Subcarrier Multiplexing Research Articles

Non-integer-oversampling digital signal processing for coherent passive optical networks

Beyond 100G passive optical networks (PONs) will be required to meet the ever-increasing traffic demand in the future. Coherent optical technologies are the competitive solutions for the future beyond 100G PON but also face challenges such as the high computational complexity of digital signal processing (DSP). A high oversampling rate in coherent optical technologies results in the high computational complexity of DSP. Therefore, DSP running in a non-integer-oversampling below 2 samples-per-symbol (sps) is preferred, which can not only reduce computational complexity but also obviously lower the requirement for the analog-to-digital converter. In this paper, we propose non-integer-oversampling DSP for meeting the requirements of coherent PON. Complexities of complex-valued multiplication of the proposed DSP working at 9/8-sps and 5/4-sps oversampling rates can be reduced by 44.04% and 40.78% compared to that working at the 2-sps oversampling rate, respectively. A 400-Gb/s-net-rate coherent PON based on digital subcarrier multiplexing was demonstrated to verify the feasibility of non-integer-oversampling DSP. There is almost no penalty on the receiver sensitivity when non-integer-oversampling DSP is adopted. In conclusion, non-integer-oversampling DSP shows great potential in the future coherent PON.

On clustering coherent optics point-to-multipoint trees for cost-effective bandwidth assignment in MANs

Point-to-multipoint (P2MP) coherent pluggable transceivers leveraging digital subcarrier multiplexing (DSCM) techniques are gaining momentum among telecommunication operators as a technology that can potentially provide broadband connectivity in metropolitan area networks (MANs) with flexible bandwidth allocation at reduced costs. This technology allows trees to be built where central metro (hub) nodes are interconnected with remote access/aggregation (spoke) nodes with dedicated and flexible bandwidth slices, enabling better resource management, simplified architectures, and reduced costs. This paper proposes a modified version of AI/ML clustering techniques that concentrates on the same P2MP tree as those low-rate leaf nodes with uncorrelated daily traffic patterns in a cost-effective manner, thus allowing 10%–18% tree reductions. This, when compared with fixed P2P transceivers, translates into potential CAPEX savings between 40% and 50% in the different scenarios here considered.

Paired-subcarrier equalization for phase noise and transmitter IQ skew in digital subcarrier multiplexing system

The digital subcarrier multiplexing (DSCM) transmission scheme is expected for future ultra-large baud rate transmission. However, the phase noise and transmitter (Tx) IQ skew tolerance are decreased due to the narrow sub-band transmission and conjugated interference from symmetric subcarrier. In this paper, we propose a paired-subcarrier equalization scheme to jointly mitigate the phase noise and Tx IQ skew. We use a phase locking loop (PLL) embedded 4 × 4 MIMO equalizer to simultaneously realize polarization demultiplexing, phase noise and Tx IQ skew compensation. The 4 × 4 MIMO can deal with the paired-subcarrier interference in the DSCM transmission. Besides, since the inner subcarrier suffers smaller interference from its symmetric subcarrier, we estimate the phase noise by inner subcarriers and share the phase noise information with other subcarriers to reduce the overall complexity. Through simulations of 100-GBaud 64-QAM DSCM coherent optical fiber transmission consisting of eight 12.5-Gbaud subcarriers and experiment of 10-GBaud four-subcarriers PM-16QAM transmission, we find that the PLL embedded equalizer for DSCM scheme exhibits better skew and phase noise compensation ability compared with other equalizers. Additionally, we compare the performance of single-carrier and DSCM schemes with the proposed equalizers in simulation. The influence of phase noise and Tx IQ skew on DSCM transmission can be largely relaxed.

