Future Optical Access Networks Research Articles

Bidirectional TFDM 200 G coherent PON with a simplified receiver at the ONU side.

Time and frequency division multiplexing (TFDM) coherent passive optical networks (PONs) are considered as a promising candidate for future optical access networks due to the advantage of high sensitivity, high spectral efficiency, and flexibility. We propose a novel, to our knowledge, bidirectional TFDM 200-Gb/s coherent PON architecture based on the digital subcarrier multiplexing (DSCM) technology. A polarization-insensitive simplified coherent receiver is achieved at the ONU side by Alamouti coding and heterodyne detection. We experimentally demonstrate this bidirectional coherent PON with a 50-Gbaud Alamouti 16 quadrature amplitude modulation (QAM) signal transmitted downstream and a 25-Gbaud dual polarization (DP)-16QAM signal transmitted upstream. By using a single sideband modulated transmitter, only one laser source is required at the optical network unit (ONU) side. The power budgets can reach 30 and 33 dB for the downstream and upstream, respectively, after the 20-km transmission over a standard single-mode fiber (SSMF).

Scheme for a WDM-PON with colorless optical network units based on a flat optical frequency comb.

A new, to the best of our knowledge, scheme for a wavelength division multiplexing passive optical network (WDM-PON) based on a flat optical frequency comb (OFC) is proposed. Using an OFC as the optical source of the system can realize the colorlessness of optical network units (ONUs), and the direct detection of the downlink data further simplifies the ONU structure. The coherent demodulation of the uplink data improves the system performance due to the coherence of the comb lines. In this research, the proposed system is studied for its performance and power budgeting. The results show the flexibility, effectiveness, and practicability of the proposed scheme, which can be applied to future high-capacity optical access networks.

Future Optical Access Networks Enabled by Digital Signal Processing and Physical Layer Softwarization

Future Optical Access Networks Enabled by Digital Signal Processing and Physical Layer Softwarization

FSK/ASK Orthogonal Modulation System Based on Novel Noncoherent Detection and Electronic Dispersion Compensation for Short-Reach Optical Communications

We propose an FSK/ASK orthogonal modulation system based on a novel noncoherent detection (NCD) scheme, aimed at expanding the system capacity for short-reach optical communications cost-effectively. In the transmitter, the FSK optical signal is generated by simple frequency modulation through a directly modulated distributed feedback laser. Subsequently, by utilizing a Mach–Zehnder modulator for ASK modulation, the FSK/ASK optical signal is obtained. The novel and low-complexity NCD receiver consists of an intensity detection branch and a frequency detection branch. The frequency detection branch is composed of an optical differentiator, a photodetector, and frequency extraction circuits. Notably, the proposed NCD scheme overcomes the limitation of the traditional FSK/ASK-NCD receiver stemming from the trade-off between the detected signal quality of the amplitude and frequency. Furthermore, electronic dispersion compensation (EDC) is available. Through numerical simulations, our findings demonstrate that the proposed FSK/ASK-NCD system, assisted by EDC, achieves a remarkable 100 km transmission span for both 40 Gbps 2FSK/2ASK and 60 Gbps 2FSK/4ASK modulation formats, which surpasses the 2ASK-DD and the 4ASK-DD systems, where the maximum achievable spans are limited to less than 20 km. These results underscore the potential of the proposed system as a robust candidate for future passive optical access networks.

Coexistence in future optical access networks from an operator’s perspective [Invited

In this paper, we address the challenges related to the simultaneous operation of legacy and future network technologies in passive optical networks (PONs) and point-to-point topologies. We explore the coexistence issues for various user categories, including residential, enterprise, factories, and mobile users. The future access networks will need to accommodate multiple technologies concurrently. Our paper demonstrates how G-PON, XGS-PON, and 50G-PON can coexist on the same optical distribution network using triple coexistence multi-PON modules. However, there are certain limitations that need to be considered, such as achieving a high optical budget and the associated costs, both in terms of monetary investment and CO2 emissions for technology deployment by operators. Additionally, we emphasize the significance of optical line terminals (OLTs) as crucial network components in the future. OLTs play a central role in supporting the traffic evolution with optimal efficiency in terms of throughput, latency, jitter, and adherence to CO2 emission constraints. Finally, we experimentally demonstrate the first assessment, to our knowledge, of a 50G-PON prototype with optical budget reaching 23.9 dB limited by the downstream (DS). Furthermore, we evaluate the potential limitations regarding coexistence by examining the effects of stimulated Raman scattering between the DS of 50G-PON and the upstream of XGS-PON for the first time with real PON systems.

