Time And Wavelength Division Multiplexed Passive Optical Network Research Articles

تأثير ثلاث خطط مختلفة على المرحلة الثانية من الشبكات الضوئية في الجيل القادم

Abstract: In this work, a simulative analysis of 40 Gb/s downstream time and wavelength division multiplexing-passive optical network (TWDM-PON) for next-generation passive optical network second stage (NG-PON2) had been designed and demonstrated. The performance of three different wavelength plans in TWDM-PON are evaluated as the solution to NG-PON2 to support 40Gbit/s bandwidth requirement. A four 10-Gb/s TWDM-PON signals are stacked using four different wavelengths in the downstream direction and externally modulated. The system performance is observed using Bit Error Rate (BER) and Optical Signal-to-Noise Ratio (OSNR) with respect to the design parameters such as distance, number of users and received optical power. Thus, the proposed system was able to support 512 number of users and 80 km transmission distance. When the system is tested upon transmission of triple play services over 80km distance with 512 users, it is shown that BER of 8.9 × 10-10, 3.9 × 10-9 and 1.3 × 10-4 are achieved for data, voice and video, respectively.

Joint Resource Allocation in TWDM-PON-Enabled Cell-Free mMIMO System

Cell-free massive multiple input multiple outputs (CF-mMIMO) is considered a promising technology for sixth-generation (6G) telecommunication systems. In the CF-mMIMO system, an extensive array of distributed small base stations (BSs) is deployed across the network, which enables us to facilitate seamless collaboration among BSs. To achieve this goal, the baseband signal from these BSs needs to be transmitted to a central server via fronthaul networks. Due to the large number of BSs, the data that needs to be transmitted is usually huge, which brings severe requirements on fronthaul networks. Time and wavelength division multiplexed passive optical networks (TWDM-PON) can be a potential solution for CF-mMIMO fronthaul due to their large capacity and high flexibility. However, how to efficiently allocate both optical and wireless resources in a TWDM-PON-enabled CF-mMIMO system is still a problem to be addressed. This paper proposes a joint scheduling method of wavelength, antenna, radio unit (RU), and radio resource block (RB) resources in the TWDM-PON-enabled CF-mMIMO system. Furthermore, an integer linear programming (ILP) model for joint resource allocation is proposed to minimize the fronthaul resource occupancy, thereby increasing network scalability. Considering the complexity of the ILP model, two heuristic algorithms are also presented to solve this model. We compare the ILP with heuristic algorithms under different scenarios. Simulation results show that the proposed algorithm can reduce the fronthaul resource occupancy to improve the network scalability of the CF-mMIMO system.

Real-time batch processing at a GPU-based edge with a passive optical network

In recent years, advances in deep learning technology have significantly improved the research and services relating to artificial intelligence. Real-time object recognition is an important technique in smart cities, in which low-cost network deployment and low-latency data transfer are key technologies. In this study, we focus on time- and wavelength-division multiplexed passive optical network (TWDM-PON)-based inference systems to deploy cost-efficient networks that accommodate several network cameras. A significant issue for a graphics processing unit (GPU)-based inference system via a TWDM-PON is the optimal allocation of the upstream wavelength and bandwidth to enable real-time inference. However, an increase in the batch size of the arrival data at the edge servers, thereby ensuring low-latency transmission, has not been considered previously. Therefore, this study proposes the concept of an inference system in which a large number of cameras periodically upload image data to a GPU-based server via the TWDM-PON. Moreover, we propose a cooperative wavelength and bandwidth allocation algorithm to ensure low-latency and time-synchronized data arrivals at the edge. The performance of the proposed scheme is verified through a computer simulation.

A Dynamic Restructuring Algorithm Based on Flexible PON Slices

In recent years, with the introduction of the concept of the Internet of Things, a large number of terminals connected to the network, the pressure of network bandwidth is increasing. The bandwidth resources wasted by the traditional fixed access network architecture have attracted more and more attention of researchers. In order to meet the different needs of different users for service quality and improve the flexibility of network, network slicing technology arises at the right moment. Based on the slicing idea of the flexible time- and wavelength-division multiplexing passive optical network (TWDM-PON), a dynamic PON slice restructuring algorithm (DRA) is proposed in this paper. The proposed algorithm avoids the influence of previous slicing on subsequent slicing in the step-by-step slicing process, slices at the global level, and is less affected by the randomness of initialization. The simulation results show that the performance of DRA is about 10~30% higher than that of the dynamic optical network units (ONUs) grouping algorithm (DGA) and the dynamic ONU slicing algorithm (DONUSA) when there are 8 OLTs, and is about 30% higher than that of DGA and 10% higher than that of DONUSA when there are 16 OLTs. Therefore, the proposed DRA has more positive significance to relieve the traffic pressure in the increasingly tight bandwidth resources.

