-OFDM modulated Gaussian-beam

Abstract The paper studies the performance of a free space optical communication link, which consists of a Gaussian Laser beam employing direct-current-biased optical frequency division multiplexing (DCO-OFDM) modulation, to carry information. While optical multi-carrier modulation is capable of alleviating multi-path fading, higher transmit power by virtue of dc-bias, helps the optical beam in partially off-setting signal intensity loss due to beam divergence in free space channel. In order to reduce the overall transmitted power apart from avoiding nonlinearities due to high peak-to-average-power-ratio (PAPR), we utilize a PAPR-dependent dc-bias, to generate a DCO OFDM light beam from a pre-clipped electrical baseband OFDM signal. The radiated optical beam encounters turbulence-induced impairments like beam-spreading, beam-wander, etc., which affect the signal-to-noise ratio (SNR) of the received data symbols. Further signal pre-clipping and optical source linearity generate clipping noise components resulting in compromised receiver performance or reduction of span length, to achieve target bit rate error (BER). In this paper, we derive an analytical model for evaluating the performance of the free-space optical (FSO) link, by modeling various impairments as noise variances and characterizing atmospheric turbulence by refractive index structure parameter under weak- and strong-turbulent conditions. Numerical results are obtained by varying system design parameters in the model, which graphically provide useful insight on the power penalties and various trade-offs involved in operating the link on a longer span, at a higher data rate, and with reduced transmitted power through pre-clipping while ensuring desired signal BER.

Precoded OVSB-OFDM transmission system using DML with Kramers-Kronig receiver

Different from the use of IQ modulator or a dual-drive Mach-Zehnder modulator, we investigate an optical vestigial sideband orthogonal frequency division multiplexing (OVSB-OFDM) transmission system using a low-cost directly modulated laser. Precoding techniques and Kramers-Kronig (KK) algorithm are employed to enhance the bit error rate (BER) performance. The KK receiver is applied to eliminate the signal-signal beat interference caused by the square-law detection of the photodetector. Discrete Fourier transform (DFT) and orthogonal circulant matrix transform (OCT) precoding techniques can effectively average the signal-to-noise ratio over the data-carrying subcarriers and improve the BER performance. Through numerical simulation, we discuss the relationship between the order and center frequency of the optical filter and the carrier signal power ratio (CSPR). Furthermore, we study the influence of the KK algorithm and precoding techniques on receiver sensitivity under different transmission distances. The results show that both KK algorithm and precoding technologies can improve receiver sensitivity and extend the transmission distance. The performance of DFT precoding is better than that of OCT precoding. When there is no KK algorithm, the transmission distance of OCT precoding or no precoding is only 20 km at the BER of 3.8e-3; while DFT precoding can extend the transmission distance up to 60 km. With the help of the KK algorithm, OCT precoding and DFT precoding can further extend the transmission distance by up to 40 and 60 km, respectively, compared to the DFT precoding and without the KK algorithm case. The DFT precoding with the KK algorithm can improve the receiver sensitivity of more than 7 dB in terms of received optical power, compared to the no precoding and without the KK algorithm case.

Dimming control scheme based on hybrid LACO-SCFDM for visible light communications

This paper proposes a hybrid layered asymmetrically clipped optical (HLACO) single-carrier frequency-division multiplexing (SCFDM) scheme for dimmable visible light communication. It designs a signal structure that combines layered asymmetrically clipped optical (LACO)-SCFDM and negative LACO-SCFDM in proportion for improving the inherent weaknesses of orthogonal frequency-division multiplexing (OFDM)-based dimmable schemes and further enhancing the system performance. Compared to the HLACO-OFDM-based dimming scheme, it obtains a lower bit error ratio and enables efficient communication over broader dimming range. Its spectral efficiency realizes 2.875 bit·s−1·Hz−1 within the dimming range of 30%–70%, and the attainable average spectral efficiency gains exceed at least 19.21% compared to other traditional dimmable schemes.

Optical MIMO-OFDM With Fully Generalized Index-Spatial LED Modulation

In this letter, we propose a novel, fully generalized index-spatial (FGIS) light emitting diode (LED) modulation scheme for the optical MIMO-orthogonal frequency division multiplexing (OFDM) system. The fully generalized index-spatial (FGIS) is a spectrally efficient (SE) visible light communication (VLC) modulation scheme in which the LED indices are exploited in an innovative way to address, not only the OFDM signal time/frequency domain shaping problem but also to provide an additional spatial modulation (SM) domain. Compared to recent OFDM-LED index modulation schemes, FGIS achieves higher SE with lower number of LEDs. The simulation results surpass the proposed FGIS in giving superior improvement in both bit error rate (BER) and achievable rate ( $R$ ) compared with the state-of-the-art OFDM-LED index modulation schemes.