PAPR Reduction and Nonlinearity Mitigation of Optical Digital Subcarrier Multiplexing Systems With a Silicon Photonics Transmitter

PAPR Reduction and Nonlinearity Mitigation of Optical Digital Subcarrier Multiplexing Systems With a Silicon Photonics Transmitter

Perturbation-Aided Fiber Nonlinearity Compensation in Subcarrier-Multiplexing Systems

A perturbation-aided advanced digital backpropagation (P-ADBP) is proposed for subcarrier-multiplexing (SCM) coherent systems. To improve the nonlinearity compensation (NLC) performance, for the first time, the first-order perturbation analysis is introduced into the conventional ADBP. Based on the fact that the perturbation-based self-subcarrier nonlinearity (PSSN) has a similar form as the cross-subcarrier nonlinearity (CSN), we approximate that PSSN and CSN can be calculated in parallel and the compensation of them can be fused together. This compensation fusion scheme saves the computational complexity effectively by sacrificing only a little performance. Furthermore, to realize the best performance, gradient descent is used to jointly optimize the perturbation coefficients and other necessary parameters in P-ADBP. Verified by 9 × 120-GBaud 1600-km wavelength-division multiplexing SCM systems with dual-polarization 16QAM modulation format, 1-Step P-ADBP provides identical Q <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{2}$</tex-math></inline-formula> improvement as 5-Step ADBP but saves about 72.3% complexity. 5-Step P-ADBP and 10-Step P-ADBP also leapfrog into the NLC level of 10-Step ADBP and 20-Step ADBP, but save about 44.4% and 46.0% complexity, respectively. The proposed algorithm helps achieve an improved performance-complexity trade-off compared to the conventional ADBP and its state-of-the-art modifications.

Hardware-Efficient and Robust DSP Scheme for Coherent DSCM System in Presence of Transmitter IQ Impairments

In this paper, a hardware-efficient and robust DSP scheme for the coherent digital subcarrier multiplexing (DSCM) system in presence of transmitter (Tx) in-phase/quadrature (IQ) impairments is introduced and demonstrated. An analytic complex model is given to describe the effects of IQ skew, IQ amplitude and phase imbalance (referred to as IQ mixing) on the DSCM signal. Based on the model, residual frequency offset, polarization and carrier phase noise are jointly recovered using frequency pilot tones (FPT) in presence of IQ mixing. This process avoids the coupling effect of phase noise and Tx IQ mixing in conventional schemes, which allows the Tx IQ mixing to be compensated for by the following joint equalization of conjugate symmetric subcarriers. Compared to the conventional scheme based on widely linear equalizer, the polarization and carrier phase tracking capability of the proposed scheme is robust to IQ mixing. Meanwhile, the proposed scheme can tolerate a wide range of Tx IQ mixing in the presence of residual frequency offset and carrier phase noise. Since polarization crosstalk is eliminated, the signals of the two polarizations can be equalized separately in the proposed scheme, thus reducing the computational complexity by half. For a 4-subcarrier DSCM system with baud rate of 32 Gbaud, experimental results show that the Q-factor degradation of the proposed scheme is less than 0.5dB in the presence of Tx IQ time skew within <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\pm$</tex-math></inline-formula> 15 ps, IQ phase imbalance within <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\pm$</tex-math></inline-formula> 20 degrees or IQ amplitude imbalance within <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\pm$</tex-math></inline-formula> 3 dB. With 1dB Q-factor degradation, the proposed scheme can track polarization variation up to 10 Mrad/s, residual frequency offset of 10 MHz or carrier phase noise with 3dB linewidth of 200 kHz in the presence of Tx IQ mixing. Therefore, the proposed scheme provides a hardware-efficient and robust DSP solution for coherent DSCM targeting access network applications.

Enabling cost-effective high-performance vibration sensing in digital subcarrier multiplexing systems.

Enabling communication networks with sensing functionality has attracted significant interest lately. The digital subcarrier multiplexing (DSCM) technology is widely promoted in short-reach scenarios for its inherent flexibility of fine-tuning the spectrum. Its compatibility with large-scale as-deployed coherent architectures makes it particularly suited for cost-sensitive integrated sensing and communication applications. In this paper, we propose a scheme of spectrally integrating the digital linear frequency modulated sensing signal into DSCM signals to achieve simultaneous sensing and communication through shared transmitter. Consequently, this cost-effective scheme has been demonstrated to achieve 100-Gb/s dual-polarization quadrature phase-shift keying (DP-QPSK) and 200-Gb/s dual-polarization 16-ary quadrature amplitude modulation (DP-16QAM) transmission with a distributed acoustic sensing sensitivity of 69 pε/Hz and 88 pε/Hz respectively, at a spatial resolution of 4 m.