DRL-assisted joint allocation of antenna, radio, and front-haul resources in TWDM-PON-based NG-RANs with mMIMO-enabled beamforming

The high spectrum efficiency of massive multiple input multiple output (mMIMO)-enabled beamforming is attractive for solving the massive number of connections in next-generation radio access networks (NG-RANs). However, the beamforming based on multiple antenna sub-arrays to support multi-connections would introduce extra 2D antenna sub-array selection and radio resource block (RB) allocation; meanwhile, considering the front-haul bandwidth consumption would reduce as much as possible the redundant data transmitting over front-haul in the NG-RANs. Therefore, a key issue is how to coordinate 2D antenna sub-array selection and radio RB allocation to minimize the front-haul bandwidth and maximize the radio RB utilization when accommodating a set of beam antenna array (BAA) requests. In this paper, we first establish a 3D BAA mapping model to jointly optimize 2D antenna sub-array selection and radio RB allocation, which would in turn affect the allocation of the front-haul wavelength bandwidth in a time and wavelength division multiplexed passive optical network (TWDM-PON)-based front-haul with the mMIMO antenna system. An integer linear programming mathematical model is formulated, and a novel deep reinforcement learning (DRL)-based algorithm is proposed to optimize the front-haul bandwidth and radio RB utilization. Besides, two heuristic algorithms are developed with different antenna sub-array selection policies as the benchmarks. The extensive simulation results show that our proposed DRL-based algorithm can attain the lowest average cost (AC) for different numbers of BAA requests considered by seeking the optimal trade-off between the front-haul bandwidth consumption and the number of the used antennas. Compared with the inter-sub-array benchmark, the DRL-based algorithm can achieve up to 7.5% AC reduction, which is mainly attributed to the 21.2% reduction of the front-haul bandwidth without increasing the number of used antennas.

Design and investigation of hybrid PON‐FSO system employing modified NZCC code under distinct weather conditions

AbstractIn this paper, a hybrid time and wavelength division multiplexing passive optical network (TWDM‐PON) and wavelength division multiplexing optical code division multiple access (WDM‐OCDMA) with free space optics (FSO) for bidirectional 160/40 Gbps transmission has been proposed. Also, to offer the information to 64 users over 60 km fiber and 600 m FSO channels under diverse climate conditions have been presented. Also, to resist the fiber impairments, interference, noise and losses under the effect of unfavorable climate conditions in hybrid fiber‐FSO link, a modified new zero‐cross correlation (NZCC) code is effectively used in the system to enhance the system ability to access shared bandwidth with users' security. The performance of the designed PON system has been analysed and investigated for diverse FSO (10–600 m) and fiber distance (0–60 km) under distinct climate conditions. Based on mathematical and simulation analysis, the designed system using modified NZCC code is shown to provide better performance than other OCDMA codes. The designed system with hybrid fiber‐FSO link improves the performance of the system in terms of high received optical power of 3 dBm and support 110 users simultaneously at acceptable BER of 10−9, under fiber impairments, climate conditions and turbulences. Additionally, on comparing the designed system's performance with the prior latest work in literature reveals its superiority than others in codes and system design.

