Division Multiplexing Passive Optical Network Research Articles

Hardware-Efficient Signal Processing Technologies for Coherent PON Systems

Future passive optical network (PON) systems supporting more than 50 Gb/s/λ present a challenge for the use of intensity modulation direct detection (IM-DD). Since coherent technology improves the receiver sensitivity over that of IM-DD, it is a promising candidate for 100 Gb/s or higher PON systems. Introducing hardware-efficient signal processing technologies tailored to PON systems will help render coherent technology suitable for PON systems. We here review hardware-efficient signal processing technologies suitable for PON systems. We introduce two types of simplified adaptive equalization (AEQ), one which sacrifices differential group delay compensation (DGDC), and another which sacrifices some chromatic dispersion compensation but does provide DGDC. Transmission experiments on a 100 Gb/s/λ-based coherent wavelength division multiplexing (WDM) PON system showed that simplified AEQ without DGDC and with DGDC exhibited only 0.2 and 1.4 dB penalty, respectively, compared with conventional DSP. The additional penalty due to the maximum possible cumulative DGD was evaluated by numerical simulation. Conventional AEQ and the simplified AEQ with DGDC showed negligible penalty, but the simplified AEQ without DGDC showed a 1.4 dB penalty. We also introduce simplified carrier phase recovery (CPR) with interpolarization phase offset estimation, and this showed the same performance as the conventional DSP, in both experiment and simulation. Taking these results into account, 100 Gb/s/λ-based coherent WDM PON systems with the simplified AEQs in combination with the simplified CPR were shown to be able to support the loss budget required for eight ONU splits over an 80 km span of single mode fiber.

A Scheme for UDWDM-PON Broadband Access Network Using a Mode Locked Laser Diode and Optical Injection Locking

Abstract This paper describes an useful scheme of ultra-dense wavelength division multiplexed-passive optical network (UDWDM-PON) meant for the bidirectional transmission of analog video signals to the users in the Optical Network Unit (ONU). Each user has a dedicated wavelength which carries a group of analog video channels. This PON scheme is bidirectional with a long distance of transmission of 100 km typically over a single optical fiber. The video channels designated for a particular user in the ONU can be continuously tuned by a post-detection tunable bandpass filter. Intermodulation distortion (IMD) resulting from a mixing of analog video channels carried by a single wavelength has been calculated for a group of three video channels which lies in the range of –30 dB to –85 dB typically for a set of values of parameters used in practice. This IMD level is fairly acceptable for analog video signal transmission and reception in practice. The novelty of this scheme is that it is long reach.

Optical Modulation Enhancement Through CW Injection Locking of a Sinusoidally Modulated Fabry-Perot Laser Diode

Abstract This paper reports an interesting phenomenon of optical modulation enhancement produced by CW optical sideband injection locking of a directly modulated Fabry-Perot Laser Diode (FPLD) lasing at 1557.1 nm wavelength, the modulation being sinusoidal in nature and effected at 300 kHz and 3 MHz. This investigation is carried out for direct modulation of the FPLD effected in the RF region at 300 kHz and 3 MHz in contradistinction to earlier studies being conducted with the modulation in the microwave frequency band. A nonlinear analysis has been carried out which corroborates experimental observation on modulation enhancement. At low level of optical Intensity Modulation (IM) index, not exceeding 20 %, typical modulation enhancement of 2 % at 300 kHz and 11 % at 3 MHz modulation frequency have been observed. Larger modulation enhancement is expected at higher values of IM indices of the slave FPLD and for medium to high power laser diodes. This circuit can have direct application in Wavelength Division Multiplexed-Passive Optical Network (WDM-PON).

Real-time bidirectional coherent ultra-dense TWDM-PON for 1000 ONUs

We propose a bidirectional coherent ultra-dense time/wavelength division multiplexing passive optical network (UD-TWDM-PON) scheme for 1000 optical network units (ONUs). Then we experimentally demonstrate a bidirectional coherent UD-TWDM-PON using real-time FPGA-based transceivers. The power budget of this system is evaluated based on 1000 downlink ultra-dense wavelength division multiplexing (UD-WDM) channels with 5 GHz channel spacing in C band. The power budgets are 28 dB for DP-QPSK format with 10 Gb/s per user and 30 dB for PS-QPSK format with 7.5 Gb/s per user after 40 km SSMF transmission. The uplink transmission over 40 km SSMF for the 1000 users is also achieved based on 200 channels with 10 Gb/s per channel under channel spacing of 12.5 GHz in L band. Time division multiplexing is performed for the uplink optical signals, where 5 ONUs share one channel in the uplink. The 24-hour real-time performance testing for the bidirectional transmission is also conducted to evaluate the feasibility of the real-time FPGA-based transceivers in the proposed UD-TWDM-PON scheme.

