Equalizer Taps Research Articles

Advancing LWIR FSO communication through high-speed multilevel signals and directly modulated quantum cascade lasers

This study investigates the potential of long-wave infrared (LWIR) free-space optical (FSO) transmission using multilevel signals to achieve high spectral efficiency. The FSO transmission system includes a directly modulated-quantum cascade laser (DM-QCL) operating at 9.1 µm and a mercury cadmium telluride (MCT) detector. The laser operated at the temperature settings of 15°C and 20°C. The experiment was conducted over a distance of 1 m and in a lab as a controlled environment. We conduct small-signal characterization of the system, including the DM-QCL chip and MCT detector, evaluating the end-to-end response of both components and all associated electrical elements. For large-signal characterization, we employ a range of modulation formats, including non-return-to-zero on-off keying (NRZ-OOK), 4-level pulse amplitude modulation (PAM4), and 6-level PAM (PAM6), with the objective of optimizing both the bit rate and spectral efficiency of the FSO transmission by applying pre- and post-processing equalization. At 15°C, the studied LWIR FSO system achieves net bitrates of 15 Gbps with an NRZ-OOK signal and 16.9 Gbps with PAM4, both below the 6.25% overhead hard decision-forward error correction (6.25%-OH HD-FEC) limit, and 10 Gbps NRZ-OOK below the 2.7% overhead Reed-Solomon RS(528,514) pre-FEC (KR-FEC limit). At 20°C, we obtained net bitrates of 14.1 Gbps with NRZ-OOK, 16.9 Gbps with PAM4, and 16.4 Gbps with PAM6. Furthermore, we evaluate the BER performance as a function of the decision feedback equalization (DFE) tap number to explore the role of equalization in enhancing signal fidelity and reducing errors in FSO transmission. Our findings accentuate the competitive potential of DM-QCL and MCT detector-based FSO transceivers with digital equalization for the next generation of FSO communication systems.

Performance Assessments of Joint Linear and Nonlinear Pre-Equalization Schemes in Next Generation IM/DD PON

Nonlinear distortions in passive optical network (PON) systems degrade signal transmission performance. Joint linear and nonlinear pre-equalizers based on the Volterra series, memory polynomials and artificial neural networks (ANNs) are utilized to mitigate linear and nonlinear impairments. Centralized pre-equalization at the transmitter side of the optical line terminal (OLT) keeps the receiver side digital signal processing (DSP) at the optical network unit (ONU) simple. In this study, performance assessments of these three different pre-equalization schemes in intensity modulation and direct detection (IM/DD)-based 25-GBaud PON systems were evaluated. The joint linear and nonlinear pre-equalization schemes were based on the weight coefficients and the model obtained by channel estimation on the receiver side. We studied and compared the performance of three pre-equalization methods under various implementation conditions, such as received optical power (ROP), number of equalizer taps, and semiconductor optical amplifier (SOA) current. Besides, we studied the equalization performance under different modulation orders, which were applied for data rate optimization to support different power budgets, being the potential solutions for future PON systems with various fiber-reach and split-ratio distributions. As a proof-of-concept, the equalization performance of the 25-Gbaud signals modulated with non-return-to-zero (NRZ), four-level pulse amplitude modulation (PAM-4) and PAM-8 formats, corresponding to 25-, 50-, and 75-Gbps data rates, respectively, were tested. Both numerical simulations and experimental demonstrations were conducted to assess the performance of different pre-equalization schemes. With ANN pre-equalization, more than 4dB improvement in power budget was achieved for 25-GBaud PAM4 and PAM8 signals after 20 km fiber transmission. ANN-based pre-equalizer performed better than ANN-based post-equalizer even when the received signals were in low SNR. The power budgets of the 25-Gbaud NRZ, PAM4 and PAM8 signals after 20 km fiber transmission were 36.2 dB, 29.5 dB and 18 dB, respectively.

1120-channel OAM-MDM-WDM transmission over a 100-km single-span ring-core fiber using low-complexity 4×4 MIMO equalization.

