Phase-conjugated pilots for fibre nonlinearity compensation in CO-OFDM transmission

In this paper, we demonstrate a novel fiber nonlinearity compensation technique for coherent optical orthogonal frequency-division multiplexing (CO-OFDM) systems based on the transmission of phase-conjugated pilots (PCPs). In this scheme, a portion of OFDM subcarriers (up to 50%) is transmitted with its phase conjugates, which is used at the receiver to estimate the nonlinear distortions in the respective subcarriers and other subcarriers, which are not accompanied by PCPs. Simulation and experimental results show that by varying the PCP overhead, a performance improvement up to 4 dB can be achieved. In addition, the proposed technique can be effectively applied in both single polarization and polarization-division multiplexed systems, in both single channel and wavelength-division multiplexing systems, thus, offering highest flexibility in implementations

Optical and digital phase conjugation techniques for fiber nonlinearity compensation

We discuss recent progress on the use of optical and digital phase conjugation techniques for nonlinearity compensation in optical fiber links. We compare the achievable performance gain of phase conjugated twin wave applied in two polarization states and time segments with mid-link optical phase conjugation and digital back propagation. For multicarrier transmission scheme such as orthogonal frequency division multiplexing, two recently proposed schemes, namely phase-conjugated pilots and phase-conjugated subcarrier coding are reviewed

A survey on fiber nonlinearity compensation for 400 Gbps and beyond optical communication systems

Optical communication systems represent the backbone of modern communication networks. Since their deployment, different fiber technologies have been used to deal with optical fiber impairments such as dispersion-shifted fibers and dispersion-compensation fibers. In recent years, thanks to the introduction of coherent detection based systems, fiber impairments can be mitigated using digital signal processing (DSP) algorithms. Coherent systems are used in the current 100 Gbps wavelength-division multiplexing (WDM) standard technology. They allow the increase of spectral efficiency by using multi-level modulation formats, and are combined with DSP techniques to combat the linear fiber distortions. In addition to linear impairments, the next generation 400 Gbps/1 Tbps WDM systems are also more affected by the fiber nonlinearity due to the Kerr effect. At high input power, the fiber nonlinear effects become more important and their compensation is required to improve the transmission performance. Several approaches have been proposed to deal with the fiber nonlinearity. In this paper, after a brief description of the Kerr-induced nonlinear effects, a survey on the fiber nonlinearity compensation (NLC) techniques is provided. We focus on the well-known NLC techniques and discuss their performance, as well as their implementation and complexity. An extension of the inter-subcarrier nonlinear interference canceler approach is also proposed. A performance evaluation of the well-known NLC techniques and the proposed approach is provided in the context of Nyquist and super-Nyquist superchannel systems.Comment: Accepted in the IEEE Communications Surveys and Tutorial

Advanced digital signal processing for coherent optical OFDM transmissions

Coherent optical orthogonal frequency division multiplexing (CO-OFDM) has been actively considered as a potential candidate for long-haul transmission and 400 Gb/s to 1 Tb/s Ethernet transport because of its high spectral efficiency, efficient implementation, flexibility and robustness against linear impairments such as chromatic dispersion and polarization mode dispersion. However, due to the long symbol duration and narrow subcarrier spacing, CO-OFDM systems are sensitive to laser phase noise and fibre nonlinearity induced penalties. As a result, the development of CO-OFDM transmission technology crucially relies on efficient techniques to compensate for the laser phase noise and fibre nonlinearity impairments. In this thesis, high performance and low complexity digital signal processing techniques for laser phase noise and fibre nonlinearity compensation in CO-OFDM transmissions are demonstrated. For laser phase noise compensation, three novel techniques, namely quasipilot-aided, decision-directed-free blind and multiplier-free blind are introduced. For fibre nonlinear compensation, two novel techniques which are referred to as phase conjugated pilots and phase conjugated subcarrier coding, are proposed. All these abovementioned digital signal processing techniques offer high performances and flexibilities while requiring relatively low complexities in comparison with other existing phase noise and nonlinear compensation techniques. As a result of the developments of these digital signal processing techniques, CO-OFDM technology is expected to play a significant role in future ultra-high capacity optical network. In addition, this thesis also presents preliminary study on nonlinear Fourier transform based transmission schemes in which OFDM is a highly suitable modulation format. The obtained result paves the way towards a truly flexible nonlinear wave-division multiplexing system that allows the current nonlinear transmission limitations to be exceeded

Digital signal processing techniques for fiber nonlinearity compensation in coherent optical communication systems

