Advanced Digital Signal Processing Techniques for High-Speed Optical Links

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Maximizing the optical network capacity

Most of the digital data transmitted are carried by optical fibres, forming the great part of the national and international communication infrastructure. The information-carrying capacity of these networks has increased vastly over the past decades through the introduction of wavelength division multiplexing, advanced modulation formats, digital signal processing and improved optical fibre and amplifier technology. These developments sparked the communication revolution and the growth of the Internet, and have created an illusion of infinite capacity being available. But as the volume of data continues to increase, is there a limit to the capacity of an optical fibre communication channel? The optical fibre channel is nonlinear, and the intensity-dependent Kerr nonlinearity limit has been suggested as a fundamental limit to optical fibre capacity. Current research is focused on whether this is the case, and on linear and nonlinear techniques, both optical and electronic, to understand, unlock and maximize the capacity of optical communications in the nonlinear regime. This paper describes some of them and discusses future prospects for success in the quest for capacity

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

Phase estimation receiver for full-field detection: a novel receiver structure for electronic dispersion compensation of metropolitan area networks

The development of ultra high speed (~20 Gsamples/s) analogue to digital converters (ADCs), and the delayed deployment of 40 Gbit/s transmission due to the economic downturn, has stimulated the investigation of digital signal processing (DSP) techniques for compensation of optical transmission impairments. In the future, DSP will offer an entire suite of tools to compensate for optical impairments and facilitate the use of advanced modulation formats. Chromatic dispersion is a very significant impairment for high speed optical transmission. This thesis investigates a novel electronic method of dispersion compensation which allows for cost-effective accurate detection of the amplitude and phase of the optical field into the radio frequency domain. The first electronic dispersion compensation (EDC) schemes accessed only the amplitude information using square law detection and achieved an increase in transmission distances. This thesis presents a method by using a frequency sensitive filter to estimate the phase of the received optical field and, in conjunction with the amplitude information, the entire field can be digitised using ADCs. This allows DSP technologies to take the next step in optical communications without requiring complex coherent detection. This is of particular of interest in metropolitan area networks. The full-field receiver investigated requires only an additional asymmetrical Mach-Zehnder interferometer and balanced photodiode to achieve a 50% increase in EDC reach compared to amplitude only detection