Spectrally-Sliced Coherent Receiver Utilizing a Gain-Switched Optical Frequency Comb

The spectrally-sliced coherent receiver is an attractive solution for future optical access network applications where cost is a sensitive factor, as it enables reduced bandwidth of the analog front-end components and analog-to-digital convertors when compared to a conventional intradyne coherent receiver. In a proof-of-concept experiment, we demonstrate a single-polarization 50 Gb/s QPSK C-band transmission experiment over 40 km of standard (G.652) fiber using receiver bandwidths as low as 6.25 GHz and also utilize an array of local oscillator lines derived from an injection locked gain-switched optical frequency comb, which can be realized as a compact photonic integrated circuit. This paper also analyses the influence of optical frequency comb properties on the quality of the reconstructed signal. The critical parameters considered in this study include comb power and flatness, mutual comb line coherence, and the comb line suppression ratio after demultiplexing the comb lines. We have shown that with practically achievable comb parameter values, the receiver can operate with less than a 1 dB sensitivity penalty compared to a conventional intradyne receiver. The scheme has the potential to provide a low-cost coherent receiver solution for next-generation access networks.

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.

Implementation of Shared Laser–LED Sources in a Free Space Optics (FSO) Network under Environmental Impact

This paper is devoted to evaluating the combined coherent and incoherent sources (CCIS) technique for different applications in the optical domain and future optical code division multiple access (OCDMA) networks. Spectral amplitude coding (SAC) has gained significant attention in optical processing systems due to its increased capabilities in dealing with multiple-access interference (MAI) efficiently. Fixed right shift (FRS) is adopted as a signature code in this study. Furthermore, performance analysis is studied in terms of bit error rate (BER) for the system using CCIS in both the free space optics (FSO) and sky mesh network using an aerial altitude platform system (AAPS). Simulation results confirmed that a CCIS design significantly improves system performance with moderate cost. An acceptable BER value of 10−9 at 1.25 Gbps data rate and 60 km, 38 km, and 6 km distances for the laser, CCIS, and LED sources, respectively, can be supported. In particular, at Q-factor ~4.5, the FSO ranges under low haze, moderate haze, and heavy haze are, respectively, 3.7 km, 2.5 km, and 1.5 km. The reason is that a CCIS design causes an increase in the effective transmitted power. It can be summarized that a CCIS design can provide reliable solutions and an affordable cost for future optical fiber and wireless network applications.

Capacity and flexibility improvement of traffic aggregation for fixed 5G: Key enabling technologies, challenges and trends

As the emergence of various high-bandwidth services and the requirements to support 5G/Wi-Fi 6 wireless networks, the next generation fixed networks, i.e. F5G, are expected to be realized in the 5G era. F5G is endowed with new characteristics, including ultra-high bandwidth, all-optical connections and optimal service experience. With the prospect of optical-to-everywhere, optical technologies are used for mobile front-haul, mid-haul, and back-haul. Optical access networks would play an important role in F5G to support radio access network and fixed access network. Low-latency PON is a key for cost effective-haul traffic aggregation. In terms of signal transmission, intensity modulation direct-detection (IM-DD) is a promising scheme due to its simple architecture. The fundamental challenge associated with direct-detection is the disappearance of the transmitted signal's phase. In access network, the flexibility and low latency are the two key factors affecting service experience. In this article, we review the evolution of PONs and the challenges of current PONs in detail. We analyze key enabling digital signal processing (DSP) techniques, including detection linearization for direct-detection and simplified coherent detection, adaptive equalizers, digital filer enabled flexible access network and low-latency inter-ONU communications. Finally, we discuss the developing trends of future optical access networks.

