Receiver Digital Signal Processing Research Articles

Alamouti-coded DSP algorithm with a simplified PTBC decoder for next-generation optical access networks

The use of Alamouti-coded polarization-time block code (A-PTBC) in combination with a simple single polarization coherent receiver enables phase-diverse coherent detection without any optical polarization tracking. However, applying this technique to high-speed single-carrier systems is not straightforward, as it requires specialized digital signal processing (DSP) algorithms for data recovery, which increases DSP complexity. In this paper, we propose a novel Alamouti-coded coherent algorithm designed to significantly reduce the complexity of the receiver DSP for data recovery. The proposed algorithm achieves the comparable performance to the conventional algorithm but requires only half the number of necessary equalizers for data recovery. We validate its performance through simulations and also experimentally demonstrate a 100 Gb/s 16-quadrature amplitude modulation (QAM) single-carrier coherent system employed the single-polarization coherent receiver over 20 km of standard single-mode fiber (SMF). Through the performance verification, the coherent system with the proposed algorithm exhibits performance comparable to that of the conventional Alamouti-coded coherent system and achieves a power budget of 34 dB when the transmit launch power is set to 7 dBm at a Bit Error Rate (BER) of 1 × 10−2 for 0-20 km fiber transmission.

Gradient-descent noise whitening techniques for short-reach IM-DD optical interconnects with severe bandwidth limitation.

Bandwidth limitation in optoelectrical components and the chromatic dispersion-induced power fading phenomenon cause severe inter-symbol interference (ISI) in high-speed intensity modulation and direct detection (IM-DD) optical interconnects. While the equalizer implemented in the receiver's digital signal processing procedure can mitigate ISI, it also inevitably enhances the noise located in the decayed frequency region, known as equalization-enhanced colored noise (EECN). Additionally, the nonlinear impairments of the modulator and photodetector also deteriorate the performance of the IM-DD system, especially for high-order modulation formats. In this work, we propose a gradient-descent noise whitening (GD-NW) algorithm to address EECN and extend it by introducing nonlinear kernels to simultaneously mitigate EECN and nonlinear impairments. The proposed algorithms are compared with conventional counterparts in terms of the achievable baud rate and the receiver optical power sensitivity. As a proof-of-concept experiment, we validate the principles of the proposed algorithms by successfully transmitting 360-GBd on-off-keying (OOK) and 180-GBd 4-level pulse-amplitude-modulation (PAM-4) signals in the back-to-back case under a 62-GHz brick-wall bandwidth limitation. 280-GBd OOK and 150-GBd PAM-4 transmissions are also demonstrated over 1-km standard single-mode fiber with a bit error rate below 7% hard-decision forward error correction aided by the proposed approach.

Multi-parameter estimation resistant to the random polarization effect for coherent optical receivers.

Optical parameter estimation based on the data obtained by coherent optical receivers is critical for optical performance monitoring (OPM) and the stable operation of the receiver digital signal processing (DSP). A robust multi-parameter estimation is intricate due to the interference of various system effects. By resorting to the cyclostationary theory, we are able to formulate a chromatic dispersion (CD), frequency offset (FO), and optical signal-to-noise ratio (OSNR) joint estimation strategy that is resistant to the random polarization effect, including polarization mode dispersion (PMD) and polarization rotation. The method uses data directly after the DSP resampling and matched filtering. Both numerical simulation and field optical cable experiment validate our method.

Ultra-High Capacity Optical Satellite Communication System Using PDM-256-QAM and Optical Angular Momentum Beams.