Hartley-Domain DD-FTN Algorithm for ACO-SCFDM in Optical-Wireless Communications

In this paper, we propose a Hartley-domain direct-detection faster-than-Nyquist (HD-DD-FTN) algorithm for asymmetrically clipped optical single-carrier frequency-division multiplexing (ACO-SCFDM) in optical-wireless communications (OWC). The HD-DD-FTN algorithm includes modified Hartley-domain equalization (HDE) and maximum likelihood sequence detection, which can simultaneously compensate serious high-frequency distortion and eliminate enhanced in-band noise for the bandwidth-limited OWC. Modified HDE can be implemented by real-valued operations, which is appropriate for dealing with real-valued signals. The simulation results show that ACO-SCFDM system using the HD-DD-FTN algorithm is more robust to inter-symbol interference than that using conventional HDE algorithm. Therefore, the bit error rate performance of the ACO-SCFDM system using HD-DD-FTN algorithm is better than that using the conventional HDE algorithm. In conclusion, the ACO-SCFDM system using the HD-DD-FTN algorithm shows great potential for bandwidth-limited OWC.

Optical single carrier-interleaved frequency division multiplexing for visible light communication systems

In this paper, a novel optical single carrier-interleaved frequency division multiplexing (OSC-IFDM) signaling scheme for an intensity modulation and direct detection (IM/DD) based visible light communications (VLC) system is presented. We show that, OSC-IFDM significantly improves the peak-to-average-power ratio (PAPR) of the DC-biased optical orthogonal frequency division multiplexing (DCO-OFDM). The reduction in the PAPR value is achieved by exploiting the symmetrical characteristics of the single carrier-interleaved frequency division multiple access (SC-IFDMA) time domain symbol, whereby the imaginary and real parts of the SC-IFDMA samples are separated and transmitted through two sub-symbols. Simulation results show that, the PAPR of OSC-IFDM is 10 dB lower than that of the DCO-OFDM scheme. We also show that the proposed scheme offers higher spectral efficiency compared to our previous work.

An Integrated High-Speed Full Duplex Coherent OFDM-PON and Visible-Light Communication System

Abstract We demonstrated a full duplex hybrid passive optical network and indoor optical wireless system employing coherent optical frequency division multiplexing. To accomplish reliable transmission in passive optical networks integrated visible-light communication (VLC), yellow light-emitting diode and infrared LED is used in downstream and upstream, respectively, for intra building network. In order to support high data rate, pulse-width reduction scheme based on dispersion compensation fiber is incorporated and system successfully covered the distance of 50 km. A data stream at the rate of 30 Gb/s is transmitted for each user out of eight users. VLC-supported users are catered with the bit rate of 1.87 Gb/s over 150 cm and in order to realize a low-cost system, visible and infrared LEDs are used in downlink and uplink, respectively.

PAPR Reduction in Coherent Optical OFDM System using Modified Sliding Norm Transformer

Orthogonal Frequency Division Multiplexing is one of the most important multiple carrier modulation format, which has many applications in wireless communication and optical communication. It is considered as an excellent method for fast optical communication inferable from its high spectral efficiency and its strength to path losses. Peak to average power ratio is regarded as one of the main problems that are experienced by the optical orthogonal frequency division multiplexing system, which directly affects the characteristics of the system. The current paper proposes an efficient technique to reduce Peak to Average Power Ratio by the modified Dursun’s method or the so called modified discrete sliding norm transformer in coherent optical orthogonal frequency division multiplexing system for the first time to the base of our knowledge consequently. The proposed technique does not need to send side information to the receiver; also, it does not degrade bandwidth. This algorithm lessens Peak to Average Power Ratio in the optical coherent Orthogonal Frequency Division Multiplexing system to about 4.15 dB at complementary cumulative distribution function (CCDF) probability of 10-3 and improves the system performance.

Optimized Layered/Enhanced ACO-SCFDM for Short-Reach Optical Interconnects

This letter is the first experimental demonstration of layered/enhanced asymmetrically clipped optical single-carrier frequency-division multiplexing (L/E-ACO-SCFDM) in short-reach optical interconnects. An optimized method is employed to significantly improve the performance of L/E-ACO-SCFDM. L/E-ACO-SCFDM has higher spectral efficiency than ACO-SCFDM, and it also has lower peak-to-average power ratio and computational complexity than L/E-ACO orthogonal frequency-division multiplexing (L/E-ACO-OFDM). In the experimental system, the low-cost electrical and optical devices are utilized, which induce serious high-frequency distortion caused by bandwidth limitation. Under the same spectral efficiency and net bit rate, the optimized L/E-ACO-SCFDM has about 4-dB and more than 3-dB receive sensitivity improvement at 7% forward error correction limit compared with the optimized L/E-ACO-OFDM and the optimized ACO-SCFDM, respectively. Therefore, L/E-ACO-SCFDM is a promising scheme for the future short-reach optical interconnects.

