Multilevel Pulse Amplitude Modulation Research Articles

Neural Network Equalisation for High-Speed Eye-Safe Optical Wireless Communication with 850 nm SM-VCSELs

In this paper, we experimentally illustrate the effectiveness of neural networks (NNs) as non-linear equalisers for multilevel pulse amplitude modulation (PAM-M) transmission over an optical wireless communication (OWC) link. In our study, we compare the bit-error-rate (BER) performances of two decision feedback equalisers (DFEs)—a multilayer-perceptron-based DFE (MLPDFE), which is the NN equaliser, and a transversal DFE (TRDFE)—under two degrees of non-linear distortion using an eye-safe 850 nm single-mode vertical-cavity surface-emitting laser (SM-VCSEL). Our results consistently show that the MLPDFE delivers superior performance in comparison to the TRDFE, particularly in scenarios involving high non-linear distortion and PAM constellations with eight or more levels. At a forward error correction (FEC) threshold BER of 0.0038, we achieve bit rates of ~28 Gbps, ~29 Gbps, ~22.5 Gbps, and ~5 Gbps using PAM schemes with 2, 4, 8, and 16 levels, respectively, with the MLPDFE. Comparably, the TRDFE yields bit rates of ~28 Gbps and ~29 Gbps with PAM-2 and PAM-4, respectively. Higher PAM levels with the TRDFE result in BERs greater than 0.0038 for bit rates above 2 Gbps. These results highlight the effectiveness of the MLPDFE in optimising the performance of SM-VCSEL-based OWC systems across different modulation schemes and non-linear distortion levels.

Synchronous time multiplexing for non-line-of-sight multiple-input multiple-output optical camera communications.

In typical multiple-input multiple-output (MIMO) optical camera communication (OCC) systems, the spatial correlation of MIMO channels is large. Optical signals between light sources can easily interfere with each other, negatively impacting the overall transmission performance. In this work, we propose a time-multiplexing integral modulation scheme for a non-line-of-sight MIMO OCC system, where each LED transmits different signals to improve both the data rate and security of the system. A genetic algorithm (GA)-based adaptive multi-threshold scheme is designed to demodulate the blurred fringes in multi-level pulse amplitude modulation. The experimental results show that at a distance of 2.5 m, a data rate of 16.4 kb/s can reach with the BER performance of 3.01 × 10-3, which validates the superiority and reliability of our proposed schemes.

A new-generation approach to PAM-4 Based on the Fano resonance effect using a multimode interference structure

The goal of this research is to come up with a new optical structure for the generation of multilevel pulse amplitude modulation (PAM-4) signalling, which is used for optical interconnects and data center networks. The Fano effect is made with a structure made up of a 4x4 MMI coupler, feedback waveguides, and phase shifters. For the production of PAM-4, we employ the Fano effect. The new proposed approach can reduce power consumption and extend the linear region of the device transfer function compared with conventional approaches based on Mach Zehnder Interferometers (MZI) and ring resonators. We show that an extreme reduction of power consumption of 3 to 30 times, compared with the conventional structure based on MZI and ring resonators, is achieved. In addition, the proposed structure based on silicon wave guides has the advantages of extreme high bandwidth, large fabrication tolerance, and a compact footprint. The proposed structure can be applied to generate the PAM-4 based on the Fano effect with low power consumption, and it is suitable for complex systems with a lot of transceivers. The device is numerically simulated and designed using the Finite Difference Method (FDM) and Finite Difference Time Difference (FDTD). The PAM-4 structure based on silicon waveguides can be compatible with CMOS’s existing technologies.

