Antonio J. Mendez

Temporal/spatial optical CDMA networks-design, demonstration, and comparison with temporal networks

The design and experimental results for a temporal/spatial (T/S) noncoherent optical code-division multiple access (CDMA), based on matrix codes, using a breadboard of passive multimode fiber-optic couplers and delay lines, is presented. It is shown that a T/S CDMA network allows for shorter bit times, given a set laser pulse width, compared to a temporal CDMA network. Also, T/S codes result in a reduction of autocorrelation sidelobes and cross-correlation peaks, and the T/S network has lower losses. It is also shown that the couplers are critical components in maintaining code integrity.<<ETX>>

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.

Strategies for realizing optical CDMA for dense, high-speed, long span, optical network applications

Since the mid 1990s, the role of optical CDMA has expanded from local area networks to longer span, telecommunication-type networks. In order to play a significant role in these longer span, denser, higher data rate networks, optical CDMA code set must (1) have at least as many codes as dense wavelength division multiplexing (WDM) (i.e., more than eight codes); (2) operate at high data rates (i.e., greater than 2.5 Gb/s); and (3) propagate with high fidelity over the installed or installable fiber links. Most approaches to optical CDMA require narrow pulses, which are more susceptible to fiber impairments and may have lower spectral efficiency than conventional WDM modulation schemes such as non-return-to-zero (NRZ), so they do not meet these new requirements. Therefore, we have formulated a strategy which simultaneously increases the number of good codes (resulting in higher density) and reduces their code length (i.e., decreasing the number of time slots required thus enabling higher data rates for a given chip time): the strategy of matrix codes. In this paper, we describe the design of a set of eight matrix codes for operation at 2.5 Gb/s and evaluate their propagation over an existing 214 km network link by means of computer simulation. The results indicate that the codes propagate well if dispersion management is used. The paper also discusses a strategy for managing the multiaccess interference (MAI) in a bursty traffic environment.

Fiber-optic digital video multiplexing using optical CDMA

The use of laser pulsing in the form of code-division multiple access (CDMA) to multiplex a set of digital video signals over a fiber network is proposed. The laser pulsing is used to generate unique code sequence addresses which identify each source, and the video data are modulated on these sequences. Digital encoding using pulse position modulation (PPM) is proposed, having advantages that permit efficient integration of optical and electronic processing. Data recovery is achieved by optical correlation for sequence recognition, followed by standard electronic PPM decoding. The relation between key system parameters of the encoding, decoding, and optics is derived, and indicates that compressed video rates between 5 and 20 Mb/s/channel, with 25-50 channels, is possible with relatively standard hardware. Experimental breadboard results being carried out at USC on this system indicate the feasibility of the CDMA concept for fiber multiplexing.

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.

Synthesis and demonstration of high speed, bandwidth efficient optical code division multiple access (CDMA) tested at 1 Gb/s throughput

Code division multiple access (CDMA) permits concurrent communication over all virtual channels (in principle), independent of the data rate and the network size. In reality, most CDMA approaches have a bandwidth penalty due to the code length and a loss penalty due to the broadcasting required by CDMA. Both of these penalties can be reduced or ameliorated by means of multi-attribute coding. Matrices constructed from relatively inefficient (0,1) pulse sequences are suitable multi-attribute non-coherent CDMA codes which are both bandwidth and broadcast efficient. We exhibit a novel approach to synthesizing matrix CDMA codes, develop a 4/spl times/4 physical model, and demonstrate concurrent communication experimentally at concurrent data rates of 100-, 150-, and 250 Mb/s per port.<<ETX>>

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.

Pulse combining and time-space coding for multiple accessing with fiber arrays

Summary form only given. The results of theoretical and experimental studies evaluating the use of pulse sequences for code-division multiple accessing for networking, data distribution, and various applications are summarized. Removal of phase noise effects is of prime importance, as they can be much more severe than the multiple accessing interference. One study has shown that the phase noise effect depends on the source linewidth, the accuracy of the correlator design, and the sequence pulse widths and that the latter should be about 3-5 times the laser coherence time. Electronic correlators avoid the phase noise problem and are generally superior to optical correlators provided accurate bit timing is maintained and the data rates are not excessive.<<ETX>>

Code division multiple access (CDMA) enhancement of wavelength division multiplexing (WDM) systems

Wavelength division multiplexing (WDM) is a mainstream technology for optical communications ranging from long haul applications to interconnections between and within high speed digital systems. WDM is, however, constrained (1) in density by the crosstalk associated with channel spacing and (2) by total number of channels which can be uniformly amplified by optical amplifiers. This number of effective WDM channels can be enhanced by overlaying code division multiple access (CDMA) on each WDM channel. The authors show by analysis and example that the number of effective WDM/CDMA hybrid channels exceeds the number of effective WDM channels, and with a reduced WDM crosstalk penalty.

Bit-parallel wavelength links for high-performance computer networks

This paper describes a new interconnect and local area network transmission concept for computer communications based on spectrally encoding one or more computer words into a wavelength datagram. At physical and data link level, this system resembles an optical ribbon cable, except that all the bits pass on one fiber optic waveguide. At the network level, such fiber optic link segments can be interconnected all-optically using 2x2 optical switches into ShuffleNet or other architectures that permit a photonic packet to pass from source to destination without being incumbered with the extra delay and bandlimiting associated with electronic switching and regeneration. Unique properties of such a system include low latency (<10ns), very high bandwidth (<100Gbit/s per port), precise time alignment (<10ps) of the individual word bits over km distances, and dynamic scalability to support cluster computing and distributed supercomputing. Novel system elements disclosed in this paper include: (J) a bit parallel wavelength (BPW) fiber optic link that uniquely maintains wavelength channel time alignment, (2) an innovative parallel stepped wavelength optical transmitter that time synchronizes each laser diode element at its optical output, (3) a spectral encoder/decoder that adds fault tolerance and optical message addressing capability, and (4) a technique for transmitting and maintaining time aligned multi-X solitons as parallel bits through fiber media. Applications to teraflop high performance parallel computing and DoD input/output (I/O) bound applications are described.