Mitigation of Non Linear Effects Using Non Linear Transform in Dispersion Managed Coherent Optical OFDM Systems

It is very important to compensate the dispersion and non linearity present in the optical backhaul so that it is compatible with upcoming technologies. Optical Frequency Division Multiplexing (OFDM) is a physical layer, Multicarrier Modulation Technique (MCT) which is being widely used to design Back Bone (BB), Back Haul (BH) and Cognitive Optical Networks (CONs). Fiber nonlinearity present in dispersion managed links limits the capacity of an OFDM link, as it gets enhanced by high Peak to Average Power Ratio (PAPR) value of the transmitted signal. This work aims to decrease the PAPR value and hence in turn reduce the non linear impairments present in the coherent OFDM optical fiber link. In this work L2-by-3 non linear transform has been implemented to mitigate various fiber non linear effects through PAPR reduction.

An improved diversity combining receiver for layered ACO-FOFDM in IM/DD systems

In this paper, an improved receiver based on diversity combining is proposed to improve the bit error rate (BER) performance of layered asymmetrically clipped optical fast orthogonal frequency division multiplexing (ACO-FOFDM) for intensity-modulated and direct-detected (IM/DD) optical transmission systems. Layered ACO-FOFDM can compensate the weakness of traditional ACO-FOFDM in low spectral efficiency, the utilization of discrete cosine transform in FOFDM system instead of fast Fourier transform in OFDM system can reduce the computational complexity without any influence on BER performance. The BER performances of layered ACO-FOFDM system with improved receiver based on diversity combining and DC-offset FOFDM (DCO-FOFDM) system with optimal DC-bias are compared at the same spectral efficiency. Simulation results show that under different optical bit energy to noise power ratios, layered ACO-FOFDM system with improved receiver has 2.86dB, 5.26dB and 5.72dB BER performance advantages at forward error correction limit over DCO-FOFDM system when the spectral efficiencies are 1 bit/s/Hz, 2 bits/s/Hz and 3 bits/s/Hz, respectively. Layered ACO-FOFDM system with improved receiver based on diversity combining is suitable for application in the adaptive IM/DD systems with zero DC-bias.

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%.

Design and Analysis of Physical Layer Security Based on Ill-Posed Theory for Optical OFDM-Based VLC System Over Real-Valued Visible Light Channel

Due to the broadcast property of the visible light beam, visible light communication (VLC) systems may suffer from malicious attack or eavesdropping in public area. In this paper, we design and analyze the physical layer security mechanism with the aid of ill-posed theory and present a channel determined subcarrier shifting (CDSS) scheme with pre-equalization. The CDSS scheme exploits the real-valued and location-sensitive channel state information (CSI) of VLC channel and performs sorting operations on the CSI, which formulates a shifting matrix with the aid of chaotic sequences, to dynamically change the available subcarrier for the optical orthogonal frequency division multiplex (OOFDM) aided VLC system. Meanwhile, we combine the CDSS scheme with the preequalization module to further improve the bit error rate (BER) performance for legitimate users. Moreover, the analytical physical layer security model for VLC system is presented using ill-posed theory, which removes the requirements of known CSI and worse channel quality of malicious users when evaluating secrecy capacity. Theoretical and simulation results are provided and compared to prove the benefits achieved by the proposed CDSS scheme, which can enhance the security with no loss of BER, while the secrecy capacity performances are also evaluated to verify the effectiveness of our design.

Photonic Circuit Topologies for Optical OFDM and Nyquist WDM

Photonic circuits are the key to advanced functionality in future optical systems, as they efficiently process terabit/s data streams. This paper reviews how photonic circuit topologies have evolved to support high-spectral efficiency modulation formats, including: all-optical optical frequency division multiplexing (AO-OFDM), discrete Fourier transform spread OFDM (DFT-S-OFDM), Nyquist wavelength division multiplexing, (NWDM), orthogonal time division multiplexing (OrthTDM, OTDM), chirped-OFDM, and quasi-Nyquist signals. The importance of the order of components such as modulators, delays, and filters within these circuits is stressed, as simple changes to the circuit topology have significant impacts on the spectra of the signals, and the required bandwidths of the modulators.