Secure Strategy for OFDM-PON Using Digital Chaos Algorithm With Fixed-Point Implementation

We propose a scheme for security improvement in the orthogonal frequency division multiplexing (OFDM) passive optical network based on a fixed-point digital chaos algorithm with a low computational precision. The dynamical degradation problem is improved by introducing the transmitted data as a perturbation source. In our demonstration, aperiodic key streams are generated from a fixed-point chaos with its trajectory disturbed by the upstream data, and then used to encrypt the downstream data. A 7.64-Gbps encrypted OFDM signal is successfully transmitted over a 25-km standard single mode fiber. The experimental results indicate that the proposed scheme suggests an effective and promising solution to meet the demands for low implementation complexity and high security level.

Joint Resource Allocation and Software-Based Reconfiguration for Energy-Efficient OFDMA-PONs

The widely deployed passive optical network (PON) is a cost-effective way to create a front/backhaul carrier for ubiquitous wireless access connectivity. To hide the difference of frame formats and multiplexing methods, an orthogonal frequency division multiplexing access PON (OFDMA-PON) was proposed. In upstream transmission, multiple optical network units (ONUs) from the same segment or residential area can transmit data through a shared group of orthogonal low-bitrate subcarriers (SCs) during different time slots (TSs) within the entire transmitting period. In downstream transmission, the optical line terminal can send control signaling of reconfiguring ONUs’ parameters so that SCs can be reallocated among segments, thus sleeping segments with empty data load while ensuring service continuity. Though energy savings can be ensured for upstream data transmission by using existing solutions, only frequency and time domains are involved. It is meaningful for us to make the flexible and energy- efficient configuration of transmitting power and modulation level assigned for ONUs in terms of improving energy efficiency. In this paper, we investigate joint resource allocation for energy-efficient upstream data transmission in OFDMA-PONs, with the help of software-based ONU reconfiguration. We formulate the problem, and the upper bound of energy saving is also derived. To solve the problem in a short span of time, we propose a novel heuristic algorithm whose effectiveness has been demonstrated by extensive simulations.

Wavelength-Insensitive Weakly Coupled FMFs and Components for the MDM-GPON

Mode division multiplexing (MDM) has been widely investigated to enhance the capacities of passive optical networks (PONs), and weakly coupled transmission schemes are highly preferred to reduce the cost and complexity. However, wavelength insensitivities are scarcely studied for such applications. In this letter, an evolution scheme of the cascaded mode division multiplexing PON (MDM-PON) and conventional Gigabit PON (GPON), which supports bidirectional 1310/1490-nm transmission, is proposed. This scheme is enabled by wavelength-insensitive weakly coupled (WIWC) few-mode fiber (FMF) and optical components. Then, the realization of both WIWC few-mode wavelength division multiplexers (FM-WMUXs) and mode-selective couplers (MSCs) is analyzed. Based on the analysis, WIWC FM-WMUXs and MSCs that support two linear polarization modes and 1310/1490-nm transmission with low modal crosstalk are designed and fabricated. Finally, the bidirectional MDM-GPON transmission scheme with 2.5 Gb/s on-off-keying modulation and simple direct detection over 10-km FMF and 10-km single-mode fiber are experimentally demonstrated. The proposed MDM-GPON evolution scheme can achieve maximum compatibility by maintaining both the optical distribution network and the optical network units.

