Future Access Networks Research Articles

Disaggregation and virtualization for future access and metro networks [Invited Tutorial

Future access and metro networks are expected to provide advanced broadband services and the evolution of mobile x-haul in a flexible manner. This paper first reviews the progress and challenges of disaggregation and virtualization technologies to meet this expectation with a focus on their application to optical access networks. Next, it describes future access and metro integrated networking in which disaggregation and virtualization technologies will play important roles.

Real-time demonstration of 64 × 200 Gbps UDWDM-PON downstream transmission based on silicon photonic integrated transceiver

Coherent detection, high-order modulation, and dense wavelength division multiplexing (DWDM) technologies provide solutions for increasing bandwidth demand of future access networks. We experimentally demonstrate a real-time 64 × 200-Gb/s coherent ultra-dense wavelength-division (UDWDM) coherent passive optical networks (PONs) at 75-GHz channel spacing. The demonstrated downlink transmission is realized with the dual-polarization QPSK (DP-QPSK) modulation scheme. The cost is reduced by using silicon photonic integrated coherent transceiver modules and wider grid AWGs. The power budget is evaluated when all 200 channels are launched at C band. The power budget is 26 dB after 20-km G.652D standard single mode fiber (SSMF) transmission without amplifier at the receiver side under soft-decision FEC (SD-FEC) threshold of 1 × 10−2, and can achieve 32.5 dB with pre-amplified semiconductor optical amplifier (SOA) used at the receiver side.

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).

K-means non-uniform-quantization digital-analog radio-over-fiber scheme for THz-band photonics-aided wireless fronthaul.

We propose a non-uniform-quantization digital-analog radio-over-fiber (NUQ-DA-RoF) scheme based on an advanced K-means NUQ algorithm and demonstrate it experimentally in a 2-m 300-GHz photonics-aided wireless fronthaul system. Results show that the NUQ-DA-RoF scheme achieves a SNR gain of ∼1.9 dB compared to the uniform-quantization DA-RoF (UQ-DA-RoF) at an equivalent Common Public Radio Interface equivalent data rate (CPRI-EDR). Remarkably, the NUQ-DA-RoF scheme exhibits an ∼1.6-dB power sensitivity enhancement over the UQ-DA-RoF at the 256-QAM SNR threshold. These findings highlight the advantages of the NUQ-DA-RoF scheme over UQ-DA-RoF in terms of power budget and SNR improvement, suggesting promising prospects for future radio access networks and wireless fronthaul.

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.

End-to-end deep learning for a flexible coherent PON with user-specific constellation optimization

A flexible coherent passive optical network (FLCS-CPON) is a promising solution for the future access network. By offering increased speed, sensitivity, and flexibility, it enables more efficient utilization of network resources and allows for serving a larger number of users. However, the past studies often overlook the flexibility of channel equalization. In the FLCS-CPON, customized rate optimization has been achieved to cater to users in different channel conditions. However, in addition to rate optimization, further performance improvement can be achieved by providing customized equalization methods. In this work, we proposed and demonstrated an end-to-end (E2E)-optimization-based FLCS-CPON in a 50 km fiber transmission. It enables tailored signal constellation shaping and equalization, maximizing system efficiency and performance. Finally, we achieved a FLCS-CPON with the net data rate (NDR) varied from 124 to 210 Gbps and the power budget of 40 and 42.4 dB in upstream and downstream, respectively; 3.7 and 2.9 dB power budgets are improved by E2E optimization. In burst-mode, the dynamic range of probabilistic shaping 32 quadrature amplitude modulation (PS-32QAM) at a line rate of 250 Gbps improved by 6.1 to 16.8 dB. Additionally, a dynamic range and net-rate product (DRNRP) of 5779 dB · Gbps is achieved.

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.

SNR improved digital-cascaded-pulse-code-modulation radio-over-fiber scheme supporting 16,777,216 QAM for mobile fronthaul

We propose a digital-cascaded-pulse-code-modulation radio-over-fiber (DCP-RoF) scheme and verify it experimentally in a coherent transmission system. The 50-Gbaud 2-order 6-bit DCP-RoF signal with a common public radio interface equivalent data rate of 279Gb/s/λ is successfully transmitted in a 10-km standard single-mode fiber link. Results show that the 2-order 6-bit DCP-RoF scheme achieves a maximum recovered signal-to-noise ratio (SNR) of 83.43 dB, which meets the requirements for 16,777,216-quadrature-amplitude-modulation (16,777,216-QAM) transmission. The DCP-RoF scheme has advantages over analog RoF and digital–analog RoF in terms of SNR improvement, as well as advantages over digital RoF (D-RoF) in spectral efficiency. Furthermore, an average SNR enhancement of 13.48 dB for each linearly increased bandwidth is achieved in the DCP-RoF scheme. Our work suggests many optimistic possibilities for the future radio access networks and mobile fronthaul.