Photonics-Assisted Millimeter-Wave Communication System Based on Low-Bit Gaussian Mixture Model Adaptive Vector Quantization

The quantization technique with a low-bit resolution can significantly reduce the cost and power consumption of analog-to-digital converter (ADC). It will play an important role in energy conservation and cost reduction for the incoming B5G millimeter-wave (MMW) communication systems. In this paper, we propose and demonstrated experimentally a low-bit Gaussian mixture model (GMM) based non-uniform adaptive vector quantization (AVQ) scheme for the low-cost intensity modulated envelope detection photonics-assisted 28 GHz MMW communication system for the first time. The principles of GMM-based one-dimensional adaptive scalar quantization (ASQ) and multi-dimensional AVQ are first introduced, and then are used to realize the low-bit non-uniform adaptive quantization for reducing the ADC bit resolution of MMW receiver. Furthermore, the performance of traditional uniform quantization, the present <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</i> -means and proposed GMM-based non-uniform ASQ/AVQ schemes are evaluated and compared in detail. Utilizing the proposed GMM-based AVQ scheme, the ADC quantization resolution in our MMW receiver can be reduced from 5 bits of the traditional uniform quantization to as low as 2 bits, without noticeable performance penalty. Moreover, as compared with the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</i> -means-based quantization scheme, the MMW receiver enabled by GMM-based ASQ/AVQ scheme can save about half of the quantization time under similar performance. This is mainly because the clustering based on probability converges faster than the Euclidean distance, which significantly reduces the number of iterations required. Therefore, the GMM-based AVQ scheme is a promising solution to realize high performance ADCs with low-bit resolution for future MMW-enabled optical fiber wireless access networks.

Dual-physical layer high-security encryption scheme for seven-core fiber FPS-OFDM-PON.

This paper proposes an encryption scheme for floating probabilistic shaping orthogonal frequency division multiplexing passive optical networks (FPS-OFDM-PON). Four chaotic sequences are generated by the 4D hyperchaotic model for floating probabilistic shaping (FPS) and bubble sort encryption scheme. An experiment is conducted to demonstrate the transmission of a 70Gb/s (7×10Gb/s) FPS-OFDM-PON signal across a 2km weakly coupled 7-core fiber. The keyspace of the 4D hyperchaotic model reaches 10120. The results show that a 1.82 dB gain in receiver sensitivity compared with the conventional uniform 16QAM-OFDM due to the introduction of FPS. When the system is assaulted by an unlawful receiver, the bit error rate (BER) can still remain at 0.49, successfully assuring the system's security. Due to its good transmission and security performance, the scheme has important application prospects in the future optical access network.

Demonstration of the high-security 3D-CAP scheme in multi-core fiber transmission

In this paper, the high-security three-dimensional (3D) carrier-less amplitude-phase (CAP) modulation based on 3D space constellation masking in multi-core fiber (MCF) transmission is proposed. Logistic Map model and Four-wing hyper-chaotic systems are employed to implement bit-level encryption, displacement masking, and space masking. This way, we achieved a key space of 1 × 1083 size, which resists violent attacks, and improves the physical layer security of the system. The superposition combined two chaotic models reduces the difficulty of implementation and enables more security for constellation encryption. A 280 Gb/s encrypted 3D-CAP32 signal transmission over 2.5 km MCF is experimentally demonstrated. The experimental results show that the proposed scheme can effectively resist illegal attack from an un-authorized optical network unit (ONU), and compared with an unencrypted 3D-CAP32 signal, the encrypted 3D-CAP32 signal has a receiver sensitivity penalty of 0.38 dB in a back-to-back configuration at a BER of 1 × 10−3. It is a strong candidate for the future highly-secured optical access network.