Performance Analysis of Different Electrical Filters in 10G Hybrid Passive Optical Network

With growing number of applications and network traffic, optic fibers are extensively used in the access part of the network. Passive Optical Networks (PON) in particular, Ethernet PON (EPON) networks based on Time Division Multiple Access (TDMA) are more prominently used in many parts of the world. Though Wavelength Division Multiplexing (WDM) PON has its own advantages, considering the cost and utilisation of such networks in the access part makes it less useful. On the other hand, Hybrid PON network combines the advantages of both EPON and WDM PON Networks. The objective of this paper is to identify suitable electrical filters for a 16-channel Hybrid Passive Optical Network with a transmission rate of 10Gbps per channel, by analysing their performance in terms of Q factor and Bit Error Rate. Different filters like the Bessel filter, Gaussian filter, Raised Cosine Filter, Rectangular filter, Butterworth filter, Chebyshev Filter are compared and their performances are evaluated. DB Modulation format that provides a longer reach is used at the transmitter to evaluate the different scenarios and the simulation is carried out using Optisystem.

Long-Reach Wavelength-Routed TWDM PON: Technology and Deployment

We present a long-reach wavelength-routed time-wavelength division multiplexing (TWDM) passive optical network (PON) architecture (LRWR-PON) and its commercial implementation, which supports up to 768 users per fiber strand and up to 50-km transmission distance. The increased reach allows central offices to become more flexible and fewer in quantity, while the increased aggregation reduces the size and number of optical cables needed, enabling smaller trenches to be used. LRWR-PON also contains eight additional point-to-point wavelengths on each fiber to support wireless sites and/or high-speed dedicated bandwidth applications, greatly simplifying converged network designs. Multiple new optical components and modules have been developed to implement our novel architecture. These include a cyclic arrayed waveguide grating to passively aggregate and distribute access wavelengths in the field, an integrated optical amplifier and multiplexer combination device to aggregate optical line terminal (OLT) channels and extend the system reach, several dense wavelength division multiplexing OLT optics, and a colorless TWDM optical network terminal employing low-cost tunable burst-mode lasers. Our analysis shows the simplification of the civil construction enabled by LRWR-PON greatly outweighs the increased optical component complexity. To date, we have conducted a successful field trial with 606 real-life customers for more than 15 months and we have been rolling out LRWR-PON in Google Fiber markets for production services.

Full-duplex broadcast RoF-WDM-PON with self-coherent detection and photonic frequency up/down-conversion using SSB pilot-carrier

A novel bidirectional broadcast radio-over-fiber-wavelength division multiplexing-passive optical network (RoF-WDM-PON) based on self-coherent detection and photonic frequency up and down-conversion is presented. In the proposed scheme, a single sideband (SSB) modulated pilot-carrier is transmitted to the optical network unit (ONU), together with the downstream signal by polarization multiplexing on the same fiber link. The SSB pilot-carrier serves as a local oscillator laser to realize coherent detection and an optical carrier of the upstream signal simultaneously. In this way, the optical carrier phase noise and frequency offset in coherent detection have been efficiently eliminated in ONU. Moreover, the detection of the downstream baseband signal and millimeter wave (MMW) signal by homodyne and heterodyne detection can be simultaneously implemented in the proposed scheme. Due to the reuse of SSB light wave, the upstream radio-frequency signal is down-converted at the optical line terminal (OLT) directly by photonic-mixing technique. The performance of proposed RoF-WDM-PON system is evaluated for 20 km signal-mode fiber full-duplex transmission of a 40 Gb/s 16-QAM baseband signal and 10 Gb/s DPSK wireless signal.

A 7-D Hyperchaotic System-Based Encryption Scheme for Secure Fast-OFDM-PON

In this paper, we propose and experimentally demonstrate a novel two-step I/Q-encryption scheme for improving physical layer security in fast orthogonal frequency division multiplexing passive optical network (fast-OFDM-PON). To meet the requirement of this encryption scheme and further enhance the security, a seven-dimensional hyperchaotic system with five positive Lyapunov exponents is designed. In this system, Walsh–Hadamard transform and discrete cosine transform are chosen to reduce the computation complexity and increase the key space simultaneously. An optional discrete Fourier transform spread is required to achieve the maximum scrambling degree. Encryption of 9.4-Gb/s 16-quadrature amplitude modulation fast-OFDM signals with a total key space of ∼10788 against the exhaustive trial has been successfully implemented over 50-km standard single mode fiber in a fast-OFDM-PON, which shows good resistance against illegal access to optical network units. All the results indicate that this proposed encryption scheme has improved security performance with relatively low computation complexity.

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.

