Real-time Digital Signal Processing for Software-defined Optical Transmitters and Receivers

A software-defined optical Tx is designed and demonstrated generating signals with various formats and pulse-shapes in real-time. Special pulse-shapes such as OFDM or Nyquist signaling were utilized resulting in a highly efficient usage of the available fiber channel bandwidth. This was achieved by parallel data processing with high-end FPGAs. Furthermore, highly efficient Rx algorithms for carrier and timing recovery as well as for polarization demultiplexing were developed and investigated

On the Probabilistic Shaping and Geometric Shaping in Optical Communication Systems

We introduce and compare typical shaping schemes suitable for optical communications. The geometrically shaped-quadrature amplitude modulation (GS-QAM) formats are characterized by the non-equidistant spacing of constellation points, transmitted uniformly, and applied to improve system capacity. On the other hand, the well-known constant composition distribution matcher (CCDM) is applied for the generation of probabilistically shaped QAM (PS-QAM) formats. Mutual information (MI) is used as a metric to analyze the performances of regular/GS/PS-MQAM formats. In a linear amplified spontaneous emission noise limited region, it can be proved by the numerical simulation that MI performances of the GS-8/16QAM are always better than regular 8/16QAM and PS-8/16QAM; the largest shaping gains can be separately reached by PS-32QAM and GS-32QAM. We continue with the experimental demonstration on the 16QAM-based transmission system, and find that GS-16QAM generally has the best MI performance. We also find that the modulation-dependent nonlinear noises of the GS-8/16/32QAM are comparable to that of the regular 8/16/32QAM and generally lower than PS-8/16/32QAM. By using the enhanced Gaussian noise model, we observe that the GS-8/16QAM formats have better performances than regular 8/16QAM and PS-8/16QAM over multi-span transmission. Meanwhile, PS-32QAM formats provide superior performance over a relatively long transmission distance.MURI Program, Office of Naval Research (ONR) [N00014-13-1-0627]Open access journal.This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Advanced optimization algorithms for sensor arrays and multi-antenna communications

Optimization problems arise frequently in sensor array and multi-channel signal processing applications. Often, optimization needs to be performed subject to a matrix constraint. In particular, unitary matrices play a crucial role in communications and sensor array signal processing. They are involved in almost all modern multi-antenna transceiver techniques, as well as sensor array applications in biomedicine, machine learning and vision, astronomy and radars. In this thesis, algorithms for optimization under unitary matrix constraint stemming from Riemannian geometry are developed. Steepest descent (SD) and conjugate gradient (CG) algorithms operating on the Lie group of unitary matrices are derived. They have the ability to find the optimal solution in a numerically efficient manner and satisfy the constraint accurately. Novel line search methods specially tailored for this type of optimization are also introduced. The proposed approaches exploit the geometrical properties of the constraint space in order to reduce the computational complexity. Array and multi-channel signal processing techniques are key technologies in wireless communication systems. High capacity and link reliability may be achieved by using multiple transmit and receive antennas. Combining multi-antenna techniques with multicarrier transmission leads to high the spectral efficiency and helps to cope with severe multipath propagation. The problem of channel equalization in MIMO-OFDM systems is also addressed in this thesis. A blind algorithm that optimizes of a combined criterion in order to be cancel both inter-symbol and co-channel interference is proposed. The algorithm local converge properties are established as well

Delta-Sigma Digitization and Optical Coherent Transmission of DOCSIS 3.1 Signals in Hybrid Fiber Coax Networks

We first demonstrate delta-sigma digitization and coherent transmission of data over cable system interface specification (DOCSIS) 3.1 signals in a hybrid fiber coax (HFC) network. Twenty 192-MHz DOCSIS 3.1 channels with modulation up to 16384QAM are digitized by a low-pass cascade resonator feedback (CRFB) delta-sigma analog-to-digital converter (ADC) and transmitted over 80 km fiber using coherent single-λ 128-Gb/s dual-polarization (DP)-QPSK and 256-Gb/s DP-16QAM optical links. Both one-bit and two-bit delta-sigma digitization are implemented and supported by the QPSK and 16QAM coherent transmission systems, respectively. To facilitate its practical application in access networks, the coherent system is built using a low-cost narrowband optical modulator and RF amplifiers. Modulation error ratio (MER) larger than 50 dB is successfully demonstrated for all 20 DOCSIS 3.1 channels, and high order modulation up to 16384QAM is delivered over fiber for the first time in HFC networks. The raw DOCSIS data capacity is 54 Gb/s with net user information ~45 Gb/s. Moreover, the bit error ratio (BER) tolerance is evaluated by measuring the MER performance as BER increases. Negligible MER degradation is observed for BER up to 1.5 × 10−6 and 1.7 × 10−4, for one-bit and two-bit digitization, respectively

