Ultra-dense Wavelength Division Multiplexing Passive Optical Network Research Articles

Towards high spectral efficiency UDWDM-PON based on the simplified coherent reception of pulsed shaped PAM-4 signals.

Enhancing spectral efficiency (SE) of ultra-dense wavelength division multiplexing passive optical network (UDWDM-PON) is vital to providing broadband access for massive users. Here, we experimentally demonstrate a high SE UDWDM-PON in the C-band, based on the simplified coherent reception of 10 Gb/s 4-level pulse-amplitude modulation (PAM-4) signals. We investigate the WDM signal reception by mathematical derivation and propose to enhance the SE by adopting both intradyne detection and pulse shaping techniques. Then, both approaches are numerically evaluated, with an identification that there occurs a trade-off between SE and power budget improvements. Finally, we experimentally achieve a SE of 0.83 (bit/s)/Hz and a power budget of 25 dB for a proof-of-concept 3 × 10 Gb/s PAM-4 downstream transmission over 20 km standard single mode fiber (SSMF).

A new full-duplex analog RoF transmission system for 5G/5G+ broadband mobile communication

Abstract A new cost-effective mobile fronthaul architecture has been proposed by designing promising analog radio over fiber (ARoF) transmission system compliant with the next generation ultra-dense wavelength division multiplexing passive optical network (UDWDM-PON). It is conceived for standalone 5G/5G+ mobile communication systems driven by broadband multi-input multi-output (MIMO) millimeter wave data signals using two high spectral efficient modulations. The feasibility of the proposed system was firstly proven in terms of spectral tracing supported by theoretical analysis that disclosed good agreement. Furthermore, the obtained simulation results regarding the system performance evaluation after 22 km bidirectional optical fiber links were revealed high reliable characteristics including error-free transmissions at low receiver sensitivities, and measured power penalties less than 1 dB. In addition, the recovered eye diagrams maintained wide openings with high-quality patterns.

Unequally spaced PAM-4 signaling enabled sensitivity enhancement of a simplified coherent receiver applied in a UDWDM-PON.

We experimentally demonstrate a 4 × 10 Gb/s cost-effective coherent ultra-dense wavelength division multiplexing passive optical network (UDWDM-PON) by the use of unequally-spaced 4-level pulse-amplitude modulation (UES-PAM-4) signaling. Because of the advantages of simple architecture and low cost, the simplified coherent receiver (SCR) based on the transmitted signal diversity (TS-D) has been reported, but its receiver sensitivity is constrained by the severe noise arising in the higher level of conventional PAM-4 signals. Here, we first experimentally demonstrate the UES-PAM-4 signaling for the SCR based on the TS-D, by altering the PAM-4 level spacing and the decision threshold through the gradient descent algorithm (GDA). Consequently, we can experimentally achieve -30.1 dBm RS for single wavelength at the bit-error ratio (BER) of 3.8 × 10-3. Compared with the conventional equally-spaced PAM-4 (ES-PAM-4) signaling, 1.3 dB RS enhancement can be secured after the 20-km standard single-mode fiber (SSMF) transmission. Meanwhile, the UES-PAM-4 signaling is experimentally verified for 4 × 10 Gb/s UDWDM-PON. An average RS of -29.6 dBm and 32.6 dB power budget are obtained after the 20-km SSMF transmission. The proposed UES-PAM-4 signaling with the RS enhancement is a promising candidate for the UDWDM-PON by utilizing the existing optical distribution network (ODN).

Wavelength synchronization technology for UDWDM-PON transmitter based on injection locking

We propose a concept of wavelength synchronization to ensure the stability of ultra-dense channels in an ultra-dense wavelength division multiplexing passive optical network (UDWDM-PON) transmitter. A mode-locked laser is used to provide wavelength references for users. By injection locking the semiconductor laser, the separation of the wavelength reference is realized in an optical line terminal. The downlink and uplink wavelength references are interlaced and distributed to facilitate the synchronization of uplink carriers. In the optical network unit, the uplink optical carriers are filtered by injection locking semiconductor lasers, which achieve wavelength synchronization for the uplink users. In this Letter, an adaptive wavelength synchronization transmitter for UDWDM-PON is realized with a channel spacing of 5 GHz.

