S. P. Majumder

Impact of fiber chromatic dispersion on the BER performance of an optical CDMA IM/DD transmission system

A theoretical analysis is presented to evaluate the impact of fiber chromatic dispersion on the bit-error-rate (BER) performance of a direct sequence optical code-division multiple-access (OCDMA) system with intensity modulation direct detection transmission link using sequence inverse keying optical correlator operating at 10 Gchip/s. The numerical results show that the system suffers a power penalty of 3.48 dB corresponding to chromatic dispersion index, /spl gamma/=0.3 for seven-chip m-sequence, at a BER of 10/sup -9/. It is further noticed that the number of simultaneous users are limited to 14 for /spl gamma/=0.1 and 10 for /spl gamma/=0.3, for 31-chip gold sequence, at 6-dB power penalty.

BER estimation of optical WDM RZ-DPSK systems through the differential phase Q

The optical return-to-zero differential phase-shift keying system is analyzed to determine the accuracy of the differential phase Q method in estimating the bit-error rate (BER). It is found that it consistently underestimates the BER though it successfully predicts the qualitative behavior of single-channel and wavelength-division-multiplexed systems for back-to-back and point-to-point configurations. A simple modification reduced the underestimation and produced accurate results.

Adaptive resource allocation based on modified Genetic Algorithm and Particle Swarm Optimization for multiuser OFDM systems

Adaptive resource allocation is one of the most challenging tasks for multiuser orthogonal frequency division multiplexing (OFDM) systems. In this paper, two evolutionary approaches, genetic algorithm (GA) and particle swarm optimization (PSO) have been applied for adaptive subcarrier and bit allocations to minimize the overall transmit power of a multiuser OFDM system. Each user will be assigned a number of subcarriers with at least one minimum subcarrier even at the worst case. Then the number of bits and the transmit power level for each subcarrier are calculated. Simulation results show that both the evolutionary approaches outperform the conventional static resource allocation schemes considerably in multiuser scenario. The results further reveal that both the algorithms can handle large allocation of subcarriers without significant performance degradation. However the performance of PSO is found to be better than the GA in terms of execution time, simplicity and convergence.

All-optical wavelength conversion based on cross-phase modulation (XPM) in a single-mode fiber and a Mach-Zehnder interferometer

An all-optical wavelength converter based on cross-phase modulation in a single-mode fiber (SMF) and a Mach-Zehnder interferometer followed by a Fabry-Perot filter is analyzed. The performance results evaluated at a bit rate of 10 Gb/s with normal and dispersion-shifted fiber (DSF) with optical filtering show that wavelength conversion is possible without significant reduction in conversion efficiency over a wavelength separation of approximately 10 nm between the input pump-and-probe signals. For pump-and-probe input powers of 10 mW each, the output converted power is found to be -6.5 dBm for normal fiber corresponding to zero wavelength separation and the corresponding output power for wavelength separation of 2 nm is -11 dBm. It is also found that the DSF converter provides higher efficiency and output power compared to the conventional SMF.

Performance of linecoded optical heterodyne FSK systems with nonuniform laser FM response

A simple theoretical analysis for evaluating the performance of linecoded optical frequency shift keying (FSK) systems is presented. It accounts for the combined effects of laser phase noise, receiver noise, and nonuniform BM response of distributed feedback lasers. A close form expression for the random frequency noise due to the combined effect of laser nonuniform FM response and phase noise is developed. The analysis is carried out for three different linecoding schemes, i.e., alternate mark inversion, Miller code or delay modulation, and Manchester coding, to investigate the efficacy of the line coding schemes in counteracting the effect of nonflat FM response. Theoretical and simulation results show that the sensitivities of linecoded FSK systems are within 0.7 and 0.4 dB for single-branch and dual-branch detection, respectively, at a bit error probability of 10/sup -9/ relative to the random non-return to zero FSK with flat FM response. >

Contention Problem in Optical Burst Switching Network

Optical burst switching (OBS) is a promising switching technology for next-generation Internet backbone networks. One of the key problems hindering the realization of optical burst switching (OBS) technology in the core networks is the losses due to contention among the bursts at the core nodes. Wavelength Conversion is an effective contention resolution technique used to reduce the number of bursts loss. In this paper, we investigate the problem of burst loss in optical burst switching (OBS) networks. The main goal of this work is to minimize burst loss rate in the network by using wavelength converters and by increasing number of channel wavelengths.