Design of Cost-Efficient Optical Fronthaul for 5G/6G Networks: An Optimization Perspective

Currently, 5G and the forthcoming 6G mobile communication systems are the most promising cellular generations expected to beat the growing hunger for bandwidth and enable the fully connected world presented by the Internet of Everything (IoE). The cloud radio access network (CRAN) has been proposed as a promising architecture for meeting the needs and goals of 5G/6G (5G and beyond) networks. Nevertheless, the provisioning of cost-efficient connections between a large number of remote radio heads (RRHs) in the cell sites and the baseband unit (BBU) pool in the central location, known as the fronthaul, has emerged as a new challenge. Many wired and wireless solutions have been proposed to address this bottleneck. Specifically, optical technologies presented by passive optical networks (PONs) are introduced as the best suitable solution for 5G and beyond network fronthaul due to their properties of providing high capacity and low latency connections. We considered time and wavelength division multiplexed passive optical networks (TWDM-PONs) as a fronthaul for 5G and beyond. Taking that into consideration, in this paper, we propose an integer linear program (ILP) that results in the optimal optical fronthaul deployment while minimizing the total cost of 5G and beyond instances. However, for larger network instances, solving the ILP problem becomes unscalable and time-consuming. To address that, we developed two heuristic-based algorithms (the K-means clustering algorithm and the one based on the genetic algorithm-GA). We evaluated the suitability of our proposed ILP and heuristic algorithms in simulations by utilizing them to plan different network instances (dense and sparse).

Real-Time Resource Allocation in Passive Optical Network for Energy-Efficient Inference at GPU-Based Network Edge

In recent years, the advances in deep learning (DL) technology have greatly improved artificial intelligence (AI)-related research and services. Among them, real-time object recognition using network cameras has become an important technology for various applications. A large number of network cameras are being deployed for real-time object detection using DL models at GPU-based edge servers. A significant issue for widely deploying this type of systems is low-cost network deployment and low-latency data transmission. A promising option for efficiently accommodating numerous network cameras is time- and wavelength-division multiplexed passive optical network (TWDM-PON), which has prevailed in optical access network systems. The key challenge in a GPU-based inference system via TWDM-PON is to optimally allocate upstream wavelengths and bandwidths to enable real-time inference. To address this problem, this article proposes the concept of an inference system in which many cameras upload image data to a GPU-based edge server via TWDM-PON. A real-time resource allocation scheme for TWDM-PON is also proposed to guarantee low latency and time-synchronized data arrival at the edge. We formulated the wavelength and bandwidth allocation problem as a Boolean satisfiability problem (SAT) for fast computation. The performance of the proposed method is verified by computer simulation. The proposed scheme contributes to the increase in the batch size of arriving data at the edge server while ensuring low-latency data transmission. As a consequence, the computational efficiency of the GPU-based inference server is greatly improved by the increase in the batch size of data.

Single-Lane 54-Gbit/s PAM-4/8 Signal Transmissions Using 10G-Class Directly Modulated Lasers Enabled by Low-Complexity Nonlinear Digital Equalization

This work presents the use of a directly modulated laser (DML) in time- and wavelength division multiplexed passive optical networks (TWDM-PONs), in which an EDFA-based booster amplification and an SOA-based pre-amplification are utilized to improve the optical power budget. We experimentally demonstrate the C-band optically amplified data transmissions and compare the performance of the non-return-to-zero on-off keying (NRZ-OOK), four and eight-level pulse amplitude modulation (PAM-4/PAM-8) using a 10-G class DML. With optical amplification, the uplink and downlink power budgets of about 28 dB and 35 dB, respectively, are achieved for the PAM-4 54-Gbit/s signals at the hard-decision forward error correction (HD-FEC) limit of 3.8x10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> after transmissions over a 25-km single-mode fiber link. To mitigate waveform distortions caused by the limited bandwidth, nonlinear dynamics, memory effects, and strong laser frequency chirp of the DML, especially during multi-level signal modulations, artificial neural network-based machine learning equalizers and low-complexity Volterra nonlinear equalizers are applied, which operate at the signal baud rate. The bit error ratio performance in conjunction with an enhanced power budget through the use of a low-complexity nonlinear equalizer can justify the validity of using a DML in the next-generation PONs.

Investigation of transmission properties of a practical double exponential pulse for communication and sensing application

In this paper, a simple generation technique and the propagation characteristics, of a practically possible double exponential pulse are proposed and numerically analyzed. Solutions of non-linear Schrödinger's equation (NLSE) has mostly been used to analyze the same in comparison with Gaussian ultrashort pulses, for various classes of optical fibers. The Double exponential pulses have shown a bandwidth-efficiency ~ 23% over Gaussian pulses, making it suitable for time-and-wavelength-division-multiplexed passive optical networks (TWDM-PON) applications in optical communication, as recommended by ITU-T G-989. Non-linear behavior of the pulse was also investigated for super-continuum (SC) generation. Super-continuum generated by Double exponential pulses was found to have ~12% higher Spectral Broadening factor compared to Gaussian pulse. Overall, the proposed double exponential pulse, generated by the proposed method, potentially displays comparable, and sometimes better sensing and communication properties, compared to the Gaussian pulses.