Flexible coherent UDWDM-PON with dynamic user allocation based on limited-tunability lasers

Coherent wavelength division multiplexing (WDM) technologies have leveraged the optical communication systems in core networks, increasing the fiber capacity by transmission with advanced modulation formats and mitigation of impairments with digital signal processing. However, these solutions are too expensive for access networks, where cost, power budget, and footprint are limited. Hence, the key technology will be developing low-cost coherent transceivers providing an excellent selectivity and giving high sensitivity, which allows high splitting ratios. This paper reports an experimental design of a low-cost coherent ultra-dense WDM passive optical network (UDWDM-PON) with 6.25 GHz channel spacing. The users’ optical network unit (ONU) is built employing coherent transceivers with two paired low-cost distributed feedback (DFB) lasers, one as the local oscillator and another as the transmitter, offering simplicity and low-cost hardware; likewise, the optical line terminal (OLT) at the central office can profit from the same design. The ONU DFB lasers have wavelengths with limited thermal tunability, controlled by a thermo-electric cooler, which is used to allocate the wavelengths. A medium access control (MAC) at the OLT manages the spectrum channel allocation for ONUs, demanding connection when activation is requested: the OLT furnishes an optical carrier wavelength for the ONU to obtain connection by a control algorithm, assigning a down-channel and another paired up-channel assigned to the ONU DFB transmitter. The MAC can reassign the channels because of interference or collision in a dynamic wavelength allocation. Measures in an activation process and in channel reassignment have been performed in environmental conditions, including control signals and the physical parameters of DFB lasers, demonstrating the practical viability of the PON scaling from 32 up to 256 wavelength channels.

100 Gb/s (4 × 25 Gb/s) real-time coherent UDWDM-PON with a large power budget

We propose a 100-Gb/s coherent ultra-dense wavelength-division multiplexing passive optical network (UDWDM-PON) structure with a large power budget by placing an erbium-doped fiber amplifier at the optical line terminal side for amplification of optical signals in both the uplink and downlink. Then we experimentally demonstrate a real-time 4 × 25 - G b / s coherent UDWDM-PON at 12.5-GHz spacing over 50-km standard single-mode optical fiber. A high sampling rate analog-to-digital converter is designed for the 25-Gb/s real-time coherent optical module. Experimental results show that 100-Gb/s bidirectional transmission can be realized. The downlink power budget can be more than 44 dB, which can support more than 1000 users based on the proposed structure. The effects of several parameters have also been investigated in the experimental demonstration. The 24-h real-time performance evaluation for bidirectional transmission is conducted to prove the feasibility of real-time field-programmable gate array based transceivers in the proposed UDWDM-PON structure. An off-line experimental demonstration is also conducted to verify the feasibility of burst-mode operation in the uplink.

Real-Time Coherent UDWDM-PON With Dual-Polarization Transceivers in a Field Trial

We propose a coherent ultra-dense wavelength-division multiplexing passive optical network (UDWDM-PON) scheme for symmetrical operations between the uplink and downlink. Then we experimentally demonstrate a field trial of coherent UDWDM-PON using real-time field programmable gate array (FPGA)-based transceivers. The optical distribution network power budget of this system is evaluated based on 40 downlink UDWDM channels with 5 GHz channel spacing in the C band. After transmission over a 40 km field-installed fiber link, the experimental demonstration can achieve power budgets of 29 dB with dual-polarization quadrature phase-shift keying formats at 10 Gb/s for each user. Twenty-four hour real-time performance testing for the field-trial transmission is also conducted to evaluate the feasibility of the real-time FPGA-based transceivers in the proposed UDWDM-PON scheme. Several parameters that may affect system performance are also investigated in the experimental demonstration.