Optimized design of filterless horseshoe networks exploiting point-to-multipoint coherent transceivers

The horseshoe topology is widely used to realize metro-aggregation networks, since it provides a natural fit to the hub-and-spoke traffic pattern present in the majority of these deployments, while enabling survivability against single link and hub failures. A filterless architecture can also be adopted to further reduce capital expenditure (CapEx) by replacing active elements, such as reconfigurable add/drop multiplexers, with simpler and passive splitters/combiners. Such an architecture can effectively host coherent-based point-to-multipoint (P2MP) transceivers enabled by digital subcarrier multiplexing (DSCM). Importantly, by carefully optimizing the deployment of amplifiers (location and gain) and splitters/combiners (type), it may be possible to reduce the total number of optical amplifiers required, further decreasing CapEx. This paper proposes an integer linear programming framework to optimize metro-aggregation filterless horseshoe networks, taking into account the specific requirements of DSCM-based P2MP coherent transceivers. The results indicate that a considerable reduction in amplifier count is possible while ensuring that end-to-end performance thresholds are met, which include the minimum required input power at the receivers, a maximum subcarrier input power difference at the hub’s receivers, and the minimum optical-signal-to-noise ratio.

Experimental Comparison of Single-Carrier and Digital Subcarrier Multiplexing Transmissions in a W-Band 200 Gb/s Fiber-Wireless System Considering Transmitter IQ Imbalance and Skew Mitigation

Experimental Comparison of Single-Carrier and Digital Subcarrier Multiplexing Transmissions in a W-Band 200 Gb/s Fiber-Wireless System Considering Transmitter IQ Imbalance and Skew Mitigation

Neural network architectures for optical channel nonlinear compensation in digital subcarrier multiplexing systems.

In this work, we propose to use various artificial neural network (ANN) structures for modeling and compensation of intra- and inter-subcarrier fiber nonlinear interference in digital subcarrier multiplexing (DSCM) optical transmission systems. We perform nonlinear channel equalization by employing different ANN cores including convolutional neural networks (CNN) and long short-term memory (LSTM) layers. First, we develop a fiber nonlinearity compensation for DSCM systems based on a fully-connected network across all subcarriers. In subsequent steps, and borrowing from the perturbation analysis of fiber nonlinearity, we gradually upgrade proposed designs towards modular structures with better performance-complexity advantages. Our study shows that putting proper macro structures in design of ANN nonlinear equalizers in DSCM systems can be crucial in development of practical solutions for future generations of coherent optical transceivers.

10.51-Tbit/s multi-channel IF-over-fiber transmission with SDM/WDM/SCM for beyond-5G mobile fronthaul accommodating ultra-high-density antennas

In beyond-5G and 6G mobile communication systems, it is expected that massive numbers of antennas are densely deployed to provide a stable and high-speed communication environment. To efficiently accommodate high-density antennas, high-capacity mobile fronthaul (MFH) is also required. Intermediate-frequency-over-fiber (IFoF) is considered as one of the attractive solutions for realizing high-capacity MFH due to its high spectral efficiency and multi-channel transmission capability. In this paper, we demonstrate the highest-capacity IFoF transmission with combinations of space division multiplexing, wavelength division multiplexing, and subcarrier multiplexing; 4608 channels of 64-quadrature amplitude modulation orthogonal frequency division multiplexing signals with a total net bit rate of 10.51 Tbit/s (4608×380.16MHz/ch×6bit/s/Hz) are successfully transmitted over a 10.1-km uncoupled 12-core multi-core fiber.