Reconfigurable Generation of PAM-4 Signal Based on Fano Effect for Optical Interconnect Systems

We present a novel low-power architecture for the generation of multilevel pulse amplitude modulation (PAM-4) signal generation. A new structure of a micro-ring resonator based on a 4´4 MMI (Multimode interference) coupler is proposed to control the coupling coefficient and Fano shapes. Based on this structure, a high linearity of the transmission is created, compared with the Mach Zehnder Interferometer (MZI) conventional structure. Here, instead of using directional couplers and MZI as shown in the previous reports, in our method only two 4´4 multimode interference structures on silicon on insulator (SOI) waveguides has been used. The new design is compatible and suitable for the current CMOS fabrication technology. In this work, the special design is as follows: one of two MMI couplers is used to be a micro-ring resonator and two segmented phase shifters are used in the micro-ring resonator to generate 4 levels of the PAM-4 signal. The micro-ring resonator is controlled to work at the over-coupled region, so the Fano effect can be generated. This proposed PAM-4 architecture uses the generated Fano effect, therefore an extreme reduction in power consumption can be achieved. A large fabrication tolerance of ±500 nm and a compact footprint of 10´500 µm2 can be carried-out. The device is simulated and optimally designed using the FDTD (finite difference time domain) and EME (Eigenmode Expansion). This architecture can be useful for optical interconnects and data center network applications.

Two-Channel OTDM System for Data-Center Interconnects: A Review.

It has been proposed to implement the >100 Gb/s data-center interconnects using a two-channel optical time-division multiplexed system with multilevel pulse-amplitude modulation. Unlike the conventional four-channel optical time-division multiplexed system which requires an expensive narrow pulse, the two-channel system can be implemented cost-effectively using a wide pulse (which can be simply generated using a single modulator). The two-channel system is expected to be practically available using an integrated transmitter in a chip due to the recent advances in photonics-integrated circuits. This paper reviews the current stage of research on a two-channel optical time-division multiplexed system and discusses possible research directions. Furthermore, it has been demonstrated that 200 Gb/s signals can be generated by using modulators with only 17.2 GHz bandwidth. Therefore, the use of the phase-alternating pulse can make the multiplexed signal robust to chromatic dispersion, enabling the 200 Gb/s 4-level pulse-amplitude-modulated signal to be transmitted over 1.9 km of standard single-mode fiber.

800 Gbit/s QSFP-DD Transceiver Based on Thin-Film Lithium Niobate Photonic Integrated Circuit

800G Ethernet is expected to be the dominant solution for the next generation inter- and intra-data-center connections. To boost the transmission capacity to 800Gb/s, utilizing multi-level pulse amplitude modulation (PAM) transmitting format and adopting multiple lanes are competitive solutions. In this paper, we demonstrate a PAM4-modulated 800G QSFP-DD transceiver integrated with two 4×100G thin-film lithium niobate (TFLN) DR4 modulator chips. The DR4 modulator chips have a 13.5 dB insertion loss (including the inherent 3 dB modulation loss and 6 dB splitting loss) and one DR4 chip requires one 17 dBm light source. The EO response has a 40 GHz bandwidth at 2.3 dB and the electric reflection keeps below -12 dB. The eight channels of 800G transceiver have a clear open eye patterns at 1.5 Vpp driving voltage under 53.125 Gbaud PAM-4 modulation with all the ERs above 4.03 dB and TDECQ's below 2.13 dB. The sensitivity of the receiver is around -8.6 dBm after 2 km and back-to-back transmission with the BER of all the channels below the DR4 forward error correcting threshold. It is demonstrated that our 800G QSFP-DD transceiver meets 400GBASE-DR4 standards and packaging requirements simultaneously. This is the first time that 4×100G DR4 modulator array based on TFLN and 800G QSFP-DD transceiver based on TFLN are reported to the best of our knowledge.

Performance evaluation of plastic optical fiber communication using micro-lens coupling and computational temporal ghost imaging algorithm.

Plastic optical fiber communication (POFC) systems are particularly sensitive to signal performance and power budget. In this paper, we propose what we belive to be a novel scheme to jointly enhance the bit-error-ratio (BER) performance and coupling efficiency for multi-level pulse amplitude modulation (PAM-M) based POFC systems. The computational temporal ghost imaging (CTGI) algorithm is developed for PAM4 modulation for the first time to resist the system distortion. The simulation results reveal that enhanced BER performance and clear eye diagrams are acquired by using CTGI algorithm with an optimized modulation basis. Experimental results also investigate and show, with CTGI algorithm, the BER performance for 180 Mb/s PAM4 signals is enhanced from 2.2 × 10-2 to 8.4 × 10-4 over 10 m POF by using a 40 MHz photodetector. The POF link is equipped with micro-lenses at its end faces by using a ball-burning technique, which helps to increase the coupling efficiency from 28.64% to 70.61%. Both simulation and experimental results show that the proposed scheme is feasible to achieve a cost-effective and high-speed POFC system with short reach.

