Forward Error Correction Limit Research Articles

Enhanced Three-Core Three-Mode Optical Transmission System Based on Probabilistic Shaping With Low Complexity MIMO Equalization Algorithm

A novel few-mode multi-core (FMMC) optical fiber transmission system based on probabilistic shaping (PS) with low complexity multiple-input multiple-output (MIMO) equalization method is proposed and demonstrated in this paper. Compared with the traditional FMMC counterpart, the total bit error rate (BER) is reduced by 3 times after 50km fiber transmission with the optical signal-to-noise ratio (OSNR) in the range of 12 to 20 dB. At a bit error rate of forward error correction (FEC) limit, the optical transmission system proposed in this paper outperforms the traditional one by at least 3dB OSNR improvement. When the OSNR is 20 dB and the BER is lower than the FEC limit, the transmission distance is doubled.

Data Rate Enhancement in Optical Camera Communications Using an Artificial Neural Network Equaliser

In optical camera communication (OCC) systems leverage on the use of commercial off-the-shelf image sensors to perceive the spatial and temporal variation of light intensity to enable data transmission. However, the transmission data rate is mainly limited by the exposure time and the frame rate of the camera. In addition, the camera's sampling will introduce intersymbol interference (ISI), which will degrade the system performance. In this paper, an artificial neural network (ANN)-based equaliser with the adaptive algorithm is employed for the first time in the field of OCC to mitigate ISI and therefore increase the data rate. Unlike other communication systems, training of the ANN network in OCC is done only once in a lifetime for a range of different exposure time and the network can be stored with a look-up table. The proposed system is theoretically investigated and experimentally evaluated. The results record the highest bit rate for OCC using a single LED source and the Manchester line code (MLC) non-return to zero (NRZ) encoded signal. It also demonstrates 2 to 9 times improved bandwidth depending on the exposure times where the system's bit error rate is below the forward error correction limit.

Digital Resolution Enhancer Employing Clipping for High-Speed Optical Transmission

Driven by the exponentially increasing demand for Internet traffic, single channel capacity in optical transmission systems continues to grow. Therefore, optical transmitter impairments such as low analog bandwidth, nonlinearities and digital-to-analog conversion with low resolution place a considerable burden on the system design. Methods aimed at compensating for the low transmitter bandwidth and transmitter nonlinearities have been introduced and recently, a digital resolution enhancer (DRE) which reduces the quantization noise added by a digital-to-analog converter (DAC) has been proposed. In this work, the interplay between DRE and clipping is introduced and studied. Experimental validation is performed by transmitting single channel 600 Gb/s 64-quadrature amplitude modulation (QAM) and wavelength-division multiplexing 460 Gb/s 128-QAM over 75 km of standard single-mode fiber. The combination of DRE and clipping allows the reduction of the DAC's physical number of bits to 3, while still maintaining performance above the forward error correction (FEC) limit. Low resolution DACs will play an important role in the implementation of low-power transmitters generating high-cardinality modulation formats.

Constellation Design for Single Photodetector Based CSK With Probabilistic Shaping and White Color Balance

Research and development of Li-Fi (Light Fidelity) in Internet-of-things (IoT) has created significant interest among researchers. The low-cost version of standard color shift keying (CSK) modulation scheme having single photodetector (PD) at the receiver can become an alternative to on-off keying (OOK) in IoT sensor networks as an M -ary CSK modulation scheme provides log 2 M times more data rate than that of OOK. This work revolves around optimizing the constellation points of single PD based CSK (CSK-1PD) to achieve a more power-efficient modulation scheme. The optimization has been done with and without average white tone constraint for uniform and non-uniform (exponential, Maxwell-Boltzmann, Pareto) source distributions. The constellation design strategy is based on maximizing the minimum distance among the differently distributed symbols. In the case of non-uniform distributions, the constellations points have been optimized by geometrically shaping the points based on their probabilistic shaping (PS). The optimized constellation points (OCPs) of CSK-1PD without white tone constraint provide higher signal-to-noise ratio (SNR) gain at forward error correction (FEC) limit symbol error rate (SER) as compared with strict white tone constraint. Further, the strict white tone constraint has been relaxed by considering the whole white light region in the CIE 1931 chromaticity diagram as white tone. The OCPs for uniform data with relaxed white tone constraint achieves SNR gain similar to without white tone constraint. However, for probabilistically shaped symbols, the SNR gain achieved from OCPs improves from strict white tone constraint but remains less than the without white tone constraint. A novel method to utilize an additional RGB LED at the transmitter side has been proposed and designed to maintain any white tone light in the white light region without degrading the maximum achieved SNR gain.