A successful transmission of 14 multiplexed orbital angular momentum (OAM) channels each carrying 80 wavelengths over a 100-km single-span ring-core fiber (RCF) is experimentally demonstrated. Each transmission channel is modulated by a 20-GBaud quadrature phase-shift keying (QPSK) signal, achieving a record spectral-efficiency-distance product of 1870 (bit/s/Hz)·km for the single-core RCF based mode division multiplexing (MDM) transmissions. In addition, only low-complexity 2×2 or 4×4 multiple-input multiple-output (MIMO) equalization with time-domain equalization tap number no more than 25 is required to deal with the crosstalk among the highly degenerate intra-MG modes at the receiving end of the demonstrated OAM-MDM-WDM system, showing great potential in large-capacity and relatively long-distance MDM transmission with low digital signal processing (DSP) complexity.

Joint equalizer and phase-locked loop for time variability in underwater acoustic communications

In this paper, we verify the performance of the equalizer and phase-locked loop (PLL) operating jointly for channel time variability in underwater acoustic communications. We apply the decision feedback equalizer in order to eliminate channel multipath and PLL to phase rotation due to Doppler effects. In the PLL, a decision-directed tan−1 type phase detector is adopted. As shown in previous papers, it is difficult to make PLL to be steady state because of time-varying ISI occurred by multipath. Also, the phase rotation incurs the equalizer tap rotation. Therefore, we can see that the two subsystems of equalizer and PLL complement each other perfectly. Another issue is the convergence speed; how quickly we can make the joint equalizer and PLL to be stable state within the preamble period (known data). In order to verify the performance, the sea experiments were conducted in the condition of moving transmitter. The transmitted signal is modulated by using PSK and consists of five burst frames that are composed of preamble of PN code and random data. As a result, if we use the initial multipath value and Doppler frequency offset, we can make the joint equalizer and PLL to be the steady state more quickly.

Reducing multipath interference in quasi-single-mode transmission via distributed Raman amplification.

In this Letter, we show numerically and experimentally that multipath interference can be reduced significantly by distributed Raman amplification (DRA) in quasi-single-mode transmission using few-mode fibers, owing to the extra differential mode attenuation induced between the fundamental mode and higher-order modes. The experimental results show that the required number of equalization taps can be reduced by 1.6 times. Moreover, by carefully optimizing the intensity overlap between the LP01 and LP11 modes through structural adjustment of the two-mode fiber, a 3.5-times reduction in the required number of taps can be obtained.

A 25 Gb/s Wireline Receiver With Feedforward and Feedback Equalizers at Analog Front-End

An important issue in wireline receivers (RX) is minimizing the area and power consumption while overcoming the channel attenuation with an equalizer. The greater the compensation for channel loss at the analog front end (AFE) of the RX, the lower the number of decision feedback equalizer (DFE) taps. Power dissipation and area can be reduced by reducing the number of DFE taps. This brief presents a technology that compensates for the channel loss with the proposed AFE based on a two-stage continuous-time linear equalizer (CTLE), low and high bandwidth amplifiers, and a gain controller. It sufficiently reduces the DC gain and increases the peak gain of the AFE by using a feedforward equalizer (FFEQ) and feedback equalizer (FBEQ). These equalizers result in an increase in the difference between the peak and DC gains and the gain difference at the fundamental frequency (f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> ) and 2nd subharmonic frequency (f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sub> ). The IC is fabricated in a 28 nm CMOS process, and the proposed architecture yields a BER less than 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−12</sup> at 25.8 dB channel attenuation. At 25 Gb/s, the area and power efficiency of the proposed AFE are 1.19 pJ/bit and 0.01 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , respectively.

