Communication systems: An introduction to signals and noise in electrical communication

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

Coherent optical communication has been well established as the technology of choice for long haul and high bit rate communication systems since a decade ago. Recent technology advances and ongoing price erosion further open the window of opportunity for the application of coherent optical transmission technology in other domains. This paper describes in detail the capabilities, design and implementation of a coherent ultra dense WDM technology for optical metro and access networks. Its capabilities enable a number of attractive options, such as variable downstream bit rates from 150 Mbit/s up to 10 Gbit/s per user, embedded OTDR and the coexistence with legacy systems such as GPON, EPON, XGPON or RF-Video in optical distribution networks. Due to its flexibility and capacity, it is also suitable for deployments in metropolitan networks, as well as for mobile front-haul and back-haul applications.

Coherent Ultra Dense WDM Technology for Next Generation Optical Metro and Access Networks

Unlike ultralong coherent optical systems that seriously suffer from fiber nonlinearities, short-reach noncoherent systems such as data center interconnections, which utilize small, cheap, and low-bandwidth components, are sensitive to nonlinearities that are mainly produced by devices responsible for electrical signal amplification, modulation, and demodulation. One of the most promising schemes for these applications is the four-level pulse amplitude modulation format combined with intensity modulation and direct detection; however, it can be significantly degraded by linear and nonlinear intersymbol interference. Linear and nonlinear signal degradation can efficiently be handled by different types of equalizers. In many cases, the straightforward linear equalizer cannot lower the error rate at the acceptable level. Therefore, much stronger equalizers based on nonlinear models such as the Volterra series are proposed. Volterra filter that can also be orthogonalized by the Wiener model is well described in the existing literature, and, in this paper, we investigate the most critical points related to high-speed Volterra filter design and implementation. Several experiments are carried out in order to indicate filter requirements/complexity, acquisition, and stability. We also provide a simple guidance for filter complexity reduction and useful hints for channel acquisition.

Volterra and Wiener Equalizers for Short-Reach 100G PAM-4 Applications

In this paper, we will address the benefits of the coherent detection in future optical access networks. The scarcity of the optical spectrum, the required flexibility, and constant evolution of requirements highlight the effectiveness of coherent techniques toward the future passive optical networks (PON). A set of architectures for coherent optical access networks will be presented and the key attributes of each scenario will be investigated. In addition, as a basis to decrease the cost of the local oscillator (LO) at customer side, we experimentally investigate the possibility of using a low-cost laser as LO with real-time detection of a Nyquist-shaped differential quadrature phase-shift keying (DQPSK) signal using simple 8-bit digital signal processing (DSP) on a field-programmable gate array. Moreover, we experimentally derive a set of optimized parameters and their impact on the network operation for coherent ultradense wavelength-division multiplexing (UDWDM) systems. The balance between the number of channels, power budget, and dynamic power range will be evaluated. Furthermore, we demonstrate a reconfigurable real-time receiver DSP for future flexible UDWDM-PON systems applying the DQPSK and D8PSK modulation formats. By reviewing some of the motivations for this technology, such as flexibility, spectral efficiency, as well as compatibility with software-defined networking, we show that this technology is approaching the required maturity.

Coherent Access: A Review

In this paper, we numerically and experimentally demonstrate a bidirectional Terabit+ ultradense wavelength-division multiplexing (UDWDM) coherent passive optical network with Nyquist shaped 16-ary quadrature amplitude modulation, offering up to 10 Gb/s service capabilities per user/wavelength in a total spectrum of 12.8 nm over 40 km of standard single-mode fiber. This paper first demonstrates the capability of Nyquist pulse shaping to mitigate crosstalk arising from back-reflections and nonlinear effects in UDWDM networks with coherent transceivers. The latter part of the paper experimentally investigates the bidirectional transmission in terms of receiver sensitivity and nonlinear tolerance under different network transmission capacity conditions, e.g., number of users and bit rate per users.

Terabit+ (192 × 10 Gb/s) Nyquist Shaped UDWDM Coherent PON With Upstream and Downstream Over a 12.8 nm Band

Coherent ultra dense wavelength division multiplexing passive optical networks

We experimentally analyze an adaptive polarization-demultiplexing (PolDemux) technique based on the representation of the state of polarization of the signal in the Stokes space. The performance and convergence speed of the Stokes PolDemux are compared with the constant modulus algorithm, for dual polarization QPSK and 16QAM modulation formats in a system with a maximum polarization rotation of 6 kHz. Moreover, the applicability of the adaptive Stokes algorithm is tested in an ultradense wavelength-division multiplexing scenario over 80 km of standard single-mode fiber. The BER results show a very small power penalty confirming the suitability of the adaptive Stokes method in systems where the accumulated chromatic and polarization-mode dispersion are small, such as high capacity optical metro and access networks scenarios.

Experimental Assessment of the Adaptive Stokes Space-Based Polarization Demultiplexing for Optical Metro and Access Networks

We propose a novel and efficient multiplierless finite-impulse response (FIR)-based filter architecture for chromatic dispersion equalization (CDE) in coherent optical communication systems. After quantizing the FIR coefficients, we take advantage of the high multiplicity of their real and imaginary parts, employing the distributive property of multiplication over addition to sharply reduce the number of multiplication operations, obtaining the distributive FIR-CDE (D-FIR-CDE). Furthermore, the implementation of multiplication operations with shifts and additions allows us to obtain a multiplierless D-FIR-CDE (MD-FIR-CDE). The proposed equalizers are experimentally validated in a 100G polarization-multiplexed (PM)-QPSK long-haul optical link and compared against benchmark FIR-CDE and frequency-domain (FD)-CDE implementations. We demonstrate computational resources savings of over 99% in number of multiplication operations and 40% in number of additions, relatively to the FIR-CDE implementation. In addition, the D-FIR-CDE is also shown to compare favorably relatively to the most widely used FD-CDE, achieving significant gains both in terms of required chip area and latency: more than 99% and 30% fewer multipliers and additions, respectively, and a latency reduction of over 90%. We have also experimentally demonstrated that the performance penalty imposed by the coefficient quantization tends to decrease with increasing propagation length, rendering it as an attractive solution for efficient and high-performance chromatic dispersion compensation in long-haul optical fiber links.

Distributive FIR-Based Chromatic Dispersion Equalization for Coherent Receivers

We experimentally characterize a bidirectional 2.5 Gb/s UDWDM-PON system based on Nyquist shaped DQPSK with digital signal processing in real time. The optical distribution network power budget of this system is evaluated and in coexistence of UDWDM channels with upstream TWDM-PON and RF video overlay signals, a set of optimum parameters and their impact on the network operation is driven. Additionally, we report the first field-trial of bidirectional coherent Nyquist UDWDM-PON using commercial real-time FPGA-based transceivers, coexisting with the deployed GPON, RF video overlay, and NG-PON2 technologies. A −44.5 dBm receiver sensitivity is achieved for 64 × 2.5 Gb/s DQPSK downstream channels and a 17 dB tolerance in dynamic power range of upstream channels is performed.

Somayeh Ziaie

Papers

Terabit+ (192 × 10 Gb/s) Nyquist Shaped UDWDM Coherent PON With Upstream and Downstream Over a 12.8 nm Band

Coherent ultra dense wavelength division multiplexing passive optical networks

Experimental Assessment of the Adaptive Stokes Space-Based Polarization Demultiplexing for Optical Metro and Access Networks

Distributive FIR-Based Chromatic Dispersion Equalization for Coherent Receivers

Real-Time Bidirectional Coherent Nyquist UDWDM-PON Coexisting With Multiple Deployed Systems in Field-Trial