High-Dimensional Feature Based Non-Coherent Detection for Multi-Intensity Modulated Ultraviolet Communications

Ultraviolet communication (UVC) has been regarded as a promising supplement for overloaded conventional wireless communications. One challenge lies in the communication deterioration caused by the UV-photon scattering induced inter-symbol-interference (ISI), which will be even worse when encountering multilevel pulse amplitude modulation (multi-PAM) symbols. To address this ISI, traditional coherent detection methods (e.g., maximum-likelihood sequence detection, MLSD) require high computational complexities for UV channel estimation and sequential detection space formation, thereby making them less attractive. Current non-coherent detection, which simply combines the ISI-insensitive UV signal features (e.g., the rising edge) into a one-dimensional (1D) metric, cannot guarantee reliable communication accuracy. In this work, a novel high-dimensional (HD) non-coherent detection scheme is proposed, leveraging a HD construction of the ISI-insensitive UV signal features. By doing so, we transform the ISI caused sequential detection into an ISI-released HD detection framework, which avoids complex channel estimation and sequential detection space computation. Then, to compute the detection surface, a UV feature based unsupervised learning approach is designed. We deduce the theoretical bit error rate (BER), and prove that the proposed HD non-coherent detection method has a lower BER than that of the current 1D non-coherent scheme. Simulation results validate our results, and more importantly, demonstrate a BER that approaches that of the state-of-the-art coherent MLSD ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$&lt; $</tex-math></inline-formula> 1 dB in SNR at BER = <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$4.5\times 10^{-3}$</tex-math></inline-formula> , the 7% overhead forward-error-correction limit), and also a reduction of computational complexity by at least two orders of magnitude.

Comparison of the Nonlinear Dynamic Pre- and Post-LED Equalization.

Visible Light Communications (VLC) have gained much popularity lately. In such a system, a white LED (Light-Emitting Diode) plays a double role as a light source and a transmitter. The main problem here is that the LED exhibits a low bandwidth and high nonlinearity, so the equalization of the LED nonlinear dynamic response is necessary. For this, various equalizers are used. This paper compares the pre- and post-equalizer performance in terms of the received signal quality for a channel that includes a nonlinear element of limited bandwidth, such as an LED. Multilevel Pulse Amplitude Modulation (PAM) was selected as the signal format, as well as a variant of the Volterra series equalizer as the compensating element. The results obtained may be used for the correction of the dynamic characteristics of LEDs applied in VLC systems. For the sake of comparison, we used Modulation Error Ratio (MER) values at the receiver output. The dynamic nonlinear behavior was modeled by a Wiener–Hammerstein device, whereas the post/pre-equalizer was based on the dynamic deviation reduction-based Volterra series. The obtained results indicate that the post- and pre-equalizer performed comparably for the linear/moderately nonlinear channels and for a high noise level. In the case of high nonlinearity and a large SNR (Signal–to–Noise Ratio) values, the post-equalizer performed somewhat better in terms of MER by a few dB at maximum.