Directionally Resolved Measurement and Modeling of THz Band Propagation Channels

Design and performance assessment of THz communications systems, which will form an essential part of 6G, require an understanding of the propagation channels the systems will operate in. This paper presents investigations of the channel characteristics in various scenarios at 145 GHz, which is the band currently envisioned for the first round of deployments. In particular, we review several extensive measurement campaigns performed by the University of Southern California in both outdoor and indoor environments. We present the measurement and evaluation methodology and sample results that illustrate the dominant propagation effects in different environments. We then summarize the parameters of the statistical channel models for path loss, delay dispersion, and angular dispersion. Based on these results, we find that even in NLoS (non-line-of-sight) situations, Gbit/s communications can be sustained over a 100 m distance; that (for an antenna gain of 20 dB), there is considerable delay dispersion, requiring tens of equalizer taps, and that angular dispersion is significant in both LoS and NLoS situations. The channel parameters can be thus used as a basis for system design and evaluation under realistic operating conditions.

ROADM-Induced Anomaly Localization and Evaluation for Optical Links Based on Receiver DSP and ML

With the advance of elastic optical networks, optical communication systems are becoming more flexible and dynamic. In this scenario, soft failures are more likely to occur due to various link impairments, of which the filter impairment caused by WSS is a major one. If these soft failures are not handled properly and timely, the quality-of-transmission (QoT) will degenerate, even leading to service disruption. During this process, it is important to know the accurate location and the magnitude of soft failure. However, it is difficult for traditional methods to accomplish this target. Fortunately, with the fast progress of the powerful machine learning (ML) algorithms, a new promising way is provided to address this problem. In this article, we propose to use artificial neural network (ANN) and Gaussian process regression (GPR) to localize the soft failure location and estimate anomaly value. The input of the ANN and GPR is extracted from the power spectrum density (PSD) and the equalizer taps, which can be easily obtained from a coherent receiver. We explore two types of soft failure caused by the WSS, including the offset of WSS's center frequency, i.e., frequency shift (FS), and the tightening of WSS's 3-dB bandwidth, i.e., frequency tightening (FT). To validate the proposed method, we perform extensive simulations. The localization accuracy of the ANN can reach 95%, and the mean-absolute-error (MAE) of the GPR can reach 0.1-GHz, demonstrating the effectiveness of the proposed method. Besides, we validate the generalization of the proposed method under different link conditions. Finally, the importance of each input feature is explored, showing the effectiveness of the selected features.

Performance optimization of OADM based DP-QPSK DWDM optical network with 37.5 GHz channel spacing

Advanced modulation formats like dual-polarization quadrature phase-shift keying (DP-QPSK) along with the digital coherent receiver are enabling optical networks to provide high spectral efficiency (SE) over long-haul transmission reach. The DP-QPSK based dense wavelength division multiplexing (DWDM) system with 37.5 GHz channel spacing can provide extra throughput of 33% compared to 50 GHz channel spacing. However, at 37.5 GHz channel spacing, optical add-drop multiplexers (OADMs) at intermediate nodes, impact the performance of WDM based optical networks in terms of increased BER, thereby reducing transmission reach drastically. In this paper, we have analyzed with the help of numerical simulations, the performance of a differentially encoded eight-channel 112 Gb/s, OADM based DWDM optical network with a channel spacing of 37.5 GHz. We propose that the selection of optimum values of system parameters can result in reducing the impact of crosstalk propagation and narrowband optical filtering introduced by a chain of OADMs at 37.5 GHz channel spacing. The paper exhibited that the selection of optimum values of parameters like filter order of multiplexer and demultiplexer, launch power per channel and filter taps of adaptive equalizer (AE) result in achieving minimum BER on all the channels over a long transmission reach up to 3600 km.

Sparse decision feedback equalization for underwater acoustic channel based on minimum symbol error rate

Abstract Underwater Acoustic Channels (UAC) have inherent sparse characteristics. The traditional adaptive equalization techniques do not utilize this feature to improve the performance. In this paper we consider the Variable Adaptive Subgradient Projection (V-ASPM) method to derive a new sparse equalization algorithm based on the Minimum Symbol Error Rate (MSER) criterion. Compared with the original MSER algorithm, our proposed scheme adds sparse matrix to the iterative formula, which can assign independent step-sizes to the equalizer taps. How to obtain such proper sparse matrix is also analyzed. On this basis, the selection scheme of the sparse matrix is obtained by combining the variable step-sizes and equalizer sparsity measure. We call the new algorithm Sparse-Control Proportional-MSER (SC-PMSER) equalizer. Finally, the proposed SC-PMSER equalizer is embedded into a turbo receiver, which perform turbo decoding, Digital Phase-Locked Loop (DPLL), time-reversal receiving and multi-reception diversity. Simulation and real-field experimental results show that the proposed algorithm has better performance in convergence speed and Bit Error Rate (BER).

