The emerging software defined networking (SDN) paradigm separates the data plane from the control plane and centralizes network control in an SDN controller. Applications interact with controllers to implement network services, such as network transport with quality of service. SDN facilitates the virtualization of network functions so that multiple virtual networks can operate over a given installed physical network infrastructure. Due to the specific characteristics of optical (photonic) communication components and the high optical transmission capacities, SDN-based optical networking poses particular challenges, but holds also great potential. In this article, we comprehensively survey studies that examine the SDN paradigm in optical networks; in brief, we survey the area of software defined optical networks (SDONs). We mainly organize the SDON studies into studies focused on the infrastructure layer, the control layer, and the application layer. Moreover, we cover SDON studies focused on network virtualization, as well as SDON studies focused on the orchestration of multilayer and multidomain networking. Based on the survey, we identify open challenges for SDONs and outline future directions.

Software Defined Optical Networks (SDONs): A Comprehensive Survey

The exponential traffic growth, demand for high speed wireless data communications, as well as incessant deployment of innovative wireless technologies, services, and applications, have put considerable pressure on the mobile network operators (MNOs). Consequently, cellular access network performance in terms of capacity, quality of service, and network coverage needs further considerations. In order to address the challenges, MNOs, as well as equipment vendors, have given significant attention to the small-cell schemes based on cloud radio access network (C-RAN). This is due to its beneficial features in terms of performance optimization, cost-effectiveness, easier infrastructure deployment, and network management. Nevertheless, the C-RAN architecture imposes stringent requirements on the fronthaul link for seamless connectivity. Digital radio over fiber-based common public radio interface (CPRI) is the fundamental means of distributing baseband samples in the C-RAN fronthaul. However, optical links which are based on CPRI have bandwidth and flexibility limitations. Therefore, these limitations might constrain or make them impractical for the next generation mobile systems which are envisaged not only to support carrier aggregation and multi-band but also envisioned to integrate technologies like millimeter-wave (mm-wave) and massive multiple-input multiple-output antennas into the base stations. In this paper, we present comprehensive tutorial on technologies, requirements, architectures, challenges, and proffer potential solutions on means of achieving an efficient C-RAN optical fronthaul for the next-generation network such as the fifth generation network and beyond. A number of viable fronthauling technologies such as mm-wave and wireless fidelity are considered and this paper mainly focuses on optical technologies such as optical fiber and free-space optical. We also present feasible means of reducing the system complexity, cost, bandwidth requirement, and latency in the fronthaul. Furthermore, means of achieving the goal of green communication networks through reduction in the power consumption by the system are considered.

Toward an Efficient C-RAN Optical Fronthaul for the Future Networks: A Tutorial on Technologies, Requirements, Challenges, and Solutions

Digital filter multiple access (DFMA) passive optical networks (PONs) are, for the first time to our knowledge, proposed and extensively investigated, where digital signal processing (DSP)-enabled, software-reconfigurable, digital orthogonal filtering is employed in each individual optical network unit (ONU) and the optical line terminal to enable all ONUs to dynamically share the transmission medium under the control of the centralized software-defined controller and the transceiver-embedded DSP controllers. The DFMA PONs fully support software-defined networking with the network control further extended to the physical layer. As digital filtering is the fundamental process at the heart of the proposed DFMA PONs, the filtering-induced tradeoffs among upstream transmission capacity, filter design flexibility, and filter DSP complexity are examined, based on which optimum filter design guidelines are identified for various application scenarios. Furthermore, the performance characteristics of the DFMA PONs are also numerically explored in terms of maximum achievable upstream transmission capacity, differential ONU launch power dynamic range, and ONU count-dependent minimum received optical power.

