Guu-Chang Yang

Optical orthogonal codes with unequal auto- and cross-correlation constraints

An optical orthogonal code (OOC) is a collection of binary sequences with good auto- and cross-correlation properties; they were defined by Salehi and others as a means of obtaining code-division multiple access on optical networks. Up to now, all work on OOCs have assumed that the constraint placed on the autocorrelation and that placed on the cross-correlation are the same. We consider-codes for which the two constraints are not equal. Specifically we develop bounds on the size of such OOCs and demonstrate constriction techniques for building them. The results demonstrate that a significant increase in the code size is possible by letting the autocorrelation constraint exceed the cross-correlation constraint. These results suggest that for a given performance requirement the optimal OOC may be one with unequal constraints. This paper also views OOCs with unequal auto- and cross-correlation constraints as constant-weight unequal error protection (UEP) codes with two levels of protection. The bounds derived are interpreted from this viewpoint. >

Multiple-wavelength optical orthogonal codes under prime-sequence permutations for optical CDMA

A new family of two-dimensional (2-D) wavelength-hopping time-spreading codes, which employs wavelength hopping algebraically under prime-sequence permutations on top of time-spreading optical orthogonal codes, is studied and analyzed. Different from other 2-D codes, our new codes allow the number of wavelengths and code length to be chosen independently and, at the same time, the code cardinality is a quadratic function of the number of wavelengths without sacrificing the maximum cross-correlation value (i.e., still at most one). They are particularly suitable for high bit-rate optical code-division multiple-access systems with broadband mode-locked lasers, in which the number of time slots is very limited, and system capacity can only be grown by increasing the number of wavelengths, rather than code length. Finally, a novel wavelength-aware detector for wavelength-hopping time-spreading codes is discussed and shown to provide improved code performance.

Design of multilength optical orthogonal codes for optical CDMA multimedia networks

To simultaneously support multimedia services with different signaling rates and quality-of-service requirements in optical code division multiple access (CDMA) networks, a new class of multilength, constant-weight optical orthogonal codes (OOCs) with good correlation properties is constructed algebraically in this paper. The performance of these new OOCs in an optical CDMA system with double-media services is analyzed. In contrast to conventional CDMA, our study shows that the performance of these multilength OOCs worsens as the code length increases, allowing prioritization in optical CDMA. Finally, an application of these multilength OOCs to integrate different types of multimedia services is briefly discussed.

Multiple-length extended carrier-hopping prime codes for optical CDMA systems supporting multirate multimedia services

Extended carrier-hopping prime codes (ECHPCs) with ideal correlation properties (i.e., zero autocorrelation sidelobes and cross-correlation values of at most 1) and significantly expanded cardinality were recently constructed for wavelength-hopping time-spreading optical code division multiple access (O-CDMA). To support multimedia services with different bit-rate requirements, a new family of multiple-length constant-weight ECHPCs with ideal correlation properties is constructed algebraically in this paper. The performance of these new codes in an O-CDMA multimedia system is analyzed. Contrary to conventional single-length codes, our study shows that the performance of these multiple-length codes improves as the code length decreases, thus supporting prioritization in O-CDMA.

Performance Analysis of Variable-Weight, Multilength Optical Codes for Wavelength-Time O-CDMA Multimedia Systems

To support multimedia services with different discrete bit-rate requirements, families of multilength optical codes, such as the carrier-hopping prime code (CHPC), extended CHPC, and multiwavelength optical orthogonal code, were recently constructed for wavelength-time optical code-division multiple-access (O-CDMA). In this paper, the performance of these multilength optical codes in a multimedia O-CDMA system with a variable-weight operation are analyzed. Our study shows that short-length codes generate stronger interference than long-length codes. This supports services prioritization in O-CDMA. Our study also shows that code weight is a more important factor than code length in determining code performance (i.e., quality of service).

2/sup n/ prime-sequence codes and coding architecture for optical code-division multiple-access

Recent study shows that optical code-division multiple-access (CDMA) networks cannot be evaluated or designed by only considering the performance (i.e., correlation properties) of the optical pseudo-orthogonal codes selected. The structures of optical encoders and decoders are another important factors to consider and are needed to coordinate with the selected optical codes as much as possible. A special family of 2/sup n/ codes, so-called 2/sup n/ prime-sequence codes, is constructed. A general theorem on the cardinality of the new codes is provided. The properties and performance of the codes are also studied. Since these codes pose the algebraic properties of both prime-sequence and 2/sup n/ codes, new optical encoding and decoding structures are designed to optimize the system parameters (e.g., power budget and cost) of these optical CDMA networks. This new configuration is particularly attractive for ultrafast optical processing and waveguide implementation for tile future high-capacity, low-loss, all-optical CDMA networks.

Extended carrier-hopping prime codes for wavelength-time optical code-division multiple access

A two-dimensional wavelength-hopping time-spreading coding scheme has been recently studied for optical code-division multiple access, owing to maturity in fiber-Bragg grating (FBG) and arrayed-waveguide grating (AWG) technologies. Most of the optical codes designed for the scheme were based on an assumption, which may not be true in high data-rate systems, that the number of chips is higher than the number of available wavelengths. To provide a flexible code design without the assumption, this paper studies and analyzes a new family of carrier-hopping prime codes with expanded cardinality and ideal correlation properties. Flexible implementation of programmable AWG- and FBG-based coding hardware by using the wavelength-shift property of the codes is discussed.

Determination of FWM products in unequal-spaced-channel WDM lightwave systems

Recently, unequal-spaced channel-allocation techniques have been studied and analyzed to reduce four-wave-mixing (FWM) crosstalk in high-capacity, long-haul wavelength-division multiplexing (WDM) transmission systems. It will be helpful to system designers if there exists a fast tool to measure the strength of FWM crosstalk in such WDM systems. In this paper, we develop two algorithms, using frequency difference triangles and frequency difference squares, that can determine the total numbers of FWM signals falling onto the operating band and each channel, respectively, of unequal-spaced WDM systems. By knowing these two numbers, one can adjust the system parameters, such as minimum channel spacing, in order to keep a good balance on the adverse effects of FWM crosstalk and interchannel interference, or avoid the assignment of channels at locations with the most severe crosstalk.

Performance analysis of extended carrier-hopping prime codes for optical CDMA

A new class of two-dimensional (2-D) carrier-hopping prime codes with very good correlation properties and significantly expanded cardinality was recently constructed for wavelength-hopping time-spreading optical code-division multiple access (CDMA). In this paper, the performance of the new codes is analyzed and compared to a carrier-grouping scheme of the original carrier-hopping prime codes. In addition, the traditional chip-synchronous assumption used in the performance analysis of optical CDMA is removed, for the first time, for 2-D optical codes with cross-correlation functions of at most one, showing improvement and better accuracy in error probabilities.

2D orthogonal spreading codes for multicarrier DS-CDMA systems

This paper presents two new constructions of two-dimensional (2D) spreading codes, here designated as 2D orthogonal variable spreading factor (OVSF) codes, for multicarrier direct-sequence code-division multiple-access (MC/DS-CDMA) systems. Orthogonality of these spreading codes is employed to improve the bandwidth efficiency and interference rejection capability of the DS-CDMA systems. The 2D OVSF codes are constructed by generalizing one-dimensional (1D) OVSF codes and they possess ideal correlation properties. As illustrated in this paper, these 2D OVSF cods preserve the orthogonality of different spreading factors in an OVSF code tree, supporting multimedia services with a variety of data rates.