Design tradeoffs and challenges in practical coherent optical transceiver implementations

This tutorial discusses the design and ASIC implementation of coherent optical transceivers. Algorithmic and architectural options and tradeoffs between performance and complexity/power dissipation are presented. Particular emphasis is placed on flexible (or reconfigurable) transceivers because of their importance as building blocks of software-defined optical networks. The paper elaborates on some advanced digital signal processing (DSP) techniques such as iterative decoding, which are likely to be applied in future coherent transceivers based on higher order modulations. Complexity and performance of critical DSP blocks such as the forward error correction decoder and the frequency-domain bulk chromatic dispersion equalizer are analyzed in detail. Other important ASIC implementation aspects including physical design, signal and power integrity, and design for testability, are also discussed.Fil: Morero, Damián Alfonso. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas, Físicas y Naturales; Argentina. ClariPhy Argentina S.A.; ArgentinaFil: Castrillon, Alejandro. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas, Físicas y Naturales; ArgentinaFil: Aguirre, Alejandro. ClariPhy Argentina S.A.; ArgentinaFil: Hueda, Mario Rafael. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Estudios Avanzados en Ingeniería y Tecnología. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Instituto de Estudios Avanzados en Ingeniería y Tecnología; ArgentinaFil: Agazzi, Oscar Ernesto. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas, Físicas y Naturales; Argentina. ClariPhy Argentina S.A.; Argentin

Compensation of Laser Phase Noise Using DSP in Multichannel Fiber-Optic Communications

One of the main impairments that limit the throughput of fiber-optic communication systems is laser phase noise, where the phase of the laser output drifts with time. This impairment can be highly correlated across channels that share lasers in multichannel fiber-optic systems based on, e.g., wavelength-division multiplexing using frequency combs or space-division multiplexing. In this thesis, potential improvements in the system tolerance to laser phase noise that are obtained through the use of joint-channel digital signal processing are investigated. To accomplish this, a simple multichannel phase-noise model is proposed, in which the phase noise is arbitrarily correlated across the channels. Using this model, high-performance pilot-aided phase-noise compensation and data-detection algorithms are designed for multichannel fiber-optic systems using Bayesian-inference frameworks. Through Monte Carlo simulations of coded transmission in the presence of moderate laser phase noise, it is shown that joint-channel processing can yield close to a 1 dB improvement in power efficiency. It is further shown that the algorithms are highly dependent on the positions of pilots across time and channels. Hence, the problem of identifying effective pilot distributions is studied.The proposed phase-noise model and algorithms are validated using experimental data based on uncoded space-division multiplexed transmission through a weakly-coupled, homogeneous, single-mode, 3-core fiber. It is found that the performance improvements predicted by simulations based on the model are reasonably close to the experimental results. Moreover, joint-channel processing is found to increase the maximum tolerable transmission distance by up to 10% for practical pilot rates.Various phenomena decorrelate the laser phase noise between channels in multichannel transmission, reducing the potency of schemes that exploit this correlation. One such phenomenon is intercore skew, where the spatial channels experience different propagation velocities. The effect of intercore skew on the performance of joint-core phase-noise compensation is studied. Assuming that the channels are aligned in the receiver, joint-core processing is found to be beneficial in the presence of skew if the linewidth of the local oscillator is lower than the light-source laser linewidth.In the case that the laser phase noise is completely uncorrelated across channels in multichannel transmission, it is shown that the system performance can be improved by applying transmitter-side multidimensional signal rotations. This is found by numerically optimizing rotations of four-dimensional signals that are transmitted through two channels. Structured four-dimensional rotations based on Hadamard matrices are found to be near-optimal. Moreover, in the case of high signal-to-noise ratios and high signal dimensionalities, Hadamard-based rotations are found to increase the achievable information rate by up to 0.25 bits per complex symbol for transmission of higher-order modulations

Digital Compensation of Transmission Impairments in Multi-Subcarrier Fiber Optic Transmission Systems