Stochastic Approximation Aided Adaptive Thresholding for Optical Detection in PAM4 Based FSO Transmission

This study proposes an adaptive thresholding optical detection technique using a stochastic approximation (SA) to enhance receiver performance in multi-level pulse amplitude modulation (PAM) based free-space optical (FSO) transmission. When the receiver detects the symbol by fixed threshold decision (FTD) scheme, the detection performance of high order modulation, including PAM4, becomes degraded due to a random fluctuation of the received signal. Owing to the slow varying characteristic of atmospheric turbulence, we used an adaptive linear prediction (ALP) filter to predict the decision threshold of the upcoming frame. The experiment was conducted on various degrees of turbulence channels. A Mach-Zehnder modulator (MZM) based turbulence channel emulator was used to experimentally investigate the turbulence-induced scintillation effect. The experiment results confirmed that the proposed technique yielded improved receiver performance compared to FTD. In the case of considering the amplified spontaneous emission (ASE) noise of erbium-doped fiber amplifier (EDFA), the adaptive threshold decision (ATD) scheme obtained about 3dB gain compared with an even-valued threshold (EVT) decision scheme when the average EDFA input optical power was below -16dBm.

Multi-User Precoder Designs for RGB Visible Light Communication Systems.

In this paper, we design linear precoders for the downlink of a visible light communication (VLC) system that simultaneously serves multiple users. Instead of using phosphor-coated white light-emitting diodes (PWLEDs), we focus on Red-Green-Blue light-emitting diodes (RGB-LEDs) that allow modulating three separate data streams on the three primary colors of the RGB-LEDs. For this system, we design a zero-forcing (ZF) precoder that maximizes the weighted sum rate for a multilevel pulse amplitude modulation (M-PAM). The precoding design in RGB-based systems presents some challenges due to the system constraints, such as the limited power, the non-negative amplitude constraints per light-emitting diode (LED), and the need to guarantee white light emission while transmitting with RGB-LEDs. For comparison purposes, we also consider the ZF design for a PWLED-based system and evaluate the performance of both a PWLED- and an RGB-based system.

Proposal of a Power Efficient $N$-Level Multipulse PPM-$L$QAM Technique

A new power efficient technique, that combines N-level multipulse pulse-position modulation with L-level quadrature-amplitude modulation, denoted by NMPPM-LQAM, is proposed. Its constrained power efficiency is derived and characterized. It is shown that the proposed combination provides a superior technique that is both power and spectral efficient simultaneously. The constrained power efficiencies of variations to this technique are considered and characterized as well. These include combining multilevel pulse-amplitude modulation with LQAM (NPAM-LQAM). The obtained constrained power efficiencies are compared numerically to that of traditional LQAM and MPPM-LQAM techniques. Our results disclose that proposed NMPPM-LQAM is the most power/spectral efficient technique. For example, at spectral efficiency constraints of 2.5 and 4.2 bs <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> /Hz/pol, the corresponding constrained power efficiencies of proposed NMPPM-LQAM technique are higher by about 4.87 and 3.62 dB, respectively, than that of traditional LQAM. In addition, the results reveal that NPAM-LQAM scheme has increased power and spectral efficiencies compared to that of traditional NPAM, but its power efficiency is less than that of traditional LQAM.

Investigation of frequency dependent nonlinearity on pulse amplitude modulation in bandlimited visible light communications

In this work, the nonlinear behavior on pulse-amplitude-modulation (PAM) and their impact on the performance of RGB light-emitting-diodes (LED) based visible light communications (VLC) is presented. Particularly, how the limited source bandwidth affects the nonlinearity of fully modulated RGB LEDs is investigated. It is found that when symbol-rate is higher than the channel bandwidth the nonlinearity gradually diminishes. It causes the symbol-rate dependent nonlinearity. When applying PAM in VLC links, a pre-distortion method is often applied to compensate for the nonlinearity. Due to this symbol-rate dependency, however, the conventional pre-distortion method has a limitation on linearizing the non-linear VLC channel, especially in the case of applying various symbol-rates according to the channel condition. Although it can be adaptively compensated by nonlinear equalizers, their complexity is considerably high. In this work, the characteristics of multi-level PAM scheme in a band-limited nonlinear channel is presented. Also, the feasibility of a low complex but efficient pre-distortion method derived from monotonically decreasing characteristics of the frequency dependent nonlinearity is investigated.