Fast-Convergence Digital Signal Processing for Coherent PON Using Digital SCM

It is foreseeable that the 100 Gb/s/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\lambda$</tex-math></inline-formula> and beyond passive optical network (PON) will be required in future optical access networks to meet the explosive growth of data traffic. The coherent optical systems could be a promising solution for the future beyond 100G PON. Coherent PON using digital subcarrier multiplexing (DSCM) can provide flexible bandwidth allocation to a large number of access subscribers by dividing subcarriers of the DSCM signal into time slots for time-and-frequency division multiple access. When the optical network unit is allocated a new subcarrier, digital signal processing (DSP) should converge fast in the allocated time slot to ensure a low handoff latency for real-time bandwidth allocation. However, the traditional coherent DSP is hard to realize fast convergence due to blind and complex algorithms. In this paper, we design a specific training sequence (TS) structure and propose data-aided DSP to achieve fast convergence for coherent PON. The feasibility of the proposed scheme is experimentally verified in an 8 Gbaud/SC×8 SCs 400 Gb/s-net-rate coherent PON using DSCM with 16 quadrature amplitude modulation. The experimental results show that fast convergence is jointly realized by the proposed TS structure and data-aided DSP using a 416-symbol TS with a 52 ns duration. The receiver sensitivity at the 20% soft-decision forward error correction limit is approximately -27 dBm and an optical power budget of about 35.5 dB is achieved with a booster amplifier.

On the Mitigation of WSS Filtering Penalties: Single- vs Multi-Carrier Modulation

The tight channel filtering imposed by long cascades of reconfigurable optical add-drop multiplexers (ROADMs) represents, nowadays, one of the major performance-limiting aspects for optically-routed coherent optical fiber systems. This makes it vital to perform a ROADM-aware optimization of the network performance at the physical layer. In particular, the use of different modulation options is known to have a strong impact on the extent of the filtering-induced penalties. In that regard, a long debate between single- and multi-carrier modulation has been taking place during the last few years, sometimes leading to apparently contradictory results. Following the open scientific discussion on this topic, in this work, we investigate by simulation and experimentally the wavelength selective switch (WSS) filtering tolerance of single-carrier (SC) and digital subcarrier multiplexing (DSCM) signals. In order to promote a fair comparison, both modulation options are carefully designed to minimize the ROADM-filtering penalties, namely resorting to the use of entropy loading together with baud rate optimization. After some preliminary numerical assessment, a comprehensive set of experiments are carried out for the transmission of 21-WDM 95–105 Gbaud SC and DSCM signals over a 2040 km straight line of fiber with regularly spaced WSSs. In general, our results allow to conclude that the two modulation options yield similar performance if the overall baud rate is optimized for each filtering scenario, keeping the baud rate at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\pm$</tex-math></inline-formula> 5% of the optimized value.

Hardware-Efficient, Ultra-Fast and Joint Polarization and Carrier Phase Tracking Scheme Based on Frequency Domain Pilot Tones for DSCM Systems

The coherent digital subcarrier multiplexing (DSCM) has recently been proposed as a cost saving solution to realize flexible and scalable point-to-multipoint (PTMP) networks. In this work, a hardware-efficient, ultra-fast and joint polarization and carrier phase tracking scheme for the DSCM systems based on frequency domain pilot tones (named as PTJ) is proposed and demonstrated via both simulations and experiments. By extracting the amplitude and phase of the pilot tones with a low-pass filter (LPF), the transmission matrix of random birefringence and carrier phase can be obtained prior to subcarrier demultiplexing and channel equalization. For the hardware-efficient implementation, a simplified LPF based on sliding window averaging (SWA) is proposed to filter out the pilot tones. Compared to previous scheme using pilot tones to track polarization and carrier phase in a cascaded manner (named as PTC), our proposed scheme shows better BER performance over wide polarization rotation rates and reduces more than 90% of computation resources by the SWA and joint tracking of polarization and carrier phase. The experimental results show that the proposed scheme can track polarization rotation rate up to 20 Mrad/s with the OSNR penalty of 0.2 dB at 7% FEC threshold. This indicates that an OSNR sensitivity gain of 2.8 dB is achieved compared with the PTC scheme. The proposed scheme provides a hardware-efficient and fast-tracking solution to recovery the polarization and carrier phase for digital subcarrier multiplexing (DSCM) and polarization division multiplexing (PDM) based optical fiber communication system.