Performance analysis of symmetrical and bidirectional 40 Gbps TWDM-PON employing m-QAM-OFDM modulation with multi-color LDs based VLC system

In this work, a full-duplex time and wavelength division multiplexing-passive optical network (TWDM-PON) system is analysed. Orthogonal frequency division multiplexing (OFDM) with m-quadrature amplitude modulation (m-QAM) is employed to improve the performance of TWDM-PON for downstream and upstream transmission. Simultaneously, multi-color (390–750 nm) laser diodes (LDs) are employed for visible light communication (VLC) using various VLC links to encourage the information rate of fiber/VLC optical network. A TWDM-PON utilizing 16-, 32- and 64-QAM OFDM with ten LDs based VLC system has been analysed for full-duplex multi-color VLC signals of the system. The impact of the LDs input current and high transmission rate in the proposed PON/VLC link has been investigated for m-QAM OFDM modulation. The results show that the 40/40 Gbps 16-, 32- and 64-QAM signals over ten 8000 m VLC links and a 50 km fiber link are successfully transmitted at the modulation input current of 9 mA under bit error rate (BER) of 3.8 × 10–3. Also, the proposed system employing 16-, 32- and 64-QAM signals over a 10 km fiber and ten 10 m VLC links provide the maximum transmission rate of 120, 100 and 80 Gbps respectively. Moreover, the measured error vector magnitudes (EVMs) and calculated BER values for 16-QAM downstream and upstream signals, are well below the required FEC limit than high-order modulation formats. Further, the numerical analysis of the proposed system reveals the superiority of the proposed fiber/VLC links.

Performance Analysis of a Full-Duplex TWDM-PON Using OFDM Modulation with Red LED Visible Light Communication System

Today’s access networks are in high demand to fulfill the high bandwidth requirement because of extensive improvement in high transmission rate applications for cloud computing, big data analytics, and other next-generation 5G smart applications. In this work, the symmetrical and bidirectional time and wavelength division multiplexing-passive optical network (TWDM-PON) system is proposed and demonstrated. Orthogonal frequency division multiplexing (OFDM) with 4-level quadrature amplitude modulation is used for downstream and upstream transmission. Also, 625 nm red light-emitting diode is utilized for visible light communication (VLC) using a wireless link to encourage the economic high transmission rate fiber/VLC optical network. The TWDM-PON employing OFDM with the VLC system has been analysed for downstream and upstream channels for variable fiber-wireless link range. Additionally, the impact of a high transmission rate and usage of digital signal processing (DSP) unit to reduce losses due to fiber nonlinearity effects in the proposed link has been investigated. The results show that the maximum faithful wireless and wired range for the proposed system is 140 m and 60 km respectively at 20 Gbps to attain the minimum 3.8 × 10−3 BER. The decrease in error vector magnitude (EVM%) by almost 52% with the use of DSP in the proposed system provides a high transmission rate of 200 Gbps in the proposed link. Further, comparative performance of the proposed system with previous latest works in literature reveals a superior performance of the proposed link. The proposed TWDM-PON system provides a next-generation access network from rural areas to urban areas.

Cost-optimal architecture design for adaptive multi-stage TWDM-PON with PtP WDM overlay