Filter Bank Multicarrier Modulation Schemes for Visible Light Communication

Filter bank multicarrier (FBMC) modulation scheme has come out as a capable contender for next generation wireless systems. This paper proposes FBMC modulation schemes for visible light communication (VLC). These modulation schemes consist of intensity modulation/direct detection (IM/DD) channel. The objective is to overcome some of the drawbacks of mostly used orthogonal frequency division multiplexing (OFDM) based schemes. Single carrier modulation technique such as multi-level pulse amplitude (M-PAM) and multi-level quadrature amplitude modulation (M-QAM) as multi carrier modulation technique are used to implement FBMC modulation scheme. Both DC bias and non-DC bias methods are used for analysis. The analytical performance is provided using parameters Bit error rate (BER) and spectral efficiency. It is found that proposed FBMC based schemes are more spectral and power efficient than optical OFDM based schemes.

On-Chip 4×10 GBaud/s Mode-Division Multiplexed PAM-4 Signal Transmission

Emerging 5G mobile networks and cloud computing applications are driving the demand for an ever-increasing capacity of short-reach optical communications. To meet this demand, mode-division multiplexing (MDM) has been proposed to scale up the bandwidth density by leveraging the spatial modes of an optical waveguide for transmitting multiple optical signals. On the other hand, the use of multi-level pulse amplitude modulation (PAM) can also increase the transmission bandwidth. Therefore, on-chip MDM in conjunction with PAM is an approach to enhance the transmission capacity in a photonic integrated circuit. In this paper, we report a silicon photonic integrated four-channel MDM circuit for high data rate on-chip communications with a low channel crosstalk and small insertion loss. To make the circuit have a small size that supports broadband operation, a mode multiplexer and demultiplexer are realized with the use of cascaded asymmetrical directional couplers on rib waveguides. By incorporating the MDM circuit in an optical communications system, the transmission of a 4 x 10 GBaud/s OOK and PAM-4 signal is experimentally demonstrated. The performance in terms of eye diagrams and power penalties is evaluated. The power penalties for the four-channel OOK transmission are 4.31, 2.38, 1.44 and 3.5 dB at a BER of 10−9. For the four-channel PAM-4 transmission, the power penalties are 7.98, 1.10, 0.66 and 5.14 dB. The required received optical power at a BER (<3.8 × 10−3) of the 7% overhead-hard decision FEC is −2.6 dBm. The key advantage of the approach is that high-capacity on-chip communications is enabled by the photonic integrated MDM circuit with a small footprint. Since the MDM circuit is implemented on a silicon photonic platform, the system holds high potential for full integration on a single chip.

Silicon Photonic Modulator Linearity and Optimization for Microwave Photonic Links

We experimentally demonstrate the influence of the DC Kerr effect in silicon photonic Mach-Zehnder modulators (MZMs) and the ability to optimize the combined effects of plasma-dispersion and DC Kerr to achieve linear transfer characteristics and thus demonstrate suitability for microwave photonic links and digital modulation formats such as multilevel pulse amplitude modulation. PN and PiN doped modulators were fabricated through AIM Photonics. Intermodulation distortion products are demonstrated to have reverse bias and MZM bias dependencies advantageous for highly linear operation. The spurious-free dynamic range, gain, and noise figure are optimized by choice of phase modulator reverse bias and MZM bias point yielding analog links with spurious-free dynamic ranges greater than 100 dB·Hz2/3. The silicon modulators demonstrate link spurious-free dynamic ranges on par with a commercial lithium niobate modulator. Furthermore, we show that simulations including the DC Kerr effect can reliably predict device performance although direct prediction of analog link metrics remains challenging, and final tuning of the device operating conditions is required to achieve optimum performance.

