Optical Network Unit Research Articles

PON transceiver technologies for ≥50 Gbits/s per λ: Alamouti coding and heterodyne detection [Invited

There has been an ongoing quest for transceiver technologies to be employed in next-generation passive optical networks (PONs) beyond 25G due to the growing number of subscribers and connected devices per subscriber and the ever-increasing bandwidth demand per device/application. Given the cost and loss/power budget requirements, the candidates seem to be digital signal processing (DSP)-aided intensity-modulation/direct detection (IM-DD) and low-complexity coherent transceivers. Here, we experimentally demonstrate a 100G coherent-lite PON using a novel transceiver DSP chain that utilizes a frequency-diverse dual-polarization RF pilot tone pair. The proposed scheme enables the implementation of Alamouti coding for single-carrier signaling, which is used in conjunction with heterodyne detection, achieving a significant complexity reduction in a coherent optical network unit (ONU) receiver. It consists of a single balanced photodiode followed by an analog-to-digital converter, offering comparable optical complexity to its DD counterpart, at the expense of DSP complexity in the ONU. The performance of a transceiver architecture facilitated by the proposed DSP was assessed in both back-to-back operation and up to 80 km standard single-mode fiber transmission using an ∼ 1 MHz linewidth distributed feedback laser as an ONU laser. At the hard-decision forward error correction (7% overhead) threshold, assumed to be 4 × 10 − 3 , a receiver sensitivity of − 29.6 dBm and a loss budget of 36.6 dB at a launch power of 7 dBm are achieved.

Efficient upstream bandwidth utilization with minimum bandwidth waste for time and wavelength division passive optical network

For an efficient bandwidth management among the optical network units in the upstream link of a passive optical network (PON), it is necessary to use a dynamic bandwidth assignment (DBA) scheme at the optical line terminal. This is really helpful for the PON service providers in meeting their delay guarantees and bandwidth commitments as per the service level agreements (SLAs). It also enables them to increase their revenue by accommodating more users in the available bandwidth. This study investigates the weaknesses of existing ITU compliant DBA schemes. The study reviews the earlier reported work on DBA with a focus on the ITU compliant schemes. The study argues that the existing reported schemes suffer from a few deficiencies such as inefficient bandwidth reporting in their polling process and borrow refund problem while utilizing the unused bandwidth (UBW). To overcome these shortcomings, an efficient bandwidth assignment (EBA) scheme is presented. Extensive simulations are performed to test the performance of the proposed scheme with both Poisson distributed and self-similar traffic patterns. The results show that the EBA scheme efficiently utilizes the UBW and RSB leading up to 80%, 90%, 60% and 99% lesser upstream delays with Poisson traffic pattern and up to 80%, 77% and 99% lesser delay with self-similar traffic compared to GREAL, IACG, and GIANT schemes. However, compared to EBU it shows up to 12% higher delay values due to reduction of its RSB share because of the utilization of UBW by T3 and T4. Overall, compared to other schemes, EBA exhibits least bandwidth waste per cycle resulting in least US delays. The bandwidth waste in EBA is also minimized due to least unallocated bandwidth ratio and frame loss rate. The maximum US delays of all traffic classes for EBA remains under 2 ms for T2 and T3 till the traffic load of 1 and till the load of 0.8 for T4 traffic class.

Strategies to increase spectral efficiency and energy saving with quality metric assurance in TWDM-PON

We propose a methodology for planning TWDM-PON that combines three lines of approach, known from literature, into a single approach that establishes a trade-off among them. That may support the network designer in finding a solution that can, simultaneously, meet high bandwidth demands, keep assurance of quality of service (QoS) level agreed with users, and increase energy saving of the network by taking into account the users’ activity and bandwidth sharing in the network. To develop the proposed algorithm, two bandwidth management techniques have been analyzed aiming at a possible migration of the end user equipment (optical network unit, ONU), whereas possible, in a more efficient distribution of ONU per wavelength pair (downstream-upstream). As a result, it becomes possible to switch off some management interfaces, thus assuring a degree of energy save. In parallel, two methods for providing an adaptive record of ONUs were aggregated to improve the performance of those ONUs that are migrating and/or entering the network, thus avoiding collision between signals and increasing the chance of their registration in the optical line terminal (OLT). The paper also addresses the fulfillment of the service level agreement (SLA), signed by operators and users, and proposes ways for designing a TWDM-PON that offers services with a quality metric assurance. Different scenarios with variable percentage of active users were considered and it was noticed that, in more realistic scenarios (50%, in average, of active ONUs), the network saved almost 50% of energy at the interfaces to be configured, with a migration period around 5 min. Additionally, it reached a SLA close to 100%, which is above the usual when it comes to compliance services. In addition, to meet bandwidth demand up to 1 Gb/s, the QoS was evaluated in four common Split Ratios (1:8 to 1:64) that the TWDM-PON can be configured, from the simplest to the most complex scenarios (active user, p, from 10% to 100%). In this way, it was verified that for the network to comply with satisfactory QoS, minimum percentage of time during which the target bit rate should be available for each connected user, the TWDM-PON should be planned at different activity levels, and the best planning are (p = 100%, 1:8), (p = 70%, 1:16), (p = 40%, 1:32) and (p = 20%, 1:64).

