Cheng-Yuan Chang

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).

Wavelength-hopping time-spreading optical CDMA with bipolar codes

Two-dimensional (2D) wavelength-hopping time-spreading coding schemes have been studied recently for supporting greater numbers of subscribers and simultaneous users than conventional one-dimensional (1D) approaches in optical code-division multiple-access (OCDMA) systems. To further improve both numbers without sacrificing performance, a new code design utilizing bipolar codes for both wavelength hopping and time spreading is studied and analyzed in this paper. A rapidly programmable, integratable hardware design for this new coding scheme, based on arrayed-waveguide gratings, is also discussed.

Spectral Efficiency Study of Two Multirate Schemes for Asynchronous Optical CDMA

Multirate transmission techniques, such as multiple-code (MC) and multicode-keying (MK) schemes, have been proposed for asynchronous optical code-division multiple-access (O-CDMA) systems supporting multimedia services. A user in the MC scheme transmits several codes at the same time to represent the transmission of several data bit ones. The number of parallel codes transmitted at a time is proportional to the users bit rate and the number of data bit ones. While the MK scheme only transmits one code at a time, each code represents a symbol and each transmitted symbol represents multiple data bits. Data rate is varied by changing the symbol size, which, in turn, changes the number of data bits per baud, similar to MODEM communications. In this paper, the spectral efficiencies (SEs) of these two asynchronous schemes are derived for the comparison of their performances and efficiencies as a whole. The SE of the conventional single-rate on-off-keying (OOK) O-CDMA scheme is also derived for a baseline comparison. Our numerical results show that the MC scheme always has a better SE than the single-rate OOK scheme, while the MK scheme can have a better SE than the single-rate OOK scheme under certain conditions.

A New Family of 2-D Codes for Fiber-Optic CDMA Systems With and Without the Chip-Synchronous Assumption

In this paper, we propose a new family of wavelength-time codes, which are based on one-dimensional optical orthogonal codes (1-D OOCs) of cross-correlation functions of at most two. By relaxing the maximum cross-correlation values to two, the new 2-D codes provide larger code cardinality for accommodating more subscribers and support heavier code weight for better code performance. It is known that the traditional chip-synchronous assumption used in the analyses of optical codes gives a pessimistic performance upper bound, while the newer chip-asynchronous assumption offers a more accurate analysis. The performance of the new 2-D codes is here analyzed under both assumptions for comparison. Under certain conditions, our results show that the new wavelength-time codes outperform our recently reported multiple-wavelength OOCs and 2-D codes, which were based on 1-D OOCs of cross-correlation functions of at most one and two, respectively.

Performance Analysis of Double-Weight Optical CDMA Under the Same-Bit-Power Assumption

To provide differential quality-of-service in optical code-division multiple access, variable-weight codes were proposed and analyzed under a conventional assumption of identical power in every optical pulses (i.e., same chip power). To account for power limit of some laser sources, we assume the use of same optical power in each bit duration (i.e., same bit power) in this paper. For illustration, we focus our study on double-weight codes under both same-chip-power and same-bit-power assumptions. A new and better approximate performance-analytical model for double-weight codes is developed and validated with computer simulation. We show that heavy-weight codes do not always perform better than light-weight codes under the same-bit-power assumption, contrary to previous findings under the same-chip-power assumption.

Frequency-Hopping CDMA With Reed--Solomon Code Sequences in Wireless Communications

In this paper, we propose the use of Reed-Solomon (RS) code sequences for modulation on top of a frequency-hopping code-division multiple access (FH-CDMA). Our analysis shows that this RS/FH-CDMA scheme supports higher data rate but worse performance than the conventional -ary frequency-shift-keying FH-CDMA scheme. These two schemes provide a tradeoff between the data rate and the performance.

Frequency-Hopping CDMA Wireless Communication Systems Using Prime Codes

In this paper, we propose a new fast frequency-hopping code-division multiple-access (FH-CDMA) scheme, in which prime sequences are used for modulation on top of FH-CDMA. The new prime/FH-CDMA scheme supports higher data rate than the conventional M-ary frequency-shift-keying FH-CDMA (MFSK/FH-CDMA) scheme, at the expense of worsening cross-correlation values. The performance and normalized spectral efficiency of both schemes are analyzed and compared. Our results show that the prime/FH-CDMA scheme, in general, utilizes similar system efficiency as the MFSK/FH-CDMA scheme and that both schemes provide a trade-off between data rate and performance

Multiple-wavelength optical orthogonal codes under prime-sequence permutations

In this paper, a new family of wavelength-time codes, called multiple-wavelength optical orthogonal codes (MWOOCs), is constructed. The new codes use prime sequences for wavelength permutations on top of a time-spreading OOC. The advantage is that the code cardinality is a quadratic function of the number of wavelengths. This property is particularly useful in high bit-rate optical CDMA applications, in which the number of time slots is very restricted. This limitation can be compensated and even improved in our MWOOCs by increasing the number of available wavelengths, through the use of lasers with broadened supercontinuum spectrum

Accurate Analysis of Double-Weight O-CDMA Scheme with the Same Bit-Power Assumption

The use of optical codes with variable weight has been considered as a desirable candidate to provide differential quality-of-service in optical code division multiple access (O-CDMA). It is because code weight plays a dominant role in determining the performance of O-CDMA codes as heavy-weight codes always outperform light-weight codes under the conventional assumption of identical power in every optical pulses (i.e., same chip power). In this paper, we assume the use of optical codes with same power in each bit duration (i.e., same bit power). In other words, heavy-weight codes have less power per chip than light-weight codes. For illustration, we focus our work on double-weight codes and compare the performance between heavy-weight and light-weight codes under both same-chip-power and same-bit-power assumptions. Most importantly, we develop a new and accurate analytical method to evaluate the performance of these double-weight codes. Validating with computer simulation, our results show that heavy-weight codes do not always perform better than light-weight codes under the same-bit-power assumption.

Study of MFSK/FH-CDMA wireless communication systems without symbol-synchronous assumption

We propose a more accurate method to analyze multilevel frequency-shift-keying frequency-hopping code-division multiple-access (MFSK/FH-CDMA) wireless communication systems using ldquoone-hitrdquo FH patterns, instead of random FH patterns. By also removing the over-optimistic symbol-synchronous assumption, the new method provides a more realistic performance analysis as the MFSK modulation may result in two hits even when one-hit FH patterns are used if symbol transmissions are not synchronized.