Optical communications systems’ performance is limited by physical layer nonlinearities inherent in some of their transmitter components. In order to overcome some of these limitations, several approaches utilizing artificial neural networks to digitally pre-distort the transmitted signal have been proposed in the last few years, alongside other more classical models. However, their performance was strongly dependent on the specific training sequence used. This work improves the performance of neural network-based direct learning approach for digital pre-distortion applications. The proposed method, which is based on the curriculum learning approach, aims to construct a training paradigm of gradually increasing complexity based on the statistical properties of the input signal, in order to achieve better convergence of the network. We prove that the proposed method defines a proper curriculum regardless of the training data distribution, considering the proposed wide-sense curriculum conditions and the case of binary weighting functions. A comparative analysis examines a pre-distorter for a coherent optical transmitter modelled by a Long Short-Term Memory neural network. An improvement of 4.8dB in terms of normalized mean-square error is demonstrated for the case of a 3-bits linear quantizer, compared to the conventional training scheme.

Analog distortion compensation based on digital signal processing methods are widely applied for transmitter or receiver impairments in various digital communication systems. Recently, several neural network methods were developed for distortion compensation. However, while digital communication systems are typically complex-valued, neural networks are mostly designed to work with real-valued inputs. Thus, adaptations of the network architecture or input data should be applied. In this article a method for using a single-input real-valued neural network for digital communication-based complex-valued signals without any modifications to the neural network is proposed. The method transforms the complex-valued signal to a real-valued one by taking the real component of a complex frequency offset applied through digital up-conversion, without affecting the distortion, therefore allowing standard neural network-based functionality with significant reduction in size. The method is tested with a multi-layer perceptron and gated recurrent unit architectures applied over a generic Wiener-Hammerstein model for the case of equalization of a coherent optical system. A reduction of about 7% and 26% in network size is shown for the gated recurrent unit-based and multi-layer perceptron-based architectures respectively, without any significant change in performance. This size reduction capability shows the high potential in applying the proposed method in neural network-based equalization and pre-distortion operations.

Electrical dispersion compensation equalizer is a key and cost-effective element in optical communication on-off-keying systems in the presence of chromatic dispersion. Here, for the first time, to the best of the author's knowledge, an analytical solution is established for the electrical equalizer coefficients in an optical communication system. The solution is based on minimum mean-square error criterion. The analytical results show a perfect match with computer simulation. In addition BER performance comparison with the adaptive least mean square (LMS) method reveals that the analytical solution performs better due to LMS excess mean-square error.

In this paper we introduce a nonlinear equalizer using the radial basis function (RBF) network with decision feedback equalizer (DFE) for electronic dispersion compensation in optical communication systems with on-off-keying and a direct detection receiver. The RBF method introduces a non-linear equalization technique suitable for optical communication direct detection systems that include nonlinear transformation at the photodetector. A bit error rate performance comparison shows that the RBF with DFE out performs the RBF without DFE and achieves similar results provided by maximum likelihood sequence estimator.

Electrical dispersion compensation equalizer is a key and cost-effective element in optical communication OOK systems in the presence of chromatic dispersion. Here, for the first time, to the best of the authors' knowledge, an analytical solution is established for the electrical equalizer coefficients in an optical communication system. The solution is based on minimum mean-square error criterion. The analytical results show a perfect match with computer simulation. In addition BER performance comparison with the adaptive least mean square (LMS) method reveals that the analytical solution performs better due to LMS excess mean-square error.

The capacity of a discrete noisy channel is achieved when the transmission source is properly matched to the channel, in the sense that the distribution of the transmitted bits maximizes the channel's mutual information. Traditional source matching assumes independency between the source and the channel, while the case of source dependent channel is usually ignored. In this article, the source dependant channel of Optical Code Division Multiple Access (OCDMA), dominated by Multiple Access Interference (MAI) is analyzed. It is shown that source dependency of MAI reverses the expected behavior of the OCDMA channel, and also that for a given value of channel condition, or mutual information, the number of concurrent users can be increased when the source is properly matched to the channel. A simple technique, which allows combining source matching with practical bit stream methods such as synchronization, error correction codes and the utilization of different detection methods, is also presented.

We study the performances of several electrical dispersion compensation (EDC) equalizers in the presence of chromatic dispersion and polarization mode dispersion for optical coherent- and direct-detection on–off keying systems. The EDCs that are analyzed include decision-feedback equalizer, linear equalizer, and maximum-likelihood sequence estimator (MLSE). We present an inclusive quantitative analysis of the performance difference between the various techniques. The MLSE gives a good indication of the best possible performance.

We study the performances of several electrical dispersion compensation (EDC) equalizers in the presence of chromatic dispersion (CD) and polarization mode dispersion (PMD) for optical coherent and direct detection on-off keying systems. The EDCs that are analyzed include the decision-feedback equalizer, linear equalizer, and maximum-likelihood sequence estimator (MLSE). We present an inclusive quantitative analysis of the performance difference between the various techniques. The MLSE gives a good indication of the best possible performance

A generalized theoretical analysis of an AC-coupled fiber-optic burst-mode receiver is presented. This receiver is designed to operate in optical burst-switching networks using DC-balanced data coding such as 8B10B. Analytic expressions for the recovery time are derived as a function of the system dynamic range, power penalty, and data coding format. The theoretical calculations are verified with a detailed simulation. It is shown that locking time of the order of nanoseconds can be achieved with commercially available AC-coupled receivers. The burst-mode receiver can adapt to large (>10 dB) amplitude variations in 30 ns with a power penalty of 2 dB at 12.5 Gb/s. An overall optimization of the transmitter-receiver link-setup time is performed for an optical burst-switching network based on tunable laser transmitters. The dark interval during laser tuning (/spl sim/50 ns) is shown to have a beneficial impact on the receiver's response time, effectively reducing its locking time to a few nanoseconds, thus resulting in an overall link setup time of about 50 ns, limited by the laser's tuning time.