Three-dimensional grid architecture supporting discretionary direct communication among optical network units in metro-access networks

In this paper, a three-dimensional grid architecture supporting discretionary direct communication among optical network units (ONUs) is proposed. By setting up one $N \times N\hspace{3pt}$N×N arrayed waveguide grating (AWG) and two coarse wavelength division multiplexers in the remote node (RN), the optimal path selection of discretionary direct communication among ONUs can be carried out. At the same time, the construction of the RN is simple. By combining with a three-band wavelength allocation, downstream and upstream signals as well as direct-communication signals can be simultaneously transmitted. By utilizing a four-level protection mechanism, three kinds of optical fiber fault protection (i.e., feeder fiber fault protection, distribution fiber fault protection, and interconnecting fiber fault protection) can be achieved. Besides, both the scalability and the reliability of the network can be greatly improved. Finally, the feasibility of the proposed architecture is demonstrated by the simulation experiment.

Power-code division non-orthogonal multiple access scheme for next-generation passive optical networks.

Advanced modulation and multiple access schemes with high spectral efficiencies are desirable to overcome the bandwidth limitation in low-cost optical and electrical devices to fulfill the high-data rate requirements in passive optical networks (PONs). We propose a non-orthogonal multiple access (NOMA) scheme, known as power-code division NOMA (PCD-NOMA), for the next-generation PON, where the optical network units (ONUs) are divided into several groups with similar path losses. The ONUs per groups are allocated in the same power domain multiplexing layer with different codebooks. We show by experimental demonstration that the proposed PCD-NOMA with a high spectral efficiency offers improved overall system performance and reduced required transmission power in the next-generation PON, particularly in flexible PON where ONUs have different path losses. For PON with a power difference loss of 14 dB, the reduced required transmission powers are 5 and 11 dB for downstream and upstream, respectively, compared with orthogonal frequency division multiple access.

Design of Novel Fiber Optical Flexible Routing System

In this paper, we propose a new versatile routing device that utilizes arrayed waveguide gratings (AWGs), optical switches, and optical circulators to implement reconfigurable optical add/drop multiplexers (ROADMs), optical interleavers, and optical cross-connect (OXC). With the development of dense wavelength division multiplexing (DWDM) technology, ROADM and OXC technologies have also been put into practical use. Thus, the optical signal can be routed directly in the optical network according to its wavelength without the need for optical-electrical-optical (OEO) conversion. Although different optical network units (ONUs) have different bandwidth requirements, the use of optical interleavers has successfully solved the connection problem between old and new systems. According to the numerical experiments of static characteristics, the proposed routing device can effectively implement three different functionalities, thereby providing greater flexibility for fiber optic network applications.

Heuristic polling sequence to enhance sleep count of EPON

Next-generation passive optical networks (PONs) demand power conservation to create a green environment. A reduction in power consumption of the traditional Ethernet passive optical network (EPON) can be achieved by increasing the sleep count in optical network units (ONUs). In this paper, this is accomplished by introducing a first-in-last-out (FILO) polling sequence in the place of a fixed polling sequence to increase the number of ONUs entering sleep mode (sleep count). In a fixed polling sequence, the optical line terminal (OLT) allocates idle time to the ONUs based on the overall load of the ONUs. This leads to a situation that whenever the idle time does not meet the wakeup time threshold of sleep mode, the ONUs are put into doze/active mode, which consumes more power. In the FILO polling sequence, the first polled ONU in the current cycle is made to be polled last in the following cycle. Polling continues in this way, and by this rearrangement, the idle time of delayed poll ONUs increases; hence, it helps to reduce the power consumption. Additionally, a modified load adaptive sequence arrangement (MLASA) method is suggested, where the ONUs are categorized into doze ONUs and sleep ONUs. A numerical simulation of the FILO polling sequence with a vertical cavity surface emitting laser (VCSEL) ONU shows a maximum reduction in power consumption of 15.5 Wand a 20% improvement in energy savings compared with the traditional fixed polling sequence. The MLASA method results in better power consumption with minimum delay than that of the proposed FILO and existing LASA methods.

