Coherent Receiver Research Articles

Fast Feed-Forward Optical and Electrical Gain Control to Extend the Dynamic Range of the Burst-Mode Digital Coherent Receiver for High-Speed TDM-PON Systems

This paper introduces a new burst-mode coherent receiver configuration that drastically widens the receiver input dynamic range. Its key advance is a fast feed forward gain control scheme for both a semiconductor optical amplifier (SOA) used as a preamplifier and trans-impedance amplifiers (TIAs) set at the coherent receiver. The two-stage optical and electrical gain control greatly decreases the received burst signal's power variations and increases the receiver dynamic range which attenuates the quantization errors in analogue-to-digital conversion (ADC). To elucidate the receiver sensitivity characteristics, bit error rate (BER) calculations are conducted on 10 Gbit/s single polarization quadrature phase shift keying (SP-QPSK) signal coherent reception. The calculations focus on two significant parameters that greatly impact the quantization noise; ADC vertical resolution and received signal's state of polarization (SOP). The calculation results confirm that the proposed configuration achieves high sensitivity, better than −44 dBm, and wide dynamic range of 30 dB in the worst SOP condition, even when 5-bit ADC is used. Transmission experiments verify the calculated sensitivities and extremely high loss budget of 48.6 dB is achieved in 40-km single mode fiber (SMF) transmission. These results show that the proposal has the potential to realize 10 Gbit/s class long-reach and high splitting ratio PONs; the extremely wide dynamic range yields 40 km reach PON systems with 512- or 1024-way splits with no change in receiver configuration.

Deep Neural Network-Based Digital Pre-Distortion for High Baudrate Optical Coherent Transmission

High-symbol-rate coherentoptical transceivers suffer more from the critical responses of transceiver components at high frequency, especially when applying a higher order modulation format. We recently proposed a neural network (NN)-based digital pre-distortion (DPD) technique trained to mitigate the transceiver response of a 128 GBaud optical coherent transmission system. In this paper, we further detail this work and assess the NN-based DPD by training it using either a direct learning architecture (DLA) or an indirect learning architecture (ILA), and compare performance against a Volterra series-based ILA DPD and a linear DPD. Furthermore, we deliberately increase the transmitter nonlinearity and compare the performance of the three DPDs schemes. The proposed NN-based DPD trained using DLA performs the best among the three contenders. In comparison to a linear DPD, it provides more than 1 dB signal-to-noise ratio (SNR) gains at the output of a conventional coherent receiver DSP for uniform 64-quadrature amplitude modulation (QAM) and PCS-256-QAM signals. Finally, the NN-based DPD enables achieving a record 1.61 Tb/s net rate transmission on a single channel after 80 km of standard single mode fiber (SSMF).

Optical Performance Monitoring for Intra-LP-Mode Dispersion in Weakly-Coupled Mode-Division Multiplexed Systems

Weakly-coupled mode-division multiplexing (MDM) technique based on linearly-polarized (LP) modes in ultralow-modal-crosstalk few-mode fiber (FMF) has been considered as a promising candidate for capacity enhancement of next-generation optical transmission systems. Similar to polarization mode dispersion in single-mode fibers (SMFs), signals in weakly-coupled FMFs suffer from the impacts of intra-LP-mode dispersion (ILMD) for both degenerate and non-degenerate LP modes.In this paper, we propose an effective method for ILMD monitoring in weakly-coupled MDM systems. The ILMD evaluation method for degenerate LP modes is designed by the analysis of channel transfer matrix in a digital coherent receiver. Then the ILMD monitoring scheme is experimentally validated for all the 6 LP mods in a weakly-coupled FMF. The influences of chromatic dispersion, optical signal to noise ratio and inter-LP-mode crosstalk to the method are also investigated.

Investigating the Performance of SCM/MMW/RoF Optical-Wireless Access System for Next Generation Communications

In this paper, we propose a model of subcarrier multiplexing millimeter wave radio-over-fiber SCM/MMW/RoF optical-wireless access system for next generation mobile communications using QPSK modulation. We then calculate signal and total noise power at the end of optical fiber link and at mobile subscribers through wireless medium. Next, system performance showed by SNR, BER at the end of system is determined. They are investigated, compared, evaluated by Matlab in many different scenarios including BER performance versus EDFA’s Gain, optical transmitter power at central office (CO), optical oscillator power in coherent receiver, wireless frequencies, wireless link distance corresponding to two wireless cases of Line of Sight (LoS) and Non Line of Sight (NLoS). Numerical results show that when number of SCM channels increase system performance becomes worse due to nonlinear effect in SCM modulation.