Secure Key Distribution Strategy in OFDM-PON by Utilizing the Redundancy of Training Symbol and Digital Chaos Technique

A secure key distribution scheme for orthogonal frequency division multiplexing (OFDM) passive optical network system is proposed and experimentally demonstrated. The training symbol (TS), which is used for time synchronization and channel estimation, is formed by adopting a noise-like chaotic sequence. The redundancy of the TS is utilized to embed secure keys. An experiment is performed, in which 7.64 Gb/s 16-quadrature-amplitude-modulation OFDM data and 28.4 Mb/s keys embedded in are successfully transmitted over 25 km standard single mode fiber. The results indicate a promising key distribution method for physical layer secure optical communication.

Reverse Bias Voltage Controlled Burst-Mode Booster SOA in λ-Tunable ONU Transmitter for High-Split-Number TWDM-PON

This paper presents a λ-tunable optical network unit (ONU) burst-mode transmitter (B-Tx) with high output power and low burst-off-level power for high-splitnumber time and wavelength division multiplexing passive optical networks (TWDM-PONs). The proposed ONU B-Tx utilizes a burst-mode booster semiconductor optical amplifier (SOA) with a control technique for reverse bias voltage that creates an optical power absorption effect in the burstoff- level state. This paper introduces the design of the biasvoltage- state control circuit, simulations of control-circuit operation, evaluations of the static optical power-absorption effect, and burst-mode operation performance in the booster SOA operated by reverse bias voltage. High output power of over +7.4 dBm and low burst-off-level power of under −73.4 dBm∕15 GHz are successfully achieved in all upstream channels. Crosstalk penalty measurements confirm a very low penalty of under 0.1 dB, which leads to a 256 splitnumber TWDM-PON.

A chaotic modified-DFT encryption scheme for physical layer security and PAPR reduction in OFDM-PON

Abstract This letter proposes a modified discrete Fourier transform (DFT) encryption scheme with multi-dimensional chaos for the physical layer security and peak-to-average power ratio (PAPR) reduction in orthogonal frequency division multiplexing passive optical network (OFDM-PON) system. This multiple-fold encryption algorithm is mainly composed by using the column vectors permutation and the random phase encryption in the standard DFT matrix, which can create ∼10 551 key space. The transmission of ∼10 Gb/s encrypted OFDM signal is verified over 20-km standard single mode fiber (SMF). Moreover, experimental results show that, the proposed scheme can achieve ∼2.6-dB PAPR reduction and ∼1-dB improvement of receiver sensitivity if compared with the common OFDM-PON.

Optical pulse width modulation based TDM-PON monitoring with asymmetric loop in ONUs

We propose a time division multiplexing passive optical network (TDM-PON) monitoring scheme based on the optical pulse width modulation (OPWM) by using a simple asymmetric loop (AL) located near the optical network unit (ONU). Various pulse widths can be easily obtained by configuring the appropriate length of the patchcord used to generate the path difference between two branches of a 1 × 2 splitter. Different wavelengths can be assigned to the same pulse width, which can further increase the network size. The optimized parameters of the AL are investigated and analysis models are established. The calculation results show that the proposed scheme has smaller correlation distance (CD) and lower multiple-customers interference probability (MCIP) than the periodic coding (PC) scheme. A simplified PON system with 8 ONUs is set up to experimentally demonstrate the feasibility of the proposed scheme. The network recognition processing used to identify the reflected monitoring signals in the proposed scheme is simple, especially compared with the reduced complexity maximum likelihood sequence estimation (RC-MLSE) used in the typical PC scheme.

Chaotic Constellation Mapping for Physical-Layer Data Encryption in OFDM-PON

A physical-layer encryption scheme is proposed and experimentally demonstrated in orthogonal frequency division multiplexing passive optical network (OFDM-PON). In the proposed multifold encryption scheme, quadratic-amplitude modulation (QAM) symbols are scrambled and distributed onto the complex plane independently. The dynamic parameters of constellation shifting are generated by a 3-D hyper digital chaos, in which a key space of ~10162 is created to enhance the security level of OFDM data encryption during transmission. An encrypted data transmission of 9.4-Gb/s, 16-QAM optical OFDM signals is successfully demonstrated over 20-km standard single-mode fiber.