Hierarchical modulation PAM4 with digital Nyquist pulse-shaped for flexible multi-ONU provisioning in NG-TDM PON

Abstract In this paper, we study the performance of digital Nyquist-shaped and hierarchical modulation four-level pulse amplitude modulation (N-HM-PAM4), which is one of the most cost-effective modulations, in the context of the intensity-modulation and direct-detection (IM/DD) enabled the next generation time division multiplexed passive optical network (NG-TDM PON). In addition, by considering the power fading (which is incurred by the chromatic dispersion (CD)) and linear and nonlinear inter-symbol interference (ISI) (which is incurred by CD and square-law photo-detection) during the transmission, vestigial side band (VSB) optical equalization, overlap frequency-domain equalization (OFDE), and iterative nonlinear ISI digital domain compensation are applied in the system to improve both bandwidth efficiency and receiver sensitivity. Simulation results show how different parameters of the N-HM-PAM4 based NG-TDM PON (i.e., optical modulation index (OMI), iterative cancellation number, power ratio, and the distance between the optical line terminal (OLT) and its optical network units (ONUs)) affect the performance of the system, which is defined as the bit error ratio (BER) and received optical power (ROP) of the ONU.

Dynamic bandwidth allocation algorithms for improved throughput and latency performance in CDMA-based next-generation ethernet passive optical networks

In this paper, we propose new scheduling algorithms to improve the performance of the code division multiple access (CDMA) enabled next generation ethernet passive optical networks (NG-EPON). The original dynamic bandwidth allocation (DBA) scheme based on a round robin scheduling algorithm was originally proposed for the first generation CDMA based EPON where the network throughput and delay requirements per optical network unit (ONU) are relatively moderate. On the other hand, in the NG-EPON standard, the network capacity is increased up to 100 Gb/s and stringent requirements are applied to the packet delay. For this reason, in this work, we propose new scheduling algorithms for NG-EPON to assign a grant size on each upstream CDMA code in order to guarantee the newly required network throughput and delay performance. The first DBA algorithm is the first in first out (FIFO) algorithm in which the ONU requests are ordered in terms of their request arrival time. The second scheme is the reservation pattern (RP) algorithm in which each ONU starts transmitting its queue content in its reserved pattern of subcycles following a pre-defined pattern. The performances of the newly proposed algorithms are evaluated and compared. Results show that both newly proposed scheduling schemes can significantly improve the network performance in terms of the packet delay, jitter, and throughput. In addition, the results are compared to many dynamic wavelength and bandwidth allocation (DWBA) algorithms. The results show that the performances are comparable and that CDMA can be a good alternative for WDM for multi-channels based NG-EPON.

A Novel Scheme for UDWDM-PON Broadband Access Network Using Injection-Locked Phase-to-Intensity Modulation Converter

Abstract In this paper, we have proposed and analyzed a novel cost-effective ultradense wavelength division multiplexed (UDWDM) passive optical network (PON) which uses phase modulation (PM) of the optical carriers in the optical line terminal (OLT) in order to achieve inherent noise immunity in the network. In the demodulation of the optical PM signal at the optical network unit (ONU), we have proposed to use a simple injection–locked Fabry–Perot (FP) laser diode (LD) for achieving PM to intensity modulation (IM) converter which is followed by a photodiode to recover the specific channel information. A detailed analysis of PM–IM conversion in an injection–locked FPLD has been carried out and the viability of the proposed scheme has been established. The proposed UDWDM–PON which is a broadband access network is cost-effective and long reach.

New Security Improvements in Next-Generation Passive Optical Networks Stage 2

Passive optical networks are currently the most promising solution for access networks. These networks rely on broadcast signal distribution in the downstream direction and unicast signal transmission in the upstream direction. The upstream direction is controlled by optical line termination (OLT). The broadcast transmission method increases security vulnerability because the attacker is able to connect his/her modified optical network unit (ONU) to the free port of the splitter (commonly in the basement). We present the concept for the activation process of ONUs based on physical unclonable function (PUF) for next-generation passive optical networks stage 2 (NG-PON2). The use of PUF increases security in the NG-PON2. Furthermore, the registration identifier (ID) is not stored in a nonvolatile memory, in comparison with the common solution defined by the International Telecommunication Union (ITU) recommendation G.989.3. An attacker cannot perform a reverse engineering attack to obtain the registration ID. For this reason, the attacker cannot clone an ONU. We proposed security improvements that involve authentication, encryption, integrity protection, and data origin verification methods in the NG-PON2. Our model uses the standard implementation of the transmission convergence layer of NG-PON2 with the new physical layer operations, administration, and maintenance (PLOAM) messages. The recommendation G.989.3 allows specifying own PLOAM messages since not all IDs are used in the current specification.