Twisted light beams such as optical angular momentum (OAM) with numerous possible orthogonal states have drawn the prodigious contemplation of researchers. OAM multiplexing is a futuristic multi-access technique that has not been scrutinized for optical satellite communication (OSC) systems thus far, and it opens up a new window for ultra-high-capacity systems. This paper presents the 4.8 Tbps (5 wavelengths × 3 OAM beams × 320 Gbps) ultra-high capacity OSC system by incorporating polarization division multiplexed (PDM) 256-Quadrature amplitude modulation (256-QAM) and OAM beams. To realize OAM multiplexing, Laguerre Gaussian (LG) transverse mode profiles such as LG00, LG140, and LG400 were used in the proposed study. The effects of the receiver's digital signal processing (DSP) module were also investigated, and performance improvement was observed using DSP for its potential to compensate for the effects of dispersion, phase errors, and nonlinear effects using the blind phase search (BPS), Viterbi phase estimation (VPE), and the constant modulus algorithm (CMA). The results revealed that the proposed OAM-OSC system successfully covered the 22,000 km OSC link distance and, out of three OAM beams, fundamental mode LG00 offered excellent performance. Further, a detailed comparison of the proposed system and reported state-of-the-art schemes was performed.

Sparse Volterra Equalizer to Mitigate the Nonlinear Distortion in High Power Budget IM-DD OFDM PON

To improve the power budget for intensity-modulation direct-detection (IM-DD) orthogonal frequency division multiplexing passive optical network (OFDM PON), a high power budget downstream scheme by using the Volterra equalizer in digital signal processing (DSP) processing is proposed in this paper. When the launch power is increased, the subcarrier-to-subcarrier beating interference (SSBI) and other nonlinear distortion are well mitigated by the Volterra equalizer. Moreover, to reduce the complexity of receiver DSP unit, the sparse Volterra equalizer is obtained through optimizing full Volterra equalizer employing orthogonal search method. Simulation and experiment results are presented to evaluate the transmission performance and analyze the distribution of taps of Volterra equalizer under different respects. The results show that the number of tap coefficient of sparse Volterra equalizer is reduced by 60% compared with that of conventional second-order full Volterra equalizer.

OAM Mode Selection for High-Speed Optical Communications: A Bit Loading Approach

Occupying more channels increases transmission rate, however, crosstalk increases to an unacceptable level long before all 24 channels can be exploited. The crosstalk is not uniform between modes, hence occupying different subsets of channels leads to vastly different achievable transmission capacities. In addition to optimizing occupied subsets of channels, we adopt a bit loading approach. We examine several resource allocation strategies in a coherent detection system, starting with typical OAM mode group granularity (all channels in a group occupied) and ending with single channel granularity. By exhaustive search at mode group granularity, we find a bit load increasing the total capacity by <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\sim$</tex-math></inline-formula> 15% compared to a minimax solution for mode group allocation. Single channel granularity imposes great computational effort to optimize bit loading. We propose search algorithms that are computationally tractable and improve capacity by <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\sim$</tex-math></inline-formula> 30% vis-à-vis the minimax solution for mode groups. Finally, we examine the impact of signal-to-noise ratio (SNR) and receiver digital signal processing (DSP) complexity on the overall capacity. We include a discussion of DSP with limited or with no multi-input, multi-output (MIMO) processing.Our algorithms could be applied to any mode multiplexing fiber, as it only relies on knowledge of the crosstalk matrix across modes.

Optical Multipath Interference Mitigation for High-Speed PAM4 IMDD Transmission System

Multipath interference (MPI) noise suppression is crucial for high-speed 4-level pulse amplitude modulation (PAM4) intensity modulation direct detection (IMDD) transmission systems. In this paper, two algorithms are proposed to eliminate the MPI noise, and these algorithms can be realized in the receiver digital signal processing (DSP) module without changing the existing IMDD system architecture. The first algorithm (A1) suppresses the MPI noise by removing the fluctuation of MPI-impaired PAM4 signals. The second algorithm (A2) can estimate the MPI noise which is intensity-dependent, and then the estimated MPI noise is subtracted from the impaired signal. The complexity of the proposed algorithms is lower than that of the traditional high-pass filter (HPF) scheme. Simulation results indicate that the proposed schemes can effectively suppress the MPI noise and improve the transmission performance under different signal-interference ratios (SIR), different laser linewidth, and random optical delay of the reflected signal. Besides, the cascade of A1 and A2 can further improve the transmission performance, and it has a higher tolerance for laser linewidth compared to the traditional HPF scheme. With the help of the proposed scheme, 56 Gb/s PAM4 signals are experimentally transmitted over 15.5 km standard single-mode fiber under the SIR of 18 dB and the laser linewidth of 100 kHz, and the measured bit error rate (BER) can be below the 7% overhead hard-decision forward error correction limit. When the laser linewidth is increased to 4 MHz, the traditional HPF scheme becomes less effective and may degrade the transmission performance, but the proposed schemes are still effective and the SIR tolerance can be improved by about 3 dB. 100 Gb/s PAM4 under optical back-to-back transmission with 24 dB SIR can be achieved, and the BER performance can be increased from 4.63e-3 to 2.06e-3 with the help of the proposed schemes.