TDD Pattern Estimation and Auto-Recovery from Estimation Error for Accommodations of Fronthaul and Secondary Services in TDM-PON

Time division multiplexing passive optical networks (TDM-PONs) are attracting attention as a cost-effective way of realizing mobile fronthaul (MFH), which is an optical link between a central unit (CU) and a distributed unit (DU) in a centralized radio access network (C-RAN). To realize an even more cost-effective network, we have studied the accommodation of MFH and secondary services in a single TDM-PON. We focus on the fact that a time division duplex (TDD) scheme will be applied to future radio access (FRA). Since a PON link is a wavelength division multiplexing (WDM) scheme, whereas a radio propagation link is a TDD scheme, unallocated intervals occur periodically in the PON link. For the above reason, we have already proposed a technique for estimating unallocated intervals in a TDD-based MFH link that involves prelearning based on a test signal transmission, and the signal of the secondary service is accommodated in the unallocated intervals. However, a scheme for recovering from an error when the estimation is wrong and the test signal is not transmitted has yet to be studied. This paper proposes an estimation technique without a test signal transmission and an estimation-error recovery scheme. The proposed estimator calculates the correlation coefficient between captured uplink traffic and the seven types of uplink and downlink ratio patterns standardized by the third generation partnership project (3GPP: http://www.3gpp.org/). We evaluate the effectiveness of the proposed estimation technique by using a numerical simulation. The recovery scheme detects the estimation error and re-allocates the bandwidth. We implement the estimator and the recovery scheme in a 10 Gigabit Ethernet PON (10G-EPON) prototype. We report the feasibility of the proposed scheme along with experimental results.

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.

A three-dimensional encryption orthogonal frequency division multiplexing passive optical network based on dynamic chaos-iteration

Orthogonal frequency-division multiple passive optical network (OFDM-PON) has emerged as one of the most promising solutions to meet the requirements for the next-generation wide-band optical access network with high capacity, strong fiber dispersion tolerance, and flexible resource allocation. However, like other optical access network in which the downstream signal is broadcasted to all the optical network units (ONUs), OFDM-PON is vulnerable to being eavesdropped. Thus the security of OFDM-PON should be taken seriously into consideration. Recently, some chaos based encryption methods, including chaotic scrambling and permutation, hyper-chaotic system and fractional Fourier transformation, chaos based IQ encryption method and chaos based two-dimensional scrambling, have been proposed to enhance the physical layer security of OFDM-PON system. Owing to the special chaos-related characteristics, such as ergodicity, pseudo randomness, and high sensitivity to the initial values, etc., these encryption methods are of high physical layer security. However, in most of these schemes, key distribution is not considered. In this paper, we propose a three-dimensional encryption OFDM-PON based on dynamic chaos-iteration. The key distribution is implemented through the dynamic chaos synchronization between the transmitter and receiver. The receiver tries to synchronize his chaos system with the transmitters' by calculating the correlation index of the synchronization sequence, which comes from the transmitter and is controlled by dynamic parameters in the parameter sets. The calculation is not very complex because the transmitter and receiver are acquainted with the parameter sets. The synchronized chaos system is used to generate keys for both encryption and decryption. In the proposed encryption scheme, one ONU is connected with four users, and the message is irrelevant to the users. Quadrature amplitude modulation (QAM) symbols from the users are mapped randomly onto the subcarriers in a flame based on the chaotic matrix M1. For the M1 is changeable, the number and position of subcarriers for different users are dynamically varying. Then the matrix M2 generated from chaos system is utilized to mask all QAM symbols. Finally the QAM symbol matrix is multiplied by an invertible chaotic matrix M3 to realize subcarrier perturbation. These three key matrixes are generated from the two-dimension logistic iteration chaos system, to which the initial sensitivity increases up to 10-15. The output sequence of the chaos system after quantification process is of good self-correlation and cross-correlation characteristic and can pass all NIST SP800-22 randomness tests. The key space of the encryption scheme is over 1086, which would be against exhaustive attack effectively. Specifically, a proof-of-principle experiment is conducted to demonstrate the aforementioned proposed scheme. In the experiment, a 13.3 Gb/s encrypted 64QAM OFDM signal transmits over 25 km standard single mode fiber in an OFDM-PON and successfully decrypts at the legal receiver. For an eavesdropper lacking correct keys, the received QAM constellation is totally in disorder and the bit error rate increases up to 0.46, which indicates that not any useful message is eavesdropped. The proposed scheme provides a promising candidate for the next-generation secure optical access networks.