Adaptive Scheduling Algorithm for the Coexistence of ONUs with Different Tuning Time in Virtual Passive Optical Network

Optical network unit (ONU)'s wavelength tuning time is a key factor that cannot be ignored in ONU scheduling algorithm for multi-wavelengths passive optical network (PON). In this paper, we propose an adaptive scheduling algorithm for the coexistence of ONUs with different tuning time in virtual PON, which is called multi-tuning-time ONU scheduling (MOS) algorithm. The simulation shows that the MOS algorithm can effectively avoid the extra queue delay caused by ONUs’ wavelength tuning and reduce the waste of bandwidth resources.

RSOA-based colorless multilevel transmitter with electrical signal predistortion

In this paper we present the design and a numerical proof-of-concept of an RSOA-based colorless multilevel transmitter, suitable for use in optical network units at user premises. Simple electrical predistortion of the M-QAM modulation signal theoretically allows the transmitter to be operated at up to 4 GBd symbol rate with 16-QAM modulation accounting for the transmission channel impairments and noise at the detection stage, if forward error correction is implemented. We also numerically confirm that 4-QAM operation is possible without the modulation signal predistortion, as long as the constellation is derotated after detection, which can be easily done using digital feed-forward algorithm.

Nyquist four-level pulse amplitude modulation scheme (PAM-4) based on hierarchical modulation in IM/DD-TDM PON with hybrid equalization

In order to achieve higher power margin in the intensity-modulation and direct detection (IM/DD) based time division multiplexed passive optical network (TDM-PON) system, at the optical line terminal (OLT) side, we propose to use a Nyquist pulse shaped four-level pulse amplitude modulation (PAM-4) by using the electric low pass filter(LPF). In addition, we apply the hierarchical modulation (HM) scheme by assigning of most-significant bits (MSBs) and the less-significant bits (LSBs) of the PAM-4 symbol to optical network units (ONUs) located at different link positions. In addition, an overlap frequency domain equalizer (OFDE) and a vestigial-side band (VSB) filter are used to mitigate the linear inter symbol interference (ISI) and power fading (both of which are incurred by chromatic dispersion (CD)), respectively. The system performance, i.e., bit error rate (BER) and receiver sensitivity for both MSB and LSB, are analyzed by varying different system configurations. Simulation results show that increasing the hierarchical power ratio, which determines the significance difference between the points in the Nyquist pulse shaped HM PAM-4 constellation diagram, can increase the MSB’s receiver sensitivity and reduce the LSB’s receiver sensitivity. Furthermore, it has been demonstrated that the electric low pass filter can reduce the system ISI. In addition, the effect of linear ISI and power fading can be migrated by utilizing OFDE and VSB filter, respectively.

Analysis of a hybrid Fixed-Elastic DBA with guaranteed fronthaul delay in XG(s)-PONs

Reusing the existing Optical Distribution Network (ODN) of Passive Optical Networks (PONs) to bring connectivity to residential households together with providing the so-called front-haul (FH) network in Cloud Radio Access Networks (C-RAN) is a promising converged fixed and mobile network architecture envisioned by network operators. This article overviews how to exploit Fixed Bandwidth Reservations allowed in ITU-T XG(S)-PONs and their suitability for the transmission of front-haul (FH) traffic, thus allowing one Remote Radio Unit (RRU) coexist with multiple residential Optical Network Units (ONUs) in the same PON. We explain how to configure such periodic reservations to improve access in the upstream channel of a PON, showing worst-case access delays below 100 μs for eCPRI’s functional split IU.

Attack-Aware Dynamic Upstream Bandwidth Assignment Scheme for Passive Optical Network

Abstract Network security is an important component of today’s networks to combat the security attacks. The passive optical network (PON) works at the medium access layer (MAC). A distributed denial of service (DDOS) attack may be launched from the network and transport layers of an Optical Network unit (ONU). Although there are various security techniques to mitigate its impact, however, these techniques cannot mitigate the impact on the MAC Layer of the PON and can cause an ONU to continuously drain too much bandwidth. This will result in reduced bandwidth availability to other ONUs and, thus, causing an increase in US delays and delay variance. In this work we argue that the impact of a DDOS attack can be mitigated by improving the Dynamic bandwidth assignment (DBA) scheme which is used in PON to manage the US bandwidth at the optical line terminal (OLT). The present DBA schemes do not have the capability to combat a security attack. Thus, this study, uses a machine learning approach to learn the ONU traffic demand patterns and presents a security aware DBA (SA-DBA) scheme that detects a rogue (attacker) ONU from its traffic demand pattern and limits its illegitimate bandwidth demand and only allows it the bandwidth assignment to it as per the agreed service level agreement (SLA). The simulation results show that the SA-DBA scheme results in up to 53%, 55% and 90% reduced US delays and up to 84%, 76% and 95% reduced US delay variance of T2, T3 and T4 traffic classes compared to existing insecure DBA schemes.