Optical filtering impairment monitoring based on model fusion for optical networks.

Reconfigurable optical add-and-drop multiplexer (ROADM) is a key element in optical networks. As several ROADMs are cascaded over long paths, the penalty induced by ROADM has become non-negligible due to the tight optical filtering. In this case, for efficient and reliable network planning and operation, accurate monitoring of optical filtering penalty is very important. In this paper, we propose a real-time optical filtering monitoring scheme based on model fusion. We combine an analytical model based on the digital communications theory of band-limited channels with linear equalization and a data-driven model implemented using artificial neural network (ANN). The scheme can achieve high accuracy and interpretability. Moreover, since the input features are extracted from configuration parameters and receiver digital signal processing (DSP), no additional devices are needed, which is attractive for practical deployment. Extensive simulations and experiments are conducted to investigate the performance of the scheme, and the results show the superior performance.

200 Gbps/λ PON Downstream C-Band Direct-Detection Links with ≥29 dB Power Budget

In this paper we present the simulative analysis of a 200 Gbps per wavelength (λ) 8-level pulse amplitude modulation (PAM-8) downstream communication over up to 20 km single mode fiber (SMF) in C-band based on direct detection (DD) achieving at least a 29 dB link power budget in a PON environment. We use chromatic dispersion digital pre-compensation (CD-DPC) and a dual-arm in-phase and quadrature Mach–Zehnder modulator (IQ-MZM) at the optical line termination (OLT) side, while preserving DD in the optical network unit (ONU). Three receiver digital-signal-processing (DSP) options are analyzed and compared: square-root-like technique (SQRT) in combination with a feed forward equalizer (FFE) and a decision feedback equalizer (DFE), the Volterra nonlinear equalizer (VNLE), and the SQRT in combination with the VNLE. The SQRT can be applied in combination with the VNLE to decrease the receiver DSP complexity while maintaining the required system performance. We show that PAM-8 with CD-DPC and the SQRT in combination with the VNLE is a feasible solution for 200 Gbps per λ downstream C-band transmission for PON.

Transceiver Imbalances Compensation and Monitoring by Receiver DSP

In-phase and quadrature (IQ) imbalances in optical transceiver, such as amplitude imbalance, phase imbalance, and IQ skew, can cause serious system performance degradation, especially when high order modulation formats are used. In this work, we present a method to isolate and separately monitor these various impairments from received signals by coherent digital signal processing (DSP). Receiver amplitude and phase imbalances could be compensated and monitored by Gram-Schmidt orthogonalization procedure (GSOP) and receiver skew could be compensated and monitored by a 4 x 4 or 4 x 2 multiple-input and multiple-output (MIMO) equalizer. Transmitter imperfections compensation and monitoring take place behind the residual frequency offset compensation and carrier phase recovery by a 4 x 4 MIMO equalizer based on minimal mean square error (MMSE) criterion. By these methods, we analyze and investigate the interaction of transceiver's impairments impact on each monitor numerically. After that, we conduct an experiment to verify the feasibility of the proposed method. All of these results show that each monitor could operate very robust even when multiple co-exist.