Virtual queueing is commonly deployed as a buffering technique in high-performance input-queued switch architectures. This paper presents the analysis for discrete-time virtual output queued switches with incoming traffic governed by a uniformly distributed Markov modulated on/off process, and geometrically distributed service times. We utilize the k-step first-passage time probability matrix to derive the probability generating function of the inter-arrival times distribution. Based on the latter, closed-form expressions for the queue size distribution and mean delay are obtained. The validity of the analysis is established through computer simulations.

In this paper a high-performance, robust and scalable scheduling algorithm for input-queued switches, called distributed sequential allocation (DISA), is presented and analyzed. Contrary to pointer-based arbitration schemes, the algorithm proposed is based on a synchronized channel reservation cycle whereby each input selects a designated output, considering both its local requests as well as global channel availability information. The distinctiveness of the algorithm is in its ability to offer high-performance when multiple cells are transmitted within each switching intervals. The subsequent relaxed switching-time requirement allows for the utilization of commercially available crosspoint switches, yielding a pragmatic and scalable solution for high port-density switching platforms. The efficiency of the algorithms and its robustness is established through analysis and computer simulations addressing diverse traffic scenarios.

This paper presents and analyzes a high-performance, robust, and scalable scheduling algorithm for input-queued switches called distributed sequential allocation (DISA). In contrast to pointer-based arbitration schemes, the proposed algorithm is based on a synchronized output reservation process, whereby each input selects a designated output while taking into consideration both local transmission requests and the availability of global resources. The distinctiveness of the algorithm lies in its ability to offer high performance when multiple cells are transmitted within each switching interval. Relaxed switching-time requirements allow for the incorporation of commercially available crosspoint switches. The result is a pragmatic and scalable solution for high port-density switching platforms. The efficiency of the scheme and its robustness in the presence of admissible traffic, without the need for speedup, is established through analysis and computer simulations. Performance results are shown for various traffic scenarios including nonuniform destination distribution, correlated arrivals and multiple classes of service.

The provision of quality-of-service (QoS) is a predominant requirement from modern high-performance switches and routers. Port densities of tens and even hundreds of ports characterize ATM and IPv6 switches where each port carries high-speed traffic with diverse statistical characteristics. In this paper a scheduling scheme for Tbit/sec, input-queued, WDM packet-switch networks is presented. The proposed architecture is contention-free, scalable, easy to implement and requires no internal "speedup". Moreover, the scheme inherently supports per-class QoS. Non-uniform destination distribution and bursty cell arrivals are studied for a switch with an aggregated capacity of 1 Tbit/sec. Simulation results show that class-differentiated low latency is achieved, yielding a powerful solution for high-performance packet-switch networks.

A new method for developing optical CDMA address code sequences is presented. The method is based on the genetic algorithm, which has already shown success in developing two-dimensional correlation functions for pattern recognition applications. The algorithm is very flexible in terms of optimization criteria, thus adequate for various optical CDMA applications, including coherent and non-coherent systems. Preliminary computer experimental results are presented, indicating very good discrimination ratios, and very fast convergence of the search process. Important implications include synchronization-independent CDMA system design, optical beat noise reduction, optical power budget improvement, and enhanced security.

STARNET is a broadband backbone optical wavelength-division multiplexing (WDM) local area network (LAN). Based on a physical passive star topology, STARNET offers all users two logical subnetworks: a high-speed reconfigurable packet-switched data subnetwork and a moderate-speed fix-tuned packet-switched control subnetwork. Thus, STARNET supports traffic with a wide range of speed and continuity characteristics. We report the analysis and implementation of an entire STARNET two-node network, from the optical to the computer layer, at the Optical Communications Research Laboratory (OCRL) of Stanford University. To implement the two logical subnetworks, we designed and implemented two different techniques: combined modulation and multichannel subcarrier multiplexing (MSCM). OCRL has already demonstrated several combined modulation techniques such as phase shift-keyed and amplitude shift-keyed (PSK/ASK), and differential phase shift-keyed and amplitude shift-keyed (DPSK/ASK), yielding combined ASK/DPSK modulation receiver sensitivities better than -32 dBm. OCRL has designed and implemented a high-speed high-performance packet-switched STARNET computer interface which enables high-throughput transfer to/from host computer, low latency switching, traffic prioritization, and capability of multicasting and broadcasting. With this interface board, OCRL has achieved average transmit and receive throughputs of 685 Mb/s and 571 Mb/s, respectively, out of the 800 Mb/s theoretical maximum of the host computer bus. The incurred packet latency due to the interface for a specified multihop network configuration has been simulated to be 24 /spl mu/s. Using simulation and experimental results, it is shown that STARNET is highly suitable for high-speed multimedia network applications.