Performance analysis on phase-encoded OCDMA communication system in dispersive fiber medium

The performance of an asynchronous phase-encoded optical code-division multiple-access system is evaluated in a dispersive fiber medium. We derive an approximate analytical expression for the root mean square width of the phase-encoded signal (pseudorandom optical signal with low intensity) propagating in linear dispersive fibers. Bit-error rate analysis of the system is performed in the case of both ordinary single-mode fiber and dispersion-shifted fiber (DSF). The numerical results demonstrate that even though system performance improves due to the smaller width of initial Gaussian optical pulse, the effect from dispersion is higher. Larger code length reduces the effect of dispersion and the use of DSF greatly increases the transmission distance.

Effect of polarization mode dispersion on optical heterodyne CPFSK system

An analytical approach is developed to determine the impact of signal phase distortion due to polarization mode dispersion (PMD) on the bit error rate (BER) performance of optical heterodyne continuous phase FSK (CPFSK) system with delay demodulation receiver. The computed results at a bit rate of 10 Gb/s show that the CPFSK system suffer a significant amount of power penalty due to the effect of PMD. At a modulation index of 0.5, the penalty at a BER of 10/sup -9/ is found to be 1.0 dB, 1.8 dB and 2.8 dB when the differential group delay is 10 ps, 50 ps and 100 ps respectively for fiber length of 100 km. At modulation index of 1, the penalty is approximate 3.75 dB for differential group delay of 10 ps.

Limitations of Soliton Transmission in a Cascaded Optical Amplifier System

The performance limitations of a soliton transmission link imposed by accumulated optical, amplifiers spontaneous emission (ASE) noise is presented. An analytical approach is developed to determine the impact of accumulated ASE-noise, dispersion, and nonlinearity on soliton propagation. The results are evaluated by computer simulation using beam propagation method for a dispersion shifted fiber. The amplified ASE noise is modelled as a zero mean Gaussian random process and the effects of ASE, dispersion and self-phase modulation are presented in terms of the signal to noise ratio at the output of the receiver filter for several values of the soliton peak power levels. The results show that the output soliton pulse is highly degraded due to the combined influence of ASE, SPM and dispersion and there is a drastic reduction in the output signal to noise ratio when the soliton pulse width is of the order of 20 ps or less with a peak power of the order of 5 mW or more.

Performance Improvement of IM-DD Free Space Optical CDMA (Attenuated by Strong Atmospheric Turbulence) with Maximal Ratio Combining

In this paper a completely new analytical approach to the performance evaluation of turbulence induced fading on free space optical (FSO) CDMA communication system is presented. The current analysis develops an expression of average SINR of FSO CDMA system using switched inversed keyed(SIK) correlator transmitter in presence of turbulence induced fading and multiple access interference(MAI). Link performance is evaluated in terms of average bit error rate which is numerically computed. The performance improvement over atmospheric turbulence fading channel is proposed by means of applying diversity using rake correlation receiver and output of rake receiver is combined using maximal ratio combining (MRC) scheme. It has been shown in numerical evaluation that BER gets improved in diversity reception than only line of sight (LOS) reception. Numerical result also shows that, applying path diversity mitigates multiple access interference with less received power. Finally, the performance is evaluated using two different sequences (Gold and Kasami ) and from performance evaluation, it is shown that KASAMI sequence exhibits less bit error performance than GOLD sequence.