Digital filter multiple access PONs with DSP-enabled software reconfigurability

Real-time optical OFDM (OOFDM) transceivers with on-line software-controllable channel reconfigurability and transmission performance adaptability are experimentally demonstrated, for the first time, utilizing Hilbert-pair-based 32-tap digital orthogonal filters implemented in FPGAs. By making use of an 8-bit DAC/ADC operating at 2GS/s, an oversampling factor of 2 and an EML intensity modulator, the demonstrated RF conversion-free transceiver supports end-to-end real-time simultaneous adaptive transmissions, within a 1GHz signal spectrum region, of a 2.03Gb/s in-phase OOFDM channel and a 1.41Gb/s quadrature-phase OOFDM channel over a 25km SSMF IMDD system. In addition, detailed experimental explorations are also undertaken of key physical mechanisms limiting the maximum achievable transmission performance, impacts of transceiver's channel multiplexing/demultiplexing operations on the system BER performance, and the feasibility of utilizing adaptive modulation to combat impairments associated with low-complexity digital filter designs. Furthermore, experimental results indicate that the transceiver incorporating a fixed digital orthogonal filter DSP architecture can be made transparent to various signal modulation formats up to 64-QAM.

Real-time experimental demonstrations of software reconfigurable optical OFDM transceivers utilizing DSP-based digital orthogonal filters for SDN PONs.

In this paper, to the best of our knowledge, this is the first time that the generation and reception of online software-reconfigurable real-time 4/16/64-quadrature amplitude modulation (QAM)-encoded and forward error correction (FEC)-enabled orthogonal frequency-division multiplexing (OFDM) signals over X-band fiber-over-radio-based upstream links has been demonstrated. A Reed–Solomon code, i.e., RS (288, 256), with symbol interleaving/deinterleaving is applied to enhance bit-error-rate (BER) performance. The experimental results show that the 4/16/64-QAM-OFDM signals with net data rates of 1.6/3.2/4.8-Gb/s after 1.5-m wireless and 2.26-km SMF-28 transmission with BERs below the hard-decision FEC threshold of 3.8e-3 can be successfully achieved without using FEC. With the help of symbol-interleaved RS code, a post-FEC BER below 1e-8 can be observed when the pre-FEC BER is around 1e-3.

Demonstration of Software-Reconfigurable Real-Time FEC-Enabled 4/16/64-QAM-OFDM Signal Transmission in an X-Band RoF System

A novel signal modulation technique, termed optical OFDM-based carrierless amplitude and phase (CAP) modulation (OOFDM-CAP) is proposed, analyzed and evaluated, for the first time, in which multiple real-valued OFDM channels are multiplexed/demultiplexed using digital orthogonal filters embedded in DSP logic. An OOFDM-CAP theoretical model is established, based on which the dependence of the required minimum oversampling factor upon the total number of OFDM channels simultaneously transmitted is identified in simple SSMF systems utilizing intensity modulation and direct detection. In such a system consisting of two OFDM channels, detailed numerical explorations are also undertaken of the impacts of major transceiver design aspects on the OOFDM-CAP transmission performance. These aspects include digital orthogonal filter characteristics, oversampling factors and OOFDM adaptability. It is shown that OOFDM-CAP not only allows the utilization of a minimum oversampling factor as low as 2, but also overcomes all fundamental limitations associated with conventional CAP modulation.

Digital Orthogonal Filter-Enabled Optical OFDM Channel Multiplexing for Software-Reconfigurable Elastic PONs

Flexible ROADMs for architecture on demand are proposed and investigated, for the first time, which offer DSP-enabled dynamic operations at wavelength, sub-wavelength and orthogonal sub-band levels, continuous add-channel frequency tuneablility, and 10dB drop-channel sensitivity improvements.

New ROADMs with DSP-enabled dynamic and flexible operations for elastic optical networks

Real-time OOFDM transceivers with software-controlled reconfigurability and adaptability are experimentally demonstrated, for the first time, utilising FPGA-based digital orthogonal filters. Impacts of transceiver's networking operations on BER performance are examined in 25km SSMF IMDD systems.

M. Bolea

Papers

Digital filter multiple access PONs with DSP-enabled software reconfigurability

Digital Orthogonal Filter-Enabled Optical OFDM Channel Multiplexing for Software-Reconfigurable Elastic PONs

Real-time experimental demonstrations of software reconfigurable optical OFDM transceivers utilizing DSP-based digital orthogonal filters for SDN PONs.

New ROADMs with DSP-enabled dynamic and flexible operations for elastic optical networks

Real-time Demonstrations of Software Reconfigurable Optical OFDM Transceivers Utilising DSP-based Digital Orthogonal Filters for Channel Multiplexing