Jingshown Wu

Construction and performance analysis of variable-weight optical orthogonal codes for asynchronous optical CDMA systems

In this paper, two construction schemes of variable-weight optical orthogonal codes (OOCs) for asynchronous optical-code-division multiple-access (O-CDMA) systems are proposed. The first scheme uses pairwise balanced design (PBD), which is a research topic in combinatorial theory. PBD produces a family of blocks with unequal block size. Therefore, PBD can be used to construct OOCs with variable code weight. The lower bound of the code size of the codes from PBD is formulated in this paper. A second scheme employes packing design with a partition to generate blocks with unequal block sizes. The variable-weight OOCs can be constructed by partitioning a larger weight codeword into a family of codes with a smaller code weight. The upper bound and lower bound of the code size of the second scheme are discussed. The bit-error-rate (BER) performances of the two proposed codes are evaluated analytically in this paper. The simulation results show that the codes from the first scheme have the same BER performance as that of conventional code, while the second scheme has a larger maximum number of simultaneous users than that of conventional codes.

Perfect difference codes for synchronous fiber-optic CDMA communication systems

In this paper, we introduce the perfect difference sets and propose a synchronous fiber-optic code-division multiple-access system (CDMA) using these sets to generate the signature codes. Two interesting properties of these codes are discussed: (1) any two different codes are cyclic-shifted with each other; and (2) the cross correlation between any two different codes is exactly one. Thus, we may treat these codes as quasi-orthogonal codes. Using the first property of these codes, we can simplify design of the transmitter. In the receiver, we can use the second property to effectively eliminate the multiple-user interference (MUI) without reducing the number of usable codes. Based on the proposed transmitter and receiver, the system performance is derived. The numerical examples reveal that the proposed system can provide reliable communication even under heavy load. We believe that the proposed system using perfect difference codes outperforms any other synchronous fiber-optic CDMA systems.

A radio-over-fiber network for microcellular system application

This paper investigates the feasibility of a radio-over-fiber network, which employs a single high-power Nd:YAG laser shared among many microcells. External amplitude modulators are employed at both base station (BS) and radio ports (RPs) to form an optical fiber distribution network. The distortions induced by the uplink remodulation are analyzed. The optimal preamplifier gain in the uplink is estimated for given sensitivity and dynamic range (DR). Appropriate modulator linearization and thermal-noise suppression schemes are utilized together to improve the DR. The clipping distortion for the predistortion-linearized modulator is also considered. The result shows that the effects of clipping and higher order distortions have little impact on the estimation of the system performance. A numerical example shows that a 100-mW Nd:YAG laser can provide for a macrocell of 5-km radius up to 16 microcells and 1600 channels when a 20-dB modulator linearization and 10-dB thermal-noise suppression are employed. The experiment setup uses a two-tone test to verify the theoretical calculation. The measurement agrees with the theoretical estimation very well.

Optimal binary training sequence design for multiple-antenna systems over dispersive fading channels

Accurate and efficient channel estimation is important in multiple-antenna communication systems in order to effectively reduce the mutual interference among different transmitting antennas. For a nondispersive channel that is modeled by a single tap for each transmitting and receiving antenna pair, the well-known Hadamard sequences can be applied to estimate the channel coefficients. However, for a dispersive channel that has multipath problem and is modeled by multiple taps, the optimal sequences must have both good autocorrelations and cross correlations. The existence of binary sequences with such good property is an open problem. In this paper, we devise an algorithm to find these sequence sets. These codes can be applied in multiple-antenna systems.

Signal light amplification by stimulated Raman scattering in an N-channel WDM optical fiber communication system

The use of forward and backward Raman amplification in an N-channel wavelength-division-multiplexing (WDM) optical-fiber communication system is analyzed. Analytical expressions for the signals, the pumps, and the amplified spontaneous scattered power (ASSP) are presented. The crosstalk among the signal channels is analyzed in terms of system parameters. It is found that the crosstalk is negligible while low pump power is used and becomes significant if high pump power is used. The signal-to-ASSP ratio can be improved by increasing pump power; however, the improvement is small when pump power is already high. A 300-km repeater spacing with more than 20-dB signal-to-ASSP ratio is calculated. >

Spectral amplitude-coding optical CDMA system using Mach-Zehnder Interferometers

In this paper, we propose a family of newly constructed codes to suppress the phase-induced intensity noise (PIIN) in spectral amplitude-coding (SAC) optical code division multiple access (OCDMA) systems. These new codes are derived from modified prime codes and their cross-correlation is not larger than one. We also present a novel SAC-OCDMA system employing the new codes together with Mach-Zehnder interferometers to eliminate the multi-user interference (MUI). Compared with the systems employing modified quadratic congruence codes (MQC codes), numerical results verify that our proposed system can more effectively suppress the PIIN and eliminate MUI. Hence, the number of simultaneously users and total transmission rate increases significantly.

Noncoherent spatial/spectral optical CDMA system with two-dimensional perfect difference codes

In this paper, the family of newly constructed codes, named 2-D perfect difference codes, is proposed to suppress the phase-induced intensity noise (PIIN) in noncoherent spatial/spectral optical code division multiple access (OCDMA) systems. A novel spatial/spectral transceiver structure employing the new codes to eliminate the multiuser interference (MUI) by using the MUI cancellation property of the new codes is also presented. Compared with the systems employing modified quadratic congruence codes (MQC codes) and maximal-area matrices codes (M-matrices codes), numerical results verify that our proposed system can more effectively suppress the PIIN and eliminate MUI. Hence, the number of simultaneous users and total transmission rate increase significantly.

Optical orthogonal codes with nonideal cross correlation

For optical code division multiple access (OCDMA) networks, many optical orthogonal codes (OOCs) with ideal auto- and cross-correlation properties had been studied widely. In this paper, we relax the cross-correlation constraint slightly and propose a new code family based on perfect difference codes. Given the same code weight and code length, the size of new codes may increase 10 times more than that of ideal OOCs. Although the maximum cross correlation of new codes is larger than one, the cross correlation is less than or equal to one, for the most part. Consequently, the performance of new codes approaches that of ideal OOCs. Numerical results show that the performance of proposed codes was almost the same as that of conventional OOCs under the same code length and code weight.

Synchronous fiber-optic CDMA using hard-limiter and BCH codes

In this paper, synchronons code division multiple access (S/CDMA) for fiber-optic local area networks is considered. The performance of the fiber-optic S/CDMA network with negligible thermal and shot noises is interference limited. Here we derive the bit error rate of the S/CDMA system as a function of code length and number of active users, and the performance characteristics are also discussed. Furthermore, we analyze the performance of the fiber-optic S/CDMA system with an ideal optical hard-limiter, and the error probability with error control coding is also derived. In addition, the optimization between the S/CDMA and BCH codes of a constant bandwidth system is presented. The results show that by using an ideal hard-limiter in conjunction with BCH codes in this system, the influence of interference arising from other users can be greatly reduced, and the number of active users can also be increased significantly. >

A synchronous fiber-optic CDMA system using adaptive optical hardlimiter

In this paper, we propose a new synchronous fiber-optic code-division multiple-access (CDMA) system using adaptive optical hardlimiter and modified prime sequence code. At the receiver, the energy in one bit duration is estimated and used to adjust the threshold value of the adaptive optical hardlimiter which is placed after the optical correlator. The bit error probability of the system is analyzed with the consideration of thermal noise, shot noise, and dark current noise of the photodetector. The results show that this system can support a larger number of simultaneous users than other systems also using modified prime sequence code under the same bit error probability constraint.