Supporting Heterogenous Traffic on Top of Point-to-Multipoint Light-Trees

New 5 G and beyond services demand innovative solutions in optical transport to increase efficiency and flexibility and reduce capital (CAPEX) and operational (OPEX) expenditures to support heterogeneous and dynamic traffic. In this context, optical point-to-multipoint (P2MP) connectivity is seen as an alternative to provide connectivity to multiple sites from a single source, thus potentially both reducing CAPEX and OPEX. Digital subcarrier multiplexing (DSCM) has been shown as a feasible candidate for optical P2MP in view of its ability to generate multiple subcarriers (SC) in the frequency domain that can be used to serve several destinations. This paper proposes a different technology, named optical constellation slicing (OCS), that enables a source to communicate with multiple destinations by focusing on the time domain. OCS is described in detail and compared to DSCM by simulation, where the results show that both OCS and DSCM provide a good performance in terms of the bit error rate (BER) for access/metro applications. An exhaustive quantitative study is afterwards carried out to compare OCS and DSCM considering its support to dynamic packet layer P2P traffic only and mixed P2P and P2MP traffic; throughput, efficiency, and cost are used here as the metrics. As a baseline for comparison, the traditional optical P2P solution is also considered in this study. Numerical results show that OCS and DSCM provide a better efficiency and cost savings than traditional optical P2P connectivity. For P2P only traffic, OCS and DSCM are utmost 14.6% more efficient than the traditional lightpath solution, whereas for heterogeneous P2P + P2MP traffic, a 25% efficiency improvement is achieved, making OCS 12% more efficient than DSCM. Interestingly, the results show that for P2P only traffic, DSCM provides more savings of up to 12% than OCS, whereas for heterogeneous traffic, OCS can save up to 24.6% more than DSCM.

Design and Performance Investigation of Intensity Modulation/Direct Detection Scheme for C- and O-band Point-to-Multipoint Optical Networks

The fifth generation (5G) and beyond mobile networks support increasing number of users with increasing bit rate per users. This has encouraged researchers to propose coherent digital subcarrier multiplexing (SCM) point-to-multipoint (P2MP) architectures to reduce the cost and complexity of optical transport networks, particularly in the metro aggregation scenario. However, coherent optical receiver is relatively costly and complexity compared with a direct-detection (DD) counterpart due to the use of a synchronized local laser. This paper addresses design issues and performance investigation of intensity modulation/direct-detection (IM/DD) P2MP optical networks. P2MP architectures are designed using digital RF multisubcarrier (MSC) waveform embedded on the intensity of continuous-wave (CW) laser beside direct-detection scheme. The design covers C- and O-band operation using a wavelength-division multiplexing (WDM) architecture. Further single- and double-polarization (SP and DP) versions are reported for each type of the networks. All the architectures are built in Optisystem version 15 environment and simulated for different network parameters, under the assumption of 25 Gbps per subcarrier data rate. The main performance measures are maximum route reach and bit rate-distance product (BDP). The simulation results indicate that DD networks can replaced the coherent counterpart when number of subcarriers per optical channel is 4. Further, the O-band P2MP networks offer high values of maximum reach and BDP than C-band counterparts.

Digital Subcarrier Multiplexing: Enabling Software-Configurable Optical Networks

The various topologies, traffic patterns and cost targets of optical networks have prevented the deployment of end-to-end solutions across multi-domains, and the optimization of the network as a whole. The consequent limitations in flexibility, scalability, and adaptability of optical networks will become increasingly important with new applications, such as 5G/6G. Coherent transceivers based on digital subcarrier multiplexing (DSCM) are proposed to address these current constraints. In particular, DSCM allows (i) the design of high-capacity point-to-point (P2P) and -multipoint (P2MP) optical networks; (ii) simplified aggregation with passive optics; and (iii) connections between low- and high-speed transceivers. Furthermore, DSCM-based networks reduce the number of opto-electro-opto stages, halve the number of bookended transceivers, and provide a better match for existing hub-and-spoke (H&S) traffic patterns in fast-growing and dynamic access/metro segments. A DSCM-based transceiver will pave the way for the deployment of next-generation flexible, adaptable, and scalable software-configurable optical networks. Key steps and elements to realize this solution are laid out, and promising applications outlined. The first real-time experimental results of coherent P2MP transceivers are presented.