Differential drive I/Q modulator based on silicon photonic electro-absorption modulators

We demonstrate up to 100 Gbaud quadrature-amplitude modulated (QAM) signal generation with monolithic silicon in-phase quadrature (I/Q)-modulator based on silicon-germanium (SiGe) electro-absorption modulators (EAM). The modulator is in a differentially driven Mach–Zehnder (MZ) interferometric configuration similar to the conventional MZ modulator (MZM) based (I/Q) modulators. In particular, single-polarization quadrature phase-shift keying (SP-QPSK) signaling at symbol rates of 100 Gbaud with BERs below the hard-decision forward error correction (FEC) limit is shown. SP-QPSK optical signal generation and transmission over 80 km single-mode fiber (SMF) are demonstrated at symbol rates of 50 Gbaud with BER of less than $\mathbf {10^{-5}}$ . The effect of the frequency roll-off used for Nyquist pulse shaping with raised cosine frequency response is studied for SP-QPSK 50 Gbaud signals. Furthermore, 16QAM signals at symbol rates of 50 Gbaud are generated.

Realization of real‐time OFDM‐based visible light communication using an InGaN/GaN multiquantum‐wells LED

AbstractThis paper presents a real‐time OFDM‐based visible light communication (VLC) system utilizing a single InGaN/GaN multiquantum‐wells (MQWs) light emitting diode (LED). An MQWs LED with 3 dB electrical bandwidth of 23.8 MHz is developed as the transmitting device. Orthogonal frequency division multiplexing (OFDM) is employed to ensure the LED bandwidth is fully used, and the system is designed and implemented on a field‐programmable gate array platform to realize real‐time communication. The real‐time OFDM‐VLC system achieves a rate of 96 Mbit/s at the distance of 2 m, and the bit error ratio (BER) is 2.25 × 10−3, which is below the forward error correction (FEC) limit.

P.507 Visual activity monitoring and social attention in autism spectrum disorder: effect of context and age

Using solar cell (or photovoltaic cell) for visible light communication (VLC) is attractive. Apart from acting as a VLC receiver (Rx), the solar cell can provide energy harvesting. This can be used in self-powered smart devices, particularly in the emerging “Internet of Things (IoT)” networks. Here, we propose and demonstrate for the first time using pre-distortion pulse-amplitude-modulation (PAM)-4 signal and parallel resistance circuit to enhance the transmission performance of solar cell Rx based VLC. Pre-distortion is a simple non-adaptive equalization technique that can significantly mitigate the slow charging and discharging of the solar cell. The equivalent circuit model of the solar cell and the operation of using parallel resistance to increase the bandwidth of the solar cell are discussed. By using the proposed schemes, the experimental results show that the data rate of the solar cell Rx based VLC can increase from 20 kbit/s to 1.25 Mbit/s (about 60 times) with the bit error-rate (BER) satisfying the 7% forward error correction (FEC) limit.

