Jen-Fa Huang

Optical CDMA network codecs structured with M-sequence codes over waveguide-grating routers

A compact optical code-division multiple-access (OCDMA) network coder-decoder (codec) of a spectral-amplitude-coding (SAC) scheme is presented. The scheme utilizes M-sequence-coded OCDMA codecs structured on arrayed-waveguide-grating (AWG) routers. By using the cyclic properties of AWG routers and M-sequence codes, the codecs pair can encode-decode multiple code words simultaneously, thus, each user can share the same hardware for coding process. The configuration not only preserves the ability of multiple-access interference cancellation in SAC-OCDMA system, but also results in a cheap system with reduced system complexity.

Fiber-grating-based optical CDMA spectral coding with nearly orthogonal M-sequence codes

An optical spectral coding scheme is proposed for fiber-optic code-division multiple access (FO-CDMA) communications. The spectral coding is based on the pseudo-orthogonality of FO-CDMA codes properly written in the fiber Bragg grating (FBG) devices. An FBG decoder is configured on the basis of orthogonal correlation functions of the nearly orthogonal maximal-length sequence codes. With such a design, an intended receiver user that operates on the defined orthogonal correlation functions will reject interfering users and obtain orthogonality between the FO-CDMA users in the system.

Optical-fiber surface-plasmon-resonance sensor employing long-period fiber gratings in multiplexing

A new type of optical-fiber surface-plasmon-resonance (SPR) sensor based on a thin metallic film and long-period fiber gratings for measuring small changes of refractive index of analyte is presented. This sensor simply employs a long-period fiber grating with a proper period to couple a core mode (HE11) to the copropagating cladding mode that can excite a surface-plasmon wave (SPW). The mainly theoretical base used to analyze this new structure is the unconjugated form of coupled-mode equations. In this new SPR sensor, the variation of the refractive index of analyte is determined by monitoring the change of the transmitted core mode power, which is calculated by unconjugated two-mode coupled-mode equations at a fixed wavelength. The numerical results have demonstrated that this new and simple configuration may be used as a highly sensitive amplitude sensor. As far as the excitation of SPW, the model of numerical simulation, and the complexity of measurement equipment are concerned, this new structure is superior to the proposed sensor, consisting of a bent polished single-mode SPR optical fiber. Furthermore, the structure can be easily adapted for a SPR fiber optical probe if a mirror is deposited on the fiber tip.

Two-dimensional M-matrices coding in spatial/frequency optical CDMA networks

A spatial/frequency encoder-decoder scheme is proposed for optical code-division multiple-access (OCDMA) communications. The coders adopt codewords with multiple-weight-per-row property and can be realized via compact hardware. With the proposed decoding scheme, an intended user will reject any interfering user and theoretically achieve quasiorthogonality between OCDMA users. The performance degradation due to various noise sources in previous discussed one-dimensional coding scheme can be improved.

Construction of quasi-cyclic LDPC codes from quadratic congruences

In this paper, we proposed a novel method for constructing quasi-cyclic low-density parity-check (QC-LDPC) codes based on circulant permutation matrices via a simple quadratic congruential equation. The main advantage is that QC-LDPC codes with a variety of block lengths and rates can be easily constructed with no cycles of length four or less. Simulation results show that the proposed QC-LDPC codes perform slight better than the random regular LDPC codes for short to moderate block lengths and have almost the same performance as Sridara-Fuja-Tanner codes.

Reductions of multiple-access interference in fiber-grating-based optical CDMA network

A fiber Bragg grating (FBG) encoder/decoder scheme based on correlation subtractions of nearly orthogonal M-sequence codes is presented. With proper coder design, a receiver can reject interfering users and obtain quasi-orthogonality between optical code-division multiple-access (CDMA) users in the network. However, optical CDMA networks may be degraded by multiple-access interference (MAI) due to nonflattened incoherent sources and nonideal FBG coders. A compensating module is therefore proposed to compensate for such MAI effects. As a result, the MAI effects induced by nonideal FBG coders can be perfectly eliminated by the compensating module. With spectral width reduction on the incoherent source, the scheme can partly compensate the MAI effects induced by nonflattened sources and further reduce the average error probability in the system performance.

Grey-based power control for DS-CDMA cellular mobile systems

The propagation channel of a mobile radio system exhibits severe signal shadowing and multipath fading, which results in wide variation of signal-to-interference ratio (SIR) at the receiver. To tackle this problem, power control is used to maintain the desired link quality and thus achieve higher capacity. In order to mitigate the channel variation effect precisely, a new application of grey theory to the power control strategy in the direct-sequence code-division multiple-access cellular mobile systems is introduced. This scheme aims to predict the SIR affected by the channel variation at the receiver and issue an appropriate control signal to the transmitter. The simulation results indicate that the grey-based scheme can offer less outage probability than the previous mechanisms.

Differentiated Service Provision in Optical CDMA Network Using Power Control

A simple scheme to use power control for differentiated service provision is proposed for the optical code-division multiple-access network. The main advantages of this scheme are: (1) Interference from other users can be eliminated theoretically. (2) The decoder with simple configuration can be used as compared to that in a previous power control scheme. (3) The design procedure of various service requirements in one network is simplified.

Compensating fiber gratings for source flatness to reduce multiple-access interferences in optical CDMA network coder/decoders

A fiber-Bragg-grating (FBG)-based optical code-division multiple-access (OCDMA) network coder/decoder (codec) is investigated for its interference suppression induced by nonflattened broad-band lightwave sources. Since each network user with different signature address code has different spectral distribution, the nonflattened light sources will cause multiple-access interference (MAI). Flatness compensation schemes are proposed to solve the MAI effects induced by nonflattened broad-band light sources. By arranging the same coding scheme but in different spectral coding band, spectral chips from FBG coder/compensator will incoherently power summed in the photodetectors to approach a more flattened power level. Signal-to-interference ratio (SIR) performances are evaluated with such compensation method for the discussed OCDMA network.

Permuted M-matrices for the reduction of phase-induced intensity noise in optical CDMA network

Two codeword families and the corresponding encoder/decoder schemes are present for spatial/frequency optical code-division multiple-access communications. These 2-D codewords have multiple weights per row and can be encoded/decoded via compact hardware. With the proposed decoding mechanism, the intended user will reject interfering users and multiple-access interference is fully eliminated. In addition, the power of the same wavelength contributed by all interfering codewords is split and detected by distinct photodiodes in the decoder. Thus the performance degradation due to the beat noise arising in the photodetecting process is improved, as compared with the traditional 1-D coding scheme, and a larger number of active users is supported under a given bit-error rate.