An improved least mean square/fourth direct adaptive equalizer for under-water acoustic communications in the Arctic

An improved least mean square/fourth direct adaptive equalizer (LMS/F-DAE) is proposed in this paper for underwater acoustic communication in the Arctic. It is able to process complex-valued baseband signals and has better equalization performance than LMS. Considering the sparsity feature of equalizer tap coefficients, an adaptive norm (AN) is incorporated into the cost function which is utilized as a sparse regularization. The norm constraint changes adaptively according to the amplitude of each coefficient. For small-scale coefficients, the sparse constraint exists to accelerate the convergence speed. For large-scale coefficients, it disappears to ensure smaller equalization error. The performance of the proposed AN-LMS/F-DAE is verified by the experimental data from the 9th Chinese National Arctic Research Expedition. The results show that compared with the standard LMS/F-DAE, AN-LMS/F-DAE can promote the sparse level of the equalizer and achieve better performance.

A 48 Gb/s PAM-4 Transmitter With 3-Tap FFE Based on Double-Shielded Coplanar Waveguide in 65-nm CMOS

A power and area-efficient pulse-amplitude modulation 4 (PAM-4) transmitter using 3-tap feed-forward equalizer (FFE) based on a slow-wave transmission line is presented. Passive delay line is adopted for generating equalizer tap to overcome the high clocking power consumption. The transmission line achieves high slow-wave factor of 15 with double floating metal shields around the differential coplanar waveguide. The physical dimensions of the transmission line are determined to have low loss and a high slow-wave factor with a small chip area by optimization with 3-D electromagnetic simulations. The transmitter includes 4:1 multiplexers (MUXs) and a quadrature clock generator for high-speed data generation in a quarter-rate system. The 4:1 MUX utilizes 2-UI pulse generator and the input configuration is determined by qualitative analysis. The chip is fabricated in 65-nm CMOS technology and occupies area of 0.151 mm2. The proposed transmitter system exhibits the energy efficiency of 3.03 pJ/b at the data rate of 48 Gb/s with PAM-4 signaling.

Suppression method of inter-symbol interference in communication system based on mathematical chaos theory

Inter-symbol interference (ISI) is prone to occur in the operation of communication system, which affects the quality of communication. Therefore, the suppression method of ISI in communication system based on mathematical chaos theory is studied to optimize the quality of communication transmission. Based on the analysis of the principle of ISI in communication system, it is known that multi-path delay can lead to ISI. The multi-path channel model of communication system is constructed, the multi-path component delay of the channel is calculated, and the inter-symbol interference is suppressed by setting a reasonable delay. The equalizer’s weight vector of communication channel is optimized by using chaotic optimization algorithm, the optimized equalizer tap of communication channel is obtained, and the reasonable multi-path component delay of channel is calculated to effectively suppress ISI. The simulation results show that the suppression method of ISI based on mathematical chaos theory can effectively overcome the ISI of multi-path channel, improve the resolution of transmission signal and the quality of communication transmission.