Experimental DCO-OFDM Optical Camera Communication Systems With a Commercial Smartphone Camera

Modern handheld mobile devices are equipped with a complementary metal-oxide-semiconductor (CMOS) image sensor. These image sensors are capable of perceiving intensity changes in optically modulated signals, driving the rapid development of optical camera communication (OCC). In this study, direct-current offset orthogonal frequency division multiplexing (DCO-OFDM) was developed for a rolling shutter OCC system in order to improve the data transmission rates of OCC systems. Two DCO-OFDM OCC signal detection algorithms, namely the parabolic column vector selection (P-CVS) algorithm and the decomposition of extremely low frequency component (D-ELFC) algorithm, were proposed, along with a frequency-domain downsampling scheme, to eliminate the influence from both the blooming effect and shot noise. The experimental results, obtained after using a self-built DCO-OFDM OCC testbed with a general commercial smartphone camera, demonstrated that the D-ELFC algorithm outperformed in the bit error rate (BER) in comparison to the P-CVS algorithm and achieved a data transmission rate >22 kbps under the conditions of the BER being below the 20% forward error correction (FEC) limit and the illuminance being higher than 510 lux.

Tunable THz Signal Generation and Radio-Over-Fiber Link Based on an Optoelectronic Oscillator-Driven Optical Frequency Comb

We propose and demonstrate experimentally a photonic THz signal generation technique based on a tunable optoelectronic oscillator (OEO), and its application in a radio over fiber (RoF) link. The OEO's frequency is tuned by varying the bandwidth of a tunable optical bandpass filter (TOBF) that is cascaded with a phase modulator (PM). The resulting tunable microwave photonic filter is used to generate OEO oscillations from 6.58 GHz up to 18.36 GHz (with a phase noise of ≤−103 dBc/Hz at 10 kHz offset from the carrier frequency). The OEO is subsequently used to drive an optical comb, generating 22 comb lines with a frequency spacing of 17.33 GHz covering a bandwidth of 360 GHz within a 20 dB envelope. By selecting two optical comb lines with a wavelength selective switch (WSS) and beating them in a uni-traveling carrier photodiode (UTC-PD), THz signals are generated at 101.5 GHz and 242.6 GHz with phase noise of −90 dBc/Hz and −78 dBc/Hz, respectively at 10 kHz offset from carrier frequency. Tunable mm-wave and THz signals can be generated either by changing the OEO oscillation frequency or the selected comb lines. Using the OEO driven OFCG, we implemented a RoF link at 242.6 GHz with a data rate of 24 Gbps over a wireless distance of 30 cm and with a bit error rate (BER) below the hard decision forward error correction (FEC) limit of 3.8 × 10−3. This method allows the creation of an all-photonic frequency reconfigurable THz signal generator and RoF system.

Iterative Algorithm for Electronic Dispersion Compensation in IM/DD Systems

A novel iterative algorithm for electronic dispersion compensation (EDC) at the transmitter is described and demonstrated for intensity modulation and direct detection (IM/DD) systems. The proposed algorithm is based on the Gerchberg-Saxton (GS) algorithm, where the link dispersion performs time-to-frequency mapping, while the unconstrained phase at the direct detection receiver acts as a degree of freedom. Simulation results for a transmitter employing a single drive Mach-Zehnder modulator (MZM) operating at 28 Gb/s non-return-to-zero on-off keying (NRZ-OOK), show pre-compensation of upto 100 km of single mode fiber (SMF), with 1 dBQ margin above the forward error correction (FEC) bit error rate (BER) limit of $3.8 \times 10^{-3}$ . It is shown that with five iterations of the proposed EDC algorithm in digital-signal processing (DSP) coupled with an 8-bit 56 GSa/s digital-to-analog converter (DAC), the transmission reach is extended by a factor of 5 in unamplified SMF links. Simulations for a 56 Gb/s four-level pulse-amplitude-modulation (PAM-4) show the compensation of upto 10 km with five iterations of the EDC algorithm at 1 dBQ above the FEC limit.