A 7D Cellular Neural Network Based OQAM-FBMC Encryption Scheme for Seven Core Fiber

This paper proposes a 7-dimensional ( <xref ref-type="disp-formula" rid="deqn6-deqn7" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7D</xref> ) Cellular Neural Network (CNN) based offset quadrature amplitude modulation filter bank multicarrier (OQAM-FBMC) encryption scheme for seven core fiber. The chaotic sequences generated by 7D CNN are applied to produce the masking vectors to encrypt the phase, carrier frequency, and time. In order to verify the performance of the encryption scheme, 70 Gb/s (7×10 Gb/s) encrypted OQAM-FBMC signal transmission over 2 km 7 core fiber is experimentally demonstrated. The key space of 7D CNN can reach 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1575</sup> and the scrambling degree can be maintained at 100% regardless of the number of symbols. The experimental results also show that when some keys are compromised, the system's bit error rate (BER) can still reach above 0.46, which effectively ensures the security of the system. Due to its good performance in security, the proposed scheme has important application prospects in future optical access network.

Estimation of four wave mixing effect in pre-, post- and in-line configuration of EDFA using advanced modulation formats for 1.28 Tbps WDM system

This paper reported the effect of different EDFA configurations on the four wave mixing (FWM) effect in wavelength division multiplexed (WDM) system which is operating at 1.28 Tbps data speed. Also, the comparative analysis of pre-, post (booster) and inline configuration of EDFA using both forward pumping and backward pumping techniques at variable input signal power and pump signal power levels. Further, the effect of various modulation formats on the signal quality of WDM system is analyzed in presence of dispersion compensated fiber. The original contribution of this investigation is to check the effect of EDFA amplification on FWM for future high-speed optical access networks and it can be observed that in line EDFA amplification configuration gives the best amplification performance among other configurations.

Mitigation of Nonlinear Effects using Machine Learning in Coherent Optical Access Networks

Introduction/Context: The use of coherent detection jointly with high-level modulation formats such as 16 and 64-QAM seems to be a convenient strategy to increment capacity of future optical access networks. However, coherent detection requires high complexity digital signal processing to mitigate different impairments. Objective: Mitigate signal distortions using nonsymmetrical demodulation techniques based on Machine Learning (ML) algorithms. Methodology: First, a single channel Nyquist m-QAM system at 28 and 32 Gbps was simulated in VPIDesignSuite software. Then, different signals modulated at 16 and 64-QAM were generated with different laser linewidth, transmission distances and launch powers. Two ML algorithms were implemented to carry out the demodulation of the generated signals. The performance of the algorithms was evaluated using the bit error rate (BER) in terms of different system parameters as laser linewidth, transmission distance, launch power and modulation format. Results: The use of ML allowed gains up to 2 dB in terms of optical signal-to-noise ratio at a BER value of for 16-QAM and 1.5 dB for 64-QAM. Also, the use of ML showed that it is possible to use a lower cost laser (100 kHz linewidth vs 25 kHz) with a better BER performance than using conventional demodulation. Conclusions: We showed that the use of both algorithms could mitigate nonlinear effects and could reduce computational complexity for future optical access networks.

Performance investigation of PM-based wavelength remodulation scheme in bidirectional TWDM-PON

Abstract Signal remodulation mechanism is a promising cost-effective choice for future optical access networks. This paper investigates the scheme of the phase modulator to remodulate upstream data in the bidirectional WDM PON and TWDM PON system. The simulation results of downstream is carried out using two data formats such as RZ and NRZ at varying lengths of single mode fibers in this scheme. In addition, the upstream transmission compares different remodulators such as Phase modulator, Mach–Zehnder modulator (MZM) and Electroabsorption modulator. By analyzing the eye diagram and Q-value, the performance of upstream and downstream transmission is obtained. It is observed that the PM remodulation TWDM-PON with NRZ modulation format has better performance, which is a rational solution for NG-PON2 cost-effectiveness.

Chaotic constant composition distribution matching for physical layer security in a PS-OFDM-PON

This paper proposes a probabilistic shaping orthogonal frequency division multiplexing passive optical network (PS-OFDM-PON) based on chaotic constant composition distribution matching (CCDM). With the implementation of a four-dimensional hyperchaotic Lv system, probabilistic shaping and chaotic encryption are realized with low complexity on the process of signal modulation, so as to enhance the system performance in the presence of bit error rate (BER) and security. An 8.9 Gb/s encrypted PS-16 quadrature amplitude modulation (QAM)-OFDM signal transmission over a 25 km standard single mode fiber (SSMF) is experimentally demonstrated. And experimental results indicate that compared with conventional uniform 16QAM-OFDM, the encrypted PS-16QAM-OFDM can obtain a 1.2 dB gain in receiver sensitivity at a BER of 10-3 under the same bit rate. Moreover, the key space of the proposed scheme is 1.98 × 1073, which is a large enough number to effectively guard against any malicious attacks from illegal optical network units (ONUs). The combined superiority of BER and security performance enables a promising prospect for the proposed PS chaotic encryption scheme in a future low-cost optical access network.