BER versus Data Rate Performance of SCM Transmission over Multimode Fibers at Low-Frequency Passbands with Linear Block Codes

The use of Multimode fibers (MMFs) in the low-frequency region has been studied. It is found that many available low-frequency passbands can be used as channels. However, these passbands are frequency-selective channels, which can negatively affect to the transmitted signal. To lessen this effect, Linear Block Code (LBC) along with a careful selection of passbands is applied. In this work, the performance in terms of BER and data rate for an MMF communication system using a 1-km length MMF at low-frequency passbands with different schemes of LBC is studied. This study uses subcarrier multiplexing (SCM) with a 3-dB modal bandwidth and other low-frequency passbands as channels. The results show that by using (31, 26) LBC and Type-I bit allocation, a data rate of 494 Mbps can be obtained with a BER lower than 10-9. Moreover, a BER lower than 10-14 can be achieved with a data rate of 399 Mbps, which is two-fold higher than the data rate obtained using only the 3-dB modal bandwidth. Additionally, the BER versus data rate for different cases in terms of applying LBC to the system are found and can be practically used as a guideline in selecting a BER and data rate that are suitable for the minimum requirement of a given communication system.

Nonlinearity aware bisection-based sphere shaping for optical digital subcarrier multiplexing systems.

In this paper, a nonlinearity-tolerant sphere shaping is proposed based on the parallel bisection structure. Compared to enumerative sphere shaping (ESS), the bisection-based sphere shaping (BS-SS) can achieve a lower average energy and rate loss at the same blocklength by selecting all preferential sequences. The nonlinearity-tolerant BS-SS (BS-SS-NLI) can mitigate the fiber nonlinearity by reducing the energy variation, which is demonstrated in the simulations with probabilistically shaped, single channel, dual-polarization 64-ary quadrature amplitude modulation (DP-64QAM) for a multi-span long-haul transmission over 2500 km of standard single-mode fiber (SSMF) applying digital subcarrier multiplexing (SCM), comparing with recent literature kurtosis-limited ESS (K-ESS) and list-encoding constant composition distribution matching (L-CCDM). The BS-SS-NLI can provide 0.25 dB effective signal-to-noise ratio (SNR) gain over ESS and 0.39 dB gain over L-CCDM. The achievable information rate (AIR) can be improved ∼0.1 bits/4D-symbol by BS-SS-NLI over ESS.

Review on Radio over Fiber Systems for Capacity Enhancement

Radio-over-fiber (RoF) system is one of the latest plans for future wideband wireless communication systems like mobile communications, suburban and residential areas. These are good nominees to build the backbone of the next generation of wireless networks. Besides, the fundamental requirement for next-generation radio optical fiber and mobile communication is the need for more capacity and speed which has a huge scope, that is further required for a better system to attain demands such as Fiber to The Home (FTTH) and 5G. The combination of RoF with spectrally-efficient modulation techniques can accomplish increasing demand for high data rates for mobile users. Currently, analog fronthaul is promoted as an energy-efficient and bandwidth solution that can reach the necessity of the 5G vision for high data rates, low latency, and energy efficiency. Mainly, this paper presents a review that focuses on various techniques which several approaches have been listed to perceive the large-capacity data transmission and describe them including Wavelength Division Multiplexing (WDM), Subcarrier Multiplexing (SCM), Mode Division Multiplexing (MDM), Polarization Division Multiplexing (PDM), Generalized Frequency Division Multiplexing (GFDM), Parallel&nbsp; Intensity Modulation (PIM)/Phase Modulation (PM) transmitter, and Millimeter-wave (Mm wave) generation techniques. Also, a comparison between them has been done