Mitigation for turbulence effects in a 40-Gbit/s orbital-angular-momentum-multiplexed free-space optical link between a ground station and a retro-reflecting UAV using MIMO equalization.

We experimentally demonstrate turbulence effect mitigation in a 100-m round-trip orbital-angular-momentum (OAM)-multiplexed free-space optical communication link between a ground transmitter and a ground receiver via a retro-reflecting hovering unmanned aerial vehicle (UAV) using multiple-input-multiple-output (MIMO) equalization. In our demonstration, two OAM beams at 1550nm are transmitted to the UAV through emulated atmospheric turbulence, each carrying a 20-Gbit/s signal. 2×2 MIMO equalization is used to mitigate turbulence-induced crosstalk between the two OAM channels. Bit error rates below the 7% overhead forward error correction limit of 3.8×10-3 are achieved for both channels.

A Novel Baseband Faster-Than-Nyquist Non-Orthogonal FDM IM/DD System With Block Segmented Soft-Decision Decoder

In this paper, we systematically investigate our recently proposed faster-than-Nyquist non-orthogonal frequency-division multiplexing (FTN-NOFDM) generation technique for intensity modulation/direct detection (IM/DD) optical systems. The proposed scheme is based on modified fractional discrete Fourier transform (mFrDFT), and presents several advantages compared to other existing generation methods. Due to the exemption of guard bands, the proposed scheme achieves higher tolerance to narrow-bandwidth filtering. Besides, without the constraints on compression factor and modulation formats, the proposed system gives better signal flexibility and compatibility to various advanced signal processing techniques. To meet the demand on a large number of FTN-NOFDM subcarriers and high-order modulation formats, we further propose an efficient block segmented soft-decision decoder (BSSDD), which does not need to adjust the signal architecture or sacrifice the spectral efficiency. A 22.24-Gb/s 4-QAM FTN-NOFDM signal with up to 25% bandwidth saving has been successfully implemented over 20-km standard single mode fiber (SSMF) in an IM/DD optical communication system while keeping the bit error ratio (BER) below the hard-decision forward error correction (HD-FEC) limit. For both 4-QAM and 16-QAM FTN-NOFDM systems, the proposed BSSDD achieves better performance than the conventional decoder. All the results indicate that the proposed FTN-NOFDM technique has great potential to be used in high-speed IM/DD optical systems.

Low-complexity and joint modulation format identification and OSNR estimation using random forest for flexible coherent receivers

A low-complexity and joint modulation format identification (MFI) and optical signal to noise ratio (OSNR) estimation scheme is proposed for flexible coherent receivers (FCRs) by using random forest (RF) combined with the amplitude histograms of pre-equalized signals. Numerical simulation and experiments demonstrate the proposed scheme can achieve good performance with very low complexity. Comprehensive simulation results of 16GBaud polarization multiplexed (PM) 4∕8∕16∕32∕64 quadrature amplitude modulation (QAM) not only prove RF-based MFI can obtain 100% accuracy at OSNR lower than that required for 20% forward error correction (FEC) limit, which is similar to deep neural network (DNN) and support vector machine (SVM), but also indicate the averaged mean absolute error (MAE) of OSNR estimation is superior to DNN and SVM. Moreover, the complexity of RF-based scheme is at least an order of magnitude lower than DNN and SVM. The performance is also experimentally investigated by 16GBaud P M−4∕16∕32 QAM wavelength division multiplexing coherent systems. 100% MFI accuracy can also be achieved at OSNR lower than that required for 20% FEC limit and the averaged MAE is lower than DNN and SVM as well. The coexistent advantages of low-complexity and good performance make the proposed scheme have the potential to be embedded in FCRs.