240 Gbit/s Silicon Photonic Mach-Zehnder Modulator Enabled by Two 2.3-Vpp Drivers

We recently reported for the first time 200 Gbit/s/ $\lambda$ net rate data transmission with a single silicon photonic modulator in an intensity modulation direct detection link. Our demonstration relied on an in-house designed, O-band segmented-electrode MZM. In this article, we detail the optimization of the modulator driving scheme, bandwidth/efficiency trade-off and modulation format enabling this result. We use two single-ended RF signals, each having 2.3-Vpp amplitude only, to drive two 1.5-mm modulator segments and transmit data at 80 Gbaud PAM-8 (240 Gbit/s) across 2 km under the 20% soft-decision forward error correction BER threshold of 2E-02. We further decrease the BER using maximum likelihood sequence detection (MLSD), and discuss the related implications. Additionally, we present the system margins regarding modulator drive voltage, received optical power and equalizer taps. At 240 Gbit/s line rate, the modulator energy consumption is only 73 fJ/bit at the SD-FEC threshold. We finally discuss practical implementation considerations for our system.

Underwater acoustic communication to an unmanned underwater vehicle with a compact vector sensor array

Although underwater acoustic (UWA) communication based on vector sensors has been intensively investigated in past years, most of these works focus on theoretical analysis and simulation, and only very limited research on the practical application of vector sensors to underwater platforms (i.e., UUVs) is available. Recently, a data transmission experiment between a surface vessel and a mobile UUV was conducted in Qiandao Lake, Zhejiang Province, China. During this experiment, a compact vector sensor array with four elements was installed on the UUV. This paper proposes a two-stage data processing scheme for the experimental data. The fractional Fourier transform (FrFT) is first used to estimate the delay-Doppler parameter of UWA channels. The delay estimation can help determine the number of equalizer taps, while the Doppler estimation can help determine the resampling frequency in the passband. Then, a new receiver called the vector multichannel decision feedback equalizer (VM-DFE) is proposed to deal with the inter-symbol interference (ISI) introduced by UWA channels. The experimental results show that the proposed two-stage data processing scheme realizes satisfactory performance and achieves a 1.8 dB gain compared with a pressure sensor-based equalizer.

A 56-Gb/s PAM4 Receiver With Low-Overhead Techniques for Threshold and Edge-Based DFE FIR- and IIR-Tap Adaptation in 65-nm CMOS

This paper presents a four-level pulse amplitude modulation (PAM4) quarter-rate receiver that efficiently compensates for moderate channel loss in a robust manner through background adaptation of the receiver thresholds and equalization taps. The front-end utilizes an input single-stage continuous-time linear equalizer (CTLE) to boost the main cursor and relax the pre-cursor cancellation requirement, requiring only a 2-tap pre-cursor feed-forward equalizer (FFE) on the transmitter side. A 2-tap decision feedback equalizer (DFE) follows that includes one finite impulse response (FIR) tap and one infinite impulse response (IIR) tap to cancel first post-cursor and long-tail inter-symbol interference (ISI), respectively. In addition to the per-slice main three data samplers, a single error sampler is utilized for background threshold control and an edge-based sampler performs both phase-locked loop (PLL)-based clock and data recovery (CDR) phase detection and generates information for background DFE tap adaptation. Fabricated in general purpose (GP) 65-nm CMOS, the 56-Gb/s receiver achieves 4.63 mW/Gb/s and compensates for up to 20.8-dB loss at a bit error rate (BER) <; 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-12</sup> when operated with a 2-tap FFE transmitter.

Dual Parallel Multielectrode Traveling Wave Mach–Zehnder Modulator for 200 Gb/s Intra-datacenter Optical Interconnects

We present a silicon photonic dual parallel multielectrode Mach-Zehnder modulator (MEMZM) based transmitter targeting 200 Gb/s four-level pulse amplitude modulation (PAM4) short reach transceivers. The MEMZMs have an average V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">π</sub> and electro-optic (EO) bandwidth of 5 V and 38 GHz, respectively. The transmitter is characterized versus receiver equalizer taps, received signal power, driving voltage swing, crosstalk voltage swing, bitrate, and reach. Results reveal that using only a three-tap equalizer at the receiver, 100 Gb/s PAM4 net rate per lane can be achieved at a bit error rate (BER) below the KP4 forward error correction (KP4-FEC) threshold of 2.4 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> . Moreover, up to 128 Gb/s can be received at a BER below the KP4-FEC threshold using only 2 Vpp and 1 Vpp driving the MEMZM segments. Then, both MEMZMs are driven simultaneously to assess the crosstalk impact on the BER performance at parallel operation. Driven by four binary signals, we demonstrate 200 Gb/s PAM4 transmission over up to 10 km of single mode fiber at a BER below the KP4-FEC threshold.

