William J. Lennon

Design and performance analysis of wavelength/time (W/T) matrix codes for optical CDMA

Two-dimensional (2-D) codes for optical CDMA (OCDMA) are increasingly important because the code set size (cardinality) of such codes is large and the codes have good spectral efficiency, especially when compared to linear or direct sequence codes. As an example, the 2-D codes described in this paper (that use intensity modulation and direct detection, IM/DD) have a cardinality of 32, and their spectral efficiency is /spl sim/0.5 bit/s/Hz when a guard-time is used to avoid intersymbol interference. The cardinality is readily increased to 64-80, using the techniques described in the paper. The next best 2-D codes of comparable cardinality that use IM/DD tend to have a lower spectral efficiency (going like 1/K, where K is the cardinality) because they do not support multiple entries per row or per column of the code matrix. To improve on the spectral efficiency of the codes described in this paper, bipolar codes must be considered. Two-dimensional codes or matrices can be generated from pseudoorthogonal (PSO) sequences by means of simple quasigraphical operations. Important results of this construction are that both the cardinality and the spectral efficiency or information spectral density of the set of matrices is higher than that of the generating set of sequences. The matrices can be interpreted (implemented) as space/time (S/T) or wavelength/time (W/T) matrix codes for OCDMA applications. The resultant matrix codes are robust, have high information spectral density, and are effective wavelength multipliers. This paper describes the design and construction of the matrices; analyzes their performance from a communications viewpoint; describes their use as codes for the asynchronous, concurrent communication of multiple users; and analyzes the bit error rate performance based on capturing and modeling a typical network topology and performing a numerical modeling of the system.

High-performance optical CDMA system based on 2-D optical orthogonal codes

Optical code-division multiple access (OCDMA) is an interesting subject of research because of its potential to support asynchronous, bursty communications. OCDMA has been investigated for local area networks, access networks, and, more recently, as a packet label for emerging networks. Two-dimensional (2-D) OCDMA codes are preferred in current research because of the flexibility of designing the codes and their higher cardinality and spectral efficiency (SE) compared with direct sequence codes based on on-off keying and intensity modulation/direct detection, and because they lend themselves to being implemented with devices developed for wavelength-division-multiplexed (WDM) transmission (the 2-D codes typically combine wavelength and time as the two dimensions of the codes). This paper shows rigorously that 2-D wavelength/time codes have better SE than one-dimensional (1-D) CDMA/WDM combinations (of the same cardinality). Then, the paper describes a specific set of wavelength/time (W/T) codes and their implementation. These 2-D codes are high performance because they simultaneously have high cardinality (/spl Gt/10), per-user high bandwidth (>1 Gb/s), and high SE (>0.10 b/s/Hz). The physical implementation of these W/T codes is described and their performance evaluated by system simulations and measurements on an OCDMA technology demonstrator. This research shows that OCDMA implementation complexity (e.g., incorporating double hard-limiting and interference estimation) can be avoided by using a guard time in the codes and an optical hard limiter in the receiver.

Bit-error-rate analysis of a 16-user gigabit ethernet optical-CDMA (O-CDMA) technology demonstrator using wavelength/time codes

This letter describes a technology demonstrator for an incoherent optical code-division multiple-access scheme based on wavelength/time codes. The system supports 16 users operating at 1.25 Gsymbols/s/user while maintaining bit-error rate (BER) <10/sup -11/ for the correctly decoded signal. Experiments support previous simulations which show that coherent beat noise, occurring between the signal and multiple access interference, ultimately limits system performance.

Effects of Optical Layer Impairments on 2.5 Gb/s Optical CDMA Transmission

We conducted a computer simulation study to assess the effects of optical layer impairments on optical CDMA (O-CDMA) transmission of 8 asynchronous users at 2.5 Gb/s each user over a 214-km link. It was found that with group velocity dispersion compensation, two other residual effects, namely, the nonzero chromatic dispersion slope of the single mode fiber (which causes skew) and the non-uniform EDFA gain (which causes interference power level to exceed signal power level of some codes) degrade the signal to multi-access interference (MAI) ratio. In contrast, four wave mixing and modulation due to the Kerr and Raman contributions to the fiber nonlinear refractive index are less important. Current wavelength-division multiplexing (WDM) technologies, including dispersion management, EDFA gain flattening, and 3 rd order dispersion compensation, are sufficient to overcome the impairments to the O-CDMA transmission system that we considered.

Simple robust receiver structure for gigabit ethernet O-CDMA using matrix codes

We have developed an optical code division multiple access (O-CDMA) technology demonstrator (TD) based on two-dimensional (2-D) codes. The 2-D codes are derived from folded optimum Golomb rulers, implemented as wavelength/time /spl lambda//T codes. The objective of the research is to develop the science and technology for high-performance, low-cost, and robust O-CDMA applications. A key ingredient of the TD is a simple receiver structure that is based on hard-limiting after optical-to-electrical (O/E) conversion and time windowing with a conventional electronic D flip-flop. With the TD, we demonstrate six asynchronous users at less than 10/sup -11/ bit error rate (BER) and up to eight users at 10/sup -8/ BER, limited by saturation in the optical preamplifier.

First field trial of optical label-based switching and packet drop on a 477 km NTON/Sprint link

We have demonstrated for the first time in our knowledge optical label-based wavelength switching and packet drop over actual field fiber. We have shown SCM-formatted optical labeling transmission with less than 2 dB power penalty for both label and payload over 476 km of fiber. The optical label switching achieves packet drop and re-transmission of the remaining packet.

Remote experimental site: a command and analysis center for 'big physics' experimentation

The next-generation of tokamaks, ITER (International Thermonuclear Text Reactor) or BPX (Burning Plasma Experiment), will be characterized by an even greater emphasis on joint operation and experimentation. Preparations are being made for such shared facilities by developing a systematic approach to remote, joint physics operation involving experimental teams at several locations. The local area network of computers used for control and data acquisition on present and future experiments can be extended over a wide area network to provide a mechanism for remote operation of subsystems required for physics experiments. The technology required for high-bandwidth (>or=45-Mb/s) connections between multiple sites either exists or will be available over the next few years. With the rapid development of high-performance workstations, network interfaces, distributed computing, and video conferencing, one can proceed with the development of a system of control and analysis sites to provide for consistent, efficient, and continuing collaborations. Early establishment of such sites could also enhance existing joint design and development efforts.<<ETX>>

Optical CDMA (O-CDMA) technology demonstrator (TD) for 2D codes

A technology demonstrator (TD) based on wavelength/time codes, configured to multiplex and transmit 32 asynchronous gigabit Ethernet data flows (GbE over O-CDMA), is described. The TD is user and data rate scalable.

Bit-error-rate performance of a gigabit Ethernet O-CDMA technology demonstrator (TD)

An O-CDMA TD based on 2-D (wavelength/time) codes is described, with bit-error-rate (BER) and eye-diagram measurements given for eight users. Simulations indicate that the TD can support 32 asynchronous users.

Performance Impact of Multiple Access Interference in a 4-ary Pulse Position Modulated Optical Code Division Multiple Access (PPM/O-CDMA) System

An O-CDMA system utilizing 4-ary pulse position modulation and a novel virtual quadrant receiver is described. Bit-error-rates show the impact of multiple access interference, as influenced by its proximity to the correctly decoded signal.