Compensation of The Nonlinear Impairments in All-Optical OFDM Systems Based on The Optical Phase Conjugation (OPC) Module

This study presents a nonlinearity mitigation technique for achieving high performance in the All-Optical Orthogonal Frequency Division Multiplexing (AO-OFDM) transmission systems. Therefore, the Optical Phase Conjugation (OPC) technique has been employed for mitigating the Nonlinear Phase Noise (NLPN) in the AO-OFDM transmission systems. The NLPN has been mitigated in the AO-OFDM transmission system by utilizing the proposed OPC at the middle point of the transmission link. The proposed system is numerically simulated by Virtual Photonics Integrated (VPI) Transmission Maker 9.0 at a symbol rate of 25 Gsymbol/s. During the simulation, 29 subcarriers were generated by Optical Frequency Comb Generator (OFCG) and modulated by 4-array Quadrature Amplitude Modulator (4-QAM). The generated signals are transmitted over 580 km fiber link and received by the coherent receiver. The transmission link contains 4 spans before and 4 spans after OPC module. Each span comprises the Standard Single-Mode Fiber (SSMF) and an EDFA (noise figure = 6 dB) to compensate for the fiber loss. The length of each span is fixed at 70 km in the system simulation. In addition, a 20 km Dispersion Compensation Fiber (DCF) has been used just before the OPC module to compensate for accumulated fiber dispersion. The Signal-to-Noise Ratio (SNR) and Error Vector Magnitude (EVM) have been used to certify the feasibility of the proposed technique. The results reveal that by employing the proposed OPC module, the SNR is improved by ~3.4 dB, and the EVMs are substantially reduced.

Coherent Multiantenna Receiver for BPSK-Modulated Ambient Backscatter Tags

Ambient backscatter communication (AmBC) is an emerging communication technology enabling green Internet of Things deployments. The widespread acceptance of this paradigm is limited by the low signal-to-interference-plus-noise ratio (SINR) of the signal impinging on the receiver antenna due to the strong direct path interference and unknown ambient signal. The adverse impact of these two factors can be mitigated by using noncoherent multiantenna receivers, which are known to require a higher SINR to reach the bit-error-rate (BER) performance of coherent receivers. However, considering the unknown ambient signal, unknown location of AmBC tags, and varying channel conditions, coherent receivers for AmBC systems are rarely studied in the literature. In this article, a coherent multiantenna receiver, which requires no prior information of the ambient signal for decoding the binary-phase-shift-keying (BPSK) modulated signal, is presented. The performance of the proposed receiver is compared with an ideal coherent receiver that has perfect phase information, and also with the performance of a noncoherent receiver, which assumes distributions for ambient signal and phase offset caused by the excess length of the backscatter path. Comparative simulation results show the designed receiver can achieve the same BER performance of the ideal coherent receiver with 1-dB more SINR, which corresponds to 5-dB or more gain with respect to noncoherent reception of on-off-keying modulated signals. Variation in the detection performance with the tag location shows that the coverage area is in the close vicinity of the transmitter and a larger region around the receiver, which is consistent with the theoretical results.

Optimizing DC restoration in Kramers-Kronig optical single-sideband receivers.

Kramers-Kronig optical single-sideband receivers remove the signal-signal beat interference (SSBI) that occurs when detecting a signal that has electrical signals mapped onto its optical field at the transmitter; such signals support electronic dispersion compensation without the need for a coherent receiver. To use the full range of the analog-to-digital converter's (ADC) range, it is best to a.c.-couple the photocurrent, to remove its DC content; however, the DC must be restored digitally before the KK algorithm is applied. Recent publications have concentrated on perfectly determining the restored DC's required level from the signal, with a view this is optimal for lowering error rates. In this paper, we investigate signal-signal beat interference (SSBI) cancellation in a single photodiode receiver using Kramers-Kronig receiver algorithm, with large variations in optical carrier-to-signal power ratio (CSPR) and DC offset level. Through simulations and experiments, we find a strategy to optimize the signal quality without the need of an extensive search for the DC offset value. We also find that a theoretically perfect determination of the original DC level does not provide best signal quality especially for low CSPRs; in order to achieve maximum cancellation of signal-signal beat interference, the level of the restored DC has an optimum value that depends on the optical CSPR. We define a digital CSPR, which is the value of the CSPR in the digital domain after DC restoration. Our measurements show that we simply need to bias the signal upwards and make the minimum signal above zero by 0.1% of the r.m.s. signal amplitude when the optical CSPR is low. For higher values of optical CSPR, the optimal digital CSPR is about 2-dB lower than the optical CSPR, and the optimal DC offset can be calculated from this digital CSPR. We find that the boundary between our low optical CSPR region and high optical CSPR region depends on the noise level in the system.