Radar-Centric Photonic Terahertz Integrated Sensing and Communication System Based on LFM-PSK Waveform

The radar-centric terahertz integrated sensing and communication (THz-ISAC) is identified as a significant application in future wireless access networks. Up to date, previously reported demonstrations regarding radar sensing performance lack sufficient support in a complex environment with a strong target masking effect. This work tacks this problem by proposing a radar-centric waveform combining linear frequency modulation (LFM) waveform and phase shift keying (PSK). We first derive sensing metrics of the LFM-PSK waveform through theoretical analysis, including range resolution, peak sidelobe ratio (PSLR), and Cramér-Rao lower bound (CRLB). Then a proof-of-concept experiment on a photonics-assisted integrated sensing and communication (ISAC) system operating at 330 with 18 GHz bandwidth is conducted to verify the performance of the proposed LFM-PSK waveform. In the experiment, the proposed waveform can reach a PSLR of up to 20.9 dB and a range resolution of 1.3 cm, simultaneously accommodating a data transmission of 6 Gbit/s. In addition, the effect of embedding symbols on sensing metrics is also discussed, and by comparing the range solution and PSLR with various data rates, around <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> 6 dB gain in the PSLR without any deterioration of range resolution is observed.

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.

10 Gbit/s optical access network system based on PAM8 modulation

In terms of high-speed optical access system, an optical access network system with low-cost and low-complexity based on PAM8 modulated signal is proposed to meet the growing demand of optical access network capacity. Based on the system scheme proposed, an experimental simulation platform of 10 Gbit/s PAM8 modulated signal optical access system is built. In this scheme of the system, the intensity modulation/direct detection (IM/DD) is used to achieve the modulation and demodulation of the signal, and the entire optical access link does not use additional signal amplifier components, reducing the complexity and cost of the system. According to the experimental results, the BER of the received signal has slight changes within a certain transmission distance. And after 20 kilometers of transmission, when the BER of the received signal is close to 1e-4, the receiving sensitivity is lower than -12 dBm. The proposed optical access system has good sensitivity and has good application prospects for future broadband access network.

Suppression of Nonlinear Optical Effects in DWDM-PON by Frequency Modulation Non-Coherent Detection

We propose a simple and cost-effective method, using a direct frequency modulation (FM) and noncoherent detection (NCD) scheme, to suppress the nonlinear optical effects in dense wavelength division multiplexed (DWDM) optical communication. The FM transmitter comprises a directly modulated distributed feedback laser and a saturable semiconductor optical amplifier. In the NCD receiver, an optical slope filter as the FM to intensity modulation (IM) signal convertor is placed before a conventional photodetector. Because the FM signal has more evenly distributed optical power, bit-pattern-dependent nonlinear effects are consequently suppressed. After analyzing the nonlinear effects in the FM-NCD system and traditional IM direct detection (IM-DD) system, we found that the minimum achievable BER of the proposed FM-NCD scheme is 40 dB smaller. Moreover, a 2 Tbps (10 Gb/s × 200 channels) capacity was achieved by the FM-NCD system in 100 km DWDM passive optical networks (PONs), which is twice the capacity of the IM-DD system (10 Gb/s × 100 channels) under the same condition. These results indicate that WDM-PONs with the cost-effective FM-NCD scheme are strong candidates for future broad access networks and show great potential for the combination of optical access and metro networks for future generations of PONs.

AI in SAGIN: Building Deep Learning Service-Oriented Space-Air-Ground Integrated Networks

In next-generation mobile communications, spaceair- ground integrated networks (SAGINs) is an emerging infrastructure in future wireless access networks. Since artificial intelligence (AI) applications become more and more important, it is essential to build a deep learning service-oriented SAGINs. In this article, we present a hierarchical intelligent computing structure focusing on processing deep learning tasks in future SAGINs. An optimization strategy is also proposed to improve the quality-of-service (QoS) of deep learning tasks in the proposed structure. We test our work in small testbed and simulations. The evaluation results show that the proposed work outperforms other offloading strategies in a SAGIN environment.

Millimeter Wave Attenuation Due to Wind and Heavy Rain in a Tropical Region.

Millimeter wave fixed wireless systems in future backhaul and access network applications can be affected by weather conditions. The losses caused by rain attenuation and antenna misalignment due to wind-induced vibrations have greater impacts on the link budget reduction at E-band frequencies and higher. The current International Telecommunications Union Radiocommunication Sector (ITU-R) recommendation has been widely used to estimate rain attenuation, and the recent Asia Pacific Telecommunity (APT) report provides the model to estimate the wind-induced attenuation. This article provides the first experimental study of the combined rain and wind effects in a tropical location using both models at a frequency in the E band (74.625 GHz) and a short distance of 150 m. In addition to using wind speeds for attenuation estimation, the setup also provides direct antenna inclination angle measurements using the accelerometer data. This solves the limitation of relying on the wind speed since the wind-induced loss is dependent on the inclination direction. The results show that the current ITU-R model can be used to estimate the attenuation of a short fixed wireless link under heavy rain, and the addition of wind attenuation via the APT model can estimate the worst-case link budget during high wind speeds.

Coherent optics for access from P2P to P2MP [Invited

Coherent optics is being re-designed to future proof access networks using point-to-point and point-to-multi-point systems. Coherent optics’ higher speeds and link budgets provide significant deployment flexibility. Here, its access optimized components, coherent access architectures, operations, and service implication benefits are discussed.

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.

The concept of PICaboo: from individual building blocks to complex photonic integrated circuits for future access and metro networks

The immense growth of data traffic that traverses modern networks, calls for immediate solutions to tackle the challenges that arise in terms of speed, capacity, cost and energy efficiency. This manuscript outlines the key technological aspects of the PICaboo project, that aims to develop novel building blocks and photonic integrated circuits for the next generation of optical metro and access networks enhanced with optical signal processing functionalities exploiting the generic foundry model approach.