1.25–2.5 Gb/s Simple Nyquist Transmitters for Coherent UDWDM-PON with Enhanced Spectral Efficiency

ABSTRACTWe present a coherent ultra-dense wavelength division multiplexing passive optical network (UDWDM-PON) with enhanced spectral efficiency. The benefit of using Gaussian or Nyquist pulse-shaping filters at the transmitter is evaluated through numerical simulations and experiments. The transmitters consist of directly phase modulated distributed feedback (DFB) lasers through beat signals whose duty-cycle and amplitude are digitally adjusted. The results show that transmitting a Nyquist-shaped signal achieves a 25% spectral saving allowing to place 2.5 Gb/s/user data in 6.25 GHz channels. Furthermore, the proposed transmitter tolerates differential link-losses of 15 dB at Rx sensitivity of ‒44 dBm at BER=10‒3 with intradyne detection.

15-dB Differential Link-Loss UDWDM-PON With Direct Beat Phase Modulated DFBs

A 15 dB differential link-loss ultra-dense wavelength division multiplexing passive optical network (UDWDM-PON) with two optical network units (ONU) spectrally spaced 6.25 GHz is experimentally implemented and tested. The ONU transmitters consist of direct phase modulated distributed feedback lasers through a digital beat signal, whose amplitude and duty cycle are optimized for maximum phase variations, avoiding the need for an analogue equalizer. We achieved receiver sensitivities of −53, −50.5, and −45 dBm for bit rates of 1.25, 2.5, and 5 Gb/s, respectively, at BER $= 4~\cdot ~10^{-3}$ with an intradyne coherent receiver.

First demonstration and field trial on multi-user UDWDM-PON full duplex PSK-PSK with single monolithic integrated dual-output-DFB-SOA based ONUs.

We demonstrate a monolithically integrated dual-output DFB-SOA, and conduct the field trial on a multi-user bidirectional coherent ultradense wavelength division multiplexing-passive optical network (UDWDM-PON). To the best of our knowledge, this is the first achievement of simplified single integrated laser-based neighboring coherent optical network units (ONUs) with a 12.5 GHz channel spaced ultra-dense access network, including both downstream and upstream, taking the benefits of low footprint and low-temperature dependence.

Cost-effective coherent ONU transceiver based on single directly modulated laser.

A cost-effective structure is proposed for the optical network unit (ONU) transceivers in coherent ultra-dense wavelength division multiplexing passive optical network (UDWDM-PON), which is based on a single directly modulated laser (DML). This is the first time that a DML is used as both optical transmitter in upstream and local oscillator (LO) for coherent detection in downstream. The impact of extinction ratio (ER) of signal from DML is investigated and optimized by adapting the driving amplitude and bias of DML. Each UDWDM grid accommodates a pair of bi-directional signal, where heterodyne detection is used due to the Rayleigh backscattering (RB) from the bi-directional transmission. The impact of frequency offset (FO) between upstream and downstream signal is also investigated. Finally, 2.5-Gb/s bi-directional transmission of OOK signal over 60-km SSMF is experimentally demonstrated within the 12.5-GHz grid, achieving about -43 and -45.5 dBm receiver sensitivity in the downstream and upstream, respectively.