Palliation of Four-Wave Mixing in Optical Fibers Using Improved DSP Receiver

A long haul optical communication system (LHOCS) is one of the key resources to fulfill the higher capacity requirements in future communication networks. To launch LHOCS, the system mainly faces high order nonlinear effects. The four-wave mixing (FWM) is one of the major nonlinear effects, which limits the transmission distance. Therefore, in this paper, an advanced duo-binary (DB) modulation scheme-based system is evaluated by employing an improved digital signal processing (IDSP) approach at the receiver side to suppress the FWM effect. In addition, an analytical analysis is also performed for the proposed system. To observe the difference between the IDSP and conventional digital signal processing (DSP), the various performance metrics such as bit error rate (BER), Q-factor, and optical signal-to-noise ratio (OSNR) parameters are evaluated. Variable channel spacing along with polarization mode dispersion (PMD) are analyzed at several ranges of input powers and fiber lengths. The analytical and simulation-based calculations exhibit the effectiveness of the proposed model and hence, FWM effect are compensated to achieve 500 km optical fiber propagation range with a BER below 10−6.

Optical Field Reconstruction of Real-Valued Modulation Using a Single-Ended Photoreceiver With Half-Symbol-Rate Bandwidth

Heterodyne detection based on a single-ended photodetector is of great interest to simplify the receiver frontend complexity compared to a full coherent one, while retaining the ability of digital chromatic dispersion (CD) compensation. Nevertheless, single-ended heterodyne detection schemes typically require a transmitter, which has similar complexity as in coherent systems. Moreover, a receiver bandwidth equal to at least the symbol rate of the signal is required for the complex-valued signal recovery when using a single-ended photodetector. In this article, we study a novel heterodyne detection scheme, which allows to use a simple transmitter structure, and to reduce the receiver bandwidth to a value well below the symbol rate, while supporting digital CD compensation. In our experiment, a real-valued signal originates from a single-drive Mach-Zehnder modulator at the transmitter. A bandwidth-limited receiver suppresses one sideband of the incoming signal, however the receiver digital signal processing (DSP) reconstructs the missing sideband based on the Hermitian symmetry of a real-valued signal spectrum. A receiver bandwidth slightly larger than half of the symbol rate is required. With our novel DSP, we successfully recover 60 Gbaud PAM-4 signal after 80 km of fiber transmission in C-band, using 33 GHz electrical bandwidth at both transmitter and receiver, without optical filtering or amplification.

Carrier Phase Estimation Softwarized on GPU Using Decision-Aided Phase Unwrapping for Flexible Optical Coherent Access Systems

We present a unique carrier phase recovery algorithm which permits the softwarization of optical coherent receiver digital signal processing (DSP). The proposed algorithm with decision-aided phase unwrapping overcomes the serial dependency issue that has, up to now, prevented the softwarization of optical coherent systems. Its performance is verified through extensive Monte Carlo (MC) simulations and hardware experiments on quadrature phase-shift keying (QPSK) signaling. The proposed algorithm is implemented on a general-purpose graphic processing unit (GPU) and achieves 5-Gbit/s throughput for real-time streaming signals, as well as −44.5-dBm receiver sensitivity. The softwarization of carrier phase recovery function is demonstrated for the first time; our enhancement of disaggregated optical coherent receivers will significantly improve system flexibility and scalability, towards the realization of a new access networking that exploits general purpose hardware and virtualized transport layer functions.

Complexity Reduction With a Simplified MIMO Volterra Filter for PDM-Twin-SSB PAM-4 Transmission

We propose and experimentally demonstrate a simplified multiple-input multiple-output Volterra filter (MIMO-VF) in a polarization division multiplexed (PDM) twin single side-band (SSB) scheme based on two offset carriers. The right side-band (RSB) and left side-band (LSB) signals of the twin-SSB in each polarization are separated by an optical filter, and the crosstalk between the RSB and LSB is mitigated in the receiver digital signal processing (DSP) by the proposed simplified MIMO-VF, which is combined with one two-stage equalizer, i.e., a linear 2 × 2 butterfly MIMO equalizer and two independent VFs. With the proposed simplified MIMO-VF, we experimentally demonstrate a 400-Gb/s four-level pulse amplitude modulation (PAM-4) signal transmission over an 80-km standard single mode fiber (SSMF). Compared with the traditional MIMO-VF, the proposed simplified MIMO-VF can retain the same transmission performance and reduce the computational complexity by approximately 57.9%.