Time and again, fiber optic medium has proved to be the best means for transporting global data traffic which is following an exponential growth trajectory. Rapid development of high bandwidth applications since the past decade based on cloud, virtual reality, 5G and big data to name a few have resulted in a sudden surge of research activities across the globe to maximize effective utilization of available fiber bandwidth which until then was supporting low speed (< 10Gbps) services. To this end, higher order modulation formats together with multicarrier super channel based fiber optic transmission systems have proved to enhance spectral efficiency and achieve multi tera-bit per second bit rates. However, spectrally efficient systems are extremely sensitive to transmission impairments stemming from both optical devices and fiber itself. Therefore, such systems mandate the use of robust digital signal processing (DSP) to compensate and/or mitigate the undesired artifacts. The central theme of this research is to propose and validate few efficient DSP techniques to compensate specific impairments as delineated in the next three paragraphs. For short reach data center and passive optical network related applications which adopt direct detection, a single optical amplifier is generally used to meet the power budget requirements in order to achieve the desired receiver sensitivity or bit error ratio (BER). Semiconductor Optical Amplifier (SOA) with its small form factor is a low-cost power booster that can be designed to operate in any desired wavelength and more importantly can be integrated with other electro-optic components. However, saturated SOAs exhibit nonlinear amplification that introduce distortions on the amplified signal. Alongside SOA, the photodiode also introduces nonlinear mixing among the signal subcarriers in the form of Signal-Signal Beat Interference (SSBI). In this research, we study the impact of SOA nonlinearity on the effectiveness of SSBI compensation in a direct detection OFDM based transmission system. We experimentally demonstrate a digital compensation technique to undo the SOA nonlinearity effect by digitally backpropagating the received signal through a virtual SOA with inverse gain characteristics, thereby effectively eliminating the SSBI. With respect to transmission sources, laser technology has made some significant strides especially in the domain of multiwavelength sources such as quantum dot passive mode-locked laser (QD-PMLL) based optical frequency combs. In the present research work, we characterize the phase dynamics of comb lines from a QD-PMLL based on a novel multiheterodyne coherent detection technique. The inherently broad linewidth of comb lines which is on the order of tens of MHz make it difficult for conventional digital phase noise compensation algorithms to track the large phase noise especially for low baud rate subcarriers using higher cardinality modulation formats. In the context of multi-subcarrier, Nyquist pulse shaped, superchannel transmission system with coherent detection, we demonstrate through measurements and numerical simulations an efficient phase noise compensation technique called “Digital Mixing” that operates using a shared pilot tone exploiting mutual phase coherence among the comb lines. For QPSK and 16 QAM modulation formats, digital mixing provided significant improvement in BER performance in comparison to conventional phase tracking algorithms. Coherent solutions for regional and long haul systems make use of in-line optical amplifiers to compensate fiber loss. Ideally, distributed amplification based on stimulated Raman effect offers enhanced optical signal to noise ratios (OSNR) compared to lumped amplification using erbium doped fiber amplifiers and semiconductor optical amplifiers. However, this benefit of enhanced OSNRs in distributed Raman amplification is offset by the transfer of intensity noise of pump laser on to both signal’s phase and intensity, resulting in performance degradation. In this work, we propose and experimentally validate a practical pilot aided relative phase noise compensation technique for forward pumped distributed Raman amplified, digital subcarrier multiplexed coherent transmission systems

Advanced DSP Techniques for High-Capacity and Energy-Efficient Optical Fiber Communications

The rapid proliferation of the Internet has been driving communication networks closer and closer to their limits, while available bandwidth is disappearing due to an ever-increasing network load. Over the past decade, optical fiber communication technology has increased per fiber data rate from 10 Tb/s to exceeding 10 Pb/s. The major explosion came after the maturity of coherent detection and advanced digital signal processing (DSP). DSP has played a critical role in accommodating channel impairments mitigation, enabling advanced modulation formats for spectral efficiency transmission and realizing flexible bandwidth. This book aims to explore novel, advanced DSP techniques to enable multi-Tb/s/channel optical transmission to address pressing bandwidth and power-efficiency demands. It provides state-of-the-art advances and future perspectives of DSP as well

Ultra-Wideband Secure Communications and Direct RF Sampling Transceivers

Larger wireless device bandwidth results in new capabilities in terms of higher data rates and security. The 5G evolution is focus on exploiting larger bandwidths for higher though-puts. Interference and co-existence issues can also be addressed by the larger bandwidth in the 5G and 6G evolution. This dissertation introduces of a novel Ultra-wideband (UWB) Code Division Multiple Access (CDMA) technique to exploit the largest bandwidth available in the upcoming wireless connectivity scenarios. The dissertation addresses interference immunity, secure communication at the physical layer and longer distance communication due to increased receiver sensitivity. The dissertation presents the design, workflow, simulations, hardware prototypes and experimental measurements to demonstrate the benefits of wideband Code-Division-Multiple-Access. Specifically, a description of each of the hardware and software stages is presented along with simulations of different scenarios using a test-bench and open-field measurements. The measurements provided experimental validation carried out to demonstrate the interference mitigation capabilities. In addition, Direct RF sampling techniques are employed to handle the larger bandwidth and avoid analog components. Additionally, a transmit and receive chain is designed and implemented at 28 GHz to provide a proof-of-concept for future 5G applications. The proposed wideband transceiver is also used to demonstrate higher accuracy direction finding, as much as 10 times improvement

Digital Linearization of High Capacity and Spectrally Efficient Direct Detection Optical Transceivers