A High Spectral Efficiency Coherent RoF System Based on OSSB Modulation With Low-Cost Free-Running Laser Sources for UDWDM-PONs

A coherent radio over fiber (RoF) system based on optical single-sideband modulation with no optical carrier (OSSB) for ultradense wavelength division multiplexing passive optical networks (UDWDM-PONs) is proposed and experimentally demonstrated. For one channel of the UDWDM-PONs, at the transmitter, the OSSB RoF signal is generated using a dual parallel Mach–Zehnder modulator (DP-MZM) with a real-valued precoded microwave vector signal which is applied to the DP-MZM via the two electrodes with one having a 90° phase shift. Then, the OSSB RoF signal is sent to a coherent receiver over a single-mode fiber (SMF). The coherent receiver with a free-running laser source as a local oscillator (LO) is used to perform the coherent detection. To recover the standard microwave vector signals from the real-valued precoded microwave vector signal that is embedded in a strong phase noise introduced by both the transmitter laser source and the LO laser source, a digital signal processing algorithm is developed to perform phase noise cancellation. An experiment is performed. The transmission of a 1.25-Gbps quadrature phase shift keying, a 1.875-Gbps eight-phase shift keying (8-PSK), and a 2.5-Gbps 16 quadrature amplitude modulation (16-QAM) microwave vector signals with a channel spacing as narrow as 3 GHz over a 25-km SMF is experimentally demonstrated. For the transmission of the 2.5-Gbps 16-QAM microwave vector signal, the received optical sensitivity at forward error correction level over a 25-km SMF link is −24.8 dBm, while for the 1.875-Gbps 8-PSK microwave vector signal, it can reach −29.8 dBm which leaves an enough margin to accommodate the splitting losses in the optical distribution networks.

Performance and Complexity of Digital Clock Recovery for Nyquist UDWDM-PON in Real Time

We analyze the performance and the complexity of a real-time digital signal processing (DSP)-based optical network unit architecture for the digital detection of Nyquist-shaped signals in ultra-dense wavelength-division multiplexing passive optical network (UDWDM-PON) scenarios. Using field-programmable gate array-based 8-bit DSP, we experimentally demonstrate in a real-time mode operation the receiver performance in terms of Nyquist filter roll-off factor for $8\times 2.5$ Gb/s quadrature phase-shift keying (QPSK) with 2.5 GHz of channel spacing. We analyze the performance of the linear and cubic interpolation for the clock recovery as well as the Gardner algorithm for the timing-error estimation, considering the amplitude and power formulations. The experimental evaluation shows that the linear interpolation along with the power formulation is an attractive solution for the QPSK-based UDWDM-PON, considering the tradeoff between the performance and the complexity.

Coherent ultra dense wavelength division multiplexing passive optical networks

In this paper, we firstly review the progress in ultra-dense wavelength division multiplexing passive optical network (UDWDM-PON), by making use of the key attributes of this technology in the context of optical access and metro networks. Besides the inherit properties of coherent technology, we explore different modulation formats and pulse shaping. The performance is experimentally demonstrated through a 12×10Gb/s bidirectional UDWDM-PON over hybrid 80km standard single mode fiber (SSMF) and optical wireless link. High density, 6.25GHz grid, Nyquist shaped 16-ary quadrature amplitude modulation (16QAM) and digital frequency shifting are some of the properties exploited together in the tests. Also, bidirectional transmission in fiber, relevant in the context, is analyzed in terms of nonlinear and back-reflection effects on receiver sensitivity. In addition, as a basis for the discussion on market readiness, we experimentally demonstrate real-time detection of a Nyquist-shaped quaternary phase-shift keying (QPSK) signal using simple 8-bit digital signal processing (DSP) on a field-programmable gate array (FPGA).

Very-High-Throughput Coherent Ultradense WDM-PON Based on Nyquist-ISB Modulation

We propose and experimentally demonstrate a very-high-throughput coherent ultradense wavelength division multiplexing passive optical network (UDWDM-PON) with its bandwidth efficiency enhanced using Nyquist-independent-sideband (N-ISB) modulation. The proposed optical line terminal (OLT) suggests 75 downlink optical subcarriers. For each optical subcarrier, there are as many as 16, 10-Gb/s N-ISB digital channels modulated on it, which can be efficiently generated using only one set of optical transmitter hardware. Experimental results show that the proposed UDWDM-PON is capable of accommodating 1200 10G users within an optical distribution network loss budget of 35.5 dB, and achieve a total throughput of 12 Tb/s and a spectral efficiency of 5.628-b/s/Hz. More importantly, the proposed system with spectral- and bandwidth-efficient N-ISB requires 16 times less downlink optical transmitters at the OLT, which can be translated into savings in cost, power consumption, and footprint. In addition, key techniques, such as N-ISB modulation in a PON system, frequency-locked optical multicarrier generation, and digital pre-equalization, are also studied.