The capacity of long-haul coherent optical communication systems is limited by the detrimental effects of fiber Kerr nonlinearity. The power-dependent nature of the Kerr nonlinearity restricts the maximum launch power into the fiber. That results in the reduction of the optical signal-to-noise ratio at the receiver; thereby, the maximum transmission reach is limited. Over the last few decades, several digital signal processing (DSP) techniques have been proposed to mitigate the effects of fiber nonlinearity, for example, digital back-propagation (DBP), perturbation based nonlinearity compensation (PB-NLC), and phase-conjugated twin wave (PCTW). However, low-complexity and spectrally efficient DSP-based fiber nonlinearity mitigation schemes for long-haul transmission systems are yet to be developed. In this thesis, we focus on the computationally efficient DSP-based techniques that can help to combat various sources of fiber nonlinearity in long-haul coherent optical communication systems. With this aim, we propose a linear time/polarization coded digital phase conjugation (DPC) technique for the mitigation of fiber nonlinearity that doubles the spectral efficiency obtained in the PCTW technique. In addition, we propose to investigate the impact of random polarization effects, like polarization-dependent loss and polarization mode dispersion, on the performance of the linear-coded DPC techniques. We also propose a joint technique that combines single-channel DBP with the PCTW technique. We show that the proposed scheme is computationally efficient and achieves similar performance as multi-channel DBP in wavelength division multiplexed superchannel systems. The regular perturbation (RP) series used to analytically approximate the solution of the nonlinear Schrödinger equation (NLSE) has a serious energy divergence problem when truncated to the first-order. Recent results on the transmission of high data-rate optical signals reveal that the nonlinearity compensation performance of the first-order PB-NLC technique decreases as the product of the transmission distance and launch power increases. The enhanced RP (ERP) method can improve the accuracy of the first-order RP approximation by partially solving the energy divergence problem. On this ground, we propose an ERP-based nonlinearity compensation technique to compensate for the fiber nonlinearity in a polarization-division multiplexed dispersion unmanaged optical communication system. Another possible solution to improve the accuracy of the PB-NLC technique is to increase the order of the RP solution. Based on this idea, we propose to extend the first-order solution of the NLSE to the second-order to improve the nonlinearity compensation performance of the PB-NLC technique. Following that, we investigate a few simplifying assumptions to reduce the implementation complexity of the proposed second-order PB-NLC technique

Visible Light Communication (VLC)

Visible light communication (VLC) using light-emitting diodes (LEDs) or laser diodes (LDs) has been envisioned as one of the key enabling technologies for 6G and Internet of Things (IoT) systems, owing to its appealing advantages, including abundant and unregulated spectrum resources, no electromagnetic interference (EMI) radiation and high security. However, despite its many advantages, VLC faces several technical challenges, such as the limited bandwidth and severe nonlinearity of opto-electronic devices, link blockage and user mobility. Therefore, significant efforts are needed from the global VLC community to develop VLC technology further. This Special Issue, “Visible Light Communication (VLC)”, provides an opportunity for global researchers to share their new ideas and cutting-edge techniques to address the above-mentioned challenges. The 16 papers published in this Special Issue represent the fascinating progress of VLC in various contexts, including general indoor and underwater scenarios, and the emerging application of machine learning/artificial intelligence (ML/AI) techniques in VLC

Decentralized Ultra-Reliable Low-Latency Communications through Concurrent Cooperative Transmission

Emerging cyber-physical systems demand for communication technologies that enable seamless interactions between humans and physical objects in a shared environment. This thesis proposes decentralized URLLC (dURLLC) as a new communication paradigm that allows the nodes in a wireless multi-hop network (WMN) to disseminate data quickly, reliably and without using a centralized infrastructure. To enable the dURLLC paradigm, this thesis explores the practical feasibility of concurrent cooperative transmission (CCT) with orthogonal frequency-division multiplexing (OFDM). CCT allows for an efficient utilization of the medium by leveraging interference instead of trying to avoid collisions. CCT-based network flooding disseminates data in a WMN through a reception-triggered low-level medium access control (MAC). OFDM provides high data rates by using a large bandwidth, resulting in a short transmission duration for a given amount of data. This thesis explores CCT-based network flooding with the OFDM-based IEEE 802.11 Non-HT and HT physical layers (PHYs) to enable interactions with commercial devices. An analysis of CCT with the IEEE 802.11 Non-HT PHY investigates the combined effects of the phase offset (PO), the carrier frequency offset (CFO) and the time offset (TO) between concurrent transmitters, as well as the elapsed time. The analytical results of the decodability of a CCT are validated in simulations and in testbed experiments with Wireless Open Access Research Platform (WARP) v3 software-defined radios (SDRs). CCT with coherent interference (CI) is the primary approach of this thesis. Two prototypes for CCT with CI are presented that feature mechanisms for precise synchronization in time and frequency. One prototype is based on the WARP v3 and its IEEE 802.11 reference design, whereas the other prototype is created through firmware modifications of the Asus RT-AC86U wireless router. Both prototypes are employed in testbed experiments in which two groups of nodes generate successive CCTs in a ping-pong fashion to emulate flooding processes with a very large number of hops. The nodes stay synchronized in experiments with 10 000 successive CCTs for various modulation and coding scheme (MCS) indices and MAC service data unit (MSDU) sizes. The URLLC requirement of delivering a 32-byte MSDU with a reliability of 99.999 % and with a latency of 1 ms is assessed in experiments with 1 000 000 CCTs, while the reliability is approximated by means of the frame reception rate (FRR). An FRR of at least 99.999 % is achieved at PHY data rates of up to 48 Mbit/s under line-of-sight (LOS) conditions and at PHY data rates of up to 12 Mbit/s under non-line-of-sight (NLOS) conditions on a 20 MHz wide channel, while the latency per hop is 48.2 µs and 80.2 µs, respectively. With four multiple input multiple output (MIMO) spatial streams on a 40 MHz wide channel, a LOS receiver achieves an FRR of 99.5 % at a PHY data rate of 324 Mbit/s. For CCT with incoherent interference, this thesis proposes equalization with time-variant zero-forcing (TVZF) and presents a TVZF receiver for the IEEE 802.11 Non-HT PHY, achieving an FRR of up to 92 % for CCTs from three unsyntonized commercial devices. As CCT-based network flooding allows for an implicit time synchronization of all nodes, a reception-triggered low-level MAC and a reservation-based high-level MAC may in combination support various applications and scenarios under the dURLLC paradigm

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