Investigation of 340-Gbps terrestrial FSO link incorporating spectral-efficient DP-QPSK-PolSK hybrid modulation scheme

A high-capacity spectral-efficient dual-polarization quadrature phase-shift keying (DP-QPSK)-polarization shift-keying (PolSK) hybrid modulation scheme for terrestrial free-space optics (FSO) transmission link is proposed and investigated. A DP-QPSK signal modulated at 300 Gbps and a PolSK signal modulated at 40 Gbps are simultaneously transmitted using a single optical carrier over the FSO link. The proposed link performance is investigated under different weather conditions, where the bit error rate metric is used to evaluate the performance of the PolSK modulated signal and the error vector magnitude parameter is used for the DP-QPSK signal. The FSO link range and the required received power are carefully explored. The conducted numerical simulations of the proposed system showed reliable 340-Gbps data transmission over link ranges varying from 1.6125 to 50 km depending on the weather conditions. The impact of the channel scintillation due to atmospheric turbulence is also investigated. The proposed high-speed FSO transmission system offers a promising solution for high-bandwidth hungry systems used for the internet of things, 5G, and smart cities. It can also be used in developing fronthaul/backhaul links for future wireless networks and optical access networks. The performance of the proposed transmission system is compared with recently published work in the literature.

Probabilistically shaped amplitude and pulse position modulation based on stepped amplitude probability distribution for optical access networks

AbstractWe propose a novel probabilistically shaped amplitude and pulse position modulation (APPM) based on stepped amplitude probability distribution for optical access networks. By adopting the probabilistic shaping (PS) technique, average signal power can be reduced and better bit error rate (BER) performance obtained. The proposed PS‐APPM scheme has been successfully verified in a 25‐km standard single‐mode fiber (SSMF). Experiment results show that at a BER of 1 × 10−3, the proposed 4 × 16 APPM‐PS1 and 4 × 16 APPM‐PS2 outperform the traditional 4 × 16 APPM by 1.54 and 2.52 dB receiver sensitivity improvement, while the proposed 8 × 8 APPM‐PS1 and 8 × 8 APPM‐PS2 excel the traditional 8 × 8 APPM by 2.1 and 3.5 dB at a BER of 1 × 10−2, suggesting the advanced nature of the proposed scheme for future optical access networks.

Analysis of all-optical wavelength converter based on FWM effect in HNLF for coherent 100 Gbps dual-polarized DQPSK signal

All optical wavelength converter (AOWC) play important functionalities to enhance the network reconfiguration, efficient wavelength routing and assignment, non-blocking capacity in future dynamic optical access networks. In optical access networks, bandwidth limitation and wavelength continuity constraints lead to inefficient utilization of wavelength resources. AOWCs are the premier option to cope with these network constraints. On the other hand, advanced modulation formats have been extensively analyzed and used to achieve the high spectral efficiency. Since advanced modulation formats are more sensitive to amplitude and phase noise, AOWC are more suitable to achieve minimum power penalty and maximum conversion efficiency. In this paper, AOWC based on four wave mixing (FWM) in highly nonlinear fiber (HNLF) for coherent 100 Gbps dual-polarized differential quadrature phase shift keying (DP-DQPSK) has been designed. The performance of AOWC is optimized at different probe input power, different pump power and wavelength to achieve the maximum conversion efficiency. After the multiwavelength conversion based on 0.7 km HNLF, the power penalty for converted 100 Gbps DP-DQPSK signals is less than 4.15 dB at BER of 10-9 and maximum conversion efficiency of −10.58 dB has been achieved.