Algorithms for Blind Equalization Based on Relative Gradient and Toeplitz Constraints

Blind Equalization (BE) refers to the problem of recovering the source symbol sequence from a signal received through a channel in the presence of additive noise and channel distortion, when the channel response is unknown and a training sequence is not accessible. To achieve BE, statistical or constellation properties of the source symbols are exploited. In BE algorithms, two main concerns are convergence speed and computational complexity. In this dissertation, we explore the application of relative gradient for equalizer adaptation with a structure constraint on the equalizer matrix, for fast convergence without excessive computational complexity. We model blind equalization with symbol-rate sampling as a blind source separation (BSS) problem and study two single-carrier transmission schemes, specifically block transmission with guard intervals and continuous transmission. Under either scheme, blind equalization can be achieved using independent component analysis (ICA) algorithms with a Toeplitz or circulant constraint on the structure of the separating matrix. We also develop relative gradient versions of the widely used Bussgang-type algorithms. Processing the equalizer outputs in sliding blocks, we are able to use the relative gradient for adaptation of the Toeplitz constrained equalizer matrix. The use of relative gradient makes the Bussgang condition appear explicitly in the matrix adaptation and speeds up convergence. For the ICA-based and Bussgang-type algorithms with relative gradient and matrix structure constraints, we simplify the matrix adaptations to obtain equivalent equalizer vector adaptations for reduced computational cost. Efficient implementations with fast Fourier transform, and approximation schemes for the cross-correlation terms used in the adaptation, are shown to further reduce computational cost. We also consider the use of a relative gradient algorithm for channel shortening in orthogonal frequency division multiplexing (OFDM) systems. The redundancy of the cyclic prefix symbols is used to shorten a channel with a long impulse response. We show interesting preliminary results for a shortening algorithm based on relative gradient

Advanced Digital Signal Processing Techniques for High-Speed Optical Links

L'abstract è presente nell'allegato / the abstract is in the attachmen

Cost-Effective Spectrally-Efficient Optical Transceiver Architectures for Metropolitan and Regional Links

The work presented herein explores cost-effective optical transceiver architectures for access, metropolitan and regional links. The primary requirement in such links is cost-effectiveness and secondly, spectral efficiency. The bandwidth/data demand is driven by data-intensive Internet applications, such as cloud-based services and video-on-demand, and is rapidly increasing in access and metro links. Therefore, cost-effective optical transceiver architectures offering high information spectral densities (ISDs > 1(b/s)/Hz) need to be implemented over metropolitan distances. Then, a key question for each link length and application is whether coherent- or direct (non-coherent) detection technology offers the best cost and performance trade-off. The performance and complexity limits of both technologies have been studied. Single polarization direct detection transceivers have been reviewed, focusing on their achievable ISDs and reach. It is concluded that subcarrier modulation (SCM) technique combined with single sideband (SSB) and high-order quadrature amplitude modulation (QAM) signaling, enabled by digital signal processing (DSP) based optical transceivers, must be implemented in order to exceed an ISD of 1 (b/s)/Hz in direct-detection links. The complexity can be shifted from the optical to the electrical domain using such transceivers, and hence, the cost can be minimized. In this regard, a detailed performance comparison of two spectrally-efficient direct detection SCM techniques, namely Nyquist-SCM and OFDM, is presented by means of simulations. It is found out that Nyquist-SCM format offers the transmission distances more than double that of OFDM due to its higher resilience to signal-signal beating interference. Following this, dispersion-precompensated SSB 4- and 16-QAM Nyquist-SCM signal formats were experimentally demonstrated using in-phase and quadrature (IQ)-modulators at net optical ISDs of 1.2 and 2 (b/s)/Hz over 800 km and 323 km of standard single-mode fibre (SSMF), respectively. These demonstrations represent record net optical ISDs over such distances among the reported single polarization wavelength division multiplexed (WDM) systems. Furthermore, since the cost-effectiveness is crucial, the optical complexity of Nyquist-SCM transmitters can be significantly reduced by using low-cost modulators and high-linewidth lasers. A comprehensive theoretical study on SSB signal generation using IQ- and dual-drive Mach-Zehnder modulators (DD-MZMs) was carried out to assess their performance for WDM direct detection links. This was followed by an experimental demonstration of WDM transmission over 242 km of SSMF with a net optical ISD of 1.5 (b/s)/Hz, the highest achieved ISD using a DD-MZM-based transmitter. Following the assessment of direct detection technology using various transmitter designs, cost-effective simplified coherent receiver architectures for access and metro networks have been investigated. The optical complexity of the conventional (polarization- and phase-diverse) coherent receiver is significantly simplified, i.e., consisting of a single 3 dB coupler and balanced photodetector, utilizing heterodyne reception and Alamouti polarization-time block coding. Although the achievable net optical ISD is halved compared to a conventional coherent receiver due to Alamouti coding, its receiver sensitivity provides significant gain over a direct detection receiver at M-ary QAM formats where M ≥16