Metropolitan area networks are experiencing unprecedented traffic growth. The provision of information and entertainment supported by cloud services, broadband video and mobile technologies such as long-term evolution (LTE) and 5G are creating a rapidly increasing demand for bandwidth. Although wavelength division multiplexing (WDM) architectures have been introduced into metro transport networks to provide significant savings over single-channel systems, to cope with the ever-increasing traffic growth, it is urgently required to deploy higher data rates (100 Gb/s and beyond) for each WDM channel. In comparison to dual-polarization digital coherent transceivers, single-polarization and single photodiode-based direct-detection (DD) transceivers may be favourable for metropolitan, inter-data centre and access applications due to their use of a simple and low-cost optical hardware structure. Single sideband (SSB) quadrature amplitude modulation (QAM) subcarrier modulation (SCM) is a promising signal format to achieve high information spectral density (ISD). However, due to the nonlinear effect termed signal-signal beat interference (SSBI) caused by the square-law detection, the performance of such SSB SCM DD systems is severely degraded. Therefore, it is essential to develop effective and low-complexity linearization techniques to eliminate the SSBI penalty and improve the performance of such transceivers. Extensive studies on SSB SCM DD transceivers employing a number of novel digital linearization techniques to support high capacity (≥ 100 Gb/s per channel) and spectrally-efficient (net ISD > 2 b/s/Hz) WDM transmission covering metropolitan reach scenarios (up to 240 km) are described in detail in this thesis. Digital modulation formats that can be used in DD links and the corresponding transceiver configurations are firstly reviewed, from which the SSB SCM signalling format is identified as the most promising format to achieve high data rates and ISDs. Following this, technical details of the digital linearization approaches (iterative SSBI cancellation, single-stage linearization filter and simplified non-iterative SSBI cancellation, two-stage linearization filter, Kramers-Kronig scheme) considered in the thesis are presented. Their compensation performance in a dispersion pre-compensated (Tx-EDC) 112 Gb/s per channel 35 GHz-spaced WDM SSB 16-QAM Nyquist-SCM DD system transmitting over up to 240 km standard single-mode fibre (SSMF) is assessed. Net ISDs of up to 3.18 b/s/Hz are achieved. Moreover, we also show that, with the use of effective digital linearization techniques, further simplification of the DD transceivers can be realized by moving electronic dispersion compensation from the transmitter to the receiver without sacrificing performance. The optical ISD limit of SSB SCM DD system finally explored through experiments with higher-order modulation formats combined with effective digital linearization techniques. 168 Gb/s per channel WDM 64-QAM signals were successfully transmitted over 80 km, achieving a record net optical ISD of 4.54 b/s/Hz. Finally, areas for further research are identified

Control de Errores en Sistemas de Comunicaciones Digitales de Alta Velocidad

Tesis (DCI)--FCEFN-UNC, 2017En los últimos 10 años, la industria de las comunicaciones ópticas ha migrado hacia esquemas de receptores coherentes utilizando conversores de señal analógica a digital (ADC, del inglés Analog-to-Digital Converter) de alta velocidad. Esto ha permitido la incorporación de códigos correctores de errores con decodificación por decisiones blandas con el objetivo de extender el alcance y la eficiencia espectral de los enlaces disponibles. Los códigos correctores de errores con decodificación por decisiones blandas tales como los turbo códigos y los de chequeo de paridad de baja densidad (LDPC, del inglés Low Density Parity Check), y en especial estos últimos, se han expandido por las más diversas áreas y constituyen en la actualidad el estándar en varias normas, tales como 10GBase-T, DVB-S2, y Mobile WiMax (802.16e). Sin embargo, en comunicaciones ópticas generalmente se los trata como algo ajeno al sistema y por lo tanto en la mayoría de la bibliografía relacionada no se han tenido en cuenta todos los efectos residuales del canal a la entrada del decodificador para evaluar su desempeño. En cambio se utiliza como métrica solo la tasa de error umbral a partir de la cual el esquema de corrección de errores empieza a corregir. Por tal motivo en la presente Tesis se aborda el tema de códigos correctores de errores con decodificación por decisiones blandas aplicados a comunicaciones ópticas coherentes con un enfoque integral. En el presente trabajo se proponen diferentes técnicas para hacer lo más efectivas posibles las transmisiones empleando códigos correctores de errores de gran ganancia frente a problemas específicos de comunicaciones ópticas como son el ruido de fase del láser, las fluctuaciones de frecuencia del láser y las no linealidades de la fibra. Se trabaja también con constelaciones multinivel tales como 16-QAM que permiten obtener una buena eficiencia espectral. Con el fin de abarcar el mayor espacio de soluciones posible, se proponen cuatro esquemas diferentes pero complementarios teniendo en cuenta las condiciones expuestas previamente. Por un lado, se propone el empleo de modulación diferencial debido a su robustez frente a variaciones en la fase y se propone una técnica para disminuir su penalidad. Por otro lado, para esquemas no diferenciales se plantea un algoritmo de compensación de desplazamientos de fase, los cuales son un fenómeno catastrófico presente en los esquemas de estimación de fase utilizados en comunicaciones ópticas. Otra de las propuestas consiste en la utilización de constelaciones híbridas intercaladas temporalmente con el propósito de generar una referencia de fase absoluta que permita evitar el problema de los desplazamientos de fase. Por último, se propone un esquema conjunto de detección y decodificación iterativo que permite realizar la estimación del ruido de fase y las fluctuaciones de frecuencia de manera conjunta con la decodificación demostrando gran robustez. También se presenta en esta Tesis una posible arquitectura de implementación del esquema conjunto iterativo que permite reducir la complejidad del mismo, incorporando una novedosa clase de códigos LDPC denominados espacialmente acoplados. Debido a su estructura estos códigos permiten disminuir la latencia de decodificación y mantener la complejidad del decodificador acotada. Resultados de simulación demuestran la eficacia del sistema propuest