Effect of optical pulse shaping and adaptive equalization on the performance of 100G DP-QPSK WDM system

Abstract Dual polarization quadrature phase shifting keying (DP-QPSK) modulation format along with coherent receiver helps in increasing the data carrying capability of existing optical networks without any major change in existing transmission infrastructure. The various linear and nonlinear fiber effects, frequency and phase errors are corrected in electrical domain at the receiver end with digital back propagation algorithms (DBP) instead of in-line compensation. In such a case the selection of optimum values of system parameters make the task easier for DBP algorithms. This paper highlights the importance of finding optimum operating point of continuous modulus algorithm (CMA) for better adaptive equalization (AE). The paper also discusses the optical pulse shaping using Gaussian optical band-pass filter to improve the spectral characteristics of DP-QPSK signal.

Synthesis of optimal coherent reception of multiposition signals in modern mobile networks

The research results of optimal coherent reception of multiposition signals in modern mobile networks are presented. The implementation of algorithms and devices provides the maximum possible noise immunity when all possible variants of the transmitted signal at the input of the demodulator are known a priori with high accuracy of the information parameter. By reducing a priori information about the received signal, it is possible to synthesize other methods of reception that are optimal under the appropriate conditions. The coherent processing algorithms of multiposition amplitude-phase modulation and amplitude-phase-difference modulation signals that are convenient for multichannel (multifrequency) systems with orthogonal channel signals are considered. In multichannel systems, for the distribution of orthogonal signals, the procedure for calculating the projections of the received signal on two mutually orthogonal oscillations with an arbitrary initial phase is used. The proposed algorithms for coherent signal processing do not require adjustment of the phase of the reference oscillations of channel signals and enable to implement both the orthogonal distribution of signals and their coherent reception.

Performance of Non-Line of Sight Underwater Optical Wireless Communication Links with Spatial Diversity

Line-of-sight (LOS) underwater optical wireless communication (UOWC) transmission may suffer blocking and are not always possible due to obstructions from sea creatures, bubbles, large suspended particles and features of the seabed, especially in coastal and turbid water environments. Thus, we present the performance of a spatially diverse non-line-of-sight (NLOS) UOWC system employing continuous phase modulation (CPM), which is shown to offer sensitivity benefits of several dBs over on–off keying (OOK) without coherent reception. We obtain the channel impulse response (CIR) by using Monte Carlo simulation, including absorption and multiple scattering. Turbulence is included by conditioning the CIR on log-normal statistics. To mitigate the resultant fading, we exploit spatial diversity with equal gain combining at the receiver side. Photon counting at the receiver is employed to accommodate shot noise. We compare the saddlepoint and Gaussian approximations for bit error rate (BER) calculations, using the latter for later calculations as it delivers excellent results and is simpler. Our results show that spatial diversity offers performance improvements, for example an 8 dB sensitivity gain at 10^-9 BER using 1 Gbps 3×1 multiple-input single-output (MISO) transmission over a 20 m link with 0.16 log-amplitude variance. We determine using an upper bound that Intersymbol Interference (ISI) has a significant impact at high bit rates, producing error floors for multiple-output arrangements.

Spectrally sliced heterodyne coherent receiver with halved electrical bandwidth

A spectrally sliced heterodyne coherent receiver (SHCR) employing four balanced photodetectors and analog-to-digital converters with half of the signal bandwidth is proposed to complete the signal reception and field recovery. We first numerically characterize the performance of SHCR compared with an intradyne coherent receiver and then validate the principle of the SHCR in a proof-of-concept single-polarization experiment. A 60 GBaud 16-quadrature amplitude modulation transmission is experimentally demonstrated over 80 km standard single-mode fiber with a bit-error-rate of 8.5×10-4 below the 7% hard-decision forward error correction threshold of 3.8×10-3. The SHCR offers a low-cost, hybrid-free, and channel-skew-tolerant candidate for data center interconnects.