Toward greenfield time and wavelength division multiplexed passive optical network (TWDM-PON) deployment in access segment, optimal network planning, and resource provisioning need to be explored in the planning phase to reduce capital expenditure (CapEx) and operational expenditure (OpEx). To address the issue, we propose an integer linear programming (ILP)-based optimization framework to design cost-efficient adaptive multi-stage TWDM-PON architecture, where an optimal number of cascaded splitting stages are used to minimize the combined expenditure comprising the CapEx and OpEx. Point-to-point WDM overlay over TWDM-PON is incorporated, wherein we reserve some wavelengths per optical line terminal (OLT) port to equip the heavily loaded (congested) optical network units (ONUs) with dedicated wavelengths. The ILP model offers the complete interconnection details from OLT to all ONUs providing the minimum combined expenditure, and selects the optimal number and locations of remote nodes (RNs) among the candidate RN locations for passive device placement, the types (either arrayed waveguide grating or power splitter) and splitting ratios of passive devices, and the number of splitting stages. Network constraints, such as the minimum guaranteed traffic rate to each ONU, the maximum number of ONUs with dedicated wavelengths per OLT port, and the maximum number of users per OLT port are taken into consideration. We compare the performance of the proposed TWDM-PON design model with the existing studies on greenfield PON design in terms of the cost savings, methodology features, and limitations. The proposed ILP model is further extended to design cost-optimal single-stage and adaptive multi-stage time division multiplexed PONs (TDM-PONs), and the expenditure for all models are estimated. We also investigate the variation in signal-to-noise ratio due to variation in the number of cascade stages. The models provide globally optimal solutions within a reasonable time bound with all constituent sub-problems and constraints jointly taken into consideration.

Joint Optical and Wireless Resource Allocation for Cooperative Transmission in C-RAN.

Cooperative multipoint transmission (CoMP) is one of the most promising paradigms for mitigating interference in cloud radio access networks (C-RAN). It allows multiple remote radio units (RRUs) to transmit the same data flow to a user to further improve the signal quality. However, CoMP may incur redundant data transmission over fronthaul network in the C-RAN. In a C-RAN employing CoMP, a key problem is how to coordinate heterogeneous resource allocation to maximize the cooperation gain while reducing the fronthaul load. In this paper, the cooperation transmission based on a multi-dimensional resource schedule (MRSCT) scheme, jointly considering user association, spectrum resource allocation, and wavelength resource allocation, is firstly envisioned in the underlying C-RAN integrating time and wavelength division multiplexing passive optical network (TWDM-PON) to maximize fronthaul efficiency. Then a two-timescale resource allocation framework including two sub-approaches is established. More specially, the first sub-approach mainly focuses on exploiting reinforcement learning to obtain a wavelength resource allocation strategy to relieve fronthaul traffic load. Moreover, the second sub-approach adopts the overlapping coalition formation game to establish a user-centric cooperative set, where spectrum resources are dynamically allocated to further alleviate the interference issue. The theoretical analysis and simulation results validate the performance of MRSCT scheme on the fronthaul efficiency, user experience, and system service capability.

Energy-Efficient vBBU Migration and Wavelength Reassignment in Cloud-Fog RAN

Cloud-Fog Radio Access Network (CF-RAN) is a new architecture that increases network capacity in Cloud RAN (CRAN) by moving some BaseBand Units (BBU) from cloud to fog nodes closer to Remote Radio Heads (RRH). However, fog nodes increases CapEx and OpEx. Moreover, tidal traffic fluctuations may lead to an energy-inefficient operation if resources becomes lightly loaded. To address this problem, BBUs of fog nodes could be dynamically activated, and following traffic fluctuations, migrated to cloud. By leveraging Network Functions Virtualization (NFV), virtualized BBUs (vBBUs) can be dynamically allocated, deallocated, and migrated from fog nodes to cloud. Moreover, considering a Time-and-Wavelength Division Multiplexed Passive Optical Network (TWDM-PON) fronthaul, traffic can be migrated among virtual Passive Optical Network (VPON) channels to optimize bandwidth usage. In this article, we propose an Integer Linear Programing (ILP) formulation and an algorithm based on its linear relaxation to solve this migration problem. Compared to an algorithm without migration capabilities, our proposals reduces blocking of RRHs demanding processing of 89% and achieves power savings of 38% by reducing activated processing resources and VPONs after migrations, while experiencing small rates of service interruption. Our relaxation-based solution approximates ILP optimality and reduces execution time of ILP up to 50 ×.