Laser Phase Noise Tolerance of Uniform and Probabilistically Shaped QAM Signals for High Spectral Efficiency Systems

We numerically and experimentally investigate the laser phase noise tolerance of probabilistically shaped (PS) and uniformly shaped (US) quadrature amplitude modulation (QAM) signals. In the simulations, we compare PS-64QAM to US-16QAM, PS-256QAM to US-64QAM, and PS-1024QAM to US-256QAM under the same information rate (IR). We confirm that a sufficient shaping gain is observed with narrow linewidth lasers, whereas degradation of the shaping gain is clearly observed when large phase noise and high order modulation formats are assumed. In our experiments, we compare polarization-division-multiplexed (PDM) 16-GBd PS-1024QAM and US-256QAM under the same IR using lasers with 0.1-kHz and 40-kHz linewidths. For carrier phase recovery (CPR), we employ a pilot-assisted digital phase locked loop. Results reveal that PS-1024QAM achieves high performance with the 0.1 kHz-laser or > 5% pilot ratio, whereas US-256QAM outperforms PS-1024QAM when lasers with 40-kHz linewidth and < 5% pilot ratio are used. We also evaluate the pilot ratio dependency of the required optical signal-to-noise ratio at the forward error correction limit and the achievable information rate. Additionally, we compare the performance of two types of CPR updating schemes: updating phase estimation at only the pilot symbol or at all symbols.

Seamless 25 GHz Transmission of LTE 4/16/64-QAM Signals Over Hybrid SMF/FSO and Wireless Link

We propose and experimentally demonstrate a photonics-assisted converged radio-over-fiber (RoF), radio-over-free-space optics (RoFSO) and millimeter-wave (MMW) wireless transmission system for use in broadband wireless access (BWA) networks. The focus is at the emerging frequency band of 25 GHz, as recommended for fifth-generation networks. As a proof-of-concept demonstration, all-optical up-converted long-term evolution test models with 4-, 16- and 64-quadrature amplitude modulation (QAM) are transmitted and evaluated over the proposed hybrid link under weak-to-strong atmospheric turbulence regimes. Link performance shows that, the error vector magnitudes are below the 3GPP standard for 4-, 16- and 64-QAM. We also show that, for all QAM signals under turbulence conditions, the bit error rate performance is below the forward error correction limit of ${10^{ - 3}}$ . Simulation analysis is also performed for the 10 Gb/s hybrid systems under turbulence for an extended FSO link up to 500 m to emulate a practical outdoor environment. Furthermore, we analytically estimate the attainable MMW wireless range for different rain rates in Prague, Czech Republic. The obtained experimental and simulation results confirm the feasibility and potential of the proposed hybrid system for next-generation last mile BWA networks.

Interband Short Reach Data Transmission in Ultrawide Bandwidth Hollow Core Fiber

We report a Nested Antiresonant Nodeless hollow-core Fiber (NANF) operating in the first antiresonant passband. The fiber has an ultrawide operational bandwidth of 700 nm, spanning the 1240–1940 nm wavelength range that includes the O-, S-, C- and L- telecoms bands. It has a minimum loss of 6.6 dB/km at 1550 nm, a loss ≤7 dB/km between 1465–1655 nm and ≤10 dB/km between 1297–1860 nm. By splicing together two structurally matched fibers and by adding single mode fiber (SMF) pigtails at both ends we have produced a ∼1 km long span. The concatenated and connectorized fiber has an insertion loss of approximately 10 dB all the way from 1300 nm to 1550 nm, and an effectively single mode behavior across the whole spectral range. To test its data transmission performance, we demonstrate 50-Gb/s OOK data transmission across the O-to L-bands without the need for optical amplification, with bit-error-rates (BERs) lower than the 7% forward error correction (FEC) limit. With the help of optical amplification, 100-Gb/s PAM4 transmission with BER lower than the KP4 FEC limit was also achieved in the O/E and C/L bands, with relatively uniform performance for all wavelengths. Our results confirm the excellent modal purity of the fabricated fiber across a broad spectral range, and highlight its potential for wideband, low nonlinearity, low latency data transmission.