Performance enhancement of overall LEO/MEO intersatellite optical wireless communication systems

SummaryThis paper has deeply investigated the performance signature of modulation techniques based low earth orbit (LEO)/medium earth orbit (MEO) intersatellite optical wireless communication systems for possible communication coverage distance of 20 000 km with possible transmission bit rate of 0.5 Tb/s. These modulation techniques that are namely multilevel quadrature amplitude modulation (M‐QAM), multilevel phase shift keying (N‐PSK), multilevel pulse amplitude modulation (H‐PAM), and finally multilevel differential phase shift keying (L‐DPSK) based on different electrical pulse generators for upgrading LEO/MEO intersatellite link operation efficiency. These pulse generators that are namely Gaussian pulse generator (GPG), hyperbolic secant pulse generator (HSPG), and raised cosine pulse generator (RCPG). The variations of maximum Q‐factor, minimum bit error rate (BER), and optical signal‐to‐noise ratio in relation to number of bits/symbol for different modulation techniques can be deeply studied in the presence of vertical cavity surface emitting laser (VCSEL). This study is done with using Optiwave system simulation version 7 for different modulation techniques, and all figures are sketched with using wizard Excel sheet set up. It is observed that maximum Q‐factor and minimum BER are optimized with using GPG and 8‐PAM, as well as 4‐DPSK with both HSPG and RCPG.

Maximizing capacity, flexibility and efficiency in G-PON networks using VCSEL-based OOK and 2/4-PAM formats

ABSTRACTThis work experimentally demonstrates the potential of multi-level pulse amplitude modulation with direct detection to maximize carrier spectral efficiency and double the Gigabit passive optical networks (G-PON) network data rate. Three scenarios have experimentally been exploited. First, a 1310 nm vertical surface-emitting laser (VCSEL) has directly been modulated with a 10 Gbps OOK data. A receiver sensitivity of −19.11 dBm is attained, and a successful error free transmission over 22 km SMF fibre achieved, with a transmission penalty of 0.46 dB. To maximize carrier spectral efficiency, 2/4 PAM modulation formats are adopted respectively. A receiver sensitivity of −14.64 and −11.63 dBm is attained for 2-PAM and 4-PAM formats respectively. However, a 3.21 km fibre transmission introduces a penalty of 0.64 and 3.30 dB for 2-PAM and 4-PAM formarts respectively. 2/4-PAM modulation formats significantly increase the aggregated data rate at different ONUs within the G-PON without expensive optics investment, though at the cost of reduced transmission reach due to the high bitrate attained. We further demonstrate the design of a software defined digital signal processing assisted receiver to recover the 2/4 PAM transmitted signal without employing costly receiver hardware.

Noise-Aided Demodulation With One-Bit Comparator for Multilevel Pulse-Amplitude-Modulated Signals

This letter proposes a demodulation method using a one-bit comparator for signals processed by multilevel pulse amplitude modulation (PAM). The proposed method is simple and provides an alternative to using an analog-to-digital converter to describe multilevel input signals. Because of the noise present in the transmitted multilevel PAM signal, the two-level output of the one-bit comparator shows different statistical behavior for each level of the signal. Thus, it is possible to detect the signal level, or perform symbol decision, based on the maximum likelihood criterion. The present theoretical analysis reveals that reliable demodulation is possible even with a one-bit comparator if the probability mass function of the two-level outputs of each received symbol plus intentionally added noise is known.

Performance of M-PAM FSO communication systems in atmospheric turbulence based on APD detector.

We characterize the performance of the optical signal propagation model of multi-level pulse amplitude modulation (M-PAM) based on avalanche photodiode (APD) detector in the free-space link for the first time. When the number of photons absorbed by the active surface of the APD is large enough, the bit error rate (BER) performance relationship of the systems based on the signal intensity and the photon characteristics are depicted. We use the Gamma-Gamma (G-G) channel model to analysis the communication systems with joint parameter constraints, and demonstrate the atmospheric turbulence intensity, link lengths, optical wavelength, symbol transmission rate, temperature of APD and pointing errors (PEs) impact on the system average bit error rate (ABER) performance. Moreover, the relationship between signal-to-noise-ratio (SNR) and ABER rate is given. The numerical results show that the 4-PAM free-space optical (FSO) communication is suitable for medium-to-weak turbulence, and the high gain of APD can mitigate the influence of ABER. The best detection condition of the 4-PAM optical signal is at least 20 dB SNR, when the ABER is under the 7% forward error correction (FEC) limit of 3.8 × 10-3. This work provides a reference for parameter designing and evaluating in M-PAM FSO communication systems.