Toward Practical Temporal Diversity Coherent Combining Based Variable Data-Rate Free Space Optical Communication Systems

Free-space optical communication (FSOC) systems with variable data-rates can support different missions with different transmission distances and ensure reliable transmission in periods of unfavorable weather conditions. The temporal diversity coherent combining (TDCC) technique can adjust the data-rate in a very large range using a software-defined manner by taking advantage of the digital coherent receivers able to recover the signal field. In this paper, we investigate the relationships between the computational complexity, optical phase alignment error, combining loss (CL) and data-rate for the TDCC-based variable data-rate coherent optical receiver. We also propose a method to minimize the computational complexity while achieving the expected output optical signal-to-noise ratio (OSNR) and the highest data-rate for an arbitrary input OSNR. Numerical simulations and experiments are carried out to validate the analytical expressions and proposed methods. The results provide an efficient tool and useful guidelines for the design of low computation complexity TDCC-based variable data-rate FSOC systems.

Using Sequence-to-Sequence Models for Carrier Frequency Offset Estimation of Short Messages and Chaotic Maps

Deep Learning methods have produced good carrier frequency offset estimations for short message sequences in comparison with methods based on the Fast Fourier Transform. However, these performance gains were observed for short ranges of frequency offsets, sequences with predefined pilot symbols and periodic modulation schemes. Chaotic modulation has an advantage over periodic signals in offering security through the continuous changes produced by parameterising the chaotic map function. However, synchronisation of chaotic map parameters in coherent receivers is dependent on the carrier recovery of phase and frequency which dramatically reduces the demodulation performance under high noise levels. This article presents a stacked sequence-to-sequence neural network architecture for blind carrier frequency offset estimation of both periodic and chaotic modulation schemes. The results obtained demonstrate better performance than conventional methods in low SNR for the Additive White Gaussian Noise channel. While this technique operates without feature engineering, the results demonstrate that data augmentation produces a higher degree of accuracy for such models, indicating the benefit of integration with conventional signal pre-processing steps as part of the deep learning pipeline. The proposed neural network architecture is shown to perform carrier frequency offset estimation, not only for the selected periodic modulations, but also in the case of highly non-linear chaotic maps. This suggests the applicability of deep learning methods for synchronisation in waveforms that employ chaotic modulation schemes for secure communication and for applications where short and sporadic messaging are required (e.g., Internet of Things).

Toward Practical Entanglement-Based Satellite FSO/QKD Systems Using Dual-Threshold/ Direct Detection

We propose a new implementation of the BBM92 protocol for satellite continuous-variable quantum key distribution (CV-QKD) systems using the dual-threshold/direct detection (DT/DD) scheme. The proposed system is less complex and therefore possibly cheaper than current discrete-variable (DV) and CV-QKD ones using coherent detection receivers. We model and analyze the performance of the proposed system in the context that a satellite distributes secret keys to two legitimate users. The analytical results are derived by considering channel loss, atmospheric turbulence-induced fading, and receiver noises. The Gaussian beam model is considered to evaluate the impact of geometric spread on the signal received by legitimate users and the possibility of being eavesdropped. Based on the design criteria for the system and analytical results, we investigate the feasibility of a case study for the Japan QKD network using the existing low-Earth orbit (LEO) satellite constellation.

Power Efficient Communications Employing Phase Sensitive Pre-Amplified Receiver

The receiver sensitivity is a very important metric in optical communication links operating at low received signal powers. Phase sensitive optical amplifiers (PSAs) can amplify optical signals without excess noise, thus providing a fundamental sensitivity improvement of 3 dB when employed as a pre-amplifier compared to conventional erbium doped fiber amplifiers (EDFA). In this letter, we investigate, both theoretically and experimentally, the sensitivities achieved using power efficient multi-dimensional modulation formats such as M-ary pulse position modulation format (M-PPM) and M-PPM combined with quadrature phase shift keying (QPSK) along with a near-noiseless PSA pre-amplified coherent intradyne receiver. We find that at high signal to noise ratios (SNRs) corresponding to low bit-error-rates (BER), M-PPM+QPSK results in the best sensitivity, which is improved with the order M, while at low SNRs corresponding to high BER (~14% where 100% overhead forward error correction codes (FEC) would be needed to recover the data), QPSK is the most sensitive format, while at the same time providing the best spectral efficiency. We report experimental sensitivities of 2.1 photons per information bit (PPB) at a pre-FEC BER = 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−3</sup> using 64-PPM+QPSK and assuming 7% FEC, and 0.8 PPB at a pre-FEC BER = 0.14 using QPSK and assuming 100% FEC.