Centralized Fiber-Distributed Data Communication and Sensing Convergence System Based on Microwave Photonics

A unified data communication and radar sensing system towards millimeter-wave (mm-wave) applications with extended coverage is proposed and experimentally demonstrated. In the unified system, the transmission and reception of mm-wave signals are implemented by microwave photonics technologies, and the orthogonal frequency division multiplexing (OFDM) signal is employed for communications and radar sensing simultaneously. The superheterodyne optical carrier suppression (HeteroOCS) is used to up-convert the intermediate frequency (IF) OFDM signal to the mm-wave band in the central office (CO). The optical mm-wave signal is transmitted to the remote antenna units (RAUs) via optical fibers. The echo signal is received by the RAU which is remotely connected to the CO by an analog photonic link based on a polarization modulator (PolM). In the transmitter and receiver links, the chromatic dispersion of optical fibers is overcome by the HeteroOCS and the PolM, respectively. The baseband units (BBU) centralized in the CO is used to process the radar sensing signal. A pre-delayed trigger signal is proposed to overcome the fiber length limitation. A proof-of-concept experiment is performed. Two targets with distances of 1.64 m and 3.40 m are successfully detected separately. The communication function of the system is also verified. 1 m wireless transmission of 1.56 Gb/s data rate is achieved below the forward error correction limit.

Performance evaluation of neural network assisted motion detection schemes implemented within indoor optical camera based communications.

This paper investigates the performance of the neural network (NN) assisted motion detection (MD) over an indoor optical camera communication (OCC) link. The proposed study is based on the performance evaluation of various NN training algorithms, which provide efficient and reliable MD functionality along with vision, illumination, data communications and sensing in indoor OCC. To evaluate the proposed scheme, we have carried out an experimental investigation of a static indoor downlink OCC link employing a mobile phone front camera as the receiver and an 8 × 8 red, green and blue light-emitting diodes array as the transmitter. In addition to data transmission, MD is achieved using a camera to observe user's finger movement in the form of centroids via the OCC link. The captured motion is applied to the NN and is evaluated for a number of MD schemes. The results show that, resilient backpropagation based NN offers the fastest convergence with a minimum error of 10-5 within the processing time window of 0.67 s and a success probability of 100 % for MD compared to other algorithms. We demonstrate that, the proposed system with motion offers a bit error rate which is below the forward error correction limit of 3.8 × 10-3, over a transmission distance of 1.17 m.

Mitigation of dispersion and turbulence in a hybrid optical fibre and free-space optics link using electronic equalisation

Transmission of a 25 Gbps non-return to zero signal over a hybrid link of 20 km standard single-mode fibre and 500 m free-space optics under weak-to-moderate turbulence regimes is demonstrated via simulation to provide a practical solution for the last-mile access networks. Even though there are several complex mitigating methods to reduce the waveform distortions caused by both the fibre chromatic dispersion and atmospheric turbulence, we provide and discuss a simple approach using an electrical equalizer at the receiver. The simulation results demonstrate that, the proposed equalization technique offer about an order of magnitude improvement in the bit error rate at the received optical power of -20 dB m and a receiver sensitivity gain of 1.5 dB at the 7% forward error correction limit.

Arbitrarily routed mode-division multiplexed photonic circuits for dense integration

On-chip integrated mode-division multiplexing (MDM) is an emerging technique for large-capacity data communications. In the past few years, while several configurations have been developed to realize on-chip MDM circuits, their practical applications are significantly hindered by the large footprint and inter-mode cross talk. Most importantly, the high-speed MDM signal transmission in an arbitrarily routed circuit is still absent. Herein, we demonstrate the MDM circuits based on digitized meta-structures which have extremely compact footprints. 112 Gbit/s signals encoded on each mode are arbitrarily routed through the circuits consisting of many sharp bends and compact crossings with a bit error rate under forward error correction limit. This will significantly improve the integration density and benefit various on-chip multimode optical systems.