A bandwidth-efficient channel allocation scheme for mitigating FWM in ultra-dense WDM-PON

In this paper, we investigate four-wave mixing (FWM) effects in the ultra-dense wavelength division multiplexing passive optical network (UDWDM-PON) system and propose an efficient channel allocation scheme to mitigate the FWM impact. This scheme is formed by grouping signal channels into several blocks with different channel spacing. Through numerical analysis and simulations, our proposed scheme is verified to be able to reduce the FWM effects and have higher bandwidth efficiency than the traditional unequal channel allocation scheme. The simulation results also demonstrate that our proposed scheme can achieve nearly 4dB increases in optical power budget of the UDWDM-PON systems at the BER of 1e−3, in comparison with the equal spaced channel scheme.

Radio-Over-Fiber Access Architecture for Integrated Broadband Wireless Services

This paper introduces two radio-over-fiber (RoF) architectures for the future broadband optical-wireless access network-all-band RoF and band-mapped 60-GHz RoF that can be integrated in ultra-dense wavelength division multiplexing passive optical network (UDWDM-PON). Legacy wireless services and multi-gigabit millimeter-wave (mm-wave) applications are integrated and delivered simultaneously under one shared infrastructure. With centralized system control and signal processing, the proposed systems provide cost-effective and protocol-transparent solutions for the next-generation multi-service bundle in heterogeneous networks (HetNets). In the all-band RoF network where wireless services are kept at their original carrier frequencies, Wi-Fi, WiMAX, and 60-GHz high-speed mm-wave services are transmitted based on subcarrier multiplexing (SCM) and dual-wavelength heterodyne beating techniques in avoidance of optical filters and large-bandwidth optoelectronic components. In the indoor environment, the band-mapped mm-wave RoF design is illustrated with real-time analog television signal, Wi-Fi, and high-speed digital baseband data-all of which are transmitted over unified optical and air links. By mapping various wireless signals into 60-GHz sub-bands, the novel architecture achieves higher spectral efficiency and lower power consumption.

8$\,\times\,$9.95-Gb/s Ultra-Dense WDM-PON on a 12.5-GHz Grid With Digital Pre-Equalization

We first experimentally demonstrate the ultra-dense wavelength division multiplexing passive optical network (UD-WDM-PON) system at 9.95 Gb/s per wavelength on a 12.5-GHz grid, adopting digital pre-equalization considering narrow-band optical filtering effect at the transmitter. Eight optical carriers with 12.5-GHz frequency spacing carry 9.95-Gb/s digital pre-equalized binary signals (on-off keying) and are optically filtered. 25- and 45-km single-mode fiber (SMF)-28 downlink transmissions are experimentally demonstrated. The bit-error ratio for each of the eight channels is less than 1.0 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-9</sup> after transmission over 25-km SMF-28.

Weighting nonlinearities on future high aggregate data rate PONs

In this paper we address high aggregate data rate coherent UDWDM-PONs (ultra-dense wavelength-division multiplexing passive optical networks) related physical impairments. Firstly, analog to digital converter resolution and laser linewidth are optimized giving the minimal signal to noise ratio penalty for UDWDM-PON systems at 10 Gb/s per user/wavelength. Secondly, inter-channel nonlinearities impact on high-order modulation formats at 1-2.5 Gb/s per channel and 100 km-reach is studied by means of Volterra series simulations.