Wavelength control method of upstream signals using AMCC in WDM-PON for 5G mobile fronthaul.

To meet the 5G mobile traffic demands, many small cells will be installed in the field. A promising candidate for reducing a large number of optical fibers connecting the central and distribution units is a tunable wavelength division multiplexing passive optical network. However, for systems in which multiple wavelengths are transmitted densely such as 100 GHz channel spacing, wavelength setting error and wavelength drift are major issues. In this paper, we describe a wavelength control method that uses an auxiliary management and control channel that complies with ITU-T G.989.3. Our method makes it possible to control the setting error of upstream signals at the initial connection between the optical line terminal and an optical network unit and also to control the wavelength drift due to the aging degradation of the laser diode. We also clarify the control conditions needed to minimize the control time.

Design of Energy-Efficient EPON: A Novel Protocol Proposal and Its Performance Analysis

Economical and environmental concerns necessitate network engineers to focus on energy-efficient access network design. The optical network units (ONUs) are predominate sources of the energy consumption of Ethernet passive optical network, which motivate us toward designing a novel protocol for saving energy at ONUs. The proposed protocol exploits different low power modes (LPMs) and opts for the suitable one using traffic prediction. This scheme provides a significant improvement of energy-efficiency especially at high load (~40%) over existing ONU-assisted protocols. A better understanding of the performance and a deeper insight into several design aspects can only be addressed through a detailed mathematical analysis. The proposed protocol involves traffic prediction, which infringes the Markovian property. However, some pragmatic assumptions along with a proper selection of observation instances and state descriptions allow us to form a discrete time Markov chain (DTMC) of the proposed algorithm. Thus, the primary objective of this paper is to propose a novel scheme for achieving energy-efficiency at ONUs and mathematically analyze the performance of it with the help of a DTMC. The analysis reveals that the energy-efficiency is more sensitive to the power consumption of doze mode as compared to other LPMs. Further, the effect of sleep-to-wake-up time of all LPMs is minor.

HMM-Based Secure Framework for Optical Fog Devices in the Optical Fog/Cloud Network

Abstract In the present optical fog/cloud computing environment, optical line terminals and optical network units are used as the most promising optical fog devices (OFDs). The inherent characteristics of fog computing provide certain granted privileges to the attacker to hack devices and make them malicious. Also, existing security solutions generate false alarms that affect the performance of the underlying network. In this paper, a secure framework is proposed that not only predicts the malicious OFDs but also reduces false alarms. Hidden Markov model and intrusion detection system are used to detect the malicious one by computing the probability of shifting (POS) and then shift it to the virtual honeypot which is kept hidden by deploying it at the optical fog layer. In addition, it also reduces the generation of false alarm and logs all malicious activities for further analysis. In the experiment section, Python is used to validate the proposed framework. Further, HMM is simulated and tested in the MATLAB to reduce the false alarm rate. Results show that the proposed framework effectively reduces the false alarms and detects the malicious one and then shifts it onto the virtual honeypot efficiently.

Comparative Analysis of High Speed 20/20 Gbps OTDM-PON, WDM-PON and TWDM-PON for Long-Reach NG-PON2

Abstract Nowadays, bandwidth demand is enormously increasing, that causes the existing passive optical network (PON) to become the future optical access network. In this paper, next generation passive optical network 2 (NG-PON2) based, optical time division multiplexing passive optical network (OTDM-PON), wavelength division multiplexing passive optical network (WDM-PON) and time & wavelength division multiplexing passive optical network (TWDM-PON) systems with 20 Gbps (8 × 2.5 Gbps) downstream and 20 Gbps (8 × 2.5 Gbps) upstream capacity for eight optical network units has been proposed. The performance has been compared by varying the input power (−6 to 27 dBm) and transmission distance (10–130 km) in terms of Q-factor and optical received power in the presence of fiber noise and non-linearities. It has been observed that TWDM-PON outperforms OTDM-PON and WDM-PON for high input power and data rate (20/20 Gbps). Also, TWDM-PON shows its superiority for long-reach transmission up to 130 km, which is a cost-effective solution for future NG-PON2 applications.