Demonstration of 400 Gb/s optical PDM-OFDM superchannel unrepeatered transmission by all optical phase-conjugated copy

Unrepeatered transmission delivers data over a few hundred kilometers without in-line active elements and it can effectively reduce the line complexity and the overall system cost. In this paper, we propose all optical phase-conjugated copy (OPC) to improve optical signal noise ratio (OSNR) margin and nonlinear tolerance for unrepeatered transmission of polarization division multiplexing (PDM) optical superchannel with EDFA only amplification. Orthogonal pumps FWM scheme is utilized to generate the optical phase-conjugated copy. The original superchannel and the phase-conjugated copy are simultaneously transmitted and received. The phases of the copy symbols are conjugated and summed with the original superchannel symbols to suppress both linear noise and nonlinear phase noise at the receiver. The proposed OPC scheme is simple and effective in phase-conjugated copy generation and digital signal processing (DSP). What is more, it is transparent to signal bit rate and modulation format, which applies to optical superchannel transmission. We experimentally verify the proposed scheme on a 400Gb/s optical polarization division multiplexing orthogonal frequency division multiplexing (PDM-OFDM) superchannel. A Q-factor improvement of 2.1dB is achieved after 180km SSMF unrepeatered transmission. The optimum launch power in OPC scheme increases from −3dBm to −2dBm. To verify the maximum reach, we extend fiber length and realize 240km SSMF unrepeatered transmission.

Hybrid OFDM experimentation based on arrayed waveguide grating for all‐optical FFT implementation in PON architecture

A hybrid optical frequency division multiplexing (OFDM) system based on the electrical generation of the OFDM signal at the transmitter and on the all-optical demultiplexing of the OFDM subcarriers is experimentally demonstrated by exploiting a purposely designed arrayed waveguide grating (AWG) with 12.5 GHz frequency spacing. The AWG passively and all-optically processes a 3-subcarrier OFDM signal with fast Fourier transform functionalities, demultiplexing it into three 12.5 Gbit/s on–off keyed data at the corresponding output ports, which are then connected to the optical network users. Transmission over a 40 km feeder link is experimentally demonstrated. Taking into account the AWG's available 16 ports and four 3-subcarrier OFDM signal generators modulating independent wavelength carriers, a passive optical network (PON) architecture can be realised serving 12 optical network units with 150 Gbit/s total aggregate downstream capacity.

Flexible OFDM-based access systems with intrinsic function of chromatic dispersion compensation

Cost-effective and tunable chromatic dispersion compensation in a fiber link are still an open issue in metro and access networks to cope with increasing costs and power consumption. Intrinsic chromatic dispersion compensation functionality of optical fractional orthogonal frequency division multiplexing is discussed and experimentally demonstrated using dispersion-tunable transmitter and receiver based on wavelength selective switching devices.

Circular Polarized Optical OFDM for Optical Wireless Communication

As one of optical wireless Orthogonal Frequency Division Multiplexing (OFDM) systems, there is Flip-OFDM, which separates an OFDM signal into positive and negative parts and transmits them. It has good power efficiency and low hardware complexity. However, the system halves transmission efficiency compared with Direct Current-biased Optical OFDM. In this paper, Circular Polarized Optical OFDM (CPO-OFDM) is presented. This system separates OFDM signals into positive and negative parts, and it converts these signals into left-handed and right-handed polarization, and it multiplexes these signals. CPO-OFDM is analyzed with an intensity modulation/direct detection channel model which considers the change of the state of polarization owing to free space propagation. As a result of the analysis, it is shown that CPO-OFDM is a flexible system like the conventional systems by using circular polarization and it has the equivalent bit error rate (BER) and the double transmission efficiency compared with Flip-OFDM. The IM/DD channel model which considers the degree of polarization (DOP) is also shown. As for the DOP, it improves by the increase of the propagation distance. Thus, we can achieve the equivalent BER obtained with a high DOP laser even if we use a low DOP laser.

Interleaved and partial transmission interleaved optical coherent orthogonal frequency division multiplexing

A novel group of interleaved orthogonal frequency division multiplexing (IL-OFDM) and partial transmission IL-OFDM (PT-IL-OFDM) schemes are proposed for optical coherent detection. In the interleaved operation, the odd (or even) subcarriers of an OFDM symbol are reserved (closed). Such an interleaved arrangement can gain the tolerance toward phase noise induced interchannel interference (PN-ICI). Meanwhile, the peak to average power ratio (PAPR) of an OFDM signal can be significantly reduced. However, half of the capacity is sacrificed for the interleaved operation. By exploring the time domain symmetry property of IL-OFDM, the concept of PT-IL-OFDM is proposed to double the capacity of IL-OFDM. Consequently, PT-IL-OFDM can gain the merits of low PAPR and a high tolerance toward PN-ICI without capacity sacrifice. Numerical simulation verified the advanced properties of IL-OFDM and PT-IL-OFDM.