Real-Time Implementation of Coherent Receiver DSP Adopting Stream Split Assignment on GPU for Flexible Optical Access Systems

We aim to maximize the flexibility of access networks by combining two trends: digital signal processing (DSP)-based physical (PHY) layer technologies and network function virtualization (NFV). We implement an advanced DSP-PHY on a general-purpose server. However, given that the server must handle high data transfer rate, the transfer time is a significant issue for softwarizing DSP-PHY. This article proposes a pipeline architecture that adopts split assignment of the transfer stream; it can execute data transfer and DSP-PHY calculation in parallel which significantly reduces the processing time. We implement on a graphic processing unit (GPU) a 5-Gbit/s coherent receiver DSP handling the output of a 40-Gbit/s analog-to-digital converter. As evaluated in an optical system, our proposal significantly relaxes the processing time requirement for DSP algorithm calculations; the processing time margin is 11.2 times greater than that of previous work. An additional digital filter is also successfully implemented in real time; it offers an optical power budget of 47.2-dB, a gain of 2-dB.

Experimental Demonstration of a Real-Time Digital Filter Multiple Access PON With Low Complexity DSP-Based Interference Cancellation

A four-channel, upstream, digital filter multiple access (DFMA) passive optical network (PON) is experimentally demonstrated for the first time in a 26 km single mode fiber (SMF) intensity modulation and direct detection system employing real-time optical network units (ONUs) and optical line terminal (OLT), which are dynamically reconfigurable in terms of channel spectral allocation and signal modulation parameters. To combat physical channel frequency response-induced cross channel interference (CCI) between spectrally overlaid orthogonal channels, a real-time, reconfigurable CCI cancellation (CCIC) function implemented in the OLT receiver digital signal processing (DSP), is demonstrated to significantly improve the DFMA PON performance and robustness in terms of significant reductions in total channel bit error rates (BERs) and individual subcarrier BERs, substantial increases in channel capacities, considerably reduced sensitivity to ONU synchronization, and excellent transparency to the interfering channel's signal modulation format. The tradeoff between the complexity of the CCIC function's filter, in terms of tap count (multiplier elements) and the DFMA PONs performance is optimized to determine the optimum tradeoff. It is shown that the CCIC filter only requires as few as 42 taps to achieve excellent CCIC performance. We also fully analyze the DSP complexity of the transmit and receive DFMA filters and the CCIC filters for varying DFMA channel counts and varying ratios of AD/DA converter sample rate to digital logic clock rate (parallelization factor). It is shown that DFMA filter complexity is independent of channel count and furthermore that for channel counts ≥ 4 the CCIC filter only needs to process ~10 interference-inducing samples per wanted signal sample, resulting in only ~10 multipliers per parallel filter in the CCIC filter. We have therefore successfully demonstrated an ultra-low complexity, real-time CCIC function, which considerably enhances the performance and robustness of DFMA PONs.

Reduced Equalization Needs of 100 GHz Bandwidth Plasmonic Modulators

As bit rates of optical interconnects increase, a large amount of complicated signal conditioning is needed to compensate for the insufficient bandwidth of current modulators. In this paper, we evaluate the reduced equalization requirements of high-bandwidth plasmonic modulators in short-reach transmission experiments. It is shown that transmission of 100 Gbit/s non-return-to-zero (NRZ) and 112 Gbit/s pulse-amplitude modulation-4 over 1 km and 2 km distance is possible without any receiver equalization. At higher bit-rates, such as 120 Gbit/s NRZ, data transmission is demonstrated over 500 m with reduced receiver equalization requirements. Transmission up to 200 Gbit/s over 1 km is also shown with more complex receiver equalization. The reduced complexity of the receiver digital signal processing is attributed to a flat frequency response of at least 108 GHz of the plasmonic modulators. All single wavelength transmissions have been performed at 1540 nm in standard single mode fiber.