High Spectral Efficiency Fiber-Optic Transmission Systems Using Pilot Tones

Modern fiber-optic communication systems combine state-of-the-art components with powerful digital signal processing (DSP) to maximize the system spectral efficiency (SE). Systems rely on wavelength-division multiplexing, including superchannel transmission, to enable transmission over the available bandwidth which reaches about 10 THz when accounting for the so-called C and L bands. A superchannel is a set of densely packed wavelength channels viewed as a single unit. By treating the channels together, they can be packed more closely than what is normally feasible and sharing of resources among the channels within the superchannel can be considered. In this thesis we focus on the special case of superchannels formed using coherent optical frequency combs. A frequency comb is a multi-wavelength light source and comb-based superchannels consists of channels which are modulated on lines originating from a common comb. Frequency combs have phase-locked carriers, meaning that in contrast to the standard case of independent lasers, the channels within a comb-based superchannel are locked on a frequency grid. Moreover, it implies that the carrier offsets originating from a non-ideal laser source are shared among all comb lines.Shared carrier offsets can be exploited to reduce the complexity of the DSP used to effectively recover the data. A frequency comb is fully characterized by knowing the state of two of its lines, meaning that if this information is transferred to the receiver, one could compensate carrier offsets for all wavelength channels within the superchannel. By transmission of optical pilot tones, self-homodyne detection of a 50x20Gbaud PM-64QAM superchannel is demonstrated with 4% spectral overhead. While two tones are required to fully phase-lock two combs, a single tone is enough to enable significant relaxation of the DSP-requirements while at the same time requiring minimal additional complexity compared to standard intradyne systems. Superchannel transmission using a single shared pilot tone is demonstrated by transmission of a 51x24Gbaud PM-128QAM superchannel with a resulting SE of 10.3bits/s/Hz. The single pilot scheme is also evaluated for distances up to 1000km showing high robustness to both noise and fiber nonlinearities. Finally, the high gain low overhead combination of the single pilot-tone scheme was used in a record demonstration reaching a SE of 11.5bits/s/Hz for fully loaded C-band transmission

Fiber-on-Chip: Digital Emulation of Channel Impairments for Real-Time DSP Evaluation

We describe the Fiber-on-Chip (FoC) approach to verification of digital signal processing (DSP) circuits, where digital models of a fiber-optic communication system are implemented in the same hardware as the DSP under test. The approach can enable cost-effective long-term DSP evaluations without the need for complex optical-electronic testbeds with high-speed interfaces, shortening verification time and enabling deep bit-error rate evaluations. Our FoC system currently contains a digital model of a transmitter generating a pseudo-random bitstream and a digital model of a channel with additive white Gaussian noise, phase noise and polarization-mode dispersion. In addition, the FoC system contains digital features for real-time control of channel parameters, using low-speed communication interfaces, and for autonomous real-time analysis, which enable us to batch multiple unsupervised emulations on the same hardware. The FoC system can target both field-programmable gate arrays, for fast evaluation of fixed-point logic, and application-specific integrated circuits, for accurate power dissipation measurements