Assessment of the algorithm for optimum coherent reception of OFDM signals with multi­position amplitude­phase modulation

The paper examines the issues of optimal coherent reception of OFDM signals with multi-position amplitude-phase modulation in conditions of a priori uncertainty of the state of the telecommunications network. It was determined that modern telecommunication systems, due to the complexity of the system’s behavior caused by the nature of random external influences, require invariant procedures for optimal reception and processing of the input signal. The paper considers the procedure for ranking input readings, which turns them into a sequence of integers — ranks that depend on the relative level of this reading among the observed set, which has the widest range of invariant properties. Based on it, algorithms for optimal coherent reception of OFDM signals with multi-position amplitude-phase modulation were developed and proposed. The proposed algorithms solve the problem of coherent reception of a multiposition AFM signal both in the presence of a special sync signal and in the absence of it. At the same time, the reception and processing of a coherent signal according to the proposed algorithm in the presence of a sync signal consists in the fact that the averaging is carried out on the interval of the sync signal. And in the case of reception without the presence of a synchronization signal — in the corresponding algorithm, averaging is carried out on a continuous interval of N parcels preceding the parcel that is being processed at the moment. Approbation of the proposed algorithms was carried out on the basis of coherent processing of 16-position OFDM signals with amplitude-phase modulation. The considered algorithms for coherent processing of multi-position AFM or AFRM signals are especially convenient for multi-channel (multi-frequency) systems with orthogonal channel signals, because in these systems the same procedure is used for the distribution of orthogonal signals, the calculation of the projections of the received signal on two mutually orthogonal oscillations with an arbitrary initial phase.

A Simplified Mode Diversity Coherent Receiver Based on Non-Mode-Selective Photonic Lantern and Kramers-Kronig Detection

A Simplified Mode Diversity Coherent Receiver Based on Non-Mode-Selective Photonic Lantern and Kramers-Kronig Detection

Error-Backpropagation-Based Background Calibration of TI-ADC for Adaptively Equalized Digital Communication Receivers

A novel background calibration technique for Time-Interleaved Analog-to-Digital Converters (TI-ADCs) is presented in this paper. This technique is applicable to equalized digital communication receivers. As shown in the literature, in a digital receiver it is possible to treat the TI-ADC errors as part of the communication channel and take advantage of the adaptive equalizer to compensate them. Therefore calibration becomes an integral part of channel equalization. No special purpose analog or digital calibration blocks or algorithms are required. However, there is a large class of receivers where the equalization technique cannot be directly applied because other signal processing blocks are located between the TI-ADC and the equalizer. The technique presented here generalizes earlier works to this class of receivers. The error backpropagation algorithm, traditionally used in machine learning, is applied to the error computed at the receiver slicer and used to adapt an auxiliary equalizer adjacent to the TI-ADC, called the Compensation Equalizer (CE). Simulations using a dual polarization optical coherent receiver model demonstrate accurate and robust mismatch compensation across different application scenarios. Several Quadrature Amplitude Modulation (QAM) schemes are tested in simulations and experimentally. Measurements on an emulation platform which includes an 8 bit, 4 GS/s TI-ADC prototype chip fabricated in 130nm CMOS technology, show an almost ideal mitigation of the impact of the mismatches on the receiver performance when 64-QAM and 256-QAM schemes are tested.

Simplified Coherent Receiver for Analogue Radio Transmission Over High Optical Budgets

The bandwidth and latency demands driven by 5G wireless networks are putting a focus on analog radio-over-fiber techniques as a promising candidate for broadband optical fronthauling. In contrast to intensity modulation / direct detection systems, as they are commonly adopted for mobile fronthaul links due to their simplicity, we are exploring a migration option towards coherent radio-over-fiber transmission. A simplified coherent homodyne receiver based on an electro-absorption modulated laser is co-integrated at the die-level with a transimpedance amplifier and evaluated for analogue coherent radio-over-fiber transmission. This low-complexity homodyne receiver can unlock channel selectivity and high receiver sensitivity inherent to coherent detection, but without the need for digital signal processing. We will demonstrate a sensitivity that allows for an optical budget of 42 dB, and thus eases the fixed-mobile convergence in power-splitting fiber plants. We further show filterless radio signal reception in presence of modulated adjacent channels. Signal integrity is confirmed through transmission of orthogonally frequency multiplexed radio signals with 16-point quadrature amplitude modulated formats, with an error vector magnitude below the corresponding antenna limit.

Real-Time 10,000 km Straight-Line Transmission Using a Software-Defined GPU-Based Receiver

Real-time 10,000 km transmission over a straight-line link is achieved using a software-defined multi-modulation format receiver implemented on a commercial off-the-shelf general-purpose graphics processing unit (GPU). Minimum phase 1 GBaud (QAM) signals are transmitted over 10,000 km and successfully received after detection with a Kramers-Kronig (KK) coherent receiver. 8-, 16-, 32-, and 64-QAM are successfully transmitted over 7600, 5600, 3600, and 1600 km, respectively.