Performance Analysis of Full-Duplex NG-PON2-RoF System with Non-linear Impairments

The motivation of this study springs from the need of exploiting the advantages of high data rate and increased mobility of the transmitted signal in the Next Generation Passive Optical Network stage 2 (NG-PON2) incorporated into Radio over Fiber (RoF) system. Nonetheless, such system is prone to degradation due to the nonlinear effects, namely, the non-linear impairments. Accordingly, this study suggests a design for a full-duplex NG-PON2-RoF system and it investigates the impact of the nonlinear effects on the performance of system, such as; Self-Phased Modulation (SPM), Cross-Phase Modulation (XPM), and Four-Wave Mixing (FWM). However, these effects would result in a significant data loss in Time and Wavelength Division Multiplexing-Passive Optical Network (TWDM-PON). To achieve the intended purpose of this investigation, both the performance of the suggested system and the nonlinear effects of SPM, XPM, and FWM are measured with reference to the qualitative parameters, such as; Bit Error Rate (BER), Q-Factor, Power link Budget, and the Eye-diagram. The results show that the system performs efficiently; however, in the presence of the nonlinear effects, the performance of the systems degrades significantly affecting the signal quality. It is worth noting that the FWM effect yields the most negative impact followed by the XPM, and the SPM effect yields the least significant impact on the system. Also, the system yields the capacity of transmitting range of 20-100 Km, and it results in data rate of 40-80 Gbps. The proposed system is simulated using the software Optisystem 15.

ITU TWDM-PON module for ns-3

Optical fiber access systems are one of the driving forces behind the success of the Internet. Time- and wavelength-division multiplexing passive optical network (TWDM-PON) is regarded as the key technology for future Internet access networks. In this paper, we propose an ITU TWDM-PON module for the ns-3 network simulator and describe its concept and design specifications. The proposed module is developed based on the XG-PON module for ns-3. It can simulate G.989 standard-compliant data packet transmission in the upstream and downstream directions using multiple wavelength channels. It enables us to evaluate the performance issues that arise with the TWDM-PON development, including various dynamic bandwidth allocation and dynamic wavelength allocation algorithms. The proposed module is expected to become a good platform for studying future access networks composed of TWDM-PON and mobile networks by enabling us to simulate dynamic wavelength and bandwidth allocation.

Fiber- and FSO-Protected Connections for Long-Reach TWDM Access Architecture With Fault Protection

In this paper, we propose a long-reach star-ring-based timeand wavelength-divisionmultiplexing passive optical network (TWDM-PON) by using fiberand free space optical (FSO)-protected architecture against fiber breakpoint. Here, the new fiber-based and FSO-based optical network unit (ONU) modules are designed in the PON system to achieve the self-protected operation. Thus, the adjacent ONU is linked to produce the fiberor FSO-based reconnections. Thus, 4 × 10 Gbit/s downstream and 10 Gbit/s upstream signals can be reached through 70 km fiber transmission and a length of free space link without signal amplification and dispersion compensation. Moreover, the relationships of splitter ratio and FSO link length in the proposed TWDM access network are also analyzed and discussed.

A learning algorithm for TWDM-PON’ DWBA with 5G fronthaul networks

Abstract Time and wavelength-division multiplexed passive optical network (TWDM-PON) is expected to be the fronthaul networks of 5G, which requires low latency and large bandwidth. In this paper, we propose a Q-learning based dynamic wavelength and bandwidth allocation (DWBA) algorithm for TWDM-PON. Simulation results show that the proposed DWBA algorithm can reduce the number of active channels by 40% and provide larger bandwidth with same wavelength number in comparation with the existing algorithms while satisfying the latency requirement of 5G fronthaul networks.

Wavelength and Bandwidth Allocation for Mobile Fronthaul in TWDM-PON

Time- and wavelength- division multiplexed passive optical network (TWDM-PON) has attracted considerable attention for the next generation optical access systems. Among potential applications of TWDM-PON, a major application is the support of mobile fronthaul streams between radio units (RUs) and distributed units (DUs) in the centralized radio access network (C-RAN) architecture, which consists of central units (CUs), DUs, and RUs. The upstream fronthaul traffic that an optical line terminal (OLT) receives is expected to become highly bursty due to the variable data rate generated by employing new functional split options and the synchronization of data transmission between neighboring RUs caused by time-division duplex (TDD). However, there has been no wavelength and bandwidth allocation scheme for TWDM-PON that is designed to efficiently accommodate fronthaul streams satisfying the strict delay requirement. Therefore, in this paper we propose a novel wavelength and bandwidth allocation algorithm that can minimize the number of active wavelength channels considering the high burstiness and delay requirement of fronthaul data transmission. Through computer simulations it was confirmed that the number of active wavelength channels can be reduced by 50% with the proposed algorithm, and thus more RUs can be efficiently accommodated using TWDM-PON.