First demonstration of a 400 Gb/s 4λ CWDM TOSA for datacenter optical interconnects.

We present the first demonstration of a 4λ transmitter optical sub-assembly (TOSA) on the coarse wavelength division multiplexing (CWDM) grid, i.e., 20 nm spacing, targeting 400G-FR4 requirements over 2 km. The TOSA is based on uncooled InP external modulated laser (EML) technology and it utilizes four EMLs followed by a CWDM multiplexer. We characterize the performance of the TOSA versus received optical modulation amplitude (OMA), number of equalizer taps, reach, modulation format, TOSA case temperature, and bit rate. Four 53 Gbaud 4-level pulse amplitude modulation (PAM4) RF signals are used to drive the TOSA achieving a net rate of 400 Gb/s. Results reveal that 400 Gb/s can be transmitted over 2 km of single mode fiber (SMF) at a bit error rate (BER) below the KP4- forward error correction (KP4-FEC) threshold (i.e., 2.4 × 10-4) using only a 5 tap feed forward equalizer at the receiver. To the best of our knowledge, this is the first demonstration of 400 Gb/s using a 4λ CWDM TOSA over 2 km of SMF. Moreover, we achieve 400 Gb/s and 600 Gb/s over 20 km and 10 km below KP4-FEC and the 7% hard-decision FEC (HD-FEC) (i.e., 3.8 × 10-3) thresholds, respectively, without optical amplification. Furthermore, we show the performance of the TOSA against temperature, where it shows no significant change in the BER performance from 20 °C to 60 °C. Finally, we compare the performance of PAM2, PAM4, and PAM8 modulation formats where we show the possibility of achieving 400 Gb/s aggregate bit rate using 42 Gbaud PAM8 modulation format at the expense of utilizing a stronger FEC.

Semi-Blind Sparse Channel Estimation Using Reduced Rank Filtering

This paper develops a novel method for semi-blind estimation of sparse channels in single-input, multiple-output and multiple-input, multiple-output single-carrier transmission systems, using single/multiple transmit and multiple receive antennas. Very little work exists on semi-blind sparse channel estimation. By a novel formulation of the received data matrix (called reduced data matrix) for sparse channels, semi-blind channel/data estimation problem has been cast into a reduced-rank filtering problem. The estimation operates on selected, non-uniformly spaced equalizer taps. Then two classes of semi-blind channel/data estimation are developed, the first being a pseudo-inverse based linear prediction method and the second based on reduced rank multi-stage Wiener filtering. The second class has the advantage of not requiring the rank of the reduced data matrix, but estimating it adaptively via stage-by-stage updates. The computational complexity of the novel reduced channel estimators are compared with the full channel estimators, which use all available equalizer taps. Simulation results show that the reduced semi-blind estimators perform much better than the full estimators and existing sparse channel estimators, with limited number of training symbols, at much reduced computational complexity.

168-Gb/s Single Carrier PAM4 Transmission for Intra-Data Center Optical Interconnects

We experimentally demonstrate 168-Gb/s single polarization four-level pulse amplitude modulation (PAM4) transmission over 10 km of single mode fiber (SMF) in the O-band below the 400-GbE task force operating bit error rate (BER) of 2 × 10 -4 . To the best of our knowledge, this rate is the highest bit rate reported for single polarization direct detection PAM4 transmission over 10 km of SMF in the O-band. We evaluate the BER performance of the PAM4 signal versus bit rate, number of equalizer taps, received power, and reach. Results reveal that up to 176-Gb/s bit rate can be transmitted over 10 km at a BER of 4 × 10 -4 and 8 dBm of received signal power. Finally, we show that the residual chromatic dispersion at up to 88 Gbaud is negligible by fixing the received power over varying reach, demonstrating that the BER is solely governed by the received signal power.