4 × 128-Gb/s PDM-DMT signal transmission over 1440-km SSMF with high phase noise tolerance.

In this paper, we propose an intensity modulation and coherent detection scheme with high phase noise tolerance for polarization division multiplexing (PDM) discrete multi-tone (DMT) signal transmission by employing Kramers-Kronig (KK) detection and digital carrier regeneration (DCR). At the transmitter side, DMT signal is modulated by a Mach-Zehnder modulator (MZM) setting bias around the null point and transmitted with the suppressed optical carrier. At the receiver side, a directly modulated lasers (DMLs) locating at the edge of DMT signal is used as the local oscillator (LO) for coherent detection. For signal recovery, KK detection is first used to reduce the signal to signal beating noise. Digital optical carrier is then regenerated by the DCR scheme and the DMT signal could be recovered by enveloping detection with the regenerated digital carrier, which can achieve high laser line-width tolerance and mitigate the residual phase noise caused by KK detection. To verify the effectiveness of the proposed scheme, we compare the KK and DCR based receiver digital signal processing (DSP) with conventional receiver DSP by using both DMLs with larger linewidth (~10 MHz) or external cavity lasers (ECLs) with smaller linewidth (less than 100 kHz) as optical carrier and LO. The results show that KK and DCR can mutually improve the system performance with ECLs as optical carrier and LO. Moreover, it is shown that the signal using DMLs cannot be recovered without the DCR method due to the high laser line-width of DML. Finally, we successfully demonstrate 4 × 128-Gb/s KK and DCR based PDM-DMT signal transmission over 1440-km SSMF by employing DMLs as optical carrier and LO.

Coherent Receiver DSP Implemented on a General-Purpose Server for a Full Software-Defined Access System

To accommodate the service paradigm shift driven by the now amazingly wide variety of services, telecom operators must overcome many challenges in redesigning their network infrastructure to attain system flexibility, scalability, and cost effectiveness. A key part of the solution is the recent advent of network virtualization based on commodity hardware. We propose a unique edge-server architecture that combines network virtualiza- tion with advanced digital signal processing (DSP) technology. Our architecture has the server's central processing unit running application layer functions and a generalpurpose accelerator running dissimilar physical layer functions, including advanced DSP. To elucidate the power budget capability of our coherent detection scheme, a hardware experiment demonstrates the optical power budget of 45.2 dB for quadrature phase-shift keying signals of 5 Gbit/s bit rate.

Transmitter and receiver DSP for 112 Gbit/s PAM-4 amplifier-less transmissions using 25G-class EML and APD.

The transmission performance of 112 Gbit/s PAM-4 signal with commercial 25 G-class EML and APD is experimentally studied by using advanced digital signal processing (DSP) algorithms, i.e. pre-equalization (Pre-EQ), error-table based pre-correction (ETC), least-mean square (LMS) based equalization, direct detection faster than Nyquist (DD-FTN) algorithm. Among them, Pre-EQ and ETC are implemented at the transmitter, and ETC is a symbol-pattern-dependent pre-compensation algorithm based on the look-up-table approach. In order to obtain these pre-compensated parameters readily, a joint equalization and error table generation (JEEG) module is proposed. Employing the combination of ETC, LMS, and DD-FTN, a single line 112 Gbit/s PAM-4 40 km amplifier-less transmission with a record receiver sensitivity of -16.6 dBm (at 7% HD-FEC threshold) is experimentally demonstrated. In addition, the computational complexities of different DSP schemes are analyzed and discussed in detail. The receiver computational complexity can be effectively reduced by employing appropriate ETC and Pre-EQ in the transmitter.